10 December 1999

Ž .Chemical Physics Letters 314 1999 558–563www.elsevier.nlrlocatercplett

The kinetics and mechanism of MgO dissolution

Jose Antonio Mejias 1, Andrew J. Berry 2, Keith Refson, Donald G. Fraser )

Department of Earth Sciences, UniÕersity of Oxford, Oxford OX1 3PR, UK

Received 12 May 1999; in final form 22 July 1999

Abstract

Reactions and atomic rearrangements at fluid–crystal interfaces play an important role in catalysis and in controlling thekinetics and mechanisms of dissolution. We have studied the attachment and reactions of water molecules at the MgO–waterinterface by combining measurements of 1H and 2D surface penetration and etch pit morphology with ab initio calculations.

Ž .These studies show that the most common MgO cleavage surface, 001 , is thermodynamically unstable when hydrated.Proton rearrangement on such surfaces precedes proton–cation exchange and provides a general mechanism for thedetachment of ions during dissolution. The kinetics of dissolution are strongly influenced by the concentration of surfacedefects and a simple model based on the ab initio results predicts a dissolution rate of 10y10 mol cmy2 sy1 for a typicalsurface defect concentration of 0.1. q 1999 Elsevier Science B.V. All rights reserved.

Dissolution at solid–liquid interfaces is a univer-sally important phenomenon and is controlled bothby the thermodynamics and the kinetics of the pro-cesses involved. In the case of silicates and oxides,re-arrangement of water molecules attached at thesurface is a crucial rate-determining step which pre-cedes hydrolysis and proton–cation exchange. Initialattempts to quantify dissolution and crystal growthwere based on chemical equilibrium and reversibilityw x1,2 . However, more recently it has become clearthat there is significant structural and kinetic control

w xof both dissolution and growth 3 . In the case of

) Corresponding author. Fax: q44-1865-272072; e-mail:thermo@ermine.ox.ac.uk

1 Present address: Facultad de Ciencias Experimentales, Uni-versidad Pablo de Olavide, E-41013 Sevilla, Spain.

2 Present address: Research School of Earth Sciences, Aus-tralian National University, Canberra, ACT, 0200, Australia.

mineral–water interfaces, a detailed mechanistic ex-planation of the hydration and transformation ofminerals in an aqueous environment is particularlychallenging because of the complexity of the pro-cesses involved, which include selective leaching,hydration, and the formation of other secondary

w xphases 3,4 . In the present Letter we focus on MgOwhich has been widely used as a model system thatcombines relative ease of computation withwidespread applicability to both natural and indus-trial systems.

MgO has the rock-salt structure and cleaves read-� 4ily along 001 . In near-neutral to acidic solutions,

the dissolution of powdered MgO is associated withŽ .the formation of a hydrated intermediate, Mg OH ,2

w xbrucite. This is apparent from X-ray diffraction 5and our transmission electron microscopy studies ofreacted powders. There is evidence both from natural

w x w xsamples 6 and calculations 7 that a topotacticŽ .relationship exists between the 111 planes of MgO

0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0009-2614 99 00909-4

( )J.A. Mejias et al.rChemical Physics Letters 314 1999 558–563 559

Ž .and the 0001 planes of the hydroxide. The fullw xhydration of MgO to brucite is exothermic 8 by

y37 kJ moly1 and the formation of a hydratedbrucite-like surface layer is thermodynamicallyfavourable. However, although there is recent theo-retical evidence for stable dissociative water adsorp-

Ž . w xtion on the normal 001 cleavage surface 9 ,chemisorption is favoured on surfaces comprisingatoms of lower coordination, with the hydroxylatedŽ . w x111 surface being of lowest energy 7,10 . Here we

Ž .show that despite the stable energetics of 001chemisorption, water does not cause penetrative hy-

Ž .dration of the MgO 001 surface. Instead the onlyprocess observed is the development of pits derivedfrom dissolution at dislocations. Ab initio calcula-tions for the MgO–water interface provide an expla-nation both for the dissolution mechanism and thekinetic barrier for the formation of brucite.

