IN SPECIES SPECIES AND STRUCTURAL PROPERTIES OF IN-EXCHANGED ZSM5 AND MORDENITE

摘要

Nitrogen oxides (NOx) produced in the combustion processes of internal combustion engines in either stationary (e.g. energy plants) or mobile sources (e.g. automobiles) aremajor atmospheric pollutants. For over a decade now, zeolites have been studied as catalysts for the Selective Catalytic Reduction (SCR) of NOx to produce innocuous N_2, indium-zeolites being possible candidates.~1 Even though the catalytic behaviour of thesesolids has been extensively studied, further research is needed to define the nature of the indium species and their relationship with the catalytic performance. Different methods have been developed to add indium to zeolites, where the method selected determines the type of indium species generated. It is recognized that the SCR NOx active specie is InO~+in the exchangeable zeolite site. Indium could also exist as In_2O_3 crystals and as highly dispersed non-stoichiometric oxides (In_xO_y)~(3,4). The active species could be generated by the solid state reaction at high temperature between zeolite protons and impregnated In_2O_3 as follows: In_2O_3 + 2 Z-H~+→ 2 Z-InO~+ + H_2O. This treatment involves the impregnation of the solid, oxide generation at 500°C, and the subsequent reaction at 700°C. However, the zeolitic structures could be affected and more detailed knowledge on the consequences and limitations of such procedures is needed.