Electro-reduction of oxygen and electro-oxidation of methanol at Pd monolayer-modified macroporous Pt electrode

With polystyrene latex spheres self-assembled on indium tin oxide-coated glass electrode as templates, highly ordered macroporous Pt was prepared by electrochemical deposition. Then, the macroporous Pt was modified by Pd monolayer involving the galvanic displacement of Cu monolayer formed by under-potential deposition on macroporous Pt. Electrocatalytic properties of the Pd-modified macroporous Pt electrode for oxygen reduction were investigated by cyclic voltammetry and chronoamperometry in O₂-saturated solution containing 0.1 M HClO₄. Methanol electro-oxidation on the Pd-modified macroporous Pt surfaces in 0.5 M H₂SO₄ containing 1 M CH₃OH was studied by cyclic voltammetry. The corresponding results showed that Pd-modified macroporous Pt electrode had negative catalytic activity for methanol oxidation in compared with macroporous Pt. However, Pd-modified macroporous Pt electrode had positive electrocatalytic activity to O₂ reduction.     

Initial stages of Pt deposition on Au(111) and Au(100)

The deposition of Pt onto unreconstructed Au(111) and Au(100) was studied with cyclic voltammetry and in-situ STM. The latter revealed that in [PtCl₄]²⁻ containing electrolytes, both surfaces are covered by an ordered adlayer of the complex. For the adsorbed [PtCl₄]²⁻ a slightly compressed (√7×√7) R19.1°-structure was assumed for Au(111) and a (3×√10) for Au(100). In both cases, a rather high overpotential for Pt deposition was observed, most probably due to the high stability of the [PtCl₄]²⁻ complex. Nucleation of Pt starts mainly at defects like step edges for low deposition rates and three-dimensional clusters are formed. Due to the high overpotential, some nuclei appear also on terraces at random sites. Higher coverages of Pt lead to a cauliflower like appearance. It is not possible to dissolve the platinum clusters at positive potentials without severely roughening the gold surface. The [PtCl₄]²⁻ complex is oxidized to the [PtCl₆]²⁻ complex at about 0.7 V, when metallic Pt is on the surface.     

Graphite Electrodes Modified with Platinum‐Nickel Nano‐Particles for Methanol Oxidation

Methanol electro‐oxidation is investigated at graphite electrodes modified with various platinum and nickel nano‐particle deposits using cyclic voltammetry. The modified electrodes are prepared by the simultaneous electrodeposition of metals from their salt solutions using potentiostatic and galvanostatic techniques. They show enhanced catalytic activity towards methanol oxidation in KOH solution. The catalytic activity of platinum nano‐particles is found to be significantly affected by the presence of relatively small amounts of nickel deposits. A comparison is made between the electrocatalytic activity of Pt/C and (Pt‐Ni)/C electrodes. The results show that the methanol electro‐oxidation current increases with an increase in the nickel content. In particular, the highest catalytic activity is achieved for platinum to nickel deposits of 95%:5% (wt.‐%), in other cases the catalytic activity decreases. It is found that Ni enhances the catalytic activity of Pt by increasing the number of active sites, as well as through an electron donation process from Ni to Pt. This process takes place once the nickel hydroxide (Ni(OH)₂)/nickel oxy‐hydroxide (NiOOH) transformation begins. The effect of the methanol concentration on the methanol oxidation reaction is investigated. The order of reaction, with respect to methanol, at the modified (Pt‐Ni)/C electrode is found to be 0.5.     

Reduction of hexavalent chromium by copper

The galvanic reaction of metallic copper in Cr(VI)-laden aqueous solutions of varying pH was examined by in situ u.v.–visible spectrophotometry, rotating disc electrode chronopotentiometry and cyclic voltammetry. The galvanic reaction in 0.2 M H₂SO₄ solutions was pseudo first order in Cr(VI) concentration. Experiments with both magnetically stirred solutions and a copper mesh or a copper film in a rotating disc electrode configuration revealed the reaction to be diffusion-controlled with respect to Cr(VI) transport to the copper surface. Finally, cyclic voltammetry data in Cr(VI)-laden media of varying pH underline the important role of protons in the galvanic reaction.     

