Thermo-driven photocatalytic CO reduction and H2 oxidation over ZnO via regulation of reactant gas adsorption electron transfer behavior

Abstract

Photothermal catalysis is a widely researched field in which the reaction mechanism is usually investigated based on the photochemical behavior of the catalytic material. Considering that the adsorption of reactants is essential for catalytic reactions to occur, in this study, the synergistic effect of photothermal catalysis is innovatively elucidated in terms of the electron transfer behavior of reactant adsorption. For the H2 + O2 or CO + H2 reaction systems over a ZnO catalyst, UV irradiation at 25 °C or heat without UV irradiation did not cause H2 oxidation or CO reduction; only photothermal conditions (100 or 150 °C + UV light) initiated the two reactions. This result is related to the electron transfer behavior associated with the adsorption of CO or H2 on ZnO, in which H2 or CO that lost an electron could be oxidized by O2 or hydroxyls. However, the electron-accepting CO could be reduced by the electron-donating H2 into CH4 under photothermal conditions. Based on the in-situ characterization and theoretical calculation results, it was established that the synergistic effect of the photothermal conditions acted on the (002) crystal surface of ZnO to stimulate the growth of zinc vacancies, which resulted in the formation of defect energy levels, adsorption sites, and an adjusted Fermi level. As a result, the electron transfer behavior between adsorbed CO or H2 and the crystal surface varied, which further affected the photocatalytic behavior. The results show that the effect of photothermal synergy may not only produce the expected kinetic energy, but more importantly, produce energy that can change the activation mode of the reactant gas. This study provides a new understanding of the CO catalytic oxidation and reduction processes over semiconductor materials.

Constructing a Channel to Regulate the Electron-Transfer Behavior of CO Adsorption and Light-Driven CO Reduction by H2 over CuO–ZnO

“…However, CH 4 was not detected over CuO-Z-2 in the presence of only CO or H 2 under UV irradiation, which proved that CH 4 came from the reduction reaction of CO + H 2 . The experimental results are consistent with our predictions and previous reports. − Moreover, the prepared pure CuO does not respond to the CO gas and is not able to catalyze the CO reduction. Also, there is no activity for the CO + H 2 reaction under dark conditions, and this proves that UV light has the driving role in the CO reduction reaction.…”

“…The bonding type and electron-transfer direction may be influenced by the d-orbital electrons of the M atom and thus it may form d−π, σ, or π back donation bonds. − Also, M atoms that have more unpaired d-electrons can more easily provide electrons to the 2σ orbital of CO molecules, whereas M atoms that have more unoccupied d-orbitals can more easily accept electrons from the anti-bonding 2π* orbital of CO molecules. Accordingly, for the adsorption of CO molecules on M atoms that have different d-electron configurations affects their electron-transfer behavior and may further the photocatalytic reaction behavior (oxidation or reduction). − For example, CO (ads) accepting electrons from a catalyst may be conducive to the reduction reaction, whereas CO (ads) donating electrons to a catalyst may be more conducive to the oxidation reaction. Therefore, we can introduce M atoms into the catalyst system that have unpaired d-electrons and unoccupied d-orbitals as active species to match the molecular orbitals of CO.…”

Synergistic Effect of the Surface Vacancy Defects for Promoting Photocatalytic Stability and Activity of ZnS Nanoparticles

“…Moreover, in situ EPR spectra are performed to investigate the change of defect types under the irradiation. 60 The in situ EPR spectra of M 3 −ZnS (Figure S13) indicate that the surface defects of M−ZnS will not change under the irradiation of a 300 W Xe lamp. The slight decrease of the intensity indicates that the degree of unsaturation of the defect structures decreases, which proves that the defect can capture photogenerated electrons.…”

“…That means there is no elemental S produced either before or after the reaction. Moreover, in situ EPR spectra are performed to investigate the change of defect types under the irradiation . The in situ EPR spectra of M 3 –ZnS (Figure S13) indicate that the surface defects of M–ZnS will not change under the irradiation of a 300 W Xe lamp.…”

Elaborated Reaction Pathway of Photothermal Catalytic CO2 Conversion with H2O on Gallium Oxide‐Decorated and ‐Defective Surfaces

“…Doping of the surface with Ga 2 O 3 facilitates competitive adsorption of CO 2 and has the same effect as the OH group in suppressing adsorption of H 2 O. According to recently reported studies, [36] the adsorption mode of the reactant affects the electron transfer behavior between it and the surface, which in turn affects the subsequent conversion reaction. From a purely thermodynamic point of view, the CO 2 adsorbed in the bent form is actually effectively activated, and the weakened C=O bond is easily protonated, as shown in Table S3.…”