The selective hydrogenation of acetylene is an important process in a naphtha cracking unit, for which Pd-Ag bimetallic catalysts prepared by an impregnation method are widely used . However, common Ag-promoted Pd catalysts have the limitation of incomplete alloy formation between two metal components because the promoter is generally located randomly over the surfaces of Pd and support. Recently, various methods have been suggested for obtaining more efficient bimetallic catalysts . In this study, Ag- or Cu-promoted Pd catalysts have been prepared using SRM for the addition of the promoter, and their surface properties and performance in acetylene hydrogenation have been compared with those of catalysts prepared by a conventional impregnation method.
The performance of Ag- or Cu-promoted Pd/Al2O3 catalysts in the selective hydrogenation of acetylene was compared for two cases that the promoter was added by a surface redox method (SRM) and by incipient wetness impregnation (IWI). Catalysts prepared by SRM showed higher acetylene conversions and ethylene selectivities than those prepared by IWI in both cases of Ag- and Cu-promotion.
According to the XPS observations of the catalysts, the electron binding energy of Pd was lowered by about 0.7 eV and 0.9 eV when Pd/Al2O3 was promoted with Ag using IWI and SRM, respectively. In the case of promotion with Cu, the binding energy was lowered by 0.2 eV when the promoter was added by SRM but remained unaffected in the case of IWI. The above results indicate that Ag was more effective than Cu in modifying the electronic structure of the Pd surface. It is known that the acetylene conversion decreases and the ethylene selectivity increases due to an increase in the electron density of Pd, which lowers the strength of acetylene adsorption on Pd . The additional lowering of the binding energy due to the promoter addition by SRM, instead of IWI, is believed to originate from the intimate contact between the promoter and the Pd surface in the former case.
IR spectra of CO adsorbed on sample catalysts show that Cu was preferentially deposited on the low-coordinated sites of the Pd surface in the case of Pd-Cu SRM, unlike the cases of two impregnated catalysts and Pd-Ag SRM that showed a more rapid decrease in the intensity of the bridged CO than that of linear CO.
Unsaturated hydrocarbons including acetylene and ethylene, known as electron-rich molecules, should be strongly adsorbed on the electron-deficient sites, such as edge or step sites, of Pd. When these molecules are strongly adsorbed on the low-coordinated Pd sites, the ethylene selectivity should decrease as a result of deep hydrogenation originating from the increased residence time of adsorbed molecules . Cu added by SRM is deposited preferentially at these detrimental sites and consequently improves the ethylene selectivity.
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