In situ FTIR spectroscopic study of ethanol oxidation on Pt(111)/Rh/Sn surface. the anion effect

Article abstract of DOI:10.1016/j.elecom.2014.12.025

In situ Fourier Transform Infrared (FTIR) spectroscopy was used to study the anion effect in ethanol oxidation on Pt (111) surface modified by rhodium and tin, Pt(111)/Rh/Sn. The in situ FTIR spectra showed that ethanol oxidation reaction pathway is strongly influenced by the nature of the electrolyte anion. In perchloric and sulfuric acid electrolytes were observed the formation of acetaldehyde, acetic acid and CO₂; however in phosphoric acid the acetic acid is not observed. The sulfuric acid is the most favorable electrolyte for acetaldehyde and CO₂ formation.

Full text of DOI:10.1016/j.elecom.2014.12.025

In situ FTIR spectroscopic study of ethanol oxidation on Pt(111)/Rh/Sn
surface. The anion effect
Marcia E. Paulino, Luiza M.S. Nunes, Ernesto R. Gonzalez, Germano
Tremiliosi-Filho
PII:
DOI:
Reference:
S1388-2481(14)00406-8
doi: 10.1016/j.elecom.2014.12.025
ELECOM 5357
To appear in:
Electrochemistry Communications
Received date:
Revised date:
Accepted date:
16 October 2014
9 December 2014
22 December 2014
Please cite this article as: Marcia E. Paulino, Luiza M.S. Nunes, Ernesto R. Gonzalez,
Germano Tremiliosi-Filho, In situ FTIR spectroscopic study of ethanol oxidation on
Pt(111)/Rh/Sn surface. The anion effect, Electrochemistry Communications (2014), doi:
10.1016/j.elecom.2014.12.025
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ACCEPTED MANUSCRIPT
In situ FTIR spectroscopic study of ethanol oxidation on
Pt(111)/Rh/Sn surface. The anion effect
Marcia E. Paulino, Luiza M.S. Nunes, Ernesto R. Gonzalez, Germano
Tremiliosi-Filho*
Instituto de Química de São Carlos, Universidade de São Paulo
Avenida Trabalhador São-Carlense 400
13569-590 - São Carlos, SP, Brazil
* Corresponding author. tel.: +55 16 33739933; fax: +55 16 33739951.
e-mail address: germano@iqsc.usp.br (G.Tremiliosi-Filho).
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Abstract
In situ Fourier Transform Infrared (FTIR) spectroscopy was used to study the
anion effect in ethanol oxidation on Pt (111) surface modified by rhodium and
tin, Pt(111)/Rh/Sn. The in situ FTIR spectra showed that ethanol oxidation
reaction pathway is strongly influenced by the nature of the electrolyte anion. In
perchloric and sulfuric acid electrolytes were observed the formation of
acetaldehyde, acetic acid and CO2, however in phosphoric acid the acetic acid
is not observed. The sulfuric acid is the most favorable electrolyte for
acetaldehyde and CO2 formation
Keywords: in situ FTIR, ethanol electro-oxidation, Pt(111)/Rh/Sn, sulfuric acid,
perchloric acid, phosphoric acid.
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1. Introduction
In recent years there has been a growing interest in the study of ethanol
oxidation because of its potentiality for application in fuel cells [1-5].
The electrocatalyst can strongly influence the ethanol oxidation reaction
pathway [6-11]. Platinum is the most studied electrocatalyst and it is well known
that this reaction on platinum electrode [12,13] can occur by different
mechanistic routes, as: (i) ethanol oxidation to acetaldehyde, with further
oxidation to acetic acid; (ii) ethanol and acetaldehyde oxidatizing to CO and
additional fragments, CHx, that can be futher oxidized to CO2 [12].
Several metals have been added to Pt to improve the electrocatalytic
activity [14-16]. Pt/Rh alloy shows hability to break the C-C bond and also
promote the break of the C-H bond following the CO2 formation [7, 12].
