Surface reactivity of Pd nanoparticles supported on polycrystalline substrates as compared to thin film model catalysts: infrared study of CH3OH adsorption

A detailed comparative study of CH3OH adsorption on two categories of supported Palladium nanoparticles is reported: (1) MgO and gamma-Al2O3-supported Pd metal catalysts prepared by impregnation techniques and characterized by different degrees of regularity and perfection and (2) single-crystal-based Pd/Al2O3 model catalysts prepared under ultrahigh vacuum (UHV) conditions. A detailed structural characterization of the supported Pd nanoparticles allows the assignment of vibrational frequencies of CH3OH and its decomposition products to well-defined types of sites on these systems. The decomposition of methanol on both types of catalysts is compared as an example for a reactivity study. On the model catalyst, infrared reflection-absorption spectroscopy (IRAs) experiments shed light on the role of two decomposition pathways, dehydrogenation and C-O bond scission. The effect of carbon contamination in the vicinity of edge and defect sites is explored. For the Pd/MgO systems studied by transmission FTIR, a mechanism for the methanol decomposition/synthesis is proposed, which involves the simultaneous formation of carbonyl species on Pd particles (at room temperature) and methoxy on the MgO matrix. The MgO matrix is basic and hence the surface OH groups can react with CO species initially formed on Pd particles and spilled onto the matrix. This reaction channel results in the formation of intermediates with a formate-type structure, which can be easily detected by IR spectroscopy. The superior activity of Pd/MgO in the decomposition/synthesis of methanol is explained on the basis of a cooperative effect between the Pd particles and the basic matrix. (C) 2004 Elsevier Inc. All rights reserved.