Elevating Photooxidation of Methane to Formaldehyde via TiO2 Crystal Phase Engineering

Surface fluorination of BiVO4 for the photoelectrochemical oxidation of glycerol to formic acid

The C-C bond cleavage of biomass-derived glycerol to generate value-added C1 products remains challenging owing to its slow kinetics. We propose a surface fluorination strategy to construct dynamic dual hydrogen bonds on a semiconducting BiVO4 photoelectrode to overcome the kinetic limit of the oxidation of glycerol to produce formic acid (FA) in acidic media. Intensive spectroscopic characterizations confirm that double hydrogen bonds are formed by the interaction of the F-Bi-F sites of modified BiVO4 with water molecules, and the unique structure promotes the generation of hydroxyl radicals under light irradiation, which accelerates the kinetics of C-C bond cleavage. Theoretical investigations and infrared adsorption spectroscopy reveal that the double hydrogen bond enhances the C=O adsorption of the key intermediate product 1,3-dihydroxyacetone on the Bi-O sites to initiate the FA pathway. We fabricated a self-powered tandem device with an FA selectivity of 79% at the anode and a solar-to-H2 conversion efficiency of 5.8% at the cathode, and these results are superior to most reported results in acidic electrolytes.

Peptide-Guided Metal-Organic Frameworks Spatial Assembly Sustain Long-Lived Charge-Separated State to Improve Photocatalytic Performance

The controlled fabrication of spatial architectures using metal-organic framework (MOF)-based particles offers opportunities for enhancing photocatalytic performances. The understanding of the contribution of assembly to a precise photocatalytic mechanism, particularly from the perspective of charge separation and extraction dynamics, still poses challenges. The present report presents a facile approach for the spatial assembly of zinc imidazolate MOF (ZIF-8), guided by β-turn peptides (SAZH). We investigated the dynamics of photoinduced carriers using transient absorption spectroscopy. The presence of a long-lived internal charge-separated state in SAZH confirms its role as an intersystem crossing state. The formation of an assembly interface facilitates efficient electron transfer from SAZH to O2, resulting in approximately 2.6 and 2 times higher concentrations of superoxide (·O2-) and hydrogen peroxide (H2O2), respectively, compared to those achieved with ZIF-8. The medical dressing fabricated from SAZH demonstrated exceptional biocompatibility and exhibited an outstanding performance in promoting wound restoration. It rapidly achieved hemostasis during the bleeding phase, followed by a nearly 100% photocatalytic killing efficiency against the infected site during the subsequent inflammatory phase. Our findings reveal a pivotal dynamic mechanism underlying the photocatalytic activity of control-assembled ZIF-8, providing valuable guidelines for the design of highly efficient MOF photocatalysts.

Ultrasonic-Induced Surface Disordering Promotes Photocatalytic Hydrogen Evolution of TiO2

Surface disordering has been considered an effective strategy for tailoring the charge separation and surface chemistry of semiconductor photocatalysts. A simple but reliable method to create surface disordering is, therefore, urgently needed for the development of high-performance semiconductor photocatalysts and their practical applications. Herein, we report that the ultrasonic processing, which is commonly employed in the dispersion of photocatalysts, can induce the surface disordering of TiO2 and significantly promote its performance for photocatalytic hydrogen evolution. A 40 min ultrasonic treatment of TiO2 (Degussa P25) enhances the photocatalytic hydrogen production by 42.7 times, achieving a hydrogen evolution rate of 1425.4 μmol g-1 h-1 without any cocatalyst. Comprehensive structural, spectral, and electrochemical analyses reveal that the ultrasonic treatment induces the surface disordering of TiO2, and consequently reduces the density of deep electron traps, extends the separation of photogenerated charges, and facilitates the hydrogen evolution reaction relative to oxygen reduction. The ultrasonic treatment manifests a more pronounced effect on disordering the surface of anatase than rutile, agreeing well with the enhanced photocatalysis of anatase rather than rutile. This study demonstrates that ultrasonic-induced surface disordering could be an effective strategy for the activation of photocatalysts and might hold significant implications for the applications in photocatalytic hydrogen evolution, small molecule activation, and biomass conversion.

Photosensitized Reduction and Polymerization for One-Step Preparation of Leucomethylene Blue Hydrogel as a Visual Indicator for Both Gaseous and Dissolved Oxygen

Oxygen is crucial for many chemical and biological processes, and its facile detection is of great significance in our daily lives. In this study, we report a leucomethylene blue (LMB)-encapsulated hydrogel visual indicator for the detection of both gaseous and dissolved Oxygen (DO). The photosensitization of methylene blue (MB) not only lead to its reduction to colorless LMB but also resulted in hydrogel polymerization, thus allowing the one-step preparation of the LMB-hydrogel. Meanwhile, the photosensitized reduction of MB was quite fast (5000-fold faster than the classical glucose reduction). In this manner, the blue color of MB could be completely decayed within only 1 min. Also, the efficient polymerization triggered by MB photosensitization ensured the rapid preparation of LMB hydrogels within 10 min. By placing the oxygen indicator in air or water, oxygen can specifically oxidize the colorless LMB-hydrogel to the blue MB-hydrogel. When coupled with a smartphone, the oxygen indicator exhibited a linear response to DO in the range 0.23-10 mg/L with a detection limit of 0.077 mg/L. The LMB-hydrogel indicator was successfully explored for visual evaluation of vacuum degree during food packaging. The LMB-hydrogel, with the advantages of low cost, ease of preparation, as well as facile use, is a promising visual indicator for both gaseous and dissolved oxygen, especially for in-house usage.

