Selective Reduction of NO by NH3over Chromia on Titania Catalyst: Investigation and Modeling of the Kinetic Behavior

Abstract

The kinetics and the parametric sensitivity of the selective catalytic reduction (SCR) of NO by NH3were investigated over a chromia on titania catalyst. The chromium oxide phase was made up predominantly of X-ray amorphous Cr2O3. High SCR activity and selectivity to N2was attained at low temperatures. The high selectivity is attributed to the absence of significant amounts of CrO2and crystalline α-Cr2O3which favor N2O formation. The selectivity to N2O increased with higher temperature. Addition of up to 6% H2O to the dry feed reduced the rate of NO conversion and decreased the undesired formation of N2O. The effect of water on the catalytic behavior was reversible. In the absence of oxygen, the reaction between NO and NH3became marginal, independently whether H2O was present or not. Small amounts of oxygen were sufficient to restore SCR activity. Admission of SO2to the SCR feed resulted in a severe loss of activity. The poisoning of the catalyst by SO2was already notable for low SO2concentrations (30 ppm) and for temperatures up to 573 K. X-ray photoelectron and FTIR spectroscopy revealed the presence of sulfate species on the catalyst surface. Analysis of the kinetic data indicated that the SCR reaction is first order in NO and zeroth order in NH3for temperatures in the range 400–520 K. The estimated activation energies for dry and wet feed amounted to 60.0±1.6 kJ/mol (95% confidence limits). For temperatures in the range 400–520 K, and for a SO2free feed, the steady-state kinetic data could be well described with a model based on an Eley–Rideal type reaction between activated ammonia surface species and gaseous or weakly adsorbed NO.