Establishing the Accuracy of Broadly Used Density Functionals in Describing Bulk Properties of Transition Metals

Determining the chemical ordering in nanoalloys by considering atomic coordination types

The energetically most favorable chemical ordering of bimetallic nanoparticles can be characterized by combining global optimization algorithms and surrogate energy models. The latter approximate the energy of nanoalloys relying on structural descriptors, training models, and data. Here, we systematically evaluate the performance of highly data-efficient topological descriptors [Kozlov et al., Chem. Sci. 6, 3868 (2015)] for predicting the energies of metal nanoalloys with different chemical orderings. We also introduce a new descriptor based on atomic coordination types, which results in a less data-efficient and interpretable approach, but improves the general accuracy and the quantification of orderings in the inner parts of nanoparticles. The capacity of both the original and new approaches in combination with a basin hopping algorithm is illustrated by generating convex hulls of PdZn nanoalloys and predicting the resulting active surface site distribution as a function of particle composition. Finally, we show how these approaches can be combined with machine-learning adsorption models in electrocatalysis studies for a fast evaluation of the reactivity landscape of targeted nanoalloys.

The Adsorption Behavior of Gas Molecules on Mn/N- and Mn-Doped Graphene

By using density functional theory (DFT), the adsorption behavior of gas molecules on defective graphene doped with manganese and nitrogen were investigated. The geometric structure, electronic structure, and magnetic properties of two substrates were calculated and the sensing mechanism was also analyzed. The results indicate that the MnSV-GP and MnN3-GP have stronger structural stability, in which Mn atoms and their coordination atoms will become the adsorption point for five gas molecules (CH2O, CO, N2O, SO2, and NH3), respectively. Moreover, at room temperature (298 K), the recovery time of the MnSV-GP sensor for N2O gas molecules is approximately 1.1 s. Therefore, it can be concluded that the MnSV-GP matrix as a magnetic gas sensor has a promising potential for detecting N2O. These results also provide a new pathway for the potential application of Mn-doped graphene in the field of gas sensors.

Electronic structure and thermodynamic approaches to the prospect of super abundant vacancies in δ-Pu

Super abundant vacancies (SAVs) have been suggested to form in the fcc phase of plutonium, δ-Pu, under a low-pressure hydrogen environment. Under these conditions, the vacancy concentration is proposed to reach 10-3 at% due to H trapping in vacancies lowering the effective vacancy formation energy. Previous density functional theory (DFT) results suggest that seven H atoms can be trapped in a single vacancy when a collinear special quasirandom magnetic structure is used to stabilize the δ phase, suggesting SAVs are a possible source of H stored in plutonium. In this report, we present DFT results for δ-Pu in the noncollinear 3Q magnetic state to study the formation of SAVs in mechanically stable δ-Pu. Together with these new simulations, we use publicly available computational and experimental data to provide further constraints on the physical conditions needed to thermodynamically stabilize SAVs in δ-Pu. Using several thermodynamic models, we estimate the vacancy concentrations in δ-Pu and discuss the limits of hydrogen driven formation of vacancies in δ-Pu. We find that, when hydrogen in the lattice is equilibrated with gaseous H2, the formation of SAVs in δ-Pu is unlikely and any excess vacancy concentration beyond thermal vacancies would need to occur by a different mechanism.

Understanding the Chemical Bond in Semiconductor/MXene Composites: TiO2 Clusters Anchored on the Ti2C MXene Surface

First-principles calculations on titania clusters (TiO2)n (n=5 and 10) supported on the pristine Ti2C (0001) surface were carried out to understand the properties of semiconductor/MXene composites with implications in (photo)-catalysis. The reported results reveal a high exothermic interaction accompanied by a substantial charge transfer with a concomitant, notorious, deformation of the titania nanoclusters. The analysis of the density of states analysis of the composite systems evidences a metallic character with titania related states crossing the Fermi level. The picture of the chemical bonds is completed by the analysis of X-Ray Photoelectron Spectra (XPS) features, evidencing clear shifts of the C(1s) and O(1s) related peaks relative to the isolated systems that have a quite complex origin. This detailed analysis provides insights to experimentalists interested in the design and synthesis of these systems with possible applications in catalysis.

The regulatory function of the d-orbital structure in TM@g-t-C4N3 for bifunctional catalysis of the oxygen evolution/reduction reaction

Highly efficient catalysts for the oxygen evolution/reduction reaction (OER/ORR) have attracted great attention in research for energy devices with high conversion efficiency. Herein, systematic first-principles investigations are performed to explore the catalytic performance of graphitic C4N3 loaded with single transition metal atoms (TM@g-t-C4N3) for the OER/ORR. The results show that Fe, Co, Ni and Rh@g-t-C4N3 exhibit fascinating bifunctional catalytic activities for both the OER and ORR. Moreover, it is observed that better activities are easily achieved when the valence d orbitals of doped TM atoms are nearly fully occupied. Further analysis reveals the volcano relationship between the OER/ORR performance and the adsorption Gibbs free energy. The adsorption free energy of intermediates in the OER/ORR process is also found to highly correlate with the electronic structures of TM@g-t-C4N3, which are mainly characterized by two quantities, one is the descriptor φ related to the electronegativity and the number of valence electrons in d orbitals, and the other is the projected d band center. The results indicate that it is possible to predict the catalytic performance of TM@g-t-C4N3 by a detailed examination of the electronic properties of the doped TM atoms to some extent. This research not only provides several highly active g-t-C4N3-based single-atom catalysts (SACs) for the OER/ORR, but also reveals some potential regularities of SAC systems.

