In-situ FTIR Spectroscopic Studies of the Adsorption and Oxidation of Small Organic Molecules at the Ru(0001) Electrode Under Various Conditions

In-situ Spectroscopic Studies of Adsorption at the Electrode and Electrocatalysis S.-G. Sun, P.A. Christensen and A. Wieckowski (Editors) r 2007 Elsevier B.V. All rights reserved

Chapter 4

In-situ FTIR Spectroscopic Studies of the Adsorption and Oxidation of Small Organic Molecules at the Ru(0001) Electrode Under Various Conditions Wen-Feng Lina, Paul A. Christensena, Jia-Mei Jina, Andrew Hamnettb a

School of Chemical Engineering and Advanced Materials, Bedson Building, University of Newcastle upon Tyne, Newcastle upon Tyne NE1 7RU, United Kingdom b The University of Strathclyde, McCance Building, John Anderson Campus, Glasgow G1 1XQ, United Kingdom

1. Introduction The adsorption and electrocatalytic oxidation of small organic molecules at platinum and related electrode materials has been the subject of numerous studies over the past three decades as a result of their relatively high energy density and hence potential application as fuels in fuel cells [1–4]. These reactions are complicated, as several elementary steps are possible en route to carbon dioxide, and the involvement of the electrode surface gives rise to geometric and electronic effects that are still relatively poorly understood and difficult to probe. Furthermore, in the reaction of surface species, such as the oxidation of adsorbed CO with surface oxide and the concomitant formation of carbon dioxide, the spatial arrangement of adsorbates can have a strong influence on the reaction rate [3,4]. The early studies of the electrochemistry of such small organic molecules in the 1960s and 1970s were essentially limited by the classical current/voltage/ time techniques then available; although providing a wealth of kinetic data [1,2], they lacked the ability to provide essential molecular information. Major 99

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breakthroughs in the understanding of mechanism at the electrode/electrolyte interface were triggered in particular by the development, at the beginning of the 1980s, of in-situ electrochemical infrared (IR) techniques [5–7], and of the evolution of straightforward experimental protocols for the production of ordered and well-defined single crystal electrodes of the Pt-group metals [8,9]. Since then, many in-situ IR studies have been reported on single crystal surfaces, generally using in-situ Fourier transform infrared (FTIR) spectroscopy [10–15] and elucidating the role of key adsorbates, such as CO or other carbonaceous fragments in electrocatalytically relevant reactions. CO can be detected on electrodes down to a few percent of a monolayer with FTIR [3,10–15], and the positions and intensities of the vibrational bands of the various adsorbed CO species provide information about the adlayer structure and interfacial environment. The overwhelming majority of in-situ FTIR spectroelectrochemical measurements to date have been carried out at room temperature, due to the fact that effective control of the temperature of an in-situ electrochemical spectroscopic system is not straightforward. 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, and a significant effect of temperature on surface electrochemistry has been observed [14–22]. Pt and Ru remain central to electrocatalysis in fuel cell applications [23,24]; in the case of methanol and CO electrocatalysis, the most active catalyst is binary PtRu [4,25,26]. Ruthenium is critical to the activity of this binary catalyst; although itself essentially inactive toward the oxidation of methanol under the conditions extant in the relatively low temperature direct methanol fuel cell (DMFC) [4], it is believed to provide oxygen-containing species at a significantly lower potential (ca. 300 mV more negative) than platinum and thus promote CO oxidation [27–30]. An electronic effect has also been postulated [31,32]. In fact, the details of the promoting effect of Ru remain essentially unclear, despite numerous studies; thus a variety of binary PtRu materials have been characterized using spectroscopic and electrochemical methods, and assessed in terms of their electrocatalytic activity toward the oxidation of small organic molecules such as CO, formic acid and methanol. These catalysts have been employed in the form of ruthenium electrodeposits on polycrystalline platinum [27,30,32,33], PtRu alloy [34–37], PtRu co-deposits [30,32,33,38], as well as supported PtRu catalysts on carbon [39–41], conducting polymers and membranes [42–44]. Some alternative means of preparation of the PtRu electrodes have also been proposed [45–47]. Recent studies of Ru deposits on Pt single crystal electrodes have been carried out by a variety of in-situ and ex situ techniques [31,48–53], and have provided new insights into surface structure/ reactivity effects. Given the importance of the proposed Ru-O species in the electrocatalytic oxidation of methanol and CO, it is surprising that attention only turned to the

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investigation of Ru electrodes having well-defined surfaces in ca. 1999. These studies focused on the application of electrochemical techniques, along with in-situ FTIR, STM, x-ray scattering and ex situ LEED/RHEED and Auger studies [13–15,21,22,54–59] on the electrochemical behavior of the Ru(0001) single crystal surface, and the surface electrochemistry of CO at the Ru(0001) electrode. Most recently, some work on the modification of the Ru(0001) by Pt deposits and the reactivity of such surfaces toward CO oxidation have been reported [60–62]. The effect of temperature on the electrooxidation of CO, formaldehyde, formic acid and methanol on the Ru(0001) has also been studied in Newcastle using in-situ FTIR [13–15,21,22,63]. A key result was the realization that the frequency and intensity of the IR absorption of adsorbed CO was a highly effective probe of the surface oxide structure of the Ru(0001) surface. These initial electrochemical results on Ru(0001) not only supported data obtained at the solid/gas interface under ultrahigh vacuum (UHV) or ‘‘real’’ conditions near atmospheric pressure [64–68], but also provided invaluable information that facilitated the elucidation of the role of ruthenium oxides in CO electrooxidation; such data are essential if further progress is to be realized in fuel cell electrocatalysis, where a major challenge of finding more effective and CO-tolerant anode catalysts remains. In this chapter, we report our studies on the adsorption and oxidation of small organic molecules at the Ru(0001) electrode under various conditions, the data so obtained are compared to those from the gas/solid interface and discussed and interpreted in terms of the surface chemistry of the Ru(0001) electrode and the implications of these data with respect to catalytic reaction mechanisms are discussed.

2. Experimental The electrolyte solutions were prepared using Millipore water (>18 MO cm), suprapure grade perchloric acid (Merck) and analytical grade (Aldrich) chemicals; 0.1 M HClO4 or 0.05 M H2SO4 was employed as supporting electrolyte. Nitrogen gas from a cryogenic boil-off was employed to de-aerate 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 disc 7.5 mm in diameter and 2 mm thick, (prepared by the Crystal Laboratory of the Fritz-Haber Institute in Berlin and oriented within 0.51). As described previously [13–15,63], the single crystal surface was freshly polished with 0.015 mm alumina (BDH), washed thoroughly with Millipore water and then immersed in Millipore water

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in an ultrasonic bath for several minutes prior to a rapid transfer into the spectroelectrochemical cell. The quality of the electrode surface was tested by cyclic voltammetry in the supporting electrolyte using the hanging meniscus configuration. The voltammogram obtained for the freshly polished electrode was identical to that observed using the UHV-prepared Ru(0001) electrode [13,57,60,69], 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
200 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 electrolyte replacement, the potential was maintained at –200 mV. Lower coverage adlayers were achieved by partial electrooxidation of the initially formed saturated CO adlayer [13]. The in-situ FTIR experiments were performed using a BioRad FTS-6000 spectrometer equipped with a Globar infrared source and a narrow-band mercury–cadmium–telluride (MCT) detector. The potentiostat was a PAR 263A, an Oxsys Micros electrochemical interface or a Sycopel AEW2 electrochemical interface. The spectroelectrochemical cell (see Fig. 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 [70]. 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 Fig. 1) consisted of a solid monolith of stainless steel fitted with 4  50 O 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 in-situ using a second such thermocouple mounted to the rear of the electrode [16]. 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 711C of the preset value. The details of the reflective working electrode and the optical bench may be found elsewhere [13,16,41].

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Fig. 1. Schematic representation of the in-situ variable temperature FTIR spectroelectrochemical 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.

