Exponential improving in the activity of Pt/C nanoparticles towards glycerol electrooxidation by Sb ad-atoms deposition

https://doi.org/10.1016/j.apcatb.2016.06.072Get rights and content

Highlights

  • Pt/C NPs are electrochemically decorated by Sb with different θSb.

  • Sb ad-atoms are leached from Pt surfaces in potentials greater than 0.7 V.

  • The performance is exponentially improved by increasing θSb.

  • The onset potential shifts 320 mV towards lower potentials.

Abstract

Tuning the chemical composition of heterogeneous nanocatalysts has emerged as an efficient strategy to improve their electroactivity and selectivity for electrolizers and fuel cells application. The stability of a catalyst is ascribed as the ability of the nanoparticles (NPs) in maintaining their chemical composition and physical structure during recycles of use. By an electrochemical procedure, here we decorated platinum nanoparticles supported on carbon with different coverage degrees of antimony ad-atom (θSb) to investigate glycerol electrooxidation reaction (GEOR). The successful decoration of Pt/C with Sb was monitored by electrochemical and physicochemical characterizations. θSb is highly dependent on the upper potential applied. Sb atoms are rapidly leached from Pt surface in potentials greater than 0.7 V in acid solution while they remain stable during several cycles in potentials lower than that. We showed that the current density of GEOR exponentially increases with the increase of Sb coverage, reaching 109 times the pseudo stationary current density of Pt/C for high θSb. Furthermore, the ad-atom facilitates the reaction by shifting the onset potential towards lower potentials. We found an improvement of −320 mV in the onset for θSb = 0.81.

Introduction

The identification of nanomaterials that can electrooxidize alcohols through an efficient pathway remains a key challenge for the commercialization of direct alcohol fuel cells (DAFCs). In general, the restricted ability of Pt nanoparticles (NPs) in cleaving Csingle bondC bond of small chain alcohols, as ethanol [1], ethylene-glycol [2] and glycerol [3], limits the performance of low-temperature DAFCs. Moreover, the poisoning of active sites of Pt due to the adsorption of partially electrooxidized molecules, compromises the activity of the catalyst. Nonetheless, some compounds produced during incomplete oxidation pathways may have commercially valuable, which is the case of glycerol partial oxidized products on Pt surfaces modified with ad-atoms [4], [5], [6].

The production of high-value molecules from glycerol (which is a surplus byproduct of biodiesel fabrication), can be tuned by the chemical composition of the catalyst and the experimental condition, as pH and applied potential [4], [6], [7], [8]. Despite many efforts, the understanding of how many high active materials act is still scarce [9]. The group of M. T. M. Koper has reported important advances in this field. Kwon et al. showed that Pb, Bi, In, Sn and Sb decorated Pt/C NPs enhance the glycerol electrooxidation reaction (GEOR) [4]. Briefly, these authors found that Pt/C covered with Sb improve GEOR and displace the reaction pathway towards the formation of dihydroxyacetone. In another work, Kwon et al. proposed a Bi-modified Pt catalyst which is 100% selective to the formation of dihydroxyacetone (not considering volatile products) [5].

Regarding the electrochemistry of shape, size and nature (chemical composition) controlled nanoparticles, the group of J. M. Feliu has reported noticeable results [10], [11], [12]. Vidal-Iglesias et al. investigated the influence of well-shaped Pt nanoparticles decorated with Sb applied to the electrooxidation of formic acid [10]. They found that Sb-decorated octahedral Pt NPs are the most active materials. Figueiredo et al. showed the electrooxidation of ethanol is sensitive to the Sb coverage on Sb-decorated PtRu/C [11]. They found that the activity of the catalysts presents a maximum for coverages around 0.47. The authors showed that direct ethanol fuel cells operating with PtRu/C-Sb as anode displays higher power densities than Pt/C [11].

Among many multimetallic Pt-based catalysts, the results observed for antimony-platinum combinations are encouraging. Hence we decide to test these materials for glycerol. Namely, herein we investigate the influence of the Sb coverage degree on Pt/C NPs towards glycerol electrooxidation through an electrochemical protocol. We have found an exponential augment of the catalytic activity up to θSb = 0.81. Besides, we show that the Sb atoms remain on the Pt surface only at potentials bellow 0.70 V.

Section snippets

Synthesis of Pt/C nanoparticles

Carbon-supported Pt NPs were prepared using the fast polyol method assisted by micro-waves [13], [14]. Briefly, proper amounts of H2PtCl2, poly(acrylic acid salt) (PA, Mw = 2100) and 20 mL of an aqueous solution of ethylene glycol (3 EG:1H2O) were sonicated during 5 min followed by the addition of Carbon Vulcan XC72® and sonication for more 20 min. The size and the distribution of NPs were controlled by adjusting the PA/metal ratio to 5.0. The metal load of the Pt/C was set to 40%. The chemical

Physical characterization of Pt/C

Pt/C NPs decorated with Sb were characterized in terms of electronic properties, crystallinity, morphology and chemical composition. The detailed physical characterization can be seen in Supplementary Materials, Section I. EDX measurement showed the identity of Pt/C (Fig. S1). XPS spectra display the characteristic peaks of Pt and amorphous carbon (Fig. S2). XPS analysis showed that most of the catalyst is in metallic form, containing 20% of oxides. Besides, the metal loading calculated by XPS

Conclusion

The electrochemical decoration procedure allows controlling the reversible deposition of Sb ad-atom on Pt surfaces and the investigation of their activities accordingly. The ability of the Sb-decorated Pt/C NPs in keeping their coverage degree (θSb) is highly dependent on the potential domain. We found that 0.7 V is the limiting working potential for Sb/Pt/C in acid media. A linear decrease in θSb is observed in function of potentials bigger than the limiting one.

Sb/Pt/C NPs are highly

Author contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Acknowledgments

The authors acknowledge financial assistance from CNPq (Grant # 454516/2014-2), FUNDECT (Grant # 026/2015), CAPES and FINEP. Authors thank LNNano-CNPEM (Campinas, Brazil) for the use of the TEM facility.

References (29)

  • M.C. Figueiredo et al.

    J. Catal.

    (2015)
  • C.R. Zanata et al.

    Appl. Catal. B: Environ.

    (2016)
  • J.H. Scofield

    J. Electron Spectrosc.

    (1976)
  • P.S. Fernández et al.

    Electrochem. Commun.

    (2012)
  • J.F. Gomes et al.

    Electrochim. Acta

    (2012)
  • J.F. Gomes et al.

    J. Catal.

    (2013)
  • A. Zalineeva et al.

    Electrochim. Acta

    (2015)
  • P.S. Fernández et al.

    Electrochim. Acta

    (2013)
  • M. Li et al.

    J. Am. Chem. Soc.

    (2013)
  • P. Bi et al.

    RSC Adv.

    (2016)
  • A. Zalineeva et al.

    J. Am. Chem. Soc.

    (2014)
  • Y. Kwon et al.

    Top. Catal.

    (2014)
  • Y. Kwon et al.

    ACS Catal.

    (2012)
  • Y. Holade et al.

    ACS Catal.

    (2013)
  • Cited by (0)

    1

    These authors contributed equally.

    View full text