Function Of Hydrogen Pressure Research Articles

Phosphorus Modification of Iron: Mechanistic Insights into Ammonia Synthesis on Fe2P Catalyst.

Ammonia (NH3) is a critical chemical for fertilizer production and a potential future energy carrier within a sustainable hydrogen economy. The industrial Haber-Bosch process, though effective, operates under harsh conditions due to the high thermodynamic stability of the nitrogen molecule (N2). This motivates the search for alternative catalysts that facilitate ammonia synthesis at milder temperatures and pressures. Theoretical and experimental studies suggest that circumventing the trade-off between N-N activation and subsequent NHx hydrogenation, governed by the Brønsted-Evans-Polanyi (BEP) relationship, is key to achieving this goal. Recent studies indicate metal phosphides as promising catalyst materials. In this work, a comprehensive density functional theory (DFT) study comparing the mechanisms and potential reaction pathways for ammonia synthesis on Fe(110) and Fe2P(001) is presented. The results reveal substantial differences in the adsorption strengths of NHx intermediates, with Fe2P(001) exhibiting weaker binding compared to Fe(110). For N-N bond cleavage, multiple competing pathways become viable on Fe2P(001), including routes involving the pre-hydrogenation of adsorbed N2 (e.g., through *NNH*). Analysis of DFT-derived turnover rates as a function of hydrogen pressure (H2) highlights the increased importance of these hydrogenated intermediates on Fe2P(001) compared to Fe(110) where direct N2 dissociation dominates. These findings suggest that phosphorus incorporation modifies the ammonia synthesis mechanism, offering alternative pathways that may circumvent the limitations of traditional transition metal catalysts. This work provides theoretical insights for the rational design of Fe-based catalysts and motivates further exploration of phosphide-based materials for sustainable ammonia production.

Kinetic Features of Light Cycle Oil Hydrodesulfurization Reactions in the Presence of a Co6–PMO12(S)/Al2O3 Catalyst

The group kinetics of reactions of organic sulfur compounds present in light cycle oil (LCO) in the hydrotreating process over a Co6–PMo12(S)/Al2O3 sulfide catalyst based on phosphomolybdic heteropoly acid as a function of hydrogen pressure (4, 5, and 6 MPa) at varying temperature (300–360°C in 20°C increments) and space velocities of 0.5, 1.0, and 3.0 h–1 has been studied. Total sulfur content in the feedstock and the hydrogenation products has been determined. The obtained data on the kinetics of hydrodesulfurization (HDS) reactions can be used to select optimum modes of the LCO hydrotreating process. Using the coefficient of determination and Fisher’s ratio test values, the adequacy of the pseudo-first-order and 1.5-order kinetic models and the Langmuir–Hinshelwood model has been determined. A kinetic model describing the hydrodesulfurization of LCO by hydrotreating in the presence of the Co6–PMo12(S)/Al2O3 catalyst has been selected, and the kinetic characteristics of the HDS of the compounds constituting LCO have been determined.

Hydrogenolysis/hydrogenation of diphenyl ether as a model decomposition reaction of lignin from biomass in pressurized CO2/water condition

