Effect of heat treatment on stability of gold particle modified carbon supported Pt-Ru anode catalysts for a direct methanol fuel cell

Carbon supported Au-PtRu (Au-PtRu/C) catalysts were prepared as the anodic catalysts for the direct methanol fuel cell (DMFC). The procedure involved simple deposition of Au particles on a commercial Pt-Ru/C catalyst, followed by heat treatment of the resultant composite catalyst at 125, 175 and 200 °C in a N₂ atmosphere. High-resolution transmission electron microscopy (HR-TEM) measurements indicated that the Au nanoparticles were attached to the surface of the Pt-Ru nanoparticles. We found that the electrocatalytic activity and stability of the Au-PtRu/C catalysts for methanol oxidation is better than that of the PtRu/C catalyst. An enhanced stability of the electrocatalyst is observed and attributable to the promotion of CO oxidation by the Au nanoparticles adsorbed onto the Pt-Ru particles, by weakening the adsorption of CO, which can strongly adsorb to and poison Pt catalyst. XPS results show that Au-PtRu/C catalysts with heat treatment lead to surface segregation of Pt metal and an increase in the oxidation state of Ru, which militates against the dissolution of Ru. We additionally find that Au-PtRu/C catalysts heat-treated at 175 °C exhibit the highest electrocatalytic stability among the catalysts prepared by heat treatment: this observation is explained as due to the attainment of the highest relative concentration of gold and the highest oxidation state of Ru oxides for the catalyst pretreated at this temperature.     

Improvement of the high energy ball-milling preparation procedure of CO tolerant Pt and Ru containing catalysts for polymer electrolyte fuel cells

Ball-milling has been used to prepare performing CO tolerant polymer electrolyte fuel cell anode catalysts that contain Pt and Ru. The catalyst precursors are obtained by milling together Pt, Ru and a dispersing agent in the atomic ratio 0.5, 0.5 and 4.0. This precursor is not easily recovered after milling because it sticks to the walls of the vial and on the grinding balls. However, the precursor is recovered as a powder when a process control agent (PCA) is added during the milling step. Various PCAs have been used. The PCA should not interfere with the electrocatalytic activity of the catalysts obtained by leaching the precursor. The best preparation of catalyst precursors are obtained by milling: (i) Pt, Ru and Al (dispersing agent) in the atomic ratio 0.5, 0.5, 4.0 + 10 wt% NaF (PCA) or (ii) Pt , Ru and MgH₂ in the 0.5, 0.5, 4.0 atomic or molecular ratio. In this case, MgH₂ plays at the same time the role of a dispersing agent and that of a PCA. The catalysts are obtained by leaching Al and NaF in (i) or MgH₂ in (ii). The CO tolerance of these catalysts is equivalent to that of Pt_0.5Ru_0.5 Black from Johnson Matthey. The ball-milled catalysts have a surface area comprised between 30 and 44 m² g^–1. As-prepared catalysts are mainly made of metallic Pt and metallic plus oxidized Ru. After fuel cell tests, Pt is completely metallic while the oxidized Ru content decreases but does not disappear. These catalysts are composed of particles with crystallites of two different sizes: in (i) nanocrystallites (4 nm) that contain essentially Pt alloyed with Al and perhaps some Ru, and larger (30 nm) crystallites that contain essentially Ru; in (ii) Pt nanocrystalline particles that may contain some Ru and larger particles that contain essentially either Ru or Pt.     

CO tolerance and CO oxidation at Pt and Pt-Ru anode catalysts in fuel cell with polybenzimidazole-H3PO4 membrane

