Noble metal-based composites

As described earlier, the DSA electrode is typically a valve metal substrate coated with a noble-metal-based composition. Either to circumvent the basic Beer patents or to achieve better and less expensive anode compositions, considerable efforts have been devoted to the development of novel anode coatings. A wide variety of compositions [14] have been described in the literature, and it is beyond the scope of this chapter to review these developments individually. A generalized formulation of anode materials can be schematically described as Sub/VM - - NM + NNM, where Sub refers to the substrate, VM to the valve metal, NM to the noble metal, and NNM to the non-noble metal. The compositions cited in the literature include the following in various combinations ... 

Identification of unknown crystal structures and determination of phase fields by X-rays can be problematical if the characteristic patterns of the various phases are quite similar, for example in some b.c.c. A2-based ordered phases in noble-metal-based alloys. However, in many cases the characteristic patterns of the phases can be quite different and, even if the exact structure is not known, phase fields can still be well established. Exact determination of phase boundaries is possible using lattice-parameter determination and this is a well-established method for identifying solvus lines for terminal solid solutions. The technique simply requires that the lattice parameter of the phase is measured as a function of composition across the phase boimdary. The lattice parameter varies across the single-phase field but in the two-phase field becomes constant. Figure 4.12 shows such a phase-boundary determination for the HfC(i i) phase where results at various temperatures were used to define the phase boundary as a fimction of temperature (Rudy 1969). As can be seen, the position of is defined exactly and the method can be used to identify phase fields across the whole composition range. 

Table 3.1-187 Basic compositions of noble-metal-based dental alloys [1.251, p. 251]...

The replacement of vanadia-based catalysts in the reduction of NOx with ammonia is of interest due to the toxicity of vanadium. Tentative investigations on the use of noble metals in the NO + NH3 reaction have been nicely reviewed by Bosch and Janssen [85], More recently, Seker et al. [86] did not completely succeed on Pt/Al203 with a significant formation of N20 according to the temperature and the water composition. Moreover, 25 ppm S02 has a detrimental effect on the selectivity with selectivity towards the oxidation of NH3 into NO enhanced above 300°C. Supported copper-based catalysts have shown to exhibit excellent activity for NOx abatement. Recently Suarez et al and Blanco et al. [87,88] reported high performances of Cu0/Ni0-Al203 monolithic catalysts with NO/NOz = 1 at low temperature. Different oxidic copper species have been previously identified in those catalytic systems with Cu2+, copper aluminate and CuO species [89], Subsequent additions of Ni2+ in octahedral sites of subsurface layers induce a redistribution of Cu2+ with a surface copper enrichment. Such redistribution... 

In addition to being able to catalyze the dissociation of O2. the material used for the cathode must be electronically conductive in the presence of air at high temperature, a property found primarily in noble metals and electronically conductive oxides. Ionic conductivity is also desirable for extending the reaction zone well into the electrode since the ions must ultimately be transferred to the electrolyte. Since precious metals are prohibitively expensive when used in quantities sufficient for providing electronic conductivity, essentially all SOFC prototypes use perovskite-based cathodes, with the most common material being a Sr-doped LaMnOs (LSM). In most cases, the cathode is a composite of the electronically conductive ceramic and an ionically conductive oxide, often the same material used in the electrolyte. 

Ce02-supported noble-metal catalysts such as Pt, Pd and Rh are of interest because of their importance in the so-called three-way converter catalysts (TWC), designed to reduce emissions of CO, NOx and uncombusted hydrocarbons in the environment and to purify vehicle-exhaust emissions. Such catalysts are also of current interest in steam reforming of methane and other hydrocarbons. Conventional practical catalysts for steam reforming consist of nickel supported on a ceramic carrier with a low surface area and are used at high temperatures of 900 C. This catalyst suffers from coke formation which suppresses the intrinsic catalyst activity. Promoters such as Mo are added to suppress coke formation. Recently, Inui etal(l991) have developed a novel Ni-based composite... 

Carbonium ions and isoparaffins are formed by hydride ion abstraction and hydride ion transfer reactions. This mechanism has been described for HF.SbFg (5). Isomerization of n-paraffins over monofunctional acidic catalysts has also been claimed for mordenite (6, 7), for sieve Y (8), and for the base of the catalyst of undisclosed composition applied in the isomerization process using a noble metal on an acidic zeolite base (3). 

For this study by Thompson et al (42), ion implantation and RBS were combined with more traditional electrochemical measurements to help establish the corrosion mechanisms of alloys in which a noble metal (Pt) was combined with an active/passive base metal (Ti). The alloys were created by ion implantation of Pt into pure Ti and were not of uniform bulk composition. Such surface alloys offer the possibilities of using a very small amount of a noble material to create a corrosion resistant coating on an otherwise chemically unstable but inexpensive metal or alloy. 

Over the past 35 years, much has been learned about the electrooxidation of methanol on the surface of noble metals and metal alloys, in particular platinum and ruthenium [2, 4, 6, 7]. Significant overpotential losses occur in the reaction due to poisoning of the alloy catalyst surface by carbon monoxide. Yet, Pt-based metal alloys are still the most popular catalyst materials in the development of new fuel cell electrocatalysts, based on the expectation that a more CO-tolerant methanol catalyst will be developed. The vast ternary composition space beyond Pt-Ru catalysts has not been adequately explored. This section demonstrates how the ternary space can be explored using the high-throughput, electrocatalyst workflow described above. 

Recently, in academic circles and in industry, a renewed interest can be observed in the development of non-noble metal catalysts, e.g., Cu-based catalysts. If lean-burning systems were to become available, oxidation would be the only reaction to be catalyzed and catalyst composition will certainly be changed accordingly. Catalytic converters are still in the development stage for diesel engines. 

Ceria affords a number of important applications, such as catalysts in redox reactions (Kaspar et al., 1999, 2000 Trovarelli, 2002), electrode and electrolyte materials in fuel cells, optical films, polishing materials, and gas sensors. In order to improve the performance and/or stability of ceria materials, the doped materials, solid solutions and composites based on ceria are fabricated. For example, the ceria-zirconia solid solution is used in the three way catalyst, rare earth (such as Sm, Gd, or Y) doped ceria is used in solid state fuel cells, and ceria-noble metal or ceria-metal oxide composite catalysts are used for water-gas-shift (WGS) reaction and selective CO oxidation. 

TPR is a routinaiy technique in the redox characterization of M/Ce02 and related catalysts. It has been very extensively used in comparative studies aimed at establishing the influence of variables like the chemical composition (281,341,343,344,350-353), or the high temperature ageing treatments (187,288,337,347,354), on the reducibility of ceria-based mixed oxides, both in the presence and in the absence of a supported noble metal. 

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