Dynamic Hydroxylation Enhances Hydrogen Atom Abstraction from Water for Nitrogen Fixation Revealed by Isotope Labeling in Situ Fourier-Transform Infrared Spectroscopy.

Employing water as a hydrogen source to participate in the hydrogen atom transfer (HAT) process is a low-cost and carbon-free process demonstrating great economic and environmental potential in catalysis. However, the low efficiency of hydrogen atom abstraction from water leads to slow kinetics of HAT for most hydrogenative reactions. Here, we prepared ultrathin Bi4O5Cl2 nanosheets where the surface can be in situ reconstructed via hydroxylation under light illumination to facilitate the abstraction of hydrogen atoms from pure water for efficient nitrogen fixation. Consequently, the isotope labeling in situ Fourier-transform infrared spectroscopy (FT-IR) involving H2O and D2O has clearly revealed that the hydroxyl groups tend to be adsorbed on the chloride vacancy sites on the Bi4O5Cl2 surface to form hydroxylated surfaces, where the hydroxylated photocatalyst surface enables partial dehydrogenation of water into H2O2, allowing the utilization of H atoms for efficient of N2 hydrogenation via HAT steps. This work elucidates the in-depth reaction mechanism of hydrogen atom extraction from H2O molecules via the light-generated chloride vacancy to promote photocatalytic nitrogen fixation, ultimately enabling the inspiration and providing crucial rules for the design of important functional materials that can efficiently deliver active hydrogen for chemical synthesis.

Targeted NO Oxidation and Synchronous NO2 Inhibition via Oriented 1O2 Formation Based on Lewis Acid Site Adjustment.

An appealing strategy for ensuring environmental benefits of the photocatalytic NO oxidation reaction is to convert NO into NO3- instead of NO2, yet the selectivity of products remains challenging. Here, such a scenario could be realized by tailoring the exposure of Lewis acid sites on the surface of ZrO2, aiming to precisely regulate the ROS evolution process for the selective oxidation of NO into NO3-. As evidenced by highly combined experimental characterizations and density functional theory (DFT) simulations, Lewis acid sites serving as electron acceptors could induce itinerant electron redistribution, charge-carrier transfer, and further oxidation of •O2-, which promotes the oriented formation of 1O2. As a result, monoclinic ZrO2 with more Lewis acid sites exhibited an outstanding NO conversion efficiency (56.33%) and extremely low NO2 selectivity (5.04%). The ROS-based reaction process and promotion mechanism of photocatalytic performance have been revealed on the basis of ESR analysis, ROS-quenching experiments, and in situ ROS-quenching DRIFTS. This work could provide a critical view toward oriented ROS formation and advance a unique mechanism of selective NO oxidation into NO3-.

A ball milling method for highly dispersed Ni atoms on g-C3N4 to boost CO2 photoreduction.

Atomically dispersed active sites can effectively enhance the catalytic activity, but the synthesis of highly dispersed single-atom active sites remains challenging. Herein, we report for the fabrication of single-atom Ni on g-C3N4 (CN) catalysts for photocatalytic CO2 reduction reaction (CO2RR) using a high-energy ball milling method. The uniformly loaded single-atomic Ni on the surface of the substrate suggests the improvement of synthetic methods. After optimizing the Ni loading, the photocatalyst containing 0.5 at% (0.32 wt%) single-atomic Ni (Ni/CN-0.5) exhibited the highest CO2 reduction performance (∼19.9 µmol·g-1·h-1) without any co-catalyst or sacrificial agent. As visualized by aberration-corrected high-angle annular darkfield scanning transmission electron microscopy (AC HAADF-STEM), the Ni atoms in the Ni/CN-0.5 photocatalyst are most uniformly dispersed for different loadings (0.1, 0.3, 0.5, 0.7, 1.0, 3.0 and 5.0 at%). These results suggest that the uniformity of the single-atom active sites plays a decisive role rather than the loading amount in the highly enhanced performance. This work provides insight into the design of photocatalysts with highly dispersed single-atom catalytic active sites for enhancing activity.