CO tolerance of H₂-air single cell with phosphoric acid doped polybenzidazole (PA-PBI) membrane was studied in the temperature range 140-180 °C using either dry or humidified fuel. Fuel composition was varied from neat hydrogen to 67% (vol.) H₂-33% CO mixtures. It was found that poisoning by CO of Pt/C and Pt-Ru/C hydrogen oxidation catalysts is mitigated by fuel humidification. Electrochemical hydrogen oxidation at Pt/C and Pt-Ru/C catalysts in the presence of up to 50% CO in dry or humidified H₂-CO mixtures was studied in a cell driven mode at 180 °C. High CO tolerance of Pt/C and Pt-Ru/C catalysts in FC with PA-PBI membrane at 180 °C can be ascribed to combined action of two factors—reduced energy of CO adsorption at high temperature and removal of adsorbed CO from the catalyst surface by oxidation. Rate of electrochemical CO oxidation at Pt/C and Pt-Ru/C catalysts was measured in a cell driven mode in the temperature range 120-180 °C. Electrochemical CO oxidation might proceed via one of the reaction paths—direct electrochemical CO oxidation and water-gas shift reaction at the catalyst surface followed by electrochemical hydrogen oxidation stage. Steady state CO oxidation at Pt-Ru/C catalyst was demonstrated using CO-air single cell with Pt-Ru/C anode. At 180 °C maximum CO-air single cell power density was 17 mW cm⁻² at cell voltage U = 0.18 V.     

Pt-Ru electrodeposited on gold from chloride electrolytes

Voltammetric behavior of submicron-thick electrodeposited Pt-Ru on gold support is studied in sulfuric acid solution as a function of deposition potential and Pt:Ru ratio in chloride bath. In contrast to Pt-Ru, deposition of pure Ru is observed only at potentials of hydrogen evolution. The reason is found to be of kinetic nature, namely an inhibition of Ru deposition in presence of chloride. Chloride ions remain adsorbed on Ru at more negative potentials than on Pt and Au because of more negative ruthenium potential of zero free charge. Cu-UPD is applied to test the surface content of the oxidized Ru on pure Ru and various Pt-Ru surfaces. An enhancement of Ru oxohydroxides reduction in presence of Pt is observed. The electrocatalytic activity of Pt-Ru in respect to methanol oxidation correlates with the content of rechargeable surface Ru oxide. Ageing and ‘training’ of Pt-Ru electrodeposits under various modes is studied in order to determine the conditions of irreversible Ru oxidation. No manifestations of Ru dissolution from Pt-Ru electrodeposits in 0.5 M H₂SO₄ are found for anodic potential limits up to 1.1 V (RHE), in agreement with thermodynamic predictions. Electrodeposited Pt-Ru can be considered as a convenient model system for the study of Ru dissolution and crossover, as well as for determining the nature of the active surface species in the real composite catalysts for methanol oxidation.     

Electrodeposition of Pt-Ru nanoparticles on fibrous carbon substrates in the presence of nonionic surfactant: Application for methanol oxidation

Liquid crystalline and micellar aqueous solutions of the nonionic surfactant Triton X-100 were used to direct the electrodeposition of Pt-Ru nanoparticles onto graphite felts, which were investigated as novel anodes for the direct methanol fuel cell. The effects of surfactant concentration, current density and deposition time in the preparation of these three-dimensional electrodes were studied in a factorial experiment and the electrodes were characterized by SEM and ICP-AES. Cyclic voltammetry, chronoamperometry and chronopotentiometry were carried out to assess the activity of the catalyzed felts for methanol oxidation. The presence of Triton X-100 (40-60 wt.%) coupled with an acidic plating solution were essential for the efficient co-electrodeposition of Ru in the presence of Pt to yield approximately a 1:1 Pt:Ru atomic ratio in the deposit. The highest mass specific activity, 24 A g⁻¹ at 298 K (determined by chronoamperometry after 180 s at 0 V versus Hg/Hg₂SO₄, K₂SO_4std), was obtained for the Pt-Ru electrodeposited in the presence of 40 wt.% Triton X-100 at 60 A m⁻², 298 K for 90 min. Surfactant mediated electrodeposition is a promising method for meso-scale (ca. 10-60 nm diameter) catalyst particle preparation on three-dimensional electrodes.     

