Support effect on the low-temperature hydrogenation of benzene over PtCo bimetallic and the corresponding monometallic catalysts

PtCo bimetallic and Co, Pt monometallic catalysts supported on γ-Al₂O₃, SiO₂, TiO₂ and activated carbon (AC) were prepared and evaluated for the hydrogenation of benzene at relatively low temperatures (343 K) and atmospheric pressure. Results from flow reactor studies showed that supports strongly affected the catalytic properties of different bimetallic catalysts. AC supported PtCo bimetallic catalysts exhibited significantly better performance than the other bimetallic catalysts, and all the bimetallic catalysts possessed higher activity than the corresponding monometallic catalysts. Results from CO chemisorption and H₂-temperature-programmed reduction (H₂-TPR) studies suggested that different catalysts possessed different properties in chemisorption capacity and reduction behavior, and AC supported PtCo catalysts possessed significantly higher CO chemisorption capacity compared to the other catalysts. Extended X-ray absorption fine structure (EXAFS) and transmission electron microscopy (TEM) analysis provided additional information regarding the formation of Pt–Co bimetallic bonds and metallic particle size distribution in the PtCo bimetallic catalysts on different supports.     

Performance and durability of PtCo alloy catalysts for oxygen electroreduction in acidic environments

The performance and durability of PtCo alloy catalysts for oxygen electroreduction in acidic environments were investigated by the electrochemical rotating disk electrode (RDE) technique, in situ electrochemical scanning tunneling microscopy (STM), and inductively coupled plasma-optical emission spectroscopy (ICP-OES). In comparison with benchmark Pt catalysts, PtCo alloy catalysts offer both enhanced catalytic activity towards molecular oxygen electroreduction and improved durability under electrochemical forces in acidic environments for short-term operation; however, PtCo alloy catalysts do not provide long-term improvement in catalytic activity and electrochemical durability due to the leaching of elemental cobalt from the alloys under intensive potential cycling. Therefore, similar to Pt, PtCo alloys are not viable cathode catalysts for low temperature fuel cells for transportation applications.     

Synthesis of an improved hierarchical carbon-fiber composite as a catalyst support for platinum and its application in electrocatalysis

A hierarchical carbon-fiber composite was synthesized based on carbon cloth (CC) modified with primary carbon microfibers (CMF) and subsequently secondary carbon nanotubes (CNT), thus forming a three-dimensional hierarchical structure with high BET surface area. The primary CMFs and the secondary CNTs are grown with electrodeposited iron nanoparticles as catalysts from methane and ethylene, respectively. After deposition of Pt nanoparticles by chemical vapor deposition from (trimethyl)cyclopentadienylplatinum, the resulting hierarchical composite was used as catalyst in the electrocatalytic oxygen reduction (oxygen reduction reaction, ORR) as specific test reaction. The modification of the CC with CMFs and CNTs improved the electrochemical properties of the carbon composite as revealed by electrochemical impedance measurements evidencing a low charge transfer resistance for redox mediators at the modified CC. X-ray photoelectron spectroscopy measurements were carried out to identify the chemical state and the surface atomic concentration of the Pt catalysts deposited on the hierarchical carbon composites. The ORR activity of Pt supported on different composites was investigated using rotating disk electrode measurements and scanning electrochemical microscopy. These electrochemical studies revealed that the obtained structured catalyst support is very promising for electrochemical applications, e.g. fuel cells.     

Importance of catalyst stability vis-à-vis hydrogen peroxide formation rates in PEM fuel cell electrodes

