Special Sites at Noble and Late Transition Metal Catalysts

An overview of recent advancements in density functional theory modeling of particularly reactive sites at noble and late transition metal surfaces is given. Such special sites include sites at the flat surfaces of thin metal films, sites at stepped surfaces, sites at the metal/oxide interface boundary for oxide-supported metal clusters, and sites at the perimeter of oxide islands grown on metal surfaces. The Newns–Anderson model of the electronic interaction underlying chemisorption is described. This provides the grounds for introducing the Hammer–N?rskov d-band model that correlates changes in the energy center of the valence d-band density of states at the surface sites with their ability to form chemisorption bonds. A reactivity change described by this model is characterized as an electronic structure effect. Br?nsted plots of energy barriers versus reaction energies are discussed from the surface reaction perspective and are used to analyze the trends in the calculated changes. Deviations in the relation between energy barriers and reaction energies in Br?nsted plots are identified as due to atomic structure effects. The reactivity change from pure Pd surfaces to Pd thin films supported on MgO can be assigned to an electronic effect. Likewise for the reactivity change from flat Au surfaces, over Au thin films to Au edges and the Au/MgO interface boundary. The reactivity enhancement at atomic step sites is of both electronic and atomic structure nature for NO dissociation at Ru, Rh and Pd surfaces. The enhancement of the CO oxidation reactivity when moving from a CO+O coadsorption structure on Pt(111) to the PtO₂ oxide island edges supported by Pt(111) is, however, identified as mainly an atomic structure effect. As such, it is linked to the occurrence of favorable pathways at the oxide island edges and is occurring despite of stronger adsorbate binding of the oxygen within the oxide edge, i.e. despite of an opposing electronic effect. As a final topic, a discussion is given of the accuracy of density functional theory in conjunction with surface reactions; adsorption, desorption, diffusion, and dissociation. Energy barriers are concluded to be more robust with respect to changes in the exchange-correlation functional than are molecular bond and adsorption energies.     

CGO-based electrochemical catalysts for low temperature combustion of propene

This study has shown that the phenomenon of electrochemical promotion of catalysis or NEMCA effect can be used to activate a metal catalyst for the propene deep oxidation in the presence of water in the feed and in both stoichiometric and lean-burn conditions. The electrochemical catalysts were based on sputtered Pt films interfaced with gadolinium-doped ceria solid electrolyte. This system allows implementing NEMCA effect at quite low temperatures and electrochemical activation of propene deep oxidation was evidenced at temperatures as low as 190 °C. In addition, a new design of electrochemical catalysts was proposed by depositing a interlayer of strontium-doped lanthanum manganite between the CGO dense support and an ultra-thin coating of Pt. This concept allows to drastically decrease the Pt loading since the electronic conductivity is ensured by the perovskite layer.     

Electrocatalysis of ethanol oxidation on Pt monolayers deposited on carbon-supported Ru and Rh nanoparticles

Pt monolayers deposited on carbon-supported Ru and Rh nanoparticles were investigated as electrocatalysts for ethanol oxidation. Electronic features of the Pt monolayers were studied by in situ XANES (X-ray absorption near-edge structure). The electrochemical activity was investigated by cyclic voltammetry and cronoamperometric experiments. Spectroscopic and electrochemical results were compared to those obtained on carbon-supported Pt–Ru and Pt–Rh alloys, and Pt E-TEK. XAS results indicate a modification of the Pt 5d band due to geometric and electronic interactions with the Ru ant Rh substrates, but the effect of withdrawing electrons from Pt is less pronounced in relation to that for the corresponding alloys. Electrochemical stripping of adsorbed CO, which is one of the intermediates, and the currents for the oxidation of ethanol show faster kinetics on the Pt monolayer deposited on Ru nanoparticles, and an activity that exceeds that of conventional catalysts with much larger amounts of platinum.     

