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.     

Synthesis and characterization of carbon-supported Pt–Ru–WOx catalysts by spectroscopic and diffraction methods

Carbon-supported Pt–Ru–WO _x /C catalysts for application in PEMFC anodes were synthesized by a modified Bönnemann method. Their electrocatalytic activity for the oxidation of H₂/CO mixtures and CH₃OH was measured by E/i-curves in PEM single cell arrangements under working conditions. Information about composition, microstructure and nanomorphology was obtained by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray fluorescence analysis (XFA) and transmission electron microscopy (TEM). X-ray diffraction data at room temperature show only one single Pt f.c.c. phase; no evidence of Ru, W and their oxides, respectively, is found. Hence, the presence of W and Ru as amorphous oxide species seems likely. Surface-sensitive XPS measurements detect Pt⁰, platinum oxide and hydroxide species, metallic Ru, ruthenium oxide, hydrous ruthenium oxide and WO₃. For the crystalline platinum phase particle sizes of less than 2 nm were determined by TEM images and XRD patterns via solving the Scherrer equation. Temperature-dependent XRD measurements were performed to show the influence of ageing on the catalyst structure.     

Evaluation of the Performance of Carbon Supported Pt–Ru–Ni–P as Anode Catalyst for Methanol Electrooxidation

This research is aimed to improve the activity and stability of ternary alloy Pt–Ru–Ni/C catalyst. A novel anodic catalyst for direct methanol fuel cell (DMFC), carbon supported Pt–Ru–Ni–P nanoparticles, has been prepared by chemical reduction method by using NaH₂PO₂ as a reducing agent. One glassy carbon disc working electrode is used to test the catalytic performances of the homemade catalysts by cyclic voltammetric (CV), chronoamperometric (CA) and amperometric i–t measurements in a solution of 0.5 mol L^–1 H₂SO₄ and 0.5 mol L^–1 CH₃OH. The compositions, particle sizes and morphology of home‐made catalysts are evaluated by means of energy dispersive analysis of X‐ray (EDAX), X‐ray diffraction (XRD) and transmission electron micrographs (TEM), respectively. TEM images show that Pt–Ru–Ni–P nanoparticles have an even size distribution with an average diameter of less than 2 nm. The results of CV, CA and i–t curves indicate that the Pt–Ru–Ni–P/C catalyst shows significantly higher activity and stability for methanol electrooxidation due to the presence of non‐metal phosphorus in comparison to Pt–Ru–Ni/C one. All experimental results indicate that the addition of non‐metallic phosphorus into the Pt–Ru–Ni/C catalyst has definite value of research and practical application for enhancing the performance of DMFC.     

The Characterization of Tungsten-Oxide-Modified Zirconia Supports for Dual Functional Catalysis

The results of catalytic titration measurements indicated that WO _x /ZrO₂ catalysts prepared by coprecipitation have higher acid site density and strength, and are more active for pentane isomerization, than catalysts prepared by impregnation. The coprecipitated WO _x /ZrO₂ contains 0.004 meq-H⁺/g-catalyst. XPS of chemisorbed 2,6-dimethylpyridine and pyridine revealed the presence of Lewis acid sites as well as strong and weak Brønsted acid sites on the surface of these catalysts. The concentration of strong Brønsted sites was close to the concentration of Lewis sites, suggesting that the high acidity of these materials may originate from a conjugate Brønsted–Lewis site. Pt/WO _x /ZrO₂ undergoes a reversible loss of hydrogen chemisorption capacity with increasing hydrogen pretreatment temperature, accompanied by a reversible loss of pentane isomerization activity. This loss can be attributed to a strong Pt-reduced tungsten oxo species interaction (SMSI). Room temperature hydrogen spillover is observed in the Pt/WO _x /ZrO₂ (16% W) catalyst, consistent with the presence of bulk WO₃ in this material as observed by TPR.     

