Supported metal nanoparticles

The properties of nanoparticles may often be significantly different from those of the smaller nanoclusters discussed above, which may have unique catalytic sites, different from those on nanoparticles (Argo and Gates, forthcoming). 

This work was supported by the Department of Energy, Office of Energy Research, Office of Basic Energy Sciences, a gift from Ford Motor Co., and the National Science Foundation (Grant CTS-9615257). 

Alexeev, O., and Gates, B. C., EXAFS characterization of supported metal-complex and metal-cluster catalysts made from organometallic precursors, Top. CataL 10,273 (2000). 

Armentrout, P. B., Gas phase organometallic chemistry, in Topics in Organometallic Chemistry (J. M. Brown and P. Hofmann, Eds.), Vol. 4, p. 1. Springer Verlag, Berlin, 1999. 

Basset, J.-M., Lefebvre, F., and Santini, C., Surface organometallic chemistry Some fundamental features including the coordination effects of the support. Coord. Chem. Rev. 180, 1703, (1998). 

---

The common underlying principle was shown in Figure 11.2. The electrochemical potential of electrons jl e(=Ep, the Fermi level) in the metal catalyst is fixed at that of the Fermi level of the support.37 This is valid both for electrochemically promoted model catalysts (left) and for seminconducting or ion-conducting-supported metal nanoparticles (right). 

In Section 2 the general features of the electronic structure of supported metal nanoparticles are reviewed from both experimental and theoretical point of view. Section 3 gives an introduction to sample preparation. In Section 4 the size-dependent electronic properties of silver nanoparticles are presented as an illustrative example, while in Section 5 correlation is sought between the electronic structure and the catalytic properties of gold nanoparticles, with special emphasis on substrate-related issues. 

The identification of structure sensitivity would be both impossible and useless if there did not exist reproducible recipes able to generate metal nanoparticles on a small scale and under controlled conditions, that is, with narrow size and/or shape distribution onto supports. Metal nanoparticles of controlled size, shape, and structure are attractive not only for catalytic applications, but are important, for example in optics, data storage, or electronics (c.f. Chapter 5). In order not to anticipate other chapters of this book (esp. Chapter 2), remarks will therefore be confined to few examples. 

The magnetron sputtering technique can prepare supported metal nanoparticles on a wide variety of support materials including WO3 and carbon. 

Supported metal nanoparticles are of great interest in catalysis, because they offer the opportunity to combine the high reactivity and selectivity of the nanosized metals with the easy separation of the catalysts from the reaction mixture and recycling. 

Size Effects in Electrocatalysis of Fuel Cell Reactions on Supported Metal Nanoparticles... 

Structure and morphology of supported metal nanoparticles may differ drastically, depending on (i) their size, (ii) their interaction with support, (iii) the (electro)chemical environment, and, (iv) since very often particles do not attain equUibrium shapes, also on the preparation conditions and sample prehistory. 

Summing up this section, we would like to note that understanding size effects in electrocatalysis requires the application of appropriate model systems that on the one hand represent the intrinsic properties of supported metal nanoparticles, such as small size and interaction with their support, and on the other allow straightforward separation between kinetic, ohmic, and mass transport (internal and external) losses and control of readsorption effects. This requirement is met, for example, by metal particles and nanoparticle arrays on flat nonporous supports. Their investigation allows unambiguous access to reaction kinetics and control of catalyst structure. However, in order to understand how catalysts will behave in the fuel cell environment, these studies must be complemented with GDE and MEA tests to account for the presence of aqueous electrolyte in model experiments. 

As the reader might have noticed, many conclusions in electrocatalysis are based on results obtained with electrochemical techniques. In situ characterization of nanoparticles with imaging and spectroscopic methods, which is performed in a number of laboratories, is invaluable for the understanding of PSEs. Identification of the types of adsorption sites on supported metal nanoparticles, as well as determination of the influence of particle size on the adsorption isotherms for oxygen, hydrogen, and anions, are required for further understanding of the fundamentals of electrocatalysis. 

Frenkel AI, Hills CW, Nuzzo RG. 2001. A view from the inside complexity in the atomic scale ordering of supported metal nanoparticles. J Phys Chem B 105 12689-12703. 