Selective oxidation of methane to C2+ products over Au-CeO2 by photon-phonon co-driven catalysis

Direct methane conversion to high-value chemicals under mild conditions is attractive yet challenging due to the inertness of methane and the high reactivity of valuable products. This work presents an efficient and selective strategy to achieve direct methane conversion through the oxidative coupling of methane over a visible-responsive Au-loaded CeO2 by photon-phonon co-driven catalysis. A record-high ethane yield of 755 μmol h-1 (15,100 μmol g-1 h-1) and selectivity of 93% are achieved under optimised reaction conditions, corresponding to an apparent quantum efficiency of 12% at 365 nm. Moreover, the high activity of the photocatalyst can be maintained for at least 120 h without noticeable decay. The pre-treatment of the catalyst at relatively high temperatures introduces oxygen vacancies, which improves oxygen adsorption and activation. Furthermore, Au, serving as a hole acceptor, facilitates charge separation, inhibits overoxidation and promotes the C-C coupling reaction. All these enhance photon efficiency and product yield.

Quasi-homogeneous photoelectrochemical organic transformations for tunable products and 100% conversion ratio

Photoelectrochemical (PEC) organic transformation at the anode coupled with cathodic H2 generation is a potentially rewarding strategy for efficient solar energy utilization. Nevertheless, achieving the full conversion of organic substrates with exceptional product selectivity remains a formidable hurdle in the context of heterogeneous catalysis at the solid/liquid interface. Here, we put forward a quasi-homogeneous catalysis concept by using the reactive oxygen species (ROS), such as ·OH, H2O2 and SO4•-, as a charge transfer mediator instead of direct heterogeneous catalysis at the solid/liquid interface. In the context of glycerol oxidation, all ROS exhibited a preference for first-order reaction kinetics. These ROS, however, showcased distinct oxidation mechanisms, offering a range of advantages such as ∼ 100 % conversion ratios and the flexibility to tune the resulting products. Glycerol oxidative formic acid with Faradaic efficiency (FE) of 81.2 % was realized by the H2O2 and ·OH, while SO4•- was preferably for glycerol conversion to C3 products like glyceraldehyde and dihydroxyacetone with a total FE of about 80 %. Strikingly, the oxidative coupling of methane to ethanol was successfully achieved in our quasi-homogeneous system, yielding a remarkable production rate of 12.27 μmol h-1 and an impressive selectivity of 92.7 %. This study is anticipated to pave the way for novel approaches in steering solar-driven organic conversions by manipulating ROS to attain desired products and conversion ratios.

High-entropy-perovskite subnanowires for photoelectrocatalytic coupling of methane to acetic acid

The incorporation of multiple immiscible metals in high-entropy oxides can create the unconventional coordination environment of catalytic active sites, while the high formation temperature of high-entropy oxides results in bulk materials with low specific surface areas. Here we develop the high-entropy LaMnO3-type perovskite-polyoxometalate subnanowire heterostructures with periodically aligned high-entropy LaMnO3 oxides and polyoxometalate under a significantly reduced temperature of 100 oC, which is much lower than the temperature required by state-of-the-art calcination methods for synthesizing high-entropy oxides. The high-entropy LaMnO3-polyoxometalate subnanowires exhibit excellent catalytic activity for the photoelectrochemical coupling of methane into acetic acid under mild conditions (1 bar, 25 oC), with a high productivity (up to 4.45 mmol g‒1cat h‒1) and selectivity ( > 99%). Due to the electron delocalization at the subnanometer scale, the contiguous active sites of high-entropy LaMnO3 and polyoxometalate in the heterostructure can efficiently activate C - H bonds and stabilize the resulted *COOH intermediates, which benefits the in situ coupling of *CH3 and *COOH into acetic acid.

Wavelength-Dependent Activity of Oxygen Species in Propane Conversion on Rutile TiO2(110)

Photocatalytic oxidative dehydrogenation of propane (C3H8) into propene (C3H6) under mild conditions holds great potential in the chemical industry, but understanding how active species participate in C3H8 conversion remains a significant challenge. Here, the wavelength-dependent activities of bridging oxygen (Ob2-) and the Ti5c-bound oxygen adatom (OTi2-) of model rutile (R) TiO2(110) in C3H8 conversion have been investigated. Under 257 and 343 nm irradiation, hole-trapped OTi- and Ob- can abstract the hydrogen atom of C3H8, forming the CH3CH•CH3 radical and C3H6. However, the rate of C3H8 conversion with hole-trapped Ob- is strongly dependent on the wavelength, primarily producing the C3H7• radical. In the case of hole-trapped OTi-, C3H6 is the main product, which is nearly independent of wavelength. The differences in the wavelength-dependent activity and product selectivity are likely due to dynamic control rather than thermodynamic control. The result provides a deeper understanding of the dynamic processes involved in the conversion of light alkanes in TiO2 photocatalysis.

