Non-Pd-based membranes

Abstract In this chapter, recent progress on palladium (Pd)-based membrane reactors (MRs) is outUned concentrating, in particular, on the production of pure hydrogen. Aspects of many reactions, as well as analysis of both the Pd-based and the amorphous membranes under study, and the governing equations are presented. Some critical aspects of non-Pd-based membranes are also discussed. All the preparation techniques for pure, alloyed, amorphous, non-Pd-based membranes used in MRs are briefly summarized and compared. Moreover, some problems related to the effect of contamination of the Pd-based membranes on the H2 flux are discussed. 

Another important class of membranes used in MRs is the so-caUed non-Pd-based membrane The interest in developing these membranes (or, at least, with a low Pd-based content) is mainly due to the high cost of Pd material, which strongly Unfits its use in wide-scale industrial use. Moreover, these membranes also show very high values of H2 permeability and resistance to Hj embrittlement (for example, an Nb-li-Ni alloy). Other non-Pd-based membranes, such as Pd-coated amorphous Zr-M-Ni (M = Ti, Hf) alloy membranes, are resistant enough in an H2 atmosphere and show stable permeability only to Hj at least in the range of 200-300°C. 

Membranes with non-traditional surfaces have been suggested [Gryaznov et al., 1975]. Pd-based membranes in the form of cellular foils were fabricated and used to carry out a hydrogen-consuming and a hydrogen-generating reaction on the opposite sides of the membrane. The foils have oppositely directed, alternate projections of hemispherical or half-ellipsoidal shapes. This type of configuration, in principle, provides uniform... 

It is well known that CO inhibits the ability of hydrogen to permeate through Pd membranes. The CO-Pd bond involves a physical adsorption phenomenon that is favored at low temperatures. Several studies on the performance decrease of Pd-based membranes and Pd-based membrane reactors due to inhibition have been carried out/ highlighting the under-estimation of the membrane area caused by the non-consideration of the inhibition by CO in the design of a reactor and/or a separation equipment. In fact, the membrane inhibition phenomenon can be relevant not only in gas separation, but also in reactive processes like the water gas shift carried out in Pd-based membrane reactors, where CO is involved as a principal reactant at a relatively low temperature (280-350 °C). 

To this end, I invited an international team of highly expert scientists from the field of membrane science and technology to write about the state-of-the-art of the various kinds of membranes (polymeric, Pd- and non-Pd-based, carbon, zeolite, perovskite, composite, ceramic and so on) used in membrane reactors, modelling aspects related to all kinds of membrane reactors, the various applications of membrane reactors and, finally, economic aspects. 

In the application of WGS in non-Pd-based MRs, Bmnetti et al. (2007) performed the WGS reaction at low-medium temperature (493—563 K) and in the pressure range of 2.0—6.0 bar by using a composite membrane composed of a silica layer supported onto a porous stainless steel disk. The support was modified to make possible the reduction of the macropores by packing with silica xerogel (500 nm) and by coating with an intermediate layer of y-alumina via boehmite sol (y-AlOOH) by a soaking-rolling procedure. Therefore, this MR made it possible to reach a CO conversion of around 95.0% at 553 K, 4.0 bar, and stoichiometric feed ratio. 

Exposure of Pd-based membranes to non-Ha components of the mixed gas can also lead to performance degradation. Most significant are S-containing compounds that are ubiquitous in fossil-fuel processing streams [17, 34]. Figure 3 (top) shows how a pure-Pd membrane can respond to H2S [26]. In this experiment, a baseline flux of pure Ha was first established across a 25 pm-thick membrane at 350 °C. Then, 1000 ppm HaS was added to the Ha. Immediately upon introduction of HaS, flux... 

It is obvious that the pure-metal membranes can be easily prepared by conventional metallurgical processes in the configurations of tube and disk, which have a thickness greater than 25 pm and can be used as unsupported H2 permeation membranes. Because of the two obvious weaknesses of embrittlement and slow surface kinetic, the preparation of these metal-based H2 permeation membranes mainly focuses on preparation of alloys in order to improve the embrittlement and modification of the surface for improving the surface kinetic. The preparation of an extra modified layer on these pure non-Pd metal membranes is the subject of the current discussion. 

Palladium is more abundant in nature and sells at half the current market price of platinum. Unlike Pt, the Pd-based electrocatalysts are more active towards the oxidation of a plethora of substrates in alkaline media. The high activity of Pd in alkaline media is advantageous considering that non-noble metals are sufficiently stable in alkaline for electrochemical applications. Importantly, it is believed that the integration of Pd with non-noble metals (as bimetallic or ternary catalysts) can remarkably reduce the cost of the membrane electrode assemblies (MEAs) and boost the widespread application or commercialization of DAFCs [1]. Palladium has proved to be a better catalyst for alcohol electrooxidation in alkaline electrolytes than Pt [2]. Palladium activity towards the electrooxidation of low-molecular weight alcohols can be enhanced by the presence of a second or third metal, either alloyed or in the oxide form [3]. 

As the most relevant aspect. Table 6.2 resumes the efforts in the last decade for developing both Pd-based MRs (from dense to composite membranes housing) and non-Pd MRs. 

Porous membranes (low H2 selectivity) are used as substrate onto which a very thin but continuous film of a selective metal, generally Pd-based, is deposited. Dense composite membranes are also fabricated by sintering together powders of highly H2 permeable metals (Pd, Pt, Nb,Ta,1i, V, Zr and their alloys) with powders of a second metal or alloy which is non-permeable to H2 (Mundschau, 2005), having the function of providing the mechanical support. Some fundamental science related to the H2 separation using dense composite membranes is reported by Mundschau (2008). 

Amorphous alloy membranes are at present being developed to tackle the problems of H2 embrittlement, sintering and cost that occur with crystalline alloy membranes, while still providing H2 permeance comparable to Pd. Fabricated from non-Pd elements, these membranes are low price and inherently resistant to embrittlement because of their amorphous structure. Much development is still required, however, to improve stability and achieve the performance of existing Pd-based crystalline alloy... 

In contrast to equilibrium-based sensing such as described above, it is also possible to use the zeolite film as a membrane controlling molecular access to an appropriate transduction mechanism. In this case, Pd-doped semiconductor gas sensors were used as a fairly non-selective sensor platform. After coating these sensors with a thin film of MFI-type or LTA-type zeolites, they were examined with respect to gas phase sensing of different analytes such as methane, propane and ethanol, at different humidity levels (Fig. 14).[121] The response of a zeolite-coated sensor towards the paraffins was strongly reduced compared to the non-coatcd sensor device, thus resulting in an increase of the sensor selectivity towards ethanol. 

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