Pt-WO3 supported on carbon nanotubes as possible anodes for direct methanol fuel cells

The carbon nanotube (CNT) synthesised by the template carbonisation of polypyrrole on alumina membrane has been used as the support for Pt-WO₃, Pt-Ru, and Pt. These materials have been used as the electrodes for methanol oxidation in acid medium in comparison with E-TEK 20 wt% Pt and Pt-Ru on Vulcan XC72R carbon. The higher electrochemical surface of the carbon nanotube (as evaluated by cyclic voltammetry) has been effectively used to disperse the catalytic particles. The morphology of the supported and unsupported CNT has been characterised by scanning electron micrograph and high-resolution transmission electron micrograph. The particle size of Pt, Pt-Ru, and Pt-WO₃ loaded CNT was found to be 1.2, 2, and 5 nm, respectively. The X-ray photoelectron spectra indicated that Pt and Ru are in the metallic state and W is in the +VI oxidation state. The electrochemical activity of the methanol oxidation electrode has been evaluated using cyclic voltammetry. The activity and stability (evaluated from chronoamperometric response) of the electrodes for methanol oxidation follows the order: GC/CNT-Pt-WO₃-Nafion>GC/E-TEK 20% Pt-Ru/Vulcan Carbon-Nafion>GC/CNT-Pt-Nafion>GC/E-TEK 20% Pt/Vulcan carbon-Nafion>Bulk Pt. The amount of nitrogen in the CNT plays an important role as observed by the increase in activity and stability of methanol oxidation with N₂ content, probably due to the hydrophilic nature of the CNT.     

Methanol oxidation at carbon supported Pt and Pt-Ru electrodes: an on line MS study using technical electrodes

Methanol oxidation at technical carbon based electrodes in 0.05 M H₂SO₄ has been investigated by cyclic voltammetry using online MS under the conditions of an acid methanol fuel cell (DMFC). 5% Pt on Norit BRX and 30% Pt/Ru (40/60) on Norit BRX were used as catalysts. It is shown that methanol oxidation at technical electrodes can be characterized by a combination of cyclic voltammetry and mass spectroscopy. The onset potentials and potential dependences of the methanol oxidation rate can be determined directly by monitoring the formation of CO₂. Onset potentials of 0.5V and 0.25 V/RHE have been measured for Pt and Pt-Ru catalysts, respectively. The onset of methanol oxidation can be shifted to even more cathodic potentials (0.2V) if the Pt-Ru electrode reduces oxygen simultaneously. Carbon monoxide gas was also purged into the methanol containing electroyte during measurement in order to investigate the catalyst performance under more adverse conditions. C¹³-labelled methanol was used to distinguish between CO₂. formed from methanol (m/e = 45) and CO-oxidation (m/e = 44). Without CO the use of C¹³-labelled methanol enabled a distinction between methanol oxidation and carbon corrosion. The methanol oxidation at the platinum catalyst is severely inhibited by the presence of CO, shifting its onset to 0.65 V/RHE. In contrast the performance of the Pt-Ru electrode is not seriously affected under these conditions. It is concluded that Pt-Ru is an excellent catalyst for a methanol anode in an acid methanol fuel cell (DMFC).     

The efficiency of methanol conversion to CO2 on thin films of Pt and PtRu fuel cell catalysts

Yields were determined for the CO₂ produced upon the electrochemical oxidation of 1.0 M methanol in 0.1 M HClO₄ at the following four fuel cell catalyst systems: Pt black, Pt at 10 wt.% metal loading on Vulcan XC-72R carbon (C/Pt, 10%), PtRu black at 50 at.% Pt, 50 at.% Ru (PtRu (50:50) black), and PtRu at 30 wt.% Pt, 15 wt.% Ru loading on Vulcan XC-72R carbon (C/PtRu, 30 wt.% Pt, 15 wt.% Ru). Samples were electrolyzed in a small volume (50 μl) arrangement for a period of 180 s keeping the reactant depletion in the cell below 1%. The dissolved CO₂ produced was determined ex situ by infrared spectroscopy in a micro-volume transmission flow cell. For the PtRu materials, the efficiencies for CO₂ formation were near 100% at reaction potentials in the range between 0.4 V (versus the reversible hydrogen electrode (RHE), V_RHE ) and 0.9 V_RHE. At the Pt catalysts, the yields of CO₂ approached 80% between 0.8 and 1.1 V_RHE and declined rapidly below 0.8 V_RHE.     

