Hcp Sites Research Articles

Understanding the effect of adsorption sites of CO at cobalt surface on its reactivity with H2/H by DFT calculations.

Cobalt (Co) is widely used in Fischer-Tropsch synthesis (FTS), converting synthesis gas, carbon monoxide + hydrogen (CO + H2), to long-chain hydrocarbons. The adsorption of CO on the Co surface is the key step in FTS. In this work, the effect of CO adsorption sites on the reactions between CO and H2 was investigated by using density functional theory (DFT). The energetics and structures of the reactions between the adsorbed CO (CO*) and H2/adsorbed H2 (H2*)/adsorbed H atom (H*) were calculated. The results show that the reaction between CO* and H2 is initiated by the molecular adsorption of H2 on the Co surface. The reactions between CO* and H2*/H* are influenced by CO adsorption sites. For the reaction system of CO* + H2*, it has the lowest reaction barrier when CO is adsorbed at the hcp site, while for CO* + H*, it has the lowest reaction barrier when CO is adsorbed on the top site. Kinetic analysis indicates that to improve the reactivity of CO + H2 in FTS, the adsorption of CO should be controlled to favour the top and bridge sites. This article is part of the theme issue 'Celebrating the 15th anniversary of the Royal Society Newton International Fellowship'.

Insights into the interaction of nitrobenzene and the Ag(111) surface: A DFT study

This study explores the potential of nitrobenzene as an anolyte material for nonaqueous redox flow batteries (RFBs) by theoretically examining its low-coverage adsorption behavior on neutral and charged Ag(111) model electrode surfaces. At the low coverage limit, DFT calculations show a preference for nitrobenzene to adsorb parallel to the surface, with the benzene ring and nitro group centered over HCP sites. Interactions between nitrobenzene and the surface were analyzed using induced charge density analysis, Bader charge analysis, and projected density of states (PDOS). It was found that nitrobenzene adsorbs primarily through van der Waals interactions with the surface. As nitrobenzene accumulates negative charge, the strength of adsorption diminishes. Understanding the electrode-electrolyte interface is crucial for enhancing RFB electrochemical performance, and this study sheds light on nitrobenzene's interaction with a model Ag electrode.

Theoretical prediction on the interfacial bonding properties of MoAlB(010)/Cu(100) interface

MoAlB ceramics, renowned for its superior mechanical characteristics, robust oxidation resistance, and exceptional thermal conductivity, has demonstrated successful applications as a reinforcing phase in metal matrix composites. However, Recent investigations have revealed that that during the sintering process, Al atoms present in MoAlB diffuse into the copper matrix, thereby compromising the performance of copper-based composites. In this work, the work of adhesion, interfacial energy and electronic structure of MoAlB/Cu interface were investigated using first-principle calculations, aiming to enhance the understanding of the diffusion phenomenon observed. The results indicate that the surface energy of Al1 (B2) and Al2 (Al1) slabs is relatively lower, and following the complete relaxation of the slabs, there is a pronounced tendency for Al atoms to detach from MoAlB. The results of adsorption work suggest that HCP sites exhibit a more stable interface stacking sequence. The calculation results pertaining to interfacial energy demonstrate that the Al/Cu interface exhibits a lower interfacial energy, thus facilitating its formation. Furthermore, a detailed analysis of the electronic structure of the Al1(Mo2)-HCP and Al2(Al1)-HCP interfaces reveals the formation of Cu–Al bonds at the interface, hinting at a potential tendency for Al atoms to diffuse into the Cu matrix.

