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Channel-Flow Cell for X-ray Absorption Spectroelectrochemistry R. J. K. Wiltshire,*,† O. Smila-Castro,† N. G. Connelly,‡ S. M. Matthews,§ A. C. Fisher,§ and T. Rayment*,† School of Chemistry, UniVersity of Birmingham, Edgbaston, Birmingham B15 2TT, U.K., School of Chemistry, UniVersity of Bristol, Bristol BS8 1TS, U.K., and Department of Chemical Engineering, UniVersity of Cambridge, Pembroke Street, Cambridge CB2 3RA, U.K. ReceiVed: July 30, 2008; ReVised Manuscript ReceiVed: NoVember 04, 2008
X-ray absorption spectroscopy has the potential to provide valuable information not obtainable by other spectroscopic techniques on the structure of intermediates in electron-transfer reactions. Microfocused X-rays have been identified as powerful tools for probing the solution in the vicinity of the working electrode, and the design of an electrochemical flow cell suitable for such measurements is described. The flowing solution, found to be essential to remove any products of beam damage, is characterized by mapping X-ray absorption changes as a function of applied potential and position within the channel. [{Fe(η5-C5H5)(CO)(µ-SPh)}2], used as a model system, undergoes two reversible, one-electron oxidations giving rise to a shortening of the Fe-Fe distance and significant changes in X-ray absorption, making it an ideal compound for trial studies. Two-dimensional numerical modeling is used to rationalize the observed absorption changes and hence solution composition. Excellent agreement between calculated and observed changes in absorbance is found in the vicinity of the electrode although there are substantial differences where forced convection dominates mass transport. These experiments represent the first steps in demonstrating that the combination of microfocused X-rays with electrochemical flow cells has the potential to be a powerful technique for opening up new possibilities for studies of intermediates in electron-transfer reactions. 1. Introduction In many synthetic and biological processes, concerted multielectron transfer is accompanied by a structural or compositional change (proton transfer, ligand binding, ion-pair formation, metal-metal bond cleavage). Electron-transfer (e.t.) reactions are frequently studied by electrochemical techniques which give insight into kinetics, reaction mechanisms, and molecular energy levels, but no structural information. This has limited its application to understanding and exploiting complex electron-transfer reactions. Where transition metal complexes are involved, the starting point for understanding the e.t. mechanism is knowledge of the number and types of atoms coordinated to the metal. While the structures of the starting material and the final products may be determined by conventional methods such as XRD or NMR, the structures of intermediates that are so readily detected by electroanalysis are frequently unknown. There has been sustained interest in the development of spectroelectrochemical methods such as UV/vis, infrared, and Raman spectroscopies, along with ESR and nonlinear optical methods, for the study of both homogeneous and heterogeneous processes.1,2 These spectroscopic methods provide valuable information about symmetry, and electronic structure, but usually fail to satisfy the requirements noted above, namely, knowledge of the number and types of atoms coordinated to the metal along with their bond lengths and bond angles. XRD is the preeminent technique for structure determination in crystalline environments, but XRD is difficult to apply to * Corresponding authors. E-mail:
[email protected]. † University of Birmingham. ‡ University of Bristol. § University of Cambridge.
[email protected] and
complex solutions and X-ray absorption spectroscopy (XAS) is much better suited for this task. XAS is a local structure probe, does not require crystalline order, and is element-specific. XAS can be used to determine both bond lengths and bond angles, the identity of neighboring atoms, and the oxidation state at the redox center.3,4 These properties coupled with the penetration depth of X-rays make it an attractive tool for in situ studies of electrode materials.5-9 In the case of electroanalysis, it is the processes taking place in solution, in the vicinity of the electrode surface, which are of interest. However, a number of technical difficulties make the application of XAS to electroanalysis nontrivial. The first of these is the concentration of the electrogenerated species, which is often dependent on their lifetimes. For longlived intermediates, bulk electrolysis can be effectively used10-12 and the concentration of the intermediate will build up in the vicinity of the working electrode to give a high and readily detectable concentration. However, short-lived intermediates will be present in very low concentrations and are likely to go undetected under these conditions.1 It is clear that the concentration of short-lived intermediates will be at a maximum next to the electrode surface and decrease as a function of distance from the electrode, the rate of which is determined by its diffusion coefficient. The development of microfocus beam lines at synchrotron facilities provides a source of high-intensity focused X-rays with spot sizes down to 5 µm in the case of thirdgeneration sources. By use of beam sizes on this scale, the region of maximum concentration next to the electrode can be probed before the intermediates have a chance either to react further or to diffuse into the bulk of the solution. A second difficulty in the study of organometallic compounds is beam damage. A molecule’s susceptibility to beam damage varies enormously and in unpredictable ways. Effects of
10.1021/jp806766q CCC: $40.75 2009 American Chemical Society Published on Web 12/11/2008
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Figure 1. Schematic of [{Fe(η5-C5H5)(CO)(µ-SPh)}2].
