Porous film catalyst support

Most industrial catalysts are supported, i.e. distributed in fine form (1-10 nm) on the surface of a porous, high surface area and usually inert support (e.g. Si02, y-Al203> Ti02).15 In this book, however, we will deal quite often with catalysts in the form of a porous film deposited on a solid electrolyte. 

Figure 11.3. Schematic of the experimental setup used (a) to induce electrochemical promotion (via YSZ) on Ir02 and Ir02-Ti02 porous catalyst films (b) to compare the electrochemical promotion induced on Pt via YSZ and via Ti02 and (c) to compare the electrochemical promotion behaviour induced by varying UWR on a Rh porous catalyst film (left) and on a fully dispersed Rh catalyst supported on porous (80 m2/g) YSZ support.22...

Both questions have been recently addressed via a surface diffusion-reaction model developed and solved to describe the effect of electrochemical promotion on porous conductive catalyst films supported on solid electrolyte supports.23 The model accounts for the migration (backspillover) of promoting anionic, O5, species from the solid electrolyte onto the catalyst surface. The... 

Two main types of catalyst layers are used in PEM fuel cells polyfefrafluo-roethylene (PTFE)-bound catalyst layers and thin-film catalyst layers [3]. The PTFE-bound CL is the earlier version, used mainly before 1990. If confains two components hydrophobic PTFE and Pt black catalyst or carbon-supported Pt catalyst. The PTFE acts as a binder holding the catalyst together to form a hydrophobic and structured porous matrix catalyst layer. This porous structure can simultaneously provide passages for reacfanf gas fransport to the catalyst surface and for wafer removal from fhe cafalysf layer. In fhe CL, the catalyst acts as both the reaction site and a medium for electron conduction. In the case of carbon-supported Pt catalysts, both carbon support and catalyst can act as electron conductors, but only Pt acts as the reaction site. 

Numerous efforfs have been made to improve existing fhin-film catalysts in order to prepare a CL with low Pt loading and high Pt utilization without sacrificing electiode performance. In fhin-film CL fabrication, fhe most common method is to prepare catalyst ink by mixing the Pt/C agglomerates with a solubilized polymer electrolyte such as Nation ionomer and then to apply this ink on a porous support or membrane using various methods. In this case, the CL always contains some inactive catalyst sites not available for fuel cell reactions because the electrochemical reaction is located only at the interface between the polymer electrolyte and the Pt catalyst where there is reactant access. 

Finally, Vroon et al. [82,97] reported the synthesis of continuous porous films of ZSM5 on top of y-alumina supported membranes (pore diameter 4 nm) by slip-casting with a zeolite crystal suspension. The porous zeolite layers (thickness 1-2.5 pm) consist of densely packed zeolite crystals with a diameter of 70-80 nm and with micropores in the zeolite and mesopores (diameter 8-24 nm) between the zeolite particles. This zeolite layer can be used as a support for further processing, e.g., pore filling of the mesopores or deposition of catalysts. First experiments by Vroon et al. to fill the mesopores by in situ crystallisation of MFI in the pores did not result in gas-tight membranes... 

Electrochemical promotion (EP) denotes electrically controlled modification of heterogeneous catalytic activity and/or selectivity. This recently discovered phenomenon has made a strong impact on modem electrochemistry/ catalysis/ and surface science. Although it manifests itself also using aqueous electrolytes/ the phenomenon has mainly been investigated in gas-phase reactions over metal and metal oxide catalysts. In the latter case, the catalyst, which is an electron conductor, is deposited in the form of a porous thin film on a solid electrolyte support, which is an ion conductor at the temperature of the catalytic reaction. Application of an electric potential on the catalyst/support interface or, which is equivalent, passing an electric current between catalyst and support, causes a concomitant change also in the properties of the adjacent catalyst/gas interface, where the catalytic reaction takes place. This results in an alteration of the catalytic behaviour, controllable with the applied potential or current. 

Extension of this methodology to porous packing elements (e.g., catalyst support pellets) is not straightforward. The challenge arises because the signal we wish to measure is associated with the liquid (water) in the bed. However, the signal intensity acquired from a specific region of water depends on its local environment, because the nuclear spin relaxation times of water in different physical environments will vary. In this system, the different environments will be (i) free water in the bulk of the inter-pellet space, (ii) water within the intra-pellet pore space, and (iii) water present in films on the surfaces of the pellets but not part of a rivulet within the inter-pellet space. 

