New catalytic functions of Pd–Zn, Pd–Ga, Pd–In, Pt–Zn, Pt–Ga and Pt–In alloys in the conversions of methanol

Pd and Pt supported on ZnO, Ga₂O₃ and In₂O₃ exhibit high catalytic performance for the steam reforming of methanol, CH₃OH+H₂OCO₂+3HH₂, and the dehydrogenation of methanol to HCOOCH₃, 2CH₃OHHCOOCH₃+2HH₂. Combined results with temperature-programmed reduction (TPR) and XRD method revealed that Pd–Zn, Pd–Ga, Pd–In, Pt–Zn, Pt–Ga and Pt–In alloys were produced upon reduction. Over the catalysts having the alloy phase, the reactions proceeded selectively, whereas the catalysts having metallic phase exhibited poor selectivities.     

Difference in the reactivity of acetaldehyde intermediates in the dehydrogenation of ethanol over supported Pd catalysts

Catalytic performances of supported Pd catalysts for the dehydrogenation of ethanol were greatly modified upon the formation of Pd alloy phases. Over Pd–Zn, Pd–Ga and Pd–In alloys, acetaldehyde was selectively produced at lower conversion levels. With the increased conversion level, ethyl acetate was produced at the expense of acetaldehyde. The selectivities for the ethyl acetate formation exceeded that over a Cu/ZnO catalyst. Over metallic Pd, the decomposition of ethanol, C₂H₅OH → CO + CH₄ + H₂, occurred to a considerable extent. It was shown that the reactivity of acetaldehyde species over the Pd alloys was markedly different from that over metallic Pd. Over the Pd alloys, acetaldehyde species were stabilized and transformed into ethyl acetate by the nucleophilic addition of ethanol. By contrast, over metallic Pd, aldehyde species were rapidly decarbonylated to methane and carbon monoxide. This revised version was published online in July 2006 with corrections to the Cover Date.     

State of platinum in zirconia and sulfated zirconia catalysts

Platinum is present in a metallic state following activation in air at 725C of both 5 wt% Pt/ZrO₂ and 5 wt% Pt/SO ₄ ^2– /ZrO₂. Reduction of either catalyst at 725C produces a Pt-Zr alloy, and these reduced catalysts, upon recalcination in air at 725C, form metallic Pt crystallites. Likewise, reduction of these uncalcined catalysts at 725C in H₂ leads to a Pt-Zr alloy formation. However, treatment of these uncalcined catalysts in H₂ at 450C does not produce Pt crystallites large enough to detect by XRD.     

Pt promotional effects on Pd–Pt alloy catalysts for hydrogen peroxide synthesis directly from hydrogen and oxygen

H₂O₂ synthesis directly from H₂ and O₂ over supported Pd–Pt alloy catalysts was carried out using a semibatch reactor under ambient conditions. As compared to pure Pd, the performance of Pd–Pt catalysts was enhanced significantly. The promotional role of Pt was studied systematically by using in situ diffuse reflectance infrared Fourier transform spectroscopy of CO adsorption (DRIFTS), quantitative powder X-ray diffraction (XRD), X-rays photoelectron spectroscopy (XPS), and temperature-programmed desorption of H₂/O₂ (H₂/O₂-TPD). The spectra of DRIFT, XPS, and XRD demonstrate the formation of Pd–Pt alloy particles, which surfaces are enriched by Pt accompanying with possible electron transfer from Pd to Pt. The addition of Pt into Pd phase was proposed to impact on reactants adsorption, stabilization of intermediates such as OOH and OH radicals, and the formation and decomposition of H₂O₂.     

Highly selective supported Pd catalysts for steam reforming of methanol

Steam reforming of methanol, CH₃OH + H₂O 3H₂ + CO₂, was carried out over various Pd catalysts (Pd/SiO₂, Pd/Al₂O₃, Pd/La₂O₃, Pd/Nb₂O₅, Pd/Nd₂O₃, Pd/ZrO₂, Pd/ZnO and unsupported Pd). The reaction was greatly affected by the kind of support. The selectivity for the steam reforming was anomalously high over Pd/ZnO catalysts.     

Kinetic Study of Metal-Catalyzed Reaction of Solid Calcium Oxide with Gas Mixtures of Methane and Water Vapor to Form Hydrogen

With noble metal catalysts (Pd, Pt, Rh, Ir) present, hydrogen is formed by the interaction of solid calcium oxide with gas mixtures of methane and water vapor, according to CaO + CH₄ + 2H₂O CaCO₃ + 4H₂. Among the metals, Ir and Rh are so active that the reaction takes place at temperatures as low as 600 K. Rate data obtained with these metals show a nearly first order with respect to CH₄ pressure, while a negative order with respect to H₂O vapor pressure. The apparent activation energies are 171 and 217 kJ/mol for the Ir- and Rh-catalyzed reactions, respectively. On the other hand, Ni does not catalyze the reaction below 733 K, probably due to its strong interaction with H₂O vapor.     

