Ab Initio Atomistic Thermodynamics Research Articles

Size-Dependent Ab Initio Atomistic Thermodynamics from Cluster to Bulk: Application to Hydration of Titania Nanoparticles.

Ab initio atomistic thermodynamics (AIAT) has become an indispensable tool to estimate Gibbs free energy changes for solid surfaces interacting with gaseous species relative to pressure (p) and temperature (T). For such systems, AIAT assumes that solid vibrational contributions to Gibbs free energy differences cancel out. However, the validity of this assumption is unclear for nanoscale systems. Using hydrated titania nanoparticles (NPs) as an example, we estimate the vibrational contributions to the Gibbs free energy of hydration (ΔGhyd(T,p)) for arbitrary NP size and degree of hydration. Comparing ΔGhyd(T,p) phase diagrams for NPs when considering these contributions (AIATnano) relative to a standard AIAT approach reveals significant qualitative and quantitative differences, which only become negligible for large systems. By constructing a size-dependent ΔGhyd(T,p) phase diagram, we illustrate how our approach can provide deeper insights into how nanosytems interact with their environments, with many potential applications (e.g., catalytic nanoparticles, biological colloids, nanoparticulate pollutants).

The Equilibrium Stability of 〖CH〗_4 and 〖CO〗_2 on the Calcite (10.4) Surface: an Atomistic Thermodynamics Investigation

The present study utilized the ab initio atomistic thermodynamics technique to assess the stability of pure carbon dioxide and pure methane on the calcite(10.4) systems. The stability of configurations 0.5 ML-A2, 0.75 ML-A2, and 0.75 ML-A1 in CH_4/calcite (10.4) systems was shown to be considerable, but only within a limited range of chemical potential. The 1.0 ML-A1 and 1.0 ML-A2 systems of CH_4/calcite (10.4) demonstrated remarkable stability throughout a wide range of chemical potentials. The predominant stable forms for CO_2/calcite (10.4) systems are the 1.0 ML-B2 and 1.0 ML-A4 structures. The surface free energy phase diagrams demonstrate that CO_2 is more favourable than CH_4 for adsorption on the calcite (10.4) surface.

First-Principles Insights into Tungsten Semicarbide-Based Single-Atom Catalysts: Single-Atom Migration and Mechanisms in Oxygen Reduction.

Understanding the structural evolution of single-atom catalysts (SACs) in catalytic reactions is crucial for unraveling their catalytic mechanisms. In this study, we utilize density functional theory calculations to delve into the active phase evolution and the oxygen reduction reaction (ORR) mechanism of tungsten semicarbide-based transition metal SACs (TM1/W2C). The stable crystal phases and optimal surface exposures of W2C are identified by using ab initio atomistic thermodynamics simulations. Focusing on the W-terminated (001) surface, we screen 13 stable TM1/W2C variants, ultimately selecting Pt1/W2C(001) as our primary model. The surface Pourbaix diagram, mapped for this model under ORR conditions, reveals dynamic Pt1 migration on the surface, triggered by surface oxidation. This discovery suggests a novel single-atom evolution pathway. Remarkably, this single-atom migration behavior is also discerned in seven other group VIII SACs, enhancing both their catalytic activity and their stability. Our findings offer insights into the evolution of active phases in SACs, considering substrate structural arrangement, single-atom incorporation, and self-optimization of catalysts under various conditions.

A DFT study towards dynamic structures of iron and iron carbide and their effects on the activity of the Fischer-Tropsch process.

The Fe-based Fischer-Tropsch synthesis (FTS) catalyst shows a rich phase chemistry under pre-treatment and FTS conditions. The exact structural composition of the active site, whether iron or iron carbide (FeCx), is still controversial. Aiming to obtain an insight into the active sites and their role in affecting FTS activity, the swarm intelligence algorithm is implemented to search for the most stable Fe(100), Fe(110), Fe(210) surfaces with different carbon ratios. Then, ab initio atomistic thermodynamics and Wulffman construction were employed to evaluate the stability of these surfaces at different chemical potentials of carbon. Their FTS reactivity and selectivity were later assessed by semi-quantitative micro-kinetic equations. The results show that stability, reactivity, and selectivity of the iron are all affected by the carbonization process when the carbon ratio increases. Formation of the carbide, a rather natural process under experimental conditions, would moderately increase the turnover frequency (TOF), but both iron and iron carbide are active to the reaction.