Catalytic hydrogenolysis of the CO bond of diphenyl ether (a lignin model compound) was investigated as a function of hydrogen pressure in scCO2 medium in the presence of water. Using commercially available Rh/C catalyst, the CO bond cleavage of diphenyl ether mainly results phenolic monomer at 80°C. Hydrogen pressure is one of the key parameters because (i) CO bond cleavage and the hydrogenation of aromatic rings are two competitive reactions; very sensitive to hydrogen pressure and (ii) hydrogen has complete solubility in scCO2. Therefore, a critical control of hydrogen pressure was essential to reach the targeted cleavage of the CO bond when the reaction was conducted in scCO2 medium under pressurized condition. Depending on the hydrogen pressure, a significant change in the ratio of monocyclic:bicyclic products from 91:9 (0.2MPa) to 58:42 (2MPa) was revealed in the shortest reaction time of 5min. Thus, low hydrogen pressure was the effective choice for the scission of the CO bond, whereas higher hydrogen pressure hydrogenate the aromatic ring due to the higher coverage of hydrogen on the catalytic surface. Amount of the catalyst (catalyst:substrate ratio) displayed a subtle effect on the breakage of the CO bond. A threshold ratio of 1:5 was preferred under the present reaction condition as the increased amount hampered the substrate:water ratio and hydrogenation of the aromatic ring occurred. In addition, as the change in temperature is associated with the change in the physical properties of scCO2, hence, the effect on the transformation of DPE was complicated and difficult to explain. Furthermore, different organic solvents as neat, along with CO2 and with water also has substantial impact on the rapture of CO bond. The obtained results from the solvent studies again proved that scCO2 along with water was the best choice for CO bond breakage and water is the driving force to mediate the reaction. In addition, a combination catalyst (Ni+Rh) was also tested for the same reaction under the similar working condition. Preliminary results suggested a synergistic effect in terms of the selectivity of monocyclic compounds.

Synthesis of ruthenium hydride

Ruthenium hydride was synthesized at a hydrogen pressure of about 14 GPa in a diamond-anvil cell. Energy-dispersive x-ray diffraction was used to monitor the ruthenium crystal structure as a function of hydrogen pressure up to 30 GPa. The hydride formation was accompanied by phase transition from the original hcp structure of the pristine metal to the fcc structure. Our results confirmed the theoretical prediction of ruthenium hydride formation under hydrogen pressure. The standard Gibbs free energy of the ruthenium hydride formation reaction was calculated assuming the pressure of decomposition as the equilibrium pressure.

Phase Equilibria, Crystal Structure and Hydriding/Dehydriding Mechanism of Nd4Mg80Ni8 Compound.

In order to find out the optimal composition of novel Nd-Mg-Ni alloys for hydrogen storage, the isothermal section of Nd-Mg-Ni system at 400 °C is established by examining the equilibrated alloys. A new ternary compound Nd4Mg80Ni8 is discovered in the Mg-rich corner. It has the crystal structure of space group I41/amd with lattice parameters of a = b = 11.2743(1) Å and c = 15.9170(2) Å, characterized by the synchrotron powder X-ray diffraction (SR-PXRD). High-resolution transmission electron microscopy (HR-TEM) is used to investigate the microstructure of Nd4Mg80Ni8 and its hydrogen-induced microstructure evolution. The hydrogenation leads to Nd4Mg80Ni8 decomposing into NdH2.61-MgH2-Mg2NiH0.3 nanocomposites, where the high density phase boundaries provide a great deal of hydrogen atoms diffusion channels and nucleation sites of hydrides, which greatly enhances the hydriding/dehydriding (H/D) properties. The Nd4Mg80Ni8 exhibits a good cycle ability. The kinetic mechanisms of H/D reactions are studied by Real Physical Picture (RPP) model. The rate controlling steps are diffusion for hydriding reaction in the temperature range of 100 ~ 350 °C and surface penetration for dehydriding reaction at 291 ~ 347 °C. In-situ SR-PXRD results reveal the phase transformations of Mg to MgH2 and Mg2Ni to Mg2NiH4 as functions of hydrogen pressure and hydriding time.

Investigation of hydrogasification of low-rank coals to produce methane and light aromatics in a fixed-bed reactor

The hydrogasification of three low rank coals (YX lignite, XD lignite and ZD subbituminous coal) was carried out in a fixed-bed reactor to investigate the effects of hydrogen pressure on the formations of gaseous, liquid and char products. The reactor was typically heated at a rate of 15 °C/min up to 700 °C with a holding time of 60 min under various hydrogen pressures up to 5 MPa. The yields of BTX (i.e., benzene, toluene, and xylene) and naphthalene increased with increasing hydrogen pressure up to 5 MPa for all three coals, but more significant for XD and ZD coals. The compositional analysis of char revealed that the chars produced by the hydrogasification had higher molar ratios of H/C and lower molar ratios of N/C than those produced by the pyrolysis in N2 atmosphere, whereas the former chars had higher or lower molar ratios of S/C than the latter chars from case to case depending principally on coals. The apparent reaction rate constants of the chars hydrogasification in the isothermal stage as a function of hydrogen pressure were found to follow a power equation of the power exponents varying with coals.

