J. Phys. Chem. C 2009, 113, 15639–15642
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First-Principles Considerations on Catalytic Activity of Pd toward Ethanol Oxidation Guofeng Cui,† Shuqin Song,‡ Pei Kang Shen,*,‡ Andrzej Kowal,§ and Claudio Bianchini*,| School of Chemistry and Chemical Engineering, Sun Yat-Sen UniVersity, Guangzhou, 510275, China, State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and Engineering, Sun Yat-Sen UniVersity, Guangzhou, 510275, China, Institute of Catalysis and Surface Chemistry, Polish Academy of Science, PL-30239 Krakow, Poland, and Istituto di Chimica dei Composti Organometallici (ICCOM-CNR) Via Madonna del Piano 10, 50019 Sesto Fiorentino, Italy ReceiVed: February 1, 2009; ReVised Manuscript ReceiVed: June 28, 2009
The ethanol oxidation on Pd electrocatalysts is dramatically affected by the pH of the aqueous ethanol solution: no reaction occurs in acidic solutions, while the reaction is fast in alkaline solutions. A rationale for the origin of this pH effect on the ethanol oxidation to acetaldehyde has been provided by density functional theory (DFT) calculations. The DFT calculations show that in acidic media continued dehydrogenation of ethanol is difficult due to the lack of OH species to instantly remove hydrogen, which inhibits the ethanol electrooxidation. Conversely, both ethanol and sufficient OH can adsorb on Pd in alkaline media, leading to continuous ethanol electrooxidation. 1. Introduction Palladium as a relatively abundant resource has been intensively studied as the Pt-alternative electrocatalyst due to its desirable activity toward the oxygen reduction reaction (ORR).1-3 It has been found that alloying Pd with other transition metals can improve its catalytic activity.4 The alloying component would play a synergistic role on the surface of Pd to affect the thermodynamics and/or the kinetics of the elementary steps of the ORR.5,6 Some researchers have also emphasized the modulation effects on the electronic structure of Pd so as to influence significantly the surface reactivity.4,7 Recent studies have demonstrated that Pd exhibits even a higher activity than Pt for alcohol electroxidation in alkaline media8-11 where the electrocatalyst stability is also higher.12 The adoption of Pdbased electrocatalysts might further reduce the cost of manufacturing electrochemical devices like fuel cells. It is well-known that Pd is inactive for alcohol electroxidation in acidic media. However, no clear-cut explanation for the origin of this pH effect has been provided so far. Herein, we report a first-principles study, based on the density functional theory (DFT), along with some experimental results, of the pH effect on the first stage of ethanol oxidation leading to acetaldehyde formation. 2. Experimental Section 2.1. Preparation of Pd Electrocatalysts. All chemicals were of analytical grade and were used as received. The Pd electrocatalysts were prepared by the pulse microwave-assisted polyol method as previously described.13 Typically, palladium dichloride (4.7 mL of 0.1 mol dm-3 PdCl2 solution) was well mixed with 20 mL of ethylene glycol (EG) in a beaker by stirring in an ultrasonic bath. XC-72 carbon powders (50.0 mg, Cabot, USA) were then added into the mixture as the support. As soon * Corresponding author. E-mail:
[email protected]. † School of Chemistry and Chemical Engineering, Sun Yat-sen University. ‡ State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and Engineering, Sun Yat-sen University. § Institute of Catalysis and Surface Chemistry, Polish Academy of Science. | Istituto di Chimica dei Composti Organometallici (ICCOM-CNR).
