Surface reaction of dialkyl phosphite on alumina and magnesia studied by inelastic electron tunneling spectroscopy

Abstract

Vibrational spectra of dimethyl, diethyl, and diphenyl phosphite on alumina and magnesia surfaces have been measured by inelastic electron tunneling spectroscopy. These dialkyl phosphites (O,O-dialkyl phosphonates) were adsorbed onto Al{sub 2}O{sub 3} and MgO surfaces from anhydrous benzene solutions by using a spin doping method. The tunneling spectra of phosphorous acid (phosphonic acid) on Al{sub 2}O{sub 3} have also been measured for comparison. Phosphorous acid reacts with surface OH groups of Al{sub 2}O{sub 3} by losing its protons and is adsorbed as phosphite anion (phosphonate dianion) onto the surface. Analysis of the tunneling spectra and comparison of the infrared and Raman spectra of the dialkyl phosphites give information about their molecular structures on Al{sub 2}O{sub 3} and MgO and a surface reaction on them. These dialkyl phosphites decompose by a reaction with the surface OH groups on Al{sub 2}O{sub 3} and MgO to give the adsorbed O-alkyl phosphonate monoanions and phosphonate dianion. The similarity of the tunneling spectra of dimethyl phosphite and that of phosphorous acid suggests that dimethyl phosphite decomposes rapidly to give preferentially the adsorbed phosphonate dianion on the surfaces.