The effect of CO2, H2O and SO2 on the kinetics of NO reduction by CH4 over La2O3

Abstract

The effect of CO2, H2O and SO2 on the kinetics of NO reduction by CH4 over unsupported La2O3 has been examined between 773 and 973K in the presence of O2 in the feed. La2O3 can maintain a stable, high specific activity (mol/(sm2)) for NO reduction with high concentrations of CO2 and H2O in the feed; however, either of these two products reversibly inhibits the activity by about one-half in the presence of excess O2. The catalyst is poisoned by SO2 at these temperatures and an oxysulfate phase is formed, but partial regeneration can be achieved at 1023K. CO2 in the feed causes the formation of lanthanum oxycarbonate, which reverts to La2O3 when CO2 is removed, but no bulk La oxyhydroxide is detected after quenching with H2O in the feed.The influence of CO2 and H2O on kinetic behavior can be described by assuming they compete with reactants for adsorption on surface sites, including them in the site balance equation, and using the rate expression proposed previously for NO reduction by CH4 in excess O2. With O2 in the feed, integral conversions of CH4 and O2 frequently occurred due to the direct combustion of CH4 by O2, although NO conversions remained differential; thus, an integral reactor model was chosen to analyze the data which utilized a recently determined rate equation for CH4 combustion on La2O3 in conjunction with a previously proposed model for NO reduction by CH4. The following rate expression described the rate of N2 formation: rN2T=k′NOPNOPCH4PO20.5/(1+KNOPNO+KCH4PCH4+KO20.5PO20.5+KCO2PCO2+KH2OPH2O)2. It gave a good fit to the experimental rate data for NO reduction, as well as providing enthalpies and entropies of adsorption obtained from the fitting parameters that demonstrated thermodynamic consistency and were similar to previous values. The heats of adsorption were altered somewhat when either CO2 or H2O was added to the feed, and the following ranges of values (kcal/mol) were obtained for the different components: NO (31–43), CH4 (16–32), O2 (35–54), and H2O (37). Apparent activation energies were typically near 30kcal/mol.