Speaker
Description
Small clusters in the gas phase are ideal model systems for investigating the fundamental aspects of complex reactions [1,2]. The good control over cluster size, composition, and charge state allows distinguishing the effects of these parameters on the studied reactions. In the high-vacuum conditions of a gas-phase experiment there are no contaminating agents that could affect the reaction mechanism. In addition, for small-sized clusters, ranging from two to a few tens of atoms, direct comparison between experiment and quantum-chemical calculations is possible.
In this talk, I will discuss two examples in which infrared multiple photon dissociation spectroscopy (IRMPD) is used to investigate the interaction of metal clusters with small molecules. First, the adsorption of CO on Mo and Nb doped cationic platinum clusters is studied with the goal to enhance understanding about the CO poisoning effect of Pt catalyst nanoparticles in proton exchange membrane fuel cells. Reactivity measurement in a low-pressure collision cell showed significant reduction in the reactivity of the doped Pt clusters compared to bare Pt clusters, which is attributed to electron transfer from the highly coordinated dopants to the Pt atoms and the concomitant lower CO binding energies [3]. Stretching frequencies of the adsorbed CO molecules reflect dopant-induced charge redistribution within the clusters [4].
Second, I will present a study of hydrogen adsorption on the V and Rh doped cationic aluminum clusters. IRMPD spectra, which provide a fingerprint of the hydrogen binding geometry, prove that H$_2$ dissociates upon adsorption for the V doped clusters. Orbital analysis shows that the activation barriers are due to an unfavorable overlap between cluster and hydrogen orbitals [5]. For Rh doped clusters, depending on the cluster sizes, either the H$_2$ binds dissociatively or it adsorbs molecularly in Kubas complexes. This size dependence is not due to kinetic impediment of the hydrogenation reaction by an activation barrier, but to binding energy differences [6].
[1] D. Justes, R. Mitrić, N. Moore, V. Bonačić-Koutecký, A. Castleman, Jr., J. Am. Chem. Soc. 125, 6289 (2003).
[2] S. M. Lang, T. Bernhardt, R. Barnett, U. Landman, Angew. Chem. Int. Ed. 49, 980 (2010).
[3] P. Ferrari, L.M. Molina, V.E. Kaydashev, J.A. Alonso, P. Lievens, E. Janssens, Angew. Chem. Int. Ed. 55, 11059 (2016).
[4] P. Ferrari, J. Vanbuel, N.M. Tam, M.T. Nguyen, S. Gewinner, W. Schöllkopf, A. Fielicke, E. Janssens, Chem. Eur. J. 23, 4120 (2017).
[5] J. Vanbuel, E.M. Fernández, P. Ferrari, S. Gewinner, W. Schöllkopf, L.C. Balbás, A. Fielicke, E. Janssens, Chem. Eur. J. 23, 15638 (2017).
[6] J. Vanbuel, M.Y. Jia, P. Ferrari, S. Gewinner, W. Schöllkopf, M.T. Nguyen, A. Fielicke, E. Janssens, Top. Catal. (2017). https://doi.org/10.1007/s11244-017-0878-x
