Climbing Image Nudged Elastic Band Research Articles

Interstitial and substitutional V5+-doped TiNb2O7 microspheres: A novel doping way to achieve high-performance electrodes

Dual improvement on electronic conductivity and ionic conductivity of TiNb2O7 (TNO) is of great significance for realizing high-performance lithium-ion batteries. In this work, the V5+-doped TNO microspheres (denoted as Vx-TNOMs, x = 0, 0.015, 0.030 and 0.045) were synthesized via a simple solvothermal approach. X-ray diffraction coupled with Rietveld refinements and Raman spectra analyses verified that V5+ not simply possessed substitution doping mode but also inserted into the interstices of the TNOMs lattice. The density functional theory (DFT) calculations indicated that the improved electronic conductivity could originate from the formation of impurity bands after the V5+ doping. Meanwhile, the climbing image-nudged elastic band (CI-NEB) demonstrated that a TNOMs framework with faster ion transport pathways is achieved by the V5+ doping. Served as anode material of lithium-ion batteries, the V0.030-TNOMs electrode presents an impressive discharge capacity of 163.5 mAh/g at 10 C and long cycle life up to 2000 cycles with a capacity fading of merely 0.014% per cycle. Furthermore, the full battery employing V0.030-TNOMs as an anode and commercial LiCoO2 as a cathode exhibits superior electrochemical performances with promising application prospect. Our present study sheds new light on constructing high-performance electrodes for electrochemical energy storage.

An automated cluster surface scanning method for exploring reaction paths on metal-cluster surfaces

Metal-cluster surfaces present a wide variety of unique coordination environments. This complexity makes it difficult to manually probe the surface reactivity of such clusters. Here, we present a simple and automated method to systematically discover reaction pathways on cluster surfaces, based on the automated cluster surface scanning (ACSS) technique for mapping out potential energy surfaces. We showcase our method on 55-atom icosahedral Cu and Ag clusters, where we determine the activation energies of four elementary steps common in heterogeneous catalysis – hydrogen recombination (H* + H* → H2* + *), oxygen recombination (O* + O* → O2* + *), water formation (OH* + H* → H2O* + *), and CO oxidation (CO* + O* → CO2* + *) – with density functional theory calculations (DFT-PBE + D3). We show that the ACSS method requires significantly less human effort than the established manually performed climbing-image nudged elastic band (MP + CI-NEB) technique and locates transition states with comparable accuracy (root-mean-squared error of 0.10 eV) and similar computational cost. Rigorous sampling of the potential energy surface with the ACSS method allows one to locate all lowest-energy reaction pathways obtained via the MP+CI-NEB approach, as well as alternative pathways that one may have missed with the MP+CI-NEB approach due to the many possible pathways available on these clusters. The accuracy and efficiency afforded by the ACSS method could enable high-throughput exploration of the diverse reactivity of metal clusters.

First-Principles Study of Effects of Combined Ti Supervalent Cations and Lithium Ion Vacancies Doping on Crystal and Electronic Structures and Conductivity in LiFePO<sub>4</sub>

Olivine-type LiFePO4 is widely considered as a cathode for lithium-ion batteries owing to its environmental friendliness and low-cost, yet its applicability in the pristine state is limited due to poor electronic and ionic conductivity. To investigate the conductivity enhancement of LiFePO4, first-principles method under the GGA+U framework is implemented to study effects of doping with Ti4+ at Fe2+ sites under the lithium-deficient environment. LiFePO4 crystal and electronic structures as well as conductivity are investigated. Ti doping creates the impurity states at the acceptor level, which are normally degenerate states, but split into multiple states by the crystal field splitting. Doping under the lithium-deficient environment induces small hole polarons localizing at the Fe atoms and creates defect states located in the intermediate band. Both phenomena combine to facilitate charge carrier hopping. The climbing-image nudge elastic band (cNEB) calculation shows that Li hopping can be promoted by doping with high Ti concentration. This co-doping mechanism therefore can enhance both the electronic and ionic conductivities, which can be beneficial benchmark for cathode-material synthesis in the future.

