PNNL

Abstract

The chemistry of acetone on the oxidized and reduced surfaces of TiO2(110) was examined using temperature programmed desorption (TPD) and high resolution electron energy loss spectroscopy (HREELS). The reduced surface was prepared with about 7% oxygen vacancy sites by annealing in ultrahigh vacuum (UHV) at 850 K, and the oxidized surface was prepared by exposure of the reduced surface to molecular oxygen at 95 K followed by heating the surface to a variety of temperatures between 200 and 500 K. Acetone adsorbs molecularly on the reduced surface with no evidence for either decomposition or preferential binding at vacancy sites. Based on HREELS, the majority of acetone molecules adsorbed in an ?¹ configuration at Ti4? sites through interaction of lone pair electrons on the carbonyl oxygen atom. Repulsive acetone-acetone interactions shift the desorption peak from 345 K at low coverage to 175 K as the first layer saturates with a coverage of ~ 1 ML. In contrast, about 7% of the acetone adlayer decomposes when the surface is pretreated with molecular oxygen. Acetate is among the detected decomposition products, but only comprises about 1/3rd of the amount of acetone decomposed and its yield depends on the temperature at which the O2 exposed surface was preheated to prior to acetone adsorption. Aside from the small level of irreversible decomposition, about 0.25 ML of acetone is stabilized to 375 K by coadsorbed oxygen. These acetone species exhibit an HREELS spectrum unlike that of ?¹-acetone or of any other species proposed to exist from the interaction of acetone with TiO2 powders. Based on the presence of extensive ¹6O/¹8O exchange between acetone and coadsorbed oxygen in the 375 K acetone TPD state, it is proposed that a polymeric form of acetone forms on the TiO2(110) surface through nucleophilic attack of oxygen on the carbonyl carbon atom of acetone, and is propagated to neighboring ?¹-acetone molecules. This process is initiated at temperatures as low as 135 K based on HREELS. Although the dominant thermal pathway of this surface species is to liberate acetone in UHV, it may be a key intermediate in acetone thermal and photolytic chemistry on TiO2 surfaces.