Dissociative Adsorption Of Hydrogen Research Articles

Surface energy reduction by dissociative hydrogen adsorption on inner surface of pore in aluminum

Surface energy reduction by dissociative hydrogen adsorption on inner surface of pore in aluminum

Macroscopic graphite felt containing palladium catalyst for liquid-phase hydrogenation of cinnamaldehyde

Developing of both effective and stable noble metal nanoparticle (NPs) catalysts with easy catalyst-product recovery is still challenging in the liquid-phase catalytic processes. Here, we report on the synthesis of a hierarchical structured catalyst that consisted of oxygen functionalized graphite felt (OGF) support for liquid-phase processes. The monolith palladium-based catalyst was used as catalytic stirrer and displays excellent stability as well as complete recyclability for liquid-phase hydrogenation of α, β-unsaturated cinnamaldehyde. The surface defects decorated with abundant oxygenated groups as well as highly accessible porous structure generated from the acid treatment of carbon support, construct a bridge between Pd and support providing the charge transfer to alter the metal-support interactions. The electron-deficient high-valent Pdδδ+ species, formed on the metal NPs, and defects on the support help to enhance the Pd dispersion and resistance to sintering and/or aggregation during both catalyst preparation and cycling tests, leading to the high and stable hydrogen dissociative adsorption for hydrogenation process.

Thermodynamic Aspects in Non-Ideal Metal Membranes for Hydrogen Purification.

In this paper, an overview on thermodynamic aspects related to hydrogen-metal systems in non-ideal conditions is provided, aiming at systematically merging and analyzing information achieved from several different studies present in the open literature. In particular, the relationships among inner morphology, dissolved hydrogen and internal stresses are discussed in detail, putting in evidence the conformation complexity and the various types of dislocations induced by the presence of H-atoms in the lattice. Specifically, it is highlighted that the octahedral sites are preferentially occupied in the FCC metals (such as palladium), whereas tetrahedral sites are more energetically favored in BCC-structured ones (such as vanadium). These characteristics are shown to lead to a different macroscopic behavior of the two classes of metals, especially in terms of solubility and mechanical failure due to the consequent induced stresses. Furthermore, starting from the expression of the chemical potential generally presented in the literature, a new convenient expression of the activity of the H-atoms dissolved into the metal lattice as a function of the H-concentration is achieved. Such an activity expression is then used in the dissolution equilibrium relationship, which is shown to be the overall result of two different phenomena: (i) dissociative adsorption of molecular hydrogen onto the surface; and (ii) atomic hydrogen dissolution from the surface to the metal bulk. In this way, the obtained expression for equilibrium allows a method to calculate the equilibrium composition in non-ideal conditions (high pressure), which are of interest for real industrial applications.

DFT study of Rh and Ti dimers decorating N-doped pyridinic and pyrrolic graphene for molecular and dissociative hydrogen adsorption

A theoretical study using density functional theory was employed to analyze the hydrogen adsorption on Rh2 and Ti2 dimers decorating pyridine and pyrrolic-like nitrogen doped graphene. First, an analysis of geometry, stability, projected density of states, overlap population and charge distribution was performed to understand the interaction between Rh and Ti metal adatoms and dimers with pyridinic and pyrrolic graphene. Charge transfer occurs from metal to substrate in all cases. An investigation of H2 adsorption on Rh and Ti metals dimers on pyridinic and pyrrolic N-doped graphene was also performed. Molecular adsorption states were observed for Ti on pyridinic graphene and for Rh on both substrates. H2 dissociative adsorption occurs on both metal supported dimers. H2s states hybridize with the Rh and Ti d band, forming localized and dispersed states in concordance with the mechanisms observed for H2 adsorption. The activation energies were calculated obtaining values smaller than 0.59 eV, indicating that the dissociation of H2 can occur spontaneously at room temperature. HSE06 hybrid functional was used to test the accuracy of PBE-D2 functional in the activation energy calculation. Adsorption in the molecular states occurs with no barriers.

