Ab Initio Molecular Dynamics Simulations of Nitrogen/VN(001) Surface Reactions: Vacancy-Catalyzed N₂ Dissociative Chemisorption, N Adatom Migration, and N₂ Desorption

Abstract

We use density-functional ab initio molecular dynamics to investigate the kinetics of N/VN(001) surface reactions at temperatures ranging from 1600 to 2300 K. N adatoms (Nad) on VN(001) favor epitaxial atop-V positions, and diffuse among them by transiting through fourfoldhollow (FFH) sites, at which they are surrounded by two V and two N surface atoms. After several atop-V FFH atop-V jumps, isolated N adatoms bond strongly with an underlying N surface (Nsurf) atom. Frequent Nad/Nsurf pair exchange reactions lead to N2 desorption, which results in the formation of an anion surface vacancy. N vacancies rapidly migrate via in-plane <110> jumps and act as efficient catalysts for the dissociative chemisorption of incident N2 molecules. During exposure of VN(001) to incident atomic N gas atoms, Nad/Nad recombination and desorption is never observed, despite a continuously high N monomer surface coverage. Instead, N2 desorption is always initiated by a N adatom removing a N surface atom or by energetic N gas atoms colliding with Nad or Nsurf atoms. Similarities and differences between N/VN(001) vs. previous N/TiN (001) results, discussed on the basis of temperature-dependent ab initio electronic structures and chemical bonding, provide insights for controlling the reactivity of NaCl-structure transition-metal nitride (001) surfaces via electron-concentration tuning.3

Ab initio molecular dynamics of atomic-scale surface reactions: insights into metal organic chemical vapor deposition of AlN on graphene

“…Density-functional ab initio molecular dynamics (AIMD) 24 yields reliable descriptions of the time-evolution of solidstate systems directly embedded in realistic environmental conditions. 25,26 AIMD simulations often reveal non-intuitive reaction pathways 27,28 and system configurations 29 while providing corresponding kinetic rates at finite temperatures. Despite its known limitations for treating electron transfer during chemical reactions and/or the energetics of transition states for molecule splitting, 30 AIMD is an excellent tradeoff between accuracy + reliability vs. computational cost for identifying chemical reaction pathways and estimating the relative occurrence of competing reactions.…”

Nonequilibrium ab initio molecular dynamics determination of Ti monovacancy migration rates in B1 TiN

“…AIMD allows for direct visualization of reaction pathways and provides corresponding kinetic rates. In addition, this technique has revealed nonintuitive system configurations and reaction pathways at finite temperatures [53][54][55]. Nevertheless, given that AIMD is highly computationally-intensive, it has been employed in very few theoretical studies of mass transport in TM nitrides [53][54][55][56][57][58], materials characterized by inherently low defect mobilities, at least within the metal sublattice [8].…”

Biomimetic Nitrogen Fixation Catalyzed by Transition Metal Sulfide Surfaces in an Electrolytic Cell

“…The hydrogen‐covered surfaces, therefore, presumably evolve hydrogen gas instead of ammonia. Recently, some transition metal nitride (TMN) surfaces were predicted to provide a solution to the problem of competitive adsorption of N 2 and protons on the catalyst surface by a Mars‐van Krevelen (MvK) mechanism, even though few TMNs were anticipated to still favor the HER . The MvK mechanism for NH 3 formation is possible for both heterogeneous gas–solid phase and liquid–solid electrochemical processes.…”

“…Ar ange of pure metal surfaces have been investigated computationally for electrochemical ammonia synthesis. [29] Even though some metallics urfaces show potentially high activity,t he majority get covered in protons under electrochemical conditions and prevent N 2 binding and its subsequent reduction to NH 3 .T he hydrogen-covered surfaces, therefore, presumably evolve hydrogen gas insteado fa mmonia.R ecently,s ome transition metal nitride (TMN) surfaces were predicted to provide as olution to the problem of competitivea dsorption of N 2 and protons on the catalyst surface [30][31][32][33][34][35] by aM ars-van Krevelen (MvK) mechanism, [36] even thoughf ew TMNs were anticipated to still favor the HER. [30,37] The MvK mechanismf or NH 3 formation is possible for both heterogeneous gas-solid phase and liquid-solide lectrochemical processes.…”