On the performance of porous platinized tungsten trioxide electrodes in the electrochemical oxidation of hydrogen

The performance of porous Pt-containing H₂ electrodes is found to be influenced by mass-transport effects. PtWO₃ electrodes are about twice as active as electrodes containing commercial PtC fuel cell catalysts in hydrogen oxidation. From our catalytic hydrogenation studies it follows that this difference in performance needs not to be caused by a synergistic effect between Pt and WO₃ with respect to H₂ oxidation. A more likely cause is a difference in effective conductivity, leading to reduced ohmic losses in PtWO₃ electrodes as compared with PtC electrodes.During this study we also investigated systems related to PtWO₃ in hydrogen oxidation. No interesting catalysts were found, however.     

Hydrogen Tungsten Bronze‐Supported Platinum as Electrocatalyst for Methanol Oxidation

Hydrogen tungsten bronze (H_xWO₃)‐supported platinum was prepared by electrodeposition and used as an electrocatalyst for methanol oxidation. The prepared electrocatalyst was characterized by Raman spectrum, X‐ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), cyclic voltammetry (CV), and chronoamperometry (CA). It is found that the support, H_xWO₃, not only reduces the platinum particle size and thus reduces the platinum loading, but also provides platinum with the anti‐poison ability to carbon monoxide and thus improves the activity of platinum toward methanol oxidation.     

Pt–NiO/C anode electrocatalysts for direct methanol fuel cells

Pt catalyst was supported on Vulcan XC-72R containing 5 wt.% NiO using NaBH₄ as a reducing agent. The prepared catalyst was heat-treated at 400 °C. XRD, TEM and EDX analyses were applied to characterize Pt–NiO/C electrocatalyst. The introduction of NiO reduces the particle size of Pt crystallites. The electrocatalytic activity of Pt–NiO/C electrocatalysts was examined towards methanol oxidation reaction in 0.5 M H₂SO₄ solution using cyclic voltammetry and chronoamperometry techniques. A three fold increment in the oxidation current density was gained at Pt–NiO/C electrocatalyst compared to Pt/C one. The corresponding chronoamperograms showed high steady state current density values suggesting better stability of Pt–NiO/C electrocatalyst towards the carbonaceous poisoning species. The enhanced electrocatalytic performance and the long-term cycle durability of Pt–NiO/C electrocatalyst are attributed to the strong interaction between Pt and NiO and the formation of small Pt crystals.     

The effect of platinum on the performance of WO3 nanocrystal photocatalysts for the oxidation of Methyl Orange and iso‐propanol

BACKGROUND: This research investigated the effect of platinum (Pt) on the reactivity of tungsten oxide (WO₃) for the visible light photocatalytic oxidation of dyes. RESULTS: Nanocrystalline tungsten oxide (WO₃) photocatalysts were synthesised by a sol‐gel process and employed for the photocatalytic degradation of Methyl Orange under visible light. For comparison commercial bulk WO₃ materials were also studied for the same reaction. These materials were fully characterised using X‐ray diffraction (XRD), UV‐visible diffuse reflection spectroscopy and transmission electron microscopy (TEM). The photocatalytic oxidation of iso‐propanol was used as a model reaction to follow the concomitant reduction of molecular oxygen. No reactions occured in the absence of platinum, which is an essential co‐catalyst for the multi‐electron reduction of oxygen. The platinised WO₃ catalysts were stable for multiple oxidation–reduction cycles. The results from the catalytic activity measurements showed that platinised nanocrystalline WO₃ is a superior oxidation photocatalyst when compared with bulk WO₃. Methyl Orange was completely decolourised in 4 h. CONCLUSIONS: The enhanced performance of nanocrystalline Pt‐WO₃ is attributed to improved charge separation in the nanosized photocatalyst. Platinum is an essential co‐catalyst to reduce oxygen. This photocatalyst could be applied to the treatment of organic pollutants in wastewater, with the advantage of using visible light compared with the widely studied TiO₂, which requires UV light. Copyright © 2011 Society of Chemical Industry     

Enhanced NO2 sensing properties of Pt/WO3 films grown by glancing angle deposition

In this work, WO₃ films were synthesized by glancing angle deposition (GLAD) and conventional planar deposition respectively. By depositing Pt on WO₃ by GLAD, the NO₂ sensitivity of WO₃ films were significantly improved. The structural characteristics and NO₂ sensing properties of the films were investigated in order to establish the enhancement mechanism. The results show WO₃ films prepared by GLAD have porous nanorod-like structure, and isolated Pt clusters are distributed on WO₃. The nanostructured Pt/WO₃ films show high sensitivity to NO₂ at 150 °C, detecting as low as 80 ppb NO₂ with a response of 1.23. Meanwhile, the films also exhibit high NO₂ selectivity against NH₃, CO, acetone and ethanol. The excellent NO₂ sensing properties of the Pt/WO₃ films can be explained due to large specific surface area of nanorod-like WO₃, catalysis of Pt and Schottky barriers at interfaces. The reliable Pt/WO₃ nanostructure prepared by GLAD could be potentially applied in low-temperature, highly sensitive NO₂ sensor for micro-electro-mechanical system (MEMS).     

