Phase relations in the ternary system Ga–Pd–Sn at 500 °C
Graphical Abstract
Introduction
Ethylene used for polyethylene production has to be free of acetylene as it acts as a poison for the polymerization catalysts. The best way to get rid of acetylene is to convert it catalytically into ethylene. Thus, semi-hydrogenation of acetylene in the excess of ethylene is an important industrial process that requires the highly selective and stable catalysts [1].
Among all transition metals of the groups 8–10 of Periodic Table that may be used as catalysts for hydrogenation of unsaturated hydrocarbons, palladium shows the highest selectivity to the corresponding alkenes [2]. In industrial processes, Pd-based substitutional alloys are used for the selective semi-hydrogenation of acetylene in the excess of ethylene [3]. Recently, intermetallic compounds Ga7Pd3, GaPd and GaPd2 have been found to be active, stable and highly selective catalysts for this process. Their high selectivity is explained by suitable adsorption of acetylene and ethylene on the Ga–Pd surfaces and absence of hydride formation. According to the active-site isolation concept [4], Pd-containing intermetallic compounds with exclusively heteroatomic coordination of Pd atoms are potential highly selective catalytic materials for the semi-hydrogenation of acetylene in the excess of ethylene. In addition, two-centre electron-deficient covalent bonding between Pd and Ga provides stability against palladium segregation with time on stream [5].
In order to find new catalytic materials among Pd-containing intermetallic compounds with element X, it is necessary to take into account the Pd–X interaction that has to provide:
- (i)
isolation of Pd atoms (high selectivity towards semi-hydrogenation);
- (ii)
modification of Pd d-band (appropriate adsorption of the reactants);
- (iii)
covalent bonding interaction (low reactivity to hydrogen and stability against segregation) [6].
Substitution of Ga by main group elements fulfills these criteria and may improve or modify the promising catalytic performance of Ga–Pd compounds. This is the reason to extend the search on the ternary systems of palladium with gallium and other p-metals or semi-metals.
Ternary systems of palladium with Ga, Sn or another p-metal or semi-metal are not well investigated. Isothermal sections of the phase diagrams have been constructed for Ga–Pd–Si [7], Ga–Ge–Pd [8], As–Ga–Pd [7,9,10], Ga–Pd–Sb [11], In–Pd–Sn [12] and As–Pd–Sn systems [13]. The Ga–Ge–Pd system was investigated experimentally in the Pd-rich region (50–100 at.% Pd) and As–Pd–Sn in the complete concentration range, while other systems – at Pd content lower than 70 at.%.
Here, we present the results of our studies on the ternary system Ga–Pd–Sn at 500 °C. The terminating binary systems Ga–Pd and Pd–Sn have been investigated experimentally for the first time in the whole concentration range by Schubert et al. [14]. At the same time, the Pd–Sn phase diagram was presented by Knight and Rhys [15]. Some regions of the Ga–Pd diagram were reinvestigated by Khalaff and Schubert (49–77 at.% Pd) [16], Wannek and Harbrecht (65–77 at.% Pd) [17]. Detailed investigation of the region with 36–43 at.% Sn in the Pd–Sn system was performed by Sarah et al. [18]. Compiled binary phase diagrams of the Ga–Pd and Pd–Sn systems are presented by Okamoto [19,20]; an assessed phase diagram of the Ga–Sn system is constructed by Anderson and Ansara [21].
Recent investigations on the catalytic properties of Ga1−xSnxPd2 showed the significant and not trivial influence of the Ga/Sn substitution on the catalytic performance [22]. High demands on sample quality in catalysis performance tests motivated us to investigate the phase relation in the Ga–Pd–Sn system in detail. The focus is on accurate determination of solubilities of the third component in the binary phases and existence regions of the phases. The localization of two- and three-phase regions and detailed microstructure characterization are of special interest, too.
Section snippets
Experimental
The samples were prepared from elemental palladium (granules, ChemPur, 99.95 wt%), gallium (pellets, ChemPur, 99.9999 wt%) and tin (granules, ChemPur, 99.999 wt%). Stoichiometric amounts of the elements with a total mass of about 1 g were weighted (±0.1 mg) and melted in the arc furnace within a glove box filled with Ar (O2 and H2O content ≤ 1 ppm). The total mass losses observed during melting were <0.5 wt%. Approximately 0.5 g of each sample were sealed in evacuated quartz glass tubes and
Results and Discussion
The phase relations in the Ga–Pd–Sn system at 500 °C are shown in Fig. 1. It is based on results of quantitative X-ray powder diffraction analysis in combination with detailed characterization of metallographic cross-sections including electron probe EDXS and EBSD analyses. Compositions of the samples were chosen to determine the solubility of the third component at the constant Pd content and to locate three-phase as well as two-phase regions. Homogeneity ranges of the binary phases were
Conclusions
The phase relations, homogeneity ranges and solubility limits of the phases in the ternary system Ga–Pd–Sn are determined by combined phase analysis using XRPD, EDXS/WDXS and – in case of Pd-rich alloys – EBSD on annealed specimens, which were all water quenched after annealing at 500 °C. Metallographic micrographs visualized an important feature of the Ga–Pd–Sn system – residues of remelted parts in Pd-poor alloys and almost as-cast microstructure in Pd-rich alloys reflect the significantly
Acknowledgements
The authors are thankful to P. Scheppan, M. Eckert, S. Kostmann for help during metallographic investigations; Yu. Prots, H. Borrmann, S. Hückmann for XRPD experiments and I. Veremchuk for fruitful discussions.
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