Electrochemical hydrogen reactions on tungsten carbide

The kinetics of hydrogen ionisation and electrochemical evolution on a smooth rotating tungsten carbide electrode have been investigated. Polarisation curves in 1 N H₂SO₄, H₃PO₄ and HCl solutions, in the temperature range of 20?80°C, have been obtained. For hydrogen ionisation at ?_r=0·3 V the apparent activation energies (Kcal/moles) were 8·1 in H₂SO₄, 7·4 in H₃PO₄ and 12·8 in HCl. At potentials of ?_r>0·4V a decrease in the hydrogen ionisation rate was observed, explained as a decrease in the number of active surface sites due to surface oxidation and adsorption of anions. However, after a moderate anodic polarisation not leading to the formation of a thick tungsten carbide oxides layer, reiterated recording of the polarisation curve yields an increase in electrode activity.In all acids the first order of reaction with respect to molecular hydrogen was found. The hydrogen ionisation rate was independent of the pH-value of the solution. These results showed adsorption, accompanied by molecular hydrogen dissociation, to be the rate-determining step.Modifying the concentration of sulphuric acid in the presence of excess normal salt or without it, as well as passing from acidic to alkaline solutions caused no change in the course of the polarisation curves. This suggests that recombination of adsorbed hydrogen atoms into molecules on tungsten carbide is the rate determinating step, ie, the same step as in the reverse process of hydrogen ionisation.     

Study of the preparation of bulk tungsten carbide catalysts with C2H6/H2 and C2H4/H2 carburizing mixtures

The influence of the preparation procedure of tungsten carbide on the mechanism of carburization is discussed. This work is focused on the reduction and the carburization of tungsten trioxide by a mixture of hydrocarbon and H₂ to form WC. Temperature-programmed reaction spectra obtained with CH₄, C₂H₆ and C₂H₄ have been measured. In presence of the CH₄-H₂ mixture, H₂ is the reducing agent and the hydrocarbon is consumed for the carburization whereas C₂H₆ or C₂H₄ participates in the reduction of the tungsten oxide. The temperatures of reduction and carburization are lower by about 150 K using C₂H₆ or C₂H₄ instead of CH₄. Such a decrease of the temperature of reduction of tungsten oxide is needed to avoid the formation of poorly reducible compounds that can occur during the preparation of supported tungsten carbide. Furthermore, the surface area of the resulting carbide is 25 m²/g with C₂H₆ and C₂H₄ and 10 m²/g with CH₄. During the carburization, the deposit of excess carbon on the WC surface is larger with the C₂ hydrocarbons than with CH₄, but it protects the carbide and can be removed by hydrogen treatment. This revised version was published online in July 2006 with corrections to the Cover Date.     

Heterogeneously assisted oxidation of adsorbates from carbonmonoxide, methanol and ethanol by hydrogen peroxide solutions on platinum electrodes in sulphuric acid

The influence of hydrogen peroxide on the adsorption and oxidation of carbon monoxide, methanol and ethanol adlayers on porous Pt electrodes were studied in 2 M sulphuric acid solution by means of cyclic voltammetry and differential electrochemical mass spectrometry (DEMS). The oxidation of adsorbed species is observed at electrode potentials far less negative than those required for electrochemical adsorbate oxidation. The oxidation by H₂O₂ is dependent on its concentration in solution, as well as on the adsorbates and their coverages. In all cases the isolated adlayers are oxidised by dissolved H₂O₂. However, the presence of H₂O₂ during adsorption partially inhibits adlayer formation from CH₃OH and C₂H₅OH, but avoids almost completely the adsorption of carbon monoxide. The removal of the residues from the surface by dissolved hydrogen peroxide probably occurs through O_ad species formed during the heterogeneous decomposition reaction of H₂O₂ on Pt.     

Adsorption of Off‐Gases from Steam Methane Reforming (H2, CO2, CH4, CO and N2) on Activated Carbon

Abstract

Hydrogen is the energy carrier of the future and could be employed in stationary sources for energy production. Commercial sources of hydrogen are actually operating employing the steam reforming of hydrocarbons, normally methane. Separation of hydrogen from other gases is performed by Pressure Swing Adsorption (PSA) units where recovery of high‐purity hydrogen does not exceed 80%.

