Surface-adsorbed species play important roles in catalysis and electrocatalysis. Identification of adsorbed species on electrode surfaces can help elucidate the mechanistic details of electrocatalysis. PtRu binary alloys have been reported to be the best catalysts for CO, H 2 , and methanol electrooxidation due to the bifunctional mechanism, in which Ru adsorbs OH adspecies at lower potentials than Pt and thus promotes the oxidation of CO and H adsorbed on Pt. [1][2][3][4][5][6][7][8][9][10] The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) also involve adsorbed OH. [11][12][13][14] However, identifying oxygen-containing species such as adsorbed OH is challenging in aqueous solutions due to the interference from adsorbed water. Nakamura et al. reported a broad infrared band at 1080 cm -1 for Pt single crystals in 0.1 M HF and assigned it to the OH bending mode of adsorbed OH. 15 In contrast, Feliu et al. did not observe this band on Pt(111) in Ar-saturated NaF/ HClO 4 solution at pH = 5.5 but observed it in O 2 -saturated solution and thus assigned it to superoxide anion (O 2 -) generated by the ORR. 16 Naguchi et al. observed an infrared band at 1095 cm -1 for the ORR on Pt in 0.1 M HClO 4 and assigned it to O 2 -. 17 Adzic et al. reported an infrared band at above 1000 cm -1 for the ORR on Pt in a pH = 11 electrolyte and assigned it to O 2 -. 18 Using electrochemical shell-isolated nanoparticle-enhanced Raman spectroscopy (EC-SHINERS) to study adsorbed species on Pt(311) in 0.1 M HClO 4 , Feliu et al. reported two bands at 860 and 1138 cm -1 at potentials between 0.25 and 0.65 V and assigned them to the bending mode [δ(OH)] of OH adsorbed on (100) steps without and with nearby adsorbed O atoms, respectively. 19 Employing SHINERS to study the ORR on platinum single crystals in 0.1 M HClO 4 , Li, Tian, and Feliu et al. found a band at ca. 1080 cm -1 on Pt(100) and Pt(110) but not on Pt(111) and assigned it to the Pt-OH bending mode [δ(PtOH)] of adsorbed OH. 13 They also found a band at 1040 cm -1 on highindex Pt(311) and Pt(211) and assigned it to adsorbed OH. 20 In contrast, Koper et al. did not observe these two bands on Pt(111) or Pt(100) over the same potential region in 0.1 M HClO 4 . Instead, they observed a band at 922 cm -1 and assigned it to the symmetric stretching ν s (Cl-O) of ClO 4
-, specifically interacting with the adsorbed OH layer. 21 Because there are contradictions about the vibrational bands of adsorbed OH species, in this work we studied the OH species adsorbed on Pt, Ru, and Ru-modified Pt electrodes in both acidic and alkaline media by using surface-enhanced infrared absorption (SEIRA) spectroscopy in a flow cell 22,23 (Figure 1a) through CO displacement and potential modulation. The stretching mode of OH adspecies was, for the first time, identified on Pt, Ru, and Ru-modified Pt, while the band at 1108 cm -1 was ascribed to hydrated OH -anions rather than adsorbed OH.
