Kinetic interactions between hydrogen and carbon monoxide oxidation over platinum

The heterogeneous chemistry coupling of H₂ and CO over platinum was investigated experimentally and numerically for H₂/CO/O₂/N₂ mixtures with overall lean equivalence ratios φ = 0.13–0.26, H₂:CO molar ratios 1:5–3:1, and a pressure of 5 bar. Experiments were performed in an optically accessible channel-flow reactor at surface temperatures 510–827 K and involved in situ Raman measurements of major gas-phase species concentrations and thermocouple measurements of surface temperatures. Emphasis was placed on the low temperature range 510–600 K, whereby H₂ inhibited the CO oxidation, and which was of particular relevance to gas turbine idling and part-load operation. Comparisons of measurements with 2-D simulations attested the aptness of the employed kinetic scheme, not only for H₂/CO fuel mixtures but also for pure CO. Measured and predicted transition temperatures below which H₂ inhibited CO oxidation agreed well with each other, showing a main dependence on the overall equivalence ratio (550 ± 5 K at φ = 0.13 and 600 ± 5 K at φ = 0.26) and a weaker dependence on the H₂:CO ratio. Furthermore, this inhibition was non-monotonically dependent on the H₂:CO ratio, becoming higher at a value of 1:1. The inhibiting kinetic effect of H₂ was an outcome of the competition between H₂ and CO/O₂ for surface adsorption and, most importantly, of the competition between the adsorbed H(s) and CO(s) for surface-deficient O(s). Finally, transient simulations in practical catalytic channels revealed the interplay between kinetic and thermal effects. While at φ = 0.13 the H₂/CO reactive mixture exothermicity was insufficient to overtake the kinetic inhibition, at φ = 0.26 catalytic ignition could still be achieved at temperatures well-below the transition temperature. The effect of H₂:CO molar ratio on the light-off times was quite strong, suggesting care when designing syngas catalytic rectors with varying compositions.     

Hydrogen production by sorption enhanced steam reforming of oxygenated hydrocarbons (ethanol, glycerol, n-butanol and methanol): Thermodynamic modelling

Thermodynamic analysis of steam reforming of different oxygenated hydrocarbons (ethanol, glycerol, n-butanol and methanol) with and without CaO as CO₂ sorbent is carried out to determine favorable operating conditions to produce high-quality H₂ gas. The results indicate that the sorption enhanced steam reforming (SESR) is a fuel flexible and effective process to produce high-purity H₂ with low contents of CO, CO₂ and CH₄ in the temperature range of 723-873 K. In addition, the separation of CO₂ from the gas phase greatly inhibits carbon deposition at low and moderate temperatures. For all the oxygenated hydrocarbons investigated in this work, thermodynamic predictions indicate that high-purity hydrogen with CO content within 20 ppm required for proton exchange membrane fuel cell (PEMFC) applications can be directly produced by a single-step SESR process in the temperature range of 723-773 K at pressures of 3-5 atm. Thus, further processes involving water-gas shift (WGS) and preferential CO oxidation (COPROX) reactors are not necessary. In the case of ethanol and methanol, the theoretical findings of the present analysis are corroborated by experimental results from literature. In the other cases, the results could provide an indication of the starting point for experimental research. At P = 5 atm and T = 773 K, it is possible to obtain H₂ at concentrations over 97 mol% along with CO content around 10 ppm and a thermal efficiency greater than 76%. In order to achieve such a reformate composition, the optimized steam-to-fuel molar ratios are 6:1, 9:1, 12:1 and 4:1 for ethanol, glycerol, n-butanol and methanol, respectively, with CaO in the stoichiometric ratio to carbon atom.     

Photocatalytic effect of ZnO on carbon gasification with CO2 for high temperature solar thermochemistry

A photocatalytic effect of ZnO on carbon gasification with CO₂ was studied using a concentrated Xe beam to enhance the gasification rate in solar/chemical energy conversion process. The sample, activated carbon impregnated with ZnO (5 wt%), was heated at 873 K by a Xe beam irradiation with UV (<400 nm). The gasification rate at 873 K increased 2 folds in comparison with the Xe irradiation without UV, but, the difference of the rate of CO evolution decreased with the increasing temperature from 873 to 1073 K. The carbothermal reduction of ZnO (ZnO+C→Zn+CO) proceeded at above 950 K, which was demonstrated by XRD analysis and thermodynamic calculation. These results indicate that the photocatalytic effect of ZnO with the UV irradiation enhance the gasification rate of carbon at low temperature (873 K).     

