Synthesis of cluster-derived PtFe/SiO2 catalysts for the oxidation of CO

Infrared (FTIR) and extended X-ray absorption fine structure (EXAFS) spectroscopy measurements were used to characterize the species formed after impregnation of Pt5Fe2(COD)2(CO)12 onto silica, before and after removal of the organic ligands. The results indicate that the Pt5Fe2(COD)2(CO)12 cluster adsorbs weakly on the SiO2 surface. Nevertheless, partial disintegration of the cluster was observed during aging even under He and at room temperature, related to the loss of CO ligands due to their interactions with silanol groups of the support. The organic ligands can be removed from a freshly impregnated cluster by thermal treatment in either He or H2, but the surface species formed in each case have different structures. Treatment in He at 350 °C leads to a complete disintegration of the Pt–Fe bimetallic core and results in the formation of highly dispersed Pt clusters with a nuclearity of six, along with surface Fe oxide-like species. In contrast, bimetallic PtFe nanoparticles with an average size of approximately 1 nm were formed when a similar H2 treatment was used. In this case, a greater degree of metal dispersion and a larger fraction of Pt–Fe interactions were observed compared to the PtFe/SiO2 samples prepared by co-impregnation of monometallic salt precursors. Electronic interactions between Pt and Fe atoms in such cluster-derived samples led to an increased electron density on platinum, as indicated by a red shift of the frequencies of FTIR bands for adsorbed NO and CO. These electronic interactions affect the strength of the CO adsorption on platinum. All bimetallic samples were found to be more active than Pt/SiO2 for the oxidation of CO in air; however, the activity depends strongly on the structure of the surface species, the fraction of Pt–Fe bimetallic contributions, the degree of electronic interactions between Pt and Fe, and the strength of the CO adsorption on platinum.