Metal powders, high-area

Samples of high area powders and of supported metals may be applied to the CaF2 support plate by a spraying technique, previously described In detall(ll). In Figure 1, we show a half plate design In which a supported metal deposit, produced by H2 reduction of metal Ions held on the support, occupies one half of the plate while the pure support occupies the other half. 

Nonmetal electrodes are most often fabricated by pressing or rolling of the solid in the form of fine powder. For mechanical integrity of the electrodes, binders are added to the active mass. For higher electronic conductivity of the electrode and a better current distribution, conducting fillers are added (carbon black, graphite, metal powders). Electrodes of this type are porous and have a relatively high specific surface area. The porosity facilitates access of dissolved reactants (H+ or OH ions and others) to the inner electrode layers. 

The pure metals are readily available to try out as catalysts, whereas the alloys are not, if a moderate surface area is required. Methods for preparing alloys as high-area powders, etc., raise questions about the unwanted introduction of promoters, e.g., chloride ions. Here, again, evaporated alloy films recommend themselves for the exploration of a whole new territory of alloy systems as catalysts in a variety of reactions. 

In the case of solid interfaces which are in the form of coarse powders, cavitation collapse can produce enough energy to cause fragmentation and activation through surface area increase. For very fine powders the partides are accelerated to high velocity by cavitational collapse and may collide to cause surface abrasion (Fig. 3.5). For some metal powders these collisions generate sufficient heat to cause particle fusion. 

When one component of a bimetallic alloy is leached out, a finely divided metal powder of high surface area results. One of the oldest of these so-called skeletal metal catalysts is Raney nickel10,11. Nickel boride is a more recently developed hydrogenation catalyst prepared by the reduction of nickel salts with sodium borohydride12-14. Bimetallic catalysts are often used to achieve selective saturation of a double bond in bifunctional unsaturated systems, e.g. in dienes. Amorphous metal alloys, a newly developed class of metal catalysts15,16, have also been applied in the hydrogenation of alkenes and dienes. 

The reactivity of oxide supported metals has received considerable attention because of the importance of such systems in heterogeneous catalysis. The morphology (structure and size) of the supported particle and its stability, the interaction of the particle with the support, and the crossover of adsorbed reactants, products and intermediates between the metal and oxide phases are all important in determining the overall activity and selectivity of the system. Because of the relative insensitivity of an optical technique such as IR to pressure above the catalysts, and the flexibility of transmission and diffuse reflection measurement techniques, vibrational spectroscopy has provides a considerable amount of information on high area (powder) oxide supported metal surfaces. Particularly remarkable was the pioneering work of Eichens and Pliskin [84] in which adsorbed CO was characterised by IR spectroscopy on... 

It should be possible to use compressed metal powders as, for example, a catalytic filter, but the present authors are not aware of any industrial examples of this. Raney metals powders, however, have been employed in some liquid-based processes in shallow beds through which reactants pass. Because Raney metals are fine grained, pressure drop can be a problem so it is more common to use them in an unstructured way in slurry reactors, as, for example, formerly in the oils and fats industry [3]. Raney metals can have high surface areas when freshly prepared, but this decreases quickly in use, particularly when exposed to elevated temperatures. Pressure drop considerations are less significant for beds of metal granules, but there is less effective use of metal than with fine powders. For granules, surface areas in the region of 30-35 cm g are typical for silver used in... 

Unsupported metals are found in a variety of forms wires, ribbons, single crystals, colloids, powders, blacks and the so called skeletal species. A black is a metallic powder obtained by reduction of a metal salt or by condensation of a metal vapor. Such materials are also sometimes referred to as powders. The blacks and powders are usually composed of relatively large particles having a low surface area. Skeletal metals are produced by leaching out one component of an alloy and leaving the active species behind in the form of a porous material having a high surface area.2... 

Reisse and co-workers [147-149] were the first to describe a novel device for the production of metal powders using pulsed sonoelectrochemical reduction. This device exposes only the flat circular area at the end of the sonic tip to the electrodeposition solution. The exposed area acts as both cathode and ultrasound emitter, named by Reisse et al. as sonoelectrode . A pulse of electric current produces a high density of fine metal nuclei. This is immediately followed by a burst of ultrasonic energy that removes the metal particles from the cathode, cleans the surface of the cathode, and replenishes the double layer with metal cations by stirring the solution. In [145], a list is given of chemically pure fine crystalline powders, mostly metals or metallic alloys, prepared by this method, with particle sizes varying between 10 and 1000 run depending on deposition conditions. 

ES S studies of catalysts are described in Chapter 7. These experiments present a particular set of problems that need to be addressed. Foremost is the fact that INS is not an intrinsically surface sensitive technique. This is overcome by using large samples to maximise the number of surface sites and hydrogenous adsorbates to give the highest possible contrast. Supported metal catalysts, zeolites and oxides often have low densities, while metal powders have high densities but low surface areas. Both situations require a large volume cell to place sufficient sample in the beam. 

In the electrochemical studies reported so far, NMR has been applied as an ex situ technique, where a powdered metal is used as an electrode in an electrochemical cell and then the metal powder is transferred, usually with electrolyte, to a NMR sample tube for observation (151-154). For example, the formation of surface CO from methanol on Pt was studied (153). High-surface-area Pt (24 m /g) was placed in a Pt boat that served as the working electrode, and a solution of 0.1 M C-enriched methanol in 0.5 M sulfuric acid was used as the electrolyte. The electrode was held at the desired potential, then a 0.2 g sample of the Pt was removed, mixed with glass beads, and placed in a glass NMR sample tube. The spectrum showed the presence of about 10 spins in the form of CO. So far only special purpose NMR instruments have been used in such studies. 

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