Chemical Transient Kinetics Applied to CO Hydrogenation over a Pure Nickel Catalyst

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.