In this work we report adsorption equilibrium and kinetics of five pure gases present in off‐gases from steam reforming of methane for hydrogen production (H₂, CO₂, CH₄, CO and N₂). Adsorption equilibrium data were collected in activated carbon at 303, 323, and 343 K between 0‐22 bar and was fitted to a Virial isotherm model. Carbon dioxide is the most adsorbed gas followed by methane, carbon monoxide, nitrogen, and hydrogen. This adsorbent is suitable for selective removal of CO₂ and CH₄. Diffusion of all the gases studied was controlled by micropore resistances. Binary (H₂‐CO₂) and ternary (H₂‐CO₂‐CH₄) breakthrough curves are also reported to describe the behavior of the mixtures in a fixed‐bed column. With the data reported it is possible to completely design a PSA unit for hydrogen purification from steam reforming natural gas in a wide range of pressures.     

A thermodesorption method for the investigation of hydrogen adsorption on electrodes; its application to tungsten carbide,along with electrochemical methods

A method for direct determination of hydrogen adsorption on large surface area electrodes or on electro-conductive powders in electrolyte solutions, at any given potential, is proposed. It consists of hydrogen desorption from the electrode by heating (thermodesorption) after complete adsorption and a quantitative chromatographic analysis of the gas collected during thermodesorption. The proposed method was used in this work to investigate hydrogen adsorption on tungsten carbide powder in a H₂SO₄ solution. The potential-dependence of adsorption was found, confirming the authors previous suggestion of an extensive coverage of tungsten carbide by chemisorbed hydrogen and of the potential region of adsorption (up to 0·35–0·4 V).The tungsten carbide surface was also investigated by the method of charging and potentiodynamic curves. Reversibility of these curves up to 0·35–0·4 V is shown. Hydrogen adsorption is correlated with the quantity of electricity calculated from the charging curve and it is shown that a considerable portion of the charge is associated with hydrogen chemisorption but some portion of the total charge is consumed even in the hydrogen region for other non-defined redox processes occurring on the tungsten carbide surface.     

Electrochemical performance for the electro-oxidation of ethylene glycol on a carbon-supported platinum catalyst at intermediate temperature

To determine the kinetic performance of the electro-oxidation of a polyalcohol operating at relatively high temperatures, direct electrochemical oxidation of ethylene glycol on a carbon supported platinum catalyst (Pt/C) was investigated at intermediate temperatures (235–255 °C) using a single cell fabricated with a proton-conducting solid electrolyte, CsH₂PO₄, which has high proton conductivity (>10⁻² S cm⁻¹) in the intermediate temperature region. A high oxidation current density was observed, comparable to that for methanol electro-oxidation and also higher than that for ethanol electro-oxidation. The main products of ethylene glycol electro-oxidation were H₂, CO₂, CO and a small amount of CH₄ formation was also observed. On the other hand, the amounts of C₂ products such as acetaldehyde, acetic acid and glycolaldehyde were quite small and were lower by about two orders of magnitude than the gaseous reaction products. This clearly shows that C–C bond dissociation proceeds almost to completion at intermediate temperatures and the dissociation ratio reached a value above 95%. The present observations and kinetic analysis suggest the effective application of direct alcohol fuel cells operating at intermediate temperatures and indicate the possibility of total oxidation of alcohol fuels.     

Analysis of sulfur poisoning on a PEM fuel cell electrode

The extent of irreversible deactivation of Pt towards hydrogen oxidation reaction (HOR) due to sulfur adsorption and subsequent electrochemical oxidation is quantified in a functional polymer electrolyte membrane (PEM) fuel cell. At 70 °C, sequential hydrogen sulfide (H₂S) exposure and electrochemical oxidation experiments indicate that as much as 6% of total Pt sites are deactivated per monolayer sulfur adsorption at open-circuit potential of a PEM fuel cell followed by its removal. The extent of such deactivation is much higher when the electrode is exposed to H₂S while the fuel cell is operating at a finite load, and is dependent on the local overpotential as well as the duration of exposure. Regardless of this deactivation, the H₂/O₂ polarization curves obtained on post-recovery electrodes do not show performance losses suggesting that such performance curves alone cannot be used to assess the extent of recovery due to sulfur poisoning. A concise mechanism for the adsorption and electro-oxidation of H₂S on Pt anode is presented. H₂S dissociatively adsorbs onto Pt as two different sulfur species and at intermediate oxidation potentials, undergoes electro-oxidation to sulfur and then to sulfur dioxide. This mechanism is validated by charge balances between hydrogen desorption and sulfur electro-oxidation on Pt. The ignition potential for sulfur oxidation decreases with increase in temperature, which coupled with faster electro-oxidation kinetics result in the easier removal of adsorbed sulfur at higher temperatures. Furthermore, the adsorption potential is found to influence sulfur coverage of an electrode exposed to H₂S. As an implication, the local potential of a PEM fuel cell anode exposed to H₂S contaminated fuel should be kept below the equilibrium potential for sulfur oxidation to prevent irreversible loss of Pt sites.     

