Palladium nanotube

Metallic nanotubes can be synthesized using hard or soft template. Notwithstanding their incomplete crystallinity, their high surface area can be exploited for various electrochemical reactions. The large surface area of the nanotube material may lead to improved kinetics of the electrochemical reactions and to a decrease in the losses due to overvoltage of the reaction. A novel utilization of this principle is demonstrated in Ref 145. Here, palladium nanotubes, a few microns long were... 

Liu, L. Park, S. Direct formation of thin-walled palladium nanotubes in nanochannels under an electrical potential. Chem. Mater. 2011, 23, 1456-1460. 

Steinhart M, JiaZH, Schaper AK, Wehrspohn RB, Gosele U, Wendorff JH (2003) Palladium nanotubes with tailored wall morphologies. Adv Mater 15 706-709... 

Wehrspohn and coworkers fabricated palladium nanotubes (Figure 9.15) through a template-assisted process [65] whereby dichloromethane or chloroform was used to form a solution of poly(D,L-lactide) (PDLLA) and palladium acetate. The solution was added to porous alumina in a 1 1 ratio under ambient conditions. Vaporization of the solvent resulted in the formation of Pd(OAc)2/PDLLA tubes, followed by annealing at 200 °C which degraded the precursor to elemental palladium. Further pyrolytic treatment at 3500°C removed PDLLA, while the alumina tern-... 

Figure 9.15 Scanning electron micrographs of poly(D,L-lactide (PDLLA). (a) An array of Pd nanotubes (pore diameter 400nm, pore aligned palladium nanotubes (b) Crossdepth lOOpm) obtained by wetting porous sectional view of an individual nanotube, alumina and after annealing at 200°C for Reproduced with permission from Ref [62] 6h and removing both the template and 2007 Wiley Interscience.

Figure 9.19 Scanning electron micrograph of palladium nanotubes after the polycarbonate membrane has been removed by rinsing the sample in methylene chloride. Reproduced with permission from Ref [70] 2005 American Chemical Society.

Several types of palladium-based hydrogen sensors have been reported in the literature. The most notable ones are based on Pd thin-film resistors, FETs, Pd nanowires, Pd nanoparticle networks, Pd nanoclusters, and Pd nanotubes as shown in Table 15.2. 

Fig. 10.15 The methods for capillary filling of nanotubes involves dispersal of the agent in a liquid capable of flowing into the nanotube followed by subsequent evaporation of the solvent to leave particles inside the tube. Nanotubes have been filled with polystyrene spheres and palladium nanocrystals using this method (Reprinted from Kim et al., 2005. With permission from American Chemical Society Reprinted from Tessonnier et al., 2005. With permission from Elsevier) (See Color Plates)...

Li, H., et al., Palladium nanoparticles decorated carbon nanotubes facile synthesis and their applications as highly efficient catalysts for the reduction of4-nitrophenol. Green Chemistry, 2012.14(3) p. 586-591. 

Mubeen, S., et al., Palladium nanoparticles decorated single-walled carbon nanotube hydrogen sensor. The Journal of Physical Chemistry C, 2007.111(17) p. 6321-6327. 

Nanomaterials can also be applied to glucose biosensors to enhance the properties of the sensors and, therefore, can lead to smaller sensors with higher signal outputs. Carbon nanotubes have been incorporated in previously developed sensors and seen to increase the peak currents observed by threefold.89 Platinum nanoparticles and single-wall carbon nanotubes have been used in combination to increase sensitivity and stability of the sensor.90,91 CdS quantum dots have also been shown to improve electron transfer from glucose oxidase to the electrode.92,93 Yamato et al. dispersed palladium particles in a polypyrrole/sulfated poly(beta-hydro-xyethers) and obtained an electrode response at 400 mV, compared to 650 mV, at a conventional platinum electrode.94... 

Carbon-based sorbents are relatively new materials for the analysis of noble metal samples of different origin [78-84]. The separation and enrichment of palladium from water, fly ash, and road dust samples on oxidized carbon nanotubes (preconcentration factor of 165) [83] palladium from road dust samples on dithiocarbamate-coated fullerene Cso (sorption efficiency of 99.2 %) [78], and rhodium on multiwalled carbon nanotubes modified with polyacrylonitrile (preconcentration factor of 120) [80] are examples of the application of various carbon-based sorbents for extraction of noble metals from environmental samples. Sorption of Au(III) and Pd(ll) on hybrid material of multiwalled carbon nanotubes grafted with polypropylene amine dendrimers prior to their determination in food and environmental samples has recently been described [84]. Recent application of ion-imprinted polymers using various chelate complexes for SPE of noble metals such as Pt [85] and Pd [86] from environmental samples can be mentioned. Hydrophobic noble metal complexes undergo separation by extraction under cloud point extraction systems, for example, extraction of Pt, Pd, and Au with N, A-dihexyl-A -benzylthiourea-Triton X-114 from sea water and dust samples [87]. 

