Reforming of methane and coalbed methane over nanocomposite Ni/ZrO2 catalyst

Nanocomposite Ni/ZrO₂-AN catalyst consisting of comparably sized Ni metal and ZrO₂ nanoparticles is studied in comparison with zirconia- and alumina-supported Ni catalysts (Ni/ZrO₂-CP and commercial Ni/Al₂O₃-C) for steam reforming of methane (SRM) and for combined steam and CO₂ reforming of methane (CSCRM). The reactions are performed under atmospheric pressure with stoichiometric amounts of H₂O and CH₄ or (H₂O + CO₂) and CH₄ at 1073 K. Under a wide range of methane space velocity (gas hourly space velocity of methane GHSV_CH4 = 12,000–96,000 ml/(h g_cat.), the nanocomposite Ni/ZrO₂-AN catalyst always shows higher activity and stability for both SRM and CSCRM reactions. The two supported Ni catalysts (Ni/ZrO₂-CP and Ni/Al₂O₃-C) exhibit fairly stable catalysis under low GHSV_CH4 but they are easily deactivated under high GHSV_CH4 and become completely inactive when they are reacted for ca.100 h at GHSV_CH4 = 48,000 ml/(h g_cat.). The CSCRM reaction is carried out with different H₂O/CO₂ ratios in the reaction feed while keeping the molar ratio (H₂O + CO₂)/CH₄ = 1.0, the results prove that the nanocomposite Ni/ZrO₂-AN catalyst can be highly promising in enabling a catalytic technology for the production of syngas with flexible H₂/CO ratios (ca. H₂/CO = 1.0–3.0) to meet the requirements of various downstream chemical syntheses.     

Oxygen permeation study of perovskite hollow fiber membranes

The first oxygen permeation data of a dense hollow fiber perovskite membrane based on BaCo_xFe_yZr_zO_3 − δ are reported. The hollow fiber was prepared by a phase inversion process. Dense fibers were obtained with the following typical geometries: outer diameter, 800–900 μm; inner diameter, 500–600 μm; length, 30 cm. The O₂-permeation through the hollow fiber perovskite membrane was studied in a high-temperature gas permeation cell under different operation conditions. The increase of the helium gas flow rate reduces the oxygen partial pressure (p_O2) on the core side and a higher oxygen permeation flux is observed. High oxygen flux of 0.73 m³ (O₂)/(m² (membrane) h) was achieved at 850 °C under the operation parameters F_air (shell side) = 150 ml/min and F_He (core side) = 30 ml/min. The oxygen partial pressure dependence of the O₂ permeation flux indicated an interplay of both surface reaction and bulk diffusion as rate limiting steps. During 5 days of permeation a high and stable oxygen flux was observed. X-ray diffraction patterns of fresh and spent membranes after the permeation measurements revealed that no degradation after oxygen permeation appears.     

Performance of a catalytic membrane reactor for the Fischer–Tropsch synthesis

The study of permeable composite monolith (PCM) membranes for the Fischer–Tropsch synthesis is continued. On the scale of membranes with outer diameter of 42 mm, it is proved that PCM can combine high productivity of hydrocarbons (>55 kg_C5+ ( h)⁻¹ at 0.6 MPa, 484 K), high selectivity towards heavy hydrocarbons (_ASF > 0.85, C₅₊ upto 0.9) as well as high heat-conductivity and high mechanical strength.     

Novel zirconia-supported catalysts for low-temperature oxidative steam reforming of ethanol

Three zirconia-supported platinum group metal (Pt, Ru and Pt–Ru) catalysts were prepared by impregnation. The activity of these catalysts toward the oxidative steam reforming of ethanol (OSRE) was examined in a fixed-bed reactor in the temperature range of 260–380 °C. The catalysts were characterized by X-ray diffraction (XRD), temperature-programmed reduction (TPR), transmission electron microscopy (TEM) and nitrogen adsorption at −196 °C. Activity results indicated that the optimized experimental conditions involved a reforming temperature of close to 300 °C and the molar ratios of O₂/EtOH and H₂O/EtOH of 0.44 and 4.9, respectively. An ethanol conversion (C_EtOH) approaching 100% and a hydrogen yield (Y_H2) exceeding 3.0 mole/mole ethanol were noticed at 280 °C over all the catalysts. Among these catalysts, the Pt–Ru/ZrO₂ catalyst was an excellent OSRE catalyst at low temperature. The maximum Y_H2 was 4.4 and the CO distribution was 3.3 mol% at 340 °C.     