Hydrogen diffusion into Pd(1 0 0) subsurface: Role of co-adsorbed bicomponent species on surface

This work examines the inherent features of co-adsorbed bicomponent species affecting microscopic mechanism of hydrogen diffusion from metal surface into its subsurface. The O + H O2 + H and OH + H were chose to separately cover on proper Pd(100) surface with unique adsorption configurations and varying coverages. The climbing image nudged elastic band (CI-NEB) method was employed to comprehensively investigate all available minimum energy pathways of hydrogen diffusion. The quantum results show that bicomponent species would change electronic structures of Pd(100) surface and then significantly affect the basic peculiarities of all diffusion routes in aspects of energy barriers adsorption and chemisorption sites. On base of total energies the lowest forward diffusion barriers of 0.082 0.125 and 0.154 eV were obtained corresponding to O + H O2 + H and OH + H adsorbates at 0.5 ML coverages of O O2 and OH respectively. Notably the O + H adsorbates demonstrate the best performance in facilitating hydrogen diffusion. Additionally the O2 + H are most conducive to reducing the forward barriers at 0.25 ML coverages of O O2 and OH. For each type of bicomponent species studied O O2 and OH severally predominate the impacts on changing Pd(100) electronic structure and then promoting diffusion process.

First-principles calculations on adsorption-diffusion behavior of Boron atom with tungsten surface

In this work, adsorption-diffusion behavior of Boron atom along and into W surface as well as the basic Boron atom absorption-diffusion behavior in W bulk were studied using first-principles density functional theory and climbing image nudged elastic band (CI-NEB) method. The result indicates that the octahedral interstice is the most stable position for B atom absorption and the energy barrier for B atom diffusing between two neighboring octahedral interstices in W bulk is 2.476 eV. For the adsorption behavior of Boron atom on W surface, Hollow site is the most stable adsorption position for W (100) surface and W (110) surface, while Hollow-II Site is the most stable adsorption position for W (111) surface. When B atom diffusing along W surface, the energy barrier for B atom diffusing along W (100) surface is the largest followed by W (110) surface and W (111) surface. During B atom diffusion into W surface, the needed energies for B atom diffusing into W (100) surface, W (110) surface and W (111) surface are 2.231 eV, 1.87 eV and 1.63 eV, respectively. After B atom diffuses to the 3rd, 3rd and 4th atomic layer below W (100) surface, W (110) surface and W (111) surface, the subsequent diffusion behavior shows the bulk diffusion characteristic. Beside, atomic diffusion coefficient D indicates that no matter diffusing along or into W surface, W (111) surface provides the best diffusion condition.

A Theoretical Study on the Interaction between Zinc Oxide Cluster (ZnO)3 and Mercury Ion (HgOH+)

The interaction between zinc oxide cluster (ZnO)3 in both pure and hydrated forms with mercury ion (HgOH+) has been investigated by using density functional theory (DFT) approach at the GGA-PBE/DZVP level of theory and climbing image – nudged elastic band (CI-NEB) method. The Fukui indices were used to predict the reactivity of the atoms. The adsorption energies were calculated. The results show that HgOH+ ion is strongly chemically adsorbed on the clusters. The adsorption process does not involve a transition state. When zinc oxide clusters were deposited on the mesoporous silica material (SBA-15), the adsorption ability of the assembly (ZnO)3/SBA-15 for HgOH+ is increased comparing to the pristine materials. Furthermore, calculation results show that the (ZnO)3/SBA-15 can adsorb HgOH+ even in the presence of a chloride ion.