Quantitative determination of hydrogen absorption by Pd cluster-assembled films using a quartz crystal microbalance

We have measured hydrogen absorption capacity for a series of Pd films grown by two deposition techniques: a) flat films made with e-beam evaporation of Pd and b) films made of assembled Pd clusters. The films range in thickness from 9.5 to 45nm. The technique implemented for measuring the amount of hydrogen absorbed by the films was a quartz crystal microbalance. The Pd films were grown on the quartz crystals and the change of mass due to the hydrogen absorption was determined from the shifting of the resonance frequency. Minute amounts of hydrogen absorbed of the order of nanograms per square centimeter can be detected by this technique. Plots of H/Pd atom ratio for each Pd film as a function of hydrogen pressure were obtained. The Pd e-beam grown films displayed a saturation H/Pd atomic ratio around 0.6 which is similar to bulk Pd. The Pd cluster film with a thickness of 14nm displayed a remarkable absorption capacity with a ratio HPd≈0.84.

Enhanced activity and selectivity in n-octane isomerization using a bifunctional SCILL catalyst

Bifunctional Solid Catalyst with Ionic Liquid Layer (SCILL) systems are presented and applied in the skeleton isomerization of n-octane in a slurry-phase reaction mode. It is demonstrated that Pt on silica, coated with a thin film of acidic chloroaluminate ionic liquid exhibits remarkable catalytic performance under mild conditions in the presence of hydrogen. Interestingly, both selectivity and activity of n-octane isomerization increase in these systems as a function of hydrogen pressure. This does not only suggest a hydrogenation activity of the catalytic Pt-centers embedded in the strongly Lewis acidic ionic liquid but also a significant increase in the proton acidity in this system as a function of the hydrogen pressure.

Modeling hydrogen attack effect on creep fracture toughness

The effect of high temperature hydrogen attack on creep crack growth rates in steels is studied by modeling the interaction between creep deformation and gaseous pressures generated by hydrogen and methane. The equilibrium methane pressure as a function of hydrogen pressure, temperature and carbide types for carbon steels and Cr–Mo steels is calculated. This gaseous driving force is incorporated into a micromechanics model for void growth along grain boundaries of a creeping solid. Growth and coalescence of voids along grain boundaries is modeled by a microporous strip of cell elements, referred to as the fracture process zone. The cell elements are governed by a nonlinear viscous constitutive relation for a voided material. Two rate sensitivities as well as two types of grain boundaries are considered in this computational study. Simulations of creep crack growth accelerated by gaseous pressures are performed under conditions of small-scale and extensive creep. The computed crack growth rates at elevated temperatures are able to reproduce the trends of experimental results.

Neutron Scattering Methodology for Absolute Measurement of Room-Temperature Hydrogen Storage Capacity and Evidence for Spillover Effect in a Pt-Doped Activated Carbon

A neutron scattering methodology is proposed to simultaneously determine the total hydrogen adsorption, the excess hydrogen adsorption, and hydrogen gas confined in the porous sample. This method is capable of an absolute measurement of the hydrogen content without need for any calibration. It involves the least amount of corrections and is not likely to be affected by the instrumental factors compared to the traditional gravimetric and volumetric methods. We used this method to study the physisorption behavior at room-temperature (RT) of a Pt-doped activated carbon sample as a function of hydrogen pressure. This method will become a simple and important tool for solving various problems arising from the traditional measurements of RT hydrogen storage capacities. It can be combined with an in situ small-angle neutron scattering to study the hydrogen spillover effect in the kinetic adsorption process. Storage capacity and spatial distribution of the hydrogen adsorbed due to spillover are concurrently revealed.

