The promoting effect of adsorbed carbon monoxide on the ... … · The promoting effect of adsorbed carbon monoxide on the oxidation of alcohols on a gold catalyst ... compounds indicated

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SUPPLEMENTARY INFORMATIONDOI: 10.1038/NCHEM.1221

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Supplementary information to

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 +31-71-5274451, E-mail: [email protected],

[email protected]

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.

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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 CH3OH 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).




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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.




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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.




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Figure S4. Transmission spectra collected in ATR configuration of the organic

compounds indicated in the figure in a 0.1 M NaOH solution




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Figure S5 FTIR spectra at different applied electrode potentials for Au(111) in 0.1

M NaOH + 2 M CH3OH recorded with p-polarized light. Eref=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 0.7-0.8 V, most




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conspicuously at 1586 cm-1 and 1652 cm-1, to be associated with the asymmetric

stretching of HCOOads 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 2740-2860 cm-1 are ascribed to asymmetric CH2 and CH3 stretches.

Finally, at most positive potentials (1.2 and 1.3 V), a band at 2347 cm-1 attributable to

CO2 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 (COtop) and

1920 cm-1 (CObridge) 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 1350-1390 and 2740-2860 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




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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 CO2 and

the regeneration of methanol [5]. The asymmetric stretching band of carbonate (1400 cm-

1), a possible reaction product of the COads or methanol oxidation, overlaps with the

bands of the formate and methylformate. However, carbonate can be converted into CO2

due to a change of the pH in the thin layer. The band corresponding to the symmetric

stretch of CO2 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, 12425 (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)




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Figure S6. FTIR spectra at different applied electrode potentials for CO-Au(111) in

0.1 M NaOH + 2 M CH3OH recorded with s-polarized light. Eref=0V vs RHE.




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Figure S7. Voltammetry of formaldehyde oxidation, 0.05 M HCHO, on Au(111) on

0.1 M NaOH; scan rate 20 mV/s.




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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 COads on ethanol oxidation is apparent from this experiment.


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The promoting effect of adsorbed carbon monoxide on the ... … · The promoting effect of adsorbed carbon monoxide on the oxidation of alcohols on a gold catalyst ... compounds indicated