Carbon nitride with encapsulated nickel for semi-hydrogenation of acetylene: pyridinic nitrogen is responsible for hydrogen dissociative adsorption

A new mechanism of catalyst has been demonstrated in this article. With the interaction between carbon nitride (CN) and encapsulated nickel, the CN in the catalyst has been endowed with new active sites for the adsorption and activation of hydrogen while nickel itself is physically isolated from the contact with reactive molecules. For the selective hydrogenation of acetylene in large amount of ethylene, the catalyst shows excellent ethylene selectivity than the nickel catalyst itself, which is almost totally unselective. Meanwhile, the CN itself is inactive for the reaction. The results of characterization demonstrate that pyridinic nitrogen doped in the carbon matrix should be the active sites for hydrogen dissociative adsorption. The theoretical calculations further confirm the results and provide with the detail in the electron transfer between nickel and CN in the catalyst. The current results supply a new concept for design of high performance catalyst.

Isotopic exchange between hydrogen from the gas phase and proton-conducting oxides: Theory and experiment

The interaction of hydrogen isotopes from the gas phase with proton-conducting oxides with the perovskite structure La1−x SrxScO3−α (x = 0; 0.04) was studied by means of the hydrogen isotope exchange with gas phase equilibration in the temperature range T = 300–800 °C and in hydrogen pressure range pH2 = 2–20 mbar.A novel kinetic model for the hydrogen isotope exchange experimental data treatment taking into account the isotopic effects was developed. This model was implemented for the obtained experimental results. The heterogeneous exchange rates of hydrogen isotopes with investigated oxides La1−xSrxScO3−α (x = 0; 0.04) were calculated. The mole fractions of hydrogen isotopes were determined for the investigated materials. It was found that deuterium saturation level is higher in comparison with protium, whereas the deuterium surface exchange coefficient for the proton-conducting oxide La0.96Sr0.04ScO3–α is smaller in comparison with the protium surface exchange coefficient. The thermodynamic isotope effect can be caused by the difference of energy of zero-point oscillations between OH- and OD-defects and molecular H2 and D2. The kinetic isotope effect can be explained by the different strength of OH and OD bonds. It is shown that the rate determining stage of hydrogen surface exchange is the process of the exchange between the forms of hydrogen in the gas phase and in the adsorption layer of the proton-conducting oxides (the stage of dissociative adsorption of hydrogen). A new statistical criterion is proposed for the first time allowing dividing the observed surface inhomogeneities caused by not only the natural surface roughness but also the presence of different isotopes of hydrogen (protium and deuterium) with different binding energies on a solid surface.The activity of the investigated proton-conducting oxides with respect to the hydrogen heterogeneous exchange is comparable to the hydrogen heterogeneous exchange activity for the oxides based on cerates and zirconates of alkaline earth metals. High catalytic activity with respect to the process of hydrogen exchange from the gas phase in reducing atmospheres allows us to consider proton-conducting oxides based on the lanthanum scandates as very promising electrolytes for numerous electrochemical applications.

Copper-cobalt catalyzed liquid phase hydrogenation of furfural to 2-methylfuran: An optimization, kinetics and reaction mechanism study

