Noble-metal impregnation

Since the catalytically active phase is frequently quite expensive (e.g. noble metals) it is clear that it is in principle advantageous to prepare catalysts with high, approaching 100%, catalyst dispersion Dc. This can be usually accomplished without much difficulty by impregnating the porous carrier with an aqueous solution of a soluble compound (acid or salt) of the active metal followed by drying, calcination and reduction.1... 

Usually noble metal NPs highly dispersed on metal oxide supports are prepared by impregnation method. Metal oxide supports are suspended in the aqueous solution of nitrates or chlorides of the corresponding noble metals. After immersion for several hours to one day, water solvent is evaporated and dried overnight to obtain precursor (nitrates or chlorides) crystals fixed on the metal oxide support surfaces. Subsequently, the dried precursors are calcined in air to transform into noble metal oxides on the support surfaces. Finally, noble metal oxides are reduced in a stream containing hydrogen. This method is simple and reproducible in preparing supported noble metal catalysts. 

By loading pre-existing support materials in the form of shaped bodies with the catalytically active phase by means of impregnation or precipitation from solution. This is the preferred method when catalyst precursors are expensive and the aim is to deposit the catalytically active phase in the form of nanometre-sized particles on the support. All noble metal catalysts are manufactured in this way. 

The catalyst impregnation on the dispersed CNFs must be carefully optimized to obtain a sufficient dispersion of noble metals. Dispersion ofthe CNF in the particular solvent is desirable for uniform impregnation ofthe catalyst. CNFs with very small diameter are very important for effective dispersion. Sophisticated procedures for dispersion must be applied. In the present study, nanodispersion at an impeller agitation of 16 500 rpm was applied to disperse the thin CNFs better. 

Figure 7-16 A highly simplified sketch of an automohile engine and catalytic converter with typical gas compositions indicated before and after the automotive catalytic converter. The catalytic converter is a tube wall reactor in which a noble-metal-impregnated wash coat on an extruded ceramic monolith creates surface on which reactions occur.

The second metal, for example, the promoter, may also be added by subsequent impregnation of binary sulfide. When a nonreactive promoter precursor, for example, metal nitrate, is used it is necessary to resulfide the impregnated sulfide in order to decompose the precursor. Another variation of this method consists in using reactive promoter precursors that will react with the surface of the binary sulfide. In this case, further treatment of the catalyst may not be required. Good precursors include metal carbonyls and metal alkyls (32, 33). The precursor decomposition approach been most widely applied to the MoS2-based systems. However, it has also been extended to the mixed noble-metal sulfides by Breysse and co-workers (34) at Lyon following the work of Passaretti et al (35). 

The mechanism and kinetics of pentane, hexane, and cyclohexane isomerization over Pd-H-mordenite have been extensively investigated by Bryant (6), Hopper (21), and Beecher (20). They assume a conventional dual function mechanism as described earlier. Bryant (6) pointed out that H-mordenite itself has a high activity for pentane isomerization and that impregnation of a noble metal does not change the rate of the isomerization reaction. This exceptional activity of mordenite has since been reported by Benesi (14) and Minachev (7) as well. In Mina-chev s paper the reaction mechanism of n-pentane isomerization over H-mordenite is discussed in some detail. The rate of reaction is inversely proportional to the hydrogen pressure, and it is concluded that the reaction proceeds according to the following scheme ... 

A comparative study of Ti02-supported noble metal catalysts, which were prepared by impregnation, was carried out for the oxidation of a low concentration of HCHO (100 ppm) by Zhang and He [102], As far as impregnation method is concerned, Au/ Ti02 is less active than Pt/Ti02, giving 90% HCHO conversion at 393 K and 100% conversion at room temperature, respectively. 

The amount of wash coat which was deposited in the testing reactors was in the same range, between 14 and 17 mg, for the rhodium, platinum and palladium samples tested. The platinum sample was calcined after impregnation at a lower temperature of 450 °C, all other samples at 800 °C. The reason for this will be explained below. The content of the active noble metal was around 5 wt.%. All noble metal-containing samples were laboratory-made catalysts. A commercial a-alumina-based catalyst containing 14 wt.% Ni was added for comparison, as nickel catalysts are applied in industrial steam reforming [52],... 

Preparation of Pt-TiOx/Pd membranes. It was also desirable to prepare metalloceramic membranes in which the catalytic activity of the ceramic phase was enhanced through the addition of a noble metal. The very low surface area of the titania films prepared as described above made them difficult to impregnate with adequate dispersion by traditional incipient wetness techniques. Instead, finely ground titania (>200 mesh) was impregnated with platinum via the incipient wetness method with a chloroplatinic acid solution. This powder was then sprinkled onto the surface of a freshly dipped membrane, which was dried and heat treated as described. These materials were activated before use at 350°C in hydrogen for three hours. 

The catalyst plays a crucial role in technology. Previously, catalysts were based on palladium of 1 to 5 wt% impregnated on silica with alkali metal acetates as activators. Modern catalysts employ as enhancers noble metals, mostly gold. A typical Bayer-type catalyst consists of 0.15-1.5 wt% Pd, 0.2-1.5 wt% Au, 4-10 wt% KOAc on spherical silica particles of 5 mm diameter [14], The reaction is very fast and takes place mainly inside a thin layer on the particle surface (egg-shell catalyst). 

Aluminas are used in various catalytic applications, a-, y-, and -aluminas are all used as support materials, the first one in applications where low surface areas are desired, as in partial oxidation reactions. The latter two, and especially y-alumina, in applications where high surface areas and high thermal and mechanical stability are required. One of the most prominent applications of y-alumina as support is the catalytic converter for pollution control, where an alumina washcoat covers a monolithic support. The washcoat is impregnated with the catalytically active noble metals. Another major application area of high-surface aluminas as support is in the petrochemical industry in hydrotreating plants. Alumina-supported catalysts with Co, Ni, and/or Mo are used for this purpose. Also, all noble metals are available as supported catalysts based on aluminas. Such catalysts are used for hydrogenation reactions or sometimes oxidation reactions. If high... 

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