Nickel-loaded lanthanum nitride (Ni/LaN) was shown in 2020 to synthesize ammonia efficiently through a dual-site mechanism in which surface nitrogen vacancies (VNV_N) on LaN bind and activate N2N_2 while Ni dissociates H2H_2, avoiding the scaling relations that limit conventional metal catalysts. The originating study left the quantitative, operando-level characterization of this concept open: how the surface VNV_N population depends on reaction conditions and evolves at steady state, how to measure accessible vacancies directly and validate the lattice-nitrogen site-count model behind reported turnover frequencies, how the nitrogen-vacancy formation energy (ENVE_{\mathrm{NV}}) can be tuned across nitride compositions, how the catalyst deactivates under impurity-containing feeds and regenerates after lattice-N consumption, and whether adjacent vacancies or metal–vacancy geometries can enable direct N2N_2 dissociation. In the later literature the validity and compositional generality of the ENVE_{\mathrm{NV}} descriptor are now substantially established (Ni/CeN, Mn-based nitrides, carbide and oxynitride-hydride analogues), and air/moisture stability has been engineered by Al doping, but the microscopic factors that set ENVE_{\mathrm{NV}} across compositions and the role of adjacent vacancies, vacancy clustering, and metal–vacancy geometries in direct N2N_2 dissociation, operando quantification of the active VNV_N population, validation of the site-count/TOF normalization, and deactivation/regeneration under realistic H2OH_2O- and CO2CO_2-containing feeds remain unresolved, defining the open core of the problem.

Background

Ammonia synthesis from atmospheric N2N_2 and H2H_2 is the backbone of fertilizer production and is run industrially by the Haber–Bosch process over promoted iron and ruthenium catalysts at high temperature and pressure. The central chemical difficulty is the N≡NN\equiv N triple bond (bond energy about 945 kJ per mole), whose cleavage on transition-metal surfaces is rate-limiting. Conventional catalyst design is constrained by scaling relations — empirical correlations tying the energetics of N2N_2 adsorption, N2N_2 dissociation, and nitrogen hydrogenation steps to one another on a single type of metal site — so that an inexpensive metal such as Ni, which dissociates H2H_2 easily, binds N2N_2 too weakly to activate it, while metals that activate N2N_2 (such as Ru) are scarce and costly.

Two families of strategies escape this constraint. Electrides — ionic crystals in which electrons act as anions and that have very low work functions — donate electrons to a loaded transition metal and lower its N2N_2-activation barrier, but the best-performing such catalysts still require Ru. Separately, metal nitrides with surface nitrogen vacancies had been reported to activate N2N_2: a nitrogen vacancy (VNV_N) is an anion site of the nitride lattice from which a lattice N atom has been removed, leaving electron density that can chemisorb and activate N2N_2. In this Mars–van Krevelen-type chemistry (a mechanism in which a lattice ion participates in the reaction cycle and is then restored from the gas-phase reactant), lattice N is hydrogenated to NH3NH_3, the vacancy is filled by dissociated or hydrogenated gas-phase N2N_2, and the cycle repeats.

Ye, Park, Lu, Li, Sasase, Kitano, Tada and Hosono (Nature 583, 391–395, 2020) combined the two ideas without any noble metal: nickel-loaded lanthanum nitride (Ni/LaN). Density functional theory (DFT) calculations in that study showed that Ni loading lowers the surface nitrogen-vacancy formation energy ENVE_{\mathrm{NV}} from 1.9 eV on bare LaN to about 1.6 eV; that a VNV_N chemisorbs N2N_2 end-on with an adsorption energy of roughly −1.7-1.7 eV and elongates the N≡NN\equiv N bond from 1.11 to 1.25 Å; and that H2H_2 dissociates without barrier on Ni, with spillover hydrogen (H* migrating from Ni to the support) hydrogenating lattice N and adsorbed N2N_2. Isotope experiments (14N^{14}N lattice, 15N^{15}N gas) confirmed that lattice N is consumed first and that gas-phase N2N_2 fills the vacancies. Substituting YN and ScN for LaN correlated catalytic activity with computed ENVE_{\mathrm{NV}}, supporting vacancy energetics as a design descriptor. The apparent activation energy was low and the activity comparable to Ru-based catalysts, with turnover frequency (TOF, ammonia molecules produced per surface site per second) estimated at 0.056 s−1^{-1} by counting all top-layer lattice N sites (3.6×10143.6\times10^{14} cm−2^{-2}) as potential active sites. Direct N2N_2 dissociation at an isolated vacancy carried computed barriers of 2.46 eV (with an adjacent vacancy) and 3.34 eV, versus a maximum barrier of about 0.76 eV for stepwise hydrogenation of adsorbed N2N_2, implying a hydrogenation-limited (associative) route. Stability was demonstrated for 100 h at 400 °C and 0.1 MPa.

