Quantitative Aspects of Normalized Differential Reflectance

Apr 5, 2014 - Normalized differential reflectance, [ΔR/R]λ (Eref =0.05 V vs RHE, left ... Some of the Optical Data Collected for Selected Systems in...
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Quantitative Aspects of Normalized Differential Reflectance Spectroscopy: Pt(111) in Aqueous Electrolytes Adriel Jebin Jacob Jebaraj and Daniel Scherson* Department of Chemistry Case Western Reserve University, Cleveland, Ohio 44106, United States S Supporting Information *

ABSTRACT: A theoretical model is herein proposed to account for changes in the normalized differential reflectance, ΔR/R, of well-defined single crystal Pt(111) surfaces|aqueous electrolyte interfaces. It assumes that ΔR/R is proportional to the area of the electrode either bare or covered by neutral and/or nominally charged species and, for a specific type of site, is modulated by the applied potential, E. Correlations between the coverage of the various species and E were obtained from data reported in the literature or by coulometric analysis of linear voltammetric scans. Excellent agreement was found for the adsorption/desorption of hydrogen and that of bisulfate from acidic electrolytes both on bare, and cyanide-modified Pt(111). Also discussed are extensions of this technique in the transient mode involving the reduction of adsorbed nitric oxide, NO, on Pt(111).

as ΔR/R, and the actual coverage of the species on the surface.28 This information was later used to monitor the rates of oxidation of adsorbed carbon monoxide, COads on quasi perfect Pt(111) facet in acidic media via a combination of timeresolved ΔR/R and second harmonic generation.29 Analyses of the results obtained supported the view that the process is initiated at defect sites, probably at the perimeter of the facet, which then propagate along the surface at rates much faster than the surface diffusion of COads.27 This work will explore extensions of both static and dynamic ΔR/R for the quantitative study of hydrogen and bisulfate adsorption on both bare and cyanide-covered Pt(111) facets in acidic electrolytes, and for the reduction of adsorbed nitric oxide, NOads on the same single crystal surface.

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xtraordinary progress has been made over the last three decades toward gaining a better understanding of the structure of well-defined single crystal electrode−electrolyte interfaces. Much of this success can be attributed to the advent of a growing number of in situ spectroscopic and structural techniques ranging from infrared reflection absorption spectroscopy (IRAS),1−4 second harmonic5−7 and sum frequency generation,8,9 and Raman scattering,10,11 to atomic force (AFM) and scanning tunneling (STM) microscopies,12−15 and synchrotron based methodologies.16−21 Application of these probes to monitor the dynamics of interfacial events with high sensitivity and specificity as well as temporal and spatial resolution, however, continues to pose formidable challenges. As has been widely emphasized, advances in this field are expected to impact significantly areas of technological and societal importance, such as energy storage, energy conversion and environmental control. Efforts in our laboratories have focused on the further development and implementation of optical methods in the ultraviolet and visible spectral regions including differential reflectance spectroscopy22,23 and second harmonic generation6,24−26 as probes of both inert and reactive adsorbates. The specific strategy we have pursued involves the use of focused laser beams impinging on the surface of welldefined facets on single crystal metal beads.27 Analyses of the static data in the case of bisulfate adsorbed on Pt(111) made it possible to establish a linear relationship between the magnitude of the normalized differential reflectance, denoted © 2014 American Chemical Society

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EXPERIMENTAL SECTION All measurements were performed at room temperature in Arpurged (Praxair 5.0) ultrapure aqueous (18.3 MΩ water, Barnstead) 0.1 M HClO4 (Ultrex, J.T. Baker) or 0.1 M H2SO4 (J.T. Baker, Suprapure) in a conventional quartz cell (10 mm optical path, Starna). A single-crystal platinum bead displaying a large Pt(111) facet was used as the working electrode. A Received: December 1, 2013 Accepted: April 5, 2014 Published: April 5, 2014 4241

