Steam reforming of propionic acid: Thermodynamic analysis of a model compound for hydrogen production from bio-oil

The thermodynamic equilibrium of steam reforming of propionic acid (HPAc) as a bio-oil model compound was studied over a wide range of reaction conditions (T = 500–900 °C, P = 1–10 bar and H₂O/HPAc = 0–4 mol/mol) using non-stoichiometric equilibrium models. The effect of operating conditions on equilibrium conversion, product composition and coke formation was studied. The equilibrium calculations indicate nearly complete conversion of propionic acid under these conditions. Additionally, carbon and methane formation are unfavorable at high temperatures and high steam to carbon (S/C) ratios. The hydrogen yield versus S/C ratio passes a maximum, the value and position of which depends on temperature. The thermodynamic equilibrium results for HPAc fit favorably with experimental data for real bio-oil steam reforming under same reaction conditions.     

Stability limits and NO emissions of premixed swirl ammonia-air flames enriched with hydrogen or methane at elevated pressures

This study reports measurements of stability limits and exhaust NO mole fractions of technically-premixed swirl ammonia-air flames enriched with either methane or hydrogen. Experiments were conducted at different pressures from atmospheric to 5 bar, representative of commercial micro gas turbines. The full range of ammonia fractions in the fuel blend, x_NH3, was considered, from 0 (pure methane or hydrogen) to 1 (pure ammonia), covering very lean (φ = 0.25) to rich (φ = 1.60) equivalence ratios. Results show that increasing pressure widens the range of stable equivalence ratios for pure ammonia-air flames. Regardless of pressure, there is a critical ammonia fraction above which the range of stable equivalence ratios suddenly widens. This is because flashback does not occur anymore when the equivalence ratio is progressively increased towards stoichiometric and rich blowout occurs instead. This critical ammonia fraction increases with pressure and is larger for ammonia-hydrogen than for ammonia-methane. Provided that enough hydrogen is blended with ammonia (x_NH3 < 0.9), flames with very lean equivalence ratios (φ < 0.7) can be stabilized and these yield competitively low NO emissions (<200 ppm), regardless of pressure. For this reason, very lean swirl ammonia-hydrogen-air flames are promising candidates for micro gas turbines. However, N₂O emissions have the potential to be unacceptably large for these operating conditions if heat loss is too large or residence time is too short. As a consequence, the post flame region must be considered carefully. Due to the lower reactivity of methane compared to that of hydrogen, very lean swirl ammonia-methane-air flames could not be stabilized and good NO performance is limited to rich equivalence ratios for ammonia-methane fuel blends. The equivalence ratio above which good NO performance depends on pressure and bulk velocity.     

Enhanced hydrogen-rich gas production from steam gasification of coal in a pressurized fluidized bed with CaO as a CO2 sorbent

Steam gasification of a typical Chinese bituminous coal for hydrogen production in a lab-scale pressurized bubbling fluidized bed with CaO as CO₂ sorbent was performed over a pressure range of ambient pressure to 4 bar. The compositions of the product gases were analyzed and correlated to the gasification operating variables that affecting H₂ production, such as pressure (P), mole ratio of steam to carbon ([H₂O]/[C]), mole ratio of CaO to carbon ([CaO]/[C]) and temperature (T). The experimental results indicated that the H₂ concentration was enhanced by raising the temperature, pressure and [H₂O]/[C] under the circumstances we observed. With the presence of CaO sorbent, CO₂ in the production gas was absorbed and converted to solid CaCO₃, thus shifting the steam reforming of hydrocarbons and water gas shift reaction beyond the equilibrium restrictions and enhancing the H₂ concentration. H₂ concentration was up to 78 vol% (dry basis) under a condition of 750 °C, 4 bar, [Ca]/[C] = 1 and [H₂O]/[C] = 2, while CO₂ (2.7 vol%) was almost in-situ captured by the CaO sorbent. This study demonstrated that CaO could be used as a substantially excellent CO₂ sorbent for the pressurized steam gasification of bituminous coal. For the gasification process with the presence of CaO, H₂-rich syngas was yielded at far lower temperatures and pressures in comparison to the commercialized coal gasification technologies. SEM/EDX and gas sorption analyses of solid residues sampled after the gasification showed that the pore structure of the sorbent was recovered after the steam gasification process, which was attributed to the formation of Ca(OH)₂. Additionally, a coal-CaO–H₂O system was simulated with using Aspen Plus software. Calculation results showed that higher temperatures and pressures favor the H₂ production within a certain range.     

