Van Marum Colloquium: Theoretical studies of the structure and catalytic activity of metal nanoclusters
- Monday 24 October 2022
2333 CC Leiden
Metal nanoclusters have various interesting catalytic properties. One example is the observations of facile CO oxidation at below room temperature on Au particles with diameter around 2-3 nm, while the activity is significantly less for both smaller and larger particles . The explanation for this and several other interesting observations regarding metal nanocluster catalysis is still controversial.
We carry out theoretical calculations with the goal of identifying the atomic scale structure of metal clusters and relating it to catalytic activity. The optimal structure of clusters with up to a few hundred atoms is determined by global optimization involving searches of saddle points on the energy surface and subsequent relaxation to new local minima. For larger clusters, up to over a thousand atoms, the search for optimal structure is carried out by starting with the three dominant structure motives: Icosahedra, decahedra and cuboctahedra, and then introducing distortions, symmetry breaking and reconstructions inspired by results on the smaller clusters. Complete shells of regular icosahedra, decahedra and cuboctahedra are often assumed to lead to particular stability and thereby 'magic numbers' of atoms, and an abrupt switch in preferred structure from icosahedra to decahedra and then to cuboctahedra as the cluster grows. Our calculations of gold clusters have revealed a more complex picture and different values for 'magic numbers' . Small clusters in the range of 55 atoms, one of the assumed magic numbers, turns out to have a mix of icosahedral and decahedral structural motives in a Janus type structure. Systematic analysis method for AC-STEM experimental data  has been developed to extract information on the atomic structure of clusters.
The catalytic activity of various active sites on large clusters is studied with DFT calculations where periodic models of the active sites provide a practical approach for the study of reaction mechanism and reaction rates of clusters with several hundreds or even thousands of atoms. Slight variations in the atomic structure can have surprizingly large effect. For example, the B-type edges on the surface of FCC Pt nanoclusters are found to be several orders of magnitude more active for the hydrogen evolution reaction than A-type edges . The catalytic activity of Pt particles is, therefore, highly dependent on the shape of the clusters, which in turn can depend on preparation conditions.
The stability of metal clusters is of major concern in practical applications such as catalysis. In addition to sintering, degradation due to chemical reactions can destroy the clusters. Even platinum clusters, the catalyst of choice for the oxygen evolution reaction, can oxidize and decompose. A possible mechanism for this has been proposed  but more work on this issue is needed.
- Operando atomic structure and active sites of TiO2(110)-supported gold nanoparticles during carbon monoxide oxidation, M-C. Saint-Lager et al., Faraday Discussions 162, 179 (2013).
- Reassignment of magic numbers for Au clusters of decahedral and FCC structural motifs, A.L. Garden et al., Nanoscale 10, 5124 (2018); Reassignment of magic numbers for icosahedral Au clusters: 310, 564, 928 and 1426, J. Kloppenburg et al., Nanoscale 14, 9053 (2022).
- Experimental evidence for fluctuating, chiral-type Au55 clusters by direct atomic imaging, Z. Wang and R. E. Palmer, Nano Letters 12, 5510 (2012).
- Catalytic activity of Pt nano-particles for H2 formation, E. Skúlason et al., Topics in Catalysis 57, 273 (2014).
- Simulations of the electrochemical oxidation of Pt nanoparticles of various shapes, B. Kirchhoff et al., J. Phys. Chem. C 126, 6773 (2022).