Anja Olafsen Sjåstad

• Afif, Abdulla Bin; Ivashenko, Oleksii; Kolstad, Alexandra Jahr & Sjåstad, Anja Olafsen (2026). Strain-Driven Honeycomb Reconstruction of Multilayered Rh/Pt(111). ACS Applied Materials & Interfaces. ISSN 1944-8244. 18(21), s. 30446–30455. doi: 10.1021/acsami.5c25647. Fulltekst i vitenarkiv
• Lunde, Vilde G. S.; Fjellvåg, Øystein S.; Döring, Allan M.; Straßheim, Marc; Pomjakushin, Vladimir & Skokov, Konstantin P. [Vis alle 12 forfattere av denne artikkelen] (2026). Influence of Ni substitution on phase transitions and magnetocaloric effect of NdCo2 at cryogenic temperatures. Physical review B (PRB). ISSN 2469-9950. 113(18). doi: 10.1103/drsr-yt3j. Fulltekst i vitenarkiv
• Pavlyuk, Volodymyr; Kordan, Vasyl; Zelinska, Oksana; Pavlyuk, Nazar; Dmytriv, Grygoriy & Silva, Bruno Hessel [Vis alle 10 forfattere av denne artikkelen] (2026). Hydrogenation properties of composite materials based on yttrium substituted Mg17Al12. Journal of Alloys and Compounds. ISSN 0925-8388. 1057. doi: 10.1016/j.jallcom.2026.186660. Fulltekst i vitenarkiv
• Muthukrishnan, S.; Ravindran, Vidya & Sjåstad, Anja Olafsen (2026). Nb2O5 and Nb10 nanocluster supported by TiO2 rutile (110) surfaces: A co-catalyst for the enhanced reduction of NO molecules by first principle study. Journal of Physics and Chemistry of Solids. ISSN 0022-3697. 208. doi: 10.1016/j.jpcs.2025.113040. Fulltekst i vitenarkiv
• Lunde, Vilde G. S.; Ofstad, Benedicte; Ohresser, Philippe; Fjellvåg, Øystein Slagtern; Sjåstad, Anja O. & Hauback, Bjørn C. [Vis alle 7 forfattere av denne artikkelen] (2025). Electronic and magnetic properties of light rare-earth cubic Laves compounds derived from x-ray magnetic circular dichroism. Physical review B (PRB). ISSN 2469-9950. 112(22). doi: 10.1103/hf2c-46t3. Fulltekst i vitenarkiv Vis sammendrag

  This work presents electronic and magnetic properties of selected members in the cubic Laves phase series Nd1−𝑥⁢Pr𝑥⁡CoNi (0 ≤ 𝑥 ≤ 1) and Ce0.25⁢Pr0.75⁡CoNi, together with the corresponding binary compositions (NdCo2, NdNi2, PrCo2, PrNi2, CeCo2, CeNi2), using soft x-ray absorption spectroscopy, x-ray magnetic circular dichroism (XMCD), density-functional theory, and crystal field multiplet calculations. All transition-metal moments saturate below 1 T, while the rare-earth moments do not saturate even at 5 T, consistent with van Vleck paramagnetic contributions and crystal field suppression. While the sum rules are widely used to extract element-specific magnetic moments from XMCD, we show that for 3⁢𝑑 transition metals, their application requires accurate estimates of the number of unoccupied 3⁢𝑑 states. We observe a finite magnetic moment on Ni, challenging the common assumption of its nonmagnetic character in Laves phases. The orbital magnetic moments were determined using the spin rules, while the spin moments were estimated from single-ion values from multiplet calculations, due to the invalidity of the spin sum rule for light rare-earth elements. The magnetic moments of Nd and Pr are found to be suppressed relative to their free-ion values, with multiplet theory indicating that this is due to crystal field effects. Our results confirm that Nd and Pr maintain localized 4⁢𝑓3 and 4⁢𝑓2 configurations, respectively, and that their element-specific magnetic moments are robust to rare-earth substitution. Ce, on the other hand, exhibits a tuneable mixed-valent ground state with both magnetic 4⁢𝑓1 and nonmagnetic 4⁢𝑓0 components. The relative fraction of these states varies with the electronegativity of the surrounding 3⁢𝑑 transition metals, revealing a pathway to tune Ce magnetism via composition. This work establishes a framework for accurately interpreting XMCD in light rare-earth-based intermetallics and provides insight for designing light rare-earth-based magnetocaloric materials.

• Fjellvåg, Øystein Slagtern; Fjellvåg, Helmer; Hessevik, Julie; Sjåstad, Anja Olafsen & Steciuk, Gwladys (2025). Crystal Structure of Nd10.67Pt4O24, a New Neodymium Platinate. ACS Omega. 10(12), s. 12487–12494. doi: 10.1021/acsomega.5c00031. Fulltekst i vitenarkiv Vis sammendrag

  A new platinate was recently discovered when Nd2O3 was explored as a platinum capture material in the Ostwald process, formed by a direct reaction between gaseous PtO2 and Nd2O3. The crystal structure of this new platinate and its composition, Nd10.67Pt4O24, are here reported for the first time. The compound is synthesized either by a direct reaction between PtO2(g) and Nd2O3 or by the citric acid chemical route. Based on 3-dimensional electron diffraction data and Rietveld refinement of high-resolution synchrotron and neutron powder diffraction data, we describe its crystal structure in space group I41/a. The compound is structurally related to the Ln11–xSrxIr4O24 (Ln = La, Pr, Nd, and Sm) phases with a double perovskite (A2BB’O6)-like crystal structure with A-site cation deficiency. Owing to the fixed oxidation state of Pt(IV), two of the four Nd sites are partly occupied to provide charge neutrality, with Nd4 taking a split position. On heating, Nd10.67Pt4O24 decomposes into Nd2O3 and Pt. A plateau in the thermogravimetric curves measured in 33 vol % O2 in N2 indicates the presence of an intermediate Pt(II) phase at around 960 °C, probably isostructural with La4PtO7.

• Lunde, Vilde Sturtzel; Møller, Amalie Bisgaard; Eggert, Bruno Guilherme Fischer; Döring, Allan M.; Grivel, Jean-Claude & Bjørk, Rasmus [Vis alle 12 forfattere av denne artikkelen] (2025). Machine learning guided discovery and experimental validation of light rare earth Laves phases for magnetocaloric hydrogen liquefaction. Acta Materialia. ISSN 1359-6454. 297. doi: 10.1016/j.actamat.2025.121282. Fulltekst i vitenarkiv Vis sammendrag

  In the search for new compounds suited for magnetocaloric hydrogen liquefaction, we have combined machine learning (ML) predictions with experiments. Two series of cubic C15 Laves phases, and (0 ≤ x ≤ 1), were selected based on Curie temperature (Tc) predictions of three ML models. A superior model performance was obtained when the dataset was developed for a specific crystal class, despite it being limited to only containing 265 compounds. The random forest regression, gradient boosting regression, and neural network ML models yielded mean absolute errors of 14, 18, and 20 K, respectively. These errors are low compared to previously reported Tc-predictions using ML. The models are powerful tools for the initial screening of new candidate materials, and the accuracy for light rare earth based cubic Laves compounds will improve as the dataset increases. The selected compounds based on the ML screening were synthesized by arc melting and characterized for potential magnetocaloric hydrogen liquefaction applications. The compositions showed magnetic ordering between 20 and 36 K, thus in the lower temperature region relevant for magnetocaloric hydrogen liquefaction. Moreover, they exhibited magnetic entropy change ranging from 6.0 to 7.2 Jkg−1K−1 and adiabatic temperature change from 2.2 to 2.6 K, for a field of 5 T. In contrast, the Ce-containing compounds exhibited a mixture of magnetic Ce and non-magnetic Ce, yielding low magnetization, thus low magnetocaloric effect, and magnetic ordering temperatures below 20 K.

• Jensen, Martin Myhre; Verne, Mathilde Ingeborg Nilsen; Kierulf-Vieira, Walace Peterle Soares; Skau, Karl Isak; Dhak, Prasanta & Sjåstad, Anja Olafsen (2025). Pt–Rh Model Nanoparticle Catalysts for Selective Oxidation of Ammonia to Nitrogen─A Systematic Screening Study. ACS Applied Nano Materials. 8(28), s. 14029–14040. doi: 10.1021/acsanm.5c01442. Fulltekst i vitenarkiv Vis sammendrag

  Abatement processes for ammonia (NH3) slip have gained renewed attention due to growing interest in using liquid NH3 as a marine fuel. A promising approach is selective catalytic oxidation (SCO) of NH3 to nitrogen (N2). This study presents the catalytic performance of five well-defined Al2O3-supported mono- and bimetallic Pt–Rh model catalysts for this reaction. Key findings show that the larger Pt nanoparticles (17 nm) exhibit a higher NH3 turnover frequency (TOF) than the smaller ones (8 nm) while maintaining similar product selectivity. Additionally, the NH3 TOF decreases in the order Pt > Pt1–xRhx solid solution > Rh. Despite this trend, the catalysts Pt, Pt1–xRhx solid solution, and Rh exhibit narrow and distinct temperature windows where the N2 selectivity exceeds 80% at 195–225, 240–250, and 300–315 °C, respectively. Thermal stability tests conducted in air for 8 h and 1 week reveal that both the Rh(core)–Pt(shell) and the Pt1–xRhx solid solution nanoparticles undergo Rh surface enrichment. These results indicate that neither elemental configuration remains stable under the SCO-NH3 conditions.

• Muthukrishnan, S.; Vidya, R. & Sjåstad, Anja Olafsen (2025). Exploring the Electronic Structure and Interfacial Phenomena of Pseudomorphic Rutile(110) Heterostructures: An Ab Initio Study. ACS Applied Engineering Materials. 3(7), s. 2141–2152. doi: 10.1021/acsaenm.5c00313. Fulltekst i vitenarkiv
• Hessevik, Julie; Carlsen, Cathinka Scheele; Bestul, Oskar K.; Waller, David; Fjellvåg, Helmer & Sjåstad, Anja Olafsen (2025). Oxides for Pt Capture in the Ammonia Oxidation Process—A Screening Study. Reactions. 6(1). doi: 10.3390/reactions6010013. Fulltekst i vitenarkiv Vis sammendrag

  Metallic Pd/Ni gauzes, located downstream of the Pt/Rh ammonia oxidation catalyst nets in the Ostwald process, is the current technology for capturing volatile gas phase platinum and rhodium species lost from the Pt/Rh combustion catalyst through evaporation. In this screening study, we explore four oxide families, ABO3 perovskites, (ABO3)n(AO) Ruddlesden–Popper (RP) phases, AO rock salt, and A2O3 sesquioxide type oxides, as alternative materials for platinum capture. It was found that all the tested nickelates, LaNiO3, NdNiO3, La2NiO4, and La4Ni3O10, captured platinum well and formed A2NiPtO6. In contrast, La0.85Sr0.15FeO3, LaFeO3, and LaCoO3 did not capture platinum. CaO, SrO, and Nd2O3 formed low-dimensional platinates such as CaxPt3O4, Sr4PtO6, and a newly discovered neodymium platinate, Nd10.67Pt4O24. Gd2O3 did not capture platinum in bench-scale experiments in dry air, but did, however, seem to capture platinum under pilot plant conditions, likely due to the co-capture of Co lost from the N2O abatement catalyst. The catalytic activity of both oxides and platinum-containing products were studied, toward NOx and N2O decomposition. None of the oxides showed significant activity toward NOx decomposition, and all showed activity toward N2O decomposition, but to different extents. An overall assessment of the screened oxides with respect to potential use in industrial Ostwald conditions is provided. All tested oxides except CaO and SrO withstood industrial conditions. From our assessments, the nickelates and A2O3 (A = Nd, Gd) stand out as superior oxides for platinum capture.

• Muthukrishnan, S.; Vidya, R. & Sjåstad, Anja Olafsen (2025). Illustrating the surface chemistry of nitrogen oxides (NOx) adsorbed on rutile TiO2 (110) with the aid of STM and AIMD simulation. Catalysis Science & Technology. ISSN 2044-4753. 15(6), s. 1850–1864. doi: 10.1039/D4CY01310G. Fulltekst i vitenarkiv Vis sammendrag

  The family of nitrogen oxides such as NO, NO2, and N2O is considered as major anthropogenic environmental pollutants. Understanding the chemical interaction between adsorbed NOx molecules and the rutile TiO2 surface is important for environmental NOx reduction. In this work, we have derived the adsorption mechanism of nitrogen oxide (NOx) molecules on the rutile TiO2 (110) surfaces by using density functional theory (DFT)-based computations. The most suitable adsorption sites for the nitrogen oxide molecules on the TiO2 (110) surfaces are identified from the adsorption energetics. The complete charge flow process is understood for various NOx adsorbed on the surface. The variation in the adsorption mechanism of NO, NO2, and N2O molecules was explored. Ab Initio Molecular Dynamics (AIMD) simulations are performed to study the stability of NOx-adsorbed TiO2 surfaces. Further insights on the ensuing modifications of the TiO2 (110) surfaces, as well as their structural and thermodynamic stability, are derived. The Scanning Tunneling Microscope (STM) images of the rutile TiO2 (110) surfaces and with the adsorbents are simulated for the first time, which helps to find the surface morphology of NOx adsorbed on the TiO2 surface. The influence of individual constituents of the adsorbed molecules and the surface is explored by analyzing the intricate features of their electronic structures. The present work provides guidelines exploring the interactions of NOx pollutants adsorbed on rutile TiO2 (110) surfaces.

