Surface Behavior of Alumina-Supported Pt Catalysts Modified with Cerium as Revealed by X-ray Diffraction,X-ray Photoelectron Spectroscopy,and Fourier Transform Infrared Spectroscopy of CO Adsorption

摘要

The effects of the carrier and pretreatment temperature on the oxidation state of Ce and Pt were evaluated in Pt/Al_2O_3,Pt/CeO_2,and Pt/CeO_2-Al_2O_3 catalyst samples with various CeO_2 loadings by applying the following spectroscopic techniques to the samples: X-ray diffraction (XRD),X-ray photoelectron spectroscopy (XPS),and Fourier transform infrared (FTIR) spectroscopy of CO adsorption.XRD showed that ceria agglomerates at high CeO_2 loadings (>3 wt %) and at a high temperature of calcination.XPS spectra showed that Ce(III) is formed more easily in the presence of Pt caused by the electron transfer between Pt and CeO_2.A variation in the intensity and a shift in the IR bands from CO adsorbed on reduced Pt were observed,as a function of the temperature of reduction and type of the support.A heterogeneous distribution of linearly adsorbed CO on Pt at terraces and defect sites was proposed for Pt/Al_2O_3 reduced at the lower temperature (623 K),while at the higher temperature of reduction (773 K) a complete reduction of the Pt was observed.The higher reduction temperature and increasing the CeO_2 loading led to a more uniform distribution of Pt on Pt/CeO_2~-Al_2O_3,related to the presence of highly unsaturated Pt caused by the strong interaction between Pt and Ce-modified alumina.The electronic effect dominates in Pt/CeO_2 samples reduced at very high temperature (973 K).