Catalysis metal deposition

The chemical and electronic properties of elements at the interfaces between very thin films and bulk substrates are important in several technological areas, particularly microelectronics, sensors, catalysis, metal protection, and solar cells. To study conditions at an interface, depth profiling by ion bombardment is inadvisable, because both composition and chemical state can be altered by interaction with energetic positive ions. The normal procedure is, therefore, to start with a clean or other well-characterized substrate and deposit the thin film on to it slowly at a chosen temperature while XPS is used to monitor the composition and chemical state by recording selected characteristic spectra. The procedure continues until no further spectral changes occur, as a function of film thickness, of time elapsed since deposition, or of changes in substrate temperature. 

Although electroless deposition seems to offer greater prospects for deposit thickness and composition uniformity than electrodeposition, the achievement of such uniformity is a challenge. An understanding of catalysis and deposition mechanisms, as in Section 3, is inadequate to describe the operation of a practical electroless solution. Solution factors, such as the presence of stabilizers, dissolved O2 gas, and partially-diffusion-controlled, metal ion reduction reactions, often can strongly influence deposit uniformity. In the field of microelectronics, backend-of-line (BEOL) linewidths are approaching 0.1 pm, which is much less than the diffusion layer thickness for a... 

Iron carbonyls have been also used to fabricate nanostructures of potential use in catalysis. In this context, the preparation at room temperature of nano-sized a-Fe single crystals over carbon micro-grid films has been reported. The particles were prepared by electron beam induced deposition using Fe(CO)s as precursor [77]. The use of a focused electron beam to induce metal deposition from carbonyl compounds opens a new route for the preparation of nano-sized metal particles. 

Catalyst particles generally consist of a metal deposited onto the surface of a support and are denoted by metal/support, e.g. Pd/C indicates palladium metal on a carbon support. Among the metals used for catalysis, Pd is often found to be the most active metal. (Augustine 1965) For example, in the aqueous hydrodechlorination of 1,1,2-trichloroethane, Pd catalysts achieved significantly more conversion than Pt or Rh catalysts. (Kovenklioglu et al. 1992) Catalyst supports can vary in shape, size, porosity and surface area typical materials include carbon, alumina, silica and zeolites. 

However, in heterogeneous catalysis, metals are usually deposited on nonconducting supports such as alumina or silica. For such conditions electrochemical techniques cannot be used and the potential of the metallic particles is controlled by means of a supplementary redox system [8, 33]. Each particle behaves like a microelectrode and assumes the reversible equilibrium potential of the supplementary redox system in use. For example, with a platinum catalyst deposited on silica in an aqueous solution and in the presence of hydrogen, each particle of platinum takes the reversible potential of the equilibrium 2H+ + 2e H2, given by Nemst s law as... 

From the standpoint of daily capacity, the greatest application of fluidized bed catalysis is to the cracking of petroleum fractions into the gasoline range. In this process the catalyst deactivates in a few minutes, so that advantage is taken of the mobihty of fluidized catalyst to transport it continuously between reaction and regeneration zones in order to maintain its activity some catalyst also must be bled off continuously to maintain permanent poisons such as heavy metal deposits at an acceptable level. 

One of the most intensively studied interfaces is the electronic conductor/ionic conductor where the interest is motivated by attempts to prevent corrosion and to improve the catalytic properties of metallic deposition. Both corrosion prevention and catalysis development can be described using an electrochemical and engineering approach, including film formation and growth and its optimization in the cell reactor. 

That the adsorption of the cyclopentane ring seems to proceed mainly flatly in deuterium exchange on films has been stated above (see Section I,D). Of considerable interest are the investigations on asymmetric catalysis initiated by Schwab et al. (273). In their work, one of the optical isomers reacted a little faster than the other in a racemic mixture. Terent yev and Klabunovskii (274, 273) carried out the catalytic asymmetric synthesis from optically inactive substances. The reactions were accomplished on metals deposited on dextro- and levorotatory quartz. Organic optically active carriers and admixtures give a still greater effect. On this problem see Klabunovskii (276). At the present time still more active catalysts for the reaction of asymmetric hydrogenation and polymerization have been revealed (277-279). 

Another complicating factor is that the surface of quartz is the only asymmetric factor in the metal-quartz catalysis and its specific areas in most cases were very small (only 44 cm /g " ). This coupled with the fact that the amount of metal deposited on quartz was rather high, so the extent of racemization of butanol during reaction would be high, which detracts from the effectiveness of the catalyst. Thus, quartz appeared not to be an effective chiral carrier for catalysis or adsorption in asymmetric experiments. Nevertheless, in general the data using quartz crystals are of interest and received positive evaluations in several publications... 

The product D is produced by the transition-metal homogeneous catalysis in at least one catalytic cycle. Contributions to the rate of formation of D may also arise from stoichiometric reactions or heterogeneous catalytic reactions on suspended colloidal metal, suspended metal particles, or metal deposited on the walls of the vessel, etc. The total rate of reaction is noted (mol/s). Since the system is time dependent, and in particular due to the need to transform the catalyst precursor to intermediates, the maximum rate of product formation Pd occurs at t > to-... 

M. Baumer, J. Libuda, and H. Freund, Metal deposits on thin well ordered oxide films Morphology, adsorption and reactivity. In Lambert, R. M., Pacchioni, G. Chemisorption and Reactivity on Supported Clusters and Thin Films Towards an Understanding of Microscopic Processes in Catalysis. Boston Kluwer Academic Publishers, NATO ASI Series E Applied Sciences-Advanced Study Institute, vol. 331, pp. 61-104, 1997. 

Finally, attention has been drawn to currents of hydrogen evolution which in the presence of some substances occurs at more positive potentials than in their absence. Such substances catalyze hydrogen evolution and result in high currents which are denoted catalytic hydrogen waves. Such waves are observed in the presence either of platinum group metals (57,58), where the catalysis is attributed to clusters of metals deposited on mercury or of compounds which possess acid-base properties. Catalytic effects of the latter type in solutions of simple buffers have been observed for low molecular weight compounds (59,60), as well as for proteins (61,62). Similar catalytic effects in ammoniacal cobalt (III)-solutions (63) found utilization in Brdicka reaction (64-66), used in cancer diagnosis. 

A typical electroless plating solution is composed of a cation provider such as nickel sulfate, a reducing agent such as ammonium hypophosphite, and additional additives tiiat help prevent the bath from decomposition, i.e., plating spontaneously. When an activated substrate is immersed in the plating bath at a temperature of 80 C and a pH around 6, nickel cation in the bath are reduced by hypophosphorous acid, and the nucleation of nickel deposition starts at the activated locations. Because nickel readily plates to itself (self-catalysis), the deposition continues and eventually fills the via locations in the dielectric with nickel metal. The reduction reaction can be expressed by the following equations ... 

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