Electride support boosts nitrogen dissociation over ruthenium catalyst and shifts the bottleneck in ammonia synthesis.

Masaaki Kitano,Hideo Hosono,Yasunori Inoue,Shinji Kanbara,Peter V Sushko,Michikazu Hara,Toshiharu Yokoyama,Navaratnarajah Kuganathan

doi:10.1038/ncomms7731

Abstract

Novel approaches to efficient ammonia synthesis at an ambient pressure are actively sought out so as to reduce the cost of ammonia production and to allow for compact production facilities. It is accepted that the key is the development of a high-performance catalyst that significantly enhances dissociation of the nitrogen–nitrogen triple bond, which is generally considered a rate-determining step. Here we examine kinetics of nitrogen and hydrogen isotope exchange and hydrogen adsorption/desorption reactions for a recently discovered efficient catalyst for ammonia synthesis—ruthenium-loaded 12CaO·7Al2O3 electride (Ru/C12A7:e−)—and find that the rate controlling step of ammonia synthesis over Ru/C12A7:e− is not dissociation of the nitrogen–nitrogen triple bond but the subsequent formation of N–Hn species. A mechanism of ammonia synthesis involving reversible storage and release of hydrogen atoms on the Ru/C12A7:e− surface is proposed on the basis of observed hydrogen absorption/desorption kinetics.

Highlights

Novel approaches to efficient ammonia synthesis at an ambient pressure are actively sought out so as to reduce the cost of ammonia production and to allow for compact production facilities
Pure alkali and alkaline earth metals drastically enhance the catalytic activity of Fe and Ru10, they are chemically unstable under ammonia synthesis conditions
As reported earlier[14], Ru/C12A7:e À exhibits an order of magnitude higher catalytic activity, as defined by the turnover frequency (TOF), for ammonia synthesis than Ru/C12A7:O2 À and the conventional Ru catalysts

Summary

Introduction

Novel approaches to efficient ammonia synthesis at an ambient pressure are actively sought out so as to reduce the cost of ammonia production and to allow for compact production facilities. The role of the catalyst is to lower the dissociation energy of the NN bond Both Fe and ruthenium (Ru) are wellknown catalysts for ammonia synthesis, and their catalytic activity is significantly enhanced by basic promoters such as alkali and alkaline earth metal oxides (Cs2 þ xO, K2O, BaOx and so on)[2,3,4,5,6]. Pure alkali and alkaline earth metals drastically enhance the catalytic activity of Fe and Ru10, they are chemically unstable under ammonia synthesis conditions. The following three possibilities for the ratedetermining step (RDS) of ammonia synthesis are suggested: It was (C12A7:e À recently reported that 12CaO Á 7Al2O3 electride ), the first room temperature stable electride, functions as an efficient electronic promoter for Ru catalyst[14]. C12A7:e À has a low work function (2.4 eV)

Methods

Results

Conclusion