The effect of CO_2 and H_2O on the kinetics of NO reduction by CH_4 over a La_2O_3/gamma-Al_2O_3 catalyst

摘要

NO reduction by CH_4 over a 40% La_2O_3/gamma-Al_2O_3 catalyst in the absence and presented of O_2 in the feed was studied.The addition of either CO_2 or H_2O to the feed produced a reversible inhibitory effect on the rate similar to that observed with unsupported La_2O_3; however, the extent of rate inhibition was considerably smaller than on unsupported La_2O_3.At 973 K, either CO_2 (9%) or H_2O (2%) in the feed decreased activity by about 35% in the absence of O_2 and by only 20% with excess O_2 in the feed. In the absence of O_2, a reaction mechanism previously proposed for La_2O_3 was altered to include competitive CO_2 and H_2O adsorption and to give the following rate expression for N_2 formation. The 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 O_2 and either CO_2 or H_2O in the feed were between 0.94 and 0.99 mumol N_2/s/g catalyst. In the presence of excess O_2, after CO_2 and H_2O adsorption were again included, a rate equation proposed earlier for La_2O_3 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 CO_2 and excess O_2 in the feed, this rate expression could not provide thermodynamically meaningufl 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 CO_ had no significant effect on NO reduction on alumina, but it inhibited this reaction of La_2O_3. A reaction model was proposed for gamma-Al_2O_3 that gave the rate expression for total CH_4 disappearance due to both combustion and NO reduction over gamma-Al_2O_3 which gave a satisfactory fit to the data along with thermodynamically consistent parameters. The second term in this equation, which represents the rate of N_2 formation, was then combined with the rate equation for N_2 formation on pure La_2O_3 in the presence of O_2 to describe overall catalyst performance, and the data were fit well, assuming that La_2O_3 composed 6.8% of the total surface area, a value close to that of 6.1% obtained from XRD line-broadening calculations.