Origin of Multiple Peaks in the Potentiodynamic Oxidation of CO

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Origin of Multi-peaks in the Potentiodynamic Oxidation of CO Adlayers on Pt and Ru-modified Pt Electrodes Hongsen Wang, and Héctor D. Abruña J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b00493 • Publication Date (Web): 17 Apr 2015 Downloaded from http://pubs.acs.org on April 20, 2015

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The Journal of Physical Chemistry Letters

Origin of Multi-peaks in the Potentiodynamic Oxidation of CO Adlayers on Pt and Ru-modified Pt Electrodes Hongsen Wang, and Héctor D. Abruña* Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301

CORRESPONDING AUTHOR: Héctor D. Abruña, Tel: 1-607-255-4720, Fax: 1-607-2559864, email: [email protected]. Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, NY 14853-1301 1 ACS Paragon Plus Environment


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ABSTRACT The study of the electrooxidation mechanism of COad on Pt based catalysts is very important for designing more effective CO-tolerant electrocatalysts for fuel cells. We have studied the origin of multiple peaks in the cyclic voltammograms of CO stripping from polycrystalline Pt and Ru modified polycrystalline Pt surfaces (Pt/Ru) in both acidic and alkaline media by differential electrochemical mass spectrometry (DEMS), DFT calculations and kinetic Monte Carlo (KMC) simulations. A new COad electrooxidation kinetic model on heterogeneous Pt and Pt/Ru catalysts is proposed to account for the multiple peaks experimentally observed. In this model, OH species prefer to adsorb at low-coordination sites or Ru sites, and thus suppress CO repopulation from high-coordination sites onto these sites. Therefore, CO oxidation occurs on different facets or regions, leading to multiplicity of CO stripping peaks. This work provides a new insight into the CO electrooxidation mechanism and kinetics on heterogeneous catalysts.

TOC GRAPHICS:

KEYWORDS: DEMS, kinetic Monte Carlo, DFT, COad electrooxidation, CO mobility, Pt, Pt/Ru

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The kinetics and mechanism of COad electrooxidation on platinum and Pt-based catalysts have been studied for several decades due to their importance in fuel cell related electrocatalysis.1-23 Regarding the mechanism of COad electrooxidation on Pt (or Ru), it is generally believed that the reaction follows the Langmuir-Hinshelwood (L-H) mechanism, which can be simply described as: + Pt + H2O → ← Pt-OH + H + e

(1)

Pt + CO → ← Pt-CO

(2)

Pt-CO + Pt-OH " CO2 + H+ + e- + 2Pt

(3)

Multiple peaks are often observed in the cyclic voltammogramms of COad electrooixdation on Pt and Pt binary catalysts in both acidic and basic media.7-10,16,24-29 Among them, the pre-peak (or pre-wave) is generally ascribed to the oxidation of CO adsorbed at defect sites.17,23,30 It is usually assumed that the occurrence of multiple peaks is due to slow CO diffusion on Pt surfaces.18,28 However, using EC-NMR, Wieckowski and co-workers estimated a high diffusion coefficient of CO on Pt black to be 3.6×10-13 cm2/s at a saturation coverage of CO, and 1.5×10-12 cm2/s at a fractional coverage of 0.36, respectively.31,32 Koper et al. reported an even higher CO diffusion coefficient of 10-11-10-12 cm2/s on stepped Pt(111) single crystals in acidic media.5 Several groups have also reported that two separate CO stripping peaks occur in the cyclic voltammograms for electrooxidation of adsorbed CO on Ru modified Pt(111).7-10 This is in contradiction with a fast CO mobility. To resolve this disparity, Koper et al. assumed that there might be a large binding energy gradient of adsorbed CO, and thus a slow “uphill” diffusion from the more distant Pt(111) sites to the sites in the vicinity of Ru islands.18 However, DFT 3 ACS Paragon Plus Environment


calculations33,34, TPD data35 and infrared data29,36 indicate that Ru modification does not significantly alter the binding energy of CO on Pt. Due to the above mentioned controversy, in this paper we revisit the oxidation of COad on Pt and Ru-modified Pt surfaces. On-line differential electrochemical mass spectrometry (DEMS) was used to follow the process of oxidation of a CO monolayer on Pt and Ru modified Pt due to its numerous advantages.17 Mass spectrometry can detect very small amounts of CO2 generated (1010

mol), hence we can use very slow scan rates or very low oxidation potentials. More

importantly is the fact that the mass spectrometric current of CO2 eliminates the interferences from double layer charging and electrode surface oxidation to the observed response.

