Influence of temperature on hydrogen electrosorption into palladium-noble metal alloys. Part 1: Palladium-gold alloys

Hydrogen electrosorption into Pd-rich (>70 at.% Pd in the bulk) Pd-Au alloys obtained by electrodeposition was studied in acidic solutions (0.5 M H₂SO₄) using cyclic voltammetry and chronoamperometry. The influence of temperature (in the range between 283 and 313 K) on the amount of absorbed hydrogen, the potential of the α-β phase transition, absorption-desorption hysteresis and the potential of absorbed hydrogen oxidation was examined. It was found that for the temperature range studied the potentials of the α → β and β → α phase transitions are shifted negatively with increasing temperature and positively with decreasing Pd content in the alloy bulk. Thermodynamic parameters (Gibbs free energy, enthalpy and entropy) of the β-phase formation and decomposition were determined from the temperature dependence of the potential of the α-β phase transition. The absolute values of enthalpy and entropy of the β-phase formation and decomposition increase with the decrease in Pd bulk concentration in Pd-Au alloys. The maximum hydrogen absorption capacity of Pd-Au alloys slightly decreases with increasing temperature. The potential of absorbed hydrogen oxidation peak is shifted negatively with increasing temperature and decreasing Pd bulk content.     

Hydrogen electrosorption into Pd-Pt-Au ternary alloys

Thin layers of Pd-Pt-Au alloys were prepared by metal codeposition at constant potential from chloride solutions. The process of hydrogen electrosorption into Pd-Pt-Au alloys was investigated in acidic solution (0.5 M H₂SO₄) using cyclic voltammetry and chronoamperometry, also coupled with the electrochemical quartz crystal microbalance. It was found that Pd alloying with both Pt and Au decreases the maximum hydrogen solubility, but improves the kinetics of absorbed hydrogen oxidation, which is mirrored in a negative shift of the potential of hydrogen desorption peak and shorter hydrogen desorption time. In Pd-Pt-Au alloys the effect of absorption/desorption hysteresis and the stresses connected with hydrogen absorption are reduced in comparison with pure Pd. After prior hydrogen absorption in Pd-Pt-Au alloys, surface oxides are formed and reduced at potentials even by 200 mV lower than before hydrogen treatment.     

Influence of adsorbed carbon dioxide on hydrogen electrosorption in palladium-platinum-rhodium alloys

Carbon dioxide electroreduction was applied to examine the processes of hydrogen electrosorption (adsorption, absorption and desorption) by thin electrodeposits of Pd-Pt-Rh alloys under conditions of cyclic voltammetric (CV) experiments. Due to different adsorption characteristics towards the adsorption product of the electroreduction of CO₂ (reduced CO₂) exhibited by the alloy components hydrogen adsorption and hydrogen absorption signals can be distinguished on CV curves. Reduced CO₂ causes partial blocking of hydrogen adsorbed on surface Pt and Rh atoms, without any significant effect on hydrogen absorption into alloy. It reflects the fact that adsorbed hydrogen bonded to Pd atoms does not participate in CO₂ reduction, while hydrogen adsorbed on Pt and Rh surface sites is inactive in the absorption reaction. In contrast, CO is adsorbed on all alloy components and causes a marked inhibition of hydrogen sorption (both adsorption and absorption)/desorption reactions.     

Electrosorption of hydrogen into palladium-rhodium alloys

Hydrogen electrosorption into Pd-Rh alloys has been studied in acidic solutions (0.5 M H₂SO₄) using cyclic voltammetry and chronoamperometry. Pd-Rh electrodes were prepared as thin alloy electrodeposits on Au wires. The influence of electrode potential on the amount of electrosorbed hydrogen was investigated. One can distinguish potential regions of α- and β-phase existence as well as the region of α-β phase transition. For alloys containing less than 20 at.% Rh in the bulk the hydrogen-to-metal ratio, H/(Pd + Rh), is greater than for pure Pd. The maximum value of the H/(Pd + Pt) ratio is 0.80 for an alloy containing in the bulk 94 at.% Pd and 6 at.% Rh. Due to a smaller lattice parameter of the Pd-Rh alloy with respect to pure Pd the α-β phase transition occurs at lower potentials than in Pd. The hysteresis is observed in chronoamperometric absorption and desorption experiments; the effect of hysteresis decreases with the increase in Rh content. Additionally, preliminary results are presented concerning the electrochemical quartz crystal microbalance measurements of hydrogen absorption/desorption into/from Pd-Rh alloys in comparison with Pd.     

