Effect of rare-earth element oxides (La_2O_3,Ce_2O_3)on the structural and physico-chemical characteristics of Pd/Al_2O_3 monolithic catalysts of nitrogen oxide reduction by methane

摘要

It is shown that introduction of additives of rare-earth element oxides (La_2O_3,CeO_2)enables regulating the structural and functional characteristics of Pd/Al_2O_3-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 Al-2O_3,as the second support,and the catalyst as a whole.Contribution of La_2O_3 and CeO_2 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 Al_2O_3,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 La_2O_3 an effect of thermal activation is observed,which is revealed by increase in catalytic activity as a result of annealing at 850 deg 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~+O_2~-and corresponding increase in concentration of active centers.