The influence of support on the low-temperature activity of Pd in the reaction of CO oxidation .3. Kinetics and mechanism of the reaction

The unsteady and steady-state kinetics of low-temperature CO oxidation over Pd supported on TiO2, SiO2, and Al2O3 were investigated by using pulse/flow microcatalytic installation. In situ Fourier-transformed IR spectroscopy was used to monitor Pd surface under reaction conditions. The kinetic parameters were found to differ considerably from those typical for the high-temperature region. Room-temperature CO oxidation was demonstrated to be structure-sensitive and to proceed via interaction between weakly bound CO and oxygen located at defect centers. Analysis by a semiempirical interacting bonds method showed weakly bound forms of CO to appear due to cooperative Pd-Pd bond rearrangement induced by CO adsorption/desorption processes, while oxygen incorporation into the subsurface layer hinders this reconstruction, thus strengthening Pd-CO bond. In the range of 298-373 K, transition to a mechanism of the usual Langmuir-Hinshelwood type occurs, affecting observed kinetic features of the reaction. (C) 1996 Academic Press, Inc.