Effect of rare-earth element oxides (La2O3, Ce2O3) on the structural and physico-chemical characteristics of Pd/Al2O3 monolithic catalysts of nitrogen oxide reduction by methane

It is shown that introduction of additives of rare-earth element oxides (La2O3, CeO2) enables regulating the structural and functional characteristics of Pd/Al2O3-catalysts (applied on ceramic monoliths of honeycomb structure) of nitrogen oxide reduction by methane. Modifying additives provide increase of thermal stability of porous structure of both highly dispersed Al2O3, as the second support, and the catalyst as a whole.Contribution of La2O3 and CeO2 in increasing the thermal stability is of an additive nature, and lanthanum oxide shows the higher efficiency than cerium one. According to X-ray phase analysis data, stabilizing action is conditioned by occurrence of rare-earth element oxides into lattice of Al2O3, which retards diffusional processes leading to phase transitions of low-temperature crystalline modifications of alumina into high-temperature ones with a low specific surface. For the catalyst samples modified with La2O3 an effect of thermal activation is observed, which is revealed by increase in catalytic activity as a result of annealing at 850 °C. Such a phenomenon, as shown by means of X-ray photoelectron spectroscopy technique, can be explained via stabilization of palladium in singly charged state in the form of groups of Pd+O2− and corresponding increase in concentration of active centers.