First Principles Study of O2 Dissociative Adsorption on Pt-Skin Pt3Cu(111) Surface

The O2 dissociative adsorption serves as a pivotal criterion for assessing the efficacy of oxygen reduction catalysts. We conducted a systematic investigation into O2 dissociative adsorption on the Pt-skin Pt3Cu(111) surface by means of the density functional theory (DFT). The computational findings reveal that the O2 adsorption on Pt-skin Pt3Cu(111) surface exhibits comparatively lower stability when contrasted with that on the Pt(111) surface. For O2 dissociation, two paths have been identified. One progresses from the t-f-b state towards the generation of two oxygen atoms situated within nearest-neighbour hcp sites. The other commences from the t-b-t state, leading to the generation of two oxygen atoms occupying nearest-neighbour fcc sites. Moreover, the analysis of the energy barrier associated with O2 dissociation indicates that O2 on the Pt-skin Pt3Cu(111) surface is more difficult to dissociate than on the Pt(111) surface. This study can offer a valuable guide for the practical application of high-performance oxygen reduction catalysts.

Identification of the optimal site for oxygen adsorption in chemisorbed and oxidized states on pseudomorphic Fe/Ru(0001) surface: A density functional theory computational investigation

The oxidation state and charge distribution of FexOy binaries, including strained monolayers on transition metal surfaces, is a topic of significant interest. The p(2 × 2) and c(4 × 2) superstructures are two stable surface structures of chemisorbed oxygen on the pseudomorphic Fe/Ru(0001) surface that may coexist at 0.25 ML oxygen coverage. Density functional theory calculations were used to investigate the possibility of these two structures occurring on the surface. The calculations considered the effect of magnetic ordering on the choice of adsorption site for oxygen. Paramagnetic ordering of the Fe monolayer favors oxygen adsorption at the hcp site, while antiferromagnetic ordering favors oxygen adsorption at the fcc site at 0.25 ML oxygen coverage. Interestingly, in the case of antiferromagnetic ordering, although the adsorption of 1 ML coverage of oxygen for the oxidation reaction energetically prefers the hcp site, the chemisorbed structures at 0.25 ML coverage are found to prefer the fcc site on the pseudomorphic Fe/Ru(0001) surface. The DFT calculations suggest that both the p(2 × 2) and c(4 × 2) structures of oxygen are exothermic, indicating the possibility of coexistence on the pseudomorphic Fe/Ru(0001) surface and occurrence at room temperature. However, the use of the Hubbard potential parameter, which is used to correctly describe the electronic band structure of FeO and other Mott insulators, also revealed that the oxidation reaction of the pseudomorphic Fe monolayer is endothermic on the Ru(0001) surface.

A study on adsorption, dissociation, penetration, and diffusion of nitrogen on and in α-Ti via First-principles calculations

A comprehensive description of the entire process of N2 molecules on Ti surface and in Ti bulk as well as the underlying nitridation mechanism were explored via first-principles calculations. The reaction process was divided into four steps and the mechanism for the determined reaction path was analyzed. Utilizing the methodology of the climbing image nudged elastic band (CI-NEB), we investigated the diffusion energy barrier at each step of the reaction path, and determined the minimum energy path (MEP) for the complete reaction path. The calculated adsorption energies of N atoms and N2 molecules show that they are most stable at the HCP site on the α-Ti (0001) surface. Charge transfer provided theoretical support for N2 dissociation. We calculated the diffusion energy barrier of the N atom by migrating from the HCP site to the octahedral (O) site, which the N atom prefers to occupy. The diffusion coefficient is highest when the diffusion channel travels from one O site to the next layer of the O site via the next H site, it takes the most energy to complete, compared to those other steps. Lastly, we determined the rate-determining steps of the reaction pathway for the entire nitridation process. This study provides valuable insight into the fundamental mechanisms governing the nitridation process of N2 molecules in titanium alloys. This study sheds light on the nitridation mechanism for N2 molecules in titanium alloys.

