Interaction of hydrogen chloride with a molybdena–silica catalyst

The interaction of HCl with a molybdena–silica catalyst has been investigated using e.p.r. and infrared spectroscopy. Exposure of reduced catalysts to HCl at room temperature causes a large (up to 20-fold) increase in the amount of MoV detected by e.p.r. which is reversed on subsequent outgassing. The accompanying changes in g- and the 95Mo hyperfine-tensor components indicate that chloride ions replace oxide ions in the coordination sphere of MoV, and the increase in MoV spin concentration is attributed to substitution of bridging oxide ligands between magnetically coupled MoV ions. Oxidized catalysts are reduced by exposure to HCl. The reaction of HCl with molybdena–silica is compared with that previously reported with molybdena–alumina catalysts.