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).

Light-Reinforced Key Intermediate for Anticoking To Boost Highly Durable Methane Dry Reforming over Single Atom Ni Active Sites on CeO2.

Dry reforming of methane (DRM) has been investigated for more than a century; the paramount stumbling block in its industrial application is the inevitable sintering of catalysts and excessive carbon emissions at high temperatures. However, the low-temperature DRM process still suffered from poor reactivity and severe catalyst deactivation from coking. Herein, we proposed a concept that highly durable DRM could be achieved at low temperatures via fabricating the active site integration with light irradiation. The active sites with Ni-O coordination (NiSA/CeO2) and Ni-Ni coordination (NiNP/CeO2) on CeO2, respectively, were successfully constructed to obtain two targeted reaction paths that produced the key intermediate (CH3O*) for anticoking during DRM. In particular, the operando diffuse reflectance infrared Fourier transform spectroscopy coupling with steady-state isotopic transient kinetic analysis (operando DRIFTS-SSITKA) was utilized and successfully tracked the anticoking paths during the DRM process. It was found that the path from CH3* to CH3O* over NiSA/CeO2 was the key path for anticoking. Furthermore, the targeted reaction path from CH3* to CH3O* was reinforced by light irradiation during the DRM process. Hence, the NiSA/CeO2 catalyst exhibits excellent stability with negligible carbon deposition for 230 h under thermo-photo catalytic DRM at a low temperature of 472 °C, while NiNP/CeO2 shows apparent coke deposition behavior after 0.5 h in solely thermal-driven DRM. The findings are vital as they provide critical insights into the simultaneous achievement of low-temperature and anticoking DRM process through distinguishing and directionally regulating the key intermediate species.

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.

Surface Hydrogen Atoms Promote Oxygen Activation for Solar Light-Driven NO Oxidization over Monolithic α-Ni(OH)2/Ni Foam.

Catalysis oxidization has been known to be an effective technique in environmental remediation. However, low efficiency for oxygen activation and difficult recovery of the catalysts in powdery form significantly limit the practical application. In this work, a new-type monolithic α-Ni(OH)2/Ni-foam was fabricated by the hydrothermal process. We found that H atoms of α-Ni(OH)2 can significantly promote oxygen activation, which endows it with favorable NO and NO2 oxidization confirmed by theoretical calculation and in situ DRIFTS. Furthermore, the introduction of Ni foam accelerated the pollutant gas transfer and charge carriers' separation because of its abundant porous structure and high conductivity and its monolithic property simplified the recycling operation. Consequently, the obtained α-Ni(OH)2/Ni-foam achieved an excellent NO oxidation (69.0%) and no toxic NO2 was detected under visible light illumination (λ > 420 nm), indicating its highly promising potential in environmental remediation. Our work provides a conceptually different fresh perception to promote oxygen activation for highly efficient gas purification.

The role of potassium in the activation of oxygen to promote nitric oxide oxidation on honeycomb-like h-BN(001) surfaces.

Two-dimensional materials have developed rapidly in the field of catalysis, which has made honeycomb-like 2D hexagonal boron nitride (h-BN) considered to be a promising material in the application of catalysis. In this study, potassium (K) modified h-BN(001) surfaces are designed for the first time for the purpose of removal of nitric oxide (NO) via dispersion corrected density functional theory (DFT) computations. The role of K in changing the electronic properties and processes of NO oxidation is investigated in detail. The introduction of K effectively narrows the band gap of h-BN and acts as the surface electron donor on the surface. Importantly, the energy barrier is decreased by 3.45 eV at most along the whole NO oxidation path on K-doped h-BN(001), which is directly ascribed to the spontaneous dissociation of O2 induced by K and efficient charge transfer across the interface. The activation of O2 may be beneficial to other analogous oxidation reactions. With the introduction of K, the energy barrier of NO2 conversion to NO3- is significantly decreased and the desorption of NO2 is inhibited. Hence, NO3- instead of NO2 becomes the main oxidation product on K-modified h-BN. This work may provide new insights into the role of alkali metals in the activation of O2 to facilitate oxidation reactions on 2D materials.

Non-noble metal plasmonic photocatalysis in semimetal bismuth films for photocatalytic NO oxidation.