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 ðR=R0 Þ vs: n cm
1 where R0 is the reference spectrum and R the spectra collected as a function of potential or time. This 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 3.1. Surface structure and electrochemical characterization of the Ru(0001) electrode in perchloric acid solution Figure 2 shows cyclic voltammograms of the freshly polished Ru(0001) electrode in 0.1 M HClO4 solution at the room temperature of 201C. The voltammograms

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Current /µA cm-2

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(2 x 2)-O(H)

(1 x 1)-O(H)

RuO2(100)

50 0 -50

-100

-200

0

200 400 600 800 1000 1200 Potential / mV vs. Ag|AgCl

Fig. 2. Cyclic voltammograms of the freshly prepared Ru(0001) electrode (0.44 cm2) immersed in N2-saturated aqueous 0.1 M HClO4 solution at 251C. Sweep rate 50 mV s
1. The surface hydroxide/oxide adlayers formed are indicated on the plot.

are identical to those observed using the UHV-prepared Ru(0001) electrode [56,60,69], and thus it was taken as confirmation that a clean and ordered surface had been obtained. The characteristic redox wave centered near
100 mV may be attributed to the formation and stripping of surface oxide species, possibly hydroxide formation/reduction [56,60,69], the cathodic wave overlapping with hydride formation [13,56,60,69]. Previous emersion studies using ex situ electron diffraction and Auger electron spectroscopy [13,54,56] showed that a well-defined and ordered oxygen-containing adlayer (2  2) – O(H)1 is present at potentials between
200 and 200 mV. The anodic feature between ca. 0 mV and +200 mV in Fig. 2 may be attributed to the increase in the size of the (2  2) – O(H) domains [13–15,56,63]. The onset of the second characteristic wave near ca. 200 mV coincides with the formation of a (1  1)–O(H) adlayer [56,63] with the increase in the anodic current between ca. 500 mV and ca. 1050 mV being attributed to a corresponding increase in the size of the (1  1) – O(H) domains[13,54,56,63]. At potentials above 1050 mV, the sharp increase of the anodic current was found to coincide with the epitaxial growth of RuO2 with its (100)-plane parallel to the Ru(0001) surface [13,54]. Figure 3 shows schematic representations of the (2  2) – O(H) and (1  1) – O(H) adlayers on the Ru(0001) substrate, as well as the RuO2(100) surface. 1

The exact natures of these adlayers remain controversial with (2  2)
O and (2  2)
OH both being postulated [13–15,21,22,54–63,69,71]; hence our designation of the adlayers as
O(H).

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O(H)

O(H)

Ru

(a) (2 x 2)-O(H) adlayer on Ru(0001)

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Ru

(b) (1 x 1)-O(H) adlayer on Ru(0001)

(c) RuO2 (100) surface

Fig. 3. (a) Schematic representations of the (2  2) – O(H); (b) (1  1) – O(H) (b) adlayers on the Ru(0001) substrate and (c) the RuO2 (100) surface.

3.2. Reactivity of the Ru(0001) electrode toward the adsorption and electrooxidation of CO in perchloric acid solution as a function of temperature The structure and reactivity of the Ru(0001) electrode toward the adsorption and electrooxidation of CO in 0.1 M HClO4 solution was studied by cyclic voltammetry (CV) and in-situ FTIRS at 10, 25 and 501C. At each temperature, CO adsorption was performed by dosing the gas at a constant potential of –200 mV for 5 min after which the CO in the solution was removed by nitrogen sparging for 20 min.The CO adsorbate stripping CVs [13,14,56] show that the electrooxidation of the COads2 at Ru(0001) took place relatively slowly at 10 and 251C (i.e. the adsorbed CO was not completely removed even after five potential sweeps at 50 mV s
1 up to 650 mV [13,14,56]), while a significant increase in the rate of the CO oxidation was obtained at 501C [14]. In order to probe the oxidation process more closely, in-situ FTIR spectra were collected from the CO-saturated surface at successively higher potentials from
200 mV to 1100 mV in intervals of 25 mV at all three temperatures. Figures 4a–c show representative spectra collected as a function of potential at the three temperatures studied. The spectra covering the CO2 absorption region (not shown, see Refs. [14,72]), 2250 cm
1 to 2450 cm
1, were normalized 2

COads is employed to represent all forms of CO adsorbed at the electrode surface.

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1100 mV

800 mV 1000 mV 750 mV

1000 mV 800 mV 900 mV

650 mV

700 mV

500 mV

700 mV

600 mV

500 mV

600 mV

450 mV 400 mV

500 mV

400 mV 200 mV

400 mV

250 mV 150 mV 200 mV

200 mV

100 mV 150 mV

100 mV

0 mV 0 mV

0 mV -100 mV

-100 mV

-100 mV -200 mV

-200 mV

COH

R/R0 COL =0.5%

2100

2000

-200 mV

1900

Wavenumber (a) 10 °C

1800

R/R0 =0.5%

2100

COH COH

COL

2000

1900

Wavenumber (b) 25 °C

1800

R/R0 =0.5%

2100

COL

2000

1900

1800

Wavenumber (c) 50 °C

Fig. 4. In-situ FTIR spectra (8 cm
1 resolution, 100 co-added and averaged scans, ca. 16 s per scanset), collected at (a) 101C, (b) 251C and (c) 501C from the Ru(0001) electrode during a potential step experiment after the adsorption of CO. At each temperature, the CO was preadsorbed at
200 mV for 5 min, after which the solution was sparged with N2 (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 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. Some of the spectra collected are omitted for the sake of clarity.

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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 to 2125 cm
1, were normalized to the spectrum taken after holding at 1100 mV for 3 min at the end of the experiment in order to ensure the complete removal of adsorbed CO. In contrast to the COads/Pt(111) system, for the COads/Ru(0001) system, it has not proved possible to separate the charge corresponding to the oxidation of the adsorbed CO from the charge associated with oxide/hydroxide formation at Ru(0001). However, at room temperature, (1) Ertl and co-workers [56] have shown that the presence of CO facilitates the formation of the (2  2) – O(H) adlayer such that, at
200 mV, the maximum coverage of the surface by the (2  2) – O(H) domains tends to 100%, corresponding to a 25% occupation of the Ru sites by O(H); (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 251C 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 ca. 0.75 from charge measurements [31,73]). In the latter experiment, the intensity of the CO2 feature was measured by stepping the potential directly from
200 mV to +1100 mV, in order to minimise 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 of ca. 0.75 at 251C. A similar value has also been obtained for CO adsorption on Pd(111) electrode [74]. Furthermore, it was found that the intensities of the CO2 feature measured in this way at 10 and 501C were within 75% of the intensity at 251C. This suggests that maximum COads coverage lies between ca. 0.65 and 0.75 over the temperature range 10–501C. A similar insensitivity of the initial coverage of COads at Pt to temperature below 501C has been reported previously by Korzeniewski and co-workers [18,20]. Both linearly bonded CO adsorbed on ‘‘on top’’ sites (COL, 2000–2045 cm
1) and threefold hollow CO coordination (COH, 1768–1805 cm
1) [13,14,75] were observed at all three temperatures. To gain a greater insight into the adsorption and electrooxidation of CO at Ru(0001) and the effect of temperature on the surface processes, 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, were plotted as a function of potential (see Figs. 5–7). For clarity, the data collected at room temperature of 251C (see Fig. 5) are discussed at first and in more detail, and compared to those obtained at 10 and 501C. 3.2.1. The FTIR data at 251C The potential dependences of the intensities of the COads and CO2 are plotted in Fig. 5a, while the corresponding COL frequency data are plotted in Fig. 5b. The electrooxidation of the CO adsorbates (COads ¼ COL+COH) to CO2 (in

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Band Intensity (a.u.)

16

12

I

II

IV

III

COL

8

4

CO2

COH

0 -200

0

200

400

600

800

1000 1200

Potential / mV vs. Ag|AgCl (b) 2040 IV III

νCO / cm

-1

2030 II 2020 I 2010

COL/Ru(0001) 2000

-200

0

200

400

600

800 1000 1200

Potential / mV vs. Ag|AgCl

Fig. 5. (a) Plots of the integrated band intensities of the COL and COH features in Fig. 4b as a function of potential (see text for details). Also plotted is the intensity of the solution CO2 feature as a function of potential; these data were taken from the same experiment as depicted in Fig. 4b, using the spectrum collected at
200 mV as the reference. (b) Plots of the frequency of the COL feature in Fig. 4b as a function of potential.

solution with an absorption near 2340 cm–1 [63,72]) started at an onset potential of ca. 200 mV (see Fig. 5a), which coincided with the formation of the (1  1) – O(H) adlayer (see above) [56,63,72]. Interestingly, both the intensities and the frequency data show four distinct potential regions: region I,
200 to +200 mV; region II, +200 to +425 mV; region III, +425 to +650 mV and region IV, +650 to +1100 mV. In region I, the intensities of the COL and COH features are potentially independent and no CO2 evolution was observed; this was attributed to the

COL/COH Band Intensity (a.u.)

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(a) COL and COH

109

Square: 10 °C Circle: 50 °C

COL 8

4 COH

CO2 Band Intensity (a.u.)

