Nanostructured catalysts for direct electrooxidation of dimethyl ether based on Bi- and trimetallic Pt–Ru and Pt–Ru–Pd alloys prepared from coordination compounds

Bi- and trimetallic platinum–ruthenium and platinum–ruthenium–palladium catalysts with specified atomic ratios Pt: Ru = 1: 1 and Pt: Ru: Pd = 1: 1: 0.1, respectively, were synthesized from the coordination compounds of the metals deposited on highly dispersed carbon black. The catalysts were characterized by powder X-ray diffraction, electron dispersive analysis, and transmission electron microscopy. According to voltammetry data, the highest activity in the dimethyl ether (DME) electrooxidation is exhibited by the catalyst Pt_0.43Ru_0.47Pd_0.1/C; hence, it may be considered as a promising anode material for direct DME fuel cells.     

Decorating Pt/C Nanoparticles with Ru by Wall‐Jet Configuration: The Role of Coverage Degree on the Catalyst Activity for Glycerol Electrooxidation

Here, we built Ru‐decorated Pt/C nanoparticles with different coverage degrees (θ_Ru) by wall‐jet configuration for the first time, and we investigated their catalytic properties towards glycerol electrooxidation in acidic medium. Moreover, we used the most active catalysts as the anode in electrolysis to produce carbonyl compounds. The use of an electrochemical cell in wall‐jet configuration allows for the controlling of electrodeposition through easily handling parameters; namely, the θ_Ru is controlled by changing the concentration of the metallic precursor, speed, and volume of injection onto a Pt/C‐modified glassy carbon electrode under applied potential. Excess of Ru on a Pt surface inhibits glycerol dissociative adsorption, which limits further electrooxidation; whereas low θ_Ru do not provide surface oxygen species to the anodic reaction. Hence, intermediates θ_Ru reveal active catalysts – namely, θ_Ru=0.42 shifts the onset potential 170 mV towards lower values and increases 1.65‐fold the current density at 0.5 V. The stability of this catalyst is also enhanced by maintaining a more constant current density during successive potential cycles in the presence of glycerol and by avoiding Ru leaching from the surface. The electrolysis on Ru‐decorated Pt/C is shown to lead the reaction towards formic acid (‘high oxidation state’), decreasing the amounts of glyceradehyde, glycolic acid, and dihydroxyacetone, as a result of the improved catalytic properties.     

Distinctive features of supported catalysts prepared from platinum carbonyl clusters

The formation of Pt/γ-Al₂O₃ and Pt/C catalysts from platinum carbonyl clusters H₂[Pt₃(CO)₆]_n (n = 2, 5) is studied. The strength of interaction between clusters (strong Lewis bases) and the support and the state of platinum in catalysts are governed by the acceptor strength of the support. The formation of a stable platinum compound with a surface of γ-Al₂O₃ (strong Lewis acid) is shown for a Pt/γ-Al₂O₃ catalyst by the method of radial distribution functions. In a Pt/C catalyst containing the same amount of Pt supported on a carbon material known to be a weaker acceptor, metallic platinum is formed along with surface-bonded platinum. Proceeding from the existence of the active phase of catalysts in the form of a surface platinum complex and platinum crystallites, the properties of catalysts are discussed in the complete oxidation of methane and the dehydrogenation of cyclohexane, as well as the high dispersity of platinum and its thermal stability     

Iridium oxide-based nanocrystalline particles as oxygen evolution electrocatalysts

Iridium-based oxides are highly active as oxygen evolving electrocatalysts in PEM water electrolyzers. In this work XRD reveals that Ir-Sn oxides contain a single rutile phase with lattice parameters between those of pure IrO₂ and SnO₂. Addition of Ru leads to the synthesis of a core-shell type material due to the strong agglomeration of Ru colloids during the preparation procedure. The shell of this material consists of an Ir-Sn-Ru oxide deficient in Ru relative to the bulk. This leads to a decrease in the surface noble metal concentration (as found by XPS), which in turn results in a significant reduction in electrochemically active surface area. Polarization analysis indicates that the addition of Ru can influence the rate-determining step or mechanism by which oxygen is evolved. In a PEM water electrolysis cell, small additions of Sn do not significantly reduce the operating performance, however larger additions cause a performance loss due to a reduction in active surface area and increased ohmic resistance. When a pure IrO₂ anode is used, a cell voltage is 1.61 V at 1 A cm⁻² and 90°C. Published in Russian in Elektrokhimiya, 2006, Vol. 42, No. 10, pp. 1260–1267. The text was submitted by the authors in English.     