Oxygen dissociation on the C3N monolayer: A first-principles study

The oxygen dissociation and the oxidized structure on the pristine C3N monolayer in exposure to air are the inevitably critical issues for the C3N engineering and surface functionalization yet have not been revealed in detail. Using the first-principles calculations, we have systematically investigated the possible O2 adsorption sites, various O2 dissociation pathways and the oxidized structures. It is demonstrated that the pristine C3N monolayer shows more O2 physisorption sites and exhibits stronger O2 adsorption than the pristine graphene. Among various dissociation pathways, the most preferable one is a two-step process involving an intermediate state with the chemisorbed O2 and the barrier is lower than that on the pristine graphene, indicating that the pristine C3N monolayer is more susceptible to oxidation than the pristine graphene. Furthermore, we found that the most stable oxidized structure is not produced by the most preferable dissociation pathway but generated from a direct dissociation process. These results can be generalized into a wide range of temperatures and pressures using ab initio atomistic thermodynamics. Our findings deepen the understanding of the chemical stability of 2D crystalline carbon nitrides under ambient conditions, and could provide insights into the tailoring of the surface chemical structures via doping and oxidation.

Ga and Zn increase the oxygen affinity of Cu-based catalysts for the COx hydrogenation according to ab initio atomistic thermodynamics

Ga and Zn increase the oxygen affinity of Cu-based catalysts for the COx hydrogenation according to ab initio atomistic thermodynamics

The surface energy phase diagrams of CO adsorption on the low index iridium surfaces and the morphology of iridium nanoparticles

The stability of molecular CO over the unreconstructed low-index surfaces was studied using ab initio atomistic thermodynamics. Results for Ir(100) in a diluted CO environment indicated that the 0.5 ML-phase was the most stable structure, whereas, in a concentrated CO medium, a 0.75 ML-phase exhibited the highest stability. Results of CO/Ir(110) systems indicated the presence of three stable configurations at 0.5, 0.75, and 1.0 ML, which are in agreement with reported experimental findings. Additional results revealed that among all CO/Ir(111) systems, the most stable structures were at 0.38, 0.44, and 0.81 ML coverages of CO. Shapes of Ir nanoparticles were predicted at different values of CO chemical potential using the Wulff construction. A truncated-tetrahedron shape was obtained for nanoparticles at low CO chemical potential values due to contributions of (100) and (111) facets. On the other hand, a cubic shape was obtained for nanoparticles at large CO chemical potential values. This was attributed to the sole contributions of the (100) surface to the Wulff construction.

An equilibrium ab initio atomistic thermodynamics investigation of hydrogen adsorption on the low index iridium surfaces and the morphology of iridium nanoparticles

The ab initio atomistic thermodynamics technique was used to calculate the surface free energy diagrams. These diagrams were obtained for hydrogen over the unreconstructed low-index iridium surfaces. Results indicated that the Ir(110) surface acquired the highest hydrogen intake among the low-index surfaces with a coverage of up to 2.33 ML. However, high-pressure hydrogen conditions were essential to maintain the stability of this phase. The Ir(100) surface required −0.81 eV of hydrogen chemical potential to produce a stable H/Ir phase with a 1.0 ML coverage. This potential represented the lowest value among the low-index surfaces. Phase diagram calculations of H/Ir(111) illustrated that the 0.75 ML phase has the highest stability at low to moderate hydrogen pressures. On the other hand, the 1.25 ML-H/Ir(111) phase became the most stable under high-pressure conditions. Finally, Wulff construction was implemented to predict the shapes of iridium nanoparticles in a hydrogen environment. Predictions indicated a truncated-octahedron shape for the nanoparticles irrespective of the value of the hydrogen chemical potential.

Relativistic Effects in Platinum Nanocluster Catalysis: A Statistical Ensemble-Based Analysis.

