An HREELS and TPD study of water on TiO2(110): the extent of molecular versus dissociative adsorption

Temperature-programmed desorption (TPD) and oxygen isotopic labeling studies were used to probe the dissociation of water on the (100) and (110) surfaces of TiO2 (rutile). Water TPD spectra from these two surfaces were distinctive. Three monolayer desorption states were observed for the (100) surface (at 205, 250, and 305 K), while only a single desorption state was observed for the (110) surface (at 270 K). TPD experiments on the surfaces enriched with 18O revealed that water desorbing in the 305 K state from TiO2(100) was isotopically scrambled with the lattice oxygen atoms, strongly suggesting that this TPD state resulted from recombination of surface hydroxyl groups. Isotopic scrambling was not observed for any other desorption state on either surface (in the absence of defects). Since very little water desorption occurred from the (110) surface in the temperature range in which exchange was observed on the (100) surface and since previous HREELS work (Henderson, M. A. Surf. Sci. 1996, 355, 151) indic...

Structural Sensitivity in the Dissociation of Water on TiO2 Single-Crystal Surfaces

A detailed thermodynamic analysis of the double layer at the interface, simple inorganic oxide-aqueous solution, is presented with special emphasis on the assumptions normally made in relating experimental observation to theoretical treatment.

An experimental study of the adsorption of potential-determining ions (H+, OH−) at the rutile (TiO2)/solution interface is reported. The results obtained by potentiometric titration of suspensions of the oxide indicate that equilibration is a two-step process with the initial fast step yielding the desired information on the surface behavior of the system. The point of zero charge (pzc) for TiO2 at room temperature was observed to lie at pH 6.0, independently of its crystalline modification, but was lowered to below pH 4 by heat treatment of the oxide prior to adsorption measurement.

The pzc was also observed to approach the neutral point (12 pKw) with increasing temperature in the temperature range, 25°C to 95°C. A thermodynamic consideration of the transfer of cationic and anionic potential-determining species from the aqueous solution to the surface at the pzc allowed the difference in the standard entropy of transfer to be estimated from this information. The entropy difference (∼2.0 cal/deg mole) for TiO2 is compared with the much larger difference of ∼42 cal/deg-mole obtained from more extensive measurements on AgI. The striking difference between the two double layer systems is to be sought in differences in the structure and composition of the interphasal region separating the bulk phases which, in turn, must be related to the variation in interaction of the polar solvent with the solids, TiO2 and AgI.

Adsorption at the rutile-solution interface: i. thermodynamic and experimental study.

IR-analysis of H-bonded H2O on the pure TiO2 surface

Publisher Summary This chapter discusses the specific poisoning of a catalyst surface. A probe molecule other than the reactant is preadsorbed as a poison and its effect on the catalytic activity studied. From the nature of interaction of this probe molecule with the surface sites one may obtain information regarding the chemical nature of these sites and from the number of poisoning molecules necessary to bring the activity to zero one may estimate an upper limit of the active site density. Besides the obvious difficulty to discriminate between surface sites that purely absorb the probe molecule and those that are involved in the catalytic process as active sites, there are a number of other limitations making it difficult to obtain clear-cut information on the nature of active sites. The chapter describes the proposals that are indispensable for a proper application of this method and the precautions that have to be kept in mind while drawing conclusions from specific poisoning experiments. The chapter illustrates two examples—namely, the dehydration of alcohols and the isomerization of olefins on alumina.

Specific Poisoning and Characterization of Catalytically Active Oxide Surfaces

An infra-red study of the surface of rutile has been carried out as a function of evacuation temperature (100–500°). Below 400° the surface was found to be composed of isolated and hydrogen bonded OH groups, molecular water being removed by evacuation at temperatures 200°. Above ca. 300° the hydrogen-bonded OH groups were progressively removed, the 450°-treated surface containing only isolated OH groups with a stretching frequency of 3740 cm–1. Absence of oxygen treatment of the surface after high-temperature evacuation markedly affects the OH region of the spectra. Rehydration experiments showed a strong interaction of the evacuated surface with water molecules but it has not been possible to decide conclusively whether surface OH groups are reformed on a 450°-treated surface.

G. D. Parfitt

Papers

Infra-red study of the surface of rutile