In Situ Dispersion of Palladium on TiO2 During Reverse Water–Gas Shift Reaction: Formation of Atomically Dispersed Palladium

The application of single‐atom catalysts (SACs) to high‐temperature hydrogenation requires materials that thermodynamically favor metal atom isolation over cluster formation. We demonstrate that Pd can be predominantly dispersed as isolated atoms onto TiO₂ during the reverse water–gas shift (rWGS) reaction at 400 °C. Achieving atomic dispersion requires an artificial increase of the absolute TiO₂ surface area by an order of magnitude and can be accomplished by physically mixing a precatalyst (Pd/TiO₂) with neat TiO₂ prior to the rWGS reaction. The in situ dispersion of Pd was reflected through a continuous increase of rWGS activity over 92 h and supported by kinetic analysis, infrared and X‐ray absorption spectroscopies and scanning transmission electron microscopy. The thermodynamic stability of Pd under high‐temperature rWGS conditions is associated with Pd‐Ti coordination, which manifests upon O‐vacancy formation, and the artificial increase in TiO₂ surface area.     

In Situ Dispersion of Palladium on TiO2 During Reverse Water–Gas Shift Reaction: Formation of Atomically Dispersed Palladium

The application of single-atom catalysts (SACs) to high-temperature hydrogenation requires materials that thermodynamically favor metal atom isolation over cluster formation. We demonstrate that Pd can be predominantly dispersed as isolated atoms onto TiO₂ during the reverse water–gas shift (rWGS) reaction at 400 °C. Achieving atomic dispersion requires an artificial increase of the absolute TiO₂ surface area by an order of magnitude and can be accomplished by physically mixing a precatalyst (Pd/TiO₂) with neat TiO₂ prior to the rWGS reaction. The in situ dispersion of Pd was reflected through a continuous increase of rWGS activity over 92 h and supported by kinetic analysis, infrared and X-ray absorption spectroscopies and scanning transmission electron microscopy. The thermodynamic stability of Pd under high-temperature rWGS conditions is associated with Pd-Ti coordination, which manifests upon O-vacancy formation, and the artificial increase in TiO₂ surface area.     

Stable Isolated Metal Atoms as Active Sites for Photocatalytic Hydrogen Evolution

The process of using solar energy to split water to produce hydrogen assisted by an inorganic semiconductor is crucial for solving our energy crisis and environmental problems in the future. However, most semiconductor photocatalysts would not exhibit excellent photocatalytic activity without loading suitable co‐catalysts. Generally, the noble metals have been widely applied as co‐catalysts, but always agglomerate during the loading process or photocatalytic reaction. Therefore, the utilization efficiency of the noble co‐catalysts is still very low on a per metal atom basis if no obvious size effect exists, because heterogeneous catalytic reactions occur on the surface active atoms. Here, for the first time, we have synthesized isolated metal atoms (Pt, Pd, Rh, or Ru) stably by anchoring on TiO₂, a model photocatalystic system, by a facile one‐step method. The isolated metal atom based photocatalysts show excellent stability for H₂ evolution and can lead to a 6–13‐fold increase in photocatalytic activity over the metal clusters loaded on TiO₂ by the traditional method. Furthermore, the configurations of isolated atoms as well as the originality of their unusual stability were analyzed by a collaborative work from both experiments and theoretical calculations.     

Modifications on reduced titanium dioxide photocatalysts: A review

A large variety of reduced titanium dioxide (TiO_2-x) materials have been reported recently. Reduced TiO₂, usually resulting from the removal of oxygen atoms or hydrogen incorporation, is proved to be efficient for achieving highly photocatalytic performance including photodegradation of organic compounds, hydrogen generation from water splitting, CO₂ reduction for CH₄ evolution, solar cells, etc. To further improve the properties and activities of TiO_2-x, a combination of the Ti³⁺ self-doping and other traditional modifications like nonmetals doping has been proposed in the past decades. This paper provides a general and critical review on the further modifications on reduced TiO₂ samples, including non-metal elements (N, B, S, F and I) doping, noble-metal (Au, Pt, Pd and Ag) and iron-group metal (Fe, Co and Ni) grafting, metal oxide compositing, carbon (nanotubes and graphene) and carbon-based-material compositing, special facets exposure (mainly dual {001}-{101} and {111}-{110} facets) of TiO_2-x and ordered structure controlling of TiO_2-x. These modifications enhance the physical and/or chemical properties of the reduced TiO₂, or create new features for the modified TiO_2-x samples, which finally leads to the enhancement of photocatalytic performance. Key examples such as N-doping, Au grafting and graphene-based compositing are discussed carefully, and the mechanisms for solar light enhancement, electron transfer and charge separation are also investigated. Finally, some challenging issues on TiO_2-x catalysts are also proposed to encourage new approaches for preparation of TiO_2-x catalysts with efficiently photocatalytic performance.     

