Study on the decomposition of trace benzene over V2O5–WO3/TiO2-based catalysts in simulated flue gas

Abstract

Catalytic decomposition of benzene was studied by using oxides of vanadium and tungsten supported on titanium oxide (TiO2) catalysts for effective reduction of emissions of toxic organic compounds from waste incineration. Experiments were conducted to evaluate the effects of the catalyst's composition, and operating conditions on benzene decomposition, and the relationship between molecular structures of catalysts and their activity was also investigated through replacing the conventional TiO2 catalyst support by nano-sized TiO2. Trace levels (1 and 10ppm) of gaseous benzene were catalytically decomposed in a fixed-bed catalytic reactor with monolithic oxides of vanadium and tungsten supported on titanium oxide (V2O5–WO3/TiO2) catalysts under conditions simulating the cooling of waste incineration flue gas. On-line monitoring of trace benzene concentrations before and after the catalyst was achieved by means of resonance enhanced multiphoton ionization-time of flight mass spectrometry (REMPI-TOFMS). Catalysts were characterized by nitrogen adsorption, X-ray diffraction, energy dispersive X-ray spectroscopy, temperature-programmed reduction, and Raman spectroscopy. The effects of several parameters, including catalyst operating temperature, space velocity (SV) and initial benzene concentration, on catalytic oxidation of benzene were investigated. Experimental results indicate that reduction of the initial benzene concentration from 10 to 1ppm either enhances or decreases the catalytic removal efficiency depending on the adsorption capability as well as the oxidation ability of the catalyst tested. The catalytic activity for benzene oxidation not simply relies on the vanadium content of the catalyst; the molecular structure of vanadium oxide, which is known to be influenced by both vanadium oxide loading and the type of support, is very important. Nano-sized TiO2 supported vanadium oxide catalyst (VWNT) with lowest V loading (0.75wt.%) provides higher catalytic activity than those of the catalysts with higher V contents (1.31 and 2.92wt.%) but supported by conventional TiO2 (VWT1 and VWT2). The relatively higher catalytic activity of VWNT may be attributed to: (a) the presence of monomeric VOx species, with one terminal VO bond as indicated by Raman spectra; (b) fewer impurities confounding active catalytic constituents (V, W and Ti); (c) pure anatase phase of TiO2 rather than a mixture phase of anatase and rutile servicing as catalyst support besides its nano-scale particle size; (d) larger specific surface area, smaller average pore diameter, and narrower pore size distribution contribute higher adsorption ability for benzene. No obvious competition effect between NOx removal and benzene oxidation was observed in this study. Reaction rate constants and activation energies for benzene catalytic oxidation tested by the catalysts were calculated. Lower activation energy obtained from VWNT than those of the two catalysts further confirming the higher catalytic activity of VWNT.