Surface reduction studied by in situ STEM on Pt-Ceria catalysts used for CO oxidation

Abstract text (incl. figure legends and references)

Heterogeneous catalyst systems used in exhaust gas treatment typically consist of precious metal nanoparticles (often Pt, Pd, Rh) supported on a metal oxide supports (often ceria, i.e. cerium IV oxide). The scarcity and cost of such catalysts demands careful design of more active and cost-efficient systems. Such theoretical predictions can happen only if there is a detailed understanding of the substrate-catalyst-gas interactions, the dynamic behavior of the catalyst system during catalysis and relevant gas atmospheres, and ultimately the reaction mechanism. One important parameter that should be ideally incorporated into such theoretical modelling, especially in systems such as CeO₂ where the surfaces are known to have oxygen vacancies[1], is the spatially resolved quantitative information about the oxygen vacancies on ceria and at Pt-ceria interface. However, this information at CO oxidation conditions is missing.

We measure electron energy loss spectroscopy (EELS) maps to find spatially resolved information of Ce oxidation state experimentally under vacuum, CO oxidation conditions and at other relevant gas conditions. Since the study aims at distinguishing the reduced surface layer (i.e. region containing Ce³⁺) to that of the bulk (Ce⁴⁺) it is important to study CeO₂ crystals with well-defined facets. Hence in this study we use CeO₂ cubes (with {100} crystal planes forming the surface facets). CeO₂ cubes were prepared by Ce(NO₃)₃.6H₂O using an acetate-acetic acid buffer-mediated hydrothermal synthesis (similar to that of reference [2]) (Fig.1 a,b). Pt nanoparticles were prepared by microfluidic synthesis (similar to [3]) (Fig. 1 c). EELS is done in a Themis Z; while the gas experiments are conducted with Atmosphere holder from Protochips.

The EELS mapping of ceria cubes in vacuum shows that there is a surface layer containing Ce³⁺ atoms (Fig.2). When oxygen is introduced into the system we observe that this surface layer with Ce³⁺ atoms becomes narrower than in vacuum at the same temperature (Fig.3). The effect on this surface layer in N₂, H₂, CO and CO+O₂ mixture at different temperatures, pressures, and waiting duration after the introduction of gas, are underway. The effect of the multiple catalytic cycles on the surface layer will also be investigated. The effect of electron beam on the EELS spectrum maps is studied to find that it causes additional surface reduction, while in the bulk region no significant changes are noticed in the spectral features within the range of electron doses we studied yet. We will attempt to study the Ce and Pt oxidation state on the samples with Pt nanoparticle loaded on CeO₂ cubes. We also intend to study the structural/morphological dynamics of the Pt-CeO₂ catalyst using in situ STEM. Since the catalyst system is extremely dynamic during catalytic activity, in situ (s)TEM can provide valuable information on the active structure and its evolution in different relevant gas environments and reaction conditions. Results about Ce oxidation state mapping and catalyst dynamics will be presented.

References

[1] Goris et al. ACS Nano 2014 (8) 10878

[2] Lyu et al. Nanoscale 2016 (8) 7923

[3] Tofighi et al. Reaction Chemistry & Engineering 2017 (2) 876

Fig. captions:

(a, b) Ceria cubes; (c) Pt nanoparticles on ceria cubes Oxidation state maps corresponding to Ce⁴⁺ and Ce³⁺ of a ceria cube in vacuum. Normalised average EELS spectra showing the Ce core loss edge in vacuum and O₂ at both surface and interior.