Skin-layer catalyst

The resulting metal membrane has a structure similar to the bulk pores of the anodic alumina but without the dense "skin layer and has straight through-pores. Nickel and platinum membranes having pore diameters in the 15 to 200 nm range can be produced this way. One of the critical steps of this method is depositing palladium catalyst on the surface, but not inside the pores, of the anodic alumina. The catalyst is used to facilitate the deposition of metal from the bottom to the surface of the cylindrical pores. 

A special case arises when the "skin" (membrane) layer of a normal composite membrane element is immobilized with a catalyst and not intended for separating reaction species. Consider the example of an enzyme, invertase, for the reaction of sucrose inversion. Enzyme is immobilized within a two<layer alumina membrane element by filtering an invertase solution from the porous support side. After enzyme immobilization, the sucrose solution is pumped to the skin or the support side of the membrane element in a crossflow fashion. By the action of an applied pressure difference across the element, the sucrose solution is forced to flow through the composite porous structure. Nakajima et al. [1988] found that the permeate direction of the sucrose solution has pronounced effects on the reaction rate and the degree of conversion. Higher reaction rates and conversions occur when the sucrose solution is supplied from the skin side. The effect on the reaction rate is consistently shown in Figure 11.6 for two different membrane elements membrane A is immobilized by filtering the enzyme solution from the support layer side while membrane B from the skin layer side. 

PtCo, and PtNi, and this was related to the formadon of a Pt-skin layer on the alloy pardcles. However, for Pt alloys with precious metals such as Ru, Os, or Re, the bifunctional mechanism is operative because of the stability of these elements in the Pt surface. With the introduction and evolution of more powerful characterization techniques such as XAS, it has been possible to perform more detailed studies of the crystalline phases present in a catalyst. The work of McBreen and Mukeijee has shown clearly that in Pt-Ru alloys, the Ru increased the Pt d-band vacancies and decreased the Pt-Pt bond distances. The wider conclusion of this work was that a fine-tuning of the electronic structure and the electrocatalysis is necessary in order to design an even more CO tolerant and active catalyst. 

Bulk Pt3Ni alloys were annealed in ultra-high vaeuum to form a single atom Pt skin layer. The challenge remains to synthesize nanosized structures of these catalysts. 

RO is the most relevant membrane-based technique for seawater desalination [98]. Similar to NF, RO is carried out using asymmetric membranes with a nonporous skin layer. Membranes can be integrally skinned or TFC. The most important technique for the preparation of such membranes is IP, which has been already described in Section 1.6.3 devoted to NF membranes. As reported by Lee et al. [99], the studies about the preparation of polymeric membranes for RO application, from 1950 to 1980, focused on the search for optimum membrane materials. Subsequently, the performance of RO membranes was improved by controlling membrane formation reactions and using catalysts and additives. 

Platinum-palladium layers, 264-265 Platinum-skin catalysts, 70-71, 77, 257-263, 321-323, 335-336 Platinum surface modification by adatoms, 208-239... 

More recently, Stamenkovic et al. [95,107] reported on the formation of Pt skins on Pt alloy electrocatalysts after high-temperature annealing. Pt skins were reported to exhibit strongly enhanced ORR activity. It was argued that the electronic properties of the thin Pt layer on top of the alloy alter its adsorption properties in such a way as to reduce the adsorption of OH from water and therefore to provide more surface sites for the ORR process (see Section 5.2 in Chapter 4 for a detailed discussion of skin catalysts, compare also Section 4.1.5 in the present Chapter). 

Although Wolfs indicated that the catalyst particles are covered by a skin of bismuth molybdate, Batist (112) recently found bismuth, molybdenum, and iron in the surface layers of multicomponent catalysts. Additional data are needed to determine if multicomponent catalysts gain their activity as a result of the formation of compounds such as bismuth iron molybdate, or by surface enhancement of an active component such as 7-phase bismuth molybdate, or by creation of low-energy electronic transitions. Of course, due to their complexity, all of these factors may be important. 

We believe that these phenomena can be explained byassuminga skin of silica on the metal surface. The fact that such a skin develops is readily acceptable for the hydrosilicates, since actually the prereduction state shows this skin in the form of Si206 layers on top of the octahedral layers containing the Ni++ ions. That, however, even co-precipitation catalysts such as 5421 in which no hydrosilicate formation could be observed by thermal analysis show inaccessibility comes somewhat as a surprise (Fco/Fr = 0.8 46% Ni removable). 

Besides activity, durability of metal electrode nano-catalysts in acid medium has become one of the most important challenges of low-temperature fuel cell technologies. It has been reported that platinum electrode surface area loss significantly shortens the lifetime of fuel cells. In recent years, platinum-based alloys, used as cathode electrocatalysts, have been found to possess enhanced stability compared to pure Pt. The phenomenon is quite unusual, because alloy metals, such as Fe, Co and Ni, generally exhibit greater chemical and electrochemical activities than pure Pt. Some studies have revealed that the surface stmcture of these alloys differs considerably from that in the bulk A pure Pt-skin is formed in the outmost layer of the alloys due to surface segrega-... 

The free chlorine acts as a catalyst and a single chlorine atom may break down tens of thousands of ozone molecules before it returns to the troposphere. In the troposphere, chlorine reacts with hydrogen and forms hydrogen chloride that is rained out. Since ozone absorbs biologically-damaging UV radiation before it reaches the earth s surface, its depletion increases the risks associated with UV exposure. Ultraviolet radiation and over-exposure are linked with skin cancers, cataracts, and suppression of immune system response. In 1985, this problem started to attract everyone s attention by the dramatic announcement of the discovery of a hole in the ozone layer over... 

The method can also be used to measure the catalytic activity of hydrogen uptake of various compounds. The metallic indicator layer has a high affinity for hydrogen but is not able to absorb it directly due to its nonactive oxide skin. This thin oxide layer does transport hydrogen to the optically active indicator layer, once the molecular hydrogen is dissociated by a catalyst (e.g. Pd-clusters [84]). On this... 

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