Post-synthetic Metalation on the Ionic TiO2 Surface to Enhance Metal-CO2 Interaction During Photochemical CO2 Reduction

During the photochemical CO2 reduction reaction, CO2 adsorption on the catalyst's surface is a crucial step where the binding mode of the [metal-CO2] adduct directs the product selectivity and efficiency. Herein, an ionic TiO2 nanostructure stabilized by polyoxometalates (POM), ([POM]x@TiO2), is prepared and the sodium counter ions present on the surface to balance the POMs' charge are replaced with copper(II) ions, (Cux[POM]@TiO2). The microscopic and spectroscopic studies affirm the copper exchange without altering the TiO2 core and weak coordination of copper (II) ions to the POMs' surface. Band structure analysis suggests the photo-harvesting efficiency of the TiO2 core with the conduction band edge higher than the reduction potential of CuII/I and multi-electron CO2 reduction potentials. Photochemical CO2 reduction with Cux[POM]@TiO2 results in 30 μmol gcat. -1 CO (79 %) and 8 μmol gcat -1 of CH4 (21 %). Quasi-in-situ Raman study provides evidence in support of CO2 adsorption on the Cux[POM]@TiO2 surface. 13C and D2O labeling studies affirm the {Cu-[CO2]-} adduct formation. Despite the photo-harvesting ability of Nax[POM]@TiO2 itself, the poor CO2 adsorption ability of sodium ions highlights the crucial role of copper ion CO2 photo-reduction. Characterization of the {M-[η2-CO2]-} species via surface tuning validates the CO2 activation and photochemical reduction pathway proposed earlier.

Steering Photooxidation of Methane to Formic Acid over A Priori Screened Supported Catalysts

Efficient methane photooxidation to formic acid (HCOOH) has emerged as a sustainable approach to simultaneously generate value-added chemicals and harness renewable energy. However, the persistent challenge lies in achieving a high yield and selectivity for HCOOH formation, primarily due to the complexities associated with modulating intermediate conversion and desorption after methane activation. In this study, we employ first-principles calculations as a comprehensive guiding tool and discover that by precisely controlling the O2 activation process on noble metal cocatalysts and the adsorption strength of carbon-containing intermediates on metal oxide supports, one can finely tune the selectivity of methane photooxidation products. Specifically, a bifunctional catalyst comprising Pd nanoparticles and monoclinic WO3 (Pd/WO3) would possess optimal O2 activation kinetics and an intermediate oxidation/desorption barrier, thereby promoting HCOOH formation. As evidenced by experiments, the Pd/WO3 catalyst achieves an exceptional HCOOH yield of 4.67 mmol gcat-1 h-1 with a high selectivity of 62% under full-spectrum light irradiation at room temperature using molecular O2. Notably, these results significantly outperform the state-of-the-art photocatalytic systems operated under identical condition.

Best Practices for Experiments and Reports in Photocatalytic Methane Conversion

Efficiently converting methane into valuable chemicals via photocatalysis under mild condition represents a sustainable route to energy storage and value-added manufacture. Despite continued interest in this area, the achievements have been overshadowed by the absence of standardized protocols for conducting photocatalytic methane oxidation experiments as well as evaluating the corresponding performance. In this review, we present a structured solution aimed at addressing these challenges. Firstly, we introduce the norms underlying reactor design and outline various configurations in the gas-solid and gas-solid-liquid reaction systems. This discussion helps choosing the suitable reactors for methane conversion experiments. Subsequently, we offer a comprehensive step-by-step protocol applicable to diverse methane-conversion reactions. Emphasizing meticulous verification and accurate quantification of the products, this protocol highlights the significance of mitigating contamination sources and selecting appropriate detection methods. Lastly, we propose the standardized performance metrics crucial for evaluating photocatalytic methane conversion. By defining these metrics, the community could obtain the consensus of assessing the performance across different studies. Moving forward, the future of photocatalytic methane conversion necessitates further refinement of stringent experimental standards and evaluation criteria. Moreover, development of scalable reactor is essential to facilitate the transition from laboratory proof-of-concept to potentially industrial production.

Catalytic and biocatalytic degradation of microplastics

In recent years, there has been a surge in annual plastic production, which has contributed to growing environmental challenges, particularly in the form of microplastics. Effective management of plastic and microplastic waste has become a critical concern, necessitating innovative strategies to address its impact on ecosystems and human health. In this context, catalytic degradation of microplastics emerges as a pivotal approach that holds significant promise for mitigating the persistent effects of plastic pollution. In this article, we critically explored the current state of catalytic degradation of microplastics and discussed the definition of degradation, characterization methods for degradation products, and the criteria for standard sample preparation. Moreover, the significance and effectiveness of various catalytic entities, including enzymes, transition metal ions (for the Fenton reaction), nanozymes, and microorganisms are summarized. Finally, a few key issues and future perspectives regarding the catalytic degradation of microplastics are proposed.

Schottky Junction Enhanced Photosynthesis of Hydrogen Peroxide by Ultrathin Porous Carbon Nitride Supported Ni Nanoparticles

Artificial photosynthesis for high-value hydrogen peroxide (H2O2) through a two-electron reduction reaction is a green and sustainable strategy. However, the development of highly active H2O2 photocatalysts is impeded by severe carrier recombination, ineffective active sites, and low surface reaction efficiency. We developed a dual optimization strategy to load dense Ni nanoparticles onto ultrathin porous graphitic carbon nitride (Ni-UPGCN). In the absence and presence of sacrificial agents, Ni-UPGCN achieved H2O2 production rates of 169 and 4116 μmol g-1 h-1 with AQY (apparent quantum efficiency) at 420 nm of 3.14% and 17.71%. Forming a Schottky junction, the surface-modified Ni nanoparticles broaden the light absorption boundary and facilitate charge separation, which act as active sites, promoting O2 adsorption and reducing the formation energy of *OOH (reaction intermediate). This results in a substantial improvement in both H2O2 generation activity and selectivity. The Schottky junction of dual modulation strategy provides novel insights into the advancement of highly effective photocatalytic agents for the photosynthesis of H2O2.

