Dispersions, noble metals

Although obviously less expensive electron conducting catalyst supports have to be sought for practical applications, this study has clearly established the technical feasibility of inducing NEMCA on finely dispersed noble metal catalysts. 

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. 

On the other hand, as already discussed in Chapter 11 in connection to the effect of metal-support interactions, it appears that a fully dispersed noble metal catalyst on porous YSZ is already at a NEMCA or electroche-mically-promoted state, i.e. it is covered by an effective double layer of promoting backspillover O2 ions. This can explain both the extreme catalytic activity ofZr02- and Ti02- supported commercial catalysts, as well as the difficulty so far to induce NEMCA on fully dispersed noble metal catalysts deposited on YSZ. 

The selective catalytic oxidation over highly dispersed noble metal catalysts in aqueous media is gaining interest (1-5). One of the main problems consists of the deactivation of the catalyst. The role of oxygen is recognized as being crucial in this matter (6,7). 

Regarding the preparation of praseodymia and teibia supported metal catalysts, the information available is rather scarce. All the reported studies have dealt with dispersed noble metal samples. Though metal vapor deposition has been applied in some cases (231), the impregnation techniques have coirstituted the most usual preparation procedure. Chlorine-containing (53,82,85,127,175,278), and chlorine-free (53,84,232,278) metal precursors have been used. As already reported, PrOCl and Tb(3CI have been identified in praseodymia and terbia supported catalysts prepared from chlorinated precursors (82). Water (82,85,127,175), and non-aqueous solvents. 

The application of transition metal oxide monolayer/support systems as carriers for platinum allows the possibility of modification of the properties of platinum in a wide range [5]. According to our previous works we adopted an optimal loading of platinum (2 %w). The reaction of platinum compound with OH groups of the support proceeded to the formation of transition metal-O-Pt bond, according to the reaction (3) - a useful tool for obtaining well dispersed noble metal on the surface of inorganic support [6,7]. 

A = electrochromism B = photoelectric conversion C = electrogenerated chemiluminescence D = highly dispersed noble metal E = catalyst F = high mechanical strength conducting polymer, G = charge-controllable transport membrane H = sensor. 

Roumanie et al. tested catalysts in a chip-like silicon microreactor for methylcy-dohexane dehydrogenation [280]. A platinum/alumina catalyst achieved 88.5% conversion, while a platinum film sputtered onto black silicon showed only 2% conversion. Low activity is frequently observed for non-dispersed noble metal surfaces. 

Adsorption and catalytic oxidation may be coupled in a vay simple way by using catalysts lowing a hi physical affinity fi>r the compound to be diminated which is the case when using ujasite zeolites [2]. Catalytic oxidation is even more eGGUaent when impregnated with dispersed noble metals like platinum or palladium [3-5]. A previous work [2] presents adsorption and diffiision data on impregnated zeolites but complete studies presenting adsorption and catalytic results for tins kind of materials are still rare. 

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