Steam reforming of methanol over Pt–Zn alloy catalyst supported on carbon black

Steam reforming of methanol over Zn-promoted Pt catalyst supported on an electrically conductive carbon black has been investigated after H₂ reduction at 873 K. X-ray diffraction measurement showed that Pt–Zn alloy was formed on the carbon black (C). The Zn-promoted Pt/C catalyst showed higher activity and selectivity to CO₂ compared with unpromoted Pt/C catalyst. Methyl formate was formed over the Zn-promoted Pt/C catalyst in decomposition of methanol (without water). This suggests that steam reforming of methanol over the Zn-promoted Pt/C catalyst can proceed via methyl formate, which is different from that of the unpromoted Pt/C catalyst.     

Formaldehyde,methanol and methyl formate from formic acid reaction over supported metal catalysts

The catalytic gas-phase decomposition of formic acid was studied over Au/Al₂O₃ and Pt/SBA-15 to investigate the formation of products other than H_2, CO₂, CO and H₂O. Formaldehyde, methanol and methyl formate were detected and identified as secondary products. Two calculation methods for H₂ selectivity (direct and indirect) were compared and assessed in terms of their validity at different temperatures, establishing that a direct quantification of H₂ is necessary for correct results. Based on selectivity trends of all the detected products a reaction scheme is proposed for the decomposition of formic acid and formation of formaldehyde, methanol and methyl formate.     

Influence of temperature on hydrogen electrosorption into palladium-noble metal alloys. Part 2—Palladium–platinum alloys

Hydrogen electrosorption into Pd-rich (>75 at.% Pd in the bulk) Pd–Pt alloys obtained by electrodeposition was studied in acidic solutions (0.5 M H₂SO₄) using cyclic voltammetry and chronoamperometry. The influence of temperature (in the range between 283 and 328 K) on the amount of absorbed hydrogen, the potential of the α–β phase transition, the extent of absorption–desorption hysteresis and the potential of absorbed hydrogen oxidation was examined. It has been found that for the temperature range studied the potentials of the α → β and β → α phase transitions are shifted negatively with both increasing temperature and decreasing Pd content in the alloy bulk. Thermodynamic parameters (Gibbs free energy, enthalpy and entropy) of the β-phase formation and decomposition were determined. With decreasing Pd bulk content the process of the β-phase formation becomes less exothermic and the thermodynamic stability of the β-phase decreases. The maximum hydrogen absorption capacity of Pd–Pt alloys decreases with increasing temperature and decreasing Pd bulk content. The potential of absorbed hydrogen oxidation is shifted negatively with increasing temperature and decreasing Pd bulk content.     

Confirmation of sulfur tolerance of bimetallic Pd–Pt supported on highly acidic USY zeolite by EXAFS

The sulfur tolerance (i.e., degree of sulfidation) of Pd and Pt in sulfided bimetallic Pd–Pt catalysts (Pd : Pt mole ratio of 4 : 1) supported on USY (ultrastable Y) zeolites (SiO₂/Al₂O₃ = 10.7, 48, and 310) was investigated using an extended X‐ray absorption fine structure (EXAFS) method. The sulfidation of the catalysts was done in a 1000 ppm H₂S–2% H₂/N₂ stream at 573 K for 0.5 h. In the Fourier transforms of Pd K‐edge and Pt L_III‐edge EXAFS spectra, both of the peaks due to metallic Pd and to metallic Pt for the Pd–Pt/USY (SiO₂/Al₂O₃ = 10.7) catalyst remained most after sulfidation. Further, the results of the Fourier transforms confirmed that the sulfur tolerance of both Pd and Pt decreased with increasing SiO₂/Al₂O₃ ratio, suggesting that Pd and Pt become sulfur‐tolerant when Pd–Pt bimetallic particles are supported on highly acidic USY zeolite. This revised version was published online in July 2006 with corrections to the Cover Date.     

Role of La2O3 in Pd-Supported Catalysts for Methanol Decomposition

Systems of Pd supported on various La₂O₃-modified -Al₂O₃ and CeO₂–Al₂O₃ catalysts were tested for catalytic methanol decomposition and characterized by means of XRD, BET, TPR, H₂-chemisorption and CO–FTIR. The addition of lanthanum significantly improved the selectivity of CO and H₂ for all the catalysts but showed a different influence on the catalytic activity in two systems. Methanol conversion decreased on La₂O₃-modified Pd/-Al₂O₃ catalysts, in line with the reduction of Pd dispersion, while the addition of La₂O₃ improved the dispersion of Pd and reinforced Pd–CeO₂ interaction for La₂O₃-modified Pd/CeO₂–Al₂O₃ catalysts, which resulted in a high production rate of CO and H₂. Thus, a synergistic effect between CeO₂ and La₂O₃ was observed on -Al₂O₃-supported Pd catalyst for methanol decomposition.     

