Computational Investigation of Mononuclear Iron Water Oxidation Catalyst Design

Abstract

Hydrogen production from non-carbon sources is an essential component of clean and sustainable technology for reducing greenhouse gas emissions from fuels. Water oxidation, which splits water molecules into hydrogen (protons) and molecular oxygen, is a thermodynamically challenging, multistep reaction achieved in photosynthetic organisms via photocatalysis by the Oxygen Evolving Complex (OEC) of Photosystem II. Mononuclear water oxidation catalysts that aim to mimic nature typically rely on heavy, rare metals such as ruthenium and iridium. Replacing these metals with iron is particularly appealing because it is abundant, benign, and inexpensive. We use density functional theory to characterize the catalytic ability of mononuclear iron photocatalysts compared with their ruthenium counterparts for 20 different ligand modifications with varying degrees of electron withdrawing behavior. We quantify the energetics, bond lengths, and charges in each of the steps leading to the highest oxidation state of the metal and necessary O-O bond formation in a mechanism determined experimentally for ruthenium catalysts and in many ways analogous to that followed by the OEC. Although many of the iron catalysts exhibited prohibitively high redox potentials in achieving the highest oxidation state required by this mechanism, a few display promising energetics and stability at each step explored. These results provide insights regarding the feasibility and performance of water oxidation catalysts using earth abundant metals as well as pinpointing mechanistic steps where catalytic ability degrades.