Photophysical and electrochemical properties of platinum(II) complexes bearing a chromophore-acceptor dyad and their photocatalytic hydrogen evolution

A series of platinum(II) complexes bearing a chromophore-acceptor dyad obtained by reacting 4-(p-bromomethylphenyl)-6-phenyl-2,2'-bipyridine or 4'-(p-bromomethylphenyl)-2,2':6',2'-terpyridine with pyridine, 4-phenylpyridine, 4,4'-bipyridine, 1-methyl-4-(pyridin-4'-yl)pyridinium hexafluorophosphate respectively, were synthesized. Their photophysical properties, emission quenching studies by Pt nanoparticles and methyl viologen, electrochemical properties and photoinduced electron-transfer reactions in a photocatalytic hydrogen-generating system containing triethanolamine and colloidal Pt without an extra electron relay, were investigated. A comparison of the rates of hydrogen production for the two photocatalytic systems, one containing a metal-organic dyad and the other comprising a 1:1 mixture of the parental platinum(II) complexes and the corresponding electron relay, showed that intramolecular electron transfer improves the photocatalytic efficiency. Compared with cyclometalated platinum(II) complexes, the related platinum(II) terpyridyl complexes exhibited poor performance for photocatalytic hydrogen evolution. An investigation into the amount of hydrogen generated by three platinum(II) complexes containing cyclometalated ligands with methyl groups located on different phenyl rings revealed that the efficiency of hydrogen evolution was affected by a subtle change of functional group on ligand, and the hydrogen-generating efficiency in the presence or absence of methyl viologen is comparable, indicating electron transfer from the excited [Pt(C^N^N)] chromophore to colloidal Pt. (1)H NMR spectroscopy of the metal-organic dyads in an aqueous solution in the presence of excess triethanolamine revealed that the dyad with a viologen unit was unstable, and a chemical reaction in the compound occurred prior to irradiation by visible light under basic conditions.     

基于曙红Y氧化性猝灭的高活性和高稳定的可见光催化产氢催化剂

以曙红Y(EY)敏化Pt/TiO₂(EY-Pt/TiO₂)光催化产氢体系为模型, 研究了电子传递剂甲基紫精(MV²⁺)的加入对该体系产氢活性和稳定性的影响, 并通过紫外-可见光(UV-Vis)吸收光谱、荧光光谱和光电化学表征手段对MV²⁺的作用机制进行了研究. 结果表明, 当以三乙醇胺(TEOA)为电子给体时, MV²⁺可使EY激发态发生氧化性和还原性淬灭, 有效降低了不稳定中间体EY^3-·的形成和积累, 促进了电子由染料分子向产氢活性位点的有效传递, 从而提高了产氢体系的活性和稳定性. 两种敏化体系瞬态光电流以及产氢活性受EY浓度影响的差异进一步证明, MV²⁺作为电子传递剂有效提高了光生电子的传递和利用效率.     

Methyl orange degradation enhanced by hydrogen spillover onto platinum nanocatalyst surface

Following a thermal reduction method, platinum nanoparticles were synthesized and stabilized by polyvinylpyrrolidone. The colloidal platinum nanoparticles were stable for more than 3 months. The micrograph analysis unveiled that the colloidal platinum nanoparticles were well dispersed with an average size of 2.53 nm. The sol–gel‐based inverse micelle strategy was applied to synthesize mesoporous iron oxide material. The colloidal platinum nanoparticles were deposited on mesoporous iron oxide through the capillary inclusion method. The small‐angle X‐ray scattering analysis indicated that the dimension of platinum nanoparticles deposited on mesoporous iron oxide (Pt‐Fe₂O₃) was 2.64 nm. X‐ray photoelectron spectroscopy (XPS) data showed that the binding energy on Pt‐Fe₂O₃ surface decreased owing to mesoporous support–nanoparticle interaction. Both colloidal and deposited platinum nanocatalysts improved the degradation of methyl orange under reduction conditions. The activation energy on the deposited platinum nanocatalyst interface (2.66 kJ mol^?1) was significantly lowered compared with the one on the colloidal platinum nanocatalyst interface (40.63 ± 0.53 kJ mol^?1).     

