Single Atom Catalysts: What Matters Most, the Active Site or The Surrounding?

Abstract

Single atom catalysts (SACs) consist of isolated metal atoms stabilized on a solid support. The name suggests that the catalytic activity is due to the nature of the metal atom, but of course the interaction with the substrate plays a role as well. But what is more important? The metal atom or its surrounding? To answer this question, we have performed numerical experiments based on density functional theory (DFT). 24 transition metal atoms have been incorporated in Nitrogen‐doped graphene, and the catalytic activity in the hydrogen evolution reaction (HER) has been studied changing the metal and keeping the N‐doped graphene matrix fixed. Then, one specific atom, Pt, has been embedded in the same matrix but the nitrogen neighbors of Pt have been systematically replaced by carbon or oxygen atoms generating more than 20 structures. The HER has thus been studied by keeping the metal center fixed but changing the surrounding. It turns out that the same great variability in chemical behavior can be achieved by changing the active site or the surrounding environment. This shows the importance of the local coordination in determining the catalytic activity. The consequences of this conclusion for modeling studies of SACs are discussed.

A Few Questions about Single-Atom Catalysts: When Modeling Helps

“…This corresponds to Δ G H going from +2.0 to −1.0 eV as a function of the transition metal. The same wide range of reactivity can be obtained by fixing the metal atom, Pt in this case, and changing the local environment (for instance, by replacing the nitrogen neighbors of Pt with carbon or oxygen atoms) . The range of adsorption free energies goes from +1.5 to −1.5 eV, showing that in SACs the metal center is as important as the surroundings.…”

“…For theoretical modeling, it is essential to know the exact coordination of a SAC in a support. Even small changes in the coordination may result in different properties and different reactivity . This problem can be addressed only by strengthening the dialogue between theory and experiment and by combining the results obtained from different spectroscopies and microscopies with computational models.…”

“…The same wide range of reactivity can be obtained by fixing the metal atom, Pt in this case, and changing the local environment (for instance, by replacing the nitrogen neighbors of Pt with carbon or oxygen atoms). 15 The range of adsorption free energies goes from +1.5 to −1.5 eV, showing that in SACs the metal center is as important as the surroundings. (In this respect, the notion of a single-atom catalyst is a bit misleading.)…”

“…This is a key aspect since the same atom can behave very differently depending on the local coordination. To show this, we recently performed a simple numerical experiment based on density functional theory (DFT) . We incorporated 24 transition-metal atoms in a nitrogen-doped graphene support, and we studied the catalytic activity in the hydrogen evolution reaction (HER) as a function of the transition metal.…”

“…To show this, we recently performed a simple numerical experiment based on density functional theory (DFT). 15 We incorporated 24 transition-metal atoms in a nitrogen-doped graphene support, and we studied the catalytic activity in the hydrogen evolution reaction (HER) as a function of the transition metal. Not surprisingly, the activity, which according to a widely used model is directly related to the free energy of H adsorption, ΔG H (see below), varies a lot, with some atoms very active and others completely inactive.…”