Catalyst Deactivation Probed by Positron Annihilation Spectroscopy
Chase N. Taylor, Jagoda Urban-Klaehn, Thuy T. Le, Radoslaw Zaleski, Jeffrey D. Rimer, and Kevin L. Gering
November 2021 - There is a widespread need to understand and improve the aging characteristics and properties of catalysts that support essential chemical reactions, including methanol-to-hydrocarbons (MTH) conversion. Here, we describe pilot measurements of a zeolite ZSM-5 catalyst with MTH as a model reaction using positron annihilation spectroscopy, which was able to identify unique signatures for different components in the catalyst mixture (i.e., zeolite and silica gel) and track the evolution of microstructures in different stages of catalyst deactivation. Samples were analyzed with both positron annihilation lifetime spectroscopy (PALS) and coincidence Doppler broadening (CDB) measurements to investigate and characterize the aging process of zeolite catalysts. PALS results show a distinct lifetime related to the presence of zeolite (active catalyst) and silica gel (diluent). Tracking these characteristic lifetimes as the catalyst deactivates shows that zeolite nanopores decrease in size as the reaction progresses, while the mesopores show an increase in size probably due to degassing. Analysis of CDB ratio curves reveals a distinct change in the defect structure of the sample that occurs between 47 and 23% conversion of the catalyst. Further examination of the shaping parameters of the Doppler broadened positron annihilation spectra suggests that the complexity of microstructures increases as the zeolite ages; notably, there is an increase in pore number density with a concomitant decrease in the pore size that suggests that catalyst coking fractionates larger pores into numerous smaller pores. Our findings suggest that the sensitivity of PALS and CDB techniques to different pore structures can potentially be useful as a characterization method for heterogeneous catalysis studies.
