Seeing the Invisible: A Hydrogen‐Centred View of Heterogeneous Catalysis Enabled by Neutron Spectroscopy

Hydrogen governs virtually every stage of heterogeneous catalytic reactions, yet its chemical identity, location, mobility, and mechanistic role are often difficult to establish by experiments. Recent advances in neutron spectroscopy have enabled direct observation of hydrogen species across increasingly complex catalytic systems, generating a rapidly growing body of mechanism insights. However, general principles of hydrogen behaviour have not been fully extracted. In this mini-review, we present a hydrogen-centred framework that reorganises recent neutron-spectroscopic studies around four fundamental questions: what hydrogen species exist, where hydrogen resides, how it moves, and how it participates in catalytic reactions. This mini-review integrates hydrogen species, spatial distribution, dynamics, and reactivity into understanding across diverse systems, and emphasises how neutron spectroscopy provides direct mechanism insights into the chemistry of hydrogen and enables discrimination between competing catalytic pathways. Finally, we discuss current limitations and future opportunities for combining neutron spectroscopy with complementary experimental and theoretical approaches to establish a more comprehensive understanding of hydrogen-controlled catalysis.