Hydrogen from Ammonia by Catalytic Spillover Membrane

<p>One of the major challenges to be overcome before hydrogen fuelled vehicles can become commonplace is to store hydrogen with sufficient storage density to be practical. One approach to overcoming this challenge involves converting the hydrogen into a secondary fuel that can be stored more easily, such as ammonia. This introduces the challenge of efficiently retrieving the hydrogen from the secondary fuel with sufficient purity to be used in a polymer electrolyte membrane fuel cell. Putting the hydrogen producing reaction inside a membrane which is capable of filtering out hydrogen creates a membrane reactor which can increase hydrogen purity and can accelerate the reaction both kinetically and thermodynamically. The most effective materials currently known for hydrogen membranes are high palladium alloys of copper and silver. These are able to absorb hydrogen on the side with high hydrogen partial pressure and desorb that hydrogen on the side with low hydrogen pressure. Palladium metal is also able to interact with some catalysts by hydrogen spillover. Hydrogen is transported from the surface of the catalyst to the palladium surface more quickly than the hydrogen can desorb from the catalyst, this potentially accelerates both the catalysis and the hydrogen filtration. This research aimed to create a catalytic spillover membrane to extend the possibility of ammonia as a secondary fuel for hydrogen transport. In this research, several methods to produce a nickel catalyst on the surface of the palladium were explored: electrodeposition with and without a lithographic template; spray coating with nanoparticles; and preshaped nickel mesh and nickel foam. These potential catalysts were tested for ammonia decomposition. Templated electrodeposition created the most effective catalyst, but the nickel foam was most easily applied to the next stage of the research. The nickel foam catalyst was subsequently retested for ammonia decomposition in three scenarios: in contact with palladium foil; in a reactor with a palladium membrane; and in contact with a palladium membrane. The presence of a palladium membrane improved decomposition more than spillover contact between nickel foam catalyst and palladium, however, the combination of spillover contact with a palladium membrane increased the ammonia decomposition further. The rate of hydrogen flux through the palladium membranes was calculated for the experimental results. These were compared to flux values predicted by a model equation. The results showed that spillover contact between nickel catalyst and palladium membrane increased the hydrogen flux through the membrane.. The research outcomes have generated new knowledge and improved understanding of the morphology and role of nickel catalysts in accelerating ammonia decomposition. The research highlights the complex relationship between reactor design, gas flow paths, catalyst presentation and catalysis chemistry, suggesting promising areas for future research.</p>

Hydrogen from Ammonia by Catalytic Spillover Membrane

<p>One of the major challenges to be overcome before hydrogen fuelled vehicles can become commonplace is to store hydrogen with sufficient storage density to be practical. One approach to overcoming this challenge involves converting the hydrogen into a secondary fuel that can be stored more easily, such as ammonia. This introduces the challenge of efficiently retrieving the hydrogen from the secondary fuel with sufficient purity to be used in a polymer electrolyte membrane fuel cell. Putting the hydrogen producing reaction inside a membrane which is capable of filtering out hydrogen creates a membrane reactor which can increase hydrogen purity and can accelerate the reaction both kinetically and thermodynamically. The most effective materials currently known for hydrogen membranes are high palladium alloys of copper and silver. These are able to absorb hydrogen on the side with high hydrogen partial pressure and desorb that hydrogen on the side with low hydrogen pressure. Palladium metal is also able to interact with some catalysts by hydrogen spillover. Hydrogen is transported from the surface of the catalyst to the palladium surface more quickly than the hydrogen can desorb from the catalyst, this potentially accelerates both the catalysis and the hydrogen filtration. This research aimed to create a catalytic spillover membrane to extend the possibility of ammonia as a secondary fuel for hydrogen transport. In this research, several methods to produce a nickel catalyst on the surface of the palladium were explored: electrodeposition with and without a lithographic template; spray coating with nanoparticles; and preshaped nickel mesh and nickel foam. These potential catalysts were tested for ammonia decomposition. Templated electrodeposition created the most effective catalyst, but the nickel foam was most easily applied to the next stage of the research. The nickel foam catalyst was subsequently retested for ammonia decomposition in three scenarios: in contact with palladium foil; in a reactor with a palladium membrane; and in contact with a palladium membrane. The presence of a palladium membrane improved decomposition more than spillover contact between nickel foam catalyst and palladium, however, the combination of spillover contact with a palladium membrane increased the ammonia decomposition further. The rate of hydrogen flux through the palladium membranes was calculated for the experimental results. These were compared to flux values predicted by a model equation. The results showed that spillover contact between nickel catalyst and palladium membrane increased the hydrogen flux through the membrane.. The research outcomes have generated new knowledge and improved understanding of the morphology and role of nickel catalysts in accelerating ammonia decomposition. The research highlights the complex relationship between reactor design, gas flow paths, catalyst presentation and catalysis chemistry, suggesting promising areas for future research.</p>

