Transient experiments performed over synthesized and commercial V2O5−WO3/TiO2 catalysts during catalyst conditioning and during step changes of the operating variables (SO2 inlet concentration and temperature) show that conditioning of the catalyst is required to attain significant and reproducible steady-state data in both the reduction of NO x and the oxidation of SO2 . The response time of conditioning for NO x reduction is of a few hours and that for SO2 oxidation is of several hours. Fourier transform infrared spectroscopy temperature programmed decomposition, and thermogravimetric measurements showed that catalyst conditioning is associated with a slow process of buildup of sulfates:  the different characteristic conditioning times observed in the reduction of NO x and in the oxidation of SO2 suggest that the buildup of sulfates occurs first at the vanadyl sites and later on at the exposed titania surface. Formation of sulfates at or near the vanadyl sites increases the reactivity in the de-NO x reaction, possibly due to the increase in the Brønsted and Lewis acidity of the catalyst, whereas the titania surface acts as SO3 acceptor and affects the outlet SO3 concentration during catalyst conditioning for the SO2 oxidation reaction. The response time to step changes in SO2 concentration and temperature is of a few hours in the case of SO2 oxidation and much shorter in the case of NO x reduction. The different time responses associated with conditioning and with step changes in the settings of the operating variables have been rationalized in terms of the different extent of perturbation of the sulfate coverage experienced by the catalyst.

No abstract

An approach to the analysis of the plate-type catalysts for high-dust selective catalytic reduction (SCR) applications is herein presented. Commercial systems were studied in the form of slabs. NO x reduction and SO2 oxidation runs were performed spanning a wide range of operating conditions. The results were analyzed on the basis of a 1D model. The estimates of the intrinsic activity parameters and of the effectiveness factors were found to differ significantly among the catalysts under study. Aspects related to scale-up were also addressed on a preliminary basis. Concerning the SO2 oxidation reaction, the lab-scale analysis of the catalysts can be directly exploited for reactor design. On the contrary, the evaluation of the DeNO x performance of the industrial plate-type monolith requires the study of the interphase mass transfer in the real scale; the NO conversion data collected in the slab-reactor were estimated as not conservative in relation to design applications.

A dynamic kinetic study of the undesired oxidation of SO2 over a commercial V2O5−WO3/TiO2 monolith catalyst for the SCR-DeNO x process is herein presented. Transient SO2 conversion data were systematically collected during experiments involving step changes of temperature, area velocity, and feed composition (SO2, O2, H2O, and NH3). Characteristic times of the system response were of a few hours:  peculiar SO3 emission peaks were apparent upon increasing the reaction temperature and the H2O feed content. The data were well interpreted by a dynamic kinetic model assuming that buildup−depletion of surface sulfate species is rate controlling. The model accounts successfully also for the transient effects resulting from the interaction between the SCR-DeNO x reaction (NO x reduction with NH3) and the oxidation of SO2.