Catalyst nanoparticles

An important question frequently raised in electrochemical promotion studies is the following How thick can a porous metal-electrode deposited on a solid electrolyte be in order to maintain the electrochemical promotion (NEMCA) effect The same type of analysis is applicable regarding the size of nanoparticle catalysts supported on commercial supports such as Zr02, Ti02, YSZ, Ce02 and doped Zr02 or Ti02. What is the maximum allowable size of supported metal catalyst nanoparticles in order for the above NEMCA-type metal-support interaction mechanism to be fully operative ... 

The catalyst inks were prepared by dispersing the catalyst nanoparticles into an appropriate amoimt of Millipore water and 5wt% Nafion solution. Then, both the anode and cathode catalyst inks were directly painted using a direct painting technique onto either side of a Nafion 117 membrane. A carbon cloth diffusion layer was placed on to top of both the anode and cathode catalyst layers [3-5]. The active cell area was 2.25cm. ... 

Andreaus B, Maillard F, Kocylo J, Savinova ER, Eikerling M. 2006. Kinetic modeling of CO monolayer oxidation on carbon-supported platinum catalyst nanoparticles. J Phys Chem B 110 21028-21040. 

Figure 11.8 Formation of ordered nanoparticles of metal from diblock copolymer micelles, (a) Diblock copolymer (b) metal salt partition to centres of the polymer micelles (c) deposition of micelles at a surface (d) micelle removal and reduction of oxide to metal, (e) AFM image of carbon nanotubes and cobalt catalyst nanoparticles after growth (height scale, 5 nm scan size, lxl pm). [Part (e) reproduced from Ref. 47].

Pristine CNTs are chemically inert and metal nanoparticles cannot be attached [111]. Hence, research is focused on the functionalization of CNTs in order to incorporate oxygen groups on their surface that will increase their hydrophilicity and improve the catalyst support interaction (see Chapter 3) [111]. These experimental methods include impregnation [113,114], ultrasound [115], acid treatment (such as H2S04) [116— 119], polyol processing [120,121], ion-exchange [122,123] and electrochemical deposition [120,124,125]. Acid-functionalized CNTs provide better dispersion and distribution of the catalysts nanoparticles [117-120],... 

The utilization of large surface areas and, to a certain extent, controllable surface properties make carbon materials an ideal support for finely dispersed catalyst nanoparticles, as discussed in Section 15.2. The special features of nanocarbons for this purpose will be highlighted in the following section. Starting with the controlled synthesis of a variety of nanocarbon-inorganic hybrids, some examples will be discussed, where the superior catalytic performance arises from the unique properties of the nanostructured support. 

In general, there are two possibilities to prepare nanocarbon-supported metal(oxide) catalysts. The in situ approach grows the catalyst nanoparticles directly on the carbon surface. The ex situ strategy utilizes pre-formed catalyst particles, which are deposited on the latter by adsorption [94]. Besides such solution-based methods, there is also the possibility of gas phase metal (oxide) loading, e.g., by sputtering [95], which is used for preparation of highly loaded systems required for electrochemical applications not considered here. 

Fig. 15.14 Illustration of selective deposition strategies for catalyst nanoparticles (left) on the inner and (right) on the outer surface of CNTs according to Ref. [Ill], For inside deposition the CNTs are (a) impregnated with an ethanolic solution of the metal precursor, followed by washing with distilled water to protect the outer surface, and (c) subsequent drying and final treatment to form the catalyst nanoparticies. For outside deposition the CNTs are (d) impregnated with an organic solvent to block the inner tubule, followed by (e) impregnation with an aqueous solution of the metal precursor and (f) subsequent drying and final treatment.

With the catalyst nanoparticles covered with an hydrophobic layer, the polar alcoholic function of the substrates remains in the aqueous phase, far from the... 

Keywords membrane catalyst, nanoparticle, carbon 1. Introduction... 

The NEMCA effect is closely related to classical promotion and to the phenomenon of metal-support interactions (MSI) with oxide supports and that MSI can be viewed as a self-driven NEMCA microsystem where the promoting O2- ions are thermally migrating from the support to the dispersed catalyst nanoparticles and replenished in the support by gaseous 02 [v]. 

