The effect of Fe on SiO2-supported Pt catalysts: Structure, chemisorptive, and catalytic properties

SiO2-supported PtFe catalysts with a wide range of Pt/Fe ratios were prepared from individual H2PtCl6 and Fe(NO3)(3) precursors and characterized by high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared (FTIR), and extended X-ray absorption fine structure (EXAFS) spectroscopies. Treatment with H-2 at 350 degrees C leads to the formation of metal particles with average sizes in the order of 2.6 nm, some of which are bimetallic in nature. The fraction of Pt-Fe bimetallic contributions in each sample, the nature and extent of electronic interactions between Pt and Fe, and the strength of the CO adsorption on Pt strongly depend on the Fe content. PtFe/SiO2 samples thus prepared were found to be active catalysts for various reactions taking place in both oxidative and reducing environments, including the oxidation of CO in air, the dehydrogenation of cyclohexane, and the selective hydrogenation of citral. The results indicate that the catalytic behavior of Pt is significantly affected by the presence of Fe. The enhancement of the catalytic activity observed for the oxidation of CO in air correlates with the fraction and degree of electronic Pt-Fe interactions and the strength of CO adsorption on Pt. Similarly, Fe in small concentrations promotes the activity of Pt for the dehydrogenation of cyclohexane and the selective hydrogenation of citral, which can be attributed to either an electronic effect and/or the presence of bimetallic Pt-Fe sites. The close proximity between Pt and Fe in such sites leads to the reduction of Fe, which can thus become active for cyclohexane dehydrogenation. Furthermore, it is possible that Pt-Fe adsorption sites favor the di-sigma(CO) mode of adsorption for alpha,beta-unsaturated aldehydes, thus promoting the selective hydrogenation of the C=O bond and the formation of alpha,beta-unsaturated alcohols. (C) 2009 Elsevier Inc. All rights reserved.