NCAFM2023 Programme Booklet

Monday 1100 - 1120

COMPREHENSIVE SINGLE-MOLECULAR CHARACTERIZATION OF COBALT PHTHALOCYANINE MOLECULES ON Ag(111) FOR SURFACE CATALYSIS APPLICATIONS

Xinzhe Wang ,1 Percy Zahl ,2 Hailiang Wang, 3 Eric Altman, 1 and Udo D. Schwarz 1,3*

1 Dept. of Mechanical Engineering and Materials Science, Yale University, New Haven, CT 06511, USA. 2 Brookhaven National Lab, Upton, NY 11973, USA. 3 Department of Chemical and Environmental Engineering, Yale University, New Haven, CT 06511, USA. Email: udo.schwarz@yale.edu

Recently, Wang et al. [1,2] have found a promising class of hybrid catalysts based on cobalt phthalocyanines (CoPcs) immobilized on carbon nanotube supports that promote the methanol production from CO2. Thereby, the binding strength of the intermediate CO to the cobalt atom at the center of the CoPcs catalyst molecule has been recognized as a key descriptor affecting catalytic efficiency, with the ideal CO-Co binding strength being neither too strong nor too weak. To study this problem systematically at the single molecule level, we present a comprehensive, three-dimensional examination of CoPc molecules on Ag(111) using low-temperature, ultrahigh vacuum scanning probe microscopy (SPM). Scanning tunneling microscopy, noncontact atomic force microscopy, 3D-SPM, and Kelvin probe force microscopy techniques were carried out to characterize the CoPc molecule. We successfully identified the geometric structure (panel a of the figure below), electron distribution, and local barrier height of a single CoPc molecule. The equilibrium distances and potential energies at equilibrium distances (panel c) were also calculated across the molecule based on the 3D-SPM we preformed (panel b). A particularly noteworthy aspect of the approach is that after characterizing the molecule, systematically changing the substituents/side chains of the CoPc or the substrate the CoPc molecules sit on will during future experimentation allow to clarify the effect of these changes on the CO-Co binding strength and eventually enable a fine tuning of the binding strength, which may open new avenues to optimize the catalytic reaction.

Fig. (a) NC-AFM image of CoPc with the structural model overlayed. At each pixel, F(z) curves were acquired. (b) Four F(z) curves obtained at the locations indicated in (a) demonstrate the ability to examine tip-sample interactions locally with high resolution using 3D-SPM; note that the contribution of the substrate to the tip-sample interaction has been removed from the data. Since the tip is terminated with a CO molecule, these curves mainly reflect the force between the CO and the CoPc. (c) By fitting the extracted force curves at each pixel, maps of the energy minima for the CO-CoPc interactions can be recovered, which allow conclusions on the location-dependent adhesion of CO to the CoPc.

References [1] Y. Wu, Z. Jiang, X. Lu, Y. Liang, and H. Wang, Nature, 2019, 575, 639. [2] X. Zhang et al., Nature Communications, 2017, 8, 14675.

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