Effects of Oxygen Adsorption on the Optical Properties of Ag Nanoparticles

Plasmonic metal nanoparticles are efficient light harvesters with a myriad of sensing- and energy-related applications. For such applications, the optical properties of nanoparticles of metals such as Cu, Ag, and Au can be tuned by controlling the composition, particle size, and shape, but less is known about the effects of oxidation on the plasmon resonances. In this work, we elucidate the effects of O adsorption on the optical properties of Ag particles by evaluating the thermodynamic properties of O-decorated Ag particles with calculations based on the density functional theory and subsequently computing the photoabsorption spectra with a computationally efficient time-dependent density functional theory approach. We identify stable Ag nanoparticle structures with oxidized edges and a quenching of the plasmonic character of the metal particles upon oxidation and trace back this effect to the sp orbitals (or bands) of Ag particles being involved both in the plasmonic excitation and in the hybridization to form bonds with the adsorbed O atoms. Our work has important implications for the understanding and application of plasmonic metal nanoparticles and plasmon-mediated processes under oxidizing environments.

Quantifying the Accuracy of Density Functionals on Transition Metal Bulk and Surface Properties

Density functional theory would be exact when the exact exchange-correlation (xc) functional would be known, but since it is regretfully not known, dozens of xc functionals have been developed in the past decades, with some of them better suited for describing certain systems and/or properties. For transition metals (TMs), recent systematic studies assessing bulk properties─shortest interatomic bond distance, δ, cohesive energy, Ecoh, and bulk modulus, B0─and surface features─surface energy, γ, work function, ϕ, and interlayer distances, δij─of 27 TM bulks and 81 TM surfaces, highlighted that generalized gradient approximation (GGA) based xc functionals are, overall, better suited than other types of xc functionals for the TMs bulk and surfaces description, such as Perdew-Burke-Ernzerhof (PBE) or Vega-Viñes (VV). Still, some basic local density approximation xc functionals were not assessed, such as the Hedin-Lundqvist (HL) and Perdew-Zunger (PZ), or GGAs such as the revised Perdew-Burke-Ernzerhof (revPBE) or the Armiento-Mattsson (AM05). Here, we expand the analysis by not only including them but also the recent meta-GGA strongly constrained appropriately normed (SCAN) xc functional, characterized by fulfilling all 17 mathematical conditions an xc must comply, plus the Bayesian error estimation functional (BEEF) xc, a functional parametrized over a large and diverse set of experimental results using machine learning. The present results reveal that none of the xc studied excel neither PBE nor VV, yet AM05 and SCAN performance is quite acceptable, while BEEF xc probably needs more shells of parametrization to reach competitive accuracy levels.

Interaction of Gallium with a Copper Surface: Surface Alloying and Formation of Ordered Structures

Alloys of gallium with transition metals have recently received considerable attention for their applications in microelectronics and catalysis. Here, we investigated the initial stages of the Ga-Cu alloy formation on Cu(111) and Cu(001) surfaces using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and low energy electron diffraction (LEED). The results show that Ga atoms deposited using physical vapor deposition readily intermix with the Cu surface, leading to a random distribution of the Ga and Cu atoms within the surface layer, on both terraces and monolayer-thick islands formed thereon. However, as the Ga coverage increases, several ordered structures are formed. The (√3×√3)R30° structure is found to be thermodynamically most stable on Cu(111). This structure remains after vacuum annealing at 600 K, independent of the initial Ga coverage (varied between 0.5 and 3 monolayers), indicating a self-limited growth of the Ga-Cu alloy layer, with the rest of the Ga atoms migrating into the Cu crystal. For Ga deposited on Cu(001), we observed a (1 × 5)-reconstructed surface, which has never been observed for surface alloys on Cu(001). The experimental findings were rationalized on the basis of density functional theory (DFT) calculations, which provided structural models for the most stable surface Ga-Cu alloys on Cu(111) and Cu(001). The study sheds light on the complex interaction of Ga with transition metal surfaces and the interfaces formed thereon that will aid in a better understanding of surface alloying and chemical reactions on the Ga-based alloys.

