Binary and ternary anode catalyst formulations including the elements W, Sn and Mo for PEMFCs operated on methanol or reformate gas

In an exploratory approach to find improved electrocatalyst formulations binary and ternary carbon supported catalysts with the elements Pt and Ru, W, Mo or Sn, respectively, amending the choice of Pt and Pt/Ru catalysts by addition of non-Pt metal cocatalysts were manufactured by impregnation and a colloid method and tested towards their activity for anodic oxidation of H₂ containing 150 ppm CO and of methanol. Membrane-electrode-assemblies with noble metal loadings of 0.4 mg cm⁻² were manufactured and tested in fuel cell operation at 75°C with H₂ fuel contaminated by CO and at 95°C for operation on methanol. Cocatalytic activities were found for the elements W and Mo for oxidation of H₂/CO and methanol while in the case of Sn a cocatalytic activity was only found for H₂/CO-oxidation. Both for oxidation of methanol and H₂/CO the system Pt/Ru/W was superior to the other systems tested. The colloid method was found to be highly suitable for synthesizing polymetallic PEM catalysts.     

The promotion of CO electro-oxidation on platinum-bismuth as a model for surface mediated oxygen transfer

The results of electrochemical studies of CO oxidation on clean and bismuth modified Pt(1 1 0)-(1×2) and Pt(1 1 1) surfaces and a supported platinum catalyst are compared. The effect of sub-monolayers of bismuth is to increase the CO oxidation potential on Pt(1 1 0)-(1×2), and decrease it on Pt(1 1 1). This primarily reflects the rather low potential for CO oxidation characteristic of the more open packed Pt(1 1 0)-(1×2) surface, and the higher potential characteristic of the close packed Pt(1 1 1) face. The oxidation potential of CO in the intermixed CO/Bi phases on the single crystals is very similar. The effect of bismuth on the supported platinum catalyst is to increase the CO oxidation potential by a similar amount to that found for Pt(1 1 0)-(1×2). In addition, modification by bismuth results in a concomitant reduction in the CO tolerance of the commercial catalyst. CO oxidation on the catalyst exhibits poisoning by bismuth in a similar fashion to Pt(1 1 0)-(1×2), and the bismuth redox couple on the catalyst is the same as that found on Pt(1 1 0)-(1×2). High coverages of bismuth on the catalyst do not appear to reduce its activity for hydrogen oxidation. These results are discussed in the context of the requirement for surface mediated oxygen transfer during CO electro-oxidation.     

Pt–Sn/C electrocatalysts for methanol oxidation synthesized by reduction with formic acid

Carbon supported Pt–Sn alloy catalysts were prepared by reduction of Pt and Sn precursors with formic acid, and their electrocatalytic activity for methanol oxidation was compared with commercial Pt/C and Pt₇₅Sn₂₅/C electrocatalysts. By X-ray diffraction analysis it was found that the Pt lattice parameter increases with the addition of Sn, indicative of alloy formation. It was confirmed that Sn exhibits cocatalytic activity for CO oxidation. The onset potential for the methanol oxidation reaction of the Pt–Sn electrode was approximately 0.1 V smaller than that on Pt both at room temperature and at 90 °C. The best performance in a direct methanol fuel cell was obtained using the Pt₇₅Sn₂₅/C alloy catalyst prepared by the formic acid method as the result of an optimal balance of Sn content, degree of alloying and metal particle size.     

Platinum/zinc oxide nanoparticles: Enhanced photocatalysts degrade malachite green dye under visible light conditions

Zinc oxide nanoparticles (ZnO) were prepared via a sol–gel method, and a photo-assisted deposition method was used to prepare platinum on zinc oxide nanoparticles (Pt/ZnO). Several techniques were used to characterize these enhanced photocatalysts: XRD, TEM, UV–vis spectra, PL spectra, XPS, and BET surface area analysis. As-prepared samples’ photocatalytic performances were studied via degradation of malachite green dye under various visible-light-only irradiation scenarios. Results demonstrated the following: platinum (Pt) was well dispersed on and in ZnO's surfaces and pores; as such, Pt/ZnO had less surface area than pure ZnO due to pore blockage; however, advantages gained from enhanced electron-hole separation and decreased band gap width more than made up for this negative effect; moreover, Pt/ZnO prepared with 0.3 wt% Pt exhibited the lowest band gap and the highest photocatalytic activity of the various samples with a solids loading of 0.8 g/l; finally, such samples were recyclable, i.e., photocatalytic performance remained stable even after five uses.     