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platinum wire and a reversible hydrogen electrode, RHE, in the same solution, served as counter and reference electrodes, respectively. Prior to the experiments, the single crystal bead was flame annealed in a hydrogen-air flame following the method originally reported by Clavilier,30 and, subsequently, allowed to cool down in a H2/Ar mixture. A drop of ultrapure deaerated water was then placed on the surface of the crystal to avoid contamination during transfer to the electrochemical cell. Cyanide-modified Pt(111) surfaces were prepared by dipping the cooled-down, flame annealed bead in a 0.1 M KCN aqueous solution and then rinsing copiously with ultrapure water.31 Measurements involving NOads reduction were performed in 0.1 M HClO4 aqueous solutions containing 10 mM NaNO2. All electrochemical measurements were carried out using an Autolab potentiostat (Metrohm, PGSTAT302N, AUT84413, Nova 1.6). Reflectance data were acquired either with a battery-powered blue diode laser (Polaris 700, λ = 447 nm, 900 mW, Laserglow) or a low power HeNe laser (λ = 633 nm, CW HeNe, JDS Uniphase, 15 mW), as the light source, using an experimental setup similar to that described elsewhere.32 As specified therein, the incident laser beam was focused at an angle of incidence of ∼45° with a glass lens onto the large Pt(111) facet with the single crystal bead fully immersed in the electrolyte. Another lens was employed to focus the reflected beam onto a batteryoperated, biased Si photodiode detector (DET-10A, Thorlabs) and its response recorded in a 0.5 GHz oscilloscope (TDS 744A, Tektronix). All the optical data are reported as normalized differential reflectance, ΔR/R = [R(E) − R(Eref)]/R(Eref), where R(E) represents the intensity of the reflected beam at an arbitrary potential E and R(Eref) that recorded at a reference potential, Eref. Theoretical Aspects. The primitive model considered herein assumes each type of surface site contributes in a linear fashion to the optical signal.33 For example, the wavelength dependent normalized differential reflectance of an electrode, [ΔR/R]λ, polarized within a region in which a single species adsorbs would be represented by [ΔR /R ]λ = Aλ (E)θ + βλ(E)(1 − θ )

Figure 1. Normalized differential reflectance, [ΔR/R]λ (Eref =0.05 V vs RHE, left ordinate) versus E, for a Pt(111) facet in 0.1 M HClO4 for λ = 633 nm (red curve) and λ = 447 nm (blue curve), recorded at a scan rate, ν = 2 V/s. Each of the optical curves represents an average of ∼200 acq. The black line is the voltammetric curve reported by Feliu and co-workers for a massive Pt(111) crystal in the same electrolyte (right ordinate), where the dotted line represents the contribution due to the charging of the double layer.

Molina and Parsons34 for the low index faces of Pt in similar electrolytes, the ΔR/R versus E curves displayed marked differences for the two values of λ selected for our studies. To test the validity of eq 1, the potential dependence of [ΔR/R]λ in the double layer region, that is, 0.425 ≤ E ≤ 0.575 V, where the surface is believed to be devoid of adsorbates other than water, was fitted with a linear function (green lines, Figure 2; see Table 1). Next, the coverage of hydrogen, θH, on Pt(111) as a function of E (see magenta line in Figure 2), was determined from the cyclic voltammetric data reported by Feliu and co-workers35 in the same electrolyte (see black line, Figure 1) after subtracting a potential independent double layer contribution (dotted line, Figure 1). Lastly, the parameters associated with the linear functions Aλ(E) and βλ(E) were found by fitting the data to eq 1 (see Table 1). An analogous analysis was performed for the “butterfly” feature in the region 0.55 ≤ E < 0.85 V attributed to the adsorption of hydroxyl ion using the expression

(1)

where θ represents the coverage of that species and, hence, (1 − θ) that of the bare surface, and the coefficients Aλ(E) and βλ(E) are, in general, linear functions that account for the potential dependence of each of the two types of sites. For some of the steps involved in the analysis, functions available in Origin’s library were used to obtain best fits to the data.

[ΔR /R ]λ = Aλ θH + βλ(E)(1 − θH − θOH) + CλθOH

(2)

where βλ(E) (green lines) represents the potential dependence of the bare sites specified above, and the term Cλ is a constant. The charge density associated with the butterfly feature was determined assuming arbitrarily a single electron transfer upon hydroxyl adsorption|desorption yielding a saturation coverage of 0.22. As evident from the results in Figure 3, and the R2 values obtained from the fit (see Table 1), the agreement between this primitive model (see black curves in Figure 3) and the experimental data (jagged lines) is indeed very good. Specific Adsorption of Bisulfate on Pt(111). The analysis presented in the previous section was extended further for a two component system, namely the adsorption of hydrogen and bisulfate from 0.1 M H2SO4 in Pt(111). The coverage of bisulfate, θHSO4, as a function of potential was obtained from data reported by Feliu et al.36 assuming in this case that the sharp spike does not contribute to θHSO4, as suggested by the optical data reported from our laboratory

