The effect of CO₂ and H₂O on the kinetics of NO reduction by CH₄ over a La₂O₃/γ-Al ₂O₃ catalyst

NO reduction by CH₄ over a 40% La₂O ₃/γ-Al₂O₃ catalyst in the absence and presence of O₂ in the feed was studied. The addition of either CO₂ or H₂O to the feed produced a reversible inhibitory effect on the rate similar to that observed with unsupported La ₂O₃; however, the extent of rate inhibition was considerably smaller than on unsupported La₂O₃. At 973 K, either CO₂ (9%) or H₂O (2%) in the feed decreased activity by about 35% in the absence of O₂ and by only 20% with excess O₂ in the feed. In the absence of O₂, a reaction mechanism previously proposed for La₂O₃ was altered to include competitive CO₂ and H₂O adsorption and to give the following rate expression for N₂ formation: r _N2=k′P_NOP_CH4(1+K_NOP _NO+K_CH4P_CH4+K_CO2P _CO2+K_H2OP_H2O)². This equation fit the data well, had apparent activation energies of 14-25 kcal/mol, and gave thermodynamically consistent enthalpies and entropies of adsorption. Stable rates at 973 K with O₂ and either CO₂ or H₂O in the feed were between 0.94 and 0.99 μmol N₂/s/g catalyst. In the presence of excess O₂, after CO₂ and H₂O adsorption were again included, a rate equation proposed earlier for La ₂O₃ again provided a good fit to the data with H ₂O in the feed as well as thermodynamically consistent parameters determined under integral reaction operation. However, with both CO₂ and excess O₂ in the feed, this rate expression could not provide thermodynamically meaningful parameters from the fitting constants even though it fit the data well. This was attributed to a major contribution from the alumina to the overall rate, because CO₂ had no significant effect on NO reduction on alumina, but it inhibited this reaction on La ₂O₃. A reaction model was proposed for γ-Al ₂O₃ that gave the rate expression for total CH ₄ disappearance due to both combustion and NO reduction over γ-Al₂O₃ (r_CH4)T=k′ _comP_CH4P_O2^0.5+k′ _NOP_NOP_CH4P_O2 ^0.5(1+K′_NO2P_NOP_O2 ^0.5+K_CH4P_CH4+K_O2^0.5P _O2^0.5+K_CO2P_CO2+K_H2OP _H2O)², which gave a satisfactory fit to the data along with thermodynamically consistent parameters. The second term in this equation, which represents the rate of N₂ formation, was then combined with the rate equation for N₂ formation on pure La₂O ₃ in the presence of O₂ to describe overall catalyst performance, and the data were fit well, assuming that La₂O ₃ composed 6.8% of the total surface area, a value close to that of 6.1% obtained from XRD line-broadening calculations.