Formation of sulfur trioxide during the SCR of NO with NH3 over a V2O5/TiO2 catalyst

Abstract

The oxidation of sulfur dioxide (SO2) to sulfur trioxide (SO3) is an undesirable reaction that occurs during the selective catalytic reduction (SCR) of nitrogen oxides (NOx) with ammonia (NH3), which is a process applied to purify flue gas from coal-fired power plants. The objectives of this work were to establish the fundamental kinetics of SO3 formation over a V2O5/TiO2 catalyst and to illustrate the formation mechanism of SO3 in the presence of NOx, H2O and NH3. A fixed-bed reactor was combined with a Fourier transform infrared (FTIR) spectrometer and a Pentol SO3 analyser to test the outlet concentrations of the multiple components. The results showed that the rate of SO2 oxidation was zero-order in O2, 0.77-order in SO2 and -0.19-order in SO3 and that the apparent activation energy for SO2 oxidation was 74.3 kJ mol-1 over the range of studied conditions. Based on in situ diffuse reflectance infrared Fourier transform (in situ DRIFT) spectroscopy, X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD) tests, the SO3 formation process is described here in detail. The adsorbed SO2 was oxidized by V2O5 to produce adsorbed SO3 in the form of bridge tridentate sulfate, and the adsorbed SO3 was desorbed to the gas phase. NOx promoted the oxidation of the adsorbed SO2 due to the promotion of the conversion of low-valent vanadium to high-valent vanadium. In addition, the desorption of the adsorbed SO3 was inhibited by H2O or NH3 due to the conversion of tridentate sulfate to the more stable bidentate sulfate or ammonium bisulfate. Finally, the mechanism of the influence of NOx, H2O and NH3 on the formation of gaseous SO3 was proposed.