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 + CH4 ⇒ 1/2(3 + α)N2 + (1 − α)CO + αCO2 + 2H2O (0 ≤ α ≤ 1) reaction were determined in low-pressure (NO/CH4/O2) mixtures ([NO] < 1 μM, [CH4] < 10[NO], [O2] ≤ [NO]; 1 μM = 82 ppm at 1 atm, 1000 K) flowing over Sm2O3 between 1000 and 1200 K. Samaria pretreated with CH4 (or H2) at reaction temperatures instantly releases N2 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 + CH4) injections to the untreated catalyst. Nitric oxide reduction rates R - NO are roughly proportional to ([CH4] × [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 CH4 and NO on the catalyst surface; (2) instead, NO is reduced to N2 by reaction with oxygen vacancies V, and with nonvolatile carbon-containing Cs species created in the heterogeneous oxidation/decomposition of CH4, 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 CH4 oxidation or with Cs species. The derived kinetic law: R - NO = k A([NO]s[CH4])1/2 + k B[CH4], 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 O2 up to F O 2 ∼ 0.3F NO but drops at larger F O 2 inflows, even if O2 is fully consumed in CH4 oxidation, is consistent with the competition of NO and O2 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.

Novel CO2 and CO gas sensor based on nanostructured Sm2O3 hollow microspheres

“…In this process, Sm 2 O 3 has been used for 180 h, at an operating temperature of 700°C [2][3][4]. Other catalytic properties of this oxide have also been investigated [5][6][7]. Moreover, its high permittivity ( r = 10) and band gap M a n u s c r i p t 3 (E g = 4.33 eV) are suitable for metal-oxide-semiconductor (MOS) devices, and resistance random access memories (RRAMs) [8][9][10].…”