For a bimolecular reaction A + B --> C obeying simple Langmuir-Hinshel
wood kinetics, the apparent activation energy E(app) is a function of
the gas-phase concentrations of each reactant; where A is the more str
ongly chemisorbed, the limiting values of E(app) at low and high press
ures of A are given respectively by (E(t) + n(A) Delta H-A(empty set)
+ n(B) Delta H-B(empty set)) and (E(t) - n(A) Delta H-A(empty set) + n
(B) Delta H-B(empty set)), where E(t) is the true activation energy, n
(A) and n(B) are the magnitudes of the orders of reaction (each having
a value of one for these limiting cases), and Delta H-A(empty set) an
d Delta H-B(empty set) are the enthalpies of adsorption of the two rea
ctants. E(t) is usually greater than E(app), and values of E(app) and
the corresponding values of In A(app) show a compensation effect. With
alkane hydrogenolysis, the opening step is the endothermic dehydrogen
ative chemisorption of the alkane; this accounts for the very high val
ues of E(app) that are often observed. In the case of Ru/Al2O3 catalys
ts, analysis of the rate dependence on H-2 pressure by an expression b
ased on the subsequent rate-limiting C-C bond breaking affords values
of E(t) that are generally approximate to 60 kJ mol(-1) and values of
Delta H-C(empty set) (C = alkane) in the range 60-80 kJ mol(-1). At Hz
pressures greater than that at which the rate is maximal, E(app) exce
eds E(t); below the maximum, the reverse is the case. Compensation bet
ween E(app) and In A(app) is again found; other literature reports con
firm the general validity of the model. Our analysis supports a recent
speculation which reported that compensation effects originate in dif
ferences in adsorption enthalpy terms, but the sign of the term for th
e alkane will be positive where its dehydrogenation initiates the reac
tion. Such compensation effects are not however true kinetic phenomena
and are better described by the term apparent. (C) 1996 Academic Pres
s, Inc.