0

1.2

Square: 10 °C Circle: 50 °C

(b) CO2

0.8

0.4

0.0

-200

0

200 400 600 800 1000 1200

Potential / mV vs. Ag/AgCl

Fig. 6. (a) Plots of the band intensities of the COL and COH features in Figs. 4a and c as a function of potential. (b) Plots of the intensity of the solution CO2 feature as a function of potential; these data were taken from the same experiments as depicted in Figs. 4a and c, using the spectrum collected at
200 mV as the reference.

inactivity of the (2  2) – O(H) phase toward the electrooxidation of COads [13,14,72]. 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 active (1  1) – O(H) phase [13,53,72]. The oxidation of the CO adlayer accelerates in region III, with the slopes of the COL, COH and CO2 intensity vs. potential plots (|q(R/R0)/qE|), see Fig. 5a increasing significantly with respect to those observed in region II; in addition, the COH species are completely stripped by ca. +575 mV. This increased activity was associated with the increase in coverage by the active (1  1) – O(H) phase [13,54,56,63]. However, as the coverage by (1  1) – O(H) increases further (region IV), the residual mixed COL in (2  2) – O(H) domains [COL+(2  2) – O(H)] are compressed into islands of relatively high local CO coverage [13]. The co-adsorbed (2  2) – O(H) 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(H) phase, via the

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νCO (cm-1)

2040

(a) COL

10 °C

2030

50 °C

2020

2010

2000

νCO (cm-1)

1800

10 °C

(b) COH

50 °C

1790

1780

1770

1760

-200

0

200 400 600 800 1000 1200

Potential/ mV vs. Ag/AgCl

Fig. 7. Plots of the frequencies of the (a) COL feature and (b) the COH feature in Figs. 4a and c as a function of potential (see text for details).

Langmuir–Hinshelwood mechanism [76]. Hence |q(R/R0)/qE| for COL decreases, as does |q(R/R0)/qE| 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 [14], so the intensity of the CO2 feature declines in this potential region. Turning now to the behavior of the COL band frequencies. The simplest possible model assumes that the frequency of the CO feature is a function of coverage and potential according to [74]: ðdn=dEÞ ¼ ð@n=@EÞy þ ð@n=@yÞE  ðdy=dEÞ

(1)

where y is the coverage, E the potential and n the frequency in cm
1. In essence, (qn/qE)y may be considered as the contribution to the frequency shift from the electrochemical Stark effect [74], or electron back donation from the metal into the 2p* orbitals of the CO group [77–79], and is positive as E increases; (qn/qy)E is the contribution from dipole–dipole coupling [80], which is positive as y increases, and dy/dE is the contribution from the coverage, which may be

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positive or negative as E increases [74]. Thus, in region I in Fig. 5, the region of constant coverage, (dn/dE) for the COL feature is constant at ca. 38 cm
1 V
1; the latter may be interpreted in terms of the domination of the (qn/qE)y term in Eq. (1), the magnitude of which suggests a high coverage and a strong chemisorption [13,31,34]. At potentials between 200 and 425 mV (region II), where the slow oxidation of the adsorbates to CO2 commences, the first term in Eq. (1) still dominates, but dy/dE is relatively small and negative, the net effect is to reduce dnCO(L)/dE slightly to 30 cm
1 V
1. In the third potential region, between 425 and 650 mV, there is a significant increase in the rate of CO oxidation, dnCO(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 dnCO(L)/dE. In region IV, +650 to +1100 mV, the oxidative stripping of the COL continues, albeit at a significantly reduced rate (see Fig. 5a). Interestingly, dnCO(L)/dE increases again to about the same value (38 cm
1 V
1) as that observed in the constant, saturated-CO coverage region (region I). This observation was taken as evidence of the growing (1  1) – O(H) layer forcing compression of the residual mixed [COL+(2  2) – O(H)] domains into distinct, compact islands of high local COL coverage, such that the (qn/qE)y term again dominates dnCO(L)/dE.

3.2.2. The FTIR data at 10 and 501C The data collected at 10 and 501C were plotted together (see Figs. 6 and 7) to allow direct comparison. Assuming constant absorption coefficients, it may be seen from Fig. 6a that high temperature favors the COL species while low temperature favors COH; however, it should be noted that the relative occupancy of the Ru(0001) surface by COL and COH cannot be deduced straightforwardly from the intensities of their bands due to intensity stealing effects [10,75,80]. It is clear from Fig. 6b that the onset potential for CO2 formation decreases with increasing temperature, from ca. +225 mV at 101C, to +200 mV at 251C (see Fig. 5a) and +125 mV at 501C; this observation coincides with that of a similar decrease in the onset potential for the formation of the active (1  1) – O(H) adlayer observed in the cyclic voltammetry experiments on increasing the temperature [14,22,56]. The removal of both forms of COads at 501C 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 101C. The similarity of the data collected at 10 and 251C is reinforced by a consideration of the potential dependence of the intensities and frequencies of the COL and COH band plotted in Figs. 6 and 7, which fall into four distinct potential regions, as was observed at 251C: region I, –200 to +225 mV; region II, +225 to +450 mV; region III, +450 to +725 mV and region IV, +725 to +1100 mV.

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At first sight, the data collected at 501C look similar to those at 10 and 251C; the plot of the intensity of the COL band as a function of potential (see Fig. 6a) shows four regions of behavior, in the first of which the COL and COH bands show an 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 significantly more marked in behavior of the COL bands may instead reflect a re-orientation of the COL, as observed by Ianniello et al. [81] and Lin and co-workers [47]. Thus, at low potentials some CO molecules lie more or less parallel to the electrode surface; as the electric field increases these then orient themselves in the direction of the electric field perpendicular to the surface. In the first region, from –200 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 101C, which also shows a linear potential dependent increase, dnCO(L)/dE, of ca. 35 cm
1 V
1. Increasing the temperature from 10 to 251C and 501C results in a steady decrease in dnCO(L)/dE in this region from 41 to 39 cm
1 V
1 and 35 cm
1 V
1, respectively. However, closer inspection of the data at 501C (see plots in Figs. 6 and 7) shows marked differences to those at 10 and 251C. First, the intensity of the CO2 feature in Fig. 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 Fig. 6a also shows three linear regions. The intensity of the CO2 feature in the first potential region in Fig. 6b, in complete contrast to the data at lower temperature, shows the highest slope, in agreement with the cyclic voltammetry data [14], which show a sharp increase in oxidative current at potentials >125 mV, in contrast to the data at 101C [14]. In the second region, the rates of COads oxidation and CO2 formation decrease only to 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 and 251C on the one hand and 501C on the other, may be seen in the plot of the frequency of the COL vs. potential, see Fig. 7a, which shows six potential regions: the first (–200 to +100 mV) shows a potential-dependent 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 dnCO(L)/dE drops from 35 to ca. 12 cm
1 V
1. At potentials between 225 and 350 mV, dnCO(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, nCO(L) drops sharply with a negative dnCO(L)/dE of ca. –50 cm
1 V
1; it is clear (see Fig. 6) that rapid oxidation of the CO adlayer takes place in this region, and that the (negative) coverage-dependent term in Eq. (1) dominates dnCO(L)/dE. At potentials between 550 and

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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 Fig. 7b. It can be seen that while there are four distinct potential regions at both 10 and 501C, significantly narrower potential ranges are observed at 501C than those at 101C. At 101C (501C), in the ranges –200 to 0 mV (
50 mV) and +200 (+50 mV) to +600 mV (+250 mV), dnCO(H)/dE ¼ 3172 cm
1 V
1 (3672 cm
1 V
1), separated by a potential region 0 (
50 mV) to +200 mV (+50 mV) in which dnCO(H)/dE ¼ 7372 cm
1 V
1 (7572 cm
1 V
1). At both 10 and 501C, the potential region showing the highest slope is that immediately before the onset of CO oxidation, this phenomenon has been observed previously [47], and was associated with the formation of surface oxide/hydroxide and/or some subsequent re-organization of the CO adlayer [13,47]. 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 101C, the blue shift of nCO(H) continues up to +600 mV and COH was detected up to +700 mV. In complete contrast, at 501C, the blue shift of nCO(H) continues only up to +250 mV and then nCO(H) levels off up to +450 mV; at higher potentials the COH is completely stripped; these data suggest that the adsorbed CO species are present as looser adlayer structures at 501C than at 101C. In addition, at potentials below the onset of CO oxidation, the frequencies of the COL and COH surface at these potentials.

3.2.3. Comparison of rate of oxidation of the CO adlayer at constant potential at 10, 25 and 501C In order 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 501C. At each temperature, the Ru(0001) surface was saturated with CO at
200 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. Figs. 8a and 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 Figs. 8a and b 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 501C. It is also clear from the plots in Fig. 8a 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

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COL Band Intensity (a.u.)

1.0

(a) COL

0.8

0.6

10 °C

0.4

25 °C

0.2

50 °C

0.0

CO2 Band Intensity (a.u.)