Effects of ozone treatment of carbon support on Pt-Ru/C catalysts performance for direct methanol fuel cell

This research is aimed to increase the activity and utilization of Pt-Ru alloy catalysts and thus to lower the catalyst loading in anodes for methanol electrooxidation. The Pt-Ru/C catalysts were prepared by chemical reduction. The support of Vulcan XC-72 carbon black was pretreated by ozone at different temperatures for different times. The specific surface area of the samples was evaluated by the standard BET method. The surface concentrations of oxygen were determined by XPS. The results showed that the surface concentrations of oxygen on the carbon were first decreased and then increased with pretreating times, and the specific surface area of the carbon was decreased with pretreating times at the same temperature. The specific surface area was increased with increasing temperature, and the surface concentration of oxygen was first decreased and then increased with increasing temperature for the same pretreating time. Pt-Ru/C catalysts supported by untreated and O₃ treated carbon black were characterized and tested for methanol electrooxidation. X-ray diffraction (XRD) was used to characterize the influence of carbon treated with ozone on Pt-Ru/C catalysts. It was found that the catalysts were composed only of f.c.c. Pt-Ru alloy particles without metallic Ru or Ru oxide. Cyclic voltammetry (CV) and Tafel curves were used for methanol electrooxidation on Pt-Ru/C catalysts in a solution of 0.5 mol/L CH₃OH and 0.5 mol/L H₂SO₄, showing that the catalytic activity of Pt-Ru/C catalysts supported by ozone treated carbon was higher than that by the untreated one. The ozone treatment time and temperature, which affect the performance of Pt-Ru/C catalysts, were discussed. Electrochemical measurements showed that the catalysts supported by the carbon after ozone treatment for 6 min at 140 °C had the best performance.     

Effect of the structure of Pt-Ru/C particles on COad monolayer vibrational properties and electrooxidation kinetics

In this paper, we combined FTIR spectroscopy and CO_ad stripping voltammetry to investigate CO_ad adsorption and electrooxidation on Pt-Ru/C nanoparticles. The Pt:Ru elemental composition and the metal loading were determined by ICP-AES. The X-ray diffraction patterns of the Pt-Ru/C indicated formation of a Pt-Ru (fcc) alloy. HREM images revealed an increase in the fraction of agglomerated Pt-Ru/C particles with increasing the metal loading and showed that agglomerated Pt-Ru/C nanoparticles present structural defects such as twins or grain boundaries. In addition, isolated Pt-Ru/C nanoparticles have similar mean particle size (ca. 2.5 nm) and particle size distributions whatever the metal loading. Therefore, we could determine precisely the effect of particle agglomeration on the CO_ad vibrational properties and electrooxidation kinetics. FTIR measurements revealed a main CO_ad stretching band at ca. , which we ascribed to a-top CO_ad on Pt domains electronically modified by the presence of Ru. As the metal loading increased, the position of this band was blue shifted by ca. 5 cm⁻¹ and a shoulder around 2005 cm⁻¹ developed, which was ascribed to a-top CO_ad on Ru domains. The reason for this was suggested to be the increasing size of Ru domains on agglomerated Pt-Ru/C particles, which lifts dipole-dipole coupling and allows two vibrational features to be observed (CO_ad/Ru, CO_ad/Pt). This is evidence that FTIR spectroscopy can be used to probe small chemical fluctuations of the Pt-Ru/C surface. Finally, we comment on the CO_ad electrooxidation kinetics. We observed that CO_ad was converted more easily into CO₂ as the metal loading, i.e. the fraction of agglomerated Pt-Ru/C nanoparticles, increased.     