Tungsten oxide in polymer electrolyte fuel cell electrodes—A thin-film model electrode study

Thin films of WO_x and Pt on WO_x were evaporated onto the microporous layer of a gas diffusion layer (GDL) and served as model electrodes in the polymer electrolyte fuel cell (PEFC) as well as in liquid electrolyte measurements. In order to study the effects of introducing WO_x in PEFC electrodes, precise amounts of WO_x (films ranging from 0 to 40 nm) with or without a top layer of Pt (3 nm) were prepared. The structure of the thin-film model electrodes was characterized by scanning electron microscopy and X-ray photoelectron spectroscopy prior to the electrochemical investigations. The electrodes were analyzed by cyclic voltammetry and the electrocatalytic activity for hydrogen oxidation reaction (HOR) and CO oxidation was examined. The impact of Nafion in the electrode structure was examined by comparing samples with and without Nafion solution sprayed onto the electrode. Fuel cell measurements showed an increased amount of hydrogen tungsten bronzes formed for increasing WO_x thicknesses and that Pt affected the intercalation/deintercalation process, but not the total amount of bronzes. The oxidation of pre-adsorbed CO was shifted to lower potentials for WO_x containing electrodes, suggesting that Pt-WO_x is a more CO-tolerant catalyst than Pt. For the HOR, Pt on thicker films of WO_x showed an increased limiting current, most likely originating from the increased electrochemically active surface area due to proton conductivity and hydrogen permeability in the WO_x film. From measurements in liquid electrolyte it was seen that the system behaved very differently compared to the fuel cell measurements. This exemplifies the large differences between the liquid electrolyte and fuel cell systems. The thin-film model electrodes are shown to be a very useful tool to study the effects of introducing new materials in the PEFC catalysts. The fact that a variety of different measurements can be performed with the same electrode structure is a particular strength.     

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.     

用于环戊烯氧化合成戊二醛反应的钨基催化剂的表征

Tungsten-containing hexagonal mesoporous silica （W-HMS） supported tungsten oxide catalysts （WOx/W-HMS） was prepared for the selective oxidation of cyclopentene with aqueous hydrogen peroxide to glutaraldehyde. X-ray diffraction （XRD） results indicated that the crystal form of the active phase （tungsten oxide） of the WOx/W-HMS catalysts was dependent on the W loading and calcination temperature. X-ray photoelectron spec- troscopy （XPS） analysis revealed that the dispersed tungsten oxides on the surface of W-HMS support consisted of a mixture of W（V） and W（VI）. It was found that a high content of amorphous W species in （5＋） oxidation state resuited in the high catalytic activity. When the W loading was up to 12% （by mass） or the catalyst precursor was treated at temperature of 623 K, the catalytic activity decreased due to the presence of WO3 crystallites and the oxidation of W（V） to W（VI） on the catalyst surface. Furthermore, NH3-temperature-programmed-desorption （NH3-TPD） analysis showed that the effects of W loading and calcination temperature on the acidity of the catalysts were related to the catalytic activity. A high selectivity of 80.2% for glutaraldehyde with a complete conversion of cyclopentene was obtained over 8%WOx/W-HMS catalyst calcined at 573 K after 14 h of reaction.     

CO oxidation and CO2 reduction on carbon supported PtWO3 catalyst

The activity of a carbon supported PtWO₃ (PtWO₃/C) catalyst in the CO oxidation and CO₂ reduction reactions was evaluated in sulfuric acid solution at room temperature.Cyclic voltammetry combined with on-line mass spectrometry shows that the oxidation of both saturated CO adlayer and dissolved CO on PtWO₃/C material commences at rather low potentials, ca. 0.18 and 0.12 V vs. RHE, respectively. However, the low-potential process seems to involve only a minor fraction of the CO adlayer, the major part of the adsorbed CO layer being oxidised at the potentials as high as those for pure Pt catalysts—ca. 0.7 V vs. RHE. PtWO₃/C material was found to reversibly de-activate upon a prolonged exposure to the CO-saturated solution due to the inhibition of the hydrogen tungsten bronze formation.The reduction of CO₂ on PtWO₃/C leads to the formation of an adsorbate - presumably CO - on the Pt sites of the catalyst. Although the rate of the adsorbate build-up on PtWO₃/C at 0.1 V is lower than that on pure Pt/C, our results indicate that upon a prolonged exposure of the PtWO₃/C electrode to a CO₂-saturated solution a complete poisoning of the Pt sites with the adsorbate is likely to occur at room temperature.     

