Oxide and carbonate surfaces as environmental interfaces: the importance of water in surface composition and surface reactivity

Environmental molecular surface science is an important and expanding area of current research. Here we give a brief review of the chemical nature of two representative oxide and carbonate surfaces, MgO(100) and CaCO3(104), under ambient conditions of temperature and relative humidity. Studies using a variety of spectroscopic techniques have shown that water readily dissociates on these surfaces under ambient conditions leaving a surface truncated with hydroxyl groups. For MgO, the surface stoichiometry is best represented as Mg(OH), whereas for CaCO3, it is Ca(OH)(CO3H). Water readily adsorbs on these hydroxylated surfaces. This adsorbed water layer plays an important role in the reactivity of MgO(100) and CaCO3 (104) under ambient conditions. The adsorption of nitric acid, a trace gas found in the troposphere. on MgO(100) and CaCO3(104) surfaces under dry (< 1% RH) and wet conditions (20-25% RH) demonstrates this point. Adsorption at higher relative humidity results in the formation of nitrate thin films that are on the order of 10-100 layers thick. Water uptake on these thin nitrate films results in several phase transitions as a function of relative humidity. Thus the results presented here are meant to demonstrate the importance of adsorbed water in the surface composition and surface reactivity of oxide and carbonate surfaces. (C) 2004 Elsevier B.V. All rights reserved.