The role of acid sites in cobalt zeolite catalysts for selective catalytic reduction of NOx

The role of the acidic support in ion-exchanged cobalt-zeolite, lean NOx catalysts has been determined by studying the individual steps in the selective reduction pathway. At a GHSV of 10,000 and reaction temperatures below 400°C, NO oxidation is not sufficiently rapid to obtain equilibrium over, for example, 1–4 wt% Co-mordenite catalysts. The NO oxidation rate increases in the order H+Co2+ Co oxide, and neither the number, nor the strength of the acid sites affects the specific rate of the Co2+ ions. For reduction of NO2 by propylene at 300°C and methane at 400°C, the formation of N2 is suggested to occur at support protons sites. In addition, the rate of N2 formation increases linearly with an increase in the number of acid sites, and the specific activity increases with an increase in acid strength. Cobalt (2+) ions do not contribute significantly to the formation of N2, but do non-selectively reduce NO2 to NO. It is proposed that the formation of N2 occurs by protonation of the reducing agent followed by attack of the carbocation by gas phase NO2. Thus, the selective reduction of NO requires two catalytic functions, metal and acid sites.