Electrocatalytic activity of Pt nanoparticles deposited on porous TiO2 supports toward methanol oxidation

Porous TiO₂ thin films were prepared on the Si substrate by hydrothermal method, and used as the Pt electrocatalyst support for methanol oxidation study. Well-dispersed Pt nanoparticles with a particle size of 5–7 nm were pulse-electrodeposited on the porous TiO₂ support, which was mainly composed of the anatase phase after an annealing at 600 °C in vacuum. Cyclic voltammetry (CV) and CO stripping measurements showed that the Pt/TiO₂ electrode had a high electrocatalytic activity toward methanol oxidation and an excellent CO tolerance. The excellent electrocatalytic performance of the electrode is ascribed to the synergistic effect of Pt nanoparticles and the porous TiO₂ support on CO oxidation. The strong electronic interaction between Pt and the TiO₂ support may modify CO chemisorption properties on Pt nanoparticles, thereby facilitating CO oxidation on Pt nanoparticles via the bifunctional mechanism and thus improving the electrocatalytic activity of the Pt catalyst toward methanol oxidation.     

Visible light induced decomposition of organic compounds on WO3 loaded PtPb co-catalysts

Tungsten oxide (WO₃)-supported ordered intermetallic PtPb nanoparticles (PtPb NPs/WO₃) were prepared through a co-reduction of Pt and Pb precursors with sodium borohydride in anhydrous methanol containing WO₃. The PtPb NPs/WO₃ were characterized based on the crystal structure obtained through powder X-ray diffraction (pXRD) as well as X-ray photoemission spectroscopy (XPS) and transmission electron microscopy (TEM). The formation of an ordered PtPb intermetallic phase on the WO₃ surface was confirmed. The PtPb NPs/WO₃ were more efficient when decomposing acetic acid (AcOH) and acetaldehyde (AcH) with visible light irradiation compared to the same process over WO₃ loaded with Pt nanoparticles (Pt NPs/WO₃).     

Is carbon-supported Pt-WOx composite a CO-tolerant material?

Pt-WO_x/C composite materials elaborated via a two-step impregnation/electrochemical reduction method have been characterized and tested for the electrooxidation of CO/H₂ mixtures. TEM and EDS measurements revealed that WO_x covered imperfectly the C particles. Nanometer-sized or agglomerated Pt particles were found on the WO_x/C surface. XRD measurements revealed the absence of diffraction peaks characteristic of crystalline WO_x and could indicate that this material is amorphous. No evidence of alloying between the Pt and W was observed. A significant improvement toward the electrooxidation of a CO_ads monolayer was observed for the composite material compared to pure Pt/C electrocatalyst, which is evidenced by a new electrooxidation peak at 0.55 V versus RHE (v=0.02 V s−1). As the electrical charge below this electrooxidation peak is sweep rate dependant, it is probably associated to the electrooxidation of CO_ads on Pt sites at the interface with the WO_x/C support. The performance of the Pt-WO_x/C material for the electrooxidation of CO/H₂ mixtures was tested by polarization curves under steady-state conditions (0.001 V s⁻¹) or potentiostatic measurements under fuel cell relevant conditions and compared with that of commercial 20 wt% Pt/C and Pt-Ru/C materials.     

The role of the WO x ad-component to Pt and PtRu catalysts in the electrochemical CH3OH oxidation reaction

High surface area carbon-supported platinum-based catalysts, Pt/C, PtWO _x /C, PtRu/C and PtRuWO _x /C, were prepared via a chemical reduction route using single metal precursor salts. The catalyst particles were found to be in the nanoscale range, and the addition of Ru clearly decreased the particle size. The Ru was found to be partially incorporated into the face centered cubic lattice of Pt and to form a single Ru catalyst component. X-ray diffraction and X-ray photon spectroscopy did not provide evidence for electronic interactions between WO _x and Pt as well as WO _x and Ru. However, the addition of tungsten to the PtRuWO _x /C catalyst resulted in a high degree of catalyst particle agglomeration. Both Ru containing catalysts showed significantly higher activities for the CH₃OH oxidation reaction in terms of Pt + Ru mass as well as electroactive Pt + Ru surface area than the Pt/C and PtWO _x /C catalysts. The addition of tungsten appeared to mainly result in some ‘physical’ modification of the catalytically active Pt and Ru surface components such as differences in electroactive surface area rather than promotion of the CH₃OH oxidation reaction via a true catalytic mechanism.     