In the present work, the hydro-conversion of biomass derived furfural (FAL) into fuel additive 2-methylfuran (2-MF) is studied over Cu–Co/Al2O3 catalyst. The influence of various operating parameters such as temperature, pressure, catalyst amount, time and FAL concentration on the conversion of FAL to 2-MF was optimized using well known Taguchi method as statistical tool. According to Taguchi method, under optimum reaction conditions viz. temperature 220°C, pressure 40bar, reaction time 5h, catalyst loading 0.75g, and FAL concentration of 1.75M, maximum 2-MF yield (87%) was obtained. The detailed kinetics of the liquid-phase hydrogenation of FAL to 2-MF in two steps was also studied in the range of temperatures from 200 to 220°C and in the range of pressures from 20 to 40bar. The initial rate of reaction for both conversion of FAL to FOL and FOL to 2-MF varied linearly with hydrogen pressure at various temperature and the catalyst loading, however, effect of reactant substrate behave distinctly. In case of FAL, rate of reaction varied linearly and order of reaction is found to be almost one, whereas, for FOL disappearance, order of reaction found to be almost zero beyond 2.25gmol/L of FOL concentration. The experimental data could also be explained using Langmuir–Hinshelwood kinetics. A dual-site mechanism with dissociative adsorption of hydrogen and surface reaction as the rate-controlling step provided the best fit for the experimental data.

Dissociative Adsorption, Dissolution, and Diffusion of Hydrogen in Liquid Metal Membranes. A Phenomenological Model

There is only limited experimental data and theoretical treatment available in the literature on hydrogen sorption and diffusion in liquid metals, in stark contrast with that in solid metals. This paper utilizes our predictive phenomenological model requiring minimal input, the Pauling Bond Valence-Modified Morse Potential (PBV-MMP) model, for estimating the thermodynamic and kinetic parameters of hydrogen solution and diffusion in liquid metals treated as quasi-crystalline. The PBV-MMP model is a refinement of the Unity Bond Index-Quadratic Exponential Potential (UBI-QEP) model for estimating the energetics of solid metal surface reactions. The sequential kinetic steps of hydrogen dissociative surface adsorption on the feed side, subsurface penetration, and atomic interstitial diffusion in the bulk, followed by these steps in reverse on the permeate side, are thus treated via the PBV-MMP model within Eyring’s transition-state theory framework, while the entropic changes are evaluated via Eyring’s free vo...

Anomalous Dependence of the Reactivity on the Presence of Steps: Dissociation of D2 on Cu(211).

Stepped metal surfaces are usually assumed to exhibit an increased catalytic activity for bond cleavage of small molecules over their flat single-crystal counterparts. We present experimental and theoretical data on the dissociative adsorption of molecular hydrogen on copper that contradicts this notion. We observe hydrogen molecules to be more reactive on the flat Cu(111) than on the stepped Cu(211) surface. We suggest that this exceptional behavior is due to a geometric effect, that is, that bond cleavage on the flat surface does not occur preferentially over a top site.

Dissociative Adsorption of Molecular Hydrogen on BN-Doped Graphene-Supported Aluminum Clusters

The present work, demonstrates dissociative adsorption of molecular hydrogen on supported and unsupported aluminium clusters (Aln, n=4 - 8, 13) using Density Functional Theory based calculations. The studies reveal that the presence of a BN doped graphene surface support reduces the dissociative adsorption barrier of the bond in molecular hydrogen on even atom clusters. In particular, supported Al6 demonstrates a barrier-less dissociative adsorption towards the H2 molecule. These results demonstrate the excellent potential of supported Al nanoparticles for hydrogen storage and also the potential of doped graphene systems are catalyzing supports.

Role of Charge Transfer in Catalytic Hydrogen Oxidation over Platinum

The chemicurrent is a flux of energetic charge carriers, which may be observed experimentally within thin metal films in response to an exothermic surface catalytic chemical reaction. This article suggests a picture behind the chemicurrent in hydrogen oxidation in a high temperature regime over a platinum surface. The picture is intrinsic to the reaction mechanism. Surface reaction intermediates, “O–”, “H–”, “OH–”, are negatively charged while the product, H2O, and reactants, H2 and O2, are neutral. Hence, charge transfers between the metal and reacting species are inevitable. After electrons are transferred from the metal to the interface in dissociative oxygen and hydrogen adsorption, translationally excited electrons in the metal arise in part as a result of decay of negatively charged transition states in reaction steps connecting surface species of different charges, “O– + H–” ⇄ “e + OH–” and “OH– + H–” ⇄ “2e + H2O”. These transition states are the lowest energy configurations possible for changing t...

