Regulating the Spin State of Single Noble Metal Atoms by Hydroxyl for Selective Dehydrogenation of CH4 Direct Conversion to CH3OH

The key scientific challenge for methane (CH₄) direct conversion to methanol (CH₃OH) is considered to be the prevention of overoxidation of target products, which is restrained by the difficulty in the well-controlled process of selective dehydrogenation. Herein, we take single noble metal atom-anchored hexagonal boron nitride nanosheets with B vacancies (M_SA/B_1-xN) as the model materials and first propose that the dehydrogenation in the direct conversion of CH₄ to CH₃OH is highly dependent on the spin state of the noble metal. The results reveal that the noble metal with a higher spin magnetic moment is beneficial to the formation of the spin channels for electron transfer, which boosts the dissociation of C-H bonds. The promoted process of dehydrogenation will lead not only to the effective activation of CH₄ but also to the easy overoxidation of CH₃OH. More importantly, it is found that the spin state of noble metals can be regulated by the introduction of hydroxyl (OH), which realizes the selective dehydrogenation in the process of CH₄ direct conversion to CH₃OH. Among them, Ag_SA/B_1-xN exhibits the best performance owing to the dynamic regulation spin state of a single Ag atom by OH. On the one hand, the introduction of OH significantly reduces the energy barrier of C-H bond dissociation by the increase in the spin magnetic moment. On the other hand, the high spin magnetic moment of a single Ag atom during the process of subsequent dehydrogenation can be modulated to nearly zero. As a result, the spin channel for electron transfer between the adsorbed CH₃OH and reactive sites is broken, which hinders its overoxidation. This work opens a new path to designing catalysts for selective dehydrogenation by tuning the spin state of local electronic structures.

Application Prospect of K Used for Catalytic Removal of NOx, COx, and VOCs from Industrial Flue Gas: A Review

NOx, COx, and volatile organic compounds (VOCs) widely exist in motor vehicle exhaust, coke oven flue gas, sintering flue gas, and pelletizing flue gas. Potassium species have an excellent promotion effect on various catalytic reactions for the treatment of these pollutants. This work reviews the promotion effects of potassium species on the reaction processes, including adsorption, desorption, the pathway and selectivity of reaction, recovery of active center, and effects on the properties of catalysts, including basicity, electron donor characteristics, redox property, active center, stability, and strong metal-to support interaction. The suggestions about how to improve the promotion effects of potassium species in various catalytic reactions are put forward, which involve controlling carriers, content, preparation methods and reaction conditions. The promotion effects of different alkali metals are also compared. The article number about commonly used active metals and promotion ways are also analyzed by bibliometric on NOx, COx, and VOCs. The promotion mechanism of potassium species on various reactions is similar; therefore, the application prospect of potassium species for the coupling control of multi-pollutants in industrial flue gas at low-temperature is described.

Dual Functions of O-Atoms in the g-C3N4/BO0.2N0.8 Interface: Oriented Charge Flow In-Plane and Separation within the Interface To Collectively Promote Photocatalytic Molecular Oxygen Activation

The photocatalytic performance of two-dimensional materials is largely limited by the fast recombination of photogenerated charges. Herein, we design and fabricate a novel g-C₃N₄/BO_0.2N_0.8 van der Waals heterostructure to realize oriented charge flow in-plane and separation within the interface. On one hand, a B-C bond forms within the g-C₃N₄/BO_0.2N_0.8 interface after the introduction of O atoms. The B-C bond as the mediator bridges g-C₃N₄ and BO_0.2N_0.8 sides to enhance the effective separation of photogenerated charges. On the other hand, the existence of O atoms promotes the formation of a B-O-O-B intermediate to realize that molecular oxygen can directionally obtain electrons from the surface to generate •O₂^-. As a result, BO_0.2N_0.8 instead of g-C₃N₄ is considered to be the main reaction side, and the energy barrier of NO₃^- generation is significantly decreased. The NO removal performance of g-C₃N₄/BO_0.2N_0.8 is enhanced and the NO₂ generation is effectively controlled compared with that of g-C₃N and g-C₃N₄/BN. This work could provide an effective and facile strategy to tune oriented charge transfer.

B-O Bonds in Ultrathin Boron Nitride Nanosheets to Promote Photocatalytic Carbon Dioxide Conversion

Limited by the chemical inertness of CO₂ and the high dissociation energy of the C═O bond, photocatalytic CO₂ conversion is highly challenging. Herein, we prepare ultrathin oxygen-modified h-BN (O/BN) nanosheets containing B-O bonds. On the O/BN surface, CO₂ can be chemically captured and is bonded with the B-O bond, leading to the formation of an O-B-O bond. This new chemical bond acting as an electron-delivery channel strengthens the interaction between CO₂ and the surface. Thus, the reactants can continuously obtain electrons from the surface through this channel. Therefore, the majority of gaseous CO₂ directly converts into carbon active species that are detected by in situ DRIFTS over O/BN. Moreover, the activated energies of CO₂ conversion are significantly reduced with the introduction of the B-O bond evidenced by DFT calculations. As a result, O/BN nanosheets present an enhanced photocatalytic CO₂ conversion performance with the H₂ and CO generation rates of 3.3 and 12.5 μmol g^-1 h^-1, respectively. This work could help in realizing the effects of nonmetal chemical bonds in the CO₂ photoreduction reaction for designing efficient photocatalysts.

The activation of oxygen through oxygen vacancies in BiOCl/PPy to inhibit toxic intermediates and enhance the activity of photocatalytic nitric oxide removal

Photocatalysis is regarded as a promising technology for indoor air purification. Despite much effort, the inhibition of toxic intermediates and the promotion of nitric oxide (NO) oxidation activity still limit real applications due to catalyst design constraints. In order to circumvent this issue, oxygen vacancies were fabricated through a strong interaction between BiOCl and polypyrrole (PPy) based on computational predictions. Oxygen vacancies worked as sites to activate O2 molecules, and the relative barrier energies of NO oxidation were significantly reduced due to the O2 activation process. With the oxygen vacancy modification, the oxidizability of BiOCl was improved, and the generation of the superoxide radical (˙O2-) was promoted on BiOCl/PPy, while the hydroxyl radical (˙OH) remained unchanged under visible light irradiation. As a result, the efficiency of NO oxidation increased from 12% to 28%, while the NO2 production was inhibited completely. Finally, in situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) investigations were conducted to shed light on the mechanism of the NO oxidation process. This work provides an in-depth understanding of the interaction between oxygen vacancies and O2 during the NO oxidation process, which offers a scheme to control the oxidation reaction.

Enhanced charge separation and increased oxygen vacancies of h-BN/OV-BiOCl for improved visible-light photocatalytic performance

h-BN/OV-BiOCl composites were prepared to improve the visible-light photocatalytic activity of OV-BiOCl.