Performance of XSCoF (X = Ba, La and Sm) and LSCrX′ (X′ = Mn, Fe and Al) perovskite-structure materials on LSGM electrolyte for IT-SOFC

X_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (X = Ba, La and Sm) and La0.75Sr0.25Cr0.5X^′0.5O3−δ (X′ = Mn, Fe and Al) mixed ionic-electronic conducting perovskite-based oxides have been tested as SOFC electrode materials on La_0.9Sr_0.1Ga_0.8Mg_0.2O_2.85 (LSGM) electrolytes under different atmospheres (air, oxygen, argon and dry and wet 5% H₂/Ar) and the area-specific resistances (ASR) were compared. Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCoF) possesses the lowest ASR values in air (0.04 Ω cm² at 1073 K) whilst La_0.75Sr_0.25Cr_0.5Mn_0.5O_3−δ (LSCrM) possesses the lowest ASR values in wet 5% H₂/Ar (0.28 Ω cm² at 1073 K). In addition, fuel cell tests were carried out using wet 5% H₂/Ar as fuel and air as oxidant. The maximum power density (∼123 mW cm⁻²) at 1073 K was reached with the electrolyte-supported system BSCoF/LSGM/LSCrM (∼1.5 mm electrolyte thickness). Furthermore, LSCrX′ materials were used simultaneously as cathode and anode in fuel cell tests and the symmetric system LSCrM/LSGM/LSCrM (∼1.5 mm electrolyte thickness) reached a maximum power density of ∼54 mW cm⁻² at 1073 K.     

Heteroepitaxial growth and characterization of 3C-SiC films on on-axis Si (1 1 0) substrates by LPCVD

Cubic SiC (3C-SiC) film has been deposited on Si (1 1 0) substrate by the low pressure chemical vapor deposition (LPCVD) with gas sources of SiH₄, C₃H₈ and carrier gas of H₂. The 3C-SiC crystalline film can be confirmed through the observations using reflection high-energy electron diffraction (RHEED) images. The X-ray diffraction (XRD) pattern and the rocking curve indicate that the (1 1 1) plane of SiC film is parallel to the surface of the Si (1 1 0) substrate and the film is of high crystallinity. The results of the field emission scanning electron microscope (FESEM) images show that the film has smooth surface morphology. Transmitted electron diffraction (TED) pattern and high resolution transmission electron microscope (HRTEM) image further confirm the high quality of the film.     

Gas evolution kinetics of two coal samples during rapid pyrolysis

Quantitative gas evolution kinetics of coal primary pyrolysis at high heating rates is critical for developing predictive coal pyrolysis models. This study aims to investigate the gaseous species evolution kinetics of a low rank coal and a subbituminous coal during pyrolysis at a heating rate of 1000 °C s^− 1 and pressures up to 50 bar using a wire mesh reactor. The main gaseous species, including H₂, CO, CO₂, and light hydrocarbons CH₄, C₂H₂, C₂H₄, C₂H₆, C₃H₆, C₃H₈, were quantified using high sensitivity gas chromatography. It was found that the yields of gaseous species increased with increasing pyrolysis temperature up to 1100 °C. The low rank coal generated more CO and CO₂ than the subbituminous coal under similar pyrolysis conditions. Pyrolysis of the low rank coal at 50 bar produced more gas than at atmospheric pressure, especially CO₂, indicating that the tar precursor had undergone thermal cracking during pyrolysis at the elevated pressure.     

Upgrading biomass pyrolysis gas by conversion of methane at high temperature: Experiments and modelling

Biomass gasification at temperatures below 1273 K produces gas which contains methane and too much tar for Fischer-Tropsch synthesis. The aim of this study is to investigate methane conversion at high temperature. Experimental tests were performed between 1273 and 1773 K, with a mixture of gas representative of wood pyrolysis at 1100 K (main components only: CO, CO₂, CH₄, H₂, H₂O). Two different kinetic schemes were used to predict the gas composition, and PAH molecules formation. For a residence time of 2 s in the reactor, the gas must be heated to at least 1650 K to reach a methane conversion rate of 90%. A parametric study was performed at 1453 K, by varying the initial methane, steam and hydrogen contents, so as to find out which components are the most influent on methane conversion and soot production.     