Synthesis of methanol from CO2 hydrogenation promoted by dissociative adsorption of hydrogen on a Ga3Ni5(221) surface.

Catalytic carbon dioxide (CO2) hydrogenation to liquid fuels including methanol (CH3OH) has attracted great attention in recent years. In this work, density functional theory (DFT) calculations have been employed to study the reaction mechanisms of CO2 hydrogenation to CH3OH on Ga3Ni5(221) surfaces. The results show that all intermediates except for the O atom prefer to adsorb on Ni sites, and dissociative adsorption of hydrogen (H2) on the Ga3Ni5(221) surface is almost barrierless and highly exothermic, favoring CO2 hydrogenation. Moreover, the presence of Ga indeed enhances the dissociative adsorption of H2, and this is verified by the projected density of states (PDOS) analysis. Importantly, three possible reaction pathways based on formate (HCOO) and hydrocarboxyl (COOH) formations and reverse water gas shift (rWGS) with carbon monoxide (CO) hydrogenation have been discussed. It is found that CO2 reduction to CH3OH in these pathways prefers to occur entirely via the Langmuir-Hinshelwood (L-H) mechanism. COOH generation is the most favorable pathway because the HCOO and rWGS with CO hydrogenation pathways have high energy barriers and the resulting HCOOH intermediate in the HCOO pathway is unstable. In the COOH reaction pathway, CO2 is firstly hydrogenated to trans-COOH, followed by the formation of COH via three isomers of COHOH, its hydrogenation to trans-HCOH, and then the production of CH3OH via a CH2OH intermediate.

New Insight into Hydrogen Oxidation Reaction on La0.3Sr0.7Fe0.7Cr0.3O3-δ Perovskite as a Solid Oxide Fuel Cell Anode

Hydrogen oxidation reaction (HOR) mechanisms on a novel perovskite anode with a nominal composition of La0.3Sr0.7Fe0.7Cr0.3O3-δ (LSFCr-3) were systematically investigated for solid oxide fuel cell (SOFC) applications. In terms of electrochemical impedance spectroscopy study of LSFCr-3-based half cells in various applied DC bias, hydrogen (pH2) and water vapor partial pressure (pH2O), the HOR on the LSFCr-3 perovskite was suggested to be rate-limited by the dissociative adsorption of hydrogen. At constant pH2O and pH2, the concentration of oxygen vacancies on the LSFCr-3 surface is much higher than that in the bulk. Importantly, a positive impact of water vapor on the process of dissociative adsorption of hydrogen was demonstrated for the perovskite anodes. This finding is explained by the hydration of surface oxygen vacancies, that formed more active oxygen anions sites. Under humid reducing atmosphere, the exact composition of the bulk of the LSFCr-3 is estimated to be La0.34Sr0.67Fe3 +0.7Cr3 +0.29O2.65 at 800°C. On the surface of the LSFCr-3 anode, the average oxidation state of B-site cations (Fe and Cr) is found to be lower than that in the bulk, resulting in a presence of Cr3+, Fe2+ and Fe3+. Cr3+ on the surface contributes to holding more active oxygen anions for the dissociative adsorption of hydrogen, due to its backbone action in the perovskite lattice structure. This effect of Cr3+ is suggested to be of great benefit to the electrochemical performance of the LSFCr-3 perovskite anode.

Excess charge driven dissociative hydrogen adsorption on Ti2O4.

The mechanism of dissociative D2 adsorption on Ti2O4-, which serves as a model for an oxygen vacancy on a titania surface, is studied using infrared photodissociation spectroscopy in combination with density functional theory calculations and a recently developed single-component artificial force induced reaction method. Ti2O4- readily reacts with D2 under multiple collision conditions in a gas-filled ion trap held at 16 K forming a global minimum-energy structure (DO-Ti-(O)2-Ti(D)-O)-. The highly exergonic reaction proceeds quasi barrier-free via several intermediate species, involving heterolytic D2-bond cleavage followed by D-atom migration. We show that, compared to neutral Ti2O4, the excess negative charge in Ti2O4- is responsible for the substantial lowering of the D2 dissociation barrier, but does not affect the molecular D2 adsorption energy in the initial physisorption step.