Low-temperature catalytic combustion of methanol and its decomposed derivatives over supported gold catalysts

Gold can be compared favorably with Pd and Pt in the catalytic combustion of CH₃OH, HCHO and HCOOH when it is deposited on some reducible metal oxides (-Fe₂O₃, TiO₂, etc.). While the supported gold catalysts are less active in H₂ oxidation, they exhibit much higher activities in CO oxidation. For Au/TiO₂, the effect of catalyst preparation was further investigated. Since the activity for CO oxidation of the gold catalysts is not depressed but enhanced by moisture, they are practically applicable to CO removal from air at room temperature. Gold supported on manganese oxide is especially effective in the selective CO removal from hydrogen, indicating its potential applicability to polymer electrolyte fuel cells using the reformed gas of methanol.     

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.     

Modification of the catalytic properties of supported noble metal catalysts by carrier doping

The influence of altervalent cation doping of TiO₂ carriers on the chemisorptive and catalytic properties of supported Pt and Rh crystallites has been investigated. It was observed that doping of the carrier with higher valence cations leads to suppression of the H₂ and CO chemisorption capacity of Pt catalysts, while their activity in hydrogenation and oxidation reactions is significantly reduced. The opposite effects were observed in the case of Rh catalysts supported on higher valence doped TiO₂. These catalysts were found to possess higher activity in CO and CO₂ hydrogenation, in aromatics hydrogenation and in CO and C₂H₄ oxidation. Their stability characteristics were also found to be superior to those of the undoped Rh/TiO₂ catalyst. These effects are believed to originate from an electronic type interaction at the metal-support interface, induced by doping, which results in electron transfer from the support to the metal crystallites.     

Fabrication of CNTs with controlled diameters and their applications as electrocatalyst supports for DMFC

A facile synthesis procedure based on chemical vapor deposition (CVD) process has been developed to fabricate carbon nanotubes (CNTs) with controlled diameters and high yields utilizing Fe-containing ordered hexagonal mesoporous silicas (HMSs) such as MCM-41 and SBA-15 having varied pore sizes as the catalysts as well as the templates. It is found that unlike Fe/HMS catalysts prepared by co-precipitation method, samples prepared by the impregnation method gave rise to multi-wall CNTs with uniform diameters, which were largely dictated by the pore size of the Fe/HMS catalysts. Among these uniform MWCNTs, sample with a larger diameter (≥ 8 nm) was found to be more favorable as support for Pt catalyst, leading to a homogeneous dispersion of metal nanoparticles. Consequently, the Pt/CNT electrocatalysts so prepared gave rise to superior methanol oxidation activities as well as tolerances for CO poisoning compared to Pt supported on commercial single-wall CNT (Pt/SWCNT) and XC-72 activated carbon (Pt/XC-72) having a similar metal loading.     

The effect of platinum and silver deposits in the photocatalytic oxidation of resorcinol

The effects of platinum (Pt) and silver (Ag) metallisation in the photocatalytic oxidation of resorcinol at pH 3 ± 0.5 have been investigated. The photocatalytic degradation of resorcinol was significantly improved by Pt/TiO₂, while the presence of Ag/TiO₂ enhanced the initial photocatalytic degradation rate of resorcinol slightly. Likewise, the photocatalytic mineralisation of resorcinol continued to be enhanced by Pt/TiO₂, but it was retarded when Ag/TiO₂ was used.

The function of Pt and Ag deposits on the surface of TiO₂ has been found to be markedly influenced by the interaction of resorcinol and its degradation products with the metal deposits. The presence of Pt or Ag on the surface of TiO₂ altered the distribution of degradation products of resorcinol as well as the production of photoactive species for the photocatalytic oxidation of resorcinol. The X-ray photoelectron spectroscopy (XPS), zeta potential and transmission electron microscopy (TEM) analyses have indicated that the contrasting effect of Pt and Ag deposits were governed by the oxidation states and the catalytic property of metal deposits. In addition to that, it has been found that the roles of metal deposits are specific and should not be generalised.     

