Fine particles, surface deactivation

Various deactivation procedures are applied to support materials. These include acid- or basewashing to remove impurities and fine particles, and treatment with a silanising agent which reacts with surface hydroxyl groups and reduces adsorptive effects. A very light coating of a polar stationary phase may also be used to increase deactivation. Commercial support materials which have been treated by these procedures are available and are usually designated by a suffix to the name, e.g. AW (acid washed) and AW HMDS (acid washed, hexamethyldisilazane treated). Deactivated supports are nearly always to be preferred, but it should be noted that the deactivation procedure may impose an upper temperature limit and may modify the polarity of the stationary phase. 

To simulate poisoning by potassium in the flue gas stream, the sample pellets were submerged in fine particles of KCl (<200 nm) and placed in a furnace for 248 hrs at 350°C in a water saturated air flow. Hereby the temperature is above the Tanmian temperature for KCl (m.p. = 771°C, TTamman= 258.5°C). Thus, the salt is surface mobile, and will diffuse into the catalyst pores deactivating any accessible active sites... 

The importance of catalyst stability is often underestimated not only in academia but also in many sectors of industry, notably in the fine chemicals industry, where high selectivities are the main objective (1). Catalyst deactivation is inevitable, but it can be retarded and some of its consequences avoided (2). Deactivation itself is a complex phenomenon. For instance, active sites might be poisoned by feed impurities, reactants, intermediates and products (3). Other causes of catalyst deactivation are particle sintering, metal and support leaching, attrition and deposition of inactive materials on the catalyst surface (4). Catalyst poisons are usually substances, whose interaction with the active surface sites is very strong and irreversible, whereas inhibitors generally weakly and reversibly adsorb on the catalyst surface. Selective poisons are sometimes used intentionally to adjust the selectivity of a particular reaction (2). 

Upon water vapor removal from the stream, only part of the dry gas-catalytic activity is recovered. This partial activity recovery may be mainly attributed to decomposition of the hydrolyzed copper complexes, and bare ion (due to the dehydration of copper complexes on the exterior surface) migration to active sites inside the zeolite cavities. Another contribution may come fi m tiie fine CuO particles on the zeolite surface, i.e., small part of active Cu cations are slowly restored by solid ion exchange with Bronsted acid sites. This hypothesis is drawn from the observation that Cu(H)-ZSM-5 with low Cu ion exchange level can be obtained by solid ion exchange between H-ZSM-5 and CuO in a vacuum at 5(X)°C as reported by Karge, et al (75). The permanent activity loss, however, is not explained. This may be attributed to irreversible CuO particle formation and deactivation, or dealumination if it happened in this study. 

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