Selective catalytic oxidation of ammonia (NH3-SCO) on iron beta zeolite catalysts prepared by ion exchange and solvated metal atom dispersion

Metal modified Beta zeolites are well-known structures, whose use as catalysts in the selective catalytic reduction of nitrogen oxides by ammonia (NH3-SCR-DeNOx) is supported by decades of studies and several commercial applications.1–3 Recently, iron modified Beta (Fe/Beta) zeolites revealed to be promising catalysts also in the selective catalytic oxidation of ammonia into nitrogen and water vapor (NH3-SCO). The latter is a common strategy for ammonia emission control, which will play an increasingly important role in the perspective to launch the NH3-SCR technology towards the goal of zero-emission of NOx from polluted gaseous.4 Indeed, to accomplish ever stricter standards for NOx emissions a possible approach could be to improve the NOx reduction efficiency by using a large excess of NH3 in the SCR reactor, and then to abate the unconverted NH3 in a second catalytic reactor, where the SCO process would occur.5 Despite the extensive body of scientific literature reporting on Fe/Beta for SCR reaction, at the current state, only few papers have appeared elucidating structure-activity relationships underlying the catalytic behaviour of Fe/zeolites in NH3-SCO.6 In this work, the role of iron deposition method on the catalytic performances of Fe/Beta catalysts in NH3-SCO has been investigated to unravel interesting relations between aggregation state of the metal phase and the corresponding catalytic performances. Besides the classical ion exchange (IE) procedure conventionally used to introduce iron phases in zeolite, solvated metal atom dispersion method (SMAD) has been considered as novel route for iron deposition on zeolite surface. The SMAD approach consists in the deposition of metal nanoparticles, named solvated metal atoms, which can be easily immobilized onto different kind of supports guarantying a high dispersion of the resulting metal phase. Low concentration (ca. 2 wt.%) of iron has been deposited either by SMAD or IE methods on Beta zeolite. In addition, iron functionalization by IE was carried out also on ZSM-5, selected as reference structure known to assure high dispersion of isolated centers. From the combination of several physicochemical characterization techniques it emerges that the zeolite topology as well as the deposition procedure strongly influenced the nuclearity of iron phase on zeolites. Iron-species distribution and dimension on the two zeolites were determined by Transmission electron microscopy (TEM) techniques combined with element maps. As expected, highly homogeneous dispersed iron species were introduced on the ZSM-5 sample by IE, while the same deposition method led to the formation of FeOx aggregates (2.5-10 nm) together with isolated iron species on Beta zeolite. On the other hands, by SMAD approach, well-formed FeOx-nanoparticles ranging 1.0 – 4.5 nm were revealed on Beta zeolite. The different iron dispersion and speciation of Fe/zeolite samples were confirmed by UV Diffuse reflectance Spectroscopy (UV-DRS), solid-gas acid-base titrations with ammonia as base probe molecule, H2-temperature programmed reduction (H2-TPR) and X-ray photoelectron spectroscopy (XPS). Ammonia oxidation activity (NH3-SCO) on iron-containing zeolites started at ca. 300°C, without no clear effect of the size of Fe on the reaction activity/selectivity. Ammonia conversion regularly increased with temperature with always very high selectivity to dinitrogen (98-100%), without any NOx or N2O formation (Table 1). Table 1 Main catalytic results Catalyst Activity a /mmolNH3 gcat-1 min-1 Selectivity to N2b / % Selectivity to NOx b / % Selectivity to N2O b / % Fe/ZSM-5IE 8.6 98.83 0 1.17 Fe/BetaIE 20.8 98.74 0 1.26 Fe/BetaSMAD 21.7 99.54 0 0.46 Fe/SA 18.4 92.70 0 7.30 a determined at 375°C; b evaluated at 90% of NH3 conversion; Fe/Beta catalyst prepared by SMAD was the best catalyst in terms of both activity (21.7 mmol g-1 min-1) and selectivity to N2 (> 99 % at 90 % conversion). Moreover, the catalyst exhibited a notable stability in reaction conditions even after four reaction runs. Only very limited increase of iron particle dimensions was observed on the used Fe-catalysts, in any case. The collected experimental results indicated that not only isolated well-dispersed iron species are associated with high activity and selectivity in the NH3-SCO reaction. SMAD-derived iron nanoparticles worked with excellent performances in the ammonia oxidation reaction with high activity in terms of conversion, selectivity to dinitrogen, and stability.