Measurement of steady-state hydrogen electrode reactions on Alloys 600 and 690 tubes

The steady-state polarization measurements for HER and HOR were carried out in order to obtain the effects of hydrogen pressure, solution pH, and temperature on the current densities of Alloys 600 and 690, respectively. Optimization was performed to obtain the electrokinetic parameters of HER and HOR on Alloys 600 and 690 such as forward and reverse transfer coefficients and equilibrium corrosion densities. From the optimization process, the activation energies, E ac, of both hydrogen reactions on the surfaces of the Alloys 600 and 690 tubes were obtained as 30.5 kJ/mole on the surface of Alloy 600 tube and 35.6 kJ/mole on Alloy 690 tube. Furthermore, the equilibrium exchange current densities of hydrogen electrode reaction, i 0(H 2), on the surface of the Alloys 600 and 690, respectively, were proposed as functions of hydrogen pressure, solution pH, and temperatures.

The very initial stage of hydride formation on polycrystalline gadolinium

The initial development of hydrides on hydrogen-exposed metallic surfaces displays a complex behavior with two possible surface-associated stages [M.H. Mintz, in: K.H.J Buschow, R.W. Cahn, M.C. Flemings, B. Ilschner, K.J. Kramer, S. Mahajan (Eds.), Encyclopedia of Materials: Science and Technology, Elsevier Sci. 2002, pp 1–9; Y. Ben-Eliyahu, M. Brill, M.H. Mintz, J. Chem. Phys. 111 (1999) 6053]: (i) Rapid formation of a dense pattern of small (sub-micrometric) hydride spots, which almost instantaneously attain their near final size and practically cease any further development (resulting in a type of “induction” or “incubation” period). (ii) Development of a few “growth centers” (GC's) of hydride “patches”, which spread over the surface and lead to the massive hydriding stage. In the present study, the features of the very initial stage (i.e., stage (i)) are characterized for the H 2/Gd (polycrystalline) system, using Hot Stage Microscopy (HSM), X-ray diffraction analysis and Atomic Force Microscopy (AFM) techniques. The main characteristics are: (1) Typical hydride nucleation patterns composed of dense overlapping “spots” aligned along certain orientations on given metallic grains. (2) Very rapid initial development rates (within seconds), followed by an exponential decay of the precipitate growth rates. (3) A strong dependence of reactivity on the orientation of the metallic grains, with some grains displaying a very high reactivity (i.e., formation of very dense nucleation patterns) and other grains displaying a long-term inertness. (4) A time delay (“induction” period) between stage (i) and the flowing stage (ii) (i.e., GC's formation), which predominantly depends on the hydriding pressure and temperature. The above features and their related kinetics were studied as a function of hydrogen pressure (in the range 6 × 10 2 to 2 × 10 4 Pa) at 373 K. Possible mechanisms controlling these reactions are discussed.

Opto-mechanical characterization of hydrogen storage properties of Mg–Ni thin film composition spreads

Thin film composition spreads of Mg–Ni were deposited by co-sputtering on micromachined Si-cantilevers. The investigated compositions range from about Mg 60Ni 40 to about Mg 80Ni 20. Structural properties as well as mechanical stress before and after hydrogenation were measured with X-ray diffraction (XRD) and laser profilometry, respectively. The composition spreads were hydrogenated in a special pressure vessel, which allows measuring optically the hydrogen-induced deflection (stress-change) of 16 cantilevers as a function of hydrogen pressure and/or temperature. It was found that the hydrogen-induced stress is correlated with the composition and microstructure of the films. Highest hydrogen-induced stress changes were found for compositions close to the crystalline Mg 2Ni phase.

