An ab initio study of dissociative adsorption of H2 on FeTi surfaces

Dissociative adsorption of H₂ on clean FeTi (001), (110) and (111) surfaces is investigated via ab initio pseudopotential-plane wave method. Adsorption energies of H atom and H₂ molecule on Fe and Ti terminated (001) and (111) and FeTi (110) surfaces are calculated on high symmetry adsorption sites. It is shown that, top site is the most stable site for horizontal H₂ molecule adsorption on (001) and (111) surfaces for both terminations. The most favorable site for H atom adsorption on these surfaces however, is the bridge site. In (110) surface, the 3-fold hollow site which is composed of a long Ti–Ti bridge and an Fe atom, (Ti–Ti)_L–Fe, and again a 3-fold hollow site this time composed of a short Ti–Ti bridge and an Fe atom, (Ti–Ti)_S–Fe, are the most stable sites for H₂ and H adsorption, respectively. With the analysis of the above favorable adsorption sites, probable dissociation paths for H₂ molecule over these surfaces are proposed. Activation energies of these dissociations are also determined with the use of the dynamics of the H₂ relaxation and climbing image nudged elastic band method. It is found that H₂ dissociation on (110) and Fe terminated (111) surfaces has no activation energy barrier. On other surfaces however, activation energies are calculated to be 0.178 and 0.190 eV per H₂ molecule for Fe and Ti terminated (001) surfaces respectively, and 1.164 eV for Ti terminated (111) surface.     

DFT investigation of high temperature water gas shift reaction on chromium–iron mixed oxide catalyst

As part of high temperature water gas shift reaction mechanism, CO adsorption and H₂O adsorption on Fe₃O₄ (111) and chromium atom substituted Fe₃O₄ (111) slab surfaces are investigated by means of periodic DFT approach using VASP. Fe₃O₄ bulk structure has been computed including the Hubbard (U) parameter. One oxygen site (Ooct1) is studied as a probable site among the six Fe₃O₄ (111) terminations. Cr atom substitution on this surface is also examined. Cr atoms prefer being on the surface rather than in the bulk structure and Cr atoms substitute on the octahedral iron atom layer (Ooct2Cr). Adsorption energies of CO on Ooct1 and Ooct2Cr are found as −96 kcal/mol and −47 kcal/mol. Water adsorption on Ooct1 surface is molecular with −54.88 kcal/mol adsorption energy. On the other hand, water adsorption on Ooct2Cr surface is dissociative with nearly same adsorption energy, −55.12 kcal/mol, indicating the catalytic effect of chromium atom.     

Platinum doped iron carbide for the hydrogen evolution reaction: The effects of charge transfer and magnetic moment by first-principles approach

In this work, the catalytic activity towards hydrogen evolution reaction (HER) was studied for hydrogen adsorption on Pt doped Fe₂C (001) surface configuration (Pt/Fe₂C) and compared with pure Pt (001). The adsorption of H on the pristine Fe₂C, Pt doped Fe₂C, and pure Pt in (001) slab was computed. The best and promising HER activity (ΔG_H1 = −0.02 eV) is obtained at the hollow site adsorption of Pt/Fe₂C (Fe₁₃Pt₃C₈) compared to the experimental value of pure Pt (ΔG_H1 = −0.09 eV) suggesting the possibility of the H₂ formation on the surface of Fe₁₃Pt₃C₈. The structural stabilities of Fe₂C and Pt/Fe₂C were investigated by the formation energy analysis. Also, it is observed that to enhance the HER mechanism, the modification of the d-electron structure of Pt atoms is essential which can be achieved by the increased Pt doping. The Bader charge analysis demonstrated the charge transfer between the substrate and the adsorbed H atoms. The density of states (DOS) of pure Fe₂C and optimal Pt/Fe₂C were calculated which revealed the magnetic and metallic nature of these materials. In addition, the adsorption and resulted activation of H₂ were facilitated by the elongation of H–H bond length in Fe₁₃Pt₃C_8. This work supports the HER over single atom catalysts (SACs) with lower Pt loading but with high catalytic activity and the maximum atom utilization of SACs.     