Synthesis and characterization of methanol tolerant Pt/TiOx/C nanocomposites for oxygen reduction in direct methanol fuel Cells

Pt/TiO_x/C nanocomposites have been synthesized by depositing hydrated titanium oxide on carbon-supported Pt (Pt/C), reducing H₂PtCl₆ with sodium formate on carbon-supported hydrated titanium oxide (TiO₂/C), and simultaneously depositing hydrated titanium oxide and reducing H₂PtCl₆ with formate on carbon support, followed by heat treatment at 500 and 900 °C in 90% Ar-10% H₂ mixture. The catalytic activity for oxygen reduction was evaluated in half cells with sulfuric acid electrolyte and in single direct methanol fuel cells (DMFC). Tolerance to methanol was studied with half cells containing sulfuric acid mixed with methanol. Charge transfer resistance and electrochemical active surface area of the Pt/TiO_x/C catalysts were studied with impedance and cyclic voltammetry measurements. Both the synthesis methods and heat treatments influence the catalytic activity, and some of the Pt/TiO_x/C composites exhibit higher catalytic activity than Pt/C. The Pt/TiO_x/C catalysts also exhibit better methanol tolerance than Pt/C. The mechanism for the enhanced catalytic activity of Pt/TiO_x/C is discussed.     

Methanol selectivity of silica-supported PtFe

Adding Fe to Pt/SiO₂ catalysts improves activity for methanol synthesis from 3H₂/CO at 523 K and 3.19 MPa. Over 90% methanol selectivity can be achieved at low conversion, depending on the metal composition and dispersion.In situ Mössbauer measurements after reduction in hydrogen at 673 K and during steady-state reaction show the presence of PtFe alloy and Fe³⁺ phases only. The amount of PtFe alloy increases as catalysts activate to produce methanol with higher activity and selectivity.     

Reduction of CuO in H2: In Situ Time-Resolved XRD Studies

CuO is used as a catalyst or catalyst precursor in many chemical reactions that involve hydrogen as a reactant or product. A systematic study of the reaction of H₂ with pure powders and films of CuO was carried out using in situ time-resolved X-ray diffraction (XRD) and surface science techniques. Oxide reduction was observed at atmospheric H₂ pressures and elevated temperatures (150-300 °C), but only after an induction period. High temperature or H₂ pressure and a large concentration of defects in the oxide substrate lead to a decrease in the magnitude of the induction time. Under normal process conditions, in situ time-resolved XRD shows that Cu¹⁺ is not a stable intermediate in the reduction of CuO. Instead of a sequential reduction (CuO Cu₄O₃ Cu₂O Cu), a direct CuO Cu transformation occurs. To facilitate the generation of Cu¹⁺ in a catalytic process one can limit the supply of H₂ or mix this molecule with molecules that can act as oxidant agents (O₂, H₂O). The behavior of CuO-based catalysts in the synthesis of methanol and methanol steam reforming is discussed in the light of these results.     

Noble metal promoted Ni0.03Mg0.97O solid solution catalysts for the reforming of CH4 with CO2

Reforming of CH₄ with CO₂ to produce syngas was studied over Ni_0.03Mg_0.97O solid solution catalyst and its bimetallic derivative catalysts which contained small amounts of Pt, Pd or Rh (the atomic ratio M/(Ni + Mg) was about 2 × 10^–4, M = Pt, Pd or Rh). It was found that although the Ni_0.03Mg{0.97}O catalyst showed an excellent stability and activity at the reaction temperature of 1123 K, it lost its activity completely within 51 h when the reaction temperature was as low as 773 K. However, both the activity and the stability at 773 K were improved significantly by adding Rh, Pt, or Pd. This synergistic effect is rationally explained by the promoted reducibility of Ni. On all these catalysts, the amount of deposited carbon during the reaction was very low, suggesting that carbon deposition was not the main cause of the deactivation. Also, the catalytic activity of bimetallic catalysts increased gradually with the noble metal loading, but after passing through a maximum, it decreased with superfluous addition. The maximum was found to be located at around the atomic ratio of M/(Ni + Mg) 0.02% (M = Pt, Pd and Rh). This phenomenon could most probably be attributed to the different composition of Pt-Ni alloy particles formed after the reduction.     