Experimental and kinetic modeling study of methyl butanoate and methyl butanoate/methanol flames at different equivalence ratios and C/O ratios

Premixed laminar methyl butanoate/oxygen/argon and methyl butanoate/methanol/oxygen/argon flames were studied with tunable synchrotron vacuum ultraviolet (VUV) photoionization and molecular-beam sampling mass spectrometry at 30 torr (4.0 kPa). Three flames were investigated in the experiment: MB (methyl butanoate) flame F1.54 (? = 1.54, C/O = 0.479), MB flame F1.67 (? = 1.67, C/O = 0.511) and MB/methanol flame F1.67M (? = 1.67, C/O = 0.479). By measuring the signal intensities at different distances from the burner surface, the mole fraction profiles of intermediates are derived. Experimental results show that the flame front shifts downstream and peak mole fractions of intermediates increase remarkably with the increase of equivalence ratio for pure MB fuel. When methanol is added, the peak mole fractions of most intermediates including those of soot precursors decrease remarkably at the same equivalence ratio, while peaks of soot precursors vary little (only slightly decreasing) at same C/O ratio. It is concluded that the formation of soot precursors is more sensitive to C/O ratio than to equivalence ratio. Besides, more CO₂ is produced near the burner surface in MB flame than that in MB/methanol flame, and this validates an early production of CO₂ in methyl ester oxidation. In addition, a modified MB detailed mechanism is used to model flame structure, and improved agreements between the experimental and predicted results are realized. Based on the simulation results, reaction flux and sensitivity are analyzed for CO₂ and C₃H₃, respectively.     

Hydrogen production from methane under the interaction of catalytic partial oxidation,water gas shift reaction and heat recovery

Hydrogen production from the combination of catalytic partial oxidation of methane (CPOM) and water gas shift reaction (WGSR), viz. the two-stage reaction, in a Swiss-roll reactor is investigated numerically. Particular emphasis is placed on the interaction among the reaction of CPOM, the cooling effect due to steam injection and the excess enthalpy recovery with heat recirculation. A rhodium (Rh) catalyst bed sitting at the center of the reactor is used to trigger CPOM, and two different WGSRs, with the aids of a high-temperature (Fe–Cr-based) shift catalyst and a low-temperature (Cu–Zn-based) shift catalyst, are excited. Two important parameters, including the oxygen/methane (O/C) ratio and the steam/methane (S/C) ratio, affecting the efficiencies of methane conversion and hydrogen production are taken into account. The predictions indicate that the O/C ratio of 1.2 provides the best production of H₂ from the two-stage reaction. For a fixed O/C ratio, the H₂ yield is relatively low at a lower S/C ratio, stemming from the lower performance of WGSR, even though the cooling effect of steam is lower. On the contrary, the cooling effect becomes pronounced as the S/C ratio is high to a certain extent and the lessened CPOM leads to a lower H₂ yield. As a result, with the condition of gas hourly space velocity (GHSV) of 10,000 h⁻¹, the optimal operation for hydrogen production in the Swiss-roll reactor is suggested at O/C = 1.2 and S/C = 4–6.     

