MULTIPLE-CYCLE TRANSIENT STUDIES APPLIED TO THE HYDROGENATION OF CO OVER NICKEL/SILICA CATALYSTS.

Temperature-programmed desorption (TPD), temperature-programmed reaction (TPR), and multiple-cycle transient analysis (MCTA) have been used to study the kinetics of hydrogen and carbon monoxide interactions with nickel/silica catalysts. The first two techniques provided valuable kinetic information of the individual steps of adsorption, surface reaction, and desorption. The third technique provided time-dependent product data under experimental conditions that are close to steady-state operation of the catalyst. A model of the reaction mechanism is proposed that is consistent with experimental results obtained over a wide range of H//2/CO ratios, flow rates, and temperatures. In the model, competitive chemisorption of H//2 and CO on the Ni surface plays an essential role in determining the rate of hydrocarbon formation. Surface CO is dissociated with the assistance of surface H to produce surface C. This is followed by hydrogenation of surface C at a rate comparable to the CO dissociation step. The model demonstrates that the surface is predominately covered by CO during the reaction, and a small fraction of the surface is covered by H, which controls the rate of methane formation.