Climbing Image Nudged Elastic Band Research Articles

Facile Synthesis of Potassium Decahydrido‐Monocarba‐closo‐Decaborate Imidazole Complex Electrolyte for All‐Solid‐State Potassium Metal Batteries

All‐solid‐state potassium metal batteries have caught increasing interest owing to their abundance, cost‐effectiveness, and high energy/power density. However, their development is generally constrained by the lack of suitable solid‐state electrolytes. Herein, we report a new complex KCB9H10·2C3H4N2, synthesized by grinding and heating the mixture of potassium decahydrido‐monocarba‐closo‐decaborate (KCB9H10) and imidazole (C3H4N2) under mild conditions, to achieve the K‐ion superionic solid‐state electrolyte. The crystal structure was revealed as an orthorhombic lattice with the space group of Pna21 by FOX software. The diffusion properties for K+ in the crystal structure were calculated using the climbing image nudged elastic band (CI‐NEB) method. KCB9H10·2C3H4N2 exhibited a high ionic conductivity of 1.3 × 10−4 S cm−1 at 30 °C, four orders of magnitude higher than that of KCB9H10. This ionic conductivity is also the highest value of hydridoborate‐based K+ conductors reported. Moreover, KCB9H10·2C3H4N2 demonstrated a K+ transference number of 0.96, an electrochemical stability window of 1.2 to 3.2 V vs. K/K+, and good stability against the K metal coated by a layer of potassium imidazolate (KIm). These great performances make KCB9H10·2C3H4N2 a promising K‐ion solid‐state electrolyte.

A first-principles study of helium diffusion in aragonite under high pressure up to 40 GPa

Helium diffusion in carbonates under mantle pressure is crucial for understanding thermal and chemical evolution of mantle. Based on the density functional theory(DFT) and the the climbing image nudged elastic band (CI-NEB) method, we performed first-principles calculations of diffusion characteristics of helium in perfect aragonite crystal under high pressure to 40 GPa. Our results show that He diffusion behaviors are controlled by pressure, temperature and crystal size. The activation energy increases, and the diffusion coefficients decrease significantly under high pressure. Ea[100] increases from 176.02 kJ/mol to 278.75 kJ/mol, and Ea[001] increases from 195.89 kJ/mol to 290.43 kJ/mol, with pressure increasing from 20 GPa to 40 GPa. At 700 K, the diffusion coefficients at 40 GPa is 7 orders of magnitude lower than that at 20 GPa; and at 1000 K it decrease 5 orders of magnitude. To ensure that at least 90 % helium is not lost, we synthesized the temperature obtained from cooling and heating processes and derive the ’stable temperature range’ for helium in aragonite. The obtained results show that the stable temperature range is 22–76 ℃ at 0 GPa and 641–872 °C at 40 GPa, for the crystal of 100–2000 μm size. Besides, the travel time of helium in aragonite under high pressure increases rapidly with pressure increasing. Our calculations indicate that helium can be quantitatively retained in aragonite in the deep mantle as long as the temperature is in the ’stable temperature range’. These results have certain implications for exploring the evolution of mantle and the storage of helium within it.

Electrochemical performance of P4Se3 as high-capacity anode materials for monovalent and multivalent ion batteries