Water–gas shift reaction in a Pd membrane reactor over Pt/Ce0.6Zr0.4O2 catalyst

A water–gas shift (WGS) Pt/Ce_0.6Zr_0.4O₂ catalyst has been prepared, which exhibits much faster kinetics than conventional high-temperature ferrochrome catalysts in the temperature range most suitable for operation of WGS Pd membrane reactors, i.e. above 623 K. The performance of the Pt catalyst was tested in a reactor furnished with a supported, 1.4 μm thick high-flux Pd membrane using feeds obtained by autothermal reforming of natural gas. CO conversion remained above thermodynamic equilibrium up to feed space velocities of 9100 l kg⁻¹ h⁻¹ at 623 K, P_total = 1.2 MPa and steam-to-carbon ratio S/C = 3, but H₂ recovery decreased from 84.8% at GHSV = 4050 l kg⁻¹ h⁻¹ to 48.7% at the highest space velocity. This rapid decline of separation performance is attributed to slow H₂ diffusion through the catalyst bed, suggesting that external mass flow resistance has a significant impact on the H₂ permeation rate in such membrane reactors. This could be minimized by the development of WGS catalysts with even faster kinetics which would allow further reduction of the catalyst bed height.     

Numerical study on NOx/CO emissions in the diffusion flames of high-temperature off-gas of steelmaking converter

The combustion of high-temperature off-gas of steelmaking converter with periodical change of temperature and CO concentration always leads to CO and NO_x over-standard emissions. In the paper, high-temperature off-gas combustion is simulated by adopting counterflow diffusion flame model, and some influencing factors of CO and NO_x emissions are investigated by adopting a detailed chemistry GRI 3.0 mechanism. The emission index of NO_x (EINO_x) decreases 1.7–4.6% when air stoichiometric ratio (SR) increase from 0.6 to 1.4, and it dramatically increases with off-gas temperature at a given SR when the off-gas temperature is above 1500 K. High-concentration CO in off-gas can result in high NO_x emissions, and NO_x levels increase dramatically with CO concentration when off-gas temperature is above 1700 K. Both SR and off-gas temperature are important for the increase of CO burnout index (BI_CO) when SR is less than 1.0, but BI_CO increase about 1% when off-gas temperature increases from 1100 K to 1900 K at SR > 1.0. BI_CO increases with CO concentration in off-gas, and the influence of off-gas temperature on BI_CO is marginal. BI_CO increases with the relative humidity (RH) in air supplied, but it increases about 0.5% when RH is larger than 30%.     

Electroactivity of high performance unsupported Pt–Ru nanoparticles in the presence of hydrogen and carbon monoxide

The electrochemical activity of high performance unsupported (1:1) Pt–Ru electrocatalyst in the presence of hydrogen and carbon monoxide has been studied using the thin-film rotating disk electrode (RDE) technique. The kinetic parameters of these reactions were determined in H₂- and CO-saturated 0.5 M H₂SO₄ solutions by means of cyclic voltammetry, including CO stripping, and RDE voltammetry. Pt–Ru/Nafion inks were prepared in one step with different Nafion mass fractions, allowing determining the ionomer influence in electrocatalytic response and obtaining the kinetic current density in absence of mass-transfer effects, being 41 and 12 mA cm² (geometrical area), for H₂ and CO oxidation, respectively. These values correspond to mass activities of 1.37 and 0.40 A mg_Pt¹ and to specific activities of 1.52 and 0.44 mA cm_Pt². The Tafel analysis confirmed that hydrogen oxidation was a two-electron reversible reaction, while CO oxidation exhibited an irreversible behavior with a charge-transfer coefficient of 0.42. The kinetic results for CO oxidation are in agreement with the bifunctional theory, in which the reaction between Pt–CO and Ru–OH is the rate-determining step. The exchange current density for hydrogen reaction was 0.28 mA cm² (active surface area), thus showing similar kinetics to those found for carbon-supported Pt and Pt–Ru electrocatalyst nanoparticles.     

