© 2015 by L M Rossi. 

Metal nanoparticles as component for the preparation of heterogeneous catalysts



Catalysis is one of the oldest commercial applications of nanotechnology! Heterogeneous catalysts may contain metal particles in a wide range of sizes, including objects in the nanometer range. The advances in characterization techniques and preparation methodologies of metal nanoparticles (NPs) open the opportunity for the preparation of heterogeneous catalysts analogues with a better control of composition, size, morphology, and surface and chemical properties than traditional methods. This high degree of control enables systematic studies of the influence of composition and structural features of metal nanoparticles on chemical reactivity and selectivity, leading to an increasingly understanding of relationships between structure and catalytic activity for the design of more efficient catalysts. One of the main distinctive features of metal nanoparticles (for the preparation of heterogeneous catalysts) remains the growing number of metallic centers and thus of potential active sites on their surface when decreasing their size, which maximizes the use of noble metals. However, nanoparticles have unique size-dependent physical and chemical properties and, in some cases, an intermediate ideal size exists for which catalytic activity is maximized. Much remains to be discovered regarding the influence of nanoparticle size, and other tuneable parameters, on catalytic activity. In this context, catalytic studies with nanoparticles are relevant only if their synthesis provides well-defined and tuneable size. The main goal of our research is the development of methods for the synthesis of metal nanoparticles supported on solids, using at least two strategies: direct deposition of metal precursors (impregnation/reduction or sputtering) or immobilization of preformed metal nanoparticles. The preparation of supported catalysts for application in liquid phase reactions is challenging and requires a strong metal-support interaction to avoid metal leaching and catalysts deactivation. In the last 10 years, we have been studying the preparation of metal nanoparticles for applications in catalysis. The catalyst supports investigated include conventional and nanomaterials with magnetic properties. The use of magnetically recoverable supports for the immobilization of NPs instead of traditional oxides, polymers or carbon based solids guarantees facile, clean, fast and efficient separation of the catalyst at the end of the reaction cycle. Magnetic separation can be considered an environmentally benign separation approach, since it minimizes the use of auxiliary substances and energy for achieving catalyst recovery (see our Review Article).


A new family of magnetically recoverable metal nanoparticle catalysts (Pd, Pt, Rh, Ru, Au, Ni, Co, Cu, etc) has been development for application in green synthesis, such as, green hydrogenation with H2 and green oxidations with O2. See more in our Account Paper and related publications (see BOX in the right side).  The immobilization of pre-formed nanoparticles opens the possibility to control the size and composition of bimetallic nanoparticles in a way not possible by co-precipitation of the two metals (see our article in ACS Catalysis). Access to core-shell bimetallic nanoparticles by the successive deposition of two metals is also a very interesting approach and allows a fine tuning of surface and chemical properties of bimetallic nanoparticles (see our article in Scientific Reports).


Concomitant with the search for new preparation methods, all the new materials are characterized by advances techniques such as transmission electron microscopy (TEM) and atom mapping techniques, XRD, XPS, XAFS (XANES, EXAFS) and their catalytic activity are studied in selected model reactions (CO oxidation, hydrogenation of olefins, oxidations of alcohols, etc.).





Rh NPs:

Jacinto, M. J. ; Kiyohara, P. K. ; Masunaga, S. H. ; Jardim, R. F. ; Rossi, L. M. Recoverable rhodium nanoparticles: Synthesis, characterization and catalytic performance in hydrogenation reactionsApplied Catalysis. A, General, v. 338, p. 52-57, 2008.


Rossi, L. M. ; Vono, L. L. R. ; Garcia, M. A. S. ; Faria, T. L. T.; Lopez-Sanchez, J. A. Screening of Soluble Rhodium Nanoparticles as Precursor for Highly Active Hydrogenation Catalysts: The Effect of the Stabilizing Agents. Topics in Catalysis, v. 56, p. 1228-1238, 2013.


Pt NPs:

Jacinto, M.J. ; Landers, R. ; Rossi, L.M. Preparation of supported Pt(0) nanoparticles as efficient recyclable catalysts for hydrogenation of alkenes and ketones. Catalysis Communications, v. 10, p. 1971-1974, 2009.


Ru NPs:

Jacinto, M. J. ; Santos, O. H. C. F. ; Jardim, R. F. ; Landers R. ; Rossi, L. M. Preparation of recoverable Ru catalysts for liquid-phase oxidation and hydrogenation reactions. Applied Catalysis. A, General, v. 360, p. 177-182, 2009.


Pd NPs:

Rossi, L. M. ; Nangoi, I. M. ; Costa, N. J. S.  Ligand-Assisted Preparation of Palladium Supported Nanoparticles: a Step toward Size Control. Inorganic Chemistry, v. 48, p. 4640-4642, 2009.

Silva, F. P.; Rossi, L. M. Palladium on magnetite: magnetically recoverable catalyst for selective hydrogenation of acetylenic to olefinic compounds. Tetrahedron, v. 70, p. 3314-3318, 2014.


Au NPs:

Oliveira, R. L. ; Kiyohara, P. K. ; Rossi, L. M. High performance magnetic separation of gold nanoparticles for catalytic oxidation of alcohols. Green Chemistry, v. 12, p. 144-149, 2010.


Ni NPs:

Costa, N. J. S. ; Jardim, R. F. ; Masunaga, S. H. ; Zanchet, D.; Landers, R.; Rossi, L. M.  Direct Access to Oxidation-Resistant Nickel Catalysts through an Organometallic Precursor. ACS Catalysis, v. 2, p. 925-929, 2012.


Ir NPs:

Jacinto, M. J. ; Silva, F. P. ; Kiyohara, P. K. ; Landers, R.; Rossi, L. M. Catalyst Recovery and Recycling Facilitated by Magnetic Separation: Iridium and Other Metal Nanoparticles. CHEMCATCHEM, v. 4, p. 698-703, 2012.


AuPd NPs:

Silva, T. A. G. ; Landers, R.; Rossi, L. M. Magnetically recoverable AuPd nanoparticles prepared by a coordination capture method as a reusable catalyst for green oxidation of benzyl alcohol. Catalysis Science & Technology, p. 2993-2999, 2013.


NiPd NPs:

Costa, N. J. S.; Guerrero, M.; Colliere, V.; Teixeira-Neto, E.; Landers, R.; Philippot, K.; Rossi, L. M. Organometallic Preparation of Ni, Pd and NiPd Nanoparticles for the Design of Supported Nanocatalysts. ACS Catalysis, v. 4, p. 1735-1742, 2014.