SUPPLEMENTARY INFORMATION

Transcription

1 DOI: /NCHEM.1221 The promoting effect of adsorbed carbon monoxide on the oxidation of alcohols on a gold catalyst Paramaconi Rodriguez, Youngkook Kwon, Marc T.M. Koper* Leiden Institute of Chemistry, Leiden University, PO Box 9502, 2300 RA Leiden, The Netherlands, FAX , rodriguezperezpb@chem.leidenuniv.nl, m.koper@chem.leidenuniv.nl Index: p.2. Figure S1 demonstrates that the effect of promotion by adsorbed CO is a sustained catalytic effect that continues during a potentiostatic (constant potential) experiment. p.3. Figure S2 demonstrates the effect of promotion by adsorbed CO of the methanol oxidation on Au(100) (red curve). p.4. Figure S3 demonstrates the absence of a promoting effect of adsorbed CO on methanol oxidation on Au(110). p.5. Figure S4 shows the transmission spectra collected for various possible products of methanol oxidation in 0.1 M NaOH. p.6-8. Figure S5 shows the reflection spectra of methanol oxidation on Au(111)-CO as obtained with p-polarized light and discusses the interpretation. p.9. Figure S6 shows the reflection spectra of methanol oxidation on Au(111)-CO as obtained with s-polarized light p.10. Figure S7 shows the high reactivity of formaldehyde on Au(111) in alkaline media. p.11. Figure S8 shows the voltammetry of the oxidation of ethanol on Au(111) and Au(111)-CO in alkaline media. NATURE CHEMISTRY 1

2 Figure S1. Steady-state catalysis experiments at 0.7 V and 1.1 V for Au(111) and Au(111)-CO in 2.5 M CH 3 OH in 0.l M NaOH, showing the current and the formaldehyde and formic acid production as determined by HPLC. These experiments clearly demonstrate that CO promotion of gold is a sustained catalytic effect. The oscillations in the current are due to the sample collection (see ref.19). NATURE CHEMISTRY 2

3 Figure S2. Cyclic voltammetric profiles of the CO-Au(100) in 0.1 M NaOH ( ). Voltammogram of the Au(100) in 0.1 M NaOH+ 2.5 M MeOH in the absence ( ) and in the presence ( ) of adsorbed CO. The blank in the same electrolyte solution ( ). Scan rate 50 mvs -1. NATURE CHEMISTRY 3

4 Figure S3. Voltammogram of the Au(110) in 0.1 M NaOH+ 2.5 M MeOH in abcense ( ) and in presence ( ) of CO adsorbed. The blank in the same electrolyte solution ( ). Scan rate 20 mvs -1. NATURE CHEMISTRY 4

5 Figure S4. Transmission spectra collected in ATR configuration of the organic compounds indicated in the figure in a 0.1 M NaOH solution NATURE CHEMISTRY 5

6 Figure S5 FTIR spectra at different applied electrode potentials for Au(111) in 0.1 M NaOH + 2 M CH 3 OH recorded with p-polarized light. E ref =0V vs RHE, (A) with CO adsorbed (B) without CO adsorbed. Figure S4 compares the evolution of the potential-dependent FTIR spectra on the Au(111) and the Au(111)-CO electrode in 0.1 M NaOH in the presence of 2 M methanol in solution. It is important to mention at the outset that these spectra were obtained in the external reflection thin-layer configuration. This setup is required for doing FTIR with single-crystal electrodes but it leads to severely restricted mass transfer conditions. Let us first discuss the FTIR spectra in the absence of CO on Au(111) in right-hand panel of Figure 2 (Fig.2B). Bands start appearing in the FTIR spectra at V, most NATURE CHEMISTRY 6

