Understanding the Decomposition of Dimethyl Methyl Phosphonate on Metal-Modified TiO 2 (110) Surfaces Using Ensembles of Product Configurations.

The decomposition of dimethyl methyl phosphonate (DMMP), a simulant for the nerve agent sarin, was investigated on Cu 4 /TiO 2 (110) and K/Cu 4 /TiO 2 (110) surfaces using a combination of near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) and density functional theory calculations (DFT). Mass-selected Cu 4 clusters and potassium (K) atoms were deposited onto TiO 2 (110) as a metal catalyst and alkali promoter to improve the reactivity and recyclability of the TiO 2 surface after exposure to DMMP. Surface reaction products resulting from decomposition of DMMP were probed by NAP-XPS measurements of phosphorus (P) 2p and carbon 1s core-level spectra. The Cu 4 /TiO 2 (110) surface is found to be very active for DMMP decomposition with highly reduced P-species observed even at room temperature (RT). The codeposition of K atoms and Cu 4 clusters further improves the reactivity with no intact DMMP detectable. Temperature-dependent measurements show that the presence of K atoms promotes the removal of residual P-species at temperatures > 600 K. Detailed DFT calculations were performed to determine the surface structures and energetically accessible pathways for DMMP decomposition on Cu 4 /TiO 2 (110) and K/Cu 4 /TiO 2 (110) surfaces. The calculations show that DMMP and P-containing reaction products preferentially bind to the TiO 2 surface, while the molecular fragments, i.e., methoxy and methyl, bind to both the Cu 4 clusters and TiO 2 . The Cu 4 clusters make the P-O, O-C, and P-C bond cleavages of DMMP markedly more exothermic. The Cu 4 clusters are highly fluxional with atomic structures that depend on the configuration of fragments bound to them. Finally, the manifold of P 2p chemical shifts calculated for a large number of energetically favorable configurations of decomposition products is in good agreement with the observed XPS spectra and provides an alternative way of interpreting incompletely resolved core-level spectra using an ensemble of observed structures.