Assessing the performance of screw deformed nanotube as potential hydrogen storage material

The hydrogen storage of screw deformed Ti-functionalized (5,2) single walled carbon nanotube is investigated by using the state of the art density functional theory calculations. The single Ti atom prefers to bind at the hollow site of the hexagonal ring with average adsorption energies per hydrogen molecule −0.56 and −0.52 eV for the un-deformed CNT-φ = 0 and deformed CNT-φ = 5 nanotubes, respectively. The hydrogen storage reactions 4H₂ + Ti-CNT-φ = 0,5 are characterized in terms of projected densities of states and statistical thermodynamics. The free energies and enthalpies meet the ultimate targets of the department of energy for minimal and maximal temperatures and pressures. The closest reactions to zero free energy exhibit surface coverage values 0.951 and 0.816 as well as (direct/inverse) rate constant ratios 6.55 and 1.5. The translational term is found to exact a dominant effect on the total entropy change with temperature, and the more promising thermodynamics are assigned to the screw deformed nanotube.     

Potassium decorated γ-graphyne as hydrogen storage medium: Structural and electronic properties

Different sites for K adsorption in γ-graphyne were investigated using density functional theory (DFT) calculations and optical and structural properties of the structures were examined. For the most stable structures, we put one H₂ molecule in different directions on the various sites to evaluate the hydrogen adsorption capability of them. Then, one to nine H₂ molecules in sequence were added to the best structure. Results show that clustering of the K atoms is hindered on the graphyne surface and the most desirable adsorption site for K atom is the hollow site of 12-membered ring with adsorption energy of 5.86 eV. Also, this site is the best site for H₂ adsorption onto K-decorated graphyne with E_das of −0.212 eV. Adding of number of H₂ molecule on this site shows that K atom can bind nine H₂ molecules at one side of the graphyne with the average adsorption energy of 0.204 eV/H₂. Therefore, for one side ca. 8.95 wt % and for both sides of the graphyne with a K atom in each side ca. 13.95 wt % of the hydrogen storage capacity can be achieved. This study shows that K-decorated graphyne can be a promising candidate for the hydrogen storage applications.     

Doping engineering: Highly improving hydrogen evolution reaction performance of monolayer SnSe

By using the first-principles approach, we explore the hydrogen evolution reaction (HER) performance of SnSe monolayer. It is found that the SnSe monolayer with or without intrinsic defects is not good HER catalyst. By doping eighteen different elements at Sn or Se sites of the SnSe monolayer, we find that the elements P and In can effectively reduce the free energies of hydrogen (H) adsorption (ΔG) to −0.1 eV and 0.21 eV, much lower than 1.45 eV of perfect monolayer SnSe. This is attributed to great dispersion of electronic density of states of absorbed hydrogen atom having small interactions with doping elements. However, strong hybridizations between H and doping elements (K and Te) increase the ΔG values of doping systems (ΔG = 2.84 eV and ΔG = 1.77 eV).     

Investigating hydrogen evolution reaction properties of a new honeycomb 2D AlC

The hydrogen due to its high mass energy density is a new renewable, economically viable and clean resource. The most eco-friendly and economical approaches for the generation of hydrogen through hydrogen evolution is electrochemical water splitting. The two-dimensional (2D) nanomaterials have been recently found as potential candidates as non-noble metal catalyst for hydrogen evolution. In this work, we have systematically studied the structural and electronic properties of the newly predicted hexagonal-aluminium carbide monolayer (h-AlC ML) under the framework of dispersion-corrected density functional theory (DFT) calculations. The calculated electronic total density of states (TDOS) of h-AlC ML predict its metallic nature in contrast to other polar honeycomb 2D materials which are either semiconducting or semimetallic. The metallic behavior of h-AlC monolayer which motivates us to investigate its HER activity results due to the presence of delocalized charge density near Fermi level. Thus, we have investigated the HER activity of h-AlC ML by calculating hydrogen (H) adsorption energy (ΔE_H) and Gibbs free energy (ΔG_H) at three different sites of the 3 × 3 and 4 × 4 supercells of h-AlC ML; top of carbon atom (E_H-C), top of aluminium atom (E_H-Al) and hollow site (E_H-Hollow). Our results show that the hollow site is most catalytically active site in both supercells of h-AlC ML. We believe that our results will inspire experimentalists to fabricate this new 2D material for achieving the desired range of HER activity.     

