On the Nature of HOPG-Supported Pt₁Ti₂O₇ and its Decomposition of a Nerve Agent Simulant: A Cluster Model of a Single Atom Catalyst Active Site

Chemical weapons, including hyper lethal nerve agents, are a persistently looming threat across the modern geopolitical landscape. There is a pressing need for the design and development of improved protective materials, which can be substantially aided by the cultivation of a fundamental molecular-level understanding of candidate systems and the corresponding decomposition chemistry. The emergence of the exciting new class of single atom catalyst (SAC) materials has enhanced the prospect of subnanoscale design tailoring in the hopes of optimizing activity and selectivity for a variety of chemical applications. Here, we apply our recently developed experimental technique for modeling the active sites of such SAC materials through the preparation of surface supported size-selected single metal-atom doped metal oxide clusters. The propensity for an SAC cluster model system for Pt₁/TiO₂ materials, Pt₁Ti₂O₇ supported on highly oriented pyrolytic graphite (HOPG), to adsorb and decompose nerve agent simulant dimethyl methylphosphonate (DMMP) was investigated through a combination of temperature-programmed desorption/reaction (TPD/R) and X-ray photoelectron spectroscopy (XPS). XPS measurements of the as-prepared Pt₁Ti₂O₇ clusters supported the successful isolation of single Pt atoms in clusters monodispersed across the HOPG surface. TPD/R experiments showed that the reactivity exhibited by the Pt₁Ti₂O₇ clusters was distinct from that of Ti₂O₇ clusters lacking the single Pt atom. It was found that DMMP decomposed over Pt₁Ti₂O₇ upon heating to as low as room temperature, and higher temperature treatments evolved exclusively H₂O, CO, and H₂, while decomposition over Ti₂O₇ evolved only methanol and formaldehyde at elevated temperatures. This indicated the promotion of C-H and PO-C bond cleavage within DMMP due to the presence of single Pt atoms in the clusters. Further, the Pt₁Ti₂O₇ clusters were found to desorb P-containing decomposition species, preventing active site poisoning; however, a change of reactivity reflecting that of Ti₂O₇ was observed following a single TPD/R cycle. This suggested the encapsulation of active Pt sites by titanium oxide during high temperature treatment and is thus an issue deserving of serious attention in the study of Pt₁/Ti₂O₇ SAC materials.