Nickel/Chromium oxide/Zirconium oxide Catalysts for Hydrogen/Deuterium Isotopic Exchange

Catalysts composed of nickel and promoted with different metal oxides proved to be suitable for H/D isotropic exchange between hydrogen and water vapour. They loss their activity due to water condensation on their surfaces. Several nickel/chromium oxide/zirconium oxide catalysts of different composition were prepared by the coprecipitation technique. The liquid phase activity of these catalysts were followed using the hydrogen peroxide decomposition on their surfaces at different temperatures. The surface characteristics of the examined catalysts were followed by applying the BET method. The results were discussed and lead to the required catalyst composition which resists the water condensation on its surface during operation and has relatively high activity in the vapour phase H/D isotopic exchange reaction between hydrogen and water.     

Adsorption, reaction and desorption of hydrogen on modified Pd(1 1 1) surfaces

The interaction of hydrogen (deuterium) with different modified Pd(1 1 1) surfaces has been investigated. The focus was put on the energy and angel dependence of the desorbing molecules from oxygen covered, potassium covered and vanadium oxide covered surfaces. Conventional adsorption/desorption as well as permeation/desorption experiments were performed. For the oxygen covered surface optimum reaction rates for water production and the energy distribution of the reaction products were determined, both for the reaction of oxygen with molecular hydrogen as well as with atomic hydrogen. Potassium on the surface enhances the activation barrier for hydrogen adsorption resulting in a hyper-thermal desorption flux and a forward focused angular distribution of desorption. Permeation/desorption of deuterium from ultra-thin vanadium oxide films yield mainly thermalized desorbing molecules or slightly hyper-thermal, depending on the oxidation state of the surface oxide.     

Effect of nitrogen and carbon dioxide as fuel impurities on PEM fuel cell performances

Polymer electrolyte membrane (PEM) fuel cells are considered to have the highest power density of all the fuel cells. They operate on hydrogen fuel, which is generally produced by reforming of hydrocarbons, and may contain large amounts of impurities such as carbon dioxide, nitrogen, and trace amounts of carbon monoxide. We studied the effect of dilution of hydrogen gas with carbon dioxide on PEM fuel cells by polarization studies. The polarization curves were different when hydrogen gas was diluted with same quantities of carbon dioxide and with nitrogen. It may be due to carbon monoxide formation by reverse shift reaction and poisoning of anode platinum catalyst. Use of Pt–Ru alloy catalyst was found to suppress the poisoning. The effects of hydrogen gas composition, temperature, current density, and anode catalyst on fuel cell performances were examined in this study.     

Sites for catalysis and electrochemistry in solid oxide fuel cell (SOFC) anode

Fuel cells represent a challenging overlap of catalysis and electrochemistry. This is illustrated by anode reactions in a solid oxide fuel cell. The sites for catalytic conversion of methane and electrochemical conversion of hydrogen on an SOFC anode appear not to be the same. The fuel (methane, hydrogen, etc.) is activated by chemisorption on the nickel surface of the anode. This is linked to the electrochemical reaction at the interface of the electrolyte and the nickel crystals converting oxygen ions into electrons and water by reactions with adsorbed hydrogen atoms resulting from the activation of the fuel. The sites for these reactions appear not to be the same. This is reflected by different sensitivities of the two steps to sulphur poisoning. The role of different sites on the nickel surface for the steam reforming reaction is well understood in terms of impact on activity for methane activation, carbon formation and sintering. The study is supplemented by an analysis of anodes having been exposed to 13000 of operation using a number of characterisation methods. PACS 82.47.Ed; 82.45.-h; 82.65.-s     

Adsorption and dissociation of molecular hydrogen on Pt/CeO2 catalyst in the hydrogen spillover process: A quantum chemical molecular dynamics study

Ultra accelerated quantum chemical molecular dynamics method (UA-QCMD) was used to study the dynamics of the hydrogen spillover process on Pt/CeO₂ catalyst surface for the first time. The direct observation of dissociative adsorption of hydrogen on Pt/CeO₂ catalyst surface as well as the diffusion of dissociative hydrogen from the Pt/CeO₂ catalyst surface was simulated. The diffusion of the hydrogen atom in the gas phase explains the high reactivity observed in the hydrogen spillover process. Chemical changes, change of adsorption states and structural changes were investigated. It was observed that parallel adsorption of hydrogen facilitates the dissociative adsorption leading to hydrogen desorption. Impact with perpendicular adsorption of hydrogen causes the molecular adsorption on the surface, which decelerates the hydrogen spillover. The present study also indicates that the CeO₂ support has strong interaction with Pt catalyst, which may cause an increase in Pt activity as well as enhancement of the metal catalyst dispersions and hence increasing the rate of hydrogen spillover reaction.     