Hydrogen adsorption on pristine and platinum decorated graphene quantum dot: A first principle study

In the ever growing demand of future energy resources, hydrogen production reaction has attracted much attention among the scientific community. In this work, we have investigated the hydrogen evolution reaction (HER) activity on an open-shell polyaromatic hydrocarbon (PAH), graphene quantum dot “triangulene” using first principles based density functional theory (DFT) by means of adsorption mechanism and electronic density of states calculations. The free energy calculated from the adsorption energy for graphene quantum dot (GQD) later guides us to foresee the best suitable catalyst among quantum dots. Triangulene provides better HER with hydrogen placed at top site with the adsorption energy as −0.264 eV. Further, we have studied platinum decorated triangulene for hydrogen storage. Three different sites on triangulene were considered for platinum atom adsorption namely top site of carbon (C) atom, hollow site of the hexagon carbon ring near triangulene's unpaired electron and bridge site over C–C bond. It is found that the platinum atom is more stable on the hollow site than top and bridge site. We have calculated the density of states (DOS), highest occupied molecular orbitals (HOMO), lowest unoccupied molecular orbitals (LUMO) and HOMO-LUMO gap of hydrogen molecule adsorbed platinum decorated triangulene. Our results show that the hydrogen molecule (H₂) dissociates instinctively on all three considered sites of platinum decorated triangulene resulting in D-mode. The fundamental understanding of adsorption mechanism along with analyses of electronic properties will be important for further spillover mechanism and synthesis of high-performance GQD for H₂ storage applications.     

Ag monolayer doped by Pt atom on WC (0001) surface: A good catalyst for H2 dissociation with high sulfur tolerance

H₂ dissociation barriers on Ag (111), Ag monolayer on WC (0001) (Ag_ML/WC), Ag monolayer doped by Pt atom on WC (0001) (Ag_MLPt-d/WC) surface are decreasing from 1.07 to 0.03 eV. The ab initio atomistic thermodynamic data shows that at typical planar SOFC operating temperatures of 923–1073 K, under 100000-ppm pH₂S/pH₂, Ag_MLPt-d/WC can be sulfur free clean surface. Therefore, compared with traditional Nickel/Yttria-stabilized zirconia (Ni/YSZ) Solid Oxide Fuel Cells (SOFCs) anode, Ag_MLPt-d/WC shows high activity towards H₂ dissociation and high tolerance of sulfur poisoning.     

Density functional theory study of reversible hydrogen storage in monolayer beryllium hydride by decoration with boron and lithium

The hydrogen storage capacity of M-decorated (M = Li and B) 2D beryllium hydride is investigated using first-principles calculations based on density functional theory. The Li and B atoms were calculated to be successfully and chemically decorated on the Surface of the α-BeH₂ monolayer with a large binding energy of 2.41 and 4.45eV/atom. The absolute value was higher than the cohesive energy of Li and B bulk (1.68, 5.81eV/atom). Hence, the Li and B atoms are strongly bound on the beryllium hydride monolayer without clustering. Our findings show that the hydrogen molecule interacted weakly with B/α-BeH₂(B-decorated beryllium hydride monolayer) with a low adsorption energy of only 0.0226 eV/H₂ but was strongly adsorbed on the introduced active site of the Li atom in the decorated BeH₂ with an improved adsorption energy of 0.472 eV/H₂. Based on density functional theory, the gravimetric density of 28H₂/8li/α-BeH₂) could reach 14.5 wt.% higher than DOE's target of 6.5 wt. % (the criteria of the United States Department of Energy). Therefore, our research indicates that the Li-decorated beryllium hydride monolayer could be a candidate for further investigation as an alternative material for hydrogen storage.     

