Catalysis Communications 12 (2011) 772–775

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Catalysis Communications

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Short Communication

Effect of deposition sequences on electrocatalytic properties of PtPd/C catalysts forformic acid electrooxidation

Ligang Feng a,c, Fengzhan Si a,c, Shikui Yao a,c, Weiwei Cai a,c, Wei Xing a,⁎, Changpeng Liu b,⁎a State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, 5625 Renmin Street, Changchun, 130022, PR Chinab Laboratory of Advanced Power Sources, Changchun Institute of Applied Chemistry, 5625 Renmin Street, Changchun, 130022, PR Chinac Graduate University of the Chinese Academy of Sciences, Beijing, 100049, PR China

⁎ Corresponding authors. Tel.: +86 431 85262223; faE-mail addresses: xingwei@ciac.jl.cn (W. Xing), liuch

1566-7367/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.catcom.2011.01.012

a b s t r a c t

a r t i c l e i n f o

Article history:Received 16 December 2010Received in revised form 10 January 2011Accepted 12 January 2011Available online 21 January 2011

Keywords:PtPd catalystFormic acid electrooxidationCatalytic characteristics

PtPd/C catalysts with different surface compositions (Pt+Pd, Pt–Pd and Pd–Pt) were synthesized withdifferent deposition sequences, and characterized by electrochemical experiments and XPS measurements.The different catalytic characteristics for formic acid electrooxidation occurred on the three PtPd/C catalystswere preliminarily discussed according to the oxidation pathway. Due to the synergistic effect between Pt andPd, especially for Pt+Pd, the catalytic stability for formic acid oxidation was greatly increased. The results arehelpful in preparing of PtPd catalyst and understanding the oxidation mechanism for formic acid oxidation.

x: +86 431 85685653.p@ciac.jl.cn (C. Liu).

ll rights reserved.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

The research on the electrocatalysts for formic acid electrooxidation(FAEO) is crucial for the development of direct formic acid fuel cell. Inrecent years, Pt based and Pd based catalysts have been extensivelydeveloped for FAEO [1–4]. It is accepted that FAEO on Pt catalystproceeds through a “dual path”mechanism, involving the “direct path”and “indirect path”. During the reaction, significant amounts of CO buildupon the catalyst surface blocking the reaction active sites. However, onPd catalyst, formic acid is oxidized to CO2 via a direct path, whichprevents the CO build-up on the catalyst surface.

The combination of Pt and Pd, for the purpose of integrating themerits of each metal, may be a good way to increase the catalyticactivity or stability for FAEO. Several preliminary works about thePtPd catalysts have been reported such as the effect of preparationmethods, the bulk composition or the surfacemodificationwith singlecrystal Pt or Pd [2,5–9]. Because of the different mechanisms of Pt andPd for FAEO, it is inferred that different surface compositions of PtPdcatalysts should have different catalytic characteristics. In order tounderstand the catalytic characteristics of PtPd catalysts, three PtPd/Ccatalysts with different surface compositions were synthesized bydifferent deposition sequences and characterized by the electrochem-ical and physical measurements. The catalysts were evaluated forFAEO and the catalytic behaviors were discussed by the oxidationpathway. The results are helpful in preparing of PtPd catalyst for directformic acid fuel cell.

2. Experimental

The PtPd/C catalysts with different surface compositions wereprepared by conventional impregnation reduction method [10,11]. Inbrief, Pt+Pd catalyst was synthesized by a co-impregnation reductionmethod. Pt–Pd and Pd–Pt catalysts were synthesized by a stepwiseimpregnation reduction method; namely, after the Pt (Pd) wasdeposited on the carbon surface, the Pd (Pt) precursor was added andreduced onto the as-prepared Pt (Pd)/C surface. The nominal weightpercentage of the metal (Pt and Pd) in the catalysts was 20% and theatomic ratio of Pd and Pt was close to 1.