Figure 53 Main types of the crystalline structure of the carbon nanofilaments produced by pyrolysis of hydrocarbons over transition metal nanoparticles coaxial cylindrical (multilayer nanotube) (A), coaxial conical (fishbone) (B), and pile (C). The nanofilaments are 10 nm in characteristic diameter. The catalyst nanoparticle behaves as a nanofilament seed.

The discussed models of the carbon nanofilaments and nanotubes forma tion allow many other thermodynamic factors to be taken into consider ation, all of which affect the shape, texture, and growth rate of the nano objects under discussion (see, e.g.. Refs. [6, 7]). It is assumed that the forma tion of the fluidized active component of the catalyst nanoparticles due to its stationary oversaturation with the crystallizing component gives rise to the possibility to synthesize nanofilaments and nanotubes from not only carbon but also from different substances, such as silicon carbide (over catalysts capable of dissolving carbon and silicon simultaneously), germanium metal (over gold metal catalysts [8]), and so on. 

The relevance of thymine/2,6-diaminotriazine interactions has been exploited by a variety of authors to effect a reversible, yet stable association of catalysts, nanoparticles and other fimctional molecules onto polymeric molecules. Thus, Shen et al. [94,95] reported on the formation of catalyst-supported structures for ATRP-polymerization via hydrogenbonding systems (Fig. 19). The relevant Cu(I)-catalyst was affixed onto a poly(styrene) gel either via the thymine/2,6-diaminopyridine or the maleimide/2,6-diaminopyridine couple. The catalyst was able to mediate a living polymerization reaction of MMA in both cases, obviously acting in its dissociated form. The catalyst could be reused, retaining about half of its catalytic activity for further use. A strong solvent effect was observed, explainable by the dissociation of the catalyst from the support upon addition of strongly polar solvents. 

Figure 20 TEM images of InP quantum wires. Mean diameters (a) 4.49 0.75 nm [ 17% grown from 9.88 0.795 ( 8.0%) tn-catalyst nanoparticles] (b) 6.6t t.03nm (c) tt.Ot 2.29mn. (Ref. 49. Reproduced by permission of Nature Publishing Group (www.nature.com))...

Figure 1. From the macroscale to the nanoscale a membrane electrode assembly has a polymer electrolyte membrane sandwiched between two catalyst layers and gas diffusion layers. The catalyst layer is composed of carbon particles impregnated with catalyst nanoparticles. Effective utilization of the catalyst particles depends on their local environment.

Schalow T, Laurin M, Brandt B, Schauermann S, Guimond S, Kuhlenbeck H, et al. (2005). Oxygen storage at the metal/oxide interface of catalyst nanoparticles. Angew Chem Int Ed Engl, 44, 7601... 

Promotion of catalyst nanoparticles, electrochemical promotion (NEMCA) of porous and of single-crystal catalyst films, and metal nanoparticle-support interactions are three, at a first glance, independent phenomena that can all dramatically affect catalytic activity and selectivity on metal and metal oxide catalyst surfaces. 

This leads us to the concept called nanocatalysis, and specifically to nanofabricated model catalysts, as an approach to bridge the structure gap. In Fig. 4.4, some examples of planar model structures of increasing complexity are depicted, which fulfill these criteria. At the top, there is a simple array of catalyst particles on an inactive support. The inactive support can be replaced by an active support (second picture from the top), meaning a support that significantly affects the properties of the nanoparticles via particle-support interactions (a clear distinction between inactive and active is not easy or not even possible—there is always some influence of the support on the supported particle). In some cases, the size of the support particle has an influence on the overall catalytic activity. This is, for example, the case when there is a spillover or capture zone for reactants or intermediates, which move by diffusion from the catalyst nanoparticle to the support or vice versa. In order to study such effects, one may want to systematically vary the radius of the... 

Control of the nanotubes diameter by means of catalyst particle with defined dimensions is an enticing perspective. The size-selective synthesis of SWNT succeeded, for instance, with iron particles 3, 9, or 13 nm wide, respectively. The resulting nanotubes had average diameters of 3, 7, or 12 nm. Obviously, a uniform size distribution of the catalyst nanoparticles is cracial here. They are usually obtained by precipitation from organometaUic precursors (e.g., iron pentacarbonyl). 

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