Ashildur Logadottir

Trends in the Exchange Current for Hydrogen Evolution

Department of Physics, Technical University Munich, D-85748 Garching, GermanyA density functional theory database of hydrogen chemisorption energies on close packed surfaces of a number of transition andnoble metals is presented. The bond energies are used to understand the trends in the exchange current for hydrogen evolution. Avolcano curve is obtained when measured exchange currents are plotted as a function of the calculated hydrogen adsorptionenergies and a simple kinetic model is developed to understand the origin of the volcano. The volcano curve is also consistent withPt being the most efficient electrocatalyst for hydrogen evolution.© 2005 The Electrochemical Society. @DOI: 10.1149/1.1856988# All rights reserved.Manuscript submitted May 10, 2004; revised manuscript received August 12, 2004. Available electronically January 24, 2005.

Electronic factors in catalysis: the volcano curve and the effect of promotion in catalytic ammonia synthesis

Abstract The activity and selectivity of heterogeneous catalysts are determined by their electronic and structural properties. In many cases, the electronic properties are determined by the choice of both the catalytically active transition metal and promoter elements. Density functional theory is used to calculate how these two factors affect the energies of the intermediates and transition states in the ammonia synthesis reaction. We show that a linear relationship exists between the activation energy for N 2 dissociation and the binding energy of adsorbed nitrogen. The ammonia synthesis activity under industrial conditions can be determined as a function of the nitrogen–surface interaction energy by combining the calculated adsorption energy–activation energy relation with a micro-kinetic model. The result is a volcano curve and we illustrate such relationships for both the non-promoted and alkali metal promoted transition metals. We conclude that promotion is most effective for the best non-promoted catalysts and that promotion will always be essential for obtaining an optimal ammonia synthesis catalyst. Analysis of the micro-kinetic model show that the best catalysts are those with the lowest apparent activation energies and they exhibit reaction orders between two asymptotic behaviors.

Ammonia synthesis over a Ru(0001) surface studied by density functional calculations

In this paper we present DFT studies of all the elementary steps in the synthesis of ammonia from gaseous hydrogen and nitrogen over a ruthenium crystal. The stability and configurations of intermediates in the ammonia synthesis over a Ru(0001) surface have been investigated, both over a flat surface and over a stepped surface. The calculations show that the step sites on the surface are much more reactive than the terrace sites. The DFT results are then used to study the mechanism of promotion by alkalies over the Ru(0001) and to determine the rate-determining step in the synthesis of ammonia over the Ru catalyst.

Ammonia synthesis at low temperatures

Density functional theory (DFT) calculations of reaction paths and energies for the industrial and the biological catalytic ammonia synthesis processes are compared. The industrial catalyst is modeled by a ruthenium surface, while the active part of the enzyme is modeled by a MoFe6S9 complex. In contrast to the biological process, the industrial process requires high temperatures and pressures to proceed, and an explanation of this important difference is discussed. The possibility of a metal surface catalyzed process running at low temperatures and pressures is addressed, and DFT calculations have been carried out to evaluate its feasibility. The calculations suggest that it might be possible to catalytically produce ammonia from molecular nitrogen at low temperatures and pressures, in particular if energy is fed into the process electrochemically.