Inducing Covalent Atomic Interaction in Intermetallic Pt Alloy Nanocatalysts for High-Performance Fuel Cells

The harsh working environments of proton exchange membrane fuel cells (PEMFCs) pose huge challenges to the stability of Pt-based alloy catalysts. The widespread presence of metallic bonds with significantly delocalized electron distribution often lead to component segregation and rapid performance decay. Here we report L1₀−Pt₂CuGa intermetallic nanoparticles with a unique covalent atomic interaction between Pt−Ga as high-performance PEMFC cathode catalysts. The L1₀−Pt₂CuGa/C catalyst shows superb oxygen reduction reaction (ORR) activity and stability in fuel cell cathode (mass activity=0.57 A mg_Pt⁻¹ at 0.9 V, peak power density=2.60/1.24 W cm⁻² in H₂-O₂/air, 28 mV voltage loss at 0.8 A cm⁻² after 30 000 cycles). Theoretical calculations reveal the optimized adsorption of oxygen intermediates via the formed biaxial strain on L1₀−Pt₂CuGa surface, and the durability enhancement stems from the stronger Pt−M bonds than those in L1₁−PtCu resulted from Pt−Ga covalent interactions.     

Improving the Electrocatalytic Activity of a Nickel-Organic Framework toward the Oxygen Evolution Reaction through Vanadium Doping

Metal-organic frameworks (MOFs) have been considered as potential oxygen evolution reaction (OER) electrocatalysts owning to their ultra-thin structure, adjustable composition, high surface area, and high porosity. Here, we designed and fabricated a vanadium-doped nickel organic framework (V_1−x−Ni_xMOF) system by using a facile two-step solvothermal method on nickel foam (NF). The doping of vanadium remarkably elevates the OER activity of V_1−x−Ni_xMOF, thus demonstrating better performance than the corresponding single metallic Ni-MOF, NiV-MOF and RuO₂ catalysts at high current density (>400 mA cm⁻²). V_0.09−Ni_0.91MOF/NF provides a low overpotential of 235 mV and a small Tafel slope of 30.3 mV dec⁻¹ at a current density of 10 mA cm⁻². More importantly, a water-splitting device assembled with Pt/C/NF and V_0.09−Ni_0.91MOF/NF as cathode and anode yielded a cell voltage of 1.96 V@1000 mA cm⁻², thereby outperforming the-state-of-the-art RuO₂⁽⁺⁾||Pt/C⁽⁻⁾. Our work sheds new insight on preparing stable, efficient OER electrocatalysts and a promising method for designing various MOF-based materials.     

组合型Pt/TiO_2催化剂用于低温催化甲苯完全氧化

将均匀分布的纳米Pt粒子直接吸附到TiO2载体上,即制得了组合型Pt/TiO2催化剂(Pt/TiO2-AS).与浸渍法制备的Pt/TiO2催化剂(Pt/TiO2-WI)比较,Pt/TiO2-AS催化剂在催化甲苯完全氧化反应中表现出了很好的催化性能,甲苯转化率为100%时的反应温度低至150°C,而且即使在较高甲苯浓度和较高气体空速下,该催化剂也能保持较好的催化性能.通过X射线衍射(XRD)、N2吸附-脱附(BET)、透射电子显微镜(TEM)、X射线光电子能谱(XPS)、氢气程序升温还原(H2-TPR)及傅里叶变换红外(FTIR)光谱等对两种Pt/TiO2催化剂的结构和表面性能进行了表征.结果表明组合型Pt/TiO2-AS催化剂粒径小(2.5 nm),活性组分主要以Pt0形式存在且分布在载体表面,而且载体表面Ti―O键活化使催化剂具有较强的催化氧化能力.另外,活性中心的价态变化(Pt0→Ptδ+)是导致Pt/TiO2-AS催化剂失活的主要原因.     

