In Situ FTIR Studies of the Effect of Temperature on the Adsorption and

At 10 °C, the in-situ FTIR data showed that the adsorbed CO species still remain in rather compact islands up to ca. 1100 mV vs Ag/AgCl as the CO oxi...
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J. Phys. Chem. B 2000, 104, 12002-12011

In Situ FTIR Studies of the Effect of Temperature on the Adsorption and Electrooxidation of CO at the Ru(0001) Electrode Surface W. F. Lin, P. A. Christensen,* and A. Hamnett Department of Chemistry, Bedson Building, The UniVersity, Newcastle upon Tyne, NE1 7RU, U.K. ReceiVed: August 15, 2000; In Final Form: October 17, 2000

The adsorption and electrooxidation of CO at a Ru(0001) electrode in perchloric acid solution have been investigated as a function of temperature, potential and time using in situ FTIR spectroscopy. This builds upon and extends previous work on the same system carried out at room temperature. As was observed at room temperature, both linear (COL) and 3-fold-hollow (COH) binding CO adsorbates (bands at 2000-2045 cm-1 and 1768-1805 cm-1, respectively) were detected on the Ru(0001) electrode at 10 °C and 50 °C. However, the temperature of the Ru(0001) electrode had a significant effect upon the structure and behavior of the CO adlayer. At 10 °C, the in-situ FTIR data showed that the adsorbed CO species still remain in rather compact islands up to ca. 1100 mV vs Ag/AgCl as the CO oxidation reaction proceeds, with oxidation occurring only at the boundaries between the COad and active surface oxide/hydroxide domains. However, the IR data collected at 50 °C strongly suggest that the adsorbed CO species are present as relatively looser and weaker structures, which are more easily electro-oxidized. The temperature-, potential-, and coverage-dependent relaxation and compression of the COL adlayer at low coverages are also discussed.

1. Introduction In situ Fourier Transform Infrared (FTIR) spectroscopy has proved to be a valuable tool for the study of surface electrochemical processes on both polycrystalline and single-crystal electrodes.1-3 One significant potential advantage of the technique is the opportunity to compare in situ FTIR data to those of the related system obtained under ultrahigh vacuum (UHV) conditions, and such studies have led to significant advances in the understanding of the electrode/electrolyte interface.4-7 In UHV studies, systems are often investigated as a function of temperature; e.g., temperature programming is used frequently to activate or restrict reaction pathways, to trap intermediates and to measure rate constants or other dynamic data.8,9 By contrast, the control of the temperature of, for instance, an in situ electrochemical spectroscopic system is not so straightforward; consequently, the majority of in-situ FTIR spectroelectrochemical measurements to date have been carried out at room temperature. However, a number of reports have appeared in the literature of late concerning in situ FTIR spectroelectrochemical measurements carried out as a function of temperature.10-13 Two of the most recent papers concern the effect of temperature on the adsorption and electro-oxidation of CO at polycrystalline Pt electrodes,12 and the electrooxidation of methanol at an irradiated TiO2 photoanode.13 The adsorption and catalytic oxidation of CO at the Ru(0001) single-crystal surface have been extensively studied in UHV conditions and in the presence of a high-pressure gas phase.14-26 Recently, we have conducted an investigation of the surface electrochemistry of CO at a Ru(0001) single-crystal electrode at room temperature, using a combination of in situ FTIR and ex situ LEED/RHEED and Auger.27 In this paper we extend our initial work to the investigation of the effect of temperature on the electro-oxidation of adsorbed * Corresponding author. Tel/Fax: +44 (191) 222 5472. E-mail: [email protected].

CO on Ru(0001) using in situ FTIR. The FTIR data reveal that the CO and O adlayer structures, surface CO bonding and diffusion, and the reactivity of the Ru(0001) surface toward the electrooxidation of adsorbed CO all show a marked dependence upon temperature. 2. Experimental Section Millipore water (>18 MΩ) and suprapure grade perchloric acid (Merck) were used for the preparation of all the electrolyte solutions. Carbon monoxide 4.7 N, (99.997, Messer Griesheim), nitrogen 5.0 N (Messer Griesheim) or cryogenic boil-off, and Argon 5.0 N (Messer Griesheim) were employed to deaerate the solutions and to maintain an air-free atmosphere over the electrolyte during the measurements. All potentials are given vs the Ag/AgCl electrode in saturated KCl solution. The working electrode was a Ru(0001) single crystal disk 7.5 mm in diameter and 2 mm thick (prepared by the Crystal Laboratory of the Fritz-Haber Institute in Berlin and oriented within 0.5°). As described previously,27 the single-crystal surface was freshly polished with 0.015 µm alumina (BDH), washed thoroughly with Millipore water and then immersed in Millipore water in an ultrasonic bath for several minutes prior to a rapid transfer into the spectro-electrochemical cell. The quality of the electrode surface was tested by cyclic voltammetry in the supporting electrolyte using the meniscus configuration. The voltammogram obtained for the freshly polished electrode was identical to that observed using the UHV-prepared Ru(0001) electrode,27,28 and was taken as confirmation that an ordered and clean surface had been obtained. CO adsorption was performed by dosing the gas at a constant potential of -100 mV vs Ag/AgCl. For this purpose, CO was directly bubbled into the 0.1 M HClO4 solution; the CO in solution was then removed by nitrogen sparging and/or a rapid exchange of electrolyte using N2-saturated base solution. Throughout this adsorption procedure and the subsequent

