Microwave-assisted polyol synthesis of carbon-supported platinum-based bimetallic catalysts for ethanol oxidation

High surface area carbon-supported Pt, PtRh, and PtSn catalysts were synthesized by microwave-assisted polyol procedure and tested for ethanol oxidation in perchloric acid. The catalysts were characterized by thermogravimetric analysis (TGA), X-ray diffraction (XRD), scanning tunnelling microscopy (STM), TEM, and EDX techniques. STM analysis of unsupported catalysts shows that small particles (～2?nm) with a narrow size distribution are obtained. TEM and XRD examinations of supported catalysts revealed an increase in particle size upon deposition on carbon support (diameter?～?3?nm). The diffraction peaks of the bimetallic catalysts in X-ray diffraction patterns are slightly shifted to lower (PtSn/C) or higher (PtRh/C) 2θ values with respect to the corresponding peaks at Pt/C catalyst as a consequence of alloy formation. Oxidation of ethanol is significantly improved at PtSn/C with the onset potential shifted for?～?150?mV to more negative values and the increase of activity for approximately three times in comparison to Pt/C catalyst. This is the lowest onset potential found for ethanol oxidation at PtSn catalysts with a similar composition. Chronoamperometric measurements confirmed that PtSn/C is notably less poisoned than Pt/C catalyst. PtRh/C catalyst exhibited mild enhancement of overall electrochemical reaction in comparison to Pt/C.     

Carbon-supported IrSn catalysts for a direct ethanol fuel cell

Carbon-supported Ir₃Sn/C and Ir/C catalysts were simply prepared with NaBH₄ as a reducing agent under the protection of ethylene glycol at room temperature. TEM and X-ray diffraction (XRD) data showed that the catalysts with small particle size exhibited the typical characteristic of a crystalline Ir fcc structure. Their electro-catalytic activities in comparison with Pt/C and Pt₃Sn/C catalysts also prepared by the NaBH₄ reduction process were characterized by cyclic voltammetry (CV), linear sweep voltammetry (LSV) and chronoamperometry (CA) techniques. The results indicated that Ir-based catalysts showed superior electro-catalytic activity towards ethanol oxidation to Pt/C and Pt₃Sn/C catalysts, mainly at low potential region. During single-cell tests at 90 °C, better performances of Ir-based catalysts as anodes were obtained compared to that of Pt/C catalyst. The comparable overall performance of Ir₃Sn/C to Pt₃Sn/C makes it a promising alternative choice of anode catalyst for direct ethanol fuel cells.     

Promotional effect of Snad on the ethanol oxidation at Pt3Sn/C catalyst

Oxidation of ethanol was studied at Sn_ad modified and unmodified Pt₃Sn/C and Pt/C catalysts. Pt₃Sn/C and Pt/C catalysts were characterized by XRD. Potentiodynamic and chronoamperometric measurements were used to establish catalytic activity and stability. High activity achieved at Sn_ad modified Pt₃Sn/C catalyst has not been observed at any bimetallic catalyst so far. Promotional effect of Sn_ad on the ethanol oxidation was related to the enhancement of CO oxidation rate in bifunctional mechanism. It was shown that electrodeposited Sn exhibited different effect on the catalytic activity compared to Sn in alloy.     

Effect of thermal treatment on phase composition and ethanol oxidation activity of a carbon supported Pt50Sn50 alloy catalyst

A carbon supported Pt–Sn electrocatalyst in the Pt/Sn atomic ratio 50:50 was prepared by the reduction of Pt and Sn precursors with formic acid and thermally treated at 200 °C (i.e., in the presence of solid tin) and 500 °C (in the presence of molten tin) in flowing hydrogen. In the absence of thermal treatment, X-ray diffraction (XRD) analysis showed a solid solution of Sn in the face centered cubic (fcc) Pt and SnO₂. After thermal treatment, the formation of a main phase of hexagonal PtSn (niggliite) and a secondary phase of cubic Pt₃Sn was observed in the Pt50Sn50 catalyst. The relative amount of the PtSn phase increased with increasing thermal treatment temperature. The presence of molten tin gave rise to the formation of some big particles during annealing at 500 °C. The activity for the ethanol oxidation reaction (EOR) of the as-prepared catalyst was higher than that of both thermally treated catalysts and Pt75Sn25/C and Pt50Ru50/C by E-TEK. The higher activity for the EOR of the as-prepared Pt–Sn catalysts was ascribed to the presence of a large amount of SnO₂. Dedicated to Teresa Iwasita’s 65th birthday.     