Engineering the Atomic Interface with Single Platinum Atoms for Enhanced Photocatalytic Hydrogen Production

It is highly desirable but challenging to optimize the structure of photocatalysts at the atomic scale to facilitate the separation of electron–hole pairs for enhanced performance. Now, a highly efficient photocatalyst is formed by assembling single Pt atoms on a defective TiO₂ support (Pt₁/def‐TiO₂). Apart from being proton reduction sites, single Pt atoms promote the neighboring TiO₂ units to generate surface oxygen vacancies and form a Pt‐O‐Ti³⁺ atomic interface. Experimental results and density functional theory calculations demonstrate that the Pt‐O‐Ti³⁺ atomic interface effectively facilitates photogenerated electrons to transfer from Ti³⁺ defective sites to single Pt atoms, thereby enhancing the separation of electron–hole pairs. This unique structure makes Pt₁/def‐TiO₂ exhibit a record‐level photocatalytic hydrogen production performance with an unexpectedly high turnover frequency of 51423 h^?1, exceeding the Pt nanoparticle supported TiO₂ catalyst by a factor of 591.     

Facile Synthesis of Thermal‐ and Photostable Titania with Paramagnetic Oxygen Vacancies for Visible‐Light Photocatalysis

A novel dopant‐free TiO₂ photocatalyst (V_o^.‐TiO₂), which is self‐modified by a large number of paramagnetic (single‐electron‐trapped) oxygen vacancies, was prepared by calcining a mixture of a porous amorphous TiO₂ precursor, imidazole, and hydrochloric acid at elevated temperature (450 °C) in air. Control experiments demonstrate that the porous TiO₂ precursor, imidazole, and hydrochloric acid are all necessary for the formation of V_o^.‐TiO₂. Although the synthesis of V_o^.‐TiO₂ originates from such a multicomponent system, this synthetic approach is facile, controllable, and reproducible. X‐ray diffraction, XPS, and EPR spectroscopy reveal that the V_o^.‐TiO₂ material with a high crystallinity embodies a mass of paramagnetic oxygen vacancies, and is free of other dopant species such as nitrogen and carbon. UV/Vis diffuse‐reflectance spectroscopy and photoelectrochemical measurement demonstrate that V_o^.‐TiO₂ is a stable visible‐light‐responsive material with photogenerated charge separation efficiency higher than N‐TiO₂ and P25 under visible‐light irradiation. The V_o^.‐TiO₂ material exhibits not only satisfactory thermal‐ and photostability, but also superior photocatalytic activity for H₂ evolution (115 μmol h^?1 g^?1) from water with methanol as sacrificial reagent under visible light (λ>400 nm) irradiation. Furthermore, the effects of reaction temperature, ratio of starting materials (imidazole:TiO₂ precursor) and calcination time on the photocatalytic activity and the microstructure of V_o^.‐TiO₂ were elucidated.     

Tuning the concentration of surface/bulk oxygen vacancies in CeO2 nanorods to promote highly efficient photodegradation of organic dyes

To enhance the photodegradation ability of CeO₂ for organic dyes, an effective strategy is to introduce oxygen vacancies (V_o). In general, the introduced V_o are simultaneously present both on the surface and in the bulk of CeO₂. The surface oxygen vacancies (V_o-s) can decrease the band gap, thus enhancing light absorption to produce more photogenerated e⁻ for photodegradation. However, the bulk oxygen vacancies (V_o-b) will inhibit photocatalytic activity by increasing the recombination of photogenerated e⁻ and V_o-b. Therefore, regulating the concentrations of V_o-s to V_o-b is a breakthrough for achieving the best utilization of photogenerated e⁻ during photodegradation. We used an easy hydrothermal method to achieve tunable concentrations of V_o-s to V_o-b in CeO₂ nanorods. The optimized CeO₂ presents a 70.2% removal of rhodamine B after 120 min of ultraviolet−visible light irradiation, and a superior photodegradation performance of multiple organics. This tuning strategy for V_o also provides guidance for developing other advanced metal-oxide semiconductor photocatalysts for the photodegradation of organic dyes.     

ZnO and TiO2 clusters as catalyst in the addition and abstraction reaction of acrylic acid with the OH radical

The present work displays the theoretical analysis on the role of metal oxide clusters as an effective catalyst in the reaction between acrylic acid and OH radical, which has an energy barrier of 12.4 kcal/mol. The formation of metal oxide cluster such as ZnO and TiO₂ with varying size from monomer to hexamer is analyzed using cohesive energy, which increases with cluster size. Adsorption of acrylic acid on clusters reveals that dimer ZnO and tetramer TiO₂ are good adsorbed entities. The dimer ZnO and tetramer TiO₂ clusters have reduced the barrier height. However, from the thermodynamical analysis of H-abstraction and OH addition reaction, the dimer ZnO cluster is found to be a good catalyst than a tetramer TiO₂ cluster. The favorable H abstraction and OH addition reactions are feasible at the active methylene group (–CH). OH addition reactions dominate over the H abstraction reaction. Further, the presence of metal oxide clusters enhances the rate of the reaction between acrylic acid and OH radical. The kinetics of the favorable reaction with a dimer ZnO cluster has a rate constant of 7.80 × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹, which is higher than the literature report (1.75 × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹). Overall, ZnO and TiO₂ metal oxide clusters can be effectively utilized as catalyst.     

