Reaction-driven Ion Exchange of Copper into Zeolite SSZ-13

Abstract

We have used several techniques to characterize Cu-SSZ-13 before and after exposure to reaction conditions relevant for NO reduction by NH3–SCR to address the increase in NH3–SCR activity observed as a function of time on stream. Specifically, we focus on characterizing copper species in samples prepared with solid state ion exchange (SSIE) having varying Cu loadings from 0.7 to 5.2 wt %. X-ray diffraction shows that CuO species that likely remain from the SSIE synthesis on the outside of the zeolite crystallites are significantly reduced by exposure to reaction conditions. High-resolution X-ray diffraction (HR-XRD) further reveals a corresponding increase of Cu2O species. X-ray photoelectron, Auger electron, and ultraviolet–visible spectroscopy show an increase in both Cu­(II) and Cu­(I) species inside the zeolite pores, with a preserved long-range order of the zeolite structure as revealed by HR-XRD.

Ammonia Desorption Peaks Can Be Assigned to Different Copper Sites in Cu/SSZ-13

“…Thereafter the temperature was raised to 800 °C at a ramping rate of 10 °C/min and then kept at 800 °C for a further 60 min. The NH 3 -TPD experiments consisted of an adsorption phase of 5 h at 70 °C during which 2000 ppm of NH 3 were flown through the bed, followed by a purge in Ar at 70 °C and a temperature ramp of 10 °C/min. Finally, ammonia oxidation experiments were also carried out in the same powder reactor.…”

“…Prior to each experiment, the catalyst was degreened as described above and then pre-treated in 8% O 2 at 500 °C for 30 min. Ammonia oxidation was carried out with 1200 ppm NH 3 and 8% O 2 . The outlet ammonia concentration was measured using the MS and steady state ammonia concentrations at each temperature were used to calculate ammonia conversions.…”

“…However, the connection between the TPD peaks and these two different Cu site location has not been extensively studied. Moreover, while there are two published studies using different copper loadings as a tool to understand the TPD peaks [2,3,7], neither correlated ammonia storage with different positions in the zeolite. This could be because Lezcano-Gonzalez et al [2] used only two Cu-loadings (ion exchange levels (IE) = 67 and 100%) and Clemens et al [3] used only one sample below 20% IE.…”

“…Moreover, while there are two published studies using different copper loadings as a tool to understand the TPD peaks [2,3,7], neither correlated ammonia storage with different positions in the zeolite. This could be because Lezcano-Gonzalez et al [2] used only two Cu-loadings (ion exchange levels (IE) = 67 and 100%) and Clemens et al [3] used only one sample below 20% IE. Yet, it is clear that low copper loadings are critical in order to assign the ammonia TPD peaks, given that Kwak et al found that a sample with IE = 20% already contains Cu sites in both 6MRs and 8MRs [6].…”

“…In the case of Cu/ SSZ-13, two peaks are typically seen at ~250-350 and ~450-500 °C. There is no consensus as to the assignment of these: the former has been attributed to ammonia stored either on Cu 2+ sites [2,3] or both Brønsted and Lewis acid sites [4] and the latter to either Brønsted acid sites [2,4,5] or copper sites [3]. It has been shown that Cu/SSZ-13 can contain copper sites in two different locations: the 6-membered rings (6MR) and chabazite cages (8-membered rings or 8MR) [6].…”