In-situ Spectroscopic Studies and Modelling of Crystallization Processes of Sulphuric Acid Catalysts.

Deactivation processes of commercial and prototype sulphuric acid catalysts have been investigated in-situ both in highly converted and unconverted feed gases, using electron paramagnetic resonance (EPR) spectroscopy. The investigation revealed that the deactivation of catalysts for SO₂ oxidation is strongly dependent on physical and chemical parameters such as the pore structure of the support, the type of the alkali promoter as well as the SO₂ and SO³ partial pressures. A hysteresis effect in catalytic activity could be observed during reheating of the deactivated catalysts. In this context indirect evidence was found of deactivation by the formation of vanadium(III) compounds. The various vanadium compounds in the oxidation state (IV) and with mixed valence (IV-V), formed during deactivation, have been characterized by EPR. The investigations showed no change in the vanadium oxidation state in the catalytic cycle under steady-state conditions, and at high SO₂ partial pressure even at low temperatures, indicating that the so-called 'associative mechanism' probably dominates in the catalytic SO₂ oxidation. A statistical lattice model was applied to describe the crystallization of binuclear complexes in the melt of sulphuric acid catalysts during deactivation while taking into account parameters such as temperature, the composition of the catalyst and SO₂ conversion.