Pt nanocrystallines/TiO2 with thickness-controlled carbon layers: Preparation and activities in CO oxidation

Pt nanocrystallines (~3 nm) covered with controllable carbon layers were synthesized by photochemical reduction method which exhibited extraordinary anti-sintering properties and different CO oxidation activities.     

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.     

Porous TiO2 Nanotubes with Spatially Separated Platinum and CoOx Cocatalysts Produced by Atomic Layer Deposition for Photocatalytic Hydrogen Production

Efficient separation of photogenerated electrons and holes, and associated surface reactions, is a crucial aspect of efficient semiconductor photocatalytic systems employed for photocatalytic hydrogen production. A new CoO_x/TiO₂/Pt photocatalyst produced by template‐assisted atomic layer deposition is reported for photocatalytic hydrogen production on Pt and CoO_x dual cocatalysts. Pt nanoclusters acting as electron collectors and active sites for the reduction reaction are deposited on the inner surface of porous TiO₂ nanotubes, while CoO_x nanoclusters acting as hole collectors and active sites for oxidation reaction are deposited on the outer surface of porous TiO₂ nanotubes. A CoO_x/TiO₂/Pt photocatalyst, comprising ultra‐low concentrations of noble Pt (0.046 wt %) and CoO_x (0.019 wt %) deposited simultaneously with one atomic layer deposition cycle, achieves remarkably high photocatalytic efficiency (275.9 μmol h⁻¹), which is nearly five times as high as that of pristine TiO₂ nanotubes (56.5 μmol h⁻¹). The highly dispersed Pt and CoO_x nanoclusters, porous structure of TiO₂ nanotubes with large specific surface area, and the synergetic effect of the spatially separated Pt and CoO_x dual cocatalysts contribute to the excellent photocatalytic activity.     

Transformation of a Pt/TiO2 catalyst from non-SMSI to SMSI studied by repeated H2-O2 titration

Transformations of Pt/TiO₂ catalyst between non-SMSI and SMSI states have been investigated by repeatedH₂–O₂ titration. The decline of capacity of H₂ and O₂ chemisorption and their reaction on Pt particles is accountable by reduction of superficial labile oxygen species in the temperature range of 298–573 K and an increase of surface oxygen vacancies on TiO₂ above 573 K, respectively.     

Multifunctional Active-Center-Transferable Platinum/Lithium Cobalt Oxide Heterostructured Electrocatalysts towards Superior Water Splitting

Designing cost-effective and efficient electrocatalysts plays a pivotal role in advancing the development of electrochemical water splitting for hydrogen generation. Herein, multifunctional active-center-transferable heterostructured electrocatalysts, platinum/lithium cobalt oxide (Pt/LiCoO₂) composites with Pt nanoparticles (Pt NPs) anchored on LiCoO₂ nanosheets, are designed towards highly efficient water splitting. In this electrocatalyst system, the active center can be alternatively switched between Pt species and LiCoO₂ for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. Specifically, Pt species are the active centers and LiCoO₂ acts as the co-catalyst for HER, whereas the active center transfers to LiCoO₂ and Pt turns into the co-catalyst for OER. The unique architecture of Pt/LiCoO₂ heterostructure provides abundant interfaces with favorable electronic structure and coordination environment towards optimal adsorption behavior of reaction intermediates. The 30 % Pt/LiCoO₂ heterostructured electrocatalyst delivers low overpotentials of 61 and 285 mV to achieve 10 mA cm⁻² for HER and OER in alkaline medium, respectively.     

