Palladium, permeability preparation

Ahmad, A.L. and N.N.N. Mustafa, Sol-gel synthesized of nanocomposite palladium-alumina ceramic membrane for H2 permeability Preparation and characterization. International Journal of Hydrogen Energy, 2007.32(12) 2010-2021. 

Ceramic and semiconductor thin films have been prepared by a number of methods including chemical vapor deposition (CVD), spray-coating, and sol-gel techniques. In the present work, the sol-gel method was chosen to prepare uniform, thin films of titanium oxides on palladium Titanium oxide was chosen because of its versatility as a support material and also because the sol-gel synthesis of titania films has been clearly described by Takahashi and co-workers (22). The procedure utilized herein follows the work of Takahashi, but is modified to take advantage of the hydrogen permeability of the palladium substrate. Our objective was to develop a reliable procedure for the fabrication of thin titania films on palladium, and then to evaluate the performance of the resulting metalloceramic membranes for hydrogen transport and ethylene hydrogenation for comparison to the pure palladium membrane results. 

Occurrence and History of Palladium—Preparation—Physical Varieties Physioal Properties—Permeability to Hydrogen--Occlusion of Canos Occlusion of Hydrogen—Chemical Properties--Catalytic Activity Crystalline Palladium—Colloidal Palladium—Spongy Palladium —Palladium Black —Uses—Atomic Weight—Alloys. 

The alloying of palladium with some other metals permits one to overcome the disadvantages of pure palladium and to prepare the materials with a hydrogen permeability above that of palladium. The insertion of a second and a third component into the palladium membrane may increase its mechanical strength, the hydrogen solubility, and catalytic activity of the membrane toward hydrogen dissociation. This was discussed in many original papers and reviews [26-36]. 

The palladium-silver alloy membrane system was successfully commercialized in the early 1960s [12], but the reduction of palladium content by the addition of silver would still not be a cost-effective alternative for large-scale processes [42] unless micron-scale films could be prepared, a goal currently being addressed by many researchers. In recent years, the Pd-Cu system has been the most heavily investigated alloy for hydrogen membrane applications due to the high permeability of select alloys [67, 90, 91], enhanced mechanical properties [92] and reported chemical resistance. The elevated permeability identifled for select Pd-Cu alloys is attributed to an increase in both the solubility and diffusivity of the B2 crystalline phase [86-88] as compared to the face-centered-cubic (fee) phase that exhibits permeability values proportional to the Pd-content [89, 91, 93]. 

In this ongoing research program, there are several issues that need to be addressed. One, mentioned briefly above, is the fact that the Pd-Cu alloy composition is critical in the as-prepared and annealed samples. Figure 11.10 shows hydrogen permeability as a function of the palladium content (in weight per cent) at 350°C, and it is evident that there is a critical composition close to 60 wt.% Pd. 

The potential of membrane reactors has been widely verified and documented for a large number of reactions. However, all the studies made are stUl confined to the laboratory scale, and their implementation in industrial systems has yet to occur. Research into new membrane materials and improvement in the properties of currently available membranes (permselectivity, resistance to poisoning, stability, reduction of palladium thickness, etc.) are always in progress. The development of procedures to deposit the catalyst within the membrane structure without changing its initial permeability and selectivity is an example of the ongoing research for the preparation of catalytic membranes. 

In a more specialized approach, IL phases have been immobilized in membrane materials. Although the primary driver of this work was the use of these materials as electrochemical devices, they have also been investigated for catalytic applications [20]. Membrane materials composed of air-stable, room-temperature ILs and poly(vinylidene fluoride)-hexafluoropropene copolymers were prepared with the incorporation of the active catalyst species in the form of palladium on activated carbon. Optical imaging revealed that the prepared membranes contained a high dispersion of the palladium catalyst particles. Studies on the materials included evaluating their gas permeability and their catalytic activity for the hydrogenation reaction of propene. 

Gryaznov V.M., Smirnov V.S., Vdovin V.M., Ermilova M.M., Gogua L.D., Pritula N.A. and Litvinov I.A., (1969), Method of preparing a hydrogen-permeable membrane catalyst on a base of palladium or its alloys for the hydrogenation of unsaturated organic compounds, US Patent 4,132,668. 

The permeation rate parameter used in this study is shown in Table 13.1. The composite palladium membrane, which was prepared by the chemical vapour deposition (CVD) technique in this study, was found to have a very large selectivity for hydrogen (ca. 10 000 of ideal selectivity for Hj/Nj at 300°C), so that the permeabilities of the other components were assumed to be zero (Itoh et al, 2007). Heat transfer through the membrane takes place in two ways that is, heat conduction, and heat exchange by the permeation. Heat conduction through the membrane is modelled by the CFD code function. Thermal conductivity of 5 W/(m K) is applied to the membrane and support material. Heat exchange through the membrane is calculated as the sum of the permeation flux and enthalpy for each species. 

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