Vanadium phosphorus oxide (VPO) catalysts have been considered as the most effective industrial catalysts for the oxidation of n-butane to maleic anhydride. The preparation methods of the catalyst, and reaction conditions and other factors will have different effects on the catalyst properties. In order to improve the performance of VPO catalyst, we often add co-catalyst: metal additives can not only improve the activity and selectivity of the catalysts to some extent, but also play important roles in reducing reaction conditions and prolonging the reaction time. The VPO catalyst precursors have been prepared through reducing pentavalent vanadium in organic phase, and then introducing the Nb element whose radius is similar to V^{4 +} ionic by  the co-precipitation method. The effect of Nb additive on the morphology of VPO catalyst was studied by SEM and TEM, and the morphology of the catalyst was compared with that of the VPO catalyst without additive in the figure below.

SEM and TEM images of VPO catalyst precursor showed sheet structure with irregular size, and the transmission diffraction pattern suggested amorphous structure in Fig.1a-a’. Although there was a small amount of hexahedral structure for the VPO activated catalyst, the main morphology was granular structure different in size. After the addition of Nb additive, configuration of VPO precursor was regular diamond-shaped as shown in Fig.1c-c’. This kind of crystal takes the thickness of about 200nm, but the crystal size is reduced to 40nm for the Nb-VPO activated catalyst. It suggests that: the broken of tabular crystal is helpful to improve the specific surface area, thus to provide more active sites for the catalytic reaction and increase the catalytic activity. In addition, the catalyst will expose more crystal defects after fragmentation, and provide necessary conditions for the separation of the catalyst active site. Therefore, crushing effect by adding Nb into the organic phase is beneficial to improve the selectivity of maleic anhydride. On the basis of the previous work, a comparative analysis on the morphology VPO catalyst before and after the addition of Nb, we can draw a conclusion: the crystal phase of VPO precursor containing the Nb additive is VOHPO₄∙0.5H₂O, which is preferred to grow along the (001) crystal face. So the rhombic structure is more regular. For the Nb-VPO activated catalyst, phase composition is still (VO) ₂P₂O₇, but there has been little VOPO₄ phase. The synergy of V⁺⁴ and V⁺⁵ species in two crystal phases is the key to improve catalytic activity of VPO catalyst. The catalytic selectivity has been greatly increased after the addition of Nb.

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EMC Abstracts - https://emc-proceedings.com/abstract/influence-of-metal-additive-nb-on-the-morphology-of-vanadium-phosphorus-oxide-catalysts/