Zinc Oxide Defect Microstructure and Surface Chemistry Derived from Oxidation of Metallic Zinc: Thin-Film Transistor and Sensor Behavior of ZnO Films and Rods

Zinc oxide thin films are fabricated by controlled oxidation of sputtered zinc metal films on a hotplate in air at temperatures between 250 and 450 °C. The nanocrystalline films possess high relative densities and show preferential growth in (100) orientation. Integration in thin-film transistors reveals moderate charge carrier mobilities as high as 0.2 cm² V⁻¹s⁻¹. The semiconducting properties depend on the calcination temperature, whereby the best performance is achieved at 450 °C. The defect structure of the thin ZnO film can be tracked by Doppler-broadening positron annihilation spectroscopy as well as positron lifetime studies. Comparably long positron lifetimes suggest interaction of zinc vacancies (V_Zn) with one or more oxygen vacancies (V_O) in larger structural entities. Such V_O-V_Zn defect clusters act as shallow acceptors, and thus, reduce the overall electron conductivity of the film. The concentration of these defect clusters decreases at higher calcination temperatures as indicated by changes in the S and W parameters. Such zinc oxide films obtained by conversion of metallic zinc can also be used as seed layers for solution deposition of zinc oxide nanowires employing a mild microwave-assisted process. The functionality of the obtained nanowire arrays is tested in a UV sensor device. The best results with respect to sensor sensitivity are achieved with thinner seed layers for device construction.     

Impact of Surface Defects on LaNiO3 Perovskite Electrocatalysts for the Oxygen Evolution Reaction

Perovskite oxides are regarded as promising electrocatalysts for water splitting due to their cost-effectiveness, high efficiency and durability in the oxygen evolution reaction (OER). Despite these advantages, a fundamental understanding of how critical structural parameters of perovskite electrocatalysts influence their activity and stability is lacking. Here, we investigate the impact of structural defects on OER performance for representative LaNiO₃ perovskite electrocatalysts. Hydrogen reduction of 700 °C calcined LaNiO₃ induces a high density of surface oxygen vacancies, and confers significantly enhanced OER activity and stability compared to unreduced LaNiO₃; the former exhibit a low onset overpotential of 380 mV at 10 mA cm⁻² and a small Tafel slope of 70.8 mV dec⁻¹. Oxygen vacancy formation is accompanied by mixed Ni²⁺/Ni³⁺ valence states, which quantum-chemical DFT calculations reveal modify the perovskite electronic structure. Further, it reveals that the formation of oxygen vacancies is thermodynamically more favourable on the surface than in the bulk; it increases the electronic conductivity of reduced LaNiO₃ in accordance with the enhanced OER activity that is observed.     

Ab Initio Calculations of Hydroxoplatinum Compounds: II. Binuclear Platinum(IV) Superoxo Complexes

The structural and spectral data have been obtained by ab initio methods for the [(OH)₄Pt(μ-O₂)(μ- OH)Pt(OH)₄]^2?, [(OH)₄Pt(μ-O₂)(μ-OH)Pt(OH)₄(OH)]^3?, [(OH)₅Pt(μ-O₂)Pt(OH)₅]^3?, and [(H₂O)(OH)₄Pt(μ- O₂)Pt(OH)₄(H₂O)]- clusters, corresponding to binuclear platinum(IV) superoxo complexes with one and two bridges. The data obtained are in good agreement with experimental data and make it possible to judge the structure of available complexes.     

One-Step Approach for Constructing High-Density Single-Atom Catalysts toward Overall Water Splitting at Industrial Current Densities

The construction of highly active, durable, and cost-effective catalysts is urgently needed for green hydrogen production. Herein, catalysts consisting of high-density Pt (24 atoms nm⁻²) and Ir (32 atoms nm⁻²) single atoms anchored on Co(OH)₂ were constructed by a facile one-step approach. Remarkably, Pt₁/Co(OH)₂ and Ir₁/Co(OH)₂ only required 4 and 178 mV at 10 mA cm⁻² for hydrogen evolution reaction and oxygen evolution reaction, respectively. Moreover, the assembled Pt₁/Co(OH)₂//Ir₁/Co(OH)₂ system showed mass activity of 4.9 A mg_{noble metal}⁻¹ at 2.0 V in an alkaline water electrolyzer, which is 316.1 times higher than that of Pt/C//IrO₂. Mechanistic studies revealed that reconstructed Ir−O₆ single atoms and remodeled Pt triple-atom sites enhanced the occupancy of Ir−O bonding orbitals and improved the occupation of Pt−H antibonding orbital, respectively, contributing to the formation of the O−O bond and the desorption of hydrogen. This one-step approach was also generalized to fabricate other 20 single-atom catalysts.     