Electrosorption of hydrogen into palladium-rhodium alloys: Part 2. Pd-rich electrodes of various thickness

Hydrogen electrosorption into Pd-rich (≥80% Pd) Pd-Rh alloy layers of various thickness has been studied in acidic solutions (0.5 M H₂SO₄) using cyclic voltammetry and chronoamperometry. Pd-Rh electrodes were prepared as limited volume electrodes (LVEs) by electrochemical deposition. The influence of electrode potential and alloy thickness on the amount of electrosorbed hydrogen was investigated. For the thickness range examined (0.11-1.55 μm) no distinct effect on alloy absorption properties was found. The influence of scan rate on hydrogen oxidation charge was also studied. The results obtained suggest that the layer of subsurface hydrogen in Pd-Rh alloys is much less developed than in pure Pd and direct hydrogen absorption/desorption path is more probable.     

Yield of hydrogen during cathodic polarisation of Al–Sn alloys

The results presented in this paper were obtained with three Al–Sn binary alloys (0.09, 0.2 and 0.4 wt.% Sn, respectively) that had been prepared on super pure (99.999%) Al base. The yield of hydrogen evolved during cathodic polarisation of Al–Sn alloys in a 2 M NaCl solution was measured volumetrically as a function of current density, temperature and tin content in the alloy. The yield was established to be somewhat below 2 at all current densities and at low temperatures, while with increasing temperature it approached the value of 4. With the increase of tin content in the alloy, the yield of hydrogen decreases, which depends on the degree of hyperactivity that occurs in Al–Sn alloys in the region of high cathodic potentials.     

Study on phase structure and electrochemical properties of Ml1−xMgxNi2.80Co0.50Mn0.10Al0.10 (x = 0.08, 0.12, 0.20, 0.24, 0.28) hydrogen storage alloys

Phase structure and electrochemical properties of the Ml_1−xMg_xNi_2.80Co_0.50Mn_0.10Al_0.10 (x = 0.08, 0.12, 0.20, 0.24, 0.28) (Ml = La-rich mixed lanthanide) alloys were studied. X-ray diffraction (XRD) analysis and Rietveld refinement show that the alloys consist mainly of LaNi₅ and (La,Mg)Ni₃ phase. Due to variation in phases of the alloys, the maximum discharge capacity, the high rate dischargeability (HRD), and the low temperature dischargeability increase first and then decrease. The maximum discharge capacity increases from 322 mAh g⁻¹ (x = 0.08) to 375 mAh g⁻¹ (x = 0.12), and then decreases to 351 mAh g⁻¹ (x = 0.28) with increasing x. As the case of x = 0.20, HRD at 1200 mA g⁻¹ and discharge capacity at 233 K reaches 41.7% and 256 mAh g⁻¹, respectively. The cycling stability is improved by substituting La with Ml and B-site multi-alloying, and the capacity retention of Ml_0.72Mg_0.28Ni_2.80Co_0.50Mn_0.10Al_0.10 at the 200th cycle is 71%.     

Quartz crystal microbalance studies on electrochemical behavior of electrodeposited Pd-Ni alloys

Electrochemical quartz crystal microbalance (EQCM) was applied in studies on electrochemical behaviour of Pd-rich Pd-Ni alloys in basic solutions. The results were compared with those obtained for pure Pd and Ni electrodes. The EQCM frequency response accompanying the desorption of the absorbed hydrogen is strongly influenced by the stresses generated during this process and depends on the alloy composition and the initial content of the absorbed hydrogen. The latter effect is much more pronounced for Pd-Ni alloys than for pure Pd electrode. The gravimmetric analysis of the oxidation processes of Ni(II) compounds on Pd-Ni electrodes and the subsequent reduction of the products is also complicated by the stresses accompanying these processes.     