Preparation and characterisation of platinum- and gold-coated copper, iron, cobalt and nickel deposits on glassy carbon substrates

Pt- and Au-coated Cu, Fe, Co and Ni deposits have been formed on glassy carbon (GC) substrates by electrodeposition of controlled amounts of the core metal onto the substrate and its subsequent partial replacement by Pt or Au upon immersion into a chloroplatinic or chlorolauric solution. This process resulted in a thin Pt or Au shell over the bimetallic particle as electrochemical evidence suggests, while indicative sputter-etch Auger Electron Spectroscopy points to the coexistence of the two metals in the particle core. SEM/EDS characterisation of the deposits revealed the existence of Pt(M) and Au(M) extensively agglomerated nanoparticles (M: Cu, Fe, Co, Ni). The surface electrochemistry of the deposits (hydrogen adsorption/desorption on Pt and oxide formation/stripping on Au) proved the complete coverage of the bimetallic particles by Pt or Au and allowed an estimate of their electroactive surface area. The study of hydrogen evolution at these deposits points to a modification of the electronic properties of the Pt and Au surface layers by the core metal (due to strain effects and/or ligand-electronic interactions) and further confirmed that these layers form a very thin outer shell.     

Electrochemical Formation of CdSe Monolayers on the Low Index Planes of Au

The electrochemical analog of atomic layer epitaxy (ALE) is being studied. ALE is a method for growing thin films of materials using a cycle of surface limited reactions. The surface limited reactions control the deposition by limiting the growth to an atomic layer at a time. In electrochemistry, a surface limited deposition is generally referred to as underpotential deposition (UPD), and UPD is used to form the atomic layers in electrochemical ALE (ECALE). The work presented here is an atomic level study of the deposition of the first few monolayers of CdSe via ECALE: by the alternated UPD of atomic layers of Se and Cd on the low index planes of Au. UPD of Se resulted in the formation of ordered structures on each of the low index planes of Au, as observed by low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). The subsequent UPD of Cd resulted in CdSe deposits which exhibited 1:1 stoichiometry, as determined by coulometry and Auger electron spectroscopy (AES). The following LEED patterns were observed for the CdSe monolayers: Au(111)(√7×√7)R19.1°, Au(111)(3×3), Au(110)(2×3), and the Au(100)(√2×2√2)R45°. Similar LEED patterns were observed on each surface for deposits formed using up to three ECALE cycles. In situ STM studies of Cd deposition on Se-covered Au(111) indicated the formation of a (3×3) structure, consistent with LEED results, and with previous TEM studies. The same LEED patterns were also observed for CdSe monolayers where Cd was deposited as the first atomic layer. AES indicated that the element deposited first remained on the bottom, and that deposited second remained on top.     

Controllable fabrication and temperature-resistance characteristics of ordered nanoporous Au and Pt films

In this work, the ordered nanoporous arrays of Au and Pt films are fabricated using anodized aluminum oxide (AAO) template based on the sputtering method. The presented synthetic strategy is scalable to large area by incorporating the deposition of a thin layer of Au or Pt. In addition, the grain size of Au and Pt nanoporous films is controlled with sputtering time. The thorough study of electrical transport properties for these metal films enables us to infer the nanoporous film morphology, and the evolution of the grain size with the change of sputtering time. In fact, the different physical behaviors are observed to occur in these metal films. The negative temperature coefficient of resistance (TCR) is visible for Pt nanoporous films, while Au nanoporous films show the positive TCR. With the increasing of sputtering time, the Pt grain size gradually becomes bigger, and the negative TCR properties weaken because the interface scattering of the electrons reduces. Therefore, the fabrication of metal nanoporous films with well-controlled physical properties might open new pathways for the growth of metal electrodes on AAO substrates for nanoelectronic devices.     

Surface relaxation at the metal/electrolyte interface

X-ray diffraction is an ideal technique for the in situ study of single crystal metal surfaces in an electrochemical environment. In this paper, measurements of the low-index surfaces of Au and Pt are described, in particular with reference to surface expansion effects. Surface expansion can be probed potentiodynamically to correlate expansion with the adsorption/desorption of solution species. In general, the results are in good agreement with recent theoretical calculations. The X-ray technique can also give insight into electrocatalytic reactions as shown by the results for the adsorption and oxidation of CO on Pt(111).     

Facile preparation of three-dimensional porous Pd–Au films and their electrocatalytic activity for methanol oxidation

Pd–Au porous foam films with three-dimensional hierarchical pores consisting of interconnected dendrite walls are obtained by using the hydrogen bubble dynamic template. The films are characterized by scanning electron microscope, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, as well as electrochemical techniques. The CV curves have confirmed that the Pd–Au catalysts have high catalytic activity toward methanol oxidation. The catalytic activities of these Pd–Au catalysts are strongly dependent on the atomic ratio of Pd/Au. Among all the Pd–Au catalysts, the Pd₁Au₁ catalyst is found to possess superior catalytic activity and stability toward methanol oxidation.     

