Si/Al Ratio Determines the SCR Performance of Cu-SSZ-13 Catalysts in the Presence of NO₂

Abstract

A comparative study was performed to investigate the NH3-selective catalytic reduction (SCR) reaction activity of Cu-SSZ-13 zeolites having Si/Al ratios (SARs) of 5, 18, and 30. Remarkably, the Cu-SSZ-13 zeolite catalysts exhibited completely opposite behaviors as a function of SAR under standard SCR (SSCR) and fast SCR (FSCR) reaction atmospheres. Under SSCR conditions, the NO x conversion increased as expected with the decreasing SAR. Under FSCR conditions, however, the NO x conversion decreased as the SAR decreased, contrary to expectations. In this study, based on characterization of the catalysts by X-ray diffraction, transmission electron microscopy, electron paramagnetic resonance, H2-temperature-programmed reduction, temperature-programmed desorption, and diffuse reflectance infrared Fourier transform spectroscopy, together with theoretical calculations, the authors found that the amount of Brønsted acid sites goes up while the SAR goes down, leading to an increase in the accumulation of NH4NO3 under FSCR reaction conditions. Moreover, the accumulated NH4NO3 is of greater stability for those low SAR Cu-SSZ-13 catalysts. These two reasons cause the FSCR performance of Cu-SSZ-13 to decrease with a decrease in SAR. As a result, the NO2 effect on SCR activity changes from promotion to inhibition as the SAR decreases.

Unexpected Promotion Effect of H2O on the Selective Catalytic Reduction of NOx with NH3 over Cu-SSZ-39 Catalysts

“…No obvious characteristic peaks related to CuO phases (2θ = 35.6 and 38.7°) are observed in the XRD patterns. HRTEM images (Figure S4) show that all of the synthesized Cu-SSZ-39 samples have a cuboidal morphology with a mean size of about 500 nm, and there are no obvious CuO particles in the interior or on the surface of the catalysts. − The negligible number and the high dispersion of CuO species in the Cu-SSZ-39 samples are mainly attributable to the low Cu loading and the strong interaction between zeolite support and active Cu ions. ,, Therefore, the active Cu species are well dispersed and mainly present as ions at the ion-exchange sites.…”

“…As shown in Figure a, the peak intensity of H 2 O adsorbed on Lewis acid sites (at 1625 cm –1 ) is obviously stronger than that on Brønsted acid sites (at 1450 cm –1 ), indicating that H 2 O prefers to coordinate at the Cu sites. After the introduction of NH 3 , the further adsorption of NH 3 causes a change in the intensity of the two peaks (Figure b). − The peak at 900 cm –1 is attributed to the framework T–O–T vibration arising from the anchoring of Cu ions, and the coordination of H 2 O and NH 3 leads to the Cu ions to be released from the framework, thus forming a negative peak. ,, After NO + O 2 introduction, the nitrate species denoted by the bands at 1500–1700 cm –1 ,,, start to accumulate and simultaneously adsorbed NH 3 are consumed (Figure c), which suggests that the activation of NO is the rate-determining step of the SCR reaction. In the experiment of H 2 O and NH 3 coadsorption (see Figure d,e), the SCR efficiency of adsorbed NH 3 reacting with NO + O 2 is significantly lower than that after the sequential preadsorption of H 2 O followed by NH 3 (Figure c), which is attributed to the decrease of H 2 O ligands at Cu sites due to the competitive adsorption of NH 3 under the coadsorption of H 2 O and NH 3 .…”

New Insights into NH3 Oxidation and N2O Formation Over Cu-SSZ-13: Comparison of NH3-SCO and NH3-SCR

“…6,20,24,25,33,35 NH 4 NO 3 is formed over the catalyst (e.g., Cu-SSZ-13) by reactions between NH 3 and surface nitrates and would decompose into N 2 O and H 2 O at temperatures higher than 200 °C. 4,23,24,29 However, it was reported that surface nitrates were observable over Cu-CHA catalyst only when sufficient NO 2 was present. 7,23,35,43 The presence of nitrate species during low-temperature standard SCR reaction is still under debate.…”

Spatial Distribution of Brønsted Acid Sites Determines the Mobility of Reactive Cu Ions in the Cu-SSZ-13 Catalyst during the Selective Catalytic Reduction of NOx with NH3

“…The catalytic performance of heterogeneous catalysts is generally determined by the structure of active sites. − The reaction atmosphere is capable of dynamically regulating the structure of active sites and forming active multinuclear species at low temperatures, thereby realizing quasi-homogeneous reactions over Cu-SSZ-13 zeolites in the selective catalytic reduction of NO x with NH 3 (NH 3 -SCR). − The chabazite ( CHA ) structure of Cu-SSZ-13 is composed of double six-membered rings ( d6r ) that form larger cha cages in three-dimensional channel structures with eight-membered rings (8-ring) of 3.8 Å × 3.8 Å dimensions . Si 4+ is replaced by Al 3+ in the framework, and a H + ion is introduced to the Al–O site to maintain framework charge equilibrium, thus generating a Brønsted acid site. − By exchanging H + ions, Cu 2+ and Cu­(OH) + cations are anchored at d6r and 8-rings, respectively, to form active Cu sites. , Under the reduction atmospheres (i.e., NH 3 or NO + NH 3 ) or the standard SCR condition (i.e., NO + NH 3 + O 2 ), a large number of linear [Cu I (NH 3 ) 2 ] + complexes are generated through the coordination with NH 3 . , Due to the solvation effect of NH 3 , the electrostatic interaction between Cu species and the negatively charged zeolite framework is weakened, leading to a high mobility of [Cu I (NH 3 ) 2 ] + within the zeolite and even allowing its intercage migration. − The Cu-NH 3 species enable the occurrence of the NH 3 -SCR reaction within cha cages in a locally homogeneous manner, hence effectively promoting the low-temperature SCR reaction. , Thus, the formation process of active dinuclear species in the channels via dynamic [Cu I (NH 3 ) 2 ] + ions is a crucial step for low-temperature SCR reactions, and enhancing the formation efficiency of dimer-Cu species is essential for the promotion of SCR activity.…”