Steady-state co oxidation kinetics over the Pd(100) single crystal surface and the c(2 × 2)-Sn/Pd(100) bimetallic surface alloy

Abstract

The kinetic rates for CO oxidation have been measured on the surface of Pd(100) surface and the c(2 × 2)-Sn/Pd(100) surface alloy ( Θ Sn = 0.5 ML) in a high pressure/ultra-high vacuum surface analysis chamber over the temperature range of 443–673 K and the pressure range of 0.3–108 Torr. An Arrhenius activation energy of 22 kcal/mole was measured for CO oxidation at the Pd(100) single crystal surface. Furthermore, the reaction order for the CO was found to decrease from -0.2 to -0.9 as the pressure was lowered from 16 to 1 Torr CO ( P O2 = 8 Tort), and the O 2 reaction order was found to increase from 0.6 to 1.0 at CO pressures of 16 and 1 Torr, respectively. These observations are in agreement with those previously reported for supported Pd/SiO 2 catalysts (Cant et al., J. Catal. 54, 372 (1978)). These changes are attributed to the dramatic variation in heat of adsorption seen on the Pd(100) surface (34 to less than 20 kcal/mole at saturation coverage (nonlinear function)). The kinetic behavior of the c(2 × 2)-Sn/Pd(100) bimetallic surface alloy was similarly studied and the activation energy for the CO oxidation reaction was found to be 12 kcal/mole. The c(2 × 2)-Sn/Pd(100) surface alloy was ascertained to have catalytic reaction orders for CO and O 2 of 0.2 and 0.1, respectively. This is attributed to an increase in the oxygen surface coverage in the form of SnO x ( x = 1–2) atop the Pd(100) template. Possible roles that the SnO x plays in accelerating this reaction are suggested. The surfaces were also characterized by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and thermal desorption mass spectroscopy (TDMS) prior to and following high pressure catalytic reactions.