A TDS study of D adsorption on terraces and terrace edges of graphite (0 0 0 1) surfaces

Adsorption of thermal (2000 K) D atoms on (0 0 0 1) surfaces of various highly oriented pyrolytic graphite (HOPG) and natural graphite substrates was studied under ultra high vacuum (UHV) conditions with thermal desorption spectroscopy (TDS). D chemisorption on terrace and terrace edge sites of graphite (0 0 0 1) surfaces was identified. Recombinative desorption of D adsorbed on terraces was observed between 400 and 600 K. The analysis of TD spectra from various graphite surfaces reveals the existence of three desorption states intrinsic to graphite (0 0 0 1), and proposed as being due to adsorbate structures composed of one (monomer) and two neighbouring (dimer) chemisorbed D atoms, and aggregates thereof (mixed). The dimer structure is supposed to exhibit higher stability than the monomer. Reaction of D with terrace edges leads to the formation of CD, CD₂ and CD₃-groups at edge C atoms. These groups decompose during heating between 790 and 1300 K via release of gaseous D₂ and CD_x, C₂D_x and C₃D_x-hydrocarbons.     

Pyrolysis products of uncoated printing and writing paper of MSW

Uncoated printing and writing paper, one of the principal waste papers in Taiwan, was pyrolyzed with a thermogravimetric analysis (TGA) reaction system. The pyrolysis experiments were carried out in nitrogen environment at a constant heating rate of 5 K min⁻¹. The gaseous products and the residues were collected at room temperature (300 K) and analyzed by gas chromatography (GC) and elemental analyzer, respectively. The major gaseous products investigated included non-hydrocarbons (H₂, CO, CO₂, and H₂O) and hydrocarbons (C_1-3, C₄, C₅, C₆, 1-ring, C_10-12, levoglucosan, C_13-15, and C_16-18). The cumulated masses and the instantaneous concentrations of gaseous products were obtained under the experimental conditions. The yields of non-hydrocarbon gases and of hydrocarbon gases were about 10.46 and 0.49% at 623 K, 33.68 and 0.89% at 700 K, 64.52 and 1.05% at 788 K, and 79.10 and 1.63% at 938 K, respectively. The estimation of the mass of tar, yielded at various pyrolysis temperatures was also made. The results of this study might be useful for the design of pyrolysis process as well as for determining the pyrolysis mechanisms of the uncoated printing and writing paper.     

Electrochemical reduction of nitrate on Pt(S)[n(1 1 1) × (1 1 1)] electrodes in perchloric acid solution

The electrochemical reduction of nitrate ion was studied by cyclic voltammetry on Pt(1 1 1) and [n(1 1 1) × (1 1 1)] stepped Pt surfaces, where n (=14, 10, 7, 6, 5, 4, 3, 2) is the number of terrace atoms, in 0.1 M HClO₄ + 10 mM KNO₃. The electrocatalytic nitrate reduction was found to hardly proceed on Pt(1 1 1) in the hydrogen adsorption region, while the electrocatalytic activity was improved with the increase in the step density. Inactivation was observed in the presence of adsorbed hydrogen or nitrate-derived reduced adsorbate, i.e. adsorbed NO, on (1 1 1) step sites. It was, therefore, concluded that the electrocatalytically active NO₃⁻ species does not adsorb on the (1 1 1) terraces but on the (1 1 1) monoatomic steps. The nitrate reduction current increased with the step density in a non-linear relationship. The overall current density at 0.21 V (RHE) corresponding to the peak potential of the main electrocatalytic nitrate reduction wave which was maximum at n = 2, abruptly increased with short terraces, i.e. n < 5, where the current wave of adsorbed hydrogen on the Pt stepped surface with comparatively narrow (1 1 1) terraces, denoted as H_nt, also appeared unmodified for n < 5 on voltammograms recorded in 0.1 M HClO₄ in the absence of nitrate.     

Hydrogen production via gasification of meat and bone meal in two-stage fixed bed reactor system