Stability and reactivity of metal nanoclusters supported on transition metal carbides

Small particles of transition metals (TM) supported on transition metal carbides (TMC) - TM_n@TMC - provide a plethora of design opportunities for catalytic applications due to their highly exposed active centres, efficient atom utilisation and the physicochemical properties of the TMC support. To date, however, only a very small subset of TM_n@TMC catalysts have been tested experimentally and it is unclear which combinations may best catalyse which chemical reactions. Herein, we develop a high-throughput screening approach to catalyst design for supported nanoclusters based on density functional theory, and apply it to elucidate the stability and catalytic performance of all possible combinations between 7 monometallic nanoclusters (Rh, Pd, Pt, Au, Co, Ni and Cu) and 11 stable support surfaces of TMCs with 1 : 1 stoichiometry (TiC, ZrC, HfC, VC, NbC, TaC, MoC and WC) towards CH₄ and CO₂ conversion technologies. We analyse the generated database to unravel trends or simple descriptors in their resistance towards metal aggregate formation and sintering, oxidation, stability in the presence of adsorbate species, and study their adsorptive and catalytic properties, to facilitate the discovery of novel materials in the future. We identify 8 TM_n@TMC combinations as promising catalysts, all of them being new for experimental validation, thus expanding the chemical space for efficient conversion of CH₄ and CO₂.

Extracting Features of Active Transition Metal Electrodes for NO Electroreduction with Catalytic Matrices

Electrocatalytic reduction of oxidized nitrogen compounds (NO_x) promises to help rebalance the nitrogen cycle. It is widely accepted that nitrate reduction to NH₄⁺/NH₃ involves NO as an intermediate, and NO hydrogenation is the potential-limiting step of NO reduction. Whether *NO hydrogenates to *NHO or *NOH is still a matter of debate, which makes it difficult to optimize catalysts for NO_x electroreduction. Here, "catalytic matrices" are used to swiftly extract features of active transition metal catalysts for NO electroreduction. The matrices show that active catalysts statistically stabilize *NHO over *NOH and have undercoordinated sites. Besides, square-symmetry active sites with Cu and other elements may prove active for NO electroreduction. Finally, multivariate regressions are able to reproduce the main features found by the matrices, which opens the door for more sophisticated machine-learning studies. In sum, catalytic matrices may ease the analysis of complex electrocatalytic reactions on multifaceted materials.

Theoretical Analysis of Magnetic Coupling in the Ti2C Bare MXene

The nature of the electronic ground state of the Ti₂C MXene is unambiguously determined by making use of density functional theory-based calculations including hybrid functionals together with a stringent computational setup providing numerically converged results up to 1 meV. All the explored density functionals (i.e., PBE, PBE0, and HSE06) consistently predict that the Ti₂C MXene has a magnetic ground state corresponding to antiferromagnetic (AFM)-coupled ferromagnetic (FM) layers. A spin model, with one unpaired electron per Ti center, consistent with the nature of the chemical bond emerging from the calculations, is presented in which the relevant magnetic coupling constants are extracted from total energy differences of the involved magnetic solutions using an appropriate mapping approach. The use of different density functionals enables us to define a realistic range for the magnitude of each of the magnetic coupling constants. The intralayer FM interaction is the dominant term, but the other two AFM interlayer couplings are noticeable and cannot be neglected. Thus, the spin model cannot be reduced to include nearest-neighbor interactions only. The Néel temperature is roughly estimated to be in the 220 ± 30 K, suggesting that this material can be used in practical applications in spintronics and related fields.

Ab initio study of the adsorption of SO2 on single-atom Cu-decorated ZnO(0001) surface

CONTEXT

The World Health Organization has cataloged sulfur dioxide (SO₂) as harmful for the human health and the environment. It also contributes to generate acid rain, which affects the ecosystems. To reduce its negative effects, new strategies to control the emissions are required. New and engineered materials are investigated to detect, capture, and eradicate toxic gases from the environment. Zinc oxide is considered a promising candidate. Here, we investigate the Cu-decorated ZnO(0001) surfaces as a single-atom catalyst (SAC) to reduce SO₂ by first-principles calculations. We propose a two-step reduction mechanism. First, one of the S-O bonds is broken on the pristine surface, with a calculated activation energy of 14.76 kcal/mol, 1.84 kcal/mol larger than the one obtained in the Cu SAC. In the second step, the SO reduction is viable only for Cu SAC, with calculated activation energy of 29.28 kcal/mol. Our results point that Cu SAC improves the SO₂ reduction, pointing it as a potentially efficient device to eradicate such harmful pollutant from the environment.

METHODS

The calculations were performed using the density functional theory, as implemented in quantum ESPRESSO package. The exchange-correlation energy was calculated within the generalized gradient approximation with the Perdew-Burke-Ernzerhof parameterization. Van der Waals dispersion-corrected interactions were considered. Spin-polarization was considered for studying dangling bonds in transition states. The minimum energy pathways were calculated by using the climbing image nudged elastic band.