Monte Carlo simulation of hydrogen absorption in palladium and palladium–silver alloys

A modified Monte Carlo (MC) simulation was performed to investigate the hydrogen absorption behavior in Pd and Pd–Ag alloys of the composition Pd_xAg_1−x (x=0.7–0.8) under H₂ pressure (0.1 MPa) at different temperatures. The present method employed can consider the dissociative adsorption of hydrogen molecule and the subsequent absorption of hydrogen atom by formalizing the relationship between the pressure of hydrogen molecule and hydrogen atom. The potential parameters were determined to reproduce the solution enthalpy of hydrogen in pure metals. The results are in good agreement with experimental findings as well as previous theoretical studies. We confirmed that our method is useful to simulate the absorption of hydrogen in metals and alloys.     

On the specific reactivity of platinum segregated layers on Pt80X20 (111) (X= Ni,Co, Fe) alloys

The influence of alloying of Pt with 20 at.% of Ni, Co and Fe has been studied in the hydrogenation of 1,3 butadiene. Iron induces the more important modifications, with both higher activity and selectivity. The results are discussed in terms of the surface segregation, the local order in surface and the electronic properties measured by photoemission of core levels.     

Influence of mercury on hydrogen overvoltage on solid metal electrodes—III. Adsorption and evolution of hydrogen on poisoned platinum

The adsorption capacity for hydrogen as well as the exchange cd of hydrogen evolution and the double-layer capacitance of a platinum electrode depend on the degree of poisoning by mercury. Mercury electrodeposited with simultaneous hydrogen evolution on smooth platinum forms droplets, the curvature radius of which is of the order 10^?6 cm, and for complete inhibition of hydrogen adsorption, an amount equivalent to several monolayers of mercury is necessary. By submerging a pure platinum electrode in a dilute solution of mercuric compounds without external polarization, a monolayer is formed and one atom of mercury replaces just one atom of hydrogen. On platinum-black electrodes, electrodeposited drops of mercury cover the orifice of the pores and one mercury atom shields on an average 2–3 active sites of the electrode surface.     

Application of an extended Tool–Narayanaswamy–Moynihan model.Part 2. Frequency and cooling rate dependence of glass transition from temperature modulated DSC

In the first part of this paper a Tool–Narayanaswamy–Moynihan-model (TNM) extended by non-Arrhenius temperature dependence of the relaxation time was applied to describe results from temperature modulated DSC (TMDSC). The model is capable to describe the features of the heat capacities measured in TMDSC scan experiments in the glass transition region of polystyrene (PS). In this part the model is applied to bisphenol A-polycarbonate (PC). Both aspects of glass transition, vitrification as well as the dynamic glass transition are again well described by the model. The dynamic glass transition above T_g can be considered as a process in thermodynamic equilibrium. The non-linearity parameter (x) of the TNM model is not needed to describe complex heat capacity as long as the dynamic glass transition is well separated from vitrification. Under such conditions the relation between cooling rate (q₀), and the corresponding frequency (ω) can be found from the two independently observed glass transitions. Fictive temperature and the maximum of the imaginary part of complex heat capacity are used for comparison here. The measurement as well as the TNM-model confirm the relation derived from Donth's fluctuation approach to glass transition, ω=q₀/aδT, where a=5.5±0.1 (confirmed previously experimentally as 6±3) and δT is mean temperature fluctuation of the cooperatively rearranging regions (CRRs).     

Impedance study of hydrogen evolution on Ni/Zn and Ni–Co/Zn stainless steel based electrodeposits

A study on the electrocatalytic performance of Ni/Zn and Ni–Co/Zn alloys for hydrogen evolution reaction (HER) in alkaline media (30 wt.% KOH solution) has been carried out. After preparing by electrodeposition on stainless steel supports, the alloys were leached of to remove part of the zinc and generate a porous layer. For the developed electrodes, the surface roughness factor, R_f, was evaluated by electrochemical impedance spectroscopy (EIS). The HER on these electrodes was evaluated by means of steady-state polarization curves and EIS. The obtained electrodes were characterized by large R_f for HER, and very low overpotentials at the current density of 250 mA cm⁻², η₂₅₀ ∼ 0.138 V at 30 °C. The high electrode activity was mainly attributed to the high surface area of the developed electrodes.     