Atomic-level insights from density functional theory and ab initio molecular dynamics calculations for oxidation mechanism of transition metal doping Nb4AlC3 (0 0 0 1) surface

Poor oxidation resistance limits the further application of Nb4AlC3 MAX phase, a high-temperature structural candidate material. Transition metal doping can significantly address this problem. In this paper, we investigated the oxidation mechanism of the Nb4AlC3 (0 0 0 1) surface with Ti doping by combining density functional theory (DFT) and ab initio molecular dynamics (AIMD). DFT results indicate that the Nb surface has a lower surface energy, making it more easily exposed and reactive towards O2 molecules. O atoms are expected to preferentially occupy the HCP and FCC hollow sites on the surface. The Ti-doped surface has a stronger adsorption capacity for O atoms. O atoms gain electrons, while Nb/Ti atoms lose electrons. The formation of O–Nb/Ti bonds is facilitated by the hybridization of O-p and Nb/Ti-d orbitals. And O–Ti bonds are more stable than O–Nb bonds. AIMD simulation results show that O2 molecules on the clean and doped surfaces gradually dissociate into O atoms over time. These O atoms then combine with surface atoms to form Nb oxides, Ti oxides, and Nb–Ti oxides. With the passage of time, the concentration of Ti oxides increases while that of Nb oxides decreases. And the oxidation rate at 1073K is higher. The O–Nb/Ti bond population gradually increases over time, indicating that the bonding between O atoms and the surface is slowly strengthened. This study provides fundamental insights into the oxidation mechanism of transition metal-doped 413 MAX phase Nb4AlC3 surfaces.

First-principles study on the adsorption and lithium storage capacities of Hf3C2 and Hf3C2T2(T = O, F, S) MXenes

Based on the theoretical framework of the first principle, the adsorption and theoretical lithium storage capacities of Hf3C2 and surface functionalized Hf3C2T2(T = O, F, S) MXenes were studied. The structural stability of the system and the influence of electrical properties on energy storage properties are analyzed when different functional groups are located in different positions. The formation energy results suggest that the three functional Hf3C2T2(T = O, F, S) structures are energetically stable, and the CCP site is the most stable among the possible sites of HCP, TOP and CCP. By calculating the energy bands and density of states, it is proved that F-functionalized Hf3C2F2 is not conducive to adsorption. During the adsorption process, the lithium intercalation process is carried out in the way of surface adsorption of atoms, in which the HCP site of bare Hf3C2 and the CCP site of functionalized Hf3C2T2 are the most favorable adsorption sites. The storage capacities of bare Hf3C2, Hf3C2O2, and Hf3C2S2 are 287.51 mAhg−1, 271.95 mAhg−1 and 258.00 mAhg−1, respectively. The Hf3C2O2 has a higher lithium storage capacity than Hf3C2S2, so both oxygen functionalized Hf3C2O2 and bare Hf3C2 are promising battery materials.

Adsorption of CO2 on the Surface of Fe(111): A First-Principles Study

First-principle is used to study the structure parameters, adsorption energy, Bader charge, electronic density of states, charge-density transformation, and surface work function of CO2 molecule at various adsorption sites on the Fe(111) surface based on Density Function Theory (DFT). Results show that the CO2 molecule is absorbed on the Fe(111) surface by combining Fe–C and Fe–O multiple bonds. The type of adsorption of most configurations is chemisorption. The most stable structure is BS-Y, with an adsorption energy of −0.8115 eV. The order of stability of adsorption sites is bridge site > hcp site > fcc site > top site. Carbon dioxide mostly reacts with the uppermost two layers of Fe atoms and just partially with the lowermost two levels. In addition, the chemical bonds between CO2 molecule and Fe atoms are covalent, and the response mechanism is the hybridization of C-2s, C-2p, O-2s, and O-2p orbitals with Fe-3p, Fe-3d, Fe-4s orbitals, forming new chemical bonds. The BS-Y configuration has the smallest increment of work function, indicating that the lowest escape energy is required for the electron to escape from the surface.

DFT study of the formation of the Sb-Ag(1 1 1) (√3×√3) – R30° surface alloy

According to performed DFT calculations, the (√3×√3) – R30° structure, in which Sb atoms occupy hcp sites on Ag(111), is more energetically favorable than the related substitutional SbAg2 surface alloy. The difference in energies is significant, 0.66 eV per unit cell, and can be attributed to a larger size of Sb atoms compared to Ag. The obtained results of modeling the substitutional process explain the formation of adsorbed Sb layer on the Ag(111) surface by means of Sb deposition instead of the formation of the substitutional surface alloy. In turn, Ag deposition onto the alloy surface will cause a floating up (segregation to the surface) of Sb atoms thus restoring the alloy surface, which may serve as a surfactant in the process of the growth of Ag crystal.