Effect of gas flow rates and Boudouard reactions on the performance of Ni/YSZ anode supported solid oxide fuel cells with solid carbon fuels

The effects of carrier gas flow rates and Boudouard reaction on the performance of Ni/YSZ anode-supported solid oxide fuel cells (SOFCs) have been studied with coconut coke fuels at 800 °C. Decreasing flow rates of carrier gas from 1000 to 50 ml min⁻¹ increased open circuit voltages and current densities from 0.71 to 0.87 V and from 0.12 to 0.34 A cm⁻², respectively. The increased cell performance was attributed to the increasing extent of electrochemical oxidation of CO, a product of Boudouard reaction. The contribution of CO oxidation to current generation was estimated to 66% in flowing inert carrier gas at 50 ml min⁻¹. The pulse transient studies confirmed the effect of flow rates on cell performance and also revealed that CO and CO₂ can displace adsorbed hydrogen on carbon fuels. Flowing CO₂ over coconut coke fuel produced CO via Boudouard reaction. The presence of CO led to a highest power density of 95 mW cm⁻², followed by a concurrent decline of power density and CO concentration. The declined power density along with decreasing CO concentration further verified contribution of gaseous CO to the power generation of C-SOFC; the decreasing CO concentration showed a typical kinetics behavior of Boudouard reaction, suggesting the loss of active sites on carbon surface for the reaction.     

Simplified correlation models for CO/H2 chemical reaction times

This paper provides simplified correlation models for CO/H₂ chemical reaction times. The procedure used for the CO/H₂ simplified modeling utilized the full chemical kinetics mechanism run over a range of temperatures from 700 to 1800 K, pressures from 0.5 to 50 atm, mixtures from 0% to 95% CO, and equivalence ratios from 0.2 to 2.0 to determine ignition (or reaction) time. The correlations for ignition times are given in formulas as functions of equivalence ratio, temperature, and pressure. Two different forms of correlations were obtained, one being a single, overall correlation and the other a two-stage correlation representing regions of high and low temperatures. These correlations are shown to work well over a range of chemical time scales spanning ten orders of magnitude. The correlations are also compared with measured data from the literature.     

Catalytic partial oxidation of coke oven gas to syngas in an oxygen permeation membrane reactor combined with NiO/MgO catalyst

A high oxygen permeability and sufficient chemical and mechanical stability mixed ion and electron conductivity membrane to withstand the hash strong oxidation and reduction working conditions is significant for the membrane reactor to commercial-scale plant. In this paper, a disk-shaped Ba_1.0Co_0.7Fe_0.2Nb_0.1O_3−δ membrane was applied to a membrane reactor for the partial oxidation of methane in coke oven gas (COG) to syngas. The reaction was carried out using NiO/MgO solid solution catalyst by feeding COG. The reforming process was performed successfully; 95% CH₄ conversion, 80% H₂ selectivity, 106% CO selectivity and 16.3 ml cm⁻² min⁻¹ oxygen permeation flux were achieved at 1148 K. The reaction has been steadily carried out for more than 100 h. The NiO/MgO catalyst used in the membrane reactor exhibited good catalytic activity and resistance to coking in the COG atmosphere. Characterization of the membrane surface by SEM and XRD after long life test showed that both the surface exposed to the air side and reaction side still preserved the Perovskite structure which is implied that the practical application of this membrane as membrane reactor for partial oxidation of COG is promising.     