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RESULTS AND DISCUSSION Pt(111) in 0.1 M HClO4. Plots of the normalized differential reflectance, [ΔR/R]λ, (Eref = 0.05 V), versus E for a quasiperfect Pt(111) facet in 0.1 M HClO4 recorded with lasers of two different wavelengths, that is, λ = 633 nm (red curve) and λ = 447 nm (blue curve), are shown in Figure 1. Each set of data represents an average of ∼200 identical acquisitions (acq.), while scanning the potential E, continuously at a rate, ν = 2 V/ s, between 0.1 V < E < 0.8 V. Although [ΔR/R]λ was found to be virtually independent of the direction of the scan, only data collected toward negative potentials are displayed in this figure (see below). As clearly evident from these results, each of the regions in the voltammogram shown in the black line in this figure elicits a unique and fairly linear optical response. Moreover, and in agreement with the results published by 4242

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Figure 2. Plots of [ΔR/R]λ vs E for a Pt(111) facet in 0.1 M HClO4 in the potential range 0.1 < E < 0.55 V vs RHE at λ = 633 nm (red curve, upper panel) and λ = 447 nm (blue curve, lower panel). See caption Figure 1, for additional details. The best linear fits to eq 2 are given by the black lines in both panels (see text for details), where βλ(E) and, θH as a function of E are given by the green and magenta lines, respectively.

Figure 3. Plot of [ΔR/R]λ vs E for a Pt(111) facet in 0.1 M HClO4 in the range 0.1 < E < 0.75 V vs RHE at λ = 633 nm (red curve, top panel) and λ = 447 nm (blue curve, lower panel). The functions βλ(E) (see green lines) are the same as those shown in Figure 2. The charge density as a function of E are given by the magenta lines and the best fits to eq 2 by the black lines in both panels (see text for details).

where θH and βλ(E) are precisely those defined in the sections above and γλ(E) was determined from the experimental data in a potential region positive to the onset of bisulfate adsorption saturation, that is, E > 0.6 V. As shown in Figure 4, excellent agreement was found for the two wavelengths (see Table 1).

previously.28 The actual data was fitted using the same primitive model, that is [ΔR /R ]λ = AθH + βλ(E)[1 − θH − θHSO−4 ] + γλ(E)θHSO−4 (3)

Table 1. Functions and Statistical Parameters for Some of the Optical Data Collected for Selected Systems in This Work functions and statistical parameters system

wavelength (nm)

Hads on Pt(111)

633 447

system

Pt(111) in 0.1 M H2SO4

system Pt(111)−CN in 0.1 M HClO4

633 447

βλ(E)

wavelength (nm) 447

Aλ −0.03513

−0.11408 × 10 −0.05017 × 10+0.07369 βλ(E)

wavelength (nm) 633 447

R2

Aλ

−0.11408 × 10−0.03513 −0.0014 −0.05017 × 10+0.07369 −0.0323 functions and statistical parameters

wavelength (nm)

Pt(111) in 0.1 M HClO4

system

βλ(E)

−0.11408 × 10 −0.05017 × 10+0.07369 β′(E) −0.0011 × 10

−0.0047 −0.33589 −0.03791 −0.08198 functions and statistical parameters

4243

0.97546 0.86478

γλ(E)

R2 −0.21

0.00904 0.0615 ×10 −0.03167 0.0012 × 10−0.098 functions and statistical parameters

A′ +0.00259

R2

Cλ

Aλ −0.03513

0.96416 0.878 14

−0.00131

σ(E) −0.03155

0.0353 × 10

0.94657 0.98799

γ

δ

R2

0.00203

−0.00038

0.99546

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Figure 5. Plot of [ΔR/R]λ vs E for a cyanide-modified Pt(111) facet in 0.1 M HClO4 at λ = 447 nm. The optical signal represents an average of ∼200 acq recorded at a scan rate ν = 1 V/s. The black curve (right ordinate) is the voltammetric curve of a cyanide-modified massive Pt(111) crystal in the same electrolyte recorded at a scan rate of 50 mV/s, reported by Cuesta and co-workers.47 Also shown in gray lines are the components of the linear scan toward positive potentials in the region 0.1 < E < 0.6 V where the dotted line represents the contribution due to the double layer, that is, 0.6 < E < 0.8.