Light-induced halogen defects as dynamic active sites for CO2 photoreduction to CO with 100% selectivity.

Dynamic defects on halide perovskite materials, caused by ion dissociation and migration under light illumination, typically result in undesirable energy dissipation and limited energy conversion efficiency. However, in this work, we demonstrated that dynamic halogen defects generated by the same process in bismuth oxyhalide (Bi5O7Cl) materials can act as active sites to promote charge separation and photocatalytic efficiency. Mechanistic studies and density functional theory calculations revealed that dynamic Cl defects affected the electronic structure of Bi5O7Cl and photocatalytic CO2 reduction process. As active sites, these defects promoted charge transfer, leading to the activation of adsorbed CO2 molecules and reduction of the energy barrier of the rate-determining step. Thus, CO2 was spontaneously converted into COOH- intermediate and finally reduced to CO with a high efficiency of 108.60 µmol g-1 and selectivity of 100% after 4-h of CO2 photoreduction. This work is highly instructive and valuable to the exploration of dynamic defects on halide-containing materials applied in solar energy conversion.

Dynamic Active Sites in Bi5O7I Promoted by Surface Tensile Strain Enable Selective Visible Light CO2 Photoreduction.

Surface defects with abundant localized electrons on bismuth oxyhalide catalysts are proved to have the capability to capture and activate CO2. However, bismuth oxyhalide materials are susceptible to photocorrosion, making the surface defects easily deactivated and therefore losing their function as active sites. Construction of deactivation-resistant surface defects on catalyst is essential for stable CO2 photoreduction, but is a universal challenge. In this work, the Bi5O7I nanotubes with surface tensile strain are synthesized, which are favorable for the visible light-induced dynamic I defects generation. The CO2 molecules absorbed on I defects are constantly reduced by the incoming photogenerated electrons from I-deficient Bi5O7I nanotubes and the successive protonation of CO2 molecules is thus highly promoted, realizing the selective CO2 conversion process via the route of CO2-COOH--CO. The efficient and stable photoreduction of CO2 into CO with 100% selectivity can be achieved even under visible light (λ >420 nm) irradiation benefited from the dynamic I defects as active sites. The results presented herein demonstrate the unique action mechanism of light-induced dynamic defects during CO2 photoreduction process and provide a new strategy into rational design of deactivation-resistant catalysts for selective CO2 photoreduction.

Unraveling the electrocatalytic reduction mechanism of enols on copper in aqueous media.

Deoxygenation of aldehydes and their tautomers to alkenes and alkanes has implications in refining biomass-derived fuels for use as transportation fuel. Electrochemical deoxygenation in ambient, aqueous solution is also a potential green synthesis strategy for terminal olefins. In this manuscript, direct electrochemical conversion of vinyl alcohol and acetaldehyde on polycrystalline Cu to ethanol, ethylene and ethane; and propenol and propionaldehyde to propanol, propene and propane is reported. Sensitive detection was achieved using a rotating disk electrode coupled with gas chromatography-mass spectrometry. In-situ attenuated total reflection surface-enhanced infrared absorption spectroscopy, and in-situ Raman spectroscopy confirmed the adsorption of the vinyl alcohol. Calculations using canonical and grand-canonical density functional theory and experimental findings suggest that the rate-determining step for ethylene and ethane formation is an electron transfer step to the adsorbed vinyl alcohol. Finally, we extend our conclusions to the enol reaction from higher-order soluble aldehyde and ketone. The products observed from the reduction reaction also sheds insights into plausible reaction pathways of CO2 to C2 and C3 products.

Insights into Dynamic Surface Bromide Sites in Bi4 O5 Br2 for Sustainable N2 Photofixation.

