Thermodynamic Constraints in Using AuM (M = Fe, Co, Ni, and Mo) Alloys as N₂ Dissociation Catalysts: Functionalizing a Plasmon-Active Metal

The Haber-Bosch process for NH₃ synthesis is arguably one of the greatest inventions of the 20th century, with a massive footprint in agriculture and, historically, warfare. Current catalysts for this reaction use Fe for N₂ activation, conducted at high temperatures and pressures to improve conversion rate and efficiency. A recent finding shows that plasmonic metal nanoparticles can either generate highly reactive electrons and holes or induce resonant surface excitations through plasmonic decay, which catalyze dissociation and redox reactions under mild conditions. It is therefore appealing to consider AuM (M = Fe, Co, Ni, and Mo) alloys to combine the strongly plasmonic nature of Au and the catalytic nature of M metals toward N₂ dissociation, which together might facilitate ammonia production. To this end, through density functional theory, we (i) explore the feasibility of forming these surface alloys, (ii) find a pathway that may stabilize/deactivate surface M substituents during fabrication, and (iii) define a complementary route to reactivate them under operational conditions. Finally, we evaluate their reactivity toward N₂, as well as their ability to support a pathway for N₂ dissociation with a low thermodynamic barrier. We find that AuFe possesses similar appealing qualities, including relative stability with respect to phase separation, reversibility of Fe oxidation and reduction, and reactivity toward N₂. While AuMo achieves the best affinity toward N₂, its strong propensity toward oxidation could greatly limit its use

Chemical Pressure-Driven Enhancement of the Hydrogen Evolving Activity of Ni₂P from Nonmetal Surface Doping Interpreted via Machine Learning

The activity of Ni₂P catalysts for the hydrogen evolution reaction (HER) is currently limited by strong H adsorption at the Ni₃-hollow site. We investigate the effect of surface nonmetal doping on the HER activity of the Ni₃P₂ termination of Ni₂P­(0001), which is stable at modest electrochemical conditions. Using density functional theory (DFT) calculations, we find that both 2<i>p</i> nonmetals and heavier chalcogens provide nearly thermoneutral H adsorption at moderate surface doping concentrations. We also find, however, that only chalcogen substitution for surface P is exergonic. For intermediate surface concentrations of S, the free energy of H adsorption at the Ni₃-hollow site is −0.11 eV, which is significantly more thermoneutral than the undoped surface (−0.45 eV). We use the regularized random forest machine learning algorithm to discover the relative importance of structure and charge descriptors, extracted from the DFT calculations, in determining the HER activity of Ni₂P­(0001) under different doping concentrations. We discover that the Ni–Ni bond length is the most important descriptor of HER activity, which suggests that the nonmetal dopants induce a chemical pressure-like effect on the Ni₃-hollow site, changing its reactivity through compression and expansion

Strong Reciprocal Interaction between Polarization and Surface Stoichiometry in Oxide Ferroelectrics

We present a systematic evaluation of the effects of polarization switchability on surface structure and stoichiometry in BaTiO₃ and PbTiO₃ ferroelectric oxides. We show that charge passivation, mostly by ionic surface reconstructions, is the driving force for the stability of the surfaces, which suggests that varying the substrate polarization offers a new mechanism for controlling surface reconstructions in polar systems and inducing highly nonstoichiometric structures. Conversely, for thin-films the chemical environment can drive polarization switching via induced compositional changes on the surface. We find that the value of the oxygen partial pressure for the positive-to-negative polar transition is in good agreement with the recent experimental value for thin-film PbTiO₃. For BaTiO₃, we show that it is harder for oxygen control to drive polar transition because it is more difficult to reduce. This study opens up the possibility of real-time control of structure and composition of oxide surfaces

