Reactivity and advanced electron microscopy of mass selected pt clusters deposited on CeO₂ supports for co oxidation catalysis

Abstract text (incl. figure legends and references)

The study of model catalyst systems with mass selected Pt, Pd and Pt-Pd clusters on nanostructured metal oxide supports like CeO₂, Al₂O₃ in exhaust gas environments can be helpful to understand the fundamental processes in catalytic reactions of exhaust gases, to unravel the complex relationship between the structure and the catalytic properties. During a catalytic reaction, the active metal clusters/ particles dynamically change their structure due to the interaction with the gas environment at elevated temperatures. In-situ TEM can be of aid to track the the dynamic transient stages involved, structure/morphology changes of clusters under catalytic reaction conditions.

We aim to study the behavior of size selected Pt clusters (50-1000 atoms) deposited on CeO₂ support, under pure gas environments (H₂, Ar, O₂, CO, CO₂) and during CO oxidation (with CO and O₂ as input gases) using in-situ TEM with Micro Electro Mechanical Systems (MEMS) based nanoreactors. Size selected Pt clusters are deposited on top of Pulsed Laser Deposition (PLD) deposited CeO₂ thin films on MEMS chips using a magnetron sputtering cluster source integrated into an Ultra High Vacuum (UHV) Cluster Ion Beam Deposition (CIBD) system¹. The atomic structural evolution and oreintation changes in the clusters, cluster-support interaction, cluster-gas interaction etc. can be studied realtime in exhaust gas environments using in-situ STEM imaging & spectroscopy. Quantitative analysis of the product gases from the MEMS based nanoreactors will be done using high sensitivity Residual Gas Analyzer (RGA).

An exemplary HAADF STEM image of CIBD deposited Pt clusters (200 atoms) on CeO₂ support is as shown in Fig. 1. The size distribution of Pt200 clusters (~1000 clusters) deposited on standard Lacey Carbon TEM grid is shown in Fig. 2. The size distribution showed a narrow peak at around 1.9 nm, matching perfectly to the expected diameter of 200-atom cluster. The second peak at around 2.5 nm could be attributed to the merging of two primary clusters. The size, three dimensional shape, orientation and atomic arrangement of size selected Pt clusters that are soft landed on the TEM grid can be determined using HAADF imaging with aberration corrected HRSTEM along with simple imaging simulations². The acquired HRSTEM image of Pt₁₀₀₀ clusters is shown in Fig. 3. The single Pt atoms displaced out from the cluster (circled in red) are due to the electron beam induced knock on damage. Moreover, the clusters seem to be unstable under electron beam with changes in the inherent structure (distortion and/ or rotation, twinning etc) with longer beam exposure. A low voltage HRTEM imaging with high senstiticve detectors is preferable for the detailed analysis of the clusters in atomic resolution to understand the stable configurations possible for each cluster size thus complementing the observations of structural evolution of the supported clusters in the exhaust gas environments from the in-situ TEM experiments.

References

A. Fischer, R. Kruk, H. Hahn, Rev Sci Instrum 2015, 86, (2), 023304. Z. Y. Li, N. P. Young, M. Di Vece, S. Palomba, R. E. Palmer, A. L. Bleloch, B. C. Curley, R. L. Johnston, J. Jiang, J. Yuan, Nature 2008, 451, 46-48.