Characterization of Cu,Ag and Pt added La0.6Sr0.4Co0.2Fe0.8O3−δ and gadolinia-doped ceria as solid oxide fuel cell electrodes by temperature-programmed techniques

Cu, Ag and Pt added La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) and gadolinia-doped ceria (GDC) were analyzed by the temperature-programmed techniques for their characteristics as either the cathode or the anode of the solid oxide fuel cells (SOFCs). Temperature-programmed oxidation using CO₂ was used to characterize the cathode materials while temperature-programmed reduction (TPR) using H₂ and TPR using CO were used to characterize the anode materials. These techniques can offer an easy screening of the materials as the SOFC electrodes. The effects of adding Cu, Ag and Pt to LSCF for the cathodic reduction activity and the anodic oxidation activity are different—Cu > Ag > Pt for reduction and Pt > Cu > Ag for oxidation. The CO oxidation activities are higher than the H₂ oxidation activities. Adding GDC to LSCF can increase both reduction and oxidation activities. The LSCF–GDC composite has a maximum activity for either reduction or oxidation when LSCF/GDC is 2 in weight.     

Exergy analysis of two phase change materials storage system for solar thermal power with finite-time thermodynamics

A mathematical model for the overall exergetic efficiency of two phase change materials named PCM1 and PCM2 storage system with a concentrating collector for solar thermal power based on finite-time thermodynamics is developed. The model takes into consideration the effects of melting temperatures and number of heat transfer unit of PCM1 and PCM2 on the overall exergetic efficiency. The analysis is based on a lumped model for the PCMs which assumes that a PCM is a thermal reservoir with a constant temperature of its melting point and a distributed model for the air which assumes that the temperature of the air varies in its flow path. The results show that the overall exergetic efficiency can be improved by 19.0-53.8% using two PCMs compared with a single PCM. It is found that melting temperatures of PCM1 and PCM2 have different influences on the overall exergetic efficiency, and the overall exergetic efficiency decreases with increasing the melting temperature of PCM1, increases with increasing the melting temperature of PCM2. It is also found that for PCM1, increasing its number of heat transfer unit can increase the overall exergetic efficiency, however, for PCM2, only when the melting temperature of PCM1 is less than 1150 K and the melting temperature of PCM2 is more than 750 K, increasing the number of heat transfer unit of PCM2 can increase the overall exergetic efficiency. Considering actual application of solar thermal power, we suggest that the optimum melting temperature range of PCM1 is 1000-1150 K and that of PCM2 is 750-900 K. The present analysis provides theoretical guidance for applications of two PCMs storage system for solar thermal power.     

Thermodynamic analysis of steam assisted conversions of bio-oil components to synthesis gas

The aim of this study is to investigate the thermodynamics of steam assisted, high-pressure conversions of model components of bio-oil – isopropyl alcohol, lactic acid and phenol – to synthesis gas (H₂ + CO) and to understand the effects of process variables such as temperature and inlet steam-to-fuel ratio on the product distribution. For this purpose, thermodynamic analyses are performed at a pressure of 30 bar and at ranges of temperature and steam-to-fuel ratio of 600–1200 K and 4–9, respectively. The number of moles of each component in the product stream and the product composition at equilibrium are calculated via Gibbs free energy minimization technique. The resulting optimization problems are solved by using the Sequential quadratic programming method. The results showed that all of the model fuels reached near-complete conversions to H₂, CO, CO₂ and CH₄ within the range of operating conditions. Temperature and steam-to-fuel ratio had positive effects in increasing hydrogen content of the product mixture at different magnitudes. Production of CO increased with temperature, but decreased at high steam-to-fuel ratios. Conversion of model fuels in excess of 1000 K favored molar H₂/CO ratios around 2, the synthesis gas composition required for Fischer–Tropsch and methanol syntheses. It was also possible to adjust the H₂/CO ratios and the amounts of CH₄ and CO₂ in synthesis gas by steam-to-fuel ratio, the value depending on temperature and the fuel type. Product distribution trends indicated the presence of water–gas shift and methanation equilibria as major side reactions running in parallel with the steam reforming of the model hydrocarbons.     

