Chemisorbed Atoms Research Articles

Mechanism of chromium poisoning on La0.5Sr0.5Co0.25Fe0.75O3 cathode and enhancing chromium tolerance using a novel anion doping strategy

Chromium poisoning the La1-xSrxCoyFe1-yO3(LSCF) cathode for solid oxide fuel cells is a severe issue that can lead to its electrochemical performance degradation. To clarify poisoning mechanism of chromium species and develop a novel anion doping strategy with enhancing chromium tolerance, the adsorption behavior of Cr species on LSCF and F-doping LSCF surface are explored by density functional theory (DFT) calculations. Our results indicate that CrO3 molecule prefers to be adsorbed on the La(Sr)O-terminated surface rather that the Co(Fe)O2-terminated LSCF(001) surface. The adsorption energies of CrO3 molecule are much larger than that of O2 molecules, implies that the CrO3 is more favorable to absorb on the LSCF surface. Bader charge analysis shows the CrO3 molecule behaves as an electron acceptor through an adsorption process. The adsorption of Cr atom is a weak physical adsorption for Sr site and La site and the Cr atom chemisorption is found to occur on hollow site and O site. In addition, F-doping efficiently improves chromium tolerance by weakening the adsorption strength of chromium species that poisons LSCF surface and this may be a feasible strategy to develop cathode for high performance solid oxide fuel cells.

HYDROGEN DEFECTS ON THE SURFACE OF LEAD-FREE FERROELECTRIC Na\(_{0.5}\)Bi\(_{0.5}\)TiO\(_3\) MATERIALS

Hydrogen atomic chemisorption and adsorption at all possible vacancy sites on the surface of perovskite Na0.5Bi0.5TiO3 (110) were explored using calculations from density functional theory. Our calculations reveal that a pristine Na0.5Bi0.5TiO3 (110) surface can exhibit direct and indirect transitions characterized by optical bandgaps of 2.68 eV and 2.75 eV, respectively. The density of states of the Na0.5Bi0.5TiO3 (110) surface indicates predominant electron transitions from O-2p valence bands to Ti-3d and Bi-6p conduction bands in the low-energy range. Hydrogen substitution at vacancies can induce magnetism, whereas hydrogen adsorption enhances both the magnetism and conductivity of the surface system. In particular, the charge density reveals electron transfer between cations and hydrogen atoms, providing insight into the variations in hydrogen bonding properties in these surface systems. These observations expand our understanding of hydrogen chemisorption and adsorption on defect sites, providing a novel avenue for investigating and potentially developing environmentally friendly multiferroic materials for advanced electronic devices.

Ab initio calculations of the chemisorption of atomic H and O on Pt and Ir metal and on bimetallic Pt x Ir y surfaces

Abstract Understanding the chemisorption of atoms on precious metal surfaces is of substantial interest for the rational design of heterogeneous and electrochemical catalysts. In this study, we report density functional theory (DFT) investigations of the chemisorption of atomic H and O on bimetallic Pt x Ir y (111) surfaces for bifunctional anode catalyst materials in polymer electrolyte membrane (PEM) fuel cells. We found that for both adsorbates, the adsorption on the Pt(111) surface is in general less exothermic than on the Ir(111) surface. Our study has revealed that chemisorption on the bimetallic surfaces becomes more stable with increasing number of Ir surface atoms at the adsorption site. While for hydrogen atoms the ONTOP sites yield the most negative adsorption energies, the chemisorption of oxygen atoms appears to be most stable on the FCC sites for both the mono- and bimetallic surfaces. Using the ab initio thermodynamics approach, we calculated phase diagrams for the chemisorption of H and O atoms on these metal surfaces in order to transfer our findings to finite temperature and pressure conditions. Our theoretical results may provide an improved understanding of the hydrogen oxidation reaction (HOR) and oxygen evolution reaction (OER) on intermetallic Pt x Ir y (111) surfaces and may be helpful for the rational design of new bifunctional PEM fuel cell anode catalyst materials.