2.1. Pt. The morphology of the Pt film, chemically deposited on the Si prism, was characterized with atomic force microscopy (AFM), and its 2-and 3-dimensional images are shown in Figures 1b and 1c. The Pt film had a thickness of ca. 90 nm and consisted of 50-100 nm nanoparticles. Cyclic voltammograms (CVs) of the Pt film exhibited the characteristic profiles which include the hydrogen region, double-layer region, and oxygen region (Figure S1). The potentialdependent SEIRA spectra of the Pt film in 0.1 M HClO 4 and 0.1 M KOH are presented in Figures 1d and 1f, respectively. In 0.1 M HClO 4 , an infrared band was observed at 1117 cm -1 , which is assigned to the symmetric stretching ν s (O-Cl-O) mode of weakly adsorbed perchlorate. 24,25 The downward band at ca. 2093 cm -1 is assigned to the stretching ν(Pt-H) mode of adsorbed H atoms at atop sites (overpotentially deposited hydrogen), formed at potentials below 0.2 V. 25,26 The downward bands at 1621 and 3510 cm -1 are assigned to the bending δ(H-O-H) mode and stretching ν(O-H) mode of H 2 O adsorbed on hydrogen-covered Pt, respectively. 25 The band, appearing at 3330 cm -1 , can be assigned to the ν(O-H) of water adsorbed on Pt. 25 Another band, which was observed at 3020 cm -1 , can be assigned to the ν(O-H) of adsorbed OH species [27][28][29] because this band frequency is below that of ice-like water, and there is no corresponding δ(H-O-H) band (also see below). In contrast, Osawa and Cuesta et al. assigned this band to ice-like water. 25,30 In ultrahigh vacuum (UHV), an OH overlayer was reported to form by dissociation of water on an oxygen-covered Pt(111) surface. However, the detailed composition and structure of the OH adlayer are still under debate. Three OH adlayer models were proposed: a pure hydroxyl overlayer, [31][32][33] a hydroxyl-water mixed overlayer, 34,35 and a hydroxyl-hydrogen mixed overlayer. 36,37 Bedurftig et al. studied the OH adlayer on Pt(111) with high-resolution electron energy loss spectroscopy (HREELS) and found that an HREELS band appeared at 431 meV (equivalent to a wavenumber of 3476 cm -1 ) for the ν(O-H) of the adsorbed OH adlayer, while no signatures for water were identified. Therefore, they proposed that OH molecules adsorbed at atop sites and were tilted from the surface normal so that they can form hydrogen bonds with each other. 31 Clay et al. observed a very weak infrared band at 3475 cm -1 and assigned it to the ν(O-H) of adsorbed OH on Pt(111). 38 In contrast to refs 31 and 32, Clay et al. proposed that a nearly flat hydroxyl-water mixed overlayer was formed with no internal O-H bonds perpendicular to the Pt surface. 38 Stuve et al. observed an HREELS band at 3250 cm -1 for OH adsorbed on Pd(100), which was red-shifted compared to the band at 3380 cm -1 for water adsorbed on Pd(100). 39 Bedurftig et al. also reported a much lower wavenumber of 3053 cm -1 for OH adsorbed on Pd(100), suggesting that strong hydrogen Journal of the American Chemical Society bonds were formed. 31 Fisher et al. observed an HREELS band at 3040 cm -1 after the adsorption of water onto an atomic oxygen layer on Pt(111) at 100 K but did not clearly assign it to adsorbed hydroxyl species. 32 Performing density functional theory (DFT) calculations, Karlberg et al. obtained a ν(O-H) band at 3589 cm -1 for a hydroxyl-water mixed overlayer on Pt(111) and two ν(O-H) bands at 3170 and 3113 cm -1 for a hydroxyl-hydrogen mixed overlayer on Pt(111). 40 36 Our observed ν(O-H) wavenumber of 3000-3050 cm -1 is close to the calculated wavenumber of a pure OH overlayer or an OH and H mixed overlayer on Pt(111). 36,40 Zhu et al. studied the Pt(100)/water interface structure at various OH coverages with ab initio molecular dynamics and found a stronger hydrogen-bonding network between OH and the interfacial water than that between water molecules. 41 This is consistent with our finding that the v(O-H) band of adsorbed OH on the Pt surface was red-shifted compared with adsorbed water.
For polycrystalline Pt, Pt(100), Pt(110), and stepped Pt(111) electrodes, the hydrogen adsorption region and hydroxyl adsorption region overlap, so that H and OH can coexist on these electrode surfaces at low potentials. 42 The OH coadsorbed with H on the Pt surface might result in a lower wavenumber for the ν(O-H) band when compared to water. 36,40 In the electrode/water interface, three-dimensional hydrogen bonds might exist for OH adsorbed on Pt, 41 unlike two-dimensional hydrogen bonds formed in UHV. 31,[33][34][35] Moreover, the polycrystalline Pt possessed heterogeneous properties, and thus, OH adsorbed on different sites would exhibit different v(O-H) frequencies. Therefore, it should be noted that the assignment of the band at 3000-3050 cm -1 to the adsorbed OH species does not rule out other ν(O-H) bands of adsorbed OH species with higher frequency, which might overlap with the water ν(O-H) bands.