Methanol synthesis and reverse water-gas shift kinetics over clean polycrystalline copper

The kinetics of simultaneous methanol synthesis and reverse water-gas shift from CO₂/H₂ mixtures have been measured at low conversions over a clean polycrystalline Cu foil at pressures of 5 bar. An absolute rate of 1.2 × 10^–3 methanol molecules produced per second per Cu surface atom was observed at 510 K, with an activation energy of 77 ± 10 kJ/mol. The rate of CO production was 0.12 molecules per second per Cu surface atom at this temperature, with an activation energy of 135 ± 5 kJ/mol. The rates, normalized to the metallic Cu surface area, are equal to those measured over real, high-area Cu/ZnO catalysts. The surface after reaction was examined by XPS and TPD. It was covered by almost a full monolayer of adsorbed formate, but no other species like carbon or oxygen in measurable amounts. These results prove that a highly active site for methanol synthesis on real Cu/ZnO catalysts is metallic Cu, and suggest that the rate-determining step in methanol synthesis is one of the several steps in the further hydrogenation of adsorbed formate to methanol.     

The positive effect of hydrogen on the reaction of nitric oxide with carbon monoxide over platinum and rhodium catalysts

The effect of adding 330–4930 ppm hydrogen to a reaction mixture of NO and CO (2000 ppm each) over platinum and rhodium catalysts has been investigated at temperatures around 200–250°C. Hydrogen causes large increases in the conversion of NO and, surprisingly, also of CO. Oxygen atoms from the additional NO converted are eventually combined with CO to give CO₂ rather than react with hydrogen to form water. This reaction is described by CO + NO +3/2H₂ CO₂ + NH₃ and accounts for 50–100% of the CO₂ formed with Pt/Al₂O₃ and 20–50% with Rh/Al₂O₃. With the latter catalyst a substantial amount of NO converted produces nitrous oxide. Comparison with a known study of unsupported noble metals suggests that isocyanic acid (HNCO) might be an important intermediate in a reaction system with NO, CO and H₂ present.     

Fischer–Tropsch synthesis using Co/SiO2 catalysts prepared from mixed precursors and addition effect of noble metals

Fischer–Tropsch synthesis in Co/SiO₂ catalysts, which were prepared by mixed impregnation of cobalt (II) nitrate and cobalt (II) acetate, was studied under mild reaction conditions (Total pressure=1 MPa, H₂/CO=2, T=513 K). X-ray diffraction indicated that highly dispersed cobalt metal was the main active sites on the catalyst prepared by the same method. It was considered that the metallic crystallines, which were readily reduced from cobalt nitrate, promoted the reduction of Co²⁺ to metallic a state in cobalt acetate by H₂ spillover mechanism during the catalyst reduction process. The reduced cobalt, from cobalt acetate, was highly dispersed one and remarkably enhanced the catalytic activity. The addition of a small amount of Ru to this type of catalyst remarkably increased the catalytic activity and the reduction degree. Its turn over frequency (TOF) increased but the selectivity of CH₄ was unchanged. However, when Pt or Pd were added into catalysts, they exhibited a higher selectivity of CH₄. Although Pt and Pd hardly exerted an effect on cobalt reduction degree, they promoted cobalt dispersion and decreased the value of TOF. Characterization of these bimetallic catalysts suggested that a different contact between Co and Ru, Pt or Pd existed. Ru was enriched on the metallic cobalt surface but, Pt or Pd dispersed well in the form of Pt–Co or Pd–Co alloy.     

XPS/SIMS studies of the promoter action in methanol synthesis over silica-supported Pd catalysts

Pd-based catalysts prepared from ultra-pure SiO₂ and doped with 5000 ppm Li, Ca and La were subjected to CO hydrogenation(H₂/CO=2, p=9 bar, T=553 K) in a flow reactor directly attached to an ultra-high vacuum chamber with facilities for X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS). The catalysts exhibited high selectivity (usually above 99%) for methanol synthesis and their activities were found to increase in the order Li<Ca<La. Subsequent analysis by SIMS revealed the presence of surface formate and methoxy on the silica support. No such species were found to be bound to the Pd particle surface. These conclusions are based on the detection of COO(H) and SiOCH₃ ions while respective Pd-based species were absent from the mass spectra. This indicates a mechanism to be in operation in which hydrogen is activated on Pd metal and subsequently spilled over to the support in order to react with formate to methanol. XPS studies of SiO₂ samples containing 10 wt% of Ca, in the absence of Pd metal, demonstrated Si 2p and O1s line shifts to lower binding energies as compared to pure SiO₂.The spectra bear strong similarities with those of wollastonite,“CaSiO₃”. A comparable behavior was found for Mg-dopedSiO₂ but not for K- or Cs-doped SiO₂, which show low activity in methanol formation. This revised version was published online in July 2006 with corrections to the Cover Date.