Mechanical milling of catalyst support for enhancing the performance in fuel cells

In this work, we studied the characteristic variations of catalyst supports caused by mechanical milling and their electrochemical application in fuel cells. Two different catalyst supports, carbon black (XC-72R) and K20 (mesoporous carbon), were crushed and dispersed by mechanical milling using a bead mill. The bead mill operated with 0.3 μm zirconia beads at the rate of 3500 rpm for 30 min. The secondary particle size of the crushed catalyst supports ranged from around 0.1 μm to 10 μm. The secondary particle size of the catalyst supports after crushing represents a decrease of approximately 10% compared with that of raw catalyst supports. To confirm the role of the catalyst supports in the direct methanol fuel cell (DMFC), Pt and Ru were loaded onto these catalyst supports using an impregnation method. In the single cell test, Pt-Ru/XC-Bead and PtRu/K20-Bead showed power densities of 135 mW/cm² and 144 mW/cm² under air at 60 °C, respectively. The performance values of these catalysts, which were fabricated using reformed catalyst supports, were 10% to 20% higher than those of raw catalyst supports. As a result, the catalyst supports crushed by the bead mill helped to improve the electrochemical performance of the direct methanol fuel cell.     

NO + H2 reaction on Pt-Ru/SiO2 catalysts: correlation between nanostructure and catalytic activity and selectivity

Nitric oxide reduction by hydrogen has been studied on Pt-Ru/SiO₂ catalysts of various Pt/Ru atomic compositions in the temperature range 298–673 K. Physical characterization showed the presence of bimetallic particles which tend to be Pt-rich. The overall activity of the bimetallic catalysts suggests a dilution of the active component (Pt) in the range 373–523 K. The addition of Ru results in a general improvement of the N₂ selectivity and significant modifications in the product distribution are observed as a function of the catalyst composition. A bimetallic particle model is proposed in which various types of surfaces are exposed including those with pure Pt atoms and/or Pt-Ru mixture. This model allows to explain the overall activity and selectivity in the whole series of catalysts.     

Characterization and electrocatalytic properties of PtRu/C catalysts prepared by impregnation-reduction method using Nd2O3 as dispersing reagent

A simple impregnation-reduction method introducing Nd₂O₃ as dispersing reagent has been used to synthesize PtRu/C catalysts with uniform Pt-Ru spherical nanoparticles. X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) analysis have been used to characterize the composition, particle size and crystallinity of the catalysts. Well-dispersed catalysts with average particle size about 2 nm are achieved. The electrochemically active surface area of the different PtRu/C catalysts is determined by the CO_ad-stripping voltammetry experiment. The electrocatalytic activities of these catalysts towards methanol electrooxidation are investigated by cyclic voltammetry measurements and ac impedance spectroscopy. The in-house prepared PtRu/C catalyst (PtRu/C-03) in 0.5 M H₂SO₄ + 1.0 M CH₃OH at 30 °C display a higher catalytic activity and lower charge-transfer resistance (R_t) than that of the standard PtRu/C catalyst (PtRu/C-C). It is mainly due to enhanced electrochemically active specific surface, higher alloying extent of Ru and the abundant Pt⁰ and Ru oxides on the surface of the PtRu/C catalyst.     

Potential dependence of segregation and surface alloy formation of a Ru modified carbon supported Pt catalyst

Ruthenium modified carbon supported platinum catalysts have been shown to have a similar activity towards carbon monoxide oxidation as conventionally prepared bimetallic PtRu alloy catalysts. In this study the effect of the applied electrode potential and potential cycles on the location and oxidation state of the Ru species in such Ru modified Pt/C catalysts was investigated using in situ EXAFS collected at both the Ru K and Pt L₃ absorption edges. The as prepared catalyst was found to consist of a Pt core with a Ru oxy/hydroxide shell. The potential dependent data indicated alloying to form a PtRu phase at 0.05 V versus RHE and subsequent dealloying to return to the Ru oxy/hydroxide decorated Pt surface at potentials greater than 0.7 V. The Ru-O distances obtained indicate that both Ru³⁺ and Ru⁴⁺ species are present on the surface of the Pt particles at oxidising potentials; the former is characteristic of the as prepared Ru modified Pt/C catalyst and following extensive periods at potentials above 0.7 V and the latter of the Ru oxide species on the PtRu alloy.     