Direct Photocatalytic Methane Oxidation to Formaldehyde by N Doping Co-Decorated Mixed Crystal TiO2

In this paper, N-doped TiO2 mixed crystals are prepared via direct calcination of TiN for highly selective oxidation of CH4 to HCHO at room temperature. The structures of the prepared TiO2 samples are characterized to be N-doped TiO2 of anatase and rutile mixed crystals. The crystal structures of TiO2 samples are determined by XRD spectra and Raman spectra, while N doping is demonstrated by TEM mapping, ONH inorganic element analysis, and high-resolution XPS results. Significantly, the production rate of HCHO is as high as 23.5 mmol·g-1·h-1 with a selectivity over 90%. Mechanism studies reveal that H2O is the main oxygen source and acts through the formation of ·OH. DFT calculations indicate that the construction of a mixed crystal structure and N-doping modification mainly act by increasing the adsorption capacity of H2O. An efficient photocatalyst was prepared by us to convert CH4 to HCHO with high yield and selectivity, greatly promoting the development of the photocatalytic CH4 conversion study.

Spatial interfacial heterojunctions of TiO2 for photocatalytic degradation of toluene: Effects of interface amorphous region and oxygen vacancy

The catalytic activity of TiO2 is contingent upon its crystal structure and the optoelectronic properties associated with defects. In this study, a one-step method was used to synthesize TiO2 with a spatial interface of rutile/anatase phases, and a simple thermal annealing process was applied to optimize the amorphous regions and oxygen vacancies at the interface between the rutile and anatase phases of TiO2. High-resolution transmission electron microscopy (HRTEM) elucidates the evolution process of the amorphous domain at the interface, skillfully introducing oxygen vacancies at the heterojunction interface by modulating the amorphous domain. The obtained photocatalyst (TiO2-350 °C) after annealing exhibits an optimal interface structure, with its photocatalytic activity and stability in degrading toluene far superior to P25. Photocurrent and photoluminescence (PL) measurements affirm that the existence of interfacial oxygen vacancies heightens the efficiency of electron transfer at the interface, while surface oxygen vacancies significantly enhance the stability and mineralization rate of toluene degradation. The improved photocatalytic properties were attributed to the combined effects of surface/interface oxygen vacancies and spatial interface heterojunctions. The one-step synthesis method developed in this work provides a novel perspective on combining spatially interfaced anatase/rutile phases with surface/interfacial oxygen vacancies.

Low-Temperature Oxidation of Methane on Rutile TiO2(110): Identifying the Role of Surface Oxygen Species

Understanding the microkinetic mechanism underlying photocatalytic oxidative methane (CH4) conversion is of significant importance for the successful design of efficient catalysts. Herein, CH4 photooxidation has been systematically investigated on oxidized rutile(R)-TiO2(110) at 60 K. Under 355 nm irradiation, the C-H bond activation of CH4 is accomplished by the hole-trapped dangling OTi- center rather than the hole-trapped Ob- center via the Eley-Rideal reaction pathway, producing movable CH3• radicals. Subsequently, movable CH3• radicals encounter an O/OH species to form CH3O/CH3OH species, which could further dissociate into CH2O under irradiation. However, the majority of the CH3• radical intermediate is ejected into a vacuum, which may induce radical-mediated reactions under ambient conditions. The result not only advances our knowledge about inert C-H bond activation but also provides a deep insight into the mechanism of photocatalytic CH4 conversion, which will be helpful for the successful design of efficient catalysts.

Humic substances mediated superior photochemical pollutant conversion on defective TiO2 in environmentally relevant matrices: The key roles of oxygen vacancy in surface interactions, oxidant activation and radical generation

Ubiquitous humic substances usually exhibit strong interfering effects on target pollutant removal in advanced water purification. This work aims to develop a photochemical conversion system on the nonstoichiometric TiO2 for pollutant removal in environmentally relevant matrices. In this synergistic reaction system, the redox-reactive humic substances and defective oxygen vacancies can serve as the organic electron transfer mediator and the key surface reactive sites, respectively. This system achieves a superior pollutant degradation in real surface water at low oxidant concentrations. Reactive oxygen vacancies on the TiO2 surface and sub-surface are of considerable interest for this photochemical reaction system. By engineering defective oxygen vacancies on high-energy {001} polar facet, the surface and electronic interactions between tailored TiO2 and humic substances are greatly strengthened for the promoted electron transfer and oxidant activation. Rendered by the strong surface affinity and molecular activation, defective oxygen vacancies thermodynamically and dynamically promote reactive chain reactions for free radical formation, including the selective O2 reduction to ·O2- and the H2O2 activation to ·OH. Our findings take new insights into environmental geochemistry, and provide an effective strategy to in-situ boost the humic substances-mediated water purification without secondary pollution. ENVIRONMENTAL IMPLICATION: Humic substances are widely distributed in aquatic environment, thus playing important roles in environmental geochemistry. For example, humic substances can achieve good surface adsorption through electrostatic adsorption, ligand exchange and electronic interactions with typical TiO2 to form reactive ligand-metal charge transfer complexes for pollutant degradation. Inspired by the unique properties of surface and sub-surface oxygen vacancies, the defective TiO2 was designed to refine the humic substances-mediated photochemical reactions. A superior reactivity was measured for pollutant degradation. Our findings provide an effective strategy to boost naturally photochemical decontamination in environmentally relevant matrices.

