Kinetics and mechanism of the heterogeneous oxidation of ethane and ethylene on samarium(III) oxide

The rates and products of the purely heterogeneous oxidations of C2H6(g) and C2H4(g) on Sm2O3 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 (C2H4 + H) or further oxidation to CO. C2H4 oxidation leads to CO as initial product, that is subsequently converted into CO2. Steady state rates are proportional to k(i)'([O-2]) x [C2Hn], with k(i)'([O-2]) = k(i) x (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 C2H6, C2H4, and CH4, the latter proportional to k(1)'[CH4], on the same Sm2O3 sample as function of [O-2] and temperature, led to the following expressions: log (k(3)/k(1)) = -(0.14 +/- 0.30) + (663 +/- 300)/T (I), log(k(1)k(1)) = (1.08 +/- 0.35) - (645 +/- 365)/T (II), log (K-1/nM(-1)) = (2.76 +/- 0.46) - (4363 +/- 458)/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-6 species generated by endothermic, exentropic O-2 chemisorption involving cooperative participation of the solid.