Vacuum sintered catalysts

Steam treatments are performed for the most part on larger samples of catalyst in a conventional metal reactor type system. The catalyst and the system are thoroughly purged with steam and then maintained in an effectively static steam atmosphere at the pressure and temperature and for the time specified in Table I. Small Pyrex bulbs are used for atmospheric pressure steam treatments. The latter treatments are performed by means of a slow stream of steam generated and preheated below the catalyst sample. The effectiveness of the steam in reducing catalyst area increases somewhat with increasing pressure. However, the particular treatments selected for this discussion were chosen primarily for purposes of convenient comparison with vacuum sintered and commercially used samples. Most of the steam treatments at elevated pressures were performed by R. I. Lindberg of our laboratories. 

Area, Pore Volume and Average Pore Radius for Representative Cracking Catalysts in the Virgin, Steam-Treated, Vacuum-Sintered,... 

Complete adsorption-desorption isotherms obtained throughout a series of vacuum sintering treatments of the TCC Bead catalyst are... 

In order that the possibility of contamination of catalysts with traces of oxides could be eliminated Campbell and Emmett (51) studied the catalytic activity of metallic films of nickel and its alloys with copper or gold. They were deposited under a high vacuum and then sintered (alloys also homogenized) in hydrogen at 5 cm Hg pressure at 350°C or 500°C. The films were subsequently allowed to cool to room temperature and only... 

The studies reported in the literature have suggested that the surface tension of Cu depends on its surrounding environment it is higher in vacuum and varies as vacuum > H2 > CO. Well-rounded particles are likely to form when the surface tension is low. In CO, the surface tension is lowered to the extent that the Cu prefers to spread out as sheets rather than as three-dimensional spherical particles. Experiments carried out on real (practical) powder catalysts are consistent with the data from the model systems. As in the model systems, sintering by Cu particles is dominant, the particles growing to several tens of nanometres. The type and extent of sintering depend on the exact composition of the bimetallic catalyst. For Cu > Ru, ETEM studies show the sintering of Cu to be primarily by particle coalescence. 

A similar comparison of rate constants for 0.6 and 5% Pt/alumina and 1,8% Ag/alumina catalysts at 673 K in oxygen atmosphere reveals that sintering rates for the Ag catalyst are roughly 40-50 times higher than for either Pt catalyst. Thus, Pt is clearly much more thermally stable than Ag under oxidizing conditions. These results are consistent with those from a model catalyst study [44] of sintering of Pt and Ag on alumina in vacuum in which it was observed that Pt/alumina was thermally stable in vacuum to about 873 K, above which temperature liquid-like particle migration was observed, while Ag/alumina was stable to only 723 K, above which temperature evaporation of the metal was observed. This latter result is... 

Aral and Nishiyama report that the pretreatment of carbon sipports significantly affects the temperature at which Pt films break up in vacuum to fc m metal crystallites. Their data summarized in Table 1 indicate that Pt films deposited on air- and H2-treated carbons break up at significantly higher temperatures (923 and 823 K) compared to that for untreated carbon (773 K). These results are consistent with those obtained for conventional Pt/caibcm catalysts showing that carbon pietreatirrent affects their sintering behavior. ... 

EssoitiaUy all of the model catalyst studies discussed thus far were conducted using smooth, low surface area supports, with one exception. Rai studied sintraing and redispersicm of a model Pt/alumina catalyst prepared using a ptxous anodized alumina with a surface area of 8 to 10 The model catalyst was sintered in vacuum by electrcMi beam heating during which... 

An Fe UFP and a precipitated Fe catalysts were used in the present work. The Fe UFP designated as Fe UFP 200 A was supplied by Vacuum Metallurgical Co., Ltd. (Chiba Prefecture, Japan) and was prepared by the gas evaporation method (refs. 4,5). Transmission electron microscopic (TEM) observation showed that the UFP was sintered in the course of preparation and was in the form of "necklace" as... 

After inserting the reactor into the vacuum system it was heated up to 773 K (lOKmin-1). Subsequently, the catalyst was subjected to H2 pulses (1017 molecules per pulse, lpulse/sec) for lh. Then, the reactor was cooled to 573 K, and stabilised for 2h. This treatment results in catalyst M2 and was meant to induce sintering of Pt particles in order to study the influence of the particle size on redox behavior by comparing catalysts Ml and M2. 

An idealized platelet picture of catalyst structure as related to possible mechanisms for sintering in vacuum and in steam is presented in Fig. 10. Extreme cases of sintering are shown schematically. Structure representations of this type may aid in the ultimate interpretation of sintering... 

The hysteresis isotherms presented in Fig. 14 illustrate very clearly for the Aerocat catalyst the differences between sintering in vacuum and... 

---