Catalyst powder deposition

Catalyst powder deposition [113] Bevers et al. described fliis method, in which all the powder components are mixed in a fast running knife mill and then are applied onto the carbon matrix. 

Catalyst powder deposition In catalyst powder deposition described by Bevers et al. (1998) the components of the catalytic layer (Vulcan XC-72, PTFE powder, and a variety of Pt/C loadings) are mixed in a fast mnning knife mill under forced cooling. This mixture is then applied onto a wet-proofed carbon cloth. Also applying a layer of carbon/PTFE mixture flattens out the roughness of the paper and improves the gas and water transport properties of the MEA. 

One should distinguish between true catalyst supports and diluents. A catalyst support (or carrier) is a material on which a thin layer of catalyst is deposited a diluent is an inert material thoroughly mixed with the catalyst to enhance the binding properties of a powdered catalyst or to assist in pelleting or extrusion fabrication procedures. 

Fig. 23. ATR spectra (left) of a 5% Pd TiO catalyst. The powder catalyst was deposited on a ZnSe IRE. Toluene saturated with various gases then flowed over the sample. At the beginning the palladium was oxidized. First, toluene saturated with argon flowed over the catalyst (bottom left, bottom spectrum). Then the flow was switched to hydrogen-saturated toluene, which led to reduction of the palladium (bottom, left). Afterwards, the flow was switched to oxygen-saturated toluene (top. left). The right graph shows the absorbance at 1700 cm" as a function of time during the treatments 49).

Fodisch et al. [156] applied electrophoretic precipitation of industrial catalyst powders at 100 V (DC). After 2 min, uniform deposition of the catalyst powder on the surface was achieved. As an alternative to impregnation methods, palladium was deposited... 

A further complication that can detract from the validity of quantitative XPS for the characterization of coke deposits Is the mobility of coke components. After Introduction into ultra-high vacuum this mobile fraction may migrate from the inside of catalyst powder grains to their boundary as analysed by XPS. For certain catalyst systems this phenomenon has been observed. It can be easily identified however since the signal intensity ratio for the coke versus support increases with time the catalyst spent in the spectrometer. For the catalysts in this study no change of the signal ratio with time in the measurement chamber was observed. 

Catalysts were obtained by pressing powders imo tablets and gently breaking the tablet. The fraction between 500 and 800 micrometers was used for the catalytic tests. The catalytic tests were carried out in convetnional fixed bed reactors at atmospheric pressure. Catalyst was deposited between two layers of glass balls in a pyrex U-iube of 8 mm internal diameter. 

Excess solution (Fp < Vg) is used for synthesis of powdered and granulated supported catalysts, if the adsorption is very intense, as well as for the preparation of powdered deposited catalysts. 

The catalyst is deposited on the walls of monolith channels as a slurry, which is then dried and calcined. Preparation of the slurry is an exacting procedure, with pH, particle size and solids content rigidly controlled. The optimal values of these parameters must be determined for each formulation. Without a high degree of adhesion to the channel wall, the catalyst can easily be turned to powder and blown out of the system, sometimes on vehicles in production. Such crises are not infrequent in the industry and must be attended to immediately. Sometimes a simple adjustment, such as changing the particle size, is effective. Several binding agents are commercially available and are often used. 

Electrophoretic deposition is a colloidal process used to coat either aluminium or stainless steel [122]. Either adhesion layers or complete coatings can be achieved. To-date mainly alumina suspensions have been used for electrophoretic deposition. Wunsch et al. coated microchannels with alumina nanopartides dissolved in oxalic add and mixed with alumina gd or glycerol [124] 2-4 pm thick adhesion layers were formed. Fbdisdi et al. applied dectrophoretic predpitation of industrial catalyst powders at 100 V (dc) [140]. After 2 min a uniform deposition of the catalyst powder on the surface was achieved. 

Self-supporting pressed discs of the pure oxide powders are prepared for in situ characterisation studies by transmission/absorption IR spectroscopy. These samples are put onto the IR beam, in an appropriate cell allowing heating, cooling, and gas/vapour manipulation. Activation is mostly performed by outgassing at relatively high temperatures. In the case of diffuse reflectance infrared Fourier transform (DRIFT) experiments the pure catalyst powder is deposited on the sample holder, with smooth pressure, and activation is mostly performed by an inert, dry gas flow. 

Samples were structurally characterised in a JEOL 2000 EX high resolution electron microscope operating at 200 kV. The catalyst powders were made suitable for TEM examination by grinding them in high purity ethanol using an agate pestle and mortar. A drop of the suspension was then deposited onto, and allowed to evaporate, on a holey carbon grid. 

Petrii and Entin [54, 57, 78, 150, 180, 243] investigated the adsorption and electrochemical characteristics of platinum and ruthenium alloys in detail. The optimum alloy compositions for oxidation of methanol at various temperatures were determined, the anodic oxidation reactions of various compounds of the alloys were investigated, the stability of the alloys after prolonged use was investigated, and the characteristics of platinum— ruthenium alloys prepared by various methods (skeletal electrodes, electro-lytically mixed deposits on a platinum and titanium carbide bases, powders deposited by sodium borohydride, smooth alloys) were compared. It was found that heat treatment of platinum - ruthenium alloys at 800 C in an atmosphere of inert gas led to loss of their high catalytic properties and formation of catalysts which behave similarly to platinum. This phenomenon is explained by diffusion of ruthenium atoms from the surface layer into the volume. 

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