7 conspicuously at 1586 cm -1 and 1652 cm -1, to be associated with the asymmetric stretching of HCOO ads or HCOO - [1] (see also Figure S3) overlapping with the bending vibration of interfacial water. Morallon et al.[2] have suggested that the band at 1650 cm - 1, which they observed during methanol oxidation on Pt single-crystal electrodes in alkaline media, could be related to changes in the acidity in the thin layer as the reaction proceeds. At slightly higher potential, bands start appearing at 1351, 1386, 2741 and 2815, 2858 cm 1. Because these bands are not observed in the absence of methanol in solution, they should be assigned to intermediates and products of the methanol oxidation. The bands at 1351 and 1386 cm -1 are attributed to the symmetric stretching of formate [2], and the features at cm -1 are ascribed to asymmetric CH 2 and CH 3 stretches. Finally, at most positive potentials (1.2 and 1.3 V), a band at 2347 cm -1 attributable to CO 2 is observed. The IR spectra of the oxidation of methanol on the CO-Au(111) electrode, as shown in the left-hand panel of Figure 2 (Fig.2A), display some significant differences compared to the bare Au(111). First of all, as shown in the inset of the figure, in the potential region below 0.5 V, the spectra show two bands at 2000 cm -1 (CO top ) and 1920 cm -1 (CO bridge ) in agreement with previous results [3,4]. Their band intensities strongly depend on the applied potential [4]. These bands are present also for Au(111)- CO in the absence of methanol in solution and are due to the irreversibly adsorbed CO on the Au(111) surface. Second, for the Au(111)-CO electrode, the two bands at 1586 and 1652 cm -1 start appearing at 0.5 V (vs. 0.7 V on the unmodified Au(111)), and the bands between and cm -1 are also observed some 0.2 V earlier than on unmodified Au(111). Again, we assign these bands to formate. The negative bands at 1460 and 1300 cm -1 can be attributed to the formation of methylformate. A weak band at NATURE CHEMISTRY 7

8 1730 cm -1 was occasionally observed overlapping with the band of the bending mode of the water (see Figure S4), which could be ascribed to the formation of methylformate, but we will not dwell on this here. The absence of bands at 1225 cm -1 and 1240 cm -1 suggests that formation dimethyl carbonate and dimethyl oxalate is difficult to detect. We note that, during the oxidation of methanol, the consumption of OH - leads a decrease of the ph in the thin layer that can induce the decomposition of the dimethylcarbonate into CO 2 and the regeneration of methanol [5]. The asymmetric stretching band of carbonate (1400 cm - 1 ), a possible reaction product of the CO ads or methanol oxidation, overlaps with the bands of the formate and methylformate. However, carbonate can be converted into CO 2 due to a change of the ph in the thin layer. The band corresponding to the symmetric stretch of CO 2 is observed from 0.9 V onwards, ca. 0.3 V earlier than on the Au(111) electrode. Herrero et al. proposed the formation of formaldehyde as an intermediate species in the methanol oxidation on gold [6]. However, Figure 2 shows no band at 1100 cm -1 as would be expected in the case of formaldehyde formation (see Figure S3). On the other hand, in the FTIR thin layer configuration, formaldehyde cannot diffuse away. Since the oxidation of formaldehyde is fast (see figure S5 in the Supporting Information), it is conceivable that under these conditions formaldehyde reacts to formate too quickly to be detected. 1. K.Nakamoto, in Infrared and Raman Spectra of Inorganic and Coordination Compounds, Wiley, New York, p.232 (1986) 2. E.Morallon, A.Rodes, J.L.Vazquez, J.M.Perez, J.Electroanal.Chem. 391, 149 (1995) 3. P. Rodriguez, J.M. Feliu, M.T.M. Koper, Electrochem.Comm.11, 1105 (2009) 4. P.Rodriguez, N.Garcia-Araez, A.A.Koverga, S.Frank, M.T.M.Koper, Langmuir 26, (2010) 5. H.Wang, B.Lu, Q.H.Cai, F.Wu, Y.K.Shan, Chin.Chem.Lett. 16, 1267 (2005) 6. J.Hernandez, J.Solla-Gullon, E.Herrero, A.Aldaz, J.M.Feliu, Electrochim.Acta 52, 1662 (2006) NATURE CHEMISTRY 8

9 Figure S6. FTIR spectra at different applied electrode potentials for CO-Au(111) in 0.1 M NaOH + 2 M CH 3 OH recorded with s-polarized light. E ref =0V vs RHE. NATURE CHEMISTRY 9

10 Figure S7. Voltammetry of formaldehyde oxidation, 0.05 M HCHO, on Au(111) on 0.1 M NaOH; scan rate 20 mv/s. NATURE CHEMISTRY 10

11 Figure S8. Voltammetry of 0.5 M ethanol oxidation in 0.15 M NaOH on Au(111) (black curve) and Au(111)-CO (red curve). Scan rate 20 mv/s. No catalytic promotion of CO ads on ethanol oxidation is apparent from this experiment. NATURE CHEMISTRY 11