The role of catalyst stability on the adverse effects of hydrogen peroxide (H₂O₂) formation rates in a proton exchange membrane fuel cell (PEMFC) is investigated for Pt, Pt binary (PtX, X = Co, Ru, Rh, V, Ni) and ternary (PtCoX, X = Ir, Rh) catalysts supported on ketjen black (KB) carbon. The selectivity of these catalysts towards H₂O₂ formation in the oxygen reduction reaction (ORR) was measured on a rotating ring disc electrode. These measured values were used in conjunction with local oxygen and proton concentrations to estimate local H₂O₂ formation rates in a PEMFC anode and cathode. The effect of H₂O₂ formation rates on the most active and durable of these catalysts (PtCo and PtIrCo) on Nafion membrane durability was studied using a single-sided membrane electrode assembly (MEA) with a built-in reference electrode. Fluoride ion concentration in the effluent water was used as an indicator of the membrane degradation rate. PtIrCo had the least fluorine emission rate (FER) followed by PtCo/KB and Pt/KB. Though PtCo and PtIrCo show higher selectivity for H₂O₂ formation than unalloyed Pt, they did not contribute to membrane degradation. This result is explained in terms of catalyst stability as measured in potential cycling tests in liquid electrolyte as well as in a functional PEM fuel cell.     

Electrocatalysis of oxygen reduction on carbon-supported PtCo catalysts prepared by water-in-oil micro-emulsion

Synthesis of carbon-supported PtCo/C using micro-emulsion method including simultaneous procedure and sequential procedures in both acid and alkaline media was reported. UV-vis and electron microscopy were used to characterize the formation, surface morphology and distribution of PtCo nanoparticles. Crystallite structure of catalysts was analyzed from XRD patterns. Catalytic properties of PtCo/C catalysts synthesized were compared with commercial Pt/C using RDE based on both mass activity (MA) and specific activity (SA). PtCo/C catalysts prepared in both acidic and basic conditions showed better performance than commercial Pt/C catalyst. High-temperature heat treatment was found useful only to PtCo/C by sequential procedure. The peroxide yield was also explored using RRDE technique. The H₂O₂ yield results were correlated with SA and R values (ratio of charge transferred about Co and Pt on the surface of catalyst) obtained from CVs in 1 M KOH solution. A sacrificial Co oxidized effect on impediment of adsorption of OH may cause higher catalytic properties and higher H₂O₂ yield to Pt base alloy catalysts.     

Influence of carbon-oxygen surface complexes on the surface acidity of tungsten oxide catalysts supported on activated carbons

Tungsten oxide catalysts supported on activated carbons were prepared by using tungsten hexacarbonyl, ammonium tungstate, and tungsten pentaethoxide as precursors. An activated carbon was obtained from olive stone by physical activation. A portion of this activated carbon was oxidized with ammonium peroxydisulfate in order to introduce different oxygen surface complexes. Subsequently, different portions of this oxidized activated carbon were heat treated in nitrogen flow at various temperatures to partially remove the oxygen surface complexes. In this way, activated carbons with different amounts of oxygen surface complexes were obtained, which were then used as supports for the tungsten oxide catalysts. Both the supports and the supported catalysts were pre-treated either in He, dry air or wet air flow at 623 K for 6 h. They were then characterized by X-ray photoelectron spectroscopy, X-ray diffraction, measurements of the pH of the point of zero charge, and activity in the decomposition reaction of isopropanol. Turnover frequencies for the formation of propene were obtained. According to these results, the oxygen surface complexes of the support have a major influence on the total acidity of the tungsten oxide supported catalysts. In some supported catalysts, W(VI) was reduced to W(V) during the decomposition reaction of isopropanol as a consequence of the hydrogen evolution. The results indicate that oxygen surface complexes of the support may play an important role in this reduction process, which was inhibited when the support had high surface oxygen content.     

Monodispersed PtCo nanoparticles on hexadecyltrimethylammonium bromide treated graphene as an effective oxygen reduction reaction catalyst for proton exchange membrane fuel cells

We report hexadecyltrimethylammonium bromide (CTAB)-functionalized graphene as a carbon support for Pt or PtCo nanoparticle (NP) catalysts for fuel cell cathodes. The CTAB treatment plays several critical roles in improving the cell performance: CTAB is non-covalently bound on the graphene surfaces and also functionalizes NP surfaces, thus minimizing aggregation between graphene sheets as well as between NPs with extremely small dimensions down to 1–2 nm for a large number of cycles. Also, unlike covalent bonding based treatments, the CTAB treatment preserves intrinsic electronic and structural properties of graphene. The increased dispersion and decreased dissolution of NP catalysts using the CTAB functionalization are reflected in various electrochemical measurements such that the CTAB-treated samples exhibit higher values in oxygen reduction reaction activity, electrochemical active surface area, and long-term durability compared to commercial catalysts and control cases with no such treatment. Finally, for the same CTAB-treated graphene, PtCo catalyst shows a higher catalytic activity than does Pt catalyst, thus confirming the known improved activities of the alloyed catalysts.     