Nanoclusters are materials of paramount catalytic importance. Among various unique properties featured by nanoclusters, a pronounced relativistic effect can be a decisive parameter in governing their catalytic activity. A concise study delineating the role of relativistic effects in nanocluster catalysis is carried by investigating the oxygen reduction reaction (ORR) activity of a Pt7 subnanometer cluster. Global optimization analysis shows the critical role of spin-orbit coupling (SOC) in regulating the relative stability between structural isomers of the cluster. An overall improved ORR adsorption energetics and differently scaled adsorption-induced structural changes are identified with SOC compared to a non-SOC scenario. Ab initio atomistic thermodynamics analysis predicted nearly identical phase diagrams with significant structural differences for high coverage oxygenated clusters under realistic conditions. Though inclusion of SOC does not bring about drastic changes in the overall catalytic activity of the cluster, it is having a crucial role in governing the rate-determining step, transition-state configuration, and energetics of elementary reaction pathways. Furthermore, a statistical ensemble-based approach illustrates the strong contribution of low-energy local minimum structural isomers to the total ORR activity, which is significantly scaled up along the activity improving direction within the SOC framework. The study provides critical insights toward the importance of relativistic effects in determining various catalytic activity relevant features of nanoclusters.

Theoretical insight into the interaction between hydrogen and Hägg carbide (χ-Fe5C2) surfaces

In heterogeneous catalysis, the interaction between adsorbates and substrate surface plays an important role on surface structure evolution and even the reaction mechanism. Herein, unified DFT calculation and ab initio atomistic thermodynamics were adopted to explore the interaction between hydrogen and multiple Fe5C2 surfaces. The computation indicates that the dissociation of hydrogen molecule is both thermodynamically and kinetically favorable on Fe5C2 surfaces. The adsorption energy of hydrogen atoms is related to the summed H bond valance (SBVH), a higher SBVH corresponding to a more stable adsorption strength. Based on calculated surface free energy with different hydrogen coverage, the equilibrium crystalline morphology evolution of Fe5C2 under dynamic conditions was constructed with Wulff theorem. C-rich facets expose more area in Wulff construction with decreasing hydrogen chemical potential. The generation of carbon vacancy through reducing to methane has also been investigated, and the reduction of (111) and (11-1) facets is more facile than other facets. We anticipate this work could complement some knowledge to help understanding the surface structure and performance of iron carbide catalysts under experimental conditions.

Ga and Zn increase the oxygen affinity of Cu-based catalysts for the CO x hydrogenation according to ab initio atomistic thermodynamics.

The direct hydrogenation of CO or CO2 to methanol, a highly vivid research area in the context of sustainable development, is typically carried out with Cu-based catalysts. Specific elements (so-called promoters) improve the catalytic performance of these systems under a broad range of reaction conditions (from pure CO to pure CO2). Some of these promoters, such as Ga and Zn, can alloy with Cu and their role remains a matter of debate. In that context, we used periodic DFT calculations on slab models and ab initio thermodynamics to evaluate both metal alloying and surface formation by considering multiple surface facets, different promoter concentrations and spatial distributions as well as adsorption of several species (O*, H*, CO* and ) for different gas phase compositions. Both Ga and Zn form an fcc-alloy with Cu due to the stronger interaction of the promoters with Cu than with themselves. While the Cu-Ga-alloy is more stable than the Cu-Zn-alloy at low promoter concentrations (<25%), further increasing the promoter concentration reverses this trend, due to the unfavoured Ga-Ga-interactions. Under CO2 hydrogenation conditions, a substantial amount of O* can adsorb onto the alloy surfaces, resulting in partial dealloying and oxidation of the promoters. Therefore, the CO2 hydrogenation conditions are actually rather oxidising for both Ga and Zn despite the large amount of H2 present in the feedstock. Thus, the growth of a GaO x /ZnO x overlayer is thermodynamically preferred under reaction conditions, enhancing CO2 adsorption, and this effect is more pronounced for the Cu-Ga-system than for the Cu-Zn-system. In contrast, under CO hydrogenation conditions, fully reduced and alloyed surfaces partially covered with H* and CO* are expected, with mixed CO/CO2 hydrogenation conditions resulting in a mixture of reduced and oxidised states. This shows that the active atmosphere tunes the preferred state of the catalyst, influencing the catalytic activity and stability, indicating that the still widespread image of a static catalyst under reaction conditions is insufficient to understand the complex interplay of processes taking place on a catalyst surface under reaction conditions, and that dynamic effects must be considered.