Automated versus Chemically Intuitive Deconvolution of Density Functional Theory (DFT)-Based Gas-Phase Errors in Nitrogen Compounds

Catalysis models involving metal surfaces and gases are regularly based on density functional theory (DFT) calculations at the generalized gradient approximation (GGA). Such models may have large errors in view of the poor DFT-GGA description of gas-phase molecules with multiple bonds. Here, we analyze three correction schemes for the PBE-calculated Gibbs energies of formation of 13 nitrogen compounds. The first scheme is sequential and based on chemical intuition, the second one is an automated optimization based on chemical bonds, and the third one is an automated optimization that capitalizes on the errors found by the first scheme. The mean and maximum absolute errors are brought down close to chemical accuracy by the third approach by correcting the inaccuracies in the NNO and ONO backbones and those in N–O and N–N bonds. This work shows that chemical intuition and automated optimization can be combined to swiftly enhance the predictiveness of DFT-GGA calculations of gases.

MXenes à la Carte: Tailoring the Epitaxial Growth Alternating Nitrogen and Transition Metal Layers

A high-throughput analysis based on density functional simulations underscores the viable epitaxial growth of MXenes by alternating nitrogen and metal adlayers. This is supported by an exhaustive analysis of a number of thermodynamic and kinetic thresholds belonging to different critical key steps in the course of the epitaxial growth. The results on 18 pristine N- and C-based MXenes with M₂X stoichiometry reveal an easy initial N₂ fixation and dissociation, where N₂ adsorption is controlled by the MXene surface charge and metal d-band center and its dissociation controlled by the reaction energy change. Furthermore, formation energies indicate the plausible formation of N-terminated M₂XN₂ MXenes. Moreover, the further covering with metal adlayers is found to be thermodynamically driven and stable, especially when using early transition metal atoms. The most restrictive analyzed criterion is the N₂ adsorption and dissociation at nearly full N-covered adlayers, which is yet achievable for almost half of the explored M₂X seeds. The present results unfold the possibility of expanding, controlling, and tuning the composition, width, and structure of the MXene family.

A computational study on the adsorption of arsenic pollutants on graphene-based single-atom iron adsorbents

Integrated gasification combined cycle (IGCC) is a promising clean technology for coal power generation; however, the high volatility and toxicity of arsenic pollutants (As₂, As₄, AsO and AsH₃) released from an IGCC coal plant cause serious damage to human health and the ecological environment. Therefore, highly efficient adsorbents for simultaneous treatment of multiple arsenic pollutants are urgently needed. In this work, the adsorption characteristics and competitive adsorption behaviors of As₂, As₄, AsO, and AsH₃ on four kinds of graphene-based single-atom iron adsorbents (Fe/GA) were systematically investigated through density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations. The results suggest that single-vacancy Fe/GA doped with three nitrogen atoms has the largest adsorption ability for As₂, As₄, AsO and AsH₃. The adsorption energies of As₂, AsO and As₄ on Fe/GA depend on both charge transfer and orbital hybridization, while the adsorption energy of AsH₃ is mainly decided by electronic transfer. The adsorption differences of As₂, As₄, AsO and AsH₃ on four Fe/GA adsorbents can be explained through the obvious linear relationship between the adsorption energy and Fermi softness. As₂, As₄, AsO and AsH₃ will compete for adsorption sites when they exist on the same adsorbent surface simultaneously, and the adsorption capacities of AsO and As₂ are relatively stronger. After the competitive adsorption between AsO and As₂, AsO occupies the adsorption site at 300-900 K. This theoretical work suggests that Fe/GA is a promising adsorbent for the simultaneous removal of multiple arsenic pollutants with high adsorption capacity and low cost.

The Adsorption Behavior of Gas Molecules on Co/N Co-Doped Graphene

Herein, we have used density functional theory (DFT) to investigate the adsorption behavior of gas molecules on Co/N3 co-doped graphene (Co/N3-gra). We have investigated the geometric stability, electric properties, and magnetic properties comprehensively upon the interaction between Co/N3-gra and gas molecules. The binding energy of Co is -5.13 eV, which is big enough for application in gas adsorption. For the adsorption of C2H4, CO, NO2, and SO2 on Co/N-gra, the molecules may act as donors or acceptors of electrons, which can lead to charge transfer (range from 0.38 to 0.7 e) and eventually change the conductivity of Co/N-gra. The CO adsorbed Co/N3-gra complex exhibits a semiconductor property and the NO2/SO2 adsorption can regulate the magnetic properties of Co/N3-gra. Moreover, the Co/N3-gra system can be applied as a gas sensor of CO and SO2 with high stability. Thus, we assume that our results can pave the way for the further study of gas sensor and spintronic devices.

