AVS 69 Session SS1+AS-ThM: Molecular Organization at Surfaces

The selection and positioning of specific functional groups will direct packing and stacking of organic building blocks, which determine the electronic and chemical properties of molecular thin films and semiconductors. Design of molecular ligands for metal-organic complexation at surfaces can address the long-standing grand challenge of high selectivity in heterogeneous catalysis. Our group is working to develop principles of on-surface molecular self-assembly¹ and of metal-organic complexation² to gain new insight into molecular layers and new chemical activity at metal single-site catalysts.³ This work involves close collaboration with multiple research groups to synergistically combine talent in design, synthesis, sample preparation, characterization, analysis, theory, and computational modeling. We use a range of surface characterization tools to interrogate these systems under well-controlled environments, including scanning probe microscopy, photoelectron spectroscopy, vibrational spectroscopy, and mass spectrometry. We investigate systems under a variety of conditions: solution/solid interface, ultra-high vacuum, and flow reactor conditions at high temperature and high pressure. Here, I will report on recent results in several aspects of this work. We have demonstrated the impact of conformational entropy in impeding self-assembly, but that this can be overcome with appropriate selection of co-solutes. Metal-organic complexes at surfaces can be designed to achieve single-site metal centers in which we can observe redox isomerism, control of metal oxidation state, transmetallation, and chemical spillover to the support. We have transferred this design concept for single-site catalysts to high-surface-area powder oxide supports and shown that these can operate as effective catalysts in solution and under gas flow conditions. Ongoing work will seek to extend understanding of these systems to achieve molecular thin films and single-site catalysts of greater complexity.

References

[1]D. L. Wisman, H. Kim, C. Kim, T. W. Morris, D. Lee, and S. L. Tait, Chemistry – a European Journal 27, 13887-13893 (2021). DOI: 10.1002/chem.202101611 [https://doi.org/10.1002/chem.202101611]

[2]T. W. Morris, D. L. Wisman, N. Ud Din, D. Le, T. S. Rahman, S. L. Tait, Surface Science712, 121888 (2021). DOI: 10.1016/j.susc.2021.121888 [https://doi.org/10.1016/j.susc.2021.121888]

[3]E. Wasim, N. Ud Din, D. Le, X. Zhou, M. S. Pape, G. E. Sterbinsky, T. S. Rahman, S. L. Tait, Journal of Catalysis413, 81-92 (2022). DOI: 10.1016/j.jcat.2022.06.010 [https://doi.org/10.1016/j.jcat.2022.06.010]

The functionality of 2D metal-organic frameworks (MOFs), crucially depends on the local environment of the embedded metal, and such details are best ascertained on 2D MOFs supported on atomically flat surfaces. Here, we present three systems which are well-defined at the atomic-scale, decoupled from the metal support and stable both in ultrahigh vacuum and in ambient conditions: M-TCNQ (M = Ni, Fe, Mn) supported on epitaxial graphene/Ir(111). We show that these systems are monophase with M₁(TCNQ)₁ stoichiometry, and we demonstrate their remarkable chemical and thermal stability. Furthermore, by a combined experimental and computational approach we study the differences between 2D MOF systems supported on graphene and on Au(111), the prototypical surface for on-surface synthesis. We show that the Fe-TCNQ on graphene is non-planar with iron in quasi-tetrahedral sites, but on Au(111) it is planarized by stronger van-der-Waals interaction. Combined with the distinct energy level alignment with the supports, this results in significant differences in the 2D MOF properties on these two surfaces. Our results outline the limitations of common on-surface approaches using metal supports and show that the intrinsic 2D MOF properties can be partially retained on graphene. The modular M-TCNQ/graphene system combines the atomic-scale definition required for fundamental studies with the robustness and stability needed for applications, thus we consider it an ideal model for research in single atom catalysis or spintronics.

Single-atom catalysts (SACs) combine the advantages of homogeneous and heterogeneous catalysts by limiting the reaction sites to isolated single metal atoms with well-defined chemical characters. Our group has developed a metal-ligand coordination method to stabilize SACs using 1,10-phenanthroline-5,6-dione (PDO) to coordinate with metal atoms such as Pt, Fe, and Cr. In order to further improve the stability of SACs and increase metal loading, we have synthesized single-layered covalent organic frameworks (SCOFs) on model surfaces under ultra-high vacuum (UHV) conditions or under ambient conditions. These two-dimensional (2D) networks with high thermal and chemical stability were used to confine single Pt atoms coordinated with ligands into SCOF pores. Under UHV conditions, the successful formation of the SCOF with regular hexagonal pores on the Au(111) surface was achieved by surface-mediated Ullmann radical coupling of 1,3,5-tris-(4-bromophenyl)benzene (TBB) and characterized by scanning tunneling microscopy (STM). Further sequential deposition of PDO ligand and Pt on the TBB-SCOF surface allowed the formation of single-site Pt catalysts by coordination interaction. STM images have proved the confinement of PDO in the SCOF pores, while X-ray photoelectron spectroscopy (XPS) has proven the oxidation state of Pt, which is an indication of the single atom character. Under ambient conditions, a 2D imine-linked SCOF was formed on the highly oriented pyrolytic graphite (HOPG) surface by a solid-vapor interface mechanism, which allows for a high quality SCOF with long-range order. STM characterization has shown that regular SCOF networks with negligible defects were formed on the HOPG surface with domain sizes greater than 1 µm × 1 µm. These systems which combine the COF and metal-ligand coordination strategy to stabilize SACs offer the possibility to achieve higher stability and greater loading in SACs.