Methanol Photo-Oxidation on Rutile TiO₂ Nanowires: Probing Reaction Pathways on Complex Materials

Abstract

Understanding photochemical processes on nanomaterials is key to developing effective photocatalysts. Herein, methanol oxidation and reduction is used to probe the thermal and photochemical properties of rutile titania nanowires grown using a hydrothermal method. The presence of oxygen vacancy defects leads to methoxy formation and subsequent disproportionation to formaldehyde and methanol at 700 K. Methane and dimethyl ether are also produced in minor quantities. Oxygen adatoms enhance the formation of methoxy, which led to an increase in the disproportionation products and dimethyl ether at high temperature and a decreased amount of methane. The thermal reactivity of the nanowires parallels that of rutile TiO2(110) single crystals. Photo-oxidation of methoxy using UV light produced formaldehyde and methyl formate. These product yields were enhanced on nanowires with oxygen adatoms, but a majority of methoxy (∼70%) is not photoactive. In contrast, all methoxy is photo-oxidized on rutile TiO2(110) when O-adatoms are present. This difference indicates that holes created in the nanowires during UV excitation do not migrate to most of the methoxya required step for methoxy photo-oxidation. This lack of activity could be due to either trapping of holes in the material or different binding of the inactive methoxy. These studies demonstrate that while charge carriers can be efficiently created in nanowires differences in chemical properties can suppress photo-oxidation.

Recent Advances in Aerobic Photo‐Oxidation of Methanol to Valuable Chemicals

“…The results from the temperature‐programmed reaction experiments suggested that the methoxy was transformed through photo‐oxidation to HCHO and HCOOCH 3 . Next, a similar phenomenon was observed on the rutile TiO 2 nanowires in the methanol photo‐oxidation [26] …”

Facile Decomposition of Organophosphonates by Dual Lewis Sites on a Fe3O4(111) Film

“…The reactivity measurements were carried out in an ultrahigh vacuum (UHV) apparatus with a base pressure of < 8×10 -11 Torr, which has been described in detail before. [27][28] The α-Fe 2 O 3 (0001) single crystal (SurfaceNet GmbH) is mounted as described previously and the sample temperature can be controlled between 130 K and 1000 3 K. 28 Crystal cleaning is accomplished by Ar + -ion sputtering (1.0 keV, 1.0 × 10 -5 Torr, 15 min, 13 µA sample current) and annealing in vacuo (1000 K, 15 min). Before every experiment, 3 cycles of sputtering and annealing are performed to remove all potential contaminants.…”

“…Temperature-programmed desorption and reaction measurements were performed with the crystal in line-of-sight of the quadrupole mass spectrometer with a constant heating rate of 1 K/s. The masses monitored were m/e 2, 15,16,18,27,28,29,30,31,32,33,35,44,46,47,79,124. The gaseous products were identified by their fragmentation patterns and quantified by correcting for mass spectrometer sensitivity the ionization cross sections.…”

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

“…With the introduction of O 2 ,O 2 -promoted CH 3 OH(a) dissociation to CH 3 O(a) [27] was observed to occur only slightly for all TiO 2 NCs under the dark condition (Supporting Information, Figure S10). Upon UV light illumination, the absorption feature of photoexcited electrons greatly decreased with the O 2 concentration increasing and became similar for all TiO 2 NCs at an oxygen concentration of 10 % (Supporting Information, Figure S11), demonstrating the efficient scavenging of photoexcited electrons by O 2 .C onsequently,p hotocatalytic oxidation of adsorbed CH 3 O(a) species to HCOO(a), HCHO(a) and carbonate species occur on all TiO 2 NCs,a se videnced by the appearance of the vibrational features of various types of HCOO(a) and carbonate species and HCHO(a) species [13,28] at the expense of those of CH 3 O(a) species (Supporting Information, Figure S12, Table S1).…”