Decrypting the photocatalytic bacterial inactivation of hierarchical flower-like Bi2WO6 microspheres induced by surface properties: Experimental studies and ab initio calculations

Bi2WO6 is considered an effective photocatalyst, even under the visible part of the solar spectrum, and recent advances in the modification of its structure leave promise for harnessing its enhanced effectiveness. Our experimental E. coli disinfection results have shown that the flower-like morphology brought considerable enhancement to the overall performance over the nanoparticle form. To clarify the photocatalytic mechanism of Bi2WO6, a combination of experimental and computational methods has been employed to investigate the surface properties of Bi2WO6 nanosheets self-assembly flower-like architecture. Although experimental evidence has determined the band edge positions of the as-synthesized samples, so as to address the photocatalytic properties, the mechanistic basis of this concept remains unclear. The calculation results demonstrated here deepen our understanding and indicate the potential of surface configuration to considerably alter the electronic structure and related photocatalytic properties of Bi2WO6 nanosheets. Firstly, we consider a range of surface slab models of Bi2WO6 (010) facet and calculate their surface Gibbs free energies. Having determined that the bi-termination is energetically more favorable by ab initio atomistic thermodynamics, hydrogen passivated termination was proved to be most stable. Through the analysis of the electronic band structure within the DFT-1/2 scheme and work function of the most stable termination, excellent agreement of experimental and theoretical predictions provided a meaningful understanding on the kinetic dependence of photocatalytic bacterial inactivation.

Defect-mediated ab initio thermodynamics of metastable γ-MoN(001).

Refractory transition metal nitrides exhibit a plethora of polymorphic expressions and chemical stoichiometries. To afford a better understanding of how defects may play a role in the structural and thermodynamics of these nitrides, using density-functional theory calculations, we investigate the influence of point and pair defects in bulk metastable γ-MoN and its (001) surface. We report favorable formation of Schottky defect pairs of neighboring Mo and N vacancies in bulk γ-MoN and apply this as a defect-mediated energy correction term to the surface energy of γ-MoN(001) within the ab initio atomistic thermodynamics approach. We also inspect the structural distortions in both bulk and surfaces of γ-MoN by using the partial radial distribution function, g(r), of Mo-N bond lengths. Large atomic displacements are found in both cases, leading to a broad spread of Mo-N bond length values when compared to their idealized bulk values. We propose that these structural and thermodynamic analyses may provide some insight into a better understanding of metastable materials and their surfaces.

Going beyond the equilibrium crystal shape: re-tracing the morphological evolution in group 5 tetradymite nanocrystals.

Nanocrystals of group 5 tetradymites M2X3 (where M = Bi and Sb, X = Se and Te) are of high technological relevance in modern topological nanoelectronics. However, there is a current lack of a systematic understanding to predict the preferred nanocrystal morphology in experiments where commonly-used equilibrium thermodynamic models appear to fail. In this work, using first-principles DFT calculations with a rationally-extended ab initio atomistic thermodynamics approach coupled to implicit solvation models and Gibbs-Wulff shape constructions, we demonstrate that this absence of predictive power stems from the limitation of equilibrium thermodynamics. By re-tracing and carefully addressing with a more realistic chemical potential definition, we illustrate this shortcoming can be overcome and afford a more rational route to size-engineer and shape-design highly-functional group 5 tetradymite nanoparticles for targeted applications.