(b) CO2 0.8

50 °C 0.6

25 °C

0.4

10 °C

0.2

0.0

0

100

200

300

400

500

Time (s)

Fig. 8. Plots of the integrated band intensities of the (a) COL and (b) CO2 features observed as a function of time at 10, 25 and 501C from the time-dependent in-situ FTIR spectra (8 cm
1 resolution, 16 co-added and averaged scans, ca. 3 s per scanset), collected at the Ru(0001) electrode during the oxidative stripping of COads at +550 mV vs. Ag/AgCl in (CO-free) aqueous 0.1 M HClO4. The CO was pre-adsorbed at
200 mV, see text for details, after which the solution was sparged with N2, 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.

unchanged up to ca. 50% of its saturated coverage during the oxidation process at 101C, it shows a steady decline at 501C (see Fig. 9). This suggests again that a looser, more open COads structure is present at 501C than is observed at 101C on the Ru(0001) electrode. 3.2.4. Surface relaxation and compression of the sub-monolayer CO adlayer The above data suggest that compact COL islands are formed on oxidizing a ‘‘saturated’’ adsorbate layer, almost certainly as a result of compaction by the

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2030

νCO (cm-1)

10 °C

2020

50 °C

2010

1.0

0.8

0.6

0.4

0.2

0.0

CO L intensity (a.u.)

Fig. 9. Plots of the frequency of the COL feature observed in the 10 and 501C experiments in Fig. 8 as a function of its band intensity.

surface oxide (e.g. (1  1) – O(H)) so formed at higher potentials; it was now decided to investigate the processes taking place after the partial stripping of a ‘‘saturated’’ adsorbate layer (at higher potentials) and stepping potential back to lower potentials (where less oxide is present). Figure 10 shows time- and potential-dependent spectra taken after a saturated CO adlayer was partially oxidized at +450 mV, and the potential stepped back to
200 mV (to ‘‘quench’’ the oxidation reaction); the coverage of the remaining adsorbed CO, COads, was estimated to be ca. 0.7ymax where ymax is the saturation coverage (ymax was estimated to be around 0.65–0.75 at temperatures between 10 and 501C, see above). This was estimated by assuming that saturation of the surface with CO gave y ¼ ymax, and measuring the ratio of the CO2 intensity vs. the CO2 intensity obtained after complete oxidation of the CO adlayer. The first spectrum (see Fig. 10) was collected at
200 mV immediately after the step down from +450 mV, and shows a feature at 1992 cm
1 corresponded to COL species at lower coverage. Interestingly, on holding the potential at
200 mV for about 1 min, the COL band split into two, with bands at 1994 cm–1 (band I) and 1974 cm–1 (band II); on stepping to higher potentials, band (II) increased in intensity at the expense of band (I) up to
75 mV, after which it decreased, while band (I) increased in intensity again at potentials >
25 mV. At potentials >50 mV, band (II) had almost disappeared; however, stepping back to
200 mV resulted in band (II) appearing again as the dominant feature.

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450 mV

225 mV

-200 mV

225 mV

50 mV

0 mV

-25 mV

-50 mV

-75 mV

-125 mV

1983

2002

-100 mV

-150 mV

-175 mV

1974

1994

-200 mV, t=1min

-200 mV, t=4s

R/R0 =0.4%

COads /Ru(0001)

2100

2000

1900

Wavenumber/cm-1

1800

In-situ FTIR Spectroscopic Studies

117

These results suggest that stepping back to
200 mV after partial oxidation of the CO adlayer at +450 mV reduces some of the surface O-layer, and so provides empty sites; this allows the migration of COL species into the vacancies, to give species showing band (II) in Fig. 10. The remaining compact islands are responsible for band (I). This ‘relaxation’ appears to be reversible; on stepping the potential up, oxide formation (e.g. at potentials >
25 mV) forces compression of the COL layer. It is interesting to note that a similar peak splitting to that observed in Fig. 10, i.e. ca. 15 cm–1, has also been observed in UHV studies of the co-adsorption of CO on oxygen pre-covered Ru(0001) surfaces, where the surface was partially covered with the (2  2) – O and (2  1) – O phases, and the coverage by CO was low [64]. At an oxygen coverage of half that expected for the completely ordered (2  2) – O surface, (yO ¼ 0.14), peak splitting was observed with both COL adsorbed on clean Ru(0001) domains (a-sites, corresponding to band II) and COL on the oxygen-modified surface (b-sites, corresponding to band I). A second study showed a similar splitting of the frequency of the COL feature due to COL on both (2  2) – O (b-sites) and COL on (2  1) – O (g-sites) [64]. Hence, the formation of distinct domains for the co-adsorption of CO and oxygen on Ru(0001) surfaces is taking place at both the electrode/electrolyte interface and the UHV/solid interface. The effect of temperature and COL coverage on the relaxation/compression process has also been studied [14], and it was found that higher temperature (e.g. 501C) facilitates the relaxation process, while at 101C a compression process was observed with the COL tending to exist as compact islands (i.e. a higher aggregation of the COL adlayer was observed at lower temperature).

Fig. 10. Time- and potential-dependent in-situ FTIR spectra (8 cm
1 resolution, 100 co-added and averaged scans, 16 s per scanset), collected from the Ru(0001) electrode having a coverage of adsorbed CO of 0.75 of maximum, (ymax). The CO was adsorbed by holding the potential of the Ru(0001) electrode at
200 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 20 min.The potential was then increased to +450 mV in a single step, and held for 1 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 the second taken at the same potential after a further 1 min. The potential was then increased in 25 mV increments to +225 mV, with spectra being collected at each step (only a representative number of the spectra are included in the figure, for clarity). The potential was then dropped back to
200 mV in a single step, and a spectrum collected, after which it was increased to 225 mV, also in a single step and a spectrum collected; after this, the potential was increased in 25 mV increments to +450 mV, with spectra being taken at each step. Only two spectra are shown from this stage, for clarity. At the end of the experiment, the potential was stepped to +1100 mV for 3 min, in order to provide a CO-free surface, and the reference spectrum taken.

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3.3. Reactivity of the Ru(0001) electrode toward the adsorption and electrooxidation of formaldehyde, formic acid and methanol as a function of temperature In contrast to the CO experiments detailed above, the organics were not removed from the in-situ FTIR cell prior to the collection of spectra in the experiments discussed below. 3.3.1. Formaldehyde electrooxidation Cyclic voltammograms recorded with the Ru(0001) electrode in N2-saturated 0.1 M HClO4+0.05 M formaldehyde at 10, 25 and 501C [21,22] show that, on the forward scan, the characteristic peaks at –100 and +300 mV observed in pure 0.1 M HClO4 (see Fig. 2) associated with H and O processes are no longer present, indicating that these processes are inhibited by the formaldehyde adsorbate formed already at –200 mV at all three temperatures. At 101C, there is very little anodic current at potentials below 400 mV, and only a small current increase up to 600 mV. In contrast, at 501C appreciable current flows at potentials greater than 150 mV during the anodic sweep. Clearly, increasing the temperature significantly enhances the reactivity of the Ru(0001) electrode surface toward formaldehyde electrooxidation. To gain an insight into the adsorbate arising from the formaldehyde formed on the surface and to further investigate the effect of temperature upon the reactivity of the Ru(0001) surface toward the oxidation of formaldehyde, in-situ FTIR spectra were collected from the Ru(0001) electrode immersed in 0.05 M HCHO+0.1 M HClO4 at 10, 25 and 501C. At each temperature, the electrode was placed in contact with the formaldehyde solution at
200 mV and spectra collected at successively higher potentials from
200 to 1100 mV at intervals of 25 mV; for clarity, not all the spectra collected are shown. Figs. 11a–c show representative spectra of the COads spectral region, which in each case were normalized to the spectrum taken after holding the potential at 1100 mV for 3 min at the end of the experiments in order to ensure the complete removal of adsorbed CO. No features attributable to adsorbates other than COads were observed over the spectral region between 2200 and 1100 cm
1. Figs. 12a and b show plots of the intensities of the CO2 features in the experiments depicted in Figs. 11a and c normalized to the spectra collected at
200 mV as a function of potential. The data taken at 251C were similar to those at 101C, and so are not shown or considered in further detail. Figs. 12a and b also show the CO2 band intensities observed in the corresponding experiments on the CO(g)/COads experiments at 10 and 501C reported above, as well as from the COads/HCOOH experiment discussed below. Considering first the data from the adsorption of formaldehyde (i.e. the HCHO/COads experiment), it can be seen from Figs. 11a–c that the dissociative

In-situ FTIR Spectroscopic Studies (a) 10 °C

119

(c) 50 °C

(b) 25 °C

800 mV 1000 mV

950 mV

900 mV

900 mV

800 mV

800 mV

700 mV

700 mV

750 mV 700 mV

600 mV

500 mV 600 mV

600 mV

400 mV 500 mV

500 mV

300 mV

400 mV

400 mV

300 mV 200 mV

300 mV

200 mV 200 mV

100 mV 100 mV

100 mV 0 mV 0 mV 0 mV -100 mV

-100 mV

-200 mV

-200 mV

-100 mV

-200 mV

R/R0 =0.3%

COL

2100 2000 Wavenumber

CO H R/R0 =0.3%

COL

2100 2000 Wavenumber

R/R0 =0.3% CO L

2100 2000 1900 1800 Wavenumber

Fig. 11. In-situ FTIR spectra (8 cm
1 resolution, 100 co-added and averaged scans, ca. 16 s per scanset), collected from the Ru(0001) electrode in 0.1 M HClO4 solution containing 0.05 M formaldehyde during potential step experiments at (a) 101C, (b) 251C and (c) 501C. At each temperature, the electrode was initially immersed in the solution at –200 mV and a spectrum taken; the potential was then stepped up to +1100 mV in 25 mV increments, with further spectra collected at each step. The reference was a single beam 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). Some of the spectra collected are omitted for clarity.

adsorption of formaldehyde to adsorbed CO species had already taken place at
200 mV at all three temperatures. However, the amount of CO adsorbate and the CO adlayer structure were both strongly dependent upon the temperature and potential. Thus, at 101C, a weak band at around 2009 cm
1 with a shoulder

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120

CO2 Band Intensity (a.u.)