Comparison of supports for the direct synthesis of hydrogen peroxide from H2 and O2 using Au–Pd catalysts

The direct synthesis of hydrogen peroxide from H₂ and O₂ using a range of supported Au–Pd alloy catalysts is compared for different supports using conditions previously identified as being optimal for hydrogen peroxide synthesis, i.e. low temperature (2 °C) using a water–methanol solvent mixture and short reaction time. Five supports are compared and contrasted, namely Al₂O₃, -Fe₂O₃, TiO₂, SiO₂ and carbon. For all catalysts the addition of Pd to the Au only catalyst increases the rate of hydrogen peroxide synthesis as well as the concentration of hydrogen peroxide formed. Of the materials evaluated, the carbon-supported Au–Pd alloy catalysts give the highest reactivity. The results show that the support can have an important influence on the synthesis of hydrogen peroxide from the direct reaction. The effect of the methanol–water solvent is studied in detail for the 2.5 wt% Au–2.5 wt% Pd/TiO₂ catalyst and the ratio of methanol to water is found to have a major effect on the rate of hydrogen peroxide synthesis. The optimum mixture for this solvent system is 80 vol.% methanol with 20 vol.% water. However, the use of water alone is still effective albeit at a decreased rate. The effect of catalyst mass was therefore also investigated for the water and water–methanol solvents and the observed effect on the hydrogen peroxide productivity using water as a solvent is not considered to be due to mass transfer limitations. These results are of importance with respect to the industrial application of these Au–Pd catalysts.     

The effect of temperature on the adiabatic laminar burning velocities of CH4−air and H2−air flames

The effect of temperature on the adiabatic laminar burning velocities of CH₄ + air and H₂ + air flames was analyzed. Available measurements were interpreted using correlation S_L = S_L0 (T/T₀)^α. Particular attention was paid to the variation of the power exponent α with equivalence ratio at fixed (atmospheric) pressure. Experimental data and proposed empirical expressions for α as a function of equivalence ratio were summarized. They were compared with predictions of detailed kinetic models in methane + air and hydrogen + air flames. Unexpected non-monotonic behavior of α was found in rich methane + air flames. Modeling results are further examined using sensitivity analysis to elucidate the reason of particular dependences of the power exponent α on equivalence ratio.     

Structural study of a series of ethylene-tetrafluoroethylene copolymers with various ethylene contents, Part 2: Phase transition behavior investigated by temperature dependent measurements of X-ray fiber diagrams

Phase transitional behavior has been investigated for a series of ethylene (E) and tetrafluoroethylene (TFE) two-component copolymers on the basis of the temperature dependent measurement of X-ray fiber diagrams taken for the uniaxially-oriented samples. The usage of such uniaxially-oriented samples has allowed us to clarify the structural changes more definitely than before. So far, the crystalline transition from the low-temperature phase to the high-temperature phase had been assumed to occur continuously, but the detailed analysis of the X-ray reflections at higher scattering angles revealed for the first time that the transition is of the thermodynamically-discontinuous first-order type between the monoclinic and the pseudohexagonal phases. This phase transition temperature shifted toward lower temperature side as the TFE content became higher. The analysis of the X-ray reflection profile allowed us to imagine that low-temperature shift of the transition point is considered to occur due to the easier thermal motion of the planar-zigzag chains around the chain axis because of looser packing of chains containing higher population of bulky CF₂CF₂ groups. From these experimental data a phase diagram has been built up to show the phase transition behaviors of a series of E/TFE copolymers in a systematic manner. The characteristic features of the structural disordering including trans-gauche conformational exchange have been also discussed on the basis of the data of polarized infrared and Raman spectra combined with the X-ray diffuse scatterings.     