• Hessevik, Julie; Sønsteby, Henrik Hovde; Fjellvåg, Helmer & Sjåstad, Anja Olafsen (2025). Reactivity of gaseous PtO2 and RhO2 with LaNiO3 thin films: a systematic XPS study. Materials Advances. 6(3), s. 1097–1106. doi: 10.1039/D4MA00974F. Fulltekst i vitenarkiv Vis sammendrag

  The reaction between volatile platinum and rhodium gas species and bulk materials (alloys or oxides) is of high practical importance in the chemical process industry. Herein, an X-ray photoelectron spectroscopy (XPS) study has been conducted to understand how PtO2(g) and RhO2(g) react with LaNiO3 (LNO) thin films grown via atomic layer deposition (ALD) at elevated temperatures (900 °C). In this study, XPS data for reference powders of La2NiPtO6, LaNi0.95Pt0.05O3, LaNi0.88Rh0.12O3 and LaRhO3 provide a library for the interpretation of oxidation states of platinum and rhodium after reaction with LNO thin films. Upon short exposure to PtO2(g) and RhO2(g), platinum appear as Pt(III) in the LNO surface, but after prolonged exposure, Pt(IV) appears. Complementary diffraction analysis shows that LaNi1−xMxO3 solid solution forms initially where Ni, Pt and Rh are predominantly +III, however, quickly accompanied by the formation of a La2Ni2−2xM2xO6 (M = Pt, Rh) double perovskite phase with its characteristic (004) peak. Rhodium appears as Rh(III) and Rh(IV) in both the R[3 with combining macron]c and the double perovskite like phases. Furthermore, platinum and rhodium depth profiles were generated by the combination of XPS and Ar+ sputtering. Platinum depth profiles reveal major differences between as-grown ALD films and post-annealed films when exposed to PtO2(g). Pt penetrates faster into the LNO material for films without post annealing, probably due to its smaller grain size and larger surface area and grain boundaries. When exposed to a mixture of PtO2(g) and RhO2(g), the Rh : Pt ratio in LNO increases upon prolonged reaction time. These results may have implications on the use of LNO as platinum and rhodium capture material in the Ostwald process.

• Brennhagen, Anders; Skautvedt, Casper; Cavallo, Carmen; Wragg, David Stephen; Koposov, Alexey & Sjåstad, Anja Olafsen [Vis alle 7 forfattere av denne artikkelen] (2024). Unraveling the (De)sodiation Mechanisms of BiFeO<inf>3</inf> at a High Rate with Operando XRD. ACS Applied Materials & Interfaces. ISSN 1944-8244. 16(10), s. 12428–12436. doi: 10.1021/acsami.3c17296. Fulltekst i vitenarkiv Vis sammendrag

  Development of new anode materials for Na-ion batteries strongly depends on a detailed understanding of their cycling mechanism. Due to instrumental limitations, the majority of mechanistic studies focus on operando materials’ characterization at low cycling rates. In this work, we evaluate and compare the (de)sodiation mechanisms of BiFeO3 in Na-ion batteries at different current densities using operando X-ray diffraction (XRD) and ex situ X-ray absorption spectroscopy (XAS). BiFeO3 is a conversion-alloying anode material with a high initial sodiation capacity of ∼600 mAh g–1, when cycled at 0.1 A g–1. It does not change its performance or cycling mechanism, except for minor losses in capacity, when the current density is increased to 1 A g–1. In addition, operando XRD characterization carried out over multiple cycles shows that the Bi ⇋ NaBi (de)alloying reaction and the oxidation of Bi at the interface with the Na–Fe–O matrix are detrimental for cycling stability. The isolated NaBi ⇋ Na3Bi reaction is less damaging to the cycling stability of the material.

• Johnsen, Hennie Marie; Pokle, Anuj; Filtvedt, Werner Olav; Hiorth, Marianne; Klaveness, Jo & Sjåstad, Anja Olafsen (2024). From silicon to silica: a green chemistry approach for hollow sphere nanoparticle formation. Nanoscale Advances. 6, s. 6196–6204. doi: 10.1039/d4na00586d. Fulltekst i vitenarkiv Vis sammendrag

  Herein we report on an environmentally friendly and scalable production route for hollow silica spheres (HSSs). The process is based on close to 100% conversion of non-crystalline solid Si nanoparticles ([D with combining macron] = 40 ± 9 nm) in mild alkaline solutions (pH ≤ 9.0) at ambient temperature. The Si nanoparticles are prepared using the centrifugal chemical vapor deposition (cCVD) method. Combining transmission electron microscopy (TEM) imaging and nanoparticle size analysis with hydrogen evolution data, elemental mapping, and nitrogen adsorption for surface area measurement, we show for the first time experimental data that document a Kirkendall type Si-to-HSS formation process. Our understanding is that the Si nanoparticles exposed to air form a SiO2 film, which is stable in the mild alkaline environment. Silicon from the Si nanoparticles is transported through the thin SiO2 film and is reacting with H2O/OH− species on the particle surface or in the already thickened SiO2 shell to form silicic acid that in turn rapidly gets converted to a sol–gel to continue the growing of the silica shell. We foresee that this green chemistry approach can be utilized for HSS preparation for use in batteries, insulation materials and drug delivery.

• Brennhagen, Anders; Skurtveit, Amalie; Wragg, David Stephen; Cavallo, Carmen; Sjåstad, Anja Olafsen & Koposov, Alexey [Vis alle 7 forfattere av denne artikkelen] (2024). (De)sodiation Mechanism of Bi2MoO6 in Na-Ion Batteries Probed by Quasi-Simultaneous Operando PDF and XAS. Chemistry of Materials. ISSN 0897-4756. 36(15), s. 7514–7524. doi: 10.1021/acs.chemmater.4c01503. Fulltekst i vitenarkiv Vis sammendrag

  Operando characterization can reveal degradation processes in battery materials and are essential for the development of battery chemistries. This study reports the first use of quasi-simultaneous operando pair distribution function (PDF) and X-ray absorption spectroscopy (XAS) of a battery cell, providing a detailed, atomic-level understanding of the cycling mechanism of Bi2MoO6 as an anode material for Na-ion batteries. This material cycles via a combined conversion-alloying reaction, where electrochemically active, nanocrystalline NaxBi particles embedded in an amorphous Na–Mo–O matrix are formed during the first sodiation. The combination of operando PDF and XAS revealed that Bi obtains a positive oxidation state at the end of desodiation, due to formation of Bi–O bonds at the interface between the Bi particles and the Na–Mo–O matrix. In addition, XAS confirmed that Mo has an average oxidation state of +6 throughout the (de)sodiation process and, thus, does not contribute to the capacity. However, the local environment of Mo6+ changes from tetrahedral coordination in the desodiated state to distorted octahedral in the sodiated state. These structural changes are linked to the poor cycling stability of Bi2MoO6, as flexibility of this matrix allows movement and coalescence of the NaxBi particles, which is detrimental to the electrochemical stability.

• Fjellvåg, Asbjørn Slagtern; Waller, David; By, Thomas & Sjåstad, Anja Olafsen (2023). Pt-Catchment Using Pd/Au Alloys: Effect of Enhanced Diffusion. Industrial & Engineering Chemistry Research. ISSN 0888-5885. 62(6), s. 2478–2493. doi: 10.1021/acs.iecr.2c02852. Fulltekst i vitenarkiv Vis sammendrag

  We have studied a series of Pd/Au alloys as Pt-catchment materials, both in lab-scale experiments and in an industrial ammonia oxidation plant. Our focus has been on how bulk Pt diffusion affects grain reconstruction in Pd/Au alloys (91/9 and 50/50 at. %) during Pt-catchment, performed by studying both polycrystalline (as produced) and quasi-monocrystalline (preannealed in vacuum) wires. The grain reconstruction is reduced when alloying Pd with Au, and it is almost absent for Pd/Au (50/50 at. %). For all Pd-containing samples with a quasi-monocrystalline grain structure, the restructuring is limited in short (<20 days) laboratory experiments but present in the 5 month industrial experiment. Notably, the Pd/Au (50/50 at. %) alloy shows a low degree of restructuring in all experiments but a reduced Pt-catchment in the industrial experiment. The mechanism for grain reconstruction of Pd/Au alloys is discussed, along with the role of the Kirkendall effect and the internal porosity on wire restructuring.

• Jensen, Martin Myhre; Kierulf-Vieira, Walace Peterle Soares; Kooyman, Patricia J. & Sjåstad, Anja Olafsen (2023). Variable temperature in situ TEM mapping of the thermodynamically stable element distribution in bimetallic Pt-Rh nanoparticles. Nanoscale Advances. 5(19), s. 5286–5294. doi: 10.1039/d3na00448a. Fulltekst i vitenarkiv Vis sammendrag

  We report here the first variable temperature in situ transmission electron microscopy (TEM) study on smaller Pt–Rh nanoparticles (≤24 nm) under vacuum conditions. Well-defined 50 at% Pt/50 at% Rh Pt–Rh solid solution and Rh(core)–Pt(shell) nanoparticles, obtained via colloidal synthesis routes, were investigated between room temperature and 650 °C to elucidate the tendency of elemental mixing/segregation. Key findings are that Pt–Rh nanoparticles <13 nm are stable in a solid solution configuration over the entire studied temperature range, whereas nanoparticles >13 nm tend to segregate upon cooling. Such a cross-over in element distribution with nanoparticle size has not been reported for the Pt–Rh system previously. The results demonstrate the technique's ability to extract valuable information concerning the intricate dynamic processes that take place in the bimetallic Pt–Rh system at the nanoscale, which may be indispensable when optimizing, e.g., the metal composition in catalytically active materials.

• Muthukrishnan, S.; Vidya, R. & Sjåstad, Anja Olafsen (2023). Band gap engineering of anatase TiO2 by ambipolar doping: A first principles study. Materials Chemistry and Physics. ISSN 0254-0584. 299. doi: 10.1016/j.matchemphys.2023.127467. Fulltekst i vitenarkiv
• Hessevik, Julie; Fjellvåg, Asbjørn Slagtern; Iveland, Oskar; By, Thomas; Skjelstad, J. & Waller, David [Vis alle 8 forfattere av denne artikkelen] (2022). LaNiO3 as a Pt catchment material in the ammonia oxidation process. Materials Today Communications. 33. doi: 10.1016/j.mtcomm.2022.104084. Fulltekst i vitenarkiv
• Jensen, Martin; Gonano, Bruno Raymond Gino; Kierulf-Vieira, Walace Peterle Soares; Kooyman, Patricia J. & Sjåstad, Anja Olafsen (2022). Innovative approach to controlled Pt-Rh bimetallic nanoparticle synthesis. RSC Advances. 12(31), s. 19717–19725. doi: 10.1039/d2ra03373a. Fulltekst i vitenarkiv Vis sammendrag

  Precise control of the elemental composition and distribution in bimetallic nanoparticles is of great interest for both fundamental studies and applications, e.g. in catalysis. We present a new innovative and facile synthesis strategy for the production of true solid solution Pt1−xRhx nanoparticles. This constitutes a development of the established heat-up method, where undesired shell formation is fully suppressed, despite utilizing metal precursors with different reaction rates. The concept is demonstrated through synthesis of selected Pt1−xRhx solid solution compositions via the polyalcohol reduction approach. In addition, we provide modified procedures, using the same surface stabilizing agent/metal precursors reaction matrix yielding controlled model Rh(core)–Pt(shell) and Pt(core)–Rh(shell) nanoparticles. Tunable bimetallic solid solution and core–shell nanoparticles with the same capping agent are of key importance in systematic fundamental studies, as functional materials properties may be altered by modifying the surface termination.

• Fjellvåg, Asbjørn Slagtern; Jørgensen, Peter Stanley; Waller, David; Wragg, David Stephen; Michiel, Marco Di & Sjåstad, Anja Olafsen (2022). Mechanism of grain reconstruction of Pd and Pd/Ni wires during Pt–catchment. Materialia. 21. doi: 10.1016/j.mtla.2022.101359. Fulltekst i vitenarkiv Vis sammendrag

  We have investigated industrial Pd and Pd/Ni (91.3/8.7 at.%) wires and gauzes used for Pt-catchment during the ammonia oxidation process, focusing on the origin of the grain reconstruction phenomena. In situ X-ray absorption tomography experiments reveal severe reconstruction of polycrystalline Pd and Pd/Ni wires over the course of 10 days, caused solely by the presence of gaseous PtO2 and dry air. A high partial pressure of PtO2 results in higher Pt-catchment rates, which in turn causes more severe reconstruction of the Pd-based wire. Additionally, the Pt-catchment efficiency is reduced when the Pt-content on the wire surface increases. Scanning electron microscopy accompanied by energy dispersive X-ray analysis reveal that Pt diffuses rapidly into the wire core via grain boundaries already after 1 day of Pt catchment. However, quasi monocrystalline (pre-annealed) Pd and Pd/Ni samples show significantly less Pt-catchment and only a little reconstruction compared to the polycrystalline samples. With support from experiments and simulations, we conclude that bulk diffusion is one of the main limitations for Pt-catchment on both poly- and quasi monocrystalline Pd and Pd/Ni wires. The diffusion limitation causes a high surface concentration of Pt, which in turn limits further Pt-catchment due to a gas-surface equilibrium. The polycrystalline wire can overcome the diffusion limitation by utilizing a rapid grain boundary diffusion, transporting both Pt to the wire core and Pd to the wire surface. However, this transport process is also the cause of grain reconstruction.