Gas phase chemistry in catalytic combustion of methane/air mixtures over platinum at pressures of 1 to 16 bar

The gas-phase combustion of fuel-lean methane/air premixtures over platinum was investigated experimentally and numerically in a laminar channel-flow catalytic reactor at pressures 1 bar?p?16 bar. In situ, spatially resolved one-dimensional Raman and planar laser induced fluorescence (LIF) measurements over the catalyst boundary layer were used to assess the concentrations of major species and of the OH radical, respectively. Comparisons between measured and predicted homogeneous (gaseous) ignition distances have led to the assessment of the validity of various elementary gas-phase reaction mechanisms. At low temperatures (900 K?T?1400 K) and fuel-to-air equivalence ratios (0.05?φ?0.50) typical to catalytic combustion systems, there were substantial differences in the performance of the gaseous reaction mechanisms originating from the relative contribution of the low- and the high-temperature oxidation routes of methane. Sensitivity analysis has identified the significance of the chain-branching reaction CHO + M = CO + H + M on homogeneous ignition, particularly at lower pressures. It was additionally shown that C2 chemistry could not be neglected even at the very fuel-lean conditions pertinent to catalytic combustion systems. A gas-phase reaction mechanism validated at 6 bar?p?16 bar has been extended to 1 bar?p?16 bar, thus encompassing all catalytic combustion applications. A reduced gas-phase mechanism was further derived, which when used in conjunction with a reduced heterogeneous (catalytic) scheme reproduced the key catalytic and gaseous combustion characteristics of the full hetero/homogeneous reaction schemes.     

Thermodynamic equilibrium calculations of hydrogen production from the combined processes of dimethyl ether steam reforming and partial oxidation

Thermodynamic analyses of producing a hydrogen-rich fuel-cell feed from the combined processes of dimethyl ether (DME) partial oxidation and steam reforming were investigated as a function of oxygen-to-carbon ratio (0.00–2.80), steam-to-carbon ratio (0.00–4.00), temperature (100 °C–600 °C), pressure (1–5 atm) and product species.Thermodynamically, dimethyl ether processed with air and steam generates hydrogen-rich fuel-cell feeds; however, the hydrogen concentration is less than that for pure DME steam reforming. Results of the thermodynamic processing of dimethyl ether indicate the complete conversion of dimethyl ether to hydrogen, carbon monoxide and carbon dioxide for temperatures greater than 200 °C, oxygen-to-carbon ratios greater than 0.00 and steam-to-carbon ratios greater than 1.25 at atmospheric pressure (P = 1 atm). Increasing the operating pressure has negligible effects on the hydrogen content. Thermodynamically, dimethyl ether can produce concentrations of hydrogen and carbon monoxide of 52% and 2.2%, respectively, at a temperature of 300 °C, and oxygen-to-carbon ratio of 0.40, a pressure of 1 atm and a steam-to-carbon ratio of 1.50. The order of thermodynamically stable products (excluding H₂, CO, CO₂, DME, NH₃ and H₂O) in decreasing mole fraction is methane, ethane, isopropyl alcohol, acetone, n-propanol, ethylene, ethanol and methyl-ethyl ether; trace amounts of formaldehyde, formic acid and methanol are observed.Ammonia and hydrogen cyanide are also thermodynamically favored products. Ammonia is favored at low temperatures in the range of oxygen-to-carbon ratios of 0.40–2.50 regardless of the steam-to-carbon ratio employed. The maximum ammonia content (i.e., 40%) occurs at an oxygen-to-carbon ratio of 0.40, a steam-to-carbon ratio of 1.00 and a temperature of 100 °C. Hydrogen cyanide is favored at high temperatures and low oxygen-to-carbon ratios with a maximum of 3.18% occurring at an oxygen-to-carbon ratio of 0.40 and a steam-to-carbon ratio of 0.00 in the temperature range of 400 °C–500 °C. Increasing the system pressure shifts the equilibrium toward ammonia and hydrogen cyanide.     