Pyrolysis of chloroplatinic acid to directly immobilize platinum nanoparticles onto multi-walled carbon nanotubes

Platinum nanoparticles supported on multi-walled carbon nanotubes (Pt/MWCNTs) were first prepared by simple pyrolysis of H₂PtCl₆ solution. The structure of Pt/MWCNTs was characterized with X-ray diffraction (XRD), scanning electron microscope (SEM), and the results showed that the diameter of the obtained platinum nanoparticles immobilized on MWCNTs was below 50 nm, although the obtained platinum nanoparticles were not well uniformly dispersed on the surface of MWCNTs. The electrocatalytic performance of Pt/MWCNTs electrode for methanol oxidation reaction (MOR) was also investigated by linear sweep voltammetry (LSV), indicating that it was possible to employ the obtained platinum nanoparticles as anode material in fuel cell. Developing a novel and simple method to prepare platinum nanoparticles onto MWCNTs is the main contribution of this letter. Published in Russian in Elektrokhimiya, 2008, Vol. 44, No. 8, pp. 1050–1053. The text was submitted the authors in English.     

Studies on the role of oxidation states of the platinum surface in electrocatalytic oxidation of small primary alcohols

The role of the oxidation state of a platinum polycrystalline surface in the electrocatalytic oxidation of C₁ to C₄ primary alcohols has been studied by using electrochemical techniques, in situ FTIR spectroscopy and X-ray photoelectron spectroscopy. The results revealed that the oxidation state of the Pt surface plays a key role in the oxidation of primary alcohols, and demonstrated that the oxidation of C₁ to C₄ primary alcohols on a Pt electrode is controlled by the formation of surface oxides on the Pt electrode at different potentials. It was found that the dependence of the reaction process on the oxidation states of the platinum surface yielded similar features in the cyclic voltammogram for oxidation of different primary alcohols at a Pt electrode. According to the effects in the oxidation of primary alcohols, the surface oxides of platinum may be classified as active and poison species. The Pt surface oxides of higher oxidation states (Pt(OH)₃ and PtO₂) formed at potentials above 1.0 V (SCE) were identified as poison species, while other lower oxidation states of Pt surface oxides such as PtOH, Pt(OH)₂ and PtO may be identified as the possible active species for primary alcohol oxidation.     

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].     

Bifunctional versus Defect-Mediated Effects in Electrocatalytic Methanol Oxidation

The most prominent and intensively studied anode catalyst material for direct methanol oxidation fuel cells consists of a combination of platinum (Pt) and ruthenium (Ru). Classically, their high performance is attributed to a bifunctional reaction mechanism where Ru sites provide oxygen species at lower overpotential than Pt. In turn, they oxidize the adsorbed carbonaceous reaction intermediates at lower overpotential; among these, the Pt site-blocking carbon monoxide. We demonstrate that well-defined Pt modified Ru(0001) single crystal electrodes, with varying Pt contents and different local PtRu configurations at the surface, are unexpectedly inactive for the methanol oxidation reaction. This observation stands in contradiction with theoretical predictions and the concept of bifunctional catalysis for this reaction. Instead, we suggest that pure Pt defect sites play a more critical role than bifunctional defect sites on the electrodes investigated in this work.     

