Evaporated metal films Catalysts, forms

Metal hydrogenation catalysts may be employed in any one of a variety of forms (a) macroscopic forms as wires, foils or granules (b) microscopic forms as powders obtained by chemical reduction, colloidal suspensions, blacks or evaporated metal films (c) supported catalysts where varying concentrations of metal are dispersed to a varying degree on a carrier such as alumina, silica or carbon. 

Complete analyses of the various deutero-ethylenes and deutero-ethanes were first obtained by Turkevich et at. (4), using a nickel wire, and this type of information has since been reported by Wilson et al. 5) for a bulk nickel catalyst and by Kemball 6) for a series of evaporated metallic films. In all cases, the ethanes produced range over the complete spectrum from do-ethane to do-ethane, and Kemball showed that it is possible to correlate the nature and the amount of the deutero-ethylenes formed over metallic films with the distribution of deuterium in the ethanes. Although the detailed mechanisms of the exchange and deuteration of ethylene are still the subject of controversy, it is clear that both are closely related and both involve the half-hydrogenated state, i.e., the adsorbed ethyl radical, as an intermediate. The main objective of the present research was to extend such studies to the benzene-deuterium system. 

Butadiene hydrogenation has been studied over most transition metals in the form of evaporated thin films6,7 and where comparisons can be made similar results are reported as for supported metal catalysts under similar conditions. Hence the mechanisms described can be considered to be reasonably widely applicable. A further interesting point that can be gleaned from the evaporated film experiments is that the yield of but-l-ene increases with Pauling electronegativity of the metal.7 This correlation was shown to apply for all three transition series. Again, this can be rationalised by the above mechanisms in that but-l-ene yield is determined by the electronic... 

Although insulators other than aluminum oxide have been tried, aluminum is still used almost universally because it is easy to evaporate and forms a limiting oxide layer of high uniformity. To be restricted, therefore, to adsorption of molecules on aluminum oxide might seem like a disadvantage of the technique, but aluminum oxide is very important in many technical fields. Many catalysts are supported on alumina in various forms, as are sensors, and in addition the properties of the oxide film on aluminum metal are of the greatest interest in adhesion and protection. 

Finely divided metal samples can also be prepared in the form of evaporated films in high vacuum, usually deposited on IR-transparent alkali halide plates (76-78). Such spectra are of interest in themselves, but tend to be much weaker than those obtained from the metal-particles-in-depth, oxide-supported catalysts. The rough surfaces of films of Cu, Ag, and Au, prepared by deposition on cold surfaces, can lead to very high-quality surface-enhanced Raman spectra (27, 28, 79, 80). The results from such experiments will be discussed in the later sections devoted to particular adsorbed hydrocarbons and metals, alongside the majority of spectra that are obtained on oxide-supported samples. 

In his lecture Taylor emphasised both the phenomenological similarities and differences that existed between adsorption processes on technical catalysts and those observed with idealised catalysts produced by the Beeck evaporated film method to form oriented or non-oriented metal surfaces. These were inherent to the two schools of surface chemistry that had emerged, there were those who studied technical catalysts (Eucken, Emmett, Brunauer) and those who favoured the clean surface approach, where particular attention was given to vacuum conditions and surface preparation and exemplified by Beeck in Emeryville and Roberts in Cambridge. Kinetic studies were dominant with Trapnell, Tompkins and Kemball following the evaporated film approach in the 1950s and sixties. " ... 

In conjunction with their studies of evaporated barium gettei film, Oda and Tanaka (97) investigated the relationships between the structuie of a nickel film evaporated on a glass plate and the conditions of its preparation. These, nickel films had a remarkable tendency to expose the (110) plane with increasing thickness even if made in a high vacuum. When the support on which the nickel vapor was condensed rvas heated, various kinds of crystal planes were observed to develop parallel to the support as a function of the temperature, e.g., the (110) plane at 100°C., the (110) plane and (200) plane at 200°C., and the (200) plane at 300°C. A non-oriented surface was formed at 350°C. From this, it seems reasonable to conclude, that even ordinary metallic catalysts, including carrier-supported catalysts, may preferentially expose crystal planes of various kinds, depending on their mode of preparation. 

Initially, the titanium substrate was eleaned and placed in an oven at 480 °C in air to form a titanium ojgmitride layer that acts as a diffusion barrier for the metal eatalyst (Fe, Ni, Co, Mo or alloys). The metal catalyst was then deposited either by eleetron-beam evaporation of a 10 nm film of (Ni or Fe) or by immersion in ethanolie solution of metal (Fe, Ni or Co) nitrate. Before growth, the substrates are exposed to a plasma of N2 (20 seem) and H2 (80 seem) at 30 Torr for 5 minutes after the substrate reaehes 760 °C. VACNT growth then proeeeds by adding CH4 (14 seem) to the N2/H2 gas mixture. The growth of VACNT was performed for 2 minutes at 30 Torr and 760 °C, with temperature measured on the backside of the substrate. 

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