Phosphate adsorption and its effect on oxygen reduction reaction for PtxCoy alloy and Aucore–Ptshell electrocatalysts

The phosphate adsorption characteristics and its effect on oxygen reduction reaction (ORR) were examined for various carbon-supported catalysts (Pt/C, Pt₃Co/C, PtCo/C, and Au_core–Pt_shell/C). Using cyclic voltammetry (CV) and the addition of phosphoric acid, the degree of phosphate adsorption for each catalyst was evaluated based on the intensity of the phosphate adsorption peaks (0.25–0.3 V and 0.5–0.65 V) and on the decrease in the platinum oxidation current (0.9 V). In the N₂O reduction technique, the surface structures were analyzed using N₂O as an electrochemical probe, which showed that as the Co content increased, (i) steps or defects were introduced by surface reconstruction, (ii) the phosphate adsorbed more strongly compared to Pt/C with a preference for the terrace sites, and (iii) the potential of zero total charge (PZTC) shifted to negative potentials. In the case of the Au_core–Pt_shell/C, the phosphate adsorption was found to be weaker than other catalysts, including Pt/C catalyst. The relative ORR activity with PA addition, normalized by that with no phosphate adsorption, was significantly smaller for Co containing alloy catalysts (PtCo/C: 18.2%) and larger for Au_core–Pt_shell (30.2%) compared with the Pt/C catalyst (27.8%), confirming the phosphate adsorption characteristics of each catalyst, as measured by CV and N₂O reduction analysis.     

CO2 Hydrogenation Studies on Co and CoPt Bimetallic Nanoparticles Under Reaction Conditions Using TEM, XPS and NEXAFS

Cobalt and platinum?Ccobalt bimetallic alloy nanoparticles of uniform size distribution where prepared and supported on MCF-17 to produce a controlled and well-characterized model catalyst which was studied under reaction conditions during CO₂ hydrogenation. Near edge X-ray absorption fine structure (NEXAFS) spectroscopy was used to elucidate the oxidation state of the catalyst under reaction conditions while the effect of reducing H₂ gas on the composition and structure of the bimetallic PtCo nanoparticles was measured using ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and environmental transmission electron microscopy (ETEM). NEXAFS indicates that Pt aids the reduction of Co to its metallic state under relevant reaction conditions, while AP-XPS and ETEM indicate that Pt is enriched at the surface by exchange with subsurface layers which become Pt deficient??in agreement with the ??Pt-like?? selectivity seen during catalytic testing of these materials.     

Synthesis and characterization of Cocore-Ptshell electrocatalyst prepared by spontaneous replacement reaction for oxygen reduction reaction

Carbon-supported PtCo nanoparticles catalysts with core-shell structure were prepared via reduction method and subsequent electroless deposition. It was found that pH has critical effect on the morphology and activity of the synthesized catalyst. The favorable pH to prevent the formation of oxide impurities and extensive Co dissolution during the replacement reaction is neutral. The intrinsic activity of the PtCo/C catalysts for oxygen reduction reaction was found to be 2-4 times higher compared with that of Pt/C. This activity enhancement may originate from the favorable Pt-Pt interatomic distance and low defect that would inhibit the formation of Pt-OH_ad at low overpotential.     