PEDOT/Palladium composite material: synthesis, characterization and application to simultaneous determination of dopamine and uric acid

Palladium (Pd) incorporated poly (3,4-ethylenedioxythiophene) (PEDOT) films were synthesized through an electrochemical route and characterized using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The electrochemical study showed catalytic oxidation of dopamine (DA) with optimum loading of Pd. DA and uric acid (UA) were detected using differential pulse voltammetry (DPV). In the presence of ascorbic acid (AA), DA-AA showed peak potential separation of 0.19 V while 0.32 V between UA-AA on Pd-incorporated PEDOT. These peak separations are large enough for sensing DA and UA in the presence of AA. DA and UA exhibited linear calibration plots and the minimum detection limits are 0.5 and 7 μM respectively. On Pd-PEDOT, the reversibility of DA oxidation was found to increase compared to bare glassy carbon electrode (GCE) and PEDOT modified GCE. Fouling effects were also found to be minimal making Pd-PEDOT composite suitable for electroanalysis.     

Platinum modification of gold and electrocatalytic oxidation of ethylene glycol on Pt-modified Au electrodes

The surface modification of gold electrodes with platinum and the electrocatalytic oxidation of ethylene glycol on Pt-modified Au electrodes are investigated by cyclic voltammetry. Platinum modification is performed by the electrochemical deposition of platinum on polycrystalline gold electrodes, and the Pt-modified Au electrodes with different amount of the deposited platinum are used for the ethylene glycol oxidation in alkaline and acidic solutions. It is shown that oxidation potential for the ethylene glycol oxidation on the Pt-modified Au electrodes shifts significantly negative compared with that on Au electrodes, nearly same oxidation potentials as that on Pt electrodes are observed, and peak current density of the ethylene glycol oxidation is dependent on the amounts of the deposited platinum on gold surface, much higher peak current densities than that on Au and Pt electrodes can be obtained. The low oxidation potential and high peak current density indicate the enhanced reactivity of Au electrodes by the platinum modification. The characteristics of the Pt-modified Au electrodes are found to be similar to that of Pt electrodes, and the reactivity of the Pt-modified Au electrodes is mainly attributed to the deposited platinum.     

Electrocatalytic applications of a vertical Au nanorod array using ultrathin Pt/Ru/Pt layer-by-layer coatings

This study compared the electro-oxidation of carbon monoxide adlayers formed on a codeposited and a layer-by-layer deposited Pt/Ru thin overlayer on a Au surface. Vertical arrays of smooth and nanoporous Au nanorods were used as a platform for the Pt/Ru overlayer-coating. The vertical arrays of Au nanorods served as high surface area templates, where the surface area could be controlled as a function of the rod length in a given geometrical surface area. A coating of the Au nanorod arrays with Pt/Ru via Cu underpotential deposition (UPD) steps allowed control of the overlayer thickness with an ultrathin film nature. The electronic modification of the Pt overlayer with a Ru underlayer became increasingly weaker with increasing Pt overlayer thickness. The thick Pt overlayer exhibited CO electro-oxidation that was similar to the electro-oxidation profile on the Pt bulk surface. Coating of the Au nanorod arrays with Pt/Ru through Cu UPD steps demonstrated an optimal electronic response between the Pt outer overlayer and the underlying Ru layer. It is expected that an ultrathin overlayer-coating strategy can be expanded to nanoporous Au nanorod arrays. These finding will facilitate better design of highly active Pt/Ru composite nanostructures for electrocatalysis.     

Structure–activity relationships in supported Au catalysts

Au-based catalysts have great potential because of their unique activity and selectivity for a variety of important reactions. The special catalytic properties of supported Au nano-particles depend critically upon the particle morphology, i.e. size, shape and thickness, as well as support effects. This paper reviews the current understanding of CO oxidation on supported Au catalysts. The electronic structure of Au particles at various nucleation sites and on different supports is summarized, and the effect these changes have on catalytic performance is discussed. Recent results from our laboratories have demonstrated the synthesis of well-ordered Au mono- and bi-layer films on a titanium oxide support and show that the active Au structure for CO oxidation is an electron-rich, Au bi-layer. In contrast, the monolayer structure, which may involve the TiO_x support, is significantly less active (by less than an order of magnitude) than the Au bi-layer. The oxidation state of the Au and how this relates to the catalytic activity are also discussed.     

