Photocatalytic Oxidation of Small Molecular Hydrocarbons over ZnO Nanostructures: The Difference between Methane and Ethylene and the Impact of Polar and Nonpolar Facets

Abstract

The development of efficient photocatalysts to oxidize small molecular hydrocarbons under atmospheric conditions is of great significance. In our previous study, it was found that nanosized ZnO can fulfill this purpose with unprecedented activity. However, the difference between the hydrocarbons and the impact of polar and nonpolar facets of ZnO are far from understood. Herein, by the successful synthesis of facet-dependent ZnO photocatalysts with predominantly (0001)- and (011̅0)-facets-exposed single-crystalline nanosheets and nanorods, it was observed that the photocatalytic reaction of CH4 over ZnO surface follows quasi-first-order kinetics, while the photocatalytic reaction of C2H4 on the highly active ZnO nanosheets surface followed two-stage linear fitting kinetics. By exploring the band edge potentials, photoelectric response in combination with Fourier transform infrared spectroscopy (FTIR) and electron paramagnetic resonance (EPR), the differences between methane and ethylene photo-oxidation and the impact of polar and nonpolar facets are systematically discussed.

Defect Engineering in Photocatalytic Methane Conversion

“…Similarly, Yi and co‐workers demonstrated that methane molecules preferred to bind to the single‐electron‐trapped surface defects (i.e., OVs) on polar facets of ZnO, leading to activation of methane under light irradiation. [ 114 ] Apart from ZnO, Wang and co‐workers reported the TiO 2 with Ti 3+ species for highly selective photocatalytic oxidation of CH 4 into CO with the assistance of external bias. [ 105 ] As investigated by in situ Raman spectroscopy and in operando attenuated total reflection Fourier‐transform infrared spectroscopy (ATR‐FTIR), they found that the presence of Ti 3+ benefited to the formation of a Ti 3+ —C bond, further resulting in selective production of CO while the absence of Ti 3+ favored the formation of a Ti 4+ —O—C bond, easily lead to the complete oxidation of CH 4 into carbonates (Figure 16b).…”

Photocatalytic Conversion of Methane: Recent Advancements and Prospects

“…[22][23][24][25][26][27] In addition, morphology control and surface modification are also able to alter the range of light absorption. [28][29][30][31] 2) Effective separation of the photogenerated electrons and holes.W hen as emiconductor is irradiated by light, the photoexcited electron transfers from the valence band (VB) to the conduction band (CB), thereby leaving ahole in the VB.T he electron-hole pairs are generated in femtoseconds,but aconsiderable number of electron-hole pairs recombine spontaneously.T he surviving electrons and holes are able to move to the trapping sites within picoseconds to nanoseconds,o rt ransfer to the adsorbed reactant molecules within microseconds to milliseconds. [32][33][34] Additionally,t he electrons and holes will also recombine in the trapping sites or in the bulk during the transfer process.T he recombined charge carriers release energy as light or heat without participation in the redox reaction.…”

“…Oxygen is anonmetallic oxidant with ahigh reactivity and low toxicity that possesses the ability to readily form various oxides with most elements and compounds.Oxygen is widely used as the oxidant in av ariety of oxidation reactions, including the photooxidation of methane. [20,21,26,30,31,36,55,[101][102][103][104] Themain reactions of the methane-oxygen system are TOM and POM. TOM offers the possibility of efficiently removing trace amounts of methane,while POM offers an eco-friendly way to produce syngas,C 2 hydrocarbons,a nd oxygenates at RT and ambient pressure.…”

“…Thephotoexcited ZnO leads to the activation and conversion of CH 4 ,w hile the LSPR effect of the Ag nanoparticles further enhance the photoactivity (Figure 15 a). Li et al applied CuO-decorated ZnO (CuO/ZnO) [26] and ZnO nanocrystals [31] for TOM-O 2 .The initial concentration of CH 4 was about 200 ppm, and aC H 4 conversion of 90 %w as achieved within 30 min over CuO/ZnO,w ith CO 2 being the only product (Figure 15 b). Similarly,t he conversions of CH 4 over ZnO nanosheets and nanorods are about 80 %and 10 %, respectively,within 2hunder simulated solar light at ambient temperature.The different catalytic performances of the ZnO nanosheets and nanorods arise from the different major exposure surfaces (Figure 15 c).…”

Photocatalytic and Thermocatalytic Conversion of Methane

“…[ 94 ] Recently, the complete photocatalytic oxidation of methane has attracted increasing attention. [ 78,95–103 ] The Yi group reported that Ag‐modified ZnO (Ag/ZnO) nanostructures showed very high photocatalytic activity for CH 4 oxidation under simulated‐sunlight illumination. [ 78 ] It was found that the built‐in electric field of a ZnO polar semiconductor promotes the separation of photogenerated electrons and holes.…”