- PhD Chemical Engineering, University of California, Berkeley USA
- BS Chemical Engineering and Biomedical Engineering (dual degrees), Carnegie Mellon University, Pittsburgh USA
In many catalytic systems, the identity of the active site remains a topic of much debate, particularly in nanoparticle/cluster systems. Are the most active sites found on the undersaturated sites, which are few in number, or are they found on the more numerous terrace sites of nanoparticles? In many catalytic systems, it is hypothesized that the undercoordinated edge and corner sites contribute to the greatest fraction of activity, while only accounting for a small fraction of the total surface sites on a metal nanoparticle/cluster. One example of the extraordinary activity of undercoordinated sites that Michael explored in his dissertation was the gold nanoparticle-catalysed reaction of resazurin to resorufin. Using kinetic poisoning experiments to poison surface sites with strongly binding ligands, he quantified the separate catalytic contributions of corner, edge, and terrace sites on a conventional Au/SiO2 heterogeneous catalyst. The corner sites were over an order of magnitude more active than the edge sites, while the terrace sites were catalytically inactive .
To create stable and electronically tunable metal clusters with a large fraction of corner and edge sites, his dissertation explored calixarene-bound gold cluster synthesis. The goal was to synthesize small clusters that due to their small size, a large fraction of undercoordinated sites on the surface would be open for binding/catalysis. Additional goals were to improve stability of the gold clusters due to the steric bulk of the surrounding calixarene ligands as well as to tune the electronic state of the gold clusters through electron-donating/withdrawing functional groups on the calixarene that are bound to the gold clusters. Their approach used large, bulky calixarene ligands that were commensurate in size to the gold cluster core, thereby creating a packing problem of the large ligand on the surface leading to accessible cluster surface atoms . Small gold clusters were synthesized via a reduction of a series of calixarene phosphine-Au(I) chloride complexes as well as by reducing calixarene N-heterocyclic Au(I) chloride complexes [2,3]. These clusters were characterized using a wide variety of techniques. (UV-Vis, MS, HAADF-STEM, TGA) Additionally, the number of open sites was quantified using a steady-state fluorescence titration procedure that demonstrated that there are open sites available for binding/catalysis present on these clusters and that the accessibility follows a general mechanical model of accessibility. This observed number of open sites did not depend on the calixarene’s functional group involved in bound to the gold surface but rather depended on the relative radii of curvature of bound ligands and the gold cluster core .
- M.M. Nigra, I. Arslan, and A. Katz,
"Gold nanoparticle-catalyzed reduction in a model system: Quantitative determination of reactive heterogeneity of a supported nanoparticle surface",
J. Catal., 2012, 295, 115.
- N. de Silva, J.-M. Ha, A. Solovyov, M.M. Nigra, I. Ogino, S.W. Yeh, K.A. Durkin, and A. Katz,
"A bioinspired approach for controlling accessibility in calixarene-bound metal cluster catalysts",
Nature Chemistry, 2010, 2, 1062.
- M. M. Nigra, A. J. Yeh, A. Okrut, A. G. DiPasquale, A. Solovyov, and A. Katz,
"Accessible gold clusters using calixarene N-heterocyclic carbene and phosphine ligands",
Dalton Trans., 2013, 42, 12762.