Incipient wetness metal dispersions

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. 

In impregnation the pores of the support are filled with a solution of the gold precursor. Where just the needful volume is used, the method is impregnation to incipient wetness, but sometimes an excess of solution is used and the solvent removed by evaporation, so that concentrated solution finally resides in the pores (Sections 4.2.1 and 4.3.1). In these methods, the metal dispersion ultimately obtained depends critically on the conditions of drying.1... 

The experimental conditions used to prepare the supported salt can have an impact on the dispersion and location of the metal in the resulting catalyst. The dispersions of Pt/Si02 catalysts prepared by incipient wetness using aqueous solutions of chloroplatinic acid are dependent more on the surface area of the support than on the average pore size distribution. There appears to be some pore blockage during the impregnation but this is somewhat reversed by calcination... 

Palladium was added to unmodified zirconium oxide and supports A and B by incipient wetness, using an aqueous solution of Pd(N03)2 as precursor, in order to obtain solids with 1% wt palladium in all the cases. The solids produced according to this procedure will be called here catalysts U, A and B, respectively. PdCfe was not used as precursor, as chlorine anions are considered poisons for the catalysts, so a wash step is necessary to eliminate them when PdCb is us as precursor. On the other hand, it is known that Pd(N03)2 leads to catalysts with higher crystallite size and lower metal dispersion, although they are more active for the oxidation of methane [7]. The catalysts thus prepared were dried overnight at 100 C after impregnation, and then calcined in air at 550°C for 2 h. 

Incipient wetness produces catalysts with higher dispersions than precipitation. The metal dispersion decreases with the severity of the oxidation. This treatment also decreases the available surface area. It is hence logical to observe lower metal dispersions for more severe oxidations. Soft oxidation seems to be the best compromise between creation of acidic functions on the support and high dispersion values. 

Soft oxidation of the support also leads to a slight increase of the metal dispersion if the catalyst is prepared by precipitation. However, during this procedure, the previously created acidity is neutralized, but it probably plays a role in anchoring the palladium on the support surface, before its precipitation under hydroxide form. When incipient wetness impregnation is considered to prepare catalysts, very high metal dispersions are obtained if the support is not or is softly oxidized, or severely oxidized but for a very short time. Increasing the severe oxidation of the support leads to decrease the metal dispersion, probably through the drastic decrease of the surface area. 

Supported nickel catalysts are widely used for hydrogenation reactions. The most common supports are silica and activated carbon, however several studies have shown that clays are rather more effective in maintaining high metal surface area and can impart useful selectivity to the catalyst.18 Deposition of nickel(II) by the incipient wetness technique [in which the volume of nickel(II) solution used is equal to the pore volume of the clay and then dried] followed by H2 reduction was found to be the most effective method of generating a high dispersion of metal particles (< 10 nm in size) within the clay. 

Incipient wetness impregnation is by far the most widely used method for the preparation of heterogeneous catalysts. This method is attractive because of its technical simplicity, low costs and limited amount of waste. Generally, a support is impregnated with a precursor-containing solution and dried. The dry product is then further treated through activation treatments (e.g. calcination and/or reduction) to obtain the desired catalyst. It is important that optimal synthesis parameters are found, since the efficiency of a catalyst is defined by the size of the active metal(oxide) particles, and their accessibility and distribution over the support. It is well-known that the use of metal nitrates as precursor salts for cobalt, iron and nickel catalysts ultimately yields poorly dispersed catalysts [1-3]. This is regrettable, as the use of nitrates as precursor is... 

The hgure also shows that loading the carbon with hnely dispersed palladium has a further accelerating effect on surface oxidation, which is most pronounced with the carbon black. This can be explained by dissociative adsorption of O2 molecules on the metal surface and oxygen spillover. Platinum had a similar effect. Palladium (200 p.mol/g) was deposited on the carbons by incipient wetness impregnation followed by H2 reduction at 523 K the dispersion was about 25% with Norit and 15% with Corax 3. 

Carbon fibrils can be produced rather easily, e.g., by exposing supported, finely dispersed iron or nickel particles to reducing carbon containing gas flows. To this end, one has to produce first finely dispersed iron or nickel particles on a support material, such as alumina or silica. The desired catalyst can be prepared, e.g., by incipient wetness impregnation of the support material with a suitable metal salt solution or by means of homogeneous deposition-precipitation of the metal ions onto the carrier. 

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