Preparation of Novel Layered High Entropy Bismuth-Based Materials and their Photocatalytic Degradation Mechanism.

We prepared BiOCl, BiO(ClBr), BiO(ClBrI), and BiO[ClBrI(CO3)0.5] materials using a simple coprecipitation method. It was found that adjusting the number of anions in the anion layer was conducive to adjusting the band structure of BiOX and could effectively promote the migration and separation of photogenerated carriers, thus improving the photocatalytic activity. We first selected methyl orange (MO) as the study pollutant and compared it with BiOCl, BiO(ClBr), and BiO(ClBrI). The first-order kinetic constants of MO degradation by BiO[ClBrI(CO3)0.5] increased by 90.3, 33.9, and 3.1 times, respectively. The photocatalytic degradation rate of methylene blue by BiO[ClBrI(CO3)0.5] was 89.5%, indicating the excellent photocatalytic performance of BiO[ClBrI(CO3)0.5]. The stability of BiO[ClBrI(CO3)0.5] was demonstrated through cyclic experiments and XRD analysis before and after the reaction. The photocatalytic degradation of MO by BiO[ClBrI(CO3)0.5] showed that h+ and 1O2 were the main active oxidizing species and •O2- was the secondary active substance. Overall, our work provides new ideas for the synthesis and degradation of organic pollutants by using two-dimensional anionic high-entropy materials.

Product Peroxidation Inhibition in Methane Photooxidation into Methanol.

Methane photooxidation into methanol offers a practical approach for the generation of high-value chemicals and the efficient storage of solar energy. However, the propensity for C─H bonds in the desired products to cleave more easily than those in methane molecules results in a continuous dehydrogenation process, inevitably leading to methanol peroxidation. Consequently, inhibiting methanol peroxidation is perceived as one of the most formidable challenges in the field of direct conversion of methane to methanol. This review offers a thorough overview of the typical mechanisms involved radical mechanism and active site mechanism and the regulatory methods employed to inhibit product peroxidation in methane photooxidation. Additionally, several perspectives on the future research direction of this crucial field are proposed.

Selective Cleavage of Chemical Bonds in Targeted Intermediates for Highly Selective Photooxidation of Methane to Methanol.

Restrained by the uncontrollable cleavage process of chemical bonds in methane molecules and corresponding formed intermediates, the target product in the reaction of methane selective oxidation to methanol would suffer from an inevitable overoxidation process, which is considered to be one of the most challenging issues in the field of catalysis. Herein, we report a conceptually different method for modulating the conversion pathway of methane through the selective cleavage of chemical bonds in the key intermediates to suppress the generation of peroxidation products. Taking metal oxides, typical semiconductors in the field of methane oxidation as model catalysts, we confirm that the cleavage of different chemical bonds in CH3O* intermediates could greatly affect the conversion pathway of methane, which has a vital role in product selectivity. Specifically, it is revealed that the formation of peroxidation products could be significantly prevented by the selective cleavage of C-O bonds in CH3O* intermediates instead of metal-O bonds, which is proved by the combination of density functional theory calculations and in situ infrared spectroscopy based on isotope labeling. By manipulating the lattice oxygen mobility of metal oxides, the electrons transferring from the surface to the CH3O* intermediates could directionally inject into the antibonding orbitals of the C-O bond, resulting in its selective cleavage. As a result, the gallium oxide with low lattice oxygen mobility shows a 3.8% conversion rate for methane with a high methanol generation rate (∼325.4 µmol g-1 h-1) and selectivity (∼87.0%) under room temperature and atmospheric pressure in the absence of extra oxidants, which is superior among the reported studies (reaction pressure: <20 bar).

Methane Photooxidation with Nearly 100 % Selectivity Towards Oxygenates: Proton Rebound Ensures the Regeneration of Methanol.

