Martin's group in the School of Chemistry published a review of cross-scale metal catalyst group effects in Nature Catalysis

Understanding the structure-activity relationship of metal catalysts is one of the important means to study the reaction mechanism and construct the
optimal catalytic system.
As the aggregation form of metals on the surface of the catalyst changes from nanoparticles, nanoclusters to atomic-level dispersion, its catalytic properties may change
dramatically.
For precious metal catalysts, limited by the scarcity of precious metal resources, how to achieve low metal dosage while ensuring high activity of the catalyst is the current research focus
.
The resulting single-atom catalysts have been shown to have good reactivity performance
in a variety of reactions.
But for many reactions, single-atom catalysts do not catalyze these processes
well.
The ensemble effect sums up this phenomenon well: for the activation or formation of certain chemical bonds, specific metal ensembles can provide matching adsorption/activation sites, so they have excellent catalytic activity
.
This concept was originally developed by W.
Sachtler et al.
proposed and experimentally verified
.
Martin's group recently studied the metal catalyst group effect of multiple reactions: cyclohexane dehydrogenation to benzene has the highest activity on Pt ensemble with Pt-Pt coordination ~2 (J.
Am.
Chem.
Soc.
2022, 144, 3535-3542）； N-heterocyclic hydrogenation is most advantageous on Pd ensemble with Pd-Pd coordination ~4 (Nat.
Catal.
2022, 5, 485-493）； Dehydrogenation of cyclohexanol to phenol requires atomically dispersed Rh ₁ and Rh ensemble sites (Rh_e; Including cluster Rh _n, nanoparticles Rh_p) can be carried out efficiently (J.
Am.
Chem.
Soc.
2022, 144, 5108–5115).

In view of the understanding of cross-scale metal catalyst structure-activity relationship and the design of efficient catalysts, Professor Martin's research group from the School of Chemistry and Molecular Engineering of Peking University was invited to publish a review article entitled "Ensemble effect for single-atom, small cluster and nanoparticle catalysts" in Nature Catalysis.
The group effect
on the activation of different chemical bonds on "nanoparticle-atomic cluster-monoatomic" cross-scale metal catalysts is summarized.
For example, metal cluster catalysts have better reactivity than larger metal nanoparticle catalysts and smaller single-atom catalysts for hydrogenation of most unsaturated chemical bonds, reactions involving C-C bond breakage, and reactions involving the simultaneous activation of multiple chemical bonds (Figure 1).

This is due to the group effect of these reactions that determines that the adsorption of their reactants and intermediates requires multiple sites; Cluster catalysts, on the other hand, provide sufficient adsorption sites (and are highly unsaturated) to activate the reactants while maximizing the surface metal ratio for optimal catalytic activity
.

Figure 1.
A catalytic reaction that requires multiple "groups" of atoms of the same metal

On the other hand, for highly active single-atom catalysts, monodisperse metal atoms often have the only decisive role in catalytic properties, and the efficient catalytic process cannot be achieved without the synergistic effect
of the surface-active components (such as carrier metal atoms, alkali metal additives, second metal atoms or other surface modification ligands) that coordinate with them.
The central metal atom and these surface-active components perform their respective functions in the catalytic cycle, and the atomic set they form can also be regarded as the metal "group"
required for the target reaction.

On the basis of determining the metal atom "group" required for the target chemical bond activation/catalytic reaction, the mass-specific activity of the catalyst can be further improved by increasing the density of the metal "group" on the catalyst surface, which is another criterion for the design of
metal catalysts.
Taking Ru-based ammonia synthesis catalysts as an example, it has been confirmed that Ru metal B_5-site is the main site
for the adsorption and activation of reactant_N2.
The_B5-site density can be changed by the following strategies: 1) the_B5-site density in Ru nanoparticles can be changed by improving the catalyst size, when the nanoparticle size is 2-3 nm, the B_5-site accounts for the highest proportion of all the particles (Figure 2a, b); 2) The active site density can be changed by changing the metal phase: the phase state of the Ru nanoparticles is changed from hexagonal phase to square phase, the proportion of surface step positions increases, and the density of B_5-site increases correspondingly (Figure 2c).

3) The authors also propose that the next way to maximize the density of B 5-site is to "transplant" atomic clusters composed of multiple B 5-sites on the surface of nanoparticles onto a carrier (Figure 2d) to make a fully exposed metal cluster composed of basically entirely B _5-site; This enables the construction of a fully high active site (in the case of Ru-based ammonia catalysts) on the basis of increasing the atomic utilization rate of the metal catalyst to 100%, so it is the ultimate goal
of metal catalyst design.

Figure 2.
Design of homoatomic metal catalysts based on "group effect"

The thesis work (Nature Catalysis 2022, 5,766–776) was supported
by the National Natural Science Foundation of China, Sinopec, Beijing National Research Center for Molecular Sciences, China Postdoctoral Fund, BMS fellowship and other projects.
Martin is the corresponding author of the work, and the joint first authors are Guo Yu, a BMS postdoctoral fellow at the School of Chemistry and Molecular Engineering of Peking University, and Wang Maolin
, a doctoral candidate.