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

Colussi, A. J. and Amorebieta, V. T. (1998) Kinetics and Mechanism of the Heterogeneous Decomposition of Nitric Oxide on Metal Oxides in the Presence of Hydrocarbons. Journal of Physical Chemistry A, 102 (44). pp. 8486-8492. ISSN 1089-5639. doi:10.1021/jp9812987.

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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.

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Colussi, A. J.0000-0002-3400-4101
Additional Information:© 1998 American Chemical Society. Received: February 25, 1998; In Final Form: April 23, 1998. This project was financially supported by the National Research Council of Argentina (CONICET), under Grant PID1131/91.
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Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET)PID1131/91
Issue or Number:44
Record Number:CaltechAUTHORS:20150623-154801496
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Official Citation:Kinetics and Mechanism of the Heterogeneous Decomposition of Nitric Oxide on Metal Oxides in the Presence of Hydrocarbons A. J. Colussi and V. T. Amorebieta The Journal of Physical Chemistry A 1998 102 (44), 8486-8492 DOI: 10.1021/jp9812987
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:58497
Deposited By: George Porter
Deposited On:28 Jul 2015 19:37
Last Modified:10 Nov 2021 22:06

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