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

Abstract

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.