In 0.1 M KOH, the bands of linearly bonded H at 2060 cm -1 , water adsorbed on H-covered Pt at 1616 and 3487 cm -1 , water adsorbed on Pt at 3286 cm -1 , and adsorbed OH at 3050 cm -1 were also observed. The band appearing at 1225 cm -1 is assigned to the stretching ν(Si-O-Si) mode of SiO 2 formed on the Si prism. 43,44 Its intensity increased with increasing potential due to the oxidation of the Si prism surface. The band observed at ca. 1108 cm -1 is likely due to the bending
in the double-layer region rather than OH adspecies 45 because this band intensity increased with increasing OH -concentration, and no Stark effect was observed.
The band intensities of adsorbed OH at ca. 3000 cm -1 in both 0.1 M HClO 4 and 0.1 M KOH are plotted vs potential in Figures 1e and 1g (blue lines), respectively. In 0.1 M HClO 4 , the intensity of the adsorbed OH band at 3020 cm -1 increased with increasing potential at first, reached a maximum at ca. 0.6 V, and then decreased. In 0.1 M KOH, the maximum intensity of adsorbed OH appeared at ca. 0.7 V and had a relatively higher intensity at high potentials, when compared to that in 0.1 M HClO 4 . The plot of integrated band intensity of adsorbed perchlorate at 1117 cm -1 vs potential exhibited a maximum at ca. 0.6 V (Figure 1e, black line), consistent with the results reported by other groups. 24,25 The integrated band intensity of the hydrated OH -anion at 1108 cm -1 increased with increasing potential due to an increase in the concentration of OH -anions in the double layer (Figure 1g, black line). In alkaline media, the potential of zero total charge (pztc) of Pt is about 0.7 V (vs RHE). 46 In the low-potential region, the Pt surface is negatively charged, and thus, smaller amounts of OH -anions were present in the double layer due to electrostatic repulsion. With increasing potential, the Pt surface became less negative and even positive at high potentials, leading to more OH -anions entering the double layer. The intensities of adsorbed water bands at ca. 3500 and 1620 cm -1 decreased with increasing potential. The water band at 3330 cm -1 overlapped with the band at 3500 cm -1 , preventing its intensity from being fully analyzed.
To further confirm that the band at ca. 3020-3050 cm -1 is related to the ν(O-H) mode of adsorbed OH, CO displacement experiments at different potentials were performed. 46 As the Ar-saturated electrolyte (0.1 M HClO 4 or 0.1 M KOH) was exchanged to a CO-saturated electrolyte at different constant potentials (≤0.5 V), time-dependent SEIRA spectra of the Pt film were recorded and are presented in Figures 2a, 2d, S2, and S3. Figures 2b and 2e show the expanded SEIRA spectra of the water band regions from Figures 2a and 2d. At 0.5 V, after introduction of the COsaturated electrolyte, the linearly bonded CO (CO ad,L ) and bridge-bonded CO (CO ad,B ) bands were observed at 2085 cm -1 (0.1 M HClO 4 ) or 2056 cm -1 (0.1 M KOH) and 1870 cm -1 , respectively, and their intensities increased with time. After ca. 1 min, the intensity of the CO ad,L and CO ad,B bands did not increase further due to the formation of a saturated CO adlayer. In 0.1 M KOH, the CO ad,L band had a lower wavenumber (2056 cm -1 ) than in 0.1 M HClO 4 (2085 cm -1 ), and relatively more CO ad,B was formed than that in 0.1 M HClO 4 . This is likely due to the positive shift of E pzfc (the potential of zero free charge vs RHE) in 0.1 M KOH when compared to that in 0.1 M HClO 4 . 46 Upon CO adsorption on Pt, two upward bands at 3650-3665 and 1630-1635 cm -1 were observed, and their intensities increased with time. They are assigned to the ν(O-H ) and δ(H-O-H) modes of water adsorbed on top of the CO adlayer. 