radiation damage can give rise to subtle changes in spectra or, in more extreme cases, destruction of the sample. Two differing strategies have been identified to overcome the problem of beam damage. The first, adopted by Best et al., is to freeze the electrolyte, trap unstable product species, and collect data at low temperatures to reduce beam damage.13-17 This is successful for moderately stable products but faces limitations for reactive species as it is dependent on the rate at which these species generated by electrolysis can be removed and quenched. The second alternative strategy is to use a channel-flow electrochemical cell. This has the advantage that damaged material flows immediately to waste. When designing cells for in situ XAS studies, the identity of the absorbing atom is an important consideration because it determines the X-ray energy and, therefore, has a profound effect upon path length. [{Fe(η5-C5H5)(CO)(µ-SPh)}2], [Fe2S2], was used as a model system (Figure 1). The interest in dimeric iron complexes stems from their importance in the electron-transfer processes of biological systems. Studies of the structural changes involved may provide models for understanding the behavior of Fe2S2 proteins and thence that of the more complex Fe4S4 ferredoxins and related FeMo proteins, where the average Fe-Fe distance may be diagnostic of the core oxidation level.18 The neutral species, [Fe2S2], undergoes two reversible, oneelectron transfers to give the cation, [Fe2S2]+, and subsequently the dication, [Fe2S2]2+.19 Both the neutral and cation species can be isolated, and literature values are available for the corresponding crystal structures. The most significant difference between the geometries is a shortening of the Fe-Fe distance from a nonbonding value of 3.39 Å20 for the neutral species to 2.925 Å for the cation species (described as a one-electron metal-metal bond).18 The shortening of the Fe-Fe distance is accompanied by a distortion in the bridging Fe-S-Fe angle. The combination of reversible electrochemistry and significant structural changes makes [Fe2S2] an ideal compound for trial studies. In this paper we report the fabrication and characterization of an electrochemical flow cell suitable for XAS. Electrochemical cell characterization was carried out using steady-state limiting current measurements. The composition of the solution within the channel was probed by mapping X-ray absorption changes with a microfocused beam while applying a series of potential steps. Two-dimensional numerical modeling was used to rationalize the observed absorption changes. 2. Experimental 2.1. Considerations for Cell Design. Spectra resulting from XAS measurements are a sum of the spectra from every Fecontaining species in the solution. Therefore, the X-ray beam would optimally pass through the solution in a position where maximum electrochemical conversion is taking place. To achieve this, the experimental arrangement shown in Figure 2a was used where the X-rays are positioned at right angles to the
Figure 2. (a) Schematic showing cell geometry. (b) Perspective drawing showing assembly of electrochemical cell.
flow and parallel to the channel surface (path A). In previous studies where optical spectroscopy has been applied to flow cells, the light path is positioned at right angles to the channel surface (path B, Figure 2a). For beams with large spot sizes, this is often the only option, due to the channel height, and for IR, UV/vis, and Raman, sufficient information can be gained in this way. However, for highly focused light sources, optical measurements could also be conducted along path A with benefits likely to be found as a result of maximizing the electrochemical conversion of material within the beam. One further difference is that the working electrode is positioned across the full width of the channel (Figure 2a). Again this helps maximize electrochemically generated species in the path of the beam. Edge effects are introduced by this geometry and will be discussed in detail in relation to cell characterization. The width of the channel is of critical importance to the cell design, and its value is ultimately dependent on the species of interest as this determines the energy and hence path length of the X-rays. The solvent is obviously present in the largest concentration and at the Fe K edge has an overriding influence on the optimal value of the path length. Based on absorption calculations at the Fe K edge, with acetonitrile as the solvent, a path length of 3 mm was found to provide a compromise between ease of construction and transmitted X-rays (∼20%). For comparison, if water was to be used as the solvent, a path length of 1 mm would be required to achieve a similar transmission. 2.2. Cell Construction. A schematic of the channel flow cell is shown in Figure 2b. The channel is defined by two sections fabricated using a machinable glass ceramic (Macor). A glassy carbon electrode was embedded with heat-setting epoxy (Bondmaster, ESP109) into a 250 µm slot. A 0.5 mm diameter hole was drilled partway through the lower section to allow an electrical contact to be made between the glassy carbon and a platinum wire. The surfaces of both Macor sections were precision-polished to give a well-defined channel with the following dimensions: length 9 mm, width 3 mm, and height controllable within the range 200-400 µm.