Synthesis of Novel Submicrometer‐Scale Flat Carbon Fibers and Application in the Electrooxidation of Methanol

Novel submicrometer‐scale flat carbon fibers (SFCF) have been synthesized by catalytic chemical vapor deposition of acetylene over an Ni‐Al layered double hydroxide (NiAl‐LDH) compound, and the electrochemical activity of Pt supported on as‐synthesized SFCF for methanol oxidation has been investigated. The materials were characterized by power X‐ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectra, and cyclic voltammetry tests. The results reveal that the active crystal facets of the NiAl₂O₄ spinel phase derived from NiAl‐LDH can deposit carbon atoms to grow SFCF, and that the co‐growing Ni nanoparticles are not catalytically active for the formation of SFCF. Furthermore, after support with Pt, the resultant Pt/SFCF electrocatalyst shows much higher activity for methanol oxidation than the Pt/C one in both acid and alkaline media, which is attributed to the combined beneficial effects of the microstructure of the SFCF support, improved electrical conductivity originating from the NiAl₂O₄ spinel catalyst embedded in SFCF, and improved dispersion of Pt particles through exposed Ni nanoparticles adhering intimately to the SFCF.     

Improving the Performance of Platinum Nanocrystal Catalysts by Square‐wave Potential Electrochemical Pretreatment

Cube‐shaped Pt nanocrystals (with the size of about 160 nm) are prepared by a square‐wave potential electrochemical pretreatment at the expense of Pt nanospheres. A cyclic voltammogram of Pt nanospheres in sulphuric acid shows two pairs of hydrogen adsorption/desorption peaks, which corresponds to the characteristics of a Pt polyoriented surface. However, a cyclic voltammogram of cubic Pt nanocrystals in sulphuric acid shows another pair of hydrogen adsorption/desorption peak at 0.22 V (vs. NHE), which corresponds to the characteristics of Pt (100) surface orientation. Cubic Pt nanocrystals show enhanced electrocatalytic activity over Pt nanospheres for methanol oxidation. The peak current density of cubic Pt nanocrystals is 1.39 mA cm^–2_Pt, which is 1.48 times that of Pt nanospheres. The poison resistant and oxygen reduction reaction (ORR) activity of cubic Pt nanocrystals are also enhanced compared with those of Pt nanospheres.     

Ethanol electrooxidation on activated graphite supported platinum-nickel in alkaline medium

In this study, activated graphite-supported platinum (Pt/C), nickel (Ni/C) and platinum-nickel (Pt–Ni/C) were prepared by electrodeposition technique using solutions containing hexachloroplatinic acid and/or nickel sulphate. Ethanol electrooxidation was investigated on Pt/C, Ni/C and Pt–Ni/C electrocatalysts by cyclic voltammetry and chronoamperometry in aqueous alkaline solution containing 1.0 M NaOH and 2.0 M C₂H₅OH. The electrocatalytic activities and stabilities of the electrocatalysts were discussed. The results showed that ethanol is oxidized anodically on Ni/C. This reaction occured simultaneously with the anodic oxidation of nickel hydroxide (Ni(OH)₂) into nickel oxy-hydroxide (NiOOH). It is also found that Pt–Ni/C electrocatalytic characteristics were better than Pt/C. The Pt–Ni/C electrocatalyst exhibited excellent activity and enhanced stability than Pt/C. It seems that Pt–Ni/C is a promising electrocatalyst toward ethanol electrooxidation in alkaline medium for fuel cells applications.     

High Electrochemical Performance and Stability of Co-Deposited Pd–Au on Phase-Pure Tungsten Carbide for Hydrogen Oxidation

Co-deposited Pd and Au nanoparticles were loaded on phase-pure tungsten mono-carbide (WC) prepared by a polymer-induced carburization method. Among the electrocatalysts, Pd₃Au/WC displayed an excellent electrochemical activity for the hydrogen oxidation reaction comparable to the state-of-the-art Pt/C catalyst both in half-cell tests and single cell tests under proton exchange membrane fuel cell (PEMFC) conditions. This unique and strong synergistic effect was not observed on the carbon support, and thus the crucial role of WC was demonstrated as a strongly interacting support as well as an active component of the electrocatalyst. Dissolution of Pd observed on the carbon support was suppressed in this Pd₃Au/WC electrocatalyst, which showed good stability in a continuous operation for 3,000?min. Thus the proposed electrocatalyst could be a potential alternative anode catalyst of lower cost for PEMFC replacing Pt/C.     

