Particles palladium-coated

Zhang and Wang (1997) studied the reaction of zero-valent iron powder and palladium-coated iron particles with trichloroethylene and PCBs. In the batch scale experiments, 50 pL of 200 pg/mL PCB-1254 in methanol was mixed with 1 ml ethanol/water solution (volume ratio = 1/9) and 0.1 g of wet iron or palladium/iron powder in a 2-mL vial. The vial was placed on a rotary shaker (30 rpm) at room temperature for 17 h. Trichloroethylene was completely dechlorinated by the nanoscale palladium/iron powders within the 17-h time period. Only partial dechlorination of PCB-1254 was observed when wet iron powder was used. 

Nanosized cobalt, copper, gold, nickel, rhodium, and silver particles have been stabilized by polyions and polymers [514, 549-553]. Particularly significant has been the simultaneous reduction of HAuC14 and PdCl2 in the presence of poly(iV-vinyl-2-pyrrolidine) to give relatively uniform, 1.6-nm-diameter, palladium-coated gold bimetallic clusters [554]. 

Electroless deposition can successfully be used in the production of various composite materials useful for the electronics applications. The examples include silver-coated copper or nickel particles, used in screen printing,43 gold-coated nickel powders, silver and/or palladium-coated polymers or glass powders used in ball grid array, etc. 

Activation is used to seed the surface of the PAA membrane with p>alladium grains to form sites for growth of the palladium coating during electroless plating. The sensitization step deposits tin hydroxide p>articles on the surface of the PAA. During immersion in the activation solution the tin hydroxide particles reduce the palladium to palladium metal which then deposits on the surface of the membrane. No tin hydroxide residue remains on the surface of the PAA after activation. The surface of an activated PAA membrane is shown in Figure 21. 

Why are the nanoscale Fe° particles coated with palladium a) To preserve the particles Palladium acts as a catalyst To decrease the reactivity of the particles None of the above... 

Precious Meta.1 Ca.ta.lysts, Precious metals are deposited throughout the TWC-activated coating layer. Rhodium plays an important role ia the reduction of NO, and is combiaed with platinum and/or palladium for the oxidation of HC and CO. Only a small amount of these expensive materials is used (31) (see Platinum-GROUP metals). The metals are dispersed on the high surface area particles as precious metal solutions, and then reduced to small metal crystals by various techniques. Catalytic reactions occur on the precious metal surfaces. Whereas metal within the crystal caimot directly participate ia the catalytic process, it can play a role when surface metal oxides are influenced through strong metal to support reactions (SMSI) (32,33). Some exhaust gas reactions, for instance the oxidation of alkanes, require larger Pt crystals than other reactions, such as the oxidation of CO (34). 

In a later study, Pfeifer et al. [30] prepared Pd/Zn catalysts by both pre- and postimpregnation of wash-coated zinc oxide particles with palladium and compared their performance in methanol steam reforming. The catalytic performance of the samples was tested at a 250 °C reaction temperature, 3 bar pressure, a S/C ratio of two and 250 ms residence time. The WHSV amounted as 0.3 Ndm3 (min gcat) 1. The thickness of the coatings was calculated to 20 pm. The formation of the PdZn alloy was proven to occur at temperatures exceeding 200 °C by XRD measurements. 

FIGURE 11 An edge-coated palladium-on-carbon catalyst. A, Photomicrograph of an irregularly shaped particle of wood carbon. B, Palladium map showing the palladium concentrated on or near the surface of the particle. C, Palladium line scan showing the distribution of the palladium at the edge of the carbon. 

The process has been improved over a period so that now very small particles of palladium may be deposited very evenly, and this has been demonstrated to give improved adhesion of the layers of coating to the substrate. 

Bimetallic particles with layered structures have opened fascinating prospects for the design of new catalysts. Schmid et al. [10m] have applied the classical seed-growth method [20] to synthesize layered bimetallic Au/Pd and Pd/Au colloids in the size range of 20-56 nm. The sequential reduction of gold salts and palladium salts with sodium citrate allows the gold core to be coated with Pd. This layered bimetallic colloid is stabilized by trisulfonated triphenylphosphane and sodium sulfanilate. More than 90% metal can be isolated in the solid state and is redispersable in water in high concentrations. 

Pure metals as well can be deposited on the surface of carbon nanotubes. The reductive precipitation of gold nanoparticles on MWNT coated with citrate ions may be given just as one example. Besides dispersing the MWNT, the citric acid is also responsible for the reductive generation of the gold particles from HAuCLt. Other metals like platinum, palladium, titanium, and iron can be deposited on the nanotube surface, too. 

Complex substrate modifications involving intermediate layers and palladium alloy deposition methods are often required for superior membrane performance. Modification of a membrane support surface before palladium deposition by sintering on smaller particles can create a smoother surface with smaller pores, facilitating the deposition of a defect-free palladium layer. Nickel microparticles have been sintered together to form a porous support that was sputter-coated with palladium and then copper [118]. Thermal treatment at 700 °C for 1 h promoted reflow to create a durable, pinhole-free membrane with a Pd-Cu-Ni alloy film. In another case, starting with commercially available PSS with a 0.5 pm particle filtration cut-ofF, submicron nickel particles were dispersed on the surface, vacnium sintered for 5 h at 800 °C, and then sputtered with UN [159]. The nickel particles created a smoother surface with smaller pores, so a thinner palladium alloy layer... 

Model catalysts (Section 2.3) permit the use of the same techniques of examination as single crystals. Palladium particles formed on alumina-coated NiAl( 100) adsorbed ethene in both the tt- and a-forms, but the latter was favoured with increase in particle size this could indeed be the basis for the weak size dependence noted previously (Section 7.2.2) in alkenehydrogenation. Hydrogen adsorbed more strongly on small particles, and in a now familiar way shifted the adsorbed states of ethene towards the r-form this reacted with weakly-held hydrogen atoms to form ethane. ... 

The particle morphology is normally assessed by scanning electron microscopy (SEM). The sample preparation depends, in the first instance, on the type of equipment and normally requires the use of aluminum stubs with a carbon conductive tape and coating with gold-palladium layer. Sometimes the particles are placed on a double-sided carbon tape that is attached to aluminum stubs, without coating requirement. The analyses are carried out by applying to sample a difference of potential between 2 and 20 kV. 

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