Surface coverage of Pt atoms on PtCo nanoparticles and catalytic kinetics for oxygen reduction

The surface coverage of Pt atoms on PtCo nanoparticles and its effect on catalytic kinetics for oxygen reduction were investigated. The PtCo nanoparticles with different surface coverage of Pt atoms were synthesized with various methods, including normal chemical method, microemulsion synthesis, and ultrasound-assisted microemulsion. A model of Pt atoms filling into a spherical nanoparticle was proposed to explain the relationship of surface metal atoms and nanoparticle size. The catalytic activity of the PtCo nano-particles is highly dependent on the synthetic methods, even if they have the same chemical composition. The PtCo nano-particles synthesized with ultrasound-assisted microemulsion showed the highest activity, which is attributed to an increase of active surface coverage of Pt atoms on the metal nanoparticles. The rate of oxygen reduction at 0.5 V (vs. SCE) catalyzed by the PtCo synthesized with ultrasound-assisted micro-emulsion was about four times higher than that of the PtCo synthesized with normal chemical method. As demonstrated with rotating-ring disk electrode measurement, the PtCo nano-particles can catalyze oxygen 4-electron reduction to water without intermediate H₂O₂ detected.     

Methanol tolerant electrocatalysts for the oxygen reduction reaction

Direct methanol fuel cells (DMFCs) represent an interesting alternative in obtaining electricity in a clean and efficient way. Portable power sources are one of the most promising applications of passive DMFCs. One of the requirements in these devices is to use high alcohol concentration, which due to methanol crossover causes a considerable loss of fuel cell efficiency. In order to develop methanol tolerant cathodes with suitable activity, different supported catalysts namely PtCo/C and PtCoRu/C, were prepared either via ethylene glycol reduction (EG) with or without microwave heating assistance (MW) or via the alloy method, the latter followed by a thermal treatment in a reducing atmosphere (N₂/H₂). All cathode-catalysts were tested to determine the role of the components in simultaneously enhancing the oxygen reduction reaction (ORR) and discouraging the methanol oxidation reaction. According to the synthesis methodology, X-ray photoelectron spectra showed that the amount of metal oxides on the surface varies, being higher on the PtCo/C EG and PtCoRu/C EG catalysts. The electrochemical characterization of the catalysts was accomplished in a three electrodes electrochemical cell with a glassy carbon rotating disk electrode covered with a thin catalytic film as working electrode. To study the ORR and the influence of different methanol concentrations, linear sweep voltammetry and cyclic voltammetry were employed. The PtCo/C EG, with an important metal oxide amount on the surface, and the PtCoRu/C MW and EG electrodes, both with RuO₂ on their surfaces, were the most tolerant to methanol presence.     

Noble metals supported on carbon black composites as catalysts for the wet-air oxidation of phenol

Pt, Pd and Ru catalysts supported on carbon black composites (CBC) were characterized in the wet air oxidation of phenol solution using a fixed-bed reactor working in a trickle-flow regime under relatively mild conditions: temperature, 393-433 K; pressure, 50-80 bar; liquid hourly space velocity (LHSV), 0.5-6 h⁻¹. The activity of the catalysts decreases in the following order: Pt/CBC>Pd/CBC≈Ru/CBC?CBC. The physicochemical properties of the CBC are affected by its reaction with oxygen during the oxidation process. Combustion of the CBC material in the aqueous phase proceeds at a lower temperature than that in the gas phase; its surface properties change according the same rules as during low temperature oxidation by gaseous air.     

PtCo/Polypyrrole‐Multiwalled Carbon Nanotube Complex Cathode Catalyst Containing Two Types of Oxygen Reduction Active Sites Used in Direct Methanol Fuel Cells

Low oxygen reduction reaction (ORR) activity and high cost of noble metal catalysts are two major challenges in direct methanol fuel cells (DMFCs). Pt‐based catalysts are considered as an ideal alternative to deal with these two problems. While the second component metals play only the role of synergy effect with Pt, they themselves are inert towards activity towards ORR. It is necessary to design a new route to ultilize the second component metal by forming CoN_x ORR active site on the base of PtM catalyst. In this paper, PtCo/polypyrrole‐multiwalled carbon nanotubes (PtCo/PPy‐MWCNTs) catalyst containing two types of ORR active site (Pt and CoN_x) was synthesized by one pot synthesis route. The effect and dynamic mechanism of the named CoN_x active site towards ORR was discussed by X‐ray photoelectron sprectroscopy and linear sweep voltammetry. PtCo/PPy‐MWCNTs cathode catalyst showed improved activity towards ORR and great potential in DMFCs.     