Gold dispersed on TiO2 and SiO2: adsorption properties and catalytic behavior in hydrogenation reactions

Titania-supported Au catalysts were given both low temperature reduction and high temperature reduction at 473 and 773 K, respectively, and their adsorption and catalytic properties were compared to identically pretreated Pt/TiO₂ catalysts and pure TiO₂ samples as well as Au/SiO₂ catalysts. This was done to determine whether a reaction model proposed for methanol synthesis over metals dispersed on Zn, Sr and Th oxides could also explain the high activities observed in hydrogenation reactions over MSI (Metal-Support Interaction) catalysts such as Pt/TiO₂. This model invokes O vacancies on the oxide support surface, formed by electron transfer from the oxide to the metal across Schottky junctions established at the metal-support interface, as the active sites in this reaction. The similar work functions of Pt and Au should establish similar vacancy concentrations, and O₂ chemisorption indicated their presence. However, these Au catalysts were completely inactive for CO and acetone hydrogenation, and ethylene hydrogenation rates were lower on the supported Au catalysts than on the supports alone. Consequently, this model cannot explain the high rate of the two former reactions over TiO₂-supported Pt although it does not contradict models invoking specialinterfacial sites.     

A novel method for the preparation of bi-metallic (Pt–Au) nanoparticles on boron doped diamond (BDD) substrate: application to the oxygen reduction reaction

A novel method was developed to synthesize bi-metallic nanoparticles (Au–Pt) on boron-doped diamond (BDD) substrate. This method consisted of (a) deposition of a small amount of gold (equivalent to a few monolayers) by sputtering on the BDD surface, (b) heat treatment of the obtained sample at 600 °C in air, resulting in the formation of stable nanoparticles on BDD (Au/BDD electrode), (c) electrodeposition of Pt on the Au/BDD surface occurring preferentially on the Au nanoparticles, and finally (d) heat treatment at 400 °C to enhance the interaction between Au and Pt. The ratio between Au and Pt nanoparticles can be modified by modifying the amount of electrodeposited Pt and was estimated using cyclic voltammetry. These Pt-Au/BDD composite electrodes were used to study oxygen reduction using both potential sweep (cyclic voltammetry) and hydrodynamic (turbine electrochemical cell) methods.     

Porous Co/Co–Ni–Pt nanostructures prepared by galvanic replacement towards methanol electro-oxidation

The nanostructured Co/Co–Ni–Pt catalyst were synthesized by electrodeposition process and galvanic replacement reaction. The alloy prepared on a copper electrode (Cu/Co/Co–Ni–Zn) was dipped in platinum containing alkaline solution to produce a porous Cu/Co/Co–Ni–Pt catalyst. The catalyst was characterized by energy dispersive X-ray and scanning electron microscopy techniques and its electrocatalytic properties were evaluated using cyclic voltammetry, electrochemical impedance spectroscopy and chronoamperometry techniques. The results showed that the Co/Co–Ni–Pt coatings are porous, and composed of discrete Pt nanoparticles with the crystallite size of about 66 nm. It was shown from cyclic voltammograms in alkaline solutions that the oxidation current of methanol on the nanostructured Cu/Co/Co–Ni–Pt electrode was much higher than that on flat platinum. Electrochemical impedance spectra on the Co/Co–Ni–Pt electrode reveal that the charge transfer resistance decreases with the increase of anodic potentials. All results show that the Co/Co–Ni–Pt catalysts can be potential anode catalysts for the direct methanol fuel cell.     

