Influence of the mobility of oxygen on the reactivity of La1-x Sr (x) MnO3 perovskites in methane oxidation

Radically different dependences of the activity of La1 - x Sr (x) MnO3 (x = 0-0.5) perovskites in methane oxidation on the degree of substitution of strontium for lanthanum are observed for low and high temperatures. Unsubstituted LaMnO3 exhibits the highest activity in the temperature range from 300 to 500A degrees C, while the sample with the maximum degree of substitution (La0.5Sr0.5MnO3) shows the highest activity at higher temperatures of 700-900A degrees C. In the low temperature region, the activity of La1t - x Sr (x) MnO3 is determined by the amount of weakly bound (overstoichiometric) oxygen, which is formed in cation-deficient lattices and is characterized by a thermal desorption peak with T (max) = 705A degrees C. At higher temperatures (800-900A degrees C), the strongly bound oxygen of the catalyst lattice is involved in the formation of the reaction products under both unsteady- and steady-state conditions. As a consequence, the catalytic activity in methane oxidation correlates with the apparent rate constant of oxygen diffusion in the oxide bulk

Investigation of unsteady-state redox mechanisms over ceria based catalysts for partial oxidation of methane and water gas shift

internationalInternational audienc

Investigation of unsteady-state redox mechanisms over ceria based catalysts for partial oxidation of methane and water gas shift

internationalInternational audienc

Kinetics of Oxygen Exchange over CeO2-ZrO2 Fluorite-Based Catalysts

International audienceThe 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 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 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

Water gas shift and partial oxidation of CH4 over CeO2–ZrO2(–La2O3) and Pt/CeO2–ZrO2(–La2O3) : performance under transient conditions

Nanocrystalline ceria-zirconia(-lanthana) based fluorite-like oxide solid solutions prepared via polymerized complex precursor route, with or without supported Pt, have been used for detailed transient kinetic studies of water gas shift (WGS) and partial oxidation of methane (POM) at 650 °C and short (∼10−2 s) contact times. It was shown that high bulk oxygen mobility and storage capacity characterizing nanocrystalline structure of ceria–zirconia and regulated by the partial substitution of Ce for La are responsible for vigorous CO2 evolution during start-up regimes. At the same time, it favors preferential methane and CO oxidation on Pt particles, thus revealing a crucial importance of Pt cluster structure

Performance of monolithic catalysts with complex active component in partial oxidation of methane into syngas: experimental studies and modeling

internationalInternational audienc