Thermal annealing ambiance effect on phosphorus passivation and reactivation mechanisms in silicon-based Schottky diodes hydrogenated by MW-ECR plasma

The main objective of this work is to investigate the effect of thermal annealing in forming gas atmosphere on the mechanism of deactivation and reactivation of phosphorus in silicon-based Schottky diodes. Firstly, the microwave plasma power, initial phosphorus concentration in the samples and hydrogen flux were fixed as 650 W, 1015 cm–3, and 30 sccm, respectively, to investigate the behavior of different working parameters of diodes, specifically the duration and temperature of hydrogenation. Secondly, few samples hydrogenated at 400 °C for 1 h were annealed under the forming gas (10% H2 + 90% N2) within the temperature range from 100 to 700 °C for 1 h. The profiles of active phosphorus concentration were monitored by evaluating the change in concentration of phosphorus after hydrogenation or thermal annealing in a forming gas environment through capacitance-voltage measurements. The obtained results depict the temperature and duration of hydrogenation, which ultimately reveals the complex behavior of phosphorous and hydrogen in silicon. However, the phosphorus passivation rate is homogeneous over all the depths measured at 400 °C. The thermal annealing in a forming gas indicates the increase in passivation rate of phosphorus as a function of annealing temperature, till the passivation rate attains saturation in the sample annealed at 400 °C. At higher temperatures, a decrease in the concentration of phosphorous-hydrogen complexes is observed due to the dissociation of these complexes and reactivation of phosphorus under thermal effect.

The Atomic Rearrangement of GaN-Based Multiple Quantum Wells in H2/NH3 Mixed Gas for Improving Structural and Optical Properties

In this work, three GaN-based multiple quantum well (MQW) samples are grown to investigate the growth techniques of high-quality MQWs at low temperature (750 °C). Instead of conventional temperature ramp-up process, H2/NH3 gas mixture was introduced during the interruption after the growth of InGaN well layers. The influence of hydrogen flux was investigated. The cross-sectional images of MQW via transmission electron microscope show that a significant atomic rearrangement process happens during the hydrogen treatment. Both sharp interfaces of MQW and homogeneous indium distribution are achieved when a proper proportion of hydrogen was used. Moreover, the luminescence efficiency is improved strongly due to suppressed non-radiative recombination process and a better homogeneity of MQWs. Such kind of atomic rearrangement process is mainly caused by the larger diffusion rate of gallium and indium adatoms in H2/NH3 mixed gas, which leads to a lower potential barrier energy to achieve thermodynamic steady state. However, when excessive hydrogen flux is introduced, the MQW will be partly damaged, and the luminescence performance will deteriorate.