Pt/Sn promotes the cleavage of O-H bond of the water molecule forming
oxygenated species that assist the ethanol oxidation reaction [4, 5, 15,17].
Ternary Pt-Rh-SnO2/C alloy exhibits good performance for ethanol
oxidation at low potential forming CO2 as major product [18].
In this context, Pt(111) modified by subsequente rhodium and tin adatom
depositions was used as a model surface to improve it electrocatalytic activity.
The interaction of anions of supporting electrolyte and the ethanol is equally
importante [19-23]. Thus, the anion effect (sulfate, perchlorate and phosphate)
was studied by in situ FTIR spectroscopy during the ethanol oxidation reaction
on Pt(111)/Rh/Sn.
2. Experimental
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All solutions were prepared with Millipore-MilliQ® water (18.2 MΩ cm).
HClO4 (Sigma Aldrich), H2SO4 (Merck Suprapur) and H3PO4 (Mallinckrodt)
acids were used as supporting electrolyte solutions in the concentration of 0.1
mol L-1. Solutions of Rh2(SO4)3 (Premion) and SnSO4 (Vetec) in a concentration
of 5x10-6 mol L-1 in 0.1 mol L-1 of acid media were used for deposition of sub-
monolayer of Rh and Sn on Pt(111) surface. Ethanol (J. T. Baker) and
acetaldehyde (Sigma Aldrich) were used in a concentration of 0.5 mol L-1. High
pure grade Argon (White Martins) was used to deareate the solutions. All
experiments were carried out at room temperature of 20 ± 1 oC.
A reversible hydrogen electrode (RHE) in different acid supporting
electrolyte solutions was used as reference systems and a platinum foil as a
counter electrode. Pt(111) single crystal electrode (MaTeck, 1 cm diameter)
modified with rhodium and tin was used as working electrode.
Pretreament of the single crystal and it transfer to the electrochemical
cell were described elsehere [24].
Three electrochemical cells were used. The first one was employed to
evaluated the quality of the single crystal surface and the purity of the
supporting electrolyte; the second one, was used to perform electrochemical
deposition of rodhium on Pt(111); and finally the third cell, was used for
electrochemical deposition of tin on Pt (111)/Rh.
Bare Pt(111) surface was characterized by cyclic voltammetry using a
meniscus configuration at potential range between 0.06 V and 0.85 V vs RHE in
acid media. Rhodium deposition on Pt(111) was performed by cycling (six
cycles) in the potential region 0.07 V – 0.8 V vs. RHE in 5x10-6 mol L-1
Rh2(SO4)3 solution in 0.1 mol L-1 acid medium at a sweep rate of 0.01 V s-1. Tin
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deposition on Pt(111)/Rh was performed by cycling (five cycles) at potential
range from 0.1 to 0.6 V vs. RHE in 5x10-6 mol L-1 SnSO4 solution in 0.1 mol L-1
acid medium at a sweep rate of 0.01 V s-1.
The electrochemical measurements were performed with a Solartron
1285 potentiostat. The FTIR equipment was a Nexus 670 – Nicolet equipped
with a MCT detector. Typically, in situ FTIR spectroscopy measurements were
performed simultaneously with cyclic voltammetry at potentials from 0.06 V to
0.9 V vs RHE at 2 mV s-1. The in situ spectroelectrochemical system consisted
of ZnSe IR transparent window attached to the bottom of the cell. Spectra were
computed from the average of 16 interferograms, and the spectral resolution
was set to 8 cm-1. Reflectance spectra were calculated as the ratio (R/Ro)
between a sample (R) and a reference (Ro) spectra. Positive bands represent
loss and negative bands is due to gain of species at the sampling potential
3. Results and discussion
Rhodium and tin coverages on Pt(111) electrode were calculated as Rh
= 0.66 and Sn = 0.25 [25, 26]. Fig.1(A) shows voltammetric cycles recorded
during ethanol oxidation on the Pt(111) and Pt(111)/Rh/Sn electrodes in
different acid electrolytes. The lowest onset potential for ethanol oxidation was
observed in sulfuric and phosphoric acid electrolytes. In perchloric acid
electrolyte, the Pt(111) without modification shows high ethanol oxidation
current density than the Pt(111)/Rh/Sn electrode.