The Comparability of Pt to Pt‐Ru in Catalyzing the Hydrogen Oxidation Reaction for Alkaline Polymer Electrolyte Fuel Cells Operated at 80 °C

The Pt‐catalyzed hydrogen oxidation reaction (HOR) for alkaline polymer electrolyte fuel cells (APEFCs) has been one of the focus subjects in current fuel‐cell research. The Pt catalyst is inferior for HOR in alkaline solutions, and alloying with Ru is an effective promotion strategy. APEFCs with Pt‐Ru anodes have provided a performance benchmark over 1 W cm^?2 at 60 °C. The Pt anode is now found to be in fact as good as the Pt‐Ru anode for APEFCs operated at elevated conditions. At 80 °C with appropriate gas back‐pressure, the cell with a Pt anode exhibits a peak power density of about 1.9 W cm^?2, which is very close to that with a Pt‐Ru anode. Even by decreasing the anode Pt loading to 0.1 mg cm^?2, over 1.5 W cm^?2 can still be achieved at 80 °C. This finding alters the previous understanding about the Pt catalyzed HOR in alkaline media and casts a new light on the development of practical and high‐power APFEC technology.     

Platinum electrocatalysts based on oxide supports for hydrogen and methanol fuel cells

Platinum electrocatalysts for fuel cells based on individual oxides Pt/SnO₂ and Pt/TiO₂ and their solid solutions Pt/Ti_1−x M _x O₂ (M = Ru, Nb) and Pt/Sn_1−x M′ _x O_2−δ(M′ = Sb, Ru) were prepared. The influence of the composition of the oxide supports on the activity of the supported platinum catalysts in electrooxidation of methanol and hydrogen in the presence of CO was studied. The prepared platinum catalysts supported on solid solutions of tin dioxide Sn_1−x M _x O_2−δ(M = Sb, Ru; x = 0.4−0.9) and Ti_1−x M _x O₂ (M = Ru, Nb; x = 0.7) exhibited higher tolerance to CO poisoning and higher activities for methanol electrooxidation than commercial Pt,Ru catalysts on carbon support. The use of the proposed oxide supported catalysts in hydrogen and direct methanol fuel cells improved their performances in comparison with that for the fuel cells with traditional Pt,Ru catalysts on carbon support.     

Traitement électrochimique d' eaux usées chargées de phénol : étude comparative sur des électrodes de dioxyde de plomb et de platine

Electrochemical treatment of waste water containing phenol: a comparative study on lead dioxide and platinum electrodes. The objective of this work was to study the efficiency of the Pb/PbO₂ electrode for decomposing the molecule of phenol, then to compare it to a platinum model electrode. Preliminary investigations by cyclic voltammetry showed that the Pb/PbO₂ anode presents a good chemical and electrochemical stability and possesses a high oxygen overvoltage. The study also showed that the electrochemical oxidation of phenol on Pt and PbO₂ in acidic media is a complex process. Long-time electrolysis was carried out using a three potential-plateau program with different values of the oxidation potentials and different concentrations of phenol. The obtained results showed that the transformation of phenol is total on the Pb/PbO₂ anode and that it is partial on Pt. On the other hand an increase in the phenol concentration decreases the rate of its conversion on the electrodes.     

Density functional calculations on CO attached to PtnRu(10−n) (n = 6–10) clusters

B3LYP and SCF‐Xα calculations have been performed on Pt_nRu_(10−n)CO (n = 6–10) clusters. The work aims to simulate the adsorption of CO on the (111) surface of platinum metal and to examine the electronic effects that arise when some Pt atoms are replaced with Ru. Adsorption energies and Pt C and C O stretching frequencies have been calculated for each cluster. Ru does affect the electronic structure of the clusters, the calculated adsorption energies, and frequencies, the Pt C frequency more than the C O. The donation‐backbonding mechanism that accompanies the shift in CO stretching frequency that occurs when CO adsorbs on platinum does not explain the differences in frequency shift observed in CO on various Pt/Ru surfaces. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 77: 589–598, 2000     

Surface characterization of Pt electrodes using underpotential deposition of H and Cu: Part I. Pt(100)

This paper is the first in a series describing the in situ surface characterization of platinum electrodes using H and Cu deposited at underpotentials. The surface of a Pt(100) electrode pretreated by simple flame annealing and quenching in aqueous sulfuric acid is shown to contain a high concentration of defects such as vacancies and self-adsorbed Pt atoms. Adsorbed hydrogen is more strongly bound at these defects than on a uniform Pt(100) surface. Potential cycling in 1 M HCl produces a higher concentration of defects, while oxide formation and reduction in 0.5 M H₂SO₄ has the opposite effect. The nature of (100)-like sites at a polycrystalline platinum electrode is also discussed.     

