Redispersion and metal-support interactionsof supported platinum catalysts

Abstract

Chemisorption and catalytic properties, sintering and redispersion phenomena, and metal-support interactions of platinum on $\gamma-Al_2O_3$, $SiO_2$, $TiO_2$, and $SiO_2-Al_2O_3$ at high temperatures up to 800$^\circ$C were experimentally studied by means of selective gas adsorption, XRD, TEM, TPD, TPR, and cyclohexene hydrogenation reaction. The platinum dispersion after treatment in all atmospheres above 600$^\circ$C decreases as treatment temperature, treatment time and initial dispersion increase. The rate of sintering is also affected by other factors, namely metal loading and preparation method. Sintering rate in oxygen is faster than that in hydrogen regardless of the supports, and sintering resistance of Pt/$TiO_2$ in hydrogen is much greater than that of other supported platinum catalysts whereas stability of the platinum in oxygen is not dependent on the supports. The increase in dispersion after treatment in oxygen below 600$^\circ$C occurs only for Pt/$\gamma-Al_2O_3$. For Pt/$TiO_2$ only the presence of chlorine during oxidation brings about a significant redispersion. For Pt/$SiO_2$ redispersion does not occur under any condition. The suggested mechanism for redispersion is the spreading of platinum oxide and trapping of migrating species. Redispersion can only occur in the presence of platinum oxide which would be stabilized by forming complex with the support. The method for determining whether redispersion may occur or not for any systems and the conditions needed for redispersion are discussed. The reduction of Pt/$TiO_2$ at high temperature results in sharp-decrease in hydrogen and carbon monoxide chemisorption capacities and catalytic activity which cannot be accounted for by metal sintering. Oxidation-reduction treatments restore the usual properties of the catalyst. The TPD spectrum of Pt/$TiO_2$ shifts to higher temperature by reduction at high temperature and shifts again to its original position by oxidation. Shift of TPD spectrum does...