Synergistic effects in heterogeneous catalysis: Status and perspectives

In many fields of science and technology dealing with the development of functional materials, a synergistic action of several components can result in unique performance which is usually superior to those of individual moieties. Despite rapidly growing interest in synergistic effects, their magic and nature are poorly understood. This review provides the recent interpretations of structure-related (geometric) and electronic synergistic effects in heterogeneous catalysis. Structure-related effects are caused by the coexistence and/or cooperation of surface species (single-atom or dual-atom species, (ultra)small clusters, nanoparticles, elemental vacancies, functional groups) and are discussed given inter- and intraparticle aspects, including facet-dependent behavior, morphology, metal-support interaction, coordination environment, proximity and spatial distribution of active sites and interfaces. Electronic effects are discussed considering electronic metal-support interaction, occupancy and hybridization of orbitals, spin and oxidation states, and position of the d-band center. Furthermore, the approaches to control and measure the synergistic effects are considered. The synergistic effects are discussed for photo- and electrocatalytic reactions (CO 2 reduction (CO 2 RR), N 2 reduction (N 2 RR), oxygen reduction (ORR), oxygen evolution (OER), hydrogen evolution (HER), water splitting, hydrazine oxidation (HzOR), hydrogen generation) as well as selective transformations (hydrogenation, amination, oxidation, epoxidation). Finally, future research perspectives are summarized.