310 J. Phys. Chem. C, Vol. 113, No. 1, 2009 The Macor assembly was mounted into the main body of the cell using small PTFE springs to separate the two sections. Two 100 µm polypropylene windows (Goodfellow Metals) were positioned on either face defining the sides of the channel. These windows were held in place by two PTFE end plates and made leaktight using a series of Kalrez O-ring seals (James Walker) mounted in recessed grooves machined into the end plates (Figure 2b). The counter electrode was a platinum wire positioned downstream of the working electrode. A commercially available Ag/AgCl electrode (Cypress) placed upstream of the working electrode was used as a reference electrode. 2.3. Syntheses.Thecomplexes[{Fe(η5-C5H5)(CO)(µ-SR)}2]21,22 and [{Fe(η5-C5H5)(CO)(µ-SR)}2][PF6]23 were prepared by modifications of the published methods and confirmed to be pure by elemental analysis (C and H), IR, and electrochemical analysis. All reactions were carried out under a nitrogen atmosphere using dried and deoxygenated solvents.18 2.3.1. [{Fe(η5-C5H5)(CO)(µ-SPh)}2]. A mixture of [{Fe(η5C5H5)(CO)2}2] (2.68 g, 7.56 mmol) and diphenyldisulfide (4.99 g, 22.9 mmol) in n-heptane (60 cm3) was heated under reflux for 3.5 h. The red-brown solution was then evaporated to dryness in Vacuo to give a brown-black solid which was dissolved in a minimum volume of CH2Cl2 and then added to an alumina/n-hexane chromatography column. Elution with n-hexane removed excess diphenyldisulfide, and elution with CH2Cl2/n-hexane (2:3) gave a brown solution which was concentrated in Vacuo to give a brown solid that was removed by filtration. The solid was then dissolved in CH2Cl2, filtered, and treated with n-hexane. Partial removal of the solvent in Vacuo gave the brown-black crystalline product, yield 2.45 g (63%). 2.3.2. [{Fe(η5-C5H5)(CO)(µ-SPh)}2][PF6]. To a brown solution of [{Fe(η5-C5H5)(CO)(µ-SPh)}2] (529 mg, 1.49 mmol) in CH2Cl2 (50 cm3) was added [Fe(η5-C5H5)2][PF6] (492 mg, 1.49 mmol). After 20 min the dark green suspension was reduced in volume to ca. 20 cm3, and the precipitate was removed by filtration and washed with n-hexane. The precipitate was then dissolved in acetone, filtered, treated with n-hexane, reduced in volume in Vacuo, and cooled to -10 °C to give a dark green solid, yield 336 mg (34%). 2.4. Electrochemistry. All electrochemical measurements were carried out using an Autolab PGSTAT 10 potentiostat. For initial cyclic voltammetry studies of [Fe2S2], a standard lowvolume three-electrode cell (Cypress) was used. The working electrode was a glassy carbon disk (3.0 mm diameter), the counter electrode was a platinum wire, and the reference was a Ag/AgCl electrode (Cypress). Solutions were 1 mM [Fe2S2] and 0.1 M [NEt4][BF4] (Aldrich, 99.9%) in anhydrous acetonitrile (Aldrich, 99.8%). Prior to assembly of the channel-flow cell, the working electrode was polished with alumina paste of decreasing particle size (1, 0.3, and 0.05 µm). After rinsing and sonication in distilled water, the working electrode was dried under vacuum. Once assembled, the electrochemical cell was filled with argonpurged anhydrous acetonitrile via a gastight delivery system. Once the cell was aligned on the beam line, an argon-purged solution of 20 mM [Fe2S2], 0.2 M [NEt4][BF4] in acetonitrile was injected into the cell using a 5 mL syringe. A well-defined smooth flow, typically between 100 and 200 µL h-1, was maintained using a microstepping Hamilton syringe pump (PHD 2000). The flow rate was chosen to maximize the changes in concentration during potential pulse experiments.