High activity PtRu/C catalysts synthesized by a modified impregnation method for methanol electro-oxidation

A modified impregnation method was used to prepare highly dispersive carbon-supported PtRu catalyst (PtRu/C). Two modifications to the conventional impregnation method were performed: one was to precipitate the precursors ((NH₄)₂PtCl₆ and Ru(OH)₃) on the carbon support before metal reduction; the other was to add a buffer into the synthetic solution to stabilize the pH. The prepared catalyst showed a much higher activity for methanol electro-oxidation than a catalyst prepared by the conventional impregnation method, even higher than that of current commercially available, state-of-the-art catalysts. The morphology of the prepared catalyst was characterized using TEM and XRD measurements to determine particle sizes, alloying degree, and lattice parameters. Electrochemical methods were also used to ascertain the electrochemical active surface area and the specific activity of the catalyst. Based on XPS measurements, the high activity of this catalyst was found to originate from both metallic Ru (Ru⁰) and hydrous ruthenium oxides (RuO_xH_y) species on the catalyst surface. However, RuO_xH_y was found to be more active than metallic Ru. In addition, the anhydrous ruthenium oxide (RuO₂) species on the catalyst surface was found to be less active.     

Ruthenium selenide catalysts for cathodic oxygen reduction in direct methanol fuel cells

The oxygen reduction reaction in sulphuric acid on commercial carbon supported platinum and ruthenium catalysts as well as on a home-made carbon supported ruthenium selenide catalysts (RuSe _x /C) was investigated. The RuSe _x /C catalysts were synthesised using similar procedures to those found in the literature. A dependency of H₂O₂ formation on the selenium content was found using the thin-film rotating ring disc electrode technique, namely that the H₂O₂ formation in the typical operation range of a Direct Methanol Fuel Cell (0.7–0.4 V) on Pt/C is below 1% and 1–4% on Ru/C and RuSe _x /C catalysts. Finally for comparing the intrinsic activities of the catalysts the electrochemically active surface areas were determined in-situ by means of copper underpotential deposition. Our results indicate a comparable activity of the present RuSe _x /C catalyst to commercial Pt/C if the activities are related to the electrochemical active areas.     

Nanosized Composite Pt‐Ru Catalysts for Production of Modern Modified Fuels

A Pt‐Ru/2 % Ce/(θ+α)‐Al₂O₃ nanosized catalyst was developed for selective catalytic oxidation of CH₄ to synthesis gas. The process was carried out entirely with the formation of synthesis gas at high selectivity by H₂ and CO with H₂:CO = 2.0 ratio only at Pt:Ru = 2:1 or 1:1 atomic ratio and short contact time on Pt‐, Ru‐, and Pt‐Ru low‐percentage catalysts. Samples, which were reduced by H₂ at high temperature, presented a mixture of Pt‐, Ru‐, and Pt‐Ru nanosized particles, its alloy in the mixed catalysts. The correlation between experimental results and data of physicochemical research was established. The activity together with physicochemical properties and quantum chemical calculations for the developed low‐percentage Pt‐Ru catalysts was investigated.     

Oxidation Reactions over Multi-Component Catalysts: Low-Temperature CO Oxidation and the Total Oxidation of CH4

A series of NM/MO _x /Al₂O₃ (NM = Pd, Ag, Pt, and Au) catalysts were prepared and tested in the oxidation of CO and CH₄. The catalysts were characterized with X-ray diffraction and transmission electron microscopy. Where addition of MO _x generally does not seem to affect the catalyst activity in CH₄ oxidation, a large enhancement in CO oxidation was observed. Fourier transform infrared spectroscopy has been used to identify the role of MO _x as a promoter for low-temperature CO oxidation. The results were found to support a Mars and van Krevelen type model.     

Radical formation in polymer electrolyte fuel cell components as studied by ESR spectroscopy

To clarify the deterioration mechanism for polymer electrolyte fuel cells, OH radical formation at the catalyst electrodes was investigated by ESR (electron spin resonance) spectroscopy using a flow cell with the catalyst electrodes. OH radicals produced from H₂O₂ were detected by a DMPO (5,5-dimethyl-1-pyrroline N-oxide) spin-trapping method for a Nafion-coated Pt/Carbon catalyst electrode under a high potential (0.85 V versus RHE) on supplying H₂ and under low potentials (lower than 0.40 V). When Pt–Ru catalysts were employed instead of Pt catalysts, the formation of OH radicals was barely detected. The results suggest the possibility of the formation of OH radicals by the oxidation of H₂O₂ at the oxidized Pt surface under a positive potential as well as the reduction of H₂O₂ at the clean Pt surface under a low potential.     