yielding for θ1, θ2, and θ3 values of 0.275, 0.275, and 0.45, respectively (see upper panel, Figure 6). The function β′(E) in eq 4 was determined by fitting a straight line in the region 0.6 V < E < 0.8 V Excellent agreement was found between the experimental data and the values predicted by the model as shown in the lower panel, Figure 6 (see Table 1). The fit in the range E < 0.75 V is shown in Figure S1 in the Supporting Information. This analysis was further extended to include the feature centered at 0.9 V in Figure 5 ascribed by Huerta et al. to the adsorption/desorption of a hydroxyl type species denoted hereafter as OH.31 Its contribution to the optical signal was then added to eq 4 to yield eq 5 where θOH is the coverage of OH (see upper panel, Figure 6) and σ(E) is a linear function which accounts for its potential dependence,

Figure 4. Plots of [ΔR/R]λ vs E for a Pt(111) facet in 0.1 M H2SO4 in the range 0.1 < E < 0.8 V vs RHE at λ = 633 nm (red curve, upper panel) and λ = 447 nm (blue curve, lower panel). The cyclic voltammogram of a Pt(111) single crystal in 0.1 M H2SO4 is shown by the magenta lines and the best fits to eq 3 by the black lines in both panels (see text for details). The optical signal is an average of ∼200 acq. at ν = 2 V/s.

Cyanide-Modified Pt(111). Pt(111)−CN in HClO4. As first reported by Huerta et al.,31 the cyclic voltammetry of cyanidemodified Pt(111) denoted as Pt(111)−CN (see Figure 5) in 0.1 M HClO4 at potentials negative to the prominent peak centered at 0.9 V exhibits two highly reversible, largely overlapping peaks in the region 0.25 to 0.6 V, and what appears to be the onset of yet another equally fast adsorption/ desorption process for E < 0.2 V. Each of these features will be ascribed to the adsorption|desorption of different forms of the same species, in all likelihood hydrogen, and their potential dependent coverages denoted as θ1, θ2, and θ3. On this basis, the primitive model would predict the following dependence of ΔR/R on the applied potential

ΔR /R = A′θ1 + β′(E)(1 − θ1 − θ2 − θ3 − θOH) + γθ2 + δθ3 + σ(E)θOH

(5)

Again, very good agreement was obtained between the experimental data and the model by assuming, rather arbitrarily, θOH = 0.5 up to 0.9 V, which corresponds to the peak maximum (see red line, lower panel, Figure 6, and Table 1). Pt(111)−CN in H2SO4. Shown in the upper panel of Figure 7 are the cyclic voltammograms obtained for a Pt(111) (red) and a Pt(111)−CN (black) electrodes in a 0.1 M H2SO4 solution. Unlike the behavior found for bare Pt(111), the voltammetry of the Pt(111)−CN in this electrolyte closely resembles that obtained in 0.1 M HClO4 (see Figure 5). This is an indication that the sites involved in the adsorption of bisulfate, are blocked by the presence of adsorbed cyanide. This explanation was suggested previously by Markovic et al.37 to account for the increased activity of Pt(111)−CN compared to a bare Pt(111) surface for oxygen reduction in sulfuric acid, who proposed that cyanide would impede adsorption of bisulfate leaving bare sites onto which the reaction can proceed at high rates. Shown in the lower panel, Figure 7 are ΔR/R vs E data recorded for a Pt(111)−CN (black) and an unmodified Pt(111) (red)

ΔR /R = A′θ1 + β′(E)(1 − θ1 − θ2 − θ3) + γθ2 + δθ3 (4)

where A′, γ, and δ, are independent of E and β′(E), are to be obtained based on the fitting of the experimental results. The functional dependences of θ1, θ2, and θ3 for data collected for λ = 447 nm were determined by first subtracting the contribution because of the double layer shown by the dotted line in Figure 5, and then deconvoluting the current in the range 0.1 < E < 0.6 V for the scan in the positive direction in terms of Gaussian functions (see gray lines Figure 5). The latter were integrated and then normalized assuming a one electron transfer process 4244

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Figure 6. Top: Plots of θ1, θ2, θ3, and θOH as a function of potential (see text for details). Bottom: Plot of [ΔR/R]λ vs E recorded from a cyanide modified Pt(111) facet in 0.1 M HClO4 in the potential range 0.1 < E < 0.9 V vs RHE. Also shown in this figure are plots of the double layer contribution to the differential reflectance (solid green line) and the calculated ΔR/R values based on the proposed model (red line).