Wiltshire et al. TABLE 1: Time Sequence of Potential Step Experiments t1 t2 t3 t4
Fe(I) - e- f Fe(II) Fe(II) + e- f Fe(I) Fe(I) + e- f Fe(0) Fe(0) - e- f Fe(I)
The choice of the concentration of [Fe2S2] was a compromise between maximizing the absorption changes and avoiding solubility issues along with increased risk of irreversibility and competing side reactions in the electrochemistry. With a concentration of 20 mM, no complications in terms of the electrochemistry were observed. The long-term stability of [Fe2S2] was an important consideration given data collection times of up to 10 h. Initial tests with [{Fe(η5-C5H5)(CO)(µSPh)}2] showed signs of decay over relatively short time periods apparent from reduced currents with applied potential. This was attributed to deposition of decay products passivating the working electrode. Subsequent tests carried out with the salt, [{Fe(η5-C5H5)(CO)(µ-SPh)}2][PF6], demonstrated improved stability over the required time period, and as a result, the cation was used for all absorption measurements. For a single-electron transfer using the neutral species as the starting material, the current would typically decay by 20% over a period of 4 h. In the case of the cation species, the rate of decay would be halved. A further reason for selecting the cation as the starting material is that complications due to comproportionation reactions are avoided. This would not be the case if the dication was generated at the electrode with the neutral complex used as the starting complex. 2.5. Microfocus X-rays. Station 9.2 at the SRS is a minifocus facility for XAS and XRF mapping. The station is on the Wiggler I beam line and consists of a water-cooled, harmonicrejecting double-crystal Si(111) monochromator with an energy range of 5.5 to 18 keV. The horizontal and vertical focusing is carried out by two Kirkpatrick-Baez (KB) mirrors consisting of an ultralower expansion (ULE) fused silica substrate coated with 15 nm Rh on 20 nm Pt. The KB mirrors directly image the source, with no secondary focusing and subsequent aperturing, to give a spot size of 50 µm.24 2.6. Data Collection and Analysis. The aim of this work is to provide data of sufficient quality for structural and electrochemical analysis. Before this can be reached, the channel flow must be well-characterized. In particular, based on the averaging nature of XAS, concentration profiles of the electrogenerated species are required. The available flux on station 9.2 of approximately 109 photons/s is relatively low in comparison to third-generation sources. As a result, for transmission experiments, data suitable for structural analysis would be difficult to obtain. However, for mapping experiments carried out over long time scales, the low flux proves advantageous as the risk of beam damage is significantly reduced. Mapping was carried out under transient conditions by applying a potential step sequence to generate all three oxidation levels of [Fe2S2]. The potential-dependent absorption was measured at a fixed energy corresponding to the top of the white line region (7.132 keV) as this gave the greatest change in absorption between the three species. This was repeated at 30 points within the channel along a grid consisting of 3 rows of 10 points with an X increment (parallel to the electrode surface) of 0.141 mm and Y increment (normal to the electrode surface) of 0.02 mm. As the observed changes in absorption with oxidation state were small, the grid was repeated 10 times and data were averaged to improve statistics.
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Figure 3. Schematic of the channel electrode. Figure 4. Finite difference grid.
3. Theory The numerical simulations used to quantify the absorption changes observed experimentally were performed using a previously reported time-dependent backward implicit finite difference technique.25 The concentration distributions of the three iron species present developing from the sequence of potential steps shown in Table 1 were calculated. For all three species the concentration distribution was determined by solving the relevant convective-diffusion equation:
∂[Fe(i)] ∂2[Fe(i)] ∂[Fe(i)] - Vx ) Di 2 ∂t ∂x ∂y
(1)