Synergy between tungsten and palladium supported on titania for the catalytic total oxidation of propane

Titania-supported palladium catalysts modified by tungsten have been tested for the total oxidation of propane. The addition of tungsten significantly enhanced the catalytic activity. Highly active catalysts were prepared containing a low loading of 0.5 wt.% palladium, and activity increased as the tungsten loading was increased up to 6 wt.%. Catalysts were characterised using a variety of techniques, including powder X-ray diffraction, laser Raman spectroscopy, X-ray photoelectron spectroscopy, temperature-programmed reduction and aberration-corrected scanning transmission electron microscopy. Highly dispersed palladium nanoparticles were present on the catalyst with and without the addition of WO_x. However, the addition of WO_x slightly increases the average palladium particle size, and there was some evidence for the Pd forming epitaxial islands on the support in the tungsten-doped samples. Surface analysis identified a combination of Pd⁰ and Pd²⁺ on a Pd/TiO₂ catalyst, whereas all of the Pd loading was found in the form of Pd²⁺ with the addition of tungsten into the catalysts. At low tungsten loadings, isolated monotungstate and some polytungstate species were highly dispersed over the titania support. The concentration of polytungstate species increased as the loading was increased, and it was also promoted by the presence of palladium. The coverage of the highly dispersed tungstate species over the titania also increased as the tungsten loading increased. Some tungstate species were also found to be associated with the palladium oxide particles, and there was an enrichment of oxidised tungsten species at the peripheral interface of the palladium oxide nanoparticles and the titania. Sub-ambient temperature–programmed reduction experiments identified an increased concentration of highly reactive species on catalysts with palladium and tungsten present together, and we propose that the new WO_x-decorated interface between PdO_x and TiO₂ particles may be responsible for the enhanced catalytic activity in the co-impregnated catalysts.     

Synergistic bimetallic Ru–Pt catalysts for the low-temperature aqueous phase reforming of ethanol

Aqueous phase reforming (APR) of ethanol has been studied over a series of Ru and Pt catalysts supported on carbon and titania, with different metal loadings and particle sizes. This study proposed that, on both metals, ethanol is first dehydrogenated to acetaldehyde, which subsequently undergoes C C cleavage followed by different paths, depending on the catalyst used. For instance, although monometallic Pt has high selectivity toward H₂ via dehydrogenation, it has a low efficiency for C C cleavage, lowering the overall H₂ yield. Large Ru particles produce CH₄ through methanation, which is undesirable because it consumes H₂. Small Ru particles have lower activity but higher selectivity toward H₂ rather than CH₄. On these small particles, CO blocks low-coordination sites, inhibiting methanation. The combination of the two metals in bimetallic Ru–Pt catalysts results in improved performance, benefiting from the desirable properties of each Ru and Pt, without the negative effects of either. © 2018 American Institute of Chemical Engineers AIChE J, 65: 151–160, 2019     

Trimerization of isobutene over WOx/ZrO2 catalysts

Trimerization of isobutene to produce isobutene trimers has been investigated over WO_x/ZrO₂ catalysts that were obtained by wet-impregnation and successive calcination at high temperatures. Very stable isobutene conversion and high selectivity for trimers are attained over a WO_x/ZrO₂ catalyst obtained by calcination at 700 °C. From the XRD study it can be understood that tetragonal ZrO₂ is beneficial for stable performance; however, monoclinic ZrO₂ is not good for trimerization. Nitrogen adsorption and FTIR experiments suggest that amorphous WO_x/ZrO₂ is inefficient catalyst even though it has high surface area and high concentration of acid sites. The observed performance with the increased selectivity and stable conversion demonstrates that a WO_x/ZrO₂ having tetragonal zirconia, even with decreased porosity and acid sites, is one of the best catalysts to exhibit stable and high conversion, high selectivity for trimers and facile regeneration.     

