Atomic-Scale Explanation of O2 Activation at the Au–TiO2 Interface

Synergy of Ag and Interfacial Oxygen Vacancies on TiO2 for Highly Efficient Photocatalytic Production of H2O2

Photocatalytic H2O2 production stands as a promising sustainable technology for chemical synthesis. However, rapid charge recombination and limited oxygen adsorption by photocatalysts often limit its efficiency. Herein, we demonstrate that the synergy of Ag and interfacial oxygen vacancies on TiO2 could overcome these challenges. The optimized Ag/TiO2-50 photocatalyst achieved an impressive H2O2 production rate of 12.9 mmol h-1 g-1 and maintained a steady-state concentration of 12.8 mM, significantly outperforming most TiO2-based photocatalysts documented in the literature. Detailed mechanistic studies, aided by TAS, in situ X-ray photoelectron spectroscopy (XPS), and in situ electron paramagnetic resonance (EPR) techniques, indicate that the oxygen vacancies at the Ag-TiO2 interface act as an interfacial hole trap, inducing a directional hole transfer. This, coupled with Ag acting as an electron acceptor, synergistically boosts the electron-hole separation. Additionally, the increased amount of oxygen vacancies at the Ag-TiO2 interface of Ag/TiO2-50 leads to enhanced O2 adsorption, thus contributing to its superior catalytic performance. This study provides valuable insights into interfacial traps in the charge transfer process and highlights the potential of interface regulation for achieving efficient photocatalytic conversion.

Tailoring surface morphology on anatase TiO2 supported Au nanoclusters: implications for O2 activation

Strong interaction between the support surface and metal clusters activates the adsorbed molecules at the metal cluster-support interface. Using plane-wave DFT calculations, we precisely model the interface between anatase TiO2 and small Au nanoclusters. Our study focusses on the adsorption and activation of oxygen molecules on anatase TiO2, considering the influence of oxygen vacancies and steps on the surface. We find that the plane (101) and the stepped (103) surfaces do not support O2 activation, but the presence of oxygen vacancies results in strong adsorption and O-O bond length elongation. Modifying the TiO2 surface with supported small Au n nanoclusters (n = 3-5) also significantly enhances O2 adsorption and stretches the O-O bond. We observe that manipulating the cluster orientation through discrete rotations results in improved O2 adsorption and promotes charge transfer from the surface to the molecule. We propose that the orientation of the supported cluster may be manipulated by making the cluster adsorb at the step-edge of (103) TiO2. This results in activated O2 at the cluster-support interface, with a peroxide-range bond length and a low barrier for dissociation. Our modeling demonstrates a straightforward means of exploiting the interface morphology for O2 activation under low precious metal loading, which has important implications for electrocatalytic oxidation reactions and the rational design of supported catalysts.

Mie Resonant Metal Oxide Nanospheres for Broadband Photocatalytic Enhancements

Metal oxides are widely used in heterogeneous catalysis as supports to disperse catalytically active nanoparticles, isolated atomic sites, or even as catalysts themselves. Herein, we present a method to produce optically active metal oxide supports that exhibit size-dependent Mie resonances based on TiO2 nanospheres with tunable size, crystalline phase composition, and optical properties. Mie resonant TiO2 nanospheres were used as supports to disperse Au, Pt, and Pd nanoparticles. We have found up to a 50-fold enhancement of the electric field at the metal oxide/metal interface corresponding to wavelength-dependent multipolar resonances in the TiO2 structure. Using Au/TiO2 as a prototypical photocatalyst, we demonstrate broadband rate enhancements between 400 and 800 nm during CO oxidation, with a noticeable increase below 500 nm. This increased reactivity at higher photon energies is due to improved photon utilization and interband absorption in the gold that results in greater secondary electron generation through electron-electron scattering processes, thus leading to higher rates in conjunction with the Mie scattering TiO2 support. This study not only highlights the potential of Mie resonant TiO2 in broadband photocatalytic enhancements but also for developing various Mie resonant metal oxide supports, such as ZnO or Cu2O, which can improve photocatalytic performance for a number of critical reactions. As the chemical and energy industries move toward conversion technologies driven by renewable energy sources, the strategy of designing optical resonances into oxide supports that are already widely used could enable a straightforward adaptation of photochemical processing based on traditional heterogeneous catalysts.