Assessment of Sieverts Law Assumptions and ‘n’ Values in Palladium Membranes: Experimental and Theoretical Analyses

Palladium and palladium alloy membranes are superior materials for hydrogen purification, removal, or reaction processes. Sieverts’ Law suggests that the flux of hydrogen through such membranes is proportional to the difference between the feed and permeate side partial pressures, each raised to the 0.5 power (n = 0.5). Sieverts’ Law is widely applied in analyzing the steady state hydrogen permeation through Pd-based membranes, even in some cases where the assumptions made in deriving Sieverts’ Law do not apply. Often permeation data are fit to the model allowing the pressure exponent (n) to vary. This study experimentally assessed the validity of Sieverts’ Law as hydrogen was separated from other gases and theoretically modelled the effects of pressure and temperature on the assumptions and hence the accuracy of the 0.5-power law even with pure hydrogen feed. Hydrogen fluxes through Pd and Pd-Ag alloy foils from feed mixtures (5–83% helium in hydrogen; 473–573 K; with and without a sweep gas) were measured to study the effect of concentration polarization (CP) on hydrogen permeance and the applicability of Sieverts’ Law under such conditions. Concentration polarization was found to dominate hydrogen transport under some experimental conditions, particularly when feed concentrations of hydrogen were low. All mixture feed experiments showed deviation from Sieverts’ Law. For example, the hydrogen flux through Pd foil was found to be proportional to the partial pressure difference (n ≈ 1) rather than being proportional to the difference in the square root of the partial pressures (n = 0.5), as suggested by Sieverts’ Law, indicating the high degree of concentration polarization. A theoretical model accounting for Langmuir adsorption with temperature dependent adsorption equilibrium coefficient was made and used to assess the effect of varying feed pressure from 1–136 atm at fixed temperature, and of varying temperature from 298 to 1273 K at fixed pressure. Adsorption effects, which dominate at high pressure and at low temperature, result in pressure exponents (n) values less than 0.5. With better understanding of the transport steps, a qualitative analysis of literature (n) values of 0.5, 0.5 < n < 1, and n > 1, was conducted suggesting the role of each condition or step on the hydrogen transport based on the empirically fit exponent value.

En Route to a Unified Model for Photoelectrochemical Reactor Optimization. II–Geometric Optimization of Perforated Photoelectrodes

Results have been reported previously of a model describing the performance of photoelectrochemical reactors, which utilize semiconductor | liquid junctions. This model was developed and verified using SnIV-doped α-Fe2O3 as photoanodes. Hematite films were fully characterized to obtain parameter inputs to a model predicting photocurrent densities. Thus, measured photocurrents were described and validated by the model in terms of measurable quantities. The complete reactor model, developed in COMSOL Multiphysics, accounted for gas evolution and desorption in the system. Hydrogen fluxes, charge yields and gas collection efficiencies in a photoelectrochemical reactor were estimated, revealing a critical need for geometric optimization to minimize H2-O2 product recombination as well as undesirable spatial distributions of current densities and “overpotentials” across the electrodes. Herein, the model was implemented in a 3D geometry and validated using solid and perforated 0.1 × 0.1 m2 planar photoanodes in an up-scaled photoelectrochemical reactor of 2 dm3. The same model was then applied to a set of simulated electrode geometries and electrode configurations to identify the electrode design that would maximize current densities and H2 fluxes. The electrode geometry was modified by introducing circular perforations of different sizes, relative separations and arrangements into an otherwise solid planar sheet for the purpose of providing ionic shortcuts. We report the simulated effects of electrode thickness and the presence or absence of a membrane to separate oxygen and hydrogen gases. In a reactor incorporating a membrane and a photoanode at 1.51 V vs RHE and pH 13.6, an optimized hydrogen flux was predicted for a perforation geometry with a separation-to-diameter ratio of 4.5 ± 0.5; the optimal perforation diameter was 50 µm. For reactors without a membrane, this ratio was 6.5 and 8.5 for a photoanode in a “wired” (monopolar) and “wireless” (photo-bipolar) design, respectively. The results and methodologies presented here will serve as a framework to optimize composite photoelectrodes (semiconductor | membrane | electrolyte), and photoelectrochemical reactors in general, for the production of hydrogen (and oxygen) from water using solar energy.