Catalyzing Artificial Photosynthesis with TiO2 Heterostructures and Hybrids: Emerging Trends in a Classical yet Contemporary Photocatalyst

Titanium dioxide (TiO2) stands out as a versatile transition-metal oxide with applications ranging from energy conversion/storage and environmental remediation to sensors and optoelectronics. While extensively researched for these emerging applications, TiO2 has also achieved commercial success in various fields including paints, inks, pharmaceuticals, food additives, and advanced medicine. Thanks to the tunability of their structural, morphological, optical, and electronic characteristics, TiO2 nanomaterials are among the most researched engineering materials. Besides these inherent advantages, the low cost, low toxicity, and biocompatibility of TiO2 nanomaterials position them as a sustainable choice of functional materials for energy conversion. Although TiO2 is a classical photocatalyst well-known for its structural stability and high surface activity, TiO2-based photocatalysis is still an active area of research particularly in the context of catalyzing artificial photosynthesis. This review provides a comprehensive overview of the latest developments and emerging trends in TiO2 heterostructures and hybrids for artificial photosynthesis. It begins by discussing the common synthesis methods for TiO2 nanomaterials, including hydrothermal synthesis and sol-gel synthesis. It then delves into TiO2 nanomaterials and their photocatalytic mechanisms, highlighting the key advancements that have been made in recent years. The strategies to enhance the photocatalytic efficiency of TiO2, including surface modification, doping modulation, heterojunction construction, and synergy of composite materials, with a specific emphasis on their applications in artificial photosynthesis, are discussed. TiO2-based heterostructures and hybrids present exciting opportunities for catalyzing solar fuel production, organic degradation, and CO2 reduction via artificial photosynthesis. This review offers an overview of the latest trends and advancements, while also highlighting the ongoing challenges and prospects for future developments in this classical yet rapidly evolving field.

Heterogeneous Photocatalytic Strategies for C(sp3 )-H Activation

Activation of ubiquitous C(sp3 )-H bonds is extremely attractive but remains a great challenge. Heterogeneous photocatalysis offers a promising and sustainable approach for C(sp3 )-H activation and has been fast developing in the past decade. This Minireview focuses on mechanism and strategies for heterogeneous photocatalytic C(sp3 )-H activation. After introducing mechanistic insights, heterogeneous photocatalytic strategies for C(sp3 )-H activation including precise design of active sites, regulation of reactive radical species, improving charge separation and reactor innovations are discussed. In addition, recent advances in C(sp3 )-H activation of hydrocarbons, alcohols, ethers, amines and amides by heterogeneous photocatalysis are summarized. Lastly, challenges and opportunities are outlined to encourage more efforts for the development of this exciting and promising field.

P-N Heterojunction Embedded CuS/TiO2 Bifunctional Photocatalyst for Synchronous Hydrogen Production and Benzylamine Conversion

The coupling of photocatalytic hydrogen production and selective oxidation of benzylamine is a topic of significant research interest. However, enhancing the bifunctional photocatalytic activity in this context is still a major challenge. The construction of Z-scheme heterojunctions is an effective strategy to enhance the activity of bifunctional photocatalysts. Herein, a p-n type direct Z-scheme heterojunction CuS/TiO2 is constructed using metal-organic framework (MOF)-derived TiO2 as a substrate. The carrier density is measured by Mott-Schottky under photoexcitation, which confirms that the Z-scheme electron transfer mode of CuS/TiO2 is driven by the diffusion effect caused by the carrier concentration difference. Benefiting from efficient charge separation and transfer, photogenerated electrons, and holes are directedly transferred to active oxidation and reduction sites. CuS/TiO2 also exhibits excellent bifunctional photocatalytic activity without noble metal cocatalysts. Among them, the H2 evolution activity of the CuS/TiO2 is found to be 17.1 and 29.5 times higher than that of TiO2 and CuS, respectively. Additionally, the yields of N-Benzylidenebenzylamine (NBB) are 14.3 and 47.4 times higher than those of TiO2 and CuS, respectively.

Photodriven Methane Conversion on Transition Metal Oxide Catalyst: Recent Progress and Prospects

Methane as the main component in natural gas is a promising chemical raw material for synthesizing value-added chemicals, but its harsh chemical conversion process often causes severe energy and environment concerns. Photocatalysis provides an attractive path to active and convert methane into various products under mild conditions with clean and sustainable solar energy, although many challenges remain at present. In this review, recent advances in photocatalytic methane conversion are systematically summarized. As the basis of methane conversion, the activation of methane is first elucidated from the structural basis and activation path of methane molecules. The study is committed to categorizing and elucidating the research progress and the laws of the intricate methane conversion reactions according to the target products, including photocatalytic methane partial oxidation, reforming, coupling, combustion, and functionalization. Advanced photocatalytic reactor designs are also designed to enrich the options and reliability of photocatalytic methane conversion performance evaluation. The challenges and prospects of photocatalytic methane conversion are also discussed, which in turn offers guidelines for methane-conversion-related photocatalyst exploration, reaction mechanism investigation, and advanced photoreactor design.

Nitrogen-Doped Titanium Dioxide for Selective Photocatalytic Oxidation of Methane to Oxygenates

Photocatalytic conversion of methane (CH4) to value-added chemicals using H2O as the oxidant under mild conditions is a desired sustainable pathway for synthesizing commodity chemicals. However, controlling product selectivity while maintaining high product yields is greatly challenging. Herein, we develop a highly efficient strategy, based on the precise control of the types of nitrogen dopants, and the design of photocatalysts, to achieve high selectivity and productivity of oxygenates via CH4 photocatalytic conversion. The primary product (methanol) is obtained in a high yield of 159.8 μmol·g-1·h-1 and 47.7% selectivity, and the selectivity of oxygenate compounds reached 92.5%. The unique hollow porous structure and substituted nitrogen sites of nitrogen-doped TiO2 synergistically promote its photo-oxidation performance. Furthermore, in situ attenuated total reflectance Fourier transform infrared spectroscopy provides direct evidence of the key intermediates and their evolution for producing methanol and multicarbon oxygenates. This study provides insights into the mechanism of photocatalytic CH4 conversion.