Novel TiO2-Supported Gold Nanoflowers for Efficient Photocatalytic NOx Abatement

In this study, we pioneered the synthesis of nanoflower-shaped TiO2-supported Au photocatalysts and investigated their properties. Au nanoflowers (Au NFs) were prepared by a Na-citrate and hydroquinone-based preparation method, followed by wet impregnation of the derived Au NFs on the surface of TiO2 nanorods (TNR). A uniform and homogeneous distribution of Au NFs was observed in the TNR + NF(0.7) sample (lower Na-citrate concentration), while their distribution was heterogeneous in the TNR + NF(1.4) sample (higher Na-citrate concentration). The UV-Vis DR spectra revealed the size- and shape-dependent optical properties of the Au NFs, with the LSPR effect observed in the visible region. The solid-state EPR spectra showed the presence of Ti3+, oxygen vacancies and electron interactions with organic compounds on the catalyst surface. In the case of the TNR + NF(0.7) sample, high photocatalytic activity was observed in the H2-assisted reduction of NO2 to N2 at room temperature under visible-light illumination. In contrast, the TNR + NF(1.4) catalyst as well as the heat-treated samples showed no ability to reduce NO2 under visible light, indicating the presence of deformed Au NFs limiting the LSPR effect. These results emphasized the importance of the choice of synthesis method, as this could strongly influence the photocatalytic activity of the Au NFs.

Unravelling the Role of Free Radicals in Photocatalysis

Free radicals are increasingly recognized as active intermediate reactive species that can participate in various redox processes, significantly influencing the mechanistic pathways of reactions. Numerous researchers have investigated the generation of one or more distinct photogenerated radicals, proposing various hypotheses to explain the reaction mechanisms. Notably, recent research has demonstrated the emergence of photogenerated radicals in innovative processes, including organic chemical reactions and the photocatalytic dissolution of precious metals. To harness the potential of these free radicals more effectively, it is imperative to consolidate and analyze the processes and action modes of these photogenerated radicals. This conceptual paper delves into the latest advancements in understanding the mechanics of photogenerated radicals.

The multivariate interaction between Au and TiO2 colloids: the role of surface potential, concentration, and defects

The established DLVO theory explains colloidal stability by the electrostatic repulsion between electrical double layers. While the routinely measured zeta potential can estimate the charges of double layers, it is only an average surface property which might deviate from the local environment. Moreover, other factors such as the ionic strength and the presence of defects should also be considered. To investigate this multivariate problem, here we model the interaction between a negatively charged Au particle and a negatively charged TiO2 surface containing positive/neutral defects (e.g. surface hydroxyls) based on the finite element method, over 6000 conditions of these 6 parameters: VPart (particle potential), VSurf (surface potential), VDef (defect potential), DD (defect density), Conc (salt concentration), and R (particle radius). Using logistic regression, the relative importance of these factors is determined: VSurf > VPart > DD > Conc > R > VDef, which agrees with the conventional wisdom that the surface (and zeta) potential is indeed the most decisive descriptor for colloidal interactions, and the salt concentration is also important for charge screening. However, when defects are present, it appears that their density is more influential than their potential. To predict the fate of interactions more confidently with all the factors, we train a support vector machine (SVM) with the simulation data, which achieves 97% accuracy in determining whether adsorption is favorable on the support. The trained SVM including a graphical user interface for querying the prediction is freely available online for comparing with other materials and models. We anticipate that our model can stimulate further colloidal studies examining the importance of the local environment, while simultaneously considering multiple factors.

