(399d) Computational Design of Active Site Structures to Circumvent Scaling in Ammonia Synthesis

The Haber-Bosch process for the reduction of atmospheric nitrogen to ammonia is one of the most optimized heterogeneous catalytic reactions, but there are aspects of the industrial process that remain less than ideal. Due to thermodynamic and kinetic limitations, the reaction proceeds appreciably only under harsh conditions (temperatures above 700K and pressures in excess of 100 bar), requiring centralized factories that are capital- and energy-intensive [1]. The discovery of catalysts enabling NH_3 synthesis in a distributed fashion under ambient conditions could be both economically and environmentally advantageous.

It has been shown that the activity of the industrially-relevant (211) transition metal stepped surfaces is limited by a Brønstedâ??Evansâ??Polanyi (BEP) scaling relationship between the N-N transition state energy (E_N-N) and the *N binding energy (E_N) [2]. E_N-N is consistently too high relative to E_N on all catalysts that satisfy this linear constraint, leading to a negligible production rate at ambient conditions and a modest rate under harsh conditions. In this work, taking inspiration from rutile oxides that bind *N onto under-coordinated metal top sites, we use DFT calculations in conjunction with mean-field microkinetic modeling to study the rate of NH_3 synthesis on model active sites that require the singly coordinated dissociative adsorption of N atoms onto transition metal atoms. Our results demonstrate that this â??on-topâ? binding of nitrogen exhibits much-improved BEP scaling behavior, which can be rationalized in terms of transition state geometries, and leads to considerably higher predicted activity. While synthesis of these model systems is likely challenging, the stabilization of such an active site could greatly reduce the temperature and pressure requirements for thermochemical ammonia synthesis.

[1] Erisman, Sutton, Galloway, Klimont, Winiwarter, Nat. Geosci. 2008, 1, 636-639.

[2] Vojvodic, Medford, Studt, Abild-Pedersen, Khan, Bligaard, Nørskov, Chem. Phys. Lett. 2014, 598, 108-112.