Hydrogen adsorption studies of TiFe surfaces via 3-d transition metal substitution

Hydrogen adsorption over TiFe surface and doped TiFe surface is investigated within density functional theory. Surface energy calculations confirm that TiFe (111) surface has the minimum value among three low index crystallographic surfaces, (100), (111) and (110). The (111) TiFe surface has two different terminations one with Fe and the other with Ti. Here both the (111) surfaces with different terminations are considered for doping with all the 3-d transition metal atoms from Sc to Zn. Furthermore, the molecular hydrogen adsorption over all the doped surfaces is investigated. V was found to be the most suitable element for doping in Fe terminated (111) surface. V doping in Fe terminated surface enhanced E_ads by 0.6 eV from ?3.30 eV (undoped) to ?3.90 eV after doping. Whereas in case of Ti terminated surface Co was found to be the best element for doping as it enhanced E_ads by ～0.5 eV from ?2.64 eV (undoped) to ?3.15 eV after doping. A significant decrease in d-band width from 1.95 eV to 1.22 eV in case of Co substitution in Ti terminated surface and from 2.42 eV to 1.33 eV in case of V substitution in Fe terminated surface enhances the hydrogen adsorption in TiFe (111) surface. Thus, even using a very small amount of dopant can influence the hydrogen adsorption properties of TiFe alloy.     

Interaction of H2S with perfect and S-covered Ni(110) surface: A first-principles study

First-Principles study based on Density functional theory (DFT) calculations are employed to investigate the dissociative mechanism of H₂S adsorption and its dissociation on perfect, and sulfur covered Ni(110) surface. On both surfaces, we probe the site preference for H₂S, HS, H, and S adsorption mechanisms. The results indicate that H₂S is energetically adsorbed on their high symmetry adsorption sites with the preferred short-bridge (SB) site on both surfaces. Furthermore, we found that chemisorption of HS is stronger in contrast to H₂S at favorable short-bridge (SB) with a binding energy of −3.59 eV on perfect Ni(110) surface, and on S-covered Ni(110) surface at the favorable hollow site having a binding energy of −3.57 eV. In the first H₂S dehydrogenation, energy barriers for S–H bond breaking over the clean surface are 0.08–0.46 eV and a little bit higher on the S-covered surface are 0.1–0.78 eV, while in second dehydrogenation the energy barrier on a clean surface is 0.19 eV. For further detail, electronic densities of states and d-band center model are used to characterize the interaction of adsorbed H₂S with both surfaces. Hence, our results show that decomposition of H₂S over perfect and S-covered Ni(110) surface is exothermic and also an easy process. However, kinetically and thermodynamically, the subsistence of surface sulfur avoids the H–S bond breaking process.     

Effect of surface carbon on the hydrogen evolution reactivity of tungsten carbide (WC) and Pt-modified WC electrocatalysts

Tungsten carbide (WC) has been previously identified as both an electrocatalyst and a support for several types of electrochemical reactions. The synthesis of WC often leads to excess surface carbon that can greatly affect its electrocatalytic activity. This work will evaluate the effect of surface carbon on WC both as a catalyst and as a support for monolayer (ML) amounts of platinum (Pt). WC thin films with no surface carbon, along with those with 1, 2, 3, or 4 equivalent ML of surface carbon, were synthesized. The hydrogen evolution reaction (HER) activity was used as a probe to test the effect of surface carbon on the electrochemical activity of WC and 1 ML Pt on WC (Pt/WC) using linear sweep voltammogram (LSV) in 0.5 M sulfuric acid. The HER activity of WC was relatively unaffected for very small amounts of surface carbon but decreased when several MLs or more of surface carbon was present. Pt/WC without surface carbon was found to have slightly higher HER activity as compared to Pt deposited on WC with surface carbon.     

