Abstract: Bimetallic systems attract the great interest of many scientific groups due to its ability to induce the significant improvement of catalytic properties compared to monometallic catalysts [1-2]. One of the most frequently studied bimetallic systems is the Pd-Au catalysts due to their high catalytic activity in a number of industrially important reactions. Although a large number of studies of Pd-Au systems have been published in recent years, the reasons of synergistic effects in different catalytic reactions have not been rationalized yet. One of the most evident proposals, which have been supported by many researchers, is the key role of surface composition of bimetallic catalysts. It is well known 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 atomic ratios in the bulk and surface [1-2]. It is also evident that surface composition can be varied under the influence of reaction mixture due to enrichment of the surface with one of the metals. Thus the detailed study of surface structure and composition of bimetallic 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. Success in such a study is impossible without development of procedures for synthesis of bimetallic Pd-Au catalysts with controlled particle size and Au/Pd ratio. Furthermore, low loading of the active metals (< 1-2 wt.%) in high surface area supported catalysts limits the application of surface sensitive techniques. Application of model catalysts, where metal particles deposited on a planar support, could help to get more reliable data concerning surface structure and chemical composition of active metals depending on different treatments [3-4]. Formation of the model bimetallic Pd-Au/HOPG (highly oriented pyrolitic graphite) catalysts has been investigated with XPS and STM [3]. 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. The prepared Pd-Au/HOPG catalysts were investigated in CO oxidation with combination of NAP XPS and MS techniques. The samples have shown catalytic activity at temperatures above 150C. The redistribution of Au and Pd on the surface depending on the reaction conditions has been demonstrated. The Pd enrichment of the bimetallic particles surface under reaction mixture has been shown. Apparently CO adsorption induces the Pd segregation on the surface. Under reaction conditions (T > 150C) decomposition of Pd-CO species with simultaneous Pd-Au alloy formation on the surface takes place. In inactive state of the catalyst (after cooling down back to RT) reversible Pd segregation due to Pd- CO formation has been observed. It has been shown that the in situ studies are necessary for the investigation of the active sites in Pd-Au bimetallic systems. Acknowledgements The authors would like to thank Russian Science Foundation (Grant No. 14-23-00146) for the financial support of this work. References 1. C.W. Yi, K. Luo, T. Wei, D.W. Goodman, J. Phys. Chem. B., 109, 18535 (2005) 2. A. Wang, X.Y. Liu, C.-Y. Mou, T. Zhang, J. Catal., 308, 258 (2013) 3. A.V. Bukhtiyarov, I.P. Prosvirin, V.I. Bukhtiyarov, Appl. Surf. Sci., 367, 214 (2016) 4. A.V. Bukhtiyarov, R.I. Kvon, A.V. Nartova, V.I. Bukhtiyarov, Russian Chem. Bull., 60, 1977 (2011)

Cite: Bukhtiyarov A.V. , Prosvirin I.P. , Saraev A.A. , Klyushin A.Y. , Knop-Gericke3 A. , Bukhtiyarov V.I.
CO Oxidation on the Model Pd-Au/HOPG Catalysts: NAP XPS and MS Study
In compilation 6th International Congress on Operando Spectroscopy. 2018. – C.33.

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