Identifying the Atomic Layer Stacking of Mo2C MXene by Probe Molecule Adsorption

A density functional theory study is presented here aimed at investigating whether the atomic stacking on the new family of two-dimensional MXene materials has an influence on their adsorption properties and whether these properties can provide information about this structural feature. To this end, the Mo₂C MXene, exhibiting two nearly degenerate crystal structures with either ABC or ABA atomic stacking, is chosen as a case study. The study of the adsorption of CO, CO₂, and H₂O on both polymorphs of Mo₂C reveals substantial differences that could be used in experiments to provide information about the atomic stacking of a given sample. Particularly, we show that the asymmetric and symmetric stretching modes of the adsorbed CO₂ and the CO stretching mode are clear features that allow one to identify the stacking of atomic layers of the Mo₂C MXene. The present finding is likely to apply to other MXenes as well.

Critical Role of Thermal Fluctuations for CO Binding on Electrocatalytic Metal Surfaces

This work considers the evaluation of density functional theory (DFT) when comparing against experimental observations of CO binding trends on the strong binding Pt(111) and intermediate binding Cu(111) and for weak binding Ag(111) and Au(111) surfaces important in electrocatalysis. By introducing thermal fluctuations using appropriate statistical mechanical NVT and NPT ensembles, we find that the RPBE and B97M-rV DFT functionals yield qualitatively better metal surface strain trends and CO enthalpies of binding for Cu(111) and Pt(111) than found at 0 K, thereby correcting the overbinding by 0.2 to 0.3 eV to yield better agreement with the enthalpies determined from experiment. The importance of dispersion effects are manifest for the weak CO binding Ag(111) and Au(111) surfaces at finite temperatures in which the RPBE functional does not bind CO at all, while the B97M-rV functional shows that the CO-metal interactions are a mixture of chemisorbed and physisorbed species with binding enthalpies that are within ∼0.05 eV of experiment. Across all M(111) surfaces, we show that the B97M-rV functional consistently predicts the correct atop site preference for all metals due to thermally induced surface distortions that preferentially favor the undercoordinated site. This study demonstrates the need to fully account for finite temperature fluctuations to make contact with the binding enthalpies from surface science experiments and electrocatalysis applications.

AgPd, AuPd, and AuPt Nanoalloys with Ag- or Au-Rich Compositions: Modeling Chemical Ordering and Optical Properties

Bimetallic nanoparticles have a myriad of technological applications, but investigations of their chemical and physical properties are precluded due to their structural complexity. Here, the chemical ordering and optical properties of AgPd, AuPd, and AuPt nanoparticles have been studied computationally. One of the main aims was to clarify whether layered ordered phases similar to L1₁ one observed in the core of AgPt nanoparticles [Pirart J.; Nat. Commun.2019, 10, 1982] are also stabilized in other nanoalloys of coinage metals with platinum-group metals, or the remarkable ordering is a peculiarity only of AgPt nanoparticles. Furthermore, the effects of different chemical orderings and compositions of the nanoalloys on their optical properties have been explored. Particles with a truncated octahedral geometry containing 201 and 405 atoms have been modeled. For each particle, the studied stoichiometries of the Ag- or Au-rich compositions, ca. 4:1 for 201-atomic particles and ca. 3:1 for 405-atomic particles, corresponded to the layered structures L1₁ and L1₀ inside the monatomic coinage-metal skins. Density functional theory (DFT) calculations combined with a recently developed topological (TOP) approach [Kozlov S. M.; Chem. Sci.2015, 6, 3868-3880] have been performed to study the chemical ordering of the particles, whose optical properties have been investigated using the time-dependent DFT method. The obtained results revealed that the remarkable ordering L1₁ of inner atoms can be noticeably favored only in small AgPt particles and much less in AgPd ones, whereas this L1₁ ordering in analogous Au-containing nanoalloys is significantly less stable compared to other calculated lowest-energy orderings. Optical properties were found to be more dependent on the composition (concentration of two metals) than on the chemical ordering. Both Pt and Pd elements promote the quenching of the plasmon.

Relating X-ray photoelectron spectroscopy data to chemical bonding in MXenes

The relationship between core level binding energy shifts (ΔCLBEs), that can be experimentally determined by X-ray photoelectron spectroscopy, and chemical bonding is analyzed for a series of MXenes, a new family of two-dimensional materials with a broad number of applications in nanotechnology. Based on first-principles calculations, the atomic and electronic structure of bare and O-terminated carbide MXene with M2C and M2CO2 (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W) stoichiometries are investigated with a focus on trends in the C(1s) and O(1s) ΔCLBEs, including initial and final state effects, along with the series. A rather good linear correlation between the available experimental and calculated C(1s) and O(1s) ΔCLBEs exists, with quantitative agreement when final state effects are included, that validates the conclusions from the present computational approach. The present study shows that ΔCLBEs of bare MXenes are governed by the initial state effects and directly correlate with the net charge on the C atoms. However, for the case of O-terminated MXenes, C(1s) and O(1s) ΔCLBEs exhibit a much less significant correlation with the net charge of either C or O atoms which is attributed to the structural changes induced on the M2C moiety by the presence of the O layers and the different stacking sequence observed depending on the MXene composition. The present study shows how and when XPS can be used to extract information regarding the nature of the chemical bond in bare or functionalized MXenes.

