Catalytic and electro-catalytic oxidation of formic acid on the pure and Cu-modified Pd(1 1 1)-surface

Available online at www.sciencedirect.com Journal of

Electroanalytical Chemistry Journal of Electroanalytical Chemistry 616 (2008) 27–37 www.elsevier.com/locate/jelechem

Catalytic and electro-catalytic oxidation of formic acid on the pure and Cu-modified Pd(1 1 1)-surface K. Brandt *,1, M. Steinhausen, K. Wandelt Institute for Physical and Theoretical Chemistry, University of Bonn, Wegelerstrasse 12, D-53115 Bonn, Germany Received 13 July 2007; received in revised form 12 December 2007; accepted 24 December 2007 Available online 11 January 2008

Abstract The electrooxidation of formic acid, potentially important for future fuel cells, was investigated by means of cyclic voltammetry (CV) and in situ infrared reflection absorption spectroscopy (IRRAS) both on a pure and on a copper-modified Pd(1 1 1)-electrode in sulphuric acid solutions. In situ IR spectra recorded under open-circuit conditions exhibit several vibrational bands in the carbonyl region characteristic of ‘‘free” and adsorbed formic acid as well as of decomposition intermediates. A detailed analysis of the intensity of the bands as a function of time leads to a reaction mechanism for the mere ‘‘catalytic” oxidation of formic acid at the Pd(1 1 1)/electrolyte interface. The ‘‘electro-catalytic” oxidation of formic acid under potential control starting above 0.35 V is investigated by following the evolution of carbon dioxide as a function of electrode potential. IR measurements at different but constant potentials point to an electronic structure of the solid/liquid interface in the open-circuit HCOOH/Pd(1 1 1)–H2SO4 system which resembles that at an electrode potential of about 0.4 V. Modification of the Pd(1 1 1) surface is achieved by copper deposition from a Cu2+/HCOOH/H2SO4 solution. Simultaneously, the influence of the foreign metal on the electro-catalytic oxidation of formic acid is studied in dependence on the potential scan direction. In the positive-going scan copper deposited on Pd(1 1 1) at cathodic potential inhibits the electro-catalytic oxidation of formic acid below 0.45 V by blocking catalytically active Pd sites. Conversely, during the negative-going scan the Cu2+/HCOOH/Pd(1 1 1)– H2SO4 system shows an increased oxidation activity between 0.6 V and 0.2 V due to a direct redox reaction between copper ions and formic acid. The amount of copper deposited on the surface was determined by ex situ Auger Electron Spectroscopy after a contamination free sample transfer into UHV. Ó 2008 Elsevier B.V. All rights reserved. Keywords: Palladium; Formic acid; Cyclic voltammetry; Infrared reflection absorption spectroscopy; Copper; Oxidation

1. Introduction Many technical processes in nanosciences, semiconductor fabrication and heterogeneous catalysis are based on reactions at solid surfaces. For instance, about 90% of all chemical products are made by using heterogeneous catalysts [1]. Besides solid/gas interfaces, also solid/liquid interfaces are receiving considerably increasing attention recently, due to their relevance in electrochemical deposi*

Corresponding author. Tel.: +49 761 203 9594; fax: +49 761 203 9585. E-mail address: [email protected] (K. Brandt). 1 Present address: Bernstein Coordination Site, University of Freiburg, Hansastrasse 9A, 79104 Freiburg, Germany. 0022-0728/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2007.12.015

tion and etching processes as well as in the direct conversion of chemical energy to electricity in fuel cells. With increasing miniaturisation of electronic devices the employed energy sources have to decrease in weight and size. Because of their smaller size and higher stored energy density fuel cells are superior to conventional batteries and are therefore developed for e.g. mobile phones, notebooks or PDAs (Personal Digital Assistant). Especially, the use of formic acid as fuel has received growing interest during the last years as it proves to be easy to handle, of higher cell voltage, of high energy and power density as well as of low fuel crossover [2–5]. Best results concerning power density have been achieved by the use of formic acid concentration of about 10 mol l1 [3].

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K. Brandt et al. / Journal of Electroanalytical Chemistry 616 (2008) 27–37

Several catalyst compositions have already been investigated [6–11]. The use of palladium nanoparticles or palladium alloys containing small amounts of platinum seems to be most successful. Pure platinum is not suitable as it is easily poisoned by irreversibly adsorbed carbon monoxide. In order to understand the macroscopic reaction behaviour detailed microscopic studies at the metal surface are of great interest. The mechanism of an electro-catalytic reaction depends on the nature of the electrode material which influences the chemical reactivity of the adsorbed reactants. Therefore the objective is to characterise both the properties of the metal surface and the elementary processes occurring on the surface in order to optimize the catalyst’s properties and the reaction conditions. Most spectroscopic methods in surface physics are employed under ultrahigh vacuum (UHV) conditions. However, as many industrial syntheses run under high pressure or even in liquid phase, UHV investigations can only possess model character. Therefore the use of surface analytical methods which are also applicable in the electrochemical environment (‘‘in situ”) means an important step towards ‘‘real” conditions. Studies on formic acid adsorption at transition metal surfaces have been performed since the 1970 s. Adsorption and reaction processes of formic acid have been investigated in UHV on poly- and single crystalline palladium [12–17] as well as on diverse other metal surfaces [18–35]. Also the adsorption and decomposition of formic acid under electrochemical conditions at different metal surfaces in sulphuric or perchloric acid with formic acid concentrations below 1 mol l1 have been described several times [36–51], including, in particular, studies on the influence of foreign metal atoms deposited on polycrystalline palladium on the oxidation of formic acid [52–55]. The impact of deposited palladium monolayers on platinum has also been the subject of several investigations during the last years [56–59]. In this paper we present the first CV (cyclic voltammetry) and FTIRS (Fourier transform infrared spectroscopy) investigations on the oxidation of highly concentrated formic acid ([HCOOH] = 10 mol l1), which shows the best performance in practical studies, on a Pd(1 1 1) single crystal surface as most promising model catalyst. A distinction is made between processes which result solely from the properties of the palladium (‘‘catalytic”) and those which are initiated by an applied potential (‘‘electro-catalytic”). The former will be shown by experiments under open-circuit conditions and the latter by studies under potential control. In order to change the chemical structure of the electrode and thereby either support or inhibit the surface reactions copper is electrochemically deposited on the Pd(1 1 1) surface. The ‘‘catalytic” results provide information on adsorbates as well as reaction intermediates of formic acid oxidation at the Pd(1 1 1) surface in dependence on time. To elucidate the dependence of the ‘‘electro-catalytic” reactiv-