An atomic study of hydrogen effect on the early stage oxidation of transition metal surfaces

Density functional theory (DFT) and tight-binding quantum chemical molecular dynamics (QCMD) have been applied to analyze the role of interstitial hydrogen in the process of oxygen adsorption to Ni (111) and Cr-doped Ni (111) surfaces and diffusion within these metal surfaces. The DFT calculations demonstrate that the fcc hollow and octahedral sites are the most favorable for hydrogen adsorption on the surface and subsurface, respectively. A clean metal surface has a slight inward relaxation in the topmost layer, whereas the metal atoms show outward relaxation (2%) due to interstitial hydrogen. The adsorption energies of oxygen and OH have decreased to 0.26 and 0.13 eV, respectively, and the metal atomic bond further extended in the range of 1–2% in order to hydrogen remained interstitial site. Hydrogen changes to a negatively charged in the interstitial site by receiving electron. The QCMD results reveal that the oxygen penetration depth increases when hydrogen occupy into interstitial octahedral site. The deeply diffused or interstitial hydrogen receives electrons from the metal. Additionally, interstitial hydrogen initiates the charge transfer and extends the metal atomic bond. The localized process weakens the metal–metal bonds and it makes the surface chemically active for further interaction. This process can help oxygen or other species to diffuse into the structure. As a result, the subsurface hydrogen accelerates the early stage of oxidation initiation.     

C在Ni(111)表面吸附行为的密度泛函理论研究

应用密度泛函理论系统地研究C在Ni(111)表面上的吸附能、吸附结构、差分电荷密度、局域态密度及Mulliken布居数,给出了覆盖度在0.167~1.0 ML内,C的吸附特性随覆盖度变化的规律。研究表明,C的稳定吸附位为三重空位(hcp,fcc),化学吸附成键的本质是C2p态和Ni3d态的耦合、杂化,使原本孤立的C原子态耦合杂化为成键态和反键态。随着覆盖度的增加,反键态电子占据增多,成键态电子减少,平均成键电子数减少,C-Ni间的相互作用减弱,吸附能降低。     

Revealing the interfacial phenomenon of metal-hydride formation and spillover effect on C48H16 sheet by platinum metal catalyst (Pt (n=1-4)) for hydrogen storage enhancement

Hydrogen is a worldwide green energy carrier, however due its low storage capacity, it has yet to be widely used as an energy carrier. Therefore, the quantum chemical method is being employed in this investigation for better understand the hydrogen storage behaviour on Pt _(n = 1-4) cluster decorated C₄₈H₁₆ sheet. The Pt_(n = 1-4) clusters are strongly bonded on the surface of C₄₈H₁₆ sheet with binding energies of ?3.06, ?4.56, ?3.37, and ?4.03 eV respectively, while the charge transfer from Pt_(n = 1-4) to C₄₈H₁₆ leaves an empty orbital in Pt atom, which will be crucial for H₂ adsorption. Initially, the molecular hydrogen is adsorbed on Pt_(n = 1-4) decorated C₄₈H₁₆ sheet through the Kubas interaction with adsorption energies of ?0.85, ?0.66, ?0.72, and ?0.57 eV respectively, while H–H bond is elongated due to the transfer of electron from σ (HH) orbital to unfilled d orbital of the Pt atom, resulting in a Kubas metal-dihydrogen complexes. Furthermore, the dissociative hydrogen atoms adsorbed on Pt_(n = 1-4) decorated C₄₈H₁₆ sheet have adsorption energies of ?1.14 eV, ?1.02 eV, ?0.95 eV, and ?1.08 eV, which are greater than the molecular hydrogen adsorption on Pt_(n = 1-4) cluster supported C₄₈H₁₆ sheet with lower activation energy of 0.007, 0.109, 0.046, and 0.081 eV respectively. To enhance the dissociative hydrogen adsorption energy, positive and negative external electric fields are applied in the charge transfer direction. Increasing the positive electric field makes H–H bond elongation and good adsorption, whereas increasing the negative electric field results H–H bond contraction and poor adsorption. Thus, by applying a sufficient electric field, the H₂ adsorption and desorption processes are can be easily tailored.     