Continuous Flow System for Highly Efficient and Durable Photocatalytic Oxidative Coupling of Methane

Photocatalytic oxidative coupling of methane (OCM) into value-added industrial chemicals offers an appealing green technique for achieving sustainable development, whereas it encounters double bottlenecks in relatively low methane conversion rate and severe overoxidation. Herein, we engineer a continuous gas flow system to achieve efficient photocatalytic OCM while suppressing overoxidation by synergizing the moderate active oxygen species, surface plasmon-mediated polarization, and multipoint gas intake flow reactor. Particularly, a remarkable CH4 conversion rate of 218.2 μmol h-1 with an excellent selectivity of ∼90% toward C2+ hydrocarbons and a remarkable stability over 240 h is achieved over a designed Au/TiO2 photocatalyst in our continuous gas flow system with a homemade three-dimensional (3D) printed flow reactor. This work provides an informative concept to engineer a high-performance flow system for photocatalytic OCM by synergizing the design of the reactor and photocatalyst to synchronously regulate the mass transfer, activation of reactants, and inhibition of overoxidation.

Spontaneous generation of singlet oxygen on microemulsion-derived manganese oxides with rich oxygen vacancies for efficient aerobic oxidation

Developing innovative catalysts for efficiently activating O2 into singlet oxygen (1O2) is a cutting-edge field with the potential to revolutionize green chemical synthesis. Despite its potential, practical implementation remains a significant challenge. In this study, we design a series of nitrogen (N)-doped manganese oxides (Ny-MnO2, where y represents the molar amount of the N precursor used) nanocatalysts using compartmentalized-microemulsion crystallization followed by post-calcination. These nanocatalysts demonstrate the remarkable ability to directly produce 1O2 at room temperature without the external fields. By strategically incorporating defect engineering and interstitial N, the concentration of surface oxygen atoms (Os) in the vicinity of oxygen vacancy (Ov) reaches 51.1% for the N55-MnO2 nanocatalyst. This feature allows the nanocatalyst to expose a substantial number of Ov and interstitial N sites on the surface of N55-MnO2, facilitating effective chemisorption and activation of O2. Verified through electron paramagnetic resonance spectroscopy and reactive oxygen species trapping experiments, the spontaneous generation of 1O2, even in the absence of light, underscores its crucial role in aerobic oxidation. Density functional theory calculations reveal that an increased Ov content and N doping significantly reduce the adsorption energy, thereby promoting chemisorption and excitation of O2. Consequently, the optimized N55-MnO2 nanocatalyst enables room-temperature aerobic oxidation of alcohols with a yield surpassing 99%, representing a 6.7-fold activity enhancement compared to ε-MnO2 without N-doping. Furthermore, N55-MnO2 demonstrates exceptional recyclability for the aerobic oxidative conversion of benzyl alcohol over ten cycles. This study introduces an approach to spontaneously activate O2 for the green synthesis of fine chemicals.

Reactivity of Group 5 and 6 Single-Site Photocatalysts for Partial Oxidation of Methane: Comparison of Chromium, Niobium, and Tungsten-Doped Mesoporous Amorphous Silica

Single-site transition-metal-doped photocatalysts can potentially be used for partial oxidation of methane (POM) at remote sites where natural gas is extracted and methane is often flared or released to the atmosphere. While there have been several investigations into the performance of vanadium, there has been no general survey of the performance of other metals. This work aims and examines Cr, Nb, and W metal oxide materials embedded in amorphous SiO2 to determine the viability of each metal in catalyzing the POM. Photoexcited states are examined to determine the nature of the photoactivated species, and then the subsequent POM reaction mechanisms are elucidated. Using the calculated energies of reaction intermediates and transition states, the rate of methanol formation is evaluated through the use of a microkinetic model. The findings indicate that all three metals are potentially more suitable for catalyzing POM than vanadium but that niobium shows the most favorable energy profile.

Elucidating the Origin of Plasmon-Generated Hot Holes in Water Oxidation

Plasmon-generated hot electrons in metal/oxide heterostructures have been used extensively for driving photochemistry. However, little is known about the origin of plasmon-generated hot holes in promoting photochemical reactions. Herein, we discover that, during the nonradiative plasmon decay, the interband excitation rather than the intraband excitation generates energetic hot holes that enable to drive the water oxidation at the Au/TiO₂ interface. Distinct from lukewarm holes via the intraband excitation that only remain on Au, hot holes from the interband excitation are found to be transferred from Au into TiO₂ and stabilized by surface oxygen atoms on TiO₂, making them available to oxidize adsorbed water molecules. Taken together, our studies provide spectroscopic evidence to clarify the photophysical process for exciting plasmon-generated hot holes, unravel their atomic-level accumulation sites to maintain the strong oxidizing power in metal/oxide heterostructures, and affirm their crucial functions in governing photocatalytic oxidation reactions.

