Structure of Pt–Co=Al2O3 and Pt–Co/NaY bimetallic catalysts: characterization by in situ EXAFS, TPR, XPS and by activity in Co (carbon monoxide) hydrogenation
Abstract
Temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and in situ extended X-ray absorption fine structure (EXAFS) studies were performed to investigate Pt–Co/NaY and Pt–Co=Al2 O3 bimetallic catalysts. The EXAFS experiments were carried out at the Pt LIII and Co K edges of the same sample. This particular approach allows a precise determination of the electronic and structural characteristics of the metallic part of the catalyst. For both systems in situ reduction under pure H2 results in the formation of nanometer-scale metallic clusters. For both Co and Pt, nearest neighbors are Co atoms. The complete set of parameters implies the presence of two families of nanometer-scale metallic clusters: monometallic Co nanosized particles and Pt–Co bimetallic clusters, in which only Pt–Co bonds exist (no Pt–Pt bonds). TPR and XPS results indicating a reduction of Co2þ ions in Pt–Co/NaY to a greater extent than in Pt–Co=Al2 O3 give evidence of a facilitated reduction. XRD also shows the presence of nanometer-scale particles with only a very small fraction of larger bimetallic particles. In subsequent mild oxidation of the reduced systems the Co nanoparticles are still present inside the supercage of NaY zeolite in bimetallic form and the oxidation of the metallic particles is slowed down. Catalytic behavior is in good agreement with the structure of the Pt–Co bimetallic system.