Methane Combustion on Perovskites

The larger A cations, usually alkaline or rare earth elements, with inert d or f electronic structure, act as structural stabilizers and do not offer much to the redox catalytic activity. The smaller B cations can be 3d, 4d, or 5d transition metal elements and are the main catalytic sites/centers in such solids due to their ability to undergo reversed redox cycles without destruction of the structure. In the vast majority of catalytic studies, 3d cations are employed due to obvious economic reasons. Nevertheless, almost 95% of the elements of the periodic table can participate in perovskites and these many possible combinations result in a plethora of diverse, and sometimes imexpected, properties of such solids, which have been nick-named chemical chameleons. 

Their chameleonian nature results from both structural and electronic reasons (i) They can stabilize unusual valence states in their fiamework like Ni in LaNi03 and Mn in CaMnOs. (ii) Such imusual valence states can coexist with usual valence states forming mixed valence ensembles in substituted system such as La Sr Nii Ni 03 and Laf Ce Ni Ni 03 and similar solids containing Co, Fe, and Mn in the B site, (iii) They can release oxygen reversibly 

Perovskites andRdatedMixed Oxides Cancels and Applicatiom, Fii EditioiL Edited by Pascal Granger, Vasile I. Parvulescu, Serge l liaguine, and Wilfrid Prellier. 

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The kinetics of methane combustion on ceramic perovskites can be almost always described by the standard equation corresponding to bimolecular Rideal-Eley mechanism. To our knowledge, the full-term bimolecular Langmuir-Hinshelwood model has not been observed dimng methane oxidation. In the RE case, oxygen molecules are adsorbed in dissociative form on the surface metal ions while methane reacts with them from the gaseous phase or from a very weakly adsorbed state, the distinction been elusive. The equation describing those results reads... 

Ferri, D. and Form, L. (1998) Methane combustion on some perovskite-like mixed orddes. Appl Catal. B, 16, 119-126. 

Saracco, G., Geobaldo, R, and Baldi, G. Methane combustion on Mg-doped LaMn03 perovskite catalysts. Appl. Catal. B Environ. 1999, 20, 277-288. 

Ciambelli, R, Cimino, S., Lasorella, G., LisL L., De RossL S., Faticanti, M., Minelli, G., and Porta, P. CO oxidation and methane combustion on LaAli Fe c03 perovskite solid solutions. Appl Catal B Environ. 2002, 37,... 

Baiker, A Marti, PE Keusch, P Fritsch, E Reller, A. Influence of the A-site cation in AC0O3 (A = La, Pr, Nd, and Gd) perovskite-type oxides on catalytic activity for methane combustion. J. Catal, 1994, Volume 146, Issue 1, 268-276. 

XRD analyses of the used catalysts Gd-Co-O and Sm-Co-O showed similar patterns to the reduced catalysts with very strong and sharp peaks for the sesquioxides Gd,0, and Sm20,. On the other hand, the XRD analysis of the La-Co-O catalyst after reaction at 1023 K for 19 h clearly showed the formation of the perovskite LaCoO,. Therefore, it is not surprising that the only reaction products observed were water and carbon dioxide. This agrees with previous works on this perovskite and other forms of cobalt oxide which have been shown to be active catalysts for methane combustion and also for CO and H, oxidation [19], The high Co/Ln surface ratio determined by XPS for the used catalyst is expected for a perovskite like surface. 

B. (2003) Effect of substitution by cerium on the activity of LaMnOa perovskite in methane combustion. Appl. Catal A ... 

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