Stable Phosphorus-Enriched (0001) Surfaces of Nickel Phosphides

In heterogeneous catalysis, catalyst synthesis precedes operation and, in most cases, is conducted in an altogether different chemical environment. Thus, determination of the structure and composition of the catalyst surface(s) due to fabrication is essential in accurately evaluating their eventual structure(s) during operation, which provides the origin of their catalytic activities and are therefore key to catalyst optimization. We explore the reconstructions of both Ni₂P­(0001) and Ni₅P₄(0001)/(0001̅) surfaces with first-principles density functional theory (DFT). Most of the stable terminations under realistic synthesis conditions are determined to be P-rich on both materials. A P-covered reconstruction of the Ni₃P₂ termination of Ni₂P­(0001) is found to be most stable, consistent with the current literature. By contrast, the most energetically favorable surfaces of Ni₅P₄ are found to be the Ni₃P₃ and Ni₄P₃ bulk-derived terminations with P-adatoms. The preferred excess P binding sites and their energies are identified on each surface. We find that the P₃ site, which is present on Ni₅P₄, and the Ni₃ site, which is present on both Ni₂P and Ni₅P₄, strongly bind excess P. Additionally, we predict the presence of stable P_<i>n</i> (<i>n</i> = 2, 4) agglomerates on Ni₅P₄ at the P₃-hollow and Ni–Ni bridge sites. This study highlights the importance of considering the aggregation behavior of nonmetal components in predicting the surface reconstruction of transition metal compounds

Chemical Pressure-Driven Enhancement of the Hydrogen Evolving Activity of Ni₂P from Nonmetal Surface Doping Interpreted via Machine Learning

The activity of Ni₂P catalysts for the hydrogen evolution reaction (HER) is currently limited by strong H adsorption at the Ni₃-hollow site. We investigate the effect of surface nonmetal doping on the HER activity of the Ni₃P₂ termination of Ni₂P­(0001), which is stable at modest electrochemical conditions. Using density functional theory (DFT) calculations, we find that both 2<i>p</i> nonmetals and heavier chalcogens provide nearly thermoneutral H adsorption at moderate surface doping concentrations. We also find, however, that only chalcogen substitution for surface P is exergonic. For intermediate surface concentrations of S, the free energy of H adsorption at the Ni₃-hollow site is −0.11 eV, which is significantly more thermoneutral than the undoped surface (−0.45 eV). We use the regularized random forest machine learning algorithm to discover the relative importance of structure and charge descriptors, extracted from the DFT calculations, in determining the HER activity of Ni₂P­(0001) under different doping concentrations. We discover that the Ni–Ni bond length is the most important descriptor of HER activity, which suggests that the nonmetal dopants induce a chemical pressure-like effect on the Ni₃-hollow site, changing its reactivity through compression and expansion

Active Role of Phosphorus in the Hydrogen Evolving Activity of Nickel Phosphide (0001) Surfaces

Optimizing catalysts for the hydrogen evolution reaction (HER) is a critical step toward the efficient production of H₂(g) fuel from water. It has been demonstrated experimentally that transition-metal phosphides, specifically nickel phosphides Ni₂P and Ni₅P₄, efficiently catalyze the HER at a small fraction of the cost of archetypal Pt-based electrocatalysts. However, the HER mechanism on nickel phosphides remains unclear. We explore, through density functional theory with thermodynamics, the aqueous reconstructions of Ni₂P­(0001) and Ni₅P₄(0001)/(0001̅), and we find that the surface P content on Ni₂P­(0001) depends on the applied potential, which has not been considered previously. At −0.21 V ≥ <i>U</i> ≥ −0.36 V versus the standard hydrogen electrode and pH = 0, a PH_<i>x</i>-enriched Ni₃P₂ termination of Ni₂P­(0001) is found to be most stable, consistent with its P-rich ultrahigh-vacuum reconstructions. Above and below this potential range, the stoichiometric Ni₃P₂ surface is instead passivated by H at the Ni₃-hollow sites. On the other hand, Ni₅P₄(0001̅) does not favor additional P. Instead, the Ni₄P₃ bulk termination of Ni₅P₄(0001̅) is passivated by H at both the Ni₃ and P₃-hollow sites. We also found that the most HER-active surfaces are Ni₃P₂+P+(7/3)H of Ni₂P­(0001) and Ni₄P₃+4H of Ni₅P₄(0001̅) due to weak H adsorption at P catalytic sites, in contrast with other computational investigations that propose Ni as or part of the active site. By looking at viable catalytic cycles for HER on the stable reconstructed surfaces, and calculating the reaction free energies of the associated elementary steps, we calculate that the overpotential on the Ni₄P₃+4H surface of Ni₅P₄(0001̅) (−0.16 V) is lower than that of the Ni₃P₂+P+(7/3)H surface of Ni₂P­(0001) (−0.21 V). This is due to the abundance of P₃-hollow sites on Ni₅P₄ and the limited surface stability of the P-enriched Ni₂P­(0001) surface phase. The trend in the calculated catalytic overpotentials, and the potential-dependent bulk and surface stabilities explain why the nickel phosphides studied here perform almost as well as Pt, and why Ni₅P₄ is more active than Ni₂P toward HER, as is found in the experimental literature. This study emphasizes the importance of considering aqueous surface stability in predicting the HER-active sites, mechanism, and overpotential, and highlights the primary role of P in HER catalysis on transition-metal phosphides