as the pH value of the mixture was adjusted above 10 by the dropwise addition of 2.0 mol dm-3 NaOH/EG solution, the welldispersed slurry was obtained after stirring in an ultrasonic bath for 30 min. Finally, the slurry was dried and heated by an intermittent microwave heating (IMH) method in a homemade microwave oven (1000 W, 2.45 GHz) for alternative heat treatment at 5 s on and 5 s pulse six times. The resulting black solid sample was filtered, washed, and dried at 80 °C for 10 h in a vacuum oven. 2.2. Characterization. The X-ray diffraction (XRD) measurements were carried out on a D/Max-IIIA (Rigaku Co., Japan) employing Cu KR1 (λ ) 0.15406 nm) as the radiation source at 40 kV and 40 mA. The transmission electron microscopy (TEM) investigation was performed on a JEOL JEM-2010 (HR) operating at 200 kV. The samples were prepared by ultrasonically suspending the particles in ethanol. A drop of the resulting suspension was deposited on a standard Cu grid covered with a carbon film and allowed to dry before being inserted into the microscope. All the electrochemical measurements were conducted on an EG&G 263A instrument in a three-electrode cell at room temperature. A saturated calomel electrode (SCE) for acidic solutions, or a Hg/HgO electrode for alkaline solutions, was used as the reference electrode, and a platinum foil was used as the counter electrode. The working electrode was prepared using a glassy carbon (GC) disk as the substrate. Typically, a mixture containing 1.0 mg of electrocatalyst, 2.0 mL of ethanol, and 0.02 mL of Nafion suspension (5 wt %, DuPont, USA) was ultrasonicated for 15 min to obtain a well-dispersed ink. A 20 µL portion of the catalyst ink was then transferred onto the surface of the GC electrode and dried under infrared lamp to obtain a catalyst thin film. Aqueous solutions of 1.0 mol dm-3 ethanol containing different electrolytes were prepared. The pHs of the solutions were controlled by changing the concentrations of HClO4 or NaOH, and the similar ionic strength was controlled by adding NaClO4. The electrolyte solutions were purged with highly pure N2 before electrochemical characterization. Cyclic voltammetry (CV) was performed in a limited potential window of 1.0 V. The start potential is shifted negatively by -0.059
10.1021/jp900924s CCC: $40.75 2009 American Chemical Society Published on Web 08/06/2009
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Figure 1. Cyclic voltammograms of ethanol electroxidation on Pd/C in different solutions containing 1.0 mol dm-3 ethanol at 50 mV s-1.
V/pH with increasing pH. All the potentials were referred to the reversible hydrogen electrode (RHE). 2.3. Theoretical Calculation. The calculations were carried out with the Gaussian 03 program,14 employing the hybrid Becke exchange and Lee, Yang, and Parr correlation (B3LYP) functional method.15-17 In the geometry optimization, singlepoint energy calculation, transition state location, and molecular orbital characteristic analysis, the 6-311G(d,p) basis set was used for hydrogen, oxygen, and carbon atoms, respectively, and the LAN2DZ basis set for palladium atoms with the Hay and Wadt effective core pseudopotential. These basis sets have also been used by Psofogiannakis et al. to investigate the methane oxidation mechanism on a Pt(111) cluster. The molecular geometry and vibrational frequency of all species were calculated on the condition of the palladium cluster structure frozen. Pd5 and Pd9 cluster models are used to represent the local structure of the Pd(111) surface since Pd(111) dominates the structure of the Pd electrocatalysts as shown in Figure 2(a). They consist of five atoms in the first layer and four atoms in the second layer. The palladium metal lattice belongs to the facecentered cubic (FCC) structure. Ethanol and H3O+ or OH- ions are presumed to be adsorbed on the top site of the central Pd atom. In all calculations, the metal cluster moieties are fixed. For characterization of the transition state (TS), the single imaginary frequency is confirmed. The energies of the TS and product are gained with respect to the energy of reactant. Since the oxidation reaction takes place in aqueous solution, a number of factors, especially solvation and catalytic activity of the palladium species, may affect the reaction. In the potential energy calculation, the solvation effect was considered using an explicit solvation model,18,19 in which hydrated hydrogen or hydroxyl ions and ethanol molecules adsorbing on the same palladium atom were used as the reactant. The molecular orbital calculation of reaction species in Figure 4 was based on considering the water solvation effect as an isodensity surface polarized continuum model (IPCM).20-23 In the IPCM calculations, the dielectric constant was set at 78.35 (in the pure water of the bulk solvent),24 and the temperature was set at 298.15 K. 3. Results and Discussion Figure 1 presents the pH corrected cyclic voltammograms of ethanol oxidation on the Pd/C nanoelectrocatalyst in different aqueous solutions containing 1.0 mol dm-3 ethanol and various electrolytes. Pd/C shows activity toward ethanol oxidation in alkaline solution (Figure 1). The presence of the oxidation peaks in both forward and reverse sweeps clearly indicates the role
Figure 2. (a) X-ray diffraction pattern and (b) transmission electron micrograph of the Pd/C electrocatalyst.