• Jensen, Martin Myhre; Gonano, Bruno; Kierulf-Vieira, Walace & Sjåstad, Anja Olafsen (2022). Innovative approach to controlled Pt–Rh bimetallic nanoparticle synthesis. RSC Advances. doi: 10.1039/D2RA03373A. Fulltekst i vitenarkiv
• Jebasty, Mariyal R.; Sjåstad, Anja Olafsen & Ravindran, Vidya (2022). Prediction of intermediate band in Ti/V doped gamma-In2S3. RSC Advances. 12, s. 1331–1340. doi: 10.1039/d0ra08132a. Fulltekst i vitenarkiv Vis sammendrag

  Materials with an intermediate energy band (IB) introduced in the forbidden gap are viable alternatives to tandem configurations of solar cells for increasing the photon-conversion efficiency. One of the aspiring designs proposed for the intermediate band concept is hyperdoped (Ti, V):In2S3. Being very important in copper indium gallium sulfide (CIGS) solar cells, indium thiospinel (In2S3) is known for its three different temperature as well as pressure, polymorphs. The most stable β-In2S3 was experimentally shown to have an isolated intermediate band (IB) and exhibits sub-band gap absorption due to the completely filled IB after V-doping. Though experimental observation holds a positive signature, recent DFT studies did not show a metallic intermediate band for the V dopant in the 3+ charge state. In order to clarify this, we have taken incentive from experimental XRD analysis that V-doped β-In2S3 shows peaks from disordered In vacancies (either α or γ), in addition to the ordered In vacancies expected. Hence, we have carried out state-of-the-art DFT based computations on pure and Ti, V-doped In2S3 in the γ-phase which has not been studied yet. We considered the Ti and V dopants in various charge states. Our theoretical study including hybrid functional, does in fact find the IB in V-doped γ-In2S3. However, at equilibrium the IB lies in between the Fermi level (EF) and conduction band minimum (CBM).

• Brennhagen, Anders; Cavallo, Carmen; Wragg, David Stephen; Ponniah, Vajeeston; Sjåstad, Anja Olafsen & Koposov, Alexey [Vis alle 7 forfattere av denne artikkelen] (2022). Operando XRD studies on Bi2MoO6 as anode material for Na-ion batteries. Nanotechnology. ISSN 0957-4484. 33(18). doi: 10.1088/1361-6528/ac4eb5. Fulltekst i vitenarkiv Vis sammendrag

  Based on the same rocking-chair principle as rechargeable Li-ion batteries, Na-ion batteries are promising solutions for energy storage benefiting from low-cost materials comprised of abundant elements. However, despite the mechanistic similarities, Na-ion batteries require a different set of active materials than Li-ion batteries. Bismuth molybdate (Bi2MoO6) is a promising NIB anode material operating through a combined conversion/alloying mechanism. We report an operando x-ray diffraction (XRD) investigation of Bi2MoO6-based anodes over 34 (de)sodiation cycles revealing both basic operating mechanisms and potential pathways for capacity degradation. Irreversible conversion of Bi2MoO6 to Bi nanoparticles occurs through the first sodiation, allowing Bi to reversibly alloy with Na forming the cubic Na3Bi phase. Preliminary electrochemical evaluation in half-cells versus Na metal demonstrated specific capacities for Bi2MoO6 to be close to 300 mAh g−1 during the initial 10 cycles, followed by a rapid capacity decay. Operando XRD characterisation revealed that the increased irreversibility of the sodiation reactions and the formation of hexagonal Na3Bi are the main causes of the capacity loss. This is initiated by an increase in crystallite sizes of the Bi particles accompanied by structural changes in the electronically insulating Na–Mo–O matrix leading to poor conductivity in the electrode. The poor electronic conductivity of the matrix deactivates the NaxBi particles and prevents the formation of the solid electrolyte interface layer as shown by post-mortem scanning electron microscopy studies.

• Periyasamy, Manimuthu; Patra, Lokanath; Fjellvåg, Øystein; Ravindran, Ponniah; Sørby, Magnus Helgerud & Kumar, Susmit [Vis alle 8 forfattere av denne artikkelen] (2021). Effect of electron doping on the crystal structure and physical properties of an n = 3 Ruddlesden-Popper compound La4Ni3O10. ACS Applied Electronic Materials. 3(6), s. 2671–2684. doi: 10.1021/acsaelm.1c00270. Fulltekst i vitenarkiv Vis sammendrag

  The physical properties of La4Ni3O10 with a two-dimensional (2D)-like Ruddlesden–Popper-type crystal structure are extraordinarily dependent on temperature and chemical substitution. By introducing Al3+ atoms randomly at Ni sites, the average oxidation state for the two nonequivalent Ni cations is tuned and adopts values below the average of +2.67 in La4Ni3O10. La4Ni3–xAlxO10 is a solid solution for 0.00 ≤ x ≤ 1.00 and is prepared by the citric acid method, with attention paid to compositional control and homogeneity at low Al level (x) concentrations. The samples adopt a slightly distorted monoclinic structure [P21/a (Z = 4)], evidenced by peak broadening of the (117) reflection. We report a remarkable effect on the electronic properties induced by tiny amounts of homogeneously distributed Al cations, with clear correspondence between resistivity, magnetization, diffraction, and density functional theory (DFT) data. DFT shows that electronically, there is no significant difference between the nonequivalent Ni atoms and no tendency toward any Ni3+/Ni2+ charge ordering. The resistivity changes from metallic to semiconducting/insulating with increasing band gap at higher Al levels, consistent with results from DFT. The metal-to-metal (M-T-M) transition reported for La4Ni3O10, which is often interpreted as a charge density wave, is maintained until x = 0.15 Al level. However, the temperature characteristics of the resistivity change already at very low Al levels (x ≤ 0.03). A coupling of the M-T-M transition to the lattice is evidenced by an anomaly in the unit cell dimensions. Moreover, there is an excellent correlation between the resistivity and magnetization data shown by the metallic and Pauli paramagnetic regime for La4Ni3–xAlxO10 with x < 0.25. The introduction of the fixed +3 oxidation state of Al atoms randomly at the Ni sites reduces the overall oxidation state of Ni2.67+ by keeping the oxygen stoichiometry unchanged. In addition, the nonmagnetic Al3+ for Ni is likely to block Ni–O–Ni exchange pathways through −Ni–O–Al–O–Ni– fragments into the network of corner-shared octahedra with the emergence of possible short-range ferromagnetic ordering of the ferromagnetic domains/clusters that are formed due to Al substitutional disorder in a paramagnetic insulating matrix.

• Saha, Dipankar; Kumar, Susmit; Gonano, Bruno Raymond Gino; Sjåstad, Anja Olafsen & Fjellvåg, Helmer (2021). Total Scattering Study of Local Structural Changes in MnAs1-xPx Influenced by Magnetic Interactions. Chemistry of Materials. ISSN 0897-4756. 33, s. 2576–2584. doi: 10.1021/acs.chemmater.1c00128. Fulltekst i vitenarkiv Vis sammendrag

  The interplay between fluctuations of the local structure and magnetic interactions is of great importance for phenomena like superconductivity, colossal magnetoresistance, and frustrated magnetism. Such local distortions are often subtle and difficult to probe. The metallic MnAs1–xPx (x = 0.06, 0.12, 0.18) solid solution represents a sensitive model system, currently analyzed by variable-temperature X-ray total scattering (TS). A second-order transition (Pnma to P63/mmc) occurs on heating, intimately coupled to magnetic ordering at low temperatures and a temperature-induced low- to high-spin transition for manganese. Intrinsic compositional fluctuations along with a particular magneto-volumetric coupling to low- and high-spin like states triggers formation of local high-symmetry nanosized domains in an orthorhombically deformed matrix that eventually converts globally into a high-temperature hexagonal arrangement. These features are only traced by TS analyses, whereas diffraction provides a regular second-order transition with, e.g., cationic displacements defined by Rietveld analysis acting as order parameter. The coexistence model provides good fits of the TS data and, on average, translates nicely into a continuous V(T) thermal expansion relation with maximum expansivity in the range where the TS data shows the largest difference between local (<20 Å) and intermediate-range (20–80 Å) structures. The degree of local distortions, Mn(As, P)6 octahedra inclusive, is a result of volume-dependent electronic properties as well as magnetic interactions. This study demonstrates how X-ray TS can conveniently give essential insights into the local symmetry fluctuations from the perspective of strong magnetic interactions.

• Ivashenko, Oleksii; Johansson, Niclas; Pettersen, Christine; Jensen, Martin; Zheng, Jian & Schnadt, Joachim [Vis alle 7 forfattere av denne artikkelen] (2021). How Surface Species Drive Product Distribution during Ammonia Oxidation: An STM and Operando APXPS Study. ACS Catalysis. 11(13), s. 8261–8273. doi: 10.1021/acscatal.1c00956. Fulltekst i vitenarkiv Vis sammendrag

  The oxidation of ammonia is a key reaction for the production of artificial fertilizers and for environmental protection. Depending on the area of application, the catalytic reaction needs to be tuned toward the production of either N2 or NO, and this selectivity is controlled by temperature, pressure, reactant ratio, and the type of catalyst. PtRh alloys are highly useful catalytic materials for the oxidation of ammonia, and they can be employed at different reaction conditions. In contrast to pure Pt and Rh catalysts, for which a large number of studies of ammonia oxidation reaction mechanism are available, for PtRh alloys, direct spectroscopic evidence for structure–performance relationship is still lacking. To understand the behavior of PtRh alloys, namely, what is their active phase under reaction conditions and how the alloy composition leads to a particular product distribution, we study the oxidation of ammonia over PtRh/Pt(111) surfaces by simultaneous operando ambient pressure X-ray photoelectron spectroscopy and mass spectrometry at 1 mbar total reaction pressure. These data are complemented by a catalyst surface characterization by scanning tunneling microscopy in ultrahigh vacuum. We establish that the predominant surface structure during NH3 oxidation strongly depends on the degree of Pt enrichment and the O2/NH3 mixing ratio. At the nanoscale, the selectivity toward N2 or NO production is driven by the surface populations of N and O species. These, in turn, are controlled by the nature of the alloying of Pt with Rh.

• Fjellvåg, Asbjørn Slagtern; Fjellvåg, Øystein; Breard, Y. & Sjåstad, Anja Olafsen (2021). Structural disorder and antiferromagnetism in LaNi1–xPtxO3. Journal of Solid State Chemistry. ISSN 0022-4596. 229. doi: 10.1016/j.jssc.2021.122181. Fulltekst i vitenarkiv Vis sammendrag

  We report on the B-site substitution of Pt for Ni in the system LaNi1–xPtxO3. The system can only be synthesized between x ​≤ ​0.50, with LaNiO3 (x ​= ​0.00) and the stoichiometric double perovskite La2NiPtO6 (x ​= ​0.50) as the end members. Higher Pt-contents (x ​> ​0.50) are unachievable due to the preference of Pt to being in oxidation state ​+ ​IV in octahedral coordination. Upon introducing Pt into LaNiO3, a phase transformation from rhombohedral (R-3c) to monoclinic (P21/n) symmetry is observed for 0.075 ​≤ ​x ​≤ ​0.125, where all monoclinic samples are B-site ordered, and Pt show a strong preference for the Pt-site (2c-site) for all compositions (x ​≥ ​0.20). Powder X-ray diffraction analysis reveal disorder of the Pt-distribution in several of the non-equimolar samples (0.20 ​≤ ​x ​≤ ​0.40), which point toward cluster formation with domains of high and low Pt-content within each sample. La2NiPtO6 further show an antiferromagnetic transition at ∼ 40 ​K. A similar transition is observed for all monoclinic samples (x ​≥ ​0.20), however, the transition is less pronounced for lower x. This is explained in terms of the structural disorder, i.e. by the coexistence of antiferromagnetic domains with long range order and paramagnetic domains dominated by short range antiferromagnetic interactions.

• Pettersen, Christine; Sjåstad, Anja O. & Ivashenko, Oleksii (2021). Near-Surface Alloys of PtRh on Rh(111) and Pt(111) Characterized by STM. Journal of Physical Chemistry C. ISSN 1932-7447. 125(45), s. 25140–25147. doi: 10.1021/acs.jpcc.1c05627. Fulltekst i vitenarkiv Vis sammendrag

  The use of Pt–Rh catalysts is central in a number of industrial processes, and to explain their performance it is essential to have a solid understanding of their nanoscopic surface structure. Here we use scanning tunneling microscopy to investigate the growth, elemental nanostructuring, and reconstruction behavior of various Pt–Rh near-surface alloys (NSA) on fcc Rh(111) and Pt(111). We document the formation of a novel lamellar island reconstruction atop Pt/Rh(111) islands as well as network reconstructions on Rh/Pt(111), similar to those previously observed for Pt/Pt(111). The extended roadmap for preparation of PtRh NSAs allows comparison with other homo/heteroepitaxial metal/fcc surfaces and provides a facile guideline for producing tailor-made model catalytic surfaces.