Catalytic and physicochemical properties of Pt/α-GeO<Subscript>2</Subscript> systems in reaction of selective hydrogenation of α,β-unsaturated aldehydes in a gas phase

Hydrogenation of crotonaldehyde in a gas phase over Pt/α-GeO₂ catalysts was investigated. The systems were characterized by BET, XRD, TPR, TEM, ToF-SIMS, and FTIR methods. The optimum pretreatment parameters were studied. The best catalytic performance shows the catalyst 5 wt % Pt/α-GeO₂ (69% selectivity to crotyl alcohol at 200 μmol s⁻¹ g _Pt ⁻¹ activity and 10% conversion of crotonaldehyde). Lower loaded catalysts (2 and 1 wt % Pt) show lower, but also promising activity and selectivity. This good catalytic performance was related to the physicochemical properties of the catalyst. GeO₂ in the presence of Pt undergoes a partial surface reduction at temperatures higher than 100°C probably leading to the creation of the active Pt-Ge centers responsible for high selectivity to crotyl alcohol. Reduction at a temperature≥200°C deactivates the catalytic systems due to the formation of inactive PtGe alloys. Published in Russian in Kinetika i Kataliz, 2007, Vol. 48, No. 4, pp. 600–605. This article was submitted by the authors in English.     

Catalysts of ethanol anodic oxidation for ethanol-air fuel cell with a proton-conducting polymer electrolyte

Synthesis techniques for binary PtSn, PdM (M = Sn, V, Mo) and ternary PtSnNi, PtRuSn catalysts of ethanol electrooxidation on highly dispersed carbon materials are suggested. The highest activity in the 0.5 M H₂SO₄ solution containing 1 M C₂H₅OH corresponds to the system of PtSn (3: 1, 40 wt % Pt) with the particle size of 2–4 nm and tin content in the alloy with platinum of about 6%. It was shown that the catalyst efficiency as regards ethanol oxidation depth decreases in the series of Pt > PtRu ≈ PtSn, and the catalyst activity by current forms the series of PtSn > PtRu > Pt. The membrane-electrode assembly (MEA) with the anodes on the basis of the PtSn (3: 1, 40 wt % Pt) catalyst had stable characteristics for 220 h at the current density of ∼50 mA/cm².     

Catalytic properties of Pt/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalysts in the aqueous-phase reforming of ethylene glycol: Effect of the alumina support

The influence of the alumina support on the catalytic activity of Pt/Al₂O₃ catalysts in aqueous phase reforming of ethylene glycol to hydrogen was studied. The catalysts were prepared by impregnation of γ-, δ-, and α-alumina with H₂PtCl₆. The highest rate of hydrogen production (452 μmol min⁻¹ g⁻¹) obtained with the Pt/α-Al₂O₃ catalyst can be related to the highest extent of dispersion of Pt on α-Al₂O₃. XPS, TEM-EDX and TPR-H₂ measurements showed the absence of chloride-containing surface complexes in the Pt/α-Al₂O₃ catalyst. However, chloride-containing entities were found on the surface of Pt/γ-Al₂O₃ and Pr/δ-Al₂O₃ catalysts. When chloride ions are removed chlorinated Pt species facilitate the sintering of Pt crystallites and in this way affect the extent of Pt dispersion. Moreover, depending upon the particular crystalline form, alumina atoms have different coordination and alumina surfaces contain varying amounts of OH groups of different nature which affect the interaction between Pt and the support.     

Gold catalysts supported on ZnO/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> for low-temperature CO oxidation

Gold catalysts with loadings ranging from 0.5 to 7.0 wt% on a ZnO/Al₂O₃ support were prepared by the deposition–precipitation method (Au/ZnO/Al₂O₃) with ammonium bicarbonate as the precipitation agent and were evaluated for performance in CO oxidation. These catalysts were characterized by inductively coupled plasma-atom emission spectrometry, temperature programmed reduction, and scanning transmission electron microscopy. The catalytic activity for CO oxidation was measured using a flow reactor under atmospheric pressure. Catalytic activity was found to be strongly dependent on the reduction property of oxygen adsorbed on the gold surface, which related to gold particle size. Higher catalytic activity was found when the gold particles had an average diameter of 3–5 nm; in this range, gold catalysts were more active than the Pt/ZnO/Al₂O₃ catalyst in CO oxidation. Au/ZnO/Al₂O₃ catalyst with small amount of ZnO is more active than Au/Al₂O₃ catalyst due to higher dispersion of gold particles.     

