Aj. Colussi et Vt. Amorebieta, KINETICS AND MECHANISM OF THE HETEROGENEOUS DECOMPOSITION OF NITRIC-OXIDE ON METAL-OXIDES IN THE PRESENCE OF HYDROCARBONS, The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory, 102(44), 1998, pp. 8486-8492
Rates and products of the (3 + alpha)NO + CH4 --> 1/2(3 + alpha)N-2 (1 - alpha)CO + alpha CO2 + 2H(2)O (0 less than or equal to alpha less
than or equal to 1) reaction were determined in low-pressure (NO/CH4/
O-2) mixtures ([NO] <1 mu M,[CH4] < 10[NO], [O-2] less than or equal t
o [NO]; 1 mu M = 82 ppm at 1 atm, 1000 K) flowing over Sm2O3 between 1
000 and 1200 K. Samaria pretreated with CH4 (or N-2) at reaction tempe
ratures instantly releases N-2 when exposed to NO. Prompt CO formation
also occurs on methane-conditioned samples. In contrast, stationary o
utflow gas compositions attain only after several reactor residence ti
mes following step (NO + CH4) injections to the untreated catalyst. Ni
tric oxide reduction rates R-No are roughly proportidnal to ([CH4] x [
NO])(1/2) but do not extrapolate to zero at [NO] - 0 and always increa
se with T. We infer that: (1) there is no direct reaction between CH4
and NO on the catalyst surface; (2) instead, NO is reduced to N-2 by r
eaction with oxygen vacancies V, and with nonvolatile carbon-containin
g C-s species created in the heterogeneous oxidation/decomposition of
CH4, respectively; (3) the entire mass, rather than just the surface,
of catalyst microparticles participate in this phenomenon. We propose
a purely heterogeneous mechanism in which physisorbed NO reacts with e
ither vacancies in equilibrium with the active oxygen OR species respo
nsible for CH4 oxidation or with C-s species. The derived kinetic law:
R-(NO) = k(A)([NO](s)[CH4])(1/2) + (k(B)[CH4], with [NO](s) = [NO]/K-
8(-1) + [NO]), in conjunction with the reported Arrhenius parameters,
closely fits rates measured under anoxic conditions. The fact that R-N
O is unaffected by O-2 UP to F-O2 similar to 0.3F(NO) but drops at lar
ger F-O2 inflows, even if O-2 is fully consumed in CH4 oxidation, is c
onsistent with the competition of NO and Oz for vacancies. The dissimi
lar observations made in experiments performed in the Torr range stron
gly suggest that solid catalysts promote combustion at such relatively
high pressures.