FINITE SIZE EFFECT ON ELECTROSORPTION AND CATALYTIC PROPERTIES OF CATALYST MONOLAYERS
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The 2D monolayer catalysts obtained by surface-limited redox replacement (SLRR) reaction have found wide applications in fuel cell technology. Their properties are very dependent on the morphology of the catalysts monolayers, i.e., the size/shape and structure of 2D nanoclusters constituents of the catalyst monolayer. This renders the need for better understanding of all processes involved in catalyst synthesis via SLRR including reaction kinetics, underpotential deposition (UPD) and catalyst nucleation. The first part of this PhD work focuses on UPD phenomenon as a critical step in development of more sophisticated protocols for catalyst monolayer synthesis using SLRR reaction. The Pb UPD has been studied on Pt and Ru submonolayer modified Au(111) surface using conventional electrochemical techniques, in-situ and exsitu Scanning Tunneling Microscopy (STM) and statistical image processing. The results suggest two distinct UPD process on Pt and Ru modified Au(111) surfaces each providing an opportunity for synthesis of more complex bi-metallic monolayer catalyst nano structures using SLRR reaction (nanoclusters). As a continuation of this effort, the new protocol for synthesis of Ru-Pt core-shell “hybrid” 2D nanocluster structures on Au(111) have been demonstrated. Their properties are compared with Ru, Pt and Pt-Ru alloy catalyst monolayers/nanoclusters on Au(111) using CO monolayer oxidation and electrosorption as the model probe molecule/reaction. The IR spectroscopy data are used for qualitative comparison of the strength of the CO-catalyst bond. These results are correlated with the CO oxidative stripping results. In the second part, the Pt nucleation during SLRR replacement of Cu UPD has been studied in order to better understand and describe relevant parameters controlling the Pt monolayer morphology (nanocluster size). The exsitu STM and statistical image processing are used to characterize the morphology of deposited Pt monolayers obtained in experiments where the coverage of Cu UPD has been varied from 0 to 1. The Pt nucleation density and the average size (area) of Pt nanoclusters are used as the main descriptors of Pt monolayer morphology. Results are discussed within the framework of analytical model developed to describe qualitatively our data. The relevance of our work for design of Pt monolayer catalyst is illustrated by our IR data illustrating CO electrosorption on Pt monolayers with different nucleation densities and average size of Pt nanoclusters. In the third part of our work, the finite-size effects (nanocluster size) in a system with strong d-orbital mixing such as Pt monolayer on Pd(hkl) were studied. IR data are used to compare qualitatively the strength of the CO-metal bond on Pt(hkl), Pd(hkl) and Pt/Pd(hkl) surfaces which were characterized by STM and statistical image processing. Our results demonstrate that the finite size of Pt nanoclusters constituents of the Pt monolayer of Pd(hkl) has a dominant effect on their electrosorption properties. The results are discussed in terms of the active strain in Pt nanoclusters, which is very different than the one expected from epitaxial relation between Pt and Pd. In an effort for better control of the morphology of Pt monolayers on Pd, the effect of citrates is studied on reaction kinetics of Pt deposition via SLRR of Cu UPD monolayer on Pd nanoparticles. The analytical model has been developed having a good predictive capability of the reaction rate constant during an industrial scale batch-reactor catalyst synthesis process.