25,47 In contrast, Watanabe et al. assigned these bands to H 2 O coadsorbed with CO. 10 Two downward bands appeared at ca. 1593-1598 and 3300-3350 cm -1 and are assigned respectively to the δ(H-O-H) and ν(O-H) modes of water adsorbed on Pt, which was displaced by CO. We also observed a downward band at ca. 3000-3050 cm -1 , which did not have a corresponding δ(H-O-H) band and thus is assigned to adsorbed OH rather than H 2 O. In addition, we integrated the intensity of the δ(H-O-H) band between 1550 and 1660/1680 cm -1 (HClO 4 /KOH) and the ν(O-H) band between 2500 and 3700 cm -1 by using a straight line between the ends of the integration region as the baseline. The integrated δ(H-O-H) band intensity was around zero or even had a positive value, while the integrated ν(O-H) band intensity at 2500-3700 cm -1 exhibited a large negative value for the CO displacement at 0.5 V (Figures 2b and 2e). However, the band intensity ratio between the ν(O-H) and δ(H-O-H) modes of adsorbed water varies greatly depending on the hydrogen-bonding structure and orientation of water molecules. Their absorption coefficients might change unproportionally before and after CO displacement so that it precluded us from quantitatively assessing the change of OH ad coverage by using the integrated band intensity.
As we increased the CO adsorption potential (Figures 2c and 2f), the intensity of the downward stretching band of adsorbed OH at ca. 3000 cm -1 increased, which is consistent with the fact that the OH coverage on Pt increases with increasing potential. 2,15,19,21,48 Meanwhile, the intensity of the downward water bending band at 1590-1600 cm -1 decreased with increasing potential, suggesting that adsorbed OH species gradually displaced the adsorbed water. Although the ν(O-H) band of the OH adspecies was obscured by those of adsorbed water on Pt surfaces, our analysis of SEIRA spectra clearly indicated that the ν(O-H) band at ca. 3000 cm -1 can be ascribed to OH adspecies. To further validate the above assignment(s), we studied SEIRA spectra of a Ru film as described below.
2.2. Ru. It is well-known that OH adspecies are formed on Ru at lower potentials than on Pt and can even be present in the hydrogen region. 49,50 To further verify that the band at ca. 3000 cm -1 originated from the ν(O-H) mode of adsorbed OH, the potential-dependent SEIRA spectra of the Ru film were also studied (Figure S4). As the potential went below 0.3 V, a large reduction peak appeared at around 0.05 V (Figure S1b). In contrast, in the positive-going scan, a small oxidation peak occurred at 0.15 V and is assigned to the oxidation of adsorbed hydrogen. The reduction peak was much larger than the oxidation peak, suggesting that not only the hydrogen adsorption but also the reduction of OH/O adspecies and even the reduction of HClO 4 contributed to the reduction peak. [49][50][51][52] In the hydrogen region, two very weak bands, which were observed at 3323 and 1600 cm -1 , are assigned to the ν(O-H ) and δ(H-O-H) modes of water adsorbed on Ru, respectively. 53 A very weak downward band appeared at ca. 2900 cm -1 in the hydrogen region, which could be assigned to the OH adspecies. The very low intensity of adsorbed water and OH bands suggests that their coverage varied very little with potential, thus precluding their potential-dependent band intensity from being quantitatively analyzed. Therefore, we further studied SEIRA spectra of the Ru film through CO displacement at constant potentials.