From Bad Electrochemical Practices to an Environmental and Waste Reducing Approach for the Generation of Active Hydrogen Evolving Electrodes

The electrodeposition of noble metals using corresponding dissolved metal salts represents an interesting process for the improvement of the electrocatalytic hydrogen evolution reaction (HER) properties of less active substrate materials. The fact that only a small fraction of the dissolved noble metals reaches the substrate represents a serious obstacle to this common procedure. We therefore chose a different path. It was found that the HER activity of Ni42 alloy drastically increased (η=140 mV at j=10 mA cm^?2; pH 1) when a platinum counter electrode was used during polarization experiments in acid. This improvement was caused by a platinum transfer from the platinum anode to the steel cathode, a process which occurred simultaneously to the hydrogen evolution. The negligible accumulation of Pt (26 μg) in the electrolyte turns this straight‐forward transfer procedure into a highly cost‐effective, environmentally friendly, and waste reducing approach for the generation of cheap, stable and effective HER electrodes.     

Size- and shape-dependent activity of metal nanoparticles as hydrogen-evolution catalysts: mechanistic insights into photocatalytic hydrogen evolution

The catalytic activity of Pt nanoparticles (PtNPs) with different sizes and shapes was investigated in a photocatalytic hydrogen‐evolution system composed of the 9‐mesityl‐10‐methylacridinium ion (Acr⁺–Mes: photocatalyst) and dihydronicotinamide adenine dinucleotide (NADH: electron donor), based on rates of hydrogen evolution and electron transfer from one‐electron‐reduced species of Acr⁺–Mes (Acr^.–Mes) to PtNPs. Cubic PtNPs with a diameter of (6.3±0.6) nm exhibited the maximum catalytic activity. The observed hydrogen‐evolution rate was virtually the same as the rate of electron transfer from Acr^.–Mes to PtNPs. The rate constant of electron transfer (k_et) increased linearly with increasing proton concentration. When H⁺ was replaced by D⁺, the inverse kinetic isotope effect was observed for the electron‐transfer rate constant (k_et(H)/k_et(D)=0.47). The linear dependence of k_et on proton concentration together with the observed inverse kinetic isotope effect suggests that proton‐coupled electron transfer from Acr^.–Mes to PtNPs to form the Pt? H bond is the rate‐determining step for catalytic hydrogen evolution. When FeNPs were used instead of PtNPs, hydrogen evolution was also observed, although the hydrogen‐evolution efficiency was significantly lower than that of PtNPs because of the much slower electron transfer from Acr^.–Mes to FeNPs.     

Transport and kinetics at microheterogeneous electrodes: Part V. The methylviologen/platinum system

Results are reported for the reaction of methylviologen radical cation, MV⁺ with platinum colloidal particles, studied by stopped flow spectrophotometry. The rate of the reaction depends on the gaseous pretreatment of the particles. For particles reduced by hydrogen, the kinetics are usually first order with respect to MV⁺. The reaction is also first order in the concentration of platinum, and is inhibited in a first order manner by the product MV²⁺. This inhibition suggests that MV²⁺ is adsorbed on the particle surfaces, and this has been confirmed by ac, ring—disc electrode studies on macroscopic platinum electrodes. At high concentrations of MV⁺ some deviation from first order kinetics is observed. These results are all explained by a kinetic model in which either the desorption of MV²⁺ or the adsorption of MV⁺ is the rate limiting process. The rate of consumption of MV⁺ on an oxidised surface is an order of magnitude faster than that on the reduced surface. Ring—disc studies show that this is because the MV⁺ is not producing H₂ but is reducing the surface oxide. The results are shown to fit a simple model which takes into account this titration of the oxide layer. The model also explains why the rate on partially oxidised surfaces will appear to have an order greater than one in [Pt].     

明胶保护的铂在联吡啶钌体系中光助催化分解水产氢的研究

以联吡啶钌络合物Ru(bipy)₃²⁺为光敏剂,甲基紫精MV²⁺为电子交换的中继物,EDTA为电子给体,氯铂酸钾为催化剂,探讨了该体系在可见光照射下,光还原水产氢的结果。提出了用明胶作为铂催化剂的稳定剂,可以提高放氢的速度和保护催化剂不受破坏。讨论了该体系光助催化分解水产氢的可能的机理。     

High‐Field Pulsed EPR Spectroscopy for the Speciation of the Reduced [PV2Mo10O40]6− Polyoxometalate Catalyst Used in Electron‐Transfer Oxidations

