Supported nanosized catalysts

Similar measurements for the cases of supported nanosized catalysts require preparation of the electrode in a specific way. Usually, catalyst is dispersed in suitably chosen media and desired amount is transfer to solid electrode serving as electrical contact [11]. This issue will be elaborated in more details later on. When stable thin catalyst layer is prepared cyclic voltammetry may be used to investigate surface electrochemical processes (Figure 4). In contrast to single-crystal and polycrystalline surface additional factor arises - namely, these processes depend also on the particle size. For example, in the case of supported Pt nanocatalysts it was observed that both hydrogen underpotential deposition and adsorption of oxygen species are clearly dependent on the particle size [12]. In specific, it was observed that smaller particles are more oxophilic and that surface oxidation is more irreversible for smaller particles. 

Figure 11.11 Linear cyclic voltammograms of carbon-supported nanosized Pt and Pt-Cr alloy catalysts with different atomic ratios (prepared using the carbonyl route [Yang et al., 2004]) recorded in 0.5 M HCIO4 saturated with pure oxygen at a scan rate of 5 mV s and a rotation speed of 2000 rev min Current densities are normalized to the geometric surface...

Recently, Chaudhari compared the activity of dispersed nanosized metal particles prepared by chemical or radiolytic reduction and stabilized by various polymers (PVP, PVA or poly(methylvinyl ether)) with the one of conventional supported metal catalysts in the partial hydrogenation of 2-butyne-l,4-diol. Several transition metals (e.g., Pd, Pt, Rh, Ru, Ni) were prepared according to conventional methods and subsequently investigated [89]. In general, the catalysts prepared by chemical reduction methods were more active than those prepared by radiolysis, and in all cases aqueous colloids showed a higher catalytic activity (up to 40-fold) in comparison with corresponding conventional catalysts. The best results were obtained with cubic Pd nanosized particles obtained by chemical reduction (Table 9.13). 

Semihydrogenation. Alkynes are hydrogenated to (Z)-alkenes with Pd on pumice. C—C Bond couplings. Different supported Pd catalysts have been tried. For example, Suzuki coupling based on Pd-thiourea resin" and nanosize Pd on poly(Af-... 

In the literature, there is much information about the adsorption of small molecules on Pt, Rh, and Pd (see, e.g., [3,13]) on such samples as single-crystal surfaces and supported metal catalysts. The FEM enables us to bridge the gap between these two extremes, because it allows a very high resolution look at sharp metal tips ( 1000 A), that are in many cases only about one order of magnitude larger than in a supported catalyst. This surface science approach, for example, permits the study of the interaction of adjacent planes on the reactivity of one another. Many of the oscillatory reactions seen on field emitters in situ are examples of such interplay of the different nanosized surfaces present [11,14]. This interaction can obviously not be studied with large single crystals and is lost in the black box techniques of the macroscopic world of the supported catalysts. 

Figure 7.16 Current—potential curves (bottom) of the carbon-supported nanosized Pt and PtNi alloy catalysts coated on a glassy carbon disk electrode in Oa-satu-rated 0.5 M HCIO4 solution (scan rate of 5 mV s and rotating speed of 2000 rpm) and the corresponding ring currents (Pt at 1.2 V vs RHE) data for the hydrogen peroxide production (upper) on Pt/C, Pt Ni(2 1)/C and Pt Ni(1 1)/C systems. Reprinted with permission from Ref. 55.

Size effect and surface effect. Metal particle size is a very important property of supported metal catalysts. The control of metal particle nanosize is of utmost importance for the performance of any industrial catalyst based on supported metals. 

Ni J, Chen L, Lin J, Kawi S (2012) Carbon deposition on borated alumina supported nanosized Ni catalysts for dry reforming of CH4. Nano Energy 1 674—686... 

With this in mind, this chapter focuses on the most recent advances on the designing of electrocatalysts for the complete electrooxidation of ethanol in add medium. The base catalyst comprises carbon supported nanosized Pt partides which, as inferred from the results given in the previous sections, lead to the preferential formation of acetic acid and acetaldehyde in acid medium. Comprehensive details about the synthesis and characterization of the electrocatalyst described in this section will be omitted and instead we shall focus on the performance of the catalyst for the ethanol electrooxidation reaction. 

Although reahstic electrocatalysts are in the form of supported nanosized metal particles measurement of ORR catalytic activity of bulky metal electrodes provides important information regarding intrinsic catalytic activity of particular metal, while in the case of single crystal electrodes information about structural sensitivity of ORR on surface orientation is obtained. These data are further used as a starting point for design of new catalysts in terms of (i) cormection between ORR catalytic activity and surface electrochemical processes and (ii) as benchmark activities. 

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. 

Molecular-level studies of mechanisms of proton and water transport in PEMs require quantum mechanical calculations these mechanisms determine the conductance of water-filled nanosized pathways in PEMs. Also at molecular to nanoscopic scale, elementary steps of molecular adsorption, surface diffusion, charge transfer, recombination, and desorption proceed on the surfaces of nanoscale catalyst particles these fundamental processes control the electrocatalytic activity of the accessible catalyst surface. Studies of stable conformations of supported nanoparticles as well as of the processes on their surface require density functional theory (DFT) calculations, molecular... 

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