Palladium, particles

Figure Bl.22.1. Reflection-absorption IR spectra (RAIRS) from palladium flat surfaces in the presence of a 1 X 10 Torr 1 1 NO CO mixture at 200 K. Data are shown here for tluee different surfaces, namely, for Pd (100) (bottom) and Pd(l 11) (middle) single crystals and for palladium particles (about 500 A m diameter) deposited on a 100 A diick Si02 film grown on top of a Mo(l 10) single crystal. These experiments illustrate how RAIRS titration experiments can be used for the identification of specific surface sites in supported catalysts. On Pd(lOO) CO and NO each adsorbs on twofold sites, as indicated by their stretching bands at about 1970 and 1670 cm, respectively. On Pd(l 11), on the other hand, the main IR peaks are seen around 1745 for NO (on-top adsorption) and about 1915 for CO (tlueefold coordination). Using those two spectra as references, the data from the supported Pd system can be analysed to obtain estimates of the relative fractions of (100) and (111) planes exposed in the metal particles [26].

MgO-supported model Mo—Pd catalysts have been prepared from the bimetallic cluster [Mo2Pd2 /z3-CO)2(/r-CO)4(PPh3)2() -C2H )2 (Fig. 70) and monometallic precursors. Each supported sample was treated in H2 at various temperatures to form metallic palladium, and characterized by chemisorption of H2, CO, and O2, transmission electron microscopy, TPD of adsorbed CO, and EXAFS. The data showed that the presence of molybdenum in the bimetallic precursor helped to maintain the palladium in a highly dispersed form. In contrast, the sample prepared from the monometallie precursors was characterized by larger palladium particles and by weaker Mo—Pd interactions. ... 

X-Ray diffraction has an important limitation Clear diffraction peaks are only observed when the sample possesses sufficient long-range order. The advantage of this limitation is that the width (or rather the shape) of diffraction peaks carries information on the dimensions of the reflecting planes. Diffraction lines from perfect crystals are very narrow, see for example the (111) and (200) reflections of large palladium particles in Fig. 4.5. For crystallite sizes below 100 nm, however, line broadening occurs due to incomplete destructive interference in scattering directions where the X-rays are out of phase. The two XRD patterns of supported Pd catalysts in Fig. 4.5 show that the reflections of palladium are much broader than those of the reference. The Scherrer formula relates crystal size to line width ... 

A strong interaction of the very small palladium particles (diameter 3.5 nm, as shown by HRTEM analysis) with the nitrogen atoms of the pyridyl groups in the polymeric support may account for this interesting result. 

Takasu Y, Zhang XG, Minoura S, Murakami Y. 1997. Size effects of ultrafine palladium particles on the electrocatalytic oxidation of CO. Appl Surf Sci 121 596-600. 

Tateishi N, Yahikozawa K, Nishimura K, Masato S, Iwanaga Y, Watanahe M, Enami E, Matsuda Y, Takasu Y. 1991. Electrochemical properties of ultra-fine palladium particles for adsorption and absorption of hydrogen in an aqueous HCIO4 solution. Electrochim Acta 36 1235-1240. 

In the case of palladium particles supported on magnesium oxide, Heiz and his colleagues have shown,29 in an elegant study, a correlation between the number of palladium atoms in a cluster and the selectivity for the conversion of acetylene to benzene, butadiene and butane, whereas in the industrially significant area of catalytic hydrodesulfurisation, the Aarhus group,33 with support from theory, have pinpointed by STM metallic edge states as the active sites in the MoS2 catalysts. 

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... 

Okitsu K, Bandow H, Maeda Y, Nagata Y (1996) Sonochemical preparation of ultrafine palladium particles. Chem Mater 8 315-317... 

The structure of 3 was confirmed by X-ray crystallography (Fig. 1). The morphology of the nanoparticles was examined by transmission electron microscopy (TEM). The two sp2-C palladacycles 1 and 4 gave what appeared to be triangular nanoparticles, in 2D, from 2-12 nm in size while the sp3-C PdCys 5 and 6 and Pd(OAc)2 exhibited more conventional morphology and were faceted palladium particles from 3-10 nm (Fig. 2). 

The intimate mechanism by which the single rhodium sites and the neighboring palladium particles interact with benzene to accelerate its reduction to cyclohexadiene remains somewhat obscure. 

Figure 7.16 AFM (above) and TEM images (below) of palladium particles evaporated onto a flat Si02 layer, left as deposited and right after cycles of heating in oxygen and hydrogen at 475 K (from Erlandsson et al. [46]).

Carbon monoxide adsorbed on sufficiently small palladium particles disproportionates to surface carbon and carbon dioxide. This does not occur on large particles. The CO-O2 reaction is shown to be structure-insensitive provided the metal surface available for the reaction is estimated correctly. 

Low pressure studies Adsorption of CO. The experiments were performed in an ultra-high vacuum system described previously (1). The data obtained on palladium particles with a size smaller than 2 nm or larger than 3 nm will be discussed in turn. 

Reaction between carbon monoxide and dioxygen. The steady-state formation of C02 was measured on palladium particles vapor... 

Since the equilibrium conversion of CO to methanol under the conditions of Table II is low, the reaction was further studied under higher pressures. The results at 535 kPa are shown in Table III. The catalysts having smaller palladium particle sizes... 

The addition of dopants is found to have beneficial effects. However, they are not restricted only to transition metals. The hydrogenation of acrylic acid can be promoted significantly by the addition of neodymium ions onto the palladium particles [142], The selective transformation of 3,4-dichloronitrobenzene to the corresponding aniline has been selected to test pre-prepared Pt hydrosols as heterogeneous catalyst precursors (see Figure 3.9) [143],... 

Amiens, C. et al., Selective synthesis, characterization, and spectroscopic studies on a novel class of reduced platinum and palladium particles stabilized by carbonyl and phosphine ligands, J. Am. Chem. Soc., 115, 11638, 1993. 

Teranishi, T., Nakata, K., Miyake, M., and Toshima, N., Promition effect of polymer-immobilized neodymium ions on catalytic activity of ultra fine palladium particles, Chem. Lett., 277, 1996. 

Bars, J.L., Specht, U., Bradley, J.S., and Blackmond, D.G., A catalytic probe of the surface of colloidal palladium particles using heck coupling reactions, Langmuir, 15, 7621, 1999. 

O2 adsorption, 38 227-228 olefin bond, strength, 30 348-349 one-component, 24 173-213 output, 24 222, 223 palladium, see Palladium particle size... 

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