Cobalt Fischer-Tropsch synthesis: Deactivation by oxidation?

Cobalt catalysts as used in the Fischer-Tropsch synthesis (FTS) are relatively expensive (as compared to iron) and need to have a high metal dispersion and long life to be able to offer a good balance between cost and performance. The oxidation of nano-sized metallic cobalt to cobalt oxide during Fischer-Tropsch synthesis has long been postulated as a major deactivation mechanism. However, to date there is no consistent picture. This paper presents an extensive overview of the literature on this topic of deactivation by means of oxidation for unsupported as well as silica-, alumina- and titania-supported cobalt catalysts. Furthermore, it presents results on the deactivation of an industrial Co/Al₂O₃ catalyst as obtained by pseudo in situ X-ray diffraction, magnetic measurements and X-ray absorption near-edge spectroscopy. These analyses were performed to study the oxidation state of spent industrial Co/Al₂O₃ catalyst samples withdrawn from a slurry reactor operating under realistic FTS conditions, and it was concluded that oxidation can be ruled out as a major deactivation mechanism. Finally, these data together with all relevant literature were used to create a common view on the oxidation behaviour of metallic cobalt during FTS. The apparent discrepancies in literature on the oxidation behaviour of cobalt are most likely due to the lack of direct characterisation of the cobalt oxidation state and due to the comparison of catalysts with varying cobalt crystallites sizes, compared at different reactor partial pressures of hydrogen and water (P_H2O/P_H2). It was shown that the oxidation of cobalt can be prevented by selecting the correct combination of the reactor partial pressures of hydrogen and water (P_H2O/P_H2) and the cobalt crystallite size.     

The beneficial role of use of ultrasound in heterogeneous Fenton-like system over supported copper catalysts for degradation of p-chlorophenol

A heterogeneous sono-catalytic system with addition of hydrogen peroxide (US_H2O2+Cat.) was employed for the degradation of 200 ppm of p-chlorophenol (4-CP) at 25 °C and 100 W of ultrasound power. One thousand and six hundred parts per million of initial hydrogen peroxide (H₂O₂) concentration and 1 g/L of catalyst loading over three heterogeneous copper catalysts, CuO, Cu/Al₂O₃ (Cu/Al) and CuO·ZnO/Al₂O₃ (Cu/Zn) was used. The benefits of ultrasound in a heterogeneous catalytic system were evaluated. A considerable synergistic effect of the US_H2O2+Cat. system was only achieved with supported catalysts (Cu/Al and Cu/Zn) possibly due to good dispersion of catalysts as a result of catalyst size reduction during ultrasound irradiation. Moreover, between the two supported copper catalysts, the Cu/Al provided promising catalytic performance by giving higher 4-CP and TOC removal accompanied with efficient H₂O₂ consumption. Experiments with a homogeneous copper catalyst revealed that use of ultrasound in a homogeneous system shows an adverse effect on decomposition of 4-CP.     

Influence of syngas composition on the transient behavior of a Fischer–Tropsch continuous slurry reactor

The influence of syngas composition on the initial behaviour of a Co/Al₂O₃ catalyst in Fischer–Tropsch reaction has been studied in a continuous perfectly mixed slurry reactor for an inlet H₂/CO ratio between 1.6 and 3.35 keeping other conditions constant (T = 220 °C, P = 2 MPa). Significantly different behaviors of initial deactivation for CO conversion have been observed with different H₂/CO ratios. It was observed that the deactivation increases with increase in H₂/CO ratio and in carbon monoxide conversion. The computed liquid concentrations of CO, H₂ and H₂O have shown that water is the most abundant species in the liquid phase of the reactor during our experiments. The concentration of the water produced by the FT reaction seems to be the key parameter responsible of the initial behavior and then of the initial deactivation. For moderate levels of water ( corresponding to P_H2O/P_H2<0.4), a simple kinetic model assuming a reversible oxidation of cobalt active sites by water in competition with their reduction by hydrogen seems to represent satisfactorily the initial behaviour of the catalyst. For higher water concentrations, the irreversible deactivation should be probably taken into consideration.     

