Kinetics of Oxygen Exchange over CeO₂−ZrO₂Fluorite-Based Catalysts

Abstract

The kinetics of 18O/16O isotopic exchange over CeO2-ZrO2-La2O3 and Pt/CeO2-ZrO2 catalysts have been investigated under the conditions of dynamic adsorption-desorption equilibrium at atmospheric pressure and a temperature range of 650-850 degrees C. The rates of oxygen adsorption-desorption on Pt sites, support surface, oxygen transfer (spillover) from Pt to the support as well as the amount of oxygen accumulated in the oxide bulk, and oxygen diffusion coefficient were estimated. The nanocrystalline structure of lanthana-doped ceria-zirconia prepared via the Pechini route with a developed network of domain boundaries and specific defects guarantees a high oxygen mobility in the oxide bulk (D = (1.5 / 2.0).10-18 m2 s-1 at 650 degrees C) and allows accumulation of over-stoichiometric/excess oxygen. For Pt/CeO2-ZrO2, oxygen transfer from Pt to support (characteristic time < 10-2 s) was shown to be responsible for the fast exchange between the gas-phase oxygen and oxygen adsorbed on the mixed oxide surface. The rate of direct exchange between the gas phase and surface oxygen is increased as well due to the increased concentration (up to 2 monolayers) of surface/near subsurface oxygen species accumulated on the oxygen vacancies (originated from the incorporation of highly dispersed Pt atoms). The characteristic time of diffusion of the oxygen localized in the subsurface layers is about 1 s. The overall quantity of over-stoichiometric oxygen and/or hydroxyl groups accumulated in the bulk can reach the equivalent of 10 monolayers, and characteristic time of oxygen diffusion within the bulk is about 20 s. All these kinetic data are required for the further step of modeling partial oxidation of hydrocarbons under steady- and unsteady-state conditions.

Final Report for "Investigation of reaction networks and active sites in bio-ethanol steam reforming over Co-based catalysts" with all publications attached.

“…The difference between the oxygen uptake values is more pronounced when the Co-loaded samples is compared, suggesting a role for Co in facilitating oxygen mobility within the support. A similar phenomenon has been previously reported over noble-metal samples [59][60][61][62]. In fact, the quantification of the results shows that part of the oxygen uptake for the ceria-supported samples is attributable to the support.…”

Nanocomposite Catalysts for Steam Reforming of Methane and Biofuels: Design and Performance

“…Downward shift of H 2 TPR peak is observed for any mechanism of the solid oxide reduction, either topochemical (such as NiO reduction) or diffusion-controlled (reduction of ceria-zirconia solid oxide solution). Facilitation of the lattice oxygen mobility due to incorporation of precious metals into domain boundaries and subsurface layers is demonstrated as well (Sadovskaya et al, 2007). Promotion of composites by Ru or Pt clearly accelerates their bulk reduction by hydrogen (Figs.…”

“…A lot of attention was paid to systematic studies of the effect of chemical composition of doped ceria-zirconia solid solutions on the real structure, oxygen mobility and reactivity in nanocomposites considered to be the most important factors controlling their performance. Information on the basic structural features and oxygen mobility/reactivity in Pt-supported ceria-zirconia solid solutions as well as their catalytic properties in transformation of methane and oxygenates into syngas studied earlier in details Sadovskaya et al, 2007;Sadykov et al, 2006bSadykov et al, , 2007aSadykov et al, -e, 2009aSadykov et al, ,d, 2010c provide required bases for understanding the catalytic properties of nanocomposites containing these constituents. Nanocomposites possessing promising performance and coking stability in target reactions were supported on anode substrates or heat-conducting metal substrates and successfully tested in realistic conditions in the in-cell methane steam reforming as well as in pilot -scale reactors of CH 4 or oxygenates transformation into syngas.…”

Methane Combustion in the Presence of Zirconia-Based Catalysts

“…Such materials are known to be effective oxidation catalysts and their oxygen storage capacity is well documented [24,25]. The powder diffraction pattern of the 6/1 material is shown in Fig.…”