Design and operation of a high pressure reaction cell for in situ X-ray absorption spectroscopy

X-ray absorption spectroscopy measurements of catalytic reactions have been instrumental in advancing the understanding of catalytic processes. These measurements require an in situ catalysis reaction cell with unique properties. Here we describe the design and initial operation of an in situ/operando catalysis reaction cell for transmission X-ray absorption spectroscopy measurements. The cell is designed: to be an ideal catalytic reactor with no mass transfer effects; to give the same conversion and selectivity under similar space velocities as standard laboratory micro-reactors; to be operational temperatures up to 600 °C and pressures up to 14 bar; to be X-ray transparent allowing XAS measurement to be collected in transmission for all elements with Z ≥ 23 (vanadium K-edge at 5.5 keV); to measure the actual catalyst bed temperature; to not use o-ring seals, or water cooling; to be robust, compact, easy to assemble, and use, and relatively low cost to produce. The heart of the cell is fabricated from an X-ray transparent beryllium tube that forms a plug flow reactor. XAFS data recorded during the reduction of a Re/γ-Al 2O 3 catalyst as a function of hydrogen pressure from 0.05 to 8 bar, and from a Pt–Sn/γ-Al 2O 3 catalyst during n-heptane reforming are given as initial examples of the versatility of the reactor.

Catalytic activity of noble metals promoting hydrogen uptake

The engineering of pure and metal alloy catalysts for hydrogen absorption is needed to improve the kinetics of hydrogen-related devices. We introduce a new route to search for alloys that can yield superior catalytic behavior for hydrogen absorption, using an optical technique to measure the catalytic activity for hydrogen sorption of thin films. The catalytic activity of the noble metals Pd, Ni, Cu, Ag, Pt, and Au is studied as a function of hydrogen pressure and temperature. The rate-limiting step is identified by the pressure dependence and the normal isotope effect for hydrogen absorption. The measured rates and activation energies are correlated to such physical properties as activation barrier for dissociation and heat of solution. The observed compensation effect is explained in the framework of interplay between surface and bulk processes. From the experimentally derived model, a guiding principle for the search for catalysts promoting hydrogen absorption is drawn.

Thermodynamics of Fe—Sm, Fe—H, and H—Sm Systems and Its Application to the Hydrogen—Disproportionation—Desorption—Recombination (HDDR) Process for the System Fe17Sm2—H2.

Abstract Thermodynamic properties of the Fe–Sm, Fe–H, and H–Sm systems have been analyzed by means of the CALPHAD method. Thermodynamic models have been defined to describe the Gibbs energy of the individual phases and the model parameters have been optimized using different experimental information: phase diagram data, calorimetric data, and equilibrium partial pressures of hydrogen. The entropy of Fe17Sm2 has been derived from experimental data on low-temperature heat capacity. The thermodynamic parameters for Fe17Sm2 have been evaluated using binary (Fe–Sm) and ternary (Fe17Sm2–H2) data simultaneously. The calculated phase diagrams and thermodynamic properties are in agreement with experiments. The thermodynamics of hydrogen in the interstitial solid solution Fe17Sm2Hδ have been described by compound energy formalism. Combining the obtained thermodynamic descriptions of the individual phases, the equilibrium temperatures of the recombination reaction: Fe+SmH2±x⇒Fe17Sm2+H2, which is an important part of the HDDR process, have been calculated as a function of hydrogen pressure.

Reactivity of the hydrocarbon C–C bonds as a function of the reaction conditions in the conversion of C 6 alkanes and methylcyclopentane over Rh catalysts