where [Fe(i)] is the concentration of species Fe(I), Fe(II), or Fe(0), Di is the diffusion coefficient of species Fe(i), x and y are the Cartesian coordinates defined in Figure 3, and Vx is the solution velocity in the x direction. In channel electrodes of high aspect ratio (width:height) and with a suitable lead in length, Vx is assumed to be parabolic and can therefore be calculated using
( ( yh ) )
Vx ) V0 1 -
2
(2)
where h is the half-height of the channel, y′ ) h - y, and V0 is the solution velocity at the center of the channel. Equation 1 was solved by the application of a twodimensional finite difference grid (shown in Figure 4) with the boundary conditions shown in Table 2, where CFe(I), CFe(II), and CFe(0) represent the normalized concentrations of species Fe(I), Fe(II), and Fe(0), respectively,25 and x0 and xf represent the start and end of the electrode as shown in Figure 4. For the purpose of the simulations, it was assumed that sufficient background electrolyte was present for migration effects to be ignored and that the iron species were all stable over the experimental time scales. Experimental absorbance measurements were obtained for 50 µm spots at a number of positions throughout the channel. In order to compare the numerical and experimental results, the simulated concentration profiles for each species were averaged
over a 50 µm diameter circle at positions corresponding to those used experimentally. The area of the circle was determined using the same algorithm previously reported for the simulation of a microdisc electrode.26 To take into account the variation in X-ray absorption, the averaged signals for each species were multiplied by a response factor and combined to give the total signal for each position. 4. Results and Discussion 4.1. Electrochemical Flow Cell Characterization. Before spectroelectrochemical studies, the flow cell was first characterized to verify that the approximations outlined in the simulations were valid. As noted above, one of the design constraints of the electrochemical cell necessitates that the electrode width, w, is equal to that of the channel, d. The hydrodynamic boundary layer on either side of the channel is likely to alter the transport properties resulting in edge effects. For one-dimensional flow in which axial convection and diffusion normal to the electrode surface are considered but axial diffusion is ignored, the limiting current is given by the Levich equation27 2⁄3 2 1⁄3 Ilim ) 0.925nF[Fe(η5-C5H5)2]bulkwx2⁄3 e D (Vf/h d)
where F is the Faraday constant, n is the number of electrons transferred, and Vf is the volume flow rate. The electrode and channel dimensions can be understood with reference to Figure 3 and are defined as follows: xe, the electrode length; h, the half-height of the channel; w, the electrode width; and d, the channel width. D corresponds to the diffusion coefficient. The one-electron oxidation of ferrocene was used for these tests, hence [Fe(η5-C5H5)2]bulk is the bulk concentration of ferrocene. Figure 5 shows the variation in limiting current with an applied potential of 0.6 V vs Ag/AgCl against the cubic root of a range of flow rates. The theoretical values show good agreement with the Levich equation and demonstrate that the variation in current density across the width of the electrode is
TABLE 2: Time-Dependent Boundary Conditions Applied t)0 all t all t all t all t t ) t1 t ) t2 t ) t3 t ) t4
0 < x < xl x)0 0 < x < x0 xf < x < xl 0 < x < xl x0 < x < xf x0 < x < xf x0 < x < xf x0 < x < xf
0 < y < h2 0 < y < h2 y)0 y ) h2 y)0 y)0 y)0 y)0 y)0
CFe(I) ) 1, CFe(II) ) 0, CFe(0) ) 0 CFe(I) ) 1, CFe(II) ) 0, CFe(0) ) 0 (∂CFe(I))/(∂y) ) 0, (∂CFe(II))/(∂y) ) 0, (∂CFe(0))/(∂y) ) 0 (∂CFe(I))/(∂y) ) 0, (∂CFe(II))/(∂y) ) 0, (∂CFe(0))/(∂y) ) 0 (∂CFe(I))/(∂y) ) 0, (∂CFe(II))/(∂y) ) 0, (∂CFe(0))/(∂y) ) 0 CFe(I) ) 0, CFe(II) ) 1, CFe(0) ) 0 CFe(I) ) 1, CFe(II) ) 0, CFe(0) ) 0 CFe(I) ) 0, CFe(II) ) 0, CFe(0) ) 1 CFe(I) ) 1, CFe(II) ) 0, CFe(0) ) 0
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Figure 5. Plot of limiting current vs Vf1/3 using a solution of 1 mM ferrocene, 0.1 M [NEt4][BF4] in MeCN. 9 represent experimental data and show the simulated behavior based on the Levich equation where xe ) 0.04 cm, 2h ) 0.04 cm, w ) d ) 0.32 cm, and D ) 2.3 × 10-5 cm2 s-1.28
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Figure 7. Normalized XANES region for 20 mM [Fe2S2] and [Fe2S2]+ in acetonitrile.
Figure 8. Schematic showing beam positions relative to the working electrode.