Palladium metallene for nitric oxide electroreduction to ammonia.

We demonstrate Pd metallene as an efficient catalyst for electrocatalytic NO reduction to NH3 (NORR), showing the maximum NO-to-NH3 faradaic efficiency of 89.6% with a corresponding NH3 yield rate of 112.5 μmol h-1 cm-2 at -0.3 V in neutral media. Theoretical calculations unveil that NO can be effectively activated and hydrogenated on the hcp site of Pd through a mixed pathway with a low energy barrier.

Ab Initio Investigation of the Adsorption and Dissociation of O2 on Cu-Skin Cu3Au(111) Surface

Surface adsorption and dissociation processes can have a decisive impact on the catalytic properties of metal alloys. We have used density functional theory to investigate the adsorption and dissociation of O2 on Cu-skin Cu3Au(111) surface. The calculated results show that the b-f(h)-b adsorption configuration is the most energetically favorable on the Cu-skin Cu3Au(111) surface. For O2 dissociation, there are two thermodynamically favorable dissociation paths. One path is from b-f-b to two O atoms in hcp sites, and the other path is from b-h-b to two O atoms in fcc sites. Moreover, the stability of O2 adsorption is higher and the dissociation energy barrier of the adsorbed O2 is lower as compared to those on the Cu(111) surface. This theoretical work provides valuable guidance for the practical application of Cu-Au alloys as highly efficient CO oxidation catalysts.

Experimental Study of Diamond-like Carbon Film on Aermet100 Steel and First-Principles Calculation of Interfacial Adhesion.

Three different types of surface-modified layers of N, C, and N+C are successfully prepared on AerMet100 steel by plasma-assisted thermochemical treatment, and diamond-like carbon (DLC) films are formed on the top surfaces of the latter two. The results show that the DLC films produced by prenitriding and then carburizing (N+C) exhibit a smoother and finer morphology and higher sp3 content than that without prenitriding (C). In addition, the wear resistance of the N+C specimen with a high hardness nitrided layer as the support for the outermost DLC films is superior to that of the C specimen. In view of the catalytic effect of the Fe3C phase on the growth of DLC films, the interfacial properties of Fe3C(001)/diamond(111) are investigated using first-principles calculations. On the basis of the most preferred Fe-terminated HCP site model, the effects of alloyed cementite (Fe2MC) on interfacial adhesion of Fe2MC(001)/diamond(111) are also investigated. Furthermore, the mechanisms of interfacial adhesion for two representative dopings (Zr weakened and V enhanced) are revealed in detail. These results are expected to provide a potential promising means for future experimental works on the preparation of high-performance DLC films on alloy steel surfaces by plasma carburizing.

First-principles analysis on the nitrogen adsorption and diffusion in Ti alloy towards clarified diffusion mechanism in nitriding

Based on first-principles calculations, the adsorption and diffusion of the nitrogen atoms in Ti bulk and Ti surface layer were studied. Al doping is considered in the study of the surface layer. We first calculated and studied N in bulk Ti and obtained that the N atom preferentially occupies the octahedron(O) interstice of the α-Ti bulk, which behaves as a metallic N-containing solid solution at a lower N concentration. And the diffusion energy barrier is the lowest in the α-Ti bulk when it diffuses between two O sites along with the （1¯ 2 1¯ 0）direction. Then we studied N adsorption and diffusion of N on the α-Ti(0 0 0 1) surface. The N atoms are predicted to be adsorbed on the FCC and HCP sites of the Ti(0 0 0 1) surface, where the FCC site has lower Ead (adsorption energy). In addition, after doping Al atoms on the Ti surface and sub-surface, the adsorption capacity of the N atom decreases. Especially when the Al atom exists on the Ti surface, this effect is very significant. And the Al atom also reduces the stability of the N atom in the subsurface O site at the same time. Finally, the diffusion process of N atoms from the surface to the subsurface was calculated and compared with and without Al doping. This paper provides fundamental insights into the diffusion mechanism in the nitriding treatment of titanium alloys. These calculation results can infer which titanium alloy is more suitable for nitriding.