Differentiating between Acidic and Basic Surface Hydroxyls on Metal Oxides by Fluoride Substitution: A Case Study on Blue TiO2 from Laser Defect Engineering

Both oxygen vacancies and surface hydroxyls play a crucial role in catalysis. Yet, their relationship is not often explored. Herein, we prepare two series of TiO₂ (rutile and P25) with increasing oxygen deficiency and Ti³⁺ concentration by pulsed laser defect engineering in liquid (PUDEL), and selectively quantify the acidic and basic surface OH by fluoride substitution. As indicated by EPR spectroscopy, the laser-generated Ti³⁺ exist near the surface of rutile, but appear to be deeper in the bulk for P25. Fluoride substitution shows that extra acidic bridging OH are selectively created on rutile, while the surface OH density remains constant for P25. These observations suggest near-surface Ti³⁺ are highly related to surface bridging OH, presumably the former increasing the electron density of the bridging oxygen to form more of the latter. We anticipate that fluoride substitution will enable better characterization of surface OH and its correlation with defects in metal oxides.

Photolysis versus Photothermolysis of N2O on a Semiconductor Surface Revealed by Nonadiabatic Molecular Dynamics

Identifying photolysis and photothermolysis during a photochemical reaction has remained challenging because of the highly non-equilibrium and ultrafast nature of the processes. Using state-of-the-art ab initio adiabatic and nonadiabatic molecular dynamics, we investigate N₂O photodissociation on the reduced rutile TiO₂(110) surface and establish its detailed mechanism. The photodecomposition is initiated by electron injection, leading to the formation of a N₂O^- ion-radical, and activation of the N₂O bending and symmetric stretching vibrations. Photothermolysis governs the N₂O dissociation when N₂O^- is short-lived. The dissociation is activated by a combination of the anionic excited state evolution and local heating. A thermal fluctuation drives the molecular acceptor level below the TiO₂ band edge, stabilizes the N₂O^- anion radical, and causes dissociation on a 1 ps timescale. As the N₂O^- resonance lifetime increases, photolysis becomes dominant since evolution in the anionic excited state activates the bending and symmetric stretching of N₂O, inducing the dissociation. The photodecomposition occurs more easily when N₂O is bonded to TiO₂ through the O rather than N atom. We demonstrate further that a thermal dissociation of N₂O can be realized by a rational choice of metal dopants, which enhance p-d orbital hybridization, facilitate electron transfer, and break N₂O spontaneously. By investigating the charge dynamics and lifetime, we provide a fundamental atomistic understanding of the competition and synergy between the photocatalytic and photothermocatalytic dissociation of N₂O and demonstrate how N₂O reduction can be controlled by light irradiation, adsorption configuration, and dopants, enabling the design of high-performance transition-metal oxide catalysts.

Synergy in Au-CuO Janus Structure for Catalytic Isopropanol Oxidative Dehydrogenation to Acetone

The controlled oxidation of alcohols to the corresponding ketones or aldehydes via selective cleavage of the β-C-H bond of alcohols under mild conditions still remains a significant challenge. Although the metal/oxide interface is highly active and selective, the interfacial sites fall far behind the demand, due to the large and thick support. Herein, we successfully develop a unique Au-CuO Janus structure (average particle size=3.8 nm) with an ultrathin CuO layer (0.5 nm thickness) via a bimetal in situ activation and separation strategy. The resulting Au-CuO interfacial sites prominently enhance isopropanol adsorption and decrease the energy barrier of β-C-H bond scission from 1.44 to 0.01 eV due to the strong affinity between the O atom of CuO and the H atom of isopropanol, compared with Au sites alone, thereby achieving ultrahigh acetone selectivity (99.3 %) over 1.1 wt % AuCu_0.75 /Al₂ O₃ at 100 °C and atmospheric pressure with 97.5 % isopropanol conversion. Furthermore, Au-CuO Janus structures supported on SiO₂ , TiO₂ or CeO₂ exhibit remarkable catalytic performance, and great promotion in activity and acetone selectivity is achieved as well for other reducible oxides derived from Fe, Co, Ni and Mn. This study should help to develop strategies for maximized interfacial site construction and structure optimization for efficient β-C-H bond activation.