Single-crystal cleavage plates of MgO werechemo-mechanically polished in an aqueous slurry of0.125 mm silica particles at pHs10.2 and reacted

Žwith either continuously pumped distilled water pH.f5.7 , or in batch mode with HNO at a pH3

w xcontrolled by a Mettler DL-77 auto-titrator 11 .Ž .Elastic recoil detection analysis ERDA used a 7.5

MeV 16O4q beam with the sample at 158 and withthe detector at 408 relative to the incident beam. Ionprobe data were collected with a Cameca IMS-4F

Ž .secondary ion mass spectrometer SIMS . Topo-graphic images were recorded by a Park ScientificInstruments SFM-BD2 atomic force microscopeŽ .AFM using a silicon nitride cantilever with anapical tip angle of 708 and a constant force of 10 nN.

Ž .A polished 001 single-crystal surface of MgOwas reacted at pH 4 for 14 h and analysed by ERDA.

Ž .No ERDA peak corresponding to Mg OH was2

observed for the reacted MgO sample and the onlypeak recorded is attributable to adsorbed surfacewater. A similar surface H O peak was observed for2

MgO samples reacted at pH 2 for 2 h, pH 5.7 for 72h, a variety of olivine crystals and hydrogen im-planted silicon. This peak was incorrectly attributed

w xin a previous study to a brucite-like layer 11 be-cause of problems with intensity calibration and

w xenergy resolution 12 .SIMS was used to analyse samples reacted at pH

2 for 2 and 6 h, pH 5.7 for 72 h, a blank handled inair, and a blank heated at 6008C and handled under

dry nitrogen. In addition, a sample was reacted inD O at pH 2. A large surface water contribution2

˚obscures results for the first 300 A. However, belowthis depth no significant differences in any of thehydrogen profiles could be determined and there isno evidence for a hydrated layer. A mass scan of anatural brucite standard indicated that 1H and 2 Dcould be easily resolved. Point mode analysis of theŽ .001 surface reacted at pH 2 for 2 h in D O2

indicated only trace amounts of 2 D. The 2 D concen-tration was below the detection limit in depth profilemode. Although some evidence of atmospheric DrHexchange in hydrated surface layers of open-struc-tured aluminosilicates has been reported on the basisof ERDA measurements, this is unlikely for atopotactic intergrowth involving reconstruction. In-deed stable isotope studies of HrD ratios in mineralswould be impossible if structurally bound hydrogenwere easily exchanged with atmospheric water. Theordered surface structure postulated here contrastswith leached silica-gel-like layers containing voidsand molecular water. The presence of a deuteratedlayer is easily distinguishable from adsorbed water,since any 2 D signal must be derived from the reactedsample. A recent synchrotron X-ray photoemission

˚study with a tens of A resolution also failed to findevidence of a brucite-like layer for samples reacted

w xin water for 10 min 13 .Ž .Scanning electron microscopy SEM and AFM

images reveal dissolution in the form of etch pitsŽ .Fig. 1 . The pits are approximately square-based

² :inverse pyramids with sides parallel to 100 , and² : ² :are distributed in rows parallel to 110 or 100 .

² :For the 100 aligned pits, the apex of the pyramidis shifted from the geometrical centre. The resulting

w x w xasymmetry is present in both the 100 and 010� 4directions, with the pit faces approximating to 01l

Ž .1F lF4 . The asymmetry is consistent with disso-² :lution along a 110 dislocation emerging at 458 to

Ž . w x ² :the 001 surface 14 . These 100 pits have beenw xreported previously 15 . However, this is the first

time the pit angles have been determined. At lowerpH, there are changes in the pit morphology and

w x ² :orientation 16 . At pHf2 110 inverted pyramidalŽ .pits are observed, while ridges on 110 and triangu-

Ž .lar pits on 111 were determined by AFM to beŽ . w xformed from 100 faces 17 . A soft surface layer

w xhas been detected 18 by surface mode AFM for

( )J.A. Mejias et al.rChemical Physics Letters 314 1999 558–563560

� 4Fig. 1. AFM image of a MgO 001 surface after reaction with H O under steady-state conditions. The largest etch pit is 1.6 mm = 1.1 mm2

= 0.25 mm.

pHs3 but was not observed in this study. Protru-w xsions on surfaces reacted with moist air or water 19

were only observed to form in the absence of a bulkfluid phase. Such precipitates began to form on etchpit surfaces with prolonged atmospheric exposure.