10.1021/jp002959e CCC: $19.00 © 2000 American Chemical Society Published on Web 11/22/2000

Electrooxidation of CO

J. Phys. Chem. B, Vol. 104, No. 50, 2000 12003 where R0 is the reference spectrum and R the spectra collected as a function of potential or time. These data manipulation results in spectra in which peaks pointing up, to +(R/R0), arise from the loss of absorbing species in R with respect to R0, and peaks pointing down, to -(R/R0), to the gain of absorbing species. 3. Results and Discussion

Figure 1. Schematic representation of the in-situ variable temperature FTIR spectro-electrochemical cell: (1) hemispherical CaF2 window, (2) retaining plate + bolts for window, (3) Teflon cushion, (4) sample compartment lid, (5) cell mounting plate, (6) magnetic seal, (7) power resistors (×4), (8) reflective working electrode, (9) Teflon cell body, (10) Teflon seal, (11) working electrode connection and thermocouple leads, (12) glass cell body, (13) cooling/heating water inlet to cell jacket, (14) counter electrode, (15) spectrometer sample compartment. The reference electrode and cell inlet/outlet ports are not shown for clarity.

electrolyte replacement, the potential was maintained at -100 mV. Lower coverage adlayers were achieved by partial electrooxidation of the initially formed saturated CO adlayer.27 The in-situ FTIR experiments were performed using a BioRad FTS-6000 spectrometer equipped with a Globar infrared source and a narrow-band MCT detector. The potentiostat was an Oxsys Micros Electrochemical Interface. The spectroelectrochemical cell, see Figure 1, was home-built and fitted with a hemispherical CaF2 window, (Alkor Technologies Co., St. Petersburg, Russia). The cell was mounted vertically on the lid of the sample compartment of the spectrometer, and was designed to allow electrolyte exchange under potential control.29 The cell was jacketed to allow careful control of the temperature of the electrolyte in the body of the cell. To ensure that this control extended to cover the electrolyte in the thin layer, and hence the temperature of the electrode, two modified versions of the plate employed to mount the cell onto the sample compartment of the spectrometer were produced. For temperatures below ambient, the plate (not shown) consisted of a hollow aluminum block fitted with inlet and outlet ports to allow the circulation of coolant from a Grant cooling/heating unit. For temperatures above ambient, the plate (see Figure 1) consisted of a solid monolith of stainless steel fitted with 4 × 50 Ω resistors, 50 W each, the current through which was controlled via home-built 30 V power supply and feedback loop, and a Eurotherm temperature controller. The temperature was monitored using a type K Ni/Cr + Ni/Al thermocouple (RS Components); the temperature of the Ru(0001) electrode was also monitored insitu using a second such thermocouple mounted to the rear of the electrode.13 Careful measurements at the rear of the electrode, and at the electrolyte/window interface confirmed that the temperature of the thin layer of electrolyte trapped between the Ru(0001) electrode and the CaF2 window agreed to within (1 °C of the preset value. The details of the reflective working electrode and the optical bench may be found elsewhere.13,27,30 The spectra presented below consisted of 100 co-added and averaged scans at 8 cm-1 resolution, ca. 16 s per scanset, unless stated otherwise, and are presented as

