From Metal Clusters to Single Atoms and about the Formation of Metal-Organic Frameworks: An X-ray Photoelectron Spectroscopy Study

Catalysis plays a crucial role in the chemical industry. However, the catalysts used for industrial processes are complex and mostly consist of various materials making it challenging to distinguish the reactivity and catalytic active sites of the systems. Thus, model catalysts with increasing complexity are studied to overcome the material gap between real-application catalysts and model systems. The model catalysts studied in this work by X-ray photoelectron spectroscopy are metal clusters on a 2D material support (graphene and h-BN). The 2D materials and their Moiré structures, which evolve on Rh(111), were employed to grow ordered, almost monodisperse nanoclusters on the surface. Starting with the investigation of Co clusters on a graphene/Rh(111) substrate, the reactive sites were narrowed down to the edges and kinks (undercoordinated sites) of the clusters using CO as a probe molecule. These reactive sites catalyzed the dissociation of CO at elevated temperatures and led to the formation of cobalt carbide and oxide. Repeating the CO adsorption experiment on the C- and O-precovered clusters revealed the blocking of edge adsorption sites by the CO dissociation products, a reduced CO adsorption capacity, and passivation of the clusters towards further CO dissociation. Furthermore, bimetallic FePt clusters were investigated to distinguish the influence of a small amount of Pt on the catalytic performance of Fe clusters. In this context, strong metal-support interaction was observed resulting in an encapsulation of the Pt by a thin iron oxide layer which evolved due to CO dissociation upon annealing of the sample. For the investigation of available adsorption sites on Pt clusters of various sizes, a Pt coverage gradient was prepared on h-BN/Rh(111) with cluster sizes ranging from 26 to only one atom per pore. For the larger clusters (12 – 26 Pt atoms), CO adsorption was observed at on-top and step sites of the clusters, while the formation of platinum carbonyls is observed for the smaller investigated Pt coverages (< 12 Pt atoms). Besides these model catalytic studies, the formation and thermal stability of metal-organic frameworks using different precursor molecules and metal coordination centers were investigated. The stability of the formed network strongly depends on the preparation route. The experiments revealed that deposition of the precursor molecules before evaporation of the metal results in a higher stability of the metal-organic framework. The higher stability of this system is explained by interactions between the precursor molecules and the metal, already at low temperatures, which prevent cluster formation. Thus, the availability of metal atoms for coordination is increased. This approach could also be especially interesting in regard to the preparation of single-atom catalysts, which recently is a hot topic in the field of catalysis because sintering of the metal atoms can be prevented by the coordination to the precursor molecules preserving the catalytic activity.