X-ray photoelectron spectroscopy (XPS) measurements werecarried out using a Kratos XSAM-800 spectrometer with Mg Kαradiator. The electrochemical measurements were performed with anEG&GModel 273A potentiostat/galvanostat and a three-compartmentelectrochemical cell. A Pt plate and an Ag/AgCl electrode were used asthe counter electrode and reference electrode, respectively. All thepotentials were quoted against the Ag/AgCl electrode. The workingelectrode was a thin layer of Nafion impregnated catalyst on a glassycarbon disk [11] and the catalysts loading was 0.38 mg cm−2. Allelectrochemical measurements were performed in 0.5 M H2SO4

solution with or without 0.5 M HCOOH deaerated by N2 at the roomtemperature. The current density was normalized to the geometricalsurface area of the glassy carbon disk (0.07 cm2).

3. Results and discussion

There were no obvious differences in the PtPd/C catalysts in theXRD patterns (not shown here) due to the similar atomic radii of Ptand Pd and the very small lattice mismatch of Pt and Pd (0.7%) [8,12].

773L. Feng et al. / Catalysis Communications 12 (2011) 772–775

The nanoparticle sizes of the three catalysts calculated according tothe peak (220) by Debye–Scherrer equation were mainly in the rangeof 4.8 to 5.3 nm. Fig. 1 shows the XPS spectra of Pt 4f, Pd 3d and C 1sfor the three PtPd/C catalysts. The peaks of the C 1s for the three PtPdcatalysts were approximately 284.6 eV. The XPS spectra of Pt 4f and Pd3d region were found to be composed of two pairs of doublets due tothe present state of the metallic and oxidized forms [13,14]. For Pt+Pd catalyst, the peaks at 71.19 and 74.46 eV were ascribed to metallicPt, whereas the peaks at 72.11 and 75.41 eV were assigned to Pt (II)species; the peaks at 335.57 and 340.83 eV were ascribed to metallicPd, whereas the peaks at 336.10 and 341.44 eV were assigned to Pd(II) species. Similarly, for Pt–Pd catalyst, the peaks at 70.70 and74.05 eV were ascribed to metallic Pt, whereas the peaks at 71.36 and74.68 eV were assigned to Pt (II) species; the peaks at 334.86 and340.25 eV were ascribed to metallic Pd, whereas the peaks at 335.73and 341.22 eV were assigned to Pd (II) species. For Pd–Pt catalyst, thepeaks at 70.49 and 73.69 eV were ascribed to metallic Pt, whereas thepeaks at 71.02 and 74.70 eV were assigned to Pt (II) species; the peaksat 333.55 and 340.33 eV were ascribed to metallic Pd, whereas thepeaks at 334.94 and 341.25 eV were assigned to Pd (II) species. It wasevident that the peaks of the binding energy for Pt+Pd catalyst were

Fig. 1. XPS spectra for Pt 4f, Pd 3d

shifted to higher binding energy values as compared to other two PtPdcatalysts, indicating a strong interaction between the Pt and Pd [14].The Pt was mainly in the form of Pt metal, but the Pd was mainly in theform of Pd oxide; it was due to that the Pd metal was easily oxidized tothe formof Pd oxide (Pd2+) at ambient conditions [14]. The atomic ratioof Pt:Pd was 1:1.21, 1:1.87 and 1:0.34 for Pt+Pd, Pt–Pd and Pd–Pt,respectively. The atomic ratio of Pt:Pd in the catalysts surface for Pt–Pdand Pd–Pt deviated from the nominal value of 1:1; it was due to thedifferent deposition sequences of Pt and Pd. For the Pt–Pd catalyst, thePdwasdeposited onto the surface of asprepared Pt/C catalyst; For Pd–Ptcatalyst, the Pt was deposited onto the surface of as prepared Pd/Ccatalyst. This indicated that the Pt–Pd catalyst or the Pd–Pt catalystwould have a Pd-rich or a Pt-rich surface, respectively. The surfacecomposition of the three PtPd/C catalysts obtained from XPS measure-ments was consistent with the expected PtPd surface composition.

