Kinetic Expression for Optimal Catalyst Electronic Configuration: The Case of Ammonia Decomposition

TL;DR

This paper develops a kinetic model linking catalyst electronic properties, specifically the Fermi level, to ammonia decomposition rates, revealing a volcano-shaped dependence and guiding catalyst optimization.

Contribution

It extends Wolkenstein electronic theory to metals using the d-band model, providing a novel kinetic framework for catalyst electronic configuration effects.

Findings

Reaction rate peaks at an optimal Fermi level.

Higher Fermi levels increase neutral nitrogen desorption.

Model aligns with experimental data on Fe, Co, and CoFe catalysts.

Abstract

A new steady-state kinetic model of ammonia decomposition is presented and analyzed regarding the electronic properties of metal catalysts. The model is based on the classical Temkin-Ertl mechanism and modified in accordance with Wolkenstein electronic theory by implementing participation of free electrons of the catalyst to change the chemical nature of adsorbed species. Wolkenstein original theory only applied to semiconductors but by including the d-band model, the electronic theory can be extended to metals. For both simplified and full reaction mechanisms, including electronic steps, we present a steady-state rate equation where the dependence on the Fermi level of the metal creates a volcano-shaped dependence. According to the kinetic model, an increasing Fermi level of the catalyst, that approaching the antibonding state with adsorbed nitrogen molecules, will increase the fraction of neutral nitrogen molecules and enhance their the desorption. Concurrently, strong chemisorption of ammonia molecules proceeds easily through participation of additional free catalyst electrons in the adsorbate bond. As a result, the reaction rate is enhanced and reaches its maximum value. A further increasing Fermi level of the catalyst that approaches the antibonding state with ammonia molecules will result in a smaller fraction of negatively charged ammonia molecules and less dehydrogenation. Concurrently, the desorption of neutral nitrogen molecules occurs without impairment. As a result, the reaction rate decreases. The detailed kinetic model is compared to recent experimental measurements of ammonia decomposition on iron, cobalt and CoFe bimetallic catalysts.