In situ investigation of the catalytic reaction H2+< ![IGNORE[O2→H2 O with second-harmonic generation

2 +O₂→H₂ O in the pressure range 0.2 Torr≤p_tot≤10 Torr on Pt(111) surface. At a catalyst temperature of T=700 K the equilibrium oxygen coverage θ_o is determined as a function of hydrogen partial pressure α. The experimentally obtained θ_o is modelled in a two step process considering the mass transport in the gas phase as well as the catalytic reaction on the surface. In this pressure range the mass transport in the gas phase changes from molecular flow conditions to laminar flow, inducing a strong modification of the gas phase present at the catalyst through different diffusivities of the reactants as well as through desorbing reaction products from the catalyst. It is shown that these gas phase alterations have to be taken into account for a proper modelling of the surface mechanism. Simulation calculations allow one to identify the sequential hydrogen addition reaction as the main reaction path for water production in this parameter range. Excellent agreement with previous investigations is obtained for the determined activation energies of the water-producing reaction steps equal to E^f _H2O≥0.7 eV. Received: 20 September 1998 / Revised version: 15 December 1998     

The effect of the temperature on the reduction kinetics of oxidized Ni(111) surfaces by hydrogen

The reduction kinetics of oxidized Ni(111) surfaces are measured in situ with ellipsometry in a temperature range between 450 and 675 K. The reaction rate is proportional to the square root of the hydrogen pressure below reduction temperatures of 525 K. The rate limiting step is the reaction between chemisorbed oxygen and dissociated hydrogen and has an experimental activation energy of 57 ± 7 kJ/mol. This reaction takes only place on the oxide free part of the Ni surface. Above 600 K, the reaction rate is proportional to the hydrogen pressure. The rate limiting step is the formation of water and has no experimental activation energy. At temperatures above 600 K the distribution of oxygen throughout the Ni crystal has a large effect on the reduction curves. A new reduction model is proposed that describes all the observed curves satisfactorily.     

钴原子插层增强磷烯纳米片析氢反应活性

本文基于第一性原理计算,证明了钴插层磷烯的析氢催化活性可以显著增强.钴插层磷烯具有金属特性,电荷从钴原子向磷烯转移,增强了磷烯的催化活性.钴插层磷烯表面的氢吸附吉布斯自由能与铂(111)表面相当,与氢覆盖度无关.研究结果表明钴原子插层提供了一种有效的方法来增强磷烯的析氢反应催化活性.     

Reactivation of Promoted Nickel Catalysts for Deuterium Exchange between Hydrogen and Water in the Vapour Phase

Binary promoted nickel – chromium oxide and ternary promoted nickel – chromium oxide – aluminium oxide mixed catalysts were prepared for use in the present study. The catalysts were prepared by co-precipitation of the corresponding metal nitrates as carbonates followed by calcination in nitrogen atmosphere at 350 °C and reduction in hydrogen atmosphere at 320 °C. To prevent spontaneous oxidation of the catalysts, bidistilled water was added followed by heating of the catalyst mixtures at about 110 °C in hydrogen atmosphere for few hours. Deactivation of catalysts was studied by measurements of the variation of their activities with the time of contact of the reacting gas mixtures with the catalyst surface in the reaction chamber. It was found that while the catalytic activity of ternary catalysts for the isotopic exchange of deuterium between hydrogen and water vapour was higher than that of the binary one, the loss in activity of the former teas faster than the latter. Reactivation of the catalysts were carried out at different temperatures between 110–160°C in hydrogen atmosphere. Catalytic activity measurements indicated that higher temperatures are better for the reactivation process.     

Analytische Fern- und Laborkontrolle bei der Wideraufarbeitung von Brennelementen aus KKW

The activity of Ni–Cr₂O₃ catalyst for the deuterium exchange reactions between hydrogen and ammonia, as well as for hydrogen and water vapor has been measured in dependence on the reaction temperature and on partial pressure of ammonia and water vapor, respectively. In both cases the activity in dependence on the partial pressure shows a maximum; the maximum of activity for H₂–NH₃ exchange is situated between partial pressures of 0.05 and 0.25, and for H₂–H₂O reaction between 0.25 and 0.5. The Ni–Cr₂O₃ catalyst is about 2.4 more active for the exchange reaction H₂–H₂O than for H₂–NH₃. For both reactions, chromia has a strong promoting effect, enhancing the activity per gram of catalyst of about three orders of magnitude in comparison with that of the Nickel black.     