Palladium Single Atoms on TiO2 as a Photocatalytic Sensing Platform for Analyzing the Organophosphorus Pesticide Chlorpyrifos

Traditional methods for analyzing organophosphorus pesticide chlorpyrifos, usually require the tedious sample pretreatment and sophisticated bio‐interfaces, leading to the difficulty for real‐time analysis. Herein, we use palladium single‐atom (PdSA)/TiO₂ as a photocatalytic sensing platform to directly detect chlorpyrifos with high sensitivity and selectivity. PdSA/TiO₂, prepared by an in situ photocatalytic reduction of PdCl₄^2? on the TiO₂, shows much higher photocatalytic activity (10 mol g^?1 h^?1) for hydrogen evolution reaction than Pd nanoparticles (1.95 mol g^?1 h^?1), and excellent stability. In the presence of chlorpyrifos, the photocatalytic activity of PdSA/TiO₂ decreases. Through this inhibition effect the platform can realize a detection limit for chlorpyrifos of 0.01 ng mL^?1, much lower than the maximum residue limit (10 ppb) permitted by the U.S. Environmental Protection Agency.     

Ultrathin Metal–Organic Framework Nanosheets with Ultrahigh Loading of Single Pt Atoms for Efficient Visible‐Light‐Driven Photocatalytic H2 Evolution

A surfactant‐stabilized coordination strategy is used to make two‐dimensional (2D) single‐atom catalysts (SACs) with an ultrahigh Pt loading of 12.0 wt %, by assembly of pre‐formed single Pt atom coordinated porphyrin precursors into free‐standing metal–organic framework (MOF) nanosheets with an ultrathin thickness of 2.4±0.9 nm. This is the first example of 2D MOF‐based SACs. Remarkably, the 2D SACs exhibit a record‐high photocatalytic H₂ evolution rate of 11 320 μmol g^?1 h^?1 via water splitting under visible light irradiation (λ>420 nm) compared with those of reported MOF‐based photocatalysts. Moreover, the MOF nanosheets can be readily drop‐casted onto solid substrates, forming thin films while still retaining their photocatalytic activity, which is highly desirable for practical solar H₂ production.     

Characterization of Pd/TiO2 embedded in multi-walled carbon nanotube catalyst with a high photocatalytic activity

Pd particles loading on TiO₂-embedded multi-walled carbon nanotubes (MWCNTs), MWCNTs, and TiO₂ particles were prepared via an impregnation method with palladium(II) chlorate solution followed by heat treatment at high temperature. To characterize the catalysts, BET surface area, scanning electron microscopy, transmission electron microscopy, X-ray diffraction, energy dispersive X-ray, Fourier transform infrared spectroscopy and ultraviolet-visible spectroscopy were employed. The prepared catalysts were tested in degradation of methyl orange under visible light. Pd/TiO₂-MWCNTs catalyst demonstrates the highest photocatalytic activity, and the phase transformation from PdO to Pd0 phase takes place at heat treatment of embedded TiO₂. The nanoparticles size of TiO₂ can be decreased by introduction of MWCNTs species. Combining structural characterization with kinetic study results we could conclude that the superior catalytic performance could arise due to the Pd/TiO₂-MWCNTs catalyst’s structure.     

High-Density and Thermally Stable Palladium Single-Atom Catalysts for Chemoselective Hydrogenations

Single-atom catalysts (SACs) have shown superior activity and/or selectivity for many energy- and environment-related reactions, but their stability at high site density and under reducing atmosphere remains unresolved. Herein, we elucidate the intrinsic driving force of a Pd single atom with high site density (up to 5 wt %) under reducing atmosphere, and its unique catalytic performance for hydrogenation reactions. In situ experiments and calculations reveal that Pd atoms tend to migrate into the surface vacancy-enriched MoC surface during the carburization process by transferring oxide crystals to carbide crystals, leading to the surface enrichment of atomic Pd instead of formation of particles. The Pd₁/α-MoC catalyst exhibits high activity and excellent selectivity for liquid-phase hydrogenation of substituted nitroaromatics (>99 %) and gas-phase hydrogenation of CO₂ to CO (>98 %). The Pd₁/α-MoC catalyst could endure up to 400 °C without any observable aggregation of single atoms.     

