CO Oxidation on the Model Pd-Au/HOPG Catalysts: NAP XPS and MS Study
Bimetallic systems attract the great interest of many scientific groups due to its ability to improve significantly catalytic properties in comparison with monometallic catalysts1-2. Good example proving this statement is the Pd-Au catalysts, which exhibit extremely high activity in a number of industrially important reactions. This motivated researchers on numerous investigations of Pd-Au systems, which have shown that not only the ratio of the introduced metals, but also temperature of calcination will affect the surface composition causing the essential difference between Au/Pd ratios in the bulk and surface1-2. Furthermore, surface composition can be varied under the influence of reaction mixture due to enrichment of the surface with one of the metals. Evidently to clarify the influence of the above-mentioned effects on catalytic properties, the detailed in situ study of surface structure and composition of Pd-Au catalysts is necessary to understand the nature of active sites and help to optimize the catalyst composition for the best activity, selectivity and stability.
Preparation of the model bimetallic Pd-Au/HOPG catalysts has been investigated with XPS and STM . Initially, model “core–shell” type Pd–Au/HOPG catalysts with similar particle size distribution (5–8 nm) were prepared. Subsequent annealing of these samples in temperature range of 300–400°C leads to formation of Pd–Au alloyed particles. Treatment of the alloyed Pd–Au/HOPG model catalysts under CO oxidation conditions destroys the alloy structure due to segregation of Pd over Pd–Au particle surface via formation of Pd-COads bonds. Heating the samples restores the alloy structure due to CO desorption even under the reaction conditions. All these changes in the particle structure were identified using NAP XPS technique.
Density functional calculations combined with calculations using topological energy expression method (TOP method) were applied to reveal the mechanism of this phenomenon and to quantify the stability of different arrangements of metal atoms in PdAu nanoparticles in the presence of CO. According to results of this computational approach, adsorption of CO already at a rather moderate coverage is sufficient to make energetically feasible segregation of Pd at terraces of PdAu nanoparticles similar in size with experimentally studied ones.