High Nitrile Yields of Aerobic Ammoxidation of Alcohols Achieved by Generating •O2- and Br• Radicals over Iron-Modified TiO2 Photocatalysts

Air-Level Oxygen Enables 100% Selectivity in Urea Synthesis via Photocatalytic C─N Coupling of CO and Ammonia

The clean-energy-driven synthesis of urea from carbon- and nitrogen-containing small molecules has garnered significant interest but remained great challenges to achieve with high selectivity. Herein, we present a photocatalytic pathway for the selective urea synthesis through the oxidative coupling between CO and NH3. The key factor in this process is the atmospheric O2 level, which plays a crucial role in controlling both the urea production rate and its selectivity. Using oxygen-deficient TiO2 under an air-level (20%) O2 atmosphere, we achieved a urea generation rate of 54.31 mg gcat -1 h-1 with 100% selectivity. This rate is 38.52 times higher than under oxygen-free conditions, while further increasing the O2 level significantly reduces selectivity. Mechanistic studies reveal that the process begins with the oxidation of NH3 to •NH2 through oxidative radicals generated on TiO2, especially the oxygen-derived O2 •-. This •NH2 radicals then couple with CO to form urea. The concentration of radicals is controlled by the O2 level, with the optimal concentration under air-level O2 enabling efficient NH3 oxidation to •NH2 while preventing over-oxidation.

Modern organic transformations: heterogeneous thermocatalysis or photocatalysis?

Organic transformation driven by heterogeneous catalysis is of crucial significance in both fundamental research and modern industrial production of fine chemicals. Thermocatalysis offers excellent applications due to its high activity and excellent scalability, yet still faces significant challenges toward the goals of high efficiency, energy-saving and sustainability. Recently, photocatalysis has emerged as a promising alternative for addressing these issues in a green and economical manner. In practice, the selection of an appropriate catalytic system is a critical factor that can influence the chemical process on multiple levels significantly. In this review, we aim to present a tutorial demonstration about the critical comparison between thermo- and photocatalytic terms for organic transformation. We begin by outlining the basic principles in thermo- and photocatalytic fundamentals, together with summarizing the general advantages and disadvantages of each. Subsequently, given the high sustainability and potentiality exhibited by the photocatalytic process, we present its representative applications including oxidation, reduction, coupling, and cleavage series. The general reaction conditions and activities observed in thermocatalysis for similar reactions are also introduced for comparison. The understanding of reaction mechanisms and the resulting regulations toward activity and selectivity are specifically discussed. Finally, future perspectives of heterogeneous photocatalytic terms for practical applications are elucidated.

Photocatalytic Formamide Synthesis via Coupling of Electrophilic and Nucleophilic Radicals over Atomically Dispersed Bi Sites

Formamide (HCONH2) plays a pivotal role in the manufacture of a diverse array of chemicals, fertilizers, and pharmaceuticals. Photocatalysis holds great promise for green fabrication of carbon-nitrogen (C-N) compounds owing to its environmental friendliness and mild redox capability. However, the selective formation of the C-N bond presents a significant challenge in the photocatalytic synthesis of C-N compounds. This work developed a photocatalytic radical coupling method for the formamide synthesis from co-oxidation of ammonia (NH3) and methanol (CH3OH). An exceptional formamide yield rate of 5.47±0.03 mmol ⋅ gcat -1 ⋅ h-1 (911.87±5 mmol ⋅ gBi -1 ⋅ h-1) was achieved over atomically dispersed Bi sites (BiSAs) on TiO2. An accumulation of 45.68 mmol ⋅ gcat -1 (2.0 g ⋅ gcat -1) of formamide was achieved after long-term illumination, representing the highest level of photocatalytic C-N compounds synthesis. The critical C-N coupling for formamide formation originated from the "σ-σ" interaction between electrophilic ⋅CH2OH with nucleophilic ⋅NH2 radical. The BiSAs sites facilitated the electron transfer between reactants and photocatalysts and enhanced the nucleophilic attack of ⋅NH2 radical on the ⋅CH2OH radical, thereby advancing the selective C-N bond formation. This work deepens the understanding of the C-N coupling mechanism and offers an intriguing photocatalytic approach for the efficient and sustainable production of C-N compounds.

Hydrogen-Bond-Mediated Formation of C-N or C=N Bond during Photocatalytic Reductive Coupling Reaction over CdS Nanosheets

Reductive amination of carbonyl compounds and nitro compounds represents a straightforward way to attain imines or secondary amines, but it is difficult to control the product selectivity. Herein, we report the selective formation of C-N or C=N bond readily manipulated through a solvent-induced hydrogen bond bridge, facilitating the swift photocatalytic reductive coupling process. The reductive-coupling of nitro compounds with carbonyl compounds using formic acid and sodium formate as the hydrogen donors over CdS nanosheets selectively generates imines with C=N bonds in acetonitrile solvent; while taking methanol as solvent, the C=N bonds are readily hydrogenated to the C-N bonds via hydrogen-bonding activation. Experimental and theoretical study reveals that the building of the hydrogen-bond bridge between the hydroxyl groups in methanol and the N atoms of the C=N motifs in imines facilitates the transfer of hydrogen atoms from CdS surface to the N atoms in imines upon illumination, resulting in the rapid hydrogenation of the C=N bonds to give rise to the secondary amines with C-N bonds. Our method provides a simple way to control product selectivity by altering the solvents in photocatalytic organic transformations.