The increasing need for advanced anode materials with superior performance in rechargeable batteries has driven investigations into cutting-edge energy storage systems. This study delves into the potential of P4Se3 Molecular Cages (MCs) as innovative anode materials for both monovalent and multivalent ion batteries, including Na ion batteries (NIBs), Mg ion batteries (MIBs), Ca ion batteries (CIBs), Al ion batteries (AIBs), and Zn ion batteries (ZIBs) through a comprehensive Density Functional Theory (DFT) and molecular dynamics (MD) based investigations. The analyses encompass the electronic structure, structural stability, electrochemical performance, charge storage mechanisms, and redox properties to offer valuable insights into the potential of P4Se3 as an anode. The DFT calculations unveil critical aspects of adsorption, diffusion, and reaction kinetics in P4Se3 elucidating its potential as high-capacity and suitable anode material. The exothermic reactions between Na, Mg, Ca, Al, and Zn with host P4Se3 highlight its suitability for the intercalation process in monovalent and multivalent ion batteries. Furthermore, calculated storage capacities for NIB, MIB, CIB, AIB, and ZIB are found as 1484.84 mAhg−1, 519.69 mAhg−1, 1410.60 mAhg−1, 593.93 mAhg−1, and 74.24 mAhg−1 respectively. The voltage profiles indicate favorable open circuit voltages (OCV) of 0.31 V, 0.27 V, 0.55 V, and 3.3 V for NIB, MIB, CIB, and AIB respectively. The study employs climbing image nudged elastic band (Cl-NEB) simulations to compute diffusion barriers faced by monovalent and multivalent ions in the host structure. The calculated minimal diffusion barriers of 0.18 eV for NIB, 0.30 eV for MIB, 0.35 eV for CIB, 0.33 eV for AIB, and 0.71 eV for ZIB indicate fast charging capabilities due to efficient ion movement. Furthermore, the respective calculated values of diffusion coefficient are found as 5.27 × 10−10 m2/s, 5.27 × 10−9, 4.0 × 10−10 m2/s, 5.27 × 10−10 m2/s and 5.27 × 10−11 m2/s and respective values of ionic conductivity σ are found as 4.13 × 10−3 S/m, 14.57 × 10−2 S/m, 30.03 × 10−3 S/m, 16.65 × 10−3 S/m and 0.20 × 10−3 S/m for NIBs, MIBs, CIBs, AIBs and ZIBs. The findings of this study point out suitability of P4Se3 as anode material in monovalent and multivalent ion batteries.

Physical Prior Mean Function-Driven Gaussian Processes Search for Minimum-Energy Reaction Paths with a Climbing-Image Nudged Elastic Band: A General Method for Gas-Phase, Interfacial, and Bulk-Phase Reactions.

The climbing-image nudged elastic band (CI-NEB) method serves as an indispensable tool for computational chemists, offering insight into minimum-energy reaction paths (MEPs) by delineating both transition states (TSs) and intermediate nonstationary structures along reaction coordinates. However, executing CI-NEB calculations for reactions with extensive reaction coordinate spans necessitates a large number of images to ensure a reliable convergence of the MEPs and TS structures, presenting a computationally demanding optimization challenge, even with mildly costly electronic-structure methods. In this study, we advocate for the utilization of physically inspired prior mean function-based Gaussian processes (GPs) to expedite MEP exploration and TS optimization via the CI-NEB method. By incorporating reliable prior physical approximations into potential energy surface (PES) modeling, we demonstrate enhanced efficiency in multidimensional CI-NEB optimization with surrogate-based optimizers. Our physically informed GP approach not only outperforms traditional nonsurrogate-based optimizers in optimization efficiency but also on-the-fly learns the reaction path valley during optimization, culminating in significant advancements. The surrogate PES derived from our optimization exhibits high accuracy compared to true PES references, aligning with our emphasis on leveraging reliable physical priors for robust and efficient posterior mean learning in GPs. Through a systematic benchmark study encompassing various reaction pathways, including gas-phase, bulk-phase, and interfacial/surface reactions, our physical GPs consistently demonstrate superior efficiency and reliability. For instance, they outperform the popular fast inertial relaxation engine optimizer by approximately a factor of 10, showcasing their versatility and efficacy in exploring reaction mechanisms and surface reaction PESs.

Out-of-Plane Single-Copper-Site Catalysts for Room-Temperature Benzene Oxidation.

Crafting single-atom catalysts (SACs) that possess "just right" modulated electronic and geometric structures, granting accessible active sites for direct room-temperature benzene oxidation is a coveted objective. However, achieving this goal remains a formidable challenge. Here, we introduce an innovative in situ phosphorus-immitting strategy using a new phosphorus source (phosphorus nitride, P3N5) to construct the phosphorus-rich copper (Cu) SACs, designated as Cu/NPC. These catalysts feature locally protruding metal sites on a nitrogen (N)-phosphorus (P)-carbon (C) support (NPC). Rigorous analyses, including X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS), validate the coordinated bonding of nitrogen and phosphorus with atomically dispersed Cu sites on NPC. Crucially, systematic first-principles calculations, coupled with the climbing image nudged-elastic-band (CI-NEB) method, provide a comprehensive understanding of the structure-property-activity relationship of the distorted Cu-N2P2 centers in Cu/NPC for selective oxidation of benzene to phenol production. Interestingly, Cu/NPC has shown more energetically favorable C-H bond activation compared to the benchmark Cu/NC SACs in the direct oxidation of benzene, resulting in outstanding benzene conversion (50.3 %) and phenol selectivity (99.3 %) at room temperature. Furthermore, Cu/NPC achieves a remarkable turnover frequency of 263 h-1 and mass-specific activity of 35.2 mmol g-1 h-1, surpassing the state-of-the-art benzene-to-phenol conversion catalysts to date.

