A model of Pt-Rh catalyst prepared by electro-chemical deposition of Rh ions on Pt(100) surface

When Pt-Rh(100) alloy surface was exposed to NO or O₂ at temperatures higher than 400 K, a characteristic p(3 × 1) LEED pattern appeared with segregation of Rh atoms on the surface. It was shown that when a Rh-deposited Pt(100) surface is heated in NO or O₂ the same 5 × 10^–8 Torr of H₂ at room temperature, the p(3 × 1) pattern disappears but is readily recovered 5 × 10^–8 Torr of H₂ at room temperature, the p(3 × 1) pattern disappears but is readily recovered by exposing to O₂ of 1 × 10^–7 Torr at room temperature. The growth of a Rh-O overlayer on Pt layer is responsible for the formation of the p(3 × 1) structure on either the Pt-Rh(100) alloy or Pt(100) surface, and this peculiar surface may correspond to the active surface of the Pt-Rh catalyst for NO _x reduction     

Oscillatory behaviour of the reduction of NO by H2 over Rh

The NO reduction by H₂ on Rh has been studied by field emission microscopy (FEM). It has been observed that this reduction shows oscillatory behaviour at 460 K andP _NO = –1.5 ×10^–1 Torr andP _H2 =1×10^-6 Torr, Unique features of FEM are the very high spatial resolution and the presence of, in principle, an indefinite number of different crystal planes. The oscillatory behaviour is reflected by periodic changes in the emission current and in the images observed. The communication between different surfaces present on the field emitter is shown on a fluorescent screen. Diffusion and gas phase coupling seem to play a role. Many of the features reported earlier for the oscillatory behaviour of the NO-H₂ and NO-NH₃ reactions over Pt(100) are observed on Rh as well, including the surface explosion. The vacancy model proposed earlier for the oscillations over Pt(100), can be applied to the reactions described in this paper as well.     

Deposition of diamond film on silicon and quartz substrates located near DC plasma

Deposition of diamond films on to both Si and quartz substrates was succeeded by means of locating the substrate near plasma, and their microstructures were investigated by using SEM and Raman spectroscopy. The plasma was generated by intermittent DC discharge in H₂–CH₄ gas mixture at high gas pressure. The deposition rate of the film was remarkably increased when distance (D) between the substrate and the plasma (discharge electrodes) decreased. When the gas pressure (P_g) was increased from 100 to 250 Torr, the deposition rate was extremely increased and the crystalline quality of the film was improved. The deposition rate, when P_g = 200 Torr and D = 5 mm, was about 2.1 and 1.7 μm/h for Si and quartz substrate, respectively.     

Hydrogen-induced CO2 formation from ethylene deposits on Pt during consecutive O2 and H2 exposures

The oxidation of C₂H₄ deposits on polycrystalline Pt when exposed to consecutive O₂ and H₂ pulses at room temperature has been investigated in a long (L = 36 mm), shallow (d = 600–700 nm) micromachined glass–SiO₂–Pt channel. Hydrogen-induced CO₂ formation from species accumulated on the Pt surface was observed. Frequent switching of the O₂/H₂ exposure pulses was found to increase the efficiency of the oxidation of the carbonaceous deposits markedly. The observations may be of general interest for the regeneration of contaminated catalysts.     

From single crystals to supported nanoparticles in experimental and theoretical studies of H2 oxidation over platinum metals (Pt, Pd): Intermediates, surface waves and spillover

