We studied CO hydrogenation over Co/MgO (10/1) model catalysts using chemical transient kinetics. Quantification of the time-dependent response during fast changes of the gas flow composition enabled the counting of surface amounts of carbon, oxygen, and hydrogen from the onset of adsorption to the steady state of the reaction and vice versa. Under the atmospheric pressure conditions of the reaction, the total amount of adsorbed species exceeded the monolayer limit on Co metal. The time response in transients and back-transients of gaseous reactants and products is in accordance with a CO insertion mechanism. Furthermore, the Anderson-Schulz-Flory chain lengthening probability is directly proportional to the CO pressure, whereas no such dependence is measured for the amounts of accumulating or fading surface carbon.

CO hydrogenation over unsupported Ni model catalysts has been studied by chemical transient kinetics (CTK) to provide insight into the time-dependent surface processes leading to hydrocarbon formation at atmospheric pressures. Buildup and backward transients were triggered by stepwise changes of the CO flow into the reactor. CTK data have been evaluated, for the first time, to allow counting of the number of surface carbon, oxygen, and hydrogen atoms from the onset of catalytic reaction conditions to steady state. In this manner, it is shown that the total amount of these atoms may considerably exceed the monolayer limit on Ni metal. Back transients from CO/H 2 to pure H 2 show that the intermediates react in two steps, of which the second occurs with a common first-order decay time for all hydrocarbons (C 1 to C 4 , in this case). This is in agreement with a chain growth mechanism, in which the C 1 most abundant surface intermediate (masi) is always of the same type. Indications have been obtained that a CO insertion mechanism is in operation to form this masi.

Carbon dioxide hydrogenation on support-free nickel model catalysts was investigated by means of a time-resolved quantitative analysis of chemical transients triggered by abrupt changes in the reactant partial pressures. It was found that carbon dioxide adsorption is strongly affected by the presence of hydrogen and by coadsorption effects and thus influences the reaction rate in the buildup and back transients. The observed transients suggest that two reaction mechanisms operate in parallel, which is consistent with previous results obtained using a Ni singlecrystal termination. The initial reaction rate involves fast direct hydrogenation of CO 2 , whereas the rate under steady-state conditions is lower due to a change in the mechanism involving an oxygen-containing intermediate.