Low-temperature H2 and N2 transport through thin Pd66Cu34Hx layers

The single gas H₂ and N₂ permeability of a 4 μm thick dense fcc-Pd₆₆Cu₃₄ layer has been studied between room temperature and 510 °C and at pressure differences up to 400 kPa. Above 50 °C the H₂ flux exhibits an Arrhenius-type temperature dependence with J_H2=(5.2±0.3) mol m⁻² s⁻¹ exp[(−21.3 ± 0.2) kJ mol⁻¹/(R·T)]. The hydrogen transport rate is controlled by the bulk diffusion although the pressure dependence of the H₂ flux deviates slightly from Sieverts’ law. A sudden increase of the H₂ flux below 50 °C is attributed to embrittlement.     

High temperature proton exchange membrane fuel cell using a sulfonated membrane obtained via H2SO4 treatment of PEEK-WC

A method for the sulfonation of PEEK-WC, a glassy poly(ether ether ketone) with sulphuric acid is presented. Depending on the reaction time, polymers with ion exchange capacity (IEC) from 0.30 to 0.76 meq_H⁺/g are obtained, as determined by titration with NaOH solutions. The thermal properties of the polymers were studied by differential scanning calorimetry, showing that the glass transition temperature increases with increasing degree of sulfonation, from 224 °C for pure PEEK-WC to 246 °C for the polymer having an IEC of 0.76 meq_H⁺/g. The sulfonated polymers were used to prepare proton exchange membranes for possible application in fuel cells. Dense membranes were prepared by solvent evaporation, using DMA as the solvent. The transport properties of the membranes were determined in terms of water uptake and permeability for hydrogen and oxygen. Electrochemical characterization was performed by measuring cell voltage and power density curves as a function of current density at different working temperatures and the results were compared with those of a commercial Nafion membrane. A power density of 284 mW/cm² was obtained for S-PEEK-WC membrane at 120 °C in H₂/air fuel cell, slightly above the corresponding value found for Nafion.     

Study of the water gas shift reaction on Fe in a high temperature proton conducting cell

The water gas shift (WGS) reaction was studied in a double-chamber high temperature proton conducting cell (HTPC). The proton conductor was a strontia–ceria–ytterbia (SCY) disk of the form: SrCe_0.95Yb_0.05O₃₋ and the working electrode was a polycrystalline Fe film. The reaction temperature and the inlet partial pressure of CO varied between 823 and 973 K, and between 1.0 and 10.6 kPa, respectively. The inlet partial pressure of steam (P_H2O) was kept constant at 2.3 kPa. An increase in the production of H₂ was observed upon “pumping” protons away from the catalyst surface. The Faradaic efficiency (Λ) was lower than unity, indicating a sub-Faradaic effect. The highest value of rate enhancement ratio (ρ) was approximately 3.2, at T = 823 K. The proton transport number (PTN) varied between 0.45 and 1.0. An up to 99% of the produced H₂ was electrochemically separated from the reaction mixture.     

Study of the reverse water gas shift (RWGS) reaction over Pt in a solid oxide fuel cell (SOFC) operating under open and closed-circuit conditions

The (electro-)kinetics of the reverse water gas shift (RWGS) reaction was studied in a solid oxide fuel cell (SOFC) of the type Pt/YSZ/Pt. The effect of imposed potentials, cell temperature (650–800 °C), H₂ (1–10 kPa) and CO₂ (1–10 kPa) partial pressures on the kinetics and mechanism of the catalytic and electrocatalytic RWGS reaction, were systematically examined. The apparent catalytic activation energy was found equal to 15.6 kcal/mol, while H₂ and CO₂ apparent reaction orders were equal to 0.5 and 0.7, respectively. At both open and closed circuit operation, the associative formate decomposition reaction mechanism was considered to describe kinetics. Under closed circuit operation, rate enhancement factor, |Λ|, values up to 10 were achieved. Finally, current density–voltage and current density–power density characteristics of the cell were recorded at various temperatures and gas mixtures of CO₂ and H₂. It was found that electrical power output of the cell was optimized by increasing temperature and decreasing CO₂/H₂ feed ratio. Maximum power density obtained was 9 mW/cm² (at 520 mV cell voltage and a current density of 17.3 mA/cm², at 800 °C and P_CO2/P_H2=0.16).     

Synthesis of ceria nanoparticles by flame electrospray pyrolysis

A flame electrospray pyrolysis is presented for synthesizing CeO₂ nanoparticles with a dense morphology, high crystallinity and nanometer size. Hydrated cerium nitrate precursor dissolved in an ethanol/diethylene glycol butyl ether mixture was injected into a CH₄/air premixed flame using an electrospray method. The number size distributions of the as-prepared particles were trimodal. It is suggested that the particles for the fine mode were formed by a Rayleigh disintegration of the charged precursor droplets during the droplet evaporation. The particles for the coarse and middle modes are surmised to come from primary and secondary droplets, respectively, which were formed simultaneously during the atomization processes. The CeO₂ nanoparticles for the coarse mode were nonspherical and composed of few crystallites. The nanoparticles for the fine and middle modes were nearly spherical and nonagglomerated. The as-prepared CeO₂ nanoparticles showed highly crystallinity.     

