Sintering/stability

Ni/Al203 catalysts prepared by coprecipitation exhibit excellent sintering stability and can be used in many catalytic reactions. The most important ones are certainly steam reforming (Section B.3.3) and metha-nation (Section B.3.6) reactions. 

For the alpha alumina catalyst (figure 8) when precalcined in a very high flow of dry nitrogen followed by treatment with moist nitrogen and intermediate cooling, upon subsequent reduction a metal surface area retention of circa 73% was observed. The same treatment applied to the novel catalyst resulted in a nickel surface area retention of 96%. Although it may be concluded from the above that NOx fumes are responsible for considerable enhancement of the hydrothermal sintering of the Ni precursor salt and their absence is a major cause for the increased sinter stability of the newly developed catalyst, it is clear from this experiment, that this effect cannot entirely explain the stability of the nickel dispersion of the novel catalyst. 

The components in catalysts called promoters lack significant catalytic activity tliemselves, but tliey improve a catalyst by making it more active, selective, or stable. A chemical promoter is used in minute amounts (e.g., parts per million) and affects tlie chemistry of tlie catalysis by influencing or being part of tlie catalytic sites. A textural (structural) promoter, on tlie otlier hand, is used in massive amounts and usually plays a role such as stabilization of tlie catalyst, for instance, by reducing tlie tendency of tlie porous material to collapse or sinter and lose internal surface area, which is a mechanism of deactivation. 

The intrinsic properties may be modified by substitution (31). Ba can be fuUy replaced by Sr or Pb and partly by Ca (<40 mol %). CaM, stabilized with 0.03 mol % La202, is also possible. The intrinsic properties of these M-ferrites vary somewhat and other factors such as sintering behavior and price of raw materials often dictate the commercial viabiUty. Large-scale production is concentrated on BaM and SrM. High quaUty magnets are generally based on SrM, and somewhat lower priced magnets are based on BaM. 

Dead-burned dolomite is a specially sintered or double-burned form of dolomitic quicklime which is further stabilized by the addition of iron oxides. Historically, it was used as a refractory for lining steel furnaces, particularly open hearths, but as of this writing is used primarily in making dolomite refractory brick (see Refractories). 

Stability, and can provide both ohmic low resistance contacts and rectifying contacts. Typically, siUcide layers are formed in situ by sputteriag a thin platiaum layer onto the siUcon surface, followed by sintering. Infrared detection is another appHcation of platiaum siUcide technology. 

Table 12 shows the physicochemical data of separators used in open stationary batteries. Since the emphasis is on low acid displacement, low electrical resistance, and high chemical stability, the phenolic resin-resorcinol separator is understandably the preferred system, even though polyethylene separators, especially at low backweb, are frequently used. For large electrode spacing and consequently high separation thickness, microporous as well as sintered... 

Other potential applications are ceramic powders coated with their sintering aids, zirconia coated withyttria stabilizer, tungsten carbide coated with cobalt, or nickel, alumina abrasive powders coated with a relatively brittle second phase such as MgAl204 and plasma spray powders without the segregation of alloying elements. 

Bulk ceramics are produced conventionally by the sintering of powders. The strength, toughness, thermal stability, and dielectric properties of the fired ceramic depend strongly on the size and uniformity of the precursor powder and on the chemical properties of the powder smface. 

Sol-gel technique has also been applied to modify the anode/electrolyte interface for SOFC running on hydrocarbon fuel [16]. ANiA SZ cermet anode was modified by coating with SDC sol within the pores of the anode. The surface modification of Ni/YSZ anode resulted in an increase of structural stability and enlargement of the TPB area, which can serve as a catalytic reaction site for oxidation of carbon or carbon monoxide. Consequently, the SDC coating on the pores of anode leads to higher stability of the cell in long-term operation due to the reduction of carbon deposition and nickel sintering. 

The stability of catalyst is one of the most important criteria to evaluate its quality. The influence of time on stream on the conversion of n-heptane at SSO C is shown in Fig. 5. The conversion of n-heptane decreases faster on HYl than on FIYs with time, so the question is Could the formation of coke on the catalyst inhibit diffusion of reactant into the caves and pores of zeolite and decrease the conversion According to Hollander [8], coke was mainly formed at the beginning of the reaction, and the reaction time did not affect the yield of coke. Hence, this decrease might be caused by some impurities introduced during the catalyst synthesis. These impurities could be sintered and cover active sites to make the conversion of n-heptane on HYl decrease faster. 

In Chapter 1 we emphasized that the properties of a heterogeneous catalyst surface are determined by its composition and structure on the atomic scale. Hence, from a fundamental point of view, the ultimate goal of catalyst characterization should be to examine the surface atom by atom under the reaction conditions under which the catalyst operates, i.e. in situ. However, a catalyst often consists of small particles of metal, oxide, or sulfide on a support material. Chemical promoters may have been added to the catalyst to optimize its activity and/or selectivity, and structural promoters may have been incorporated to improve the mechanical properties and stabilize the particles against sintering. As a result, a heterogeneous catalyst can be quite complex. Moreover, the state of the catalytic surface generally depends on the conditions under which it is used. 

Sufficient thermal stability against sintering, structural change or volatilization inside the reaction environment (e.g. when steam is a byproduct of the reaction). 

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