Surface science studies of hydroformylation on a model single-atom catalyst

Abstract

Catalysis is a highly relevant field in the chemical industry. Two groups into which catalysts can be divided are homogeneous and heterogeneous catalysts. Homogeneous catalysts typically are more efficient and selective compared to heterogeneous catalysts, but are also more difficult to separate from the products, and more expensive. “Single-atom” catalysts are bound to a solid support, and thus belong to the heterogeneous group, yet are intended to bridge this gap, increasing selectivity and efficiency, while reducing the mass of the involved catalyst and therefore also the price. Hydroformylation (alkene + CO + H2 ! aldehyde) is an important industrial reaction typically performed in solution using highly-selective mononuclear complexes (i.e. homogeneously). Recently, rhodium-based “single-atom” catalysts have been shown to catalyze this reaction heterogeneously with similar levels of selectivity, suggesting single-atom catalysis can be a strategy to heterogenize problematic reactions. In this work, surface science methods such as temperature-programmed desorption and X-ray photoelectron spectroscopy are used to study the critical component of coadsorption of C2H4 and CO on isolated rhodium adatoms on the Fe3O4(001) surface, in context of hydroformylation. Reference data of C2H4 on the clean surface are also gathered. The results indicate that a monolayer coverage of C2H4 is equal to four molecules per unit cell. This is likely due to the four octahedrally coordinated iron atoms on the surface of the unit cell serving as adsorption sites. The data also show that 2-fold coordinated rhodium adatoms on the Fe3O4(001) surface do coadsorb C2H4 and CO. Simultaneously they also show that the 5-fold coordinated rhodium adatoms incorporated in the surface layer do not adsorb C2H4. In conclusion, a method exists to manipulate the active site geometry such that coadsorption is possible, a vital step towards hydroformylation. Gaining control of the active site geometry is key to the development of highly-selective single-atom catalysis.