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.     

Gold supported on thin oxide films: from single atoms to nanoparticles

[Figure: see text]. Historically, people have prized gold for its beauty and the durability that resulted from its chemical inertness. However, even the ancient Romans had noted that finely dispersed gold can give rise to particular optical phenomena. A decade ago, researchers found that highly dispersed gold supported on oxides exhibits high chemical activity in a number of reactions. These chemical and optical properties have recently prompted considerable interest in applications of nanodispersed gold. Despite their broad use, a microscopic understanding of these gold-metal oxide systems lags behind their application. Numerous studies are currently underway to understand why supported nanometer-sized gold particles show catalytic activity and to explore possible applications of their optical properties in photonics and biology. This Account focuses on a microscopic understanding of the gold-substrate interaction and its impact on the properties of the adsorbed gold. Our strategy uses model systems in which gold atoms and clusters are supported on well-ordered thin oxide films grown on metal single crystals. As a result, we can investigate the systems with the rigor of modern surface science techniques while incorporating some of the complexity found in technological applications. We use a variety of different experimental methods, namely, scanning probe techniques (scanning tunneling microscopy and spectroscopy, STM and STS), as well as infrared (IR), temperature-programmed desorption (TPD), and electron paramagnetic resonance (EPR) spectroscopy, to evaluate these interactions and combine these results with theoretical calculations. We examined the properties of supported gold with increasing complexity starting from single gold atoms to one- and two-dimensional clusters and three-dimensional particles. These investigations show that the binding of gold on oxide surfaces depends on the properties of the oxide, which leads to different electronic properties of the Au deposits. Changes in the electronic structure, namely, the charge state of Au atoms and clusters, can be induced by surface defects such as color centers. Interestingly, the film thickness can also serve as a parameter to alter the properties of Au. Thin MgO films (two to three monolayer thickness) stabilize negatively charged Au atoms and two-dimensional Au particles. In three dimensions, the properties of Au particles bigger than 2-3 nm in diameter are largely independent of the support. Smaller three-dimensional particles, however, showed differences based on the supporting oxide. Presumably, the oxide support stabilizes particular atomic configurations, charge states, or electronic properties of the ultrasmall Au aggregates, which are in turn responsible for this distinct chemical behavior.     

Electrochemical catalysts for hydrocarbon combustion

Two series of electrochemical catalysts were prepared from sputtered Pt thin films onto two kinds of electrolyte membranes, 8 mol% Y₂O₃-stabilized ZrO₂ (YSZ), an O²⁻ conducting oxide and Na₃Zr₂Si₂PO₁₂ (NASICON), a Na⁺ one; respectively. The thickness of the Pt films varied from 8 to 120 nm. Therefore, the Pt loading was extremely low. The catalytic activity of Pt/YSZ and Pt/NASICON systems has been investigated between 200 and 500 °C for propane and propene, respectively. In spite of the low Pt loading, the Pt/YSZ electrochemical catalysts exhibited high activity for propane combustion. Furthermore, the catalytic activity can be in-situ controlled by applying electrical polarisation with high Faradaic efficiency (10³). The catalytic rate of propene deep oxidation on Pt/NASICON electrochemical catalyst was found to be limited by the number of active sites, which is low on very thin Pt films. Moreover, initial anodic polarisation indicate that Na⁺ ions are already present on the top surface of Pt, probably proceeding from the preliminary stabilisation treatment of Pt in the reactive mixture. Nevertheless, polarisation allows the tuning of the catalytic activity of the electrochemical catalysts for propene oxidation. Finally, for both kinds of electrochemical catalysts, our results have evidenced that the measurement of the open-circuit voltage during catalytic process can be an indicator of the hydrocarbon conversion.     