An innovative approach to electro-oxidation of dopamine on titanium dioxide nanotubes electrode modified by gold particles

Au/TiO₂/Ti electrodes were prepared by galvanic deposition of gold particles from an acidic bath containing KAu(CN)₂ in the presence of a citrate buffer onto TiO₂ nanotubes layer on titanium substrates. Titanium oxide nanotubes were fabricated by anodizing titanium foil in a DMSO fluoride-containing electrolyte. The morphology and surface characteristics of Au/TiO₂/Ti electrodes were investigated using scanning electron microscopy and energy-dispersive X-ray, respectively. The results indicated that gold particles were homogeneously deposited on the surface of TiO₂ nanotubes. The nanotubular TiO₂ layers consist of individual tubes of about 40–80 nm diameters. The electro-catalytic behavior of Au/TiO₂/Ti electrodes for the dopamine electro-oxidation was studied by cyclic voltammetry and differential pulse voltammetry. The results showed that Au/TiO₂/Ti electrodes exhibit a considerably higher electro-catalytic activity toward the oxidation of dopamine. The catalytic oxidation peak current showed a linear dependence on dopamine concentration and a linear calibration curve was obtained in the concentration range of 0.5–2.5 mM of dopamine.     

The origin of high activity of Pt–Au surfaces in the formic acid oxidation

We have investigated the oxidation of HCOOH on Au-modified Pt (Au@Pt) and Pt-modified Au (Pt@Au) electrodes to establish the role of the ensemble and the electronic effect in the catalysis of this reaction. The surfaces were modified by the electrodeposition of sub-monolayers. Cyclic voltammetry in the supporting electrolyte and CO_ads stripping have shown that the electronic Pt–Au interactions are insignificant on Au@Pt, but considerable on Pt@Au. Oxidation of CO_ads on Au@Pt commenced at the same potentials as on bare Pt, whereas on Pt@Au the oxidation was delayed for 0.2 V. Both types of Pt–Au surfaces were found to be more active toward the oxidation of HCOOH compared to bare Pt, exhibiting the maximum activity at Pt fraction between 0.15 and 0.25 with the enhancement factor of more than two orders of magnitude. On bimetallic surfaces the amount of CO_ads formed by HCOOH dehydration was low and decreased with decreasing Pt fraction. It was concluded that the high activity of Pt–Au surfaces is caused by the increased selectivity toward the HCOOH dehydrogenation. Similar behavior of Pt–Au surfaces in the HCOOH oxidation, regardless on the Pt–Au electronic interactions, leads to the conclusion that the ensemble effect plays dominant role in the catalysis of HCOOH oxidation.     

Au-Promoted Pt nanoparticles supported on MgO/SBA-15 as an efficient catalyst for selective oxidation of glycerol

A systematic study of Au-promoted and unpromoted Pt/MgO/SBA-15 catalyst is developed to separate the promoter effect from electron transfer effect between Au and Pt. Multi-characterizations revealed that Au and Pt metals in these bimetallic catalysts mainly exist in the form of alloy, and the main role of Au is to reduce the size of AuPt alloy nanoparticles, thus enhancing the adsorption and activation of intermediate products. Through the optimization of various factors (including MgO content, Au/Pt molar ratio, reaction temperature and time), the Au₁Pt₂/MgO/SBA-15 (0.05) catalyst exhibits excellent catalytic activity and glyceric acid selectivity for the selective oxidation of glycerol. Density functional theory calculation confirmed that the synergistic effect between Pt and Au active sites could facilitate the oxidation of primary hydroxyl group by promoting the activation of C H bond and the oxidation of aldehyde group. The results may give insights on designing effective Pt based bimetallic catalyst for selective oxidation of glycerol.     

