Oxygen Vacancy-Rich TiO₂ as an Efficient Non-noble Metal Catalyst toward Mild Oxidation of Methane Using Hydrogen Peroxide as the Oxidant

Abstract

Direct functionalization of methane remains a key challenge, especially for using non-noble metal catalysts. We demonstrated that TiO 2 nanorods with abundant oxygen vacancies enabled mild oxidation of methane by H 2 O 2 into formaldehyde (HCHO) without light irradiation. The activity of TiO 2 nanorods with the concentration gradient of oxygen vacancies (V O ) increased with the V O concentration. In H 2 O 2 aqueous solution under 30 bar of CH 4 at 70 °C for 1 h, the TiO 2 nanorods with the most abundant V O exhibited a total oxygenate yield of 40.80 μmol, among which the selectivity for HCHO was 64.1%. On the basis of the catalytic and spectroscopic data, we identified the reaction intermediates and accordingly mapped the reaction scheme. Specifically, H 2 O 2 is activated on Ti atoms near V O to form surface peroxo intermediates, followed by the activation of CH 4 to produce methoxy groups. The methoxy group can react either with water to form methanol or with hydroxyl radicals to form CH 3 OOH. Methanol is attacked by hydroxyl radicals and dehydrated to form •CH 2 OH that further reacts with hydroxyl radicals and is dehydrated to HCHO. CH 3 OOH directly undergoes dehydration to engender HCHO.

CO-Assisted Methane Oxidation into Oxygenates over Surface Platinum–Titanium Alloyed Layers

“…15 To overcome the spin-forbidden limitation, the highly reactive metal center has been adopted to transfer electrons to molecular oxygen, leading to the excitation of the triplet to the singlet state, 14 but it is difficult to form highly reactive metal−oxo sites under mild conditions. To this end, various oxidants have been utilized, such as H 2 O 2 , 12,16,17 sulfur trioxide (SO 3 ), 13 and corrosive acids (e.g., oleum 18 and hydrobromic acid 19 ). The expensive nature of strong oxidants currently impedes their utilization.…”

Wavelength-Dependent Activity of Oxygen Species in Propane Conversion on Rutile TiO2(110)

“…Although TiO 2 photocatalysis may be a promising method for selective light alkane conversion under mild conditions, − the active oxygen species for light alkane conversion via photocatalysis remains under debate. A series of experimental works conducted under ambient conditions support the idea that the hole-trapped lattice oxygen is the active species for the initial C–H bond activation of methane (CH 4 ) over ZnO, TiO 2 , and MgO catalysts. ,− However, mechanistic studies of light alkane (C1–C3) conversion on the model rutile (R) TiO 2 (110) surface ,, suggest that the hole-trapped oxygen adatom at the five-coordinated Ti 4+ site (Ti 5c , O Ti – ) is the active species for the initial C–H bond activation of light alkanes, which occurs via a hydrogen atom transfer (HAT) process .…”

“…Since the first discovery of photoelectrocatalytic water (H 2 O) splitting into hydrogen (H 2 ) and oxygen (O 2 ) with Pt and TiO 2 semiconductor electrodes in 1972 by Fujishima and Honda, TiO 2 photocatalysis has intrigued a broad array of researchers for decades because of its potential applications in renewable clean energy. − In thorough research, a TiO 2 -based catalyst has shown its potential in light alkane conversion under mild conditions. − Although an enormous amount of attention has been focused on the fundamental processes of TiO 2 photocatalysis, − a deep understanding of the fundamental processes (including charge/energy relaxation and transfer, breaking and formation of chemical bonds, etc.) underlying photocatalytic light alkane conversion is still lacking.…”