ity on the applied potential and the chemical surface structure the evolution of carbon dioxide is used as a ‘‘probe” for the oxidation rate of formic acid. 2. Experimental All measurements were performed in a home-built transfer chamber [60] which allows the contamination free transfer of the sample from the electrochemical cell located in an argon filled glove box into a UHV chamber (2  1010 mbar) and vice versa. The Pd(1 1 1) crystal was cleaned under UHV conditions in several cycles of 15 min Ar+-sputtering at room temperature and a target current of 2.5 lA cm2, and subsequent annealing for 30 min at 1050 K. Still remaining traces of carbon or sulphur were oxidised for 2 min at 750 K in an oxygen atmosphere (p = 8  108 mbar). Finally the sample was flashed to 1150 K in order to remove excess oxygen. The surface cleanliness was controlled by AES and LEED. The palladium sample (working electrode) was placed into the electrochemical cell with a Pt wire as a counter electrode and a reversible hydrogen electrode (RHE) as reference electrode in such a way, that only the (1 1 1) surface was in contact with the electrolyte. Cyclic voltammograms were recorded with a scan rate between 10 mV s1 and 50 mV s1. For the FTIR measurements a Nicolet magna 560 spectrometer was used with a nitrogen cooled MCT detector. The IR measurements were performed under external reflection conditions. The spectro-electrochemical cell is designed as to establish a thin layer configuration. Therefore the cell window is a CaF2 prism beveled at 60° from the surface normal [61]. The spectra were recorded with a resolution of 8 cm1 and 128 scans were accumulated. The polarisation of the light can be switched between p- and s-polarisation to allow a differentiation between molecules on the surface and in the thin layer solution. An interaction between a molecule and the IR light can only take place if the dipole moment of the molecule and the electrical field vector vibrate in the same plane. As the electric field vector of s-polarised light nearly vanishes at the surface upon reflection [62], s-polarised is not suitable to detect adsorbed species but only species in the thin layer solution, while conversely, with p-polarised light molecules both on the surface and in the solution can be identified (‘‘surface selection rule”). The used electrolytes were 10 M HCOOH + 1 mM H2SO4 and 1 mM CuSO4 + 10 M HCOOH + 1 mM H2SO4 prepared from high purity water (Milli-Q purification system, >18 MX cm) and pure chemicals (Merck, HCOOH: 98–100%, H2SO4: suprapur, CuSO45H2O: 99%, pro analysi). All solutions were flushed with argon which was passed through a dual gas purification in order to reduce the residual oxygen in the argon gas to a level where no oxygen reduction currents could be detected in the voltammograms anymore. To this end argon is purified

K. Brandt et al. / Journal of Electroanalytical Chemistry 616 (2008) 27–37

over a highly dispersed copper catalyst at 80 °C for removal of any trace of oxygen and passed through a ChemisorbÒ trap for removal of volatile organic compounds. 3. Results and discussion Table 1 shows a survey of the vibrational modes and the corresponding frequencies of formic acid in the liquid (l) and in the gas (g) phase. The gas phase data are listed both for monomeric ({g}) and dimeric (2{g}) formic acid. Due to hydrogen-bridge bonds in the liquid the frequency of the OH-stretching vibration (6) has decreased compared to that in the gas phase. The most characteristic band, i.e. the most intense vibrational band, is the C@O-stretching vibration (4) between 1669 cm1 and 1777 cm1. This feature will be used to identify adsorbates and reaction intermediates. The bands below 1500 cm1 correspond to the CAOAH- (2) and the HACAO-deformation vibrations (3) which are coupled with the CAO-stretching vibration (1). Below 1000 cm1 there exist three deformation modes (I–III) which are less characteristic. In this paper we mark vibrations of molecules located in the thin layer solution by a dash (0 ) in order to distinguish them from those of molecules adsorbed on the Pd(1 1 1)surface. 3.1. Catalytic oxidation of formic acid Fig. 1a shows a series of IR spectra in 10 M HCOOH + 1 mM H2SO4 without applied potential. After immersing the prepared sample into the solution it was slowly lowered onto the prism at the bottom of the electrochemical cell. Then recording of an IR spectrum was started every 2 min as given in the figure. For each spectrum 128 scans were accumulated resulting in a recording time of approximately 1 min per spectrum. The polarisation of the IR-beam was altered after each spectrum. The first spectra taken with p- and s-polarisation, respectively, were chosen as reference spectra. Subtraction of the respective reference spectrum from each particular sample spec-

