Imaging a Haber-Bosch Catalysis Precursor at the Atomic Scale

Abstract

The low ionization potentials make alkali atoms obliging electron donors in industrial reduction chemistry as important, general cocatalysts. Interaction between potassium atoms and nitrogen molecules co-adsorbed on metal surfaces is of interest as a model of catalytic promotion of dissociation of a reluctant precursor, N2, which is considered the rate-limiting step in the Haber-Bosch ammonia synthesis. Although the atomic-scale insight into the co-adsorption of K and N2 on metals is fundamentally important for understanding the alkali promotion chemistry, the high mobility of K atoms, the weak chemisorption of N2, and the extreme pressures and temperatures in the Haber-Bosch process obstruct investigation of their atomic-scale interactions. By scanning tunneling microscopy and density functional theory, we characterize the N2 adsorption, collective interactions, and tunneling electron-induced desorption on K/Ag(111) surface. Our study reveals the fundamental collective interactions of alkali promoters with the N2 feedstock of consequence to heterogeneous catalysis.