Oxygen and Chlorine Dual Vacancies Enable Photocatalytic O2 Dissociation into Monatomic Reactive Oxygen on BiOCl for Refractory Aromatic Pollutant Removal

Room-temperature molecular oxygen (O₂) dissociation is challenging toward chemical reactions due to its triplet ground-state and spin-forbidden characteristic. Herein, we demonstrate that BiOCl of oxygen and chlorine dual vacancies can photocatalytically dissociate O₂ into monatomic reactive oxygen (•O^-) for the ring opening of aromatic refractory pollutants toward deep oxidation. The electron-rich and geometry-flexible dual vacancies of oxygen and chlorine remarkably lengthen the O-O bond of adsorbed O₂ from 1.21 to 2.74 Å, resulting in the rapid O₂ dissociation and the subsequent •O^- formation. During the photocatalytic degradation of sulfamethazine, the in situ-formed •O^- plays an indispensable role in breaking the critical intermediate of pyrimidine containing a stubborn aromatic heterocyclic ring, thus facilitating the overall mineralization. More importantly, BiOCl of oxygen and chlorine dual vacancies is also superior to its monovacancy counterparts on the degradation of other refractory pollutants containing conjugated six-membered rings, including p-chlorophenol, p-chloronitrobenzene, p-hydroxybenzoic acid, and p-nitrobenzoic acid. This study sheds light on the importance of sophisticated defects for regulating the O₂ activation manner and deliveries a novel O₂ activation approach for environmental remediation with solar energy.

Single-Atom High-Temperature Catalysis on a Rh1O5 Cluster for Production of Syngas from Methane

Single-atom catalysts are a relatively new type of catalyst active for numerous reactions but mainly for chemical transformations performed at low or intermediate temperatures. Here we report that singly dispersed Rh₁O₅ clusters on TiO₂ can catalyze the partial oxidation of methane (POM) at high temperatures with a selectivity of 97% for producing syngas (CO + H₂) and high activity with a long catalytic durability at 650 °C. The long durability results from the substitution of a Ti atom of the TiO₂ surface lattice by Rh₁, which forms a singly dispersed Rh₁ atom coordinating with five oxygen atoms (Rh₁O₅) and an undercoordinated environment but with nearly saturated bonding with oxygen atoms. Computational studies show the back-donation of electrons from the d_z² orbital of the singly dispersed Rh₁ atom to the unoccupied orbital of adsorbed CH_n (n > 1) results in the charge depletion of the Rh₁ atom and a strong binding of CH_n to Rh₁. This strong binding decreases the barrier for activating C-H, thus leading to high activity of Rh₁/TiO₂. A cationic Rh₁ single atom anchored on TiO₂ exhibits a weak binding to atomic carbon, in contrast to the strong binding of the metallic Rh surface to atomic carbon. The weak binding of atomic carbon to Rh₁ atoms and the spatial isolation of Rh₁ on TiO₂ prevent atomic carbon from coupling on Rh₁/TiO₂ to form carbon layers, making Rh₁/TiO₂ resistant to carbon deposition than supported metal catalysts for POM. The highly active, selective, and durable high-temperature single-atom catalysis performed at 650 °C demonstrates an avenue of application of single-atom catalysis to chemical transformations at high temperatures.

High Catalytic Performance of Au/Bi2O3 for Preferential Oxidation of CO in H2

Preferential oxidation (PROX) of CO in hydrogen is of great significance for proton exchange membrane fuel cells (PEMFCs) that need a CO-free hydrogen stream as fuel. The key technical problem is developing catalysts that can efficiently remove CO from the H2-rich stream within the working temperature range of PEMFCs. Herein, we design a Au/Bi2O3 interfacial catalyst for PROX with excellent catalytic performance, which can achieve 100% CO conversion in the PROX reaction over a wide temperature window (70-200 °C) and is perfectly compatible with the operating temperature window (80-180 °C) of PEMFCs. Moreover, the catalyst also demonstrates excellent high flow performance and long-term stability. Density functional theory (DFT) calculations reveal that the electrons transferring from Bi2O3 to Au and then to adsorbed perimeter CO and O2 molecules promote the activation of CO and O2, thus enhancing the catalytic performance of PROX.

