INFRARED STUDY OF THE DYNAMICS OF ADSORBED SPECIES DURING CO HYDROGENATION

The dynamics of adsorbed species on Rh/SiO2 catalyst during CO hydrogenation to form methane was studied using in situ infrared (IR) spectroscopy combined with steady-state isotopic transient and pulsing CO methods. In situ IR spectra and transient responses reveal that gaseous CO rapidly exchanges with adsorbed CO and hydrogenation of CHx intermediate is the rate-determining step under steady-state conditions. Compartment modeling and site distribution analysis of the transient responses show that CHx is hydrogenated to CH4 with a bimodal distribution of the rate constant. When CO is pulsed into H-2 flow, CO dissociation is the rate-determining step and linear CO depletes faster than bridged CO. The rate-determining step for methanation is a delicate balance of intermediate concentrations, surface intrinsic properties, and reaction conditions. Mechanistic and kinetic studies of catalytic reactions have to be performed under practical reactor conditions. (C) 1995 Academic Press, Inc.