Modified Cathode Materials for Garnet Based All-Solid-State Lithium Batteries

All solid-state batteries (ASB) are an emerging energy storage technology, which is expected to improve the safety on the cell level and to increase the battery energy density. One of important issues in the development of bulk-type ASBs with a high energy density is a possibility of processing thick composites cathodes with percolating pathways for ion and electron transport. For the ceramic electrolytes such as garnet-type Ta-substituted LLZ (Li6.6La3Zr1.6Ta0.4O12; LLZ:Ta) the practical realization of composite cathodes is however challenging due to the necessity of high temperature processing, raising the issues of material compatibility and the formation of reaction interphases influencing the total cell resistance. So far, the thick garnet-based composite cathodes could be made only with LiCoO2 (LCO) as an active material. For this kind of ASB, high areal capacities of up to 1.63 mAh/cm² were obtained. However, the utilization of high capacity cathode materials like LiNixCoyMn1–x–yO2 (NCM) was so far not possible due to the lower thermal stability of NCM as compared to LCO and the resulting enhanced reactivity between LLZ:Ta and NCM.Usually NCM materials were only optimized for liquid electrolyte based batteries. Therefore, an optimization of these active materials with respect to their integration into ASBs is intended in this work. Possible strategies for this optimization are doping or substitution, surface coatings or core-shell structures.We present the synthesis of optimized cathode materials for bulk-type, fully inorganic ASBs based on ceramic materials. A detailed material screening of NCM materials with different compositions and modifications is performed by evaluating the compatibility with cubic LLZ:Ta during co-sintering at elevated temperatures. The compatibility is tested by in situ high temperature X-ray diffraction (HT-XRD), differential thermal analysis/thermogravimetry (DTA/TG), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). The experimental work is supported by simulation studies using ab-initio-based approaches such as density functional theory (DFT), ab initio molecular dynamics (AIMD) as well as ab initio atomistic thermodynamics and kinetics approaches. These simulation based methods enable the prediction of lattice parameter of NCM at different stages of charge. Additionally, the calculation of interchange energies needed for the cation interchange between NCM and cubic LLZ:Ta during co-sintering is possible.Boron doping of NCM was investigated as a possible strategy to improve its compatibility with LLZ:Ta, as this strategy was demonstrated to be very efficient for liquid electrolytes. The theoretical and experimental results on B-doped NCM with different compositions are already available. However, the formation of secondary phases after co-sintering that was predicted by us theoretically and confirmed experimentally using Rietveld refinement discard boron doping as a way to enhance thermal stability of NCM during the co-sintering with cubic LLZ:Ta. Introduction of other dopants and surface coatings is currently investigated to improve the compatibility of NCM with cubic LLZ:Ta during co-sintering. The combined experimental and theoretical approach is expected to result in an optimized cathode material for ASBs with enhanced performance.

Stable CO/H2 ratio on MoP surfaces under working condition: A DFT based thermodynamics study

By performing a systematic DFT calculation and applying the atomistic thermodynamics analysis, the adsorption configurations, stable concentrations of nCO + mH2 co-adsorption on three representative MoP surfaces [(101), (001)-Mo and (001)-P] were investigated. Our results show that CO adsorbs much stronger than dissociative H2 on (101) and (001)-Mo surfaces but competitively with dissociative H2 on the (001)-P surface, and the hydrogen saturation coverage decreases with increasing CO pre-coverage. Ab initio atomistic thermodynamics analysis indicates the quite different CO and H2 co-adsorption manners on three surfaces under syngas atmosphere, i.e., at the equilibrium co-adsorption state, the Mo/P-terminated (101) surface and the Mo-terminated (001) surface have more versatile surface CO and H2 ratios, which are entirely different from that in the gas phase. However, the P-terminated (001) surface has only hydrogen adsorption at a wide range of conditions, which plays a role of hydrogen reservoir. Such investigations reveal that surface CO/H2 ratio could be altered by manipulating the pressures of the gas phase and temperatures, which would be beneficial to modify the syngas conversion reactivity as well as different product distributions on solid catalyst surfaces.

Theoretical insights of codoping to modulate electronic structure of hbox {TiO}_2 and hbox {SrTiO}_3 for enhanced photocatalytic efficiency

hbox {TiO}_2 and hbox {SrTiO}_3 are well known materials in the field of photocatalysis due to their exceptional electronic structure, high chemical stability, non-toxicity and low cost. However, owing to the wide band gap, these can be utilized only in the UV region. Thus, it’s necessary to expand their optical response in visible region by reducing their band gap through doping with metals, nonmetals or the combination of different elements, while retaining intact the photocatalytic efficiency. We report here, the codoping of a metal and a nonmetal in anatase hbox {TiO}_2 and hbox {SrTiO}_3 for efficient photocatalytic water splitting using hybrid density functional theory and ab initio atomistic thermodynamics. The latter ensures to capture the environmental effect to understand thermodynamic stability of the charged defects at a realistic condition. We have observed that the charged defects are stable in addition to neutral defects in anatase hbox {TiO}_2 and the codopants act as donor as well as acceptor depending on the nature of doping (p-type or n-type). However, the most stable codopants in hbox {SrTiO}_3 mostly act as donor. Our results reveal that despite the response in visible light region, the codoping in hbox {TiO}_2 and hbox {SrTiO}_3 cannot always enhance the photocatalytic activity due to either the formation of recombination centers or the large shift in the conduction band minimum or valence band maximum. Amongst various metal-nonmetal combinations, hbox {Mn}_text {Ti}hbox {S}_text {O} (i.e. Mn is substituted at Ti site and S is substituted at O site), hbox {S}_text {O} in anatase hbox {TiO}_2 and hbox {Mn}_text {Ti}hbox {S}_text {O}, hbox {Mn}_text {Sr}hbox {N}_text {O} in hbox {SrTiO}_3 are the most potent candidates to enhance the photocatalytic efficiency of anatase hbox {TiO}_2 and hbox {SrTiO}_3 under visible light irradiation.