The present paper reviews our investigations concerning the mechanism of H₂ + O₂ reaction on the metal surfaces (Pt, Pd) at different structures: single crystals (Pt(1 1 1), Pt(1 0 0), Pd(1 1 0)); microcrystals (Pt tips); and nanoparticles (Pd–Ti³⁺/TiO₂). Field electron microscopy (FEM), field ion microscopy (FIM), high-resolution electron energy loss spectroscopy (HREELS), XPS, UPS, work function (WF), TDS and temperature-programmed reaction (TPR) methods have been applied to study the kinetics of H₂ oxidation on a nanolevel. The adsorption of both O₂ and H₂ and several dissociative products (H_ads, O_ads, OH_ads) was studied by HREELS. Using the DFT technique the equilibrium states and stretching vibrations of H, O, OH, H₂O, adsorbed on the Pt(1 1 1) surface, have been calculated depending on the surrounding of the metal atoms. Sharp tips of Pt, several hundreds angstroms in radius, were used to perform in situ investigations of the dynamic surface processes. The FEM and FIM studies on the Pt-tip surface demonstrate that the self-oscillations and waves propagations are connected with periodic changes in the surface structure of nanoplane (1 0 0)-(hex) ↔ (1 × 1), varying the catalytic property of metal. The role of defects (Ti³⁺-□_O) in the adsorption centers formation, their stabilization by the palladium nanoparticles, and then the defects participation in H₂ + O₂ steady-state reaction over Pd–Ti³⁺/TiO₂ surface have been studied by XPS, UPS and photodesorption techniques (PhDS). This reaction seems to involve the “protonate” hydrogen atoms (H⁺/TiO_x) as a result of spillover effect: diffusion of H_ads atoms from Pd particles on a TiO_x surface. The comprehensive study of H₂, O₂ adsorption and H₂ + O₂ reaction in a row: single crystals → tips → nanoparticles has shown the same nature of active centers over these metal surfaces.     

Development of the Eco-Cascade- cell reactor

The development of multicompartment rotating cylinder electrode reactors for the removal of metal from aqueous solutions is described. Such reactors approximate to a cascade of continuously stirred tank reactors and the results illustrate that, for electrodeposition of copper powder from acid sulphate solutions, high overall conversions (about 98%) may be realised, with low exit metal concentrations (about 1 mg dm^–3) and reasonable current efficiencies (65–87%).Nomenclature A electroactive surface area (cm²) - C _in inlet concentration of metal (mg dm^–3) - C _out outlet concentration of metal (mg dm^–3) - C _reactor reactor concentration of metal (mg dm^–3) - f _R fractional conversion - (f _R)n overall fractional conversion - F Faraday=96 500 (C mol^–1) - I _L limiting current (A) - k _l mass transfer coefficient (cms^–1) - m weight of metal (g) - M molecular weight of metal - n number of reactor elements in the cascade - N volumetric flow rate (cm³ s^–1) - z electron change - dm/dt rate of removal of metal (gs^–1) This paper was presented, in part, at the Electrochemical Reaction Engineering Symposium, Southampton University, April (1979).     

Promoting Effect of Pt Addition to V2O5/ZrO2 Catalyst on Reduction of NO by C3H6

The effect of Pt addition to a V₂O₅/ZrO₂ catalyst on the reduction of NO by C₃H₆ has been studied by FTIR spectroscopy as well as by analysis of the reaction products. Pt loading promoted the catalytic activity remarkably. FTIR spectra of NO adsorbed on the catalysts doped with Pt show the presence of two different types of Pt sites, Pt oxide and Pt cluster, on the surface. The amount of these sites depends on Pt contents and the catalyst state. Pt atoms highly disperse on the surface as Pt oxide at low Pt content, being aggregated into Pt metal clusters by increasing Pt amount or reducing the catalysts. The spectral behavior of V=O bands on the surface also supports the formation of Pt clusters. It is concluded that Pt promotes the NO–C₃H₆ reaction through a reduction–oxidation cycle between its oxide and cluster form.     

