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Site specific heterogeneous catalysis : CO and C₂H₄ bonding with Fe-Terpyridine on a Ag(111) surface studied by scanning probe microscopy DeJong, Miriam Dieneke


Heterogeneous catalysis is a key process in the manufacturing of chemicals, clean fuels, plastics, and pharmaceuticals, and often involves gaseous reactants catalyzed by solid surfaces producing gaseous products. At the solid-gas interface, a variety of potential nucleation sites and reaction pathways for chemical transformations exist. To track these different pathways and distinguish the main reaction from side reactions, it is essential to explore the surface site-by-site, at the atomic scale. Experimental techniques offering the ability to probe surfaces on a site- specific, atomic scale are limited, and rigorous knowledge of reaction mechanisms in many heterogeneous catalysis processes is lacking. In this work, Scanning Probe Microscopy (SPM) techniques including Scanning Tunneling Microscopy (STM), Scanning Tunneling Spectroscopy (STS), and non contact Atomic Force Microscopy (ncAFM) were utilized to examine the site-by-site surface transformation of small gaseous adsorbates at the single-molecule and submolecular scales. SPM techniques are able to access site-specific information from surfaces on the atomic scale, which is vital to elucidating reaction mechanisms in heterogeneous catalysis. For this work, an Fe-terpyridine coordination complex supported on a Ag(111) surface was tested as a catalyst for the transformation of gaseous carbon monoxide (CO) and ethylene (C₂H₄). Using SPM techniques, bonding was measured between CO and Fe-terpyridine active sites, and between C₂H₄ and the active sites at temperatures of T ≤ 30K. Physical structural changes in ncAFM and STM images as well as shifts in electronic structure obtained through STS measurements were used to characterize the bonds, and both Fe-CO and Fe-C₂H₄ bonds were preceded by metastable reaction intermediates. The measured bonding activity presented in this work demonstrates the potential of surface-supported Fe-terpyridine complexes as catalysts in the transformation of small hydrocarbons and oxides, and the fundamental mechanistic insight obtained through this SPM investigation opens the door to future optimization of these heterogeneous transformations.

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