DFT Study on Atomic Layer Deposition of Cerium Dioxide on Hydroxylated Titanium Dioxide As Efficient Photocatalyst

Design of photocatalyst materials for hydrogen production is of great interest due to a lack of cost-effective candidates. Titanium dioxide (TiO2) is a well-known photocatalyst with good stability, reasonable photoactivity, and low cost. To improve the efficiency and selectivity, we apply a surface modification approaches using nanostructured Cerium dioxide (CeO2) deposited onto TiO2 by atomic layer deposition (ALD). The deposition of CeO2 onto TiO2 uses the precursor Tetrakis(2,2,6,6-tetramethyl-3,5-heptanedionato)cerium (IV) (Ce(TMHD)4) and co-reactant ozone.The atomic layer deposition (ALD) of CeO2 on TiO2 was carried out by experiments with an atmospheric pressure fluidized bed ALD reactor. The TiO2 substrate was pre-treated using a combination of ozone and humidified nitrogen flow pulses to investigate the effect of the increase of surface functional groups on CeO2 deposition. The physicochemical properties of the obtained CeO2/TiO2 photocatalysts were characterized using different methods and their photocatalytic activity was evaluated during photocatalytic hydrogen evolution reaction.We have also performed density functional theory (DFT) calculations to investigate the reaction mechanism of metal precursors Ce(TMHD)4 and tris(methylcyclopentadienyl)cerium (III) (Ce(MeCp)3) on hydroxylated rutile and anatase TiO2 surfaces. The hydroxyl saturation coverage on different TiO2 facets was first addressed. These facets include anatase a-TiO2(001) and a-TiO2(101) and rutile r-TiO2(100) and r-TiO2(110). Hydroxyl groups OH and H are initially binding to surface Ti and O atoms, respectively. On a-TiO2(001) hydroxyl recombination to form water results in a final coverage of 0.81ML, due to the shortest neighboring HO-H distance on all surfaces. The Gibbs energy is then calculated to elucidate the effect of pressure and temperature on the hydroxylated termination of TiO2 surfaces. The final hydroxyl coverage is as follows: 0.81ML on a-TiO2(001) surface, 1ML on a-TiO2(101) and r-TiO2(100) surfaces, and 0.75ML on r-TiO2(110) surface.Two precursors (Ce(TMHD)4 and Ce(MeCp)3) are adsorbed on hydroxylated TiO2 surfaces. After precursor adsorption and relaxation, the surface hydroxyl coverage changes with water formation. After Ce(THMD)4 adsorbed on r-TiO2(100) surface, the hydroxyl coverage is decreased from 1ML to 0.25ML with 12 H2O molecules formation. After Ce(MeCp)3 adsorbed on a-TiO2(101) surface, although the hydroxyl coverage is not changed, the hydroxyl orientation is modified, breaking the symmetry. For Ce(TMHD)4, direct ligand dissociation is applied to study the mechanism of eliminating the ligands. For Ce(MeCp)3, the ligand can be eliminated by hydrogen transfer. The barrier for hydrogen transfer is calculated using climbing image nudged elastic band (CI-NEB) method. The results show that for Ce(MeCp)3, the reaction is endothermic on a-TiO2(101) surface, but it is overall exothermic on the other three surfaces. The facets play an important role in the eliminating the ligand. We present DFT results on the activity of ceria-modified rutile TiO2 towards water dissociation and CO2 activation showing that this heterostructured catalyst can promote both reactions. This work will be important to reveal the mechanism and feasibility of atomic layer deposition of ceria on different TiO2 surface facets.

Modelling the Atomic Layer Deposition of Cobalt and Ruthenium on NHx-Terminated Metal Surfaces