Tungsten carbide-based electrochemical sensors for hydrogen determination in gas mixtures

An amperometric study of gas-diffusion electrodes (GDE) catalysed by two types of tungsten carbide, WC₍₁₎ and WC₍₂₎, which differ considerably in their specific surface area (0.5 and 6 m² g^–1), was carried out. The H₂–air gas mixture (H₂ 1–4%) measurements show that for this range of hydrogen concentration the hydrogen limiting diffusion current (i _d(H2)) may be attained so that a curve of limiting current density against hydrogen concentration can be obtained. The response and stability of the electrode performance were compared to those of platinum catalysed GDEs. The most promising for use in amperometric hydrogen sensing is the WC₍₁₎ catalyst of small specific surface area. Electrodes catalysed with this catalyst show inferior response time in comparison to electrodes catalysed with the other two catalysts (WC₍₂₎ and Pt) but their overall stability is much better.     

Infrared spectroscopy and microcalorimetry studies of CO adsorption on sulfated zirconia supported platinum catalysts

Carbon monoxide adsorption has been investigated on Pt particles supported on a high surface area zirconia and sulfated zirconias. The accessibility of the Pt surface determined from the comparison of H₂ chemisorption and transmission electron microscopy depends on two parameters: the temperature of treatment in air used to dehydroxylate sulfated zirconia, and the temperature of reduction. An oxidative pretreatment at 823 K yields a poor accessibility of Pt (0.03 < H/Pt < 0.05) whatever the temperature of reduction, whereas a Pt dispersion of 0.6 can be obtained by oxidation at 673 K followed by a mild reduction at 473 K. FTIR spectroscopy of adsorbed CO on Pt/ZrO₂ shows besides the normal linear species at 2065 cm^–1, a band at 1650 cm^–1 which is attributed to CO bridged between Pt and Zr atoms. On Pt/ZrO₂-SO ₄ ^2– , all bridged species tend to disappear, as well as the dipole-dipole coupling andv _CO is shifted by 57 cm^–1 to higher frequencies. These results are attributed to sulfur adsorption on Pt which decreases the electron back-donation from Pt to the 2 ^* antibonding orbital of CO. The lower initial heat of CO adsorption observed on Pt/ZrO₂-SO^4/2– supports this proposal.     

Kinetics of heptane reforming on Pt/L zeolite

The kinetics of heptane reforming over 0.64% Pt/KBaL have been measured over a wide range of conditions from 390 to 475 ° C, 0.05 to 1.00 atm heptane, and 0.2 to 25.0 atm hydrogen. Below about 6 atm H₂, the catalyst deactivates due to carbon fouling of the platinum particles. The reaction rate increases with hydrogen pressure under these conditions, presumably because this accelerates the rate of carbon hydrogenation off the metal surface. Above 6 atm H₂, no deactivation occurs. The activation energies and reaction orders in heptane and hydrogen at high H₂ pressure are: 39 kcal/mol, 0.7 and –1.9 for hydrogenolysis; 60 kcal/mol, 0.6 and –2.8 for isomerization; and 58 kcal/mol, 0.4 and –2.7 for dehydrocyclization. These kinetics are the same as those observed over platinum on nonacidic supports, and indicate that the reaction mechanism on Pt/KBaL is no different from that on monofunctional Pt catalysts.     

Low-temperature SCR of NO with NH3 over noble metal promoted Fe-ZSM-5 catalysts

We have reported previously the excellent performance of Fe-exchanged ZSM-5 for selective catalytic reduction (SCR) of NO with ammonia at high temperatures (300–400 °C). In this work, we found that the reaction temperature could be decreased to 200–300 °C when a small amount of noble metal (Pt, Rh, or Pd) was added to the Fe-ZSM-5. The SCR activity follows the order Pt/Fe-ZSM-5 > Rh/Fe-ZSM-5 > Pd/Fe-ZSM-5 at 250 °C. On the Pt promoted Fe-ZSM-5, 90% NO conversion was obtained at 250 °C at GHSV = 1.1 × 10⁵ h^–1. Moreover, the noble metal improved the resistance to H₂O and SO₂. The presence of H₂O and SO₂ decreased the SCR performance only very slightly.     

