Structure-Activity Relationships of SSZ-13 and Other Small Pore Zeolites for Catalytic NOx Abatement

Jonatan D Albarracin Caballero, Purdue University

Abstract

Environmental regulations on diesel automobiles have promoted the development of exhaust after-treatment technologies that can abate Nitrogen Oxides (NOx) i.e. Ammonia Selective Catalytic Reduction (NH3-SCR). Recently, the Cu-exchanged SSZ-13 (Cu-SSZ-13) zeolite catalyst has been commercialized due to its efficient reduction of NO x and superior hydrothermal stability in light and heavy-duty diesel vehicles. Nevertheless, a fundamental understanding of this catalyst and the NH3-SCR reaction mechanism are still under debate. In this work, kinetic and characterization studies were performed to gain insight of the active sites on Cu-SSZ-13 and contribute to the understanding of NH3 -SCR mechanism. Additionally, hydrothermal stability studies were carried out on small pore zeolites i.e. SSZ-13, SSZ-39, and RTH, to evaluate potential catalysts for the reduction of NOx using NH3-SCR. NH3-SCR kinetic measurements at 473 K taken on Cu-exchanged SSZ-13 Si/Al = 15 samples, with copper varying from Cu/Al = 0.02 to Cu/Al= 0.44, revealed two distinct active sites that can populate SSZ-13 as a function of the zeolite composition. A previous study done by our group identified isolated Cu2+ as the active species for NH3-SCR for SSZ-13 Si/Al = 4.5. Similarly, it was predicted that for an SSZ-13 Si/Al = 15 the maximum amount of active sites would be found in a sample with Cu/Al = 0.09. However, the linear correlation between the SCR rates and the Cu loadings up to Cu/Al = 0.44 found in this work demonstrate that an additional Cu configuration was equally active as the proposed isolated Cu2+. Ex-situ UV-Vis spectroscopy was used to show that both species are indistinguishable under ambient conditions, displaying a unique absorption band assigned to the hydrated Cu2+ configuration. Further chemical and spectroscopic characterization demonstrated that the [Cu-OH]+ species, associated with 1 Al sites, can be exchanged in SSZ-13 once the isolated Cu2+ species saturate the 6-membered ring (6MR). Additionally, kinetic parameters presented in this contribution suggest that both sites undergo a similar redox reaction mechanism, which depending on the zeolite composition and gas conditions can be limited by either the oxidation or reduction half-cycle. [Cu-OH]+ species interact after O2 activation to produce static dimers that were able to oxidize NO to NO2 under dry conditions at 573 K. Dimer species were quantified using CO-TPR, and the fraction of Cu titrated by CO correlates linearly with the dry NO oxidation rate per mol of Cu at 573 K. To elucidate the structure of Cu dimers after O2 activation in-situ UV-Vis spectroscopy was used. Four absorption bands in the d-d transition (11 200 cm–1, 13 900 cm –1, 16 500 cm–1 and 19 700 cm–1 ), were observed after O2 activation for samples with [Cu-OH] + species exchanged in SSZ-13 framework. By synthesis methods and in situ UV-Vis methods, the bands at 11 200 cm–1, 13 600 cm–1 were tentatively assigned to be a characteristic signature of [Cu-OH]+ species. Using the enzyme literature, bands center at 19 700 and 27 300 cm–1 were assigned to belong a (µ-η2: η2-peroxo)–dicopper, further confirmed by the absence of the peak at 19 700 cm–1 in a highly Al disperse zeolite where the formation of Cu dimers was restricted. Hydrothermal stability of small pore zeolites was also explored. It was found dimensionality of the pore system in the zeolite would influence the catalytic performance of small pore zeolites. By using a step-wise approach, the impact of the steaming process and SCR gasses was deconvoluted to explain the zeolite deactivation. It was observed that the steaming process hydrolyze the residual Brønsted acid sites to produce Extra Framework Al (EFAl) species. This species could further interact with Cu active sites that are mobilized by the influence of NH3-SCR at 473 K. The reduction in the catalytic activity was assigned to occurred because the formation of inactive CuAlxOy phase after the interaction of Cu and EFAl species.

Degree

M.S.Ch.E.

Advisors

Ribeiro, Purdue University.

Subject Area

Engineering|Chemical engineering

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