Size-selected nanoparticles (atomic clusters) represent a new class of model systems for catalysis research. However very rarely have their structures been obtained from direct experimental measurements. The availability of aberration-corrected HAADF STEM is transforming our approach to this structure challenge [1,2]. I will address the atomic structures of size-selected Au clusters, deposited onto standard carbon TEM supports from a mass-selected cluster beam source. Specific examples considered are the “magic number clusters†such as Au20, Au55, Au309, Au561, and Au923. The results expose, for example, the metastability of frequently observed structures [3], the nature of equilibrium amongst competing isomers, and the cluster surface and core melting points as a function of size. The experimental data also provide a benchmark for theoretical modelling. Moreover the cluster beam approach is applicable to more complex nanoparticles too, such as oxides and sulphides [4].
A second major challenge is to translate the beautiful physics and chemistry of clusters into applications, notably catalysis [5]. Compared with the (powerful) colloidal route, the nanocluster beam approach involves no ligands, particles can be size selected by amass filter, and particles containing challenging combinations of metals can readily be produced. However, the cluster approach has been held back by extremely low rates of particle production, only 1 microgram per hour, sufficient for surface science studies but well below what is desirable even for research-level realistic reaction studies. In an effort to address this scale-up challenge, I will discuss the development of a new kind of nanoparticle source, the “Matrix Assembly Cluster Source†(MACS) [5,6]. The results suggest cluster beam yields of grams per hour may ultimately be feasible ; mg scale has been demonstrated. Some practical catalysis applications [5,7], both gas and liquid phase, will be presented, showing attractive activities and especially selectivities.
[1] Z.Y. Li et al, Nature 451 46 (2008).
[2] Z.W. Wang and R.E. Palmer, Phys. Rev. Lett. 108 245502 (2012).
[3] D.M. Foster, R. Ferrando and R.E. Palmer, Nature Communications (2018), in press.
[4] D. Escalera-Lopez, Y.B. Niu, J.L. Yin, K. Cooke, N.V. Rees, R.E. Palmer, ACS Catalysis 6 6008 (2016).
[5] P.R. Ellis et al, Faraday Discussions 188 39 (2016).
[6] R.E. Palmer, L. Cao and F. Yin, Rev. Sci. Instrum. 87 046103 (2016).
[7] R. Cai, P.R. Ellis, J.L. Yin, J. Liu, C.M. Brown, R. Griffin, G. Chang, D. Yang, J. Ren, K. Cooke, P.T. Bishop, W. Theis and R.E. Palmer, Small, in press (2018).