On the mechanism of the SCR reaction on Fe/HBEA zeolite

The mechanism of the so-called standard SCR reaction on an efficient HBEA zeolite catalyst modified with 0.25wt.% Fe (0.25Fe/HBEA) was elucidated. The studies were systematically performed by using DRIFTS, TPD, XANES and kinetic studies. SCR kinetics were examined between 175 and 275°C employing a gradient-free loop reactor. DRIFTS and NOx-TPD showed formation of Fe3+-NO species as well as their reaction with NH3. But, only minor adsorption and conversion of NOx on the zeolite substrate occurred. On the contrary, DRIFTS and NH3-TPD showed mainly adsorption of NH3 on the zeolite, whereas XANES and kinetic examinations indicated NH3 adsorption on the Fe3+ sites as well. Moreover, the XANES studies evidenced deNOx on the iron component which is considered to be the major pathway in standard SCR. From our experimental investigations we suggest a dual site mechanism implying the adsorption and reaction of NO and NH3 on neighbouring Fe3+ sites. The uptake of NH3 results in partial reduction of Fe3+ sites which are finally recycled by O2. However, some contribution from a single site mechanism cannot be ruled out completely. Numerical modelling of the NH3-TPD pattern provides kinetic parameters of the adsorption and desorption of NH3 on 0.25Fe/HBEA differentiating the specific adsorption sites of the substrate. As a result, the uptake and release of NH3 on the zeolite is found to be much faster than that of SCR. In connection with a previous SSITKA study, this comparison suggests that NH3 undergoes several adsorption/desorption cycles on the substrate before adsorbing and reacting on the Fe3+ sites.