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

Andrey V Bukhtiyarov,Igor P Prosvirin,Valery I Bukhtiyarov,Alexander Yu Klyushin,Alexey Yu Fedorov,Maxim A Panafidin,Axel Knop-Gericke,Yan V Zubavichus

doi:10.3390/nano11123292

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.

Highlights

The combination of NAP XPS and mass spectrometry (MS) was applied to study the CO oxidation over model alloyed Pd–Au/highly oriented pyrolytic graphite (HOPG) catalysts with different metal ratios (Pd/Au) on the surface
The catalytic activities were found to be different for two samples with different initial Pd/Au atomic ratios
At temperatures below 100 ◦ C for both samples, the exposure to the mixture of carbon monoxide and oxygen gives rise to a disturbance of the alloy structure due to surface segregation of palladium atoms accumulating on the surface and depleting subsurface layers

Summary

Introduction

The PdAu system is among most studied due to its enhanced catalytic properties in a number of reactions, such as vinyl acetate synthesis [11], low-temperature CO oxidation [12,13], NO reduction [14], direct formation of hydrogen peroxide from an H2 + O2 mixture [15] and others. Nanomaterials 2021, 11, 3292 unambiguously by the amount of initially introduced metals, but is strongly affected by the calcination temperature. A redistribution of the metals in a particle can occur directly during the catalytic reaction, under the influence of the temperature and reaction mixture [6,7,8,9,10].

Methods

Results

Conclusion