Supported Nickel-Cobalt Oxide Catalysts for the Isotopic Exchange of Deuterium between Hydrogen and Water Vapour

The specific catalytic activity of supported nickel-cobalt (II) oxide catalysts for the isotopic exchange of deuterium between hydrogen and water in the vapour phase was tested. The specific surface area of the catalysts was evaluated by nitrogen adsorption at ?195.8 °C and application of the BET-equation. The specific metallic surface area for these catalysts was carried out at liquid nitrogen temperature by hydrogen adsorption. Comparison between the specific activity of the catalysts and the specific surface area and specific metallic surface area teas made. The results of this study indicated that nickel catalysis supported with 15 – 20 mole% CoO exhibit a relatively high catalytic activity for the isotopic exchange reaction between hydrogen and water vapour, high specific surface area and high specific metallic surface area.     

Role of graphene in improving catalytic behaviors of AuNPs/MoS2/Gr/Ni-F structure in hydrogen evolution reaction

The efficient production of hydrogen through electrocatalytic decomposition of water has broad prospects in modern energy equipment. However, the catalytic efficiency and durability of hydrogen evolution catalyst are still very deficient, which need to be further explored. Here in this work, we prove that introducing a graphene layer (Gr) between the molybdenum disulfide and nickel foam (Ni-F) substrate can greatly improve the catalytic performance of the hybrid. Owing to the excitation of local surface plasmon resonance (LSPR) of gold nanoparticles (NPs), the electrocatalytic hydrogen releasing activity of the MoS₂/Gr/Ni-F heterostructure is greatly improved. This results in a significant increase in the current density of AuNPs/MoS₂/Gr/Ni-F composite material under light irradiation and in the dark at 0.2 V (versus reversible hydrogen electrode (RHE)), which is much better than in MoS₂/Gr/Ni-F composite materials. The enhancement of hydrogen release can be attributed to the injection of hot electrons into MoS₂/Gr/Ni-F by AuNPs, which will improve the electron density of MoS₂/Gr/Ni-F, promote the reduction of H₂O, and further reduce the activation energy of the electrocatalyst hydrogen evolution reaction (HER). We also prove that the introduction of graphene can improve its stability in acidic catalytic environments. This work provides a new way of designing efficient water splitting system.     

Methanol-water co-electrolysis for sustainable hydrogen production with PtRu/C-SnO<Subscript>2</Subscript> electro-catalyst

Hydrogen at present is mainly produced from fossil fuels for use in ammonia synthesis, the petrochemical industry, and chemical production. In the future, hydrogen will be increasingly used as an energy vector. Although water electrolysis to produce hydrogen with renewable electricity offers a clean process, the approach is energy intensive, requiring a large renewable resource footprint. Methanol-water co-electrolysis can reduce the energy input by >?50%; its electrochemical oxidation poses complex issues such as poisoning of the catalyst, sluggish oxidation kinetics, and degradation over time. The addition of nano-sized SnO₂ to PtRu/C catalyst, to reduce noble metal loading, has been shown here to reduce catalyst leaching and increase the chemical, micro-structural, and performance stability of the methanol-water co-electrolysis process during extended periods of testing. The electrochemical characterization, analysis of the methanol solution, and exit gases, post-cell testing, revealed complete oxidation of methanol with little performance degradation. This is further supported by the stability of the catalyst composition and structure as revealed by the post-mortem XRD and XPS analysis of the cell. The energy balance calculations show that methanol-water co-electrolysis can significantly reduce the renewable energy footprint, and the process can become carbon neutral if bio-methanol is used with renewable electricity.     

H/D Isotopic Exchange between Hydrogen and Water Vapour on Ni/Cr2O3/ThO2 Catalysts

Thoriumdioxide was selected as a second promoter for nickel catalyst in addition to chromium oxide which was proved to be suitable to accelerate the H/D isotopic exchange reaction between hydrogen and water vapour. A series of Ni/Cr₂O₃/ThO₂ catalysts were prepared by the co-precipitation technique. The amount of Ni was 70 … 90 mol%, while that of Cr₂O₃ was 0 … 20 mol% and that of ThO₂ ranges from 0 … 30 mol%. This type of catalysts is sensitive for water condensation on its surface. The total surface area, total pore volume and pore radius of the catalysts were calculated from nitrogen adsorption on their surfaces at 77 K and application of the BET-equation.     