Hydrogen production for fuel cells by autothermal reforming of methane over sulfide nickel catalyst on a gamma alumina support

Experimental and modelling studies have been conducted on catalytic autothermal reforming (ATR) of methane for hydrogen production over a sulfide nickel catalyst on a gamma alumina support. The experiments are performed with different feedstock under thermally neutral conditions. The results show that the performance of the reformer is dependent on the molar air-to-fuel ratio (A/F), the molar water-to-fuel ratio (W/F) and the flowrate of the feedstock mixture. The optimum conditions for high methane conversion and high hydrogen yield are A/F = 3–3.5, W/F = 2–2.5 and a fuel flowrate below 120–250 l h⁻¹. Under these conditions, a methane conversion of 95–99% and a hydrogen yield of 39–41% on a dry basis can be achieved and 1 mole of methane can produce 1.8 moles of hydrogen at an equilibrium reactor temperature of not exceeding 850 °C.A two-dimensional reactor model is developed to simulate the conversion behaviour of the reactor for further study of the reforming process. The model includes all aspects of the major chemical kinetics and the heat and mass transfer phenomena in the reactor. The predicted results are successfully validated with experimental data.     

Simulation of steam reforming of biogas in an industrial reformer for hydrogen production

The paper aims to investigate the steam reforming of biogas in an industrial-scale reformer for hydrogen production. A non-isothermal one dimensional reactor model has been constituted by using mass, momentum and energy balances. The model equations have been solved using MATLAB software. The developed model has been validated with the available modeling studies on industrial steam reforming of methane as well as with the those on lab-scale steam reforming of biogas. It demonstrates excellent agreement with them. Effect of change in biogas compositions on the performance of industrial steam reformer has been investigated in terms of methane conversion, yields of hydrogen and carbon monoxide, product gas compositions, reactor temperature and total pressure. For this, compositions of biogas (CH₄/CO₂ = 40/60 to 80/20), S/C ratio, reformer feed temperature and heat flux have been varied. Preferable feed conditions to the reformer are total molar feed rate of 21 kmol/h, steam to methane ratio of 4.0, temperature of 973 K and pressure of 25 bar. Under these conditions, industrial reformer fed with biogas, provides methane conversion (93.08–85.65%) and hydrogen yield (1.02–2.28), that are close to thermodynamic equilibrium condition.     

Ignition delay study of moist hydrogen/oxidizer mixtures using a rapid compression machine

Autoignition of moist hydrogen/oxidizer mixtures has been studied experimentally using a rapid compression machine (RCM). This work investigated the effect of water addition on ignition delays of stoichiometric hydrogen/oxidizer mixtures in the end of compression temperature range of T_C = 907–1048 K at three different end of compression pressures viz. P_C = 10 bar (1 MPa), 30 bar (3 MPa), and 70 bar (7 MPa). RCM experiments were conducted with 0%, 10%, and 40% molar percentages of water in the reactive mixture. At P_C = 30 bar and 70 bar, the presence of 10% and 40% water vapor was shown to promote autoignition. However, at P_C = 10 bar, water addition (10%) was seen to retard the reactivity, thereby increasing the ignition delay. Comparison with different reaction kinetic mechanisms reported in literature shows widely different results of simulated ignition delays for the temperature and pressure range studied, although most of the mechanism predictions demonstrate similar trend in ignition delay with water addition. A recent chemical kinetic mechanism, which shows good agreement with the present experiments at higher pressure but some discrepancy at lower pressure, was used for brute force sensitivity analysis in order to identify the important reactions for the dry mixtures in the temperature and pressure window investigated. An important reaction identified was further adjusted within the uncertainty limit as an attempt to improve the results from mechanism prediction for the ignition delay at low pressure (P_C = 10 bar) without water addition. In addition, the modification in the reaction rate leads to good agreement between the experiment data and the mechanism prediction for the moist mixtures at varying compressed pressures.     