Mechanistic Investigation of Co Containing NO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Traps

Co-containing NO _x storage and reduction catalysts were investigated to identify the mechanism of Co promotion. X-ray diffraction and temperature programmed reduction demonstrated that Co exists in a highly oxidized state (Co₃O₄) and that the surface oxygen could be removed from the catalyst a typical operating conditions around 300 °C. Electron microscopy showed that Co is more uniformly distributed over the catalyst surface, as compared to Pt, with particle sizes ranging between 20 and 80 nm. In situ IR studies illustrated that NO _x storage occurs on Co-containing NSR catalyst via formation of nitrites and nitrates as surface intermediates. Finally, it was found that, similar to Pt, the addition of Co to Ba catalysts enhances the nitrite to nitrate transition rate and also increases the overall formation of nitrates. Therefore, the promotional effect shown by Co is the result of the combination of increased NO to NO₂ oxidation and improved surface area for NO₂ spillover to the Ba storage sites.     

Characterization of methanol-tolerant Pd–WO3 and Pd–SnO2 electrocatalysts for the oxygen reduction reaction in direct methanol fuel cells

Palladium (Pd) catalysts containing nanosized metal oxides, tungsten oxide (WO₃) and tin oxide (SnO₂), supported on carbon black (Pd–MO_x/C) were synthesized, and the effect of the metal oxide on the oxygen reduction reaction (ORR) in a direct methanol fuel cell (DMFC) was investigated. The SEM images showed that the Pd nanoparticles were highly dispersed on the carbon black, and the metal oxide particles were also distributed well. Pd/C and Pd–WO₃/C catalysts as cathode materials for the ORR in DMFCs showed activity similar to or better than that of Pt/C, whereas Pd–SnO₂/C showed no improvement in catalytic activity.     

Pt-Ru catalysts prepared by high energy ball-milling for PEMFC and DMFC: Influence of the synthesis conditions

High energy ball-milling was used to prepare several unsupported Pt-Ru anode catalysts for PEM- and direct methanol fuel cells. Pt and Ru with a 50:50 nominal Pt/Ru ratio were ball-milled at various ball-to-powder weight ratios (from 4/1 to 12/1) and with various Pt:Ru:MgH₂ proportions (from 1:1:2 to 1:1:10), where MgH₂ is a leacheable dispersive agent. The presence of MgH₂ is necessary to obtain unsupported catalysts with a specific surface area of between 50 and 75 m² g⁻¹. The ball-milling parameters greatly affected the relative proportions of the three phases constituting the catalysts. These phases are: Pt(Ru) alloy nanocrystallites, unalloyed Ru crystallites and nanocrystallites. The best CO tolerant catalyst is obtained by using a 12/1 ball-to-powder ratio and a 1:1:8 Pt:Ru:MgH₂ proportion of dispersive agent. It is made of 57 at.% of a nanocrystalline (3 nm) Pt₈₀Ru₂₀ alloy, 42 at.% of a nanocrystalline (3 nm) Ru phase and 1 at.% of a crystalline (∼40 nm) Ru phase. This catalyst has the lowest Pt/Ru surface ratio (0.9), the highest content in nanocrystalline Ru, and the highest ratio of oxidized/metallic Ru (3.3). Both Pt-Ru alloy and nanocrystalline Ru participate to the CO tolerance. The best CO tolerant catalyst is, however, not the best catalyst in DMFC. The latter is obtained by using a 4/1 ball-to-powder ratio and a 1:1:6 Pt:Ru:MgH₂ proportion. Within the starting 50:50 Pt-Ru nominal atomic ratio, no specific correlation was found between catalyst performance in DMFC and atomic surface Pt/Ru ratio, nor nanocrystalline Ru content, nor oxidized/metallic Ru ratio. Performances of the best ball-milled catalysts are compared to those of commercial unsupported catalysts in PEMFC and DMFC.     

Preparation and characterization of core–shell structured catalysts using PtxPdy as active shell and nano-sized Ru as core for potential direct formic acid fuel cell application

Carbon-supported core–shell structured Ru@Pt_xPd_y/C catalysts with Pt_xPd_y as shell and nano-sized Ru as core are prepared by a successive reduction procedure. The catalysts are extensively characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The formic acid oxidation activity of Ru@Pt_xPd_y/C varies with the varying Pt:Pd atomic ratio. The peak oxidation potential on Ru@Pt₁Pd₂/C shifts negatively for about 200 mV compared with that of Pd/C. The higher electro-catalytic activity toward formic acid oxidation on core–shell structured Ru@Pt_xPd_y/C catalyst than that on Pt_xPd_y/C suggests the high utilization of noble metals. In addition to the enhanced noble metal utilization, Ru@Pt_xPd_y/C catalyst also shows improved stability as evidenced by chronoamperometric evaluations.