• Karthikeyan, Jayakumar; Fjellvåg, Helmer; Bundli, Silje & Sjåstad, Anja Olafsen (2021). Efficient Exfoliation of Layered Double Hydroxides; Effect of Cationic Ratio, Hydration State, Anions and Their Orientations. Materials. 14(2). doi: 10.3390/ma14020346. Fulltekst i vitenarkiv
• Fjellvåg, Asbjørn Slagtern; Fjellvåg, Øystein; Kumar, Susmit; Ruud, Amund & Sjåstad, Anja Olafsen (2021). Interplay of valence states and magnetic interactions in the perovskite system LaNi1–xRhxO3. Journal of Solid State Chemistry. ISSN 0022-4596. 298. doi: 10.1016/j.jssc.2021.122124. Fulltekst i vitenarkiv Vis sammendrag

  We report on the structural, magnetic and electronic properties of Rh substituted LaNiO3, LaNi1–xRhxO3 (0 ​≤ ​x ​≤ ​1). Increased Rh contents are associated with increased structural distortion, and we observe a phase transformation and an immiscibility gap between 0.15 ​≤ ​x ​≤ ​0.25, separating the R-3c and Pnma perovskite structure regimes. Structural evaluation by synchrotron X-ray diffraction, X-ray absorption spectroscopy and DC magnetic measurements suggest variations in oxidation state for the B-site cations, with Ni(II)/Rh(IV), in addition to Rh(III), dominating for high Rh-contents. Although ordering of Ni(II)/Rh(IV) is possible, no B-site ordering between Ni and Rh is observed for any composition. A metal-to-insulator transition occurs upon Rh-substitution, correlating with the structural transition. DC- and AC magnetic measurements reveal that several of the compositions are magnetically frustrated with both antiferro- and ferromagnetic behaviour at low temperatures. The antiferromagnetic interactions are dominating, while the ferromagnetic interactions increase with increasing Rh-content, to a maximum at x ​= ​0.80 and x ​= ​0.90 in LaNi1–xRhxO3. The mixed valence states are assumed to contribute with a ferromagnetic super-exchange interaction between Ni(II) and Rh(IV).

• Zacharaki, Eirini; Bremmer, Marien G.; Vajeeston, Ponniah; Kalyva, Maria Evangelou; Fjellvåg, Helmer & Kooyman, Patricia J. [Vis alle 7 forfattere av denne artikkelen] (2020). One-pot synthesis of cobalt-rhenium nanoparticles taking the unusual beta-Mn type structure. Nanoscale Advances. 2, s. 1850–1853. doi: 10.1039/d0na00097c. Fulltekst i vitenarkiv Vis sammendrag

  Using a facile one-pot colloidal method, it is now possible to obtain monodisperse Co1−xRex nanoparticles (NPs), with excellent control of Re stoichiometry for x < 0.15. Re-incorporation in terms of a solid solution stabilizes the β-Mn polymorph relative to the hcp/ccp variants of cobalt. The NPs are thermally stable up to 300 °C, which may make them attractive as model catalysts.

• Arstad, Bjørnar; Blom, Richard; Håkonsen, Silje Fosse; Pierchala, Joanna; Cobden, Paul & Lundvall, Fredrik [Vis alle 10 forfattere av denne artikkelen] (2020). Synthesis and Evaluation of K-Promoted Co3-xMgxAl-Oxides as Solid CO2 Sorbents in the Sorption-Enhanced Water−Gas Shift (SEWGS) Reaction. Industrial & Engineering Chemistry Research. ISSN 0888-5885. 59(40), s. 17837–17844. doi: 10.1021/acs.iecr.0c02322. Fulltekst i vitenarkiv Vis sammendrag

  Hydrogen is essential in a variety of large-scale chemical processes. As a carbon-free energy carrier, hydrogen has a potential for wide use within power production and transportation. However, most of the recent production methods involve the release of CO2 as a by-product. Hence, decarbonization of hydrogen production is one step to reduce CO2 emission into the Earth’s atmosphere. Several process schemes have been suggested for low-carbon emission production of hydrogen. In this work, we show how to improve solid sorbents for the sorption-enhanced water–gas shift (SEWGS) process, which is a process that exploits a solid sorbent in the water–gas shift reactor to capture CO2 in situ and drive the process toward an improved hydrogen yield. We report herein a series of CoxMg3-xAl materials based on hydrotalcites, promoted with various loadings of K. The materials have been characterized by BET, XRD, and NMR and tested for their CO2 adsorption performance in three adsorption–desorption cycles in a lab-scale fixed-bed reactor (20–22 bar, CO2 + steam as reactant gas, and isothermal conditions at 375 and 400 °C). The most promising material was subjected to a long-term test (120 adsorption–desorption cycles at similar conditions). This test indicates that a K-promoted Co1.5Mg1.5Al (22 wt % of added K2CO2 to the oxide) material has a higher cyclic capacity for CO2 than standard reference cases. We have estimated that the volumetric capacity (in mol/L unit) of this sorbent will be 23–26% higher than a standard reference material at 400 °C and 30–39% higher at 375 °C. This would, in fixed-bed columns, lead to significant reduction in the needed column volumes in the final process and reduce costs.

• Duparc, Marion Jeanne Louise; Sønsteby, Henrik Hovde; Nilsen, Ola; Sjåstad, Anja Olafsen & Fjellvåg, Helmer (2020). Atomic Layer Deposition of GdCoO3 and Gd0.9Ca0.1CoO3. Materials. 13(1), s. 1–10. doi: 10.3390/ma13010024. Fulltekst i vitenarkiv Vis sammendrag

  Thin films of the catalytically interesting ternary and quaternary perovskites GdCoO3 and Gd0.9Ca0.1CoO3 are fabricated by atomic layer deposition using metal β-diketonates and ozone as precursors. The resulting thin films are amorphous as deposited and become single-oriented crystalline on LaAlO3(100) and YAlO3(100/010) after post-annealing at 650 °C in air. The crystal orientations of the films are tunable by choice and the orientation of the substrate, mitigated through the interface via solid face epitaxy upon annealing. The films exhibit no sign of Co2+. Additionally, high-aspect-ratio Si(100) substrates were used to document the suitability of the developed process for the preparation of coatings on more complex, high-surface-area structures. We believe that coatings of GdCoO3 and Gd1−xCaxCoO3 may find applications within oxidation catalysis.

• Periyasamy, Manimuthu; Fjellvåg, Øystein; Fjellvåg, Helmer & Sjåstad, Anja Olafsen (2020). Coupling of magnetoresistance switching and glassy magnetic state at the metal–insulator transition in Ruddlesden-Popper manganite Ca4Mn3O10. Journal of Magnetism and Magnetic Materials. ISSN 0304-8853. 511, s. 1–8. doi: 10.1016/j.jmmm.2020.166949. Fulltekst i vitenarkiv Vis sammendrag

  Electronic phase separation is a frequent ingredient of the physics of manganites for which the colossal magnetoresistance (CMR) phenomenon is at debate. We investigate the switchable magnetoresistance (MR) effect of the Ruddelsden-Popper (RP) oxide Ca4Mn3O10 (CMO), which shows a metal-to-insulator transition (MIT) at 70 K (TMIT). The static (DC) and dynamic (AC) magnetic susceptibilities of CMO indicate an unusual magnetic phase coexistence below the Néel temperature (TN) i.e., at TMIT, which we describe as a glassy magnetic behavior. Furthermore, by means of synchrotron X-ray diffraction, we observed a structural anomaly directly associated with the MIT. Hence, the onset of glassiness correlates with a slight structural distortion. Interestingly, CMO displays a large positive magneto-resistance (PMR) effect (up to 550% at 4 K) in the metallic state below TMIT, while a small negative magneto-resistance (NMR) effect (-3.5% at 112 K) in the insulating region above TMIT. At TMIT, the PMR switches to NMR behavior. PMR is probably induced via enhancing the exchange interaction by localization of itinerant moment resulting in formation of magnetic polarons, whereas NMR is possibly due to reduced electron-spin scattering. This appears to be a special type of magnetoresistance, different from regular observations, and may open a new avenue in searching for exotic phenomena in manganese-based CMR materials.

• Fjellvåg, Øystein; Nygård, Kristin Hubred; Vajeeston, Ponniah & Sjåstad, Anja Olafsen (2019). Advances in the LiCl salt flux method and the preparation of phase pure La2−xNdxLiHO3 (0 ≤ x ≤ 2) oxyhydrides. Chemical Communications. ISSN 1359-7345. 55(26), s. 3817–3820. doi: 10.1039/c9cc00920e. Fulltekst i vitenarkiv Vis sammendrag

  The LiCl salt flux method is an established aid in oxyhydride synthesis. By operating the flux below its melting point, we have obtained phase pure Nd2LiHO3 for the first time. Further, the suitability of the flux method is shown to be dictated by a delicate balance between the thermal stability of the oxyhydride in question and the ionic mobility of the reactants.

• Fjellvåg, Øystein; Krzystyniak, Matthew; Vajeeston, Ponniah; Sjåstad, Anja Olafsen & Armstrong, Jeff (2019). A combined deep inelastic neutron scattering andab initiolattice dynamics study of the hydride anion dynamics and bonding in La2LiHO3 oxyhydride. Journal of Physics Communications. 3(10). doi: 10.1088/2399-6528/ab4be6. Fulltekst i vitenarkiv Vis sammendrag

  Kobayashi et al (6279) (Science 2016, 351) reported recently the existence of pure H- conductivity in the oxyhydride La2−x−y Sr x+y LiH1−x+y O3−y , while demonstrating its functionality through a prototype solid-state Ti/La2LiHO3/TiH2 battery. In this study, we probe the atomistic motion of La2LiHO3 obtained by the promising halide salt flux method, via a combination of deep inelastic neutron scattering (DINS) and ab initio lattice dynamics (LD) calculations verified by vibrational inelastic neutron spectroscopy (INS). We successfully describe the measured momentum distributions from DINS via our LD calculations, without observing any diffusion activation over the temperature range reported by Kobayashi et al. This observation is corroborated by model predictions from our LD study, which reveals that the hydride anions remain bound within a 3D-harmonic potential. We conclude that with the current synthesis parameters, the method produces a vacancy free lattice, and that a necessary ingredient for diffusive motion of H- is the presence of a large population of vacancies. Based on the harmonic prediction for the hydrogen kinetic energy, we derive a picture of the evolution of the effective bonding potential for the hydride anions, and link this to the dynamics associated with decomposition of the oxyhydride.

• Bundli, Silje; Dhak, Prasanta; Jensen, Martin; Gunnæs, Anette Eleonora; Nguyen, Phuong Dan & Fjellvåg, Helmer [Vis alle 7 forfattere av denne artikkelen] (2019). Controlled alloying of Pt-Rh nanoparticles by the polyol approach. Journal of Alloys and Compounds. ISSN 0925-8388. 779, s. 879–885. doi: 10.1016/j.jallcom.2018.11.301. Fulltekst i vitenarkiv
• Kumar, Susmit; Waller, David; Fjellvåg, Helmer & Sjåstad, Anja Olafsen (2019). Development of custom made bimetallic alloy model systems based on platinum-rhodium for heterogeneous catalysis. Journal of Alloys and Compounds. ISSN 0925-8388. 786, s. 1021–1029. doi: 10.1016/j.jallcom.2019.02.026. Fulltekst i vitenarkiv Vis sammendrag

  The current study extends the insight into solid-state diffusion in alloys of the Platinum Group Metals (PGMs). Thereby, we pave the way for tailor making surface compositions of binary platinum (Pt) – rhodium (Rh) model catalysts, with deposited rhodium layers on the surface of Pt single crystals (SCs) as the starting point. Platinum single crystals with (100), (110) and (111) orientations were studied by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). In addition, X-ray Photoelectron spectroscopy (XPS) characterization was carried out on Pt(110). The crystallographic orientation has a prominent effect on diffusion characteristics. The slowest moving Kirkendall – plane was observed for Pt(110) at xK = 250 nm from reference point x0, while, the fastest for Pt(100) was at xK = 450 nm, indicating that the inter-diffusion rate with respect to orientation increases in the sequence Pt(110) < Pt(111) < Pt(100). The diffusion coefficient of Rh into Pt is of the order ~ 1017 – 1018 m2/s, whereas it was slightly less than ~ 1019 m2/s for Pt into Rh; albeit comparable to the vacancy diffusion rate from Pt into Rh. Our experimental findings show good agreement with calculated inter-diffusion coefficients using Darken analysis. A theoretical direction is proposed for improved fitting of data.

• Kumar, Susmit; Fjellvåg, Øystein; Sjåstad, Anja Olafsen & Fjellvåg, Helmer (2019). Physical properties of Ruddlesden-Popper (n = 3) nickelate: La4Ni3O10. Journal of Magnetism and Magnetic Materials. ISSN 0304-8853. 496. doi: 10.1016/j.jmmm.2019.165915. Fulltekst i vitenarkiv Vis sammendrag

  Nickelates, just like their isoelectronic cousins (the cuprate superconductors), are candidates for high temperature superconductivity. As charge density waves (CDW) and superconductivity might share a similar microscopic origin and that CDW behaviour is earlier suggested for La4Ni3O10, we here provide a detailed physical characterization of the magnetic, electronic and thermodynamic properties of stoichiometric polycrystalline La4Ni3O10 at and below 300 K. By variable temperature, synchrotron powder X-ray diffraction, we observe noticeable expansion in the b-axis and a contraction in c-axis coinciding with the electronic transition. Magnetic measurements indicate Pauli paramagnetic behaviour for the entire temperature range of 4–300 K. Transport data suggest electron–phonon interaction based conduction from 150 to 300 K, at 138 K the we observe an anomaly of second order nature, previously reported as a CDW anomaly. Below this anomaly, we observe no indications of electron–phonon interaction, but rather, Fermi liquid (FL) based T2 dependence with resistivity minima at 20 K, below which T1/2 dependence occurs for the resistivity indicating pure electron–electron interactions (EEI) based transport. Specific heat data indicates that mass enhancement of La4Ni3O10 similar to that of the normal state of the cuprates and the entropy change at anomaly suggests close similarities to other quasi-two dimensional systems, such as molybdenum bronzes and manganites.

