Hydrogen reduction metal deposition

The catalyst is also employed in the form of the finely-divided metal deposited upon activated carbon (usually containing 5 or 10 per cent. Pd) two methods of preparation are described, in one reduction is effected with alkaline formaldehyde solution and in the other with hydrogen ... 

Hydrogen reduction has a major advantage in that the reaction generally takes place at lower temperature than the equivalent decomposition reaction. It is used extensively in the deposition of transition metals from their halides, particularly the metals of Groups Va, (vanadium, niobium, and tantalum) and Via (chromium, molybdenum, and tungsten). The halide reduction of Group IVa metals (titanium, zirconium, and hafnium) is more difficult because their halides are more stable. 

The hydrogen reduction of the metal halides, described in Sec. 1.2, is generally the favored reaction for metal deposition but is not suitable for the platinum-group metals since the volatilization and decomposition temperatures of their halides are too close to provide efficient vapor transport. 1 1 For that reason, the decomposition of the carbonyl halide is preferred. The exception is palladium which is much more readily deposited by hydrogen reduction than by the carbonyl-halide decomposition. 

Bonding by CVD is a relatively simple process whereby a layer of boron is deposited on a metal substrate, followed by heat treatment.P] The boron can be deposited by the hydrogen reduction... 

Borides of Group IVa. UB2, ZrB2, and HfB2 are readily deposited by the hydrogen reduction of the metal halide, usually the chloride. Atypical reaction is as follows ... 

Molybdenum. Molybdenum is another refractory metal with low resistivity (5-7 iohm-cm) now under investigation for metallization of IC s. It is usually deposited by the decomposition of the carbonyl, Mo(CO)6, or by the hydrogen reduction of the halide (M0CI5 or MoFg). These reactions are described in Ch. 6. 

More recently, Ikeda et a/.108 have examined C02 reduction in aqueous and nonaqueous solvents using metal-deposited p-GaP and p-InP electrodes under illumination. Metal coatings on these semiconductor electrodes gave much improved faradaic efficiencies for C02 reduction. In an aqueous solution, the products obtained were formic acid and CO with hydrogen evolution at Pb-, Zn-, and In-coated electrodes, while in a nonaqueous PC solution, CO was obtained with faradaic efficiencies of ca. 90% at In-, Zn-, and Au-coated p-GaP and p-InP, and a Pb coating on a p-GaP electrode gave oxalate as the main product with a faradaic efficiency of ca. 50% at -1.2 V versus Ag/AgCl. 

In this paper we report the application of bimetallic catalysts which were prepared by consecutive reduction of a submonolayer of bismuth promoter onto the surface of platinum. The technique of modifying metal surfaces at controlled electrode potential with a monolayer or sub-monolayer of foreign metal ("underpotential" deposition) is widely used in electrocatalysis (77,72). Here we apply the theory of underpotential metal deposition without the use of a potentiostat. The catalyst potential during promotion was controlled by proper selection of the reducing agent (hydrogen), pH and metal ion concentration. 

When metal ion M"+ is deposited by the controlled-current method, the electrode potential during the electrolysis changes in the order T, 2, 3, 4, 5, 6 in Fig. 5.33 and the next reduction process occurs near the end of the electrolysis. If the solution is acidic and the next reduction process is hydrogen generation, its influence on the metal deposition is not serious. However, if other metal is deposited in the next reduction process, metal M is contaminated with it. In order that two metal ions M"1+ and M "21 can be separated by the controlled-current method, the solution must be acidic and the reduction of hydrogen ion must occur at the potential between the reductions of the two metal ions. An example of such a case is the separation of Cu2+ and Zn2+ in acidic solutions. If two metal ions are reduced more easily than a hydrogen ion (e.g. Ag+ and Cu2+), they cannot be separated by the controlled-current method and the controlled-potential method must be used. 

