Palladium Deposition Methods

Thorough cleaning and the avoidance of any extraneous debris are essential in any membrane fabrication scheme. After EP, a composite membrane can be soaked in hot water or dilute ammonia to help remove any impurities trapped in the porous support that may be detrimental to the palladium film during high temperature operation [72, 73]. However, traces of impurities from the EP bath such as chlorine, sodium, and carbon, inevitably become incorporated into the metal film. Membrane defects can be a consequence of preparation conditions. Fabrication in a dean-room environment has resulted in increased permselectivity [140]. Porous stainless steel (PSS) must be cleaned and pickled before electrodeposition or else activated for electroless plating. 

Although it is somewhat more complicated in terms of equipment, CVD can promote transport of metal directly into pores for deposition [141]. Precursors such as paUadium(ll) chloride, palladium(II) acetylacetonate [Pd(acac)2], and palladium bis(hexafluoroacetylacetonato) [Pd(hfacac)2] are volatilized and entrained to the heated reaction zone, where the material is preferentially deposited within the pores. Counterdiffusion of the reducing gas (hydrogen) can aid in depositing metal more predsely [142]. Alloys may be deposited from a single mixed source. 

It is important to ensure the metal-organic precursor is completely reacted in order to avoid carbon incorporation into the film [143]. 

Volumetric steady-state gas permeation tests at elevated temperatures are typically used to characterize the performance of paUadium membranes. Electrochemical methods are also effective, even at high temperatures [90, 166]. Permeation through palladium depends on a solution-diffusion mechanism, including the steps of chemisorption and dissociation into atoms, absorption into the metal, diffusion through the metal lattice, transfer from the bulk metal to the opposite side, and recombination into molecules for desorption [167, 168]. Difiusion of molecular hydrogen through boundary layers adjacent to the surface is also necessary. 

The equation used to describe flux is derived from a combination of Pick s and Sieverts laws where the difference between the square root of hydrogen partial pressure on the feed and permeate sides of the membrane creates the driving force for hydrogen flux [168,169]  

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S. Van, H. Maeda, K. Kusakabe, and S. Moro-Oka, Thin palladium membrane formed in support pores by metal-oiganic chemical vapor deposition method and application to hydrogen separation, IruL Eng, Chem. Res. Ji 616 (1994). 

Palladium deposition at the water-DCE interface has been described Attempts have been made to correlate the experimental current response for deposition with nucleation models allowing for two-phase transport and interfacial nucleation [191, 192]. Relatively poor agreement was observed, a fact attributed to the growing particles lateral motion and resultant tendency to aggregate. This problem led to the use of micropipette methods (see Sections III and IV), applied to silver deposition at the ITIES in an attempt to produce a finite number of particles [193]. Another approach to the restriction of lateral aggregation involved the use of porous membranes palladium and platinum particles formed at the ITIES were 10nm in diameter [145, 194, 195]. The form of typical experimental current-time responses for the deposition process is consistent with the classical nucleation response seen for deposition on solid electrode surfaces (see Fig. 25) [50]. The quantitative analysis of this... 

For industrial application usually such metals as palladium, platinum, iron, ruthenium, cobalt, molybdemun, nickel, either alone or as bimetallic catalyst are used. They are introduced using ion exchange, excess solution impregnation, incipient-wetness impregnation or physical vapor deposition methods. 

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... 

For a composite membrane that is relatively free of pinholes, there is apparently a limiting palladium thickness that depends primarily on support quaUty and somewhat on deposition method. For example, unmodified PSS requires at least 10 pm of palladium to bridge a majority of the pores [149, 227]. Invariably, a few macrodefects remain that preclude the attainment of perfect hydrogen permselectivity. On porous asymmetric a-alumina supports with 100-200 nm pore size on the surface, approximately 7 pm appears to be the limiting thickness that is proba-... 

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. 

Barbieri et al. [523] prepared palladium membranes by solvated metal atom deposition, a deposition method that created a 0.1-pm thick film of palladium on an alumina tube. Another membrane was prepared from a palladium/silver alloy (21 wt.% silver), which had a thickness of 10 pm. Both membranes suffered from pinholes and cracks and thus did not show infinite hydrogen selectivity. However, conversion exceeding the thermodynamic equilibrium could still be achieved for methane steam reforming at temperatures exceeding 400 °C. 

Skital PM (2014) The mathematical modelling of the palladium deposition/dissolution process by cyclic voltammetry method. Int 1 Electrochem Sci 8 2589-2602... 

The synthesis of Pt metal nanowires and nanotubes electrochemically deposited through nanoporous membranes was reported by Ichikawa (2000). Palladium deposits with high specific surface areas, up to 50 m g", have been synthesized by Tsirlina et al. (2002) from palladium chloride solutions with additions of polyethylene glycol and polyvinyl pyrrolidone. Details of the globular structine of the deposits depend on the polymer additives. A comparison with electrochemically determined true surface areas demonstrates the coalescence of nanoparticles. Wang et al. (2003) electrochemically synthesized thin films composed of ordered arrays of palladium nanowires using silica mesoporous channels. Mesoporous channels were deposited on a conductive glass substrate. In this method, nanowires with face-centered cubic crystal structure are continuously deposited from the conductive substrate upward until the mesoporous channels are filled. 

Guazzone F. and Ma Y.H., Leak growth mechanism in composite palladium membranes prepared via electroless deposition method, AfC// /., 54 (2008) 487-494. 

Platinum catalysts were prepared by ion-exchange of activated charcoal. A powdered support was used for batch experiments (CECA SOS) and a granular form (Norit Rox 0.8) was employed in the continuous reactor. Oxidised sites on the surface of the support were created by treatment with aqueous sodium hypochlorite (3%) and ion-exchange of the associated protons with Pt(NH3)42+ ions was performed as described previously [13,14]. The palladium catalyst mentioned in section 3.1 was prepared by impregnation, as described in [8]. Bimetallic PtBi/C catalysts were prepared by two methods (1) bismuth was deposited onto a platinum catalyst, previously prepared by the exchange method outlined above, using the surface redox reaction ... 

Thus, silver nanoparticles grow gradually during UV light irradiation (processes al-a3 in Figure 2). Nanoparticles of other noble metals such as gold, copper, platinum, and palladium can also be deposited by this method. 

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