Thermal Chemistry of Trimethyl Acetic Acid on TiO2(110)

Abstract

On the basis of temperature-programmed desorption and isothermal reaction mass spectrometry, the thermal surface chemistry of trimethyl acetic acid, (CH3)3CCOOH, dosed onto a well-characterized single-crystal TiO2(110) surface is described. Deprotonation occurs at or below 300 K to form trimethyl acetate, (CH3)3CCOO-, and hydroxide, OH-. (CH3)3CCOO- is bound to exposed Ti4+ cations, and OH- involves a bridging oxygen atom of the substrate. On the basis of temperature-programmed desorption and isothermal reaction mass spectrometry, the desorbing products include (CH3)3CCOOH, isobutene (i-C4H8), carbon monoxide, and water accompanied by smaller amounts of other products including methyl isopropenyl ketone (CH2C(CH3)C(O)CH3), isobutane (i-C4H10), and di-tert-butyl ketone, (CH3)3CC(O)C(CH3)3. Much of the (CH3)3CCOO- is relatively stable and decomposes to release mainly carbon monoxide and isobutene above 550 K with a maximum rate at 660 K. Thermal desorption to 750 K leaves a carbon-free surface that is indistinguishable from the initially clean surface. During dosing at 550 K, a steady-state reaction condition is realized with about half the adsorption sites being occupied at any instant.