In order to understand the presence of orientedpits and the absence of deep hydration we havestudied the hydration energetics of several represen-tative interface planes using density functional the-

w xory and the plane-wave pseudopotential method 20within the generalized gradient approximation for the

w xexchange-correlation potential 21 . This methodyields bond lengths accurate to 2% and energieswithin 10 kJ moly1 in the MgO–water system. Wehave included the effect of water physisorption onto

Ž . Ž .terraces on 001 and 013 , as well as waterchemisorption at edge and corner sites. The calcu-lated reconstructionq chemisorption energies arelisted in Table 1. We find that reconstruction of the

Ž .hydrated 001 surface with its ordered water mono-

w x Ž .layer 9 into a hydrated 111 plane is energeticallyŽ y1 .favoured D Esy20 kJ mol in agreement with

w xearlier calculations 7 . This stable hydrated plane isnot the dominant site of dissolution as is shown byits absence in our AFM measurements. In contrast,

Ž .reconstruction energies to form hydrated 01l sur-faces are slightly positive, but not distinguishablefrom zero within the approximations of the calcula-tions. These hydrated surfaces are of intermediate

Table 1Ž .Energies of the reaction of MgO 001 with an ordered monolayer

of adsorbed water to form reconstructed hydrated surfaces

Surface D Ey1Ž .kJ mol

Ž .001 q130Ž .110 q5Ž .310 0 to q12Ž .111 y20

( )J.A. Mejias et al.rChemical Physics Letters 314 1999 558–563 561

stability and it is these which undergo ready dissolu-Ž .tion leading to the 01l pit faces shown in Fig. 1.

Ž .The presence of unreacted 001 planes, the ab-Ž .sence of 111 planes, and the lack of penetrative

hydration are evidence of an interface far from equi-librium and whose surface features are kineticallycontrolled. The kinetic factors depend on the mecha-nism of ion detachment from the topmost layer,providing low-energy pathways for surface recon-struction or dissolution. We have investigated thismechanism by ab initio simulations of the reaction ofwater molecules with valleys and edges at steps on

Ž . Ž .the 001 surface Fig. 2 . The dissociation of a H O2Ž y1 .molecule in the valley is exothermic y83 kJ mol .

The OHy occupies a site bridging two valley Mg2q

ions and the Hq attaches to a valley oxygen. Asecond water molecule will dissociate at the stepedge but there are two local minima depending onwhere the resulting Hq attaches to the surface. The

Ž y1 .lower energy y145 kJ mol is obtained if bothy q Ž .the OH and the H attach to ions at the edge E

Ž Ž . . qwe call this the E, E structure . If the H instead

Ž . Ž Ž .attaches to an oxygen in the plane P the E, P. y1structure , the energy change is q42 kJ mol . In

the latter case, the edge Mg2q begins to detach from˚the step edge and moves outwards by )1 A. This

Ž . 2qE, P structure is unrealistic since the Mg is onlycoordinated to two oxygens instead of the usual 5 or6. When two further H O molecules are added the2

Mg2q becomes fivefold coordinated and this config-uration is only 26 kJ moly1 higher in energy than

Ž .the E, E state, where the additional waters merelyhydrogen-bond to the OHy groups.

Ž . Ž .The evolution of these E, E and E, P states wasexplored using ab initio molecular dynamics simula-tions. A timestep of 0.5 fs was used, the simulatedtime was 1 ps, and the temperature was 300 K. In the

Ž .simulation started from the E, E geometry, the edge2q ˚Mg does not move more than 0.2 A from its

Ž .initial site. In contrast, in the E, P simulation this˚movement increases to )1 A, indicating a looserŽ .bonding to the surface. The E, P configuration,

with its semi-detached Mg2q is a clear candidate fora dissolution intermediate state. The hydrated Mg

Ž .Fig. 2. Chemisorption of water on a MgO 001 stepped surface and the formation of a dissolution intermediate.