The influence of the CO inhibition effect on the estimation of the H2 purification unit surface

The CO inhibition effect on H₂ permeance through commercial Pd-based membranes was analysed by means of permeation measurements at different CO compositions (0–30% molar) and temperatures (593–723 K) with the aim to determine the increase of the membrane area in order to compensate the H₂ flux reduction owing to the CO inhibition effect. The permeance of H₂ fed with carbon monoxide was observed to decrease with respect to the case of pure hydrogen. At 647 K the H₂ permeance of a pure feed of 316 μmol m⁻² s⁻¹ Pa^−0.5 reduces progressively until 275 μmol m⁻² s⁻¹ Pa^−0.5 when 15% or more of CO is present in the system, until it reaches a plateau at 20%. The inhibition effect occurring when CO is present in the feed stream reduces with the progressive temperature increase; the reduction of the permeance decreases exponentially by 23% at 593 K and by 3% at 723 K with 10% of CO. The inhibition effect is seen to be reversible. An H₂ flux profile in a Sieverts' plot shows the effect produced by the increase of the CO composition along the Pd-based membrane length. The H₂ flux profile allows the area of a Pd-based membrane to be evaluated in order to have the same permeate flow rate of H₂ when it is fed with CO or as a pure stream. Moreover, a qualitative comparison between the H₂ flux profiles and a previously proposed model has been carried out.     

Characterisation of the electrochemical water gas shift reactor (EWGSR) operated with hydrogen and carbon monoxide rich feed gas

Water gas shift units are used to raise the H₂ yield of reforming processes by converting CO to CO₂ and additional H₂. Additional subsequent processes are required to separate H₂ from the product gas stream to obtain pure H₂. This study investigates a novel electrochemical membrane reactor, where the water gas shift reaction occurs electrochemically. The reactor is operated at 393 K and 403 K with electrical energy to enable hydrogen purification in terms of electrochemical pumping, as well as a simultaneous electrochemical CO oxidation to increase the yield of purified H₂. The experimental results show the influence of several operation parameters upon its operation characteristics (e.g. cell voltage, electrochemical CO oxidation, the energy demand, etc.). The process yielded high overall exergy efficiencies of, e.g. 78.3%, whereas the anodic outlet stream contributed with 35%-units, and the purified hydrogen with 43.3%-units.     

Relationship between carbon corrosion and positive electrode potential in a proton-exchange membrane fuel cell during start/stop operation

Fuel starvation during start-up and shut-down processes can adversely affect the performance of proton-exchange membrane fuel cells. In this study, fuel starvation is induced intentionally by supplying hydrogen and air to the negative electrode (anode) side alternately, and the individual electrode potential is measured in situ using a dynamic hydrogen electrode. The positive electrode (cathode) potential is increased to 1.4 V when air/hydrogen boundaries developed on the anode side. The development of a high cathode potential causes oxidation of the carbon support with the amount of CO₂ evolution proportional to the cathode potential above 1.0 V. Above ∼1.2 V, CO and SO₂ are generated electrochemically or chemically and the rate of CO production is higher than that of SO₂. Although a higher cathode potential is induced irrespective of the cell temperature, oxidation of the carbon support is retarded significantly at low temperatures.     

Decontamination of industrial cyanide-containing water in a solar CPC pilot plant

The aim of this work was to improve the quality of wastewater effluent coming from an Integrated Gasification Combined-Cycle (IGCC) power station to meet with future environmental legislation. This study examined a homogeneous photocatalytic oxidation process using concentrated solar UV energy (UV/Fe(II)/H₂O₂) in a Solar Compound Parabolic Collector (CPC) pilot plant. The efficiency of the process was evaluated by analysis of the oxidation of cyanides and Total Organic Carbon (TOC). A factorial experimental design allowed the determination of the influences of operating variables (initial concentration of H₂O₂, oxalic acid and Fe(II) and pH) on the degradation kinetics. Temperature and UV-A solar power were also included in the Neural Network fittings. The pH was maintained at a value >9.5 during cyanide oxidation to avoid the formation of gaseous HCN and later lowered to enhance mineralization. Under the optimum conditions ([H₂O₂] = 2000 ppm, [Fe(II)] = 8 ppm, pH = 3.3 after cyanide oxidation, and [(COOH)₂] = 60 ppm), it was possible to degrade 100% of the cyanides and up to 92% of Total Organic Carbon.     