(R/R0) vs ν/cm-1

3.1. Cyclic Voltammetry Study. Figure 2a,b show cyclic voltammograms of the Ru(0001) electrode in 0.1 M HClO4 collected at 10 and 50 °C, respectively, both of which sets of data are similar to those observed at room temperature.27 At all three temperatures, two characteristic redox waves can be seen centered near -100 and +290 mV, respectively. A similar feature to that centered near -100 mV has been observed by Hubbard and co-workers31 and Ertl and co-workers,28 and has been attributed to the formation and stripping of surface hydride. However, as may be seen from Figure 2a,b, increasing the positive potential limit increases the peak cathodic and anodic currents of this wave. This dependence of the peak currents in the hydride region on the positive potential limit of the voltammogram suggests an appreciable overlap between the processes of surface oxidation and subsequent reduction with those of hydride formation and stripping. Adzic and coworkers32 have also observed two oxide stripping processes, one of which occurs at high overpotentials, at Ru(0001) electrodes having extended coherence lengths. At room temperature, ex-situ (emersion) LEED/RHEED and AES studies27 showed that a (2 × 2)-O adlayer was formed at potentials between -80 and 200 mV, i.e., before the second anodic wave in Figure 2,b. The steady increase in the anodic current from ca. 0 to +200 mV in Figure 2a may be attributed to a corresponding increase in the size of the (2 × 2)-O domains. The onset of the second wave near c. 225 mV coincided with the formation of (3 × 1)-O and (1 × 1)-O adlayers. The same work showed that, at room temperature, the (2 × 2)-O adlayer surface is not active toward the electrooxidation of COads, while the formation of the (3 × 1)-O and (1 × 1)-O adlayers activates the surface toward CO oxidation.27 It may be seen from Figures 2a,b that the small anodic wave corresponding to the formation of the (3 × 1)-O layer27 is shifted ca. 100 mV to lower potentials on increasing the temperature from 10 °C to 50 °C, onsets at 225 and 125 mV, respectively. Hence, given that the (3 × 1)-O surface is active for COads oxidation, the onset of this process would be expected to decrease as the temperature is increased. The small anodic wave corresponding to the formation and reorganization of the (2 × 2)-O layer27 also appears to show a similar shift to lower potentials as the temperature is increased, with the onset beneath the hydride stripping peak and terminating at +100 mV. The cyclic voltammograms in Figure 3a,b show the oxidative stripping of the preadsorbed CO at 10 and 50 °C, respectively. It can be seen from Figure 3a that, at 10 °C, there is very little anodic current at potentials below 400 mV, and only a small fraction of the COads is stripped during the first potential sweep up to 620 mV. In contrast, Figure 3b shows that, at 50 °C, appreciable current flows at potentials greater than 150 mV, even during the first anodic sweep. Clearly, increasing the temperature significantly enhances the reactivity of the Ru(0001) electrode surface toward COads oxidation. In situ electrochemical FTIR spectroscopy was employed in order to investigate further the effect of temperature upon the reactivity of the Ru(0001) surface toward the oxidation of adsorbed CO.

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Figure 2. Cyclic voltammograms of the 0.44 cm2 Ru(0001) electrode immersed in N2-saturated aqueous 0.1 M HClO4 solution at (a) 10 °C and (b) 50 °C. The numbers correspond to the number of cycles, increasing the positive potential limit with each new cycle. Sweep rate 50 mV s-1. The arrow indicates the onset potential for the formation of the (3 × 1)-O adlayer.

Figure 3. Cyclic voltammograms showing the oxidative stripping of the preadsorbed CO adlayer from the Ru(0001) electrode in N2-saturated aqueous 0.1 M HClO4 (CO-free) electrolyte at (a) 10 °C and (b) 50 °C. At each temperature, the CO was adsorbed from the CO-saturated electrolyte at -100 mV for 5 min, after which the solution was replaced with CO-free electrolyte. The numbers correspond to the number of cycles. Sweep rate 50 mV s-1.

3.2. In Situ FTIR Experiments. In-situ FTIR spectra were collected from the CO-saturated surface at 10, 25, and 50 °C. At each temperature, CO was adsorbed at -100 mV for 5 min, after which the electrolyte was replaced with CO-free solution while holding the potential at -100 mV, the potential was stepped to -200 mV and spectra collected at successively higher potentials from -200 to 1100 mV in intervals of 25 mV. Figures 4 and 5 show spectra collected as a function of potential at 10 and 50 °C, respectively. The spectra covering the CO2 absorption

region, 2250-2450 cm-1, were normalized to the spectrum collected at -200 mV, i.e., prior to the formation of CO2. The spectra covering the COads spectral region, 1725 cm-1 - 2125 cm-1, were normalized to the spectrum taken after holding at 1100 mV for 3 min at the end of the experiments in order to ensure the complete removal of adsorbed CO. As already noted in the previous report,27 in contrast to the COads/Pt(111) system, it has not proved possible to separate the charge corresponding to the oxidation of the adsorbed CO from

Electrooxidation of CO

Figure 4. In situ FTIR spectra collected from the Ru(0001) electrode at 10 °C during a potential step experiment after the adsorption of CO. The CO was preadsorbed at -100 mV, see text for details. The potential was then stepped up to +1100 mV in 25 mV increments, with further spectra collected at each step. The spectra showing the CO adsorption region were normalized to a spectrum taken after holding the potential at +1100 mV for 3 min at the end of the experiment, to ensure the electrode surface was free of adsorbed CO. The spectra showing the CO2 absorption were normalized to the first spectrum, collected at -200 mV. Some of the spectra collected are omitted for clarity.

the charge associated with oxide/hydroxide formation at Ru(0001). However, at room temperature, (1) Ertl and co-workers33 have shown that the presence of CO facilitates the formation of the (2 × 2)-O adlayer such that, at -100 mV, the maximum coverage of the surface by the (2 × 2)-O domains tends to 100%, corresponding to a 25% occupation of the Ru sites by O or OH; (2) the intensity of the (solution) CO2 gain feature near 2340 cm-1 observed on complete stripping of the COads layer at maximum coverage at 25 °C was found to be almost identical to that observed on complete oxidation of the maximum coverage COads layer at a Pt(111) electrode having the same geometric area, (where the coverage has been shown to be c. 0.75 from charge measurements7,27,34). In the latter experiment, the intensity of the CO2 feature was measured by stepping the potential directly from -100 to +1100 mV, to minimize any loss of CO2 from the thin layer by diffusion. Hence, it does not seem unreasonable to postulate that saturation of the Ru(0001) surface by adsorbed CO corresponds to a maximum coverage

