Alloy palladium-base

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 . 

Palladium-based dense metallic membranes have been known to be completely selective for hydrogen permeation and are used in commercially available small-scale hydrogen purification units (e.g., Johnson Matthey, 2007 REB Research, 2007 Power + Energy, 2007 ATI Wah Chang, 2007). These hydrogen purification units typically use palladium-alloy... 

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). 

Bimetallic nanoparticles constitute suitable model systems for studying alloying effects in catalysis. In particular, palladium-based bimetallic systems were developed in order to increase the selectivity... 

V. M. Gryaznov and his co-workers (e.g. IGryaznov, 1986]) have extensively explored the permselective properties of palladium and its alloys as dense membranes and membrane reactors. While their studies will be discussed in later chapters, it suffices to say that the palladium-based membranes have reached the verge of a commercialization potential for the process industry. 

Electroplating. Basically in electroplating, a substrate is coated with a metal or its alloy in a plating bath where the substrate is the cathode and the temperature is maintained constant Membranes from a few microns to a few millimeters thick can be deposited by carefully controlling the plating time, temperature, current density and the bath composition. Dense membranes made of palladium and its various alloys such as Pd-Cu have been prepared. Porous palladium-based membranes have also been made by deposition on porous support materials such as glass, ceramics, etc. 

As mentioned earlier, two compatible reactions may be coupled or conjugated properly by a shared membrane through which the species (as a product on one side of the membrane and a reactant on the other) common to both reactions selectively passes. Summarized in Table 8.5 are some documented studies of reaction coupling using dense palladium-based membranes with the alloying component ranging from nickel, ruthenium, rhodium to silver. 

Effects of alloying metal on the hydrogen permeability of a palladium-based membrane... 

Dense palladium-based membranes. Shown in Table 10.1 are modeling studies of packed-bed dense membrane shell-and-tube reactors. All utilized Pd or Pd-alloy membranes except one [Itoh et al., 19931 which used yttria-stabilized zirconia membranes. As mentioned earlier, the permeation term used in Ihe governing equations for the tube and shell sides of the membrane is expressed by Equation (10-51b) with n equal to 0.5 [c.g., Itoh, 1987] or 0.76 [e.g., Uemiya et al., 1991]. 

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. 

I. Y. Machikina, G. A. Barannikova, V. A. Gol tsov, V. V. Latyshev, S. K. Kurum-china, N. I. Timofeev, Mechanical and diffusion characteristics of palladium-base type V-3 phase-hardened alloys for hydrogen diffusion filters, Sov. Mater. 

N. I. Timofeev, F. N. Berseneva, V. M. Makarov, New palladium-based membrane alloys for separation of gas mixtures to generate ultrapure hydrogen, Int.]. Hydrogen Energy 1994, 39(11), 895-898. 

Depending on the type of catalyst adopted, the operating temperatures can vary between 600 K (low-temperature shift catalyst) and 700 K (high-temperature shift catalyst). Because of the relatively high temperatures employed, studies reported in the literature usually refer to the use of inorganic membrane reactors [13-16]. In particular, because palladium and its alloys are extremely selective to hydrogen, many reports discuss work on palladium-based membrane reactors. 

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