Cl electrosorption on Ag(1 0 0): Lateral interactions and electrosorption valency from comparison of Monte Carlo simulations with chronocoulometry experiments

We present Monte Carlo simulations using an equilibrium lattice-gas model for the electrosorption of Cl on Ag(1 0 0) single-crystal surfaces. Fitting the simulated isotherms to chronocoulometry experiments, we extract parameters such as the electrosorption valency γ and the next-nearest-neighbor lateral interaction energy ϕ_nnn. Both coverage-dependent and coverage-independent γ were previously studied, assuming a constant ϕ_nnn [I. Abou Hamad, Th. Wandlowski, G. Brown, P.A. Rikvold, J. Electroanal. Chem. 554–555 (2003) 211]. Here, a self-consistent, entirely electrostatic picture of the lateral interactions with a coverage-dependent ϕ_nnn is developed, and a relationship between ϕ_nnn and γ is investigated for Cl on Ag(1 0 0).     

Spin design of iron complexes on Fe-ZSM-5 zeolites

The spin state of iron ions in Fe-ZSM-5 zeolites can be purposefully varied by adsorption of gaseous probe molecules. The resulting Fe complexes with half-integer spin (, and ) can be reliably identified by electron paramagnetic resonance (EPR). A good correlation has been found between the concentration of surface sites active in low-temperature nitrous oxide decomposition and the concentration of low-spin () nitrosyl complexes of Fe formed after adsorption of NO molecules. Based on the analysis of the formation of such complexes under varying conditions, we conclude that these active sites contain a binuclear iron complex with S = 0 and three adsorbed NO molecules. An approach to investigate various Fe-containing sites in oxidation catalysts is discussed.     

Methanol reformate treatment in a PEM fuel cell-reactor

The treatment of methanol reformate, containing up to 2500 ppm CO, by the anode of a PEM fuel cell, operating as a preferential oxidation (PROX) reactor, was investigated in order to examine the possibility of electrochemically promoting the water–gas-shift (WGS) reaction and thus making the gas mixture suitable for anodic oxidation. It was found that the electrochemical promotion effect plays a significant role in a normal fuel cell operation (air at the cathode) but not in a hydrogen pumping operation (H₂ at the cathode). This implies that the role of oxygen crossover in the electropromotion (EP) of the WGS reaction and in the CO oxidation is vital. During fuel cell operation, the increase in the rate of CO consumption over a Pt/C anode is 2.5 times larger than the electrochemical rate, I/2F of CO consumption, while for oxygen bleeding conditions (fuel mixture + 1% O₂ at the anode) the increase is up to five times larger than I/2F, i.e. the Faradaic efficiency is up to 5. This shows that the catalytic properties of the Pt anode are significantly modified by varying catalyst potential and by the extent of O₂ crossover.

The effect of temperature, gas composition, membrane thickness and Pt anode alloying with Cu was studied. It was found that the rate of CO consumption is significantly enhanced by increasing T, p_H2 and increasing O₂ crossover rate. Also the Faradaic efficiency reaches even higher values (up to 9) when using PtCu/C anodes. However, the Faradaic efficiency drops in general below 100% at high current densities and CO conversion levels.     

Fractal growth modelling of nanoparticles

The kinetics of iodine oxide aerosol production and growth was studied in an aerosol flow reactor by the photolysis of I₂ in an excess of O₃, at a temperature of 295 K and a total pressure of 1 atm. The time-resolved evolution of the particle size distribution was fitted using a model which assumes that the initial period of particle growth (in the free molecular flow regime) is dominated by collision-coalescence, maintaining spherical shape and compact structure. This leads to the formation of primary particles of about 3 nm radius, which trigger fractal (agglomerative) growth in the transition regime resulting in particle aggregates characterised by lower mass fractal dimensions (D_f) in the range 2.2–2.5. Enhancement of the particle pair collision kernels due to competing van der Waals and hydrodynamic forces is treated within the model. The densities of the fractal aggregates are lower than that of the bulk material, recently identified as I₂O₅ [Saunders, R. W., & Plane, J. M. C. (2005). Formation pathways and composition of iodine oxide ultrafine particles. Environmental Chemistry, 2, 299], as a result of internal void space within the aggregate structures.     

