Surface area and the mechanism of hydroxylation of ionic oxide surfaces

Jones, Colin F.; Reeve, Robyn A.; Rigg, Rupert; Segall, R. L.; Smart, Roger St. C.; Turner, Peter S.

doi:10.1039/f19848002609

Cited by 65 publications

(24 citation statements)

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“…Previous ab initio calculations have shown that water molecules dissociate at surface defects, particularly steps and corners 12,14 , corroborating the observation that the reconstruction is more rapid if the surface is damaged 8 . The activation barrier for ion transport may be estimated given that the inverse reconstruction, the annealing of {100} facets on a (111) surface, is observed to occur at 1000K…”

Section: −supporting

confidence: 55%

“…They showed that dissociative chemisorption is energetically unfavourable and that physisorption of intact water molecules is preferred. This was confirmed by a CarParrinello ab initio molecular-dynamics study 12 instead at low co-ordinated sites such as steps, corners and other defects 8,13 and dissociation was demonstrated computationally at steps and corners 12,14 . However one might expect this to simply saturate the defect sites.…”

mentioning

confidence: 55%

“…1a) as postulated by Coluccia et al 7 . Some striking hydroxylation experiments were reported by Jones et al who studied surface roughening on (001) faces of nanocrystalline MgO in a transmission electron microscope 8 . The remarkable affinity of MgO for water is demonstrated by their in situ observation of hydration-induced surface roughening over 10 minutes under vacuum with P H 2 O < 10 −5 Pa.…”

mentioning

confidence: 99%

“…Dissociation at defects does not explain the monolayer coverage of the surface by OH groups 7 , the whole-surface roughening observed in the TEM 8 nor the progressive transformation of the entire (001) surface and incipient bulk hydroxylation observed by i.r. spectroscopy 8,9 and in our dissolution experiments.…”

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confidence: 99%

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Water chemisorption and reconstruction of the MgO surface

Refson¹,

Wogelius²,

Fraser³

et al. 1995

Phys. Rev. B

243

158

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The observed reactivity of MgO with water is in apparent conflict with theoretical calculations which show that molecular dissociation does not occur on a perfect (001) surface. We have performed ab-initio total energy calculations which show that a chemisorption reaction involving a reconstruction to form a (111) hydroxyl surface is strongly preferred with ∆E = −90.2 kJ mol −1 . We conclude that protonation stabilizes the otherwise unstable (111) Magnesium oxide has long provided a prototype for the study of surface structure and chemical reactions of oxides. Naturally occurring MgO, known by its mineral name of periclase, is not a common crustal mineral, but its simple structure makes it an excellent example for the investigation of mineral surface chemistry.Reactions at mineral surfaces are responsible for much of the chemical change which occurs in the Earth's crust. Weathering reactions control the erosion of rocks and the consequent evolution of surface topography thus providing an opposing mechanism to the more dramatic process of mountain building. Aqueous reactions in sedimentary basins are responsible for the diagenetic processes which transform unconsolidated sediments into rocks. In this work we study the nature of a simple mineral surface when exposed to an aqueous environment and the chemical interaction of water with that surface. This is both a prerequisite to studying the interaction with aqueous solutions and a tractable first step towards ligand-exchange reactions in more complex silicate minerals.We have performed experiments on single-crystals of MgO prepared with high-quality (001) faces which were reacted with acidic solutions. The experiments and results are reported in detail elsewhere 1 , the main feature being the development of an altered surface layer. Elastic Recoil Detection Analysis (ERDA)2 shows protonation to a depth of 900Å with a H/Mg ratio close to 2 giving a probable chemical composition of magnesium hydroxide. Indeed brucite (the mineralogical name for Mg(OH) 2 ) is the most common alteration product of periclase in the natural environment 4 and well-crystallized intergrowths of brucite on periclase have been reported 5 . The initial stage in the reaction is hydroxylation of the surface. MgO has the cubic rocksalt structure with (001) cleavage planes. This is the most stable surface and is the only one seen experimentally 6 . The simplest possibility for a hydroxylated surface is obtained by dissociating a water molecule and placing the OH group above each magnesium ion and the H above each oxygen of the (001) surface (see Fig. 1a) as postulated by Coluccia et al.7 . Some striking hydroxylation experiments were reported by Jones et al. who studied surface roughening on (001) faces of nanocrystalline MgO in a transmission electron microscope 8 . The remarkable affinity of MgO for water is demonstrated by their in situ observation of hydration-induced surface roughening over 10 minutes under vacuum with P H 2 O < 10 −5 Pa. The presence of surface hydroxyl groups on MgO powde...

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Section: −supporting

confidence: 55%

mentioning

confidence: 55%

mentioning

confidence: 99%

mentioning

confidence: 99%

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Water chemisorption and reconstruction of the MgO surface

Refson¹,

Wogelius²,

Fraser³

et al. 1995

Phys. Rev. B

243

158

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“…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.…”

Section: O>(s) + H + (Aq) -+ Oh(s)mentioning

confidence: 99%

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

Gerson

Jones

Simpson

et al. 2001

Ionics

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Abstract. Over the past 20 years quantum-chemical methods have been developed sufficiently so that they can now be applied to the elucidation of the complex mechanistic processes that occur during metal oxide dissolution and catalysis reactions. Many of the reactions occurring during these processes are not directly accessible to experimental techniques and therefore quantum-chemical modelling can be applied to probe the individual reaction steps involved in the overall mechanism.Quantum chemistry provides the means of calculating the electronic properties of solids (e.g. band structures) structural properties of solids and surfaces (for instance surface relaxation and rumpling) heats of formation and reaction, activation energies, spectroscopic excitation energies and vibrational frequencies. Three case studies are described, which have been chosen to cover a range of quantum chemical applications and methodologies. These case studies are a) the dissolution mechanism of MgO, b) the parameterisation of titanium dioxide for the determination of electronic properties and c) the mechanism and energetics of adsorption of Pd onto rutile. These case studies utilise Hartree-Fock semiempirical and ab initio quantum-chemical methods as well as density functional methodologies. A range of model types are used, namely cluster models embedded in pseudo-atoms, 3-dimensional periodic models and 2-dimensional periodic surface models.

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