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Motivation
Molybdenum, vanadium and tungsten containing suboxides are widely used as partial oxidation catalysts [1-3] e.g. the partial oxidation of acrolein to acrylic acid is performed on such a system [4, 5]. Recently (MoVW)5O14 was synthesised as a single phase. Compounds with identical XRD patterns however, show different catalytic activity. This is due to different “real structures” of the active catalyst. The aim of this work is to suggest a reaction mechanism in solution leading to a precursor. Due to his molecular structure this precursor is vital to give the Mo5O14 type structure single phase after calcination [6]. Further, the generation of defects in the solid, which might be important for the catalytic process could be monitored and finally controlled.
Experimental
A detailed calcination procedure is described in the literature [6]. Respective amounts of AHM, AMT and vanadyl oxalate are dissolved. mixed and spray-dried. This is followed by the calcination procedure. The structural changes in solution are followed by UV/Vis, Raman and ESR spectroscopy.
Results
A significant increase in the absorption is monitored in the region between 30000 cm-1 and 50000 cm-1 when the pH is lowered from pH = 5.5 to pH = 3. This change in the metal ligand charge transfer region is caused by a protonation of oxygen atoms. The addition of AMT enhances this effect.
Additionally, a band in the region between 18000 cm-1 and 19000 cm-1 emerges when vanadyl oxalate is added stepwise. The blue shift of this band at higher vanadyl concentrations can be explained by an increasing degree of polymerisation. The band position speaks for a intervalence charge transfer transition [7].
This result is corroborated by EPR experiments. The higher the vanadyl concentration the less isolated vanadyl species are present. Further, EPR shows that oligomeric or polymeric units are present.
Discussion
Reaction pathways of transition metal compounds such as molybdenum, vanadium or tungsten have recently been reviewed by Cruywagen [8]. Depending on the pH Mo7O246-, [HMo7O24]5-, and Mo8O264- species prevail. In strongly acidic media [Mo36O112(H2O)16] is found. The replacement of molybdenum by tungsten in hepta molybdenum species has been reported as well [9].
These species can be interlinked by vanadyl bridges and form a polymeric network. Comparison with the literature shows that in the respective pH range [HMo7O24]5- and Mo8O264- species are very likely. A good reason for the [Mo36O112(H2O)16] species would be the pentagonal bipyramide as a structural element which also appears in Mo5O14.