Electrochemical performances of the as-melt La0.75−xMxMg0.25Ni3.2Co0.2Al0.1 (M = Pr,Zr; x = 0, 0.2) alloys applied to Ni/metal hydride (MH) battery

The partial replacement of La by M (M = Pr, Zr) has been performed in order to ameliorate the electrochemical hydrogen storage performances of La–Mg–Ni-based A₂B₇-type electrode alloys. For this purpose, we adopt melt spinning technology to prepare the La_0.75−xM_xMg_0.25Ni_3.2Co_0.2Al_0.1 (M = Pr, Zr; x = 0, 0.2) electrode alloys. Then systemically investigate the effects that the preparation methods and M (M = Pr, Zr) substitution have on the structures and electrochemical hydrogen storage characteristics of the alloys. The analysis of XRD and TEM reveals that the as-cast and spun alloys hold a multiphase structure, containing two main phases (La, Mg)₂Ni₇ and LaNi₅ as well as a trace of residual phase LaNi₂. Besides, the as-spun (M = Pr) alloy displays an entire crystalline structure, while an amorphous-like structure is detected in the as-spun (M = Zr) alloy, implying the replacement of La by Zr facilitates forming amorphous phase. Based upon electrochemical measurements, an impact engendered by melt spinning on the electrochemical performances of the alloys appears to be evident. The cycle stabilities monotonously augment with the growing of the spinning rate. The discharge capacity and high rate discharge ability (HRD), however, exhibit difference. For the (M = Pr) alloy, they first mount up and then fall with the rising of the spinning rate, whereas for the (M = Zr) alloy, they always decline as the spinning rate elevates. Furthermore, the replacement of La by M (M = Pr, Zr) considerably enhances the cycle stability of the alloys and the replacement of La by Pr clearly increases the discharge capacity, but the Zr replacement results in an adverse impact.     

A multi-membrane reformer for the direct production of hydrogen via a steam-reforming reaction of methane

We designed and prepared a multi-membrane reformer (MMR) for the direct production of hydrogen via a steam-reforming (SR) reaction of methane. The MMR consisted of two single modules containing coin-shaped nickel metal catalysts and Pd-based membrane. The SR reaction was performed in the MMR for relatively high-pressure operation ranges (P₂ = ∼21 bar) without sweep gas and the methane conversion and hydrogen production rate were observed under various experimental conditions. It was found that the high-performance of the Pd-based membrane and the porous metal catalyst and their configuration in the MMR guaranteed a high rate of hydrogen production. For instance, the methane conversion, the rate of hydrogen separation and the hydrogen purity were 75%, 30.6 L/h and 99.95%, respectively, under the experimental conditions of 540 °C, S/C = 3.0 and del-P = 20 bar. The design and performance of MMR show potential advantages, such as the simple preparation of a compact membrane reformer able to operate in relatively high-pressure ranges and easy enlargement of the hydrogen production capacity by stacking the modules, which is possible due to the disk-type shape of the metal catalyst and the membrane.     

Hydrogen-rich gas generation via the exhaust gas-fuel reformer for the marine LNG engine

Reformed exhaust gas recirculation technology has attracted great attention in internal combustion engines. A platform of an exhaust gas-fuel reformer connected with the marine LNG engine was set up for generating on-board hydrogen. Based on the platform, effects of the methane to oxygen ratio (M/O) and reformed exhaust gas ratio (R_EG) from the reformer and excess air ratio (λ) from the engine on the components, hydrogen yield, thermal efficiency and reforming process of the reformer were experimentally investigated. Results shown that hydrogen-rich gases (reformate) can be generated by reforming the mixture of engine exhaust gas (about 400 °C) and methane supplied via the reformer with Ni/Al₂O₃ catalyst, and the hydrogen concentration of reformate was between 6.2% and 12.6% by volume. The methane supplied rate and λ affected the components and temperature of the reactant in the reformer, while R_EG changed the gas hour space velocity during the exhaust gas-fuel reforming processes, resulting in the difference in the components of the reformate and thermal efficiency. At the present experimental condition, the highest H₂ concentration reformate was generated under the M/O of 2.0, λ of 1.55 and R_EG of 6%.     

