Characterisation and Reaction Studies of Supported Metal Catalysts

[摘要] A series of silica-, molybdenum trioxide- and tungsten trioxide-supported platinum catalysts (1.1% w/w metal) have been prepared by impregnation of the appropriate oxide support with an aqueous solution of chloroplatinic acid. Series of silica-, molybdenum trioxide-, and tungsten trioxide-supported rhodium catalysts (1.5% w/w metal) have been prepared by impregnation using either aqueous rhodium (III) nitrate or aqueous rhodium (III) chloride solutions. Each of the catalysts has been characterised by a) temperature programmed reduction, b) chemisorption and temperature programmed desorption of carbon monoxide, to determine variations in the nature of the chemisorbed state of carbon monoxide and the metal areas of the different catalysts, c) thermo-gravimetric analysis and differential scanning calorimetry, to obtain information relevant to the chemical nature of the supported salt and its mode of reduction, and d) physical adsorption of nitrogen to determine total surface areas, to obtain information regarding the influence of (i) the support and (ii) the metal salt precursor on the physical and chemical characteristics of the catalyst. Evidence for an effect of both of these variables has been found. Thus, differences have been observed in the temperature programmed reduction profiles and hence in the reduction/activation temperatures of the catalysts, and in the metal areas, and hence dispersions, as determined by carbon monoxide chemisorption and transmission electron microscopy. Thermo-gravimetric analysis and temperature programmed reduction have shown that in both the molybdenum trioxide- and tungsten trioxide-supported metals hydrogen bronze formation occurs. The composition of these hydrogen bronzes have been determined as: Pt/MoO3(a) → H0.9MoO3 by T.G.A., H0.3MoO3 by T.P.R. Pt/WO3(a) → H0.3WO3 by T.P.R. Rh/MoO3(b) →H0.3MoO3 by T.P.R. Rh/WO3(b) → H0.7WO3 by T.P.R. Rh/MoO3(c) → H0.7MoO3 by T.G.A. and by T.P.R. Rh/WO3(c) → H0.36 WO3 by T.P.R. where (a), (b) and (c) denote catalysts prepared using chlroplatinic acid, rhodium(III) nitrate and rhodium(IH) chloride, respectively. Temperature programmed reaction studies of the hydrogenation of carbon monoxide over the supported platinum catalysts and molybdenum trioxide-supported rhodium have shown that the molybdenum trioxide-supported catalysts are particularly good for the methanation reaction and have provided strong evidence for the existence of metal-support inter- actions with this particular support. Buta-1,3-diene hydrogenation studies over the various catalysts have shown that whilst the selectivity is dependent upon the support, the metal and the metal salt precursor, the distribution of the three isomeric n-butenes formed as initial products is, particularly with rhodium, almost independent of the nature of the support and the metal salt precursor used in the preparation. Analysis of the pressure fall against time curves and of variations in the activity of the catalyst as a function of catalyst usage provide indirect evidence for the formation of surface carbonaceous residues and their involvement in the reaction. Deactivation of each of the catalysts during usage has a greater effect on the rate of butene hydrogenation than on the rate of buta-1,3-diene hydrogenation, as witnessed by the disappearance of the acceleration point in the pressure/time curves. The product distributions and their variations with conversion and temperature, activation energies and kinetic orders are interpreted in terms of a mechanism in which (i) but-2-ene formation occurs by both 1,4-addition of hydrogen to adsorbed buta-1,3-diene and by the subsequent isomerisation of but-l-ene formed by 1,2-addition and (ii) the formation of n-butane by a direct route not involving the intermediate formation of butenes. Examination of the turnover frequencies for buta-1,3-diene hydrogenation shows that the activities decrease in the order: (i) Pt/WO3(a) >Pt/MoO3(a) > Pt/SiO2(a) (ii) Rh/WO3(b) > Rh/SiO2(b) and (iii) Rh/MoO3(c) > Rh/SiO2(c) > Rh/WO3(c) although the metal particle sizes as determined by carbon monoxide chemisorption are in the order: (i) Pt/WO3(a) > Pt/SiO2(a) > Pt/MoO3(a) (ii) Rh/SiO2(b) > Rh/WO3(b) and (iii) Rh/WO3(c) > Rh/SiO2(c) > Rh/MoO3(c) suggesting that whilst the reaction is structure-insensitive in terms of metal dispersion, the support plays a significant role in determining the overall activity.

[发布机构] University:University of Glasgow

[关键词] Inorganic chemistry