Initial dissolution kinetics of ionic oxides

Abstract

It is shown th at factors previously recognized, but not regarded as critical, can dominate dissolution kinetics of ionic oxides. The use of the nearly perfect {100} MgO surfaces of smoke cubes to obtain very precise values of dissolution rates per unit surface area, in dilute HC1, HC1O 4 and HNO 3 , has shown th at rates extrapolated to zero dissolution are almost independent of pH in the range 2.0- 3.5. Dissolution rates were measured by monitoring solution pH as a function of time. This revealed increasing rates with increasing pH up to about 5 % total dissolution, followed ultimately by a return to the linear relation between Ig(rate) and pH (slope ca. — 0.5) normally expected. The initial increase in rate is due to increasing [Mg 2+ ] in solution and is observed with [Mg 2+ ] as low as 1 % of the [H + ]. A linear relation between lg(rate) and [Mg 2+ ] is found during the early stages of dissolution. Other cations (Al 3 +, Na + ) also increase the initial rate, to a similar extent. Electron-microscope observations of the cubes show alteration of the surfaces to a castellated structure (of {100}-based projections and intrusions) on wetting before dissolution, and the development of facets having an average (110}-natureduring dissolution. The results are in conflict with current theoretical models, and a qualitative account of the mechanism of the establishment of a ‘ stable ’ solution double layer is given.

Quantum chemical modelling case studies relevant to metal oxide dissolution and catalysis

“…This dependence is not observed during the initial kinetic regime of MgO dissolution [11,12]. Protonation of oxide sites (eqs.…”

“…A dramatic restructuring of the smooth, nearly perfect (100) MgO smoke faces occurs before any significant dissolution (i.e. < 1 monolayer) can be measured [11,12]. It seems clear that surface diffusion of cationic and anionic species across the surface must be a rapid process in comparison to dissolution during the initial stage of interaction with water or acid.…”

“…In each case, it was assumed that the density of reactive sites per unit area is constant and independent of the extent of dissolution or of pH. * This assumption is certainly not true of the initial stages of dissolution of MgO where the reactivity is govemed by the rapidly changing structure of the oxide surface [11,12,14]. A dramatic restructuring of the smooth, nearly perfect (100) MgO smoke faces occurs before any significant dissolution (i.e.…”

Dissolution kinetics of periclase in dilute hydrochloric acid

“…The overall rate can be controlled by diffusion of liquid reactants/products or by the surface reaction, dependent on the reaction conditions (Vermilyea, 1969;Segall et al, 1978Segall et al, , 1988Jones et al, 1981;Fruhwirth et al, 1985;Blesa et al, 1994;Wogelius et al, 1995;Jordan et al, 1999). However, there are situations referred to by some authors when the use of different theoretical approaches resulted in determining various potential ratecontrolling steps for identical reaction conditions, with no clear basis for discrimination between these alternatives (Vermilyea, 1966;Diggle, 1973;Gorichev and Kyprianov, 1984).…”

“…FeO, Fe 3 O 4 , CuO, CoO, Cr 2 O 3 or NiO) in which the electronic properties of the solid in particular, charge-carrier concentration and mobilities, and the extent of charge depletion layers at the surfaces, may be of prime importance in the dissolution rate, in the case of a predominantly ionic oxide such as MgO, the charge characteristics are not rate-determining; so the influence on the dissolution kinetics of atomic surface detail, overall surface morphology and bonding of both surface atoms and adsorbed species can then be studied (Segall et al, 1978;Jones et al, 1984). (b) MgO is one of the few oxides that allow investigation of dissolution rates and mechanisms at near-room temperature-this fact turned out to be crucial for studying the dissolution kinetics of MgO intensively using various modern (and complementary) experimental techniques, from continuous kinetic measurements using a pH-meter (Vermilyea, 1969;Segall et al, 1978Segall et al, , 1988Jones et al, 1981;Fruhwirth et al, 1985) to atomic force microscopy or elastic recoil detection analysis (Wogelius et al, 1995;Suárez and Compton, 1998;Jordan et al, 1999;Mejias et al, 1999;Simpson et al, 2003). (c) Periclase forms a rock salt structure and has a relatively simple electronic structure that can be modelled using ab initio calculations (Mejias et al, 1999;Al-Abadleh and Grassian, 2003;Simpson et al, 2003).…”