Synthesis and Characterization of Pt/C Nanocatalysts Using Room Temperature Ionic Liquids for Fuel Cell Applications

Carbon supported Pt nanocatalysts are prepared using different room temperature ionic liquids (RTILs) as the solvent and conventional preparation techniques, based on wet impregnation methods. The Pt/C nanocatalysts are characterized by XRD, TEM, EDX, and XPS. The results of the analyses show that the Pt/C catalysts, using different RTILs as solvents, are homogeneously dispersed with a narrow size distribution. The electro‐oxidation of liquid methanol on these catalysts is investigated at room temperature by cyclic voltammetry and chronoamperometry. The results have shown that the Pt/C catalysts prepared using RTILs are more active than the other Pt/C catalysts prepared by the authors. Surface area measurements of the Pt metal, conducted by electro‐oxidation of preadsorbed CO, indicate that catalysts prepared using RTILs as the solvents have higher surface area. The Pt/C nanocatalysts, prepared using RTILs, exhibit enhanced activity for the methanol oxidation reaction, compared with the Pt/C catalysts prepared by the impregnation method and commercial Pt/C catalysts.     

XPS and reactivity study of bimetallic nanoparticles containing Ru and Pt supported on a gold disk

We report a new method of immobilization of catalytic metal/alloy nanoparticles on a gold disk for transfer from an electrochemical cell to UHV (without sample exposure to air) for XPS analyses. Using this immobilization approach, several samples were examined: a core-shell Pt-on-Ru catalyst prepared from Ru black onto which Pt was spontaneously deposited, commercial Pt/Ru 50:50 nanoparticle alloy, as well as single metal Ru and Pt nanoparticle samples. The catalysts were characterized for the Ru oxidation state and for the methanol electrooxidation activity (as Pt was always metallic). For all bimetallic samples, we found that the reduced nanoparticles were more active towards methanol oxidation than the fully or partially oxidized samples. Regardless the Ru oxidation state however, the activity was lower than that previously reported for Ru decorated Pt nanoparticle catalysts (Ru-on-Pt). Possible reasons for the reactivity differences are discussed.     

The effects of SO2 on the oxidation of CO and propane on supported Pt and Au catalysts

The catalytic activities of Pt and Au supported on TiO₂ were compared with respect to the oxidation of CO and propane. While the Au catalysts showed higher activities for CO oxidation, the Pt catalysts were more active for propane combustion. A strong de-activation of the CO oxidation activity by SO₂ was observed only over the TiO₂-supported Au catalyst, indicating that SO₂ can block the active sites for CO oxidation over Au catalysts. The results are consistent with a model in which the perimeter sites have a special role in the CO oxidation reaction over Au catalysts.     

Superior catalytic behaviour of Pt-doped Pd catalysts in the complete oxidation of methane at low temperature

The catalytic combustion of methane at low temperature under lean conditions was investigated over bimetallic palladium-platinum catalysts supported on alumina. Pd-Pt catalysts with constant 2 wt.% metal loading and varying compositions in Pt and Pd were prepared by successive impregnations of the metal salts. The catalysts were characterised by powder X-ray diffraction, transmission electron microscopy/electron dispersion X-ray spectroscopy (TEM/EDX), volumetry of H₂ chemisorption, FTIR study of CO adsorption and temperature-programmed oxidation (TPO). In the absence of water added to the feed, the methane conversion over Pd-rich bimetallic catalysts (Pt/Pt + Pd molar ratios less than 0.3) was found to be the same as that of the reference Pd/Al₂O₃ catalyst. Interestingly, under wet conditions, these bimetallic catalysts exhibited an improved performance with respect to Pd/Al₂O₃. This effect was found to be maintained upon mild steam ageing. An interaction between both metals was suggested to explain the enhanced activity of bimetallic catalysts. This was confirmed by TPO experiments indicating that formation and decomposition of PdO is affected upon Pt addition even for very low amounts of Pt. The adsorption of CO on reduced catalysts studied by FTIR revealed new types of adsorbed CO species, suggesting again an interaction between two metals.     

