Characterizing metal nanoparticle catalytic activity by the work function – REACTIVITY

While nc-AFM is used to reveal the surface morphology determined by the size, shape, distribution and sintering of NPs, the task of KPFM is to monitor the local electronic structure determined by changes in the NP's work function (WF) providing a quantitative measure for adsorbed molecular species or dissolved atomic species in NPs. The techniques is established as calibrated standard tools in heterogeneous model catalysis involving metal NPs. Local measurements by nc-AFM and KPFM are supported by an analysis with X-ray photoelectron spectroscopy (XPS) and Ultraviolet photoelectron spectroscopy (UPS) to obtain the chemical fingerprints and average WF of the molecule-NP-surface system. Experiments are further backed by density functional theory (DFT) analysing and predicting the adsorption of reactants on NPs and related WF changes. The ultimate and challenging goal is to observe and to quantify simple reactions at single NPs, namely the oxidation of CO and the hydrogenation of hydrocarbons.

For the project, we focus on pure NPs of platinum group materials (PGMs), in particular on palladium (Pd) and gold (Au), commonly used for industrial catalytic converters. Furthermore, we consider bi-metallic NPs with the focus on Pd and Au. We optionally consider also other metals like Fe, Co or Cu. The support for the NPs will be ultra-thin and thick films of cerium oxide (ceria).

1. On the Pt(111) surface, we studied ultra-thin ceria films, which are used as a support for our nanoparticles (NP). The surface morphology, alloying phenomena between Ce and Pt and work function (WF) changes induced by the ceria film were characterized.

2. We studied carbon phenomena at palladium NP: (1) we could show that during the growth of the NPs on HOPG, a small amount of carbon is detached from HOPG and build into the subsurface regions below the NP's facets. (2) Upon higher carbon content, graphene is formed on the NP's facets (G@PdNPs). Experiments are supported by DFT calculations.

The perspective is to study the adsorption of, e.g., oxygen, hydrogen, CO or water on as-prepared NPs and G@PdNPs and to study reactions on the two systems.

Henrik Grönbeck and Clemens Barth
“Revealing Carbon Phenomena at Palladium Nanoparticles by Analyzing the Work Function“
Journal of Physical Chemistry C 123 (2019) 4360

Gabriele Gasperi, Paola Luches and Clemens Barth
“Stability of Ultrathin Ceria Films on Pt(111) Exposed to Air and Treated in Redox Cycles“
Journal of Physical Chemistry C 122 (2018) 25954

REACTIVITY is a fundamental research project, at the technology readiness level 1. It is submitted to the Other-Knowledge Challenge as we will develop a new, untouched and challenging experimental strategy in heterogeneous model catalysis for describing reactivity related phenomena: we will demonstrate that the combination of non-contact atomic force microscopy (NC-AFM) and Kelvin probe force microscopy (KPFM) can be used to quantify the adsorption/desorption characteristics of fundamental gases in heterogeneous catalysis at the single nanoparticle (NP) level as a function of NP size, shape, composition and temperature, and how properties are affected by the oxide support. Furthermore, we will explicitly demonstrate that KPFM is capable of quantifying phenomena of contamination and dissolution at single NPs that is most important for many catalytic processes.

While NC-AFM will be used to reveal the surface morphology determined by the size, shape, distribution and sintering of NPs, the task of KPFM will be to monitor the local electronic structure determined by changes in the NP's work function (WF) providing a quantitative measure for adsorbed molecular species or dissolved atomic species in NPs. The techniques will be established as calibrated standard tools in heterogeneous model catalysis involving metal NPs. Local measurements by NC-AFM and KPFM will be supported by an analysis with X-ray photoelectron spectroscopy (XPS) and Ultraviolet photoelectron spectroscopy (UPS) to obtain the chemical fingerprints and average WF of the molecule-NP-surface system. Experiments will be backed by density functional theory (DFT) analysing and predicting the adsorption of reactants on NPs and related WF changes. The ultimate and challenging goal is to observe and to quantify simple reactions at single NPs, namely the oxidation of CO and the hydrogenation of hydrocarbons.

For the project, we will first focus on pure NPs of platinum group materials (PGMs), in particular on palladium (Pd) and gold (Au), commonly used for industrial catalytic converters. Furthermore, we consider bi-metallic NPs with the focus on Pd and Au. During the course of the project, we optionally consider also other metals like Fe, Co or Cu. The support for the NPs will be ultra-thin and thick films of cerium oxide (ceria).

The adsorption and desorption of CO, oxygen, hydrogen and simple hydrocarbon gases (ethylene) will be investigated in detail as this is relevant for the reaction to be studied and these species are known to adsorb at, e.g., PdNPs. Contamination experiments will focus in particular on carbon contamination and dissolution, phenomena most relevant for many catalytic reactions like the hydrogenation of hydrocarbons, Fischer-Tropsch process and in the synthesis of carbon structures like graphene or nanotubes.