Molybdenum oxide based partial oxidation catalyst: 1. Thermally induced oxygen deficiency, elemental and structural heterogeneity and the relation to catalytic performance

Abstract

A mixed oxide catalyst containing Mo, V, and W was used for the partial oxidation of methanol. The relation between the structure and the degree of reduction of this mixed oxide catalyst and its catalytic performance was investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray (EDX), Rutherford backscattering (RBS), XPS, ion scattering spectroscopy (ISS), UPS, X-ray diffraction (XRD) and Raman microspectroscopy. Thermal activation of the MoVW mixed oxide led to an increase in the methanol conversion by a factor of 3 and an increase in selectivity to formaldehyde from 66% to 80%. SEM–EDX characterization of the untreated catalyst revealed the presence of at least two different phases in the sample on a micron range, one with a high V concentration, and another with all three metals present. TEM–EDX showed a homogeneous element distribution in the submicron regime. The thermally activated mixed oxide revealed an inhomogeneous element distribution in the micron and submicron regime as shown by SEM–EDX and TEM–EDX. The activation led to a reduction of the bulk oxide as determined by RBS and its surface as shown by XPS, ISS, and UPS. The formation of Mo 4+ and V 4+ centers was detected at the catalyst surface upon thermal activation. XRD of the starting material showed the presence of nanocrystalline material which was identified as being a mixture of a majority of Mo 5O 14 and a minority of MoO 3-type MoVW oxides. Confocal Raman microspectroscopy confirmed the presence of two different components. The major component could be identified as amorphous Mo 5O 14-type MoVW mixed oxide. The second, minor component was similar to an amorphous MoO 3-type MoVW oxide. XRD showed that the thermally activated mixed oxide consisted of a mixture of a majority of crystalline Mo 5O 14-type oxide and of small amounts of crystalline MoO 3-type and MoO 2-type oxides. The Raman spectrum of the Mo 5O 14-type phase could be identified by statistical data evaluation of 1000 spectra and by comparison with the XRD result. Raman microscopy confirmed the presence of a minority of MoO 3- and MoO 2-type oxide. The formation of Mo 5O 14-type oxide upon loss of oxygen is discussed with respect to the remarkable increase in the catalytic activity and selectivity.