The electrochemical behavior of cobalt phthalocyanine/platinum as methanol-resistant oxygen-reduction electrocatalysts for DMFC

The electrochemical behavior of cobalt phthalocyanine/platinum as methanol-resistant oxygen-reduction electrocatalyst for DMFC was investigated. Platinum was chemically deposited on the carbon-supported cobalt phthalocyanine (CoPc), and then it was heat-treated in high purity nitrogen at 300 °C, 635 °C and 980 °C. In order to evaluate the electrocatalytic behavior of CoPc-Pt/C, the PtCo/C and Pt/C as reference catalysts were employed. TGA, XRD, EDAX, XPS and electrochemical experiments were used to study the thermal stability, crystal structure, physical characterization and electrochemical behavior of these catalysts. These catalysts exhibited similar electrocatalytic activity for oxygen reaction in 0.5 M H₂SO₄ solution. In methanol tolerance experiments, Pt/C, PtCo/C and CoPc-Pt/C heated at 980 °C were active for the methanol oxidation reaction (MOR). The presence of Co did not improve resistance to methanol poisoning. However, the CoPc-Pt/C after 300 °C or 635 °C heat-treatment demonstrated significant inactivity for MOR, hence they have a good ability to resist methanol poisoning. The current study indicated that the macrocyclic structure of phthalocyanine is the most important factor to improve the methanol tolerance of CoPc-Pt/C as the oxygen-reduction reaction (ORR) electrocatalyst. The CoPc-Pt based catalyst should be a good alternation for oxygen electro-reduction reaction in DMFC.     

Dissolved oxygen removal by combination with hydrogen using wetproofed catalysts

Dissolved oxygen in water at parts per million levels could be reduced to a few parts per billion by reaction with hydrogen using Pt catalysts supported on carbon and stainless steel in random and structured bed configurations. The carbon supported catalyst was Teflon coated to wetproof it. Both gas phase and liquid phase reactions occurred simultaneously under trickle bed operation, resulting in higher oxygen removal efficiency for this mode of operation than for the liquid-filled condition. The structured catalyst bed yielded greater hydraulic capacity than the random bed, and with wetproofed catalyst it gave the best oxygen removal efficiency. Since the gas phase reaction rate could be increased by reducing the wetted fraction of the catalyst through wetproofing, wetproofed catalysts offer a unique advantage over conventional hydrophilic catalysts.     

Activity,selectivity, and methanol tolerance of novel carbon-supported Pt and Pt<Subscript>3</Subscript>Me (Me = Ni,Co) cathode catalysts

The activity, selectivity, and methanol tolerance of novel, carbon supported high-metal loading (40 wt.%) Pt/C and Pt₃Me/C (Me = Ni, Co) catalysts for the O₂ reduction reaction (ORR) were evaluated in model studies under defined mass transport and diffusion conditions, by rotating (ring) disk and by differential electrochemical mass spectrometry. The catalysts were synthesized by the organometallic route, via deposition of pre-formed Pt and Pt₃Me pre-cursors followed by their decomposition into metal nanoparticles. Characteristic properties such as particle sizes, particle composition and phase formation, and active surface area, were determined by transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. For comparison, commercial Pt/C catalysts (20 and 40 wt.%, E-Tek, Somerset, NJ, USA) were investigated as well, allowing to evaluate Pt loading effects and, by comparison with the pre-cursor-based catalyst with their much smaller particle sizes (1.7 nm diameter), also particle size effects. Kinetic parameters for the ORR were evaluated; the ORR activities of the bimetallic catalysts and of the synthesized Pt/C catalyst were comparable and similar to that of the high-loading commercial Pt/C catalyst; at typical cathode operation potentials H₂O₂ formation is negligible for the synthesized catalysts. Due to their lower methanol oxidation activity the bimetallic catalysts show an improved methanol tolerance compared to the commercial Pt/C catalysts. The results indicate that the use of very small particle sizes is a possible way to achieve reasonably good ORR activities at an improved methanol tolerance at DMFC cathode relevant conditions.     