Band Structure Engineering of Single Pt Atoms on Fe−TiO2 for Enhanced Photocatalytic Performance

Single atomic site catalysts display the maximal atom-utilization efficiency, unique structural properties, and remarkable enhancements on catalytic activity. Herein, single Pt atoms loaded Fe−TiO₂ catalysts were prepared. Fe³⁺ doping leads to the formation of oxygen vacancies and improve the interaction between TiO₂ and Pt. Single Pt atoms are thus anchored and effectively modify the local energy band structure of TiO₂. The optimized local band structures improve the intrinsic photoexcitation of Pt/Fe−TiO₂, promote the separation of photogenerated carriers, and extend the lifetime of photogenerated carriers. Meanwhile, the electrons transfer from the excited dyes to the conduction band edge of Pt/Fe−TiO₂ is also facilitated due to the shift-down of the conduction band edge. Therefore, with the increase of the Pt content (till up to 0.6 wt%), the photocatalytic performance of Pt/ Fe−TiO₂ with the confined single Pt atoms is significantly boosted in either the intrinsic or the sensitized photocatalytic process.     

Universal Sub-Nanoreactor Strategy for Synthesis of Yolk-Shell MoS2 Supported Single Atom Electrocatalysts toward Robust Hydrogen Evolution Reaction

The coordination structure determines the electrocatalytic performances of single atom catalysts (SACs), while it remains a challenge to precisely regulate their spatial location and coordination environment. Herein, we report a universal sub-nanoreactor strategy for synthesis of yolk-shell MoS₂ supported single atom electrocatalysts with dual-anchored microenvironment of vacancy-enriched MoS₂ and intercalation carbon toward robust hydrogen-evolution reaction. Theoretical calculations reveal that the “E-Lock” and “E-Channel” are conducive to stabilize and activate metal single atoms. A group of SACs is subsequently produced with the assistance of sulfur vacancy and intercalation carbon in the yolk-shell sub-nanoreactor. The optimized C−Co−MoS₂ yields the lowest overpotential (η₁₀=17 mV) compared with previously reported MoS₂-based electrocatalysts to date, and also affords a 5–9 fold improvement in activity even comparing with those as-prepared single-anchored analogues. Theoretical results and in situ characterizations unveil its active center and durability. This work provides a universal pathway to design efficient catalysts for electro-refinery.     

Regulating the Coordination Environment of MOF‐Templated Single‐Atom Nickel Electrocatalysts for Boosting CO2 Reduction

The general synthesis and control of the coordination environment of single‐atom catalysts (SACs) remains a great challenge. Herein, a general host–guest cooperative protection strategy has been developed to construct SACs by introducing polypyrrole (PPy) into a bimetallic metal–organic framework. As an example, the introduction of Mg²⁺ in MgNi‐MOF‐74 extends the distance between adjacent Ni atoms; the PPy guests serve as N source to stabilize the isolated Ni atoms during pyrolysis. As a result, a series of single‐atom Ni catalysts (named Ni_SA‐N_x‐C) with different N coordination numbers have been fabricated by controlling the pyrolysis temperature. Significantly, the Ni_SA‐N₂‐C catalyst, with the lowest N coordination number, achieves high CO Faradaic efficiency (98 %) and turnover frequency (1622 h^?1), far superior to those of Ni_SA‐N₃‐C and Ni_SA‐N₄‐C, in electrocatalytic CO₂ reduction. Theoretical calculations reveal that the low N coordination number of single‐atom Ni sites in Ni_SA‐N₂‐C is favorable to the formation of COOH* intermediate and thus accounts for its superior activity.     

Photocatalytic Activity of Supported Metal Nanoparticles and Single Atoms

Photocatalysis has been known as one of the promising technologies due to its eco-friendly nature. However, the potential application of many photocatalysts is limited owing to their large bandgaps and inefficient use of the solar spectrum. One strategy to overcome this problem is to combine the advantages of heteroatom-containing supports with active metal centers to accurately adjust the structural parameters. Metal nanoparticles (MNPs) and single atom catalysts (SACs) are excellent candidates due to their distinctive coordination environment which enhances photocatalytic activity. Metal-organic frameworks (MOFs), covalent organic frameworks (COFs) and carbon nitride (g-C₃N₄) have shown great potential as catalyst support for SACs and MNPs. The numerous combinations of organic linkers with various heteroatoms and metal ions provide unique structural characteristics to achieve advanced materials. This review describes the recent advancement of the modified MOFs, COFs and g-C₃N₄ with SACs and NPs for enhanced photocatalytic applications with emphasis on environmental remediation.     

