1H MAS NMR study of the coordination of hydroxyl groups generated upon adsorption of H 2O and CD 3OH on clean MgO surfaces

Abstract

The coordination of OH groups on MgO has been characterized by 1H MAS NMR after appropriate pre-treatment conditions to obtain surfaces free from carbonates and with a controlled degree of hydroxylation. On MgO-sol–gel sample treated at 1023 K under flowing nitrogen, hydroxylated with water and evacuated at 673 K for 2 h, at least six lines have been identified at 1.2, 0.7, 0.0, −0.4, −0.7 and −1.8 ppm. Upon dehydroxylation at higher temperature, the signals become sharper and a relative decrease of intensity of the lines at δ ≥ −0.4 and at −1.8 ppm is observed. After adsorption of CD 3OH and further desorption at 473 K, an OH signal with two main lines at 0.4 and 0.0 ppm and a broad contribution around 1.0 ppm are observed. Thus deuterated methanol dissociation generates only OH groups at high chemical shift. These observations are related to the coordination of the oxygen atom of the OH groups. It is thus suggested that, when water dissociates on the surface, the OH groups coming from protonation of O 2− ions and being the more coordinated, give a signal at a higher chemical shift than the OH groups formed by coordination of a hydroxide ion to a Mg 2+. The link with the acido-basic properties is then discussed on the basis of literature data.