25

(a) 10 °C

20 CO

15

HCHO

10

5

0

HCOOH

-200

0

200

400

600

800

1000 1200

Potential / mV vs. Ag /AgCl

CO2 Band Intensity (a.u.)

120

(b) 50 °C

90

60

HCOOH

30

HCHO CO

0

-200

0

200

400

600

800

1000 1200

Potential / mV vs. Ag/AgCl

Fig. 12. The integrated band intensities of the CO2 features observed during the experiments shown in Fig. 11 plotted as a function of potential (m) at (a) 101C and (b) 501C. Also shown are the corresponding band intensities from the CO stripping experiments () described above and those from the HCOOH experiments (’), for comparison (see text for details). In each case, the spectrum collected at –200 mV, where no CO2 was formed, was employed as the reference.

at around 1990 cm
1 was observed which, according to the above studies on the CO/Ru(0001) system, may be associated with a low coverage CO adlayer that consists of linear bonded CO (COL) co-adsorbed on the (2  2) – O(H) phases (the b sites) and also COL adsorbed on the oxide-free Ru(0001) domain (a sites, which show a ca. 20 cm
1 lower CO frequency). On increasing the potential, the CO adsorbed at a sites decreased due to the further formation of oxide, the

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121

oxidation of the CO adlayer was very slow and CO adsorbate was detected up to ca. 1000 mV. At 501C, there was significantly more of the COL adsorbate formed, and the threefold hollow binding COH (band at 1770–1790 cm
1) was also clearly observed; the intensities of the COL and COH features are very close to those observed in the CO(g)/COads experiments, see Table 1. The onset of CO2 formation at 101C was ca. 225 mV, see Fig. 12a, close to the value obtained for the CO/Ru(0001) system at the same temperature, and in agreement with the decrease in COL intensity. On increasing the potential up to ca. 1050 mV, CO2 was evolved somewhat slowly, together with a little formic acid produced as a result of the partial oxidation of formaldehyde, with three features near 1736, 1396 and 1211 cm
1 being observed (not shown) which may be attributed to the CQO, C–O stretch and C–H bending modes [10,11,43,70], of formic acid. The slow oxidation reaction in this potential region may be as a result of the relatively densely packed (1  1) – O(H) layer formed on the surface blocking any further significant formaldehyde adsorption. Interestingly, at potentials >1050 mV there was a marked increase in the evolution of CO2 and production of formic acid in the potential region where the Ru(0001) electrode exhibits a marked increase in anodic current in the cyclic voltammetry, see the discussion above, suggesting that the oxide layer formed on the Ru(0001) in this potential region (e.g. RuO2(100) islands exposing bridging oxygen atoms, Ru2O, and coordinatively unsaturated Ru atoms not capped by oxygen, Rucus) [54,67,68] is relatively active toward the oxidation of formaldehyde. No adsorbed CO was observed in this higher potential range, suggesting that formaldehyde (and formic acid) oxidation is either via species having very low surface concentrations, or via species with very low IR extinction coefficients, since no other features attributable to adsorbed species were observed over the spectral range between 2200 and 1150 cm
1. Table 1 Comparison of the intensities of CO adsorbed at linear (COL) and threefold hollow (COH) sites at the Ru(0001) electrode at –200 mV in experiments in which the adsorbate was formed after adsorption of CO gas (CO(g)/COads), after the chemisorption of HCHO (HCHO/COads) and after the chemisorption of HCOOH (HCOOH/COads); see text for details Experiment

CO(g)/COads HCHO/COads HCOOH/COads

Adsorbate

Integrated band intensity at 101C (cm
1)

Integrated band intensity at 501C (cm
1)

COL COH COL COH COL COH

9.0 3.5 4.1 0 1.0 0

9.9 2.1 9.1 2.5 8.0 1.0

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The data obtained at 501C are markedly different to those collected at 10 and 251C in terms of the amount of both CO2 and formic acid formed, and the adsorbed COL and COH species observed. At 501C, it is clear from Fig. 11c that thermal activation significantly facilitates the dissociative adsorption of formaldehyde to CO adsorbates at lower potentials (e.g.
200 mV). As was stated above, we observed the loosening of the CO adlayer on Ru(0001) and the activation of surface oxide on increasing the temperature [14,22], and these thermal effects may open CO2-producing pathways at relatively lower potentials, e.g. the onset of CO2 evolution at 501C was ca. 100 mV lower than at 101C, see Figs. 12a and b. After the initial oxidation/stripping of CO adsorbates, which may free up some of the surface sites, partial oxidation of formaldehyde to formic acid was also clearly observed at potentials above 200 mV. Again, there was a similar increase in CO2 evolution and formic acid production at potentials up to 525 mV at 501C, (compared to a maximum of CO2 at ca. 750 mV at 101C), followed by a slight decline due to diffusion out of the thin layer [13] and then the same rapid increase at the postulated RuO2 sites at potentials above 950 mV. Holding the potential at 1100 mV (i.e. increasing the amount of active RuO2), the CO2 features continued to grow while the formic acid absorptions decreased, indicating that the formic acid was further oxidized to CO2 at a rate greater than its formation. Since formic acid was detected during formaldehyde electrooxidation, it was decided to study its behavior at the Ru(0001) electrode. 3.3.2. Formic acid electrooxidation Cyclic voltammograms recorded with the Ru(0001) in 0.05 M HCOOH+0.1 M HClO4 at 10, 25 and 501C [15] show that, as was observed for the formaldehyde experiments discussed above, the H and O processes are inhibited by chemisorption of formic acid at
200 mV at all three temperatures. At 101C, there is very little anodic current at potentials below 400 mV, and only a small current increase up to 620 mV. In contrast, at 501C appreciable current flows at potentials greater than 150 mV during the anodic sweep. It was clear from the voltammetry that, increasing the temperature significantly enhances the reactivity of the Ru(0001) electrode surface toward formic acid oxidation. To investigate this further, the in-situ electrochemical FTIR spectroscopic studies detailed below were carried out. In-situ FTIR spectra were collected from the Ru(0001) electrode immersed in 0.05 M HCOOH+0.1 M HClO4 at 10, 25 and 501C. At each temperature, the electrode was placed in contact with the formic acid solution at
200 mV and spectra collected at successively higher potentials from
200 to 1100 mV in intervals of 25 mV; for clarity, not all the spectra collected are shown. Figs. 13a–c show representative spectra of the COads spectral region, which were normalized in each case to the spectrum taken after holding the potential at

In-situ FTIR Spectroscopic Studies (a) 10 °C

(b) 25 °C

123

(c) 50 °C

800 mV 950 mV

950 mV

700 mV 900 mV

900 mV

600 mV 800 mV

800 mV

500 mV 700 mV

700 mV

400 mV

600 mV

600 mV

500 mV

500 mV

300 mV

400 mV

200 mV 400 mV

300 mV 300 mV

150 mV

200 mV 200 mV

100 mV

100 mV 100 mV

0 mV

0 mV 0 mV -100 mV -100 mV -150 mV

-100 mV

-150 mV -200 mV

-200 mV

-200 mV

COH R/R0 =0.1%

COL

R/R0 =0.1%

COL

2100 2000

2100 2000

Wavenumber

Wavenumber

R/R0 COL =0.5%

2100 2000 1900 1800 Wavenumber

Fig. 13. In-situ FTIR spectra (8 cm
1 resolution, 100 co-added and averaged scans, ca. 16 s per scanset), collected from the Ru(0001) electrode in 0.1 M HClO4 solution containing 0.05 M formic acid during potential step experiments at (a) 101C, (b) 251C and (c) 501C. At each temperature, the electrode was initially immersed in the solution at
200 mV and a spectrum taken; the potential was then stepped up to +1100 mV in 25 mV increments, with further spectra collected at each step. A single beam spectrum taken after holding the potential at +1100 mV for 3 min at the end of the experiment where the electrode surface was free of adsorbed CO was employed as the reference.