Thermodynamic Cyclic Voltammograms Based on Ab Initio Calculations: Ag(111) in Halide-Containing Solutions

Cyclic voltammograms (CVs) are a central experimental tool for assessing the structure and activity of electrochemical interfaces. Based on a mean-field ansatz for the interface energetics under applied potential conditions, we here derive an ab initio thermodynamics approach to efficiently simulate thermodynamic CVs. All unknown parameters are determined from density functional theory (DFT) calculations coupled to an implicit solvent model. For the showcased CVs of Ag(111) electrodes in halide-anion-containing solutions, these simulations demonstrate the relevance of double-layer contributions to explain experimentally observed differences in peak shapes over the halide series. Only the appropriate account of interfacial charging allows us to capture the differences in equilibrium coverage and total electronic surface charge that cause the varying peak shapes. As a case in point, this analysis demonstrates that prominent features in CVs do not only derive from changes in adsorbate structure or coverage but can also be related to variations of the electrosorption valency. Such double-layer effects are proportional to adsorbate-induced changes in the work function and/or interfacial capacitance. They are thus especially pronounced for electronegative halides and other adsorbates that affect these interface properties. In addition, the analysis allows us to draw conclusions on how the possible inaccuracy of implicit solvation models can indirectly affect the accuracy of other predicted quantities such as CVs.

The structure of metallic melts in eutectic alloys based on the Wulff cluster model: theory meets experiment

In the present work, the Wulff cluster model, which has been proved to be successful for pure metals and homogeneous alloys, has been extended to eutectic alloys (Ag-Cu and Al-Si). In our model, the shapes of the clusters in melts were determined by the interfacial energy calculated by density functional theory (DFT) of different facet families based on Wulff theory. The cluster size was given by the pair distribution function (PDF) g(r), which was converted from experimental high-temperature X-ray diffraction (HTXRD). The simulated XRD curves in the high temperature region were in good agreement with the experimental results. For the Al-Si alloy, a deviation of the intensity and position of the second peak near the eutectic temperature was observed. The simulated results after structure and composition modification corresponded to the experimental ones. It indicates that the deviation is mainly related to the significant change of the cluster size during Si clusters' growth processes before nucleation. Differently, there are no such nucleation processes at temperatures near the eutectic point due to the relatively high nucleation barriers of the two components in the Ag-Cu alloy.

A computational study of the properties of low- and high-index Pd, Cu and Zn surfaces

We report a detailed Density Functional Theory (DFT) based investigation of the structure and stability of bulk and surface structures for the Group 10-12 elements Pd, Cu and Zn, considering the effect of the choice of exchange-correlation density functional and computation parameters. For the initial bulk structures, the lattice parameter and cohesive energy are calculated, which are then augmented by calculation of surface energies and work functions for the lower-index surfaces. Of the 22 density functionals considered, we highlight the mBEEF density functional as providing the best overall agreement with experimental data. The optimal density functional choice is applied to the study of higher index surfaces for the three metals, and Wulff constructions performed for nanoparticles with a radius of 11 nm, commensurate with nanoparticle sizes commonly employed in catalytic chemistry. For Pd and Cu, the low-index (111) facet is dominant in the constructed nanoparticles, covering ∼50% of the surface, with (100) facets covering a further 10 to 25%; however, non-negligible coverage from higher index (332), (332) and (210) facets is also observed for Pd, and (322), (221) and (210) surfaces are observed for Cu. In contrast, only the (0001) and (10-10) facets are observed for Zn. Overall, our results highlight the need for careful validation of computational settings before performing extensive density functional theory investigations of surface properties and nanoparticle structures of metals.

Structural, electronic, and magnetic properties of Ni nanoparticles supported on the TiC(001) surface

Metals supported on transition metal carbides are known to exhibit good catalytic activity and selectivity, which is interpreted in terms of electron polarization induced by the support. In the present work we go one step further and investigate the effect that a titanium carbide (TiC) support has on the structural, electronic, and magnetic properties of a series of Ni nanoparticles of increasing size exhibiting a two- or three-dimensional morphology. The obtained results show that three-dimensional nanoparticles are more stable and easier to form than their homologous two-dimensional counterparts. Also, comparison to previous results indicates that, when used as the support, transition metal carbides have a marked different chemical activity with respect to oxides. The analysis of the magnetic moments of the supported nanoparticles evidences a considerable quenching of the magnetic moment that affects mainly the Ni atoms in close contact with the TiC substrate indicating that these atoms are likely to be responsible for the catalytic activity reported for these systems. The analysis of the electronic structure reveals the existence of chemical interactions between the Ni nanoparticles and the TiC support, even if the net charge transfer between both systems is negligible.