Palladium Encapsulated by an Oxygen-Saturated TiO2 Overlayer for Low-Temperature SO2-Tolerant Catalysis during CO Oxidation

The development of oxidation catalysts that are resistant to sulfur poisoning is crucial for extending the lifespan of catalysts in real-working conditions. Herein, we describe the design and synthesis of oxide-metal interaction (OMI) catalyst under oxidative atmospheres. By using organic coated TiO₂, an oxide/metal inverse catalyst with non-classical oxygen-saturated TiO₂ overlayers were obtained at relatively low temperature. These catalysts were found to incorporate ultra-small Pd metal and support particles with exceptional reactivity and stability for CO oxidation (under 21 vol % O₂ and 10 vol % H₂O). In particular, the core (Pd)-shell (TiO₂) structured OMI catalyst exhibited excellent resistance to SO₂ poisoning, yielding robust CO oxidation performance at 120 °C for 240 h (at 100 ppm SO₂ and 10 vol % H₂O). The stability of this new OMI catalyst was explained through density functional theory (DFT) calculations that interfacial oxygen atoms at Pd−O−Ti sites (of oxygen-saturated overlayers) serve as non-metal active sites for low-temperature CO oxidation, and change the SO₂ adsorption from metal(d)-to-SO₂(π*) back-bonding to much weaker σ(Ti−S) bonding.     

PdO/TiO2 and Pd/TiO2 Heterostructured Nanobelts with Enhanced Photocatalytic Activity

Heterostructures play an important role not only in the manufacture of semiconductor devices, but also in the field of catalysis. Herein, we report the synthesis of PdO/TiO₂ and Pd/TiO₂ heterostructured nanobelts by means of a simple co‐precipitation method, followed by a reduction process using surface‐modified TiO₂ nanobelts as templates. The as‐obtained heterostructures were characterized by transmission electron microscopy, X‐ray photoelectron spectroscopy, and UV/Vis diffuse reflectance spectroscopy. PdO and Pd nanoparticles with a size of about 1.3 and 1.6 nm were assembled uniformly on the surface of TiO₂ nanobelts, respectively. Compared with TiO₂ nanobelts, PdO/TiO₂ and Pd/TiO₂ hybrid nanobelts exhibit enhanced photocatalytic activity upon UV and visible‐light irradiation. Photoelectrochemical technology was used to study the heterostructure effect on enhanced photocatalytic activity. Our mechanistic investigation revealed that energy‐band matching is the major factor in the observed enhancement of photocatalytic activity.     

Preparation and characterization of M1-Nx-Cy based single atom catalysts for environmental applications

Single atom catalysts(SACs) have become the frontier research fields in catalysis. The M1-Nx-Cybased SACs, wherein single metal atoms(M1) are stabilized by N-doped carbonaceous materials, have provided new opportunities for catalysis due to their high reactivity, maximized atomic utilization, and high selectivity. In this review, the fabrication methods of M1-Nx-Cybased SACs via support anchoring strategy and coordination design strategy are summarized to help the readers understand the interact...     

Controlling the Oxidation State of Pt Single Atoms for Maximizing Catalytic Activity

Single-atom catalysts (SACs) have emerged as promising materials in heterogeneous catalysis. Previous studies reported controversial results about the relative level in activity for SACs and nanoparticles (NPs). These works have focused on the effect of metal atom arrangement, without considering the oxidation state of the SACs. Here, we immobilized Pt single atoms on defective ceria and controlled the oxidation state of Pt SACs, from highly oxidized (Pt⁰: 16.6 at %) to highly metallic states (Pt⁰: 83.8 at %). The Pt SACs with controlled oxidation states were then employed for oxidation of CO, CH₄, or NO, and their activities compared with those of Pt NPs. The highly oxidized Pt SACs presented poorer activities than Pt NPs, whereas metallic Pt SACs showed higher activities. The Pt SAC reduced at 300 °C showed the highest activity for all the oxidations. The Pt SACs with controlled oxidation states revealed a crucial missing link between activity and SACs.     