The conversion of 3-methylpentane (3MP), 2-methylpentane (2MP), n-hexane ( nH), and methylcyclopentane (MCP) was investigated on 0.3 and 10% Rh/Al 2O 3 and 5% Rh/SiO 2 as a function of hydrogen pressure and temperature. The catalysts were prepared by the incipient wetness method. Metal accessibility was 57, 18 and 36%, which corresponded to mean particle size of 1.5, 5 and 2.5 nm, respectively. The hydrogenolytic cleavage of hydrocarbon C–C bonds was the main reaction. Skeletal isomers (up to 20%) were formed from methylpentanes on 10% Rh/Al 2O 3. C 5-cyclization was a minor reaction (less than 10%) and was promoted by low p(H 2). Particle size effect was clearly observed in the non-degradative reaction path; this route was favored by larger Rh particles. Single splitting of C–C bonds was catalyzed at high hydrogen coverage. Decreasing p(H 2) caused “deepening” of the hydrogenolysis and the catalysts lost much of their activity. The hindrance in the re-hydrogenation of the surface intermediate of fragmentation was proposed to explain the positive hydrogen order. The role of further hydrogenolysis of particular fragments or ring opening intermediates was significant at low p(H 2). Reaction conditions governed the desorption or the further reactions of the surface intermediates. For instance, fragments were produced in the conversion of MCP mainly from branched ring opening intermediates. The fragmentation patterns of hexane isomers were successfully applied for modeling the fragment distribution of MCP.

Thermodynamics of Fe–Sm, Fe–H, and H–Sm systems and its application to the hydrogen–disproportionation–desorption–recombination (HDDR) process for the system Fe 17Sm 2–H 2

Thermodynamic properties of the Fe–Sm, Fe–H, and H–Sm systems have been analyzed by means of the CALPHAD method. Thermodynamic models have been defined to describe the Gibbs energy of the individual phases and the model parameters have been optimized using different experimental information: phase diagram data, calorimetric data, and equilibrium partial pressures of hydrogen. The entropy of Fe 17Sm 2 has been derived from experimental data on low-temperature heat capacity. The thermodynamic parameters for Fe 17Sm 2 have been evaluated using binary (Fe–Sm) and ternary (Fe 17Sm 2–H 2) data simultaneously. The calculated phase diagrams and thermodynamic properties are in agreement with experiments. The thermodynamics of hydrogen in the interstitial solid solution Fe 17Sm 2H δ have been described by compound energy formalism. Combining the obtained thermodynamic descriptions of the individual phases, the equilibrium temperatures of the recombination reaction: Fe+SmH 2± x ⇒Fe 17Sm 2+H 2, which is an important part of the HDDR process, have been calculated as a function of hydrogen pressure.

The water formation rate on platinum and palladium as a function of the surface hydrogen pressure from three-dimensional hydrogen pressure distributions

The water formation rate on platinum and palladium as a function of the reactant pressures at the catalytic surface has been determined from measurements of the three-dimensional hydrogen pressure distributions over the catalytic surface. Measurements were made for gas mixtures containing less than 1% of hydrogen and 20% of oxygen in argon at atmospheric pressure. The experiments were performed in a reactor with a simple and well-defined geometry which makes it possible to calculate the hydrogen pressure distribution in the entire region where measurements are made. The water formation rate as a function of the hydrogen pressure at the catalytic surface is obtained by comparing calculated hydrogen pressure distributions to measured ones. It is concluded that three-dimensional reactant pressure distributions can be used to accurately determine the reaction rate on a catalytic surface as a function of the reactant pressures at the surface. This is valuable for studies in the viscous pressure regime, where mass transfer limitations may be difficult to avoid.

On the effect of hydrogen on the palladium-catalyzed formation of benzene from acetylene

An infrared spectrum of a Pd(111) surface collected in the presence of 5 Torr of acetylene as a function of hydrogen pressure reveals that the ethylidyne coverage increases with hydrogen pressure (P(H2) between zero and 20 Torr). The amount of CO that can be accommodated onto the surface at a pressure of 5 Torr, measured after evacuating the acetylene and hydrogen, increases linearly with hydrogen pressure, and this effect is ascribed to the presence of a more open surface produced by the formation of ethylidyne. It is found that acetylene adsorbs in ultrahigh vacuum on ethylidyne-covered Pd(111) and reacts to form benzene, where the benzene desorbs at ∼280 K. This effect is mirrored in the catalytic chemistry where the rate of benzene formation from acetylene in the presence of hydrogen increases linearly with hydrogen pressure.