• Kishore, M. R. Ashwin; Sjåstad, Anja Olafsen & Ponniah, Ravindran (2019). Influence of hydrogen and halogen adsorption on the photocatalytic water splitting activity of C2N monolayer: A first-principles study. Carbon. ISSN 0008-6223. 141, s. 50–58. doi: 10.1016/j.carbon.2018.08.072. Fulltekst i vitenarkiv
• Lundvall, Fredrik; Kalantzopoulos, Georgios N.; Wragg, David; Arstad, Bjørnar; Blom, Richard & Sjåstad, Anja Olafsen [Vis alle 7 forfattere av denne artikkelen] (2019). Characterization and evaluation of synthetic Dawsonites as CO2 sorbents. Fuel. ISSN 0016-2361. 236, s. 747–754. doi: 10.1016/j.fuel.2018.09.057. Fulltekst i vitenarkiv Vis sammendrag

  The sorbent enhanced water-gas shift (SEWGS) process has great potential for CO2 capture and is thereby a realistic alternative to conventional post-combustion capture by gas emission scrubbing. Nevertheless, better performing materials are required to make SEWGS competitive against state-of-the-art scrubbing technologies. Dawsonite, NaAl(OH)2CO3, has a high carbonate content and forms in situ during CO2 capture on alkali metal promoted alumina, thereby showing potential in variable temperature capture processes. For the isothermal SEWGS process the performance in pressure swing absorption is essential. We evaluate here synthetic Dawsonites [MAl(OH)2CO3, M = K, Na] as CO2 sorbents for the SEWGS process based on data from in situ X-ray diffraction, combined thermogravimetric analysis and mass spectrometry, magic-angle spinning nuclear magnetic resonance spectroscopy, as well as high pressure fixed bed reactor tests at isothermal conditions. The results show that synthetic Dawsonites are promising sorbents for the SEWGS process, however, with moderate cyclic CO2 capacities (∼0.3–0.7 mmol CO2/g sorbent) in comparison to alkali-metal promoted hydrotalcites. On the other hand, the materials exhibit relatively fast sorption rates at low temperatures (280–310 °C), favoring the water-gas shift reaction in the SEWGS process.

• Fjellvåg, Asbjørn Slagtern; Waller, David; Skjelstad, Johan & Sjåstad, Anja Olafsen (2019). Grain Reconstruction of Pd and Pd/Ni Alloys for Platinum Catchment. Johnson Matthey Technology Review. 63(4), s. 236–246. doi: 10.1595/205651319X15597236291099. Fulltekst i vitenarkiv
• Fjellvåg, Øystein; Øygarden, Vegar; Sørby, Magnus Helgerud & Sjåstad, Anja Olafsen (2019). Crystal structure of LaSr3Fe3O9 and its phase relation with LaSr3Fe3O10. Journal of Solid State Chemistry. ISSN 0022-4596. 275, s. 56–62. doi: 10.1016/j.jssc.2019.03.050. Fulltekst i vitenarkiv Vis sammendrag

  The Ruddlesden-Popper phase LaSr3Fe3O10 and its reduced derivatives with nominal composition LaSr3Fe3O10–x, with 0 < x < 1.21, are described in detail with respect to crystal structure, magnetic order, thermal stability and likely vacancy mechanisms. The crystal structure of the LaSr3Fe3O9 (x = 1) phase is described. The LaSr3Fe3O10-δ phase prevails for x < 0.51, whereas LaSr3Fe3O9±ε prevails for x > 0.85. The phases are separated by a two-phase region 0.51 < x < 0.85. By combined Rietveld refinements of high-resolution powder synchrotron and neutron diffraction, the crystallographic and magnetic structure of LaSr3Fe3O9 is described. LaSr3Fe3O9 takes a layer like RP3 type structure with features of the brownmillerite structure in the triple perovskite type slabs; unit cell dimensions a = 28.7562(13) Å, b = 5.5280(2) Å, c = 5.4583(2) Å, space group Cmcm. LaSr3Fe3O9 is an antiferromagnet with TN > 350 K and with a G-type magnetic structure, magnetic space group PCbcm. Based on unit cell volume considerations, local oxygen vacancy ordering schemes are suggested.

• Zheng, Jian; Ivashenko, Oleksii; Fjellvåg, Helmer; Groot, Irene M. N. & Sjåstad, Anja Olafsen (2018). Roadmap for Modeling RhPt/Pt(111) Catalytic Surfaces. Journal of Physical Chemistry C. ISSN 1932-7447. 122(46), s. 26430–26437. doi: 10.1021/acs.jpcc.8b08124. Fulltekst i vitenarkiv
• Mom, Rik V.; Ivashenko, Oleksii; Frenken, Joost W. M.; Groot, Irene M. N. & Sjåstad, Anja Olafsen (2018). Nucleation, Alloying, and Stability of Co–Re Bimetallic Nanoparticles on Al2O3/NiAl(110). Journal of Physical Chemistry C. ISSN 1932-7447. 122(16), s. 8967–8975. doi: 10.1021/acs.jpcc.8b00993. Fulltekst i vitenarkiv
• Slawinski, Wojciech Andrzej; Zacharaki, Eirini; Fjellvåg, Helmer & Sjåstad, Anja Olafsen (2018). Structural Arrangement in Close-Packed Cobalt Polytypes. Crystal Growth & Design. ISSN 1528-7483. 18(4), s. 2316–2325. doi: 10.1021/acs.cgd.7b01736. Fulltekst i vitenarkiv
• Fjellvåg, Øystein Slagtern; Armstrong, Jeff; Vajeeston, Ponniah & Sjåstad, Anja Olafsen (2018). New Insights into Hydride Bonding, Dynamics, and Migration in La2LiHO3 Oxyhydride. The Journal of Physical Chemistry Letters. s. 353–358. doi: 10.1021/acs.jpclett.7b03098. Fulltekst i vitenarkiv Vis sammendrag

  Hydride anion-conducting oxyhydrides have recently emerged as a brand new class of ionic conductors. Here we shed a first light onto their local vibrations, bonding mechanisms, and anion migration properties using the powerful combination of high-resolution inelastic neutron scattering and a set of rigorously experimentally validated density functional theory calculations. By means of charge-density analysis we establish the bonding to be strongly anisotropic; ionic in the perovskite layer and covalent in the rock salt layer. Climbing nudged elastic band calculations allow us to predict the hydride migration paths, which crucially we are able to link to the observed exotic ionic–covalent hybrid bonding nature. In particular, hydride migration in the rock salt layer is seen to be greatly hindered by the presence of covalent bonding, forcing in-plane hydride migration in the perovskite layer to be the dominant transport mechanism. On the basis of this microscopic insight into the transport and bonding, we are able to propose future candidates for materials that are likely to show enhanced hydride conductivity.

• Kalyva, Maria Evangelou; Wragg, David; Fjellvåg, Helmer & Sjåstad, Anja Olafsen (2017). Engineering Functions into Platinum and Platinum?Rhodium Nanoparticles in a One-Step Microwave Irradiation Synthesis. ChemistryOpen. 6(2), s. 273–281. doi: 10.1002/open.201600163. Fulltekst i vitenarkiv

Se alle arbeider i NVA

• Kosler, Josef; Sjåstad, Anja Olafsen; Fjellvåg, Øystein Slagtern; Frommen, Christoph & Hauback, Bjørn Christian (2026). Study of Magnetic and Structural Properties of Ho1-xNdxVO3 for Hydrogen Liquefaction. Fulltekst i vitenarkiv
• Kumar, Abhishek; Fjellvåg, Helmer; Deledda, Stefano; Hauback, Bjørn Christian & Sjåstad, Anja Olafsen (2026). Composition-Tuned Phase Evolution in BCC (TiVZrNb)1-xMx HEAs (M = Ni, Fe, Mn, Co, Cr). Fulltekst i vitenarkiv
• Lunde, Vilde Sturtzel; Møller, Amalie Bisgaard; Eggert, Bruno Guilherme Fischer; Döring, Allan; Grivel, Jean-Claude & Bjørk, Rasmus [Vis alle 12 forfattere av denne artikkelen] (2025). Machine Learning Guided Discovery of Light Rare Earth Laves Phases for Magnetocaloric Hydrogen Liquefaction. Fulltekst i vitenarkiv
• Lunde, Vilde Sturtzel; Møller, Amalie Bisgaard; Eggert, Bruno Guilherme Fischer; Döring, Allan; Straßheim, Marc & Grivel, Jean-Claude [Vis alle 15 forfattere av denne artikkelen] (2025). Machine Learning Guided Discovery Light Rare Earth Laves Phases for Magnetocaloric Hydrogen Liquefaction. Fulltekst i vitenarkiv
• Lunde, Vilde Sturtzel; Fjellvåg, Øystein Slagtern; Pomjakushin, Vladimir; Sjåstad, Anja Olafsen; Hauback, Bjørn Christian & Frommen, Christoph (2025). Influence of Ni-Substitution on Structural and Magnetic Transitions of NdCo2. Fulltekst i vitenarkiv
• Lunde, Vilde Sturtzel; Fjellvåg, Øystein Slagtern; Frommen, Christoph; Sjåstad, Anja Olafsen & Hauback, Bjørn Christian (2025). Influence of Ni-Substitution on Structural and Magnetic Transitions of NdCo2. Fulltekst i vitenarkiv
• Brennhagen, Anders; Skurtveit, Amalie; Skautvedt, Casper; Cavallo, Carmen; Wragg, David Stephen & Vajeeston, Ponniah [Vis alle 9 forfattere av denne artikkelen] (2024). Revealing the (de)sodiation mechanisms of Bi-metallates through operando X-ray characterisation. Fulltekst i vitenarkiv Vis sammendrag

  Na-ion batteries is entering the battery market as an alternative to Li-ion batteries. To improve their overall performance it is crucial to develop new types of anode materials with high capacity and long cycle life. Materials combing conversion and alloying mechanisms (CAMs) are promising anodes with their high capacity, but obtaining good cycling stability is still challenging. A comprehensive understanding of the cycling and degradation mechanism of these materials is crucial to improve their performance. Bi-metallates, with a general formula of Bi−TM−O (TM = transition metal) is a group of ternary CAMs. Their general cycling mechanism consists of an irreversible conversion reaction forming nanoparticles of Bi-metal embedded in a Na-TM-O matrix during the first sodiation, followed by a reversible two-step alloying reaction forming Na3Bi with NaBi as an intermediate phase. In this work, we have used operando X-ray diffraction (XRD), pair distribution function (PDF) analysis and X-ray absorption spectroscopy (XAS) to investigate the desodiation mechanisms of Bi2MoO6 and BiFeO3. Through this work, we discovered that Bi2MoO6 forms the cubic version of Na3Bi (c-Na3Bi) while BiFeO3 forms hexagonal Na3Bi (h-Na3Bi) in addition to c-Na3Bi during the first sodiation. In the desodiated state, the Bi-particles are partially oxidised, while still maintaining the Bi-metal structure, indicating that it is only the Bi atoms at the interface between the Bi nanoparticles and the Na−TM−O matrix that is oxidised. During cycling the NaxBi particles grow larger thus increasing the distance between them and increasing the impedance in the material. This is considered to be the main driver for the capacity degradation that was observed during the first 20 cycles. The operando XAS data also revealed that Mo6+ in Bi2MoO6 does not change oxidation state during cycling, but changes coordination between tetrahedral and distorted octahedral coordination during cycling. The cycling and degradation mechanisms of Bi2MoO6 is summarised in Figure 1.

• Brennhagen, Anders; Skautvedt, Casper; Cavallo, Carmen; Skurtveit, Amalie; Wragg, David Stephen & Koposov, Alexey [Vis alle 9 forfattere av denne artikkelen] (2023). Revealing the Cycling and Degradation Mechanism of Bi-Metallates as Anode Materials fro Na-ion Batteries. Fulltekst i vitenarkiv Vis sammendrag

  Na-ion batteries (NIBs) represent an emerging alternative to modern Li-ion batteries. To improve the overall performance of NIBs it is crucial to develop new types of anode materials with high capacity and long cycle life. Among the anode candidates, materials operating under combined conversion and alloying mechanisms (CAMs) are considered as promising. They are capable of delivering high capacity, however, despite theoretical predictions the cycling stability is still below expectations. Therefore, a comprehensive understanding of the cycling and degradation mechanisms of CAMs is crucial to improve their performance. However, these materials undergo multiple chemical transformations, which often involves formation of amorphous and nanocrystalline phases during cycling. Such chemical complexity substantially impedes their full characterization and understanding. Bismuth metallates, with a general formula of Bi-TM-O (TM = transition metal), is a group of ternary CAMs. Their general cycling mechanism consists of an irreversible conversion reaction, which leads to formation of Bi nanoparticles embedded in a Na-TM-O matrix during the first sodiation. This is followed by a reversible two-step alloying reaction leading to formation of Na3Bi with NaBi as an intermediate phase. In this work, we have used operando X-ray diffraction over the course of ∼30 cycles to study the cycling and degradation mechanisms of two CAMs - Bi2MoO6 and BiFeO3. With the help of ex situ X-ray absorption spectroscopy and pair distribution function (PDF) analysis, we have revealed new insights into these materials in addition to confirming the general cycling mechanism. Both materials form the cubic phase of Na3Bi (c-Na3Bi) in the sodiated state with some noticeable differences. While, the hexagonal Na3Bi (h-Na3Bi) phase starts to form after 10 cycles in Bi2MoO6, a combination of c-Na3Bi and h-Na3Bi can be observed after the first cycle in BiFeO3. The formation of c-Na3Bi is kinetically favorable in these systems, while h-Na3Bi is the more thermodynamically stable and, therefore, forms later in the cycling process. Both materials shows three distinct regions of capacity degradation, where the first is initiated by a growth in the crystallite sizes of Bi-particles. This is followed by a disappearance of the Bi-phase as the second step. The last degradation phase is related to the disappearance of the NaBi-phase and locking the system in the sodiated state. The present talk will discuss the mechanisms of these degradation steps in view of the future development of CAMs in greater details.