Yuan Ch, G., Zhang, Y., Wang, S., Chang, A. Separation and preconcentration of palladium using modified multi-walled carbon nanotubes without chelating agent. Microchim. Acta 173, 361-367 (2011)... 

To summarize, in this chapter we have discussed medium-sized supermolecules with unique shapes. Some of them (fullerenes and carbon nanotubes) are found in nature, while others (dendrimers, rotaxanes and catenanes) can be synthesized via artificial molecular design. As seen from palladium-based molecular capsule formation, spontaneous association can sometimes also provide highly sophisticated supramolecular structures. Indeed, as... 

The aim of the present article is to report the large scale (several hundred grams per gram of active phase) synthesis of uniform carbon nanofibers (average diameter ranging between 40 and 60 nm) by the catalytic decomposition of a mixture of ethane and hydrogen over a nickel catalyst supported on carbon nanotubes. To illustrate their catalytic potential, the as-synthesized carbon nanofibers are subsequently used as catalyst support for palladium in the hydrogenation of nitrobenzene in a liquid phase reaction. 

Palladium metal particles with an average diameter of ca. 5 nm were homogeneously dispersed inside carbon nanotubes. Such nanostructured material was an extremely active and selective catalyst for the hydrogenation of the C=C bond of cinnamaldehyde. The high external surfece area of the carbon nanotubes could explain the high reactivity of the catalyst despite its relatively low specific surfece area, i.e. 20 m. g". On the other hand, the high selectivity towards the C=C bond hydrogenation was attributed to the absence of a microporous network and of residual acidic sites in the carbon nanotube catalyst as compared to a commercial activated charcoal. 

The aim of the present article is to report the use of carbon nanotubes as eatalyst support for a palladium active phase in the selective C=C hydrogenation of cinnamaldehyde in liquid-phase. Such reaction is of interest especially in the fine chemieal domain where speeific hydrogenation is actively sought. The catalytic performance was evaluated by comparing the observed activity and selectivity with those of a commercial catalyst supported on a high surfece area activated chareoal. The influenee of the support morphology and microstructure on the hydrogenation activity and selectivity will also be discussed. 

Fig. 1 presents the TEM images of the palladium nanoparticles deposited in the CNTs. According to the TEM observations almost all the metal particles were located inside the CNTs with an average particle size centered at around 5 nm. It should be noted that some palladium particles were observed on the outer surface of the nanotube especially next to the tube tip. This could be explained by some of the palladium salt which was located next to the tube tips having been physically transported from the inner tubide to the outer surfece during the evaporation process. 

Fig. 1. TEM images of the palladium deposited inside the carbon nanotube tubule. The palladium was almost exclusively located inside the tubule of the support and the average palladium particle size distribution was around 5 nm.

Fig. 2. High-resolution TEM images of the palladium particles supported on carbon nanotubes with the unreactive basal planes exposed. The metal particles were in a round shaped form.

Such a difference in terms of product selectivity was attributed to the complete absence of any acidic sites on the carbon nanotubes sur ce and also to the absence of micropores which could induce re-adsorption and consecutive reaction [16]. The presence of micropores could artificially increase the contact time and as a consequence, modify the hydrogenation pathway. The influence of the support nature on the electronic properties of the metallic phase eould also be put forward to explain these results. Depending on the metal-support interaction, the metal particles could exhibit different exposed faces and as a consequence, significantly modify the chemisorption of the reactant on their surface. According to the interaction between the C=C bond and the laces exposed by the palladium particles, the residence time and the desorption of the intermediate could be different and thus, lead to a different selectivity. The presence of palladium aggregates on the activated charcoal as compared to the individual palladium dispersion on the CNTs could be the illustration of this difference in exposed crystalline feces. 

Chun YS, Shin JY, Lee SC et al (2008) Palladium nanoparticles supported onto ionic carbon nanotubes as robust recyclable catalysts in an ionic liquid. Chem Commun 8 942-944... 

---