Rate Oscillations and Chemiluminescence in the Selective Catalytic Reduction of NO by Propane over H-ZSM5

Abstract

The selective catalytic reduction (SCR) of nitric oxide by propane was investigated in the presence of oxygen over a H-ZSMS catalyst. The concentrations of reactants were varied from 0% to 2% NO, 0% to 2% C3H8, and 0% to 35% 02. Reactions were carried out at temperatures from 25 to 600 "C at a space velocity of 9600 h-l. Periodic oscillations were observed in the NO concentrations of products near 400 "C when the feed contained a minimum of 2% NO, 1% C3H8, and 10% 0 2 . Random oscillations and chemiluminescent flashes were observed between 520 and 600 "C. The random oscillations occurred when the feedstock contained 2% NO, 2% C3H8, and 10-35% 0 2 , and the chemiluminescence occurred when the feedstock contained at least 2% NO, 1% C&, and 35% 0 2 . Results suggest that the SCR reaction proceeds via two different reaction pathways below and above 500 "C. The low-temperature reactions are possibly homogeneousheterogeneous processes involving NO2 as the key reactant to form radical surface intermediates. At high temperatures, homogeneous radical reactions involving both NO and NO2 may be rate determining.

Mechanistic Studies and Design of Highly Active Cuprate Catalysts for the Direct Decomposition and Selective Reduction of Nitric Oxide and Hydrocarbons to Nitrogen for Abatement of Stack Emissions

“…Since NO 2 is a very reactive moiety compared to NO and the oxidation of NO to NO 2 is typically faster than the direct interaction of NO and hydrocarbons over most SCR catalysts, formation of NO 2 is widely regarded as initial reaction step in SCR processes [1,2,[49][50][51][52]. Such mechanisms have commonly been proposed for the SCR of NO over In-based catalysts as well [17][18][19][20][21].…”

“…However, it is important to note here that the reaction of (1% Comparative data in Table 1 and Table 2 also indicate that Et-OH and NO can mutually affect each other's reactivities which is typical for radical processes [50,57].…”

“…While dearth of appreciable catalytic activity in reactions of (NO + H 2 + CO + O 2 ) is typical for SCR catalysts [1,2,38,43,44], they are usually highly active for the oxidation of NO to NO 2 which is believed to be a major reaction step in the SCR process over most catalysts [2,19,20,[45][46][47][48][49][50][51][52]. Data in Table 1 demonstrate that the In-SMMC is only slightly active for the oxidation of NO even in the presence of a large excess of O 2 .…”

“…It is possible that these reactivity and selectivity changes for NO are related to selectivity changes in converting Et-OH to aldehyde or CO x which also depend on the concentration of oxygen over the In-SMMC [28]. [50,57]. The degradation of aldehydes to peroxy radicals and their low temperature interactions with NO are well known atmospheric processes [60].…”