Design of a "high-efficiency" $NH_3$-SCR reactor for stationary applications : a kinetic study of $NH_3$ oxidation and $NH_3$-SCR over V-based catalysts

In this work, the kinetics of the NH3/NO/O2 reacting system over V2O5–WO3/TiO2 catalysts was studied under conditions of NH3/NO ratio ≥ 1 and temperature up to 400 °C, where some ammonia oxidation is expected. NH3-SCR and NH3-oxidation tests were performed over powdered catalysts, at varying V-loads. Over a high V-load home-made catalyst, NO conversion passed through a maximum at increasing temperature, as commonly observed in SCR tests at the onset of ammonia oxidation; however, since the experiments were performed under excess ammonia, the high temperature decrease of NO conversion could not be associated with a lack of surface NH3. The experiments were interpreted as the evidence of an unselective NH3-oxidation route, giving rise to some NO production. NH3-oxidation tests, though, showed low selectivity to NO (which is in line with the current literature); however, a quantitative analysis of data showed that the observed product distribution is fully in line with an indirect-consecutive reaction scheme from NH3 to N2, wherein NO is formed by NH3 oxidation and is then rapidly converted to N2 via NH3-SCR. The kinetic investigation was extended to commercial catalysts with low and medium V-loads. NH3-SCR tests showed increasing trends of NO conversion with temperature within the range 250–400 °C, thus they did not show any clear evidence for NH3 oxidation. However, NH3-oxidation tests showed that the reaction was active above 350 °C; notably, over 95% selectivity to N2 was observed. A quantitative analysis of the NH3-oxidation tests, based on the independent estimate of the intrinsic rate of NH3-SCR, showed that the observation of few ppm NO slip is fully in line with the assumption of a two-step kinetic scheme. A reaction path analysis shows that by neglecting the unselective nature of ammonia oxidation on V-catalysts the rate of this reaction is overestimated by a factor of 2. Preliminary evaluations suggest that the impact of NH3-oxidation on the integral performance of monolith reactors (operating at NH3/NO ratios close to 1) is non negligible, so that the correct identification of the reaction stoichiometry is crucial