Fig. 1(B) shows in situ FTIR spectra recorded during ethanol oxidation on
Pt(111)/Rh/Sn in HClO4 electrolyte. The formation of acetaldehyde, acetic acid
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and CO2 [27,28] bands at ca 930, 1282 and 2341 cm-1, respectively, confirm the
ethanol oxidation [9-12]. The band at 2983 cm-1 is attributed to the C-H
fragment [4], which appears at the potential of 0.86 V vs. RHE. This C-H
fragment can be correlated with the =COH―CH3, ―OCH2―CH3, ―CO―CH3–
CH2 species formed during the course of ethanol oxidation at high potentials as
suggested by Iwasita and collaborators [13, 29]. The band at 1110 cm-1 is
-
atributed to Cl-O stretching [30] correlated to ClO4 . The band at 1716 cm-1 is
atributed to C=O stretching [28] correlated to COOH and/or CHO which ocurrs
in potentials higher than 0.56 V vs. RHE. The inset shows a low intensity band
characteristic of the C–O stretch of a CHO group of the aldehyde at ca. 1258
cm- 1. Nevertheless, this band was not observed on bare Pt(111) [29]. Thus, the
Rh and Sn deposit promoted the appearing of this band.
In H2SO4 electrolyte the main products formed from ethanol oxidation on
Pt(111)/Rh/Sn were similar to those obtained in HClO4 electrolyte. In H3PO4
solution, the acetic acid band at ca. 1282 cm-1 is absent at the in situ FTIR
spectra (data not shown).
In general, during the ethanol oxidation reaction it is usually observed the
formation of adsorbed carbon monoxide on bare platinum catalyst surface [12,
29]. However, in this study CO is not formed on Pt(111)/Rh/Sn indicating that
this specie is not a reaction intermediate, independently of the anion.
The acetaldehyde and CO2 band intensities were chose for monitoring
the anion effect of the supporting electrolyte on the ethanol oxidation, see Figs.
1(C) and 1(D). The acetaldehyde, Fig. 1(C), and CO2, Fig. 1(D), band intensities
were direct calculated from the bands at ca. 930 and 2341 cm-1, respectively.
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The potentials observed for the initial formation of acetaldehyde occur in
the order H2SO4 < HClO4 < H3PO4, see Fig.1(C),
In the case of CO2 formation, see Fig. 1(D), the initial potentials occur at
0.46 V vs RHE in both H2SO4 and H3PO4 electrolytes, with higher band
intensities in H2SO4. In HClO4 electrolyte, the lowest CO2 band intensity, exhibit
an initial formation potential at 0.66V vs RHE. Kowal et al. [17] observed in this
same electrolyte an initial potential of 0.48 V vs. RHE on RhSnO2/Pt(111)
prepared by thermal decomposition at 200 ◦C of SnCl4 on Rh/Pt(111).
In summary, the best performance for the ethanol oxidation is observed
in the H2SO4 electrolyte due to: (i) higher band intensities associated to
formation of acetaldehyde and CO2, and (ii) lowest initial formation potentials,
0.36 V vs. RHE for acetaldehyde and 0.46 V vs. RHE for CO2, see Figs. 1(C)
and 1(D). It is important to observe that the initial formation potential for CO2 in
H2SO4 is the same one observed in H3PO4 electrolyte, however, the band
intensity in phosphoric acid is much less intense than in sulfuric acid.