Understanding electronic structure tunability by metal dopants for promoting MgB2 hydrogenation

Hydrogen is a promising energy carrier, but its onboard application is limited by the need for compact, low-pressure storage solutions. Solid-state complex metal hydride systems, such as MgB2/Mg(BH4)2, offer high storage capacities but suffer from sluggish kinetics and poor reversibility. One avenue for improving reactivity is to introduce metal dopants to alter electronic and atomic properties, but the role of these chemical additives remains poorly understood, particularly for the hydrogenation reaction. In this work, we used density functional theory calculations on model MgB2 systems to rationalize the potential role of metal dopants in destabilizing B–B bonding within the MgB2 lattice. We carried out detailed electronic structure analyses for 28 different metal dopant adatoms to identify properties that contribute to a dopant’s efficacy. Based on the simulation results, we propose that an intermediate ionic and covalent character of the bonds between adatoms and B atoms is desirable for facilitating charge redistribution, disrupting the B–B bond network, and promoting H2 dissociation and H atom chemisorption on MgB2.

First-principles study of the oxidation susceptibility of WS2, WSe2, and WTe2 monolayers.

The environmental stability of two-dimensional (2D) transition metal dichalcogenide monolayers is of great importance for their applications in electronic, photonic, and energy storage devices. In this study, we focus on understanding the susceptibility of WS2, WSe2, and WTe2 monolayers to oxygen exposure in the form of atomic oxygen and O2 and O3 molecules, respectively. Calculations based on the van der Waals-corrected density functional theory predicted that O2 and O3 molecules are weakly adsorbed on these monolayers, although atomic oxygen prefers chemisorption accompanied by a significant charge transfer from the surface to oxygen. In the physisorbed molecular configurations consisting of O2 and O3, the partially oxidized monolayers retain their geometrical and electronic structures. The calculated transition path as the oxygen approaches the surface shows a high-energy barrier for all cases, thus explaining the photo-induced formation of the oxidized configurations in the experiments. Furthermore, oxidizing the WS2 monolayer is predicted to modify its electronic structure, reducing the band gap with increasing oxygen coverage on the surface. Overall, the calculated results predict the resilience of WS2, WSe2, and WTe2 monolayers against oxygen exposure, thus ensuring stability for devices fabricated with these monolayers.

Chemisorption of gas atoms on one-dimensional transition-metal halides

We study computationally the bonding of hydrogen and oxygen to one-dimensional materials with the formula XY3, where X represents Ti, Zr, or Hf and Y represents a halogen atom Cl, Br, or I. The materials consist of chains of transition metal atoms surrounded by halogen atoms with a three-fold rotation symmetry. The chemisorption leads to a reduction in the number of d-electrons in the materials accompanied by significant changes in Fermi energy. The chemisorption energies are seen to correlate with the work functions of the clean systems supporting the view that electron transfer plays a major role in the bonding.

On the role of chemisorption in the formation of the target surface compound layer during reactive magnetron sputtering

Modeling of the reactive sputter processes requires a description of the target processes which results in the formation of a compound layer on the target. The sputter removal of the formed compound is balanced by compound formation mechanisms. Chemisorption of reactive gas atoms and molecules was initially identified as one of these processes. Further improvement of these chemisorption based models was achieved by the inclusion of processes such direct reactive ion implantation, knock-on implantation of chemisorbed species, and the redeposition of sputtered material on the target. In a recent study, the importance of ion implantation was questioned and an alternative model proposed. The results of this model are confronted with existing literature to investigate its validity.

