Physico-chemical properties of VOx-TiO2 catalysts prepared by flame pyrolysis

The catalytic properties of titanium dioxide are being considered, among others, for photocatalytic splitting of water into hydrogen and oxygen and for degradation of contaminants in wastewater and air [1]. The highly efficient use of TiO2 is sometimes prevented by its wide band gap: the band gap of bulk TiO2 lies in the UV range (3.0 eV for the rutile phase and 3.2 eV for the anatase), which is only a small fraction of the sun energy (<10%) [2]. It follows that to improve the performance of TiO2 – based systems, one should increase their optical activity by shifting the absorption onset to the visible region. Previous studies observed red-shifts in the adsorption thresholds of TiO2 after doping with transition-metals (V, Cr, Mn, Fe and Ni), obtaining the following order in terms of red-shifts: V > Cr > Mn > Fe > Ni [2,3]. In this work, a series of VOx-TiO2 systems (5 ÷ 15 wt.% V loading) was obtained by flame pyrolysis (FP), a high temperature synthesis technique allowing high V-dispersion to be achieved. The prepared samples were characterized by powder XRD, SEM microscopy, DR UV-Vis, IR, Raman and XPS spectroscopies. As a whole, X-ray diffraction patterns of all VOx-TiO2 systems showed the presence of ca. 50:50 anatase : rutile phases, although the amount of rutile was observed to progressively increase with V-content (up to 60%). SEM micrographs showed uniform particles size, although better homogeneity and smaller particles (diameter of ca. 100 nm) were obtained at higher V loadings. Micro-Raman spectroscopy with both 5 and 10 wt.% VOx-TiO2 showed the presence of isolated or low-polymeric VOx species (bands 1030 and 930 cm-1, respectively), whereas micro-crystalline V2O5 (sharp peak at 990 cm-1) was detected at the highest V content (15 wt.% VOx-TiO2) [4]. Figure 1 shows DR UV-Vis spectra of (hydrated) powders. As compared with the spectrum of pure TiO2, an additional absorption is observed in the 400-600 nm range (maximum at ca. 400 nm), suggesting the incorporation of Vanadium [5]. XPS analysis, used to investigate surface VOx species, showed that all VOx-TiO2 samples exhibited a main V 2p3/2 peak at ca. 517 eV, corresponding to V5+ species, and a less intense peak at ca. 516 eV due to V4+ species (Figure 2), with an estimated ratio V5+:V4+ close to 4:1. The latter finding is particularly important for oxidation reactions, and therefore these materials could be attractive catalysts for many photocatalytic applications and their activity in both the splitting of water and in the degradation of organic pollutants under both UV and visible light irradiation are currently under investigation. References [1] E. Nilsson, Y. Sakamoto, A.E.C. Palmqvist, Chem. Mater. 23 (2011) 2781. [2] X. Chen, S.S. Mao, Chem. Rev. 107 (2007) 2891. [3] M.A. Henderson, Surface Science Reports 66 (2011) 185. [4] G. Ertl, H. Knözinger, F. Schüth, J. Weitkamp, Handbook of Heterogeneous Catalysis , 2nd ed., Wiley, John & Sons Incorporated, 2008, p. 550. [5] R. Akbarzadeh, S.B. Umbarkar, R.S. Sonawane, S. Takle, M.K. Dongare, Appl. Catal. A: Gen. 374 (2010) 103.