In-operando study of a model catalyst constituted by Cu2O nanoparticles on alumina

Although the principles of catalysis are well known, it remains difficult to precisely describe the mechanisms that take place during a catalytic reaction on a substrate. Indeed, the complexity and often the inhomogeneity of catalysts used in the industry make difficult their characterization and their understanding. It is therefore interesting to synthetize model catalyst with lower catalytic activity but for which the understanding of the structure, the chemical or physical properties and the reactivity will be easier using spectroscopic techniques.

In this project, the catalyst is constituted by Cu₂O nanoparticles (Copper with an oxidation state of +I) deposited on an alumina substrate. In the optic of reducing the use of noble metals as well the cost of materials, the use of metals such as copper is promising. Furthermore, Cu₂O can be used for several catalytic reactions (propylene epoxidation, CO oxidation…) as well as for photocatalytic reactions (CO₂ reduction, water splitting…). However, the deactivation of the catalyst due to its oxidation into Cu(II) makes its use delicate.

Nanoparticle’s reactivity depends greatly on their shape and size. The smaller a nanoparticle will be, the greater the ratio between surface atom and bulk atom will be. It means more surface atoms on facets (that can be (100), (111), (110)… depending on the nanoparticle shape), edges, corners or other defects. These surface atoms are undercoordinated sites which make them very reactive. The goal of this project is to study the evolution of Cu₂O nanoparticles properties as a function of their shape and size in the presence of a gas phase and during a catalytic reaction. How nanoparticles will be modified? Will they keep their initial structure? Can we identify the structure of the active site? To answer these questions, we propose to develop three axes: first the synthesis of Cu₂O nanoparticles in solution in order to control their shape (nanocubes, octahedra…) and their size (with a small size dispersion) from 15 to 500 nm. These nanoparticles will afterwards be deposited on alumina single crystal surfaces with different orientations in order to investigate the interaction between the nanoparticle and the substrate. The second step will consist in following, in real time and in-operando, the structural and chemical modifications induced by the presence of a gas phase (under an atmosphere such as O₂ or H₂ …). Finally, the third step will extend the previous one by investigating the modifications on a nanoparticles during a catalytic reaction. In particular, we will start by a simple and well documented model reaction: the oxidation of CO.

During this thesis, the synthetized nanoparticles will be characterized by Scanning Electronic Spectroscopy (SEM), X-Ray Diffraction (DRX) and X-ray Photoelectron Spectroscopy (XPS). In the case of very small nanoparticles, the use of Atomic Force Microscopy (AFM) will be considered in order to have a better understanding of the interactions between the nanoparticle and the substrate. In-operando measurements will be performed by Near-Ambient Pressure XPS (NAP-XPS) coupled with mass spectroscopy.