Table 1 Vibrational modes and frequencies of formic acid Number Mode type

Wavenumber (cm1) HCOOH{l} [63]

Wavenumber (cm1) HCOOH{g} [64–66]

Wavenumber (cm1) (HCOOH)2{g} [65]

I II III 1 2+3 4 5 6

– – – 1200 1300–1400 1700 2940 2500–3300

625 642 1033 1104 1223/1381 1777 2942 3569

677 – 1059 1214 1374/1415 1669 2948 –

dJCO sOH cCH mC–O dCOH/dHCO mC@O mC–H mO–H

29

trum provides the background compensated final spectra which are shown in the figures. A positive band (‘‘profit band”) indicates more absorption, i.e. a higher concentration of the absorbing species in the sample than in the reference spectrum. Conversely, a negative band (‘‘deficit band”) describes the inverse situation. The frequencies read from Fig. 1a are given in Table 2. They are numbered and assigned according to Table 1. The index M describes vibrational bands of monomeric formic acid. Fig. 1b shows a fit of the feature in the carbonyl region by five Lorentz curves. A negative band at 1643 cm1 which is assigned to the HAOAH-deformation vibration of water as well as four bands above 1700 cm1 which characterise different C@O-containing species can be observed. Following Table 1 the negative band at 1720 cm1 (40 ), which can be detected with p- as well as with s-polarised light describes the C@O-stretching vibration of formic acid in the thin layer solution. It corresponds to the deficit bands at 1199 cm1 (10 ) and 1400 cm1 (30 ) which characterise the m(CAO)- (10 ) and the coupled d(CAOAH)-/d(HACAO)-vibrations (30 ). The bands of the CAH-stretching vibration at about 2940 cm1 (50 ) and of the OAH-stretching vibration between 2900 cm1 and 3400 cm1 (60 , max. 3190 cm1, s-polarisation) complete the spectrum of formic acid in the thin layer solution. The profit carbonyl band at 1757 cm1 ð40M Þ appears also with p- and s-polarised light and is therefore assigned to a species in the thin solution layer. Its relatively high frequency points to the fact that there exist small amounts of monomeric formic acid. As shown in Fig. 2c the intensity of this band increases over a period of up to 12 min after immersion, then remains constant for 8 min until a further increase is observed. Due to their very weak intensity corresponding deformation vibrations of monomeric formic acid cannot be detected. Furthermore, Fig. 1a and b show a negative C@Ostretching vibration band at 1794 cm1 (4M) arising from the Pd(1 1 1) surface. The clearly increased band position and the lower band width, compared to the ‘‘free” acid in the solution, suggest that formic acid is adsorbed as monomer after breaking the hydrogen-bridge bonds. This corresponds to a shift of the OAH-stretching vibration (6M) to higher wavenumbers as its intensity maximum shifts to 3270 cm1 (6M, p-polarisation). Additional vibrations of formic acid located on the surface can be observed at 1230 cm1 (1M) and 1431 cm1 (3M). The fit functions which lead to the assigned wavenumbers are shown in Fig. 1c. According to Table 1 these vibration frequencies can be interpreted as coupled HACAO-/CAOAH-deformation (3M) and CAO-stretching (1M) vibrations. The integrated band intensities of formic acid on the surface (Fig. 2a: 1M, 3M, 4M) and in the thin layer solution (Fig. 2b: 10M , 30M , 40M ) are displayed in Fig. 2 as a function of time. Due to overlap of the CAH- and OAH-bands a distinct analysis of the region above 2900 cm1 is not possible. At the beginning of the experiment a small increase of the strongest surface vibration at 1794 cm1 (4M) is observed.

K. Brandt et al. / Journal of Electroanalytical Chemistry 616 (2008) 27–37

(a) 1’ 1M 3’ 3M

4’M 4M 7 4’

8’

5’

6’ 6M

s-polarisation

H2 O

1757

(b) 30 min.

(4M’ )

absorbance / a.u.

30

22 min. 14 min. 6 min.

1643 1804 (7)

1794 1720

1×10-2

(4M)

(4’ )

absorbance / a.u.

28 min.

1600 1700

1800 1900

wavenumber / cm-1

20 min. 16 min. 12 min. 8 min. 4 min.

1500

2000

2500

absorbance / a.u.

24 min.

(c) 1199

1431

(1’ )

(3M)

1230 (1M)

1400 (3’ )

1200

1300

1400

1500

wavenumber / cm-1

3000

wavenumber / cm-1 Fig. 1. (a) Series of FTIR spectra in 10 M HCOOH + 1 mM H2SO4 without applied potential, reference 0 min, p- and s-polarisation. (b) Fit of the absorption feature between 1600 cm1 and 1800 cm 1 by superposition of Lorentzian functions, sample time: 24 min, p-polarisation. (c) Fit of the vibrational bands at 1200 cm1 and 1400 cm 1 by superposition of Lorentzian functions, sample time: 24 min, p-polarisation.