Out‐of‐Plane Single‐Copper‐Site Catalysts for Room‐Temperature Benzene Oxidation

AbstractCrafting single‐atom catalysts (SACs) that possess “just right” modulated electronic and geometric structures, granting accessible active sites for direct room‐temperature benzene oxidation is a coveted objective. However, achieving this goal remains a formidable challenge. Here, we introduce an innovative in situ phosphorus‐immitting strategy using a new phosphorus source (phosphorus nitride, P3N5) to construct the phosphorus‐rich copper (Cu) SACs, designated as Cu/NPC. These catalysts feature locally protruding metal sites on a nitrogen (N)‐phosphorus (P)‐carbon (C) support (NPC). Rigorous analyses, including X‐ray absorption spectroscopy (XAS) and X‐ray photoelectron spectroscopy (XPS), validate the coordinated bonding of nitrogen and phosphorus with atomically dispersed Cu sites on NPC. Crucially, systematic first‐principles calculations, coupled with the climbing image nudged‐elastic‐band (CI‐NEB) method, provide a comprehensive understanding of the structure‐property‐activity relationship of the distorted Cu−N2P2 centers in Cu/NPC for selective oxidation of benzene to phenol production. Interestingly, Cu/NPC has shown more energetically favorable C−H bond activation compared to the benchmark Cu/NC SACs in the direct oxidation of benzene, resulting in outstanding benzene conversion (50.3 %) and phenol selectivity (99.3 %) at room temperature. Furthermore, Cu/NPC achieves a remarkable turnover frequency of 263 h−1 and mass‐specific activity of 35.2 mmol g−1 h−1, surpassing the state‐of‐the‐art benzene‐to‐phenol conversion catalysts to date.

Theoretical study of NO/CO/O2 gas separation using graphdiyne and boron-doped graphdiyne membranes – A new corresponding state principle

The adsorption and permeance of O2, NO and CO gases on graphdiyne (GDY) and boron-doped graphdiyne (B-doped GDY) surfaces are modeled and studied. Dispersion-corrected density functional theory (DFT) calculation combined with the Climbing Image Nudged Elastic Band (CI-NEB) method was used to elucidate the gas/membrane adsorption energy, equilibrium distance and energy barrier of the studied gases passing through the membranes. It is revealed that the selectivity of B-doped GDY to NO is much greater than that of GDY. Based on selectivity and permeability values, high separation efficiency of NO can be achieved by lowering B-doped GDY temperature below 100 K. In addition, using the corresponding principle, a general expression for the dimensionless permeance of gases in terms of reduced temperature is introduced, which helps us to find an industrially acceptable value of permeance for each gas at a given pressure.

Modeling the Atomic Layer Deposition of Metal Thin Films: Studying the Surface Reaction Mechanism By Density Functional Theory Simulations

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. The atomic scale understanding of ALD is vital and essential to design and optimize the deposition process, where density functional theory (DFT) calculations play an important role in providing detailed reaction mechanism, theoretical screening of suitable precursors and estimated growth-per-cycle (GPC).In this study, we show our recent works on DFT calculations of the growth mechanism study of Cobalt (Co) thin film by plasma-enhanced ALD1,2. We first addressed the surface reaction mechanism at the metal precursor pulse and plasma half-cycle on NHx-terminated Co surfaces, which corresponds to the steady growth for the PE-ALD. The adsorption and reactions of metal precursors (CoCp2) on NHx terminated metal surfaces were investigated with the inclusion of van der Waals corrections. The plausible reaction pathways include: precursor adsorption, hydrogen transfer, CpH formation and CpH desorption. The direct Cp dissociation mechanism is not considered on these NHx-terminated metal surfaces due to experimentally observed minimal C impurities at the deposited metal thin films, which indicates that most of the Cp ligand is removed completely. The barrier for proton transfer was calculated using climbing image nudged elastic band (CI-NEB) method.The reactions at the initial stages on typical H:Si(100) surface are investigated to gain atomic insight on the effect of different substrates on the elimination of Cp ligands. Our work is important to reveal the mechanism and feasibility of atomic layer deposition of metals using N-plasma.In addition, we present our recent works3, 4 on the thermal ALD of Co using reducing agent Zn(DMP)2 and thermal ALD of Fe4Zn9 thin film with diethyl zinc (DEZ). Our DFT results show that by selecting suitable reducing agent, we can control the deposited thin films to be Zn-free high quality Co thin film or desired intermetallic thin film with high Zn content.These DFT results are important to understand the reaction mechanism for ALD and provides new knowledge in depositing the thin films by adjusting the reducing agents.

