Abstract:
Effects of pretreatment with oxygen and oxygen-containing compounds (NO, N[subscript 2]O, CO and CO[subscript 2]) on Pd-Ag catalysts for the selective hydrogenation of acetylene have been studied. Enhancement of the catalyst activity for C[subscript 2]H[subscript 2] removal has been found after pretreatment. However, only pretreatment with N[subscript 2]O and NO have revealed the improved ethylene gain. Catalysts have been characterised either on the bulk or surface aspects by various techniques, i.e., XRD, TEM, CO-adsorption, TPR, TPD, TPO, XPS and FT-IR. XRD and TEM suggest bulk phase of Pd-Ag alloy with diameter ca. 6.7 nm after reduction. Modification in the reducibility and hydrogen sorption properties of palladium was influenced by the presence of Ag. Surface analysis by XPS has confirmed the existence of surface Pd-Ag alloy with Ag enrichment after reduction. Pretreatment results in dilution of surface Ag enrichment, indicating from the smallerPd:Ag ratio (Palladium fraction becomes slightly increased 3-13.6%). Significant shift of the Ag 3d binding energy is revealed after NO and N[subscript 2]O pretreatment. The surface after reaction shows no state change of either Pd or Ag compared to those measured prior to reaction, therefore surface modification occurs after pretreatment and is retained after reaction for 8 h. No carbonaceous deposits is formed after 8 h on stream from TPO measurement and XPS. Infrared spectroscopy of NO and N[subscript 2]O treated samples shows the linear complex Pd-NO, nitrate and nitrite species on Ag-Al[subscript 2]O[subscript 3]. C[subscript 2]H[subscript 2] adsorption displays ethylidyne species which is sufficiently decreased by N[subscript 2]O pretreatment. Pretreatment with NO and N[subscript 2]O results in blockage of Pd sites responsible for direct ethane formation via ethylidyne species, ethylene gain is thus increased as a consequence. On the other hand, pretreatment with O[subscritp 2], CO or CO[subscript 2]increases dispersion of Pd on the support, which increases C[subscript 2]H[subscript 2] hydrogenation activity
