LOSS OF ALKALI PROMOTER BY DESORPTION FROM PROMOTED VANADIUM-OXIDE CATALYSTS

Experiments have been done with molecular beam mass spectrometric methods to characterize the desorption and emission processes of alkali promoter from a commercial vanadium oxide catalyst. It is a potassium and cesium promoted catalyst used for SO3 production, with a K, Cs/V molar ratio of 2.5. An excited state signal is detected, as in our previous studies of other oxide catalysts. Desorption of both K and Cs is also detected. All signals are observed at temperatures above 910 K, which is just above the process temperature range. The values found for atomic desorption are 2.66 +/- 0.16 eV for K and 3.89 +/- 0.27 eV for Cs, while the excited state emission has a barrier of 0.89 +/- 0.08 eV. The difference in desorption energies between Cs and K is larger than expected. This is probably mainly due to different rate limiting steps in the release and diffusion processes. After an approximate compensation for this, Cs still has a larger activation barrier equal to 3.0 +/- 0.3 eV towards desorption than K, which indicates a slower loss for Cs from the catalyst.