Experimental and numerical study of the laminar burning velocity of NH3/H2/air premixed flames at elevated pressure and temperature

Ammonia (NH₃) is a carbon-free fuel that shows great research prospects due to its ideal production and storage systems. The experimental data of the laminar burning velocity of NH₃/H₂/air flame at different hydrogen ratios (X_H2 = 0.1–0.5), equivalent ratios (φ = 0.8–1.3), initial pressures (P = 0.1–0.7 MPa), and initial temperatures (T = 298–493 K) were measured. The laminar burning velocity of the NH₃/H₂/air flame increased upon increasing the hydrogen ratios and temperature, but it decreased upon increasing the pressure. The equivalent ratio of the maximum laminar burning velocity was only affected by the proportion of reactants. The equivalence ratio value of the maximum laminar burning velocity was between 1.1 and 1.2 when X_H2 = 0.3. The chemical reaction kinetics of NH₃/H₂/air flame under four different initial conditions was analyzed. The less NO maximum mole fraction was produced during rich combustion (φ > 1). The results provide a new reference for ammonia as an alternative fuel for internal combustion engines.     

High pressure palladium membrane reactor for the high temperature water-gas shift reaction

The water-gas shift (WGS) catalytic membrane reactor (CMR) incorporating a composite Pd-membrane and operating at elevated temperatures and pressures can greatly contribute to the efficiency enhancement of several methods of H₂ production and green power generation. To this end, mixed gas permeation experiments and WGS CMR experiments have been conducted with a porous Inconel supported, electroless plated Pd-membrane to better understand the functioning and capabilities of those processes. Binary mixtures of H₂/He, H₂/CO₂, and a ternary mixture of H₂, CO₂ and CO were separated by the composite membrane at 350, 400, and 450 °C, 14.4 bar (P_tube = 1 bar), and space velocities up to 45,000 h⁻¹. H₂ permeation inhibition caused by reversible surface binding was observed due to the presence of both CO and CO₂ in the mixtures and membrane inhibition coefficients were estimated. Furthermore, WGS CMR experiments were conducted with a CO and steam feed at 14.4 bar (P_tube = 1 bar), H₂O/CO ratios of 1.1-2.6, and GHSVs of up to 2900 h⁻¹, considering the effect of the H₂O/CO ratio as well as temperature on the reactor performance. Experiments were also conducted with a simulated syngas feed at 14.0 bar (P_tube  = 1 bar), and 400-450 °C, assessing the effect of the space velocity on the reactor performance. A maximum CO conversion of 98.2% was achieved with a H₂ recovery of 81.2% at 450 °C. An optimal operating temperature for high CO conversion was identified at approximately 450 °C, and high CO conversion and H₂ recovery were achieved at 450 °C with high throughput, made possible by the 14.4 bar reaction pressure.     

Methane membrane steam reforming: Heat duty assessment

Membrane reactors are an innovative technology with huge application potentialities for equilibrium limited endothermic reactions. Assembling a membrane selective to a reaction product avoids the equilibrium conditions to be achieved, supporting the reactions at lower operating temperatures. Taking as an example the natural gas steam reforming, a methane conversion around 98% can be reached imposing an operating temperature of 823 K, much lower than that of the traditional process. In the present paper, a stringent analysis of heat power requirement needed to carry out the natural gas steam reforming process by applying a membrane reactor is made. The simulations allows to understand how the main operating parameters (inlet temperature, inlet methane flow-rate, steam to carbon ratio, ratio between sweeping steam and inlet methane, operating reaction pressure) influence the total heat power required by the process, divided among power contributions for the reaction heat duty, reactant steam and permeation steam generation and preheating. Moreover, the specific thermal energy per mole of pure H₂ is computed and assessed. Optimizing the operating conditions set, a specific thermal energy per mole of pure hydrogen of 92.3 kWh kmol⁻¹ is obtained corresponding to a total thermal power of 687.4 kW required to convert, in a single membrane reactor, a methane flow-rate of 2 kmol h⁻¹ (GHSV = 9.590 h⁻¹) with a conversion around 98%.     

