Heterogeneous Catalysis for Sustainable Chemical and Energy Technologies: an Atomistic View

The molecular transformation of carbon dioxide to value-added chemicals and synthetic fuels is one of the most promising approaches towards sustainable chemical and energy technologies starting from economical and renewable feedstock. Heterogeneous catalysis is of the most effective technologies capable of activation and chemical transformation of CO₂ to a broad class of compounds such as alcohols, different types of oxygenates, alkenes. An atomistic-level understanding of heterogeneously catalysed processes of CO₂ activation and chemical transformations is an important prerequisite for the rational design of new effective catalysts. Towards this goal, we follow a rigorous surface science approach to obtained a detailed understanding of the reaction mechanisms, kinetics and thermodynamics of processes involved in chemical conversion of carbon dioxide on well-defined model oxide-based surfaces. We employ a unique combination of the state-of-the-art experimental tools including multi-molecular beam techniques, infrared and polarization-modulation infrared reflection-absorption spectroscopy, scanning tunnelling microscopy, single crystal adsorption calorimetry to address these major issues and derive precise structure-reactivity relationships.<br/><br/>In my talk, I will focus on the interaction of CO₂ and H₂O with different types of well-defined oxides – CoO, Co₃O₄ and Fe₃O₄ – prepared by epitaxial growth on metal single crystals under ultra-high vacuum conditions. Specifically, it will be shown that CO₂ can be effectively activated on these oxide surfaces and react with H originating from the co-adsorbed hydroxyl groups formed upon adsorption and dissociation of water. The mechanistic picture of the latter process will be discussed in more detail for water interaction with model Fe₃O₄(111)/Pt(111) oxide.¹ Combining single crystal adsorption calorimetry, infrared spectroscopy on the isotopically labelled iron oxide and theoretical studies, we show that water dissociates readily on iron oxide surfaces forming a dimer-like hydroxyl-water complex and prove that the generally accepted model of water dissociation to two individual OH groups is incorrect.<br/><br/>In the second part, I will introduce a concept of ligand-directed heterogeneous catalysis and provide an example on how purposeful functionalization of a catalytic surface with co-adsorbed organic molecules can dramatically enhance chemoselectivity in hydrogenation of multi-unsaturated hydrocarbons. Specifically, I will present a mechanistic study on formation and dynamic changes of a ligand-based heterogeneous Pd catalyst for chemoselective hydrogenation of α,β-unsaturated aldehyde acrolein.² Deposition of allyl cyanide as a precursor of a ligand layer renders Pd highly active and nearly 100 % selective toward propenol formation by promoting acrolein adsorption in a desired configuration via the C=O end. Employing a combination of real space microscopic (STM) and <i>in operando</i> spectroscopic (IRAS) surface sensitive techniques, we show that an ordered active ligand layer is formed under operational conditions, consisting of stable butylimin species, which turns the surface highly selective toward the desired product. Obtained atomistic-level insights into the formation and dynamic evolution of the active ligand layer under operational conditions provide important input required for controlling chemoselectivity by purposeful surface functionalization.<br/><br/><i>1. Dementyev, P. et al, <i>Angew. Chem. Int. Edit. 54</i> (<b>2015</b>) 13942</i><br/><i>2. Schröder, C. et al., Angew. </i><i>Chem. Int. Edit. <b>2021,</b> 60 (30), 16349</i>