The electrochemical oxidation of organics using tungsten oxide based electrodes

High surface area tungsten oxide (WO_x) based electrodes containing centers of Pt, Sn or Ru were synthesized. The WO_x electrodes were found to display good capacitive behavior and relatively high specific capacitance values of up to 180 F g⁻¹. The oxidation behavior of particularly HCOOH and (COOH)₂, using the WO_x electrodes containing Pt and Sn centers (Pt/WO_x and Sn/WO_x, respectively), was studied in detail in aqueous solutions at high potentials, i.e. at which O₂ is evolved. Both HCOOH and (COOH)₂ appear to be oxidized following 1st order kinetics. The (COOH)₂ oxidation reaction is faster than the HCOOH reaction using otherwise the same experimental conditions. The reaction mechanism of both the HCOOH and (COOH)₂ oxidation was found to most likely involve the adsorptive interaction of the two organics with the anode surface. The WO_x based anodes appear to be promising catalysts for the anodic oxidation of both (COOH)₂ and HCOOH.     

Pt-CeO2/C anode catalyst for direct methanol fuel cells

In order to develop a cheaper and durable catalyst for methanol electrooxidation reaction, ceria (CeO₂) as a co-catalytic material with Pt on carbon was investigated with an aim of replacing Ru in PtRu/C which is considered as prominent anode catalyst till date. A series of Pt-CeO₂/C catalysts with various compositions of ceria, viz. 40 wt% Pt-3–12 wt% CeO₂/C and PtRu/C were synthesized by wet impregnation method. Electrocatalytic activities of these catalysts for methanol oxidation were examined by cyclic voltammetry and chronoamperometry techniques and it is found that 40 wt% Pt-9 wt% CeO₂/C catalyst exhibited a better activity and stability than did the unmodified Pt/C catalyst. Hence, we explore the possibility of employing Pt-CeO₂ as an electrocatalyst for methanol oxidation. The physicochemical characterizations of the catalysts were carried out by using Brunauer Emmett Teller (BET) surface area and pore size distribution (PSD) measurements, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) techniques. A tentative mechanism is proposed for a possible role of ceria as a co-catalyst in Pt/C system for methanol electrooxidation.     

Unique and facile solvothermal synthesis of mesoporous WO3 using a solid precursor and a surfactant template as a photoanode for visible-light-driven water oxidation

Mesoporous tungsten trioxide (WO₃) was prepared from tungstic acid (H₂WO₄) as a tungsten precursor with dodecylamine (DDA) as a template to guide porosity of the nanostructure by a solvothermal technique. The WO₃ sample (denoted as WO₃-DDA) prepared with DDA was moulded on an electrode to yield efficient performance for visible-light-driven photoelectrochemical (PEC) water oxidation. Powder X-ray diffraction (XRD) data of the WO₃-DDA sample calcined at 400°C indicate a crystalline framework of the mesoporous structure with disordered arrangement of pores. N₂ physisorption studies show a Brunauer-Emmett-Teller (BET) surface area up to 57 m² g^-1 together with type IV isotherms and uniform distribution of a nanoscale pore size in the mesopore region. Scanning electron microscopy (SEM) images exhibit well-connected tiny spherical WO₃ particles with a diameter of ca. 5 to 20 nm composing the mesoporous network. The WO₃-DDA electrode generated photoanodic current density of 1.1 mA cm^-2 at 1.0 V versus Ag/AgCl under visible light irradiation, which is about three times higher than that of the untemplated WO₃. O₂ (1.49 μmol; Faraday efficiency, 65.2%) was evolved during the 1-h photoelectrolysis for the WO₃-DDA electrode under the conditions employed. The mesoporous electrode turned out to work more efficiently for visible-light-driven water oxidation relative to the untemplated WO₃ electrode.     

Pt-CeO2/C anode catalyst for direct methanol fuel cells

In order to develop a cheaper and durable catalyst for methanol electrooxidation reaction, ceria (CeO₂) as a co-catalytic material with Pt on carbon was investigated with an aim of replacing Ru in PtRu/C which is considered as prominent anode catalyst till date. A series of Pt-CeO₂/C catalysts with various compositions of ceria, viz. 40 wt% Pt-3–12 wt% CeO₂/C and PtRu/C were synthesized by wet impregnation method. Electrocatalytic activities of these catalysts for methanol oxidation were examined by cyclic voltammetry and chronoamperometry techniques and it is found that 40 wt% Pt-9 wt% CeO₂/C catalyst exhibited a better activity and stability than did the unmodified Pt/C catalyst. Hence, we explore the possibility of employing Pt-CeO₂ as an electrocatalyst for methanol oxidation. The physicochemical characterizations of the catalysts were carried out by using Brunauer Emmett Teller (BET) surface area and pore size distribution (PSD) measurements, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) techniques. A tentative mechanism is proposed for a possible role of ceria as a co-catalyst in Pt/C system for methanol electrooxidation.     

