Adatoms nanoparticles

In the previous Sections (2.1-2.3) we summarized the experimental and computational results concerning on the size-dependent electronic structure of nanoparticles supported by more or less inert (carbon or oxide) and strongly interacting (metallic) substrates. In the following sections the (usually qualitative) models will be discussed in detail, which were developed to interpret the observed data. The emphasis will be placed on systems prepared on inert supports, since - as it was described in Section 2.3 - the behavior of metal adatoms or adlayers on metallic substrates can be understood in terms of charge transfer processes. 

The FTIR studies revealed that the formation of CO2 is only detected when the CO starts to be oxidized (Fig. 6.18). Therefore, it was proposed that the mechanism has only one path, with CO as the C02-forming intermediate [Chang et al., 1992 Vielstich and Xia, 1995]. This has two important and practical consequences. First, methanol oxidation will be catalyzed by the same adatoms that catalyze CO oxidation, mainly ruthenium. Second, since the steric requirements for CO formation from methanol are quite high, the catalytic activity of small (<4nm) nanoparticles diminishes [Park et al., 2002]. 

It has been often stressed that low eoordinated atoms (defeets, steps, and kink sites) play an important role in surfaee ehemistry. The existenee of dangling bonds makes steps and kinks espeeially reaetive, favoring the adsorption of intermediate species on these sites. Moreover, smdies of single-crystal surfaces with a eomplex geometry have been demonstrated very valuable to link the gap between fundamental studies of the basal planes [Pt( 111), Pt( 100), and Pt(l 10)] and applied studies of nanoparticle eatalysts and polycrystalline materials. In this context, it is relevant to mention results obtained with adatom-modified Pt stepped surfaces, prior to discussing the effect of adatom modification on electrocatalysis. 

The formation of Au-OHad or surface oxides on gold in alkaline electrolyte was in fact proposed to explain some of the electrocatalytic properties observed for a gold electrode (e.g., incipient hydrous oxide/adatom mediator model ). Our previous measurement of the interfacial mass change also indicated the formation of Au oxides (AU2O3, AuOHorAu(OH)3) on gold nanoparticle surfaces. A detailed delineation of the catalytic mechanism is part of our on-going work. 

The explanation given (Scheme 3.23) suggests that the surface nanoparticles of rhodium are supposedly able to coordinate the aromatic moiety as a ii -ligand whereas when the surface (low index planes) of Rh is covered by tin adatoms the only function that can be adapted on the surface is the carbonyl via its lone pair. 

Using Phi as the substrate, the reaction mixture turned from a light yellow solution to a dark brown suspension after 20 min. However, no conversion was observed by GC analysis. We assumed that Pd ions, oxidised from the anode, were in turn reduced to adatoms at the Pt cathode and formed Pd° nanoparticles, ca. 11 nm in diameter (10). After 8 h, the Phi was totally consumed, giving 80% biphenyl and 20% benzene. Weighing the electrodes before and after the reaction showed difference of 2.5 mg in the Pd anode, equivalent to 0.1 mol% of the aryl halide substrate. This corresponds to a TON of 1000 at least (assuming that all the missing Pd participates in the catalysis). 

Nanoparticles have different morphologies than flat, bulk surfaces. Perez et al. have considered the activation of water and COads + OHads reactions on Pt and PtRu clusters including the effects of solvation." They found that the presence of under-coordinated Ru adatoms on the Pt cluster surfaces enhances the production of OHads from water compared to Ru alloyed into the nanoparticle surfaces. More significantly, they found that the presence of an aqueous environment simulated by up to six water molecules dramatically stabilized the transition state and products of the reactions. For example, in a gas-phase environment they calculated a water dissociation barrier of 20 kcal/mol whereas in the solvated environment the barrier was reduced to 4.5 kcal/mol on the alloy surface. The barrier for water dissociation on the Ru adatom in the aqueous environment was only 0.9 kcal/mol. Although their results are for an adatom on a near flat (111) surface, they may have significance in describing the catalytic properties of undercoordinated Ru atoms at edge and corner sites on nanoparticles. 

This different behavior was the result of an electron transfer from Fe to Pd, and of a deposition of Fe adatoms on the surface of Pd nanoparticles by the redox reaction while the co-impregnated catalysts would lead to alloy formation with a strong Pd surface segregation. 

Scheme 18.12 Explanation of the selectivity in citral hydrogenation (Left) small coverage of Rh nanoparticles by adatoms favors the V coordination of the conjugated double bonds of citral. (Right) high coverage of Rh nanoparticles by =Sn (n-butyl) favors the coordination of the carbonyl.