Synthesis of dimethyl ether from CO and H<Subscript>2</Subscript>

Thermodynamic conditions for synthesizing dimethyl ether from synthesis gas are determined. The optimum conditions of the process are as follow: T ∼ 300°C at p = 3 MPa for two catalysts loaded into the reactor: methanol synthesis catalyst (Katalco-58) and catalyst of methanol dehydration to dimethyl ether (γ-Al₂O₃). The changes that occur with the catalysts during this process are demonstrated by electron scanning microscopy.     

Pt4ZrO2/C cathode catalyst for improved durability in high temperature PEMFC based on H3PO4 doped PBI

In this paper, Pt₄ZrO₂/C was prepared and compared with commercial Pt/C (46.6 wt.% TKK) in terms of the durability as cathode catalyst in a high temperature proton exchange membrane fuel cell (PEMFC) based on H₃PO₄ doped polybenzimidazole (PBI) by a potential sweep test. The catalysts before and after the potential sweep test were characterized by rotating disk electrode (RDE), X-ray diffraction (XRD), transmission electron microscopy (TEM) and inductively coupled plasma-atomic emission spectroscopy (ICP-AES). After 3000 cycles potential sweep test, the overall performance loss of the Pt₄ZrO₂/C membrane electrode assembly (MEA) was less than that of the Pt/C MEA. The RDE results demonstrated that the oxygen reduction reaction (ORR) activity of the as-prepared Pt₄ZrO₂/C is nearly the same as TKK-Pt/C. The XRD and TEM results showed that Pt₄ZrO₂/C catalyst presented higher sintering resistance than commercial Pt/C catalyst during the potential sweep test. This may be attributed to the addition of ZrO₂, which acts an anchor to inhibit the adjacent platinum particles to agglomerate. The ICP-AES results of Pt₄ZrO₂/C cathode catalyst before and after the potential sweep test showed that the composition of Pt and Zr were very near the nominal atomic ratio of Pt:Zr, which reflected that Pt₄ZrO₂/C catalyst had a good stability during the potential sweep test. In brief, the preliminary results indicate that Pt₄ZrO₂/C catalyst is a good candidate of Pt/C catalyst in high temperature PEMFC based on H₃PO₄ doped PBI for achieving longer cell life-time and higher cell performance.     

Investigation of the Pt–Ni–Pb/C ternary alloy catalysts for methanol electrooxidation

This research is aimed to increase the activity of anodic catalysts and thus to lower noble metal loading in anodes for methanol electrooxidation. The Pt–Ni–Pb/C catalysts with different molar compositions were prepared. Their performance were tested by using a glassy carbon disk electrode through cyclic voltammetric curves in a solution of 0.5 mol L⁻¹ CH₃OH and 0.5 mol L⁻¹ H₂SO₄. The performances of Pt–Ni–Pb/C catalyst with optimum composition (the molar ratio of Pt/Ni/Pb is 5:4:1) and Pt/C (E-Tek) were also compared. Their particle sizes and structures were determined by means of X-ray diffraction (XRD). The XRD results show, compared with that of Pt/C, the lattice parameter of Pt–Ni–Pb (5:4:1)/C catalyst decreases, its diffraction peaks are shifted slightly to a higher 2θ values. This indicates the formation of an alloy involving the incorporation of Ni and Pb atoms into the fcc structure of Pt. The electrochemical measurement shows the activity of Pt–Ni–Pb/C catalyst with an atomic ratio of 5:4:1 for methanol electrooxidation is the best among all different compositions. The activity of Pt–Ni–Pb (5:4:1)/C catalyst is much higher than that of Pt/C (E-Tek).     

Enhanced electrochemical activity for ethanol oxidation on the carbon-supported Pt3Te nanocatalysts by addition of Ru

Ternary Pt–Te–Ru catalysts with different atomic ratios were synthesized by reducing the precursor with formic acid. The physical and electrochemical characterization of the Pt₃TeRu_0.25/C catalyst was performed by transmission electron microscopy (TEM), X-ray diffraction, energy-dispersive X-ray spectroscopy equipped with TEM (TEM-EDX), X-ray photoelectron spectrometer, ethanol oxidation, and CO stripping. In TEM images, the Pt₃TeRu_0.25/C nanoparticles with an average particle size of around 2.9 nm were well dispersed on the carbon support. The Pt₃TeRu_0.25/C catalyst was superior to the Pt₃Te/C catalyst in respect of catalytic activity, durability, and CO tolerance. The positive effect of the Ru presence in the Pt₃TeRu_0.25/C catalyst was ascribed to the interactions of Ru or Ru oxides.     