Table 2 Vibrational modes and frequencies according to Figs. 1 and 2 Number

Mode type

Wavenumber (cm1)

10 1M 30 3M 40 40M 4M 50 6M 60 7 80

mC–O mC–O dCOH/dHCO dCOH/dHCO mC@O mC@O mC@O mC–H mO–H mO–H mC@O masO@C@O

1199 1230 1400 1431 1720 1757 1794 2940 2900–3500 2900–3400 1804 2345

After 8 min formic acid disappears from the surface which results in increasing deficit bands at 1230 cm1 (1M), 1431 cm 1 (3M) and 1794 cm1 (4M). Since the bands become negative formic acid has to be adsorbed on the surface even at the reference spectrum. That means formic acid adsorbed immediately on the surface while the sample was immersed into the electrolyte. It has to be emphasized that the CAO-stretching vibration which is normally weak shows more intensity than the carbonyl stretching vibration. This suggests that the CAO-bond is more tilted with respect to the surface than the C@O-bond. After 16 min even the vibrations of ‘‘free” formic acid in the thin layer

solution show increasing deficit bands at 1199 cm1 (10 ), 1400 cm 1 (30 ) and 1720 cm1 (40 ). The very small, negative-going band at the high frequency end of the carbonyl region at 1804 cm1 (Fig. 1a: 7) can be assigned to the m(C@O)-stretching vibration of an anhydride [67]. As already the first sample spectrum displays this negative band, this species has to be formed spontaneously when the crystal is immersed into the electrolyte and lowered onto the prism. The concentration of this surface species drops continuously during the first 16 min (Fig. 2c). A strong positive band at 2345 cm1 (80 ) in Fig. 1a which is assigned to the asymmetric O@C@O-stretching vibration of carbon dioxide shows that CO2 evolution starts immediately after the surface has been dipped into the electrolyte and characterises the catalytic oxidation of formic acid on the Pd(1 1 1) surface in the absence of an external potential. Fig. 2d shows the integral intensity of this asymmetric O@C@O-stretching vibration (80 ) as a function of time. The intensity increases for the first 12 min in a fairly linear way. Afterwards the slope of the curve becomes lower. Summarizing the observed results the catalytic oxidation of formic acid on the Pd(1 1 1) surface can be described as follows. Immersing the sample into the electrolyte formic acid adsorbs from the thin layer solution (bands 10 , 30 , 40 , 50 , 60 ) as monomer (bands 1M, 3M, 4M, 6M) on the surface. Beside this, hydrogen as well as water separation leads to

K. Brandt et al. / Journal of Electroanalytical Chemistry 616 (2008) 27–37

time /min 0 0.4

4

(a)

8

12

16

20

24

28

ν = 1794 cm-1 (4M) ν = 1431 cm-1 (3M)

0.0

31

observed it can be concluded that the decomposition of the monomers occurs via dehydrogenation rather than dehydratisation [45,46]. This implies that formic acid is bound more weakly on palladium than for example on platinum thereby disfavouring cleavage of the CAO bond which would result in water separation.

-0.4

3.2. Electro-catalytic oxidation of formic acid

(b)

ν = 1199 cm-1 (1’ )

0.0

ν = 1400 cm-1 (3’ )

intensity / a.u.

-0.4

ν = 1720 cm-1 (4’ )

-0.8 -1.2 0.4

(c)

ν = 1757 cm-1 (4M’ )

0.0 -0.4

ν = 1804 cm-1 (7)

(d) 0.4

ν = 2345 cm-1 (8’ )

0.2 0.0 0

4

8

12

16

20

24

28

time / min Fig. 2. 10 M HCOOH + 1 mM H2SO4, without applied potential, reference 0 min, p-polarisation: time dependence of the integral intensities of the formic acid absorption bands (a) on the surface and (b) in the thin layer solution. Time dependence of the integral intensities of (c) the bands at 1757 cm1 ð40M Þ and 1804 cm1 (7) and (d) the CO2 band at 2345 cm1.

an adsorbed anhydridic species (band 7). During the measurements equilibrium is established between the electrode surface and the thin layer solution which results in an addition reaction between water and the anhydride, the latter dissociating in carbon dioxide (band 80 ) and monomeric formic acid (band 40M ). Therefore the presence of water avoids formation of irreversibly adsorbed CO upon decomposition of the anhydride. By contrast, results from experiments after immersion of a Pd(1 1 1) electrode in pure formic acid without any water being present showed that the anhydride decomposes in carbon dioxide and carbon monoxide which remains adsorbed on the surface [63]. After the dissociation of the anhydride has been completed CO2 is formed by hydrogen abstraction from the adsorbed monomers on the surface which leads to a depletion of formic acid on the surface and in the nearest solution layers. Therefore formic acid has to diffuse from the thin layer solution to the surface where it is oxidised immediately without forming adsorbed species. As in the course of the catalytic reaction process no CO formation could be

