CHROMYL CHLORIDE CHEMISTRY ON THE TIO2(110) SURFACE

The surface chemistry of chromyl chloride (CrO2Cl2) on TiO2(110) was e xamined with TPD, AES, Delta Phi, measurements, SSIMS, and XPS. CrO2Cl 2 adsorbs on TiO2(110) at 130 K with a constant, coverage-independent sticking probability, which is presumably near unity based on TPD. Del ta Phi data suggest the molecule is adsorbed with the oxygens bound to the surface and the chlorines pointing into vacuum. In TPD, multilaye r CrO2Cl2 desorption occurs at 156 K, and monolayer desorption states occur at about 295 and 430 K. Approximately 0.09 ML. (1 ML = 5.2 x 10( 14) sites/cm(2)) desorbs in each of the latter states. The remaining a dsorbed CrO2Cl2 (0.48 ML) decomposes irreversibly above 500 K, eventua lly resulting in CrCl2 desorption above 700 K. No other decomposition products are observed in TPD. Adsorption of CrO2Cl2 at 585 K results i n multilayer CrCl2 formation, while adsorption at or below 500 K resul ts in a saturated monolayer. Between 500 and 700 K, XPS results sugges t that the Cr deposited from CrO2Cl2 decomposition is reduced, possibl y as the result of charge transfer from the defects in the n-doped TiO 2(110). SSIMS experiments conducted on the O-18-enriched TiO2(110) ind icate that O-16 brought in by CrO2Cl2 is incorporated into the surface , whereas TPD indicates that only a small fraction of this incorporate d O-16 is due to isotopic exchange into the desorbing parent at 400 K. Since no oxygen-containing decomposition products were observed in TP D, the decomposition of CrO2Cl2 on TiO2(110) involves reaction at redu ced surface sites (oxygen vacancies). As evidence for this, SSIMS indi cates that the reaction of O-18(2) With (TiO2)-O-16(110) resulting in O-18 incorporation proceeds above 520 K presumably by the formation of oxygen vacancies from O-16(2) desorption. These results suggest that the reduction and potential immobilization of Cr(VT) species on TiO2 m aterials may occur thermally if the appropriate surface defect sites a re present.