Dimethyl methylphosphonate decomposition on fully oxidized and partially reduced ceria thin films

摘要

The thermal decomposition of dimethyl methylphosphonate (DMMP) on crystalline ceria thin films grown on Ru(0001) was studied by temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS) and infrared absorption reflection spectroscopy (IRAS). TPD experiments show that methanol and formaldehyde desorb as the two main products at 575 K, while water, formaldehyde and CO are produced above 800 K. IRAS studies demonstrate that DMMP adsorbs via the phosphoryl oxygen at 200 K, but the P=O bond converts to a bridging O-P-O species at 300 K. DMMP decomposition initially occurs via P-OCH_3 bond scission to form methyl methylphosphonate (MMP) and methyl phos-phonate (MP) between 300 and 500 K; XPS and IRAS data are consistent with a methoxy intermediate on the surface at these temperatures. The more stable P-CH_3 bonds remain intact up to 700 K, and the only surface intermediate at higher temperatures is believed to be PO_x. Although the presence of PO_x decreases activity for DMMP decomposition, some activity on the ceria surface remains even after 7 cycles of adsorption and reaction. The ceria films become reduced by multiple DMMP adsorption-reaction cycles, with the Ce~(+4) content dropping to 30% after seven cycles. Investigations of DMMP reaction on reduced ceria surfaces show that CO and H_2 are produced in addition to methanol and formaldehyde. Furthermore, DMMP decomposition activity on the reduced ceria films is almost completely inhibited after only 3 adsorption-reaction cycles. Similarities between DMMP and methanol chemistry on the ceria films suggest that methoxy is a key surface intermediate in both reactions.