Abstract
Reactions of cyclohexanone, cyclohexenone, and p-benzoquinone were investigated on TiO2(001) single crystal surfaces. On the reduced, sputtered surfaces, the following reactions were observed: partial reduction of the cycloketones to their corresponding alcohols and of p-benzoquinone to phenol; reductive coupling to form C6H10=C6H10 and C6H8=C6H8 from cyclohexanone and cyclohexenone respectively; and reductive coupling/reduction of p-benzoquinone to benzene, biphenyl, and terphenyl. The reduced surface was oxidized upon adsorption of the ketones as evidenced by an increase of the intensity of the XPS Ti(2p(3/2)) peak at 459.1 eV corresponding to Ti+4 cations. The population of Ti+4 cations in the region sampled by XPS increased by 6.8%, 11.2%, and 16.8% upon adsorption at 300 K of cyclohexanone, cyclohexenone, and p-benzoquinone, respectively. The activity of the reduced surface for reductive coupling was highest for p-benzoquinone and lowest for cyclohexanone. The observation of reductive coupling on single crystal surfaces in ultrahigh vacuum suggests that gas-solid contact catalysis of this reaction may be feasible.