Simulating photosynthesis has long been one of the ideas for realizing the conversion of solar energy into industrial chemicals. Heterogeneous N2 photofixation in water is a promising way for sustainable production of ammonia. However, a mechanistic understanding of the complex aqueous photocatalytic N2 reduction is still lacking. In this study, a light-dependent surface hydrogenation mechanism and light-independent protection of catalyst surface for N2 reduction are revealed on ultrathin Bi4 O5 Br2 (BOB) nanosheets, in which the creation and annihilation of surface bromine vacancies can be controlled via a surface bromine cycle. Our rapid scan in situ FT-IR spectra verify that photocatalytic N2 reduction proceeds through an associative alternating mechanism on BOB surface with bromine vacancies (BrV-BOB). This work provides a new strategy to combine light-dependent facilitated reaction with light-independent regeneration of catalyst for advancing sustainable ammonia production.

The Crystal Plane is not the Key Factor for CO2 -to-Methane Electrosynthesis on Reconstructed Cu2 O Microparticles.

Cu2 O microparticles with controllable crystal planes and relatively high stability have been recognized as a good platform to understand the mechanism of the electrocatalytic CO2 reduction reaction (CO2 RR). Herein, we demonstrate that the in situ generated Cu2 O/Cu interface plays a key role in determining the selectivity of methane formation, rather than the initial crystal plane of the reconstructed Cu2 O microparticles. Experimental results indicate that the methane evolution is dominated on all three different crystal planes with similar Tafel slopes and long-term stabilities. Density functional theory (DFT) calculations further reveal that *CO is protonated via a similar bridge configuration at the Cu2 O/Cu interface, regardless of the initial crystal planes of Cu2 O. The Gibbs free energy changes (ΔG) of *CHO on different reconstructed Cu2 O planes are close and more negative than that of *OCCOH, indicating the methane formation is more favorable than ethylene on all Cu2 O crystal planes.

Synergistic Effect of Cu Single Atoms and Au-Cu Alloy Nanoparticles on TiO2 for Efficient CO2 Photoreduction.

The synergy between metal alloy nanoparticles (NPs) and single atoms (SAs) should maximize the catalytic activity. However, there are no relevant reports on photocatalytic CO2 reduction via utilizing the synergy between SAs and alloy NPs. Herein, we developed a facile photodeposition method to coload the Cu SAs and Au-Cu alloy NPs on TiO2 for the photocatalytic synthesis of solar fuels with CO2 and H2O. The optimized photocatalyst achieved record-high performance with formation rates of 3578.9 for CH4 and 369.8 µmol g-1 h-1 for C2H4, making it significantly more realistic to implement sunlight-driven synthesis of value-added solar fuels. The combined in situ FT-IR spectra and DFT calculations revealed the molecular mechanisms of photocatalytic CO2 reduction and C-C coupling to form C2H4. We proposed that the synergistic function of Cu SAs and Au-Cu alloy NPs could enhance the adsorption activation of CO2 and H2O and lower the overall activation energy barrier (including the rate-determining step) for the CH4 and C2H4 formation. These factors all enable highly efficient and stable production of solar fuels of CH4 and C2H4. The concept of synergistic SAs and metal alloys cocatalysts can be extended to other systems, thus contributing to the development of more effective cocatalysts.

Doping and facet effects synergistically mediated interfacial reaction mechanism and selectivity in photocatalytic NO abatement.

The surface atomic coordination and arrangement largely determine photocatalytic properties. Whereas, the intrinsic impact of surface microstructures on the reaction mechanism and pathway is still unclear. Herein, via constructing N-doped Bi2O2CO3 photocatalysts with diverse exposed facets, (1 1 0) and (0 0 1) facet, we testify that the pivotal roles of crystal facet and doping effect on the intermediate production and reactivity for photocatalytic nitric oxide (NO) abatement. The photoreactivity of N-doped Bi2O2CO3 is documented to be higher than that of the pure samples because of the enhanced light absorption and charge transfer. Further in situ probing experiments and theoretical calculations verify that the unique adsorption patterns and activated intermediates on the (1 1 0) facet facilitate the formation of final products and inhibit the generation of toxic NO2 by-product in terms of thermodynamics. More importantly, we found that the selective and nonselective oxidation processes are emerged over (1 1 0) and (0 0 1) facets of Bi2O2CO3, respectively.