Atomic layer deposition (ALD) is widely used in microelectronics and semiconductor industry to deposit thin films as part of device fabrication in nano- or subnano-dimensions. The key advantages of ALD are the conformality and precise thickness control at the atomic scale, which are difficult for physical or traditional chemical vapor deposition methods. Cobalt (Co) and Ruthenium (Ru) are used as seed layers for metallization of interconnects. They are also potential materials for the electrode in dynamic random-access memory (DRAM) capacitors and metal-oxide-semiconductor field-effect transistors (MOSFETs). Plasma-enhanced ALD (PE-ALD) is used for low-temperature thin film growth by alternating exposures of metal precursors and plasma reactants. During the PE-ALD growth of metals, N-plasma, for example, NH3 or a mixture of N2 and H2, has been applied to avoid surface metal oxidation. The PE-ALD of Ru and Co has been experimentally investigated using metal precursors such as RuCp2, Ru(EtCp)2, CoCp2, and CoCp(CO)2. However, the reaction mechanism is not clear and theoretical studies on the reaction mechanism is entirely lacking.In this presentation, we study the PE-ALD growth of Co and Ru by first principle calculations. The (001) surface of both metals, with a hexagonal structure, is the most stable and the (100) surface with a zigzag structure is less stable but has high reactivity. These two surfaces allow the study of the influence of the surface facet. The surface saturation coverage was studied by considering individual adsorption and co-adsorption of NH and NH2 to terminate both surfaces. On the (001) surface, the zero K saturation coverage for NH is 1ML on Ru and 6/9ML on Co. For NH2, the saturation coverages are 6/9ML on Co and Ru surfaces. On the (100) surface, the saturation coverages are 2ML for NH on Ru and Co surfaces, 1.33 ML for NH2 on Ru, and 1ML for NH2 on Co surface. The higher saturation coverage on (100) surface is attributed to the unique trench structure, which provides more available surface sites than that of (001) surface. We also consider co-adsorption of NH and NH2 on (001) and (100) surfaces. The results are then analyzed with ab initio thermodynamics by calculating the Gibbs free energy. Both the ultra-high vacuum (UHV) condition and standard ALD operating condition are used to elucidate the effect of pressure and temperature on the termination of metal surfaces.The adsorption and reactions of metal precursors (CoCp2 and RuCp2) on NHx terminated metal surfaces were investigated with the inclusion of van der Waals corrections. Two possible adsorption structures, namely the precursor lying parallel and perpendicular to the NHx-terminated surfaces, were considered at zero K and ALD operating conditions. Possible reactions include: precursor adsorption, first hydrogen transfer, first Cp ligand dissociation, first Cp ligand desorption, second hydrogen transfer, and second Cp ligand desorption. The barrier for proton transfer was calculated using climbing image nudged elastic band (CI-NEB) method. Our results show that (100) surface has higher activity than (001) surface. In addition, the Cp ligand elimination of CoCp2 has lower barrier than RuCp2, regardless of surface facet. After the metal precursor pulse, the surface is terminated with MCp fragment or M atom depending on surface facet, where M is Ru or Co. The surface N atom and H atom can be eliminated during the following plasma step by forming NH3, N2 or H2. After a full cycle, the surface is an NHx-terminated metal surface and ready for the next cycle. This work will be important to reveal the mechanism and feasibility of atomic layer deposition of metals using N-plasma.

Nghiên cứu lý thuyết phản ứng methane hóa CO2 trên xúc tác Ni5/AC bằng phương pháp phiếm hàm mật độ. Phần I: Giai đoạn hấp phụ và hoạt hóa

The adsorption and activation processes of CO2 and H2 on Ni5 catalyst supported on activated carbon (Ni5/AC) were investigated by using density functional theory at GGA-PBE/DZP level of theory and climbing image – nudged elastic band (CI-NEB) method. The adsorption energy, charges on atoms, bond orders and geometry parameters were calculated and analyzed. The most favourable adsorption configurations were determined. The results show that H2 and CO2 are chemically adsorbed on Ni5/AC. The adsorption process does not involved a transition state. CO2 is strongly activated on Ni5/AC system.

Investigating the solution and diffusion properties of hydrogen in [formula omitted]-Uranium by first-principles calculations

The stability, solution and diffusion properties of an interstitial hydrogen atom in uranium metal have been firstly investigated by first-principles calculations. In energy, the octahedral site is more favorable for hydrogen to occupy than tetrahedra site with neglectable anisotropic perturbation. Besides, the effects of temperature on solution energy are quantified, which demonstrate the solution energy decreases fast with temperature. The calculated density of states and electronic charge re-distribution are analyzed. It is found the conductivity of metal uranium remains well after hydrogen occupied the interstitial position with lower concentration. The minimum migration pathways of interstitial hydrogen in uranium lattice are characterized by the climbing image nudged elastic band (CINEB) method. The obtained energy barriers are 0.239 eV, 0.298 eV and 0.313 eV with respect to O⟷T, O⟷O and T⟷T pathways with feeble structural deterioration. We believe our results for hydrogen diffusion in such a complex f-electron system not only provide en evidence for uranium corrosion but also supports the future experiments on measuring the hydriding rate and their interpretations.

