In Situ and Operando Tools and Methods for Characterization of Heterogeneous Catalysts

Abstract

In situ and operando spectroscopic characterization of catalysts is a powerful tool to explore the nature of species that may be involved in a heterogeneous catalytic cycle. In combination with methodologies to discriminate passive and active species, it can inform researchers on the presence of spectator and true reaction intermediate species. Towards this goal, this dissertation encompasses novel tools and methods to detect adsorbed species, differentiate spectator species from likely intermediate species, and their application via in situ and operando spectroscopic methods at relevant reaction conditions. First, we introduce a commercial in situ diffuse reflectance (DR) mirror optics cell that was modified for use at high temperatures with fiber optics and with reduced void volume. Such design enabled the development of a technique named oxygen gold plasmon sensing (O2-GPS). The O2-GPS combines in situ/operando DR UV-Vis spectroscopy and the determination of gold surface plasmon resonance (Au SPR) peak shifts as a sensor for adsorption of oxygen on gold catalysts via a simple correlation based on Drude’s free electron model for gold nanoparticles. Such method allowed, for the first time, the observation via in situ/operando UV-Vis spectroscopy of O2 adsorbed at the gold-support interface during O2 flow and under CO oxidation reaction conditions. This work also describes a unique and powerful mathematical framework for the discrimination of spectator species and likely intermediate species. The method is called modulation excitation-phase sensitive detection-diffuse reflectance Fourier infrared spectroscopy (ME-PSD-DRIFTS) as it is applied to in situ/operando DRIFTS data that has been collected under periodic changes of surface species coverages and processed via phase sensitive detection methods that employ Fourier analysis. Such approach allowed the collection of infrared spectra with enhanced signal-to-noise ratio of reacting species while avoiding the presence of spectator species. Here, it is also shown how ME-PSD-DRIFTS can be used to study surface reacting and likely intermediate species on a Co-Al2O3 catalyst with enhanced properties for ethanol dehydration. Along with ex situ characterization and kinetics measurements, ME-PSD-DRIFTS demonstrated the enhanced hydrophobic surface properties of this catalyst which reduced inhibition by water typical of the parent γ-Al2O3. The technique also allowed the sensitive detection of adsorbed ethanol and ethoxide species as well as terminal and bridging hydroxyls bonded to octahedral and tetrahedral Al on Al2O3 (100) and (110) facets as likely reaction intermediates in the conversion of ethanol to diethyl ether and ethylene.