Figure 7. Top: Cyclic voltammetry recorded from a bare (red) and cyanide modified Pt(111) single crystal (black) in 0.1 M H2SO4, recorded at ν = 1 V/s. Bottom: Plots of ΔR/R (Eref = 0.05 V vs RHE) versus E for a Pt(111) (red) an a Pt(111)−CN (black) facet collected at λ = 447 nm in the potential range indicated. The optical data represents an average of ∼200 acq.

collected with λ = 447 nm. As clearly evidenced from the data, the characteristic drop in ΔR/R induced by bisulfate adsorption can only be seen for the bare surface (see red lines) affording evidence that bisulfate does not adsorb on Pt(111)-CN. Adsorption and Reduction of NO on Pt(111). Yet another illustration of the versatility of differential reflectance spectroscopy as a monitoring probe of adsorbed species is provided by the adsorption and reduction of nitric oxide, NO, on Pt(111) in acidic electrolytes. Voltammetric data collected in 0.1 M HClO4 for a saturated layer NOads on Pt(111), in the absence of NO in solution (see red line, upper panel, Figure 8), revealed that the reduction of NOads occurs in two potential ranges, namely 0.5 < E < 0.25 V, and 0.25 V down to the onset of hydrogen evolution. These have been ascribed to two different NOads species denoted in what follows as NOIads and NOIIads, respectively.38 Corresponding data reported by Feliu and co-workers39 collected in the presence of NO in solution is shown in the blue line in the same panel. The [ΔR/R]447 (Eref = 0.05 V) versus E plot recorded at λ = 447 nm in 0.1 M HClO4 containing 10 mM NaNO2 (blue scattered symbols, lower panel, Figure 8) displays an increase in the potential region associated with the reduction of NOIIads; however, no distinct changes in the optical signal could be discerned for NOIads,which indicates a lack of sensitivity to its presence at this wavelength. It should be pointed out that the in situ

potential difference IRAS spectrum of NOads on Pt(111) in the absence of NO in solution showed a single sharp peak associated with NOIads,without clear evidence for the presence of NOIIads because of interference with the solvent features.40 An aspect that demands close scrutiny is the sign of ΔR/R in the potential region associated with the reduction of NOIIads. Specifically, ΔR/R increases for E < 0.25 V, which is opposite to what would be expected if that process was associated solely with hydrogen adsorption, as the results in black symbols in this panel indicate. It may thus be surmised that the reductive desorption of NOads leads to an increase in ΔR/R, for which the magnitude would be larger than that derived from the simultaneous adsorption of hydrogen leading to a net increase in the optical signal as shown by the data. Although Rosca et al.38 claim to have found no clear effects associated with hydrogen coadsorption on the current response in their NOads reduction chronoamperometric studies, the inability of hydrogen to adsorb on sites on the surface made available by the partial reductive desorption of the NOads layer seems highly unlikely. Lastly, the increase in ΔR/R at about 0.6 V appears to be associated with the desorption of adsorbed nitrite, an explanation partially supported by the in situ surface enhanced infrared absorption spectroscopy, SEIRAS, and Raman scattering data for Pt (poly) in solutions containing nitrite.41,42 It should be stressed that the reduction of NOads on Pt(111) has been found to be rather slow43,44 and, therefore, it is 4245

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Figure 9. Plot of [ΔR/R]447 versus time for a Pt(111) facet in the presence of 10 mM NaNO2 in a 0.1 M HClO4 solution. The potential was held at Eads = 0.7 V vs RHE for 5 s before stepping (see arrow) to the various Efinal as specified in the figure.

to more negative potentials, that is, 0.15 and 0.05 V, where a clear increase in ΔR/R could be discerned (see magenta and green lines in Figure.9). The time dependent response for these two potentials is characterized by a very fast rise, on the order of 10−20 ms, which is then followed by a much slower increase. This behavior strongly suggests that a fraction of the NOads reduces much faster than the rest. This phenomenon may be caused by the migration of NOads to sites made available by the partial stripping of the NOads layer where their reduction would proceed at significantly slower rates. Support for this view has been obtained from experiments performed in solutions devoid of NO in which the potential of the Pt(111) surface bearing a saturated layer of NOads was stepped from Eads = 0.7 down to Efinal = 0.10 V and shortly thereafter stepped back to 0.7 V. As evidenced by a subsequent linear scan toward negative potentials, a clear peak was found for E > ∼0.38 V, that is, more positive than those associated with peaks I and II and thus consistent with the presence of NOads. It is interesting to note that a fast, followed by slow process, was also observed by monitoring ΔR/R while stepping the potential from 0.05 V af ter allowing suf ficient time for the NOads layer to undergo f ull reduction to 0.7 V, a value at which the NOads layer would fully assemble (see Figure 10). This phenomenon is currently under investigation in our laboratory using an array of spectroscopic techniques the results of which will be reported in due course. Monitoring the Dynamics of Assembly of a √19 × √19)R23.4°−13CO Adlaye r on Pt(111) Facets. A quantitative analysis of in situ differential reflectance spectroscopy can also afford insight into interfacial processes involving formation of well-defined molecular layers in cases in which the currents associated with adsorption would be too small to be measured reliably. As a means of illustration, Figure 11 (black line) displays a plot of ΔR/R for an experiment performed in CO-saturated 0.1 M H2SO4 aqueous solutions in which the potential of a Pt(111) facet of a faceted microsphere was stepped from Eini = 0.98 V vs RHE, that is, positive enough to fully oxidize COads on Pt(111), down to Efin = 0.6 V, a value at which a full (√19×√19)R23.4°−13CO adlayer is expected to assemble.45 As evidenced from the data collected, ΔR/R increases monotonically until it reaches a steady value for saturation conditions for which θ = 0.685. Under strict diffusion

