New nonstoichiometric molybdate, tungstate, and vanadate catalysts with the scheelite-type structure
Phases of the formula A 1- xф xMO 4 with the scheelite-type structure are described where ф represents a vacancy at the A cation site and M is Mo 6+, W 6+, and/or V 5+. Many different univalent, divalent, and trivalent A cations were used in this study. The phases with no defects, i.e., x = 0, were known except for those of the type A 1+.5A 3+.5MO 4 where A 1+ is Ag or Tl and M is Mo 6+ or W 6+. Phases with x > 0 are generally new and were prepared for catalytic studies. An excellent correlation between catalytic properties and defect concentration has been observed.