• Brennhagen, Anders; Cavallo, Carmen; Wragg, David Stephen; Koposov, Alexey; Ponniah, Vajeeston & Sjåstad, Anja Olafsen [Vis alle 7 forfattere av denne artikkelen] (2023). Tracing the (De)sodiation of Bi2MoO6 Through Good and Bad Times With Operando XRD. Fulltekst i vitenarkiv Vis sammendrag

  Na-ion batteries (NIBs) generally have slightly lower energy densities than Li-ion batteries (LIBs), but the abundancy of Na could make NIBs cheaper and more sustainable. The search for new anode materials is essential for further development of NIBs. Anode materials combining conversion and alloying mechanisms (CAMs) are promising because of their high capacity and high rate capabilities, but their complex cycling mechanisms involving amorphous phases are challenging to study [1]. Bi2MoO6 is an example of a CAM that has high initial capacity, but struggles with cycling stability and undergoes major changes during cycling. Detailed understanding of its cycling and degradation mechanism will hopefully enable us to improve the cycling stability of Bi2MoO6 and CAMs in general. Therefore, we have studied Bi2MoO6 using a combination of operando X-ray diffraction (Fig. 1), pair distribution function, X-ray absorption spectroscopy and transmission electron microscopy [3]. This revealed that during the first sodiation, an irreversible conversion of Bi2MoO6 occurs, creating Bi nanoparticles embedded in an amorphous Na-Mo-O matrix. The Bi particles then reversibly alloy with Na, forming cubic Na3Bi leading to a specific capacity close to 300 mAh g−1 for the 10 first cycles. During these cycles, we observe a growth in the crystallite size of the alloying particles, which is probably the leading cause of the rapid capacity decay after cycle 10 where the sodiation of Bi becomes irreversible leaving several inactive Na3Bi particles.

• Brennhagen, Anders; Cavallo, Carmen; Wragg, David Stephen; Koposov, Alexey; Ponniah, Vajeeston & Sjåstad, Anja Olafsen [Vis alle 7 forfattere av denne artikkelen] (2022). Tracing the (De)sodiation of Bi2MoO6 Through Good and Bad Times With Operando XRD. Fulltekst i vitenarkiv Vis sammendrag

  Anode materials combining conversion and alloying mechanisms (CAMs) are promising for Na-ion batteries, but their complex cycling mechanisms are challenging to study. Understanding the (de)sodiation mechanism is crucial and, in several cases, requires advanced synchrotron characterization. Operando synchrotron studies are generally limited to one or two sodiation cycles, which are not fully comprehensive for CAMs. Therefore, herein, we studied the sodiation and desodiation of Bi2MoO6-based anodes with laboratory-based operando X-ray diffraction exceeding more than 30 cycles to have a complete overview of our CAM’s mechanism. This revealed important aspects of the cycling and degradation mechanisms in the material. During the first sodiation, an irreversible conversion of Bi2MoO6 occurs, creating Bi nanoparticles embedded in an amorphous Na-Mo-O matrix. The Bi particles then reversibly alloy with Na forming cubic Na3Bi leading to a specific capacity close to 300 mAh g−1 for the 10 first cycles. This is followed by a rapid capacity decay where the sodiation of Bi becomes irreversible leaving several inactive Na3Bi particles. To the best of our knowledge, this is due to the observed crystal growth of the Bi particles accompanied by structural changes in the insulating Na-Mo-O leading to poor conductivity in the electrodes. The poor electronic conductivity of the matrix deactivates the NaxBi particles and prevents the formation of the solid electrolyte interface layer as shown by post-mortem scanning electron microscopy studies.

• Brennhagen, Anders; Cavallo, Carmen; Wragg, David Stephen; Ponniah, Vajeeston; Sjåstad, Anja Olafsen & Koposov, Alexey [Vis alle 7 forfattere av denne artikkelen] (2022). Tracing the (de)sodiation of Bi2MoO6 through good and bad times with operando XRD. Fulltekst i vitenarkiv Vis sammendrag

  Anode materials combining conversion and alloying mechanisms (CAMs) are promising for Na-ion batteries, but their complex cycling mechanisms are challenging to study1. Understanding the (de)sodiation mechanism is crucial and, in several cases, requires advanced synchrotron characterization. Operando synchrotron studies are generally limited to one or two sodiation cycles, which are not fully comprehensive for CAMs2. Therefore, herein, we studied the sodiation and desodiation of Bi2MoO6-based anodes with laboratory-based operando X-ray diffraction exceeding more than 30 cycles to have a complete overview of our CAM’s mechanism (Figure 1)3. This revealed important aspects of the cycling and degradation mechanisms in the material. During the first sodiation, an irreversible conversion of Bi2MoO6 occurs, creating Bi nanoparticles embedded in an amorphous Na-Mo-O matrix. The Bi particles then reversibly alloy with Na forming cubic Na3Bi leading to a specific capacity close to 300 mAh g−1 for the 10 first cycles. This is followed by a rapid capacity decay where the sodiation of Bi becomes irreversible leaving several inactive Na3Bi particles. To the best of our knowledge, this is due to the observed crystal growth of the Bi particles accompanied by structural changes in the insulating Na-Mo-O leading to poor conductivity in the electrodes. The poor electronic conductivity of the matrix deactivates the NaxBi particles and prevents the formation of the solid electrolyte interface layer as shown by post-mortem scanning electron microscopy studies.

• Brennhagen, Anders; Cavallo, Carmen; Wragg, David Stephen; Ponniah, Vajeeston; Sjåstad, Anja Olafsen & Koposov, Alexey [Vis alle 7 forfattere av denne artikkelen] (2022). Corrigendum: Operando XRD studies on Bi2MoO6 as anode material for Na-ion batteries (Nanotechnology (2022)33 (185402) DOI: 10.1088/1361-6528/ac4eb5). Nanotechnology. ISSN 0957-4484. 33(35). doi: 10.1088/1361-6528/ac72b2. Fulltekst i vitenarkiv
• Brennhagen, Anders; Wragg, David; Fjellvåg, Helmer; Vajeeston, Ponniah; Sjåstad, Anja Olafsen & Cavallo, Carmen (2021). Beyond Insertion: Conversion alloying Bi/RGO-based materials as novel anodes for Na-ion batteries. Fulltekst i vitenarkiv
• Hessevik, Julie; Fjellvåg, Asbjørn Slagtern; Iveland, Oskar; By, Thomas; Skjelstad, Johan & Waller, David [Vis alle 8 forfattere av denne artikkelen] (2021). LaNiO3 as a Pt catchment material in the ammonia oxidation process. Fulltekst i vitenarkiv
• Brennhagen, Anders; Cavallo, Carmen; Wragg, David Stephen; Sjåstad, Anja Olafsen; Ponniah, Vajeeston & Fjellvåg, Helmer (2021). Operando XRD studies on Bi2MoO6 as anode material for Na-ion batteries. Fulltekst i vitenarkiv Vis sammendrag

  Na-ion batteries (NIBs) could be a good alternative to Li-ion batteries (LIBs) due to the large abundance and availability of sodium resources. The search for good anode materials is one of the big challenges since graphite, which is the most common anode material for LIB, does not work well in NIBs. We are developing Bi2MoO6 as an anode material for NIBs, and will explain the main cycling mechanism. By utilizing a combination of conversion and alloying reactions, the material achieves high capacity while still maintaining a decent cycling stability. The complex cycling mechanism also makes it an interesting and challenging material to characterize during cycling. In this poster, we present high quality operando XRD data of the material during several cycles, acquired in the home lab. The data clearly shows that the reversible alloying reaction of Bi-metal to cubic Na3Bi gives the main capacity contribution, after the initial conversion reaction. This indicates formation of nanocrystalline Bi-particles, as nanocrystalline Bi-metal has shown the same reaction mechanism while microcrystalline Bi instead forms the hexagonal phase of Na3Bi. The molybdenum goes into an amorphous matrix that is only partially electrochemically active. The phases formed with molybdenum during cycling are difficult to characterize, due to their amorphous nature, and remain an unsolved mystery.

• Hessevik, Julie; Fjellvåg, Asbjørn Slagtern; Iveland, Oskar; By, Thomas; Skjelstad, Johan & Waller, David [Vis alle 8 forfattere av denne artikkelen] (2021). LaNiO3 as a Pt catchment material in the ammonia oxidation process. Fulltekst i vitenarkiv
• Ivashenko, Oleksii; Pettersen, Christine; Johansson, Niclas; Jensen, Martin; Schnadt, Joachim & Sjåstad, Anja Olafsen (2021). Operando (N) APXPS study of PtRh alloys for NH3 oxidation. Fulltekst i vitenarkiv
• Ivashenko, Oleksii; Pettersen, Christine; Johansson, Niclas; Jensen, Martin; Schnadt, Joachim & Sjåstad, Anja Olafsen (2021). How surface species drive product distribution during ammonia oxidation: an operando APXPS Study. Fulltekst i vitenarkiv
• Brennhagen, Anders; Cavallo, Carmen; Wragg, David Stephen; Koposov, Alexey; Ponniah, Vajeeston & Sjåstad, Anja Olafsen [Vis alle 7 forfattere av denne artikkelen] (2021). Operando XRD study on Bi2MoO6 as anode material for Na-ion batteries. Fulltekst i vitenarkiv Vis sammendrag

  Na-ion batteries (NIBs) could be a good alternative to Li-ion batteries (LIBs) due to the large abundance and availability of sodium resources. The search for good anode materials is one of the big challenges since graphite, which is the most common anode material for LIB, does not work well in NIBs. We are developing Bi2MoO6 as an anode material for NIBs, and will explain the main cycling mechanism. By utilizing a combination of conversion and alloying reactions, the material achieves high capacity while still maintaining a decent cycling stability. The complex cycling mechanism also makes it an interesting and challenging material to characterize during cycling. In this poster, we present high quality operando XRD data of the material during several cycles, acquired in the home lab. The data clearly shows that the reversible alloying reaction of Bi-metal to cubic Na3Bi gives the main capacity contribution, after the initial conversion reaction. This indicates formation of nanocrystalline Bi-particles, as nanocrystalline Bi-metal has shown the same reaction mechanism while microcrystalline Bi instead forms the hexagonal phase of Na3Bi. The molybdenum goes into an amorphous matrix that is only partially electrochemically active. The phases formed with molybdenum during cycling are difficult to characterize, due to their amorphous nature, and remain an unsolved mystery.