In situ ATR-FTIR study of bulk CO oxidation on a polycrystalline Pt electrode

Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) measurements were conducted to analyze the electrochemical oxidation of CO dissolved in bulk electrolyte solution on a polycrystalline Pt electrode during linear sweep (cyclic) voltammetry (CV) in 1% and 100% CO-saturated 0.1 M HClO₄. A CO adlayer was first formed on Pt at 0.05 V vs. the reversible hydrogen electrode (RHE) during CO bubbling in each electrolyte, followed by the CV measurement. In 1% CO-saturated 0.1 M HClO₄, a well-developed prepeak was found, showing an onset and a peak at around 0.25–0.3 V and 0.42 V, respectively. A major contribution of bridge-bonded CO, CO(B), to the prepeak is concluded based on a decrease in the corresponding band intensity. In 100% CO-saturated 0.1 M HClO₄, the onset of bulk CO oxidation takes place around 0.25–0.3 V, which is closely associated with a band intensity decrease of CO(B), whereas atop (linear) CO, CO(L), did not exhibit intensity change in this potential region. This suggests that vacant sites made available upon oxidation of CO(B) serve as active sites for bulk CO oxidation. The oxidation of CO(B) at such low potentials is interpreted in terms of an adsorption energy on Pt that is lower than that for CO(L) and also of the specific structure of an adlayer that consists of intermixed CO(L) and CO(B). The bulk CO oxidation becomes diffusion-limited by dissolved CO above ca. 0.72 V, where we observed hardly any infrared spectral features ascribed to reaction intermediates. During a negative-going scan back to 0.05 V from 1.2 V, a steep decrease of the bulk CO oxidation current takes place around 0.66 V, at which preferential adsorption of CO(L) is observed. A rigid CO(L) island formation is strongly suggested from its high CO stretching frequency vs. its very small initial coverage and from its subsequent dependence on potential, with a linear Stark shift characterized by a slope of −29 cm⁻¹/V. Such an island formation is in marked contrast to the adlayer structure with intermixed CO(L) and CO(B) initially formed at 0.05 V during CO bubbling. It is concluded that the Pt surface saturated with the CO adlayer formed initially at 0.05 V exhibits a low overpotential for bulk CO oxidation owing to its adlayer structure with intermixed atop and bridge-bonded CO.     

Electrochemical reduction of O<Subscript>2</Subscript> and NO on Ni,Pt and Au

The electrochemical reduction of oxygen and nitric oxide was studied using cyclic voltammetry on point electrodes of Ni, Pt and Au in the temperature range 400–600 °C. All the materials were more active towards the reduction of oxygen than towards the reduction of nitric oxide, except Pt at 400 °C. As a general trend it was observed that the activity of the materials towards the reduction of oxygen increases more than the activity of the materials towards the reduction of nitric oxide with increasing temperature. The Ni wire was covered with a layer of NiO. The NiO layer inhibits the reduction of both nitric oxide and oxygen. No sign of the formation of an oxide layer on the platinum and gold wires was observed. Pt was the most active electrode material towards the reduction of nitric oxide. Au showed almost no activity towards the reduction of nitric oxide, but Au was active towards the reduction of oxygen. This implies that the triple-phase boundary (3PB) catalyses the reduction of oxygen but not the reduction of nitric oxide, as Au is a catalytic inactive metal. All the materials were more active towards oxidation than towards reduction that is evolution of oxygen or oxidation of nitric oxide to nitrogen dioxide was more pronounced than the reduction reactions.     

Direct Electrochemistry and Electrocatalysis of Hemoglobin on Bimetallic Au–Pt Inorganic–Organic Nanofiber Hybrid Nanocomposite and Mesoporous Molecular Sieve MCM-41

A glassy carbon electrode modified with MCM-41 and bimetallic inorganic–organic nanofiber hybrid nanocomposite was prepared and used for determination of trace levels of hydrogen peroxide (H₂O₂). The direct electron transfer (DET) and electrocatalysis of hemoglobin (Hb) entrapped in the MCM-41 modified Au–Pt inorganic–organic nanofiber hybrid nanocomposite electrode (Au–PtNP/NF/GCE) were investigated by using cyclic voltammetry in 0.1 M pH 7.0 phosphate buffer solution. Due to its uniform pore structure, high surface area and good biocompatibility, the mesoporous silica sieve MCM-41 provided a suitable matrix for immobilization of biomolecules. The MCM-41 modified Au–Pt inorganic–organic nanofiber hybrid nanocomposite electrode showed significant promotion to DET of Hb, which exhibited a pair of well-defined and quasi-reversible peaks for heme Fe(III)/Fe(II) with a formal potential of ?0.535 V (vs. Ag/AgCl). Additionally, the Hb immobilized on the MCM-41 modified electrode showed excellent electrocatalytic activity toward H₂O₂ reduction.     