A powerful azomethine ylide route mediated by TiO2 photocatalysis for the preparation of polysubstituted imidazolidines

Lewis- and Brønsted-acid catalyzed 1,3-dipolar cycloaddition between azomethine ylides and unsaturated compounds is an important strategy to construct five-membered N-heterocycles. However, such a catalytic route usually demands substrates with an electron-withdrawing group (EWG) to facilitate the reactivity. Herein, we report a TiO2 photocatalysis strategy that can conveniently prepare five-membered N-heterocyclic imidazolidines from a common imine (N-benzylidenebenzylamine) and alcohols along the route of 1,3-dipolaron azomethine ylide but without pre-installed EWG substituents on the substrates. Our EPR results uncovered the previously unknown mutual interdependence between an azomethine ylide and TiO2 photo-induced hvb+/ecb- pair. This transformation exhibited a broad scope with 21 successful examples and could be scaled up to the gram level.

Influence of oxygen and water content on the formation of polychlorinated organic by-products from catalytic degradation of 1,2-dichlorobenzene over a Pd/ZSM-5 catalyst

Understanding the generation and influence mechanism of polychlorinated organic by-products during the catalytic degradation of chlorinated volatile organic compounds (CVOCs) is essential to the safe and environmentally friendly treatment of those pollutants. In this study, a systematic investigation of the catalytic oxidation of 1,2-dichlorobenzene (1,2-DCB) was conducted using various oxygen and water contents over a Pd/ZSM-5(25) catalyst. It was found that decreasing the oxygen content and increasing the water content resulted in the improvement of the 1,2-DCB catalytic activity, while the amount and variety of polychlorinated organic by-products decreased. More importantly, when water was the sole oxidant, the Pd/ZSM-5(25) catalyst also demonstrated high activity towards 1,2-DCB catalytic degradation. Only chlorobenzene and 1,3-dichlorobenzene were detected as by-products. X-ray photoelectron spectra (XPS) and UV-vis DRS spectra results indicated that the polychlorinated organic by-products were suppressed mainly due to inhibition of the chlorination of the palladium species by regulating the oxygen and water content in the reaction atmosphere. Similar surface species were formed under aerobic and anaerobic atmospheres via the study of the in situ FTIR spectra. We therefore proposed that 1,2-DCB undergoes similar catalytic degradation reaction mechanisms under both aerobic and anaerobic conditions.

Fe1 /TiO2 Hollow Microspheres: Fe and Ti Dual Active Sites Boosting the Photocatalytic Oxidation of NO

Recently, single-atom catalysts have aroused extensive attention in fields of clean energy and environmental protection due to their unique activity and efficient utilization of the active atoms. It is of great importance but still remains a great challenge to unveil the effect of single atoms on precise catalysis. Herein, it is reported that doping TiO₂ hollow microspheres (TiO₂ -HMSs) with single atomic Fe can boost the photoreactivity of TiO₂ -HMSs towards NO oxidation due to the synergistic effects of atomically dispersed Fe and bonded Ti atom which act as dual active sites. The atomically dispersed Fe atoms occupy the subsurface Ti vacancies, and the interaction between Ti 3d and Fe 3d orbitals result in the formation of FeTi bond. Single atomic Fe modulates the electronic structure of the bonded Ti atoms by electron transfer, which facilitates the adsorption and activation of NO and O₂ at Fe and bonded Ti sites, respectively. In addition, the introduction of single atomic Fe sharply suppresses the production of toxic NO₂ byproduct. The synergistic effects of the dual active sites then cause a drastic promotion in photocatalytic oxidation of NO.