Gibbs free energy minimization as a way to optimize the combined steam and carbon dioxide reforming of methane

Gibbs free energy minimization was applied to study thermodynamic equilibrium of the combined steam and carbon dioxide reforming of methane. Coke deposition, the content of methane and carbon dioxide in syn-gas as well as H₂/CO ratio were investigated as a function of CO₂/CH₄ and H₂O/CH₄ mole ratios at different temperatures and pressures. The ranges of the molar ratios CH₄/CO₂/H₂O in the feed with H₂/CO = 2.1-2.2 were identified at which reforming of methane is not complicated by coke deposition. For each range optimized CH₄/CO₂/H₂O molar ratios characterized by the lowest content of methane and carbon dioxide in syn-gas were found.     

Hydrogen production via CaO sorption enhanced anaerobic gasification of sawdust in a bubbling fluidized bed

This paper presents the experimental results of CaO sorption enhanced anaerobic gasification of biomass in a self-design bubbling fluidized bed reactor, aiming to investigate the influences of operation variables such as CaO to carbon mole ratio (CaO/C), H₂O to carbon mole ratio (H₂O/C) and reaction temperature (T) on hydrogen (H₂) production. Results showed that, over the ranges examined in this study (CaO/C: 0-2; H₂O/C: 1.2-2.18, T: 489-740 °C), the increase of CaO/C, H₂O/C and T were all favorable for promoting the H₂ production. The investigated operation variables presented different influences on the H₂ production under fluidized bed conditions from those obtained in thermodynamic equilibrium analysis or fixed bed experiments. The comparison with previous studies on fluidized bed biomass gasification reveals that this method has the advantage of being capable to produce a syngas with high H₂ concentration and low CO₂ concentration.     

Fast in-situ photodeposition of Ag and Cu nanoparticles onto AgTaO3 perovskite for an enhanced photocatalytic hydrogen generation

The M (M = Ag, Cu) nanoparticles were deposited by a fast in-situ photoreduction method onto the AgTaO₃ photocatalyst surface using 1, 2, and 5 wt % ratios, in order to investigate their photocatalytic properties for hydrogen production. The obtained results indicated a nanoparticles growth <20 nm in diameter during 10 min of photoreduction process for both nanoparticles. The M (M = Ag, Cu) NPs/AgTaO₃ exhibited superior photocatalytic activity than bare AgTaO₃, with an efficiency increment of around 11 and 30 times at 2 wt % ratio of Ag and Cu nanoparticles, respectively. The excellent photocatalytic activity could be related to the surface plasmon resonance effect of nanoparticles, preventing the electron-hole recombination. Additionally, the optical and photoelectrochemical characterization revealed the presence and the effect of oxidized species of the nanoparticles, with a direct impact on the transport of photogenerated charge carriers for the improvement of the photocatalytic activity.     

Decomposition and coupling of methane over Pd–Au/Al2O3 catalysts to form COx-free hydrogen and C2 hydrocarbons

Catalytic decomposition of methane (CDM; CH₄ → C + 2H₂) is expected to be used for clean hydrogen production because CDM does not emit carbon dioxide. Recently, it was reported that Pd–based catalysts promotes CDM, simultaneously facilitating coupling of CH₄ to form C₂ hydrocarbons. In this study, varieties of supported Pd–M alloy catalysts (M = Fe, Co, Ni, Cu, Zn, Ga, In, Sn, Au, Pb, and Bi) were synthesized and their activities for the CDM and CH₄ coupling were examined. The catalytic activity for CH₄ strongly depended on the types of Pd–M. Pd–M/Al₂O₃ (M = Ni, Fe, Co, Au) showed high activity for CDM. In addition to the production of hydrogen by the CDM, Pd–Au/Al₂O₃ formed C₂ hydrocarbons such as ethane and ethylene via the coupling of CH₄. Effects of Pd/Au ratio and reaction temperatures were examined and the role of Au for the CH₄ conversion reaction was discussed.