Influence of nitrogen vacancies on selective oxidation of aromatic alcohols on g-C3N4: A comparative DFT study

The development of new metal-free catalysts that effectively oxidise alcohols to aldehydes or ketones is one of the fundamental goals of research on environmental photocatalysis. In this study, the influence of nitrogen vacancies on the catalytic process was investigated. We performed calculations using the density functional theory (DFT) to elucidate the mechanism of selective oxidation of benzyl alcohol to benzaldehyde with molecular O2 on g-C3N4. Through modelling of the pathways of the reaction, we found that vacancy defects strongly promote the formation of aldehyde. Our results clearly depicted that there are different interaction patterns that determine the reaction energy profiles for defective and defect-free systems. Furthermore, we estimated the energy barrier for the postulated rate-limiting step for a few alcohols by using the climbing image nudge elastic band (CI-NEB) method. The obtained values agreed very well with the benzaldehyde yield reported by Ding et al. (2018) [23]. In addition to the energetic effects, the electronic properties of the nitrogen-vacancy-containing g-C3N4 were also discussed. Our results demonstrated the underlying mechanism of selective oxidation of alcohols to aldehydes on both the catalysts in a comprehensive and systematic manner. Thus, by knowing the individual reaction steps, further increase in the substrate conversion may be attained by modification of the catalyst structure.

H2/CO2 separations in multicomponent metal-adeninate MOFs with multiple chemically distinct pore environments.

Metal–organic frameworks constructed from multiple (≥3) components often exhibit dramatically increased structural complexity compared to their 2 component (1 metal, 1 linker) counterparts, such as multiple chemically unique pore environments and a plurality of diverse molecular diffusion pathways. This inherent complexity can be advantageous for gas separation applications. Here, we report two isoreticular multicomponent MOFs, bMOF-200 (4 components; Cu, Zn, adeninate, pyrazolate) and bMOF-201 (3 components; Zn, adeninate, pyrazolate). We describe their structures, which contain 3 unique interconnected pore environments, and we use Kohn–Sham density functional theory (DFT) along with the climbing image nudged elastic band (CI-NEB) method to predict potential H2/CO2 separation ability of bMOF-200. We examine the H2/CO2 separation performance using both column breakthrough and membrane permeation studies. bMOF-200 membranes exhibit a H2/CO2 separation factor of 7.9. The pore space of bMOF-201 is significantly different than bMOF-200, and one molecular diffusion pathway is occluded by coordinating charge-balancing formate and acetate anions. A consequence of this structural difference is reduced permeability to both H2 and CO2 and a significantly improved H2/CO2 separation factor of 22.2 compared to bMOF-200, which makes bMOF-201 membranes competitive with some of the best performing MOF membranes in terms of H2/CO2 separations.

Density functional investigation of oxygen reduction reaction on Pt3Pd alloy electrocatalyst

We have investigated the surface structure and electronic properties of Pt segregated Pt3Pd (111) surface within the framework of density-functional theory. Surface adsorption of the oxygen reduction reaction (ORR) mediators including H, O, OH, O2, OOH, H2O2 and H2O and the related reaction pathways have then been thoroughly examined via determining the corresponding activation energies and reaction heats using climbing-image nudged elastic band (CI-NEB) calculations. It is found that the ORR preferably proceeds via OOH dissociation mechanism on the Pt3Pd (111) surface. Surface corrosion resistivity has also been investigated via evaluating the electrochemical potential shift of the surface Pt atoms. Results verify an enhanced stability against pure Pt (111) in the presence of an oxygen atom and nearly the same stability compared to clean pure Pt (111) surface.