Electrochemical and infrared study of electro-oxidation of dimethyl ether (DME) on platinum polycrystalline electrode in acid solutions

Electro-oxidation of dimethyl ether (CH₃OCH₃, denoted as DME below) on Pt polycrystalline electrode has been investigated by electrochemical and in situ infrared (IR) measurements in acid solutions. A reaction intermediate species, (CH₃OCH₂-)_ad, has been observed in the low potential region as an initial product for dehydrogenation process of DME on Pt electrode surface. This species is subsequently decomposed to adsorbed carbon monoxide (CO) and finally oxidized to carbon dioxide (CO₂) in higher potential region. The time-resolved IR measurement is employed to follow the transient process of the formation and decomposition of the intermediate on Pt electrode surface. Based on these electrochemical and IR spectroscopic results, a reaction scheme for DME electro-oxidation process is proposed.     

Mechanism studies on CVD of boron carbide from a gas mixture of BCL3, CH4, and H2 in a dual impinging‐jet reactor

Nearly pure boron carbide free from impurities was produced on a tungsten substrate in a dual impinging‐jet chemical vapor deposition reactor from a BCl₃, CH₄, and H₂ mixture. The Fourier Transform Infrared (FTIR) analysis proved the formation of reaction intermediate BHCl₂, which is proposed to occur mainly in the gaseous boundary layer next to the substrate surface. Among a large number of reaction mechanisms proposed only the ones considering the molecular adsorption of boron carbide on the substrate surface gave reasonable fits. In the proposed mechanism dichloroborane is formed in the gas phase only as a by‐product. Boron carbide, on the other hand, is formed through a series of surface reactions involving adsorbed boron trichloride, adsorbed methane and gas phase hydrogen. The simultaneous fit of the experimental rate data to the model expressions gave correlation coefficient values of 0.977 and 0.948, in predicting the B₄C and BHCl₂ formation rates, respectively. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Adsorption,oxidation and effect of CO on hydrogen adsorption at smooth rhodium electrodes in acid solution

The adsorption of carbon monoxide was studied under steady state conditions on smooth rhodium electrodes in 0.5 M H₂SO₄ at room temperature. The coverage with carbon monoxide is equal to saturation coverage between 0.1 and 0.5 V vs a hydrogen electrode in the same solution. Above 0.5 V it decrease to 0 within 0.2 V. The oxidation of chemisorbed carbon monoxide at constant current is accompanied by the formation of the oxygen layer above 0.6 V. A correction for the contribution of the oxygen layer formation to the anodic charge has to be applied in the determination of the CO_ad coverage even at small current densities.The coverage with H_ad was determined as a function of potential at constant coverage with preadsorbed carbon monoxide from anodic charging curves. These curves were taken at a sufficiently small current density so that the equilibrium of the Volmer reaction remained practically established. The coverage of CO_ad, computed from the charge for the oxidation of CO_ad, agreed well with that obtained from the decrease of the length of the hydrogen branch of the charging curves. The isotherms of hydrogen adsorption are of the Temkin type at CO_ad coverages above 0.4. The characteristic parameters of the Temkin isotherm were determined as a function of the coverage with CO_ad.     

Selective nonanal molecular recognition with SnO2 nanosheets for lung cancer sensor

SnO₂ nanosheets were developed to detect nonanal gas in the order of ppb which was a marker of lung cancer. The nanosheets showed higher resistance change in nonanal gas than that in carbon monoxide (CO), nitrogen dioxide (NO₂), acetone (CH₃COCH₃), hydrogen (H₂), ethanol (C₂H₆O), ammonia (NH₃), hydrogen sulfide (H₂S), formaldehyde (HCHO), acetaldehyde (CH₃CHO), or butanal (C₄H₈O). Crystal surfaces of the nanosheets would be effective for adsorption of nonanal molecules. Furthermore, it was shown that resistance changed with an increase in carbon number in aldehyde. The nanosheets had molecular selectivity for a series of aldehyde molecules. Molecular recognition of the nanosheets gave us a great advantage to detect nonanal gas which was produced by lung cancer.     