Optical properties of Pt-TiO<Subscript>2</Subscript> catalyst and photocatalytic activities for benzene decomposition

In order to improve the photocatalytic decomposition activity of benzene, which has been regarded as a typical volatile organic compound in air, TiO₂ catalysts modified with metals (Pt, Cu, and Fe) were prepared and tested. Certain correlations between the photocatalytic activities and the optical properties of those catalysts were also found and discussed by using UV-visible spectroscopy and a photoluminescence spectroscopy. Among the metal impregnated TiO₂, the Pt impregnated TiO₂ showed the best activity and it was even better than that of P-25 which is widely used in commercial applications. For the various metal impregnated TiO₂ samples, certain proportional relationships were found between the observed photoluminescence values and photocatalytic activities. On the other hand, in UV-visible spectra for metal impregnated TiO₂ samples, the transmittance value was reduced depending upon the loading of metals. It was thought that photocatalytic activity increases from initial reaction state because the number of photoexcited electrons, which exist at Pt surface augment due to the band gap energy change of Pt and TiO₂ by sintering and light energy-absorbed electrons excited easily to conduction. In conclusion, it was confirmed that the enhanced photocatalytic activity for high metal loading on TiO₂ is related with the high concentration of excited electrons, which could be monitored through UV-visible spectra.     

Facile Preparation of an Excellent Pt/RuO2‐MnO2/CNTs Nanocatalyst for Anodes of Direct Methanol Fuel Cells

RuO₂‐MnO₂ complex supported by multi‐wall carbon nanotubes (CNTs) was firstly synthesised by the oxidation–reduction precipitation of RuCl₃ and KMnO₄ in one step. Then Pt was loaded onto the obtained RuO₂‐MnO₂/CNTs to fabricate a novel anodic catalyst Pt/RuO₂‐MnO₂/CNTs for direct methanol fuel cells (DMFCs). The catalyst was characterised by transmission electron microscopy (TEM), X‐ray diffraction (XRD), temperature programmed reduction (TPR), X‐ray photoelectron spectroscopy (XPS) and BET specific surface areas (BET). Pt nanoparticles were found uniformly dispersed on the surface of CNTs, with the average diameter of about 2.0 nm. The activities of methanol and CO electrocatalytic oxidation were analysed, and the reaction mechanism of methanol electro‐oxidation on Pt/RuO₂‐MnO₂/CNTs catalyst was discussed. The MnO₂ in the catalysts improves the proton conductivity and electrochemical active surface area (EAS) for the catalysts. RuO₂ improves the CO oxidation activity and Pt dispersion. CNTs provide effectively electron channels. Thus, the Pt/RuO₂‐MnO₂/CNTs catalyst has high utilisation of the noble metal Pt, high CO oxidation ability and excellent methanol electro‐oxidation activity, being an outstanding anode catalyst for DMFC.     

Selective CO oxidation in the presence of hydrogen over supported Pt catalysts promoted with transition metals

Selective CO oxidation in the presence of excess hydrogen was studied over supported Pt catalysts promoted with various transition metal compounds such as Cr, Mn, Fe, Co, Ni, Cu, Zn, and Zr. CO chemisorption, XRD, TPR, and TPO were conducted to characterize active catalysts. Among them, Pt-Ni/γ-Al₂O₃ showed high CO conversions over wide reaction temperatures. For supported Pt-Ni catalysts, Alumina was superior to TiO₂ and ZrO₂ as a support. The catalytic activity at low temperatures increased with increasing the molar ratio of Ni/Pt. This accompanied the TPR peak shift to lower temperatures. The optimum molar ratio between Ni and Pt was determined to be 5. This Pt-Ni/γ A1₂O₃ showed no decrease in CO conversion and CO₂ selectivity for the selective CO oxidation in the presence of 2 vol% H₂O and 20 vol% CO₂. The bimetallic phase of Pt-Ni seems to give rise to stable activity with high CO₂ selectivity in selective oxidation of CO in H₂-rich stream.     

