Catalyst coalescence

Nice Rh dispersions of nano-sized Rh particles (1 to 3 nm, 75% metal dispersion) are obtained by reduction of fresh spinel-supported catalysts exhibiting amorphous Rh-O species (j.e. low initial calcinations temperature). Active phases calcined at higher temperatures (from 720°C to 1000°C) exhibit larger Rh particles. Nano-sized Rh particles observed on fresh RhS500 catalyst coalesce upon aging at 900°C in water-rich atmosphere metal dispersion is decreased to 19%. 

The in situ process is simpler because it requires less material handling (35) however, this process has been used only for resole resins. When phenol is used, the reaction system is initially one-phase alkylated phenols and bisphenol A present special problems. As the reaction with formaldehyde progresses at 80—100°C, the resin becomes water-insoluble and phase separation takes place. Catalysts such as hexa produce an early phase separation, whereas NaOH-based resins retain water solubiUty to a higher molecular weight. If the reaction medium contains a protective coUoid at phase separation, a resin-in-water dispersion forms. Alternatively, the protective coUoid can be added later in the reaction sequence, in which case the reaction mass may temporarily be a water-in-resin dispersion. The protective coUoid serves to assist particle formation and stabUizes the final particles against coalescence. Some examples of protective coUoids are poly(vinyl alcohol), gum arabic, and hydroxyethjlceUulose. 

An unstabilized high surface area alumina siaters severely upon exposure to temperatures over 900°C. Sintering is a process by which the small internal pores ia the particles coalesce and lose large fractions of the total surface area. This process is to be avoided because it occludes some of the precious metal catalyst sites. The network of small pores and passages for gas transfer collapses and restricts free gas exchange iato and out of the activated catalyst layer resulting ia thermal deactivation of the catalyst. 

Even when small nanoparticles (<10 nm) are used, they may first coalesce to form larger ones at a temperature below the synthetic temperature of SiNW and the temperature at which the small nanoparticle catalysts begin to melt. This may be why... 

SMSI is also thought to affect methanation catalysts (normally transition metal or noble metals supported on alumina), which are used in the producton of substitute natural gas (SNG). In general, heating in H2 causes sintering on alumina and silica supports and heating in O2 or steam can cause dispersion and particle coalescence at 200 °C (Rukenstein and Lee 1984,1987, Nakayama et al 1984). The data have been based on ex situ EM studies. Here EM methods, especially under dynamic reaction conditions, can provide a wealth of new insights into metal-support interactions under reaction conditions. 

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. 

Metal-oxide catalysts and support suffer a decrease in the surface area and porosity upon exposure to high temperatures due to the coalescence and growth of the bulk oxide crystallites. 

Catalyst-supporting materials are used to immobilize catalysts and to eliminate separation processes. The reasons to use a catalyst support include (1) to increase the surface area of the catalyst so the reactant can contact the active species easily due to a higher per unit mass of active ingredients (2) to stabilize the catalyst against agglomeration and coalescence (fuse or unite), usually referred to as a thermal stabilization (3) to decrease the density of the catalyst and (4) to eliminate the separation of catalysts from products. Catalyst-supporting materials are frequently porous, which means that most of the active catalysts are located inside the physical boundary of the catalyst particles. These materials include granular, powder, colloidal, coprecipitated, extruded, pelleted, and spherical materials. Three solids widely used as catalyst supports are activated carbon, silica gel, and alumina ... 

In supported catalysts there is evidence that particle morphology is affected by the nature of the support, and by the methods of preparation and pretreatment. Coalescence and reconstruction of clean particles should be extremely rapid. The fact that in many cases small particles in contact do not combine into a single coherent particle suggests that the surface of supported metal particles may be relatively highly contaminated. When this occurs it must affect catalytic properties and correlations between activity and structure. 

These results contradict Jacobsen s earlier mechanistic theories, which would have predicted a top-on" approach for the sterically demanding tetrasubstituted olefins (Figure 1) and thus inferior results compared to the less-substituted olefins, which were assumed to approach from a skewed side-on" disposition. Furthermore, his observation that trisubstituted olefins were epoxidized in an opposite stereochemical sense compared to other olefins required invoking a stepwise mechanism, wherein the radical intermediate is steered by the pendant chiral catalyst [94JOC4378], At the current time, these results fail to coalesce into a clear unified predictive model. 

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