ISOTOPIC EXCHANGE BETWEEN HYDROGEN FROM THE GAS PHASE AND PROTON-CONDUCTING OXIDES: THEORY AND EXPERIMENT

The interaction of gaseous hydrogen isotopes from the gas phase with proton-conducting oxides with the perovskite structure La 1−x Sr x ScO 3− α (x = 0; 0.04) was studied by means of the hydrogen isotope exchange with gas phase equilibration in the temperature range T = 300−800 ºC and in hydrogen pressure range pH 2 = 2−20 mbar. A novel kinetic model for the hydrogen isotope exchange experimental data treatment taking into account the isotopic effects was developed. This model was implemented for the obtained experimental results. The heterogeneous exchange rates of hydrogen isotopes with investigated oxides La 1−x Sr x ScO 3− α (x = 0; 0.04) were calculated. The mole fractions of hydrogen isotopes were determined for the investigated materials. It was found that deuterium uptake is higher in comparison with protium, whereas the deuterium surface exchange coefficient for the proton-conducting oxide La 0.96 Sr 0.04 ScO 3– α is smaller in comparison with the protium surface exchange coefficient. The thermodynamic isotope effect can be caused by the difference of energy of zero-point oscillations between OH- and OD-defects and molecular H 2 and D 2 . The kinetic isotope effect can be explained by the different strength of OH and OD bonds. The rate determining stage of hydrogen exchange is shown to be the process of exchange between the forms of hydrogen in the gas phase and in the adsorption layer of the proton-conducting oxide (the stage of dissociative hydrogen adsorption). For the first time, a new statistical criterion is proposed that allows dividing the observed surface inhomogeneities caused by not only the natural surface roughness but also the presence of different isotopes of hydrogen (protium and deuterium) with different binding energies on a solid surface. The activity of the investigated proton-conducting oxides with respect to the hydrogen heterogeneous exchange is comparable to the heterogeneous hydrogen exchange activity for oxides based on cerates and zirconates of alkaline earth metals. High catalytic activity with respect to the process of hydrogen exchange from the gas phase in reducing atmospheres allows us to consider the proton-conducting oxides based on the lanthanum scandates as the very promising electrolytes for numerous electrochemical applications.

Manipulation of electrical properties in CVD-grown twisted bilayer graphene induced by dissociative hydrogen adsorption

We report hydrogen adsorption on twisted bilayer graphene (tBLG). Raman spectroscopy and the electrical transport properties (electrical resistance and thermoelectric power) confirm the electron doping by hydrogen adsorption, in agreement with the previous report involving exfoliated bilayer graphene (BLG). Common electron doping behaviors were observed at various twist angles (0°, 5°, 12.5°, and 30°), and the adsorptions follow the first-order Langmuir-type adsorption model. Specifically, we analyzed the off-state currents, with band-gap openings of around 13 meV in tBLG with twist angle of 0°, as in Bernal-stacked BLG.

MODELING STYRENE HYDROGENATION KINETICS USING PALLADIUM CATALYSTS

Abstract - The high octane number of pyrolysis gasoline (PYGAS) explains its insertion in the gasoline pool. However, its use is troublesome due to the presence of gum-forming chemicals which, in turn, can be removed via hydrogenation. The use of Langmuir-Hinshelwood kinetic models was evaluated for hydrogenation of styrene, a typical gum monomer, using Pd/9%Nb 2 O 5 -Al 2 O 3 as catalyst. Kinetic models accounting for hydrogen dissociative and non-dissociative adsorption were considered. The availability of one or two kinds of catalytic sites was analyzed. Experiments were carried out in a semi-batch reactor at constant temperature and pressure in the absence of transport limitations. The conditions used in each experiment varied between 16 – 56 bar and 60 – 100 oC for pressure and temperature, respectively. The kinetic models were evaluated using MATLAB and EMSO software. Models using adsorption of hydrogen and organic molecules on the same type of site fitted the data best. Keywords