Microstructure and properties of sol-gel derived Pb(Zr<Subscript>0.3</Subscript>Ti<Subscript>0.7</Subscript>)O<Subscript>3</Subscript> thin films on GaN/sapphire for nanoelectromechanical systems

Highly (111) oriented, phase-pure perovskite Pb(Zr_0.3Ti_0.7)O₃ (or PZT 30/70) thin films were deposited on single-crystal, (0001) wurtzite GaN/sapphire substrates using the sol-gel process and rapid thermal annealing. The phase, crystallinity, and stoichiometry of annealed PZT films were evaluated by X-ray diffraction and Rutherford backscattering spectroscopy. The atomic force microscopy revealed a smooth PZT surface (rms roughness ∼1.5 nm) with striations and undulations possibly influenced by the nature of the underlying GaN surface. The cross-sectional field-emission scanning electron microscopic images indicated a sharper PZT/GaN interface compared to that of sol-gel derived PZT on (111) Pt/TiO₂/SiO₂/(100) Si substrates. The capacitance-voltage (C-V) characteristics for PZT in the Pt/PZT/GaN (metal-ferroelectric-semiconductor or MFS) configuration were evaluated as a function of annealing temperature and applied voltage. The observed C-V hysteresis stemmed from trapped charge at defect sites within PZT. Also, the lower capacitance density (C/A = 0.35 μF/cm², where A is the area of an electrode) and remnant polarization (P _r ∼ 4 μC/cm²) for PZT in the MFS configuration, compared to the values for PZT in the MFM configuration (Pt/PZT/Pt), were attributed to the high depolarization field within PZT.     

Silica‐Encapsulated Pt‐Sn Intermetallic Nanoparticles: A Robust Catalytic Platform for Parahydrogen‐Induced Polarization of Gases and Liquids

Recently, a facile method for the synthesis of size‐monodisperse Pt, Pt₃Sn, and PtSn intermetallic nanoparticles (iNPs) that are confined within a thermally robust mesoporous silica (mSiO₂) shell was introduced. These nanomaterials offer improved selectivity, activity, and stability for large‐scale catalytic applications. Here we present the first study of parahydrogen‐induced polarization NMR on these Pt‐Sn catalysts. A 3000‐fold increase in the pairwise selectivity, relative to the monometallic Pt, was observed using the PtSn@mSiO₂ catalyst. The results are explained by the elimination of the three‐fold Pt sites on the Pt(111) surface. Furthermore, Pt‐Sn iNPs are shown to be a robust catalytic platform for parahydrogen‐induced polarization for in vivo magnetic resonance imaging.     

Laws of selective CO oxidation over a Ru/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalyst in the surface ignition regime: II. Transition states

The kinetics of selective CO oxidation (or individual CO or H₂ oxidation) over ruthenium catalysts are considerably as affected by the heat released by the reaction and specifics of the interaction of ruthenium with feed oxygen. In a reactor with reduced heat removal (a quartz reactor) under loads of ∼701 g_Cat⁻¹ h⁻¹ and reagent percentages of ∼1 vol % CO, ∼1 vol % O₂, ∼60 vol % H₂, and N₂ to the balance, the reaction can be carried out in the catalyst surface ignition regime. When catalyst temperatures are below ∼200°C, feed oxygen deactivates metallic ruthenium, the degree of deactivation being a function of temperature and treatment time. Accordingly, depending on the parameters of the experiment and the properties of the ruthenium catalyst, various scenarios of the behavior of the catalyst in selective CO oxidation are realized, including both steady and transition states: in a non-isothermal regime, a slow deactivation of the catalyst accompanied by a travel of the reaction zone through the catalyst bed along the reagent flow; activation of the catalyst; or the oscillation regime. The results of this study demonstrate that, for a strongly exothermic reaction (selective CO oxidation, or CO, or H₂ oxidation) occurring inside the catalyst bed, the specifics of the entrance of the reaction into the surface ignition regime and the effects of feed components on the catalyst activity should be taken into account.     

Ethanol electrocatalytic oxidation on highly dispersed Pt-TiO<Subscript>2</Subscript>/C catalysts

Carbon supported Pt-TiO₂(Pt-TiO₂/C) catalysts were prepared with colloidal and chemical reduction method on different conditions and used for ethanol electrooxidation in acidic solution. The size of TiO₂ and Pt crystal varied on the catalysts prepared by different methods and the Electrochemical specific surface area (ESA) also showed different on these catalysts. The catalytic activity on these carbon supported Pt-TiO₂ catalysts showed some different behaviors. Published in Russian in Elektrokhimiya, 2009, Vol. 45, No. 7, pp. 785–790. The article is published in the original.     