Rare-Earth Single-Atom La-N Charge-Transfer Bridge on Carbon Nitride for Highly Efficient and Selective Photocatalytic CO2 Reduction.

Photocatalytic CO2 conversion into valuable solar fuels is highly appealing, but lack of directional charge-transfer channel and insufficient active sites resulted in limited CO2 reduction efficiency and selectivity for most photocatalytic systems. Herein, we designed and fabricated rare-earth La single-atoms on carbon nitride with La-N charge-transfer bridge as the active center for photocatalytic CO2 reaction. The formation of La single-atoms was certified by spherical aberration-corrected HAADF-STEM, STEM-EELS, EXAFS, and theoretical calculations. The electronic structure of the La-N bridge enables a high CO-yielding rate of 92 µmol·g-1·h-1 and CO selectivity of 80.3%, which is superior to most g-C3N4-based photocatalytic CO2 reductions. The CO production rate remained nearly constant under light irradiation for five cycles of 20 h, indicating its stability. The closely combined experimental and DFT calculations clearly elucidated that the variety of electronic states induced by 4f and 5d orbitals of the La single atom and the p-d orbital hybridization of La-N atoms enabled the formation of charge-transfer channel. The La-N charge bridges are found to function as the key active center for CO2 activation, rapid COOH* formation, and CO desorption. The present work would provide a mechanistic understanding into the utilization of rare-earth single-atoms in photocatalysis for solar energy conversion.

Nature-inspired CaCO3 loading TiO2 composites for efficient and durable photocatalytic mineralization of gaseous toluene.

The accumulation of intermediates or final products on TiO2 during photocatalytic volatile organic compounds (VOCs) degradation is typically neglected, despite the fact that it could result in the block of active sites and the deactivation of photocatalysts. Inspired from the natural formation of stalactite (CaCO3 + H2O + CO2 â†” Ca(HCO3)2), we fabricated CaCO3 loading TiO2 composites (CCT21) to realize the spontaneously transfer of accumulated final products (CO2 and H2O). Efficient and durable performance for gaseous toluene removal has been demonstrated and the cost of photocatalyst is greatly reduced by the comparison of specific activity. The introduction of CaCO3 induces the interaction between TiO2 and CaCO3 to stimulate abundant activated electrons for the improvement on the adsorption and activation of reactants and the transformation of photogenerated carriers, and most importantly, facilitates the transfer of final products to release active sites and thus suppress the deactivation of TiO2. Furthermore, we develop a facile method to immobilize CCT21 powder on flexible support, which greatly reduces the loss of photocatalysts and correspondingly enables the practical application of TiO2-based products. Therefore, this work presents a novel nature-inspired strategy to address the challenge of deactivation, and advances the development of photocatalytic technology for environmental remediation.

Graphene oxide mediated co-generation of C-doping and oxygen defects in Bi2WO6 nanosheets: a combined DRIFTS and DFT investigation.

In order to efficiently control air pollutants using photocatalytic technology, the co-generation of C-doping and oxygen vacancies (OVs) in Bi2WO6 (BWO) nanosheets was achieved by a graphene oxide (GO)-mediated hydrothermal method. The photocatalytic performance was highly improved with the synergistic effects of C-doping and OVs. The experimental characterization and DFT calculations were closely combined to reveal that the C element could serve as both an electron acceptor and channel for charge transfer to promote charge separation. Meanwhile, the OVs could induce the formation of a defect level in the band gap which increases the production of ˙OH as the primary reactive species by introducing more light-generated holes into the valence band. Meanwhile, the OVs could enhance the generation of ˙O2- species via the promotion of O2 adsorption and activation on the catalyst surface. Moreover, the reaction intermediates were monitored and the mechanism of photocatalytic NO oxidation was proposed based on in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The design concept of photocatalyst modification with C-doping and OVs could offer a novel strategy to enhance the performance for environmental applications.

Promoting ring-opening efficiency for suppressing toxic intermediates during photocatalytic toluene degradation via surface oxygen vacancies.