Electrochemical supercapacitors based on industrial carbon blacks in aqueous H2SO4

It is shown that industrial carbon blacks (CBs) are interesting materials for electrochemical supercapacitors (ECSCs). The specific areas A _s ranged from 28 to 1690 m² g^–1. The highest values were realized through activation in CO₂ at 1100 °C. Precompacted carbon black electrodes with 5–10 wt% PTFE as a binder in the pellet in 10–12 M H₂SO₄ were characterized by constant current cycling, CCC, j = 20–50 mA cm^–2. Voltage–time curves showed nearly pure capacitive behaviour. Specific capacitance of single electrodes, C _s,1, could be derived therefrom. A plot of C _s,1 against A _s shows a linear behavior according to C _s,1 = C _A,DL A _s, where C _A,DL is the Helmholtz double layer capacitance per atomic surface area. Best fit was obtained with C _A,DL = 16 F cm^–2. The highest experimental values, C _s,1 = 250 F g^–1, are due to 60% of the theoretical maximum, which corresponds to an A _s calculated from both faces of isolated graphene layers. Only marginal pseudocapacitances are observed. Model cells for ECSCs (with microporous Celgard^TM separators) could be extensively cycled (CCC). A monopolar cell endured Z > 2000 cycles. Bipolar cells (5 units) allowed 700 cycles. Practical problems such as the development of electrode holders and of carbon black filled polypropylene composites for current collectors are discussed. It is concluded that entirely metal-free ECSCs with low cost can be produced.     

Electrolytic reduction of trichloroethylene and chloroform at a Pt- or Pd-coated ceramic cathode

Trichloroethylene (TCE) and chloroform (CF) were electrolytically dechlorinated in a two-compartment cell in which the working electrode (cathode) consisted of an Ebonex ceramic sheet plated with platinum (Pt) or palladium (Pd). The halogenated targets were not reduced using a cathode of untreated Ebonex. Under typical experimental conditions (e.g., cathode potentials E _C = –0.3 V to –1.4 V vs SHE, pH 7.0), transformations were first order in TCE and CF. Reaction kinetics were mass transport limited at E _C < –1.4 V. Transport-limited rate constants were 0.45 cm min^–1 for TCE reduction and 0.42 cm min^–1 for CF. The primary products of CF reduction were methane and hydrochloric acid. For TCE reduction, major products were ethane, ethylene and hydrochloric acid. Carbon and chlorine mass balances were within 5–10%. Current efficiencies ranged from nearly 100% at E _C = –0.5 V (both reactants) to 24.4% for TCE and 16.6% for CF at E _C = –1.4 V. Rate constants for TCE and CF transformations were inversely related to pH in the range 2 < pH < 11. Pt–Ebonex resisted sulfate and chloride poisoning. The Pd–Ebonex electrode quickly lost activity (50% loss in 5–10 min) in 0.1 M K₂SO₄ electrolyte (cathode potential, E _C = –1.15 to –1.4 V vs SHE).     

Measurement of the Metal Surface Acidity/Electronegativity of Pt(110)

In previous work, it has been found that a hydrogen-covered Pt(110) surface is acidic, but quantification of the acidity has not yet been done. In this paper a spectroscopic method is developed to measure the acidity of a metal surface for the first time. The technique involves measuring the intensity of the N–H stretch from the C₅H₄XNH⁺ that forms when hydrogen coadsorbs with pyridine, 2-fluoropyridine and 3-fluoropyridine. The Bethe approximation is then used to estimate the metal surface acidity/electronegativity (MSAEL). The proton affinity/MSAEL of Pt(110) has been determined to be 907 ± 4 kJ/mol at high coverage. This is the first time the MSAEL has been measured on a metal surface. Implications for fuel cell catalysis are discussed.     

