Tracking Minority Species in Redox Reactions: A Quantitative Combined XAS and Modulation Excitation Study

Abstract

Understanding nature of intermediates/active species in reactions is a major challenge in chemistry. This is because spectator species typically dominate the experimentally derived data and consequently active phase contributions are masked. Transient methods offer a means to bypass this difficulty. In particular, modulation excitation with phase‐sensitive detection (ME‐PSD) provides a mechanism to distinguish between spectator and reacting species. Herein, modulation excitation (ME) time‐resolved (energy dispersive) X‐ray absorption spectroscopy, assisted by phase sensitive detection (PSD) analysis, has been applied to the study of a liquid phase process; in this case the classic ferrocyanide/ferricyanide redox couple. Periodic switches of the electrical potential (anodic/cathodic) enabled the use of the ME approach. Structural changes at fractions as low as 2% of the total number of electroactive species were detected within the X‐ray beam probe volume containing ~30 pmol of Fe(II)/Fe(III).