Crystal-Phase Engineering in Heterogeneous Catalysis

The performance of a chemical reaction is critically dependent on the electronic and/or geometric structures of a material in heterogeneous catalysis. Over the past century, the Sabatier principle has already provided a conceptual framework for optimal catalyst design by adjusting the electronic structure of the catalytic material via a change in composition. Beyond composition, it is essential to recognize that the geometric atomic structures of a catalyst, encompassing terraces, edges, steps, kinks, and corners, have a substantial impact on the activity and selectivity of a chemical reaction. Crystal-phase engineering has the capacity to bring about substantial alterations in the electronic and geometric configurations of a catalyst, enabling control over coordination numbers, morphological features, and the arrangement of surface atoms. Modulating the crystallographic phase is therefore an important strategy for improving the stability, activity, and selectivity of catalytic materials. Nonetheless, a complete understanding of how the performance depends on the crystal phase of a catalyst remains elusive, primarily due to the absence of a molecular-level view of active sites across various crystal phases. In this review, we primarily focus on assessing the dependence of catalytic performance on crystal phases to elucidate the challenges and complexities inherent in heterogeneous catalysis, ultimately aiming for improved catalyst design.

Modification engineering of TiO2-based nanoheterojunction photocatalysts

Titanium dioxide (TiO2)-based photocatalysts have gained increasing attention for their versatile applications in organic degradation, hydrogen production, air purification, and CO2 reduction. Various TiO2-based heterojunction structures, including type I, type II, Schottky junction, Z-scheme, and S-scheme, have been extensively studied. The current research frontier is centered on the engineering modifications of TiO2-based nanoheterojunction photocatalysts, such as defect engineering, morphological engineering, crystal phase/facet engineering, and multijunction engineering. These modifications enhance carrier transport, separation, and light absorption, thereby improving the photocatalytic performance. Remarkably, this aspect has been less addressed in existing reviews. This review aims to fill this gap by focusing on the engineering modifications of TiO2-based nanoheterojunction photocatalysts. We delve into specific topics like oxygen vacancies, n-p homojunctions, and double defects. The review also systematically discusses the applications of multidimensional heterojunctions and examines carrier transport pathways in heterophase/facet junctions and their interactions with heterojunctions. A comprehensive summary of multijunction systems, including multi-Schottky junctions, semiconductor-based heterojunction-attached Schottky junctions, and multisemiconductor-based heterojunctions, is presented. Lastly, we outline future perspectives in this promising research field. This paper will assist researchers in constructing more efficient TiO2-based nanoheterojunction photocatalysts.

Al2 O3 -coated BiVO4 Photoanodes for Photoelectrocatalytic Regioselective C-H Activation of Aromatic Amines

Photoelectrochemistry is becoming an innovative approach to organic synthesis. Generally, the current photoelectrocatalytic organic transformations suffer from limited reaction type, low conversion efficiency and poor stability. Herein, we develop efficient and stable photoelectrode materials using metal oxide protective layer, with a focus on achieving regioselective activation of amine compounds. Notably, our photoelectrochemistry process is implemented under mild reaction conditions and does not involve any directing groups, transition metals or oxidants. The results demonstrate that beyond photocatalysis and electrocatalysis, photoelectrocatalysis exhibits high efficiency, remarkable repeatability and good functional group tolerance, highlighting its great potential for applications.

Photocatalytic ethane conversion on rutile TiO2(110): identifying the role of the ethyl radical

Oxidative dehydrogenation of ethane (C2H6, ODHE) is a promising approach to producing ethene (C2H4) in the chemical industry. However, the ODHE needs to be operated at a high temperature, and realizing the ODHE under mild conditions is still a big challenge. Herein, using photocatalytic ODHE to obtain C2H4 has been achieved successfully on a model rutile(R)-TiO2(110) surface with high selectivity. Initially, the C2H6 reacts with hole trapped OTi- centers to produce ethyl radicals , which can be precisely detected by a sensitive TOF method, and then the majority of the radicals spontaneously dehydrogenate into C2H4 without another photo-generated hole. In addition, parts of the radicals rebound with diversified surface sites to produce C2 products via migration along the surface. The mechanistic model built in this work not only advances our knowledge of the C-H bond activation and low temperature C2H6 conversion, but also provides new opportunities for realizing the ODHE with high C2H4 efficiency under mild conditions.

Visible light photoreforming of greenhouse gases by nano Cu-Al LDH intercalated with urea-derived anions

The accumulation of anthropogenic greenhouse gases (GHGs) in the atmosphere causes global warming. Global efforts are carried out to prevent temperature overshooting and limit the increase in the Earth's surface temperature to 1.5 °C. Carbon dioxide and methane are the largest contributors to global warming. We have synthesized copper-aluminium layered double hydroxide (Cu-Al LDH) catalysts by urea hydrolysis under microwave (MW) irradiation. The effect of MW power, urea concentration, and MII/MIII ratios was studied. The physicochemical properties of the prepared LDH catalysts were characterized by several analysis techniques. The results confirmed the formation of the layered structure with the intercalation of urea-derived anions. The urea-derived anions enhanced the optical and photocatalytic properties of the nano Cu-Al LDH in the visible-light region. The photocatalytic activity of the prepared Cu-Al LDH catalysts was tested for greenhouse gas conversion (CH4, CO2, and H2O) under visible light. The dynamic gas mixture flow can pass through the reactor at room temperature under atmospheric pressure. The results show a high conversion percentage for both CO2 and CH4. The highest converted amounts were 7.48 and 1.02 mmol mL-1 g-1 for CH4 and CO2, respectively, under the reaction conditions. The main product was formaldehyde with high selectivity (>99%). The results also show the stability of the catalysts over several cycles. The current work represents a green chemistry approach for efficient photocatalyst synthesis, visible light utilization, and GHGs' conversion into a valuable product.

