RESUMO
Based on the precise phase control V species adjustment of vanadium phosphorus oxides (VPOs), a series of metal oxides (Nb2O5, MoO3, WO3, and Bi2O3) were selected as modification agents to further enhance the catalytic activity and retain the excellent durability of VPO-TiO2-based catalysts for the new procedure of producing acrylic acid via acetic acid-formaldehyde condensation. At an elevated liquid hourly space velocity (LHSV), the (AA + MA) selectivity reached 92.3% with a (MA + AA) formation rate of 63.8 µmol-1 gcat -1 min-1 over the Nb-decorated catalyst (catalyst VTi-Nb), and it maintained good durability for up to 100 h. The detailed characterization results of XRD, Raman, XPS, NH3-TPD, CO2-TPD, and H2-TPR, demonstrated that the addition of Nb2O5 could observably enhance the catalytic efficiency of the VPO-TiO2 catalyst. It not only improved the catalyst durability by enhancing prereduction of the V5+ species, but also enhanced the active site density to improve the catalytic activity.
RESUMO
High-purity anatase TiO(2) nanoparticles were prepared using a low-temperature sol-gel route. The as-prepared sample was characterized by X-ray diffraction, transmission electron microscopy, infrared spectroscopy, thermogravimetric analysis, UV-vis spectroscopy, and photoluminescence. It is shown that the as-prepared sample crystallized in a pure anatase phase with an average crystallite size of about 7 nm, and the surfaces were highly hydrated. These nanoparticles were stabilized as a water suspension via the cooperation of DLVO force and surface hydration force. These suspensions showed characteristic band-gap emission at 397+/-1.5 nm, which is a little red-shifted compared with the band-gap energy of indirect electronic transition measured in the UV-vis absorption spectrum. These observations were explained by the light-induced relaxation of polar water molecules in the surface hydration layer.