J. Phys. Chem. B, Vol. 104, No. 50, 2000 12005

Figure 5. In situ FTIR spectra, (8 cm-1 resolution, 100 co-added and averaged scans, c. 16 s per scanset), collected from the Ru(0001) electrode during a potential step experiment after the adsorption of CO. Both the adsorption of CO and the subsequent FTIR spectral data collection were performed at 50 °C. Other conditions as for Figure 4.

of ca. 0.75 at 25 °C. Furthermore, it was found that the intensities of the CO2 feature measured in this way at 10 and 50 °C were within ( 5% of the intensity at 25 °C. This suggests that maximum COads coverage lies between ca. 0.65 and 0.75 over the temperature range 10-50 °C. A similar insensitivity of the initial coverage of COads at Pt to temperature below 50 °C has been reported previously by Korzeniewski and coworkers.11,12 Both linearly-bonded CO adsorbed on “on top” sites (COL, 2000-2045 cm-1), and 3-fold hollow CO coordination (COH, 1768-1805 cm-1)27,35 were observed at all three temperatures. To gain a greater insight into the effect of temperature on the adsorption and electro-oxidation of CO at Ru(0001), the band intensities and frequencies of both the COL and COH species, as well as the intensity of the solution CO2 feature near 2340 cm-1, at the two temperatures of 10 and 50 °C were plotted as a function of potential (see Figures 6 and 7). The data collected at 25 °C are omitted for clarity, and may be found in our previous paper.27 Assuming constant absorption coefficients, it may be seen from Figure 6a that high temperature favors the COL species while the low temperature favors COH; however, it should be noted that relative occupancy of the Ru(0001) surface by COL

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Figure 6. Plots of the integrated band intensities of the COL and COH features (a), and CO2 feature (b), in Figures 4 and 5 as a function of potential, (see text for details).

and COH cannot be deduced straightforwardly from the intensities of their bands, due to intensity stealing effects.1,35,36 It is clear from Figure 6b that the onset potential for CO2 formation decreases with increasing temperature, from ca. +225 mV at 10 °C, to +200 mV at 25 °C (not shown), and +125 mV at 50 °C, in agreement with the decrease in the onset potential for the formation of the (3 × 1)-O adlayer observed in the cyclic voltammetry experiments shown in Figure 2a,b. The removal of both forms of COads at 50 °C is complete by 750 mV, whereas residual COL is still present at 1100 mV for at least 2 min in the experiments carried out at 10 and 25 °C.27 3.2.1 The FTIR Experiments at 10 °C. The similarity of the data collected at 10 and 25 °C may also be observed in terms of the potential dependence of the intensities and frequencies of the COL and COH features, and the intensity of the CO2 band plotted in Figures 6 and 7, which fall into 4 distinct potential regions, as observed at 25 °C: region I, -200 to +200 mV; region II, +200 to +450 mV; region III, +450 to +725 mV and region IV, +725 to +1100 mV. The corresponding regions observed at 25 °C: region I, -200 to +175 mV; region II, +175 to +425 mV; region III, +425 to +600 mV and region IV, +600 to +1100 mV. In region I, the intensities of the COL and COH features are potential-independent and no CO2 evolution is observed; this was attributed in the experiments at 25 °C27 to the inactivity of the (2 × 2)-O surface toward the electro-oxidation of COads. In region II, the oxidation of the CO adlayer commences with the concomitant appearance and growth of the CO2 feature near 2340 cm-1; the activity of the surface in this region was attributed to the formation of the more active (3 × 1)-O oxide phase.27 The oxidation of the CO adlayer accelerates in region III, with the slopes of the COL, COH and CO2 intensity vs potential plots (|∂(R/R0)/∂E| for COL, COH, and CO2, see Figure 6) increasing significantly with respect to those observed in region II; in addition, the COH species are completely stripped by ca. +650 mV. This increased activity was associated with the appearance of the (1 × 1)-O oxide phase.27 However, as the coverage of this phase increases (region IV), the residual

Figure 7. Plots of the frequencies of the (a) COL feature and (b) the COH feature in Figures 4 and 5 as a function of potential. (See text for details.)

mixed COL in (2 × 2)-O domains are compressed into islands of relatively high local CO coverage. The coadsorbed (2 × 2) oxide is inactive toward the oxidation of the COL, and so further oxidation can only occur at the perimeter of the islands, adjacent to the active (1 × 1)-O phase. Hence |∂(R/R0)/∂E| for COL decreases, as does |∂(R/R0)/∂E| for CO2, and the residual COL persists even at potentials as high as +1100 mV. The rate of formation of CO2 is now less than the rate of its diffusion out of the thin layer, and hence out of the optical path, so the intensity of the CO2 feature declines in this potential region.7,27 Turning now to the behavior of the COL and COH band frequencies. The simplest possible model assumes that the frequency of the CO feature is a function of coverage and potential according to27,37