After we have investigated the catalytic processes under open-circuit conditions the following part will display the electrochemical experiments with potential control. Fig. 3 shows a cyclic voltammogram of the Pd(1 1 1) electrode in 10 M HCOOH + 1 mM H2SO4 solution recorded with a scan rate of 0.05 V/s in the spectro-electrochemical cell (black line). A cyclic voltammogram in pure 1 mM H2SO4 (Fig. 3 dotted line) is also displayed for comparison. In the negative-going scan it can be observed that formic acid hinders the hydrogen ad- and desorption on the Pd(1 1 1) surface, respectively, as the corresponding current peak is shifted to more cathodic potential. Above 0.35 V in both the positive- and the negative-going scans the formic acid containing solution shows different current-potentialcharacteristics from that in pure sulphuric acid solution. A strong anodic current is observed which originates from formic acid oxidation. The shape of the curve remains stable for many potential cycles which suggests that no poisoning of the surface by irreversibly adsorbed decomposition products occurs. In order to further investigate the influence of an externally applied potential on the oxidation of formic acid the following FTIR experiments were carried out under potential control. The spectra strongly differ from those at opencircuit. Especially the complex superimposition of various vibrational bands in the carbonyl (1600–1800 cm1) and hydroxyl region (2800–3400 cm1) does not allow an unambiguous analysis of the spectra regarding the mechanism of the formic acid oxidation. Therefore, we restrict the analysis of our results to the presentation of the carbon dioxide evolution as a function of potential and time as a ‘‘probe” for the rate of formic acid oxidation. Irreversibly adsorbed intermediates like carbon monoxide are not found on the surface. This suggests that, e.g. in current density / Acm-2

-1 ν = 1230 cm (1M)

-0.8

80 40 0 -40 0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

potential / V versus RHE Fig. 3. Cyclic voltammograms of the Pd(1 1 1) electrode in 10 M HCOOH + 1 mM H2SO4 (——) and in 1 mM H2SO4 (), dE/ dt = 50 mV s1, potentials referred to RHE.

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K. Brandt et al. / Journal of Electroanalytical Chemistry 616 (2008) 27–37

contrast to platinum, palladium prefers the dehydrogenation reaction of formic acid resulting in carbon dioxide formation rather than dehydratisation leading to irreversibly adsorbed carbon monoxide [12,21,36,45,46]. This includes that in analogy to the catalytic process the electro-catalytic oxidation reaction proceeds via weakly bound intermediates which do not allow any CAO cleavage. Fig. 4a displays a typical series of p-polarised IR spectra of the Pd(1 1 1) surface in 10 M HCOOH + 1 mM H2SO4. After immersing the prepared sample into the solution at a potential of 0.2 V it was slowly lowered onto the prism at the bottom of the electrochemical cell. The potential was then changed in steps of 0.05 V in the anodic direction until a final value of 0.7 V and IR spectra were recorded at each ‘‘sample” potential as given in the figure. After each potential step the following measurement was started with a delay of 10 s. The reference spectrum was chosen at 0.2 V.

(a) 2*10

-2

(b) 4*10

-2

0.20 V 0.25 V 0.70 V 0.30 V

absorbance / a.u.

0.65 V

0.35 V

0.60 V

0.40 V

0.55 V 0.50 0.45 0.40 0.35

0.45 V

V V V V

0.50 V 0.55 V 0.60 V

0.30 V

0.65 V

0.25 V 2200

2400

2200

2400

wavenumber / cm-1 ad b)

intensity / a.u.

1.6

(c)

1.2 0.8 0.4 0.0

ad a) 0.2

0.3

0.4

0.5

0.6

0.7

potential / V versus RHE Fig. 4. Series of potential-modulated FTIR spectra in 10 M HCOOH + 1 mM H2SO4, p-polarisation. (a) Immersion at 0.2 V, reference at 0.2 V. (b) Immersion at 0.7 V, reference at 0.7 V. (c) Integral intensities of the asymmetric O@C@O-stretching vibration bands at 2345 cm1 depending on the applied potential.

All spectra in Fig. 4a show a positive band at 2345 cm1 which grows with increasing potential and which is attributed to the asymmetric stretching vibration of CO2. Carbon dioxide desorbs from the palladium surface at temperatures above 90 K. Therefore it is not necessary to compare p- and s-polarised spectra as CO2 is only detected in the thin layer solution after desorption from the electrode. In order to gain more information about the relative accumulation of CO2 in the thin layer solution the integrated band intensities are displayed in Fig. 4c as a function of electrode potential. With increasing potential the intensity starts to grow from 0.2 V to 0.35 V in a fairly linear way. Afterwards an exponential growth of intensity is observed which, according to Fig. 3, marks the beginning of the electro-catalytic oxidation of formic acid. Compared to the experiments in 10 M HCOOH + 1 mM H2SO4 at open-circuit (Figs. 1 and 2) CO2 evolution is little weaker below 0.4 V. Above 0.4 V the oxidation of formic acid is clearly accelerated. In addition to the measurements mentioned above we now discuss a series of spectra with anodic immersion potential in order to emphasize the possibility of influencing the formic acid oxidation at the surface by changing the starting point. Fig. 4b shows a series of in situ IR-spectra in 10 M HCOOH + 1 mM H2SO4 solution with ppolarisation. The immersion potential, which was also taken as reference potential, was 0.7 V. The spectra were recorded in the potential modulation mode by varying the potential in constant steps to the final value of 0.2 V. A positive asymmetric O@C@O-stretching vibration band appears at 2345 cm1 which characterises a strong CO2 evolution leading to a maximum carbon dioxide concentration in the thin layer solution at 0.45 V (Fig. 4c). As the potential is further decreased more CO2 disappears from the thin layer solution than is formed at the surface. This results in a weak decrease in intensity towards lower potentials. Below 0.35 V the electro-catalytic oxidation of formic acid stops (Fig. 3) which leads to a stronger decrease in the CO2 intensity. In order to combine the investigations of time and potential dependence of formic acid oxidation, several spectral series, each with a given constant applied potential, were recorded. Fig. 5 shows three series (a–c) of IR spectra in 10 M HCOOH + 1 mM H2SO4 at 0.2 V, 0.4 V and 0.5 V. To this end the prepared sample was immersed at the respective potential into the solution and lowered onto the prism at the bottom of the electrochemical cell. The reference spectrum was recorded immediately at t = 0 min. Then recording of IR spectra was started every 2 min. Only every second spectrum is displayed. All three spectral series show the positive band of the asymmetric O@C@O-stretching vibration at 2345 cm1 which characterises formic acid oxidation. The integral intensities of these three bands and therefore their time dependent evolution are presented in Fig. 5d. The gradient of the curves becomes considerable lower as time passes.

