Dynamics of CrO3–Fe2O3 catalysts during the high-temperature water-gas shift reaction: molecular structures and reactivity

A series of supported CrO3/Fe2O3 catalysts were investigated for the high-temperature water-gas shift (WGS) and reverse-WGS reactions and extensively characterized using in situ and operando IR, Raman, and XAS spectroscopy during the high-temperature WGS/RWGS reactions. The in situ spectroscopy examinations reveal that the initial oxidized catalysts contain surface dioxo (O═)2Cr6+O2 species and a bulk Fe2O3 phase containing some Cr3+ substituted into the iron oxide bulk lattice. Operando spectroscopy studies during the high-temperature WGS/RWGS reactions show that the catalyst transforms during the reaction. The crystalline Fe2O3 bulk phase becomes Fe3O4 ,and surface dioxo (O═)2Cr6+O2 species are reduced and mostly dissolve into the iron oxide bulk lattice. Consequently, the chromium–iron oxide catalyst surface is dominated by FeOx sites, but some minor reduced surface chromia sites are also retained. The Fe3–-xCrxO4 solid solution stabilizes the iron oxide phase from reducing to metallic Fe0 and imparts an enhanced surface area to the catalyst. Isotopic exchange studies with C16O2/H2 → C18O2/H2 isotopic switch directly show that the RWGS reaction proceeds via the redox mechanism and only O* sites from the surface region of the chromium–iron oxide catalysts are involved in the RWGS reaction. The number of redox O* sites was quantitatively determined with the isotope exchange measurements under appropriate WGS conditions and demonstrated that previous methods have undercounted the number of sites by nearly 1 order of magnitude. The TOF values suggest that only the redox O* sites affiliated with iron oxide are catalytic active sites for WGS/RWGS, though a carbonate oxygen exchange mechanism was demonstrated to exist, and that chromia is only a textural promoter that increases the number of catalytic active sites without any chemical promotion effect

Surface characterization and properties of ordered arrays of CeO2 nanoparticles embedded in thin layers of SiO2

We demonstrated the surface composite character down to the nanometer scale of SiO2-CeO2 composite high surface area materials, prepared using 5 nm colloidal CeO2 nanoparticle building blocks. These materials are made of a homogeneous distribution of CeO2 nanoparticles in thin layers of SiO2, arranged in a hexagonal symmetry as shown by small-angle X-ray scattering and transmission electron microscopy. Since the preparation route of these composite materials was selected in order to produce SiO2 wall thickness in the range of the CeO2 nanoparticle diameter, these materials display surface nanorugosity as shown by inverse chromatography. Accessibility through the porous volume to the functional CeO2 nanoparticle surfaceswasevidenced throughanorganic acid chemisorption technique allowing quantitative determination of CeO2 surface ratio. This surface composite nanostructure down to the nanometer scale does not affect the fundamental properties of the functional CeO2 nanodomains, such as their oxygen storage capacity, but modifies the acid-base properties of the CeO2 surface nanodomains as evidenced by Fourier transform IR technique. These arrays of accessible CeO2 nanoparticles displaying high surface area and high thermal stability, along with the possibility of tuning their acid base properties, will exhibit potentialities for catalysis, sensors, etc

Novel sol-gel prepared zinc fluoride: synthesis, characterisation and acid-base sites analysis

The fluorolytic sol-gel route sets a milestone in the development of synthesis methods for nanoscopic fluoride materials. They exhibit fundamentally distinct properties in comparison to classically prepared metal fluorides. To broaden this area, we report in this paper the first fluorolytic sol-gel synthesis of ZnF₂. The obtained sol was studied with dynamic light scattering (DLS). The dried ZnF₂ xerogel was investigated with elemental analysis, thermal analysis, powder X-ray diffraction (XRD), solid-state MAS NMR, and N₂ adsorption-desorption measurements. The characterisations revealed a remarkably high surface area of the sol-gel prepared ZnF₂. To determine key parameters deciding its prospects in future catalytic applications, we studied the surface acidity-basicity by using in situ FTIR with different probe molecules. Compared to the previously established MgF₂, weaker Lewis acid sites are predominant on the surface of ZnF2 with some base sites, indicating its potential as a heterogeneous catalyst component. In short, we believe that the successful synthesis and detailed characterisation of nanoscopic ZnF₂ allow follow-up work exploring its applications, and will lead to studies of more metal fluorides with similar methods

On the use of advanced IR spectroscopy for porous material investigation

International audienc

Catalysts under reaction conditions. An experimental view

Plenary LectureInternational audienc

Advanced vibrational spectroscopy for porous materials investigation and design: applications to pollutant removal

Keynote lectureInternational audienc

Catalysts under reaction conditions. An experimental view

PlenaryInternational audienc

From the study of reaction mechanisms to catalyst design

International audienc

Studio di ossidi misti per applicazioni tecnologiche

Dottorato di ricerca in chimica per l'ingegneria. 9. ciclo. Coordinatore V. Lorenzelli. Relatore G. Busca. Correlatore G. RamisConsiglio Nazionale delle Ricerche - Biblioteca Centrale - P.le Aldo Moro, 7, Rome; Biblioteca Nazionale Centrale - P.za Cavalleggeri, 1, Florence / CNR - Consiglio Nazionale delle RichercheSIGLEITItal