Sintered catalysts

Non-noble metal molybdenum containing carbonyls were studied for ORR activity.189-191 Mox(CO) and Mo,Sr(C 0) were studied side by side.189 190 These high-nuclearity carbonyl compounds were prepared by sintering. It was found that the Mox(CO) was amorphous by XRD, however, as S was added, a polycrystalline/ amorphous combination was formed in the catalyst. While neither Mox(CO) or Mo SyCO), compounds were found to be very active for ORR, the addition of S to Mox(CO) appeared to nullify any activity towards ORR.189 190 MoxSeJ,(CO) was produced by both screen printing and sintering191 as well as chemical synthesis.192 The screen printed catalysts were more active than the sintered catalysts, but as the amount of Se increased, the ORR activity decreased.191 192... 

The earlier investigations, previously referred to, used empirical models to understand sintering. Here, the dispersion ( ) ) of a sintered catalyst (e.g. Pt/alumina) is correlated with the dispersion of the fresh catalyst ( >q), using a power law ... 

Based on this, it is apparent that the exothermic catalyst reactivation reactions need to be appropriately controlled to avoid Ni sintering/catalyst deactivation. 

Catalyst composition also depends on the type of reactor used. Fixed-bed iron catalysts are prepared by precipitation and have a high surface area. A silica support is commonly used with added alumina to prevent sintering. Catalysts for fluidized-bed application must be more attrition-resistant. Iron catalysts produced by fusion best satisfy this requirement. The resulting catalyst has a low specific surface area, requiring higher operating temperature. Copper, another additive used in the preparation of precipitated iron catalysts, does not affect product selectivity, but enhances the reducibility of iron. Lower reduction temperature is beneficial in that it causes less sintering. 

In general, CTL intensity depends on the catalytic reaction rate, so that a catalyst with a large surface-to-volume ratio is preferable. In this sense, the catalyst powder or a sintered layer of porous particles is used as the sensor material. As the CTL catalyst should be heated to a working temperature, the catalyst powder is pressed in a ceramic pot with a heating wire, or the sintered catalyst layer is formed on a substrate with an electric heater. 

Pt-Ir Temperature, gas flow rate, oxychlorination studied. Careful control necessary. Sintered catalysts could be redispersed somewhat by oxychlorination, especially with higher Cl levels.89,90 ... 

In Ni/Al203 catalysts prepared by impregnation there is a weaker interaction between the impregnating salt and the support than there is when the catalyst is prepared by deposition-precipitation. Calcination of the former catalysts gave large nickel oxide crystallites and, on reduction, the supported metal was rather easily sintered. Catalysts prepared by precipitation-deposition, on the other hand, were resistant to sintering because of the strong interaction... 

Boudart and Ptak (77) confirmed this enhancement of selectivity in isomerization on the (111) faces by comparing the reactions of neopentane on two 1% Pt/carbon black catalysts of exactly the same dispersion, 0.35. One of the catalysts had been fired at high temperature (900°C), and such a treatment, according to the work of Lyon and Somorjai (188), is expected to favor the formation of randomly distributed (111) facets. It is remarkable that the selectivity in isomerization was five times larger on the sintered catalyst than on the nonsintered one. 

Various solid state reactions are involved in the process. In addition to those governing sintering, catalyst-support interactions may alter the nature and catalytic activity of the solid and may stabilise or destabilise the solid towards sintering. Thus, for example, the formation of nickel aluminate, NiAlgO, is well established in steam reforming catalysts [21,22], and this compound is catalytically inactive. However, its presence may affect the thermal stability of the solid [23], as is the case in cobalt-molybdenum and nickel-molybdenum based catalysts supported on alumina and used for hydro-treating [24]. 

The hot gas recycle process made it necessary to use iron catalysts of adequate mechanical strength. Sintered catalysts showed better resistance against the erosive influence of fast moving gases than highly active precipitation catalysts. 

This model reaction was carried out "in situ" on fresh or sintered catalysts (0X800 and RED900). 

Table I PtRh/AhO3 fresh and sintered catalysts characteristics...

Catalysts %Pt %Rli Fresh catalysts Relative dispersion D/Do of sintered catalysts at ... 

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