K. Brandt et al. / Journal of Electroanalytical Chemistry 616 (2008) 27–37

(a) 0.2 V

(c) 0.5V

-2

absorbance / a. u.

2*10

(b) 0.4 V

t / min 28 24 20 16 12 8 4 2340

intensity / a. u.

1.6

2340 2340 -1 wavenumber / cm

(d) 0.5 V

1.2 0.8

open-circuit

0.4

0.4 V

0.0

0.2 V

0

4

8

12 16 time / min

20

24

28

Fig. 5. Series of FTIR spectra in 10 M HCOOH + 1 mM with constant potential, reference at 0 min, p-polarisation, immersion at (a) 0.2 V, (b) 0.4 V and (c) 0.5 V. (d) Time dependence of the integral intensities of the asymmetric O@C@O-stretching vibration bands at 0.2 V (N), 0.4 V (h), 0.5 V (j) and at open-circuit (d).

The relatively strong difference in intensity between 0.4 V and 0.5 V reveals that oxidation increases very strongly in this potential region due to the onset of electro-catalytic oxidation. Additionally, Fig. 5d shows the integral intensity of the CO2 band without applied potential. Comparing the data it can be concluded that the electronic state of the solid/liquid interface in the open-circuit HCOOH/Pd(1 1 1)–H2SO4 system corresponds to that at about 0.4 V. 3.3. Influence of copper deposition An additional parameter, besides the electronic properties of the palladium metal itself (‘‘catalytic oxidation”) and the applied potential (‘‘electro-catalytic oxidation”), which may influence the formic acid oxidation, has been introduced by electrochemically depositing a second metal on the Pd(1 1 1) surface and, thereby, altering the elemental composition of the electrode surface. The adsorbed metal should be ad-, respectively, desorbed in the potential region around 0.4 V between hydrogen absorption and palladium dissolution which is the critical regime where the electro-

33

chemical oxidation of formic acid is initiated. This allows to study the oxidation reaction upon simultaneous metal deposition or dissolution. Additionally, in order to avoid a mixture between the catalytic effects of palladium with those of the foreign metal, copper as a widely inactive component regarding formic acid oxidation is used to modify the Pd(1 1 1)-surface. Electrochemically upd-deposited copper forms a complete pseudomorphic (1  1) monolayer on Pd(1 1 1) [68–70]. Bulk deposition is determined by layerby-layer growth for the first 10 layers [70]. Fig. 6 a shows the cyclic voltammograms of Pd(1 1 1) in 10 M HCOOH + 1 mM CuSO4 + 1 mM H2SO4 (black line) and, for comparison, in formic acid free 1 mM CuSO4 + 50 mM H2SO4 solution (dotted line) with a scan rate of 0.01 V/s. The cyclic voltammogram in 1 mM CuSO4 + 50 mM H2SO4 displays two cathodic (K1, K2) and two anodic peaks (A1, A2). The under potential deposition of one monolayer copper starts below 0.45 V in the cathodic direction (K2). The transferred charge density can be calculated to approximately 410 lC cm2 which corresponds to about 85% of a complete pseudomorphic monolayer copper on Pd(1 1 1) if two electrons are transferred. The monolayer dissolution starts above 0.45 V in the positive-going scan (A2). At potentials below 0.2 V bulk copper is deposited (K1) and desorbs again above 0.2 V in the positive-going scan (A1). The shape of the current–voltage curve changes dramatically if the Pd(1 1 1) electrode is immersed into 10 M HCOOH + 1 mM CuSO4 + 1 mM H2SO4. During the negative-going scan a broad feature (KFA) between 0.45 V and 0.2 V can be observed. After changing the potential sweep FA direction three anodic current waves (AFA and AFA 1 , A2 3 ) FA are detected. As K is referred to copper deposition it has to be emphasized that the cyclic voltammogram does not offer any possibility to distinguish between monolayer and bulk deposition under these conditions. Compared to the cyclic voltammogram in the formic acid free, copper containing solution, copper deposition in the formic acid containing solution shows a maximum in current density at higher potential (than K1) and converges with decreasing potential toward a current density limit. The anodic features AFA (max. 0.33 V) and AFA (max. 1 2 0.4 V) correspond to Cu bulk dissolution and monolayer desorption, respectively. Due to the width and the extended overlap of these two peaks an exact determination of the charge is not meaningful. The observed dissolution of bulk copper at potentials more positive than in the formic acid free electrolyte indicates that the adsorbate layer is stabilized in the presence of formic acid. In contrast the copper monolayer desorbs at more cathodic potential. After dissolution of the copper adsorbate layers the current density decreases. Above 0.48 V the current density increases again ðAFA 3 Þ as formic acid oxidation starts. The potential where formic acid oxidation starts has shifted to a higher value (compare Fig. 3). This points to the fact that copper, even

34

K. Brandt et al. / Journal of Electroanalytical Chemistry 616 (2008) 27–37

potential / V versus RHE 0.2

(a)

current density / Acm-2

0.4

multilayerCV

200

0.5

0.6

mono layerCV

A1

FA

A1

FA

A2

FA

A3

A2

100 0 -100 -200

ICu /(ICu+I Pd)

0.3

K

FA

K2

K1

(b)

multilayerA ES

0.06 0.04

mono layerA ES

0.02 0.2

0.3

0.4

0.5

0.6

emersion potential / V versus RHE

intensity / a.u.