Study of the local structure of supported nanostructural platinum catalysts

The possibility of controlling the state of platinum deposited on the support surface via minor changes in the catalyst preparation procedure is demonstrated using a series of highly dispersed Pt/γ-Al₂O₃ catalysts with different particle size of the active component. Dispersity, local structure and electronic state of supported platinum were examined by a combination of high resolution transmission electron microscopy and X-ray absorption spectroscopy (EXAFS/XANES). It was shown that various platinum species can be obtained on the surface of the support: bulk or surface Pt(II) or Pt(IV) oxides, mixed metal-oxide structures, bulk particles of metallic platinum, and two-dimensional surface Pt⁰ particles strongly interacting with the support.     

Electrochemical Oxidation of 8‐Oxo‐2′‐Deoxyguanosine on Glassy Carbon,Gold, Platinum and Tin(IV) Oxide Electrodes

Cyclic voltammetry was used to investigate the oxidation of 8‐oxo‐2′‐deoxyguanosine (8‐oxo‐dG) on the glassy carbon (GC), platinum, gold and SnO₂ electrodes over a range of the sweep rate, 8‐oxo‐dG concentration and the solution pH. Reaction mechanism that is common to all these electrodes involves the two‐electron two‐proton charge transfer step followed by the irreversible chemical reaction(s). Rate of the charge transfer reaction decreases with the increasing solution pH (GC, Pt, Au), and depends on the nature of the electrode material following the sequence GC>Pt, Au>>SnO₂. These effects can be related to the degree of oxidation of the electrode surface (Pt, Au, SnO₂), or to the density of the active surface sites (GC). Any of these electrodes can be used for the fabrication of an amperometric detector for 8‐oxo‐dG .     

Electrocatalysis on binary alloys: II. Oxidation of molecular hydrogen on supported Pt+Ru alloys

The electrocatalytic properties of Pt+Ru alloys supported on graphitized carbon have been studied using oxide-free metal alloys that have been well characterized for phase identification, specific metal surface area, and surface composition. The CO tolerance of the Pt+Ru alloys for the oxidation of CO contaminated hydrogen in hot concentrated H₃PO₄ increases monotonically with Ru content of the surface and is a direct result of a decreasing coverage of the alloy by adsorbed CO. Furthermore, the strength of bonding of adsorbed CO with the metal surface decreases dramatically with increasing Ru content in the surface. The absolute activity of Pt+Ru alloys for the oxidation of CO contaminated hydrogen is a complex function of temperature and electrode potential. At 160°C, pure Pt is the most active catalyst at all potentials, but at temperatures lower than 120°C the reaction-limiting current for pure Ru exceeds that of pure Pt. At any temperature from 110–160°C or any electrode potential from 0–0.3V (HE), the variation of electrocatalytic activity with alloy composition indicates only dilution of the activity of the more active component.     

Oxidation of formic acid on platinum electrodeposited on polished and oxidized glassy carbon

Formic acid oxidation at platinum electrochemically deposited on polished (GC/Pt) and oxidized glassy carbon (GC_ox/Pt) was examined with the objective of studying the effect of electrochemical treatment of the support on deposition of platinum and on the activity of Pt catalyst. The electrodes were characterised by STM and XPS techniques. The oxidative treatment of the support leads to deposition of smaller Pt particles in comparison with the one on the polished substrate. The XPS spectra indicated the increased fraction of functional (acidic) groups on the treated support as well as the higher fraction of oxygen containing species on Pt catalyst deposited on oxidised referring to Pt deposited on polished substrate.The activity of GC_ox/Pt electrode is increased by the factor of 2–4 for formic acid oxidation compared to the activity of GC/Pt electrode. This result is explained by the oxidative removal of CO_ad species leading to enhanced amount of Pt free sites available for direct formic acid oxidation to CO₂.     

