CO Oxidation on the Model Pd-Au/HOPG Catalysts: NAP XPS and MS Study
6th International Congress on Operando Spectroscopy
15-19 Apr 2018
||NAP XPS, bimetallic Pd-Au nanoparticles, CO oxidation
|| Bukhtiyarov Andrey Valerievich
, Prosvirin Igor Petrovich
, Saraev Andrey Aleksandrovich
, Klyushin Alexander
, Knop-Gericke Axel
, Bukhtiyarov Valerii Ivanovich
Boreskov Institute of Catalysis SB RAS
Novosibirsk State University
Fritz Haber Institute of the Max Planck Society
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 . 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 300–400C 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 150C. 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 > 150C)
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.
The authors would like to thank Russian Science Foundation (Grant No. 14-23-00146) for the financial
support of this work.
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