The hydrogen adsorption/desorption curves of the Pt black, Pdblack and the PtPd catalysts are shown in Fig. 2. The potential value ofhydrogen desorption peak for Pt black, Pd black, Pt+Pd, Pt–Pd andPd–Pt catalysts was −0.148, −0.120, −0.165, −0.122 and −0.134 V,respectively. The hydrogen adsorption strength for Pt+Pd catalystwas obviously weakened. The reduction peak potential of the oxygen-

and C 1s of PtPd/C catalysts.

Fig. 2.Hydrogen adsorption/desorption curves of Pt black, Pd black and PtPd/C catalystsin 0.5 M H2SO4 with a scan rate of 50 mV s−1.

774 L. Feng et al. / Catalysis Communications 12 (2011) 772–775

containing species for Pt black and Pd black was 0.545 and 0.498 V,respectively; it was 0.528 V for Pt+Pd catalyst, which was betweenthe potentials of 0.486 and 0.552 V for Pt–Pd and Pd–Pt catalysts. TheCOad stripping voltammograms of the catalysts are shown in Fig. 3. ForPt+Pd catalyst, the peak potential of COad oxidation was 0.65 V,which was negatively shifted by about 70 mV and 50 mV as comparedto Pt–Pd (0.72 V) and Pd–Pt (0.70 V) catalysts, respectively. Theelectrochemical surface area of Pt+Pd catalyst estimated from COad

stripping charge was the largest among the three catalysts. This wasan indication that there was a larger synergistic effect between Pt andPd, and it was helpful in weakening the CO adsorptive bond on the

Fig. 3. COad stripping voltammograms of PtPd/C catalysts in 0.5 M H2SO4 with a scanrate of 20 mV s−1.

active sites. From the above discussion, it was evident that the PtPdcatalysts with different surface compositions were successfullysynthesized.

Fig. 4 shows the cyclic voltammograms for FAEO on the PtPd/Ccatalysts, Pt black and Pd black. For Pt black, there were three obviousoxidation peaks in the positive direction, which were attributed to theoxidation of hydrogen at −0.15 V, formic acid at 0.41 V and CO-intermediate at 0.70 V. In some research, it was proposed that theFAEO in the potential region of 0.2–0.6 V was via a direct path [15]. Infact, the current in that potential region was very small, and it may bedue to the poisoning effect of the accumulation of CO-intermediate[16]. There was only one obvious peak on Pd black in the positivedirection which was assigned to FAEO through the direct path. It wasevident that Pd had much higher catalytic activity than Pt for FAEO.

The peak current was an indication of the catalytic activity of thecatalysts [3,4]. For Pt–Pd catalyst, the peak current catalyzed by Pdwas 26.7 mA cm−2 at 0.15 V; For Pd–Pt catalyst, the peak currentcatalyzed by Pt was 14.4 mA cm−2 at 0.48 V. There were two formicacid oxidation peaks for Pt+Pd catalyst with the current of11.3 mA cm−2 at 0.05 V catalyzed by Pd and 20.6 mA cm−2 at 0.41 Vcatalyzed by Pt, respectively. It was evident that the correspondingpeak potential for FAEO on Pt+Pd catalyst was negatively shifted ascomparedwith Pt–Pd and Pd–Pt catalysts. The catalytic characteristicsof Pd–Pt catalyst were similar to the Pt black in the positive scandirection. The oxidation region in the potential of 0.6–0.8 V wasassigned to the oxidation of the CO-intermediate; the potential regionof 0.2–0.6 V was attributed to FAEO catalyzed by Pt [17]. For Pd blackand Pt–Pd catalyst, a similar oxidation peak appeared in 0–0.3 V dueto the FAEO through the direct pathway catalyzed by Pd. There was aCO-oxidation peak on the Pt–Pd catalyst indicating the existence of anindirect oxidation path, but the current of FAEO catalyzed by Pt wasnot obvious in the potential region of 0.3–0.6 V. The oxidation

Fig. 4. Cyclic voltammograms of PtPd/C catalysts, Pt black and Pd black in 0.5 M H2SO4+0.5 M formic acid with a scan rate of 20 mV s−1.