CO oxidation over promoted Pt catalysts

A study of CO oxidation by O₂ over Pt catalysts, promoted by MnO_x and CoO_x, is described. The activities of Pt/SiO₂, Pt/MnO_x/SiO₂ and Pt/CoO_x/SiO₂ are compared with commercial Pt/Al₂O₃, Pt/Rh/Al₂O₃ and Pt/CeO_x/Al₂O₃ catalysts. Since these catalysts differ in dispersion and weight loading of platinum, the turnover frequencies are also compared. The following order in activity in CO oxidation after a reductive pretreatment is found: Pt/CoO_x/SiO₂ > Pt/MnO_x/SiO₂, Pt/CeO_x/Al₂O₃ > Pt/Al₂O₃, Pt/Rh/Al₂O₃, Pt/SiO₂. Over Pt/CoO_x/SiO₂ CO is already oxidised at room temperature. Possible models to account for the high activity of Pt/CoO_x/SiO₂ in the CO/O₂ reaction are presented and discussed. Partially reduced metal oxides are necessary to increase the activity of the Pt/CoO_x/SiO₂, Pt/MnO_x/SiO₂ or Pt/CeO_x/Al₂O₃ catalysts. It was shown that mild ageing treatments did not affect the activity of the Pt/CoO_x/SiO₂ catalyst in CO oxidation.     

Investigation of CO oxidation and NO reduction on three-way monolith catalysts with transient response techniques

The kinetics of CO oxidation and NO reduction reactions over alumina and alumina-ceria supported Pt, Rh and bimetallic Pt/Rh catalysts coated on metallic monoliths were investigated using the step response technique at atmospheric pressure and at temperatures 30–350°C. The feed step change experiments from an inert flow to a flow of a reagent (O₂, CO, NO and H₂) showed that the ceria promoted catalysts had higher adsorption capacities, higher reaction rates and promoting effects by preventing the inhibitory effects of reactants, than the alumina supported noble metal catalysts. The effect of ceria was explained with adsorbate spillover from the noble metal sites to ceria. The step change experiments CO/O₂ and O₂/CO also revealed the enhancing effect of ceria. The step change experiments NO/H₂ and H₂/NO gave nitrogen as a main reduction product and N₂O as a by-product. Preadsorption of NO on the catalyst surface decreased the catalyst activity in the reduction of NO with H₂. The CO oxidation transients were modeled with a mechanism which consistent of CO and O₂ adsorption and a surface reaction step. The NO reduction experiments with H₂ revealed the role of N₂O as a surface intermediate in the formation of N₂. The formation of NN bonding was assumed to take place prior to, partly prior to or totally following to the NO bond breakage. High NO coverage favors N₂O formation. Pt was shown to be more efficient than Rh for NO reduction by H₂.     

Effect of the nature of the support on the catalytic performance of noble metal catalysts for the water–gas shift reaction

The catalytic activity of supported noble metal catalysts (Pt, Rh, Ru, and Pd) for the WGS reaction is investigated with respect to the physichochemical properties of the metallic phase and the support. It has been found that, for all metal-support combinations investigated, Pt is much more active than Pd, while Rh and Ru exhibit intermediate activity. The turnover frequency (TOF) of CO conversion does not depend on metal loading, dispersion or crystallite size, but depends strongly on the nature of the metal oxide carrier. In particular, catalytic activity of Pt and Ru catalysts, is 1-2 orders of magnitude higher when supported on “reducible” (TiO₂, CeO₂, La₂O₃, and YSZ) rather than on “irreducible” (Al₂O₃, MgO, and SiO₂) metal oxides. In contrast to what has been found in our previous study over Pt/TiO₂ catalysts, catalytic activity of dispersed Pt does not depend on the structural and morphological characteristics of CeO₂, such as specific surface area or primary crystallite size.     

Experimental and Theoretical Study of CO Oxidation on PdAu Catalysts with NO Pulse Effects

The effect of NO on CO oxidation was studied for Pt, Pd and PdAu catalysts. It was found that NO inhibits significantly the CO oxidation reactivity on both Pt and Pd catalysts. On PdAu catalyst, however, the presence of NO resulted in an enhancement of CO oxidation activity. In order to gain an atomistic understanding of this effect, density-function theory (DFT) calculations were performed on the adsorption and reaction properties of NO and CO on these metal surfaces. We have identified that the inhibition effects on Pt and Pd catalysts are due to stronger NO binding, and that the enhanced reactivity on PdAu is due to the reduced NO oxidation barrier on PdAu leading to NO₂ formation.