The originating paper stated these results but did not measure the vacancy population under reaction conditions, did not test impurity-containing feeds, and did not resolve the dissociation pathway question. Subsequent work has built directly on the concept: Ye et al. (J. Am. Chem. Soc. 2020) extended it to CeN with an ENVE_{\mathrm{NV}}-based design rule; Yiliguma et al. (ACS Catal. 2021) transferred the vacancy idea to C2C_2 vacancies in rare-earth dicarbides; Lu et al. (Angew. Chem. 2022) made the LaN family air-stable by Al doping; Liu et al. (J. Phys. Chem. Lett. 2021) computed interfacial vacancy sites on Mn4_4N-supported Ni3_3/Fe3_3 clusters; Zhang et al. (Nat. Chem. 2025) and Dai et al. (J. Am. Chem. Soc. 2025) engineered anion-vacancy activation in oxynitride-hydrides and carbides; and Shao et al. (Chem. Eng. J. 2026) developed operando powder neutron diffraction with 15N^{15}N labelling for metal nitride catalysts, the technique closest to the missing vacancy-quantification measurement.

Problem Statement

For nickel-loaded rare-earth nitride ammonia-synthesis catalysts of the Ni/LaN family, whose dual-site mechanism — surface nitrogen vacancies (VNV_N) activating N2N_2 while Ni dissociates H2H_2 — is established at the level of DFT energetics, isotope labelling, and steady-state kinetics, establish the quantitative, operando-level characterization needed to make the concept measurable and durable in practice. Specifically, determine: (1) the density and spatial distribution of catalytically accessible surface VNV_N sites under reaction conditions, and how this population reaches and maintains its dynamic steady state as a function of temperature, pressure, and H2H_2 partial pressure during long-term operation; (2) whether the site-count model used to normalize activity — all top-layer lattice N atoms (3.6×10143.6\times10^{14} cm−2^{-2} on LaN) counted as potential active sites, giving TOF = 0.056 s−1^{-1} at 400 °C and 0.1 MPa — is correct, or what the true active-site count and per-site turnover frequency are; (3) the deactivation behavior of these catalysts under industrially relevant impurity-containing feeds (water, CO2CO_2, oxygenates), and whether activity can be regenerated after extensive lattice-N consumption; and (4) the factors that determine the nitrogen-vacancy formation energy ENVE_{\mathrm{NV}} across nitride (and, by extension, anion-vacancy) compositions, so that vacancy energetics can be tuned deliberately, including the role of adjacent vacancies, vacancy clustering, and metal–vacancy interfacial geometries in lowering the barrier for direct N2N_2 dissociation relative to the hydrogenation-mediated route. The single overarching objective is to convert the demonstrated vacancy-enabled dual-site concept from a phenomenon characterized by indirect DFT and isotope evidence into a catalyst whose active-vacancy population is measurable under operating conditions, whose activity normalization is experimentally validated, whose vacancy energetics — including the role of adjacent vacancies, vacancy clustering, and metal–vacancy interfacial geometries — are understood well enough to be tuned by deliberate design, and whose lifetime under realistic feeds is understood and recoverable. All four determinations above are required.

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