Formation of sulfur trioxide during the SCR of NO with NH over a VO/TiO catalyst.

The oxidation of sulfur dioxide (SO) to sulfur trioxide (SO) is an undesirable reaction that occurs during the selective catalytic reduction (SCR) of nitrogen oxides (NO ) with ammonia (NH), 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 SO formation over a VO/TiO catalyst and to illustrate the formation mechanism of SO in the presence of NO , HO and NH. A fixed-bed reactor was combined with a Fourier transform infrared (FTIR) spectrometer and a Pentol SO analyser to test the outlet concentrations of the multiple components. The results showed that the rate of SO oxidation was zero-order in O, 0.77-order in SO and -0.19-order in SO and that the apparent activation energy for SO oxidation was 74.3 kJ mol over the range of studied conditions. Based on diffuse reflectance infrared Fourier transform ( DRIFT) spectroscopy, X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD) tests, the SO formation process is described here in detail. The adsorbed SO was oxidized by VO to produce adsorbed SO in the form of bridge tridentate sulfate, and the adsorbed SO was desorbed to the gas phase. NO promoted the oxidation of the adsorbed SO due to the promotion of the conversion of low-valent vanadium to high-valent vanadium. In addition, the desorption of the adsorbed SO was inhibited by HO or NH due to the conversion of tridentate sulfate to the more stable bidentate sulfate or ammonium bisulfate. Finally, the mechanism of the influence of NO , HO and NH on the formation of gaseous SO was proposed.