It should be noted that a wide range of external field behavior of the specific conductivity o( ) for nanotubes with hydrogen adatoms has the same qualitative nonhnear dependence as for the ideal case of nanoparticles, which was discussed in detail in Ref [11], In general, the dependence of conductivity on the electric field has a characteristic for semiconductors form tends to saturate and decreases mono-tonically with increasing intensity. This phenomenon is associated with an increase in electrons fill all possible states of the conduction band. Behavior of electrical conductivity under the influence of an external electric field is typical for semiconductor stmctures with periodic and limited dispersion law [17],... 

Catalyst activity towards formic acid electrooxidation is strongly influenced by preparation method and nanoparticle size. As discussed in the previous chapter, the optimal sizes for Pt/C and Pd/C are 4 nm and 5.2-6.5 imi, as determined by Park et al. [14] and Zhou et al. [15], respectively. This chapter is segregated into two sections bimetallic catalysts and catalyst supports. The section on bimetallic catalysts is subdivided into adatoms, alloys, and intermetallics. 

The most commonly investigated substrates have been Pt and Pd, ranging from well-defined single-crystal surfaces to nanoparticles. Bismuth (Bi) has been the most extensively tested adatom [18-28]. Other adatoms that have also exhibited performance enhancements are lead (Pb) [29-31], antimony (Sb) [2,22,29,32,33], arsenic (As) [34, 35], gold (Au) [36], tellurium (Te) [37, 38], selenium (Se) [39], ruthenium (Ru) [40], and palladium (Pd) [5,40,41]. Researchers have seen that, for the various adatoms, higher coverages promote the direct reaction pathway. 

The driving force for small nanoparticle catalysts is reduced cost by minimizing inactive non-surface atoms, which is the basis of most low Pt approaches. Yu and Pickup investigated the coverage dependence of Pb and Sb on commercial 40 wt% Pt supported on carbon in situ in a formic acid/02 fuel cell [29]. They found optimal coverages of 0.7 for both types of adatoms. The performance of both PtSb/C and PtPb/C far exceeded that of Pt/C. After nearly a 2 h hold at 0.6 V under fuel cell operation, the performance increase over Pt/C was 15- and 12.8-fold, respectively. Figure 4.2 is a comparison of fuel cell performance at 0.6 V as a function of adatom... 

Wieckowski s group has studied formic acid electrooxidation on Pt nanoparticles decorated with controlled amounts of Pd and Pd-l-Ru adatoms [41]. They reported two orders of magnitude increase in the reactivity of the Pd-decorated catalyst compared to pure Pt towards formic acid oxidation. Also, it was concluded that the impact of COads on the Pt/Pd catalyst through the dual pathway mechanism is much lower even though the potential required to remove COads from the surface was the highest. 

Bismuth has attracted significant interest as a Pt/C modifier for formic acid electrooxidation [21, 24, 26, 27]. A wide range of stable and well-characterized electrode surfaces modified by irreversible Bi adatom adsorption on Pt have been reported in the literature for a range of Bi coverages 6). Chen et al. have explored Bi adatom decoration on 81 nm tetrahexahedral Pt nanoparticles that while composed of (100) and (110) facets that are the least active for formic acid electrooxidation, they are boimd by 730 and vicinal high-index facets that are extremely active [18]. They have measured current densities of 10 mA cm for Bi coverages up to 0.9 at 0.4 V in 0.25 M formic acid and 0.5 M H2SO4 solution see Fig. 4.4. They also showed steady-state activity at 0.3 V of 2.8 mA cm after 1 min vs. 0.0003 mA cm for the non-modified Pt baseline. 

To improve the electrocatalytic activity of platinum and palladium, the ethanol oxidation on different metal adatom-modified, alloyed, and oxide-promoted Pt- and Pd-based electrocatalysts has been investigated in alkaline media. Firstly, El-Shafei et al. [76] studied the electrocatalytic effect of some metal adatoms (Pb, Tl, Cd) on ethanol oxidation at a Pt electrode in alkaline medium. All three metal adatoms, particularly Pb and Tl, improved the EOR activity of ft. More recently, Pt-Ni nanoparticles, deposited on carbon nanofiber (CNE) network by an electrochemical deposition method at various cycle numbers such as 40, 60, and 80, have been tested as catalysts for ethanol oxidadmi in alkaline medium [77]. The Pt-Ni alloying nature and Ni to ft atomic ratio increased with increasing of cycle number. The performance of PtNi80/CNF for the ethanol electrooxidation was better than that of the pure Pt40/CNF, PtNi40/CNF, and PtNi60/CNF. 

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