The role of alkali sulphates in vanadium catalysts for sulphur dioxide oxidation

The chemical composition of alkali metal sulphate‐vanadium oxide mixtures in equilibrium with SO3‐SO2‐air mixtures has been investigated. These gas mixtures were prepared from a 5% SO2–95% air mixture, the ratio of SO2 to SO3 being maintained at the equilibrium value corresponding to the temperature of each run. Experiments have been carried out with mixtures of V2O3 and one alkali metal sulphate (Na, K, Rb, or Cs) in the temperature range 380–600°. In the temperature range 440–600° a liquid is produced, consisting of vanadium compounds dissolved in an alkali pyrosulphate‐sulphate mixture. The melting point of the mixture formed decreases with increasing atomic weight of alkali metal used. The weight increases observed with an alkali sulphate/V2O6 molecular ratio of 2.5/1 suggest the formation of normal pyrosulphate, M2S2O7, and VOSO4. With the Rb2SO4 and Cs2SO4 mixtures there is also evidence for the part formation of a higher sulphate, M2S3O10. The extent of reduction of the V2O5 is lower in the presence of alkali metals of higher atomic weight. Thus potassium is more effective in stabilizing the vanadium in the pentavalent state than is sodium, and caesium is even better. This may account for the preference for the potassium salt over the sodium salt as a promoter in commercial catalysts.