SELECTIVE REDUCTION OF NO BY NH3 OVER CHROMIA ON TITANIA CATALYST - INVESTIGATION AND MODELING OF THE KINETIC-BEHAVIOR

The kinetics and the parametric sensitivity of the selective catalytic reduction (SCR) of NO by NH3 were investigated over a chromia on tita nia catalyst. The chromium oxide phase was made up predominantly of X- ray amorphous Cr2O3. High SCR activity and selectivity to N-2 was atta ined at low temperatures. The high selectivity is attributed to the ab sence of significant amounts of CrO2 and crystalline alpha-Cr2O3 which favor N2O formation. The selectivity to N2O increased with higher tem perature. Addition of up to 6% H2O to the dry feed reduced the rate of NO conversion and decreased the undesired formation of N2O. The effec t of water on the catalytic behavior was reversible. In the absence of oxygen, the reaction between NO and NH3 became marginal, independentl y whether H2O was present or not. Small amounts of oxygen were suffici ent to restore SCR activity. Admission of SO2 to the SCR feed resulted in a severe loss of activity. The poisoning of the catalyst by SO2 wa s already notable for low SO2 concentrations (30 ppm) and for temperat ures up to 573 K. X-ray photoelectron and FTIR spectroscopy revealed t he presence of sulfate species on the catalyst surface. Analysis of th e kinetic data indicated that the SCR reaction is first order in NO an d zeroth order in NH3 for temperatures in the range 400-520 K. The est imated activation energies for dry and wet feed amounted to 60.0 +/- 1 .6 kJ/mol (95% confidence limits). For temperatures in the range 400-5 20 K, and for a SO2 free feed, the steady-state kinetic data could be well described with a model based on an Eley-Rideal type reaction betw een activated ammonia surface species and gaseous or weakly adsorbed N O. (C) 1997 Academic Press.