Surface Chemistry of Trimethyl Phosphate on α-Fe2O3

Abstract

The chemistry of trimethyl phosphate (TMP) was examined on the (012) crystallographic face of hematite (α-Fe2O3) using temperature-programmed desorption (TPD), high resolution electron energy loss spectroscopy (HREELS), static secondary ion mass spectrometry (SSIMS), and Auger electron spectroscopy (AES). TMP adsorbed at Fe3+ sites on the clean α-Fe2O3(012) surface through lone pair electrons on the P═O oxygen atom. A small portion of adsorbed TMP desorbed without decomposition; however, the majority of adsorbed TMP decomposed on the clean surface in a two-step process. The first step, occurring at or below room temperature, involved displacement of one methoxy group of TMP to form a surface methoxy and adsorbed dimethyl phosphate (DMP). In the second step, adsorbed DMP decomposed above 500 K to a 1:1 ratio of gaseous methanol and formaldehyde leaving phosphate on the surface. The phosphate was stable on the α-Fe2O3(012) surface to 950 K. Identification of these steps was assisted by using the chemistry of methanol on the clean surface. Coadsorption of TMP and water led to a small degree of hydrolysis between these two molecules in the multilayer but no significant changes in the chemistry of TMP molecules adsorbed on the surface.