Novel Pt-Ru nanoparticles formed by vapour deposition as efficient electrocatalyst for methanol oxidation: Part II. Electrocatalytic activity

The methods developed and described in paper—part I are employed to prepare nanometer size Pt-Ru particles on a Vulcan^® XC72R substrate with controlled metal loading. Transmission Electron Microscopy (TEM) confirmed uniform particles size (average diameter 2 nm) and homogeneous dispersion of the particles over the substrate. Energy Dispersive X-ray absorption (EDX) analysis confirmed the compositional homogeneity. The catalytic activity of these supported nanoparticles with regard to methanol electrooxidation is investigated using cyclic voltammetry (CV), chronoamperometry (CA) and CO-stripping voltammetry techniques at temperatures between 25 and 60 °C. Such investigation concerns supported catalysts prepared with ca. 10 and 18 wt.% overall metal loading (Pt + Ru) onto the Vulcan^® XC72R substrate. Comparative testing of our catalysts and a commercial Pt-Ru/Vulcan reveals markedly superior activity for our catalysts. In fact, we observe for the latter a five-fold increase of the oxidation current as compared to a commercial Pt-Ru/Vulcan with equal metal loading. One of the reasons for the greater activity is found to be the very high dispersion of the metals over the substrate, i.e. the large surface area of the active phase. Other reasons are plausibly ascribable to the varied Pt/Ru composition and/or reduced presence of contaminants at the catalyst surface.     

Investigation of carbon-supported Pt and PtCo catalysts for oxygen reduction in direct methanol fuel cells

Carbon-supported Pt and Pt₃Co catalysts with a mean crystallite size of 2.5 nm were prepared by a colloidal procedure followed by a carbothermal reduction. The catalysts with same particle size were investigated for the oxygen reduction in a direct methanol fuel cell (DMFC) to ascertain the effect of composition. The electrochemical investigations were carried out in a temperature range from 40 to 80 °C and the methanol concentration feed was varied in the range 1-10 mol dm⁻³ to evaluate the cathode performance in the presence of different conditions of methanol crossover. Despite the good performance of the Pt₃Co catalyst for the oxygen reduction, it appeared less performing than the Pt catalyst of the same particle size for the cathodic process in the presence of significant methanol crossover. Cyclic voltammetry analysis indicated that the Pt₃Co catalyst has a lower overpotential for methanol oxidation than the Pt catalyst, and thus a lower methanol tolerance. Electrochemical impedance spectroscopy (EIS) analysis showed that the charge transfer resistance for the oxygen reduction reaction dominated the overall DMFC response in the presence of high methanol concentrations fed to the anode. This effect was more significant for the Pt₃Co/KB catalyst, confirming the lower methanol tolerance of this catalyst compared to Pt/KB. Such properties were interpreted as the result of the enhanced metallic character of Pt in the Pt₃Co catalyst due to an intra-alloy electron transfer from Co to Pt, and to the adsorption of oxygen species on the more electropositive element (Co) that promotes methanol oxidation according to the bifunctional theory.     

Anodic catalysts for polymer electrolyte fuel cells: the catalytic activity of Pt/C,Ru/C and Pt-Ru/C in oxidation of CO by O2

The catalytic activity for oxidation of CO by O₂ was investigated on commercial Pt/C, Pt-Ru/C (Pt/Ru atomic ratio = 20, 3, 1, 1/3) and Ru/C. All samples contained 20 wt.% metal. Assuming equal surface and bulk composition, the number of surface Pt and Ru atoms was calculated from the average size of the supported metal particle as determined by TEM. On Pt-Ru/C alloys, the turnover frequency per Ru atom, N_Ru/molecules s⁻¹ Ru-atom⁻¹, was independent of chemical composition. This finding suggests that the active site in these alloys is Ru. In the temperature range 300–400 K, the turnover frequency per active metal atom was 50–300 times higher on Pt-Ru/C than on Pt/C. The turnover frequency was 400 times higher on Ru/C than on Pt/C at 313 K and 90 times higher at 353 K. Addition of water vapor to the reactant mixture left the catalytic activity of Ru/C unchanged but slightly increased the activity of Pt/C. On both catalysts the activation energy and reaction orders were nearly the same as in dry atmosphere. Conversely, the addition of water markedly decreased the activation energy for Pt-Ru(1 : 1)/C alloy (from 19 to 11 kcal mol⁻¹). These findings suggest that fuel cells equipped with Pt-Ru/C anodes perform better than cells with Pt/C anodes. They do so because Ru effectively oxidizes the carbon monoxide present as an impurity in the H₂-reformed fuel.     

