THERMAL DECOMPOSITION OF MO(CO)? ON OXIDIC SUPPORTS : MECHANISTIC STUDIES TOWARDS THE PREPARATION OF HIGHLY DISPERSED METAL PARTICLES

Abstract

The aim of this thesis is to study the interaction of Mo(CO)6 with oxidic supports using the technique - temperature programmed decomposition (TPDE). In particular, the influence of the extent of hydroxylation of the support on the surface mediated decarbonylation is addressed. Different CO-evolution spectra were obtained for different supports which can be explained based on the density of OH groups and the strength of Lewis acid sites on the supports. Activation energies are derived from the CO peak maxima of the deconvoluted TPDE spectra by means of the Redhead equation. The desorption signal can be deconvoluted into individual signals corresponding to the successive removal of one CO after the other from the complex. With the TPDE technique, intermediate subcarbonyls can be identified on hydroxylated surfaces, while evidence for multinuclear cluster formation can be gathered on highly dehydroxylated surface. Increasing dehydroxylation of the support lowered the temperature for initial CO elimination and increased the temperature for complete decarbonylation. On a hydroxylated surface the mechanism for decarbonylation is that of a nucleophilic ligand exchange between CO and surface OH. On a dehydroxylated surface, Lewis acid sites dominate and assist in decarbonylation via a trans-CO labilisation mechanism. Since the decarbonylation of Mo(CO)6 is highly sensitive to the nature of support surface, the TPDE technique may be useful as a probe for surface acid-base properties. The amount of CO and H2 formed during the decomposition reaction is used to calculate the Mo loading and oxidation state on the support. H2 formation is the result of a redox reaction between Mo and surface OH groups, indicating that after the removal of all CO groups, not metallic Mo, but an oxidised species is obtained. The final oxidation state depends on the number of OH groups present on the surface as well as the Mo loading. A low OH:Mo ratio is essential for obtaining zerovalent Mo particles on the dehydroxylated supports. Besides the dependence of TPDE spectra on the nature of the support and the degree of dehydroxylation of the surface, it also depends on the loading of Mo(CO)6. Adsorbed Mo(CO)6 is highly mobile at the surface, and at low concentration, it will become adsorbed at deep trap sites like kinks or missing atoms within terraces. The energetics of the decomposition reaction over these sites as revealed by TPDE are different from the situation at the more abundant step or terrace sites. By carrying out TPDE at different Mo loading, it is possible to "titrate" certain types of sites. A home-made volumetric adsorption apparatus is built for determination of the dispersion of Mo/support by CO chemisorption at room temperature. The dispersion is found to depend on Mo loading and the method of preparation of the supported metal particles. Higher loading lead to greater extend of clustering (revealed in TPDE) and lower dispersion (revealed in CO chemisorption). A dry prncess or subliming Mo(CO)6 on the support gives lower dispersion than a wet impregnation process using a pentane solution of Mo(CO)6.