Investigations on molybdenum phosphide surfaces for CO2 adsorption and activation

The activation of CO 2 molecule over a catalyst surface is an imperative step to tackle the depletion of fossil fuels and global warming effects. Here, we present density functional calculations to explore sensitivity of molybdenum phosphide (MoP) surfaces for this purpose. Despite well documented efficacy of MoP as a catalyst for electrochemical hydrogen evolution, the fundamental understanding of their structural and electronic analysis remains incomplete. We consider four types of surfaces, (001), (100), (101) and (110) that have varied distribution pattern of metal atoms (Mo and P) that allowed us to investigate the role of p anions in CO 2 activation. Various adsorption sites over each surface were taken into account, the CO 2 molecule adsorbed over the hcp site of MoP (001) surface adopted bent geometry with elongation of C-O bond length to 1.31 Å with adsorption energy 1.50 eV. The electron localization plots and charge transfer as computed via Density Derived Electrostatic and Chemical (DDEC6) approach provide significant overlap of 0.75 e charge density between adsorbate and adsorbent. The activated CO 2 is investigated in detail through charge density difference plots, Projected Density of states (PDOS) and red shift in the symmetric and antisymmetric stretching frequency. The study provides conclusive evidence for catalytic potential of MoP(001) surface towards CO 2 activation.

Exploring structural evolution and graphitization of the interface between tungsten and diamond (1 1 1) surface: A DFT study

In the ultra-precision machining field, tungsten (W) is a promising material as a polishing disc for machining diamond, which is easier to obtain high-quality diamond surfaces than cast iron. The removal mechanism of the diamond during high-speed polishing is considered surface graphitization. In this work, the diamond graphitization mechanism induced by tungsten is revealed by the density functional theory (DFT). The results show that the W adatoms at the fcc sites can activate the interlayer C-C bonds, causing the surface carbon atoms to separate. Notably, the electron transfer of W atoms and the structure contraction of the diamond surface cause the graphitization. Moreover, the movement of W adatoms intensifies the diamond surface structure transformation and graphitization process. It is worth noting that the W adatoms of the fcc and hcp sites have different contributions to the surface carbon atom separation during the interfacial sliding process. When the W atoms cross the hcp sites, the pulling force of the W-C bonds promotes the reconfiguration of the separated surface carbon atoms. The diamond-to-graphite transition is attributed to catalyst activation by W adatoms, diamond surface structure contraction, and mechanical tensile stress by the W-C bonds.

Adsorption and diffusion behavior of hydrogen on the M-doped (M=Zr, Mo, Y, Cu, Pd, Ir, Mg, Al, Si) Ti(0001) surfaces: A first-principles study

The adsorption and diffusion behaviors of hydrogen on M-doped Ti(0001) surfaces are investigated by first-principles calculations. The surface energies of M-doped surfaces follow the ordering of Mo > Ti > Zr > Cu > Mg > Y > Al > Pd > Ir > Si. In all the M-doped systems, H atom adsorbed at the next nearest neighboring site is more stable than the nearest neighboring site. All involved M dopants prevent the H adsorption at the nearest neighboring site, and the Si dopant has a significant repulsive effect on it. Both the surface stability and H adsorption behavior are interpreted successfully by the doping effects of s-d hybridization (Al, Si), D-band contribution (Pd, Cu, Ir), and the relative electronegativity (Mo, Zr, Y, Mg). H prefers to penetrate from the surface hcp site to the subtet site and finally to the suboct site, except for the Mo-doped and clean Ti surfaces with the path from the surface fcc site directly to the suboct site. Among these dopants, Mo promotes the H in-plane surface diffusion and Pd promotes the H penetration process most significantly.

