Hydrothermal sintering

Water vapor produced during reduction is particularly problematic as it can inhibit the reaction or lead to hydrothermal sintering. 

Hydrothermal crystal growth Hydrothermal treatment Hydrothermal alteration Hydrothermal dehydration Hydrothermal extraction Hydrothermal reaction sintering Hydrothermal sintering Corrosion reaction Hydrothermal oxidation... 

Two final words of caution are necessary, (i) It is essential to analyse the finished cafalysf chemically, because nol all of the active components will have ended up where you would like - on the support, (ii) It is desirable to measure the total surface area and porosity of the finished catalyst, because they may have been changedby the preparation. This is particularly necessary when silica is the support, because it readily undergoes hydrothermal sintering during drying, calcination and reduction, leading to the sealing off of internal pores. 

Coke is deposited in the first reactor, particularly when the flame temperature is low, if hydrocaibons are not completely oxidized in the furnace. This can be avoided with proper fiunace design. Hydrothermal sintering of the catalyst is... 

HYDROTHERMAL SINTERING OF THE ACTIVE PHASE IN ALUMINA SUPPORTED FIXED BED NICKEL CATALYSTS DURING REDUCTION... 

The results obtained after various treatments show that sintering of the nickel metal crystallites and of the support play only a minor role. The predominant mechanism is hydrothermal sintering of the Ni species present on the support prior to their reduction. The novel catalyst proved far more stable than the other two. This could partly be ascribed to the fact that nitric oxide vapours evolved with the nitrate prepared catalysts enhanced Ni sintering considerably. 

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 major mechanism leading to poor metal dispersion of alumina supported nickel catalysts is hydrothermal sintering of Ni precursors prior to reduction. Sintering of the support or metal crystallites once formed are relatively unimportant. 

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