Fritz-Haber-Institut / Inorganic Chemistry |
|Catalytic Studies of Nanostructured Molybdenum Oxides by Temperature Programmed Reaction Spectroscopy|
|Authors:||Timpe, Olaf; Othman, Diana; Abdullah, Norly; Knobl, Stefan; Niemeyer, Dirk; Abd Hamid, Sharifah Bee; Schlögl, Robert|
|Research Context:||Combicat: Selective Oxidation of C3 molecules|
|Name of Conference/Meeting:||Jahrestagung deutscher Katalytiker|
|Place of Conference/Meeting:||Weimar|
|(Start) Date of Event |
|End Date of Conference/Meeting |
|Intended Educational Use:||No|
|Abstract / Description:||Temperature programmed reaction spectroscopy with propylene has been carried out on different Mo oxides, obtained by precipitation from heptamolybdate precursors. These experiments have shown that all three oxides are catalytically active. Supramolecular Mo oxides without any dopants showed a similar activity to the Mo(VW) oxide benchmark material, however, it was converted into inactive orthorhombic MoO3 after exposure to 700K. The benchmark material was not, because V and W dopants stabilize the active structure.
High throughput experimentation (HTHE) and combinatorial methods are now established tools in catalysis research to solve the problem of lacking success of empirical optimisation techniques. However, it becomes clear that also a knowledge-based strategy is required to define compositional libraries and experimental procedures with a reasonable success rate. Such a strategy is currently put into operation; its objective is to map out the synthetic parameter space for defining the synthesis of chemically simple but structurally complex oxidic nanoparticles suitable for selective oxidation catalysis. Structural complexity is considered to be crucial for good catalytic activity; it is currently created by incorporation of dopants such as tungsten or vanadium. The new approach presented in this work is to create the same structural complexity by careful adjustment of the preparation conditions. Molybdenum oxide was chosen as a model system because it is also commonly used in many partial oxidations reactions. As MoO3 is virtually inactive the common approach is to synthesize many different modifications of Mo oxides. Temperature programmed reaction spectroscopy (TPRS) was applied as a quick method to determine the catalytic activity of a well-known sample over a wide range of temperatures.
Molybdenum oxides with a variety of defects were obtained by precipitation from ammonium molybdate solutions. By carefully adjusting the preparation conditions, hexagonal MoO3 and supramolecular Mo oxides are obtained. Vanadium and tungsten doped Mo oxide was obtained by spraydrying a mixture of ammonium heptamolybdate, vandyl oxalate and ammonium metatungstate solution . It was investigated as benchmark. TPRS was carried out as follows. A temperature ramp of 5K/min was applied up to a temperature of 773 K, this temperature was maintained for one hour. Subsequently the temperature was ramped down to 423K at 5K/min and again up to 773K for another 30 minutes. 50 mg of sample was taken and diluted with the same amount of boron nitride and 200 mg of SiC. It was placed into a reactor with 7.5 mm in diameter and exposed to a gas feed of 10% propylene, 10% oxyen and 80% helium at a rate of 100 ml/min. The length of the catalyst bed in the tube was about 10 mm.
RESULTS AND DISCUSSION
The CO2 signal of the three samples showed three different onsets of catalytic activity. The crystalline MoO3 with the hexagonal phase showed the lowest catalytic activity. Production of carbon dioxide started at about 600K, local maxima were achieved at 630K and 670K. During the second temperature ramp the CO2 signal was diminished due to conversion of the catalyst into orthorhombic MoO3. The supramolecular phase also showed a CO2 signal between 400 and 500K. The oxygen signal at this temperature was unaffected so the oxygen necessary for carbon dioxide formation must have been donated by the catalyst. Such a reduction of the sample occurred again at about 700K and 730K. During the second temperature ramp, it showed almost identical behaviour as the hexagonal sample, indicating a conversion of both materials into the orthorhombic form. The tungsten and vanadium doped Mo oxide showed a large CO2 peak between 400 and 500K, similar to the supramolecular material. A further peak was observed at 620K. During the second temperature ramp the onset of catalytic activity was at about 620K.
This trend in catalytic activity is also reflected during partial oxidation into acrolein. The onset of partial oxidation occurs at about 650K for the cases of the mixed oxide and the supramolecular phase and at about 700K for the hexagonal MoO3.
The experiments presented in this work have clearly shown that temperature programmed reaction spectroscopy is a valuable tool to assess the catalytic activity of a sample. It has also been demonstrated that structural complexity can be achieved without the need for increased chemical complexity as undoped supramolecular Mo oxides achieve similar conversion rates than doped Mo oxides. The major effect of tungsten and vanadium additions is to maintain the structural complexity at high temperatures by stabilizing the active phase against conversion into inactive o MoO3.
1. Knobl, S., Zenkovets, G., Kryukova, G., Ovsitser, O., Niemeyer, D., Mestl,. G., Schlögl, R., J. Catal., Journal of Catalysis, 215, 177-187 , 2003
|Communicated by:||Robert Schlögl|
|Affiliations:||Fritz-Haber-Institut/Inorganic Chemistry/Inorganic Chemistry|
|External Affiliations:||Combinatorial Technology and Catalysis Research Centre (COMBICAT) UNIVERSITI MALAYA
50603 Kuala Lumpur, Malaysia, firstname.lastname@example.org
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