Figure 3. ELUMO and EHOMO of the Pd5 and Pd9 clusters and of the reacting species in (a) acidic solution and (b) alkaline solution. The reacting species are: S1, H2O; S2, CH3CH2OH; S3, H3O+; S4, OH-; S5, (H2O + OH-); S6, (H2O + CH3CH2OH); S7, (CH3CH2OH + H3O+); and S8, (CH3CH2OH + OH-), respectively. The molecular orbitals of the reacting species whose orbital energies approach that of the Pd cluster are also shown in the figure.
of labile bonded oxygenated species in determining the overall ethanol oxidation in alkaline solution. However, the activity is tremendously decreasing with reducing solution pH values from pH 14 (in 1 M NaOH solution) to pH 12 (in 0.01 M NaOH solution) at comparable ionic strength (NaClO4 was used as the electrolyte to keep the ionic strength constant). This shows the strong dependence of the activity for ethanol electrooxidation on pH. The formation of oxygenated species on Pd is limited at pH < 12, and there is not any onset of ethanol oxidation in natural and acidic solutions. The Pd on the carbon electrocatalyst used in this work was characterized by XRD and TEM. The Pd particles are small with the average particle size of less than 4 nm and well dispersed on the surface of carbon as evident in Figure 2.
Catalytic Activity of Pd toward Ethanol Oxidation
Figure 4. Potential energy profile of the initial reaction processes of ethanol oxidation on Pd5 and Pd9 in alkaline solution. Blue, gray, red, and white spheres represent the Pd, carbon, oxygen, and hydrogen atoms, respectively. The blue arrows represent the vibration mode at the imaginary frequency in the transition state.
Considering the importance of the metallic surface orbitals in the chemisorption processes, we have calculated the molecular orbital energies and the corresponding orbital plot of all the species present which may be present in either acidic or alkaline solutions (Figure 3). Commonly, the adsorption of a species on a metal is directly linked to the energy difference between EHOMO and ELUMO,25-27 in which the HOMO represents the highest occupied molecular orbital and the LUMO represents the lowest unoccupied molecular orbital. The adsorption is stronger if the EHOMO and ELUMO of the reacting species approach the ELUMO and EHOMO of the metal, respectively. Only in this case, the electrocatalytic process runs smoothly, and the electrons of the reacting species can enter the metallic orbitals without any barrier. In this study, we preliminarily considered Pd5 and Pd9 clusters which correspond to the atoms in the first layer and in the first two layers of a single face-centered cubic cell, respectively. In this report, we emphasize the importance of the interaction between Pd clusters, water, and hydrated species. In acidic solution, we ignored the effect of the conjugated bases such as ClO4- and SO42-. Figure 3 shows the molecular orbital energies of the Pd clusters and of the reacting species in either acidic or alkaline solutions. As shown in Figure 3a, only S3 has a lower ELUMO relative to the EHOMO of Pd9 and Pd5 clusters in acidic solution, indicating that the strongest interaction can occur between H3O+ and the Pd clusters, no other species being able to interact with the Pd clusters in acidic condition. In fact, the oxidation of ethanol can only proceed via S6 or S8 processes. The results indicate that the oxidation of ethanol in acidic solution is difficult due to the lack of adsorbed OH species. In the case of alkaline solution, however, the EHOMO values of both hydrated hydroxyl (S5) and alcoholated hydroxyl (S8) approach the ELUMO of the Pd cluster, as shown in Figure 3b. Consequently, these two species have similar adsorption capability on the Pd surface, in agreement with the fact that OH- can accelerate the ethanol oxidation reaction.28 Figure 4 shows the potential energy profile of the initial reaction processes of ethanol oxidation on the Pd5 and Pd9 clusters in alkaline solution, while details of the reaction mechanism on the Pd9 cluster are shown in Figure 5. The whole process includes two single reactions. First, the R-H atom of ethanol is cleaved and then combines with the OH- group chemically adsorbed on Pd to give water, one electron, and the CH3CHOHads intermediate. TS5 and TS6 in Figure 4 represent
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Figure 5. Mechanism of the initial reaction process of ethanol on a Pd9 cluster in alkaline solution, obtained at the B3LYP/6-311G(d,p) level for main group element atoms and at the LANL2DZ level for Pd. Blue, gray, red, and white spheres represent the Pd, carbon, oxygen, and hydrogen atoms, respectively. The blue arrows represent the vibration mode at the imaginary frequency at the transition state.
the energy barriers of the transition states on the Pd5 and Pd9 clusters for this process. Subsequently, the dehydrogenation keeps going catalytically through the extraction of the hydroxyl hydrogen atom of CH3CHOHads leading to the formation of acetaldehyde. In this process, the transition states are TS7 and TS8 for the Pd5 and Pd9 clusters, respectively. The oxidation process is similar to the oxidation pathway of hypophosphite catalyzed by nickel atoms reported by Homma29,30 and discussed in detail in our previous paper31 as follows.
