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Whilst much research effort was spent on optimisation of catalyst performance, less attention has been drawn to problems concerning catalyst preparation. It is commonly known that catalyst research is facing two problems: the Materials gap and the Pressure gap. The “Materials gap” describes the discrepancy between commercial catalyst material that is often too complex to be successfully characterised and (single crystal) model catalysts that are often not able to achieve good product rates. The “Pressure gap” addresses the problem that surface investigations are commonly performed under UHV conditions whereas commercial processes are carried out at ambient or high pressure. As a consequence information of reaction mechanisms or the “real structure” under reaction conditions is very limited.
Although catalysis experiments on single crystals led to new information about catalyst behaviour, it is now commonly believed that the main catalytic processes happen on centres with a high- but often unidentified- number of defects. A major task for catalyst preparation is therefore to produce highly defective metastable material. New syntheses have to be developed that fulfil many more requirements such as to ensure high reproducibility and products easy to characterise. Whilst the former can be achieved by monitoring each reaction step in-situ, the latter is taken care of by preparing thin films on a substrate.
These new preparation methods will be demonstrated on the example of MoVW supported catalysts, which are used in industry for the synthesis of acrylic acid [1-5]. Despite this industrial importance, there is still a lack of information concerning structure formation during synthesis and the atomic arrangements with respect to different preparation routes and element ratios. Earlier work [6-9] showed a significant increase in selectivity for partial oxidation products in the presence of a Mo5O14 type structure. This structure, which was first identified by Kihlborg et.al. [10], is built up by pentagonal bipyramids and octahedrally coordinated metal centres [Figure 1]. It is metastable until crystallisation and oxidative decomposition into binary oxide phases occurs under high oxygen partial pressure (air and above). The element ratio is (Mo0.68V0.23W0.09)5O14. At the same time binary molybdenum based oxides doped with different elements such as Nb, W and Ta have been synthesised and their structure was identified as that of the Mo5O14-type [11, 12]. These phases were found to be stable at a wide temperature range. For the synthesis of this oxide, solutions of ammonium heptamolybdate, ammonium metatungstate, and vanadyl oxalate were spray-dried and subsequently calcined in air and helium. The Mo5O14 structure is an idealised endpoint that is formed under reduced oxygen partial pressure during the organisation process of a mixture of oligo anions, which are generated in solution. It is therefore necessary to characterise not only the structure itself but also the full preparation process with all intermediates. It seems plausible that different thermal treatments of the precursor solutions affect a) the composition of the usually mixed phase catalysts and b) the crystallite sizes of the different constituting phases. Thus, the understanding of the aqueous precursor chemistry is required to control the preparation of such mixed oxide catalysts. Furthermore, subsequent drying and activation procedures from the liquid precursor to the active and selective catalyst are of paramount importance for the development of the optimal catalytic performance. A preparation that is based on understanding of the system would allow precise control of the phase composition of the mixed oxide catalyst, the crystallite size, the crystallinity, and the morphology of the active phase. A developed synthesis routine thus could lead to defined crystallite sizes or even nano-crystalline (MoVW)5O14 mixed oxide catalysts. Moreover, it offers a versatile path to control its elementary composition. Effects of crystallite size / morphology and elemental composition could be studied separately on the catalytic performance.
To this end, some steps of the developed aqueous preparation procedure are characterised by in situ micro Raman spectroscopy. The important, subsequent drying process as well as further activation and formation procedures are investigated by in situ Raman spectroscopy, HREM and XRD. Comparison with Raman spectra of well defined, single-crystalline reference oxides [13] can be used to assign the obtained spectra during these catalyst preparation routes to certain oxides, such as MoO2, Mo4O11, Mo8O23, MoO3, or Mo5O14.
A different approach is currently carried out to synthesize the MoVW oxide by a Sol gel method. The Sol-gel chemistry is widely used to synthesize metal oxides by inorganic polymerisation of molecular precursors in organic media (alcohols, hydrocarbons). The low synthesis temperatures often lead to the formation of oxides with amorphous or metastable phases, which are not observed using other synthesis routes. The sol-gel synthesis of molybdenum oxides has received little attention, especially in comparison with transition metal oxides such as TiO2, V2O5 and WO3. The overall aim of this work is the rational preparation of molybdenum-based oxides via sol-gel synthesis of alkoxide precursors. The work concentrates on the mechanisms of solid formation from solution by in-situ measurements (Raman and UV-vis) in order to find new synthesis methods for high surface molybdenum oxides.