Time-dependent SEIRA spectra of the Ru film at 0.5 and 0.1 V, after switching the electrolyte from 0.1 M HClO 4 to CO saturated 0.1 M HClO 4 , are presented in Figures 3A and S5A, respectively. The integrated band intensities are plotted vs time in Figures 3B and S5B. Upon CO adsorption at 0.5 V, two CO bands developed at 2020 and 1816 cm -1 , which are assigned to CO ad,L and CO ad,B , respectively. 10,53,54 Their intensities increased with time, and the adsorption of CO on Ru was much slower than on Pt, likely due to the blockage of adsorbed OH/O species. OH/O adsorption and desorption on Ru were sluggish as indicated by the irreversible oxidation/reduction peaks (Figure S1b) and thus suppressed CO adsorption. Meanwhile, two upward bands appeared at 3630 and 1621 cm -1 , and their intensity increase was parallel to the increase of the CO ad,L and the CO ad,B band intensities. Thus, they are assigned to the ν(O-H ) and δ(H-O-H) modes of water adsorbed on top of the CO adlayer. 25,53 A downward band developed at 3000 cm -1 , and its intensity increased with time due to the displacement of CO. It did not have a corresponding δ(H-O-H) band and thus is assigned to OH adspecies on Ru rather than adsorbed water. As for CO adsorption at 0.1 V, we observed time-dependent SEIRA spectra similar to those at 0.5 V (Figure S5). The downward δ(H-O-H) and ν(O-H) bands of water adsorbed on bare Ru were barely observed, suggesting that at 0.1 and 0.5 V, the Ru surface was mainly covered with OH/O adspecies, consistent with previous reports. 49,50 The ν(O-H) band of OH adsorbed on Ru was red-shifted when compared to that on Pt, which parallels the fact that according to DFT calculations OH is adsorbed on Ru more strongly than on Pt. 5 However, the direct correlation between the vibrational frequencies of OH adspecies and their adsorption energy on different metal
surfaces might be less clear/direct and needs further investigation.
Studying the adsorption of H 2 O on Ru(0001) in UHV, Thiel et al. observed an HREELS band at ca. 2935 cm -1 , and this band remained at high temperature of 220 K while other high-frequency v(O-H) bands of water disappeared. 55 This band is likely attributed to the decomposition of water to form adsorbed OH species. 56 Our observed infrared band at 2900-3000 cm -1 for OH adsorbed on the Ru film is in good agreement with the findings of Thiel et al. 55 PtRu binary catalysts are generally considered to be the most effective for CO electrooxidation due to the so-called bifunctional mechanism, 1 in which OH adspecies are formed on Ru sites at low potentials and react with CO molecules adsorbed on Pt sites, enabling the electrooxidation of CO adsorbed on Pt sites at lower overpotentials when compared to pure Pt. Identification of OH adspecies on PtRu surfaces would advance our understanding of the bifunctional mechanism. Thus, in the following section, we further studied the SEIRA spectra of adsorbed OH species on a Ru-modified Pt film.