Hydrogen adsorption on and diffusion through MoS2 monolayer: First-principles study

We investigate the hydrogen adsorption on and diffusion through the MoS₂ monolayer based on density-functional theory. We show that the hydrogen atom prefers to bond to the S atom at the monolayer, leading to enhanced conductivity. The hydrogen atom can also adsorb at the middle of the hexagon ring by overcoming an energy barrier of 0.57 eV at a strain of 8%. Also, we show that the MoS₂ monolayer is flexible and any mechanical deformation of the monolayer is reversible because the extension of the Mo–S bond is much smaller than the applied strain. The monolayer can block the diffusion of hydrogen molecule from one side to the other due to a high energy barrier (6.56 eV). However, the barrier can be reduced to 1.38 eV at a strain of 30% and even totally removed by creating S vacancies and applying a strain of 15%. The MoS₂ monolayer may find applications in sensors to detect hydrogen, and as mechanical valve to control the concentration of hydrogen gas.     

Atom doping in α-Fe2O3 thin films to prevent hydrogen permeation

Prevention of hydrogen (H) penetration into passive films and steels plays a vital role in lowering hydrogen damage. This work reports effects of atom (Al, Cr, or Ni) doping on hydrogen adsorption on the α-Fe₂O₃ (001) thin films and permeation into the films based on density functional theory. We found that the H₂ molecule prefers to dissociate on the surface of pure α-Fe₂O₃ thin film with adsorption energy of −1.18 eV. Doping Al or Cr atoms in the subsurface of α-Fe₂O₃ (001) films can reduce the adsorption energy by 0.03 eV (Al) or 0.09 eV (Cr) for H surface adsorption. In contrast, Ni doping substantially enhances the H adsorption energy by 1.08 eV. As H permeates into the subsurface of the film, H occupies the octahedral interstitial site and forms chemical bond with an O atom. Comparing with H subsurface absorption in the pure film, the absorption energy decreases by 0.01–0.22 eV for the Al- and Cr-doped films, whereas increases by 0.82–0.96 eV for the Ni-doped film. These results suggest that doping Al or Cr prevents H adsorption on the surface or permeation into the passive film, which effectively reduces the possibility of hydrogen embrittlement of the underlying steel.     

Theoretical investigation of the reactivity of flat Ni (111) and stepped Ni (211) surfaces for acetic acid hydrogenation to ethanol

Reactivity of two types of Ni surfaces-flat (111) and stepped (211) surfaces for acetic acid hydrogenation to ethanol was investigated using density functional theory method. The most stable configurations of the reactants, intermediates and products were obtained by investigating all the possible adsorption sites. Results showed that the adsorption of all the studied molecules on the Ni (211) surface are stronger than that on the Ni (111) surface, except for H atom (similar adsorption strength of H atom on the both surfaces was found). In addition, most of the molecules on the Ni (211) surface preferred to adsorb at the step edge, indicating that different coordination numbers of Ni atoms could result in different adsorption strength. Moreover, the elementary reactions with energy barriers related to ethanol and ethyl acetate formations were studied. The most favorable pathways for ethanol formation on the Ni (111) and (211) surfaces are CH₃COOH → CH₃CO → CH₃CHO → CH₃CHOH→ CH3CH₂OH and CH₃COOH → CH₃CO → CH₃COH → CH₃CHOH → CH₃CH₂OH, respectively. The direct decomposition of acetic acid molecule to form acetyl species was the rate-determining step on the both surfaces. Slight difference for the rate-determining step barriers was observed (1.04 eV vs. 1.13 eV). However, the elementary step of ethyl acetate formation by CH₃CO and CH₃CH₂O became much more difficult on the Ni (211) surface than that on the Ni (111) surface (1.06 eV vs. 0.67 eV). These results suggests that the Ni (211) surface is more likely to inhibit ethyl acetate formation compared with the Ni (111) surface. Meanwhile, the results of the rate constants and the effective barriers indicates that the Ni (211) surface presents a higher probability for higher ethanol selectivity.