Facile and Large-Scale Fabrication of Sub-3 nm PtNi Nanoparticles Supported on Porous Carbon Sheet: A Bifunctional Material for the Hydrogen Evolution Reaction and Hydrogenation

Facile and large-scale preparation of materials with uniform distributions of ultrafine particles for catalysis is a challenging task, and it is even more difficult to obtain catalysts that excel in both the hydrogen evolution reaction (HER) and hydrogenation, which are the corresponding merging and splitting procedures of hydrogen, respectively. Herein, the fabrication of ultrafine bimetallic PtNi nanoparticles embedded in carbon nanosheets (CNS) by means of in situ self-polymerization and annealing is reported. This bifunctional catalyst shows excellent performance in the hydrogen evolution reaction (HER) and the hydrogenation of p-nitrophenol. Remarkably PtNi bimetallic catalyst with low metal loading (PtNi₂@CNS-600, 0.074 wt % Pt) exhibited outstanding HER activity with an overpotential as low as 68 mV at a current density of 10 mA cm⁻² with a platinum loading of only 0.612 μg_Pt cm⁻² and Tafel slope of 35.27 mV dec⁻¹ in a 0.5 m aqueous solution of H₂SO₄, which is comparable to that of the 20 % Pt/C catalyst (31 mV dec⁻¹). Moreover, it also shows superior long-term electrochemical durability for at least 30 h with negligible degradation compared with 20 % Pt/C. In addition, the material with increased loading (mPtNi₂@CNS-600, 2.88 % Pt) showed robust catalytic activity for hydrogenation of p-nitrophenol at ambient pressure and temperature. The catalytic activity towards hydrogen splitting is a circumstantial evidence that agrees with the Volmer–Tafel reaction path in the HER.     

CO oxidation over supported Pt clusters at different CO coverage

CO adsorption and oxidation over supported Pt₁₄ with different CO coverage on TiO₂(110) surface were investigated using density functional theory (DFT) calculations and thermodynamic analysis. According to the phase diagram, Pt₁₄/TiO₂(110) and 11CO@Pt₁₄/TiO₂(110) were chosen to represent the low and high CO coverage of Pt clusters, respectively. Our study shows that the high coverage of CO can induce the structural change of supported Pt clusters and weaken the interaction between Pt clusters and TiO₂ support. The CO adsorption and oxidation mechanism depends on the CO coverage, which is determined by the experimental reactant composition, pressure, and temperature. At low CO coverage, the dissociated oxygen is active specie to form CO₂ by reacting with CO. At high coverage, the molecular oxygen can directly react with CO via the formation of OOCO intermediate. Our proposed mechanisms provide useful information for understanding the CO oxidation over Pt clusters with different CO coverage. © 2016 Wiley Periodicals, Inc.     

Hierarchical Architectures Based on Ru Nanoparticles/Oxygen-Rich-Carbon Nanotubes for Efficient Hydrogen Evolution

Highly active and durable electrocatalysts are essential for producing hydrogen fuel through the hydrogen evolution reaction (HER). Here, a uniform deposition of Ru nanoparticles strongly interacting with oxygen-rich carbon nanotube architectures (Ru-OCNT) through ozonation and hydrothermal approaches has been designed. The hierarchical structure of Ru-OCNT is made by self-assembly of oxygen functionalities of OCNT. Ru nanoparticles interact strongly with OCNT at the Ru/OCNT interface to give excellent catalytic activity and stability of the Ru-OCNT, as further confirmed by density functional theory. Owing to the hierarchical structure and adjusted surface chemistry, Ru-OCNT has an overpotential of 34 mV at 10 mA cm⁻² with a Tafel slope of 27.8 mV dec⁻¹ in 1 M KOH, and an overpotential of 55 mV with Tafel slope of 33 mV dec⁻¹ in 0.5 M H₂SO₄. The smaller Tafel slope of Ru-OCNT than Ru-CNT and commercial Pt/C in both alkaline and acidic electrolytes indicates high catalytic activity and fast charge transfer kinetics. The as-proposed chemistry provides the rational design of hierarchically structured CNT/nanoparticle electrocatalysts for HER to produce hydrogen fuel.