Model Catalysis with HOPG-Supported Pd Nanoparticles and Pd Foil: XPS, STM and C2H4 Hydrogenation

Md Abdul Motin; Andreas Steiger-Thirsfeld; Michael Stöger-Pollach; Günther Rupprechter

doi:10.1007/s10562-021-03868-2

Model Catalysis with HOPG-Supported Pd Nanoparticles and Pd Foil: XPS, STM and C₂H₄ Hydrogenation

Catal Letters. 2022;152(10):2892-2907. doi: 10.1007/s10562-021-03868-2. Epub 2021 Dec 6.

Authors

Md Abdul Motin^{1

2}, Andreas Steiger-Thirsfeld³, Michael Stöger-Pollach³, Günther Rupprechter¹

Affiliations

¹ Institute of Materials Chemistry, Technische Universität Wien, Getreidemarkt 9/BC/01, 1060 Vienna, Austria.
² Present Address: Department of Chemistry, University of California, Riverside, 501 Big Springs Road, Chemical Sciences #141, Riverside, CA 92521 USA.
³ University Service Center for Transmission Electron Microscopy (USTEM), Technische Universität Wien, Wiedner Hauptstraße 8-10, 1040 Vienna, Austria.

Abstract

A surface science based approach was applied to model carbon supported Pd nanoparticle catalysts. Employing physical vapour deposition of Pd on sputtered surfaces of highly oriented pyrolytic graphite (HOPG), model catalysts were prepared that are well-suited for characterization by X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Analysis of the HOPG substrate before and after ion-bombardment, and of Pd/HOPG before and after annealing, revealed the number of "nominal" HOPG defects (~ 10¹⁴ cm^-2) as well as the nucleation density (~ 10¹² cm^-2) and structural characteristics of the Pd nanoparticles (mean size/height/distribution). Two model systems were stabilized by UHV annealing to 300 °C, with mean Pd particles sizes of 4.3 and 6.8 nm and size/height aspect ratio up to ~ 10. A UHV-compatible flow microreactor and gas chromatography were used to determine the catalytic performance of Pd/HOPG in ethylene (C₂H₄) hydrogenation up to 150 °C under atmospheric pressure, yielding temperature-dependent conversion values, turnover frequencies (TOFs) and activation energies. The performance of Pd nanocatalysts is compared to that of polycrystalline Pd foil and contrasted to Pt/HOPG and Pt foil, pointing to a beneficial effect of the metal/carbon phase boundary, reflected by up to 10 kJ mol^-1 lower activation energies for supported nanoparticles.

Keywords: Carbon; Ethylene hydrogenation; Kinetics; Metal-support interaction; Model catalysts; Palladium; Photoelectron spectroscopy; Platinum; Scanning tunneling microscopy.