RESUMO
The decomposition of dimethyl methyl phosphonate (DMMP), a simulant for the nerve agent sarin, was investigated on Cu4/TiO2(110) and K/Cu4/TiO2(110) surfaces using a combination of near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) and density functional theory calculations (DFT). Mass-selected Cu4 clusters and potassium (K) atoms were deposited onto TiO2(110) as a metal catalyst and alkali promoter to improve the reactivity and recyclability of the TiO2 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 Cu4/TiO2(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 Cu4 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 Cu4/TiO2(110) and K/Cu4/TiO2(110) surfaces. The calculations show that DMMP and P-containing reaction products preferentially bind to the TiO2 surface, while the molecular fragments, i.e., methoxy and methyl, bind to both the Cu4 clusters and TiO2. The Cu4 clusters make the P-O, O-C, and P-C bond cleavages of DMMP markedly more exothermic. The Cu4 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.
RESUMO
The decomposition of dimethyl methylphosphonate (DMMP, (CH3O)2P(O)(CH3)), a simulant to the toxic nerve agent Sarin, on the rutile TiO2(110) surface has been studied with temperature programmed desorption (TPD) and density functional theory (DFT) calculations. The reactivity of the TiO2(110) surface for DMMP decomposition is shown to be low, with mainly molecular desorption and only a small fraction of methanol and formaldehyde decomposition products seen from TPD at around 650 K. In addition, this amount of products is similar to the number of O vacancies on the surface. DFT calculations show that O vacancies are key for P-OCH3 bond cleavage of DMMP, lowering the barrier by 0.7 eV and enabling the reactive process to occur at around 600 K. This is explained by the closer position of DMMP with respect to the surface in the presence of O vacancies. Calculations show that the produced methoxy groups can transform into gas phase formaldehyde and methanol at the considered temperature (600 K), in agreement with experiments. O-C bond cleavage of DMMP is also a viable pathway at such a high temperature (600 K) for DMMP decomposition on r-TiO2, even in the absence of O vacancies, but the formation of a gas phase product is energetically unfavorable. O vacancies hence are the active sites for decomposition of DMMP into gas phase products on r-TiO2(110).