Recently, non-noble metals with plasmonic properties have attracted great attention due to their potential applications in photocatalytic solar-energy conversion. However, in contrast to the well-studied plasmonic noble metals (mainly Au and Ag), which have distinct absorption peaks, the understanding of light absorption and the photocatalytic reaction mechanism of non-noble metals is far less. In this study, semimetal bismuth films are deposited on fluorine-doped tin oxide substrates by a dc magnetron sputtering method. Both theoretical calculation and UV-vis absorption spectra confirm that the field enhancement and location of plasmonic resonance peaks are strongly correlated with the size of Bi particles. Through tuning the sputtering power, for the first time, four distinct absorption peaks are observed over isolated Bi particles. Moreover, it is found that the energy barrier for the conversion of NO into NO2 over Bi is even lower than that with Au nanoclusters. Thus, Bi films are highly active for photocatalytic oxidation of NO. Moreover, the low NO2 desorption energy over Bi indicates that Bi films can be the main active sites during the reaction process and that they possess good stability.

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.

Boosting charge-transfer in tuned Au nanoparticles on defect-rich TiO2 nanosheets for enhancing nitrogen electroreduction to ammonia production.

The electrocatalytic nitrogen reduction reaction (eNRR) to ammonia (NH3) has been recognized as an effective, carbon-neutral, and great-potential strategy for ammonia production. However, the conversion efficiency and selectivity of eNRR still face significant challenges due to the slow transfer kinetics and lack of effective N2 adsorption and activation sites in this process. Herein, we designed and fabricated defect-rich TiO2 nanosheets furnished with oxygen vacancies (OVs) and Au nanoparticles (Au/TiO2-x) as the electrocatalyst for efficient N2-fixing. The experimental results demonstrate that OVs act as active sites, which enable efficient chemisorption and activation of N2 molecules. The Au nanoparticles loaded on the OVs-rich TiO2 nanosheets not only accelerate charge transfer but also change the local electronic structure, thus enhancing N2 adsorption and activation. In this work, the optimal Au/TiO2-x electrocatalyst displays a considerable NH3 yield activity of 12.5 µg h-1 mgcat.-1 and a faradaic efficiency (FE) of 10.2 % at -0.40 V vs reversible hydrogen electrode (RHE). More importantly, the Au/TiO2-x exhibits a stable N2-fixing activity in cycling and it persists even after 80 h of consecutive electrolysis. Density functional theory (DFT) calculations reveal that the OVs serve as the active sites in TiO2, while Au nanoparticles are crucial for improving N2 chemisorption and lowering the reaction energy barrier by facilitating the charge transfer for eNRR with a distal hydrogenation pathway. This research offers a rational catalytic site design for modulating charge transfer of active sites on metal-supported defective catalysts to boost N2 electroreduction to NH3.

Resonant Lasing Emission in Undoped and Mg-Doped Gallium Nitride Thin Films on Interfacial Periodic Patterned Sapphire Substrates.

In this work, low-threshold resonant lasing emission was investigated in undoped and Mg-doped GaN thin films on interfacial designed sapphire substrates. The scattering cross-section of the periodic resonant structure was evaluated by using the finite difference time domain (FDTD) method and was found to be beneficial for reducing the threshold and enhancing the resonant lasing emission within the periodic structures. Compared with undoped and Si-doped GaN thin films, p-type Mg-doped GaN thin films demonstrated a better lasing emission performance. The lasing energy level system and defect densities played vital roles in the lasing emission. This work is beneficial to the realization of multifunctional applications in optoelectronic devices.

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 (MSA/B1-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, AgSA/B1-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.

In Situ Growth of Fe2O3 Nanorod Arrays on Carbon Cloth with Rapid Charge Transfer for Efficient Nitrate Electroreduction to Ammonia.

Electrochemical reduction of nitrate to ammonia (NH3), a green NH3 production route upon combining with renewable energy sources, is an appealing and alternative method to the Haber-Bosch process. However, this process not only involves the complicated eight-electron reduction to transform nitrate into various nitrogen products but simultaneously suffers from the competitive hydrogen evolution reaction, challenged by a lack of efficient catalysts. Herein, the in situ growth of Fe2O3 nanorod arrays on carbon cloth (Fe2O3 NRs/CC) is reported to exhibit a high NH3 yield rate of 328.17 µmol h-1 cm-2 at -0.9 V versus RHE, outperforming most of the reported Fe catalysts. An in situ growth strategy provides massive exposed active sites and a fast electron-transport channel between the carbon cloth and Fe2O3, which accelerates the charge-transport rate and facilitates the conversion of nitrate to NH3. In situ Raman spectroscopy in conjunction with attenuated total reflection Fourier transform infrared spectroscopy reveals the catalytic mechanism of nitrate to NH3. Our study provides not only an efficient catalyst for NH3 production but also useful guidelines for the pathways and mechanism of nitrate electroreduction to NH3.

Ultrafast charge transfer dynamics in 2D covalent organic frameworks/Re-complex hybrid photocatalyst.

