Improvement of Ethanol Catalytic Activity by Mn Doped Pt and Pt-Sn/C- Based Catalysts

Introduction

Great attention has been given in the literature for studies to develop an efficient catalyst for ethanol electrooxidation, since the pure platinum is not stable for long-term fuel cell operation. It is well known that PtSn catalysts are the best compositions for ethanol oxidation [1,2]; however, this composition still need improvement to access commercial operational conditions, usually an co-catalyst metal is required to improve the efficiency of the process.

In this context, manganese and manganese oxides presents interesting catalytic proprieties to the oxidation of organic small molecules, such as the excellent proton conductivity, the increase of catalyst utilization and the synergistic effect between catalysts and manganese oxides, which make this compound a promising material for co-catalyst or carrier support for fuel cell[3,4].

Experimental

The Mn, PtSn, PtMn and PtSnMn catalysts supported on carbon with 40 % wt. metal loading were synthesized by chemical reduction with NaBH₄in water/ethylene glycol solution pH 13 and were characterized by DRX, EDX, for composition and structural information as well as by voltammetry cyclic, chronoamperometry, and CO-stripping to evaluate their performance toward ethanol electrooxidation.

Results and Discussion

Pt₅₁Mn₄₉ and Pt₅₄Sn₄₆ showed experimental composition close to nominal one, that should be Pt:M (60:40). However, the ternaries PtSnMn presented a huge deviation from the nominal catalysts. Reducing Mn in presence of Sn was really difficult even using a stronger reducing agent as NaBH₄. All ternary PtSnMn catalysts presented just small amount of Mn, which lies between 1 and 7.5 molar %, very different from the expected nominal compositions (10 to 30 %). All catalysts presented a FCC crystalline structure of Pt with small crystallite sizes, 2.0 to 3.5 nm. The diffraction pattern exhibited a significant displacement of diffraction planes of Pt by the addition of Sn, Mn suggesting a structure modification of the catalyst. Indeed, we observed an increase in lattice parameter when Sn and SnMn were added. Mn catalysts are composed by a mixture of its oxides, MnO₂ and Mn₂O₃identified by the characteristic peaks of these species.

Electrochemical characterization by cyclic voltammetry in presence of ethanol 1.0 mol L^-1 showed that   ternary compositions have lower E_onset for ethanol electrooxidation 0.16 – 0.21 V vs. HRE compared to 0.3-0.4V observed to binary PtSn composition. Surprisingly, Mn/C catalyst is very active to ethanol electrooxidation exhibiting an E_onset of 0.27 V and a catalytic activity comparable to binary Pt₅₄Sn₄₆. The best catalytic performance, determined by chronoamperometry at 0.4V vs RHE was found to Pt_79,1Sn_19,6Mn_1,3 and Pt_68,6Sn_29,9Mn_1,6 which presented mass activities of 4.5 and 3.0 A g_Pt^-1, respectively. The binaries compositions and Mn/C presented catalytic activity less than 0.5 A g_Pt^-1. It shows that even in small amount the presence of Mn plays an important contribution to the catalytic activity for ethanol electrooxidation.

The electrochemical active area (EAA) determined by CO-stripping are in agreement with the electrochemical results where Pt_79,1Sn_19,6Mn_1,3 and Pt_68,6Sn_29,9Mn_1,6 had the better EAA, ~30 m² g_Pt^-1, which corroborate to the highest catalytic performance obtained. The other catalysts presented an EAA closer to 10 m² g_Pt^-1_.

Conclusion

The use of NaBH₄ as a reducing agent is efficient to obtain catalysts with small crystallite size. However, this reagent was not efficient when reducing Pt, Sn and Mn simultaneously. The catalytic activity for ethanol electrooxidation showed ternaries PtSnMn as the best catalysts, exhibiting low E_onset and higher catalytic activity. Nevertheless, the results suggest the Mn, even in low amount, plays a crucial role on catalytic behavior of PtSn, improving its performance to the ethanol oxidation.

 

Acknowledge

The authors acknowledge FAPESP, Capes, and CNPq (Brazil) for financial support. T.S Almeida acknowledges FAPESP foundation under contract number 2009/15034-2 for the PhD scholarship.

 

References

[1] Antolini, E. et al. Electrochem. Commun. (9) 2007,  398.

[2] Colmati, F. et al. Journal of Power Sources (157) 2006, 98.

[1] Zhao Y. et al . J Power Sources (30) 2012, 218.

[2] Liu W. et al. J Power Sources (64) 2013, 238.