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

The effect of CO₂, H₂O and SO₂ on the kinetics of NO reduction by CH₄ over unsupported La₂O₃ has been examined between 773 and 973 K in the presence of O₂ in the feed. La₂O₃ can maintain a stable, high specific activity (mol/(s m²)) 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 1023 K. 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: rN_2T = k′_NOP_NOP_CH4P_O2 ^0.5/(1 + K_NOP_NO + K_CH4P_CH4 + K_O2^0.5P_O2^0.5 + K_CO2P_CO2 + K_H2OP_H2O)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 30 kcal/mol.