Preparation of supported PtRu/C electrocatalyst for direct methanol fuel cells

In this work, high-surface supported PtRu/C were prepared with Ru(NO)(NO₃)₃ and [Pt(H₂NCH₂CH₂NH₂)₂]Cl₂ as the precursors and hydrogen as a reducing agent. XRD and TEM analyses showed that the PtRu/C catalysts with different loadings possessed small and homogeneous metal particles. Even at high metal loading (40 wt.% Pt, 20 wt.% Ru) the mean metal particle size is less than 4 nm. Meanwhile, the calculated Pt crystalline lattice parameter and Pt (2 2 0) peak position indicated that the geometric structure of Pt was modified by Ru atoms. Among the prepared catalysts, the lattice parameter of 40-20 wt.% PtRu/C contract most. Cyclic voltammetry (CV), chronoamperometry (CA), CO stripping and single direct methanol fuel cell tests jointly suggested that the 40-20 wt.% PtRu/C catalyst has the highest electrochemical activity for methanol oxidation.     

Characteristics of adsorbed CO and CH3OH oxidation reactions for complex Pt/Ru catalyst systems

Pt/Ru powder catalysts of the same nominal Pt to Ru composition were prepared using a range of methods resulting in different catalyst properties. Two PtRu alloy catalysts were prepared, one of which has essentially the same surface and bulk Pt to Ru composition, while the second catalyst is surface enriched with Ru. Two powders consisting of non-alloyed Pt phases and surfaces enriched with Ru were also prepared. The oxidation state of the surface Ru of the latter two catalysts is mainly metallic Ru or Ru-oxides. The catalyst consisting of Ru-oxides was formed at 500 °C. Part of this catalyst was then reduced in a H₂ atmosphere under “mild” conditions, thus catalyst properties such as particle size are not changed, as they are locked in during previous high temperature treatment. The oxidation kinetics of adsorbed CO (CO_ads) and solution CH₃OH were studied and compared to the Ru ad-metal state and Pt to Ru site distribution of the as-prepared catalysts. The kinetics of the CO_ads oxidation reaction were observed to be slower for the catalyst containing Ru-oxides as opposed to mainly Ru metal. The CH₃OH oxidation activities measured per Pt surface area, i.e., the catalytic activities are better (by ca. seven times) for the alloy catalysts than the non-alloyed Pt/Ru catalysts. The latter two catalysts showed essentially the same catalytic CH₃OH oxidation activities, i.e., independent of the Ru ad-metal oxidation state of the as-prepared catalysts. Furthermore, it is shown that CO_ads oxidation experiments can be used to extract characteristics that allow the comparison of catalytic activities for the CO_ads oxidation reaction and Pt to Ru site distribution for complex catalyst systems.     

High energy ball-milled Pt and Pt–Ru catalysts for polymer electrolyte fuel cells and their tolerance to CO

High energy ball milling, an industrially amenable technique, has been used to produce CO tolerant unsupported Pt–Ru based catalysts for the oxidation of hydrogen in polymer electrolyte fuel cells. Nanocrystalline Pt_0.5–Ru_0.5 alloys are easily obtained by ball-milling but their performances as anode catalysts are poor because nanocrystals composing the material aggregate during milling into larger particles. The result is a low specific area material. Improved specific areas were obtained by milling together Pt, Ru and a metal leacheable after the milling step. The best results were obtained by milling Pt, Ru, and Al in a 1:1:8 atomic ratio. After leaching Al, this catalyst (Pt_0.5–Ru_0.5 (Al₄)) displays a specific area of 38 m²g^–1. Pt_0.5–Ru_0.5 (Al₄) is a composite catalyst. It consists of two components: (i) small crystallites (4 nm) of a Pt–Al solid solution (1–3 Al wt%) of low Ru content, and (ii) larger Ru crystallites. It shows hydrogen oxidation performance and CO tolerance equivalent to those of Pt_0.5–Ru_0.5 Black from Johnson Matthey, the commercial catalyst which was found to be the most CO tolerant one in this study.