Acid gas adsorption on zeolite SSZ-13: Equilibrium and dynamic behavior for natural gas applications

In developing new adsorption separation processes, it is necessary to study both the equilibrium and dynamic adsorption properties of potential materials. Experimental determination of isotherms and dynamic breakthrough properties aid in the development of modeling new adsorption systems toward process development. Here, the equilibrium adsorption properties of a small-pore zeolite, Na-SSZ-13, are studied for its natural gas separation potential. Using volumetric, gravimetric, and dynamic column breakthrough adsorption techniques, the adsorption properties of CO₂, CH₄, C₂H₆, H₂S, and H₂O are determined. High-pressure breakthrough experiments demonstrate the mixed gas separation performance of Na-SSZ-13 in mixtures containing CO₂, CH₄, C₂H₆, and H₂S. Simulations of these breakthrough experiments show that ideal adsorbed solution theory adequately describes the mixed gas adsorption modeling for this zeolite. In gas mixtures containing both CO₂ and H₂S, there is an observed acid gas reaction that results in elution of carbonyl sulfide, COS.     

The role of bulk H and C species in the chain lengthening of Fischer-Tropsch synthesis over nickel

This work documents the possible role of bulk species (dissolved H and C atoms) in the Fischer-Tropsch reaction. This has led us to study the sorption (adsorption or absorption) of hydrogen when increasing the pressure from 0.1 to 5 MPa, the selectivity toward C₂₊ hydrocarbons in carbon monoxide hydrogenation at 5 MPa total pressure, and the amount of nickel carbide formed in the course of the reaction over unsupported nickel, Ni/SiO₂, Ni/TiO₂ and Ni/Cr₂O₃ catalysts. Increasing the hydrogen pressure results in a subsequent sorption which is found to be proportional to the nickel dispersion, and can be attributed either to a completion of the surface hydrogen adlayer (8%) or to a formation of a hydrogen sublayer. No relationship could be found between the selectivity towards C₂₊ hydrocarbons and the quantity of sorbed hydrogen. A correlation between this selectivity and the amount of nickel carbide formation was demonstrated, suggesting that the active phase which leads to C₂₊ hydrocarbons is nickel carbide, or that carbon atoms of subsurface sites participate in the homologation reaction. This revised version was published online in July 2006 with corrections to the Cover Date.     

Catalytic and inhibitory effects of dissolved Cd2+ on C3H8 electro-oxidation

Studies have been made of the effect of Cd²⁺ on the oxidation rate of C₃H₈ in 80% H₃PO₄ at 130 C on platinized Pt electrodes.Dissolved Cd²⁺ discharges, probably as Cd⁰, and forms a stable sub-monolayer on a Pt electrode at potentials as high as 0·5 V versus RHE. Although this Cd⁰ layer is apparently substantially desorbed in favor of adsorbed hydrocarbon, C₃H₈ adsorption is inhibited in its presence. C₃H₈ oxidation is also inhibited. However, the C₃H₈ oxidation rate is observed to increase under some circumstances, and this appears to be because Cd isabsorbed into the Pt lattice. Co-plating of Cd and Pt leads to an electrode which, once the excess Cd has been removed, appears to be stable, and which catalyses C₃H₈ oxidation by about a factor of three at 0·25 V. Above 0·3 V, this absorbed Cd somewhat inhibits C₃H₈ oxidation.     

Formation and gasification of carbon deposits on NiSiO2 catalysts

The deposition of carbon from CH₄ and CO on $\text{Ni}\text{SiO}{}_2$ catalysts was studied in pulse-flow experiments as well as volumetrically with a low-field magnetic permeameter. It was found that carbon, deposited from CH₄ according to: CH₄ → C + 4H, gave rise to the formation of nickel carbide, Ni₃C, only at the surface of the nickel particles (T< 300 °C). However, carbon, deposited from CO according to: 2CO → C + CO₂, led to the formation of a bulk nickel carbide as well as dissolution of carbon interstitially. The reactivity of the carbon thus deposited was studied with both H₂ and H₂O. The rate of reaction with hydrogen appeared to be a function of temperature: the rate passed through a maximum at 200 °C and dropped steeply above 300 °C. The only product of the reaction was CH₄. The reaction with H₂O produced besides CH₄, CO₂ and (at low carbon surface coverages) H₂.     