2.3. Pt/Ru. The Ru-modified Pt film was obtained by constant potential deposition from a freshly prepared solution containing 5 mM RuCl 3 and 0.1 M H 2 SO 4 at 0.6 V for 5 min. 57 A Ru coverage of about 0.25 was obtained. 57 Potentialdependent SEIRA spectra of the Ru-modified Pt film in 0.1 M HClO 4 are presented in Figure S6. A ν(O-H) band for adsorbed OH was observed at ca. 2926 cm -1 and exhibited a maximum intensity at ca. 0.7 V. Time-dependent SEIRA spectra of the Ru-modified Pt film (Θ Ru ≈ 0.25) at 0.5 and 0.05 V, after switching the electrolyte from 0.1 M HClO 4 to COsaturated 0.1 M HClO 4 , are presented in Figures 4 and S7, respectively. Upon CO adsorption at 0.5 V, two CO ad,L bands were observed at 2073 and 2008 cm -1 and are assigned to CO adsorbed at Pt and Ru sites, respectively. 8,10 A CO ad,B band appeared at 1875 cm -1 , mainly assigned to CO adsorbed at Pt sites. A downward ν(O-H) band of adsorbed OH was observed at ca. 2900 cm -1 due to the CO displacement. Meanwhile, two upward bands, occurring at 3622 and 1633 cm -1 , are assigned to the ν(O-H) and δ(H-O-H) modes of water adsorbed on the top of the CO adlayer, respectively. 25,53 In contrast to pure Pt, an upward band (or two overlapped bands) was observed at ca. 3292 cm -1 (Figure S8), which is likely due to water coadsorbed with CO at Ru sites. A downward band at ca. 1598 cm -1 became much smaller when compared to that of the Pt film and even disappeared, further suggesting that CO adsorption displaced adsorbed OH species and promoted water coadsorption at Ru sites. CO and water coadsorption on Pt/Ru might also contribute to the enhancement of CO oxidation on Pt/Ru. 10 2.4. CO Stripping from Pt, Ru, and Pt/Ru. SEIRA spectra of CO stripping on Pt and Ru in 0.1 M HClO 4 and 0.1 M KOH are presented in Figures S9-S11. The oxidation of CO ad on Pt in 0.1 M HClO 4 onset at ca. 0.4 V and exhibited a prepeak at 0.5 V and a main peak at 0.65 V with a shoulder on the positive side. In 0.1 M KOH, the oxidation of CO ad was ca. 100 mV negatively shifted. The CO adlayer can be completely oxidized on Pt at potentials below 0.7 V in 0.1 M KOH or 0.8 V in 0.1 M HClO 4 in the first positive-going scan. In contrast, the oxidation of CO ad on Ru in 0.1 M HClO 4 , which onset at ca. 0.4 V, exhibited only a broad main peak at ca. 0.58 V with a long tail on the positive side. The CO adlayer cannot be completely oxidized on the Ru film, even up to 1.0 V in the first positive-going scan. Ru is prone to OH species and has higher OH coverage at low potentials than Pt, which benefits CO oxidation. However, the kinetics of CO oxidation are sluggish, likely due to the too strong OH adsorption on Ru. On the Ru-modified Pt, two main CO ad oxidation peaks occurred at 0.49 and 0.51 V, corresponding to Ru and Pt sites, respectively (Figure S12). 2,6,7 They were negatively shifted compared to pure Ru and Pt, respectively. This suggests that the kinetics of CO oxidation at both the Pt and Ru sites are significantly enhanced. The enhancement is generally attributed to the synergistic effect/bifunctional mechanism, in which Ru provides OH adspecies at lower potentials, while Pt promotes the reaction between adsorbed CO and OH. 1,2,58 Our infrared data suggest that water coadsorbed with CO at Ru sites might also benefit CO electrooxidation.
ATR-SEIRA spectroscopy together with a spectroelectrochemical flow cell was employed to identify OH species adsorbed on Pt, Ru, and Pt/Ru in both acidic and alkaline media. SEIRA spectra were studied through potential modulation and CO displacement. The stretching band of OH species adsorbed on Pt, Ru, and Pt/Ru surfaces were identified for the first time at ca. 2900-3050 cm -1 , though they were very close to the water stretching band. The band intensity of OH species adsorbed on Pt exhibited a maximum at ca. 0.6 V in acidic media and 0.7 V in alkaline media. The Ru surface was mainly covered with adsorbed OH/O in the potential region studied (0.1-0.8 V), thus exhibiting a stronger and better resolved ν(O-H) band of OH adspecies than on Pt during CO adsorption. The band at ca. 1108 cm -1 in alkaline media was ascribed to the bending mode of hydrated OH -anions in the double layer rather than the OH adspecies. Ru-modified Pt enhanced CO oxidation due to a synergistic effect and water coadsorption with CO at Ru sites. These new spectroscopic evidence are enabling a more complete understanding of the mechanism of fuel cell related electrocatalysis.
https://doi.org/10.1021/jacs.3c04785 J. Am. Chem. Soc. 2023, 145, 18439-18446
Journal of the American Chemical Society