First Principles Modeling of Metal Nanoislands on Metal Substrates

The catalytic activity of metal-supported metal nanoparticles and overlayers is known to differ from that of the corresponding bulk surfaces (1, 2). Moreover, metal-supported metal nanoparticles exhibit a higher reactivity than their overlayer counterparts (1). Differences in reactivity between bulk and overlayered surfaces arise from lattice strain and ligand effects. Yet, these effects do not suffice for rationalizing the high reactivity of such systems. Thus other aspects of metal-supported nanoparticles are often invoked, e.g., particle size, the density of low-coordinated atoms, and the direct involvement of the substrate in a catalytic cycle. However, a concise rationalization of these aspects has been inhibited, in part, due to the lack of systematic computational studies on metal-supported metal nanoparticles.

Recently, we explored geometric, energetic, and electronic properties of metal-supported Pd and Pt clusters employing small triangular model species (3, 4). Such small clusters are far from realistic metal-supported metal nanoparticles that feature diameters of a few nanometers (1). Yet, comparison of Au- and Cu-supported Pt₃ and Pd₃ clusters with the corresponding monoatomic overlayers brought insights into the electronic properties of larger metal-supported planar metal clusters, i.e., monolayer nanoislands. We found the d-band centers, relative to the Fermi energy, of Au-supported Pt₃ and Pd₃ clusters to be similar to those of the corresponding overlayers. In contrast, the d-band centers of the same clusters on Cu are ~1 eV closer to the Fermi energy than those of the corresponding overlayers (3). The difference arises from a compensation of counteracting effects in Au-supported Pt₃ and Pd₃clusters and the corresponding overlayers: lateral interactions (intra-layer coordination) vs. strain due to the support (3). These effects result in downward and upward shifts, respectively, of the d-band centers of monolayer nanoislands on Au surfaces as the size of the islands increases. For similar systems on Cu, both lateral interactions and compressive strain effects shift the d-band centers of nanoislands downward only (3).

We have extended our calculations to study nanoislands of hexagonal shape and nanoscale diameters (Figure 1). We systematically explore how geometric and energetic properties of these systems behave as function of size. The results will help, on the one hand, in rationalizing the observed high reactivity of metal-supported metal nanoparticles, and on the other hand, in assessing the accuracy of full overlayers as models of supported nanoparticles.

Acknowledgments. J.A.S. work was supported by a research fellowship of the Alexander von Humboldt Foundation and in part by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division.

Reference:

1. H. Wolfschmidt, O. Paschos and U. Stimming, in Fuel Cell Science: Theory, Fundamentals, and Biocatalysis, A. Wieckowski and J. K. Nørskov Editors, p. 1, Wiley, Chichester, U.K. (2010).

2. L. A. Kibler, Electrochimica Acta, 53, 6824 (2008).

3. J. A. Santana and N. Rösch, Journal of Physical Chemistry C, 116, 10057 (2012).

4. J. A. Santana and N. Rösch, Physical Chemistry Chemical Physics, 14, 16062 (2012).

Figure 1. Planar models of metal-supported nanoislands with increasing diameter.