• Brennhagen, Anders; Cavallo, Carmen; Wragg, David; Vajeeston, Ponniah; Sjåstad, Anja Olafsen & Fjellvåg, Helmer (2021). Bi4V2O11/RGO composite as conversion and alloying anode for Na-ion batteries. Fulltekst i vitenarkiv
• Fjellvåg, Øystein; Armstrong, J; Krzystyniak, Matthew; Vajeeston, Ponniah & Sjåstad, Anja Olafsen (2020). Structure and dynamics of La2LiHO3. Fulltekst i vitenarkiv
• Fjellvåg, Asbjørn Slagtern & Sjåstad, Anja Olafsen (2020). Platinum Catchment on Noble Metal Alloys. Fulltekst i vitenarkiv
• Ivashenko, Oleksii; Johansson, Niclas; Pettersen, Christine; Jensen, Martin; Zheng, Jian & Schnadt, Joachim [Vis alle 7 forfattere av denne artikkelen] (2020). How surface species drive product distribution during ammonia oxidation, STM and APXPS study. Fulltekst i vitenarkiv
• Fjellvåg, Asbjørn Slagtern; Waller, David; Skjelstad, Johan & Sjåstad, Anja Olafsen (2019). Ni-loss and Grain Reconstruction of Pd/Ni alloys. Fulltekst i vitenarkiv
• Sunding, Martin Fleissner; Jensen, Ingvild Julie Thue; Svenum, Ingeborg-Helene; Ivashenko, Oleksii; Redekop, Evgeniy & Wells, Justin [Vis alle 11 forfattere av denne artikkelen] (2019). National Surface and Interface Analysis Laboratory (NICE II): From (near) in-situ to operando XPS analysis. Fulltekst i vitenarkiv
• Ivashenko, Oleksii; Zheng, Jian; Johansson, Niclas; Groot, Irene M.N.; Schnadt, Joachim & Sjåstad, Anja Olafsen (2018). Role of Rhodium in RhPt catalysts for ammonia oxidation revealed by operando STM and NAP-XPS. Fulltekst i vitenarkiv
• Sjåstad, Anja Olafsen (2018). Bimetallic PtRh catalysts for intermediate temperature ammonia oxidation - Combining traditional catalysis with surface science. Fulltekst i vitenarkiv
• Ivashenko, Oleksii & Sjåstad, Anja Olafsen (2018). Role of Rhodium in RhPt catalysts for ammonia oxidation revealed by operando STM and NAP-XPS. Fulltekst i vitenarkiv
• Kumar, Susmit; Nagell, Marius Uv; Sjåstad, Anja Olafsen & Fjellvåg, Helmer (2017). Low-dimensional Ruddlesden-Popper nickelates La4(NixCo1-x)3O10: Effect of cobalt doping on physical properties and dimensionality. Fulltekst i vitenarkiv
• Sjåstad, Anja Olafsen; Zacharaki, Eirini; Bundli, Silje; Jensen, Martin; Kalyva, Maria Evangelou & Dhak, Prasanta [Vis alle 10 forfattere av denne artikkelen] (2017). Nanoparticles for heterogeneous catalysis of industrial relevant processes. Fulltekst i vitenarkiv
• Ivashenko, Oleksii; Zheng, Jian; Fjellvåg, Helmer & Sjåstad, Anja Olafsen (2017). Operando Investigations of Surfaces for NOx Abatement Catalysis. Fulltekst i vitenarkiv
• Periyasamy, Manimuthu; Fjellvåg, Helmer & Sjåstad, Anja Olafsen (2017). Role of defects on the functional properties of mixed transition metal oxides. Fulltekst i vitenarkiv
• Lundvall, Fredrik; Kalantzopoulos, Georgios N.; Wragg, David; Arstad, Bjørnar; Blom, Richard & Sjåstad, Anja Olafsen [Vis alle 7 forfattere av denne artikkelen] (2017). Thermogravimetric analysis – a viable method for screening materials for the SEWGS process. Fulltekst i vitenarkiv
• Fjellvåg, Øystein Slagtern & Sjåstad, Anja Olafsen (2017). Oxyhydrides, a new material class of complex mixed anion compounds. Fulltekst i vitenarkiv
• Lundvall, Fredrik; Kalantzopoulos, Georgios N.; Wragg, David; Arstad, Bjørnar; Blom, Richard & Sjåstad, Anja Olafsen [Vis alle 7 forfattere av denne artikkelen] (2017). Dawsonite as sorbent for the SEWGS process. Fulltekst i vitenarkiv
• Arstad, Bjørnar; Blom, Richard; Håkonsen, Silje Fosse; Pierchala, Joanna; Lundvall, Fredrik & Kalantzopoulos, Georgios N. [Vis alle 8 forfattere av denne artikkelen] (2017). Preparation and evaluation of new sorbent materials for the Sorption Enhanced Water-Gas Shift (SEWGS) reaction. Fulltekst i vitenarkiv
• Sjåstad, Anja Olafsen (2017). Catalyst Preparation. Fulltekst i vitenarkiv
• Lunde, Vilde Sturtzel; Hauback, Bjørn Christian; Sjåstad, Anja Olafsen & Frommen, Christoph (2026). Light Rare-Earth Based Laves Phases for Magnetocaloric Hydrogen Liquefaction. Universitetet i Oslo. Fulltekst i vitenarkiv Vis sammendrag

  Given the pressing issue of climate change, there is a need to transition to more sustainable and lower-emission technologies. Hydrogen is a renewable energy carrier, but suffers from low volumetric energy density, which makes it challenging to store and transport. Cooling hydrogen from gas to liquid greatly reduces its volume, but current liquefaction technologies are highly energy-intensive and expensive as one approach the liquefaction point of hydrogen (20 K or -253 °C). Magnetic cooling utilizes the magnetocaloric effect of magnetic materials and offers a potentially more efficient alternative. The currently known most efficient magnetocaloric materials rely on expensive and scarce heavy rare-earth elements. Therefore, this work studies the effects of substituting these with light rare-earth elements. Machine learning models trained on compounds reported in the literature were successfully used to screen light rare-earth based compounds. Promising candidates were experimentally synthesized and characterized to examine their magnetocaloric effect. Furthermore, advanced characterization techniques including neutron scattering and synchrotron-based techniques were used to understand the materials and how to optimize them for magnetic refrigeration applications.

• Brennhagen, Anders; Cavallo, Carmen; Wragg, David Stephen; Ponniah, Vajeeston; Sjåstad, Anja Olafsen & Fjellvåg, Helmer (2024). Bi metallates as conversion-alloying anodes for Na-ion batteries. Universitetet i Oslo. Fulltekst i vitenarkiv Vis sammendrag

  Rechargeable batteries are essential for the green energy transition and Li-ion batteries (LIBs) are currently dominating the marked. Li and other elements used in LIBs are limited resources on the planet, which makes them relatively expensive. Na-ion batteries (NIBs) work similarly to LIBs, but with slightly lower capacities due to the higher mass of Na compared to Li. However, the much higher and widespread abundance of Na could make NIBs cheaper and more sustainable that LIBs, perfect for large scale stationary energy storage. One of the major challenges with NIBs is to find a good anode material. In this work, we have not been able to do that. However, we have shown how a group of anode materials works during charge and discharge to a much greater detail than before, and understood why they do not perform so well. We used these results to improve the performance of the materials, but not enough to be of commercial interest. Nevertheless, the methods, ideas and knowledge developed in this thesis can be used for other materials and lead to a great improvements of NIBs in the future.

• Fjellvåg, Asbjørn Slagtern; Sjåstad, Anja Olafsen; Waller, David & Warner, Maximillian (2022). Platinum Catchment by Noble Metal Alloys and structural studies of Pt- and Rh-containing perovskites. Universitetet i Oslo. Fulltekst i vitenarkiv Vis sammendrag

  One of the steps in the production of synthetic nitrogen-based fertilizers is the oxidation of ammonia (NH3) to nitric oxide (NO). In this reaction, Pt/Rh (95/5 wt. %) is used as a catalyst and a significant Pt-loss occurs due to the high temperature and aggressive chemical conditions. The Pt-loss is unavoidable without compromising the catalytic selectivity. Mitigation of the Pt-loss is performed by Pt-catchment, utilizing a Pd/Ni (95/5 wt. %) net placed downstream of the Pt/Rh catalyst, catching the evaporated Pt. A re-alloying process of Pd/Ni with Pt occurs during Pt-catchment, causing the Pd/Ni wires to swell and the net to grow into something more similar to a porous plate. Alloying Pd with Au reduces the swelling, and the nets keep their shape during operation, only at the cost of a small reduction in Pt-catchment. For the future of industrial chemistry, oxide materials are seen as a cheaper and more environmentally friendly alternative to noble metals. In this mind-set, selected Pt- and Rh-containing perovskite oxides were synthesized for fundamental studies. The stability of the perovskite structure depend strongly on the oxidation states for the present elements and their size, allowing only certain compositions with Pt- and Rh- to synthesized.

• Skautvedt, Casper; Fjellvåg, Helmer; Koposov, Alexey; Brennhagen, Anders; Cavallo, Carmen & Sjåstad, Anja Olafsen (2022). Syntese og karakterisering av BiFeO3 som anodemateriale i Na-ionebatterier. Universitetet i Oslo. Fulltekst i vitenarkiv Vis sammendrag

  Na-ion batteries (NIBs) are currently an immature technology but have gained interest in research in the last years. A big field within NIBs is on the anode since there is none good enough yet. In this work, we have looked at BiFeO3 as an anode material for NIBs. Bi-based anodes have been investigated in the group before this work and have shown promising results. The choice of BiFeO3 was based on the interesting mechanism that occurs during charging and discharging when used as an anode. The mechanism is a conversion-alloying reaction and combines conversion and alloying reactions when BiFeO3 is sodiated. We have investigated this mechanism in detail with combined X-ray diffraction (XRD) and galvanostatic cycling (GS) with operando-XRD in the RECX-lab at the Department of Chemistry. BiFeO3 was synthesized by ceramic solid-state synthesis and sol-gel synthesis as part of the work. It has previously been synthesised with these syntheses but has proven to be difficult to be phase pure. The product of the syntheses has been characterized with XRD and refined with the Rietveld method. The ceramic solid-state synthesis gave a low yield of BiFeO3 and several impurities, but the sol-gel synthesis gave a high yield of BiFeO3 and few impurities. Various treatments of BiFeO3 were investigated, including ball milling, leaching and doping, to provide increased performance as a battery material. These tests showed that ball milling of the material gave the best results, but it was still not possible to find an optimisation that avoided a large drop in capacity during the first 30 cycles. To characterise BiFeO3 as an anode material, we test it in half-cells against Na metal. These half-cells were electrochemically analyzed by GS and cyclic voltammetry (SV). We also tested different mixtures of electrode composition and electrolytes to optimise battery properties. These showed that NaPF6 was the best salt and propylene carbonate (PC) + 5 wt% fluoroethylene carbonate (FEC) gave the most stable battery cells. The data from the electrochemical characterization and operando-XRD were used together to interpret the reaction mechanisms of BiFeO3. We saw from these that there were similarities in the alloying reaction between Na and Bi with other Bi-based anodes, with NaBi and Na3Bi as different stages in the reaction. However, there were no indications of the phase(s) of Fe after the reaction.

• Kierulf-Vieira, Walace Peterle Soares; Sjåstad, Anja Olafsen & Jensen, Martin (2022). Solid Solution PtxRh1–x Nanoparticles Synthesis, Characterization, Catalyst Preparation and Thermal Stability Evaluations. Universitetet i Oslo. Fulltekst i vitenarkiv
• Iveland, Oskar & Sjåstad, Anja Olafsen (2021). Ammonia Oxidation Based Reactions and their Catalysts. Universitetet i Oslo. Fulltekst i vitenarkiv
• Tollin, Hallvard Rise; Sjåstad, Anja Olafsen & Fjellvåg, Øystein (2021). Syntese og karakterisering av hydridoksider. Universitetet i Oslo. Fulltekst i vitenarkiv Vis sammendrag

  Denne masteroppgaven har fokus på syntese og karakterisering av hydridoksidene med sammensetningen Ln2LiHO3 (Ln = La, Nd), Sr substituert La2LiHO3, LaSr3Fe3HxOy og HoHO. Motivasjonen for valg av de nevnte hydridoksidsystemene er som følgende: Ln2LiHO3 (Ln = La, Nd) er velstudert og derfor en god kandidat for å få mekanistisk innsikt i hvordan hydridoksider nukleerer og gror i en saltfluks. Sr substituert La2LiHO3 har potensiale som et ioneledende materiale med mobilitet av hydridaniont, mens for HoHO er ikke trend for ordning av anioner forstått relativt til Ln-ioneradius (Ln &lt; Nd gir uordnet aniongitter) og dette må verifiseres. LaSr3Fe3HxOy er interessant som hydridoksid da det aldri tidligere er rapportert hydridoksid inneholdende det 3d redoks-aktive jern-kationet, eller som inntar RP3-struktur. Systemene Ln2LiHO3 (Ln = La, Nd) og Sr substituert La2LiHO3, ble syntetisert med fluksmetoden. Det ble også utviklet metodikk for in –situ pulver-røntgendiffraksjon studier av nukleering og vekst av Nd2LiHO3. LaSr3Fe3HxOy ble syntetisert ved topotaktisk ionebytte og HoHO med kerammetoden. Strukturanalyse ble gjort med røntgendiffraksjon, og i tillegg ble det foretatt termisk stabilitetsanalyser med TGADSC. Forbindelsene LaSr3Fe3HxOy og HoHO ble studert med nøytrondiffraksjon. For første gang er Sr substituert La2LiHO3 fremstilt med fluksmetoden. Srsubstitusjonsgraden er lavere enn forventet (ca. 20 % av nominell sammensetning), men det foreligger gode bevis for at en Sr-rik tetragonal fase stabiliseres i fluks ved høy konsentrasjon av Sr (x + y &gt; 0,5 i La2-x-ySrx+yLiH1+2yO3-(x/2)-y). For ortorombisk Sr substituert La2LiHO3 er substitusjonsmekanismen aliovalent anionsubstitusjon. HoHO tar en F-43m type struktur og er anion-ordnet med identisk bindingsavstand for Ho-O og Ho-H. Basert på TGA-DSC, strukturanalyse fra nøytrondata og BVS er hydridoksidet LaSr3Fe3H1,07O8,6 fremstilt. Det er allikevel usikkerhet knyttet til det faktum at forbindelsen er et hydridoksid, så dette må verifiseres.

