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.
Aminopolymer functionalization with fluoroalkyl chains and resulting void volume afford 4.1-fold increase in amine efficiency and 2.8-fold increase in CO2 capture capacity in the fluorinated material.
The techniques of positron annihilation spectroscopy have been applied to the study of well-characterized reservoir rock samples. In this article we focus primarily on sandstone and carbonate samples. We report (a) measurements of the Doppler broadening (DB) parameters and mean lifetime values (〈τ〉) of dry samples; (b) measurement of the DB parameters as a function of temperature from room temperature to 200 °C; and (c) observation of the outgassing of heated samples simultaneously with the DB measurements. We interpret the measurements in terms of positron annihilation with both valence and core electrons and discuss how the DB parameters depend on the major chemical constituents (lithology) and to a lesser degree on the structure of the samples. The major result of this work is that the positron parameters such DB parameters (S, SW, and W) and mean lifetime values are shown to be reproducible and reliable bulk properties of well-characterized rock samples and can be used to categorize rocks of geophysical interest in the same way as the more familiar bulk properties such as density, porosity, resistivity, etc. These results provide the first set of data for and a physical description of some of the processes involved in the interaction of positrons with rocks. This research provides a basis for further laboratory studies and is necessary for the eventual development of a new nuclear well-logging tool based on Doppler broadening of the positron annihilation gamma ray.
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