Sequential Photo-oxidation of Methanol to Methyl Formate on TiO₂(110)

Abstract

Methyl formate is produced from the photo-oxidation of methanol on preoxidized TiO(2)(110). We demonstrate that two consecutive photo-oxidation steps lead to methyl formate using mass spectrometry and scanning tunneling microscopy. The first step in methanol oxidation is formation of methoxy by the thermal dissociation of the O-H bond to yield adsorbed CH(3)O and water. Formaldehyde is produced via hole-mediated oxidation of adsorbed methoxy in the first photochemical step. Next, transient HCO is made photochemically from formaldehyde. The HCO couples with residual methoxy on the surface to yield methyl formate. Exposure of the titania surface to O(2) is required for these photo-oxidation steps in order to heal surface and near-surface defects that can serve as hole traps. Notably, residual O adatoms are not required for photochemical production of methyl formate or formaldehyde. All O adatoms react thermally with methanol to form methoxy and gaseous water at rt, leaving a surface devoid of O adatoms. The mechanism provides insight into the photochemistry of TiO(2) and suggests general synthetic pathways that are the result of the ability to activate both alkoxides and aldehydes using photons.

Methyl Formate Production on TiO2(110), Initiated by Methanol Photocatalysis at 400 nm

“…In a recent study employing temperature programmed desorption (TPD) and scanning tunneling microscopy (STM), Friend and co-workers have deduced the mechanism of CH 3 OH photooxidation on a preoxidized TiO 2 (110) surface. 23 They concluded that coadsorbed oxygen atoms first reacted thermally with CH 3 OH to produce adsorbed CH 3 O and H 2 O. Then CH 3 O underwent photo-oxidation to HCOOCH 3 in a two-step process where the CH 3 O dissociated to CH 2 O and a crosscoupling reaction involving an HCO intermediate led to the formation of HCOOCH 3 .…”

Coverage Dependence of Methanol Dissociation on TiO2(110)

“…17,18 In other studies, molecular methanol was considered to undergo photoinduced dissociation to methoxy through the breaking of the O-H bond, 18,21,33 and then further photooxidized into formaldehyde and methyl formate. 22,23,33,34 Thus, a stepwise breaking of C-H bond, or stepwise photooxidation of methoxy was suggested. However, methoxy was identified to be a photoactive species while the molecularly adsorbed methanol was considered to be photoinactive.…”

Production of Hydrogen: Photocatalytic Decomposition of Dimethyl Ether over Metal-Promoted TiO<sub>2</sub> Catalysts

“…The highest yield of methyl formate was measured for Pt/TiO 2 (62.2) and the lowest one for Ru/TiO 2 (26.0). Recent studies performed under UHV conditions on preoxidized TiO 2 (110) disclosed that methyl formate is produced from the photo-oxidation of methanol even at ~200 K [41]. The finding that methyl formate is produced in the photocatalytic decomposition of DME further supports the idea that CH 3 O species is involved in its photoreaction The formation of methyl formate can be ascribed to the recombination of CH 2 O formed in the dissociation of CH 3 O (Equation (3)):…”