CO oxidation over Pd and Cu catalysts III. Reduced Al 2O 3-supported Pd

Abstract

CO oxidation by O 2 over Pd was studied between 300 and 450 K and at relatively high partial pressures (up to 100 Torr CO and 180 Torr O 2). As the temperature increased, the apparent activation energy increased, the pressure dependency on CO changed from near 0 to −1, and above 50 Torr the O 2 dependency increased from 0.2 to 0.7. Below 400 K a first-order O 2 dependence was observed at lower O 2 pressures. IR spectra were obtained under steady-state reaction conditions to monitor the CO species present on the Pd surface. The spectra were consistent with the negative-order dependence on CO and higher rates were observed when the surface concentration of CO decreased, and they also provided evidence for compressed layers of CO below 400 K. At the higher O 2 pressures employed, a normal Langmuir-Hinshelwood (L-H) model appears applicable in the regime above 400 K, whereas a reaction between CO and O atoms at the perimeters of their respective islands seems more appropriate below 400 K. The rate parameters derived from the two rate equations are extremely consistent with this transformation as the activation energy of the rate-determining step shifted from 15 kcal/mole in the island regime to 23 kcal/mole in the normal L-H regime, in agreement with the shift from 14 to 25 kcal/mole reported by Engel and Ertl on a Pd(111) surface. Furthermore, the heats of adsorption derived from the K CO and K O values in the L-H rate expression are similar to Q ad values reported on nearly saturated Pd surfaces, while the values obtained from the island rate expression are lower, which is expected if compression occurs. Under conditions where O 2 adsorption appeared to be the slow step, this elementary step appeared to remain nonactivated.