Recent Progress on Phase Engineering of Nanomaterials

As a key structural parameter, phase depicts the arrangement of atoms in materials. Normally, a nanomaterial exists in its thermodynamically stable crystal phase. With the development of nanotechnology, nanomaterials with unconventional crystal phases, which rarely exist in their bulk counterparts, or amorphous phase have been prepared using carefully controlled reaction conditions. Together these methods are beginning to enable phase engineering of nanomaterials (PEN), i.e., the synthesis of nanomaterials with unconventional phases and the transformation between different phases, to obtain desired properties and functions. This Review summarizes the research progress in the field of PEN. First, we present representative strategies for the direct synthesis of unconventional phases and modulation of phase transformation in diverse kinds of nanomaterials. We cover the synthesis of nanomaterials ranging from metal nanostructures such as Au, Ag, Cu, Pd, and Ru, and their alloys; metal oxides, borides, and carbides; to transition metal dichalcogenides (TMDs) and 2D layered materials. We review synthesis and growth methods ranging from wet-chemical reduction and seed-mediated epitaxial growth to chemical vapor deposition (CVD), high pressure phase transformation, and electron and ion-beam irradiation. After that, we summarize the significant influence of phase on the various properties of unconventional-phase nanomaterials. We also discuss the potential applications of the developed unconventional-phase nanomaterials in different areas including catalysis, electrochemical energy storage (batteries and supercapacitors), solar cells, optoelectronics, and sensing. Finally, we discuss existing challenges and future research directions in PEN.

Electron-Delocalization-Stabilized Photoelectrocatalytic Coupling of Methane by NiO-Polyoxometalate Sub-1 nm Heterostructures

The oxidative coupling of methane to C2 oxygenates merits great scientific and technological potential yet remains a challenge due to its inferior selectivity. Subnanomaterials (SNMs) with "p-n-p-n"-type heteroconstructions feature enhanced external field coupling properties and tunable electronic structures, serving as promising catalysts for the selective partial oxidation of methane. Here we develop NiO-polyoxometalate (POM) subnanocoils with a thickness of 1.8 nm, showing excellent catalytic activity toward photoelectrochemical coupling of methane into a C2 product under mild conditions (1 bar, 25 °C) with a notable productivity (up to 4.48 mmol gcat-1 h-1) and a high selectivity (>99%). Under photoelectrochemical coupling, C-H bonds can be activated by NiO, and the resulted *COOH intermediates are stabilized by the delocalized electrons in POM clusters. The contiguous active sites of NiO and POM at the molecular level allow the in situ coupling of *COOH into oxalate. This work points out an economic way for the oxidation of methane under mild conditions and may enlighten the design of functional SNMs from fundamental standpoints.

Synergy of Bulk Defects and Surface Defects on TiO2 for Highly Efficient Photocatalytic Production of H2O2

Artificial photosynthesis of H2O2 by TiO2-based semiconductors is a promising approach for H2O2 production. However, the efficiency of pristine TiO2 is still limited by rapid charge separation and low O2 adsorption capacity. Here, we found that the synergy between bulk and surface defects on TiO2 could overcome this demanding bottleneck. The introduced bulk defects act as hole acceptors to induce directional hole transfer, efficiently boosting electron-hole separation. Furthermore, the adsorption of O2 is strengthened by the introduced surface defects. Consequently, this synergy of bulk and surface defects on TiO2 significantly improves the photocatalytic performance, with a H2O2 production rate of 4560 μmol h-1 g-1, outperforming most reported TiO2-based photocatalysts. This work not only provides a new insight into the mechanism of surface/bulk defects in photocatalysis but also highlights that surface/bulk regulation holds great promise for achiveing efficient photocatalytic conversion.

Crystal Engineering of TiO2 for Enhanced Catalytic Oxidation of 1,2-Dichloroethane on a Pt/TiO2 Catalyst

Crystal engineering of metal oxide supports represents an emerging strategy to improve the catalytic performance of noble metal catalysts in catalytic oxidation of chlorinated volatile organic compounds (CVOCs). Herein, Pt catalysts on a TiO₂ support with different crystal phases (rutile, anatase, and mixed phase (P25)) were prepared for catalytic oxidation of 1,2-dichloroethane (DCE). The Pt catalyst on P25-TiO₂ (Pt/TiO₂-P) showed optimal activity, selectivity, and stability, even under high-space velocity and humidity conditions. Due to the strong interaction between Pt and P25-TiO₂ originating from the more lattice defects of TiO₂, the Pt/TiO₂-P catalyst possessed stable Pt⁰ and Pt²⁺ species during DCE oxidation and superior redox property, resulting in high activity and stability. Furthermore, the Pt/TiO₂-P catalyst possessed abundant hydroxyl groups, which prompted the removal of chlorine species in the form of HCl and significantly decreased the selectivity of vinyl chloride (VC) as the main byproduct. On the other hand, the Pt/TiO₂-P catalyst exhibited a different reaction path, in which the hydroxyl groups on its surface activated DCE to form VC and enolic species, besides the lattice oxygen of TiO₂ for the Pt catalysts on rutile and anatase TiO₂. This work provides guidance for the rational design of catalysts for CVOCs.

