Revealing the Formation and Reactivity of Cage-Confined Cu Pairs in Catalytic NO_x Reduction over Cu-SSZ-13 Zeolites by In Situ UV–Vis Spectroscopy and Time-Dependent DFT Calculation

Abstract

The low-temperature mechanism of chabazite-type small-pore Cu-SSZ-13 zeolite, a state-of-the-art catalyst for ammonia-assisted selective reduction (NH 3 -SCR) of toxic NO x pollutants from heavy-duty vehicles, remains a debate and needs to be clarified for further improvement of NH 3 -SCR performance. In this study, we established experimental protocols to follow the dynamic redox cycling (i.e., Cu II ↔ Cu I ) of Cu sites in Cu-SSZ-13 during low-temperature NH 3 -SCR catalysis by in situ ultraviolet− visible spectroscopy and in situ infrared spectroscopy. Further integrating the in situ spectroscopic observations with timedependent density functional theory calculations allows us to identify two cage-confined transient states, namely, the O 2 -bridged Cu dimers (i.e., μ-η 2 :η 2 -peroxodiamino dicopper) and the proximately paired, chemically nonbonded Cu I (NH 3 ) 2 sites, and to confirm the Cu I (NH 3 ) 2 pair as a precursor to the O 2 -bridged Cu dimer. Comparative transient experiments reveal a particularly high reactivity of the Cu I (NH 3 ) 2 pairs for NO-to-N 2 reduction at low temperatures. Our study demonstrates direct experimental evidence for the transient formation and high reactivity of proximately paired Cu I sites under zeolite confinement and provides new insights into the monomeric-to-dimeric Cu transformation for completing the Cu redox cycle in low-temperature NH 3 -SCR catalysis over Cu-SSZ-13.

Transition-Metal-Promoted Cu-SSZ-13 Catalysts for Continuous Selective Oxidation of Methane to Methanol Using Molecular Oxygen

“…The best-performed Cu-Fe-SSZ-13 with 0.2 wt % Fe was investigated as a representative to unravel the promoting effect of TM doping using various characterization methods, including inductively coupled plasma optical emission spectroscopy (ICP-OES), X-ray powder diffraction (XRD), N 2 physisorption, X-ray absorption spectroscopy (XAS), temperature-programmed desorption, and diffuse reflectance infrared Fourier transform spectroscopy using NH 3 as a probe molecule (NH 3 -TPD and NH 3 -DRIFTS). N 2 physisorption results (Table S2) suggest that the Fe or Zn introduction did not alter significantly the surface area and porosity of the SSZ-13 zeolite, which agrees well with our previous studies. − Characteristic diffraction peaks for the CHA zeolite were detected by XRD at 2θ = 9.5°, 14.0°, 16.1°, 17.8°, 20.7°, 25.0°, 26.2°, and 30.7° in all the catalysts (Figure S2), , which confirms that the introduction of Cu, as well as Fe and Zn, has no detectable influence on the zeolite framework structure. ICP-OES (Table S3) revealed that the Cu contents in the zeolites were reduced after TM introduction, likely because of the fact that these TM ions occupied a portion of the ion-exchange sites and, thus, replaced partially the introduced Cu ions .…”

“…38 Weak absorption in the 400−500 nm range was also detectable over Cu-Fe-SSZ-13, which can be attributed to small FeO x clusters typically present in Fe zeolites. 37 Over the NH 3 -saturated zeolite catalysts, an intensified contribution was recorded at ∼350 nm in the in situ DRUVS spectra for Cu-Fe-SSZ-13, which, according to our previous studies on Cu-SSZ-13 NH 3 -SCR catalysts, 21,22,38 is due to a higher population of highly dispersed binuclear Cu−O−Cu moieties (Figure 2c). The transformation of isolated Cu 2+ to [Cu(OH)] + in Cu-Fe-SSZ-13 was also confirmed by in situ DRIFTS, which revealed a higher band intensity at ∼917 cm −1 (for the T−O−T skeletal vibrations perturbated by [Cu(OH)] + ), while a lower band intensity was determined at 850 cm −1 (for the T−O−T skeletal vibrations perturbated by Cu 2+ ) than that of Cu-SSZ-13 (Figure 2d).…”

“…As compared with that of Cu-SSZ-13 (Figure 3c,d), the LMCT band intensity of Cu-Fe-SSZ-13 decreased more dramatically during CH 4 exposure and increased more significantly after O 2 exposure (Figure 3e,f), thereby implying that Fe promoted the reaction-driven Cu 2+ ↔ Cu + redox cycling. 21,22 Such promotion is believed to originate from a favored formation of [Cu(trans-μ-1,2-O 2 )Cu] 2+ dimers, which can be easily excited because of a small energy difference between its ground and excited states. 42 Finally, MTM mechanisms over the Cu zeolites were interrogated by in situ DRIFTS in a CH 4 -O 2 -H 2 O mixture similar to that in Figure 1.…”

“…Surface acidity of the zeolite catalysts was investigated by using NH 3 -TPD (Figures a and S6) and NH 3 -DRIFTS (Figure b). Strong peaks at ∼110 °C in the NH 3 -TPD profiles correspond to the desorption of NH 3 on weak acid sites. − The peaks at 240 to ∼280 °C are related to the desorption of NH 3 adsorbed on Lewis acid sites (LASs), which are associated with the Cu and Fe species . The high-temperature peaks at around 350 °C are attributed to the desorption of NH 3 from strong Brønsted acid sites (BASs) of the zeolites. − Intensities of the low-temperature desorption peaks are similar for both samples, thereby demonstrating that the introduction of Fe species had negligible impact on the abundance of weak acid sites .…”

“…In situ diffuse reflectance ultraviolet–visible spectroscopy (DRUVS), in situ DRIFTS, and in situ Raman spectroscopy were combined to investigate the nature of Cu and/or Fe species in various zeolite catalysts. − For Cu-SSZ-13, Fe-SSZ-13, and Cu-Fe-SSZ-13 activated in O 2 , in situ DRUVS spectra (Figures S4 and S5) revealed dominant ligand-to-metal charge transitions (LMCT) from the framework O to Cu 2+ or [Cu­(OH)] + species (coordinated within the six- or eight-membered rings, i.e., 6MR or 8MR, of the CHA framework) corresponding to the strong absorption bands centered at ∼263 nm. , Notably, isolated Fe 3+ ions with characteristic absorptions below 300 nm (Figure S4) may contribute to this band, as well. , For Cu-Fe-SSZ-13, three broad yet clearly visible absorption bands were observed at 497, 555, and 748 nm, respectively, which can be ascribed to the characteristic quadruplet d–d transitions for [Cu­(OH)] + species (the fourth band is expected at ∼910 nm, which is beyond the measuring range of the instrument) . On the contrary, intensities of these bands for [Cu­(OH)] + are clearly lower in case of Cu-SSZ-13 (Figure S4), which suggests a prevalence of Cu 2+ in the catalyst .…”