Peter Ng'ang'a Njoroge

• Njoroge, Peter Ng'ang'a; Solemsli, Bjørn Gading; Wijerathne, Asanka; Berdiell, Izar Capel; Seremak, Agnieszka Patrycja & Chiesa, Mario [Vis alle 16 forfattere av denne artikkelen] (2025). Influence of Aluminum Distribution in Cu-MOR Systems on Methane-to-Methanol Conversion: A Combined Experimental and Theoretical Study. Journal of Physical Chemistry C. ISSN 1932-7447. 129(44), s. 20044–20056. doi: 10.1021/acs.jpcc.5c06045. Fulltekst i vitenarkiv Vis sammendrag

  A series of copper-mordenite (MOR) samples of different provenances were investigated in the methane-to-methanol (MTM) reaction after preparing their copper-exchanged analogues. Noticeable activity improvements were observed when biasing the Al framework distribution of the confined side-pocket toward 12-ring openings (T2 and T4 enrichment) over 8-ring openings (T1 and T3), achieved by using K+ or Na+ in the synthesis gel, respectively. This was rationalized by performing a geometry optimization algorithm using density functional theory (DFT) simulations, which revealed distortions in the structure of the pores among different idealized zeolite models. From this, effects on the copper species were observed, as evidenced from both electron paramagnetic resonance (EPR) spectroscopy and operando X-ray absorption spectroscopy (XAS), which suggested varying monomeric [Cu]2+/[CuOH]+ concentrations with intrinsic copper reducibility differences. Monte Carlo simulations on selected MOR structures of the experimental series exposed dimeric structures with more acute Cu–O–Cu angles, thereby suggesting a more reactive system for Cu-MOR based on K+ in the synthesis gel, in line with the experimental finding. The combined insights from simulations, calculations, and experiments have enabled us to establish a synthesis-structure–activity relationship for mordenite in methane conversion, highlighting the reactive interplay between pore geometry and copper speciation.

• Aunan, Erlend; Finelli, Valeria; Prodinger, Sebastian; Cao, Ning; Garetto, Beatrice & Deplano, Gabriele [Vis alle 13 forfattere av denne artikkelen] (2024). Partial oxidation of cyclohexene over histidine-modified Cu-UiO-66 under aerobic conditions. Journal of Catalysis. ISSN 0021-9517. 438. doi: 10.1016/j.jcat.2024.115722. Fulltekst i vitenarkiv Vis sammendrag

  Enzyme-inspired catalysis is an attractive approach for developing selective hydrocarbon partial oxidation processes. Here, histidine was introduced into defective UiO-66 metal–organic framework by ligand exchange, followed by Cu(II) ion insertion. Density functional theory modeling combined with X-ray Absorption spectroscopy revealed that Cu(II) has a higher affinity for open Zr-O node sites than for histidine, leading to a series of materials where Cu(II) ions are predominantly anchored to the Zr6O4(OH)4 nodes, while only minority fractions of Cu(II) are anchored to histidine. Catalytic testing for aerobic cyclohexene oxidation with tert-butyl hydroperoxide (t-BuOOH) in trifluorotoluene solution at 25 °C showed activity for cyclohexene oxidation to cyclohexenol and cyclohexenone, with high selectivity. Benchmark tests revealed that Cu and t-BuOOH are both essential for the reaction, yet t-BuOOH consumption was far inferior to product formation rates, suggesting a radical reaction pathway involving O2.

• Usoltsev, Oleg; Stoian, Dragos; Skorynina, Alina; Kozyr, Elizaveta; Njoroge, Peter Ng'ang'a & Pellegrini, Riccardo [Vis alle 9 forfattere av denne artikkelen] (2024). Restructuring of Palladium Nanoparticles during Oxidation by Molecular Oxygen. Small. ISSN 1613-6810. doi: 10.1002/smll.202401184. Fulltekst i vitenarkiv Vis sammendrag

