Vapor pressure measurements on molybdena-alumina
1987; Elsevier BV; Volume: 108; Issue: 1 Linguagem: Inglês
10.1016/0021-9517(87)90164-3
ISSN1090-2694
Autores Tópico(s)Thermal and Kinetic Analysis
ResumoThe common monolayer model of MoO3 on Al2O3 implies a decrease in free energy when a monolayer is formed from a crystallite of MoO3. The increased stability of the monolayer should be reflected in a decrease in MoO3 vapor pressure above MoO3Al2O3, by comparison with unsupported MoO3. This prediction is confirmed: the apparent vapor pressure above MoO3Al2O3 is at least an order of magnitude less than that above unsupported MoO3, at comparable temperatures. The vapor pressure has been studied for two alumina supports, two particle sizes, and two MoO3 loadings as a function of temperature, carrier gas velocity, and time of the experiment. Particle size has little effect, suggesting that the values are not limited by the rate of diffusion through pores. Loading is an important variable, suggesting energetic heterogeneity of the sites on the alumina surface. The apparent vapor pressures are highest in the initial period of measurement. Experiments with predried samples and with the addition of water vapor to the feed indicate that this behavior is partly caused by evolution of water vapor (by surface dehydroxylation of the alumina) at the beginning of a run, and partly by severe loss of surface area during the first few hours.
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