Structural and Chemical Evolution of an Inverse CeO_x/Cu Catalyst under CO₂ Hydrogenation: Tunning Oxide Morphology to Improve Activity and Selectivity

Abstract

Small nanoparticles of ceria deposited on a powder of CuO display a very high selectivity for the production of methanol via CO2 hydrogenation. CeO2/CuO catalysts with ceria loadings of 5%, 20%, and 50% were investigated. Among these, the system with 5% CeO x showed the best catalytic performance at temperatures between 200 and 350 °C. The evolution of this system under reaction conditions was studied using a combination of environmental transmission electron microscopy (E-TEM), in situ X-ray absorption spectroscopy (XAS), and time-resolved X-ray diffraction (TR-XRD). For 5% CeO x /Cu, the in situ studies pointed to a full conversion of CuO into metallic copper, with a complete transformation of Ce4+ into Ce3+. Images from E-TEM showed drastic changes in the morphology of the catalyst when it was exposed to H2, CO2, and CO2/H2 mixtures. Under a CO2/H2 feed, there was a redispersion of the ceria particles that was detected by E-TEM and in situ TR-XRD. These morphological changes were made possible by the inverse oxide/metal configuration and facilitate the binding and selective conversion of CO2 to methanol.

Observing Chemical and Morphological Changes in a Cu@TiOx Core@Shell Catalyst: Impact of Reversible Metal-Oxide Interactions on CO2 Activation and Hydrogenation

“…Our studies indicate that metal–support interactions had a strong impact on the structural properties of Cu@TiO 2 catalysts. On purpose, following previous works for core–shell nanostructured catalysts, , we started with a Cu@TiO 2 configuration that mimicked the inverse oxide/metal configuration seen for several CO 2 hydrogenation catalysts. − , However, this pristine core@shell configuration was not stable under H 2 or CO 2 /H 2 mixtures. The oxide shell cracked and copper was present in three regions of the catalyst: (i) as a nanowire encapsulated by a TiO 2 shell, (ii) as a component in a Cu–Ti–O x mixed oxide, and (iii) as metal particles on top of the oxide shell that were decorated by TiO x clusters.…”

“…In this study, before doing any catalytic test, we investigated the stability of these Cu@TiO x core–shell structures under the standard pretreatment in O 2 /H 2 used for CO 2 hydrogenation catalysts, ,,, following the behavior of the sample with a combination of XRD, XAS, AP-XPS and TEM. Treatment in O 2 at 350 °C induced a migration of Cu into the titania to form a Cu–Ti–O x solid solution ,,, and the characteristic diffraction lines of Cu or TiO 2 in XRD disappeared (Figure d).…”

“…Nowadays, the trapping and conversion of CO 2 is a major issue in environmental chemistry. − Metal-oxide interfaces are frequently used for the catalytic conversion of CO 2 into into fuels and high-value chemicals. − There have been a lot of studies focused on identifying the active phase of metal/oxide catalysts that are active for the hydrogenation of CO 2 to methanol. − , High-resolution transmission electron microscopy (HR-TEM) has been used to study the morphology of powder Cu/ZnO/Al 2 O 3 catalysts after exposure to pure H 2 or the CO 2 /H 2 reactants. , The microscopy results showed that the active phase of the catalyst involved an inverse oxide/metal configuration produced as a consequence of strong metal–support interactions. , The addition of ZnO to copper opens new reaction channels for the dissociation of H 2 and conversion of CO 2 to CO or oxygenates. ,, Cu–TiO 2 interfaces also display superior activity and selectivity during CO 2 hydrogenation and are receiving a lot of attention. − They can exhibit a lot of interesting features. − In particular, the trapping of Cu centers into the titania lattice can prevent admetal sintering and improve long-term activity and stability for CO 2 hydrogenation . Furthermore, the formation of Cu–Ti–O x units can facilitate H 2 dissociation and the hydrogenation of CO 2 into oxygenates. , Recent theoretical calculations have predicted that a Cu/TiO 2 interface binds CO 2 much better than isolated copper or titania, and spontaneous dissociation of CO 2 to CO has been observed on defective surface of Cu­(I)/TiO 2– x nanoparticles at room temperature .…”

Reactant-Induced Dynamic Stabilization of Highly Dispersed Pt Catalysts on Ceria Dictating the Reactivity of CO Oxidation

“…On the other hand, the as-prepared Pt 1 species was reported to be regenerated in situ as a highly active Pt C during CO oxidation . The formation of highly active sites at the Pt–CeO 2 interface was attributed to the high activity of Pt C /CeO 2 . ,, Nevertheless, the lack of uniformity in preparing Pt/CeO 2 catalysts, often in a mixture of single atoms, clusters, and nanoparticles, which might change and transform dynamically depending on the reaction conditions, surface orientations, and defects, , prevents the identification of the true active species responsible for the observed activity. Investigating the functions of Pt 1 , Pt C , and Pt NP catalysts under operating conditions, their restructuring, and the authentic active centers for CO oxidation is a formidable task from both experimental and theoretical points of view.…”

MgO Nanostructures on Cu(111): Understanding Size- and Morphology-Dependent CO2 Binding and Hydrogenation

“…In this article, we investigate the reactivity of inverse MgO/Cu(111) model catalysts toward CO 2 , H 2 , and CO 2 /H 2 mixtures. Inverse oxide/metal catalysts are known to be active systems in the hydrogenation or reduction of CO 2 . − An industrial Cu/ZnO/Al 2 O 3 powder catalyst adopts an inverse ZnO/Cu configuration during methanol synthesis . Images of high-resolution transmission electron microscopy (HR-TEM) show that the active phase in the catalyst consists of a layer of graphitic ZnO on top of the copper particles .…”