Kinetics and Mechanism of the Heterogeneous Decomposition of Nitric Oxide on Metal Oxides in the Presence of Hydrocarbons

Abstract

Rates and products of the (3 + α)NO + CH_4 ⇒ 1/2(3 + α)N_2 + (1 − α)CO + αCO_2 + 2H_2O (0 ≤ α ≤ 1) reaction were determined in low-pressure (NO/CH_4/O_2) mixtures ([NO] < 1 μM, [CH_4] < 10[NO], [O_2] ≤ [NO]; 1 μM = 82 ppm at 1 atm, 1000 K) flowing over Sm_2O_3 between 1000 and 1200 K. Samaria pretreated with CH_4 (or H_2) at reaction temperatures instantly releases N_2 when exposed to NO. Prompt CO formation also occurs on methane-conditioned samples. In contrast, stationary outflow gas compositions attain only after several reactor residence times following step (NO + CH_4) injections to the untreated catalyst. Nitric oxide reduction rates R-NO are roughly proportional to ([CH_4] × [NO])^(1/2) but do not extrapolate to zero at [NO] → 0 and always increase with T. We infer that:  (1) there is no direct reaction between CH_4 and NO on the catalyst surface; (2) instead, NO is reduced to N_2 by reaction with oxygen vacancies V, and with nonvolatile carbon-containing C_s species created in the heterogeneous oxidation/decomposition of CH_4, respectively; (3) the entire mass, rather than just the surface, of catalyst microparticles participate in this phenomenon. We propose a purely heterogeneous mechanism in which physisorbed NO reacts with either vacancies in equilibrium with the active oxygen OR species responsible for CH_4 oxidation or with C_s species. The derived kinetic law:  R-NO = k_A([NO]_s[CH4])^(1/2) + k_B[CH_4], with [NO]_s = [NO]/(K_8^(-1) + [NO]), in conjunction with the reported Arrhenius parameters, closely fits rates measured under anoxic conditions. The fact that R-NO is unaffected by O_2 up to F_(O_2) ∼ 0.3F_(NO) but drops at larger F_(O_2) inflows, even if O_2 is fully consumed in CH_4 oxidation, is consistent with the competition of NO and O_2 for vacancies. The dissimilar observations made in experiments performed in the Torr range strongly suggest that solid catalysts promote combustion at such relatively high pressures.