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Kinetics and Mechanism of the Heterogeneous Oxidation of Ethane and Ethylene on Samarium(III) Oxide

Amorebieta, V. T. and Colussi, A. J. (1996) Kinetics and Mechanism of the Heterogeneous Oxidation of Ethane and Ethylene on Samarium(III) Oxide. Journal of the American Chemical Society, 118 (42). pp. 10236-10241. ISSN 0002-7863. doi:10.1021/ja961273q.

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The rates and products of the purely heterogeneous oxidations of C_2H_6(g) and C_2H_4(g) on Sm_2O_3 in the presence of O_2(g) were investigated in a very low-pressure flow reactor by on-line molecular beam mass spectrometry, about 1000 ± 100 K. Ethane is oxidized to ethyl radicals, which undergo unimolecular decomposition into (C_2H_4 + H) or further oxidation to CO. C_2H_4 oxidation leads to CO as initial product, that is subsequently converted into CO_2. Steady state rates are proportional to k_i‘([O_2]) × [C_2H_n], with k_i‘([O_2]) = k_i × (K_i[O_2])^(1/2)/{1+(K_i[O_2])^(1/2)} (i = 3, 4 for n = 6, 4, respectively), which is consistent with the direct oxidation of hydrocarbons on surface oxygen species in dissociative equilibrium with O_2(g). Alternate or simultaneous measurement of the oxidation rates for C_2H_6, C_2H_4, and CH_4, the latter proportional to k_1‘[CH_4], on the same Sm_2O_3 sample as function of [O2] and temperature, led to the following expressions:  log (k_3/k_1) = −(0.14 ± 0.30) + (663 ± 300)/T (I), log(k_4/k_1) = (1.08 ± 0.35) − (646 ± 365)/T (II), log (K_1/nM^(-1)) = (2.76 ± 0.46) − (4363 ± 468)/T (III), log (K_3/nM^(-1)) = (1.85 ± 0.22) − (4123 ± 260)/T (IV), log(K_4/nM^(-1)) = (5.31 ± 0.65) − (6480 ± 647)/T (V) (nM = 10^(-9)M), that are independent of catalyst mass, active area, or morphology. Equations I−V imply that ethane and ethylene are oxidized faster than methane at all relevant temperatures. Although the activation energies, E_4 > E_1 > E_3, correlate with the corresponding BDE(C−H) energies suggesting a common H-atom abstraction mechanism, the A-factor for the oxidation of ethylene is about tenfold larger. Oxidations occur on distinguishable O_s species generated by endothermic, exentropic O_2 chemisorption involving cooperative participation of the solid.

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Colussi, A. J.0000-0002-3400-4101
Additional Information:© 1996 American Chemical Society. Received April 18, 1996. This project was financially supported by CONICET/Argentina, under Grant PID/1131-91.
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Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET)PID/1131-91
Issue or Number:42
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Official Citation:Kinetics and Mechanism of the Heterogeneous Oxidation of Ethane and Ethylene on Samarium(III) Oxide V. T. Amorebieta and A. J. Colussi Journal of the American Chemical Society 1996 118 (42), 10236-10241 DOI: 10.1021/ja961273q
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:58504
Deposited By: George Porter
Deposited On:24 Jul 2015 20:17
Last Modified:10 Nov 2021 22:06

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