First-principles study on the half-metallic properties of the VA group atoms adsorbed on WS2 monolayer

Adsorption of atoms on the surface of two-dimensional (2D) materials is one of the most effective ways to induce magnetic properties. In this study, the atomic structure, electronic structure, magnetic properties, and strain effects of VA group atoms (N, P, As, Sb and Bi) adsorbed on a WS2 monolayer are systematically studied using a first-principles method. After calculating the adsorption energy, it was determined that all of the VA group atoms showed a preference for being directly adsorbed above the S atoms. Based on the analysis of the orbital projection density of states and charge transfer, it appears that the group VA atoms chemisorb onto the WS2 layer. The adsorption of the VA group atoms on a WS2 monolayer will introduce 1 μB magnetic moment into the system. It is exciting that WS2 monolayer adsorbed with P, As, Sb or Bi is half-metallic with 100% spin polarization at the Fermi level. Furthermore, the magnetic properties are robust in the range of 10% strain and the magnetic moment of the system can be effectively controlled by tensile strain. In addition, when two or four atoms are adsorbed on a monolayer WS2 supercell, the adatoms show a tendency towards alignment in terms of their local magnetic moments, which may indicate a potential for ferromagnetic ordering in the system. After the adsorption of VA group atoms, monolayer WS2 exhibits structural stability, tunable magnetism under strain, 100% spin polarizability, and potential for ferromagnetism, making it a promising material for spintronic device applications.

The initial oxidation of the 4H-SiC (0001) surface with C-related point defects: Insight by first-principles calculations

The initial oxidation of 4H-SiC (0001) surfaces with C-related point defects (SurfCD) is studied by using first-principles coupled with thermodynamics calculations to gain deeper insights into the effects of C-related point defects on the formation of SiO2 and SiO2/SiC interface. Three types of SurfCD, including a surface with C interstitial defect (SurfCi), a surface with antisite defect (SurfCsi) and a surface with C vacancy (SurfVc) are considered. The initial oxidation of SurfCD commences on the chemisorption of oxygen atoms at the bridge sites between the Si atoms adjacent to the C-related point defects. As the oxygen coverage increased, the effects of C-related point defects on the adsorption sites and energies of oxygen atoms, the diffusion of oxygen atoms and the interfacial lattice strain are elucidated. The thermodynamic diagrams of O/4H-SiC structures are performed to investigate the stability of surface during the actual oxidation conditions for SurfCi, SurfCsi and SurfVc. Thermodynamically stable intermediate structures are observed on these surfaces during the initial formation of SiO2, the 1 ML structures are the most stable; the Si dangling bonds, Si-O-C structure and C-clusters are observed in the 1 ML structures. Moreover, the evolution mechanisms and electronic properties of the defects are further investigated.

Hydrogen Atoms on Zigzag Graphene Nanoribbons: Chemistry and Magnetism Meet at the Edge.

Although the unconventional π-magnetism at the zigzag edges of graphene holds promise for a wide array of applications, whether and to what degree it plays a role in their chemistry remains poorly understood. Here, we investigate the addition of a hydrogen atom─the simplest yet the most experimentally relevant adsorbate─to zigzag graphene nanoribbons (ZGNRs). We show that the π-magnetism governs the chemistry of ZGNRs, giving rise to a site-dependent reactivity of the carbon atoms and driving the hydrogenation process to the nanoribbon edges. Conversely, the chemisorbed hydrogen atom governs the π-magnetism of ZGNRs, acting as a spin-1/2 paramagnetic center in the otherwise antiferromagnetic ground state and spin-polarizing the charge carriers at the band extrema. Our findings establish a comprehensive picture of the peculiar interplay between chemistry and magnetism that emerges at the zigzag edges of graphene.

Molecular chemisorption: a new conceptual paradigm for hydrogen storage

Developing efficient hydrogen storage materials and the corresponding methods is the key to successfully realizing the “hydrogen economy”. The ideal hydrogen storage materials should be capable of reversibly ab-/desorbing hydrogen under mild temperatures with high hydrogen capacities. To achieve this target, the ideal enthalpy of adsorption is determined to be 15-50 kJ/mol for hydrogen storage. However, the current mainstream methods, including molecular physisorption and atomic chemisorption, possess either too high or too low enthalpy of hydrogen adsorption, which are not suitable for practical application. To this end, hydrogen storage via molecular chemisorption is perceived to regulate the adsorption enthalpy with intermediate binding energy between the molecular physisorption and atomic chemisorption, enabling the revisable hydrogen ad-/desorption possible under ambient temperatures. In this perspective, we will elaborate the molecular chemisorption as a new conceptual paradigm and materials design to advance future solid-state hydrogen storage.

