Photocatalytic Methane Conversion to C1 Oxygenates over Palladium and Oxygen Vacancies Co‐Decorated TiO₂

Abstract

As the principal constituent of natural/shale gases, methane (CH 4 ) is a promising industrial feedstock for manufacturing value-added chemicals. [1] However, efficient CH 4 conversion is still of a great challenge owing to its high C-H bond energy (439 kJ mol À1 ), low electron affinity (À1.9 eV), and high ionization energy (12.6 eV). [2] The current industrial CH 4 conversion via dry/steam-reforming [3] and subsequent Fischer-Tropsch synthesis [4] is an energy-intensive and indirect route, where high temperature (>700 °C) is required. [5] Accordingly, direct CH 4 conversion under mild conditions is highly desired.Photocatalysis has emerged as the green pathway to activate CH 4 under mild conditions through the injection of a photoinduced charge carrier instead of thermal energy. [6] The key to efficient photocatalytic CH 4 conversion lies in the development of a suitable photocatalyst. Recently, ZnO loaded with noble metal was reported to convert CH 4 into liquid oxygenates, with oxygen (O 2 ) as the oxidant. [7] Au 1 -BP promoted CH 4 conversion into CH 3 OH with the reactive hydroxyl radicals (OH), which are formed by O 2 with the assistance of water under light irradiation. [8] It is clear that the predominant challenge lies in simultaneous regulation of both activation of CH 4 and selectivity of desired products. Suitable co-catalysts like Au and Pd were reported to be the hole/ electron acceptors to promote charge separation, [9] as well as accelerating H 2 O oxidation and O 2 reduction to generate reactive oxygen species. Such encouraging advances then provide to some extent understanding of both charge dynamics and surface kinetics during photocatalytic CH 4 activation. Besides co-catalysts modification, surface engineering is the other way to promote charge dynamics. [10] It was reported that oxygen vacancies (OVs) and metastable Ti 3þ played a vital role in determining the photocatalytic performance of TiO 2 , especially due to the n-type doping and the improved carrier density. [10c,11] Moreover, interfacial resistance could also be regulated through surface engineering. [12] In parallel, surface kinetics could also be optimized by the introduction of surface defects by providing additional chemical adsorption and reactive sites. [13] Given these aforementioned attractive potentials of co-catalyst and OVs modification, the synergy of both would largely promote charge separation and surface reactions.Besides the design of suitable photocatalysts, reaction conditions including oxidant, solvent, pressure, and reaction time during CH 4 conversion are also important taking into account the reaction kinetics. Though it is difficult to gain an efficient activity due to the low solubility of CH 4 in H 2 O, H 2 O oxidation into •OH radicals was reported to be essential in the activation of CH 4 . [14] Meanwhile, H 2 O could also promote the desorption of the oxygenate products and avoid over-oxidation of CO 2 . [15] In parallel, a high pressure would increase the concentration of reactants, and a long rea...

Highly Selective Photocatalytic Methane Oxidation to Methanol Using CO2 as a Soft Oxidant

“…To further confirm the existence of oxygen vacancy, low-temperature EPR (Figure d) was performed. The paramagnetic signal centered at g = 2.003, which can be assigned to the typical response of O V , was identified for various catalysts. , The peak intensity was enhanced after the addition of Cu and reached the highest value for 2Cu/CeO 2 . The introduction of metals has been reported to promote the formation of oxygen vacancies.…”

Highly Selective Conversion of CH4 to High Value‐Added C1 Oxygenates over Pd Loaded ZnTi‐LDH

“…In the EPR spectrum of 0.5Pd‐ZnTi‐LDH‐A200 shown in Figure 4d, the signal peak at g = 2.076 is assigned to the adsorbed oxygen, indicating the efficient adsorption of O 2 molecules. [ 40,49 ] The signal peak at g = 2.028 is attributed to the oxygen radicals of O −· (Ti 4+ O 2− Ti 4+ O −· ), [ 42,54,55 ] which was reported to be formed by the trapping of photoproduced holes. ERP tests indicate that Pd loading is conductive to the adsorption of O 2 molecules on the defective site of ZnTi‐LDH, which may result in the generation of more reactive oxygen species and enhanced CH 4 oxidation.…”

“…In Figure 3c, the original ZnTi-LDH without calcination and Pd loading shows very poor performance for CH 4 conversion with only a slow rate of HCHO production (299.95 μmol g −1 h −1 ). It is widely reported that co-catalyst loading is beneficial to improve the activity of photocatalysts, [40,49] herein Pd was photo deposited on the surface of the prepared ZnTi-LDH. However, the activity improvement for C 1 products is very limited by Pd loading, which is only 1.8 times higher (557.13 μmol g −1 h −1 ) than the unloaded ZnTi-LDH.…”

“…This may be because of different role of Pd for the photocatalytic activity enhancement. Here, Pd likely acts as the hole acceptor, [45,49] while Au, Ag and Pt act as electron acceptor, [50] which will be discussed later. Most importantly, the activity of used photocatalysts for highly selective CH 4 conversion can be recovered by simple calcination for 3 h at the temperature of 200 °C.…”

Solar Photocatalysis of A Pesticide in A Tubular Reactor on Titaniferous Sand as A New Semi-Conductor

“…This can be explained by the fact that the presence of oxygen in the solution limits the risks of hole/electron recombination. Indeed, the oxygen molecules, absorbed on the surface of the photocatalyst, act as very effective traps of electrons from the conduction band, thus totally or partially suppressing the recombination of surface electron-hole pairs [22]. As a result, the formation of hydroxyl radicals (˚OH) and other reactive species, responsible for the degradation of the pollutant, will be improved.…”