RESUMO
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.
RESUMO
Chemical warfare agents (CWAs) are a persistent threat facing civilians and military personnel across the modern geopolitical landscape. The development of the next generation of protective and sensing materials stands to benefit from an improved fundamental understanding of the interaction of CWA molecules with the active components of such candidate materials. The use of model systems in well-controlled environments offers a route to glean such information and has been applied here to investigate the fundamental interaction of a nerve agent simulant molecule, dimethyl methylphosphonate (DMMP), with a small cluster model of a single atom catalyst (SAC) active site. The cluster models, Pt1Zr2O7, were prepared by depositing mass-selected cluster anions synthesized in the gas phase onto a 100 K highly oriented pyrolytic graphite (HOPG) substrate surface prepared in ultra-high vacuum (UHV) at sub-monolayer coverage. Upon deposition, the cluster anions lost their charge to the electrically conductive surface to yield free-standing neutral clusters. The HOPG-supported clusters were characterized by X-ray photoelectron spectroscopy (XPS) to determine the oxidation states and chemical environment of the metal atoms present within the clusters. The reactivity of the clusters with DMMP was investigated via temperature-programmed desorption/reaction (TPD/R) and XPS experiments in which the clusters were exposed to DMMP and incrementally heated to higher temperatures. In contrast to two other HOPG-supported clusters, (ZrO2)3 and Pt1Ti2O7, recently investigated in our laboratory, Pt1Zr2O7 decomposed DMMP to primarily evolve a methane species, which was completely absent for the other clusters.
RESUMO
The selective dehydrogenation of hydrocarbons and their functionalized derivatives is a promising pathway in the realization of endothermic fuel systems for powering important technologies such as hypersonic aircraft. The recent surge in interest in single atom catalysts (SACs) over the past decade offers the opportunity to achieve the ultimate levels of selectivity through the subnanoscale design tailoring of novel catalysts. Experimental techniques capable of investigating the fundamental nature of the active sites of novel SACs in well-controlled model studies offer the chance to reveal promising insights. We report here an approach to accomplish this through the soft landing of mass-selected, ultrasmall metal oxide cluster ions, in which a single noble metal atom bound to a metal oxide moiety serves as a model SAC active site. This method allows the preparation of model catalysts in which monodispersed neutral SAC model active sites are decorated across an inert electrically conductive support at submonolayer surface coverage, in this case, Pt1Zr2O7 clusters supported on highly oriented pyrolytic graphite (HOPG). The results contained herein show the characterization of the Pt1Zr2O7/HOPG model catalyst by X-ray photoelectron spectroscopy (XPS), along with an investigation of its reactivity toward the functionalized hydrocarbon molecule, 1-propanamine. Through temperature-programmed desorption/reaction (TPD/R) experiments it was shown that Pt1Zr2O7/HOPG decomposes 1-propanamine exclusively into propionitrile and H2, which desorb at 425 and 550 K, respectively. Conversely, clusters without the single platinum atom, that is, Zr2O7/HOPG, exhibited no reactivity toward 1-propanamine. Hence, the single platinum atom in Pt1Zr2O7/HOPG was found to play a critical role in the observed reactivity.