直接乙醇燃料电池PtSn/C电催化剂的合成表征和性能

采用多元醇法制备了n(Pt)/n(Sn)比为2:1,3:1,4:1的PtSn/C电催化剂.通过XRD,TEM、循环伏安和氢化学吸附技术对催化剂进行了表征.TEM和XRD结果表明,不同比例的PtSn/C金属粒子的平均粒径均小于4nm,且粒径分布较窄;该系列催化剂中Pt具有fcc结构;PtSn间的相互作用使Pt晶格参数增大.循环伏安和氢化学吸附实验结果表明,加入Sn可抑制Pt对氢的吸附,Pt3Sn/C对乙醇的氧化电流比Pt4Sn/C高约1倍.用不同n(Pt)/n(Sn)比的催化剂作为直接醇类燃料电池阳极电催化剂,在相同条件下,随着Sn含量的增加,单电池最大输出功率逐渐增大,当Sn含量继续增大时,单池性能反而下降.导致不同比例PtSn催化剂活性差别的原因可能是由于Sn与Pt间的合金化程度不同和催化剂粒子尺寸效应及Sn含量对电池阻抗等几方面因素所致.对40h寿命测试前后的阳极Pt3Sn/C催化剂的分析(EnergydispersiveX-rayanalysis,EDX)结果表明,PtSn含量在测试前后均有所降低,PtSn催化剂的寿命尚有待改善.     

Distinctive features of supported catalysts prepared from platinum carbonyl clusters

The formation of Pt/γ-Al₂O₃ and Pt/C catalysts from platinum carbonyl clusters H₂[Pt₃(CO)₆]_n (n = 2, 5) is studied. The strength of interaction between clusters (strong Lewis bases) and the support and the state of platinum in catalysts are governed by the acceptor strength of the support. The formation of a stable platinum compound with a surface of γ-Al₂O₃ (strong Lewis acid) is shown for a Pt/γ-Al₂O₃ catalyst by the method of radial distribution functions. In a Pt/C catalyst containing the same amount of Pt supported on a carbon material known to be a weaker acceptor, metallic platinum is formed along with surface-bonded platinum. Proceeding from the existence of the active phase of catalysts in the form of a surface platinum complex and platinum crystallites, the properties of catalysts are discussed in the complete oxidation of methane and the dehydrogenation of cyclohexane, as well as the high dispersity of platinum and its thermal stability     

Bimetallic Rh-Co/ZrO<Subscript>2</Subscript> catalysts for ethanol steam reforming into hydrogen-containing gas

The properties of supported bimetallic Rh-Co/ZrO₂ catalysts in ethanol steam reforming into hydrogen-containing gas were studied. The particles of Rh-Co solid solutions on the catalyst surface were prepared by the thermal decomposition of the double complex salt [Co(NH₃)₆][Rh(NO₂)₆] and the solid solution Na₃[RhCo(NO₂)₆]. It was found that the bimetallic Rh-Co/ZrO₂ catalysts exhibited high activity in the reaction of ethanol steam reforming. The equilibrium composition of reaction products was attained at 500–700°C and a reaction mixture space velocity of 10000 h⁻¹.     

Study on chemisorption of H<Subscript>2</Subscript>, O<Subscript>2</Subscript>, CO and C<Subscript>2</Subscript>H<Subscript>4</Subscript>on Pt-Ag/SiO<Subscript>2</Subscript>catalysts by microcalorimetry and FTIR

Adsorption microcalorimetry has been employed to study the interaction of ethylene with the reduced and oxidized Pt-Ag/SiO₂catalysts with different Ag contents to elucidate the modified effect of Ag towards the hydrocarbon processing on platinum catalysts. In addition, microcalorimetric adsorption of H₂, O₂, CO and FTIR of CO adsorption were conducted to investigate the influence of Ag on the surface structure of Pt catalyst. It is found from the microcalorimetric results of H₂and O₂adsorption that the addition of Ag to Pt/SiO₂leads to the enrichment of Ag on the catalyst surface which decreases the size of Pt surface ensembles of Pt-Ag/SiO₂catalysts. The microcalorimetry and FTIR of CO adsorption indicates that there still exist sites for linear and bridged CO adsorption on the surface of platinum catalysts simultaneously although Ag was incorporated into Pt/SiO₂. The ethylene microcalorimetric results show that the decrease of ensemble size of Pt surface sites suppresses the formation of dissociative species (ethylidyne) upon the chemisorption of C₂H₄on Pt-Ag/SiO₂. The differential heat vs. uptake plots for C₂H₄adsorption on the oxygen-preadsorbed Pt/SiO₂and Pt-Ag/SiO₂catalysts suggest that the incorporation of Ag to Pt/SiO₂could decrease the ability for the oxidation of C₂H₄.