(dν/dE) ) (∂ν/∂E)θ + (∂ν/∂θ)E(dθ/dE)

(1)

where θ is the coverage, E the potential, and ν the frequency in cm-1. In essence, (∂ν/∂E)θ may be considered as the contribution to the frequency shift from the Electrochemical Stark Effect,37 or electron back-donation from the metal into the 2π* orbitals of the CO group,38-40 and is positive as E increases; (∂ν/∂θ)E is the contribution from dipole-dipole coupling,36 which is positive as θ increases, and dθ/dE is the contribution from the coverage which may be positive or negative as E increases.37 Thus, in region I in Figure 7a, the region of constant coverage (dν/dE) for the COL feature is constant at ca. 41 cm-1 V-1, which is very close to the value of ca. 39 cm-1 V-1 observed at 25 °C; the latter was interpreted in terms of the domination of the (∂ν/∂E)θ term in eq 1, the magnitude of which suggests a high coverage and a strong chemisorption.27,41 At potentials between 225 and 575 mV, where the slow oxidation of the adsorbates to CO2 commences, the first term

Electrooxidation of CO in eq 1 still dominates, but dθ/dE is relatively small and negative, the net effect is to reduce dνCO(L)/dE slightly to 33 cm-1 V-1. In the third potential region, between 575 and 700 mV, there is a significant increase in the rate of CO oxidation, dνCO(L)/dE falls sharply to 5 cm-1 V-1. It is clear that significant oxidation of the CO adlayer takes place in this region, and that the (negative) coverage-dependent terms in eq 1 dominate dνCO(L)/dE. In region IV, +700 to +1100 mV, dνCO(L)/dE returns to that observed in region I, i.e., 41 cm-1 V-1. In the room-temperature experiments,27 this was taken as evidence of the compression of the residual mixed COL in (2 × 2)-O domains into islands of high local COL coverage. 3.2.2. The FTIR Experiments at 50 °C. At first sight, the data collected at 50 °C looks similar to those at 10 and 25 °C; the plot of the intensity of the COL band as a function of potential (see Figure 6a) shows four regions of behavior, in the first of which the COL and COH bands show a small increase and very small decrease, respectively, in intensity. This may be due to some interchange between the forms of adsorbed CO; however, the fact that the effect is somewhat more marked in terms of the COL may also reflect a small reorientation of the COL, as postulated by Ianniello et al.42 and Lin and co-workers.41 Thus, at low potentials some CO molecules may lie at an angle a little away from the surface normal; as the electric field increases these then orient themselves in the direction of the electric field perpendicular to the surface. In this first region, from -200 mV up to +100 mV, i.e., the region of constant CO coverage (before CO oxidation), the frequency of the COL feature is generally slightly higher than that at 10 °C, which also shows a linear potential dependent increase, dνCO(L)/dE, of ca. 35 cm-1 V-1. Increasing the temperature from 10 °C to 25 °C and 50 °C results in a steady decrease in dνCO(L)/dE in this region from 41 to 39 and 35 cm-1 V-1, respectively. However, closer inspection of the data at 50 °C (see plots in Figures 6 and 7) shows marked differences to those at 10 and 25 °C. First, the intensity of the CO2 feature in Figure 6b shows three linear regions over the range of increasing CO2 band intensity: +125 to +225 mV, +225 to +400 mV, and +400 to +500 mV. The corresponding region of the COL and COH plots, see Figure 6a, also shows three linear regions. The first CO2 region, in complete contrast to the data at lower temperature, shows the highest slope, in agreement with the cyclic voltammetry data in Figure 3b, which show a sharp increase in oxidative current at potentials >125 mV, in contrast to the data at 10 °C, see Figure 3a. In the second region the rates of COads oxidation and CO2 formation decrease and then they increase again in the third region. After this region, i.e., at potentials above +500 mV, the rate at which CO2 is produced is clearly lower than the rate at which it diffuses out of the thin layer, this rate also clearly increasing with temperature. The second marked difference between the data at 10, 25, and 50 °C, may be seen in the plot of the frequency of the COL vs potential, see Figure 7a, which shows six potential regions: the first (-200 mV to +100 mV) shows a potential independent slope of 35 cm-1 V-1. At potentials between 125 and 225 mV, a relatively rapid oxidation of the CO adsorbates to CO2 takes place and dνCO(L)/dE drops from 35 to ca. 12 cm-1 V-1. At potentials between 225 and 350 mV, dνCO(L)/dE increases to ca. 30 cm-1 V-1, but it decreases again to ca. 20 cm-1 V-1 at potentials between 350 and 450 mV. At potentials between 450 and 550 mV, νCO(L) drops sharply with a negative dνCO(L)/dE of ca. -50 cm-1 V-1; it is clear (see Figure 6) that rapid oxidation of the CO adlayer takes place in this region, and that