Effect of W on PtSn/C catalysts for ethanol electrooxidation

Binary and ternary Pt-based catalysts were prepared by the Pechini–Adams modified method on carbon Vulcan XC-72, and different nominal compositions were characterized by TEM and XRD. XRD showed that the electrocatalysts consisted of the Pt displaced phase, suggesting the formation of a solid solution between the metals Pt/W and Pt/Sn. Electrochemical investigations on these different electrode materials were carried out as a function of the electrocatalyst composition, in acid medium (0.5 mol dm⁻³ H₂SO₄) and in the presence of ethanol. The results obtained at room temperature showed that the PtSnW/C catalyst display better catalytic activity for ethanol oxidation compared to PtW/C catalyst. The reaction products (acetaldehyde, acetic acid and carbon dioxide) were analyzed by HPLC and identified by in situ infrared reflectance spectroscopy. The latter technique also allowed identification of the intermediate and adsorbed species. The presence of linearly adsorbed CO and CO₂ indicated that the cleavage of the C–C bond in the ethanol substrate occurred during the oxidation process. At 90 °C, the Pt₈₅Sn₈W₇/C catalyst gave higher current and power performances as anode material in a direct ethanol fuel cell (DEFC).     

Anion-modified zirconia: effect of metal promotion and hydrogen reduction on hydroisomerization of n-hexadecane and Fischer–Tropsch waxes

The effect of metal promoters on the activity and selectivity of tungstated zirconia (8 wt.% W) for n-hexadecane isomerization in a trickle bed continuous reactor is studied by using different metals (Pt, Ni, and Pd) and, in one case, by varying metal loading. Platinum is found to be the best promoter. The effect of hydrogen reduction is investigated using platinum-promoted tungstated zirconia catalysts (Pt/WO₃/ZrO₂, 0.5 wt.% Pt and 6.5 wt.% W). Pretreatment at temperatures between 300 and 400°C for 3 h in hydrogen is found to be slightly beneficial for achieving high yields of isohexadecane. A platinum promoted sulfated zirconia (Pt/SO₄/ZrO₂) is compared with a Pt/WO₃/ZrO₂ catalyst for the hydroisomerization of n-hexadecane in the same reactor at the same n-hexadecane conversion. The former is a good cracking catalyst and the latter is suitable for use as a hydroisomerization catalyst. In a 27-ml microautoclave reactor, studies of the hydroisomerization and hydrocracking of two Fischer–Tropsch (F–T) wax samples are carried out. Severe cracking can be effectively suppressed using a Pt/WO₃/ZrO₂ catalyst so as to obtain branched isomers in the diesel fuel or lube-base oil range.     

Exfoliated graphene-supported Pt and Pt-based alloys as electrocatalysts for direct methanol fuel cells

To greatly improve the electrocatalytic activity for methanol oxidation, high-quality exfoliated graphene decorated with uniform Pt nanocrystals (NCs) (3 nm) have been prepared by a very simple, low-cost and environmentally benign process. During the entire process, no surfactant and no halide ions were involved, which not only enabled very clean surface of Pt/graphene leading to excellent conductivity, but also greatly improved the electrocatalyst tolerance to carbon monoxide poisoning (Pt/graphene, I_f/I_b = 1.197), compared to commercial Pt/C (I_f/I_b = 0.893) catalysts. To maximize the electrocatalytic performance and minimize the amount of precious Pt, Pt–M/graphene (M = Pd, Co) hybrids have also been prepared, and these hybrids have much larger electrochemically active surface areas (ECSA), which are 4 (PtPd/graphene) and 3.3 (PtCo/graphene) times those of commercial Pt/C. The PtPd/graphene and PtCo/graphene hybrids also have remarkably increased activity toward methanol oxidation (I_f/I_b = 1.218 and 1.558). Furthermore, density functional theory (DFT) simulations demonstrate that an electronic interaction occurred between Pt atoms and graphene, indicating that graphene substrate plays a crucial role in regulating the electron structure of attached Pt atom, which confirmed that the increased efficiency of methanol oxidation was due to the synergetic effects of the hybrid structure.