Structural and optical properties of textured ZnO:Al films on glass substrates prepared by in-line rf magnetron sputtering

Textured ZnO:Al films with excellent light scattering properties as a front electrode of silicon thin film solar cells were prepared on glass substrates by an in-line rf magnetron sputtering, followed by a wet-etching process to modify the surface morphologies of the films. Deposition parameters and wet etching conditions of the films were controlled precisely to obtain the optimized surface features. All as-deposited films show a strong preferred orientation in the [0 0 1] direction under our experimental conditions. The microstructure of the films was significantly affected by working pressure and film compactness was reduced with increasing working pressure, while the effect of a substrate temperature on the microstructure is less pronounced. A low resistivity of 4.25×10⁻⁴ Ω cm and high optical transmittance of above 80% in a visible range were obtained in the films deposited at 1.5 mTorr and 100 °C. After wet etching process, the surface morphologies of the films were changed dramatically depending on the microstructure and film compactness of the initial films. By controlling the surface feature, the haze factor and angular resolved distribution of the textured ZnO:Al films were improved remarkably when compared with commercial SnO₂:F films. The textured ZnO:Al and SnO₂:F films were applied as substrates for a silicon thin film solar cells with tandem structure of a-Si:H/μc-Si:H. Compared with the solar cells with the SnO₂:F films, a significant enhancement in the short-circuit current density of the μc-Si:H bottom cell was achieved, which is due to the improved light scattering on the highly textured ZnO:Al film surfaces in the long wavelength above 600 nm.     

Plasma-induced TCO texture of ZnO:Ga back contacts on silicon thin film solar cells

This paper considers texturing of ZnO:Ga (GZO) films used as back contacts in amorphous silicon (a-Si) thin film solar cells. GZO thin films are first prepared by conventional methods. The as-deposited GZO surface properties are modified so that their use as back contacts on a-Si solar cells is enhanced. Texturing is performed by simple dry plasma etching in a CVD process chamber,at power=100 W, substrate temperature=190 °C (temperature is held at 190 °C because thin film solar cells are damaged above 200 °C), pressure=400 Pa and process gas H₂ flow=700 sccm. Conventional a-Si solar cells are fabricated with and without GZO back contact surface treatment. Comparison of the with/without texturing GZO films shows that plasma etching increases optical scattering reflectance and reflection haze. SEM and TEM are used to evaluate the morphological treatment-induced changes in the films. Comparison of the a-Si solar cells with/without texturing shows that the plasma treatment increases both the short-circuit current density and fill factor. Consequently, a-Si solar cell efficiency is relatively improved by 4.6%.     

Hydrogen production by steam-gasification of petroleum coke using concentrated solar power—II Reactor design,testing, and modeling

The solar chemical reactor technology for the steam-gasification of petcoke is presented. The reactor features a continuous vortex flow of steam laden with petcoke particles confined to a cavity receiver and directly exposed to concentrated solar radiation. A 5 kW prototype reactor tested in a high-flux solar furnace in the range 1300–1800 K yielded up to 87% petcoke conversion in a single pass of 1 s residence time. The solar-to-chemical energy conversion efficiency attained 9% without accounting for the product's sensible heat, and 20% when the sensible heat is recovered for steam generation and pre-heating. A steady-state process model that couples radiative heat transfer with the reaction kinetics is validated with experimental data and used for optimization and scale-up of the reactor design.     

Functionalization of electrochemically prepared titania nanotubes with Pt for application as catalyst for fuel cells

Titania nanotubes (TiNTs) were prepared by electrochemical anodization and were used as a support for depositing Pt. After annealing the TiNTs changed to crystalline anatase phase and were doped with carbon. The TiNTs/Pt/C was tested as electrode for electrochemical catalysis of methanol oxidation. The composite catalyst activities were measured by cyclic voltammetry in 1 M CH₃OH + 1 M H₂SO₄. The results demonstrated that TiNTs/Pt/C can greatly enhance the catalytic activity of methanol oxidation. The CO stripping led to the increase in the current peak of methanol oxidation due to activating the catalyst surface by point defect formation. Moreover, the higher ratio of the forward anodic peak current to the reverse anodic peak current indicates more effective removal of the poisonous species.