An in‐depth spectroscopic EPR investigation of a key intermediate, formally notated as [PV^IVV^VMo₁₀O₄₀]^6? and formed in known electron‐transfer and electron‐transfer/oxygen‐transfer reactions catalyzed by H₅PV₂Mo₁₀O₄₀, has been carried out. Pulsed EPR spectroscopy have been utilized: specifically, W‐band electron–electron double resonance (ELDOR)‐detected NMR and two‐dimensional (2D) hyperfine sub‐level correlation (HYSCORE) measurements, which resolved ⁹⁵Mo and ¹⁷O hyperfine interactions, and electron–nuclear double resonance (ENDOR), which gave the weak ⁵¹V and ³¹P interactions. In this way, two paramagnetic species related to [PV^IVV^VMo₁₀O₄₀]^6? were identified. The first species (30–35 %) has a vanadyl (VO²⁺)‐like EPR spectrum and is not situated within the polyoxometalate cluster. Here the VO²⁺ was suggested to be supported on the Keggin cluster and can be represented as an ion pair, [PV^VMo₁₀O₃₉]^8?[V^IVO²⁺]. This species originates from the parent H₅PV₂Mo₁₀O₄₀ in which the vanadium atoms are nearest neighbors and it is suggested that this isomer is more likely to be reactive in electron‐transfer/oxygen‐transfer reaction oxidation reactions. In the second (70–65 %) species, the V^IV remains embedded within the polyoxometalate framework and originates from reduction of distal H₅PV₂Mo₁₀O₄₀ isomers to yield an intact cluster, [PV^IVV^VMo₁₀O₄₀]^6?.     

Electrode reactions of tungstate ions in the presence of carbon monoxide

Tungstate ions may be reversibly reduced at platinum, rhodium and mercury electrodes in phosphoric acid according to the reaction WO₄^2-+e^- ? WO₄^3-. The specific rate constants (k_s) on Pt, Rh and Hg are 1.2.10^-2, 7.0.10^-3, and 6.5.10^-4 cm/sec, respectively. In the presence of carbon monoxide, hydrogen evolution at Pt and Rh is blocked while the electron transfer for tungstate reduction is unhindered. This is used as a criterion for a surface dissociation or recombination step in an electrochemical reaction. Two methods may be used with platinum or rhodium electrodes for the determination of tungstate, either rotating the electrode at a constant speed and measuring the diffusion current, or measuring the reduction peak height at a constant potential scan rate.     

Viologen-modified platinum clusters acting as an efficient catalyst in photocatalytic hydrogen evolution

A highly efficient photocatalytic system for hydrogen evolution with dihydronicotinamide coenzyme (NADH) as a sacrificial agent in an aqueous solution has been constructed by using water-soluble platinum clusters functionalized with methyl viologen-alkanethiol (MVA2+) and a simple electron-donor dyad, 9-mesityl-10-methylacridinium ion (Acr+-Mes), which is capable of fast photoinduced electron transfer but extremely slow back electron transfer. The mean diameter of the platinum core was determined as R(CORE) = 1.9 nm with a standard deviation sigma = 0.5 nm by transmission electron microscopy (TEM). As a result, the hydrogen-evolution rate of the photocatalytic system with MVA2+-modified platinum clusters (MVA2+-PtC) is 10 times faster than the photocatalytic system with the mixture of the same amount of MVA2+ and platinum clusters as that of MVA2+-PtC under otherwise the same experimental conditions. The radical cation of NADH has been successfully detected by laser flash photolysis experiments. The decay of the absorbance due to NAD*, produced by the deprotonation from NADH*+, coincides with the appearance of the absorption band due to Acr*-Mes. This indicates electron transfer from NAD* to Acr+-Mes to give Acr*-Mes, which undergoes the electron-transfer reduction of MVA2+-PtC, leading to the efficient hydrogen evolution.     

Oscillatory gas evolution during the visible-light-induced cleavage of water in platinum-free clay-hydroxide suspensions

The visible light irradiation of a mixed colloidal system prepared from sepiolite—Ru(bpy)₃²⁺RuO₂ (bpy  2,2′-bipyridine) composite particles and Al_xEu_1−x(OH)₃ gels leads to the oscillatory evolution of hydrogen and oxygen. With respect to the previously reported results obtained with Al_xEu_1−x(OH)₃Pt, the following differences were obtained: (i) the amplitude of the oscillations and the total amount of gas are much smaller; (ii) the descending part of the oscillations is more gradual; (iii) the oscillations are no longer damped. All these features are related to the catalytic properties of platinum. The origin of the oscillations (either physical or chemical) is briefly discussed.     

Atomically Dispersed Copper–Platinum Dual Sites Alloyed with Palladium Nanorings Catalyze the Hydrogen Evolution Reaction

Designing highly active catalysts at an atomic scale is required to drive the hydrogen evolution reaction (HER). Copper–platinum (Cu‐Pt) dual sites were alloyed with palladium nanorings (Pd NRs) containing 1.5 atom % Pt, using atomically dispersed Cu on ultrathin Pd NRs as seeds. The ultrafine structure of atomically dispersed Cu‐Pt dual sites was confirmed with X‐ray absorption fine structure (XAFS) measurements. The Pd/Cu‐Pt NRs exhibit excellent HER properties in acidic solution with an overpotential of only 22.8 mV at a current density of 10 mA cm⁻² and a high mass current density of 3002 A g⁻¹_(Pd+Pt) at a −0.05 V potential.     

