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Polyoxymetallates, UV/Vis, Precursor, defect structure
8 Mixed V/W/Mo oxides for selective oxidation
Abstract:
Motivation
Molybdenum containing suboxides are widely used as partial oxidation catalysts e.g. the partial oxidation of acrolein to acrylic acid is performed on such a system. These systems usually contain additional elements such as V, W, Nb etc. The role of these elements is quite unclear. Their intrinsic properties could be either important for the catalytic process, or they are structural promoters leading to a certain defect structure which is vital for catalysis [1].
To clarify this a precipitation method was chosen, leading to a controlled variation of solid precursors for catalysts without having to change the chemical composition. In this work the pH dependency of a mixed ammonium heptamolybdate (AHM) and ammonium metavanadate (AMV) solution is investigated. Chemical transformations are monitored as well as the precipitation. This will offer a detailed insight in the processes occurring in solution. The aim is to control as many parameters as possible to ensure a reproducible preparation. Further, a profound understanding of the process should lead to a rational preparation of defects, which are important for catalysis.
Experimental
Respective amounts of either ammonia, sodium or lithium tungstates or molybdates were dissolved in bi-distilled water. Nitric acid was added at a controlled rate. the pH values were monitored online by a calibrated glass electrode and in situ UV/Vis spectroscopy was performed with a fibre (configuration 8 around one).
Results
The chemical transformations occurring in solutions can by followed by the changes of the spectra in between 380 nm and 420 nm (Fig. 1).
The counter cations present in the solution are involved in the nucleation and the precipitation process. The larger the cation, the easier the precipitation. Starting from AHM and K2MoO4, spontaneous precipitation is observed at about pH 2, with Na2MoO4 precipitation sets in within 2 hours at elevated temperature. Solutions of Li2MoO4 stay stable at pH=2 without any precipitation over weeks.
The spectra reveal:
Þ a continuing alteration of the freshly generated precipitate dispersed in the solution with time;
Þ a continuing (red) shift for the absorption of the fresh precipitate with increasing V content;
Þ the occurrence of a transient intermediate state just before the precipitation sets on;
Þ the nature of oligomeric species formed in the acidified solutions depends on the V-concentration.