Determination of overall mass transport coefficients in gas diffusion electrodes

Gas diffusion electrodes are used for many purposes, for example in fuel cells, in synthesis and as anodes in electrodeposition processes. The behaviour of gas diffusion electrodes has been the subject of many studies. In this work the transport of gas in the gas diffusion electrode, characterized by the overall mass transport coefficient, has been investigated using hydrogen-nitrogen mixtures. A reactor model for the gas compartment of the gas diffusion electrode test cell is proposed to calculate the concentration of hydrogen in the gas compartment as a function of the input concentration of hydrogen and the total volumetric gas flow rate. The mass transport coefficient is found to be independent of variations in hydrogen concentration and volumetric gas flow rate. The temperature dependence of the mass transport coefficient has been determined. A maximum was found at 40°C.Notation Agd geometric electrode surface area (m²) - C _in concentration of reactive component at the inlet of the gas compartment (mol m^–3) - c _out concentration of reactive component at the outlet of the gas compartment (mol m^–3) - E potential (V) - E _e equilibrium potential (V) - E _t upper limit potential (V) - F _v volumetric flow rate (m^–3 s^–1) - F _v,H volumetric flow rate of hydrogen (m^–3 s^–1) - F _v,N volumetric flow rate of nitrogen (m^–3 s^–1) - F _vin volumetric flow rate at the inlet of the gas compartment (m^–3 s^–1) - F _v,out volumetric flow rate at the outlet of the gas compartment (in ^–3 s^–1) - F _v,reaction volumetric flow rate of reactive component into the gas diffusion electrode (m^–3 s^–1) - Faraday constant (A s mo^–1) - I _gd current for gas diffusion electrode (A) - i _gd current density for gas diffusion electrode (A m^–2) - I _gd,1 diffusion limited current for gas diffusion electrode (A) - i _gd,1 diffusion limited current density for gas diffusion electrode (A m^–2) - I _gd,1,calc calculated diffusion limited current for gas diffusion electrode (A) - i _gd,1,calc calculated diffusion limited current density for gas diffusion electrode (A m^–2) - I _hp current for hydrogen production (A) - k _s mass transport coefficient calculated from c _out (m s^–1) - n number of electrons involved in electrode reaction - T temperature (°C) - V _m molar volume of gas (m³ mol^–1) - overpotential (V)     

States of surface hydrogen under reaction conditions of the Fischer-Tropsch synthesis

The reaction of hydrogen with Fe surfaces was observed by the ellipsometric method at 77 K to 500 ° C under H₂ pressures of 10^–3 Torr (0.1 Pa) to 500 Torr (7×10⁴ Pa). The ellipsometric analysis reveals no existence of adsorbed hydrogen on the surface above 400 °C; hydrogen seems to be alternatively absorbed into the subsurface region of two to three atomic layers. It is concluded that the subsurface hydrogen is specific to the high temperature and the high pressure owing to strained and roughened Fe surfaces as well as the equilibrium with gas phase hydrogen. Absence of adsorbed hydrogen is indicated as well in the ellipsometric response to the hydrogenation reaction of surface carbon species on Fe surfaces.     

Electrochemically assisted organosol method for Pt-Sn nanoparticle synthesis and in situ deposition on graphite felt support: Extended reaction zone anodes for direct ethanol fuel cells

Two electrochemically assisted variants of the Bönneman organosol method were developed for Pt-Sn nanoparticle synthesis and in situ deposition on graphite felt electrodes (e.g. thickness up to 2 mm). Tetraoctylammonium triethylhydroborate N(C₈H₁₇)₄BH(C₂H₅)₃ was employed as colloid stabilizer and reductant dissolved in tetrahydrofuran (THF). The role of the electric field at a low deposition current density of 1.25 mA cm⁻² was mainly electrophoretic causing the migration and adsorption of N(C₈H₁₇)₄BH(C₂H₅)₃ on the graphite felt surface where it reduced the PtCl₂-SnCl₂ mixture. Faradaic electrodeposition was detected mostly for Sn. Typical Pt-Sn loadings were between 0.4 and 0.9 mg cm⁻² depending on the type of pre-deposition exposure of the graphite felt: surfactant-adsorption and metal-adsorption variant, respectively. The catalyst surface area and Pt:Sn surface area ratio was determined by anodic striping of an underpotential deposited Cu monolayer. The two deposition variants gave different catalyst surfaces: total area 233 and 76 cm² mg⁻¹, with Pt:Sn surface area ratio of 3.5:1 and 7.7:1 for surfactant and metal adsorption, respectively. Regarding electrocatalysis of ethanol oxidation, voltammetry and chronopotentiometry studies corroborated by direct ethanol fuel cell experiments using 0.5 M H₂SO₄ as electrolyte, showed that due to a combination of higher catalyst load and Pt:Sn surface ratio, the graphite felt anodes prepared by the metal-adsorption variant gave better performance. The catalyzed graphite felt provided an extended reaction zone for ethanol electrooxidation and it gave higher catalyst mass specific peak power outputs compared to literature data obtained using gas diffusion anodes with carbon black supported Pt-Sn nanoparticles.     