Restrained by uncontrollable dehydrogenation process, the target products of methane direct conversion would suffer from an inevitable overoxidation, which is deemed as one of the most challenging issues in catalysis. Herein, based on the concept of a hydrogen bonding trap, we proposed a novel concept to modulate the methane conversion pathway to hinder the overoxidation of target products. Taking boron nitride as a proof-of-concept model, for the first time it is found that the designed N-H bonds can work as a hydrogen bonding trap to attract electrons. Benefitting from this property, the N-H bonds on the BN surface rather than C-H bonds in formaldehyde prefer to cleave, greatly suppressing the continuous dehydrogenation process. More importantly, formaldehyde will combine with the released protons, which leads to a proton rebound process to regenerate methanol. As a result, BN shows a high methane conversion rate (8.5 %) and nearly 100 % product selectivity to oxygenates under atmospheric pressure.

Dual-Function Reaction Center for Simultaneous Activation of CH4 and O2 via Oxygen Vacancies during Direct Selective Oxidation of CH4 into CH3OH.

The direct oxidation of methane (CH4) to methanol (CH3OH) has been a focus of global concern and is quite challenging due to the thermodynamically stable CH4 and uncontrolled overoxidation of the products. Here, we provided a new viewpoint on the role of oxygen vacancies that induce a dual-function center in enhancing the adsorption and activation of both CH4 and O2 reactants for the subsequent selective formation of a CH3OH liquid fuel on two-dimensional BiOCl photocatalysts at atmospheric pressure. The key for the favorable activity lies in the simultaneous ability of the electron-trapped Bi atom in activating CH4 and the formation of •O2- radicals due to the activation of O2 at the adjacent oxygen vacancy site, which immediately attack the activated CH4 to directly produce CH3OH, in initiating the oxidation reaction. What is more, the relatively low reaction barriers and the easy desorption of CH3OH concertedly facilitate the highly selective conversion of CH4 up to 85 µmol of CH3OH, with a small amount of CO2 and CO as the byproducts over the BiOCl nanosheets with an oxygen vacancy concentration of 2.4%. This work could broaden the avenue toward the application of oxygen-defective metal oxides in the photocatalytic selective conversion of CH4 to CH3OH and offer a disparate perspective on the role of oxygen vacancy acting as the surface electron transfer channel in enhancing the photocatalytic performance.

Modulating electron density of vacancy site by single Au atom for effective CO2 photoreduction.

The surface electron density significantly affects the photocatalytic efficiency, especially the photocatalytic CO2 reduction reaction, which involves multi-electron participation in the conversion process. Herein, we propose a conceptually different mechanism for surface electron density modulation based on the model of Au anchored CdS. We firstly manipulate the direction of electron transfer by regulating the vacancy types of CdS. When electrons accumulate on vacancies instead of single Au atoms, the adsorption types of CO2 change from physical adsorption to chemical adsorption. More importantly, the surface electron density is manipulated by controlling the size of Au nanostructures. When Au nanoclusters downsize to single Au atoms, the strong hybridization of Au 5d and S 2p orbits accelerates the photo-electrons transfer onto the surface, resulting in more electrons available for CO2 reduction. As a result, the product generation rate of AuSA/Cd1-xS manifests a remarkable at least 113-fold enhancement compared with pristine Cd1-xS.

Visible-Light-Driven Selective Oxidative Coupling of Amines to Imines by Bismuth-Rich Bismuth Oxybromide in Water.

Photocatalytic selective oxidative coupling of amines is a promising method for imine synthesis. Bismuth-rich bismuth oxybromide photocatalysts have been found to exhibit improved conversion and selectivity for the visible-light-driven selective oxidative coupling of amines to imines in water. Benzylamines with either electron-withdrawing or electron-donating groups undergo good conversion (94-99 %) and excellent selectivity (93-98 %) to the corresponding imines. The average apparent quantum efficiency at 455 nm is 57.4 %. Active species trapping experiments, isotope experiments, and in situ infrared spectroscopy reveal that water both serves as solvent and participates in the coupling reaction as a reactant. This work provides a green and environmentally friendly method for the efficient synthesis of imines with bismuth-rich bismuth oxyhalides and reveals water is involved in the oxidative coupling mechanism of benzylamines.