A DFT study on nitrogen enhancing mechanism of SRF niobium cavity performance

High-temperature nitriding has been shown to improve the quality factor (Q) of the superconducting radio frequency (SRF) cavity recently, while its inherent promoting mechanism is still unclear. Therefore, based on first-principles calculations, the adsorption and dissociation processes of nitrogen molecules on the Nb(110) surface, and the adsorption, diffusion processes of nitrogen atoms on the Nb(110) surface, were studied in this paper to explain the principle of its performance improvement. To establish the reaction mechanism, adsorption preference sites were determined. All possible reaction paths were investigated for each step including the energy barrier based on the climbing image nudged elastic band (CI-NEB) method. We establish multiple reaction paths and the only optimal one was found. It is found that the Nb(110) surface can facilitate the dissociation of nitrogen molecules (with only a 1.14 eV energy barrier), and nitrogen atomic energy can readilyreach the subsurface (only a 1.73 eV energy barrier). Still, the high energy barrier (up to 6.64 eV) prevents it from diffusing inward. This revelation provides the nitriding mechanism of pure niobium and the rationale for enhancing niobium cavity performance after nitrogen mixing.

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.

Atomistic simulations of dislocation activity in Si nanofibers in Al-Si eutectics

Atomistic simulations were used to elucidate the mechanisms of localized plasticity and fracture in Si nanofibers embedded in Al matrix, with uniaxial tensile straining along the fiber axis. The Si nanofibers within the Al matrix exhibited large non-linear elastic strains followed by localized plasticity with the latter accounting for approximately 5 % permanent length increase in the fibers. On the other hand, simulations with aligned Si nanofibers in vacuum (no Al matrix) exhibited elastic deformation only followed by cleavage fracture. Localized plasticity in Si nanofibers is accomplished by gliding of “zonal” partial dislocations with Burgers vector b=aSi3〈112〉, which leads to synchronous shear of two shuffle-sets. Local stress concentration in Si nanofibers due to non-screw segments of dislocation loops around fibers triggers yielding in the nanofibers through the activation of dislocation sources at or near the interfaces but only below some critical fiber diameter, highlighting the role of nanoscale refinement in activating plasticity in embedded Si nanofibers in Al matrix. In addition, the kinetic barriers associated with the glide of edge/screw zonal dislocation in the perfect crystal and twinned crystal are computed using climbing-image nudge elastic band method. A higher mobility of the zonal dislocation in the twinned crystal suggests that twin boundaries can assist in dislocation glide parallel to the boundary. On the other hand, a nano-crack is nucleated at the interaction position of a glide partial dislocation with an intersecting growth twin boundary suggesting that intersecting twins serve to block glide and provide sites for crack nucleation.

Prediction of primary knock-on damage during electron microscopy characterization of lithium-containing materials

To fulfill power and energy demands, lithium-ion battery (LIB) is being considered as a promising energy storage device. For the development of LIBs, high-resolution electron microscopy characterization of battery materials is crucial. During this characterization, the interaction of beam-electrons with Li-containing materials causes damage through several processes, especially knock-on damage. In this study, we investigated this damage by determining the probability of knock-on damage and performing Monte Carlo simulation. For this objective, the threshold displacement energies (TDEs) were computed using sudden approximation technique for three sets of materials, including pure elements, LiX (X = F, Cl, Br), and Li2MSiO4 (M = Fe, Co, Mn). By including the Climbing-Image Nudge Elastic Band (CI-NEB) method into the sudden approximation approach, it was found that the accuracy of the predicted TDEs could be improved. Results also indicated that at moderate electron energies, the knock-on damage for Li in both its elemental and compound forms maximized. In addition, it was shown that the TDE should be the principal parameter for assessing the Li sensitivity to knock-on damage across similar structures. Nonetheless, other parameters, including cross-section, density, weight fraction, atomic weight, and atomic number, were found to impact the knock-on damage.