J. Phys. Chem. B, Vol. 104, No. 50, 2000 12007 the (negative) coverage-dependent term in eq 1 dominates dνCO(L)/dE. At potentials between 550 mV and 750 mV, the oxidative stripping of COL continues, but at a reduced rate. The frequency of the COL feature levels off, and the COL has been completely stripped off by 750 mV. The potential dependence of the COH frequency is shown in Figure 7b. It can be seen that while there are four distinct potential regions at both 10 and 50 °C, significant narrower potential ranges are observed at 50 °C than those at 10 °C. At 10 °C (50 °C), in the ranges -200 to 0 mV (-50 mV) and +200 mV (+50 mV) to +600 mV (+250 mV), dνCO(H)/dE ) 31 ( 2 cm-1 V-1 (36 ( 2 cm-1 V-1), separated by a potential region 0 mV (-50 mV) to +200 mV (+50 mV) in which dνCO(H)/dE ) 73 ( 2 cm-1 V-1(75 ( 2 cm-1 V-1). At both 10 and 50 °C, the potential region showing the highest slope is that, immediately before the onset of CO oxidation, this phenomenon has been observed previously,41 and was associated with the formation of surface oxide/hydroxide and/or some subsequent reorganization of CO adlayer.27,41 It would appear that the frequency of the COH species is significantly more sensitive to the structure of the surface oxide/hydroxide on the Ru(0001) surface than is that of the COL adsorbate. At 10 °C, the blue shift of νCO(H) continues up to +600 mV and COH was detected up to +700 mV. In complete contrast, at 50 °C, the blue shift of νCO(H) continues only up to +250 mV and then νCO(H) levels off up to +450 mV, at potentials above which, the COH had been completely stripped; these data suggest again that the adsorbed CO species are present as looser adlayer structures at 50 °C than at 10 °C. This postulate is in agreement with the work of Korzeniewski and co-workers on CO adsorption at Pt.12 In addition, at potentials below the onset of CO oxidation, the frequencies of the COL and COH absorptions are higher (corresponding to lower metal-2π* back-donation)38-40 at 50 °C than at 10 °C, suggesting weaker bonding to the Ru surface at these potentials. 3.3. The Rate of Oxidation of the CO Adlayer at Constant Potential. To gain an insight into the effect of temperature on the rate of oxidation of the COL adlayer at constant potential, time-dependent experiments were performed at 10, 25, and 50 °C. At each temperature, the Ru(0001) surface was saturated with CO at -100 mV, as described above, after which the potential was stepped to +550 mV, and spectra (16 co-added and averaged scans, 3 s per scanset) were collected as a function of time. Figure 8 shows representative spectra obtained at 10 and 50 °C, and Figure 9a,b show plots of the intensities of the COL and CO2 features as a function of time at all three temperatures. It is clear from Figures 8 and 9 that both the rate of oxidation of COL, and the rate of diffusion of CO2 out of the thin layer are significantly accelerated on increasing the temperature from 10 to 50 °C. It is also clear from the plots in Figure 9a that the oxidation of COL is initially rapid, up to ca. 50 s, and is followed by a significantly slower process, in agreement with the postulated formation of compact COL islands as the oxidation proceeds due to an increase in surface oxide formation. In addition, whereas the frequency of the COL remains largely unchanged up to ca. 50% of its saturated coverage during the oxidation process at 10 °C, it shows a steady decline at 50 °C (see Figure 10). This suggests again that a looser, more open COads structure is present at 50 °C than is observed at 10 °C on the Ru(0001) electrode. 3.4. Surface Relaxation and Compression of the Submonolayer CO Adlayer. Previous room-temperature experiments27 showed relaxation and compression processes took place

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cm-1

Figure 8. Time-dependent in situ FTIR spectra (8 resolution, 16 co-added, and averaged scans, ca. 3 s per scanset), collected at (a) 10 °C and (b) 50 °C from the Ru(0001) electrode showing the oxidative stripping of CO at +550 mV vs. Ag/AgCl in (CO-free) aqueous 0.1 M HClO4; all the spectra were normalized to the reference taken at +1100 mV at the end of the experiment. The CO was preadsorbed at -100 mV, see text for details, after which the solution was replaced with CO-free electrolyte, the potential stepped to 550 mV, and the first spectrum taken; further spectra were then collected at the same potential as a function of time. The COL spectra were all normalized using the spectrum taken after 3 min at +1100 mV at the end of the experiment as reference; the CO2 spectra employed the spectrum collected at -200 mV at the beginning of the experiment as reference.

after freeing up part of the surface by partial oxidation of the CO adlayer. It was now decided to investigate how these phenomena would be influenced by temperature. Figure 11 shows time-dependent spectra collected at -200 mV and at 10, 25, and 50 °C after the saturated CO adlayer had been partially oxidized at +450 mV and the potential stepped back to -200 mV. The potential was held at 450 mV for sufficient time to give the CO coverages given on the figures. The coverages of the submonolayer CO were estimated from the CO2 intensities as described previously,27 and are expressed in terms of θmax, the maximum coverage (θmax ) 0.65-0.75). Turning first to the data at 25 °C (see Figure 11)). At a CO coverage of 0.7θmax (see region I), the first spectrum collected immediately after stepping down from +450 mV shows a feature near 1992 cm-1, corresponding to COL species at this submonolayer coverage.27 On holding the potential at -200 mV for 30 s, this feature starts to diminish in intensity, while a new

Lin et al.