Carbon-supported Pt(Cu) electrocatalysts for methanol oxidation prepared by Cu electroless deposition and its galvanic replacement by Pt

Bimetallic Pt–Cu carbon-supported catalysts (Pt(Cu)/C) were prepared by electroless deposition of Cu on a high surface area carbon powder support, followed by its partial exchange for Pt; the latter was achieved by a galvanic replacement process involving treatment of the Cu/C precursor with a chloroplatinate solution. X-ray diffraction characterization of the Pt(Cu)/C material showed the formation of Pt-rich Pt–Cu alloys. X-ray photoelectron spectroscopy revealed that the outer layers are mainly composed of Pt and residual Cu oxides, while metallic Cu is recessed into the core of the particles. Repetitive cyclic voltammetry in deaerated acid solutions in the potential range between hydrogen and oxygen evolution resulted in steady-state characteristics similar to those of pure Pt, indicating the removal of residual Cu compounds from the surface (due to electrochemical treatment) and the formation of a compact Pt outer shell. The electrocatalytic activity of the thus prepared Pt(Cu)/C material toward methanol oxidation was compared to that of a commercial Pt/C catalyst as well as of similar Pt(Cu)/C catalysts formed by simple Cu chemical reduction. The Pt(Cu)/C catalyst prepared using Cu electroless plating showed more pronounced intrinsic catalytic activity toward methanol oxidation than its counterparts and a similar mass activity when compared to the commercial catalyst. The observed trends were interpreted by interplay between mere surface area effects and modification of Pt electrocatalytic performance in the presence of Cu, both with respect to methanol oxidation and poisonous CO removal.     

用磷钼酸修饰甲醇燃料电池的铂电极

近年来以杂多化合物为基础的催化体系受到广泛的关注.为了研究杂多酸与铂电极对甲醇电催化氧化的协同效应,通过循环伏安扫描法制备了磷钼酸（H3PMo12O40）修饰铂电极.通过循环伏安和计时电流法研究了该修饰电极对甲醇氧化的电催化活性和抗中间产物的毒化作用,并比较了该修饰电极与其单酸盐（Na2MoO4）修饰铂电极的性能,测试结果表明：磷钼酸修饰铂电极能够提高对甲醇氧化反应的催化活性,基本上同其单酸盐Na2MoO4修饰铂电极的催化活性相当,并且这种促进作用主要是由Mo原子价态变化引起的.同时计时电流曲线测试结果表明,该修饰电极具有一定的抗毒化作用,但不如钼酸钠好.     

Improved surface-enhanced Raman scattering based on Ag-Au bimetals prepared by galvanic replacement reactions

In this work, the improved surface-enhanced Raman scattering (SERS) of Rhodamine 6G (R6G) adsorbed on Ag-Au bimetals synthesized via galvanic replacement of Ag with Au was first investigated. First, silver substrates were roughened by triangular-wave oxidation-reduction cycles (ORC) in aqueous solutions containing 0.1 M KCl. At the same time, Cl⁻ and Au-containing nanocomplexes in solutions were prepared by treating gold substrates with the similar electrochemically roughening procedures. Then the roughened Ag substrates were incubated in the Cl⁻ and Au-containing solutions for different couples of minutes to undergo the galvanic replacement reactions. Encouragingly, the SERS of R6G adsorbed on this roughened Ag substrate modified by the replacement of Ag with Au for 3 min exhibits a higher intensity by one order of magnitude and a better resolution, as compared with the SERS of R6G adsorbed on an unmodified roughened Ag substrate. The increased SERS effect can be ascribed to the compositions of complexes formed on the substrates. In this optimum condition, the atomic ratio of Ag to Au is ca. 6.6.     

