Adsorption of water and organic vapours on hydroxylated rutile

Abstract

The interaction of water vapour with the surface of rutile (∼4 m2 g–1) has been studied by a combination of three methods: (i) determination of adsorption isotherms, (ii) temperature-programmed desorption (TPD), and (iii) infra-red spectroscopy. A distinction is drawn between dissociative chemisorption of water, which exhibits a TPD peak at 370°C and a heat of desorption of 107kJ mol–1, a strong form of molecular adsorption (with a TPD peak at high coverage at 250°C) and a weak molecular adsorption. It is concluded that the dissociative adsorption occurs heterolytically on Ti—O pairs until ∼50 % of the surface is hydroxylated. Molecular adsorption is considered to occur on isolated titanium ions (strong) and on isolated oxygen ions (weak). Non-dissociatively adsorbed water can be totally removed by evacuation at 325°C, leaving the rutile surface partially hydroxylated. The properties of this surface towards the adsorption of isopropanol and of acetone are described. The vapours are adsorbed to give a type I isotherm, the saturation value being the same in both cases. Desorption has been investigated by TPD. Models are proposed to account for the localized adsorption of the alcohol and ketone on this hydroxylated rutile.The relative strengths of adsorption of water (W), acetone (A) and isopropanol (I) have been further examined by studying the six combinations: W(ads)+A(g); W(ads)+I(g); A(ads)+W(g); A(ads)+I(g); I(ads)+W(g); I(ads)+ A(g). The compositions of the adsorbed and gas phases at equilibrium have been deduced from gas chromatographic analysis. These displacement studies have enabled the pattern for the mutual adsorption characteristics of the three species to be obtained. The results have a particular significance for the rutile-photocatalyzed oxidative dehydrogenation of isopropanol, a process in which the adsorptions of alcohol, ketone and water occur simultaneously.