Microkinetic model validation for Fischer-Tropsch synthesis at methanation conditions based on steady state isotopic transient kinetic analysis

A Single-Event MicroKinetic (SEMK) model has been extended towards the simulation of Steady State Isotopic Transient Kinetic Analysis (SSITKA) data for Co catalyzed Fischer-Tropsch Synthesis (FTS). The extended model considers two types of sites and both direct and H-assisted CO dissociation. Regression of the model parameters to the data obtained from 17 steady state and 11 SSITKA experiments resulted in physicochemically meaningful estimates for the activation energies and atomic chemisorption enthalpies. The application of the phenomenological UBI-QEP method allows to physically interpret the nature of the two site types considered in the model, i.e., terrace and step sites. A reaction path analysis shows that over 80 percent of the CO reacts on the step sites. Furthermore, chain growth exclusively occurs on these sites. The terrace sites are less reactive for CO dissociation and are identified as responsible for methane production. A fraction of the alkenes, produced on the step sites, is hydrogenated to alkanes on the terrace sites. Based on model simulations, the metal particle size effect, i.e., a lower TOF, higher methane selectivity and increasing alkane to alkene ratio with decreasing metal particle size, is attributed to an increasing relative importance of the terrace sites on the reaction kinetics.

Density Functional Theory and Machine Learning Description and Prediction of Oxygen Atom Chemisorption on Platinum Surfaces and Nanoparticles.

Elucidating chemical interactions between catalyst surfaces and adsorbates is crucial for understanding surface chemical reactivity. Herein, interactions between O atoms and Pt surfaces and nanoparticles are described as a linear combination of the properties of pristine surfaces and isolated nanoparticles. The energetics of O chemisorption onto Pt surfaces were described using only two descriptors related to surface geometrical features. The relatively high coefficient of determination and low mean absolute error between the density functional theory-calculated and predicted O binding energies indicate good accuracy of the model. For Pt nanoparticles, O binding is described by the geometrical features and electronic properties of isolated nanoparticles. Using a linear combination of five descriptors and accounting for nanoparticle size effects and adsorption site types, the O binding energy was estimated with a higher accuracy than with conventional single-descriptor models. Finally, these five descriptors were used in a general model that decomposes O binding energetics on Pt surfaces and nanoparticles. Good correlation was achieved between the calculated and predicted O binding energies, and model validation confirmed its accuracy. This is the first model that considers the nanoparticle size effect and all possible adsorption sites on Pt nanoparticles and surfaces.

On the use of recombination rate coefficients in hydrogen transport calculations

The commonly accepted picture for the uptake of hydrogen isotopes (HIs) from the gas phase across the surface into a metal with an endothermic heat of solution for HIs is that of dissociation followed by thermalisation in a chemisorbed surface state and finally overcoming a surface barrier to enter the metal bulk where the HIs occupy interstitial solute sites. To leave the metal bulk the HIs first transition to the chemisorbed surface state from which they then enter gas phase by recombining into a diatomic molecule. This model is generally attributed to the work of Pick and Sonnenberg from 1985. They clearly distinguish surface states and subsurface solute sites where the recombination flux is proportional to the square of the concentration of chemisorbed atoms due the diatomic nature of this Langmuir–Hinshelwood process. In an effort to compare their extended model with an earlier surface model by Waelbroeck, which uses an expression for the recombination flux proportional to the square of the sub-surface interstitial solute concentration, they derive an effective recombination coefficient. However, also with the so-derived Pick and Sonnenberg recombination coefficient, the Waelbroeck model is only applicable under certain conditions. But, due to its simplicity, it is often used in boundary conditions of diffusion trapping type calculations, generally ignoring whether or not these conditions are met. This paper will use the full Pick and Sonnenberg model implemented in the TESSIM-X code and in simplified algebraic approximations, to show the limits of applicability of the Waelbroeck–Ansatz in modelling hydrogen transport in metals foreseen for the first wall of magnetic confinement fusion devices.