  An interplay between Pd and PdO and their spatial distribution inside the particles are relevant for numerous catalytic reactions. Using in situ time-resolved X-ray absorption spectroscopy (XAS) supported by theoretical simulations, a mechanistic picture of the structural evolution of 2.3 nm palladium nanoparticles upon their exposure to molecular oxygen is provided. XAS analysis revealed the restructuring of the fcc-like palladium surface into the 4-coordinated structure of palladium oxide upon absorption of oxygen from the gas phase and formation of core@shell Pd@PdO structures. The reconstruction starts from the low-coordinated sites at the edges of palladium nanoparticles. Formation of the PdO shell does not affect the average Pd‒Pd coordination numbers, since the decrease of the size of the metallic core is compensated by a more spherical shape of the oxidized nanoparticles due to a weaker interaction with the support. The metallic core is preserved below 200 °C even after continuous exposure to oxygen, with its size decreasing insignificantly upon increasing the temperature, while above 200 °C, bulk oxidation proceeds. The Pd‒Pd distances in the metallic phase progressively decrease upon increasing the fraction of the Pd oxide due to the alignment of the cell parameters of the two phases.

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• Njoroge, Peter Ng'ang'a; Olsbye, Unni; Prodinger, Sebastian & Svelle, Stian (2026). Rational Design of Mordenite: Linking Synthesis Parameters, Framework Properties, Copper Speciation and Methane-to-Methanol Performance. Universitetet i Oslo. Fulltekst i vitenarkiv Vis sammendrag

  The selective conversion of methane to methanol (MTM) under mild conditions remains one of the major challenges in heterogeneous catalysis due to the C-H bond strength in methane and hence, the tendency of methanol to undergo overoxidation. Among the catalyst systems investigated for this transformation, Cu-exchanged zeolites, particularly Cu-mordenite (Cu-MOR), have emerged as promising materials because of their ability to stabilize reactive copper-oxo species within confined microporous environments. Despite extensive investigation, substantial differences in methanol productivity are reported for nominally similar Cu-MOR catalysts, highlighting the absence of consistent and transferable synthesis-structure-activity relationships. In particular, the influence of zeolite synthesis history and framework formation on copper speciation and subsequent catalytic performance remains insufficiently understood. This thesis addresses this challenge through a synthesis-driven investigation of mordenite zeolites, focusing on how hydrothermal synthesis parameters such as structure directing agents and aging time influence framework formation, aluminium siting, copper incorporation, and methane activation. Rather than relying on commercially sourced materials, all zeolites investigated in this work were synthesized in-house under systematically controlled conditions. The work specifically examines the influence of inorganic structure-directing agents (ISDA), gel aging, and isomorphous framework substitution on the resulting framework topology, pore architecture, acidity, and local coordination environments. A combination of advanced characterization techniques, including diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), electron paramagnetic resonance (EPR), X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), solid-state nuclear magnetic resonance (ssNMR), Raman spectroscopy, nitrogen adsorption, and MTM testing, was employed to establish direct synthesis-structure-activity relationships. The obtained results demonstrate that variations in synthesis conditions lead to significant differences in aluminium distribution and framework ordering, which subsequently govern copper speciation as well as the nature of the active Cu species. The findings reveal that synthesis-induced framework variations strongly influence the stabilization of mono-, and di-meric, copper species within the MOR framework and thereby determine extent of methane activation and thus methanol productivity. Differences in aging conditions and inorganic structure-directing agents were found to alter aluminium siting and Brønsted acidity, leading to measurable differences in copper incorporation and catalytic behaviour during methane-to-methanol conversion. Furthermore, isomorphous site substitution using heteroatoms such as Ga and Fe demonstrated how subtle modifications to framework composition influence crystallinity, copper speciation, and catalytic performance. Collectively, the results demonstrate that methane-to-methanol performance in CuMOR cannot be optimized solely through adjustment of copper loading or reaction conditions but is intrinsically governed by the synthesis history and directly linked to the framework formation. The zeolite framework must therefore be regarded not as passive support, but rather as an integral component of the catalytic system that dictates active-site formation, copper reactivity, and methane activation behaviour. Ultimately, this work establishes a unified synthesis-framework-activity relationship for Cu-MOR based materials and provides design principles for the rational development of improved methane activation materials and other framework-controlled catalytic systems.

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