Palladium alloy model systems forunderstanding the surface properties of Pd-based membranes and catalysts

Abstract

Palladium has high solubility, permeability and selectivity for hydrogen, thus being a suitable membrane material for hydrogen separation. In addition, palladium is a versatile catalyst with applications in industrial processes such as CO removal in car exhaust or total oxidation of hydrocarbons. In this thesis, surface science techniques such as X-ray photoelectron spectroscopy, scanning tunneling microscopy, and low energy electron diffraction, as well as theoretical tools including solid state kinetic analysis and density functional theory, have been utilized for atomic level investigations of Pd(100) and PdAg(100) surfaces as model systems for Pd-based membranes and catalysts. The thesis aims at improving the understanding of the influence of the alloying element Ag and surface oxides on the catalytic properties of palladium and palladium alloys, by studying model system Pd(100) and PdAg(100) crystals surfaces.

Clean Pd(100) and PdAg(100) have distinct surface signatures. While for Pd(100) the contribution from the surface region in the Pd 3d5/2 core level is observed at lower binding energy relative to the bulk Pd component, for Pd75Ag25(100) the surface contribution is observed at higher binding energies relative to the bulk Pd75Ag25 component, which is found to be a signature of Pd embedded in Ag. The composition of the clean Pd75Ag25(100) surface shows enrichment of Ag at the top most layers. On both surfaces a (√5×√5)R27º surface oxide structure can be formed. The oxidation of the Pd75Ag25(100) surface by molecular oxygen results in a (√5×√5)R27º surface oxide structure similar to the one reported for Pd(100). The interface layer between the surface oxide and the bulk Pd75Ag25 was found to be rich in Ag. The calculated core level shifts for the oxidized surface are in good agreement with the experimental observations.

The reduction of the (√5×√5)R27º surface oxide structure on the Pd(100) and Pd75Ag25(100) surfaces by CO was found to proceed faster with increasing temperature. Kinetic analysis indicates that the reduction process is phase boundary controlled for Pd(100) in the temperature range from 30° C to 120º C. On Pd75Ag25(100) the surface oxide reduction is significantly slower compared to Pd(100). A phase boundary controlled surface oxide reduction is observed at temperatures of 120º C and above, while at and below 70º C the reduction is found to be diffusion limited. Density functional theory calculations show that the presence of silver in the outermost surface layer significantly increases the CO diffusion barriers on the reduced areas, supporting a diffusion limited reduction process for Pd75Ag25(100) at lower temperatures. Compared to the CO case, reduction by H2 is significantly more complex. The reduction of the (√5×√5)R27º surface oxide on Pd(100) and Pd75Ag25(100) using H2 show a complex non monotonic temperature dependent behavior. For Pd(100) the surface oxide reduction is rather independent of temperature, while for Pd75Ag25(100) is slowest at 30º C, increases at intermediate temperatures, and decreases at 170º C. The dependence in temperature of the reduction rates for Pd75Ag25(100) correlates with the amount of surface Pd atoms. As with CO, reduction proceeds slower on Pd75Ag25(100) compared to Pd(100).

In situ high-pressure x-ray photoelectron spectroscopy of the catalytic CO oxidation over Pd(100) at different partial pressures of O2 and CO form both O-covered Pd(100) and a (√5×√5)R27º surface oxide as stable, highly active phases. It was concluded that at near stoichiometric O2/CO pressure ratios a total pressure of 1.3 mbar is at the edge of the "pressure gap", above which formation of oxide phases on Pd(100) should be observed. In comparison no oxide phase is observed on Pd75Ag25(100) at similar pressure. The catalytic CO and H2 oxidation on Pd(100) and Pd75Ag25(100) surfaces under oxygen rich conditions at near-realistic pressures show that below a critical temperature of about 185º C and 200º C for Pd(100) and Pd75Ag25(100), respectively, the oxidation activity is low, showing no significant CO2 and H2O production due to CO poisoning of the surface. Above the critical temperature the reaction enters a high activity regime, which is accompanied by formation of a (√5×√5)R27º surface oxide on Pd(100) and a chemisorbed oxygen phase on Pd75Ag25(100). Additionally, for Pd75Ag25(100), above the critical temperature 200º C, further increasing the temperature leads to decay in CO2 production, while H2O formation is not affected. The presence of silver in the outermost surface layer significantly affects the surface chemistry during these reactions and thereby the reaction mechanism.