Figure 9. Plots of the integrated band intensities of the (a) COL and (b) CO2 features observed in the experiments in Figure 8 as a function of time at 10, 25, and 50 °C.

Figure 10. Plots of the frequency of the COL feature observed in the 10 °C and 50 °C experiments in Figure 8 as a function of its band intensity.

feature appears near 1974 cm-1. On holding the potential at -200 mV up to 180 s, the intensity of the latter grows at the expense of that of the former, with no further change at longer times. We have previously assigned these features, on the basis of the work of Hoffmann and co-workers,16 to COL species

Electrooxidation of CO

Figure 11. Time-dependent in-situ FTIR spectra, collected from the Ru(0001) electrode having (total) COads coverages of (I) 0.7θmax and (II) (a) 0.3θmax; (b) 0.4θmax and (c) 0.5θmax at 10, 25, and 50 °C, respectively. At each temperature, the CO was preadsorbed at -100 mV in CO-saturated aqueous 0.1 M HClO4 for 5 min, after which the CO was removed from solution by sparging with nitrogen gas for 25 min, with the electrode held at -100 mV. The potential was stepped first to -200 mV, then to +100 mV and two spectra collected to ensure no CO2 had been evolved. The potential was then increased to +450 mV in a single step, and held for sufficient time to partially oxidize the CO adlayer to give the CO coverages as indicated in the figures, (see text for details), after which it was stepped back to -200 mV; further spectra were then collected at the times indicated after the step to -200 mV. At the end of the experiment, the potential was stepped to +1100 mV for 3 min, to provide a CO-free surface, and the reference spectrum taken.

coadsorbed on (2 × 2)-O phases (β-sites, 1994 cm-1); and to COL species adsorbed on oxide-free Ru(0001) domains (R-sites, 1974 cm-1), in other words, to COL species that have migrated away from the compact [COL in (2 × 2)-O] islands. The growth of the R-band at the expense of the β-band simply reflects the “freeing up” of the surface due to partial oxidation of the CO adlayer via the Langmuir-Hinshelwood mechanism43 at 450 mV, followed by further reduction of oxide at -200 mV and the concomitant migration of COL. Not surprisingly, as the coverage of CO is decreased, the rate and extent of the migration increase until at θCO ) 0.4 θmax, where migration is complete after 4 s at -200 mV and no islands remain. At 50 °C (see Figure 11c), a similar migration process is observed, albeit with a lower β band frequency, (1987 cm-1 cf. 1994 cm-1 at 25 °C), and a decreased splitting between the R and β bands, (15 cm-1 cf. 20 cm-1 at 25 °C). As may be expected, the rate of migration increases with temperature; thus, for θ ) 0.7θmax at 10 °C, there is no observable migration up to ca. 180 s; at 25

J. Phys. Chem. B, Vol. 104, No. 50, 2000 12009 °C, migration is observable after ca. 20 s, and at 50 °C migration is clearly observed within ca. 10 s. These data support the above postulated existence of islands, but with looser, more open structure at 50 °C. Interestingly, at 10 °C (see Figure 11a), no migration of COL was observed at θCO of 0.7θmax and 0.5θmax up to 180 s, with only the β band observed, corresponding to compact islands of [COL in (2 × 2)-O].16,27 As expected, when the coverage of CO was decreased to ca. 0.3θmax, migration did take place and was complete after c. 4 s at -200 mV, yielding a broad R band near 1967 cm-1, corresponding to COL islands on the oxidefree Ru(0001) domain.16,27 Surprisingly, on holding the potential at -200 mV for 40 s, this R band starts to diminish in intensity, while a β band appears near 1990 cm-1. On holding the potential at -200 mV up to 180 s, the intensity of the β band was observed to grow at the expense of the R band, with little further change at longer times. The frequency of the R band observed at 10 °C and a coverage of 0.3θmax, 1967 cm-1, is higher than the value of 1960 cm-1 observed at 25 °C and 50 °C at higher coverages of 0.4θmax and 0.5θmax. These results suggest that, at -200 mV and a COads coverage of 0.3θmax, submonolayer COL species show a higher mean aggregation size at 10 °C than at 25 °C or 50 °C on the oxide-free Ru(0001) domains. The higher aggregation of COL on the oxide-free Ru(0001) domains, and the [COL in (2 × 2)-O] island structures as well, would appear to account for the very slow oxidation of the CO adlayer at 10 °C. To try and gain more insight into the submonolayer CO structure at 10 °C, potential-dependent FTIR spectra were recorded at 0.3θmax. Figure 12 shows representative spectra taken at potentials from -200 to +1100 mV. On holding the potential at -200 mV for 180s, it is clear from Figure 12 that there is some small redistribution of intensity between the R- and β-bands, with the latter growing slightly at the expense of the former. However, on stepping the potential up to +200 mV there is a significant increase in intensity of the β-band which cannot be accounted for in terms of a concomitant decrease in the R-band, or additional CO adsorption, as there was no free CO present in the experiment. The most likely interpretation of these data is the reorientation of the COL species driven both by the increasing electric field41,42 and surface oxide formation.27 This combination of drivers forces the compression of the COL species into islands, a postulate supported by the observed CO2 evolution onset of +225 mV, which is the same as that observed for the maximum coverage COads layer suggesting that oxidation only takes place at the perimeter of the CO islands.27 The fact that two bands are observed at 1100 mV in Figure 12 suggests that the COL species can still exist on at least two domains even at such high potentials, although it is not clear that the domains are necessarily the same as those present at lower potentials. Thus, ex situ HREED and Auger data have shown that, holding the potential of the Ru(0001) electrode at 1100 mV for 2 min results in the formation of RuO2 clusters, while part of the surface still remains flat.27,28 While Over et al.44 observed that CO adsorbed on a RuO2(110) overlayer on Ru(0001) did so at coordinatively unsaturated Ru sites under UHV conditions. 4. Concluding Remarks In situ electrochemical FTIR spectroscopy has been employed to study the effect of temperature on the adsorption, electrooxidation, and surface migration of CO at the Ru(0001) electrode