The structure of metallic melts in binary homogenous alloys: a thermodynamical understanding from the Wulff cluster model

In the present work, a new liquid metal model (Wulff cluster model) which has been proved to describe the structures of pure metal melts has been extended to binary homogeneous alloy melts (Cu-Ni and Ag-Au). The shapes of the nano-particles are determined by surface energies of different families of crystal planes, calculated by density functional theory (DFT), whereas the size was given by the pair distribution function (PDF) g(r) which was converted from experimental high-temperature X-ray diffraction (HTXRD). We demonstrated that the simulated X-ray diffraction (XRD) curves from present models match the experimental results quite well at high temperatures above the liquid-solid two-phase region, including not only the position and width of the peaks but also the relative intensity of the first and second peaks. Moreover, when the temperature is near the liquid-solid two-phase region, our model also fits the experimental strength curve well after modification using the solid XRD pattern of a relatively high melting point metal instead of its nano-particle. The agreement indicates the nucleation processes in homogeneous alloy melts.

Adsorption processes on a Pd monolayer-modified Pt(111) electrode

Specific adsorption of anions is an important aspect in surface electrochemistry for its influence on reaction kinetics in either a promoted or inhibited fashion. Perchloric acid is typically considered as an ideal electrolyte for investigating electrocatalytic reactions due to the lack of specific adsorption of the perchlorate anion on several metal electrodes. In this work, cyclic voltammetry and computational methods are combined to investigate the interfacial processes on a Pd monolayer deposited on Pt(111) single crystal electrode in perchloric acid solution. The “hydrogen region” of this Pd_MLPt(111) surface exhibits two voltammetric peaks: the first “hydrogen peak” at 0.246 V_RHE actually involves the replacement of hydrogen by hydroxyl, and the second “hydrogen peak” H_II at 0.306 V_RHE appears to be the replacement of adsorbed hydroxyl by specific perchlorate adsorption. The two peaks merge into a single peak when a more strongly adsorbed anion, such as sulfate, is involved. Our density functional theory calculations qualitatively support the peak assignment and show that anions generally bind more strongly to the Pd_MLPt(111) surface than to Pt(111).

Specific adsorption of anions is an important aspect in surface electrochemistry for its influence on reaction kinetics in either a promoted or inhibited fashion.

Electronic effects of transition metal dopants on Fe(100) and Fe5C2(100) surfaces for CO activation

Spin polarized density functional theory computations were performed to elucidate electronic effects based on first-row transition metal doped Fe(100) and Fe₅C₂(100) surfaces for CO dissociation.

Influence of Van der Waals Interactions on the Solvation Energies of Adsorbates at Pt-Based Electrocatalysts

Solvation can significantly modify the adsorption energy of species at surfaces, thereby influencing the performance of electrocatalysts and liquid-phase catalysts. Thus, it is important to understand adsorbate solvation at the nanoscale. Here we evaluate the effect of van der Waals (vdW) interactions described by different approaches on the solvation energy of *OH adsorbed on near-surface alloys (NSAs) of Pt. Our results show that the studied functionals can be divided into two groups, each with rather similar average *OH solvation energies: (1) PBE and PW91; and (2) vdW functionals, RPBE, PBE-D3 and RPBE-D3. On average, *OH solvation energies are less negative by ∼0.14 eV in group (2) compared to (1), and the values for a given alloy can be extrapolated from one functional to another within the same group. Depending on the desired level of accuracy, these concrete observations and our tabulated values can be used to rapidly incorporate solvation into models for electrocatalysis and liquid-phase catalysis.

van der Waals exchange-correlation functionals over bulk and surface properties of transition metals

The correct description of van der Waals (vdW) interaction forces is required for accurately describing dispersion bonded systems. Several approaches have been proposed to include London dispersion in density functional theory exchange-correlation functionals, where the family of so-called van der Waals (vdW-DF) exchange-correlation functionals have shown a better performance than local or semi local exchange-correlation functionals for describing molecular adsorption on metals. Despite the numerous benchmarks performed with these functionals, their performance in predicting bulk properties of transition metals has hitherto not been investigated in detail. We have therefore tested five vdW-DF exchange-correlation functionals, vdW-DF2, optPBE-vdW, BEEF-vdW, optB88-vdW and C09 _x -vdW to assess their performance in the prediction of lattice constants, bulk moduli, cohesive energies and surface energies of bulk Ni, Cu, Rh, Pd, Ag, Ir, Pt and Au (in fcc crystal structure). These transition metals are commonly used for benchmarking density functionals because they are important for applications in catalysis. The results are compared with experimental data and the PBE exchange-correlation functional. We found that both the optB88-vdW and the C09 _x -vdW exchange-correlation functionals estimate all properties with high accuracy, in better agreement with experimental data than PBE and other considered vdW functionals. The C09 _x -vdW functional clearly outperforms all other exchange-correlation functionals for surface energies for the (1 1 1) termination of different metals. We have also evaluated the interatomic electron density emerging from different functionals, and concluded that the observed differences are a result of the predicted lattice parameter, rather than a direct consequence of the functional form. Plane-wave and real-space grid-based expansions of the electron density are also compared, revealing good agreement between the two approaches for lattice parameters, cohesive energies, and surface energies, but more severe differences in bulk moduli. On the basis of our results, we recommend using the C09 _x -vdW for studying bulk properties and surface energies of transition metals.

