Loss of alkali promoter by desorption from promoted vanadium oxide catalysts

Abstract

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.