Effect of electrolyte environment and Pt crystallite size on hydrogen adsorption—V

The role of Pt crystallite surface morphology on hydrogen adsorption isotherms in H₂SO₄ and alkaline electrolytes was examined by a potentiodynamic sweep technique. By varying the crystallite size (40–280Å) of highly dispersed Pt electrocatalysts, the relative concentrations of edges, vertices and crystallite faces which contribute to the surface morphology are changed. The potentiodynamic i-V profiles for adsorbed hydrogen oxidation on highly dispersed Pt electrocatalysts in 0.05 and 1 M H₂SO₄ showed similar changes with Pt crystallite size. Only two states of adsorbed hydrogen on highly dispersed Pt were observed in 0.05 M H₂SO₄, compared to four states reported by Angerstein-Kozlowska et al on smooth polycrystalline Pt electrodes.In 1 M NaOH and 35 wt % KOH, less than a monolayer of adsorbed hydrogen was present on highly dispersed Pt electrocatalysts at the reversible hydrogen potential. Two states of chemisorbed hydrogen were observed at 23–91°, while at low temperature (?47°) in 35 wt % KOH, an additional adsorbed hydrogen species was evident in the potentiodynamic i-V curves. A Pt crystallite size effect on the adsorption of hydrogen on highly dispersed Pt in alkaline electrolytes was not deduced.     

Computational study of linear carbon chains on gold and silver surfaces

Density-functional-theory calculations were carried out for hydrogen capped linear carbon chains, polyynes and cumulenes, adsorbed dissociatively on the (1 1 1) and (2 1 1) surfaces of gold and silver. In the studied adsorption reactions, carbon–hydrogen bonds are broken and covalent carbon–metal bonds are created. The adsorption of cumulenes is highly endothermic, whereas the adsorption of polyynes is near thermoneutral. Also, the hydrogenation of adsorbed polyynyl radicals (·C_nH) into adsorbed cumulene carbenes (:C_nH₂) was investigated, which was found to be exothermic on both metals. Vibrational calculations were conducted on the adsorption systems, and the results were compared with experimental surface enhanced Raman scattering spectra. An interpretation is proposed for the spectra of polyyne–silver solutions, and features of polyyne–gold spectra are predicted.     

Structural study of carbon nanomaterials prepared by chlorination of tungsten carbide and bis(cyclopentadienyl)tungsten dichloride

Hollow carbon nanobags have been obtained by the chlorination of bis(cyclopentadienyl)tungsten dichloride (W(C₅H₅)₂Cl₂) at 400 and 900 °C. Transmission electron microscopy images indicate an incipient graphitisation at higher reaction temperatures and an increase in the average dimensions of the particles. When using tungsten carbide (WC) as precursor, carbide-derived carbon has been observed at 900 °C, whereas at lower temperatures core-shell-like structures have been found as intermediate reaction steps. In both type of materials, electron energy-loss spectroscopy shows a very similar sp² carbon bonding content (∼94%). Textural studies show Type 1 adsorption isotherms with surface areas of 1250 and 1320 m²/g for WC and W(C₅H₅)₂Cl₂ respectively at the higher temperature treatment.     

Theoretical investigation of ethylene adsorbed on Ni(100) surface by the multiple-scattering cluster method

In this paper the carbon K-edge near edge X-ray absorption fine structure spectra (NEXAFS) of adsorption system C₂H₄/Ni(100) are calculated using the multiple-scattering cluster method. By a comparison between the theoretical results and experimental spectra, the chemisorption geometry of this system has been determined. The result shows that the molecule is adsorbed on the perpendicular bridge site, and the distance between the C atom and the nearest Ni atom is 1.70 Å, while the molecular plane tilting to the surface is 50°. It is found that the interaction between hydrogen atom and Ni substrate plays an important role in the formation of the adsorption structure. The above results are supported by other evidences.     

One-step purification of ethylene from acetylene and carbon dioxide by ultramicroporous carbons

The one-step adsorptive separation of high-purity ethylene (C₂H₄) from a ternary gas mixture (C₂H₂/C₂H₄/CO₂) is challenging and has not been reported on porous carbons. Herein, we report camphor seeds husk-derived ultramicroporous carbons (CSHs) show high affinities toward acetylene (C₂H₂) and carbon dioxide (CO₂) over C₂H₄. The optimized CSH-2-700 with high heteroatom contents and centered pore size distributions shows high C₂H₂ adsorption capacity (2.24 mmol g⁻¹) and record ideal adsorbed solution theory (IAST) C₂H₂/C₂H₄ selectivity (10.2) among one-step C₂H₄ purification adsorbents. Meanwhile, CSH carbons are the only carbon adsorbents that preferentially adsorb CO₂ over C₂H₄, with a CO₂/C₂H₄ selectivity of 1.9 under ambient conditions. Furthermore, dynamic breakthrough experiments verified its feasibility for one-step C₂H₄ purification from a three-component C₂H₂/C₂H₄/CO₂ gas-mixture.