Hydrogen Spillover and Its Relation to Hydrogenation: Observations on Structurally Defined Single‐Atom Sites**

Hydrogen spillover, involving the transfer of H atoms from metal sites onto the catalyst support, is ubiquitous in chemical processes such as catalytic hydrogenation and hydrogen storage. Atomic level information concerning the kinetics of this process, the structural evolution of catalysts during hydrogen spillover, as well as the nature of participation of the spilled over H in catalysis, remain vastly lacking. Here, we provide insights to those questions with a solubilized polyoxometalate-supported single-atom catalyst which allows for the use of characterization techniques generally inaccessible to the study of heterogeneous catalysts. Hydrogenation kinetics together with poisoning studies further reveal that hydrogen spillover can be either detrimental or beneficial for catalysis, the direction and magnitude of which depends mostly on the nature of the reducible functional group. Similar trends were observed on one of the most prototypical hydrogen spillover catalysts-Pt/WO3 .