Palladium-based alloys

Multiphase gold or palladium-based alloys never show dissolution of Au or Pd but often exhibit progressive surface ennoblement due to selective dissolution of copper or silver from the outer 2-3 atomic layers Heat treatment often decomposes multicomponent alloys into a Pd-Cu rich compound and an Ag-rich matrix with corrosion of the latter phase in deaerated artificial saliva and S -containing media . Au-Cu-rich lamellae have similarly been observed, again with preferential attack on Ag-rich phases or matrix. These effects presumably arise from the ability of the noble alloy phases to catalyse the cathodic reduction of oxygen . 

F.L. Chen, Y. Kinari, F. Sakamoto, Y. Nakayama and Y. Sakamoto, Hydrogen Permeation through Palladium-Based Alloy Membranes in Mixtures of 10% Methane and Ethylene in the Hydrogen , Int. J. Hydrogen Energy, 21 [7] 555-61 (1996). 

Because of the high activity of palladium in electrocatalysis, palladium-based alloys, which contain one or more platinum-group metals along with titanium as well as boron and/or silicon, have been thoroughly investigated. 

In this respect palladium is superior to all other platinum group metals, but it cannot be used because of its fast dissolution under working conditions. On the other hand, it has been known that certain amorphous alloys have extremely high corrosion resistance (S). These facts led to the studies of amorphous palladium-based alloys as anode materials in the electrolysis of soda. 

The surface activation consisting of zinc deposition, heat treatment, and subsequent leaching of zinc (63, 64) was applied to different amorphous iron-, cobalt-, nickel-, and palladium-based alloys (63, 64). SEM measurements indicated the formation of a porous surface layer. Cyclic voltammetric examinations suggested an increase of surface area by about two orders of magnitude. Heat treatments at higher temperatures resulted in thicker, more porous surface layers and higher electrocatalytic activities (Table II). Palladium-phosphorus alloys with Ni, Pt, Ru, or Rh proved to be the best specimens. Pd-Ni-P with 5% Ni, after treatment at 573 K, exhibited even higher activity than that of the Pt-Pt electrode (Table II). These amorphous alloy electrodes were active in the oxidation of methanol, formaldehyde, and sodium formate. 

F. Sakamoto, Effect of carbon monoxide on hydrogen permeation in some palladium-based alloy membranes, Int. J. Hydrogen Energy 1996, 21(11/12), 1017-1024. 

F. N. Berseneva, 1. A. Mikhaylova, N. 1. Timofeyev, The influence of hydrogen on the phase composition and physico-mechanical properties of the membrane palladium-based alloy V-1, Phys. Met. Metall. 1992, 72(2), 151-156. 

Palladium or its alloys are the most practical membrane materials, due to their high hydrogen permeability and stability at high temperatures. The membrane reformer is composed of a steam reformer equipped with palladium-based alloy modules in its catalyst bed, and can perform steam reforming reaction and hydrogen separation processes concurrently with no help from shift converter and PSA, as shown in Fig. 12.1. 

L0vvik OM (2005) Surface segregation in palladium based alloys from density-functional... 

Burkhanov GS, Gorina NB, Kolchugina NB, Roshan NR. Palladium-based alloy membranes for separation of high purity hydrogen from hydrogen-containing gas mixtures. Platinum Mat Rev 2011 55 3-12. 

As has been demonstrated, the fields of quartz crystal microbalance (QCM)- and surface acoustic wave (SAW)-based gas sensors are also of interest in metal film application (Miura 1991 Jakubik etal. 2003 Jakubik and Urbanczyk 2005). When the palladium or palladium-based alloy layer absorbs hydrogen, both its mass density and electrical conductivity change, and this produces a detectable change in the frequency of the SAW and resonance frequency of (JCM. Devices were able to detect hydrogen gas in a range of 1.5-4.0% concentration in air. 

The introduction of other metals to form palladium based alloys has had promising results. In particular doping of the palladium with silver has been shown to improve the stability of the film and increase the solubility of hydrogen. Further, the temperature above which the a palladium hydride occurred was lowered with increasing silver content (Uemiya et al.,1991 Kikuchi Uemiya, 1991). The hydrogen permeability was optimized when the silver content of the alloy was aroimd 23 wt%. Silver occupies interstitial sites in the palladium lattice and so moderates the lattice expansion and contraction due to hydrogen absorption/desorption. 

The purpose of this section is twofold. First it will be shown that experimental data on hydrogen absorption in palladium-based alloys can be understood within the framework of the density-of-sites model presented here. Second, numerical results will be used to analyze the relative importance of the various ingredients entering the model. 

The model gives a good description of the pressure composition isotherms of palladium-based fcc-alloys. In this work we considered in some detail Pd Ag H and Pd Cu H because these systems have been thoroughly inveXti aSed in tne pa t Results for other palladium-based alloys shall be presented elsewhere. 

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