Surface Thermodynamics and Kinetics of MgO(100) Terrace Site Hydroxylation

Abstract

Surface thermodynamic and kinetic models are applied to MgO hydroxylation data obtained under adsorption–desorption water vapor conditions using ambient pressure X-ray photoelectron spectroscopy. Experimental conditions range from 0.67–67 Pa and 270–600 K (10–5 to 20% relative humidity). The resulting standard state enthalpy (ΔH°) and entropy (ΔS°) for terrace site hydroxylation are −40 ± 1 kJ mol–1 and −50 ± 3 J mol–1 K–1, respectively, and independent of OH coverage. At 298 K, this gives a Gibbs free energy of ΔG° = −25 kJ mol–1. The bulk reaction of water vapor with MgO to form Mg(OH)2 (brucite) is ΔG° = −36 kJ mol–1, suggesting that the hydroxylation of the top layer of MgO is a metastable state. Hydroxylation (adsorption) follows zero order kinetics with respect to surface site concentrations, suggesting water is a precursor prior to dissociative adsorption. Dehydroxylation (desorption) kinetics are first order, with a frequency factor of ν = 1.4 (±1.2) × 1011 s–1, suggesting that recombinative desor...