Atomically Strained Metal Sites for Highly Efficient and Selective Photooxidation

Strain engineering is an attractive strategy for improving the intrinsic catalytic performance of heterogeneous catalysts. Manipulating strain on the short-range atomic scale to the local structure of the catalytic sites is still challenging. Herein, we successfully achieved atomic strain modulation on ultrathin layered vanadium oxide nanoribbons by an ingenious intercalation chemistry method. When trace sodium cations were introduced between the V₂O₅ layers (Na⁺-V₂O₅), the V-O bonds were stretched by the atomically strained vanadium sites, redistributing the local charges. The Na⁺-V₂O₅ demonstrated excellent photooxidation performance, which was approximately 12 and 14 times higher than that of pristine V₂O₅ and VO₂, respectively. Complementary spectroscopy analysis and theoretical calculations confirmed that the atomically strained Na⁺-V₂O₅ had a high surficial charge density, improving the activation of oxygen molecules and contributing to the excellent photocatalytic property. This work provides a new approach for the rational design of strain-equipped catalysts for selective photooxidation reactions.

Transport Mediating Core-Shell Photocatalyst Architecture for Selective Alkane Oxidation

The high activation barrier of the C-H bond in methane, combined with the high propensity of methanol and other liquid oxygenates toward overoxidation to CO₂, have historically posed significant scientific and industrial challenges to the selective and direct conversion of methane to energy-dense fuels and chemical feedstocks. Here, we report a unique core-shell nanostructured photocatalyst, silica encapsulated TiO₂ decorated with AuPd nanoparticles (TiO₂@SiO₂-AuPd), that prevents methanol overoxidation on its surface and possesses high selectivity and yield of oxygenates even at high UV intensity. This room-temperature approach achieves high selectivity for oxygenates (94.5%) with a total oxygenate yield of 15.4 mmol/g_cat·h at 9.65 bar total pressure of CH₄ and O₂. The working principles of this core-shell photocatalyst were also systematically investigated. This design concept was further demonstrated to be generalizable for the selective oxidation of other alkanes.

Edge-oriented phosphatizing engineering of 2D Ni-MOFs with a tailored d-band center for boosting catalytic activity

Metal-organic framework (MOF)-based heterostructures have aroused widespread interest owing to their extensive compositional tunability and interesting catalytic properties. However, the precise edge-oriented growth of transition metal compounds at the edges of 2D MOFs to construct edge mode heterostructures remains a great challenge due to their inherent thermodynamic instability. Here, edge-oriented growth of Ni₂P at the edges of a 2D Ni-MOF was achieved for the first time by precisely tuning the phosphorus source content and phosphating temperature. Owing to the formation of the edge mode Ni-MOF/Ni₂P heterostructure, the as-prepared heterostructure showed upregulated d-band center, more robust 4-nitrophenol (4-NP) adsorption capacity, lowered energy barrier of the rate-determining step (RDS), and higher specific surface area, resulting in the best performance of the hydrogenation reduction of 4-NP to 4-aminophenol (4-AP) in the presence of non-precious metal catalysts.

Enabling Specific Photocatalytic Methane Oxidation by Controlling Free Radical Type

Selective CH₄ oxidation to CH₃OH or HCHO with O₂ in H₂O under mild conditions provides a desired sustainable pathway for synthesis of commodity chemicals. However, manipulating reaction selectivity while maintaining high productivity remains a huge challenge due to the difficulty in the kinetic control of the formation of a desired oxygenate against its overoxidation. Here, we propose a highly efficient strategy, based on the precise control of the type of as-formed radicals by rational design on photocatalysts, to achieve both high selectivity and high productivity of CH₃OH and HCHO in CH₄ photooxidation for the first time. Through tuning the band structure and the size of active sites (i.e., single atoms or nanoparticles) in our Au/In₂O₃ catalyst, we show alternative formation of two important radicals, ^•OOH and ^•OH, which leads to distinctly different reaction paths to the formation of CH₃OH and HCHO, respectively. This approach gives rise to a remarkable HCHO selectivity and yield of 97.62% and 6.09 mmol g^-1 on In₂O₃-supported Au single atoms (Au₁/In₂O₃) and an exceptional CH₃OH selectivity and yield of 89.42% and 5.95 mmol g^-1 on In₂O₃-supported Au nanoparticles (Au_NPs/In₂O₃), respectively, upon photocatalytic CH₄ oxidation for 3 h at room temperature. This work opens a new avenue toward efficient and selective CH₄ oxidation by delicate design of composite photocatalysts.

Atomically Dispersed Nickel Anchored on a Nitrogen-Doped Carbon/TiO2 Composite for Efficient and Selective Photocatalytic CH4 Oxidation to Oxygenates

Direct photocatalytic oxidation of methane to liquid oxygenated products is a sustainable strategy for methane valorization at room temperature. However, in this reaction, noble metals are generally needed to function as cocatalysts for obtaining adequate activity and selectivity. Here, we report atomically dispersed nickel anchored on a nitrogen-doped carbon/TiO₂ composite (Ni-NC/TiO₂ ) as a highly active and selective catalyst for photooxidation of CH₄ to C1 oxygenates with O₂ as the only oxidant. Ni-NC/TiO₂ exhibits a yield of C1 oxygenates of 198 μmol for 4 h with a selectivity of 93 %, exceeding that of most reported high-performance photocatalysts. Experimental and theoretical investigations suggest that the single-atom Ni-NC sites not only enhance the transfer of photogenerated electrons from TiO₂ to isolated Ni atoms but also dominantly facilitate the activation of O₂ to form the key intermediate ⋅OOH radicals, which synergistically lead to a substantial enhancement in both activity and selectivity.