The oxidation of CO by Au/Fe(2)O(3) and Au/ZnO catalysts is compared in the very early stages of the reaction using a temporal analysis of products (TAP) reactor. For Au/Fe(2)O(3) pre-dosing the catalyst with (18)O labelled water gives an unexpected evolution order for the labelled CO(2) product with the C(18)O(2) emerging first, whereas no temporal differentiation is found for Au/ZnO. High pressure XPS experiments are then used to show that CO bond cleavage does occur for model catalysts consisting of Au particles deposited on iron oxide films but not when deposited on ZnO films. DFT calculations, show that this observation requires carbon monoxide to dissociate in such a way that cleavage of the CO bond occurs along with dynamically co-adsorbed oxygen so that the overall process of Au oxidation and CO dissociation is energetically favourable. Our results show that for Au/Fe(2)O(3) there is a pathway for CO oxidation that involves atomic C and O surface species which operates along side the bicarbonate mechanism that is widely discussed in the literature. However, this minor pathway is absent for Au/ZnO.
Catalysts H 2000Synthesis and Characterization of Vanadyl Hydrogen Phosphite Hydrate. -VOHPO3·1.5H2O is synthesized by solvothermal reaction of V2O5 and H3PO3 in H2O-free PrOH (autoclave, 150°C, 24 h). In the presence of water, VOHPO3·H2O is the unique product. The samples are characterized by TGA, powder XRD, and IR and Raman spectroscopy. On heating the samples in flowing N2 at >700°C they transform into VPO 4 via an internal oxidation-reduction process. A catalyst derived from VOHPO3·H2O and activated in situ under the reaction conditions shows higher intrinsic activity for the selective oxidation of n-butane to maleic anhydride than that derived from VOHPO3·1
Temperature programmed reduction (TPR) and oxidation (TPO) are used extensively in catalyst characterisation. In this paper, we examine the use of TPR/TPO cycles for the characterisation of a range of molybdates and single oxides. In particular we observe that the first cycle differs from that of subsequent analyses, even when the maximum temperature is limited to that used in the catalytic reaction. The effect is independent of heating rates and cooling atmospheres and has been demonstrated using different bed configurations. This observation has significance when these oxides are used in periodic flow reactors that involve many cyclical reduction/oxidation.
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