CH3CH2OH + OH- f CH3CHOH- + H2O
(1)
CH3CHOH- f · CH3CHOH + e-
(2)
CH3CHOH f CH3CHO + · H
(3)
The formation of acetaldehyde is the first step during the ethanol oxidation, and it could be further oxidized. In our recent work on in situ FTIR measurements, we found that the main product of ethanol oxidation is acetic salt without the evidence of C-C bond cleavage in high pH solutions. The cleavage of the C-C bond is favored on the Pd electrode in dilute NaOH solutions to result in the complete oxidation of ethanol, but the catalytic activity for ethanol oxidation is poor, indicating the limited conversion of ethanol in such conditions. The highest energy barriers are 116.9 kJ mol-1 (TS6) for the Pd9 cluster and 161.0 kJ mol-1 (TS5) for the Pd5 cluster, and the reaction can proceed smoothly. Both the R-H atom and the H atom on the hydroxyl group of C2H5OH can take part in the ethanol oxidation on Pd in the presence of OH-. Since the energy barrier is lower on the Pd9 cluster than that on the Pd5 cluster in both acidic and alkaline solution, one can reasonably conclude that the second layer of Pd atoms has a promoting action on the catalytic oxidation of ethanol. The reaction trajectory between the Pd clusters and the reacting species in acidic solution has been calculated, and the results are presented in Figure 6. The relative energies of the transition states TS1, TS2, TS3, and TS4 in Figure 6 correspond to the H on the hydroxyl group in C2H5OH attacking the Pd5 cluster, the R-H in C2H5OH attacking the Pd5 cluster, the H on the hydroxyl group of C2H5OH attacking the Pd9 cluster, and the R-H atom of a C2H5OH molecule attacking the Pd9 cluster. The corresponding energy barriers
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Figure 6. Potential energy profile of the initial reaction processes of ethanol oxidation on Pd5 and Pd9 clusters in acidic solution. Blue, gray, red, and white spheres represent the Pd, carbon, oxygen, and hydrogen atoms, respectively. The blue arrows represent the vibration mode at the imaginary frequency at the transition state.
for the dehydrogenation of H3O+ are rather low, ranging from 145.3 (TS3) to 176.8 (TS2) kJ mol-1. These results indicate that neither the R-H atom nor the hydroxyl H atom of ethanol is extracted by Pd, and consequently ethanol is not oxidized. It is reasonable to conclude that there is no blocking effect due to hydrogen adsorption at potentials higher than the onset potential for ethanol oxidation. Therefore, the difficulty of dehydrogenation of ethanol is most likely due to the lack of OH species in acidic solution since adsorbed OH species are mandatory for ethanol oxidation, which is in good agreement with the electrochemical measurements shown in Figure 1b. The energy barriers of the transition states on the Pd9 surface are lower than those on the Pd5 surface, as shown in Figure 6, indicating that the Pd atoms on the second layer promote the adsorption of hydrogen dissociated from H3O+. 4. Conclusion In conclusion, we have rationalized for the first time the origin of the pH effect on the activity of nanostructured Pd for ethanol electrooxidation, a reaction which is inhibited in neutral or acidic media. The DFT calculations show that in acidic media continued dehydrogenation of ethanol is difficult due to the lack of OH species to instantly remove hydrogen, which inhibits the ethanol electrooxidation. Conversely, both ethanol and sufficient OH can adsorb on Pd in alkaline media, leading to continuous ethanol electrooxidation. Acknowledgment. This work was supported by the Guangdong Sci. & Tech. Key Projects (2007A010700001, 2007B090400032), Guangzhou Sci. & Tech. Key Projects (2007Z1-D0051¬SKT[2007]1711), and NNSFC (50801070). Dr. G. F. Cui gratefully acknowledges the financial support by the National Natural Science Foundation of China (50801070). Dr. S.Q. Song gratefully acknowledges the financial support by the Scientific Research Foundation for Young Teachers of the Sun Yat-Sen University (2006-30000-1131148).
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