How to Avoid the Oxidation of Silicene/Ag(111)? Reaction Mechanisms of O2 with Chlorinated Silicene

Ab-initio atomistic thermodynamics and ab-initio molecular dynamics simulations were performed to investigate the reaction of O2 with hydrogenated and chlorinated silicene supported on Ag(111) in o...

Atomically dispersed platinum on low index and stepped ceria surfaces: phase diagrams and stability analysis.

Through the combination of density functional theory calculations and ab initio atomistic thermodynamics modeling, we demonstrate that atomically dispersed platinum species on ceria can adopt a range of local coordination configurations and oxidation states that depend on the surface structure and environmental conditions. Unsaturated oxygen atoms on ceria surfaces play the leading role in stabilization of PtOx species. Any mono-dispersed Pt0 species are thermodynamically unstable compared to bulk platinum, and oxidation of Pt0 to Pt2+ or Pt4+ is necessary to stabilize mono-dispersed platinum atoms. Reduction to Pt0 leads to sintering. Both Pt2+ and Pt4+ prefer to form the square-planar [PtO4] configuration. The two most stable Pt2+ species on the (223) and (112) surfaces are thermodynamically favorable between 300 and 1200 K. The most stable Pt4+ species on the (100) surface tends to desorb from the surface as gas phase above 950 K. The resulting phase diagrams of the atomically dispersed platinum in PtOx clusters on various ceria surfaces under a range of experimentally relevant conditions can be used to predict dynamic restructuring of atomically dispersed platinum catalysts and design new catalysts with engineered properties.

Metastability triggered reactivity in clusters at realistic conditions: a case study of N-doped (TiO2) n for photocatalysis

We present a strategy, by taking a prototypical model system for photocatalysis (viz. N-doped (TiO2) n clusters), to accurately design photocatalyst with enhanced reactivity at a given environmental conditions (i.e. temperature (T), pressure () and doping (µ e )). Since free energy potential energy surface consists of many competing isomers even in a small energy window, computational design of specific metastable photocatalysts with enhanced activity is extremely challenging. This requires fixing various parameters as follows; (i) favorable formation energy, (ii) low fundamental gap, (iii) low excitation energy, (iv) high vertical electron affinity (VEA) and (v) low vertical ionization potential (VIP) to drive the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) for water splitting. Here, we show that by integrating several first principles based methodologies that consideration of the global minimum structure alone can severely underestimate the activity. First, we have used a suite of genetic algorithms (namely, searching clusters with conventional minimum total energy ((GA)); searching clusters with specific property i.e. high VEA ((GA)), and low VIP ((GA))) to model the N-doped (TiO2) n ( n = 4–10, 15, 20) (meta)stable clusters. Next, we have identified its free energy using ab initio atomistic thermodynamics to confirm that the metastable structures are not too far from the free energy global minima. We find that N-substitution ((N)) prefers to reside at highly coordinated oxygen site, whereas N-interstitial ((NO)) and split-interstitial ((N) favor the dangling oxygen site. (NO) and (N doped states are thermodynamically stable at finite temperature (T) and pressure (). Interestingly, each types of defect (viz. substitution, interstitials) reduce the fundamental gap and excitation energy substantially. However, (N doped clusters are found to be less probable in the Pourbaix phase diagram, whereas (N) and (NO) doped metastable clusters show significant occupance probability near the phase boundaries. The latter ensures much higher electrocatalytic activity for water splitting than the most stable configurations.