Amorebieta, V. T. and Colussi, A. J. (1995) Kinetics and Mechanism of the Heterogeneous Oxidation of Methyl Radicals on Samarium(III) Oxide. Implications for the Oxidative Coupling of Methane. Journal of the American Chemical Society, 117 (13). pp. 3856-3861. ISSN 0002-7863. doi:10.1021/ja00118a022. https://resolver.caltech.edu/CaltechAUTHORS:20150623-154804397
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Abstract
The purely heterogeneous oxidation of CH_3(g) on Sm_2O_3, in the presence and absence of O_2(g), was investigated in a very low pressure flow reactor by molecular beam mass spectrometry, between 1000 and 1200 K. In the presence of O_2, CH_3 is quantitatively oxidized to H_2O and CO_x at steady state rates proportional to [CH3]Γ_5-([O_2]) = [CH_3]k_5K_^(s1/2)[O_2]^(1/2)/(1 + K_5^(1/2)[O_2]^(1/2)). Alternate or simultaneous measurement of oxidation rates for CH_3 and CH_4, the latter proportional to [CH_4]Γ_4([O_2]), on the same Sm_2O_3 sample as a function of [O_2] and temperature, led to the following expressions: log(k_5/k_4) = -(2.18 ± 0.35) + (3210 ± 301)/T (1), log(K_4/10^9 M) = (1.89 ± 0.25) - (4170 ± 260)/T (2), log(K_5/10^9 M) = (5.65 ± 0.11) - (6480 ± 130)/T (3). Equations 1-3 imply that (1) methyl radicals are oxidized faster than methane on Sm_2O_3 below 1500 K and (2) each reaction occurs on distinguishable active sites generated by endothermic, but highly exentropic, O_2 chemisorptive processes involving cooperative participation of the solid. Transient experiments provide evidence on the relative timing of O_2 chemisorption, CH_3 oxidation, and CO_2 release. Sm_2O_3 is almost inert under anoxic conditions. Present results impose an irreducible floor to CO_x yields in the steady state oxidation of methane on Sm_2O_3 at low pressures, but open up the possibility of disengaging CH_3 and CH_4 oxidations under other conditions.
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Additional Information: | © 1995 American Chemical Society. Received October 25, 1994. This project was financially supported by CONICET/Argentina, under Grant PID/1131-91. | |||||||||
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Issue or Number: | 13 | |||||||||
DOI: | 10.1021/ja00118a022 | |||||||||
Record Number: | CaltechAUTHORS:20150623-154804397 | |||||||||
Persistent URL: | https://resolver.caltech.edu/CaltechAUTHORS:20150623-154804397 | |||||||||
Official Citation: | Kinetics and Mechanism of the Heterogeneous Oxidation of Methyl Radicals on Samarium(III) Oxide. Implications for the Oxidative Coupling of Methane V. T. Amorebieta and A. J. Colussi Journal of the American Chemical Society 1995 117 (13), 3856-3861 DOI: 10.1021/ja00118a022 | |||||||||
Usage Policy: | No commercial reproduction, distribution, display or performance rights in this work are provided. | |||||||||
ID Code: | 58508 | |||||||||
Collection: | CaltechAUTHORS | |||||||||
Deposited By: | George Porter | |||||||||
Deposited On: | 26 Jun 2015 15:14 | |||||||||
Last Modified: | 10 Nov 2021 22:06 |
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