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 spect rometry, about 1000 +/- 100 K. Ethane is oxidized to ethyl radicals, w hich undergo unimolecular decomposition into (C2H4 + H) or further oxi dation to CO. C2H4 oxidation leads to CO as initial product, that is s ubsequently 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 cons istent with the direct oxidation of hydrocarbons on surface oxygen spe cies in dissociative equilibrium with O-2(g). Alternate or simultaneou s measurement of the oxidation rates for C2H6, C2H4, and CH4, the latt er 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) (n M 10(-9)M), that are independent of catalyst mass, active area, or mor phology. Equations I-V imply that ethane and ethylene are oxidized fas ter 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-fa ctor for the oxidation of ethylene is about tenfold larger. Oxidations occur on distinguishable O-6 species generated by endothermic, exentr opic O-2 chemisorption involving cooperative participation of the soli d.