Effect of borohydride as reducing agent on the structures and electrochemical properties of Pt/C catalyst

In this paper, we found that boron deposited on the surface of support when sodium borohydride used as reducing agent during the preparation of Pt/C catalyst. The deposition of boron markedly reduces particle size of Pt, raises electrochemical active surface (EAS) area of catalyst and electrochemical activity for hydrogen evolution or oxygen reduction reaction (ORR) compared with which prepared using other reducing agents (hydrogen and formaldehyde).     

DFT study on H2O activation by stepped and planar Rh surfaces

In this research the effect of steps (lower coordinated surface atoms) and the presence of pre-adsorbed oxygen on the activation energy of water are studied with DFT. Without oxygen water activation is found to be structure insensitive. When oxygen is adsorbed on the surface and acts as the acceptor for the hydrogen at the step edge, the barrier will decrease significantly.     

A molecular-beam investigation of the reaction H2 + 12O2 → H2O on Pd(111)

Using molecular-beam relaxation techniques and isotopic exchange experiments, the water-formation reaction on Pd(111) has been shown to proceed via a Langmuir-Hinshelwood mechanism. The reaction product H₂O is emitted from the surface with a cosine distribution. The rate-determining step is the formation of OH_ad in the reaction O_ad + H_ad → OH_ad. The activation energy for this step is 7 kcal/mole with a pre-exponential factor, v, of 4 × 10^?8 cm² atom^?1 sec^?1. This value for v lies well below that observed for simple second-order desorption of dissociatively adsorbed diatomic gases, but is roughly of the order of that obtained for the oxidation of CO on Pd(111). The formation of H₂O proceeds differently under conditions of excess O₂ or H₂. In an excess of H₂, the kinetics is dominated by the transport of atomic hydrogen between the bulk and the surface as was found for the H?D exchange reaction on Pd(111). In an excess of O₂, diffusion of hydrogen into the bulk is blocked by adsorbed oxygen and the hydrogen reservoir available for reaction at the surface is decreased by several orders of magnitude. This results in a drastic reduction of the reaction rate which can be reversed by increasing the partial pressure of H₂.     

Hydrogen oxidation and proton transport at the Ni–zirconia interface in solid oxide fuel cell anodes: Quantum chemical predictions

We explore the hydrogen anode reaction chemistry at the Ni–zirconia triple phase boundary in solid oxide fuel cells by using hybrid density functional quantum chemistry calculations and cluster models. The activation energy for H spillover is calculated to be the same order of magnitude as experimental estimates at the reversible potential. Proton transport on the oxide surface is shown to be activated by strongly held hydrogen-bonded water molecules: in the absence of H₂O the activation energy is calculated to be 4.98 eV and the water molecule reduces the activation energy to 0.25 eV. Substitutional Y³⁺ (for Zr⁴⁺) is shown to slow proton diffusion when present in the zirconia surface.     

Influence of nano-AL2O3-catalyst sizes on hydrogen formation at the water radiolysis under ionizing radiation

The aim of this study was to investigate the regularities of molecular hydrogen formation from water dispersing Al₂O₃ nanoparticles irradiated with gamma ray. It was established that formed molecular hydrogen’s yield changed depending on the size of the catalyst, so that yield of molecular hydrogen formed on the surface with small size is 1.4–1.6 times greater than the one with big size. Equal distribution of nanocatalyst in water medium and much more adsorption of water molecule on the catalyst surface result in more efficient radiolysis process.     

Influence of methanol impurity in hydrogen on PEMFC performance

Proton exchange membrane fuel cells [PEMFC] have become highly attractive for stationary as well as mobile energy applications due to their good efficiency compact cell design and zero emissions. PEM fuel cells mainly consist of anode and cathode containing platinum/platinum alloy electrocatalysts and Nafion membrane as the electrolyte. They operate on hydrogen fuel, which is generally produced by reforming of hydrocarbons, alcohols such as methanol and may contain large amounts of impurities such as methanol, carbon dioxide, trace amounts of carbon monoxide, etc. The studies on the effect of methanol impurity in hydrogen on fuel cell performance and methods of mitigation of poisoning are very important for the commercialization of fuel cells and are described in a limited number of papers only. In this paper, we present the studies on the influence of methanol impurity in hydrogen for the PEM fuel cells. The effect of various parameters such as methanol concentration, cell voltage, current density, exposure time, reversibility, operating temperature, etc. on the cell performances was investigated using pure hydrogen. Various methods of methanol poisoning mitigation were also investigated.