Thermal ageing of Pt on low-surface-area CeO2–ZrO2–La2O3 mixed oxides: Effect on the OSC performance

This work aims at exploring the thermal ageing mechanism of Pt on ceria-based mixed oxides and the corresponding effect on the oxygen storage capacity (OSC) performance of the support material. Pt was supported on low-surface-area CeO₂–ZrO₂–La₂O₃ mixed oxides (CK) by impregnation method and subsequently calcined in static air at 500, 700 and 900 °C, respectively. The evolutions of textural, microstructural and redox properties of catalysts after the thermal treatments were identified by means of X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (TPR) and high-resolution transmission electron microscope (HRTEM). The results reveal that, besides the sintering of Pt, encapsulation of metal by the mixed oxides occurs at the calcination temperature of 700 °C and above. The burial of Pt crystallites by support particles is proposed as a potential mechanism for the encapsulation. Further, the HRTEM images show that the distortion of the mixed oxides lattice and other crystal defects are distributed at the metal/oxides interface, probably indicating the interdiffusion/interaction between the metal and mixed oxide. In this way, encapsulation of Pt is capable to promote the formation of Ce³⁺ or oxygen vacancy on the surface and in the bulk of support. The OSC results show that the reducibility and oxygen release behavior of catalysts are related to both the metal dispersion and metal/oxides interface, and the latter seems to be more crucial for those supported on low-surface-area mixed oxides. Judging by the dynamic oxygen storage capacity (DOSC), oxygen storage capacity complete (OSCC) and oxygen releasing rate, the catalyst calcined at 700 °C shows the best OSC performance. This evident promotion of OSC performance is believed to benefit from the partial encapsulation of Pt species, which leads to the increment of Ce³⁺ or oxygen vacancies both on the surface and in the bulk of oxides despite a loss of chemisorption sites on the surface of metal particles.     

Preparation of Pt-Co catalysts on mesoporous carbon and effect of alloying on catalytic activity in oxygen electro-reduction

Mesoporous carbon (MC)-supported PtCo catalysts were prepared by a sodium borohydride (NaBH₄) reduction method. To increase the alloy degree of PtCo catalyst, the heat treatment was carried out at various temperatures (300–700°C). The heat-treated PtCo catalysts (PtCo/MC-x) had the higher degrees of Pt-Co alloy than that of as-synthesized PtCo catalyst (PtCo/MC). MC supported-PtCo catalyst (PtCo/MC-500) that was treated at 500°C, had the highest activity and lowest overpotential in oxygen electro-reduction (ORR) among the prepared PtCo catalysts. The high alloy degree and favorable chemical states of PtCo/MC-500 are believed to be responsible for the superior activity in oxygen electroreduction compared to the other PtCo catalysts. This paper was presented at the 11th Korea-Japan Symposium on Catatysis held at Seoul, Korea, May 21–24, 2007.     

The effect of preparation variables on the dispersion of supported platinum catalysts

The effects of several preparation variables on the dispersion of supported platinum catalysts were examined in this study. Supported catalysts were prepared using two different platinum salts, four high surface area support materials, and two aqueous deposition techniques. The catalysts were characterised by hydrogen chemisorption, oxygen chemisorption, and the hydrogen titration reaction. In addition, X-ray diffraction measurements were conducted on all catalysts. Results from this study show that one of the platinum salts, chloroplatinic acid, always gave an equally or more highly dispersed catalyst than the other salt, tetraammine platinum (II) nitrate. The incipient wetness preparation technique produced equal or better dispersions than did the excess water method, with the possible exception of the silica-alumina support. The highest dispersions were attained with alumina, and the lowest with carbon. In some samples, a lack of good agreement existed between X-ray diffraction and chemisorption measurements of crystallite size which indicated broad or bimodal particle size distributions. However, these X-ray data were qualitatively helpful in understanding the unusual chemisorption behaviour of Pt/C catalysts, one of which had a dispersion higher than any previous reported Pt/C catalyst prepared by aqueous impregnation techniques. Experiments were also conducted which showed that either a 1 h or a 12 h treatment in hydrogen at 723 K was sufficient to reduce the platinum salt, and 1 h or 12 h evacuations at 698 K gave similar results. Finally, it was found that approximately one-half of the hydrogen monolayer on supported Pt could be removed by evacuating for 1 h at 300 K.