Fig. 5. Chronoamperometric curves of PtPd/C catalysts in 0.5 M H2SO4+0.5 M formicacid.

775L. Feng et al. / Catalysis Communications 12 (2011) 772–775

behaviors on Pt+Pd catalyst between 0.2 and 0.6 V were similar tothose of Pt black and Pd–Pt catalyst; so it can be assigned to FAEOcatalyzed by Pt. The oxidation region of −0.1 to 0.1 V for the Pt+Pdcatalyst was much negatively shifted as compared to Pd black and Pt–Pd catalyst, which was due to the FAEO catalyzed by Pd in the directpath. In thenegative-scandirection, the catalytic characteristics of Pt–Pdand Pd–Pt were very interesting. There was a spike in the curve for thePd–Pt catalyst and the current was lower compared to other PtPd/Ccatalysts. The current oscillation behavior was not frequently observedfor FAEO, so itwasa surprise to observe theoscillation current on thePt–Pd catalyst. During the initial negative scan, the noble metals Pt and Pdin the catalyst should be in the state ofmetal oxide, and themetal oxideswere gradually reduced with decreasing the potentials. The Pt–Pdcatalystwithdifferentmetal oxides ormetal reduced statesmaypossessdifferent catalytic activities for FAEO, thereby leading to the currentoscillation.

According to the contributions of the different compositions in thecatalyst to the catalytic activity, the potentials of 0.2 V and 0.4 V werechosen to compare the catalytic stability catalyzed by Pd and Pt,respectively. For Pt–Pd catalyst, the current at 0.4 V was low, as wasthe Pd–Pt catalyst at 0.2 V; therefore, it was insignificant to evaluatethe catalytic activity at that potential. According to the similarcontributions to the catalytic activity, the potential of 0.2 V waschosen to compare the catalytic stability catalyzed by Pd between Pt–Pd and Pt+Pd catalysts, and 0.4 V was chosen to compare thecatalytic stability catalyzed by Pt between the Pd–Pt and Pt+Pdcatalysts. The corresponding chronoamperometric curves of the PtPdcatalysts are shown in Fig. 5. It was evident that the Pt+Pd catalysthad much higher stable current than that of the Pd–Pt catalyst at 0.4 Vand it was also more stable than that of the Pt–Pd catalyst at 0.2 V. At0.2 V, the contribution to current was mostly from Pd; the initialcurrent of FAEO was high, but it was not stable even after about1000 s. At 0.4 V, the contribution to current was mostly from Pt; a

relative stable current was obtained after about 300 s. According tothe above results, the FAEO at 0.4 V may mainly occur on the Ptsurface via indirect path, and at 0.2 V it may mainly occur on the Pdsurface through the direct path. Therefore, the introduction of Pt intoPd is helpful for the stability of FAEO. For the Pt+Pd catalyst, thoughthe initial oxidation activity may reduce to some extent, the stabilityhas been largely improved.

4. Conclusions

Three PtPd/C catalysts with different surface compositions weresuccessfully prepared and characterized by electrochemical experi-ments. Due to the large synergistic effect between Pt and Pd, theoxidation of hydrogen, COad and formic acid became much easier onthe Pt+Pd catalyst. For the FAEO, especially in the negative scandirection, the three PtPd/C catalysts had different catalytic character-istics. The combination of Pt and Pd would cause a decrease in theinitial catalytic activity, but the catalytic stability was largelyincreased for FAEO.

Acknowledgements

This work was supported by the National Natural ScienceFoundation of China (20703043, 20876153, 21073180, 20933004,and 21011130027), Science & Technology Research Programs of JilinProvince (20102204), High Technology Research Program (863program, 2007AA05Z159 and 2007AA05Z143) of the Science andTechnology Ministry of China.

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