Effect of solute Nb and Sn on self-interstitial atom defect in zirconium-based alloys by first-principles calculations

Zirconium based alloys are viewed as one of the most important fuel cladding materials, and alloy elements play a crucial role in improving its irradiation resistance, especially Nb and Sn. In present work, self-interstitial atom formation energy, binding energy between dopant Nb/Sn and the self-interstitial atom, and electronic structure are systematically investigated with the density functional theory. The diffusion barriers of the self-interstitial atom under the effect of Nb/Sn are calculated with the climbing image nudged elastic band (CI-NEB) method. Depending on the calculation results, the most stable configuration is BT and BS after doping Nb and Sn atom, respectively, different from BO for the Zr crystal without dopant. In the doped structure, the orbital hybridization of Nb atom and self-interstitial atom was obvious, while the orbital overlap of Sn atom with matrix atom and self-interstitial atom was less. Orbital hybridization of Nb atom and zirconium atoms mainly occurs between d orbits. After doping Nb, the diffusion barrier of the interstitial atoms increases, inhibits their diffusion or cluster growth. And the dopant Sn makes the formation of interstitial atom more difficult depending on the larger formation energy. The calculation results may explain the mechanism of improving its irradiation resistance by Nb and Sn doping.

The role of oxygen incorporation in Ni (111) substrates on the growth of hexagonal boron nitride monolayers

Reliable and controllable growth of two-dimensional (2D) hexagonal boron nitride (h-BN) is essential for its wide range of applications. Substrate engineering is one of the critical factors that influence the growth of the epitaxial h-BN films. Here, we report the growth of monolayer h-BN on Ni (111) substrates incorporated with oxygen atoms via molecular beam epitaxy. It was found that the increase of incorporated oxygen concentration in the Ni substrate through a pretreatment process prior to the h-BN growth step would have an adverse effect on the morphology and growth rate of 2D h-BN. Under the same growth condition, h-BN monolayer coverage decreases exponentially as the amount of oxygen incorporated into Ni (111) increases. Density functional theory calculations and climbing image nudged elastic band (CI-NEB) method reveal that the substitutional oxygen atoms can increase the diffusion energy barrier of B and N atoms on Ni (111) thereby inhibiting the growth of h-BN films. As-grown large-area h-BN monolayer films and fabricated Al/h-BN/Ni (MIM) nanodevices were comprehensively characterized to evaluate the structural, optical and electrical properties of high-quality monolayers. Direct tunneling mechanism and high breakdown strength of ∼11.2 MV cm−1 are demonstrated for the h-BN monolayers grown on oxygen-incorporated Ni (111) substrates, indicating that these films have high quality. This study provides a unique example that heterogeneous catalysis principles can be applied to the epitaxy of 2D crystals in solid state field. Similar strategies can be used to grow other 2D crystalline materials, and are expected to facilitate the development of next generation devices based on 2D crystals.

Ab initio study of Li-shrouded Si-doped [formula omitted]-Graphyne nanosheet as propitious anode in Li-ion batteries

Li-ion batteries (LIBs) have become a house-hold name in the energy-storage research community in the wake of their noteworthy attributes. With increasing energy demands, the urgency to develop high performance LIBs is also rising rapidly which has compelled the researchers to design and fabricate new high capacity materials. Bearing that in mind, we have investigated a carbon–silicon based anode, i.e. Si-doped γ-Graphyne (SiG) nanolayer for its applicability as LIB electrode. Carbon and Silicon are two complementary choices for anode material due to the reason that the weakness of one is complemented by the other i.e. the extensive volume changes of Si anodes is compensated by the stable carbon matrix and the lower Li affinity of carbon is complemented by large capacities of Si anode. The Density Functional Theory (DFT) based studies reveal that SiG nanolayer has a dynamically and thermally stable structure with reliable mechanical properties. Density of states (DOS) calculation substantiates the good conductivity of the nanolayer. The binding energy of the Li atom (adatom) is −1.33 eV resulting from a charge transfer of 0.88e from Li to the nanolayer. The SiG anode achieved a specific capacity of 1005.05 mAhg−1 (almost 2.7 times the Graphite anode used in commercial LIBs). Climbing Image-Nudged Elastic Band (CI-NEB) calculations depict an easy migration of the adatoms through the anode with an energy barrier of 0.67 eV having diffusion coefficient (D) as 8.91 × 10−14 cm2/s. A low working voltage of 0.44 V is obtained advocating a safe operation and good cyclability. So, the theoretical investigation presents favorable outcomes for Si-doped γ-Graphyne to be applicable as a progressive anode in next-generation LIBs.