The Bi-promoted catalysts were prepared by consecutive deposition of Bi onto a commercial 5 wt% Pt/alumina (Engelhard E 7004, Pt dispersion 0.30 determined by TEM) [7]. The reduction of bismuth-nitrate was carried out in a dilute aqueous acidic solution O10"6 M, pH = 3-4) by hydrogen. The metal content of the catalysts was determined by inductive coupled plasma atomic emission spectroscopy (ICP-AES). Preferential deposition of Bi onto Pt particles has been confirmed by TEM, combined with energy dispersive X-ray analysis (EDX) [7]. Pb-, Sn- and Ag-promoted catalysts were prepared similarly. 

Under these conditions the metal ions play the role of hydrogen atoms, as above explained. They discharge themselves in the cathode boundary surface and, depending upon their reaction velocities, affect the reduction of the depolarizer and the metallic deposition. With a great reduction velocity, therefore, no metal whatever is deposited on the cathode so long as sufficient quantities of the depolarizer are present.1... 

In bimetallic catalysts prepared by catalytic reduction of copper by hydrogen, copper is deposited as three-dimensional agglomerates which are located, at low copper loadings, on the edges, corners, and rims of the parent metallic particles. The mechanism of deposition can be transferred from that proposed in corrosion and involving a local electrochemical cell ... 

CFD [Chemical Fluid Deposition] A process for depositing thin films on solid surfaces by a chemical reaction in a liquid such as supercritical carbon dioxide. Superior to CVD in being capable of operation at almost ambient temperatures. Demonstrated for depositing platinum metal on silicon wafers, polymer substrates, and porous solids by hydrogen reduction of an organo-platinum compound at 80°C. 

Otherwise changes in metal deposition behavior with pH could be involved. Due to the competition between reduction of metal ions and hydrogen ions at the cathode the pH affects metal deposition. The current efficiency70 of nickel deposition was seen to decrease markedly below pH 2 in the presence of SiC particles. Unfortunately, it was not determined if this effect is accompanied by a decrease in particle content below pH 2. 

An example of the use of the technique to obtain information during a cyclic voltammagram is shown in Fig. 113 for n-ZnO [192], As the crystal was cycled to very negative potentials, a large cathodic current was found due both to hydrogen evolution and Zn-metal deposition from the reductive decomposition of the ZnO. Re-oxidation of the Zn metal is seen as an anodic peak in the CV, but it is very difficult, in the presence of both cathodic and anodic currents, to form a clear impression from the CV of the amount of Zn... 

Several transition metals such as V, Nb, Ta, and Pd can form stable bulk hydrides, so-called interstitial hydrides the bonding in the hydride phase is not ionic but mostly metallic in character, and the hydrogen to metal ratio is not necessarily stoichiometric. Especially, nanoparticles of noble metals such as Pd are relatively easy to prepare by various methods, such as vapor phase deposition on substrates, reductions of salts in solution (electrochemically or electroless), and the inverse micelle templated growth. They are not easily oxidized, and, in recent years, several methods have been developed to precisely control the size of the particles or clusters. Furthermore, growth in solution in the presence of surfactants and stabilizers allows control over the shape of the final particles [35, 36, 42]. 

R.L. Van Hemert, L.B. Spendlove, and R.E. Sievers, Vapor Deposition of Metals by Hydrogen Reduction of Metal Chelates, Journal of Electrochemical Society, Vol.112, 1965, pp.l 123-1126. 

Again, the degree of deactivation can be expected to depend on the temperature and conversion level, as expressed by Equations 4 and 5. Indeed, coke profiles over isothermal laboratory reactors (72) show such differences, primarily due to a reduction in hydrogen partial pressure. Metals deposition over residue catalysts beds show a decrease with conversion simply because of depletion of the reactant 2,13,14). 

The electroless deposition of metals on a silicon surface in solutions is a corrosion process with a simultaneous metal deposition and oxidation/dissolution of silicon. The rate of deposition is determined by the reduction kinetics of the metals and by the anodic dissolution kinetics of silicon. The deposition process is complicated not only by the coupled anodic and cathodic reactions but also by the fact that as deposition proceeds, the effective surface areas for the anodic and cathodic reactions change. This is due to the gradual coverage of the metal deposits on the surface and may also be due to the formation of a silicon oxide film which passivates the surface. In addition, the metal deposits can act as either a catalyst or an inhibitor for hydrogen evolution. Furthermore, the dissolution of silicon may significantly change the surface morphology. 

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