Spectroscopic and kinetic study of copper-exchanged zeolites for the selective catalytic reduction of NOx with ammonia

Shane Adam Bates, Purdue University

Abstract

The recent application of metal-exchanged, small-pore zeolites for use in the selective catalytic reduction (SCR) of NOx with ammonia NH3 for automotive deNOx applications has been a great stride in achieving emission standard goals. Copper-exchanged SSZ-13 (Cu-SSZ-13), the small-pore zeolite in this study, has been shown to be very hydrothermally stable and active under conditions presented in the exhaust of the lean-burn diesel engine. In this work, detailed studies were performed to identify many aspects of the active site(s) in Cu-SSZ-13 in order to learn about the standard SCR mechanism. A series of seven Cu-SSZ-13 samples were created with silicon to aluminum atomic ratios (Si:Al) of 4.5 and varying Cu loadings ranging from a copper to aluminum atomic ratio (Cu:Al) of 0 to 0.35. Standard SCR kinetics were collected over each with the standard conditions being 320 ppm NO, 320 ppm NH3, 10% O2, 8% CO2, and 6% H2O at 473 K. A linear increase in rate per gram was observed as the Cu:Al ratio increased up to Cu:Al = 0.2, indicating the active Cu species was being populated in that region of Cu loading. Further characterization of the Cu in this region with operando X-ray absorption spectroscopy (XAS) revealed this Cu species to be a mix of isolated Cu(I) and Cu(II) under reaction conditions. Density functional theory (DFT) and statistical simulations were able to identify the location for the active Cu species as exchanged into the six-membered ring of the SSZ-13 structure. Ambient ultraviolet–visible–near infrared spectroscopy (UV-Vis-NIR) and XAS showed the precursor to the active Cu species was a hydrated Cu(II) which tracked with the standard SCR rate. Above Cu:Al = 0.2, ambient UV-Vis-NIR and XAS showed a CuxO yspecies began to form which was not active for standard SCR. Thus, the active Cu was identified to be an isolated Cu exchanged into the six-membered ring of the SSZ-13 and after Cu:Al = 0.2, a new CuxOy species began to form. Additionally, the number of Brønsted acid sites was determined to not track with the standard SCR rate per gram, indicating the kinetically relevant steps in these samples took place on the Cu. The active state of the isolated Cu was also explored. Under reaction conditions, our group has previously observed a mix of Cu(I) and Cu(II) to be present. Additionally, from DFT, the most stable Cu species under various oxidizing conditions in SCR were a two-coordinate Cu(I) and four-coordinate Cu(II). From these observations, we proposed a redox cycle between isolated Cu(I) and isolated Cu(II). To probe this redox cycle and refute other hypotheses, the proposed half-reactions were removed in order to drive the Cu into primarily a Cu(I) state or Cu(II) state. When O2 was removed, the Cu was driven into primarily an isolated Cu(I) state. When NO or NH3 were removed, the Cu became completely Cu(II). Thus, we illustrated the redox cycle existed in the active Cu and should be incorporated into proposed standard SCR mechanisms. Finally, techniques for determining the accurate number of Brønsted acid sites were developed. Three techniques using NH3 as a titrant were proven to selectively titrate Brønsted acid sites when compared to site counts from n-propylamine decomposition over H-ZSM-5. In Cu-ZSM-5, the results also matched, suggesting the NH3 procedures could also avoid titrating Cu sites. In H- and Cu-SSZ-13, the NH3 titrations gave close to four times higher counts of Brønsted acid sites, suggesting the small pore openings cause n-propylamine to be mass transfer limited and the close proximity of Brønsted sites to cause n-propylamine to block itself. We suggest that NH3 is an ideal titrant to use for small-pore zeolites. Thus, these procedures may be attractive for deNOx applications where NH3 is a reactant in the selective catalytic reduction reactions.

Degree

Ph.D.

Advisors

Delgass, Purdue University.

Subject Area

Inorganic chemistry|Chemical engineering

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