Kovalyov EV, Elokhin VI, Myshlyavtsev AV. 2008. Stochastic simulation of physicochemical processes performance over supported metal nanoparticles. J Comput Chem 29 79-86. 

It is also observed in Fig. 5.3 that Pd(II) ions are partly adsorbed on AI2O3 before ultrasonic irradiation the concentration of Pd(II) just before irradiation becomes ca. 0.8 mM, although 1 mM Pd(II) was added in the sample solution. From a preliminary adsorption experiment, the rate of Pd(II) adsorption on A1203 was found to be slow compared with those of Pd(II) reduction in the presence of alcohols. Therefore, it is suggested that the sonochemical reduction of Pd(II) in the presence of alcohols mainly proceeds in the bulk solution. The mechanism of the Pd/Al203 formation is also described in the section of sonochemical synthesis of supported metal nanoparticles. 

By using sonochemical reduction processes, supported metal nanoparticles on metal oxides such as Au/Si02, Au/Fe203, Pd/Fe203, Pt/Ti02, etc. can be synthesized [38 -1],... 

Campelo JM, Luna D, Luque R, Marinas JM, Romero AA (2009) Sustainable preparation of supported metal nanoparticles and their applications in catalysis. ChemSusChem 2 18—45... 

Zheng, N.F. and Stucky, G.D. (2006) Ageneral synthetic strategy for oxide-supported metal nanoparticle catalysts. Journal of the American Chemical Society, 128 (44), 14278-14280. 

Traditional Routes to Supported Metal Nanoparticle Catalysts... 

The emphasis here is principally on metals rather than the other materials, because metals are simpler, more widely investigated, and better understood than the others. Supported metal nanoparticles are considered here in only a few summary statements, and nanolayers are beyond the scope of the chapter. 

In typical surface science experiments as presented previously, oxide-supported metal nanoparticles are deposited onto a clean oxide surface by physical vapor deposition. The precursor in this process is metal atoms in the gas phase, which impinge on the surface, diffuse until they eventually get trapped (either at surface defects or by dimer formation), and then act as nuclei for the growth of larger particles. These processes are well understood for ideal model systems under ultrahigh vacuum (UHV) conditions [56, 57]. In contrast, most realistic supported metal catalyst... 

In summary, the way is paved to look at oxide-supported metal nanoparticles, prepared in solution, and to understand the formation of MNPs through calcination and reduction. However, there is still a way to go to identify the elementary steps in the interaction of the species from solution at the solid-liquid interface. Of course, this is what we really want. 

EBL was used to fabricate uniform platinum nanoparticle arrays on Si02 (mean platinum particle diameter 30-1000 nm 52,53,106,107,398)), and evaporation techniques were used to prepare smaller particles and a continuous platinum film. The EBL microfabrication technique allows the production of model catalysts consisting of supported metal nanoparticles of uniform size, shape, and interparticle distance. Apart from allowing investigations of the effects of particle size, morphology, and surface structure (roughness) on catalytic activity and selectivity, these model catalysts are particularly well suited to examination of diffusion effects by systematic variations of the particle separation (interparticle distance) or particle size. The preparation process (see Fig. 1 in Reference 106)) is described only briefly here, and detailed descriptions can be found in References 53,106,399). 

Bezerra et al extensively reviewed heat treatment and stability effects of various Pt/C, Pt-M/C, and C-supported Pt-free alloy catalysts, taking into account particle sizes and stiuctural parameters. Appropriate heat treatment of Pt/C catalysts improves ORR activity by stabilizing the carbon support against corrosion, which in turn increases the cathode life time. Depositing mixed-metal Pt monolayers on carbon-supported metal nanoparticles or Pt monolayers on noble/non-noble core-shell nanoparticles leads to enhanced electrode performance. RRDE experiments on the catalytic activity of Pt-M (M = Au, Pd, Rh, Ir, Re or Os) monolayers on carbon-supported Pd nanoparticles showed that an 80 20 PtiM ratio for the nuxed monolayers performs better than commonly nsed Pt/C catalysts. ... 

---