Water dissociation on the low-coordinated sites of MgO nanopowders
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FOCUS ISSUE
UNDERSTANDING WATER-OXIDE INTERFACES TO HARNESS NEW PROCESSES AND TECHNOLOGIES
Water dissociation on the low-coordinated sites of MgO nanopowders Fabio Finocchi1,a), Francia Haque1, Stéphane Chenot1, Jacques Jupille1, Slavica Stankic1,b) 1
Sorbonne Université, CNRS-UMR 7588, Institut des Nanosciences de Paris, F-75252 Paris Cedex 05, France Address all correspondence to these authors. e-mail: fabio.fi[email protected] b) e-mail: [email protected] a)
Received: 10 July 2018; accepted: 12 November 2018
The configurations associated with the dissociative adsorption of water on a variety of low-coordinated sites of MgO(100) surfaces, including corners, steps, MgO vacancies, and kinks on 〈010〉 steps, have been studied and assigned by combining infrared spectroscopy and ab initio calculations. Three kinds of MgO powders were examined: powders of very high specific surface area prepared by chemical vapor synthesis and well-defined cubic smoke particles obtained by combustion in either 20:80 or 60:40 O2:Ar mixtures, the latter one involving less defects and smaller particles. It appears that an imperative requirement to obtain a precise characterization of the reactive behavior of defects is to keep the samples in ultra–high vacuum conditions and to control the water partial pressure finely.
Introduction The dissociative adsorption of water molecules on MgO surfaces has been a case study for years. Despite the apparent simplicity of the system, many issues have not been solved yet. The expected full dissociation of isolated water molecules on low-coordinated sites is one of the pending questions. At partial pressure #105 mbar, water vapor only dissociates on low-coordinated sites of MgO surfaces [1], including steps, corners, and vacancies. In such conditions, the hydroxyl coverage determined on the defective (100) face of cleaved crystals was estimated to a few percent [2]. Flat MgO(100) terraces do not dissociate isolated H2O molecules, even in the case of thin supported films [3], provided they are stoichiometric [4, 5], but at pressures .105 mbar, the (100) terraces progressively hydroxylate [2, 6, 7]. Consistently, microgravimetric analyses performed on MgO powders obtained by thermal decomposition of the hydroxide have evidenced a full monolayer OH coverage of the oxide by exposure to saturated vapor pressure at room temperature [8]. The dissociative adsorption of H2O on monoatomic h010i MgO steps was previously studied by vibrational spectroscopy on exposure of MgO smoke and MgO films to water vapor [9, 10]. Water splitting at 3-fold coordinated sites, 3C, such as kinks and surface vacancies at monatomic and diatomic steps, has been the objective of a theoretical study [11]. However, little is
ª Materials Research Society 2019
known on the relative proportion of those sites when the preparation conditions of the powders vary. The previous findings indicate that a fine analysis of the reactivity of defective MgO surfaces and nanopowders with respect to water vapor imperatively re
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