Density Functional Theory Evaluation of Cation-doped Bismuth Molybdenum Oxide Photocatalysts for Nitrogen Fixation

TL;DR

This study uses density functional theory to evaluate how cation doping in bismuth molybdenum oxide influences its ability to catalyze nitrogen fixation, revealing optimal compositions and electronic property modifications.

Contribution

It provides new insights into the effects of Fe, La, and Yb doping on the electronic structure and nitrogen fixation activity of Bi$_{x}$M$_{y}$MoO$_6$ photocatalysts, identifying optimal doping levels.

Findings

Yb and Fe doping decrease Mo-O binding and increase N/H affinity.

Band gap energy shifts by approximately 0.2 eV due to orbital hybridization.

Optimal composition predicted as orthorhombic (Bi$_{0.75}$Fe$_{0.25}$)$_2$MoO$_6$ with a 1.4 eV energy barrier.

Abstract

This study investigates the photocatalytic nitrogen fixation on a cation-doped surface (Bi$_{x}$M$_{y}$)$_2$MoO$_6$ where (M = Fe, La, Yb) in both the orthorhombic and monoclinic configurations using a density functional theory (DFT) approach with experimentally validated model inputs. The proceeding discussion focuses on the Heyrovsky-type reactions for both the associative and dissociative reaction pathway related to nitrogen reduction. Key fundamental insight in the reduction mechanism is discussed that relates the material properties of the substitutional ions to the nitrogen and hydrogen affinities. Physical insight is gathered through interpretation of bound electronic states at the surface. Compositional phases of higher Fe and Yb concentrations resulted in decreased Mo-O binding and increased affinity between Mo and the N and H species on the surface. The modulation of the Mo-O binding is induced by strain as Yb and Fe are implemented, this, in turn, shifts energy levels and modulates the band gap energy by approximately 0.2 eV. This modification of Mo-O bond as substitution occurs is a result of the orbital hybridization of M-O (M = Fe, Yb) that causes a strong orbital interaction that shifts states. The optimal composition was predicted to be an orthorhombic configuration of (Bi$_{0.75}$Fe$_{0.25}$)$_2$MoO$_6$ with a predicted maximum thermodynamic energy barrier of 1.4 eV. This composition demonstrates effective nitrogen and hydrogen affinity that follows the associative or biological nitrogen fixation pathway.