Effects of Zn, Mg and Cu Doping on Oxidation Reaction of Al (111) Surface

Aiming at the performance degradation of lithium-ion batteries due to shell corrosion, the doping of alloy elements Zn, Mg and Cu on Al (111) surface and the effect on oxidation reaction of Al (111) surface were studied by the first-principles calculation method. The results show that Zn, Mg and Cu atoms stably combine with Al atoms, and the surface smoothness is slightly different due to their different radii and electronegativity. The dissociative adsorption of O2 molecules is related to the surface doping atoms and O2 coverage, while the electron tunneling of underlying metal promotes O2 adsorption on the surface. As O2 coverage increases, the O atoms adsorbed on the hcp site gradually migrate to the subsurface layer. Zn, Mg, Cu and vacancy defect hinder the migration of the surrounding O atoms to subsurface layer, resulting in different structures and thicknesses of the oxide film near the doped atoms. At the same time, Zn, Mg, and Cu atoms differ in their ability to gain or lose electrons compared with Al atoms, resulting in their different positions on the surface. In addition, the surface work function rises with the increase of O2 coverage, and Zn and Cu atoms make the work function increase faster. Finally, according to the research results, it can be inferred that Zn and Mg are the unfavorable factors for the oxidation reaction of Al surface.

Revealing the interface characteristic of the semi-coherent Co(111)/WC(0001) interface: a first principles investigation

ABSTRACT In the present work, based on the first-principles calculations, we systematically discuss the electronic structure, adhesion work, interface energy, chemical bonding strength and interfacial mechanical failure forms of semi-coherent Co(111)/WC(0001) interface. After relaxation, the interface configurations of the middle top (MT) sites migrate toward the hexagonal close-packed hollow top sites (HCP), indicating that the MT occupancy is unstable. The atoms at the interface are more inclined to the HCP sites. The calculated results indicate that the C-terminated HCP1 site interface configuration has the largest adhesion work and the smallest interface energy, which depends on the formation of strong C-Co covalent bonds at the interface. Finally, the strong Co/WC interface configuration formed in the WC-Co composites can effectively carry out load transfer and reduce stress concentration, improving the strength and plasticity of the composites.

Compatibility of Environmentally Friendly Insulating Gases CF3I and c-C4F8 with Cu Contacts

The gas-solid compatibility between environmentally friendly insulating gas and copper contacts is worth studying. In this paper, based on density functional theory, the adsorption calculation of CF3I, c-C4F8, five typical decomposition gases, and Cu (1 1 1) surface was carried out. The adsorption energies, transferred charges, charge densities, and densities of states were calculated for different adsorption configurations. Research indicates that there is no obvious charge transfer between the I atom and the Cu atom in the four adsorption sites of Cu (1 1 1) for the CF3I molecule. There is a charge transfer between the F atoms and the Cu top surface. The electrons lost by Cu are transferred to F atoms. In the configurations of different adsorption positions on CF3I and Cu (1 1 1) planes, the top and bridge adsorption energies are −0.835 eV and −0.993 eV, respectively, which are chemical adsorption. Therefore, CF3I is most likely to form adsorption at the top or bridge site of the Cu (1 1 1) surface. The adsorption energy of c-C4F8 gas on Cu (1 1 1) surface is similar to that of CF3I at fcc and hcp sites. The absolute values are all less than 0.8 eV, and the van der Waals force is the main force. The adsorption energies of C2F4 and C3F6 in the five decomposed gases are −1.315 eV and −1.204 eV, respectively. The charge transfer is −0.32 eV and −0.45 eV, respectively. Their values are larger than those of the other gases studied, which belong to chemical adsorption. The smaller values of the remaining three gases belong to physical adsorption. All molecular structures and Cu (1 1 1) planes were not significantly deformed. From a microscopic point of view, the gas can better exist on the copper surface.