Gasification of meat and bone meal followed by thermal cracking of tar was carried out at atmospheric pressure using a two-stage fixed bed reaction system in series. The first stage was used for the gasification and the second stage was used for thermal cracking of tar. In this work, the effects of temperature (650-850 °C) of both stages, equivalence ratio (actual O₂ supply/stoichiometric O₂ required for complete combustion) (0.15-0.3) and the second stage packed bed height (40-100 mm) on the product (char, tar and gas) yield and gas (H₂, CO, CO₂, CH₄, C₂H₄, C₂H₆, C₃H₆, C₃H₈) composition were studied. It was observed that the two-stage process increased hydrogen production from 7.3 to 22.3 vol.% (N₂ free basis) and gas yield from 30.8 to 54.6 wt.% compared to single stage. Temperature and equivalence ratio had significant effects on the hydrogen production and product distribution. It was observed that higher gasification (850 °C) and cracking (850 °C) reaction temperatures were favorable for higher gas yield of 52.2 wt.% at packed bed height of 60 mm and equivalence ratio of 0.2. The residence time of tar and product gases was varied by varying the packed bed height of second stage. The tar yield decreased from 18.6 wt.% to 14.2 wt.% and that of gas increased from 50.6 wt.% to 54.6 wt.% by changing the packed bed height of second stage from 40 to 100 mm while the gross heating value (GHV) of the product gas remained almost constant (16.2-16.5 MJ/m³).     

Kinetics of the surface reaction Had ↔ OHad on n-Ge(1 1 1)

Different surface states (GeOH and GeH) were found on n-Germanium. These two surface states can be controlled by potential (GeH at U<−0.2 V, GeOH at U>0.1 V in 0.5 M H₂SO₄). The surface reaction H_ad↔OH_ad is reversible and can be described by:

(1)     

Chemical-looping combustion in a 300 W continuously operating reactor system using a manganese-based oxygen carrier

Chemical-looping combustion (CLC) is a method for the combustion of fuel gas with inherent separation of carbon dioxide. This technique involves the use of two interconnected reactors. A solid oxygen carrier reacts with the oxygen in air in the air reactor and is then transferred to the fuel reactor, where the fuel gas is oxidized to carbon dioxide and water by the oxygen carrier. Fuel gas and air are never mixed and pure CO₂ can easily be obtained from the flue gas exit. The oxygen carrier is recycled between both reactors in a regenerative process. This paper presents the results from a continuously operating laboratory CLC unit, consisting of two interconnected fluidized beds. The feasibility of the use of a manganese-based oxygen carrier supported on magnesium stabilized zirconia was tested in this work. Natural gas or syngas was used as fuel in the fuel reactor. Fuel flow and air flow was varied, the thermal power was between 100 and 300 W, and the air ratio was between 1.1 and 5.0. Tests were performed at four temperatures: 1073, 1123, 1173 and 1223 K. The prototype was successfully operated at all conditions with no signs of agglomeration or deactivation of the oxygen carrier. The same particles were used during 70 h of combustion and the mass loss was 0.038% per hour, although the main quantity was lost in the first hour of operation. In the combustion tests with natural gas, methane was detected in the exit flue gases, while CO and H₂ were maintained at low concentrations. Higher temperature or lower fuel flows increases the combustion efficiency, which ranged from 0.88 to 0.99. On the other hand, the combustion of syngas was complete for all experimental conditions, with no CO or H₂ present in the gas from the fuel reactor.     

Experimental electrical resistivity of liquid carbon in the temperature range from 4800 to ∼20,000 K

Specimens of high density pyrolytic graphite (2.2 g/cm³) were placed inside thick-walled sapphire tubes and heated over several microseconds by an electric current of 68 kA. The electrical resistivity of the liquid carbon was measured in a constant volume heating process. The transition of liquid carbon from semi-metal properties (resistivity decreasing with increase of input energy) to metal-like behavior (resistivity increasing with increase of input energy) was obtained at a high input energy (25-75 kJ/g) and at a high, but not measured, pressure. The transition temperature, T, was roughly estimated through the C_V value (heat capacity under constant volume). The relationship between the density and the transition temperature is as follows: for 1.88 g/cm³ density, the transition temperature T = 6300 K, for 1.76 g/cm³, T = 10,100 K, and for 1.1 g/cm³, T = 13,500 K. The estimated temperature at the maximum input energy (75 kJ/g) for liquid metal-like carbon (just before the destruction of the sapphire tube) is 23,000 K, with a corresponding measured electrical resistivity of 3000 μΩ cm.     