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.

Direct Photoreforming of Real-World Polylactic Acid Plastics into Highly Selective Value-Added Pyruvic Acid under Visible Light

Although polylactic acid (PLA) represents a pivotal biodegradable polymer, its biodegradability has inadvertently overshadowed the development of effective recycling techniques, leading to the potential wastage of carbon resources. The photoreforming-recycling approach for PLA exhibits significant potential in terms of concepts and methods. However, the reaction faces enormous challenges due to the limited selectivity of organic oxidation products as well as the increased costs and challenging separation of organic products associated with alkali-solution-assisted prehydrolysis. Herein, we report an alkali-free direct-photoreforming pathway for real-world PLA plastics utilizing the Pd-CdS photocatalyst under visible-light illumination, obviating the need for chemical pretreatment of PLA. The devised pathway successfully produces H2 at a rate of 49.8 μmol gcat.-1 h-1, sustained over 100 h, and exhibits remarkable selectivity toward pyruvic acid (95.9% in liquid products). Additionally, experimental findings elucidate that Pd sites not only function as a typical cocatalyst for enhancing the photocatalytic evolution of H2 but also suppress competitive side reactions (e.g., lactic acid coupling or decarboxylation), consequently augmenting the yield and selectivity of pyruvic acid and H2. This investigation provides a straightforward and sustainable direct-photoreforming route capable of simultaneously mitigating and repurposing plastic waste into valuable chemicals, thus offering a promising solution to the current environmental challenges.

A fatty acid photodecarboxylase-mimicking photonanozyme with defect-induced enzymatic substrate-binding pockets

Hydrocarbon synthesis hints at the significance of in-depth investigations and detailed explanations of mimicking fatty acid photodecarboxylase (FAP). Considering the importance of photodecarboxylases in hydrocarbon synthesis, we present the potential of defective semiconductor nanomaterials as a novel type of photonanozymes (PNZs) that mimic enzyme-like performance, serving as alternatives to FAP. Ferrum-doped titanium dioxide (Fe-TiO2) was synthesized to introduce appropriate amounts of surface defects including reduced Ti3+ sites and oxygen vacancies, which reduce the band gap of TiO2 and enhance the visible-light absorption, thereby facilitating efficient charge trapping. Notably, the surface defects of Fe-TiO2 PNZs singularly act as enzymatic substrate-binding pockets that enable efficient carboxylic acid adsorption during the dark process, conversely facilitating the formation of more defects and boosting the FAP-like activity for photocatalytic decarboxylation reactions. This work provides a creative strategy for designing substrate-dependent higher-concentration defects as enzyme-like binding sites on promising PNZs that mimic natural photoenzymes.

Photosynthesis of Benzonitriles on BiOBr Nanosheets Promoted by Vacancy Associates

Photocatalytic organic functionalization reactions represent a green, cost-effective, and sustainable synthesis route for value-added chemicals. However, heterogeneous photocatalysis is inefficient in directly activating ammonia molecules for the production of high-value-added nitrogenous organic products when compared with oxygen activation in the formation of related oxygenated compounds. In this study, we report the heterogeneous photosynthesis of benzonitriles by the ammoxidation of benzyl alcohols (99 % conversion, 93 % selectivity) promoted using BiOBr nanosheets with surface vacancy associates. In contrast, the main reaction of catalysts with other types of vacancy sites is the oxidation of benzyl alcohol to benzaldehyde or benzoic acid. Experimental measurements and theoretical calculations have demonstrated a specificity of vacancy type with respect to product selectivity, which arises from the adsorption and activation of NH3 and O2 that is required to promote subsequent C-N coupling and oxidation to nitrile. This study provides a better understanding of the role of vacancies as catalytic sites in heterogeneous photocatalysis.

Bi/Mn-Doped BiOCl Nanosheets Self-Assembled Microspheres toward Optimized Photocatalytic Performance

Doping engineering of metallic elements is of significant importance in photocatalysis, especially in the transition element range where metals possess empty 'd' orbitals that readily absorb electrons and increase carrier concentration. The doping of Mn ions produces dipole interactions that change the local structure of BiOCl, thus increasing the specific surface area of BiOCl and the number of mesoporous distributions, and providing a broader platform and richer surface active sites for catalytic reactions. The combination of Mn doping and metal Bi reduces the forbidden bandwidth of BiOCl, thereby increasing the absorption in the light region and strengthening the photocatalytic ability of BiOCl. The degradation of norfloxacin by Bi/Mn-doped BiOCl can reach 86.5% within 10 min. The synergistic effect of Mn doping and Bi metal can change the internal energy level and increase light absorption simultaneously. The photocatalytic system created by such a dual-technology combination has promising applications in environmental remediation.