Direct probing of atomically dispersed Ru species over multi-edged TiO2 for highly efficient photocatalytic hydrogen evolution

A cocatalyst is necessary for boosting the electron-hole separation efficiency and accelerating the reaction kinetics of semiconductors. As a result, it is of critical importance to in situ track the structural evolution of the cocatalyst during the photocatalytic process, but it remains very challenging. Here, atomically dispersed Ru atoms are decorated over multi-edged TiO₂ spheres for photocatalytic hydrogen evolution. Experimental results not only demonstrate that the photogenerated electrons can be effectively transferred to the isolated Ru atoms for hydrogen evolution but also imply that the TiO₂ architecture with multi-edges might facilitate the charge separation and transport. The change in valence and the evolution of electronic structure of Ru sites are well probed during the photocatalytic process. Specifically, the optimized catalyst produces the hydrogen evolution rate of 7.2 mmol g^-1 hour^-1, which is much higher than that of Pt-based cocatalyst systems and among the highest reported values.

Single Ni Atoms Anchored on Porous Few-Layer g-C3 N4 for Photocatalytic CO2 Reduction: The Role of Edge Confinement

It is greatly intriguing yet remains challenging to construct single-atomic photocatalysts with stable surface free energy, favorable for well-defined atomic coordination and photocatalytic carrier mobility during the photoredox process. Herein, an unsaturated edge confinement strategy is defined by coordinating single-atomic-site Ni on the bottom-up synthesized porous few-layer g-C₃ N₄ (namely, Ni₅ -CN) via a self-limiting method. This Ni₅ -CN system with a few isolated Ni clusters distributed on the edge of g-C₃ N₄ is beneficial to immobilize the nonedged single-atomic-site Ni species, thus achieving a high single-atomic active site density. Remarkably, the Ni₅ -CN system exhibits comparably high photocatalytic activity for CO₂ reduction, giving the CO generation rate of 8.6 µmol g^-1 h^-1 under visible-light illumination, which is 7.8 times that of pure porous few-layer g-C₃ N₄ (namely, CN, 1.1 µmol g^-1 h^-1 ). X-ray absorption spectrometric analysis unveils that the cationic coordination environment of single-atomic-site Ni center, which is formed by Ni-N doping-intercalation the first coordination shell, motivates the superiority in synergistic N-Ni-N connection and interfacial carrier transfer. The photocatalytic mechanistic prediction confirms that the introduced unsaturated Ni-N coordination favorably binds with CO₂ , and enhances the rate-determining step of intermediates for CO generation.

Facile Fabrication of Amorphous Molybdenum Oxide as a Sensitive and Stable SERS Substrate under Redox Treatment

Amorphous MoO_3-x nanosheets were fabricated by the room-temperature oxidation of molybdenum powder with H₂ O₂ , followed by light-irradiation reduction in methanol. When applied as a substrate for surface-enhanced Raman spectroscopy (SERS), these nanosheets exhibit higher sensitivity than the crystalline counterpart for a wide range of analytes. Moreover, the SERS activity remains stable on repeated oxygen insertion/extraction. In contrast, the performance of crystalline MoO_3-x continuously deteriorates on successive redox treatments. This unique SERS activity allows the recycling of the substrate through an H₂ O₂ -based Fenton-like reaction. More importantly, the non-invasive SERS was unprecedentedly used for the self-diagnosis of amorphous MoO_3-x as a more selective photocatalyst than its crystalline counterpart.

The synergistic role of the support surface and Au–Cu alloys in a plasmonic Au–Cu@LDH photocatalyst for the oxidative esterification of benzyl alcohol with methanol

The acid–base pairs of the support synergistic with Au–Cu alloy NPs could drive the oxidative esterification of benzyl alcohol with methanol.

Cl2 Production by Photocatalytic Oxidation of HCl over TiO2

We studied the photocatalytic aerobic oxidation of HCl over TiO₂ for producing Cl₂ . Steady-state Cl₂ production rates were determined with a photocatalytic fixed-bed gas-phase reactor equipped with UV light-emitting diodes (LEDs) using iodometric titration as online analytics. We found stable Cl₂ production rates of up to 16 mmol h^-1  m^-2 for commercial anatase TiO₂ Hombikat UV100. The rate increased linearly with temperature from 21 to 140 °C, indicating the acceleration of the limiting desorption rate of the coupled product water. Comparing different TiO₂ polymorphs revealed that anatase possesses higher activity than rutile. The adsorption of HCl was monitored in situ by IR spectroscopy. The IR spectra indicated that HCl chemisorption chlorinates the surface of TiO₂ under the reaction conditions, suggesting it to be the first step of the reaction mechanism. High stability opens up the opportunity of developing a promising photocatalytic process of HCl recycling at lower temperatures suitable for reaching full conversion.