A new type of hydrogen sensor for molten metals usable up to 1600 K

In order to simplify the structure of the galvanic cell-type hydrogen sensor using α-alumina as a solid electrolyte, the availability of employing air as the reference was checked by measuring the electromotive force (emf) of a gas concentration cell in the temperature range from 1073 to 1573 K. The emf was found to depend only on the hydrogen activity of the working electrode when air of normal humidity was used as the reference gas.The hydrogen activities in molten copper melted in a closed chamber were measured by immersing this structure-type hydrogen sensor at 1423 K. The measured values agreed with the hydrogen activities of the molten copper produced by supplying a H₂/Ar gas mixture over the surface of the melt. The performance of the hydrogen sensor was also tested for molten copper melted in the open air. Reasonable emf responses were obtained by the melt treatments such as the enrichment of hydrogen by covering the surface with charcoal or the reduction of hydrogen by exposing the surface to air.These results suggest that this type of hydrogen sensor is currently the best one for the real time detecting of the amount of hydrogen in molten metal up to 1600 K.     

Effects of temperature and composition on the laminar burning velocity of CH4 + H2 + O2 + N2 flames

This work summarises available measurements of laminar burning velocities in CH₄ + H₂ + O₂ + N₂ flames at atmospheric pressure performed using a heat flux method. Hydrogen content in the fuel was varied from 0% to 40%, amount of oxygen in the oxidiser was varied from 20.9% down to 16%, and initial temperature of the mixtures was varied from 298 to 418 K. These mixtures could be formed when enrichment by hydrogen is combined with flue gas recirculation. An empirical correlation for the laminar burning velocity covering a complete range of these measurements is derived and compared with experiments and other correlations from the literature.     

Measurement and correlation of the solid-liquid-gas equilibria for carbon dioxide + normal chain saturated aliphatic hydrocarbon systems

The pressure p-temperature T projections of solid-liquid-gas (S-L-G) three-phase coexistence lines for the carbon dioxide + tetradecanoic acid (C₁₄H₂₈O₂) system, the carbon dioxide + hexadecanoic acid (C₁₆H₃₂O₂) system, and the carbon dioxide + 1-hexadecanol (C₁₆H₃₄O) were measured by the first melting point method in which the initial appearance of the liquid phase was observed. The profiles of the p-T projections of the S-L-G lines for the carbon dioxide + acid systems are similar to each other, the S-L-G equilibria for the carbon dioxide + acid systems are, however, different from that for the carbon dioxide + 1-hexadecanol systems. The experimental p-T projections of the S-L-G lines were also correlated by the Peng-Robinson equation of state and the van der Waals type mixing rules with two binary interaction parameters introduced into attraction term and size terms, respectively. The present model gave good correlation results for all of the experimental S-L-G lines with maximum average absolute relative deviations of 0.075% for the carbon dioxide + tetradecanoic acid system, 0.14% for the carbon dioxide + hexadecanoic acid system and 0.28% for the carbon dioxide + 1-hexadecanol system, respectively.     

Structural evolution, sintering behavior and microwave dielectric properties of (1 − x)Li3NbO4-xLiF (0 ≤ x ≤ 0.9)

Sintering behavior, microstructure and microwave dielectric properties of (1 − x)Li₃NbO₄-xLiF (0 ≤ x ≤ 0.9) ceramics have been studied in this paper. Continuous solid solution with rock salt structure formed across the entire compositional range. Phase transformed from ordered body-centered cubic phase to short range ordered face-centered cubic phase with the addition of LiF. The x > 0.4 compositions could be densified at 850 °C/2 h. The optimized Q × f value for each composition increased with the increase of LiF content and saturated at about 75,000 GHz when x ≥ 0.15, whereas the optimized dielectric permittivity decreased with the increase of LiF content. All specimens exhibited negative τ_f value. The chemical compatibilities with copper (Cu) in the case of x = 0.4 composition and silver (Ag) in the case of x = 0.5 composition were also investigated, respectively. No chemical reaction has taken place between the matrix phase and Ag or Cu after sintering at 850 °C/2 h or 950 °C/2 h, respectively.     

Kinetics of adenine adsorption on Au(1 1 1) electrodes: An impedance study

The adsorption/desorption kinetics of adenine on Au(1 1 1) electrodes is studied by Electrochemical Impedance Spectroscopy (EIS) in 0.5 M NaF solutions at four adenine concentrations. The experimental procedure is designed in order to obtain impedance data unaffected by surface reconstruction on the entire potential region of adsorption. The frequency dispersion of the impedance at potentials of the adsorption region has been analysed according to the Frumkin-Melik-Gaykazyan adsorption theory without any “a priori” assumption about the potential dependence of the adsorption rate constant. The analysis provides the values of the adsorption capacitance, C_ad, adsorption resistance, R_ad and the Warburg coefficient, σ_ad, at every potential, and from them the relaxations times τ_H and τ_D. A mixed adsorption-diffusion control has been detected and the specific rate constant of adsorption has been obtained in a wide potential region.     

