Adsorption and Decomposition of Dimethyl Methylphosphonate on Size-Selected Zirconium Oxide Trimer Clusters

In recent years, zirconium hydroxide powder and zirconium-based metal organic frameworks have found promising applications as chemical warfare agent (CWA) decomposition materials. While bulk zirconium oxide (ZrO2) has proven to be relatively inactive for such purposes, well-controlled fundamental studies investigating the potential CWA decomposition propensity of subnanoscale zirconium oxide, in which undercoordinated metal centers abound, are still severely lacking. Herein, the adsorption and decomposition of the nerve agent simulant dimethyl methylphosphonate (DMMP) on size-selected zirconium oxide trimer, that is, (ZrO2)3, clusters supported on highly oriented pyrolytic graphite (HOPG), have been investigated via the combination of X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption/reaction (TPD/R). XPS measurements acquired for the DMMP-adsorbed, HOPG-supported clusters at a preparation temperature of 298 K, and also after annealing to several successively higher temperatures of 473, 573, and 673 K, elucidated the uptake of DMMP to the (ZrO2)3 clusters, with one DMMP molecule adsorbed per cluster and virtually no thermal molecular desorption observed up to 673 K. These measurements also showed dissociative adsorption of DMMP at room temperature on some clusters, likely via scission of a P–OCH3 bond in DMMP, with further decomposition accompanying an increase in temperature above 473 K. TPD/R experiments showed the evolution of methanol as a major reaction product via two distinct pathways, with desorption peaks centered around 410 and 575 K. Evolution of dimethyl ether and formaldehyde as minor reaction products was also observed with desorption peaks centered around 560 and 620 K, respectively. A second TPD/R cycle following cluster-induced DMMP decomposition resulted in no detected decomposition chemistry, showing DMMP decomposition on the (ZrO2)3 clusters to be stochiometric and non-catalytic, whereby the remaining P-containing species poisoned the clusters.