Oxidation of propylene over Pd 551 Temperature hysteresis induced by carbon deposition and oxygen adsorption

The oxidation of propylene over a Pd(5 5 1) single crystal surface has been studied by X-ray photoelectron spectroscopy (XPS) and temperature-programmed reaction spectroscopy during both heating and cooling in various oxygen/propylene mixtures. In all the experiments, the partial pressure of propylene was approximately 1 × 10^-7 mbar and the partial pressure of oxygen was varied to achieve molar oxygen/propylene ratios of 1, 3, 10, and 100. Under all these conditions, we observed a temperature hysteresis: temperature dependences of the catalyst activity and product distribution were different during heating and cooling. It was shown that the temperature hysteresis was due to concurrent accumulation of carbon and oxygen atoms on the palladium surface. At low temperatures, a high concentration of carbonaceous deposits detected by XPS resulted in a low catalytic activity due to blocking of the palladium surface. Increasing temperature led to full dehydrogenation of the carbonaceous species and dissolution of carbon atoms into subsurface palladium layers. As a result, even under oxygen-rich conditions, the formation of a PdC_x phase was detected by XPS at 373 K. This process had no influence on the selectivity in the oxidation of propylene at least under UHV conditions. A shift of selectivity toward CO₂ was found to result from an increase in the oxygen concentration on the palladium surface. The state with a low catalytic activity in the oxidation of propylene was associated with palladium in the metallic state covered by carbon deposits. The high-active state of palladium was associated with palladium in the metallic state with a high concentration of chemisorbed oxygen and a moderate concentration of a surface oxide. Bulk PdO was not detected by XPS under all conditions used