Additionaly, the acetaldehyde start to appears only at high potentials, ca. 0.76 V
vs RHE in H3PO4, and coincides with the phosphate desorption potential [31,
32]. For comparison, the initial potentials for acetaldehyde and CO2 formation in
HClO4 electrolyte, are 0.56 V vs. RHE and 0.66 V vs. RHE, respectively, see
Fig. 1(C) and 1(D).
During the ethanol oxidation, its adsorption occurs at 0.3 V on Pt with a
subsequent formation of the following adsorbed species: =COH―CH3,
―OCH2―CH3, ―CO―CH3 and ―CO, that can be futher oxidized to CO2,
according to DEMS results [29]. Thus, it is possible to infer that similar
adsorbed species, except COads, may be formed in H2SO4, H3PO4 and HClO4
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electrolytes, and can be oxidized to CO2, without goes through the COads. The
Rh/Pt, Sn/Pt, prepared by deposition of Rh or Sn on Pt surface and
RhSnO2/Pt(111), prepared by thermal decomposition of SnCl4 on Rh/Pt(111),
favor the C–C bond break with formation of adsorbed CO [8,16,17] that is
further oxidized to CO2. This mechanistic step occurs at a low onset potential,
ca 0.3 V vs RHE, and involves surface adsorbed oxygen donor as, adsorbed
OH, following a well established Langmuir-Hinshelwood mechanism. On the
other hand, for the Pt(111)/Rh/Sn the CO2 is formed without previous formation
of adsorbed CO, see Fig. 1(B). Additionally, the initial potentials for CO2
formation on this material are 150 mV in H2SO4 and H3PO4 and 350 mV in
HClO4 more positives than the observed for Pt/Rh, Pt/Sn and RhSnO2/Pt(111)
electrode. These are evidences that other kind of reaction intermediates as, e.g.
=COH―CH3 and ―CO―CH3, are involved in the CO2 formation. Acetaldehyde
formation on Pt(111)/Rh/Sn electrode occurs in lower potentials than the ones
for CO2 in H2SO4 and HClO4 electrolytes, indicating that the first mechanistic
step of the ethanol oxidation involves the formation of acetaldehyde, that is
futher oxidized to CO2. However, the formation of acetaldehyde in H3PO4
medium follows a different mechanistic route that possible involves the specie
=COH―CH3, before CO2 formation.
The ethanol oxidation in phosphoric acid solution on Pt(111)/Rh/Sn
prepared in: (a) phosphoric acid solution and, (b) sulfuric acid solution, showed
similar behavior in terms of acetaldehyde and CO2 band intensities, see Figs
1(C) and 1(D) (open and closed triangles). These results suggest that the anion
used in the electrolyte for the electrode deposition does not exerce any
influence on the Rh and Sn distribution on Pt(111) electrode.
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Figure 1(E) exhibits the band intensities calculated at 1282 cm-1 for the
acetic acid formation on Pt(111)/Rh/Sn electrode during the ethanol oxidation.
The formation of this reaction product was observed only in H2SO4 and HClO4
electrolytes. Acetic acid is a natural oxidation product resulted from
acetaldehyde oxidation and involves 2 electrons in this mechanistic step [18]. In
both eletrolytes, the initial formation potentials occur at the same potential, 0.66
V vs. RHE. The band intensities are similar and increase with the potential
augmentation.
The proposed mechanism in H2SO4 and HClO4 involves the C―H
dissociation of the –C with formation of adsorbed acetaldehyde, that can be
futher oxidized to acetic acid or CO2:
The FTIR measurements in H3PO4 show that the acetic acid generation
is negligible during ethanol oxidation yielding only CO2 and acetaldehyde with a
predominant formation of CO2 in low potentials:
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Fig. 2(A) shows voltammetric curves of acetaldehyde oxidation on
Pt(111)/Rh/Sn electrode in different electrolytes. The initial acetaldehyde
oxidation potential values observed in voltammograms (Fig. 2(A)) were in
accordance with acetaldehyde bands order observed in in situ FTIR ethanol
oxidation spectra in each acid electrolyte medium, see Figs. 2 (B), 2(C) and
2(D). In the phosphoric acid electrolyte (Fig. 2(B)), the acetaldehyde band
intensity increase up to potential of 1.2 V vs. RHE. Similar behavior is observed
in perchloric and sulfuric acid, Figs. 2(C) and 2(D), respectively.
