In-situ and operando molecular spectroscopy for the characterization of mixed metal oxide catalysts in redox reactions
Creators: name: Nguyen, Dinh Thu
Publisher: name: No Publisher Supplied
Year: 2023

description: Metal oxides play a vital role in wide range of industrial applications ranging from solid catalysts to pigments, ceramics, semiconductors, energy storage, gas sensor, and so on. Such significance of this class of materials posts a strong demand for research to characterize their physicochemical properties, with which fine-tuned synthesis can be guided. Especially in the field of heterogeneous catalysis and photocatalysis, understandings of the material physical chemistry are also important in exploring the reaction mechanism and optimizing the reaction conditions Among different material characterization techniques, spectroscopic techniques such as vibrational spectroscopy and UV-vis spectroscopy have been recognized to be both convenience and informative in identifying structures of metal oxides at the molecular level.Even though UV-vis spectroscopy is an attractive characterization technique due to its short data collection time and lower safety requirements for user, a consistent and reliable quantitative analysis method for UV-vis spectra to determine optical bandgap energy is still evasive in the literature. To address this need, a derivative peak fitting method was modified using the exponentially modified Gaussian (EMG) as a fitting function. This modification allowed for the extension of the diffuse-reflectance UV-vis spectroscopy (DPR) method to all phases of titania and other metal oxides commonly used in catalysis and photocatalysis, including CeO2, ZnO, SnO2, V2O5, MoO3, Y2O3, Ta2O5, and Nb2O5. The similarity between the distribution of electronic excitation by UV-vis photon and the EMG function was demonstrated to be physically justified. The peak parameters intrinsic to each oxide phase were then used to identify and quantify phase composition in mixtures with high sensitivity and minimal deviation. Additionally, response factors for each of the aforementioned metal oxides, relative to anatase TiO2, were recorded and shown to be reusable, even in higher order mixtures.The application of metal oxides in the heterogeneous catalysis has been attracting more attention especially in the production of highly demanding chemicals such as light olefins. Recently, the catalytic oxidative dehydrogenation of ethane (ODHE) has become increasingly popular as a substitute for direct thermal dehydrogenation in the production of olefins. Modern catalytic processes for ODHE demand a shift in focus towards the rational design of materials, with the goal of achieving high selectivity to olefins by improving redox properties, limiting coke formation, and enhancing in-situ coke removal pathways. Previous studies have also recognized the potential of utilizing carbon dioxide (CO2) as an oxidant in ODHE to increase selectivity towards desired products and prevent total oxidation of ethylene. Dispersed metal oxides such as MoOx, WOx, and VOx supported on TiO2 have been identified as catalytically active materials for C-H activation, which is the rate-limiting step in ODHE of ethane. This study aims to investigate the effect of active sites, specifically MoOx species, as well as the composition of a mixed CeO2-TiO2 support, which has good redox properties, on CO2-assisted ODHE of ethane. Possible competing ethane conversion pathways and how CO2 affects the performance of different catalysts are extensively discuss through the implementation of in-situ Raman, and IR spectroscopy.In order to gain insight into the surface structure of supported metal oxide at the molecular level, this study also developed a transient operando Raman spectroscopy framework and showcased by applying on the study of structural behavior of supported Mo oxide on 2 types of support: pure anatase TiO2 and mixed CeO2-TiO2. The study employs an advanced synthesis route known as the equilibrium deposition filtration method for loading active sites on the surface. The surface of the samples is then characterized using in-situ/operando Raman spectroscopy in combination with an isotope 18O/16O exchange technique for molecular level identification of Mo surface configurations. The study reveals that pH control during the synthesis process has an impact on the distribution of Mo structural species on the support surface. Specifically, lower pH leads to the formation of more 6-coordination and oligomeric Mo structures on the surface. The study also confirms that the dominant configuration of EDF-synthesized Mo on both pure anatase TiO2 and CeO2-TiO2 is mono-oxo arrangement. Moreover, the study identifies the spectral behaviors of two-component peaks under the ~900 cm-1 band for supported Mo on CeO2-TiO2, which are assigned to anchoring oxygen - Mo-O-support of monomers (915 cm-1) and anchoring oxygen - Mo-O-supp of oligomers (920 cm-1). The study also employs pulse experimentation coupled with operando Raman spectroscopy to distinguish the reactivity among oxygen sites. The results show that upon exposure to ethane, the order of reactivity and tendency to produce CO2 of surface oxygen sites follows support-O &gt; Mo=O &gt; Mo-O-X. While less reactive, anchoring/bridging oxygen also participates in H2 combustion.