Effect of pre-treatment and modification conditions of natural zeolites on the decomposition and reduction of N2O

Abstract

The decomposition of N 2 O and reduction of N 2 O with NH 3 on natural and modified zeolites were studied by a steady state reaction. Two different natural zeolites having different phase compositions were modified by ion exchange (0.5 M NH 4 NO 3 ) and acid leaching (1 M HCl) and treated under low temperature, high temperature and steam conditions. It was observed that the surface modification of natural zeolites depends strongly on their structure and composition. The modification of zeolites by ion exchange and acid leaching increases the decomposition activity of N 2 O because of the formation of isolated iron and Fe-O-Al species. However, these modifications insignificantly affect the reduction of N 2 O with NH 3 . High temperature and steam treatments lead to dealumination, loss of crystallinity, sintering of phases and the formation of amorphous material in zeolites, resulting in a significant decrease in the decomposition and reduction activity of N 2 O. The kinetic evaluation for N 2 O decomposition predicts that the rate-limiting step is the recombinative desorption of molecular oxygen.

N2O Decomposition over Fe-ZSM-5: A Systematic Study in the Generation of Active Sites

“…These "α-sites" are able to accommodate adsorbed atomic oxygen (typically called "α-oxygen") [5][6][7][8][9], which, in a second step, can (i) recombine forming O 2 , or (ii) react with other compounds (e.g., CO [4], CH 4 [10], benzene [5,11], propane [12]). The formation of this active oxygen can be controlled by iron species, which, in turn, can be modified by the pre-treatment conditions (e.g., activation temperature, final isothermal hold time, and/or gas composition) [13][14][15][16][17], although the effect of each parameter remains a matter of debate. Indeed, catalyst activation by thermal treatment at high temperature can involve (i) calcination [18][19][20], (ii) steaming [11,13,[21][22][23][24], or (iii) heating in an inert [25]/H 2 stream [26] or under vacuum conditions [20], where exposure to different atmospheres has been shown to impact on the activity of Fe-ZSM-5 [20], attributed to changes in the amount of extra-framework iron species [27].…”