Surface properties of VOx-SiO2 and VOx-Al2O3 catalysts: a spectroscopic study by FT-IR, Micro-Raman, XPS and EPR techniques

Recently, the flame pyrolysis method (FP) has been proposed for the preparation of VOx-SiO2 and VOx-Al2O3 catalysts (nominal content of V2O5 5-50 % by weight) effective in the oxidative dehydrogenation (ODH) of propane to propylene [1,2]. As a whole, catalytic tests revealed that VOx-Al2O3 samples were more active whereas VOx-SiO2 exhibited higher selectivity to propylene. Specifically, the most selective catalyst resulted to be that with a 10 % nominal content of V2O5 (V10Si). As expected, the catalytic performances of such catalysts may be correlated to their physico-chemical properties, as studied by FT-IR, Micro-Raman, XPS and EPR techniques. IR spectra of VOx-SiO2 samples (Figure 1) showed a band at 930 cm-1, due to the vibration of SiO44- groups strongly polarized by interaction with vicinal vanadium atoms: such [SiOδ-…Vδ+] species were not observed with a sample prepared by impregnation (V10Si-i), indicating that V-incorporation into the silica framework takes place during FP. No evidence of V introduction in the framework of alumina was instead detected in VOx-Al2O3 systems. Micro-Raman analysis showed the presence of isolated VOx species (signals 1027 and 512 cm-1) only with sample V10Si, whereas with all other VOx-SiO2 and VOx-Al2O3 systems, and with samples prepared by impregnation, only bands of crystalline V2O5 were detected. Adsorption of CO at nominal -196 °C on V10Si sample showed the presence of different OH species, with the following relative acidity scale: isolated SiOH < H-bonded SiOH < V-OH. Likewise, the Brønsted acidity was studied by means of NH3 adsorption on samples outgassed at 500 °C. The blue-shift (ΔνNH4+) of the band due to the bending vibration of NH4+ species with respect to that of free NH4+ (1410 cm-1) can be used as a semi-quantitative measure of the acidic strength of Brønsted sites: the smaller the shift the higher the Brønsted acidity. A good correlation has been observed between the acidic strength of Brønsted sites and the selectivity to propylene for both VOx-SiO2 and VOx-Al2O3 systems: for both sets of catalysts, the selectivity increases with the decreasing of the Brønsted acidic strength, which favors side-reactions catalyzed by acids leading to a higher amount of COx [3]. X-ray photoelectron spectroscopy (XPS) was used to investigate the electronic structure of VOx species. Indeed, XPS can provide information on the V oxidation states by considering the V 2p3/2 binding energies (BE). Interestingly, the V10Si sample exhibited a broad peak at higher V 2p3/2 BE as compared to other VOx-SiO2 systems, probably due to the presence of highly dispersed V5+-OH groups partially incorporated into the silica matrix and interacting poorly with one another [4,5]. A similar spectra was recorded on V5Si, although the latter exhibited an additional signal at ca. 516 eV, suggesting also the presence V3+ species at the surface. The latter result is in fair agreement with the worse catalytic performances of V5Si towards the ODH of propane, kinetically modeled through a MvK mechanism based on V5+ reduction and subsequent reoxidation. On the contrary, at higher V-contents and with V10Si-i sample peaks at low V 2p3/2 BE appeared, typically assigned to extra-framework V2O5. EPR spectra revealed weaker V=O bond in VOx- Al2O3 systems: being V=O bond strength an index of oxygen availability, this could explain their higher activity towards this reaction. In conclusion, for FP-prepared catalysts: (i) a better dispersion of the active phase can be obtained with silica rather than with alumina; (ii) selectivity to propylene depends on Brønsted acidity; (iii) stronger Brønsted acidity leads to higher COx production. Moreover, the best catalyst (V10Si) was characterized by highly dispersed (isolated) V species, less acidic Brønsted sites, higher V oxidation states and stronger V=O bonds. References: [1] I. Rossetti, L. Fabbrini, N. Ballarini, F. Cavani, A. Cericola, B. Bonelli, M. Piumetti, E. Garrone, H. Dyrbeck, E. A. Blekkan, L. Forni, J. Catal. 256 (2008) 45-61. [2] I. Rossetti, L. Fabbrini, N. Ballarini, C. Oliva , F. Cavani, A. Cericola, B. Bonelli, M. Piumetti, E. Garrone, H. Dyrbeck, E.A. Blekkan, L. Forni, Catal. Tod., 141 (2009) 271-281. [3] K. Chen, A. Khodakov, J. Yang, A. T. Bell, E. Iglesia, J. Catal. 186 (1999) 325-333. [4] C. Hess, R. Schlögl, Chem. Phys. Lett. 432 (2006) 139-145. [5] C. Hess, J. Catal. 248 (2007) 120-123.