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X-ray spectroscopy

A study on structural features of bimetallic Pd-M/C (M: Zn, Ga, Ag) catalysts by EXAFS

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  • Anna TSAPINA

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Nowadays, palladium supported on different carriers is one of the most intensively studied catalytic systems. Pd-based catalysts exhibit excellent activity in such important processes as hydrogenation of organic acids and oils, selective hydrogenation of acetylene to ethylene, oxidation of methane and carbon monoxide, etc. An enhancement of catalytic properties is achieved by using Pd-based bimetallic systems [1]. In the case of bimetallic catalysts for the selective hydrogenation of acetylene to ethylene, a key role is played by a dilution of Pd atoms with atoms of modifying metal [1-2]. Doping the Pd-based catalysts with second metal leads to an increase in the distance between neighboring atoms of palladium and the creation of the new type of active sites that facilitate an improvement of the process selectivity thus increasing the yield of target product. At the same time, the contribution of electron interaction of Pd with modifier is of significant importance. Atoms of second metal act as donors of electron density for vacant d-orbitals of palladium that suppresses an affinity of Pd towards adsorption of unsaturated compounds. It facilitates ethylene desorption and impedes its complete hydrogenation to ethane.

The aim of the present research was to study in detail the local structure of carbon-supported bimetallic Pd-Ag, Pd-Zn and Pd-Ga catalysts by Extended X-Ray Absorption Fine Structure spectroscopy (EXAFS). The EXAFS method offers the possibility to obtain the interatomic distances and near neighbor coordination numbers of the supported metal nanoparticles. The supported bimetallic catalysts were prepared via an incipient wetness impregnation of the pretreated by a solution of nitric acid porous carbonaceous graphite-like material Sibunit used as a support with solutions containing corresponding metal nitrates. Then, the samples were dried in air and reduced in a hydrogen flow at 500°C. The palladium loading was 0.5 wt%, and Pd:M ratio was 1:1. The research was performed at the “Structural Materials Science” beamline of the Kurchatov synchrotron radiation source (SRC “Kurchatov Institute”, Moscow).

According to the EXAFS data, the Pd-Ag/C catalyst contains separate nanoparticles of metallic palladium and silver as well as bimetallic particles of PdxAgy nanoalloy with stoichiometry close to Pd:Ag = 1:1. The analysis of the Pd and Zn K-edge absorption indicates that the Pd-Zn/C catalyst contains the bimetallic nanodispersed PdZn phase, which characterized by the reduced Pd-Pd coordination number; the Zn-doped catalyst contains metallic Pd, PdO and ZnO oxide formed as a result of contact with air. The composition of PdZn phase is deviated from the stoichiometry towards its enrichment with zinc atoms and the surface segregation of zinc from the bimetallic structure. As it follows from the Pd and Ga K-edge EXAFS data, the amount of the joint Pd-Ga phase in the Ga-doped catalyst is quite small. Some part of the alloyed Pd-Ga particles as well as nanoparticles of metallic palladium can be blocked by a layer of oxidized gallium, which goes out on the surface owing to segregation.

This work supported by the Russian Foundation for Basic Research (grant 17-33-50019). Authors are grateful to E. V. Khramov for his support in the XAS measurements.

[1] S.A. Nikolaev, V.V. Smirnov, L.N. Zanaveskin, K.L. Zanaveskin, V.A. Averyanov, Russ. Chem. Rev. 78 (2009) 231-247.

[2] A.J. McCue, J.A. Anderson, Front. Chem. Sci. Eng. 9 (2015) 142-153.