SELECTIVE CATALYTIC REDUCTION OF NO WITH METHANE ON GALLIUM CATALYSTS

Gallium species supported on H-ZSM-5 and on H-mordenite are active catalysts for NO reduction with methane in the presence of excess O2. Over Ga-H-ZSM-5, the NO reduction rate is first order in NO with a variable, fractional order in CH4. The distinct feature of these gallium-based catalysts is their very high CH4 selectivities for NOx reduction. Ga-H-Y, on the other hand, is a poor catalyst for this reaction. The preparation method, ion exchange or wet impregnation, does not appear to be critical for the performance of a Ga/H-ZSM-5 catalyst. Gallium-exchanged and impregnated H-ZSM-5 have similar activities for the NO reduction. Comparable Brønsted acid sites were found on the H-ZSM5 and the H-ZSM-5 supported gallium catalysts via temperature-programmed desorption measurements with ammonia and isopropylamine and by infrared measurements. On the other hand, the presence of H+ in zeolite is important; Ga/Na-ZSM-5 was inactive for the NO reduction. It is apparent that a synergism exists between gallium species and H+, and this serves to activate CH4 as a reductant for NO in the presence of O2. While Ga-H-ZSM-5 is more selective than Co-ZSM-5 for NO reduction, it is more sensitive to water, which makes it less attractive for commercial applications. © 1994 Published by Elsevier Inc.