Studies of the process of catalytic hydrogen evolution on the epoxy-resin-impregnated graphite-based mercury film electrode

The phenomenon of catalytic hydrogen evolution in the presence of platinum salts on the epoxy-resin-impregnated graphite-based mercury film electrode was studied. Two measurements were performed: (i) the deposition of catalyst on the IGE surface from HCl solution containing H₂PtCl₆ and HgCl₂; (ii) the voltammetric curve of a solution of HCL + KCl was recorded. The effect of some parameters on the catalytic hydrogen evolution peak potential was studied. The investigations performed have shown that the four following parameters have a decisive effect on the form of the CHE curve: (1) Hg²⁺ concentration in the deposition step; (2) deposition potential value; (3) deposition time; (4) Pt(IV) concentration during the deposition step. It was established that, with increasing amount of Pt deposited on the IGE, a considerable shift of the CHE peak potential towards positive values occurred. A linear potential dependence of the CHE peak on log c_Pt was obtained when the Pt(IV) concentration was changed from 4×10⁻⁹M to 2×10⁻⁶M. That dependence can be employed well for analytical purposes.     

Au@Pt Core-Shell Nanoparticle Bioconjugates for the Therapy of HER2+ Breast Cancer and Hepatocellular Carcinoma. Model Studies on the Applicability of 193mPt and 195mPt Radionuclides in Auger Electron Therapy

^193mPt and ^195mPt radionuclides are therapeutically attractive Auger electron emitters with notably high Auger electron yield per decay. The present paper summarizes the first step of research on the applications of core-shell (Au@Pt) nanoparticles for electron Auger therapy of HER2+ (human epidermal growth factor receptor 2) breast cancer and hepatocellular carcinoma. Gold nanoparticles (30 nm) were synthesized covered with a platinum shell at high efficiency (>80%) and were further evaluated for in vitro studies such as binding affinity, internalization and cytotoxicity. To find the mechanism(s) responsible for platinum cytotoxicity in HepG2 cells, the platinum concentration in isolated cell nuclei and cytoplasm was determined using ICP-MS (inductively coupled plasma mass spectrometry). Lack of platinum in cell nuclei suggests that the cytotoxic effect is associated with the generation of reactive oxygen species (ROS) and reactive nitrogen species (RNS). Studies carried out on the SKOV-3 cell line with the use of a synthesized targeting bioconjugate (Au@Pt-PEG-trastuzumab) revealed a high affinity of this preparation to HER2+ cells, its internalization, its placement in the perinuclear area and partial intranuclear location. The specific binding for HER2 negative cells, MDA-MB-231, was negligible and Au@Pt-PEG-trastuzumab did not enter these cells. The results obtained are promising and warrant future investigation of Auger electron therapy using ^193mPt and ^195mPt based radiopharmaceuticals.     

Hydrogen evolution from water by use of viologen polymers as electron transfer catalyst

The behavior of viologen polymer (P-V²⁺) as an electron transfer catalyst in the reaction of hydrogen generation was studied. In the photoirradiation system, which contains triethanolamine (TEA), Ru(bpy)³⁺₃, and P-V²⁺, the amount of hydrogen evolution was less than methyl viologen (MV²⁺); P-V²⁺, however, was more effective in sodium dithionite as the electron donor and showed higher initial rates than MV²⁺.     

Platinum Oxide Nanoparticles for Electrochemical Hydrogen Evolution: Influence of Platinum Valence State

Electrochemical hydrogen generation is a rising prospect for future renewable energy storage and conversion. Platinum remains a leading choice of catalyst, but because of its high cost and low natural abundance, it is critical to optimize its use. In the present study, platinum oxide nanoparticles of approximately 2 nm in diameter are deposited on carbon nitride (C3N4) nanosheets by thermal refluxing of C3N4 and PtCl₂ or PtCl₄ in water. These nanoparticles exhibit apparent electrocatalytic activity toward the hydrogen evolution reaction (HER) in acid. Interestingly, the HER activity increases with increasing Pt⁴⁺ concentration in the nanoparticles, and the optimized catalyst even outperforms commercial Pt/C, exhibiting an overpotential of only −7.7 mV to reach the current density of 10 mA cm⁻² and a Tafel slope of −26.3 mV dec⁻¹. The results from this study suggest that the future design of platinum oxide catalysts should strive to maximize the Pt⁴⁺ sites and minimize the formation of the less active Pt²⁺ species.     