Rhenium(I)-carbonyl-diimine complexes have emerged as promising photocatalysts for carbon dioxide reduction with covalent organic frameworks recognized as perfect sensitizers and scaffold support. Such Re complexes/covalent organic frameworks hybrid catalysts have demonstrated high carbon dioxide reduction activities but with strong excitation energy-dependence. In this paper, we rationalize this behavior by the excitation energy-dependent pathways of internal photo-induced charge transfer studied via transient optical spectroscopies and time-dependent density-functional theory calculation. Under band-edge excitation, the excited electrons are quickly injected from covalent organic frameworks moiety into catalytic RheniumI center within picosecond but followed by fast backward geminate recombination. While under excitation with high-energy photon, the injected electrons are located at high-energy levels in RheniumI centers with longer lifetime. Besides those injected electrons to RheniumI center, there still remain some long-lived electrons in covalent organic frameworks moiety which is transferred back from RheniumI. This facilitates the two-electron reaction of carbon dioxide conversion to carbon monoxide.

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.

Ti3C2 MXene modified g-C3N4 with enhanced visible-light photocatalytic performance for NO purification.

As for photocatalytic oxidation of nitric oxide (NO), the low NO oxidation activity and generation of by-products (cf. NO2) are the urgent challenge. To tackle these issues, a simple in-situ growing stragegy was employed to constructe Ti3C2/g-C3N4 system. The fabricated Ti3C2/g-C3N4 composite (TC-CN) presented enhanced photocatalytic NO removal efficiency and inhibited toxic NO2 generation. Density functional theory (DFT) calculations and experimental characterizations were combined to demonstrate the presence of strong interface effect between Ti3C2 and g-C3N4, which could greatly improve photo-generated charge carrier separation via the construction of electron transfer channels, and further activate oxygen molecules adsorbed on the side of Ti3C2 layer. As a result, nitrite and nitrate instead of NO2 were the final products during the photocatalytic reaction process through the monitoring by in situ diffuse reflectance infrared spectroscopy (DRIFTS).

Surface modification to control the secondary pollution of photocatalytic nitric oxide removal over monolithic protonated g-C3N4/graphene oxide aerogel.

Recently, photocatalytic NOx treatment has attracted great attention on account of the use of environmental-friendly and tremendous energy source. However, the difficult recovery of most reported powdery photocatalysts and the high generation rate of toxic NO2 byproduct limit its application. Here, we designed a novel monolithic protonated g-C3N4/graphene oxide aerogel through a direct frozen-drying method. A remarkable surface electric charge change of negative g-C3N4 to positive protonated g-C3N4 can be observed after the protonating treatment, which connects with negative graphene oxide nanosheets through the formation of strong electrostatic self-assembly to accelerate the photogenerated charge carriers transfer. Graphene oxide aerogel acts as a monolithic substrate, which provides abundant porous structure, enhanced visible-light absorption, and electrons transport pathway to improve photocatalytic activity. Importantly, the introduction of H atoms on the N atoms of p-C3N4 promotes the activation of oxygen atoms, thus improving the oxidization of NO2 to nitrate. As a result, protonated g-C3N4/graphene oxide aerogel reveals an excellent NO removal ratio (46.1%) and low NO2 generation (2.4%), demonstrating its excellent promising for air pollution purification. Our current work affords novel innovative insight for the construction of monolithic photocatalysts to control the secondary pollution for environmental remediation.

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/BO0.2N0.8 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/BO0.2N0.8 interface after the introduction of O atoms. The B-C bond as the mediator bridges g-C3N4 and BO0.2N0.8 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, BO0.2N0.8 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/BO0.2N0.8 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 visible-light response of metal-free doped bulk h-BN as potential efficient photocatalyst: a computational study.

We have provided a straightforward route to screen a series of metal-free doped bulk h-BN as potential visible-light photocatalysts via the first-principle computations. Various nonmetal dopants are considered including Si, P, C, S, Cl, O, and F atoms according to increasing electronegativity. The results show that the introduction of nonmetals leads to small lattice distortions but significant modifications of band structures, electron transition paths and chemical bonding interactions. Generally, all doped h-BN except Si doping have an active response to the visible-light, and dopants with higher electronegativity can significantly narrow the band gaps, which could induce easier optical transition under visible-light excitation. Based on the electronic structures and absorption spectra, three different mechanisms of enhanced visible-light response for the doping effect are proposed. It is expected that F, Cl, and S-doped h-BN could be used as potential efficient visible-light driven photocatalysts. This study could aid in the design of novel efficient h-BN photocatalysts. Graphical Abstract The mechanisms of the enhanced visible-light response of metal-free doped bulk h-BN.