MVS-derived Nanostructured Metal Catalysts

MVS-derived Nanostructured Metal Catalysts

Metal vapour synthesis (MVS) technique provides a valuable synthetic route to mono- and heterometallic catalysts. This research line involves the synthesis of mono- and bimetallic nanoparticles featuring strictly controlled dimension and composition by the innovative MVS protocol. Metal atoms, produced by resistive heating of the bulk metal under high vacuum, are co-condensed at liquid nitrogen temperature (-196°C) with weakly stabilizing organic solvents (e.g. mesitylene or acetone) on the cooled wall of a commercial reactor available at ISTM-CNR unit. The obtained solid matrix will yield, upon melting, metal nanoclusters weakly stabilized by the solvent molecules, named solvated metal atoms (SMA). The interaction of the metal vapour with the solvent matrix quench very quickly the kinetic energy of metal atoms. As a result, during the further melting stage, metal nanoparticles with unusual distorted geometries, thermodynamically not stable, are formed. The nucleation and growth processes in SMA solutions can be controlled by different parameters such as kind of organic solvent, metal/solvent ratio, storage temperature and presence of additional stabilizing ligands.
SMA are suitable precursors to prepare mono- and heterometallic supported nanoparticles simply by mixing the SMA with a solid support. The interaction of the metal with the organic ligand, acting as solvent, is so weak that they can be properly regarded as naked metal clusters.
The method is applied, as an alternative to the well-known traditional methods, to the deposition of metals on different kind of supports, which include: pristine inorganic/organic materials in powder form (e.g. silica, alumina, carbon, organic polymers…), monolith supports or 2D support such as surfaces or ceramic membranes. (Image1)

Figure 1 

The preparation of supported heterometal catalysts is affordable either by depositing two metals sequentially from two different metal atom solutions, or by vaporizing two metals at the same time in a multi-electrode reactor (Figure) [16], employing a suitable stabilizing solvent or a mixture of them.
The metal vapour route to supported metal particles can be conducted on lab scale (vaporization of 100 ÷ 500 mg of metal) (Figure 1.1).

This synthetic approach to obtain metal particles has generally the following advantages over traditional metal particle synthesis which foresees a reduction step of the oxidized metal precursor: (i) The final metal-content can be adjusted by the concentration of the solvated metal particles in solution; (ii) M-NPs of comparable size are accessible, regardless of the support employed; moreover, the size of the metal particles can be controlled upon the different metal clusters’ growth in different solvents; (iii) The supported NPs contain only metal in its reduced form, so that, calcination and activation processes of the conventional wet deposition method are not required.

Figure 2. Multi-electrode static glass reactor for simultaneous vaporization of two metals (A); example of condensation of solvent vapors (B) and co-condensation of metal vapors with solvent vapors (C).

Refs:

• Klabunde, K.J. (1994) Free Atoms, Clusters and Nanoscale Particles, (Academic Press, New York).
• Evangelisti, C. et al. (2016) in “Gold Catalysis: Preparation, Characterization, and Applications” Eds. L. Prati, A. Villa, “Solvated Metal Atoms in the Preparation of Supported Gold Catalysts” (Pan Stanford Publishing Pte. Ltd.) pp. 73-92.
• G. Vitulli, G.; Evangelisti, C. et al. (2008) in “Metal Nanoclusters in Catalysis and Materials Science: the Issue of Size-Control”, Eds. B. Corain, G. Schmid, N. Toshima, “Metal Vapor-Derived Nanostructured Catalysts in Fine Chemistry: The Role Played by Particle Size in the Catalytic Activity and Selectivity” (Elsevier) pp. 437-452.
• Pitzalis, E.; Evangelisti, C. et al. (2011) in “Membrane for Membrane Reactors”, Eds. A. Basile and F. Gallucci, “Solvated Metal Atoms in the Preparation of Catalytic Membranes”, (Wiley, UK) pp. 371-380.