Pt/Au(111)表面合金电化学稳定性的第一性原理研究

利用密度泛函理论(DFT)对Pt/Au (111)表面合金的电化学稳定性进行了初步研究。形成能计算结果表明,Au与Pt不易在块体中形成合金,但能在Pt (111)面形成表面合金。溶解电位计算结果进一步表明,Pt/Au (111)面上Pt原子的溶解电位与其第一近邻Au原子数有很好的线性关系,而Au对第二近邻及更远近邻的Pt溶解电位的影响可忽略。这些结果意味着可建立表面配位环境与表面原子溶解电位间的标度关系,为揭示表面合金的"结构-电化学稳定性"构型关系奠定了基础。     

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.     

Bimetallic and Ternary Alloys for Improved Oxygen Reduction Catalysis

Using a combination of density functional theory (DFT) calculations and an array of experimental techniques including in situ X-ray absorption spectroscopy, we identified, synthesized, and tested successfully a new class of electrocatalysts for the oxygen reduction reaction (ORR) that were based on monolayers of Pt deposited on different late transition metals (Au, Pd, Ir, Rh, or Ru), of which the Pd-supported Pt monolayer had the highest ORR activity. The amount of Pt used was further decreased by replacing part of the Pt monolayer with a third late transition metal (Au, Pd, Ir, Rh, Ru, Re, or Os). Several of these mixed Pt monolayers deposited on Pd single crystal or on carbon-supported Pd nanoparticles exhibited up to a 20-fold increase in ORR activity on a Pt-mass basis when compared with conventional all-Pt electrocatalysts. DFT calculations showed that their superior activity originated from the interaction between the Pt monolayer and the Pd substrate and from a reduced OH coverage on Pt sites, the result of enhanced destabilization of Pt–OH induced by the oxygenated third metal. This new class of electrocatalysts promises to alleviate the major problems of existing fuel cell technology by simultaneously decreasing materials cost and enhancing performance.     

Preparation of catalytic films of platinum on Au substrates modified by self-assembled PAMAM dendrimer monolayers

In this work we demonstrate the preparation of highly catalytically active Pt formed by the galvanic replacement of the copper adlayer on Au substrates, modified by the self-assembly of fourth generation amine terminated PAMAM dendrimer (G4NH₂). The copper adlayer was formed on the dendrimer-modified gold substrate by chemical preconcentration of copper ions followed by electrochemical reduction. The Pt overlayer was characterized by SEM, XPS and by cyclic voltammetry. The catalytic efficiency of the modified film thus prepared through soft route was evaluated by the electro catalytic oxidation of methanol using cyclic voltammetry, chronoamperometry and AC impedance techniques. This work also demonstrates that the copper adlayer formed on the dendrimer-modified electrode can undergo galvanic replacement by nobler metals like Au and Ag, besides Pt. An elegant soft route involving a new three-step protocol to build the concentration of active Pt on the Au surface has been developed. Concentration of the same metal (Pt) or two different metals (Pt–Au) can be built at the interface in a stepwise manner at ambient temperature.     

Three-dimensional crumpled graphene-based platinum–gold alloy nanoparticle composites as superior electrocatalysts for direct methanol fuel cells

High performance of electrocatalysts for direct methanol fuel cells was demonstrated by three-dimensional (3D) graphene (GR) decorated with platinum (Pt)–gold (Au) alloy nanoparticles (3D-GR/PtAu). The 3D-GR/PtAu composite with a morphology like a crumpled paper ball was synthesized from a colloidal mixture of GR and Pt–Au alloy nanoparticles with aerosol spray drying. The 3D-GR/PtAu had a high specific surface area and electrochemical surface area of up to 238 and 325 m²/g(Pt), respectively, and the electrocatalytic applications of the 3D-GR/PtAu were examined through methanol oxidation reactions. The 3D-GR/PtAu had the highest electrocatalytic activity for methanol oxidation reactions compared with commercial Pt–carbon black and Pt-GR. The 3D-GR/PtAu was also highly sensitive electrocatalytic activity in the methanol oxidation reaction compared with the 2D-GR/Pt–Au. Furthermore, the electrocatalytic activity of the 3D-GR/PtAu had the highest performance among the catalysts containing Pt, Au, and GR for the methanol oxidation reactions. The increased electrocatalytic activity is attributed to the high specific surface area of the 3D formation and the effective surface structure of the Pt–Au alloy nanoparticles.