(c)

734 778 842

110 192 247 82 283 47 66

942 922

333

100 200 300 400 500 600 700 800 900

kinetic energy / eV Fig. 6. (a) Cyclic voltammograms of the Pd(1 1 1) electrode in 1 mM CuSO4 + 10 M HCOOH + 1 mM H2SO4 (——) and in 1 mM CuSO4 + 50 mM H2SO4 (), dE/dt = 10 mV s1, potentials referred to RHE. (b) Normalized Auger intensities of the 922 eV Cu-peak depending on the emersion potential, emersion from 1 mM CuSO4 + 10 M HCOOH + 1 mM H2SO4 (j) and 1 mM CuSO4 + 50 mM H2SO4 (h). (c) Auger spectrum of the copper covered Pd(1 1 1) surface after emersion at 0.25 V from 1 mM CuSO4 + 10 M HCOOH + 1 mM H2SO4.

at residual coverages in the submonolayer regime, lowers the electro-catalytic activity of the surface. At about 0.55 V the current–voltage curve approaches a current density of about 100 lA cm2. Immediately after potential inversion the curves do not deviate much. At 0.47 V and 0.55 V the anodic and the cathodic curves cross each other indicating some hysteresis effects. As already mentioned in the experimental section the advantage of the FTIR transfer chamber consists in the combination of surface analytical methods in solution and spectroscopic methods in UHV. After the cyclovoltammetric measurements presented in Fig. 6a the sample was emersed at a certain potential in the

negative-going scan, rinsed with 1 mM H2SO4 at 0.2 V and transferred back to UHV. In order to determine the amount of copper which has been deposited an Auger spectrum of the copper covered surface was recorded immediately after the transfer. In Fig. 6c an Auger spectrum after emersion at 0.25 V is shown and the signals of the palladium substrate at 47 eV, 82 eV, 192 eV, 247 eV, 283 eV and 333 eV are observed [71]. Evidence for sulphur (153 eV) and oxygen (510 eV) can also be found. Particularly noteworthy are the copper signals at 66 eV, 110 eV 734 eV, 778 eV, 842 eV, 922 eV and 942 eV (Fig. 6c: italic). In order to correlate the potential and the deposited amount of copper on the Pd(1 1 1) surface the Auger intensity of the largest copper signal at 922 eV is normalized to the largest palladium signal at 333 eV and plotted versus the corresponding emersion potential (Fig. 6b). The open squares in Fig. 6b display the Auger intensities after emersion from 1 mM CuSO4 + 50 mM H2SO4 and the full squares those from 10 M HCOOH + 1 mM CuSO4 + 1 mM H2SO4. The analysis of the normalized Auger intensities at emersion potentials of 0.25 V and 0.3 V from the formic acid containing solution shows that at both potentials there is already bulk copper deposited on the surface. Therefore, it can be concluded that formic acid supports the deposition of copper following reaction (1). This effect prevents that a defined Cu upd layer is formed. Cu2þ þ HCOOH ! Cu þ CO2 þ 2Hþ

ð1Þ

In order to study the efficiency of formic acid oxidation at the electrode during ad- and desorption of copper, respectively, IR spectra were recorded using the potential modulation technique. Fig. 7a displays a typical series of p-polarised IR spectra of the Pd(1 1 1) surface in 10 M HCOOH + 1 mM CuSO4 + 1 mM H2SO4 solution. The prepared sample was immersed into the solution at a potential of 0.2 V. The potential was then changed in steps of 0.05 V in the anodic direction and IR spectra were recorded at each ‘‘sample” potential as given in the figure. The reference was chosen at 0.2 V. All spectra display the band of the O@C@O-stretching vibration of carbon dioxide in the thin layer solution at 2345 cm1. In Fig. 7c the relative integrated intensities are plotted as a function of the applied potential. Between 0.2 V and 0.3 V no CO2 evolution is observed. As concluded from the cyclic voltammogram, copper bulk deposition occurs in this potential region and leads to a complete inhibition of the formic acid oxidation. As the copper multilayer desorbs between 0.3 V and 0.35 V a very small CO2 band is detected. During copper monolayer desorption between 0.35 V and 0.45 V formic acid oxidation starts. Above 0.45 V the CO2 evolution rises exponentially. For comparison Fig. 7c shows again the potential dependent intensity of the CO2 band in copper free 10 M HCOOH + 1 mM H2SO4 which is always greater than in the copper containing electrolyte. It can be concluded that even a small number of copper atoms on the palladium sur-

