CO oxidation over nonstoichiometric nickel manganite spinels

Nonstoichiometric nickel–manganese spinel oxides, NixMn3−x ▭3δ/4O4+δ (1≥x≥0), have been synthesized by calcination in air of mixed oxalates at 623 K. These materials are shown to be highly reactive for CO oxidation, some conversion being observed at room temperature for the most active solid (x=1.0). The interaction of CO and O2 with these oxides has been studied by in situ IR spectroscopy under steady-state and transient reaction conditions. A detailed mechanism is proposed wherein CO reacts with coordinatively unsaturated cations to give carbonyl complexes which in turn react with surface oxygen activated on anionic vacancies. Adsorbed and gaseous CO2 also undergo much slower side reactions with lattice oxygen or surface hydroxide groups to give more stable hydrogen carbonate and carbonates species, which lead to catalyst deactivation. Marked effects of pretreatment are explained on the basis of the observed kinetics and the proposed mechanis

Modeling of transient studies on the reaction kinetics over catalysts with lattice oxygen mobility : dry reforming of CH 4 over a Pt/PrCeZrO catalyst

Dynamics of red-ox reactions occurring over catalysts with active oxide support is described by mathematical modeling. Numerical analysis is applied to transients from an initially oxidized state of a Pt/PrCeZrO catalyst to a partially reduced steady state present during CH4 dry reforming. Oxygen transport to the surface from adjacent regions in the catalyst lattice is considered to quantify the impact on the transient behavior in the model red-ox reaction over the catalyst with a high lattice oxygen mobility. Chemical transformations and coverages at the catalyst surface are largely affected by the internal transport of oxygen species, while the overall character and shape of transient curves remain defined by the specificity of the reaction kinetic scheme. Detailed analysis of CH4 dry reforming over a Pt/PrCeZrO catalyst at contact times of 4.7, 8, and 15 ms allowed to (1) clarify the factors that control dynamic system behavior and catalytic properties, (2) discriminate kinetic schemes, (3) confirm a high efficiency of cationic Pt species in CH4 dissociation, and (4) underpin that CO2 transformation may occur via carbonate intermediates located on oxidized Pt^(n+)-Pr^(4+)-O surface sites. Direct estimation of bulk oxygen diffusion rate as well as kinetic parameters was carried out. Findings are consistent with the characteristics of the catalyst surface state and oxygen mobility in the surface/bulk layers

Catalytic conversion of biomass: ethanol reforming and pyrolytic bio-oils co-cracking. Coupling between reactivity, diffusion and ageing for heterogeneous catalysts

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