1100 mV for 3 min at the end of the experiments in order to ensure the complete removal of adsorbed CO. No features other than those attributable to COads species were observed in the spectral region between 2200 and 1100 cm
1. Plots of the intensities of the CO2 features in the experiments in Figs. 13a and c as a

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function of potential, and normalized to the corresponding spectra collected at
200 mV, are shown (together with the data obtained from the CO(g) and HCHO systems) in Figs. 12a and b. The data taken at 251C were very similar to those at 101C, and so are not shown or considered in further detail. It can be seen from Figs. 13a–c that the dissociative adsorption of formic acid to adsorbed CO had already taken place at
200 mV at all three temperatures. Again, the amount of CO adsorbate and the CO adlayer structure were both strongly dependent upon the temperature and potential. At 101C, only a weak and broad band around 1990–1967 cm
1 was observed, which may be attributed to a low coverage CO adlayer that consists of linear bonded CO (COL) coadsorbed on the (2  2) – O(H) phases (b sites) and also COL adsorbed on the oxide-free Ru(0001) domain (a sites). On increasing the potential, the CO adsorbed at a sites decreased due to the further formation of oxide, the oxidation of the CO adlayer was very slow and CO adsorbates were detected up to ca. 900 mV. At 501C, there was significantly more of the COL adsorbate formed but still very little, if any, COH. The onset of CO2 formation was ca. 200 mV, see Fig. 12a, close to the value obtained for the CO/Ru(0001) system at the same temperature, and in agreement with the decrease in COL intensity. On increasing the potential up to ca. 950 mV, CO2 was evolved somewhat slowly, again perhaps as a result of the (1  1) – O(H) layer blocking any further significant HCOOH adsorption. Again, at potentials >1000 mV there was a significant increase in CO2 evolution in the potential region over which the Ru2O(100) phase is expected to form. No COads or other adsorbed species were observed in this higher potential range as was the case in the HCHO experiments discussed above, confirming that the formic acid oxidation is also either via species having a very low surface concentrations, or via species with very low IR extinction coefficients. At 501C, the onset of CO2 evolution was ca. 100 mV lower than at 101C, see Figs. 12a and b; while there was a similar increase in CO2 evolution at lower potentials up to 550 mV (compared to a maximum at ca. 750 mV at 101C), followed by a slight decline due to diffusion out of the thin layer and then the same rapid increase at the postulated RuO2 sites. These similarities aside, the data obtained at 501C are markedly different from those collected at 10 and 251C in terms of both the amount of CO2 and adsorbed COL species observed. On increasing the temperature from 10 to 501C, there was a ca. 10-fold increase in the maximum CO2 evolution observed below 950 mV, and a ca. sixfold increase in the CO2 observed at 1100 mV, after allowing for the differing path lengths (between 2.2 and 3.0 mm) as judged from the intensity of the 1640 cm
1 water feature in the single beam spectra collected at
200 mV, and using an average of the extinction coefficients for this feature at 3 and 701C reported by Fox and Martin [82]. At 501C, there is also a ca. eightfold increase in the intensity of the COL feature at
200 mV, compared to that seen at 101C.

In-situ FTIR Spectroscopic Studies

125

In order to try and rationalize the above data, it is useful to compare them to the analogous CO(g)/COads data obtained at the same temperatures, see Figs. 12a and b that represent the stripping of a fixed amount of CO adsorbates. The intensities of the COL and COH adsorbate bands observed at
200 mV in both the HCOOH/COads and CO(g)/COads experiments are given in Table 1. At 101C there is ca. 10-fold less COL present in the HCOOH/COads experiment at
200 mV than that was observed in the CO(g)/COads experiment, see Table 1; however, the amount of CO2 evolved is only ca. fourfold less than in the CO(g)/COads experiment, see Fig. 4a, suggesting some turnover of HCOOH is occurring, even at this low temperature and these low potentials. At 501C, while the intensities of the COL features at
200 mV in the HCOOH/COads and CO(g)/COads experiments are comparable, (see Table 1), the CO2 evolved in the former experiment is ca. fourfold that observed in the latter, and",4,0,4?>10fold that observed in the HCOOH/COads experiment at 101C, again suggesting that turnover of formic acid is taking place even at the lower potentials, becoming significantly more rapid at potentials >1000 mV, with the oxidation on both the RuO2- and RuO2-free surfaces being significantly increased on increasing the temperature above 251C. The data at potentials below the onset of the ‘active’ RuO2 surface are most simply interpreted in terms of two mechanisms in which either: (i) HCOOH is being oxidized by an additional pathway involving intermediates at very low coverage, or having low extinction coefficients in the infrared, as we are not detecting any adsorbed species other than CO, e.g. the ‘‘dual pathway’’ mechanism at Pt [3], or (ii) the COL species formed on dissociative adsorption of the HCOOH are significantly more mobile than from the adsorption of CO(g), and hence migrate across the surface more rapidly. Given that, in contrast to the CO(g)/COads experiments, the HCOOH was present in solution throughout the IR spectral data collection, this then leads to the observed more rapid turnover of the HCOOH. It is clear that with the increase of molecular size, e.g. from CO to HCHO and HCOOH, the amount of CO adsorbate formed on Ru(0001) is significantly reduced, see Table 1. 3.3.3. Methanol electrooxidation The cyclic voltammetric response of the Ru(0001) electrode immersed in 0.05 M CH3OH+0.1 M HClO4 between
200 and 600 mV is identical to that observed in the pure supporting electrolyte, which suggests that neither the adsorption nor oxidation of methanol takes place in this potential region; a similar lack of activity toward methanol has been observed at polycrystalline Ru [4,37,38]. When the potential was increased up to 1100 mV, a significant anodic current was obtained at potentials >950 mV both in the absence (see Fig. 2) and presence of methanol, and so it was not clear if oxidation of the organic was taking

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place at these higher potentials; in-situ FTIR experiments were carried out therefore to probe if the Ru(0001) surface showed any activity toward methanol oxidation. Our in-situ FTIR studies on the electrooxidation of methanol at Ru(0001) [15,63] clearly showed that no Faradaic reaction takes place at 10–501C at the Ru(0001) electrode at potentials o950 mV. At potentials >950 mV, some small CO2 evolution was observed, while at potentials >1000 mV, weak features attributable to HCOOH appeared. Holding the potential at 1100 mV to generate the active RuO2 surface resulted in a significant increase in the amount of CO2 produced. No adsorbed CO species were observed at any temperature or potential. Increasing the temperature from 10 to 501C, or increasing the methanol concentration from 0.05 to 0.5 M, resulted in a small increase in CO2 evolution, and a somewhat larger increase in the amount of HCOOH (which reacts with methanol in the solution to produce methyl formate). In order to provide a direct link between the above studies and the extensive literature on CO oxidation at Ru(0001) under UHV conditions, it was decided to investigate if, and to what extent, COads oxidation took place at the Ru(0001) interface under open circuit conditions. As was shown above, the electrooxidation of COads on the Ru(0001) electrode takes place via reaction with the active (1  1) – O(H) phase formed at higher potentials, and that the (2  2) – O(H) phase formed at lower potentials is inactive. There are three likely mechanisms for the oxidation of adsorbed CO: Ru
OH þ Ru
CO ! 2Ru þ CO2 þ Hþ þ e


(2)

Ru
O þ Ru
CO ! 2Ru þ CO2

(3)

2Ru
OH þ Ru
CO ! 3Ru þ CO2 þ H2 O

(4)

The processes shown as Eqs. (3) and (4) were postulated by Watanabe and Motoo on the basis of simple open circuit potential (OCP) measurements in 1975 [83], but this appears to have dropped out of the thinking in this area since, and so reactions (3) and (4) are not generally considered; to this end, it would be of fundamental interest to see if the oxidation of the COads to CO2 can take place under open circuit conditions, and if so, to determine the difference between the potentially controlled and open circuit oxidation of COads. 3.4. Reactivity of the Ru(0001) toward the oxidation of CO adsorbates under open circuit conditions 3.4.1. The open circuit oxidation of CO adsorbates on the Ru(0001) in perchloric acid solution Figure 14 shows in-situ FTIR spectra collected from the COads/Ru(0001) electrode in 0.1 M HClO4 solution at open circuit (stepped from
200 mV) as a

In-situ FTIR Spectroscopic Studies CO2 signal

127

CO signal

540 s 15 min.