Density-functional-based tight-binding parameterization of Mo, C, H, O and Si for studying hydrogenation reactions on molybdenum carbide

Hydrogenation reactions catalyzed by transition-metal-containing nanoparticles represent an important type of reaction in chemical industry. However, the modeling of these reactions in their working conditions requires much longer simulation times than what could usually be achieved with ab initio or first-principle methods. To address this problem, in this work, the density-functional-based tight-binding (DFTB) method was parameterized for hydrogenation reactions on molybdenum carbide catalysts, involving the elements C, H, Mo, O and Si. The overall quality of the DFTB parameters was tested with band structure/molecular orbital energies, molecular/crystal structures, chemisorption bond strengths, hydrogen adsorption energies, hydrogenation reaction energies, molecular vibrational frequencies, energy barriers and the structures of the transition states for systems of interest. The parameterized DFTB method gave errors of <1.45 % for bond distances of hydrocarbons and 4.86 % for non-hydrocarbons. It could reproduce the structure and vibrational frequencies (with errors of about 100 cm−1) of selected hydrocarbon–molybdenum carbide complexes obtained from DFT calculations. Good agreement was reached between DFTB and DFT on the dissociative adsorption of hydrogen on the α-Mo2C (0001) surface. For most of the hydrogenation reactions examined, DFTB showed errors of ~2 kcal/mol compared to DFT/PBE, with a few exceptions of ~5 kcal/mol. It could also describe the reaction energies, the forward and reverse energy barriers and the transition-state structures for the benzene hydrogenation reaction on a Mo38C19 cluster.

Density functional theory calculations of hydrogen dissociative adsorption on platinum-involved alloy surfaces

Density functional theory calculations of the H2 molecule over the Pt(111), Pt4Pd5(111), Pt3Ir6(111), and Pt8Ru1(111) surfaces were carried out to derive key properties involving interactions and electronic state of each atom. From the calculations, H2 dissociative adsorption shows the lowest barrier in case of the Pt3Ir6(111) surface. Pt4Pd5(111) and Pt8Ru1(111) surfaces show a small energy barrier, while the Cu(111) surface is the highest energy barrier. The difference in the reactivity of H2 molecule with the surface is pointed out by the differences in the valence electron configuration of approaching hydrogen which is also verified from the density of state curve. The electronic structure plots illustrate the substituted atoms can interact with molecular H2 projected on the surface d-orbital.

Dissociative Adsorption of H2 on PtRu Bimetallic Surfaces

We present a theoretical study of the dissociative adsorption of molecular hydrogen on PtRu bimetallic surfaces based on density functional theory (DFT) calculations. We focus on the reactivity of a pseudomorphic Pt monolayer deposited on Ru(0001), Pt1ML/Ru(0001), for which we have obtained a minimum activation energy barrier for H2 dissociation, Eb = 0.32 eV, i.e., ∼0.3 and 0.26 eV higher than on Ru(0001) and Pt(111), respectively. Accordingly, the initial sticking probability for low energy impinging molecules (Ei ≲ 0.1 eV) derived from classical trajectory calculations is various orders of magnitude smaller than on Ru(0001) in apparent contradiction with available experimental data. However, undercoordinated Pt atoms in the borders of Pt pseudomorphic islands and isolated Ru atoms or small Ru aggregates in a Pt-rich two-dimensional PtxRu1–x surface alloy provide active sites allowing nonactivated H2 dissociative adsorption. These two possible defects in a full pseudomorphic Pt monolayer deposited on Ru...