V–P mixed oxide catalysts prepared by evaporation method was made up of a mixture of an amorphous phase with P/V = 2.0 and a crystalline phase of (VO)2P2O7. The amorphous phase, separated from the (VO)2P2O7 particles by dissolving the precursor in boiled water, was found to show essentially the same selectivity and activity per unit surface area for the maleic anhydride formation from butane as the unseparated catalyst.
The influence of B on the state and dispersion of Mo phases on A1203 have been investigated by bulk and surface characterization techniques, e.g., XRD, XPS and TPR, respectively. In the absence of B, the dispersion of Mo phases Is in good agreement with the theoretical value calculated from Kerkhof-Moulijn model, suggesting that Mo atoms disperse easily on A1203 surface. XPS results indicate that B addition to A1203 leads the Mo dispersion to decrease in comparison with the theoretical mondayer value. This result is also confirmed by XRD analysis, which shows the formation of crystalline Moo3 particles with B addition. TPR profiles for Mo/A1203 consists of two reduction peaks, a low temperature peak (400°C-4500C) and a high temperature one (77OoC-810"C), respectively. The peak intensity of the higher temperature decreases rapidly with B addition, whereas that of lower temperature increases wi.h increasing B contents. Consequently, these results suggest that B addition weakens the interaction between Mo species and the A1203 surface.
INTRODUCTIONCatalysts based on molybdenum or tungsten sulfide and promoted by nickel and/or cobalt have been used extensively for hydrotreating in petroleum refining such as hydrodesulfurization (HDS), hydrodenitrogenation (HDN) and hydrodemezallization (HDM) for many years. Many studies have been focused on both elucidation of the structure of active phases and enhancement of HDS, HDN and HDM activities.The chemistry and structure of CoMolAlnOs catalysts have been well studied. Several models have been proposed to describe the interactlon between Mo and Co species. These models include the mondayer model [ l , 21, Co-Mo-S complex model [3-51, intercalation model [6] and contact synergy (remote control) model [7]. Recently, the Co-Mo-S model is generally believed as the active structure for the CoMolAlnOs catalysts.In order to take advantage of the relatively high surface area of Al203, a number of studies have been carried out in which the A203 support was modified by such oxides, e.g., MgO [8]
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