Aromatic ring-opening process is well recognized as the rate-determining step for catalytic toluene degradation. In photocatalytic toluene degradation, the toxic intermediates with harmful effects may be generated. To clarify the precise reaction mechanism and control the toxic intermediates generation, a closely combined in situ DRIFTS and DFT calculation is utilized to address these important issues. We construct the BiOCl with oxygen vacancies (OVs) and reveal the structure of OVs. The defect level caused by oxygen vacancies could promote the light adsorption and charge separation, which further boosts the activation of ring-opening species and enhances the generation process of free radicals. The reaction energy barriers of four possible ring-opening processes on defective BiOCl (OVBOC) are all declined in comparison with perfect BiOCl (BOC). The existence of oxygen vacancies could smooth the rate-determining step so the ring-opening efficiency of photocatalytic toluene degradation is highly increased. Most importantly, the methyl species would be further oxidized and tend to open the benzene-ring at benzoic acid on BOC while the ring would be broken at the benzyl alcohol on OVBOC. These results indicate that the toluene degradation pathway is shortened via the surface OVs, which enables the production of radicals with high oxidation capability for the accelerated chain scission of the ring-opening intermediates. Finally, the efficiency of the key ring-opening process could be enormously improved and toxic intermediates are effectively restrained. The present work could provide new insights into the design of high-performance photocatalysts for efficient and safe degradation of VOCs in air.

Simultaneous introduction of oxygen vacancies and Bi metal onto the {001} facet of Bi3O4Cl woven nanobelts for synergistically enhanced photocatalysis.

In order to relieve air pollution problems via photocatalytic NO purification, we prepared Bi3O4Cl (BOC) woven nanobelts with exposed {001} facets by a one-pot hydrothermal method. The as-prepared catalysts were modified by a simple chemical reduction to introduce oxygen vacancies (OVs) and plasmonic Bi metal on their surfaces. The photocatalytic performance was strongly dependent on the OVs and surface plasmon resonance (SPR) effect induced by Bi metal. DFT calculations and in situ DRIFTS study were closely combined to show that the OVs could not only induce the formation of a middle-gap in BOC which increases the production of the ˙OH species by introducing more holes on the valence band, but also promote the generation of ˙O2- species by activating the adsorbed O2 on the surface. Meanwhile, the Bi metal can serve both as an electron donor and a bridge for charge transfer to promote charge separation. As a result, the OVs and Bi metal could synergistically tailor the charge transfer pathway and enhance the photocatalytic performance significantly. Moreover, the reaction intermediates were revealed and the mechanism of photocatalytic NO oxidation was proposed based on in situ DRIFTS spectra. The design concept of modifying the catalyst surface via simultaneous introduction of OVs and Bi metal on the catalyst surface could offer a novel strategy to enhance the photocatalytic performance of other nanomaterials for environmental and energy-related applications.

In situ FT-IR investigation on the reaction mechanism of visible light photocatalytic NO oxidation with defective g-C3N4.

The g-C3N4 with different structures was prepared by heat treatment using urea (CN-U) and thiourea (CN-T) as precursors under the same conditions. The microstructure and optical properties of the photocatalyst were analyzed with advanced tools. The results showed that the CN-U has a porous structure, a high specific surface area and a wide band gap in comparison with CN-T. The in situ FT-IR technique was used to monitor the adsorption and reaction process of visible photocatalytic NO oxidation on g-C3N4. The corresponding reaction mechanism was proposed based on the results of reaction intermediate observation and electron paramagnetic resonance (EPR) radical scavenging. It was revealed that (1) the presence of defective sites favored the adsorption of gas molecules and electronically compensated it leading to promoted formation of the final products; (2) the high separation efficiency of photogenerated electron-hole pairs enhanced the production of radicals during the photocatalytic reaction; (3) the hydroxyl radicals (OH) are not selective for the decomposition of pollutants, which are favorable to the complete oxidation of the reaction intermediates. The above three aspects are the main reasons for the CN-U possessing the efficient visible light photocatalytic activity. The present work could provide new insights and methods for understanding the mechanism of photocatalysis.