Silver as a capturing material for iodine released from lead\u2013bismuth eutectic in various conditions

The usage of silver as a filtering material for removal of iodine from the gas phase of a lead–bismuth eutectic based nuclear reactor was investigated in various atmospheres representing normal operation as well as accident conditions. Thermochromatography experiments were performed to quantify the retention experienced on a silver surface by iodine species evaporated from a lead–bismuth eutectic sample. Measured adsorption enthalpies ranged from −171 to − 208 kJ mol−1 with observed differences attributed to various surface effects rather than a change in iodine speciation. The postulated adsorption mechanism is chemisorption of iodine atoms on the silver surface. Metallic silver fulfills the desired criteria for a capturing material in water-free filtering systems to be used as an alternative to traditional alkaline scrubbers commonly used in LWR systems.

Unravelling the influence of catalyst properties on light olefin production via Fischer–Tropsch synthesis: A descriptor space investigation using Single-Event MicroKinetics

Kinetic models constitute a useful tool to provide fundamental insights for catalyst development. Single-Event MicroKinetic modelling (SEMK) is a versatile strategy to assess complex reactions with a limited number of parameters. Particularly for Fischer–Tropsch synthesis SEMK modelling has focused on explaining the performances of individual catalysts within a wide range of operating conditions. In this work, we extend the capabilities of the SEMK modelling approach to investigate the influence of variation in catalyst properties i.e. catalyst descriptors, on the yield of desired component, light olefins (C2−C4=). We explore the catalyst descriptor space around three literature-reported iron-based catalysts. The three catalyst descriptors, i.e. atomic chemisorption enthalpies of hydrogen (QH), carbon (QC), and oxygen (QO) in the SEMK modelling approach have a combined effect on the conversion, whereas the selectivity to light olefins is found to be less sensitive to QO. These effects can be rationalized in terms of relative surface coverages of different species, leading to different dominant reaction pathways, and thus resulting in product yield variations. Using this approach, a “promising catalyst” with catalyst descriptors, QH≈ 234 kJ/mol, QC≈ 622 kJ/mol and QO≈ 575 kJ/mol resulting in 55% light olefins yield with lower methanation and long-chain hydrocarbon formation, is identified.

Ab initio study of lithium decoration of popgraphene and hydrogen storage capacity of the hybrid nanostructure

The successful realization of extended ‘5-5-8’ line defects in graphene in a controlled way has suggested the possible formation of a new 2D carbon allotrope consisting in pentagonal-octagonal-pentagonal carbon rings. The mechanical and thermal stability of this nanostructure, called popgraphene, has recently been confirmed on the basis of first-principles calculations. Moreover, it has been proposed as a promising anode material for use in Li-ion batteries with fast charge/discharge rates. In the present paper we perform density functional theory calculations to investigate the hydrogen storage ability of popgraphene. Our calculations show that popgraphene can be decorated in a very stable way by placing Li atoms on the pentagonal carbon rings of both sides of the sheet, leaving free the octagonal carbon rings. In that condition, the Li-decorated popgraphene sheet can bind up to four H2 molecules per unit cell, with an average adsorption energy in a range between physisorption and atomic chemisorption, which would allow for reversible hydrogen storage at moderate temperatures and pressures. Moreover, the gravimetric density of the Li-decorated popgraphene is 4.24 wt%, which is almost equal to the threshold specified by the U.S. Department of Energy for novel hydrogen-storage materials. All these results show that Li-decorated popgraphene nanostructures could be good materials for hydrogen storage.

