McGeouch, Carrie-Anne, Peruffo, Massimo, Edwards, Martin A., Bindley, Lucy A., Lazenby, Robert A., Mbogoro, Michael M., McKelvey, Kim M. (Kim Martin) and Unwin, Patrick R. (2012) Quantitative localized proton-promoted dissolution kinetics of calcite using scanning electrochemical microscopy (SECM). Journal of Physical Chemistry C, Volume 116 (Number 28). pp. 14892-14899. doi:10.1021/jp301382e

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Abstract

Scanning electrochemical microscopy (SECM) has been used to determine quantitatively the kinetics of proton-promoted dissolution of the calcite (101̅4) cleavage surface (from natural “Iceland Spar”) at the microscopic scale. By working under conditions where the probe size is much less than the characteristic dislocation spacing (as revealed from etching), it has been possible to measure kinetics mainly in regions of the surface which are free from dislocations, for the first time. To clearly reveal the locations of measurements, studies focused on cleaved “mirror” surfaces, where one of the two faces produced by cleavage was etched freely to reveal defects intersecting the surface, while the other (mirror) face was etched locally (and quantitatively) using SECM to generate high proton fluxes with a 25 μm diameter Pt disk ultramicroelectrode (UME) positioned at a defined (known) distance from a crystal surface. The etch pits formed at various etch times were measured using white light interferometry to ascertain pit dimensions. To determine quantitative dissolution kinetics, a moving boundary finite element model was formulated in which experimental time-dependent pit expansion data formed the input for simulations, from which solution and interfacial concentrations of key chemical species, and interfacial fluxes, could then be determined and visualized. This novel analysis allowed the rate constant for proton attack on calcite, and the order of the reaction with respect to the interfacial proton concentration, to be determined unambiguously. The process was found to be first order in terms of interfacial proton concentration with a rate constant k = 6.3 (± 1.3) × 10–4 m s–1. Significantly, this value is similar to previous macroscopic rate measurements of calcite dissolution which averaged over large areas and many dislocation sites, and where such sites provided a continuous source of steps for dissolution. Since the local measurements reported herein are mainly made in regions without dislocations, this study demonstrates that dislocations and steps that arise from such sites are not needed for fast proton-promoted calcite dissolution. Other sites, such as point defects, which are naturally abundant in calcite, are likely to be key reaction sites.

Item Type:	Journal Article
Subjects:	Q Science > QD Chemistry
Divisions:	Faculty of Science > Chemistry
Library of Congress Subject Headings (LCSH):	Electrochemistry, High resolution electron microscopy, Electron microscopy, Calcite , Chemical kinetics
Journal or Publication Title:	Journal of Physical Chemistry C
Publisher:	American Chemical Society
ISSN:	1932-7447
Official Date:	19 July 2012
Dates:	Date Event 19 July 2012 Published
Volume:	Volume 116
Number:	Number 28
Page Range:	pp. 14892-14899
DOI:	10.1021/jp301382e
Status:	Peer Reviewed
Publication Status:	Published
Access rights to Published version:	Restricted or Subscription Access
Funder:	European Research Council (ERC), Engineering and Physical Sciences Research Council (EPSRC), Unilever (Firm), Imperial College, London, Advantage West Midlands (AWM), European Regional Development Fund (ERDF), Science City Research Alliance
Grant number:	ERC-2009-AdG247143-QUANTIF (ERC), EP/H023909/1 (ICL) (U),

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