Electro-Oxidation of Ethanol at Low and Intermediate Temperature Investigated By on-Line Mass Spectrometry

Introduction

 

The search for renewable energy sources is motivated by a decrease in the reserves of fossil fuels and rising environmental degradation. Researchers have studied the use of ethanol as a sustainable fuel, due to its several positive characteristics, such as low toxicity and wide availability. However, the ethanol oxidation reaction (EOR) has parallel reaction routes that release lower number of electrons than result from CO₂ formation [1]. To address this challenge, ethanol electroxodiation at intermediate temperatures using solid acid electrolytes has been proposed [2]. In the present work, the EOR was investigated at low temperature, using aqueous H₂SO₄, and at intermediate temperature, using solid CsH₂PO₄, as electrolytes. The reaction products for the catalysts, carbon-supported combinations of Pt, Ru and Sn, were investigated by on-line mass spectrometry using a Dual Thin Layer Flow Cell for room temperature, and a tip inlet setup for the mass spectrometry measurements at intermediate temperature.

Experimental

 

The Pt₃(SnO_xRuO_x)₁/C electrocatalysts were synthesized via thermal decomposition of the precursor metal salts, adapting a previously published method [3]. Pt/C (E-TEK) was also studied for comparison. The atomic composition was estimated by X-ray energy dispersive spectroscopy (X-EDS) analysis, and structural features were investigated by X-ray diffraction (XRD) measurements. Synthesis of CsH₂PO₄ and fabrication of the CsH₂PO₄-based electrochemical cell, with Pt/C serving as the cathode, followed previously published procedures [2]. A dynamic hydrogen electrode was used as reference (humidified H₂). The m/z signals of 22 (CO₂), 29 (acetaldehyde), 44 (CO₂ + acetaldehyde) and 15 (acetaldehyde and methane) was utilized during the DEMS measurements.

Results and Discussion

Fig. 1 presents the faradaic currents (a) and the mass signals (b) m/z = 22 and (c) m/z = 29 (insets) obtained during potentiostatic measurements of ethanol electro-oxidation (0.1 mol L^-1) in the DEMS equipment, at room temperature, catalyzed by Pt₃(SnO_xRuO_x)/C and Pt/C. One can be observed a deactivation of the currents (faradaic or ionic) as a function of the time, for both electrocatalysts, with the CO₂ signal (m/z = 22) dropping to zero. The stripping of the adsorbed reaction fragments (Fig. 1(d)), revealed that the poisoning species are CO and CH_x. These species may accumulate during the course of the ethanol oxidation reaction in this potential domain.

Fig. 1: (a) Room temperature potentiostatic measurements presenting the faradaic and ionic currents (inset) obtained during on-line DEMS experiments (Dual Thin Layer Flow Cell) and (b) reaction fragment stripping obtained DEMS measurements. Ethanol: 0.1mol L^-1 . Scan rate of 10 mVs^-1.

On the other hand, for the measurements conducted at 245^oC, and at 0.5V and 0.8V, the faradaic current and the ionic signal for CO₂ formation underwent no decay (Fig. 2). Additionally, the trimetallic electrocatalyst, presented much higher activity for the EOR than did Pt alone. While the calculation of the faradaic current efficiency for CO₂ formation at intermediate temperature is still in progress, initial results showed much higher values for both electrocatalysts when compared to the values obtained at room temperature.

Fig 2: (a) Elevated temperature potentiostatic measurements and (b) CO₂ ionic signal (tip inlet DEMS setup) obtained during the EOR. Ethanol: 36 wt.% in H₂O. Cell temperature = 245 ^oC.

 

Acknowledgements

The authors thank FAPESP, CNPq and LiOx Power, Inc.

References

 

[1] Lai, S.C.S.; Kleijn, S.E.F.; et al. Catalysis Today, 154, n. 1-2, (2010) 92–104.

[2] Uda, T.; Boysen, D.A.; et al. Electrochemical and Solid-State Letters, 9, n. 6, (2006) A261-A264.

[3] Sao-Joao, S.; Giorgio, S.; et al. The journal of physical chemistry. B, 109, n. 1, (2005) 342-347.