The synthesis of hydrocarbons from CO and H2 over well-characterized supported PtFe catalysts

A combination of Mössbauer spectroscopy, chemisorption measurements and kinetic studies of the synthesis of hydrocarbons from CO and H₂ has been employed to investigate alumina-supported Fe, Pt and PtFe catalysts. Three different chemical states of Fe have been identified in the reduced catalysts: ferrous ions, ferromagnetic Fe metal and PtFe bimetallic clusters. The relative amounts of these species depend upon the Pt:Fe ratio and the total metal loading.At low concentrations (0.1%) iron was not reduced by hydrogen beyond the ferrous state at 773 K, and the ferrous ions had no catalytic activity in the synthesis reaction. However, catalysts containing 1.75 wt.% Pt and varying amounts of Fe showed marked differences. The addition of this amount of platinum to iron on alumina led to the formation of PtFe clusters on reduction. At high Pt:Fe ratios (∼5) all of the iron combined with platinum to form clusters. The iron in the PtFe clusters was catalytically inert in the CO/H₂ synthesis reaction and the clusters exhibited essentially the catalytic behaviour of platinum. At low Pt:Fe ratios (0.1) ferromagnetic iron metal, in addition to Fe²⁺ ions and PtFe clusters, was produced. The ferromagnetic iron dominated the activity and selectivity pattern of the catalyst because of the higher specific activity of the iron compared to Pt or PtFe clusters.The inert behavior of the iron in PtFe clusters is attributed to a decrease in the electron density on the iron as indicated by the Mössbauer isomer shift. The direction of electron transfer is opposite to that proposed for alkalimetal-promoted Fe catalysts on which the production of higher molecular weight species and the activity was enhanced.Finally, this investigation shows that bimetallic alloys with stoichio-metries identical to the metal loading are not necessarily formed in reduced supported catalysts.     

N-alkyl-4, 4'-Bipyridyl. A new efficient electron carrier in the photochemical hydrogen producing system

Various pyridine derivatives were found to be effective electron carriers in the photochemical hydrogen generating system consisting of ascorbic acid as a reproducible electron donor, zinc meso-tetra (4-hydroxysulphophenyl) porphyrin as a photocatalyst. and colloidal platinum. Among them the system using an N-alkyl-4, 4'-bipyridyl, especially the n-hexyl derivative (C₆Py⁺Py), was most effective, where the quantum yield for the hydrogen generation was 0.1 and the theoretical recycling number of the photocatalyst reached 100,000. The mechanism of the hydrogen generating reaction is discussed. Based on a number of standard experiments, C₆Py⁺Py is shown to be effective only in the specific combination with the present components.     

Broadband Light Harvesting and Unidirectional Electron Flow for Efficient Electron Accumulation for Hydrogen Generation

The efficiency of solar hydrogen evolution closely depends on the multiple electrons accumulation on the catalytic center for two‐electron‐involved water reduction. Herein, we report an effective approach to enable broadband light absorption and unidirectional electron flow for efficiently accumulating electrons at active sites for hydrogen evolution by rationally engineering the nanostructure of Pt nanoparticles (NPs), TiO₂, and SiO₂ support. In addition to Schottky‐junction‐driven electron transfer from TiO₂ to Pt, Pt NPs also produce hot electrons by recycling the scattered visible and near‐infrared (vis‐NIR) light of the support. Unidirectional electron flow to active sites is realized by tuning the components spatial distribution. These features collectively accumulate multiple electrons at catalytic Pt sites, thereby affording enhanced activity toward hydrogen evolution under simulated sunlight.     

Photocatalytic generation of hydrogen from water using a platinum(II) terpyridyl acetylide chromophore

The cationic complex [Pt(tolylterpyridine)(phenylacetylide)]+ has been used as a photosensitizer for the reduction of aqueous protons in the presence of a sacrificial electron donor to make H2. In this system, triethanolamine (TEOA) acts as the sacrificial reducing agent, methyl viologen (MV2+) serves as an electron transfer agent, and colloidal Pt stabilized by polyacrylate functions as the catalyst for H2 generation. The Pt(II) chromophore undergoes both oxidative and reductive quenching, but H2 is only seen when both TEOA and MV2+ are present. Irradiation of the reaction solution for 10 h with lambda > 410 nm leads to 85 turnovers and an overall yield of 34% based on TEOA. While H2 evolution is maximized for the system at pH 7, it is also seen at pH 5 and 9, in contrast with earlier reports using Ru(bpy)32+ as the photosensitizer. This is the first time that a Pt diimine or terpyridyl complex has been used as the photosensitizer for H2 generation from aqueous protons.