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Free keywords:
PdGa; acetylene hydrogenation; selectivity; stability; in-situ investigation; XPS; FTIR; CO adsorption.
Intermetallic catalysts
Abstract:
Selective acetylene hydrogenation to ethylene is an important method for removing traces of acetylene in the ethylene feed for the production of polyethylene. Typical catalysts, like Pd dispersed on metal oxides are widely used for this reaction and show a limited selectivity and long-term stability. This can be attributed to the presence of active-sites ensembles on the catalyst surface. These drawbacks can be overcome by using the intermetallic compound PdGa which only possesses isolated palladium atoms in the crystal structure and shows higher selectivity as well as an increased long-term stability compared to the commercial catalysts. In the present work the surface of the intermetallic compound PdGa was probed by in situ XPS as well as CO adsorption using FTIR spectroscopy.
Investigations by UHV XPS revealed a significant modification of the Pd electronic state in the intermetallic compound due to covalent bonding: the Pd3d5/2 peak is shifted by 1 eV to higher binding energy compared to the metallic palladium. In situ measurements showed a high stability of the Pd surface states without appearance of any additional components or significant shifts of the Pd3d5/2 peak when applying a reactive atmosphere and temperature (1.0 mbar of H2 + 0.1 mbar of C2H2, t = 120ºC).
The adsorption of CO on the PdGa compound at room temperature results in appearance of only one band with a maximum at 2047 cm-1, which corresponds to linear Pd–CO carbonyls. The absence of bands due to bridged carbonyls in the observed spectra and the fact that the observed band is not coverage dependent, indicated that the active sites in PdGa are really isolated.
Characterization of PdGa by FTIR and in situ XPS revealed high surface stability during the reaction of acetylene hydrogenation and confirms the isolation of the active Pd site on the surface. In combination with modified electronic Pd states due to covalent bonding in the intermetallic compound it leads to superior catalytic properties like high selectivity and long-term stability in acetylene hydrogenation.