1.5

(a)

0.1 mV/s (x100) 1 mV/s (x10) 10 mV/s 100 mV/s (x0.1) 500 mV/s (x0.02)

(b) Peak II Peak I Prepeak

0.9

Ep / V (RHE)

2.0

IMS / nA

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1.0

m/z=44 0.5

129 mV

0.8 0.7

56 mV

0.6

63.5 mV

113 mV

0.5

50 mV 0.0

0.0

0.4

0.2

0.4

0.6

0.8

1.0

1.2

-1

0

1

2

3

-1

log(υ / mV s )

E / V (RHE)

Figure 1. (a) MSCVs of CO2 at m/z=44 for the oxidation of a saturated CO adlayer on a polycrystlalline platinum (pc-Pt) electrode in a 0.1M H2SO4 solution at different potential scan rates. CO was adsorbed at 0.1V (vs. RHE) from a CO saturated 0.1M H2SO4 solution. The potential scan rates are indicated in the figure. (b) Peak potentials plotted vs. the logarithm of potential scan rates.

The scan rate dependent mass spectrometric cyclic voltammograms (MSCVs) of CO2 at m/z=44 for the oxidation of COad on polycrystalline Pt in 0.1M H2SO4 are shown in Figure 1a. CO 4 ACS Paragon Plus Environment

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stripping on polycrystalline Pt exhibits a pre-peak (or pre-wave) at low potentials and a main peak at high potentials, in agreement with previously reported results for CO stripping on polycrystalline Pt in acidic media.16,27,37-39 The origin of the pre-peak has been discussed in a previous paper,17 so here we mainly focus on the splitting of the main peak. At low scan rates, such as 0.1 and 1 mV/s, an additional sharp peak also occurs on the positive side of the main peak. This additional sharp peak is likely due to COad oxidation on large Pt(100) facets.16,24,28 The Tafel plots of the pre-peak, the first main peak and the second main peak are presented in Figure 1b. It should be noted that in general, the current (for a surface confined species) is proportional to the scan rate, and thus a plot of Ep vs. log (scan rate) is equivalent to a plot of Ep vs. log (current). There are two Tafel slopes for each of the three peaks: a small Tafel slope (around 59 mV) at low scan rates, and a large Tafel slope (around 118 mV) at high scan rates. Based on dynamic Monte Carlo simulations, Koper et al. found nonlinear slopes of Ep vs. logν, which were 40 mV at low scan rates and 120 mV at high scan rates. In the medium scan rate regime, the slope was between 40 and 120 mV. Our experimental data is in quite good agreement with Koper’s findings.6 Some authors have assumed that the change in Tafel slope means a change of the rate-determining step in CO oxidation.19 We believe that the change in Tafel slope with scan rates does not necessarily mean a change in the reaction mechanism, but rather a change in OH coverage with potential.6 Simulations of the voltammograms with Pt(111) electrodes have shown that the changes in the Tafel slope are indeed due to changes in the OH coverage.20 Assuming simple L-H kinetics (a mean-field approximation), the reaction rate for CO electrooxidation (R) can be expressed as: R = k2θCOθOH = k2θCO(1-θCO) 5 ACS Paragon Plus Environment


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Here θOH = (1-θCO) Where k1, k-1 and k2 denote the rate constants of OHad formation, OHad desorption and reaction between COad and OHad, respectively. θCO represents the coverage of adsorbed CO. It is assumed that k1, k-1 and k2 obey the Butler-Volmer law:

k1 = k-1 = k2 =

exp exp exp

Assuming that OHad formation and desorption are in equilibrium, e.g. k1, k-1 >> k2θCO, θOH = (1θCO) Then, R = k2θCO(1-θCO) In the low potential region, k-1 >> k1, and thus

θOH = (1-θCO) R = k2θCO(1-θCO)

==

θ CO(1-θCO)exp

Tafel slope: 2.3RT/1.5F = 39 mV ( In the high potential region, k1 >> k-1, and thus

θOH = (1-θCO)