Advances in Sustainable Catalysis: A Computational Perspective

The enormous challenge of moving our societies to a more sustainable future offers several exciting opportunities for computational chemists. The first principles approach to "catalysis by design" will enable new and much greener chemical routes to produce vital fuels and fine chemicals. This prospective outlines a wide variety of case studies to underscore how the use of theoretical techniques, from QM/MM to unrestricted DFT and periodic boundary conditions, can be applied to biocatalysis and to both homogeneous and heterogenous catalysts of all sizes and morphologies to provide invaluable insights into the reaction mechanisms they catalyze.

Hybrid Organic-Inorganic Perovskites as Promising Substrates for Pt Single-Atom Catalysts

Single-atom catalysts (SACs) combine the best of two worlds by bridging heterogeneous and homogeneous catalysis. The superior catalytic properties of SACs, however, can hardly be exploited without a suitable substrate. Here, we explore the possibility of using hybrid organic-inorganic perovskites as supporting materials for single transition-metal atoms. By means of first-principles calculations, we predict that single Pt atoms can be incorporated into methylammonium lead iodide surfaces by replacing the methylammonium groups at the outermost layer. The iodide anions at the surface provide potentially uniform anchoring sites for the Pt atoms and donate electrons, generating negatively charged Pt_{1}^{δ-} species that allow for preferential O_{2} adsorption in the presence of CO. Such Pt sites are able to catalyze CO oxidation and may also play a role in CO_{2} reduction. The fundamental understanding generated here will shed light on potential applications of hybrid perovskites in the field of (photo)catalysis.

Implicit solvent effects in the determination of Brønsted–Evans–Polanyi relationships for heterogeneously catalyzed reactions

The Brønsted–Evans–Polanyi relationship derived for the water dissociation reaction within an implicit solvent approach is similar to that without such effects.

Robustness of surface activity electronic structure-based descriptors of transition metals

Efficient yet simple electronic structure-based descriptors of transition metal surfaces are key in material design for many scientific fields in research and technology. Density functional theory-based methods provide the framework to systematically explore the performance and transferability of such descriptors. Using appropriate surface models and the Vosko-Wilk-Nussair (VWN), Perdew-Burke-Ernzerhof (PBE), PBE adapted for solids (PBEsol), revised PBE (RPBE), and Tao-Perdew-Staroverov-Scuseria (TPSS) exchange-correlation functionals, we study the transferability of three descriptors: the d-band centre, the width-corrected d-band centre, and the Hilbert transform highest peak, among the low-index Miller surfaces for the metals of transition elements. We show that the d-band centre and the width-corrected d-band centre descriptors are almost independent of the functional used whereas a dependency is seen in the Hilbert transform highest peak. Moreover, it is seen that the differences between the surface descriptor values and predictions from the bulk ones are affected by the presence of surface states. Interestingly, a direct relation between the surface coordination number and the d-band centre electronic descriptor is found when surface states are absent.

Can the state of platinum species be unambiguously determined by the stretching frequency of an adsorbed CO probe molecule?

The paper addresses possible ambiguities in the determination of the state of platinum species by the stretching frequency of a CO probe, which is a common technique for characterization of platinum-containing catalytic systems. We present a comprehensive comparison of the available experimental data with our theoretical modeling (density functional) results of pertinent systems - platinum surfaces, nanoparticles and clusters as well as reduced or oxidized platinum moieties on a ceria support. Our results for CO adsorbed on-top on metallic Pt(0), with C-O vibrational frequencies in the region 2018-2077 cm(-1), suggest that a decrease of the coordination number of the platinum atom, to which CO is bound, by one lowers the CO frequency by about 7 cm(-1). This trend corroborates the Kappers-van der Maas correlation derived from the analysis of the experimental stretching frequency of CO adsorbed on platinum-containing samples on different supports. We also analyzed the effect of the charge of platinum species on the CO frequency. Based on the calculated vibrational frequencies of CO in various model systems, we concluded that the actual state of the platinum species may be mistaken based only on the measured value of the C-O vibrational frequency due to overlapping regions of frequencies corresponding to different types of species. In order to identify the actual state of platinum species one has to combine this powerful technique with other approaches.

Density functional theory study of thermodynamic and kinetic isotope effects of H2/D2 dissociative adsorption on transition metals

Thermodynamic/kinetic isotope effects for H₂/D₂ dissociative adsorption calculated on metal surfaces offer a means to identify active sites.

Bulk and surface properties of metal carbides: implications for catalysis

We present a comprehensive study of the bulk and surface properties of transition metal carbides with rock salt structures and discuss their formation energies, electronic structure and potential catalytic activity.