DFT Analysis of Elementary N2 Electro-Reduction Kinetics on Transition Metal Surfaces

Presented on October 31, 2018 during the NH3 Fuel Conference 2018

Authors Sharad Maheshwari Yawei Li Michael Janik Pennsylvania State University

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Abstract

Ammonia is currently produced through the catalytic Haber Bosch process (HB) at temperatures of about 300 to 500 °C and pressure of about 200-300 atm. In a future with plentiful renewable electricity from distributed sources, an electro-chemical system to produce ammonia could efficiently generate ammonia on site and on demand. Possible heterogeneous catalysts for electro-chemical nitrogen reduction are currently marred by the poor rate and selectivity due to difficulty in activating the strong N-N bond and to the competing hydrogen evolution reaction (HER), resulting in infeasible faradaic efficiency. To develop more selective and active catalysts, better understanding of the mechanistic and kinetic aspects of nitrogen electro-reduction is essential. We use density functional theory (DFT) methods to examine the elementary kinetics of the possible associative mechanisms for electro-reduction of N2. A comprehensive electro-reduction mechanism with elementary activation barriers for each step will be presented on iron (Fe) surfaces. Key step barriers are also evaluated across a series of late transition metal catalysts to evaluate the reliability of scaling and Bronsted-Evans-Polanyi relationships to predict catalyst performance. We compare the “kinetic over-potential” to the thermodynamic over potential to emphasize the need to explicitly evaluate barriers in predicting catalyst performance. We then examine how near-surface additives can act as shuttling agents to alter the kinetic barriers of the proton coupled electron transfer, acting as co-catalysts to improve activity and selectivity.

Topics Electrochemical Ammonia

Authors sum458@psu.edu lywiailyw@gmail.com mjanik@engr.psu.edu reza.rostamikia@gmail.com greenlee@uark.edu julie.renner@case.edu

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