K. Brandt et al. / Journal of Electroanalytical Chemistry 616 (2008) 27–37

(a)

strongly multi-bound, species, e.g. xxxCOH, can be excluded as, if they existed on the surface, copper adsorption should induce an increase of formic acid oxidation due to the lack of adjacent adsorption sites [45,46,53]. Fig. 7b shows a series of IR spectra in 10 M HCOOH + 1 mM CuSO4 + 1 mM H2SO4 with anodic start potential. The immersion potential which was also taken as reference potential was 0.7 V. The spectra were recorded in the potential modulation mode by varying the potential in constant steps to a final value of 0.2 V. The vibrational band of carbon dioxide at 2345 cm1 characterises the immense electro-catalytic formic acid oxidation which achieves a maximum at 0.4 V. Further lowering of the potential leads to a decrease in the CO2 concentration in the thin layer. Compared to the copper free formic acid solution the curves are identical until 0.55 V before copper deposition starts. Below 0.55 V a clear superiority in reactivity of the copper containing electrolyte is observed which also confirms the conclusion that formic acid directly reacts with copper ions at the palladium surface (reaction 1) to form carbon dioxide and adsorbed copper.

(b) 2×10-2

1×10-2

0.20 V

absorbance / a.u.

0.25 V 0.30 V

0.70 V 0.65 V

0.35 V

0.60 V

0.40 V

0.55 V

0.45 V

0.50 V 0.45 V 0.40 V 0.35 V 0.30 V

0.50 V 0.55 V 0.60 V 0.65 V

0.25 V 2300

2300

2400

wavenumber / cm-1

intensity / a.u.

1.6

(c)

35

2400

wavenumber / cm-1

ad b)

1.2

4. Conclusion

0.8 0.4 0.0

ad a) 0.2

0.3

0.4

0.5

0.6

0.7

potential / V versus RHE Fig. 7. Series of potential-modulated FTIR spectra in 1 mM CuSO4 + 10 M HCOOH + 1 mM H2SO4, p-polarisation. (a) Immersion at 0.2 V, reference at 0.2 V. (b) Immersion at 0.7 V, reference at 0.7 V. (c) Integral intensities of the asymmetric O@C@O-stretching vibration bands in 1 mM CuSO4 + 10 M HCOOH + 1 mM H2SO4 (j) and in 10 M HCOOH + 1 mM H2SO4 (s) depending on the applied potential.

face block reactive adsorption sites needed for cleavage of the CAH-bond and for adsorption of hydrogen and oxidation intermediates. The observed inhibition of formic acid oxidation by copper at potentials around 0.4 V leads to the conclusion that the electrochemical oxidation proceeds predominantly via weakly adsorbed intermediates like single-bound xCOOH-fragments which are then directly converted into hydrogen and carbon dioxide. Other, more

The adsorption and oxidation of formic acid on the Pd(1 1 1) surface in sulphuric acid solution was studied by means of cyclic voltammetry, in situ FTIR Spectroscopy and Auger Electron Spectroscopy. The ‘‘catalytic” results, especially the intensive vibrational bands in the carbonyl region, provide information on adsorbates as well as reaction intermediates of spontaneous formic acid oxidation at the Pd(1 1 1) surface. Analysis of the in situ IR spectra recorded under open-circuit conditions leads to a reaction mechanism for the ‘‘catalytic” oxidation of formic acid which is summarised in Fig. 8. While lowering the sample into the electrolyte formic acid adsorbs as monomer as well as anhydridic species on the surface. During the experiment the system aims at an equilibrium state between the electrode surface and the thin layer solution. This results in an addition of water to the anhydride, which decomposes into carbon dioxide and monomeric formic acid. Afterwards hydrogen abstraction from the adsorbed monomers on the surface starts and CO2 is formed. Thereby, formic acid on the surface and in the adjacent solution layers is consumed, so that further acid diffuses from the thin layer solution to the surface

- HCOOH + H 2O - HCOOH, - 2 Had

[(HCOOH)2 ] - 2 Had - H2O

[HCOOH]ad

[(OC)2O]ad

+ H 2O - Had, - e-, - H3O+

CO2

+ H 2O

HCOOH Fig. 8. Reaction mechanisms: catalytic oxidation of formic acid on Pd(1 1 1) at open-circuit potential.

36

K. Brandt et al. / Journal of Electroanalytical Chemistry 616 (2008) 27–37

where it is oxidised immediately. No poisoning of the surface by strong adsorbing intermediates has been observed. In order to investigate the ‘‘electro-catalytic” oxidation of formic acid FTIR experiments were carried out with potential control. The complex structure of the spectra demands a concentration on the carbon dioxide evolution as a function of potential and time. In the potential-modulated experiments, an exponential growth of the intensity of the CO2 band at 2345 cm1 above 0.35 V determines the onset of the electro-catalytic oxidation of formic acid. IR measurements at constant potentials show that the electronic state of the solid/liquid interface in the open-circuit HCOOH/Pd(1 1 1)–H2SO4 system corresponds to that at about 0.4 V. Copper deposition determines the degree of formic acid oxidation on Pd(1 1 1) depending on the potential scan direction. Cyclic voltammograms as well as the IR experiments show that inhibition due to copper blocking active Pd sites occurs in the positive-going scan below 0.45 V until copper has completely desorbed from the surface. It can be concluded that formic acid is electrochemically oxidised via weakly bound intermediates which then directly decompose in hydrogen and carbon dioxide. During the negative-going scan the system shows an even increased formic acid oxidation between 0.6 V and 0.2 V. This comes along with a stronger copper deposition due to a direct reaction between formic acid and copper ions. AES data were used to analyse the amount of copper deposition on the surface.

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