540 s 360 s 360 s

270 s 270 s 240 s

188 s 240 s

126 s 188 s 90 s 126 s 1s

90 s

COH

R/R0 =0.5%

10 s

COL

2400

2300

Wavenumber/

cm-1

2100

2000

1900

Wavenumber/

1800

cm-1

Fig. 14. In-situ FTIR spectra (64 co-added and averaged scans, 16 s per scanset) collected from the Ru(0001) electrode in 0.1 M HClO4 solution at 251C during an open circuit experiment after the adsorption of CO. The CO was pre-adsorbed at
200 mV, after which the solution was sparged with N2, and the first spectrum collected at
200 mV. The circuit was then broken and further spectra collected as a function of time. The spectra showing the CO2 absorption were normalized to the first spectrum, collected at
200 mV. The spectra showing the CO absorption 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. Some of the spectra collected are omitted for the sake of clarity.

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128

function of time; the COads was pre-adsorbed at
200 mV (see above), the solution CO was removed by N2 sparge, the electrode pressed against the cell window and the reference spectrum collected, after which the connection to the working electrode was broken by physical disconnection and further spectra collected as a function of time. The initial OCP experiments were carried out using O2 sparging to remove the solution CO after CO adsorption at
200 mV and to saturate the solution with O2; however, the oxidation of the COads under these conditions was too fast to allow any meaningful FTIR data collection. A similar rapid OCP rise in oxygen saturated sulfuric acid solution was reported by Watanabe and Motoo [83]. Hence, the open circuit experiments were carried out under N2 saturated solution (i.e. at a low O2 concentration of 1.4  10
5 mol dm
3, about one hundredth of that under O2 saturated solution [84]) as depicted in Fig. 14. As can be seen from Fig. 14 there is a steady decrease in the intensities of the features near 2010 and 1780 cm
1 corresponding to the loss of COL and COH, and a concomitant gain in the intensity of the 2340 cm
1 CO2 feature. Fig. 15 shows plots of the intensities of the COL and CO2 features in Fig. 14 as a function of time under the open circuit condition. Figs. 14 and 15 strongly suggest that the loss of the CO adsorbates correlates with their oxidation to CO2 via the reactions such as those represented by Eqs. (3) and (4).

14

COL/CO2 Intensity (a.u.)

12 10

(a): COL

8 6 4 2 0

(b): CO2

0

200

400

600

800

1000

Time /s

Fig. 15. Plots of the band intensities of the COL and CO2 features in Fig. 14 as a function of time under open circuit conditions. The data at 0 s are those obtained from the spectrum collected
200 mV before the circuit was broken; see text for details.

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129

Careful analysis of the data in Fig. 15 reveals that there is a ca. 50 s induction time before the onset of the COads oxidation to CO2; to determine the reason for this, the COL frequency was employed as a means of estimating the open circuit potential. Thus, Fig. 16 shows the detailed variation in the COL frequency in Fig. 14 with time at open circuit; as can be seen, the C-O frequency (in wavenumbers) rises to a maximum value of ca. 2027 cm
1 before staying approximately constant; by comparing this variation with that observed in the corresponding potential-controlled experiment as described above (e.g. regions (I) and (II) in Fig. 4), the open circuit potential was estimated as a function of time as shown in Fig. 17. The data in Fig. 17 were checked by comparison with an experiment in which the OCP was actually measured using the Sycopel electrochemical system, and reasonably good agreement was obtained (measurements showed the OCP to reach a constant value of ca. 570 mV cff the 550 mV obtained in Fig. 17). It appears from Fig. 17 that the reason for the induction period observed in Fig. 15 is that the potential has to rise to a value above which the formation of the active (1  1)–O(H) phase takes place (i.e. +200 mV, see discussion above). This postulate was supported by the measurements carried out at 501C, where the onset of the (1  1)–O(H) phase occurs somewhat lower in potential, i.e. at ca. +125 mV (see above), and similar experiment to that depicted in Fig. 14 showed that the oxidation of the surface COads commenced at approximately the same onset potential (ca. +125 mV).

2035 2030

νco/cm-1

2025 2020 2015 2010 2005 2000 0

200

400

600

800

1000

Time / s

Fig. 16. Plots of the frequency of the COL feature in Fig. 14 as a function of time. The data at 0 s are those obtained from the spectrum collected –200 mV before the circuit was broken.

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130

OCP / m V vs. Ag|AgCl

600

400

200

0

-200

0

50

100

150

200 250 Time / s

300

350

Fig. 17. Plots of the open circuit potential (OCP) of the Ru(0001) electrode in 0.1 M HClO4 solution at 251C during the open circuit experiment depicted in Fig. 14 as a function of time. The OCP was estimated from the frequency of the COL feature in Fig. 16, see text for details.

In order to determine how significant the open circuit (OC) oxidation reaction in relation to the electrooxidation of adsorbed CO to CO2 under potential control, the experiment depicted in Fig. 18 was carried out. Thus, after adsorption of CO at
200 mV, and removal of the solution CO by N2 sparging, the reference spectrum was taken, the potential stepped to +500 mV (i.e. approximately the equilibrium potential at open circuit) and three spectra taken over the first 25 s; the circuit was then broken, and further spectra collected as a function of time. As can be seen from the figure, the rate of CO2 evolution (i.e. COads oxidation) does not change on breaking the circuit, showing that the OC reaction may make a very significant contribution to the observed rate of (electro)oxidation. An analysis of the COL frequency shows that very little change in the frequency of the COL band around 2026 cm
1 on switching from +500 mV to OC, suggesting very little change in the potential of the electrode on switching from +500 mV to OC. It is generally reported in the literature that there is negligible specific adsorption of perchlorate ion on the Ru(0001) surface [54–62,69], and hence the presence of perchlorate anion does not seem to protect/inhibit the surface from oxidation. In contrast, Adzic and co-workers [58,59,62] have reported that bisulfate adsorbs at potentials even as low as the hydrogen evolution potential, and this adsorption protects the surface from oxidation. Consequently, it was

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131

14

COL/CO2 intensity (a.u.)

12 10

(b) : CO2

-200 mV

8 6 4 (a): COL

Open Circuit

2

500 mV -200 mV

0

0

100

200

300

Time /s

Fig. 18. Plots of the band intensities of the (a) COL and (b) CO2 features as a function of time from in-situ FTIR spectra (16 co-added and averaged scans, 5 s per scanset) collected during an experiment as follows: the CO was pre-adsorbed on the Ru(0001) electrode in 0.1 M HClO4 (251C) at
200 mV, after which the solution was sparged with N2 and the first spectrum collected at
200 mV. The potential was then stepped to +500 mV and three spectra taken over the first 25 s, the circuit broken, and further spectra collected as a function of time. The CO2 spectra were normalized to the spectrum collected at
200 mV where no CO2 was produced. The COL spectra were normalized to the 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.

decided to investigate if, and to what extent, the specific adsorption of bisulfate ion [58,59] has any effect on the oxidation of the Ru(0001) surface and hence on the reactivity toward CO oxidation at open circuit. To this end, the experiments in 0.1 M HClO4 described above were repeated using 0.05 M H2SO4 solution. 3.4.2. The open circuit oxidation of CO adsorbates on the Ru(0001) electrode in sulfuric acid solution Both COL and COH were detected on the Ru(0001) surface in sulfuric acid solution with the former showing a similar Stark effect to that observed in the perchloric acid solution, i.e. of 37 cm
1/V. However, in contrast to the results from perchloric acid solution, no oxidation of the COads to CO2 took place in the first 9 min on switching from
200 mV to open circuit; thereafter, only a slow oxidation of the COads was observed, see Fig. 19. The open circuit potential was estimated using the C-O frequency of the COL as a function of time

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14

COL /CO2 Intensity (a.u.)

12 10

(a): COL

8 6 4 2 0

(b): CO2

0

200

400

600

800

1000

Time /s

Fig. 19. Plots of the band intensities of the (a) COL and (b) CO2 features measured as a function of time under open circuit conditions from in-situ FTIR spectra (64 co-added and averaged scans, 16 s per scanset) collected from the Ru(0001) electrode immersed in 0.05 M H2SO4 at 251C during an open circuit experiment identical to that depicted in Fig. 14 after the adsorption of CO. The data at 0 s are those obtained from the spectrum collected
200 mV before the circuit was broken; see text for details.