Electrodeposition of copper on Ru(0 0 0 1) in sulfuric acid solution: Growth kinetics and nucleation behavior

The electrodeposition of Cu on Ru(0 0 0 1) from 0.1 M CuSO₄/0.5 M H₂SO₄ solution has been studied by cyclic voltammetry, current-time transient measurements, and by in situ electrochemical atomic force microscopy (EC-AFM). Cyclic voltammetry measurements show that the as-prepared Ru(0 0 0 1) electrode exhibits a UPD peak, while EC-AFM data indicate a broadly terraced surface with step heights of atomic dimensions. Kinetic data show that the electrodeposition/nucleation process is not well described by 3D or 2D nucleation models. The EC-AFM data show that at potentials near the OPD/UPD threshold, Cu crystallites exhibit pronounced growth anisotropy, with lateral dimensions greatly exceeding vertical dimensions. AFM data also show that deposition at more cathodic potentials result in smaller crystallites.     

Reaction of layered carbon fluorides CxF (x = 2.5-3.6) and hydrogen

The layered carbon fluorides C_xF (x = 2.5, 2.8, 3.6), generally classified as fluorine-graphite intercalation compounds, were heat-treated in hydrogen gas. These fluorides are more reactive with hydrogen compared to (CF)_n and (C₂F)_n. Reduction of C_xF to graphite-like carbon starts at about 573 K, and proceeds gradually along with the elevation of temperature. Fluorine atoms in C_xF are eliminated as HF in the reduction process without being substituted by hydrogen atoms. Systematic difference was not found in the average crystallite sizes of the carbon material prepared from C_xF by the reduction with hydrogen and that by the pyrolysis in vacuum. On the other hand, interlayer distance and fluorine content of the former are smaller than those of the latter. In the case that the C_xF precursor maintains a large particle size, the reduced carbon as well as the pyrolytically prepared carbon possesses a foam-like shape due to the exfoliation during the heat treatment.     

Surface X-ray scattering of high index plane of platinum containing kink atoms in solid-liquid interface: Pt(3 1 0) = 3(1 0 0)-(1 1 0)

Surface structure of Pt(3 1 0) = 3(1 0 0)-(1 1 0), which contains kink atoms in the step, has been determined with the use of in situ surface X-ray scattering (SXS) in the double layer region (0.50 V(RHE)) in 0.1 M HClO₄. Clean Pt(3 1 0) surface has pseudo (1 × 1) structure on which lateral displacements of 2-9% and 0.3-1% are found along a and b directions, respectively, whereas the surfaces of Pt(1 1 0) = 2(1 1 1)-(1 1 1) and Pt(3 1 1) = 2(1 0 0)-(1 1 1) are reconstructed to (1 × 2) according to previous reports. Interlayer spacing between the first and the second layers d₁₂ is contracted about 5% compared with the bulk spacing, whereas those between underlying layers are expanded down to fourth layer. Fully adsorbed CO has no effect on the surface structure of Pt(3 1 0). This result differs from that on Pt(1 1 1), where d₁₂ is expanded after CO adsorption.     

Oxygen electroreduction on Ag(1 1 1): The pH effect

The rotating ring disk method (RRDE) is applied to investigate the pH effect on oxygen reduction reaction (ORR) on Ag(1 1 1) single crystal surface in 0.1 M KOH and 0.1 M HClO₄. In 0.1 M KOH, the ORR proceeds through 4e⁻ reaction pathway with a very small (0.5-2.5%) peroxide formation in the entire potential range. In 0.1 M HClO₄ the onset potential for the ORR is shifted for ca. 400 mV toward the higher overpotentials compared to the 0.1 M KOH solution. At the low overpotentials, in 0.1 M HClO₄ the ORR proceeds entirely as a 2e⁻ process, i.e, 100% H₂O₂ formation. At higher overpotentials, the initial mixed a 2e⁻ and 4e⁻ reduction is followed by the potential region where the ORR proceeds entirely as a 4e⁻ process, with H₂O formation as a final product. The pH dependent shift in the onset of the ORR as well as the reaction pathway has been explained based on both: a thermodynamic analysis of pH independent rate determining step, and on the pH dependent change in availability of surface active sites and adsorption energies of molecular oxygen and reaction intermediates.