Platinum–polytyramine composite material with improved performances for methanol oxidation

Polytyramine (PTy) is shown to be a possible alternative to other conducting polymers as a support material for fuel cell electrocatalysts such as platinum. In this work, a Pt–PTy composite was prepared via potentiodynamic deposition of polytyramine on graphite substrate, followed by the electrochemical deposition of Pt nanoparticles. The material obtained by this straightforward method exhibited, for platinum loadings as low as ca. 0.12 mg cm⁻², a specific electrochemically active surface area of the electrocatalyst of ca. 54 m² g⁻¹, together with a good electrocatalytic activity for methanol oxidation in acidic media, thus ensuring better efficiency of Pt utilization. The system Pt–PTy appears to be worthy of development for methanol fuel cell applications also because the results suggested that, when deposited as small particles in a PTy matrix, platinum is less sensitive to fouling during CH₃OH oxidation.     

Binding energy of ruthenium submonolayers deposited on a Pt(111) electrode

We investigated the 3d_5/2 core-level binding energy of Ru in Ru nanoislands spontaneously deposited on a Pt(111) electrode [Pt(111)/Ru], and the binding energies of 3d_5/2 iodine and 1s CO adsorbed on Pt(111)/Ru by the use of X-ray photoelectron spectroscopy. Both iodine and CO were used as surface probes of the electronic properties of Pt(111)/Ru. Little difference was found in the binding energy of Ru in Pt(111)/Ru and in Ru(0001). However, the addition of Ru to Pt(111) induces major changes in the core-level binding energies of chemisorbed iodine and CO as referenced to those adsorbed on Ru(0001). We conclude that the iodine 3d_5/2 and CO 1s C core levels experience higher electronic charge on Pt(111)/Ru than on Ru(0001), suggesting a charge transfer from Pt to Ru, or to a Ru-I surface molecule within the deposit. The charge transfer from Pt to Ru is in agreement with the result of previous in situ electrochemical NMR investigations [P.K. Babu, H.S. Kim, A. Wieckowski, E. Oldfield (2003) J. Phys. Chem. B 107:7595] and confirms the general trend of reduction in the density of states of Pt due to alloying with Ru [J. McBreen, S. Mukerjee (1995) J. Electrochem. Soc. 142:3399]. Theoretical calculations are in progress to further interpret the origin of the binding-energy shifts observed in this study.Dedicated to Zbigniew Galus on the occasion of his 70th birthday     

Effect of titanium,antimony, and ruthenium doping on tin dioxide adsorption properties. Quantum-chemical modeling

The influence of the composition of oxides supports on the specific electroactive surface area of Pt in the catalysts, the platinum nanoparticles dispersion, and Pt contents in the catalysts was studied. The Sb-doped SnO₂ oxides with various Sb-doping levels were prepared as a supports of platinum catalysts in polymer electrolyte membrane fuel cells. Density functional theory simulation of Ti, Sb, and Ru doping of tin dioxide and interaction of the doped surfaces with platinum cluster Pt₁₉ have been carried out. All calculations were performed in PBE exchange–correlation functional, with periodic boundary conditions and projector-augmented waves (PAW) basis set. The calculation results were compared with the experimental data X-ray diffraction and transmission electron microscopy (TEM). It was shown that Sb doping of tin dioxide (in quantity of less than 10%, that is, the quantity which cannot provoke significant defects of crystal structure of the supports) leads to a significant increase in a number of platinum clusters adsorbed from the colloidal solution onto the supports surface which results to an increase of the platinum cluster interaction with the supports. The calculated and experimental results are in close fit.