Reductive Dissolution Mechanisms at the Hematite-Electrolyte

Subscriber access provided by YORK UNIV

Article

Reductive Dissolution Mechanisms at the HematiteElectrolyte Interface Probed by In Situ X-ray Scattering Martin E. McBriarty, Joanne E. Stubbs, Peter J Eng, and Kevin M. Rosso J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07413 • Publication Date (Web): 18 Dec 2018 Downloaded from http://pubs.acs.org on December 18, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Reductive Dissolution Mechanisms at the HematiteElectrolyte Interface Probed by In Situ X-ray Scattering Martin E. McBriarty,1,† Joanne E. Stubbs,‡ Peter J. Eng,‡ Kevin M. Rosso*,†

†Physical

Sciences Division, Pacific Northwest National Laboratory, Richland,

Washington 99354, U.S.A.

‡Center

for Advanced Radiation Sources, University of Chicago, Argonne, Illinois

60439, U.S.A.

ABSTRACT

The electron-catalyzed dissolution and reprecipitation of iron (oxyhydr)oxides constitute critical steps in natural geochemical iron cycling. However, the atomic-scale mechanisms of reductive dissolution and oxidative precipitation of Fe2+ remain poorly understood because they are difficult to directly experimentally probe. Using in situ synchrotron X-ray 1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

scattering and a novel electrochemical cell, we interrogate the interfacial structure between the hematite (-Fe2O3) (1102) surface and acidic aqueous solution (5 mM Na2SO4, pH 4.0) under controlled electrochemical potential from open circuit to cathodic bias as the reductive dissolution potential is approached and then exceeded. The crystalline order of the surface improves under mild reducing conditions, and the surface Fe stoichiometry changes with cathodic bias. After significant reductive dissolution occurs and cathodic bias is removed, dissolved Fe is re-deposited, forming a disordered interface. Unlike at circumneutral pH, water layers at the hematite interface with acidic solution are poorly ordered, likely due to the adsorption of sulfate from the electrolyte. These results provide a novel atomic-scale view into the behavior of reducible transition metal oxide surfaces under fluctuating (electro)chemical conditions.

I.

INTRODUCTION

Iron (oxyhydr)oxides (FeOHs) are naturally abundant minerals which play critical roles in biogeochemical phenomena such as carbon cycling and the transport of toxic metals.1-2

2


Page 2 of 41

Page 3 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The important environmental properties of FeOHs, such as dissolution, growth, and crystal phase transformation, are closely linked to the Fe2+/Fe3+ redox couple. For example, aqueous Fe2+ catalyzes transformations of nano-FeOHs to more stable phases by donating an electron to less soluble Fe3+ in FeOHs, which initiates an auto-catalytic flux of iron atoms between solid and solution phases across the interface.3-5 However, such transformation pathways involve multiple charge-transfer steps that are not easily followed experimentally, and the end products are strongly influenced by local chemical conditions. Predicting FeOH mineral transformations in arbitrarily complex systems therefore requires a deeper understanding of the fundamental behavior of reactive FeOH interfaces under far-from-equilibrium conditions. The presence of an external electrochemical potential, either through chemical reductants in solution or an applied voltage, can induce reductive dissolution (RD) of FeOHs. Donated electrons localize on Fe3+ sites in FeOHs, reducing them to Fe2+.6-8 Fe2+ is unstable in the lattice of ferric minerals relative to the aqueous ion and therefore is released; such is the case of hematite RD according to Equation 1. RD kinetics are 3



controlled by the structure and chemistry of the mineral surface9 and the behavior of protons, ligands, and water near the surface.10-12 The RD rate is accelerated by low solution pH, cation-coordinating ligands, light, and the presence of additional electron donors such as generated aqueous Fe2+ which can be recycled as a donor.13-15 In all cases, for RD to proceed, Fe2+ ions must be solvated at the FeOH-water interface. However, determining the molecular-scale behavior of water at interfaces is difficult, especially at oxide surfaces which are topographically and chemically complex.16 To date, the dissolution of hematite has not been directly observed at the atomic scale.17 (1)

Fe2O3(s) + 6H+ + 2e-  2Fe2+(aq) + 3H2O

The structure of FeOH surfaces and their interfaces with water have been studied extensively over the past two decades, with much of this work focusing on the surfaces of hematite, the most stable iron oxide.18 The surface structures of dry hematite19-22 and the ordering of water at the surfaces of hematite12, 23-26 and other iron oxide minerals27-31 have been measured using surface- and interface-sensitive techniques, often paired with chemical simulation. In particular, X-ray scattering methods such as crystal truncation rod

4


Page 4 of 41

Page 5 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(CTR) analysis can be used to resolve atomically ordered structures at crystal-liquid interfaces.32 This method has been extensively applied to investigate the interfaces between hematite and water23-26 as well as the atomic-scale details of contaminant adsorption33-34 and the influence of Fe2+ by oxidative adsorption and/or electron-induced dissolution.35-37 We recently used CTR analysis to determine that cathodic bias applied to a hematite (1102) electrode in 5 mM Na2SO4 (pH 7.4) induces a change in the interfacial water ordering which appears to slow RD.12 However, these conditions represent only a narrow slice of the enormous parameter space of the geochemical environments in which hematite may reside. Low solution pH accelerates the rate of RD (Equation 1) and promotes the adsorption of sulfate,38-39 a naturally abundant anion. Thus, we investigate structural changes that accompany the application of cathodic bias to hematite in 5 mM Na2SO4 at pH 4.0. We observe the depletion of surface Fe upon application of cathodic bias, indicating RD of Fe into solution. At cathodic potentials near the onset of RD, the crystal surface is smoothed, an effect akin to electropolishing. However, when the potential is then reversed 5



to values more positive of RD, oxidative re-adsorption of Fe2+ precipitates Fe3+, yielding a rougher, more defect-rich surface. The atomic-scale information enabled by these in

situ observations, from initial depletion of surface and near-surface Fe to subsequent reprecipitation, is the first that is directly tied to the natural process of FeOH mineral redox cycling. II.

EXPERIMENTAL METHODS

All experiments were performed using a naturally conductive hematite crystal (Bahia, Brazil), cut into a 1 mm thick, 3 mm diameter cylinder. The crystal was polished such that the basal surface of the cylinder was parallel to the (1102) (r-cut) crystal plane within 0.2°, then underwent chemical-mechanical planarization using a 20 nm colloidal silica suspension (Buehler MasterMet 2, Lake Bluff, IL, USA). Trace metal and silica residue was cleaned from the crystal by soaking in NaOH (pH 10) followed by HNO3 (pH 2) solutions, followed by a final rinse of acetone and then methanol or isopropanol. This polishing and cleaning procedure yields atomically smooth terraces of several tens of nanometers in width (see Refs. 12, 26). 6


Page 6 of 41

Page 7 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Electrochemical biasing and measurements were performed using a CHI 660C potentiostat in a three-electrode configuration. The crystal was mounted as the working electrode in a purpose-built electrochemical mini-cell12 for CTR measurements, where the exposed surface area of the crystal was approximately 7.1 mm2. Electrical connection was made to the crystal using Ag paint. The cell contained a Pt counter electrode and an AgCl-coated Ag wire as the reference electrode. 0.005 M Na2SO4 electrolyte was prepared from anhydrous Na2SO4 (A.C.S. Grade, Fisher Scientific) and titrated to pH 4.0 with H2SO4. All solution preparation was performed in an anoxic glovebox (

Reductive Dissolution Mechanisms at the Hematite-Electrolyte

Recommend Documents