Beryllium carbide as diffusion barrier against Cu: First-principles study**Project supported by the National Natural Science Foundation of China (Grant Nos. 11974253 and 11774248) and the National Key Technology Research and Development Program of the Ministry of Science and Technology of China (Grant No. 2017YFA0303600).

Beryllium carbide is used in inertial confinement fusion (ICF) capsule ablation material due to its low atomic number, low opacity, and high melting point properties. We used the method of climbing image nudged elastic band (CINEB) to calculate the diffusion barrier of copper atom in the crystal of beryllium and beryllium carbide. The diffusion barrier of copper atom in crystal beryllium is only 0.79 eV, and the barrier in beryllium carbide is larger than 2.85 eV. The three structures of beryllium carbide: anti-fluorite Be2C, Be2C-I, and Be2C-III have a good blocking effect to the diffusion of copper atom. Among them, the Be2C-III structure has the highest diffusion barrier of 6.09 eV. Our research can provide useful help for studying Cu diffusion barrier materials.

Ab initio study of the adsorption and dissociation of NO2 on pristine and Cu decorated ZnO(0001)-3 × 3

Due to the importance of reducing environmental pollution, especially NO2, we have studied the adsorption of this contaminant to eliminate it from the atmosphere. We perform ab initio calculations of the NO2 adsorption on pristine and copper decorated zinc oxide (0001)-3 × 3 surface. Several high symmetry sites have been tested of the NO2 molecule adsorption. The structural and electronic properties of the most stable configurations are reported. The adsorption energies values show that the molecule is chemisorbed. The minimum energy pathways (MEP), to study the NO2 dissociation process, were calculated by the climbing image nudged elastic band (CI-NEB) method. The activation energy of the first stage NO2 dissociation on pristine ZnO surface is 0.14 eV, 1.83 eV lower than the 1.97 eV on the Cu decorated system. On the other hand, for the second dissociation process, the activation energy when Cu is present has a value of 1.16 eV which is 0.8 eV lower than the 1.96 eV needed on the pristine surface. Our results suggest that NO2 adsorption and first dissociation is possible on pristine ZnO(0001), but copper decoration allows a second dissociation stage providing a possible mechanism of the NO2 transformation into harmless substances.

First principles study of H2O adsorption on U2Ti (1 1 0) surface

U-Ti alloy is a kind of metallic nuclear fuel that has been of interest to the nuclear power industry. Nuclear fuel is exposed to an aggressive physical, chemical and radiative environment, and may suffer corrosion damage caused by irradiation or chemicals like water vapour. Surface adsorption is the very start of the corrosion process. In this work, basing on the density functional theory (DFT) corrected for on-site Coulomb interactions (DFT + U), we investigate the adsorption behavior of H2O on U2Ti surface. U2Ti (1 1 0) surface is predicted to be the most stable one using DFT + U calculations. Eight possible sites are chosen to conveniently demonstrate the chemical circumstance on the (1 1 0) surface. Our study shows that the adsorption of H2O on U2Ti (1 1 0) surface is a mixed adsorption involving both physisorption and chemisorption. Physisorption can be observed on all these sites with an equilibrium distance, while chemisorption can only occur on some sites, mostly represented as dissociative adsorption. Density of states and deformation charge density are also calculated to examine the charge transfer between surface atoms as well as atoms in the bulk. A climbing image nudged elastic band (CI-NEB) method is used to determine the energy barriers for H2O migrating from the equilibrium position of physisorption to the states of dissociative adsorption. Bader charge analysis was performed to determine the numerical values of charge on each adsorbed atom.