Figure 6. Potential step sequence used for mapping and corresponding current response for 20 mM [{Fe(η5-C5H5)(CO)(µ-SPh)}2][PF6], 0.2 M [NEt4][BF4] in MeCN with a flow rate of 3.5 × 10-5 cm3 s-1.
not significant and that the 2D approach utilized in the CFD simulations is valid. 4.2. Potential Step Sequence. Figure 6 shows the potential step sequence and corresponding current response used for the channel mapping. With a solution of 20 mM [Fe2S2]+ flowing at 3.5 × 10-5 cm3 s-1, the start potential was set at 0.5 V vs Ag/AgCl where any current is solely due to double-layer charging (cyclic voltammetry, Supporting Information). A series of potential steps using potentials of 0.8 and 0.2 V vs Ag/AgCl were then used to generate the dicationic and neutral species, respectively. After each step change, the potential was returned to 0.5 V vs Ag/AgCl. The pulse time was set to 30 s to generate sufficient concentrations of electrochemical species and, as a result, noticeable changes in absorption. A time delay of 50 s was used before repeating the potential step sequence to allow electrochemically generated species to clear the channel. 4.3. XANES (X-ray Absorption Near-Edge Structure). Figure 7 shows the normalized XANES region for both the [Fe2S2] and [Fe2S2]+ pure species. The spectra were collected in transmission using a liquid cell with a path length of 3 mm. The solvent was argon-purged acetonitrile, and a concentration of 20 mM was used in both cases. The variation in absorption between the two species allowed an energy to be selected for the channel mapping corresponding to the greatest change in absorption with oxidation state (7.132 keV).
4.4. Mapping. The schematic in Figure 8 shows the grid of 30 sampling positions within the channel relative to the working electrode. The absorption change as a function of position within the channel and applied potential is shown in Figure 9. The X-ray energy was fixed to the maximum in the white line region (7.132 keV). An increase in absorption relative to the monocation corresponds to the generation of the dication, while a drop in absorption below the monocation baseline indicates the presence of the neutral species. This change in transmission is consistent with an increase in the energy of the absorption edge with oxidation number. In Figure 9a, positions 1 and 2 correspond to positions upstream of the electrode, and as a result, no change in absorption is observed. By position 3, changes in absorption are observed with an offset relative to the potential step sequence corresponding to the time it takes the electrochemically generated species to reach the path of the beam. This offset increases with distance downstream of the electrode. The relative changes in absorption are at a maximum close to the electrode center (position 4) and, as with the time offset, decrease both downstream of this position and in moving perpendicular to the channel surface. This is a result of the electrochemically generated species diffusing into the bulk solution. The absorption change and hence apparent oxidation state do not always track the potential step sequence. Variations are particularly noticeable over the first two potential changes and at positions close to the electrode surface. For example, position 4 in row 1 shows an initial drop in absorption at 20 s despite the potential having been stepped from 0.5 to 0.8 V vs Ag/ AgCl, generating the higher oxidation state. This is followed by a steady increase in absorption which levels off after 30 s.
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Figure 10. Comparison between experimental (row 1, position 4) and simulated absorption changes close to the electrode surface with xe ) 0.04 cm, 2h ) 0.027 cm, w ) d ) 0.32 cm, Vf ) 3.5 × 10-5 cm3 s-1, and D ) 2.43 × 10-6 cm2 s-1. Response factors: monocation ) 0.13, dication ) 0.16, neutral ) 0.12.
Figure 9. Absorption at the Fe K edge (7.132 keV) as a function of beam position and potential. (a) corresponds to row 1 (closest to channel surface), while (b) and (c) correspond to rows 2 and 3 positioned at 20 µm increments into the bulk solution. The potential step sequence and current response are given in Figure 6. xe ) 0.04 cm, 2h ) 0.027 cm, w ) d ) 0.32 cm, and Vf ) 3.5 × 10-5 cm3 s-1.