Copper (hydr)oxide modified copper electrode for electrocatalytic oxidation of hydrazine in alkaline media

Stable copper (hydr)oxide modified copper electrode was prepared by cyclic voltammetry in 0.1 M NaOH solution in the potential range of −300 to 800 mV. In the first cycle the oxidation peaks of copper were observed but in the second and next cycles, they were omitted and a clean background was obtained. This indicates that an irreversible electrochemical transformation has been achieved during the first cycle and a stable layer of hydr(oxide) formed on the surface of the copper electrode. This layer protects the electrode from corrosion. This electrode can be used for electrochemical studies in the potential range of −300 to 800 mV without any interfering effects by the oxidation peaks of copper. The modified electrode was used for electrocatalytic oxidation of hydrazine. Results showed that on the bare copper electrode the oxidation peak of 10 mM hydrazine appear at 380 mV while on the copper (hydr)oxide modified copper electrode, it appear at 260 mV. About 120 mV negative shift of the peak potential indicated the catalytic activity of (hydr)oxide layer for hydrazine. The kinetic parameters were investigated by using cyclic voltammetry and chronoamperometry.     

Nanostructured Pt-M/C (M = Fe and Co) catalysts prepared by a microemulsion method for oxygen reduction in proton exchange membrane fuel cells

Nanostructured Pt-M/C (M = Fe and Co) catalysts have been synthesized by a microemulsion method and a high-temperature route. They have been characterized by cyclic voltammetry in 1 M H₂SO₄ and for oxygen reduction in proton exchange membrane fuel cells (PEMFC). The Pt-M alloy catalysts synthesized by the microemulsion method show higher electrochemical active surface area than those prepared by the high-temperature route, and some of them exhibit improved catalytic activity towards oxygen reduction compared to pure Pt. Among the various alloy catalysts investigated, the Pt-Co/C catalyst prepared by the microemulsion method shows the best performance with the maximum catalytic activity and minimum polarization loss. Mild heat treatment of the catalysts prepared by the microemulsion method at moderate temperatures (200 °C) in reducing atmosphere is found to improve the catalytic activity due to a cleaning of the surface and an increase in the electrochemical surface area.     

Enhanced electrochemical activity of Pt nanowire network electrocatalysts for methanol oxidation reaction of fuel cells

In this work, Pt nanowire networks supported on high surface area carbon (Pt NWNs/C) are synthesized as electrocatalysts for direct methanol fuel cells (DMFCs). The electrocatalytic behavior of Pt NWNs/C catalysts for the methanol and adlayer CO oxidation reactions is investigated and the results are compared with the Pt nanoparticles (NPs) supported on carbon (Pt NPs/C). The results indicate that Pt NWNs are characterized by interconnected nanoparticles with large number of grain boundaries, downshifted d-band center and reduced oxophilicity, which results in the enhanced surface mobility of oxygen-containing species such as CO_ads and OH_ads. The enhanced surface mobility of CO_ads and OH_ads in turn facilitates the removal of intermediate CO species during the methanol oxidation. The activity of the Pt NWNs/C electrocatalyst for the methanol oxidation reaction and electrooxidation of adsorbed CO is also evaluated by cyclic voltammetry, CO stripping, and kinetic analysis. The results show that Pt NWNs/C catalysts have a significantly higher electrocatalytic activity for the methanol oxidation reaction as compared to Pt NPs/C catalysts. The enhanced electrocatalytic activity of Pt NWNs/C catalysts is mainly due to the existence of large number of the grain boundaries of the interconnected nanoparticles of the unique Pt NWN structure.