Theoretical Model of Oxidative Adsorption of Water on a Highly Reduced Reconstructed Oxide Surface

Highly reduced surface reconstructions of BaTiO₃ (001) have been found to be composed of a TiO₂ surface covered with Ti adatoms occupying surface interstitial sites. We predict the reactivity of these highly oxophilic and reduced surface Ti species through density functional theory, where we calculate the adsorption of H₂O on the (√5 × √5)<i>R</i>26.6° TiO₂–Ti_3/5 reconstruction. H₂O serves as the primary O source and oxidizing agent. We demonstrate that H₂O oxidizes some of the Ti adatoms, shifting their occupied 3d states to the surface conduction band edge. We find that, due to the high concentration of reduced Ti species on the surface, a dissociative adsorption of water on the reconstructed surface can also lead to the formation of surface hydrides, which serve as a precursor for H₂ evolution. This suggests that the reconstructed surface may be an attractive single-phase hydrogen evolution catalyst

Chemical Pressure-Driven Enhancement of the Hydrogen Evolving Activity of Ni₂P from Nonmetal Surface Doping Interpreted via Machine Learning

The activity of Ni₂P catalysts for the hydrogen evolution reaction (HER) is currently limited by strong H adsorption at the Ni₃-hollow site. We investigate the effect of surface nonmetal doping on the HER activity of the Ni₃P₂ termination of Ni₂P­(0001), which is stable at modest electrochemical conditions. Using density functional theory (DFT) calculations, we find that both 2<i>p</i> nonmetals and heavier chalcogens provide nearly thermoneutral H adsorption at moderate surface doping concentrations. We also find, however, that only chalcogen substitution for surface P is exergonic. For intermediate surface concentrations of S, the free energy of H adsorption at the Ni₃-hollow site is −0.11 eV, which is significantly more thermoneutral than the undoped surface (−0.45 eV). We use the regularized random forest machine learning algorithm to discover the relative importance of structure and charge descriptors, extracted from the DFT calculations, in determining the HER activity of Ni₂P­(0001) under different doping concentrations. We discover that the Ni–Ni bond length is the most important descriptor of HER activity, which suggests that the nonmetal dopants induce a chemical pressure-like effect on the Ni₃-hollow site, changing its reactivity through compression and expansion

Coexisting Surface Phases and Coherent One-Dimensional Interfaces on BaTiO₃(001)

Coexistence of surface reconstructions is important due to the diversity in kinetic and thermodynamic processes involved. We identify the coexistence of kinetically accessible phases that are chemically identical and form coherent interfaces. Here, we establish the coexistence of two phases, <i>c</i>(2 × 2) and <i>c</i>(4 × 4), in BaTiO₃(001) with atomically resolved Scanning Tunneling Microscopy (STM). First-principles thermodynamic calculations determine that TiO adunits and clusters compose the surfaces. We show that TiO diffusion results in a kinetically accessible <i>c</i>(2 × 2) phase, while TiO clustering results in a kinetically and thermodynamically stable <i>c</i>(4 × 4) phase. We explain the formation of domains based on the diffusion of TiO units. The diffusion direction determines the observed 1D coherent interfaces between <i>c</i>(2 × 2) and <i>c</i>(4 × 4) reconstructions. We propose atomic models for the <i>c</i>(2 × 2), <i>c</i>(4 × 4), and 1D interfaces