Water effect on the conductivity behavior of NH4PO3-based electrolytes for intermediate temperature fuel cells

The electrical conductivity at intermediate temperature of 150–250 °C and the activation energy for conductivity of composite proton conductors, 2NH₄PO₃–(NH₄)₂Mn(PO₃)₄ and 2NH₄PO₃–(NH₄)₂SiP₄O₁₃, were investigated as a function of water vapor pressure, P_H2O. The activation energy decreased linearly with the natural logarithm of P_H2O, indicating that water is chemically adsorbed to the electrolytes. The decrease in activation energy is possibly caused by formation of hydrogen bonds between the adsorbed water and the electrolytes. In addition, the pre-exponential factor of Arrhenius equation, σ₀, increased with P_H2O. This suggests that the adsorbed water may generate additional mobile protons for the composite electrolyte. Therefore, the enhancement in the electrical conductivity of a NH₄PO₃-based electrolyte in a water-vapor rich atmosphere originates possibly from the decrease in activation energy as well as the increase in mobile proton concentration.     

Modelling a catalytic membrane reactor with plug flow pattern and a hypothetical equilibrium gas-phase reaction with Δn ≠ 0

The scope of this paper is to present a theoretical study of a catalytic polymeric dense membrane reactor (CPDMR) assuming isothermal conditions, plug flow pattern without pressure drop in both retentate and permeate sides, shell side feed and cocurrent operation. The conversion enhancement over the thermodynamic equilibrium value is studied for a gas-phase reaction of the type aAbB, considering two different stoichiometric ratios: Δn > 0 and Δn < 0, where Δn = b − a. For each of these cases, the influence of the relative sorption and diffusivity of the reaction species is studied. It is concluded that the conversion of a reversible reaction can be significantly enhanced when the diffusivity of the reaction products is higher than that of the reactants and/or the sorption is lower. It is also concluded that, even for equal sorption and diffusion coefficients, the conversion could also be improved for reactions with Δn ≠ 0. The extension of this enhancement depends on the reaction stoichiometry, the overall concentration inside the membrane, the Thiele modulus, and the contact time.     

Enhanced degradation of organic wastewater containing p-nitrophenol by a novel wet electrocatalytic oxidation process: Parameter optimization and degradation mechanism

This paper explored a novel advanced oxidation process (AOP) for wastewater treatment—wet electrocatalytic oxidation (WEO), which introduced a small current into wet air oxidation (WAO) to promote the formation of hydroxyl radical and accelerate the oxidation of organic pollutants. The results showed that this novel process could couple the advantages of both WAO and electrochemical oxidation (EO). The effects of major operational factors of WEO were studied at relatively moderate condition (T = 100–160 °C, C = 1000 mg L⁻¹, P_O2 = 4 TOD, P_N2 = 0.50 MPa, current density = 0–7.08 mA cm⁻² and pH 2.7–10.6) aiming to p-nitrophenol (PNP) degradation, TOC and COD removal. The cost-effective operational parameters were found and the current efficiencies were highly beyond 100% with the maximum value of 386%. The degradation mechanism of WEO was carefully studied based on the formation of free radical. After the detail analysis of Hammett theory and quantitative structure-activity relationships (QSAR), the reaction pathway of hydroxyl radical with PNP was analyzed with the help of Gaussian 03W and the probable degradation pathway of PNP by WEO was deduced.     

The morphology of MoS2, WS2, Co–Mo–S, Ni–Mo–S and Ni–W–S nanoclusters in hydrodesulfurization catalysts revealed by HAADF-STEM

HAADF-STEM studies have provided detailed morphological insight regarding MoS₂, WS₂, Co–Mo–S, Ni–Mo–S and Ni–W–S nanostructures in graphite-supported catalysts. It is found that the technique allows the catalytically active edges to be imaged even for single layer metal sulfide structures. Unpromoted MoS₂ and WS₂ are predominantly present as slightly truncated triangular clusters containing only a single S–M–S layer (M = Mo, W). The addition of promoter atoms results in more heavy truncations consistent with the expected tendency for the Co–Mo–S structures to expose promoted S-type edges at the expense of unpromoted Mo-type edges. However, the HAADF-STEM (High-Angle Annular Dark-field Scanning Transmission Electron Microscopy) results show for the first time that Co–Mo–S and Ni–W–S may also expose extended high index truncations.