First-Principles Study of Atomic Diffusion by Vacancy Defect of the L12-Al3M (M = Sc, Zr, Er, Y) Phase.

Atomic diffusion by the vacancy defect of L12-Al3M (M = Sc, Zr, Er, Y) was investigated based on a first-principles calculation. The point defect formation energies were firstly evaluated. Then, the migration energy for different diffusion paths was obtained by the climbing-image nudged elastic band (CI-NEB) method. The results showed that Al atomic and M atomic diffusions through nearest-neighbor jump (NNJ) mediated by Al vacancy (VAl) were, respectively, the preferred diffusion paths in Al3M phases under both Al-rich and M-rich conditions. The other mechanisms, such as six-jump cycle (6JC) and next-nearest-neighbor jump (NNNJ), were energetically inhibited. The order of activation barriers for NNJ(Al-VAl) was Al3Zr < Al3Y < Al3Er < Al3Sc. The Al3Sc phase had high stability with a high self-diffusion activation barrier, while the Al3Zr and Al3Y phases were relatively unstable with a low self-diffusion activation energy. Moreover, the atomic-diffusion behavior between the core and shell layers of L12-Al3M was also further investigated. Zr atoms were prone to diffusion into the Al3Y core layer, resulting in no stable core-shelled Al3(Y,Zr), which well agreed with experimental observation.

Theoretical insight into the promotion effect of potassium additive on the water-gas shift reaction over low-coordinated Au catalysts.

How to elucidate the effect of alkali metal promoters on gold-catalyzed water-gas shift reaction intrinsically remains a challenging, because that the complex synergy effects such as strong metal-support interactions, interfacial effects, and charge transfer of supported metal catalysts makes people difficulty in the understanding the alkali promotion phenomenon in nature. Herein, we report a systematically study of whole water-gas shift reaction mechanism on pure and the K-modified defected-Au(211) (i.e., by removing one surface Au atom from perfect Au(211) and make one model with the Au-Au coordination number is six) by using the microkinetic modeling based on first principles. Our results indicate that the presence of K can increase the adsorption ability of oxygen-containing species via the attractive coulomb interaction, has no significant effect on the adsorption of H species, but inhibits the adsorption of CO due to the steric effect. K promoter stabilizes the water adsorption by ~0.3 eV, which results in one order increasing of whole reaction rate. Interestingly, the strong promotion effect of the K can be assigned to the significant direct space interaction between K and the adsorbate H2O* through the inducted electric field, which can be further confirmed by the posed negative electric field on the unpromoted D-Au(211). Microkinetic modeling results revealed that the carboxyl mechanism is the most likely to occur, redox mechanism is the next one, and the formate mechanism is the least likely to occur. For different kinds of alkali metal additives, the adsorption strength of water molecules gradually weakens from Li to Cs, but Na shows the best promoter behavior at the low temperature. By considering the effect of K contents on the reactivity of water-gas shift reaction, we found that the K with the medium coverage (~0.2~0.3 ML) has the strongest promoting effect. It is expected that the conclusion of this work can be extended to other WGSR catalytic systems like Cu(or Pt). All calculations were performed by using the plane-wave based periodic method implemented in Vienna ab initio simulation package (VASP, version 5.4.4), where the ionic cores are described by the projector augmented wave (PAW) method. The exchange and correlation energies were computed using the Perdew, Burke and Ernzerhof functional with the vdw correction (PBE-D3). The transition states (TSs) were searched using the climbing image nudged elastic band (CI-NEB) method. Some electronic structure properties like work function was predicated by the DS-PAW software. Microkinetic simulation was carried out using MKMCXX software.