Bimetallic nickel complexes of trimethyl phenyl linked salicylaldimine ligands: Synthesis, structure and their ethylene polymerization behaviors

Bimetallic salicylaldimine-nickel complexes, 2,4,6-Me₃-1,3-{[NCH–(3′-R-5′-Y-2′-O–C₆H₃)-κ²-N,O]Ni(Ph) (PPh₃)}₂ [R = tert-Bu, Y = Me, 1b; R = Ph, Y = H, 2b] were prepared and their catalytic behaviors of ethylene polymerization were investigated. The bimetallic complex 2b shows higher activities (2.9 × 10⁵ g PE mol⁻¹ Ni h⁻¹) for ethylene polymerization and affords polymer with high molecular weight (M_w = 1.41 × 10⁵) and broad molecular weight distribution (M_w/M_n = 6.1) than its mononuclear matrix, {[(2,6-Me₂C₆H₃)–NCH–(3′-Ph-2′-O–C₆H₃)-κ²-N,O]Ni(Ph)(PPh₃)} (3) (Activity = 5.5 × 10⁴ g PE mol⁻¹ Ni h⁻¹; M_w = 1.86 × 10⁴; M_w/M_n = 2.8).     

Promotional effect of H2 on CO oxidation over Au/TiO2 studied by operando infrared spectroscopy

The oxidation of carbon monoxide in the presence of various concentrations of molecular hydrogen has been studied over a Au/TiO₂ reference catalyst by combining diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and mass spectrometry. It is shown for the first time that H₂ enhances the CO oxidation rate on Au/TiO₂ without leading to any major loss of selectivity. Increasing the H₂ pressure induces higher CO and H₂ oxidation rates. Under H₂-free conditions, the surface species detected are Au^δ+–CO, Ti⁴⁺–CO, carbon dioxide and carbonates. Upon the addition of H₂, Au⁰–CO, water and hydroxyl groups become the main surface species. The occurrence of a preferential CO oxidation mechanism involving H_xO_y species under the present experimental conditions is proposed.     

Mass transfer behaviour of gas-evolving particulate-bed electrode

Rates of mass transfer for the cathodic reduction of potassium ferricyanide at a particulate bed of graphite supported on a horizontal nickel disc were studied under H₂-evolving conditions. Variables studied were: H₂ discharge rate, particle size and bed height. The rate of mass transfer was found to increase to a maximum in the presence of the bed which was about 4.5 times compared to that of the supporting disc. The rate of mass transfer was found to increase with H₂ discharge rate, particle size and bed height. Polarization was measured for beds of different particle size and it was found that the presence of the bed increased polarization especially at relatively high current densities, the increase in polarization was independent of particle size of the bed. Comparison with an O₂-evolving particulate electrode was made and possible practical applications were pointed out.Symbols K mass transfer coefficient (cm s^–1) - V H₂ discharge rate (cm³cm^–2s^–1) - I current consumed in reducing potassium ferricyanide (A) - A supporting disc area (cm²) - F Faradays constant (96 500 C mol^–1) - C Potassium ferricyanide concentration (mol cm^–3) - Z number of electrons involved in the reaction