Reversible and cooperative photoactivation of single-atom Cu/TiO2 photocatalysts

The reversible and cooperative activation process, which includes electron transfer from surrounding redox mediators, the reversible valence change of cofactors and macroscopic functional/structural change, is one of the most important characteristics of biological enzymes, and has frequently been used in the design of homogeneous catalysts. However, there are virtually no reports on industrially important heterogeneous catalysts with these enzyme-like characteristics. Here, we report on the design and synthesis of highly active TiO₂ photocatalysts incorporating site-specific single copper atoms (Cu/TiO₂) that exhibit a reversible and cooperative photoactivation process. Our atomic-level design and synthetic strategy provide a platform that facilitates valence control of co-catalyst copper atoms, reversible modulation of the macroscopic optoelectronic properties of TiO₂ and enhancement of photocatalytic hydrogen generation activity, extending the boundaries of conventional heterogeneous catalysts.

Wet-Chemistry Strong Metal-Support Interactions in Titania-Supported Au Catalysts

Classical strong metal-support interactions (SMSI), which play a crucial role in the preparation of supported metal nanoparticle catalysts, is one of the most important concepts in heterogeneous catalysis. The conventional wisdom for construction of classical SMSI involves in redox treatments at high-temperatures by molecular oxygen or hydrogen, sometimes causing sintered metal nanoparticles before SMSI formation. Herein, we report that the aforementioned issue can be effectively avoided by a wet-chemistry methodology. As a typical example, we demonstrate a new concept of wet-chemistry SMSI (wcSMSI) that can be constructed on titania-supported Au nanoparticles (Au/TiO₂-wcSMSI), where the key is to employ a redox interaction between Au^δ+ and Ti³⁺ precursors in aqueous solution. The wcSMSI is evidenced by covering Au nanoparticles with the TiO _x overlayer, electronic interaction between Au and TiO₂, and suppression of CO adsorption on Au nanoparticles. Owing to the wcSMSI, the Au-TiO _x interface with an improved redox property is favorable for oxygen activation, accelerating CO oxidation. In addition, the oxide overlayer efficiently stabilizes the Au nanoparticles, achieving sinter-resistant Au/TiO₂-wcSMSI catalyst in CO oxidation.

Homojunction and defect synergy-mediated electron–hole separation for solar-driven mesoporous rutile/anatase TiO2 microsphere photocatalysts

The photocatalytic hydrogen evolution of TiO₂ is deemed to be one of the most promising ways of converting solar energy to chemical energy; however, it is a challenge to improve the photo-generated charge separation efficiency and enhance solar utilization. Herein, black mesoporous rutile/anatase TiO₂ microspheres with a homojunction and surface defects have been successfully synthesized by an evaporation-induced self-assembly, solvothermal and high-temperature surface hydrogenation method. The H500-BMR/ATM (HX-BMR/ATM, where X means the different hydrogen calcination temperatures) materials not only possess a mesoporous structure and relatively high specific surface area of 39.2 m² g⁻¹, but also have a narrow bandgap (∼2.87 eV), which could extend the photoresponse to the visible light region. They exhibit high photocatalytic hydrogen production (6.4 mmol h⁻¹ g⁻¹), which is much higher (approximately 1.8 times) than that of pristine mesoporous rutile/anatase TiO₂ microspheres (3.58 mmol h⁻¹ g⁻¹). This enhanced photocatalytic hydrogen production property is attributed to the synergistic effect of the homojunction and surface defects in improving efficient electron–hole separation and high utilization of solar light. This work proposes a new approach to improve the performance of photocatalytic hydrogen production and probably offers a new insight into fabricating other high-performance photocatalysts.

Mesoporous rutile/anatase TiO₂ microspheres with surface defects are fabricated and exhibit excellent solar-driven photocatalytic performance due to synergistic effect of the homojunction and surface defects favoring efficient e–h separation.