= 1-θCO

R = k2θCO(1-θCO) =

θCO(1-θCO)exp

Tafel slope: 2.3RT/0.5F = 118 mV ( In the medium potential region, k1 +k-1 is likely to be relatively constant, and thus 6 ACS Paragon Plus Environment

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θOH = (1-θCO) R = k2θCO(1-θCO)

=

θCO(1-θCO)exp

Tafel slope: 2.3RT/F = 59 mV ( The peak potential corresponding to the Tafel slope transition from 59 mV to 118 mV indicates that beyond that potential, the OH coverage is saturated on CO-free sites, e.g. θOH = 1- θCO. For defect sites contributing to the pre-peak, the OH coverage can reach saturation at very low potentials of around +0.45 V, while for sites related to the first main peak, the OH coverage reaches saturation at about +0.70 V. For the large Pt(100) facets, corresponding to the second main peak, it is difficult to determine at which potential (which should be higher than +0.70 V) the OH coverage reaches saturation, since it overlaps with the first main peak at high scan rates.

3.0

1 mV/s (x10) 10 mV/s 100 mV/s (x0.1) 500 mV/s (x0.02)

(a)

2.5

0.8

Ep / V (RHE)

1.5

m/z=44

1.0 0.5

Peak II Peak I Prepeak

79 mV

111 mV

0.6

0.5

120 mV

0.4

0.0 0.0

(b)

0.7

2.0

IMS / nA

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0.3 0.2

0.4

0.6

0.8

1.0

1.2

0

1

2

3

-1

E / V (RHE)

log(υ / mV s )

Figure 2. (a) MSCVs of CO2 at m/z=44 for the oxidation of a saturated CO adlayer on pc-Pt electrode in a 0.1M KOH solution at different potential scan rates. CO was adsorbed at 0.1V (vs. RHE) from a CO saturated 0.1M KOH solution. The potential scan rates are indicated in the figure. (b) Peak potentials plotted vs. the logarithm of potential scan rates.



In basic media, the additional sharp peak is more significant (Figure 2). The Tafel slope transitions suggest that in basic media, the OH coverage on Pt surfaces can reach saturation at much lower potentials than those in acidic solution, which is consistent with lower potentials of COad oxidation in basic media. If the potential is reversed at the minimum between the two separate main peaks, in the subsequent positive-going scan, the pre-peak and the first main peak disappear and only the second main peak can be observed (Figure S2 in the Supporting Information section). Some authors have proposed that CO diffusion is very fast in acidic media, while it becomes very slow in basic media due to carbonate poisoning.5,28 However, CO stripping on polycrystalline Pt in both acidic and basic media exhibits two main peaks at low scan rates. This suggests that CO mobility on Pt surfaces seems to be restricted in both media. This is very similar to the case of CO stripping on Ru modified Pt surfaces.7-10,18

(a)

0.1 mV/s (x100) 1 mV/s (x10) 10 mV/s 100 mV/s (x0.1) 500 mV/s (x0.02)

2.0 1.5

m/z=44

1.0

0.8

(b) Peak I (Ru) Peak II (Pt)

0.7

Ep / V (RHE)

2.5

IMS / nA

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127 mV 0.6

133 mV

45 mV 0.5

0.5

69 mV

0.4 0.0 0.0

0.2

0.4

0.6

0.8

1.0

0.3

1.2

-1

E / V (RHE)

0

1

2

3

-1

log (υ / mV s )

Figure 3. a) MSCVs of CO2 at m/z=44 for the oxidation of a saturated CO adlayer on a Ru modified pc-Pt electrode with a Ru coverage of ~0.25 (Pt/Ru-0.25) in a 0.1M H2SO4 solution at different potential scan rates. CO was adsorbed at 0.1V (vs. RHE) from a CO saturated 0.1M H2SO4 solution. The potential scan rates are indicated in the figure. (b) Peak potentials plotted vs. the logarithm of potential scan rates.


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The scan rate dependent MSCVs of CO2 at m/z=44 for the oxidation of COad on Ru modified polycrystalline Pt with a Ru coverage of ~0.25 (Pt/Ru-0.25) in 0.1M H2SO4 are shown in Figure 3a. The pre-peak cannot be observed, likely due to an overlap with the first main peak. Two main stripping peaks are observed on Pt/Ru-0.25 at slow scan rates (

Origin of Multiple Peaks in the Potentiodynamic Oxidation of CO

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