(see Fig. 20) and the data were checked by comparison with an experiment in which the OCP was actually measured using the Sycopel electrochemical interface. It was found that the OCP rises significantly more slowly in H2SO4 than was observed in HClO4, apparently reaching an equilibrium potential of 200 mV, i.e. significantly less than the ca. 550 mV observed in the absence of specific anion adsorption. It does not seem unreasonable to postulate that the specific adsorption of bisulfate anions inhibits the formation of the oxygencontaining adlayer on the Ru(0001) surface, in agreement with the work of Adzic et al. [58,59], and particularly the densely packed but active (1  1)–O(H) phase [21,22,63], so leading to the low rate of the COads oxidation (see Fig. 19). 3.5. The possible mechanisms of CO oxidation under open circuit conditions: a comparison of the data obtained at gas/solid and electrolyte/solid interfaces The (CO and/or O)/Ru(0001) systems have been well documented by numerous gas phase/UHV studies [64–68]. In contrast to the Ru(0001)/electrolyte interface, under UHV conditions, it is generally accepted that the oxygen-containing

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133

OCP / mV vs. Ag|AgCl

600

400

200

0

-200 0

200

400 Time /s

600

800

1000

Fig. 20. Plot of the open circuit potential (OCP) of the Ru(0001) electrode immersed in 0.05 M H2SO4 at 251C during the open circuit experiment depicted in Fig. 19.

adlayer on the Ru(0001) is Ru-O rather than Ru-OH [64–68,71]. Under UHV conditions, at coverages up to 1 monolayer (1 ML), oxygen has been found to occupy hexagonal closely packed sites and to form ordered layers having (2  2), (2  1) and (1  1) symmetries [64–68]; at coverages in excess of 1 ML, oxygen is located in the subsurface region [85]. In the case of the co-adsorption of CO and oxygen on Ru(0001), a model comprising linearly bonded CO occupying top sites and ‘bridge’-bonded CO in threefold (face centred cubic) sites has been proposed [64–66,86,87]. CO oxidation at oxygen precovered Ru(0001) surfaces at low oxygen coverages (i.e. o1 ML, (2  2)–O and (2  1)–O overlayers), gives an extremely low CO oxidation rate [88,89]. Oxidation is believed to take place via a Langmuir-Hinshelwood mechanism [76]; for this to occur, the Ru-O bond needs to be activated [89,90], after which the reaction with the co-adsorbed CO takes place. However, the energy required for CO desorption (Ea ¼ 0.83 eV) [89] is considerably lower than the barrier to CO2 formation (Ea ¼ 1.8 eV) [89], so that attempts to drive the reaction thermally fail because CO is desorbed before the O is activated to react [89]. The strength of the Ru-O bond decreases as the oxygen coverage increases [91], thus in principle favouring reaction with CO; however, CO adsorption actually becomes inhibited by the densely packed O–(1  1) adlayer, such that a Ru(0001) surface with an O–(1  1) overlayer is still essentially unreactive toward CO oxidation.

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Studies of the Ru(0001)/electrolyte interface have shown that the oxidation of the Ru(0001) electrode surface in perchloric acid electrolyte solution results in the formation of ordered (2  2)–O(H) and (1  1)–O(H) adlayers, with the exact nature of the adlayer, i.e. whether Ru-O or Ru-OH, as yet unclear [13–15,54–59,69,71]. The open (2  2)–O(H) phase is inactive toward CO (electro)oxidation, and both COL and COH can be co-adsorbed with the (2  2)–O(H) on the Ru(0001) electrode. The reactivity of the surface toward COads (both COL and COH) oxidation starts from the formation of the active (1  1)–O(H) phase. In contrast to the situation at the UHV/Ru(0001) interface, it appears that COads is more stable and the (1  1)–O or (1  1)–OH species have weaker Ru-O bonds rendering them more active. Hence, COads cannot be removed by desorption but can be removed by oxidatively stripping through reaction with (1  1)–O(H). The adsorbed CO species remain in islands as COads oxidation proceeds, with the oxidation rate decreasing as the COads coverage decreases. Obviously, the oxidation of COads can only take place via the Langmuir-Hinshelwood mechanism at the perimeters between the COads islands and the active (1  1)–O(H) phase. In HClO4 solution where there is negligible specific adsorption of ClO–4 on the Ru(0001) surface, significant oxidation of the COads takes place on switching from
200 mV to open circuit. However, the OC oxidation of the COads was inhibited by specific adsorption of HSO
4 ions in sulfuric acid solution. For the oxidation of COads on Ru(0001) at the gas/solid interface, the mechanism may simply comprise reaction between Oads and COads to produce CO2, as shown in Eq. (2) above [71,76]. In contrast, it is clear that oxygen is the oxidant in the open circuit process at the Ru(0001)/electrolyte interface, suggesting reactions of the sort equation (2) coupled with Ru þ O2 þ 3Hþ þ 3e
! Ru
OH þ H2 O

(5)

giving an overall reaction: O2 þ 2Ru
OH þ 3Ru
CO ! 5Ru þ 3CO2 þ H2 O

(6)

The above studies of the OC oxidation of COads on Ru(0001) were extended to RuPt electrodes [92] where our in-situ FTIR data show that adsorbed CO is present on both the Ru and Pt domains and can be oxidized by the oxygencontaining adlayer on the Ru in a chemical process to produce CO2 under OC conditions. Surprisingly, there is a free exchange of COads between the Ru and Pt sites. Oxidation of COads may take place at the edges of the Ru islands, but COads transfer, at least on the time scale of these experiments, allows the two different populations to maintain equilibrium. Oxidation is limited in this region by the rate of supply of oxygen to the surface of the catalyst.

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4. Concluding remarks We have developed a highly sensitive in-situ FTIR system in Newcastle; this may be deduced from, for example, the data in Fig. 4 where the COL band near 2000 cm
1 shows a signal-to-noise ratio>500 for a data collection time of only 16 s at 8 cm
1 resolution. Our work has shown the power of in-situ IR spectroscopy with respect to the investigation of the surface oxide structure of Ru(0001) by using adsorbed CO as a probe molecule. This approach has highlighted the fact that (i) different oxide adlayers form on Ru(0001) as a function of potential and (ii) only the (1  1) – O(H) phase is active toward the oxidation of adsorbed CO; i.e. not all surface oxides on Ru are active. At potentialso200 mV, CO co-adsorbs with the oxygen-containing adlayers to form mixed [CO+(2  2) – O(H)] domains. The oxidation of the Ru(0001) surface precovered with saturated amounts of CO leads to the formation of active (1  1) – O(H); oxidation of adsorbed CO then takes place at the perimeter of these latter domains via the Langmuir–Hinshelwood mechanism. As the oxidation of the surface proceeds at the higher potentials, the remaining adsorbed CO is compressed by the growing (1  1) – O(H) phases into small [CO+(2  2) – O(H)] domains. However, if the potential is stepped back to –200 mV to quench the oxidation after a partial oxidation of the COads layer at a higher potential (e.g. 450 mV), removal of the (1  1) – O(H) adlayer through reaction with COads frees up the Ru(0001) surface, allowing COads present in the (2  2) – O(H) domains to spread out over the oxide-free part of the surface. If the potential is then stepped up to a value o200 mV again, the formation of more (2  2) – O(H) forces the COads to retreat back into the (2  2) – O(H) domains. At 101C, the adsorbed CO is present as rather compact islands, and this remains the case as the oxidation of the adsorbate is initiated by the formation of the active (1  1) – O(H). In contrast, at 501C, the COads is present as a relatively looser and weaker adlayer. Higher temperature was also found to facilitate the surface diffusion and oxidation of COads. No dissociation or electrooxidation of methanol is observed at potentials below ca. 950 mV, suggesting that the surface oxide phases physically block the Ru(0001) with respect to the adsorption of methanol. However, both formaldehyde and formic acid undergo dissociative adsorption at the Ru(0001) surface to form COH and predominantly COL adsorbates; higher temperature was also found to facilitate both the dissociative adsorption and the direct oxidation of HCHO/HCOOH, as well as the oxidation of CO adsorbates. A surprising observation was the high activity of the Ru(0001) surface at high anodic potentials due to the formation of RuO2 (100) phase, the RuO2 exposes bridging O atoms (Obr) and Ru atoms not capped by oxygen (Rucus, coordinatively unsaturated sites, see Fig. 3c), both of which can act as active sites and

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thus open a novel pathway for the adsorption and oxidation of all organics studied. Our work has shown, for the first time, that significant oxidation of the adsorbed CO takes place at open circuit driven by oxygen as the oxidant, however, this reaction is inhibited by the specific adsorption of bisulfate anions. Future work will focus upon the addition of Pt to the Ru(0001) surface, and will investigate the effect of Pt on the electrooxidation of fuel molecules as a function of Pt coverage, potential and temperature. More insight into the surface processes involved in the electrocatalysis of fuel molecules can be obtained by combined ex situ UHV and in-situ FTIR spectroscopy studies.

Acknowledgments The authors would like to thank Prof. G. Ertl and Dr. M. S. Zei for the helpful discussions. Financial support from the EPSRC is gratefully acknowledged.

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