Hydrogenation of alkynyl substituted aromatics over rhodium/silica

The cascade reactions of phenylacetylene to ethylcyclohexane and 1-phenyl-1-propyne to propylcyclohexane were studied individually, under deuterium and competitively at 343 K and 3 barg pressure over a Rh/silica catalyst. Both systems gave similar activation energies for alkyne hydrogenation (56 ± 4 kJ mol−1 for phenylacetylene and 50 ± 4 kJ mol−1 for 1-phenyl-1-propyne). Over fresh catalyst the order of reactivity was styrene > phenylacetylene ≫ ethylbenzene. Whereas with the cascade hydrogenation starting with phenylacetylene, styrene hydrogenated much slower phenylacetylene even once all the phenylacetylene was hydrogenated. The activity of ethylbenzene was also reduced in the cascade reaction and after styrene hydrogenation. These reductions in rate were likely due to carbon laydown from phenylacetylene and styrene. Similar behavior was observed with the 1-phenyl-1-propyne cascade. Deuterium experiments revealed similar positive KIEs for phenylacetylene (2.6) and 1-phenyl-1-propyne (2.1). Ethylbenzene hydrogenation/deuteration gave a KIE of 1.6 obtained after styrene hydrogenation in contrast to the inverse KIE of 0.4 found with ethylbenzene hydrogenation/deuteration over a fresh catalyst, indicating a change in rate determining step. Competitive hydrogenation between phenylacetylene and styrene reduced the rate of phenylacetylene hydrogenation but increased selectivity to ethylbenzene suggesting a change in the flux of sub-surface hydrogen. In the competitive reaction between 1-phenyl-1-propyne and propylbenzene, the rate of hydrogenation of 1-phenyl-1-propyne was increased and the rate of alkene isomerization was decreased, likely due to an increase in the hydrogen flux for hydrogenation and a decrease in the hydrogen species active in methylstyrene isomerization.

Temperature Dependence of Hydrogen Solubility and Diffusivity in Hydrogen Permeable Membrane of Pd-Cu Alloy with B2-Type Crystal Structure

The temperature dependence of hydrogen solubility and diffusivity of Pd–53mol%Cu alloy membrane with the B2–type crystal structure has been investigated. The hydrogen permeation tests are performed using ultra–pure hydrogen (more than 9N) purified by a Pd–Ag alloy membrane to avoid any influences of impurities. It is found that the hydrogen permeability decreases significantly at low temperatures, especially near room temperature. The time dependence of hydrogen flux is monitored and found that the hydrogen flux decreases gradually during about 4 ~ 5 days after rapid cooling down to room temperature from 623 K.The results of the temperature dependence of the hydrogen permeability are analyzed in view of the consistent description of hydrogen permeation based on hydrogen chemical potential, where the hydrogen flux is proportional to the product of the mobility for hydrogen diffusion, B, and the PCT factor, fPCT. In this study, the pressure–composition–isotherms (PCT curves) for Pd–53Cu alloy with B2 structure are measured for the first time by the in–situ XRD–PCT method, and they are applied to estimate the PCT factors. Then, the temperature dependence of the PCT factor and the mobility for hydrogen diffusion is evaluated. It is revealed that the decrement in hydrogen permeability at low temperatures is mainly attributable to the decrement of the mobility for hydrogen diffusion.According to the positron annihilation experiments, the defects density is considered to be small in Pd–53Cu alloy with the B2 structure even at room temperature, suggesting that the excess Cu atoms in Pd–53Cu alloy occupy the positions of Pd sublattice, at which the Cu atoms form a local BCC–Cu unit. The diffusion of Cu atoms corresponds to the diffusion of BCC–Cu units in the B2 structure. Therefore the diffusion of Cu atoms and the configuration of BCC–Cu units in B2 structure could be a key to understand the gradual transition of hydrogen diffusivity at low temperatures.