A second anomaly appears at 50 s when the potential is stepped back to 0.5 V vs Ag/AgCl. Instead of the expected decrease in absorption, the absorption rises before returning to the baseline level corresponding to the cation. The possible causes will be discussed in the following section with reference to the simulated absorption. 4.5. Comparison with Simulations. The diffusion coefficients used in the simulations were calculated experimentally from potential step measurements in a standard low-volume three-electrode cell. Using the cation, 1 mM [Fe2S2]+, as the
starting material and 0.1 M [NEt4][BF4] as the supporting electrolyte, the potential was stepped to generate the dication (0.8 V vs Ag/AgCl) and the current transient recorded over a period of 10 s. This was repeated using a potential step of 0.075 V vs Ag/AgCl to generate the neutral species. Both potential steps correspond to a single-electron transfer, and the calculated diffusion coefficients relate to that of the cation. The data were analyzed with plots of I versus t1/2, and the values obtained for the diffusion coefficients from repeat measurements for the two potential steps agreed to within 5%. An average value was taken ((2.43 ( 0.05) × 10-6 cm2 s-1) and used for the simulations. The simulations calculate the composition of the solution within a 50 µm spot size. From this the absorption can be determined by multiplying the contribution from each species by a response factor (see Theory section). Two approaches were taken to determine the response factor. Values were obtained experimentally by measuring the absorption through the channel at the Fe K edge under conditions of no flow and with an applied potential corresponding to the neutral, monocationic, or dicationic species. A relatively long equilibration time of 15 min was used to avoid incomplete conversion arising from solution resistance. Despite efforts to maximize conversion within the path of the beam, the absorption changes observed in the XANES were smaller than expected when compared to that of the pure species (Figure 7). By comparison of the absorption for measurements made in the channel and for the two available reference compounds, a conversion of 8% was determined for the cation to neutral electrolysis. Further evidence of incomplete electrolysis came from a measure of the charge transferred under applied potential. The poor electrochemical activity was attributed to a passivation of the working electrode preventing 100% electrolysis of the monocation to the neutral or dicationic species. Given the difficulty in obtaining experimental values, a second approach was taken where the response factors were determined by adjusting the input values in the simulation to give the closest fit at a position next to the electrode surface (Figure 9a, row 1, position 4). The values obtained were as follows, monocation ) 0.13, dication ) 0.16, neutral ) 0.12, and were applied to all other positions within the channel. The simulated absorption changes show excellent agreement when compared to those determined experimentally. Figure 10
314 J. Phys. Chem. C, Vol. 113, No. 1, 2009 shows a plot of both the experimental and simulated data at position 4 for row 1. Moving downstream of the electrode and further into solution, the quality of the fit was found to decrease. This suggests that some of the approximations made in the theory are not correct. In the vicinity of the electrode, the absorption changes are significant, and as a result, the sensitivity to any deviation from the approximations is low. This is not the case further from the electrode where, in particular, variations in the geometry of the flow cell would have a significant effect. Small variations in absorption between grid points within the channel provide evidence that the cell width is not constant. Bowing of the polypropylene windows would cause this variation and is difficult to correct without the use of thicker windows, which would not be viable at the Fe K edge due to a reduction in transmission. The comparisons between experimental and simulated data suggest that within the diffusiondominated region close to the electrode the agreement is high. Away from the electrode where forced convection is the overriding means of mass transport, the approximations made in the simulations no longer hold. The closeness of fit between the experimental and simulated absorption in the vicinity of the electrode allows the composition of the solution to be reliably determined within this region. For the position identified above (position 4, row 1), the normalized concentration of the individual species at a given time can be calculated. For example, at a time of 44 s, corresponding to the time at which the dication concentration is at its greatest (see Figure 6), the composition within the 50 µm spot size is as follows: monocation ) 0.914, dication ) 0.086, and neutral ) 0. Simulations at reduced flow rates show an increase in conversion. For example, halving the flow rate from 3.5 × 10-5 cm3 s-1 to 1.75 × 10-5 cm3 s-1 gives nearly a doubling of the dication concentration. We now return to consideration of the sharp features observed in the absorption that are not explained by these simulations. From Figure 9, the first point to note is that there is a time delay between the potential change and absorption spike. This implies that the effect cannot be caused by mechanical or electrokinetic effects since the time delay is too long. In moving from row 1 to 3, very little change is observed in the spike position. Rows 1 and 2 appear identical within error, while for row 3 the spike position is shifted to longer times, ca. 1 s. Given a distance of 20 µm between rows, this result is expected from diffusion processes perpendicular to the electrode. Estimates using the Einstein equation, t ) d2/2D (where t is the diffusion time, d the diffusion layer thickness, and D the diffusion coefficient), give a time of ca. 0.8 s for the molecules to move 20 µm. This would be difficult to detect experimentally given the noise levels and sampling rate. A further important observation is that the effect is more pronounced over the electrode surface. From position 7 and further downstream, the absorption spike is difficult to detect. This implies that the spike must be induced by processes taking place at the electrode. A number of possibilities arise including: (1) products of electron transfer, (2) decay products (EC-type mechanism), (3) electrode adsorption processes, or (4) migration. Taking into account each possibility in turn, and with the results available, we can narrow down the options. (1) The simulations are based solely on electron transfer of the [Fe2S2] species. As the absorption spike is not mapped, these particular e.t. processes can be ruled out. Simulations with varying diffusion coefficients for the individual [Fe2S2] species have also been carried out (not shown), but no improvement in fit was observed.
