Real-Space Identification of Non-Noble Single Atomic Catalytic Sites within Metal-Coordinated Supramolecular Networks

Abstract

With regard to the development of single atom catalysts (SACs), non-noble metal–organic layers combine a large functional variability with cost efficiency. Here, we characterize reacted layers of melamine and melem molecules on a Cu(111) surface by noncontact atomic force microscopy (nc-AFM), X-ray photoelectron spectroscopy (XPS) and ab initio simulations. Upon deposition on the substrate and subsequent heat treatments in ultrahigh vacuum (UHV), these precursors undergo a stepwise dehydrogenation. After full dehydrogenation of the amino groups, the molecular units lie flat and are strongly chemisorbed on the copper substrate. We observe a particularly extreme interaction of the dehydrogenated nitrogen atoms with single copper atoms located at intermolecular sites. In agreement with the nc-AFM measurements performed with an O-terminated copper tip on these triazine- and heptazine-based copper nitride structures, our ab initio simulations confirm a pronounced interaction of oxygen species at these N–Cu–N sites. To investigate the related functional properties of our samples regarding the oxygen reduction reaction (ORR), we developed an electrochemical setup for cyclic voltammetry experiments performed at ambient pressure within a drop of electrolyte in a controlled O2 or N2 environment. Both copper nitride structures show a robust activity in irreversibly catalyzing the reduction of oxygen. The activity is assigned to the intermolecular N–Cu–N sites of the triazine- and heptazine-based copper nitrides or corresponding oxygenated versions (N–CuO–N, N–CuO2–N). By combining nc-AFM characterization on the atomic scale with a direct electrochemical proof of performance, our work provides fundamental insights about active sites in a technologically highly relevant reaction.

A Monolayer Carbon Nitride on Au(111) with a High Density of Single Co Sites

“…48 This type of synthesis was shown to form a catalytic Cu−N 2 unit. 49 Importantly, our results also demonstrate that well-defined monolayer carbon nitride phases that differ significantly from g-C 3 N 4 in their structure and capability to coordinate Co species can be realized. Notably, the cobalt carbon nitride is formed already at 450 K, which is far lower than the temperature used in typical pyrolysis methods for g-C 3 N 4 .…”

“…Similar reactions have also been performed by reacting melamine with a Cu(111) substrate . This type of synthesis was shown to form a catalytic Cu–N 2 unit …”

Local structural environment of single-atom catalysts

“…[175][176][177][178][179][180][181][182][183] This has prompted intense efforts toward the precise design of tailored structures by engineering the coordination environment with coordination atoms, from the first coordination shell to the second shell or higher for SACs. [184][185][186][187][188][189][190][191][192][193] The first coordination shell refers to those atoms directly bonded to the central MSAs, while the second coordination shell refers to atoms bonded to the firstshell atoms but not to the central MSAs. The engineering of the first or higher coordination shells by precisely tailoring the heterogeneity of coordination atoms offers a great opportunity to tune the properties of SACs.…”

Structural Phase Transition of Quinone-Containing PAH Derivatives on Au(111) at High Coverage

“…The controllable fabrication of desired well-ordered nanoarchitectures on surfaces has received great interest due to its potential in the design of functionalized nanomaterials. An effective and appealing strategy to regulate the surface nanostructure is delicate control over the structural phase transition by which thermal stabilities, electronic properties, and other functions − may be modulated to meet the demand for specific functionalization. In this sense, the introduction of metal adatoms, especially intrinsic metal adatoms generated directly from surfaces, is considered to be an effective and appealing strategy to steer the structural phase transition, as (1) intrinsic metal adatoms can be easily introduced into the organic system by simple heating treatment and coordinate with specific functional groups to form relatively stable nanoarchitectures − and (2) metal-involved nanostructures can enrich the function of the nanoarchitectures, such as the electronic, catalytic, or spintronic properties, ,,, which is of more general interest for potential applications.…”

“…An effective and appealing strategy to regulate the surface nanostructure is delicate control over the structural phase transition by which thermal stabilities, electronic properties, and other functions − may be modulated to meet the demand for specific functionalization. In this sense, the introduction of metal adatoms, especially intrinsic metal adatoms generated directly from surfaces, is considered to be an effective and appealing strategy to steer the structural phase transition, as (1) intrinsic metal adatoms can be easily introduced into the organic system by simple heating treatment and coordinate with specific functional groups to form relatively stable nanoarchitectures − and (2) metal-involved nanostructures can enrich the function of the nanoarchitectures, such as the electronic, catalytic, or spintronic properties, ,,, which is of more general interest for potential applications. It should be noted that such a kind of phase transition mostly occurs at relatively low surface coverage, in which situation organic molecules are able to diffuse in surfaces freely to coordinate with the target metal atoms to form energetically favorable nanostructures. ,− However, the structural phase transition at a higher molecular concentration is still challenging, as the molecular migration ability on surfaces is greatly limited with the increase of the molecular concentration and the phase transition might be hindered to some extent.…”