12010 J. Phys. Chem. B, Vol. 104, No. 50, 2000

Lin et al. structures, which are more easily electro-oxidized, yielding a lower onset potential of 125 mV, i.e., 100 mV lower than that observed at 10 °C, with all the adsorbed CO oxidized at 750 mV. The surface relaxation and compression processes of the COL adlayer at low coverages were also found to be temperature-, as well as potential- and coverage-, dependent. At high potentials, the growing oxide/hydroxide causes the COL to compress into tightly packed islands. At lower potentials, the oxide/hydroxide layer is partially stripped, creating space on the surface for COL expansion. It was also found that, while higher temperatures facilitate the relaxation process, at 10 °C a compression process was observed with the COL tending to exist as compact islands. In summary, higher temperatures not only facilitate active surface oxide formation and weaken the CO adsorption, they also facilitate the relaxation of the CO adlayer. All of which contributes to the observed higher surface reactivity of the Ru(0001) toward CO electrooxidation at higher temperature. Future work will focus upon the addition of Pt to the welldefined Ru(0001) surface, and will investigate the effect of Pt on the electrooxidation of small organic molecules such as methanol, formic acid, and CO as a function of Pt coverage, potential and temperature. Acknowledgment. Financial support from the EPSRC is gratefully acknowledged.

Figure 12. Time- and potential-dependent in-situ FTIR spectra, collected from the Ru(0001) electrode having a coverage of adsorbed CO of 0.3θmax at 10 °C. The CO was adsorbed by holding the potential of the Ru(0001) electrode at -100 mV in CO-saturated aqueous 0.1 M HClO4 for 5 min, after which the CO was removed from solution as described in Figure 11. The potential was stepped first to -200 mV, and then to +100 mV and two spectra collected to ensure no CO2 had been evolved. The potential was then increased to +450 mV in a single step, and held for ca. 10 min to partially oxidize the CO adlayer, after which it was stepped back to -200 mV. After 4 s, the first spectrum was collected, and then several spectra taken at the same potential over a further 3 min. The potential was then increased in 50 mV increments up to +1100 mV, and spectra collected at each step (only a representative number of the spectra are included in the figure, for clarity). The potential was then held at +1100 mV for 3 min, during which time several further spectra were collected, to provide a CO-free surface, and the reference spectrum taken.

in 0.1 M HClO4 solution over the range 10 °C to 50 °C. The results so obtained clearly show that increasing the temperature from ambient to that approaching those typical of lowtemperature fuel cells can have a significant effect upon the electrocatalytic process. At temperatures between 10 and 50 °C, adsorption of CO at the Ru(0001) electrode at -100 mV vs Ag/AgCl for 5 min in CO-saturated 0.1 M HClO4 results in a saturated CO adlayer with a total CO coverage between 0.65 and 0.75. While both linear (COL) and 3-fold-hollow (COH) binding CO adsorbates were detected on the Ru(0001) electrode at these temperatures, significant differences in terms of the structure and reactivity of the CO adlayer were observed. At 10 °C, the in situ FTIR data show that the adsorbed CO species remain in rather compact as the CO oxidation proceeds up to 1100 mV, suggesting that the oxidation occurs only at the boundaries between the COad and active surface oxide/hydroxide domains. The rate of the COad oxidation reaction decreases as the reaction proceeds as a result of the decrease in the perimeter of the CO islands. In contrast, the IR data at 50 °C show that adsorbed CO species are present as relatively looser and weaker adlayer

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