Kinetics and Mechanism Study of Low-Temperature Selective Catalytic Reduction of NO with Urea Supported on Pitch-Based Spherical Activated Carbon

The kinetics and mechanism of selective catalytic reduction (SCR) of NO with urea supported on pitch-based spherical activated carbons (PSACs) were studied at low temperatures. NO oxidation to NO(2) catalyzed by the 0.5-0.8 nm micropores in PSACs was found to be the rate-limiting step in urea-SCR reaction, which was confirmed by both the apparent activation energy calculations and the kinetics results of urea-SCR reaction and NO oxidation on PSAC. These two reactions gave very similar negative apparent activation energies (-16.5 kJ/mol for urea-SCR reaction and -15.2 kJ/mol for NO oxidation), indicating that the adsorption of reactants on PSAC is of key importance in these two reactions. Moreover, these two reactions were both approximately first-order with respect to NO and one-half order with respect to O(2). It was found that NO(3) from the disproportionation of the produced NO(2) was quickly reduced by supported urea into N(2). After the complete consumption of supported urea, NO(2) started to release, and the carbon surface was gradually oxidized by adsorbed NO(x) species. NO(3) was found to be stably adsorbed on the oxidized carbon surface. On the basis of the results obtained, a reaction mechanism of low-temperature urea-SCR reaction on PSAC was proposed and discussed.