Tuning the catalytic activity of Nb2C MXenes via surface functionalization and defects

Herein, we investigated the pathways to modulate the electronic properties and improve the activity of Nb2C MXenes by functionalizing their surface with the terminating groups and creating surface defects. Through an in-depth Density Functional Theory (DFT) study, the activity of Nb2C MXenes terminated with F, Cl, S, and their combinations, denoted as Nb2C-T, were explored, and their activities were tested for H2O, O2, N2, and CO2 adsorption and dissociation. It was found that the work function of Nb2C-T MXenes varies from 3.79 to 5.07 eV, and Nb d-band center ranges between -2.53 to -1.53 eV. This provides the opportunities to tune their activities for specific chemical reactions since most catalytic reactions depend on the electronic properties of materials. To improve the catalytic activity of terminated Nb2C-T further, Nb2C-S and Nb2CCl, having the highest and lowest work functions, respectively, were analyzed with defects created by removing one surface terminating group, representing partially functionalized surfaces (Nb2C-Tx-1). The Climbing Image Nudge Elastic Band (CI-NEB) calculations for pristine and defected Nb2C-S and Nb2CCl surfaces confirmed that defected surfaces have a lower dissociation barrier for O2, H2O, N2, and CO2, and therefore, could prove as better catalysts, as compared to the pristine functionalized Nb2C surfaces. Specifically, Nb2C-Sx-1 showed a dissociation barrier as low as 0 eV for O2 and CO2, and 0.16 and 0.17 eV for H2O and N2, respectively, proving it to be an attractive candidate for reactions involving the dissociation of O2, H2O, N2, and CO2. Overall, this work indicates that there is a potential to improve the catalytic performance of Nb2C MXenes by altering its terminating groups. By choosing the right synthesis method, functionalized Nb2C MXenes with the desired electronic properties could be prepared in the future for critical chemical reactions.

Influencing mechanism of carbon monoxide on the hydrogen absorption performance of Zr2Fe

Zr2Fe is considered as the promising hydrogen isotopes trapping material to control the quantity of tritium generated in deuterium–tritium fusion reaction. However, the vulnerability to poisoning by impurity gases such as carbon monoxide is the bottleneck for their applications. In this study, we show from our DFT calculations that the Zr2Fe(101) surface is saturated with 13CO molecules and the hollow site is the most stable CO adsorption site. The adsorption energy of CO is much lower than that of hydrogen, resulting in the preferential adsorption of CO on Zr2Fe(101). The dependence of the Gibbs free energies of CO on the temperatures under UHV conditions were also obtained. The climbing-image nudged elastic band results reveal that CO pre-coverage would block the hydrogen adsorption sites and affect the hydrogen diffusion. The Sieverts analysis demonstrates that even the low concentration CO is a fatal poison to the hydrogenation process of Zr2Fe at ambient temperature. Hydrogen absorption may occur at a higher temperature with a lower absorption rate and capacity, as CO starts to desorpt from Zr2Fe(101) with the increase of temperature and thus making H2 absorption possible, even though the H diffusion energy barriers are relatively high. Our results provide the basis for exploring the influence mechanisms of the impurity gas CO on the hydrogen adsorption properties of the Zr-based alloys, which would be beneficial for the applications of metal getter of tritium-containing waste gas.

New insights into the effect of zirconium dopant on tritium release from Li2TiO3 (001) surface from first-principles calculation

The influence of zirconium (Zr) dopants on tritium release from Li2TiO3 (001) surface was researched by density functional theory (DFT) calculations. The energy barriers for T2 and T2O molecules formation on undoped and doped surfaces were calculated and compared by using climbing-image nudged elastic band (CI-NEB) method. It was found that T2O is more easily generated than T2 on the surfaces. And Zr dopants can significantly promote the production and release for T2O on the surface of Li2TiO3. Especially when Zr atoms doping ratio is to be 3.12 atom % (Zr3), the barrier of T2O formation changed from 0.97 eV on undoped surface to spontaneous formation on doped surface. The energy barrier for T2O desorption on the surface also decreases to a smaller energy barrier of about 0.42 eV. Although the catalytic effect of Zr doping on T2 formation on the surface is relatively weak. The electronic characteristics show that the doping of Zr atoms can influence the charge transfer of surface atoms, making OT group more likely to appeal to adjacent T atom, which may be the reason for promoting the formation of T2O by Zr dopants. This study can provide useful details for the effect mechanism and catalytic behavior of Zr dopants on the release of tritium from Li2TiO3 surface.