层流冷却传热系数对金属物料温度场的影响

层流冷却的冷却强度决定着金属物料微观组织结构,从而影响产品的性能,而传热系数直接影响着冷却强度的大小。本工作提出了一种获得层流冷却传热系数的方法,通过工艺分析得到其分布形式为以喷头为中心由半波正弦和直线构成的分段函数,给出了通过结构参数、运行参数和实验数据获得函数特征参数的方法。以实验台验证了该方法的准确性,同时分析了特征参数变化对金属物料温度场的影响规律。结果表明,振幅增加1 W/(m2?K),金属物料表面温度相对下降量在最后一个区域达到最大值,为?0.36 m2?K2/W,断面温差最大相对变化量为0.07 m2?K2/W;周期减小1 s,金属物料表面温度相对增加量在最后一个区域达到最大值,为13.24 K/s,断面温差呈现周期性变化,变化范围为?2.5~0 K/s。     

Effect of Ca on the microstructural and electrochemical properties of La2.3−xCaxMg0.7Ni9 hydrogen storage alloys

The microstructural and electrochemical properties of La_2.3−xCa_xMg_0.7Ni₉ hydrogen storage alloys have been studied systematically. The microstructure examined by XRD, SEM and EDX shows that the alloys consist of multi-phases, which are (La, Mg)₂Ni₇ phase, LaMgNi₄ phase, (La, Mg)Ni₃ phase and LaNi₅ phase. It is can be found that Ca does not appear to segregate. This phenomenon is different from Mg. With increasing Ca content, the main phase varies from (La, Mg)₂Ni₇ phase (x = 0) to (La, Mg)Ni₃ phase (x = 0.3), LaNi₅ phase (x = 0.6, 0.8) and (La, Mg)Ni₃ phase (x = 1.0, 1.3). The maximum discharge capacities of the alloy electrodes increase from 244.6 mAh/g (x = 0) to 380 mAh/g (x = 1.0), and then decrease to 353.6 mAh/g (x = 1.3). The discharge capacities of the alloys are related to phase content. Cell volumes of LaNi₅ phase, (La, Mg)₂Ni₇ phase and (La, Mg)Ni₃ phase all decrease and the high rate dischargeability (HRD) is improved by adding Ca. The alloy electrodes also show relative good cycling stability up to 100 cycles.     

The role of graphene and europium on TiO2 performance for photocatalytic hydrogen evolution

Highly efficient Eu-TiO₂/graphene composites were synthesized by a two-step method such as sol-gel and hydrothermal process. The synthesized photocatalysts were characterized by XRD, TEM, XPS, UV–vis diffuse reflectance spectroscopy and photoluminescence (PL) spectroscopy. The results confirmed that anatase Eu-TiO₂ nanoparticles with average 10 nm sizes were successfully deposited on two-dimensional graphene sheets. The UV–visible spectroscopy showed a red shift in the absorption edge of TiO₂ due to Eu doping and graphene incorporation. Moreover, effective charge separation in Eu-TiO₂/graphene composites was confirmed by PL emission spectroscopy compared to TiO₂/graphene, Eu-TiO₂ and pure TiO₂. The photocatalytic activity for H₂ evolution over prepared composites was studied under visible light irradiation (λ ≥ 400 nm). The results demonstrate that photocatalytic performance of the photocatalysts for hydrogen production increases with increasing doping concentration of Eu upto 2 at%. However, further increase in doping content above this optimum level has decreased the performance of photocatalyst. The enhanced photocatalytic performance for H₂ evolution is attributed to extended visible light absorption, suppressed recombination of electron-hole pairs due to synergistic effects of Eu and graphene.     

Simulation of SO2 absorption into sulfuric acid solutions containing hydrogen peroxide in the fast and moderately fast kinetic regimes

Absorption of sulfur dioxide into aqueous solutions containing hydrogen peroxide results in the irreversible oxidation to sulfuric acid. As shown in previous works, the absorption rate increases with the concentration of oxidizing agent but decreases with the rise of acidity. This paper presents a thorough kinetic investigation of the liquid-phase reaction of SO₂ for a wide range of H₂O₂ and H₂SO₄ concentrations.Absorption of sulfur dioxide was carried out at 20 °C in a pilot-scale packed column. The experimental data were modelled thanks to the theory of absorption in the fast or moderately fast chemical reaction regimes defined by classical Hatta numbers and depending on the H₂O₂ concentration at a given acidity. For both regimes, a satisfactory agreement was obtained between experimental results and predictions of models including original values of kinetic parameters.