Chemical synthesis of Ni/TiO<Subscript>2</Subscript> nanophotocatalyst for UV/visible light assisted degradation of organic dye in aqueous solution

The Ni/TiO₂ nanoparticles with different Ni dopant content were prepared by a modified sol–gel method. The structure and photoinduced charge properties of the as-prepared catalysts were determined using X-ray diffraction, transmission electron microscopy, UV–vis diffuse reflectance spectroscopy and surface photovoltage spectroscopy techniques, and the photocatalytic efficiency of these catalysts was tested using an organic dye. It was shown that Ni modification could greatly enhance the photocatalytic efficiency of these nanocomposite catalysts by taking the photodegradation of methyl orange as a model reaction. With appropriate ratio of Ni and TiO₂, Ni/TiO₂ nanocomposites showed the superior photocatalytic activity than the single TiO₂ nanoparticles. Surface photovoltage spectra demonstrated that Ni modification could effectively inhibit the recombination of the photoinduced electron and holes of TiO₂. This electron–hole pair separation conditions are responsible for the higher photocatalytic performance of Ni/TiO₂ nanocomposites in the visible region of electromagnetic spectrum.     

Selective hydrogenation of citral over Au-based bimetallic catalysts in supercritical carbon dioxide

Selective hydrogenation of citral was investigated over Au-based bimetallic catalysts in the environmentally benign supercritical carbon dioxide (scCO₂) medium. The catalytic performances were different in citral hydrogenation when Pd or Ru was mixed (physically and chemically) with Au. Compared with the corresponding monometallic catalyst, the total conversion and the selectivity to citronellal (CAL) were significantly enhanced over TiO₂ supported Pd and Au bimetallic catalysts (physically and chemically mixed); however, the conversion and selectivity did not change when Ru was physically mixed with Au catalyst compared to the monometallic Ru/TiO₂, and the chemically mixed Ru-Au/TiO₂ catalyst did not show any activity. The effect of CO₂ pressure on the conversion of citral and product selectivity was significantly different over the Au/TiO₂, Pd-Au/TiO₂, and Pd/TiO₂ catalysts. It was assumed to be ascribed to the difference in the interactions between Au, Pd nanoparticles and CO₂ under different CO₂ pressures.     

Thermolysis of Noble Metal Nanoparticles into Electron‐Rich Phosphorus‐Coordinated Noble Metal Single Atoms at Low Temperature

Noble metal single atoms coordinated with highly electronegative atoms, especially N and O, often suffer from an electron‐deficient state or poor stability, greatly limiting their wide application in the field of catalysis. Herein we demonstrate a new PH₃‐promoted strategy for the effective transformation of noble metal nanoparticles (MNPs, M=Ru, Rh, Pd) at a low temperature (400 °C) into a class of thermally stabilized phosphorus‐coordinated metal single atoms (MPSAs) on g‐C₃N₄ nanosheets via the strong Lewis acid–base interaction between PH₃ and the noble metal. Experimental work along with theoretical simulations confirm that the obtained Pd single atoms supported on g‐C₃N₄ nanosheets exist in the form of PdP₂ with a novel electron‐rich feature, conceptionally different from the well‐known single atoms with an electron‐deficient state. As a result of this new electronic property, PdP₂‐loaded g‐C₃N₄ nanosheets exhibit 4 times higher photocatalytic H₂ production activity than the state‐of‐art N‐coordinated PdSAs supported on g‐C₃N₄ nanosheets. This enhanced photocatalytic activity of phosphorus‐coordinated metal single atoms with an electron‐rich state was quite general, and also observed for other active noble metal single atom catalysts, such as Ru and Rh.     

RE/TiO2光催化剂的谱学特性与气相光催化性能的研究

采用改进的sol-gel法和浸渍法制备了TiO₂掺杂稀土离子La³⁺、Y³⁺、Gd³⁺、Er³⁺、Nd³⁺、Pr³⁺的RE/TiO₂光催化剂,运用FTIR、XRD、TEM、低温氮吸附/脱附、TG/DTA、UV-Vis DRS、表面光电压谱(SPS)等进行表征,以气相光催化降解乙烯、溴代甲烷作为探针反应,阐明了RE/TiO₂光催化剂的谱学特性与气相光催化性能的关系。结果显示,稀土离子掺杂后,TiO₂的锐钛矿含量增加,比表面积增大,粒径变小,吸收边发生蓝移,表面光电压的响应阈值增大,此外,Pr³⁺除外的其它稀土离子掺杂的TiO₂的表面光电压信号增强;光催化降解实验表明,与纯TiO₂相比,La³⁺、Y³⁺、Gd³⁺、Er³⁺、Nd³⁺掺杂TiO₂样品上乙烯、溴代甲烷的光催化活性均有不同程度的增强,而且表现出较强的矿化能力。但是,掺杂Pr³⁺的TiO₂的光催化性能降低恰好对应较弱的表面光电压信号。所以,本文认为提高光生电子-空穴对的分离效率是改善光催化性能的关键因素。