The production of synthetic ammonia remains dependent on the energy- and capital-intensive Haber–Bosch process. Extensive research in molecular catalysis has demonstrated ammonia production from dinitrogen, albeit at low production rates. Mechanistic understanding of dinitrogen reduction to ammonia continues to be delineated through study of molecular catalyst structure, as well as through understanding the naturally occurring nitrogenase enzyme. The transition to Haber–Bosch alternatives through robust, heterogeneous catalyst surfaces remains an unsolved research challenge. Catalysts for electrochemical reduction of dinitrogen to ammonia are a specific focus of research, due to the potential to compete with the Haber–Bosch process and reduce associated carbon dioxide emissions. However, limited progress has been made to date, as most electrocatalyst surfaces lack specificity towards nitrogen fixation. In this Review, we discuss the progress of the field in developing a mechanistic understanding of nitrogenase-promoted and molecular catalyst-promoted ammonia synthesis and provide a review of the state of the art and scientific needs for heterogeneous electrocatalysts. The artificial synthesis of ammonia remains one of the most important catalytic processes worldwide, over 100 years after its development. In this Review, recent developments in enzymatic, homogeneous and heterogeneous catalysis towards the conversion of nitrogen to ammonia are discussed, with a particular focus on how mechanistic understanding informs catalyst design.

Catalysts for nitrogen reduction to ammonia

Publisher Summary This chapter presents an account of the main results of the studies of the kinetics of some industrial heterogeneous catalytic reactions. The phenomenon of catalysis is a subject of chemical kinetics. The accumulation of data on the kinetics of concrete catalytic reactions favors the progress of the theory of catalysis. The chapter discusses mass transfer in heterogeneous catalysis. Purely physical phenomena––namely, the diffusion of reactants to the catalyst surface (in the majority of cases, to the inner surface of pores), the diffusion of products from this surface, and the transfer of heat evolved or absorbed in the course of the reaction––are indispensable components of the heterogeneous catalystic process. A purely experimental criterion of the kinetic region––namely, independence of the experimentally observed reaction rate on the size of catalyst grains––is used.

The Kinetics of Some Industrial Heterogeneous Catalytic Reactions

New bimetallic molybdenum nitride catalysts were prepared and their kinetic analyses were done. The nitride catalysts were prepared by nitriding the corresponding oxide precursors with ammonia gas through a temperature-programmed reaction up to 973 K. The standard rate of ammonia synthesis was measured with a 0.4 g of catalyst at 588–673 K under 0.1–3.1 MPa with a flow rate of 60 ml min − 1 of N 2 +3H 2 . The rates over molybdenum nitride catalysts were increased with increasing the reaction pressure. The rates over Co 3 Mo 3 N at 673 K under 3.1 MPa (4.8 mmol h −1  g-cat. −1 ) was much higher than the corresponding molybdenum bimetallic nitrides with nickel or iron prepared in a similar way to Co 3 Mo 3 N. Apparent activation energies of these catalysts (11–14 kcal mol −1 ) were on the same level. Alkali promoted Co 3 Mo 3 N catalysts and a doubly promoted iron catalyst were tested. The rate of ammonia synthesis over Co 3 Mo 3 N catalyst with 2 mol% Cs added was remarkably high under high pressure (15.0 mmol h −1  g-cat. −1 under 3.1 MPa). This catalyst gave higher productivity of ammonia than the doubly promoted iron catalyst (4.6 mmol h −1  g-cat. −1 under 3.1 MPa). The kinetic parameters for Co 3 Mo 3 N catalysts were similar to that of the iron catalyst, however, the rate on Co 3 Mo 3 N was more retarded by the product ammonia. Interestingly, the strong ammonia retardation on the Co 3 Mo 3 N surface was relieved by the alkali promoter addition.

Cobalt molybdenum bimetallic nitride catalysts for ammonia synthesis: Part 2. Kinetic study

Cesium-promoted cobalt molybdenum bimetallic nitride Co 3 Mo 3 N catalyst was found to give higher productivity of ammonia than a doubly promoted iron catalyst. Molybdenum nitride catalysts were prepared by nitridation of the corresponding oxide precursors with ammonia gas via a temperature-programmed reaction up to 973 K. The precursors were molybdenum trioxide or molybdenum bimetallic oxide prepared via a mixed solution of ammonium molybdate and metal (Co, Ni, and Fe) nitrate. Ammonia synthesis was carried out with a 0.4 g of catalyst at 673 K under 0.1 MPa with a flow rate of 60 ml min − 1 of N 2 +3H 2 . The simple Mo 2 N was less active (35 μmol h −1  g-cat −1 ) and deactivated soon. Co 3 Mo 3 N prepared from cobalt molybdate hydrate was active (179 μmol h −1  g-cat −1 ) and stable. The activity of Co 3 Mo 3 N was increased (652 μmol h −1  g-cat −1 ) after being treated with the reactant gas at 873 K for 6 h after the passivation with 1% of O 2 at 298 K. The corresponding molybdenum bimetallic nitrides with nickel or iron were also prepared; however, they were not as active as Co 3 Mo 3 N. The activity of Co 3 Mo 3 N was increased by the addition of alkali promoter. Cesium was a more effective promoter than potassium, suggesting an electronic effect. 2 mol% Cs addition was found to give the maximum activity after the reactant gas treatment at 873 K for 6 h (986 μmol h −1  g-cat −1 ).

Cobalt molybdenum bimetallic nitride catalysts for ammonia synthesis: Part 1. Preparation and characterization

https://pubs.rsc.org/en/content/articlepdf/2020/gc/d0gc02058c

Plasma-driven catalysis: green ammonia synthesis with intermittent electricity

The reaction rate in stoichiometric mixtures of nitrogen and hydrogen or deuterium has been measured on two different doubly promoted iron catalysts, between 218 and 302 °C, ⅓ and 1 atm and over a 300-fold range of efficiencies. The kinetic data as well as the isotope effect indicate that the rate-determining step is the chemisorption of nitrogen on a surface mainly covered with NH radicals. The presence on the surface of NH radicals instead of nitrogen atoms opens new perspectives on the kinetics and mechanism of ammonia synthesis.

F. R. S. Sir Hugh Taylor

Papers

Kinetics and mechanism of the ammonia synthesis