Ligand-directed heterogeneous catalysis for controlling chemoselectivity of multi-pathway surface reactions: towards mechanistic understanding via surface science approach.

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The selectivity of multi-pathway surface reactions depends on subtle differences in the activation barriers of competing reactions, which is difficult to control. One of the most promising strategies to overcome this problem is to introduce a specific selective interaction between the reactant and the catalytically active site, directing the chemical transformations towards the desired route. This interaction can be imposed via functionalization of a catalyst with ligands, promoting the desired pathway via steric constrain or electronic affects. Recently, a number of highly selective ligand-functionalized powdered catalysts was developed, opening up a new field of ligand-directed heterogeneous catalysis. The microscopic-level understanding of the underlying surface processes, however, is still largely missing.With the proposed research we are aiming at an atomistic-level understanding of ligand-directed heterogeneous catalysis occurring at the metal surfaces functionalized with organic ligand assemblies. The focus of this study will be at the atomistic-level characterization of the geometric and chemical structure of the ligand assemblies as well as at the exploring the molecular reaction mechanisms and kinetics of multi-pathway reactions occurring at these complex interfaces.We will apply a unique combination of surface-sensitive techniques on well-defined model catalysts functionalized with ligands – both metal single crystals and metallic nanoparticles supported on model oxides– to explore the origins of ligand-directed catalysis at the atomistic level. Our experimental approach includes a combination of scanning tunneling microscopy for structural characterization of the ligand layers; infrared spectroscopy enabling the chemical identification and in operando monitoring of the surface species and molecular beam techniques allowing for highly controlled kinetic studies.We will systematically change the chemical and the geometric structure of the ligand assemblies to tune the surface confinement and electronic effects and find exact correlations between these structural and chemical properties of the ligand-layer and the adsorption and reactive processes, such as the adsorption geometries of the reactants and the reaction intermediates over a ligand-layer, the reaction mechanisms, the activity and selectivity towards competing reaction pathways and reaction kinetics. Two types of reactions will be investigated: partial selective hydrogenation of multi-unsaturated carbonyl compounds and alkynes.The combination of these state-of-the-art experimental techniques will be employed for the first time in the fundamental-level studies on the model ligand-directed catalysis and will allow to obtain new atomistic-level insights into the underlying elementary surface processes. The outcome of this research holds a great potential for developing new concepts towards rational-based design of surfaces with tailor-made catalytic properties.