• Christensen, Ida Anette; Sjåstad, Anja Olafsen; Kooyman, Patricia J. & Jensen, Martin (2020). Syntese og karakterisering av Ag-Pt nanopartikler for antibakterielle formål. Universitetet i Oslo. Fulltekst i vitenarkiv Vis sammendrag

  Dette master‒prosjektet ble initiert for å etablere metodikk og erfaring med studier av sølvbaserte nanopartikler for antibakterielle formål rettet mot drikkevann. I første del av oppgaven ble Ag1‒x‒Ptx (x ≤ 0,15) nanopartikler blitt syntetisert i ikke‒vandige systemer via to «burst nukleering» strategier; varm injisering og oppvarming, hvor AgNO3 og Pt(acac)2 ble brukt som metallforløpere. Fra litteratur var det lite studier på syntetiserte Ag‒Pt nanopartikler via varm injiseringsmetoden i ikke‒vandige systemer slik at syntesen i dette arbeidet ble basert på en opprinnelig Ag‒Rh syntese. Bimetalliske Ag1‒x‒Ptx nanopartikler (x ≤ 0,15) basert reagensmatrisen; etylenglykol som løsnings‒ og reduksjonsmiddel og PVP10 som surfaktant. Metallforløperne i syntesene ble gitt ulik tilberedningshistorikk. Dette ga ulik utslag på partikkelstørrelsen og størrelsesfordelingen, og i kombinasjon av en observert segregert grunnstoffsfordeling av Ag og Pt fra en HAADF‒STEM‒EDS analyse, ble det antatt metallforløperne hadde ulik reaksjonskinetikk. Undersøkelsen av AgNO3 og Pt(acac)2 sin reaksjonskinetikk i et reaksjonskinetikk eksperiment viste at Ag‒forløperen reduseres raskere enn Pt‒forløperen noe som støtter opp hypotesen om dannelse av segregert kjerne‒skall partikler. Basert på disse resultatene og den brede størrelsesfordelingen ble disse partiklene dermed ikke brukt videre i oppgaven. Videre ble det syntetisert Ag1‒xPtx (x ≤ 0,10) via oppvarmingsmetoden hvor synteseparameterne ble holdt likt i alle syntesene. DFE fungerte her som et løsningsmiddel, 1,2‒HDD som reduksjonsmiddel og OA som surfaktant, med AgNO3 og Pt(acac)2 som metallforløpere Totalt 0,08 mmol metallkationer, 0,48 g 1,2‒HDD, 0,6 mL OA og 5 ml løsningsmiddel ble brukt i syntesene og dette ble varmet opp til en reaksjonstemperatur å 260˚C i 60 minutter. Prøvene med de nominelle sammensetningene Ag1‒xPtx (x=0,10‒ 0,05‒ 0) er videre analysert med ulike karakteriseringsteknikker som SEM, TEM‒EDS og XRD, som viser at de ulike sammensetningene hadde en midlere partikkelstørrelse på 6‒7 nm og segregerte struktur. Videre deponeres på filterpapir og analyser i forhold til sølvoppløsningsprosessen. Kommersielle 10 nm nanopartikler fra Sigma‒Aldrich (S‒A) ble benyttet for å utviklet en deponeringsmetodikk på glassfiber‒ og cellulose filterpapir ved hjelp av to deponeringsmetoder; vannstrålepumpe og bløtlegging etterfulgt av inndamping. Den mest optimale deponeringen sett ut fra SEM avbildninger var på cellulose filterpapir ved hjelp av bløtlegging og inndamping. Denne metoden ble dermed tatt med videre når det ble foretatt deponering av egenproduserte nanopartikler. Under deponeringen av egenproduserte Ag1‒xPtx med nominelle sammensetningen Ag1‒xPtx (x=0,10‒ 0,05‒ 0) ble ulike mengder deponert (5‒7 vekt % og 35‒43%). Det ble foretatt SEM og ICP‒OES analyse av filtrene for å få undersøke fordelingen av partikler på filteret og for å være sikre på at mengde Ag og Pt stemte overens med forventet mengde fra den nominelle sammensetningen, bestemt med EDS analyse. Videre i arbeidet ble en selektiv ISE elektrode kjøpt inn og installert før den ble benyttet til å måle Ag+ konsentrasjonen i Ag‒baserte nanopartikler. Fra kalibreringer av elektroden ble det utarbeidet en konverteringsfunksjon for å konvertere målt mV verdi til ppm Ag+ verdi. De kommersielle S‒A 10 nm nanopartiklene ble også her benyttet for å utvikle en sølvoppløsningsmetodikk for å få en forståelse av hvordan Ag‒basert NP oppførte seg i kontakt med vann. Eksperimentert med ulik pH (2 og 7) og oksygenrike vs. inerte betingelser ble gjennomført. Denne metodikken ble delvis videreført for å undersøke sølvoppløsningen til de egenproduserte Ag‒Pt nanopartiklene som var deponert på filterpapir. Grunnen til at metodikken delvis ble videreført var at partiklene i S‒A suspensjonen var stabilisert sterisk med sitrationer hvorav egenproduserte partikler Ag1‒xPtx nanopartikler var stabilisert ved hjelp av polymer molekyler. For å undersøke oppførselen til sølv i de legerte Ag‒Pt nanopartiklene deponert på cellulose filterpapiret ble det benyttet avionisert vann (type 2), oksygenrikt miljø og en pH = 2. Effekten av Pt innlegert og mengde sølv deponert ble undersøkt og filterpapirene ble avbildet i SEM etter måling for å undersøke om de hadde noen visuelle endringer. Hovedfunnet mitt var at mengden Pt innlegert ville bidra til å øke sølvoppløsningsprosessen.

• Pettersen, Christine; Sjåstad, Anja Olafsen & Ivashenko, Oleksii (2020). Preparation, Characterization and Oxidation of Nanostructured Pt-Rh Surfaces. Universitetet i Oslo. Fulltekst i vitenarkiv
• Nesteng, Hanne; Sjåstad, Anja Olafsen; Kooyman, Patricia J. & Jensen, Martin (2020). Syntese og karakterisering av Ag100-xAux nanopartikler for antibakterielle formål. Universitetet i Oslo. Fulltekst i vitenarkiv Vis sammendrag

  Overordnet mål i dette M. Sc prosjektet har vært å etablere nødvendig metodikk for å studere Ag+-oppløsningsprosessen av sølvbaserte nanopartikler i vann for antibakterielle formål. Mono- og bimetalliske nanopartikler med nominell sammensetning Ag100–xAux (x = 0, 5, 10 og 50) har blitt syntetisert via termisk reduksjon i vandig miljø. HAuCl4×3H2O og AgNO3 ble brukt som metallforløpere, type II vann som løsningsmiddel og natriumsitrat som reduksjonsmiddel og surfaktant. I en typisk syntese ble det brukt en total metallionkonsentrasjon på 0,25 eller 2,50 mM, 50 eller 100 mL løsningsmiddel, 5 mL 1% natriumsitratløsning og en syntesetemperatur og –tid på henholdsvis 100°C og 30 minutter. Det ble utført enkle visuelle reduksjonskinetikkeksperimenter av metallforløperne HAuCl4×3H2O og AgNO3, for å studere den relative reaksjonskinetikken. Ut fra visuell observasjon og BF-STEM avbildning, ble det klart at gullforløperen hadde en betydelig raskere reduksjonskinetikk enn sølvforløperen. HAADF-STEM-EDS grunnstoffkartlegging ble anvendt på utvalgte Ag100–xAux (x = 5, 10 og 50) nanopartikkelprøver for å studere grunnstoffordelingen. Det ble observert en kjerne(Au)-skall(Ag) grunnstoffordelingstype for samtlige prøver, som samsvarte med forventningen fra kinetikkeksperimentene. For Ag100–xAux (x = 0, 5, 10 og 50) nanopartikler syntetisert med en høy metallionkonsentrasjon, ble det observert dannelse av kubeformede agglomerater og stavformede partikler. I tillegg ble det observert tilstedeværelse av Cl i prøven med nominell sammensetning Ag50Au50, som kan ha ført til utfelling av AgCl. Utfelling av salter og dannelse av agglomerater ble videre unngått ved bruk av en lav metallionkonsentrasjon. Syntetiserte Ag100–xAux nanopartikler ble karakterisert med SEM (BF-STEM) avbildning for å analysere partikkelstørrelsesfordelingen ved fremstilling av histogrammer. Histogramanalyse av størrelsesfordelingen viste en monomodal størrelsesfordeling, der størrelsen avtok med økende gullinnhold for bimetalliske nanopartikler. Monometalliske Ag nanopartikler fulgte ikke størrelsestrenden, da nukleringen ble styrt av reduksjonskinetikken til Ag-metallforløperen og ikke Au-metallforløperen som i de bimetalliske syntesene. Videre ble nanopartikler med nominell sammensetning Ag50Au50 karakterisert med pulverrøntgendiffraksjon for faseidentifikasjon, hvor det ble observert tilstedeværelse av en ccp-fase som er i full overensstemmelse med at både Ag og Au krystalliserer med en ccp-type struktur og har tilnærmet like enhetscelledimensjoner. Det kunne derfor utelukkes andre krystallinske faser som for eksempel Ag2O. Det ble deponert kommersielle Sigma-Aldrich Ag nanopartikler på to forskjellige bærematerialer i pulverform (γ-alumina og hydroksylapatitt), for å gjøre en første iterasjon på å fremskaffe materialer for studie av Ag+-oppløsning av sølvbaserte nanopartikler. Deponering av 1 vekt % Ag nanopartikler på γ-alumina viste tydelig via BF- og HAADF-STEM avbildning at Ag partiklene både var lokalisert på aluminapulveret og på karbonfilmen, hvilket betyr at partiklene ikke heftet på pulveret. Videre ble det deponert 0,5 vekt % Ag nanopartikler på hydroksylapatitt som heftet til pulveret, men hvor Ag partiklene agglomererte under prosessen. Resultatene ble bekreftet via SEM-avbildning og EDS-analyse. Utfordringer knyttet til heft og agglomerering må derfor løses før metoden kan anvendes til Ag+-oppløsningsstudier av syntetiserte Ag100–xAux nanopartikler. Det ble derfor bestemt å studere Ag+-oppløsningen i suspensjoner i det videre arbeidet. Det ble kjøpt inn en sølvion-selektiv elektrode og utviklet metode for kalibrering og Ag+-konsentrasjonsmålinger, som videre skulle brukes til å studere Ag+-oppløsningsprosessen i egensyntetiserte Ag100–xAux nanopartikler. Under metodeutviklingen til kalibrering av elektroden ble det utført fem like kalibreringer over en lengre tidsperiode (30 dager), som viste at målingene var relativt reproduserbare og at elektroden oppførte seg stabilt over tid. Fra metodeutviklingen til Ag+-konsentrasjonsmålinger hvor det ble brukt kommersielle Sigma-Aldrich Ag nanopartikler, ble syretilsats (HNO3) med og uten tilgang på oksygen undersøkt. Det ble funnet at Ag+-konsentrasjonen var i stor grad pH-avhengig og at oksygentilførselen påvirket Ag+-ion oppløsningsprosessen i mindre grad. Videre Ag+-konsentrasjonsmålinger av egensyntetiserte nanopartikler skulle derfor foregå med tilgang på oksygen med og uten syretilsats i suspensjonene, da effekten av å jobbe i en Ar-atmosfære var liten. Metodeutviklingen fra kalibrering og Ag+-konsentrasjonsmålinger, ble videre brukt til Ag+-ion oppløsningsstudier av kommersielle og egensyntetiserte Ag100–xAux (x = 0, 5 og 10) nanopartikler, der med og uten surgjøring av nanopartikkelsuspensjonene til henholdsvis pH = 2 - 3 og pH = 7 ble anvendt. Ag+-ion oppløsningseksperimentene av nanopartiklene viste kompliserte kurveforløp, der det ble observert partikkelagglomerering under alle eksperimentene, men som var klart tydeligst i de surgjorte suspensjonene. Ag+-konsentrasjonen gikk gjennom en maksimumskonsentrasjon før den avtok i alle tilfellene. Som en forklaring på den observerte reduksjonen i Ag+-konsentrasjon, ble det foreslått en vekselvirkning mellom frigjorte Ag+-ioner og sitrationer fra den elektrostatiske stabiliseringen som førte til dannelse av sølvsitrat. Som forventet, hadde de surgjorte nanopartikkelsuspensjonene en høyere Ag+-konsentrasjon til sammenligning med prøvene ved pH = 7. Tilstedeværelsen av innlegert Au i nanopartiklene førte til en økning i Ag+-ion oppløsningen med økende gullinnhold, som kom tydeligst frem i de surgjorte suspensjonene der Ag, Ag95Au5 og Ag90Au10 nanopartikler ble undersøkt.

• Yavasheva, Galina Tenkova; Sjåstad, Anja Olafsen & Jensen, Martin (2019). A Fundamental Study of Pt100-xNix Nanoparticles: Synthesis and Characterization. Universitetet i Oslo. Fulltekst i vitenarkiv
• Fjellvåg, Øystein; Sjåstad, Anja Olafsen & Gunnæs, Anette Eleonora (2019). Oxyhydrides – Synthesis, Crystal Structure and Properties. Universitetet i Oslo. Fulltekst i vitenarkiv Vis sammendrag

  This thesis focus on the research of the new and scarcely explored material class oxyhydrides, where the anionic lattice consists of both oxide and hydride anions. Selected oxyhydrides described by the formula La2‑xNdxLiHO3 (0 ≤ x ≤ 2) have been obtained by a halide salt flux method in large quantities (gram scale). We describe key factors for preparation of oxyhydrides by this approach, and we discover lithium chloride to serve as an excellent flux also below its melting temperature, opening for a completely new temperature range for synthesis of oxyhydrides. Phase pure Nd2LiHO3 was in this work produced phase pure for the first time. Neutron diffraction was employed to study the long range and local structure of La2LiHO3, and neutron spectroscopy combined with computational methods yields new insight the migration of hydride anions in the compound based on evaluation of the chemical bonding. LaSr3Fe3O9 is found to be a new layered compound with a G-type antiferromagnetic structure which is immiscible with LaSr3Fe3O10‑δ. Preliminary neutron diffraction experiments on corresponding oxyhydrides prepared by topotactic anion-exchange with CaH2, indicate successful preparation of an oxyhydride with the composition LaSr3Fe3H1.1O8.9.

• Zacharaki, Eirini; Sjåstad, Anja Olafsen & Fjellvåg, Helmer (2018). Synthesis and Characterization of Colloidal Co, Co-Re and Ni Nanoparticles for Catalysis. Universitetet i Oslo. Fulltekst i vitenarkiv
• Bakke, Ingrid Marie Bergh; Fjellvåg, Helmer & Sjåstad, Anja Olafsen (2018). Struktur og magnetiske egenskaper til de perovskittliknende oksidene ACo0,5B0,5O3 (A = Y, La, Nd; B = Ti, Fe, Ni). Universitetet i Oslo. Fulltekst i vitenarkiv

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