Tuning the surface termination of ceria under gaseous environments in a Cs‐corrected Environmental TEM

Abstract

Surface termination of solids varies according to the environment to which they are submitted. This is very important in catalysis since surface modifications have a strong influence in the way thatadsorption at the surface followed by eventual dissociation, diffusion and recombination of the molecular occur. Gaining information at atomic level on the structural and chemical variations of the surface is thus essential to understand the mechanisms of catalytic reactions and ultimately obtain a better control on the design of catalysts with specific controlled properties (activity, selectivity, stability). This information can only be obtained (at local level) by high resolution transmission electrom microscopy (HRTEM and thus requires the use of a Cs‐corrected (image) Environmental TEM capable of very high resolution observations of the nanometric materials in presence of controlled gaseous environments. In this work we study, in a ETEM, ceria which is a catalytic material used generally in oxidation reactions both as support of active phases or as an active phase itself by taking advantage of its redox properties and oxygen mobility. We focused our interest on the surface atomic termination of ceria nanocubes under different controlled environments (high vacuum, oxygen, carbon dioxide) important for the understanding of CO 2 ‐valorization related reactions. The studies were performed in the Ly‐EtTEM (Lyon Environmental and tomographic Transmission Electron Microscope), a 80–300 kV TITAN objective lens Cs‐corrected Environmental TEM from FEI equipped with a GATAN high resolution Imaging Filter (GIF) [1]. The analysis was carried out at room temperature varying the gas pressures from 10 −6 mbar (vacuum) up to 2 mbar. The HRTEM images in Figure 1 show the termination of a {100} facet of a ceria nanocube under high vacuum (HV) and in presence of molecular oxygen and carbon dioxyde. Under HV conditions (Figure 1 – left image) the incident electron beam induces a partial reduction of ceria [2]. As a consequence cerium surface atoms become highly mobile as it can be readily observed on {100} facets and electron energy‐loss spectroscopy (EELS) shows that the characteristic features related with Ce 4+ species rapidly decrease in intensity both on the O‐K and Ce‐M 4,5 edges. In the presence of molecular oxygen (O 2 pressure in the mbar range; Figure 1 – center image) this beam‐induced reduction is “healed” and the surface of ceria NPs is stabilized as it can be confirmed by the absence of mobility of surface cerium atoms on {100} facets as well as by the in situ real‐time dynamic monitoring of the O‐K and C‐M 4,5 edges by EELS that now remain unaltered under irradiation. Finally, under CO 2 pressure (10 −2 ‐ 1 mbar; Figure 1 – right image), the {100} facets become more stable from both structural (oxygen surface termination) and chemical (stable non‐reduced state revealed by EELS) points of view. We notice that the surface terminations of the ceria nanoparticles under vacuum are different from those observed under O 2 or CO 2 (Figure 2). Under HV the terminal contrasts observed are strong and clearly correspond to the formation of a terminal cerium plane. A different contrast can be noted in the case of the sample exposed to O 2 (Figure 2 – left image) where the presence of terminal oxygen in accordance with the ceria modelis imaged. Exposure to CO 2 (Figure 2 – right image) yields a still different terminal contrast close to the expected position of termina oxygen hinting at the presence of stabilized species at these position of oxygen atoms at the surface termination; the stabilization of surface oxygen atoms is likely related to CO 2 adsorption and the consequent formation of carbonate surface species.