Surface area and its accessibility are important both in catalysis and gas cleanup. Nano-structured micro-porous catalysts or catalyst supports offer intensified catalysis since they provide an enhanced surface area which is accessible to the reactants and products through a network of arterial channels feeding into the regions of catalytic activity. In non-structured catalysts, although the surface area might be large, as determined by gas adsorption, they are often not accessible as a result of surface fouling and the diffusion resistance can slow down the rates of reactions. Catalysts are either deposited as a thin film on a support or they are used as pellets. These two techniques have certain drawbacks in coated systems, catalyst adhesion can be non-uniform and weak while the accessibility of the active sites within the interior of the catalyst is hindered due to low porosity. 

Experiment has shown that Pd and Pt can be deposited on the surface of a sensor platform in the form of films (Cavicchi et al. 2004) (see Fig. 11. lb). However, such a configuration does not provide the required rate of combustion due to small active surfece area. As a result, sensors have low sensitivity. Therefore, as a rule, catalysts used Pt and Pd nanoparticles dispersed on a porous metal oxide support (Debeda et al. 1995 Lee et al. 1997 Katti et al. 2002 Fuijes et al. 2005) (Fig. 11.1a). It was found... 

Barhieri et al. developed a membrane reactor for water-gas shift 544]. A palladium/ silver film containing 23 wt.% silver, which was between 1- and 1.5-pm thick was produced hy sputtering. This film was coated onto a porous stainless steel support. This patented production method allowed a much higher ratio of pore size to film thickness compared with conventional methods. Tubular membranes of 13-mm outer diameter, 10-20-mm length and 1.1-1.5-pm thickness, respectively, were prepared. A commercial Cu based catalyst supplied by Haldor-Topsoe was used for the water-gas shift reaction. At 210 °C a permeating flux of 4.5 L (m s) was determined for pure hydrogen at 0.2-bar pressure drop. At a reaction temperature of 260-300 °C, and 2085 h gas hourly space velocity, the thermodynamic equilibrium conversion could be exceeded by 5-10% with this new technology. 

Monofunctional organosilanes terminating the surfaces of porous xerogels or coatings can be used as catalyst supports, membranes, or sensors [241]. For example, Fig. 16a shows that incorporation of aminosilanes in a porous silicate coating results in significant COj adsorption, despite rather low surface areas. In order to reduce the influence of water vapor, hydro-phobic components can be incorporated. Figure 16b shows the effect of a propylsilane addition on the water uptake of an aminosilane-silicate film. 

SO3 is a key ingredient in making H2S04( ). It is produced rapidly and efficiently by oxidizing SO2 to SO3 in a molten V, K, Na, Cs, S, O catalyst film, 400-600 °C. The molten film is supported on a solid, porous silica substrate. 

This study, in conjunction with that discussed in 12.2.1.2, show that when using aqueous electrolytes or Nafion saturated with H20, the induction of NEMCA on finely dispersed noble metal catalysts is rather straightforward. The role of the electronically conducting porous C support is only to conduct electrons and to support the finely dispersed catalyst. The promoting species can reach the active catalyst via the electrolyte or via the aqueous film without having to migrate on the surface of the support, as is the case when using ceramic solid electrolytes. 

Figure 6. Immobilization of Chiral Ruthenium Hydrogenation Catalyst in a Thin Hydrophilic Film on a Porous Glass Support...

Realistic model systems. Some techniques become much more informative if suitable model systems are used. Examples are the thin-film oxides used as conducting model supports, which offer much better opportunities for surface analysis than do technical catalysts. Another example is provided by the non-porous, spherical supports that have successfully been employed in electron microscopy. It is important that the model systems exhibit the same chemistry as the catalyst they represent. 

Functionalized polymers are of interest in a variety of applications including but not limited to fire retardants, selective sorption resins, chromatography media, controlled release devices and phase transfer catalysts. This research has been conducted in an effort to functionalize a polymer with a variety of different reactive sites for use in membrane applications. These membranes are to be used for the specific separation and removal of metal ions of interest. A porous support was used to obtain membranes of a specified thickness with the desired mechanical stability. The monomer employed in this study was vinylbenzyl chloride, and it was lightly crosslinked with divinylbenzene in a photopolymerization. Specific ligands incorporated into the membrane film include dimethyl phosphonate esters, isopropyl phosphonate esters, phosphonic acid, and triethyl ammonium chloride groups. Most of the functionalization reactions were conducted with the solid membrane and liquid reactants, however, the vinylbenzyl chloride monomer was transformed to vinylbenzyl triethyl ammonium chloride prior to polymerization in some cases. The reaction conditions and analysis tools for uniformly derivatizing the crosslinked vinylbenzyl chloride / divinyl benzene films are presented in detail. 

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