Particles Pd

PD = particle diameter from intercept on ordinate axis at percent indicated... 

The longtime stabihty of surfactant-coated Pd nanoparticles in w/o microemulsions has been investigated. It has been proven that under suitable conditions, the use of the functionalized surfactant Pd(AOT)2 allows very stable nanosize Pd particles to be obtained and to finely control their average size [229],... 

Pd on Carbon. The catalyst analyzed here is a commercial hydrogenation catalyst with 5% Pd supported on activated carbon (Alfa). The catalyst was ground in a mortar and pestle and dispersed dry onto a carbon coated Cu grid. While x-ray spectra from heavy metal particles down to 2nm in diameter can be obtained (O by manually directing the electron beam to the particle, digital images of Pd particles at high resolution have not been obtained previously. 

Figure 5. Pd supported on carbon a) annular dark-field STEM image revealing particles as small 2nm, b) processed digital x-ray image showing that Pd particles 4nm and larger can be imaged using Pd x-rays alone.

Two examples of the application of transmission IR methods will be presented. The first, dealing with the chemisorption of CO on a Pd/S102 catalyst surface. Illustrates the first observation of a local stoichiometric surface species Interconversion process which occurs among chemisorbed CO species at high CO coverages. Evidence for the operation of the local stoichiometric process has been obtained on 75A Pd particles. These particles seem to show close similarities as well as to differ In some respects from a Pd(lll) single crystal surface Insofar as their Interaction with CO Is concerned. 

Pd(lll). However, Pd(lll) shows little or no evidence for the stoichiometric 2Bi + L + 3L process. This could be due to the presence of longer range order on the single crystal than on the Pd particles, leading to processes more akin to two dimensional phase transitions on the Pd(lll) crystal surface, rather than a more local species conversion on the small metal crystallites. 

It proves possible to anchor catalysts of H2 evolution to the outer and inner surface of the vesicle membrane. These catalysts are finely dispersed (10-20 A in diameter) metal Pt or Pd particles formed via reduction of appropriate salts in vesicle suspension (see [15, 16] and refs, therein). Among the viologen-type electron carriers a promising one is p-bis (1,2,5-triphenyl-4-pyridil)benzene which possesses reduction potential low enough for water reduction at neutral pH. Recently, using this mediator we succeeded in H2 evolution conjugated with PET... 

The present study was initiated to understand the causes of large differences in perfonnance of various catalyst formulations after accelerated thermal aging on an engine dynamometer. In particular, we wished to determine whether performance charaderistics were related to noble metal dispersion (i.e. noble metal surface area), as previous studies have suggested that the thermal durability of alumina-supported Pd catalysts is due to high-temperature spreading or re-dispersion of Pd particles [20-25]. 

At 295 K the current increases linearly with the increasing voltage. This means nothing, but that the 15nm Pd particle follows the Ohm s law at room temperature and so clearly demonstrates bulk behaviour. However, at... 

Figure 11. (a) SEM image of a ligand-protected 17 nm Pd particle between two Pt tips, (b) The current voltage behaviour of this system... 

Figure 1. Electrosteric stabilization of a Pd particle by the tetra(octyl)ammomum halide stabilizer. (Reprinted from Ref. [51], 2004, with permission from Elsevier.)...

In some cases shape-control has also been achieved tetra( -octyl)ammonium glycolate transforms Pd(N03)2 predominantly into trigonal Pd particles [186]. Recent work has confirmed that the colloidal protective agents not only prevent particle agglomeration but even provide control of the crystal growth during particle synthesis (see e.g., Ref. [187-191]). The drawbacks of this route are the restriction to noble metal salts and the limited industrial availability of A-(octyl)j RC02. 

In the chemical preparation of unprotected metal colloids, the metal concentration usually has a significant influence on the particle size of obtained metal nanoclusters. For example, when increasing Pd concentration from 0.1 to 1.0 mM in the preparation of Pd metal colloids by the thermal decomposition of Pd acetate in methyl isobutyl ketone, the average Pd particle size increased from 8 to 140nm [6,7]. However, in the alkaline EG synthesis method, the size of metal nanoclusters was only slightly dependent on the metal concentration of the colloidal solution. The colloidal Pt particles prepared with a metal concentration of 3.7 g/1 had an average diameter of... 

Figure 3.3. Schematic representation of the adsorption, surface diffusion, and surface reaction steps identified by surface-science experiments on model supported-palladium catalysts [28]. Important conclusions from this work include the preferential dissociation of NO at the edges and defects of the Pd particles, the limited mobility of the resulting Nads and Oads species at low temperatures, and the enhancement in NO dissociation promoted by strongly-bonded nitrogen atoms in the vicinity of edge and defect sites at high adsorbate coverages. (Figure provided by Professor Libuda and reproduced with permission from the American Chemical Society, Copyright 2004).

The same group has looked into the conversion of NO on palladium particles. The authors in that case started with a simple model involving only one type of reactive site, and used as many experimental parameters as possible [86], That proved sufficient to obtain qualitative agreement with the set of experiments on Pd/MgO discussed above [72], and with the conclusion that the rate-limiting step is NO decomposition at low temperatures and CO adsorption at high temperatures. Both the temperature and pressure dependences of the C02 production rate and the major features of the transient signals were correctly reproduced. In a more detailed simulation that included the contribution of different facets to the kinetics on Pd particles of different sizes, it was shown that the effects of CO and NO desorption are fundamental to the overall behavior... 

Zero-valent Pd complexes in organic solutions (e.g., THF) decomposed under CO in the presence of PPh3 to give novel PPh3/CO-stabilized Pd particles which were studied by spectroscopic methods 942 although the Pd particles were relatively unstable and were subject to size variations in solution, three distinct size-selected distributions were obtained with observed mean... 

Table 5.2 shows the average size of the formed Pd nanoparticles. As described in the previous section, the added alcohol acts as a precursor for the reduction of Pd(II) under ultrasonic irradiation and affects the rate of the reduction. Therefore, it is considered that smaller Pd particles are formed when the rate of Pd nucleation (corresponding to the rate of Pd(II) reduction) is faster. In addition, it can be seen from Table 5.2 that, in the presence of 1-propanol, the size of the formed Pd particles and its standard deviation become smaller with increasing amount of... 

Fig. 5.12 TEM images of sonochemically prepared Pd/Al203 and size distribution of Pd particles formed by 30 min irradiation of ImM Pd(II) and 20 mM alcohol. Conditions 200kHz, Ar, 20°C. (a) 1 wt% Pd/Al203 formed in the presence of 1-propanol, (b) 5 wt% Pd/Al203 formed in the presence of 1-propanol, (c) 5 wt% Pd/Al203 formed in the presence of methanol [30]...

Table 5.2 Average size and standard deviation of Pd particles formed on the A1203 surface and rate of Pd(II) reduction by ultrasonic irradiation [30, 31] ...

Irradiation time 30 min of 200 kHz ultrasound, Pd(II) 1 mM, alcohol 20 mM. bWeight ratio of Pd/A12Q3, Standard deviation of size of Pd particles, dAverage rate for initial 5 min irradiation. 

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