Vt. Amorebieta et Aj. Colussi, KINETICS AND MECHANISM OF THE HETEROGENEOUS OXIDATION OF ETHANE AND ETHYLENE ON SAMARIUM(III) OXIDE, Journal of the American Chemical Society, 118(42), 1996, pp. 10236-10241
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.