Near-Ambient Pressure XPS and MS Study of CO Oxidation over Model Pd-Au/HOPG Catalysts: The Effect of the Metal Ratio

Abstract

In this study, the dependence of the catalytic activity of highly oriented pyrolytic graphite (HOPG)-supported bimetallic Pd-Au catalysts towards the CO oxidation based on the Pd/Au atomic ratio was investigated. The activities of two model catalysts differing from each other in the initial Pd/Au atomic ratios appeared as distinctly different in terms of their ignition temperatures. More specifically, the PdAu-2 sample with a lower Pd/Au surface ratio (~0.75) was already active at temperatures less than 150 °C, while the PdAu-1 sample with a higher Pd/Au surface ratio (~1.0) became active only at temperatures above 200 °C. NAP XPS revealed that the exposure of the catalysts to a reaction mixture at RT induces the palladium surface segregation accompanied by an enrichment of the near-surface regions of the two-component Pd-Au alloy nanoparticles with Pd due to adsorption of CO on palladium atoms. The segregation extent depends on the initial Pd/Au surface ratio. The difference in activity between these two catalysts is determined by the presence or higher concentration of specific active Pd sites on the surface of bimetallic particles, i.e., by the ensemble effect. Upon cooling the sample down to room temperature, the reverse redistribution of the atomic composition within near-surface regions occurs, which switches the catalyst back into inactive state. This observation strongly suggests that the optimum active sites emerge under reaction conditions exclusively, involving both high temperature and a reactive atmosphere.

The Enhancement of CO Oxidation Performance and Stability in SO2 and H2S Environment on Pd-Au/FeOX/Al2O3 Catalysts

“…Figure 6 shows the Au4f binding spectra of both catalysts. The peaks of Au/FeO x /Al 2 O 3 at 87.3 eV and 83.5 eV were assigned to Au4f 5/2 and Au4f 7/2, respectively, both of which corresponded to Au 0 ( Figure 6 a) [ 42 , 43 ]. After vulcanisation, the peaks corresponding to Au4f 5/2 and Au4f 7/2 increased in energy, appearing at 87.5 eV and 83.7 eV, respectively.…”

Characterization of model and real catalysts by APXPS

“…One of the explored routes is its simultaneous combination with other in-situ techniques, ensuring the same sample conditions and direct correlation of the information from both sources. Gas product characterization by mass spectrometry, proton transfer-reaction mass spectrometry, or gas chromatography allows to confirm that the reaction is taking place and to monitor activity and selectivity [11].…”

Room-Temperature CO Oxidation over Au–Pd Monometallic and Bimetallic Nanoparticle-Supported MgO

“…Because noble metals are expensive, many authors have proven that even when using noble metals in low contents, highly efficient catalysts can be obtained. In addition, many researchers used solo and/or bi-noble metals when manufacturing catalysts to obtain highly effective, durable, and stable catalysts for CO oxidation under optimum conditions. ,,− The efficiency of noble metals as catalysts is very sensitive to many factors including the type of metal oxide support, preparation method, activation temperature, noble metal content, dispersion, and particle size. ,,− In a variety of industrial applications, gold and palladium bimetallic catalysts have demonstrated outstanding catalytic activity and selectivity. ,, …”