On the Mechanism of Methyl Formate Production Initiated by Photooxidation of Methanol on Rutile TiO₂(110) and TiO₂(011)-(2 × 1) Surfaces

Abstract

The formation of HCOOCH 3 initiated by photooxidation of CH 3 OH on TiO 2 surfaces is interesting, but the mechanism remains ambiguous. Using thermal desorption spectroscopy and X-ray photoelectron spectroscopy, we herein have comparatively studied the adsorption of HCOOCH 3 on the rutile TiO 2 (110) surface, the coadsorption and photooxidation of CH 3 OH and HCHO on the rutile TiO 2 (110) surface, and the adsorption and photooxidation of CH 3 OH on the rutile TiO 2 ( 011)-(2 × 1) surface. The HCOOCH 3 formation from the photooxidation of CH 3 OH was identified as a relay reaction of photocatalytic reactions, followed by thermal reactions. CH 3 OH is photocatalytically oxidized to CH 3 O and HCHO species, and HCHO is photocatalytically oxidized to HCO species, and then the CH 3 O and HCO species undergo thermal coupling reactions to produce HCOOCH 3 during the subsequent heating process. The TiO 2 surface structure was observed to affect not only the photocatalytic oxidation process of CH 3 OH to produce CH 3 O and HCO species but also the subsequent thermal coupling reaction of CH 3 O and HCO species to produce HCOOCH 3 . CH 3 OH on the TiO 2 (110) surface is more photoreactive than on the TiO 2 ( 011)-(2 × 1) surface. A higher fraction of HCHO is produced for CH 3 OH/TiO 2 (011)-(2 × 1) than for CH 3 OH/TiO 2 (110), while a smaller faction of HCOOCH 3 is produced. These results broaden our fundamental understanding of the reaction mechanism of CH 3 OH photocatalysis on TiO 2 surfaces.

Photocatalytic Production of Methyl Formate by Methanol Self-Coupling: From Oxidative Dehydrogenation to Direct Dehydrogenation

“…(ii) HCO* from the further photoreaction of HCHO reacts with CH 3 O ad * 31,32 In order to examine the possibility of MF production by the direct photocatalytic dehydrogenation of methanol and also to clarify the mechanism of MF formation, a series of dehydrogenation experiments and in situ DRIFTS measurements was conducted in an Ar atmosphere over the Pd/TiO 2 catalyst. For comparison, experiments of the oxidative dehydrogenation in a CO 2 atmosphere have been conducted also, and the influence of the atmosphere on MF production is discussed in detail in the present work.…”

“…On the dehydrogenation of methanol, previous experiments and theories have confirmed that first methanol undergoes two steps of dehydrogenation to produce formaldehyde but after that there is a controversy on how formaldehyde is further converted to MF. It has been proposed by different authors that MF may form via different pathways. Dimerization of HCHO , HCO* from the further photoreaction of HCHO reacts with CH 3 O ad * , Reaction of CH 3 O ad * with HCHO, which does not involve the HCO* intermediate − …”

Site Sensitivity of Interfacial Charge Transfer and Photocatalytic Efficiency in Photocatalysis: Methanol Oxidation on Anatase TiO2 Nanocrystals

“…The CH 3 O(a) Ti5c species is photocatalytic active to undergo the C−H bond breaking to form the HCHO(a) species; [7–9] moreover, the photo‐induced oxidative coupling reaction between co‐adsorbed CH 3 O(a) and HCHO(a) at the Ti 5c sites were observed to produce methyl formate (HCOOCH 3 ) [10] . The CH 3 O(a) Ti5C is more photoreactive on rutile TiO 2 (110) surface than on rutile TiO 2 (011)‐(2×1) surface [10d, e] . On anatase TiO 2 (001)‐(1×4) surface, photocatalytic oxidation was observed to occur specifically for the CH 3 O(a) species at the Ti 4c site (CH 3 O(a) Ti4c ) without the further formation of HCOOCH 3 [11] .…”

In Situ Generated Ti3+-Mediated Photocatalytic Methanol Decomposition to Carbon Monoxide and Hydrogen on a Rutile TiO2(100) Surface

“…These demonstrate that CH 3 O(a) is more photoactive than CH 3 OH-(a) and is photooxidized to HCHO(a). With the irradiation time increasing to 20 min, the CH 3 O(a) coverage only decreases slightly, while the CH 3 OH(a) coverage decreases greatly; meanwhile, the HCHO(a) coverage increases slightly, and the HCOO TiO 2 species with the C 1s binding energy at 289.9 eV 17,30,31 16 This can be attributed to the very small production of HCOOCH 3 .…”

“…Evolutions of surface species of a 0.55 ML CH 3 OH-covered TiO 2 (100) surface at 110 K under Hg light irradiation were monitored by C 1s XPS (Figure 3A), and variations of observed surface species as a function of the irradiation time were plotted (Figure 3B). Upon a 10 min irradiation, the CH 3 O(a) coverage decreases greatly, while the CH 3 OH(a) coverage decreases slightly; meanwhile, the HCHO(a) species with the C 1s binding energy at 288.7 eV 14,16,17,30,31 emerges. These demonstrate that CH 3 O(a) is more photoactive than CH 3 OH-(a) and is photooxidized to HCHO(a).…”