( )J.A. Mejias et al.rChemical Physics Letters 314 1999 558–563562

complex after 1 ps is coordinated to two OHy

groups, two H O molecules and one surface oxygen,2

giving a total coordination number of 5.The reconstruction of the cubic MgO surface into

Ž .trigonal Mg OH requires substantial concerted2

mass transport by means of mobile intermediates.w xFor a detachment limited dissolution rate 22–24 ,

Ž . Ž .E, E and E, P are in equilibrium, and the probabil-Ž .ity of the E, P intermediate is given by exp

Ž . y5yD Erk T f10 at ambient temperature. In theB

presence of aqueous solution, the low concentrationof this complex is an impediment for aggregation

Ž . 2qinto a Mg OH layer. Since the Mg is already2˚)2 A from its original edge site it would only

require the replacement of the surface OH coordina-tion by water molecules to completely detach thehydration complex into solution. Screw dislocationsprovide a constant source of steps from which dis-

Ž . Ž .solving hydrated 01l faces develop. Such 01lŽ .surfaces with l)1 may be viewed as stepped 001

surfaces. Detachment at edges causes unidirectionalŽ .migration of the steps in the 001 plane. This self-

Ž .generating process creates a new 01l surface, iden-tical to the original, but which has retreated with

Ž .respect to the initial 01l plane. In contrast, dissolu-Ž .tion of the 011 surface is self-limited by the forma-

Ž .tion of a stable unfaceted hydroxylated 011 plane.Ž .This mechanism predicts the presence of only 01l

Ž .planes l)1 for a steady-state surface, in fullagreement with the experimental observations.

We conclude that the joint effect of chemisorptionand dissolution results in a steady-state surface whichdoes not evolve into deep hydroxylation providedthat the concentration of defects is low. If the de-tached complexes are unable to diffuse sufficientlyrapidly into a bulk liquid phase, surface reconstruc-tion or precipitation of a hydroxide phase may occur.This provides a ready explanation for precursor me-diated adsorption and the kinetic coupling betweenterrace and defect sites in desorption experimentsw x25 as well as the surface roughening of abraded

w xMgO smoke cubes exposed to water 26 .The proposed mechanism allows the dissolution

rate to be estimated. The fast rate of waterŽ .chemisorption implies that the E, E concentration

equals the surface defect density, u . For the calcu-Ž .lated E, P probability, the surface concentration of

detaching complexes, C, is u=6.25=109 cmy2 . If

detachment occurs at a similar rate to water ex-Ž .2q w xchange for Mg H O in aqueous solution 16 ,2 6

rf105 sy1, the steady-state dissolution rate is RfC=rfu=10y9 mol cmy2 sy1 f10y10 mol cmy2

sy1, for us0.1 as estimated from AFM images.This value is in excellent agreement with experimen-

w xtal data 11,27 .We have shown that the MgO–water interface is

largely controlled by kinetic factors. Chemical equi-librium favours the hydration and reconstruction ofŽ . Ž .001 into 111 planes, and the formation of a

Ž .Mg OH layer. Single-crystal surfaces exposed to2

aqueous environments for several days show no evi-dence of hydration. In contrast, highly defectivesurfaces and powders convert readily to brucite. Theformation of the detaching complexes arises from areconstruction to a metastable state involving protonrearrangement. This explains the kinetic barrier forreconstruction and provides a dissolution mecha-nism. This mechanism differs from that proposed forquartz dissolution for which chemisorption occurs

w xconcurrently with Si–O bond cleavage 28 .The importance of these results extends to other

mineral–water systems. Firstly, a quantitative de-scription of the energetics and structure of oxidesurfaces in aqueous environments is important for an

w xunderstanding of many catalytic systems 29 . Sec-ondly, the dissolution rates of orthosilicates scale ina fashion similar to rates of water exchange around

w xdissolved divalent cations 30,22–24 . This processresembles the replacement of the surface oxygen by

Ž .a water molecule to completely detach the E, Pcomplex into solution. Finally, the reconstruction ofpolymeric silicate anions during mineral leachingdoes not require previous dissolution of silicon as

w xsilicic acid 31 . Mobile intermediates, similar to ourŽ .E, P complex, will facilitate this reconstruction.

Acknowledgements

J.A.M. thanks the European Commission for aMarie Curie Fellowship. D.G.F. and K.R. acknowl-edge support from the NERC under grantsGR3r8310 and GR3r8970, respectively. We alsothank EPSRCrUKCP for T3D time, J.A. White forthe CETEP code, J.A. Craven for the SIMS data,G.W. Grime for assitance with the ERDA experi-

( )J.A. Mejias et al.rChemical Physics Letters 314 1999 558–563 563

ments, and R.A. Wogelius for helpful discussions.D.G.F. thanks the Division of Geological and Plane-tary Sciences and the Beckmann Institute at Caltechfor their support and hospitality and Oxford Univer-sity for generous sabbatical leave.

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