Kinetics of metal-catalyzed reactions of alkanes and the compensation effect

Journal Article · Tue Oct 01 04:00:00 EDT 1996 · Journal of Catalysis

DOI:https://doi.org/10.1006/jcat.1996.0333· OSTI ID:501842

Bond, G C; Hooper, A D; Slaa, J C; Taylor, A O ^[1]

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1. Brunel Univ., Uxbridge (United Kingdom)

For a bimolecular reaction A + B {yields} C obeying simple Langmuir-Hinshelwood kinetics, the apparent activation energy E{sub app} is a function of the gas-phase concentrations of each reactant; where A is the more strongly chemisorbed, the limiting values of E{sub app} at low and high pressures of A are give respectively by (T{sub t} + n{sub A}{Delta}H{sub A}{sup {theta}} + n{sub {beta}}{Delta}H{sub {Beta}}{sup {theta}}) and E{sub t} - n{sub A}{Delta}H{sub A}{sup {theta}} + n{sub {Beta}}{Delta}H{sub {Beta}}{sup {theta}}, where E{sub t} is the true activation energy, n{sub A} and n{sub B} are the magnitudes of the orders of reaction (each having a value of one for these limiting cases), and {Delta}H{sub A}{sup {theta}} and {Delta}H{sub {Beta}}{sup {theta}} are the enthalpies of adsorption of the two reactants. E{sub t} is usually greater than E{sub app}, and values of E{sub app} and the corresponding values of In A{sub app} show a compensation effect. With alkane hydrogenolysis, the opening step is the endothermic dehydrogenative chemisorption of the alkane; this accounts for the very high values of E{sub app} that are often observed. In the case of Ru/Al{sub 2}O{sub 3} catalysts, analysis of the rate dependence on H{sub 2} pressure by an expression based on the subsequent rate-limiting C-C bond breaking affords values of E{sub t} that are generally {approx}60 kJ mol{sup -1} and values of {delta}H{sub C}{sup {theta}} (C = alkane) in the range 60-80 kJ mol{sup -1}. At H{sub 2} pressures greater than that at which the rate is maximal, E{sub app} exceeds E{sub t}; below the maximum, the reverse is the case. Compensation between E{sub app} and ln A{sub app} is again found; other literature reports confirm the general validity of the model. 28 refs., 11 figs.

• OSTI ID: 501842
• Journal Information: Journal of Catalysis, Journal Name: Journal of Catalysis Journal Issue: 2 Vol. 163; ISSN JCTLA5; ISSN 0021-9517
• Country of Publication: United States
• Language: English

Related Subjects

02 PETROLEUM
40 CHEMISTRY
ACTIVATION ENERGY
ALKANES
ALUMINIUM OXIDES
CATALYTIC EFFECTS
CHEMICAL REACTION KINETICS
HETEROGENEOUS CATALYSIS
HYDROGENATION
RUTHENIUM