Permeation of chemisorbed hydrogen through graphene: A flipping mechanism elucidated

The impermeability of defect-free graphene to all gases has been recently contested since experimental evidence (see Nature 579, 229–232 (2020)) of hydrogen transmission through a two-dimensional carbon layer has been obtained. By means of density functional theory computations here we elucidate a flipping mechanism which involves the insertion of a chemisorbed hydrogen atom in the middle of a C–C bond via a transition state that is relatively stable due to a sp2 rehybridization of the implicated carbon atoms. Present results suggest that transmission for hydrogenated graphene at low local coverage is highly unlikely since other outcomes such as hydrogen diffusion and desorption exhibit quite lower activation enthalpies. However, at high local coverage, with a given graphenic ring tending to saturation, the proposed flipping mechanism becomes competitive leading to a significantly exothermic process. Moreover, for a specific arrangement of four neighboring chemisorbed hydrogen atoms the flipping of one of them becomes the most likely outcome with a low activation enthalpy (about 0.8 eV), which is in the range of the experimental estimation. The effect of charge doping is investigated and it is found that electron doping can help to reduce the related activation enthalpy and to slightly enhance its exothermicity. Finally, an analysis of corresponding results for deuterium substitution is also presented.

Solvents forming redox mediators

Surface Chemistry The role of solvents participating directly in thermal catalytic reactions is clearer for homogeneous catalysis than for heterogeneous catalysis. Adams et al. studied the formation of hydrogen peroxide from hydrogen and oxygen on palladium nanoparticles by measuring the kinetic isotope effect and performing density functional theory simulations in aqueous and organic solvents. Methanol formed chemisorbed hydroxymethyl intermediates. These surface redox mediators transferred electrons and protons to adsorbed oxygen species and were regenerated by oxidizing chemisorbed hydrogen atoms. However, water molecules heterolytically oxidized hydrogen to generate solvated protons and surface electrons that reduced oxygen. Science , this issue p. [626][1] [1]: /lookup/doi/10.1126/science.abc1339

Electrodeposited Transitional Metal-Based Electrocatalysts for Hydrogen Evolution Reaction

As the fossil fuel is continuously declining, the necessity of a sustainable and clean source of energy is increasing. Hydrogen has been considered as a potential alternative due to its “zero emission” when reacting with oxygen in electrochemical cells. In addition, hydrogen is also a promising energy carrier/ storage to compensate the intermittent nature of other renewable energy sources (e.g. sunlight, wind, and tides). However, majority of hydrogen production comes from the combustion of fossil fuel which doesn’t make hydrogen a practical sustainable energy. Electrocatalytic hydrogen evolution reaction (HER) in water electrolysis therefore has drawn considerable attention owing to water and oxygen as the “green” reactant and by-product respectively.Early works with electrocatalytic HER involved noble metals (e.g. Pt) which is not cost-effective for industrial scale production. Therefore, transitional metal-based electrocatalysts (TMEs) have been intensively studied owing to the affinity of unpaired d-band electrons for chemisorption of hydrogen atoms. Among different strategies to improve the intrinsic properties of TMEs, coupling with other materials synergistically promotes the kinetics of HER. Nickel with minimum energy for hydrogen adsorption among various nonprecious metals exhibits the most efficient catalytic activities as a binary alloy with molybdenum in alkaline HER. Hydrogen spillover and averaging effect between the hydrogen adsorption energies of nickel and molybdenum facilitate the catalytic activity of nickel molybdenum alloy. On the other hand, transitional metal phosphides, especially cobalt with similar energy for hydrogen adsorption as nickel, have been considered as promising TMEs for acidic HER. With high electronegativity, phosphorus atoms can trap the positively charged protons and consequently enhance the dissociation of hydrogen. Moreover, dissolution of phosphorus-doped cobalt is less thermodynamically favored than that of cobalt, resulting in better electrocatalytic stability. To further enhance the HER catalytic performance, both types of electrocatalysts are synthesized at non-equilibrium by electrodeposition techniques to introduce chemical disorders and metaphases as well as to widen the range of composition. Different properties of deposits (e.g. composition, morphology, and grain size) can be tuned by controlling electrolyte composition and electrodeposition parameters.