Wiltshire et al. Figure 9 shows that the relative absorption changes for the [Fe2S2] species decrease when moving into the bulk solution while the relative absorption changes remain constant for the absorption spikes. Mass transport perpendicular to the electrode is expected to be diffusion-controlled suggesting that a species very differenet from [Fe2S2] is involved. This provides further evidence that the absorption spike is not directly related to the [Fe2S2] species and that any secondary e.t. product generated at the electrode must be an X-ray absorbing species with a high diffusion coefficient. (2) The true nature of a secondary e.t. product is difficult to determine with the experimental data collected thus far. Any secondary product from competing side reactions may go on to decay via an EC-type mechanism. For meaningful simulations of an EC- type mechanism, experimentally determined rate constants are required. (3) As mentioned previously, data collection times of up to 10 h were required, and the current was found to deteriorate with time. Subsequent electrochemical tests carried out on the electrodes showed reduced performance indicating that decay products had adsorbed on the surface. As a result, it is possible that a potential-induced adsorption/desorption process could lead to the absorption spikes observed in the experimental data. (4) Ions present in the background electrolyte include [NEt4]+, [BF4]-, and [PF6]- have varying absorption cross sections with the fluorinated ions absorbing more strongly. For each potential step, there must be a change in the movement of these ions with the passage of current. This could account for the absorption spikes observed in the experimental data. 5. Conclusions and Future Directions An electrochemical flow cell suitable for XAS was successfully characterized by mapping X-ray absorption changes as a function of applied potential and position within the channel. The results presented here show that in the region of the working electrode, variation in the composition of the solution can be closely mapped by simulation. Discrepancies between experimental and simulated values were found in regions where forced convection dominates mass transport. This was attributed to variations in channel geometry which were difficult to control due to constraints imposed by X-ray absorption measurements. The experiments also show the existence of subtle phenomenon close to the electrode. Though as yet unexplained, the power of the technique is demonstrated. For successful EXAFS analysis of mixtures, knowledge of the solution composition is required. Channel-flow cells provide a well-defined flow regime that allows accurate simulation of the relative concentration of the species present. The conversion from monocation to dication within a 50 µm spot at a distance of 75 µm from the electrode was found to be ∼9%. The low conversion rates highlight the difficulties in making absorption measurements using a 50 µm probe. For data of sufficient quality to allow structural characterization of an electrochemically generated species, far higher conversion rates are required. This can be achieved in two ways, first, by reducing the flow rate and hence increasing the thickness of the diffusion layer. Second, a reduction in the X-ray beam size would allow far smaller distances of approach. An added benefit is that the time scale of the experiment is reduced and electrochemical intermediates with short life times could be detected. Microfocus beam lines at third-generation synchrotron sources can routinely produce X-ray beams with spot sizes on the order of 1-5 µm. The expected conversion based on simulations for a 5 µm spot size at a distance of 5 µm from the electrode surface
Channel-Flow Cell for XAS is approximately 95%. Conversion rates of this magnitude open up possibilities for the study of intermediates in electron-transfer reactions with the potential of providing structural information unobtainable by other techniques. Despite smaller spot sizes allowing regions with high concentrations of intermediates to be probed, it is important not to understate the challenges remaining before the goal of obtaining structural information can be reached. Low electrochemical conversion due to passivation of the working electrode is a significant problem and may not be overcome unless the solute concentration can be significantly reduced. Collecting XAS in the fluorescence mode offers a possible solution as the contrast in signal is greater than for transmission at low concentrations. Work to develop a suitable flow cell for such measurements is now in progress and will be reported in the near future. Acknowledgment. We acknowledge the staff of the Synchrotron Radiation Facility, Daresbury, especially Steven Fiddy and Andrew Bennett, for their help and support. Financial support of the project by the EPSRC is also gratefully acknowledged. Supporting Information Available: Cyclic voltammetry of 20 mM [Fe2S2]+, 0.2 M [NEt4][BF4] in MeCN. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Neudeck, A.; Marken, F.; Compton, R. G. UV/Vis/NIR Specotroelectrochemistry; Springer, New York, 2002. (2) Wolfgang, K.; Klein, A. Spectroelectrochemistry; Royal Society of Chemistry: Cambridge, U.K., 2008. (3) Crozier, E. D. Nucl. Instrum. Methods Phys. Res., Sect. B 1997, 133, 134. (4) Lee, P. A.; Citrin, P. H.; Eisenberger, P.; Kincaid, B. M. ReV. Mod. Phys. 1981, 53, 769. (5) Mukerjee, S.; McBreen, J. J. Electrochem. Soc. 1999, 146, 600.
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