The Ab Initio Calculations on the Areal Specific Resistance of Li‐Metal/Li7La3Zr2O12 Interphase

AbstractThe solid–solid interfacial impedance between the garnet electrolyte and Li metal anode is one of the major challenges for garnet's application in all‐solid‐state batteries. The areal specific resistance (ASR) is investigated by ab initio calculations in this article, to predict the intrinsic ASR as lower limitation. The Li ion migration across the interphase is divided into two steps: 1) Li "intercalation" from Li‐metal to the LLZO, forming a Li‐rich interphase beneath the surface of LLZO, and 2) Li migration in the Li‐rich interphase within LLZO. The first step is investigated by climbing image nudged elastic band (CI‐NEB), resulting in barrier energy lower than 0.37 eV compared to experimental 0.34 eV of the garnet bulk phase. The second step is investigated by ab initio molecular dynamic simulations (AIMD), indicating that the Li‐rich interphase's conductivity is lower by about 1–2 orders of magnitude compared to the bulk phase. As a result, the sum theoretical intrinsic ASR is as low as 0.01 Ω cm2, suggesting high ASR in practicable battery arises from surficial impurity Li2CO3 rather than intrinsic ionic resistance. However, difficulties of removing Li2CO3 lie in that Li7La3Zr2O12 can thermodynamically decompose into reactive Li2O to form high resistant Li2CO3.

First-Principles Study of the Influence of Zirconium on the Diffusion of Uranium Defects in Uranium Dioxide

The migration properties of uranium vacancies and interstitials in zirconium-doped uranium dioxide are studied by using density functional theory (DFT) and the climbing-image nudge elastic band (CI-NEB) method. The strong correlations among uranium 5f electrons were described by using a spherically averaged Hubbard parameter. In the model, the zirconium atoms are introduced by replacing the uranium atoms at the nearest and the next nearest neighbor sites along the diffusion path of uranium defects. The doping with zirconium obviously reduces the migration barriers for defects in uranium dioxide. The effect of doping with zirconium on the diffusion of uranium defects decreases with increasing distance between the zirconium dopant and the uranium defects. Further, we investigated the lattice distortion and the electron transfer associated with the migration of uranium defects, and we analyzed the physical origin of the reduction in the migration barriers caused by zirconium doping.

Insight into activation of CO and initial C2 oxygenate formation during synthesis of higher alcohols from syngas on the model catalyst K2O/Cu(111) surface

Alkali-metal-modified Cu-based catalysts are important in low-temperature synthesis of methanol and higher alcohols from syngas. Spin-polarized density functional theory calculations with periodic slab models were performed for a K2O/Cu(111) model catalyst to investigate the interactions between K2O and the Cu(111) surface, and activation of CO. The results show that K2O interacts strongly with the Cu(111) surface via electron transfer between K2O and the Cu(111) surface. This changes CO activation and modulates the mechanism of formation of the initial C2 oxygenates. Although K2O significantly promotes CO direct dissociation, H-assisted dissociation of CO is still the most energetically favorable route. The optimal reaction pathway for C2 oxygenate formation is proposed based on a climbing-image nudged elastic band calculation. The reactions starts with CO hydrogenation to yield the preferred monomers, i.e., CHO, CH2O, and CH2, through the pathways CO + H → CHO + H → CH2O + H → CH2 + OH, with reaction barriers of 0.92, 0.97, and 0.58 eV. The reaction of CH2 with CH2O is the main C2 oxygenate formation route on the K2O/Cu(111) surface. These results clarify the effects of K2O on Cu-based catalysts and elucidate the mechanism of C2 oxygenate formation during alcohol synthesis from syngas.

Effects of Mo alloying on stability and diffusion of hydrogen in the Nb16H phase: a first-principles investigation.

First-principles calculations and the method of climbing-image nudged elastic band were used to investigate the effects of Mo alloying on the structural stability, mechanical properties, and hydrogen-diffusion behavior in the Nb16H phase. The Nb12Mo4H phase (26.5 at% Mo) was found to be the most thermodynamically stable structure, with a low ΔHf value (−0.26 eV) and high elastic modulus. Calculations revealed that the tetrahedral interstitial site (TIS) was the predominant location of H in both Nb16H and Nb12Mo4H phases. The calculated H-diffusion energy barrier and the diffusion coefficient of the Nb12Mo4H phase were 0.153 eV and 5.65 × 10−6 cm2 s−1 (300 K), respectively, which suggest that the addition of Mo would lead to a lower energy barrier and high diffusion coefficients for the Nb16H phase, thus improving the hydrogen-permeation properties of Nb metal.