Figure 8. Top: Voltammetric data collected for a bare Pt(111) (black line) and a saturated layer of NOads on Pt(111) in 0.1 M HClO4 in the absence of NO in solution (see red line, ν = 2 mV/s). The blue line represents corresponding data collected in the presence of nitrite in solution. Bottom: Plots of [ΔR/R]447 (Eref = 0.05 V) vs E recorded at ν = 20 mV/s in 0.1 M HClO4 before (black symbols) and after addition of 10 mM NaNO2 (blue symbols) in the potential range 0.1 < E < 0.9 V vs RHE. These data were collected while scanning the potential in the negative direction and represent an average of ∼40 acq.

conceivable that a small amount of NOads remained on the surface even at the most negative potentials. For this reason, conclusions derived from the analysis of optical data acquired during linear potential cycles should be regarded as largely qualitative. It may be argued that slower scan rates may mitigate this problem; however, under such conditions, the time required to collect a sufficient number of cycles to render a good signal-to-noise ratio would be unacceptably long rendering such strategy highly impractical. The approach we have implemented to gain insight into the dynamics of NOads reduction involves measurements in which both the current and ΔR/R were monitored simultaneously following a potential step from E = 0.7 V, a value at which a saturated layer of NOads would assemble, to a potential at which NOads would be expected to undergo reduction. Shown in Figure 9, is a plot of ΔR/R vs time collected for potential steps from Eads = 0.7 V to values Efinal down to 0.05 V. In accordance with the linear potential scan measurements in the upper panel, Figure 8, the steps to 0.5, 0.4, and 0.25 V yielded only a small increase in ΔR/R, supporting the view that the process being monitored does not correspond to the reduction of NOIads. This behavior is unlike that found for steps 4246

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ASSOCIATED CONTENT

S Supporting Information *

Additional material as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a grant from the National Science Foundation CHE-0911621. REFERENCES

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Figure 10. Plot of ΔR/R vs time for a Pt(111) facet in 10 mM NaNO2 in a 0.1 M HClO4 solution following a potential step from 0.05 to 0.7 V vs RHE. The data represents an average of ∼100 acquisitions.

Figure 11. Plot of ΔR/R (0.98 V) vs time, collected using λ = 633 nm, following application of a potential step from Eox (= Eref) = 0.98 V down to Eads= 0.60 vs RHE (black curve) V in CO-saturated 0.1 M H2SO4. The black and red lines represent the average of ∼700 acquisitions and the best fit to the optical data, respectively.

control, the flux toward a perfect microsphere, is given by DC*0 /r, where, in our case, D is the diffusion coefficient of CO, that is, 1.5 × 10−5 cm2/s,46 C0* its bulk concentration, that is, 9 ×10−7 mol/cm3, and r is the radius of the microsphere, ∼3.0 × 10−3 cm, to yield a value of 4.5 nmol/cm2 s. On this basis, the time required for a monolayer of CO to form on the surface of Pt, assuming a unit sticking coefficient and a saturation coverage of 0.685 is about 0.36 s, and thus very close to the value found in the optical data presented here (see Figure 11).

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CONCLUSIONS The results presented in this study afford strong evidence that the theoretical model herein proposed successfully describes the optical behavior found for the various interfaces selected for analysis opening new prospects for monitoring quantitatively the dynamics of a wide class of interfacial events. The coupling of this technique with strategies aimed at achieving fast control of the potential across the interface might be expected to gain access to redox transfer processes in the nanosecond domain and thus contribute to a better understanding of electrocatalytic processes of both fundamental and technological importance. 4247

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