4. Conclusions
The anion effect was evaluated during the formation of acetaldehyde,
acetic acid and CO2 by in situ FTIR spectroscopy during ethanol oxidation
reaction on Pt(111)/Rh/Sn electrode. The highest band intensity and the lowest
formation potential for acetaldehyde and CO2 was obtained in sulfuric acid.
The adsorbed CO was not detected as a reaction intermediate in all
studied electrolytes. In sulfuric and perchloric acids the acetaldehyde was
formed previously the appearance of CO2. However, in phosphoric acid the
production of CO2 starts in 0.46 V vs RHE while the acetaldehyde shows up in
much higher potential, 0.76 V vs RHE. It was concluded that the phosphate
anions compete for adsorption sites with ethanol showing strong influence in the
reaction pathway. Additionally, in this electrolyte was not observed formation of
acetic acid during the ethanol oxidation reaction.
Two different mechanisms were proposed according to the anion effect in
the ethanol oxidation reaction. In sulfuric and perchloric acids, the mechanism
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involves the acetaldehyde adsorption as the first oxidation step, while in
phosphoric acid, the ethanol is direct oxidized to CO2 at low potentials and also
to acetaldehyde in high potentials.
Acknowledgments
Financial supports: Fundação de Amparo à Pesquisa do Estado de São
Paulo (FAPESP - process (2012/18807-5) and Conselho Nacional de
Desenvolvimento Científico e Tecnológico (CNPq – process 150885/2012-1).
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Figure Captions:
Figure 1. (A) Cyclic voltammograms for ethanol oxidation, 0.5 mol L-1, in 0.1
mol L-1 H3PO4, H2SO4 and HClO4, at 10 mV s-1, on Pt(111) (black line) and
Pt(111)/Rh/Sn (gray line). (B) In situ FTIR spectra recorded during ethanol
oxidation, 0.5 mol L-1, in 0.1 mol L-1 HClO4 electrolyte on Pt(111)/Rh/Sn
electrode. Insert: Bands of CHO at ca 1258 cm-1 and COOH at ca 1282 cm-1.
(C) acetaldehyde, (D) carbon dioxide and (E) acetic acid band intensities as a
function of potential in 0.1 mol L-1 acid electrolytes: (o) H2SO4, (□) HClO4 and
(∆, ▲) H3PO4. The empty and full triangle symbols corresponde to
Pt(111)/Rn/Sn prepared in phosphoric and sulfuric acid, respectively.
Figure 2. (A) Voltammetric profiles for acetaldehyde oxidation, 0.5 mol L-1, on
Pt(111)/Rh/Sn, recorded at 10 mV s-1, in 0.1 mol L-1 H2SO4 (solid line), HClO4
(dotted line) and H3PO4 (dashed line). In situ FTIR spectra recorded during
ethanol oxidation, 0.5 mol L-1, on Pt(111)/Rh/Sn in 0.1 mol L-1 (B) H3PO4, (C)
HClO4 and (D) H2SO4. The acetaldehyde band intensity occurs in 930 cm-1.
Figures
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Fig.1.
Fig. 2
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Graphical abstract
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Highlights
 The effect of the anions, sulfate, perchlorate and phosphate, on ethanol
electro-oxidation was demonstrated.
 The analysis was performed by in situ FTIR.
 Surface modified Pt (111) by Rh and Sn was used as electrode material.
 In situ FTIR spectroscopic data showed that H2SO4 is the most favorable
to acetaldehyde and CO2 formation.
20