Ethanol Electro-oxidation at Room and Intermediate Temperature: Quantitative Study of Reaction Vias by On-line Mass Spectrometry

Author(s)

Adriana Coêlho Queiroz

• Document type: Doctoral Thesis
• Press: São Carlos.
• Institution: Universidade de São Paulo (USP). Instituto de Química de São Carlos (IQSC/BT)
• Defense date: 2016-03-22

Examining board members

Fabio Henrique Barros de Lima; Mauro Coelho dos Santos; Giuseppe Abíola Câmara da Silva

• Advisor: Fabio Henrique Barros de Lima

Abstract

In the first part of this study were investigated active materials to electro-oxidize ethanol and acetaldehyde selective for the C2 route (Carbon 2), besides active to electro-oxidize molecular hydrogen, in order to apply into indirect hydrogen fuel cells. In this type of cell, ethanol can be dehydrogenated in the external fuel processor and the products generated in this reaction, containing H2, acetaldehyde and, possibly, unreacted ethanol are used to feed the fuel cell anode. Therefore, the anode electrocatalyst has to be active to electro-oxidize residual ethanol and acetaldehyde, however, it has to catalyze the reaction via C2 route aiming to avoid the species formation that poison the catalyst surface (CO and CHx), in the other words, the C-C bond should remain intact. The bimetallic electrocatalysts were formed by W, Ru and Sn-modified Pt nanoparticles. The reaction products were followed by on-line differential electrochemical mass spectrometry (DEMS) experiments. The results showed that Ru/Pt/C and Sn/Pt/C presented higher overall reaction rate when compared to the other studied materials, however, they were non-selective. On the other hand, W/Pt/C with high W content was more selective to the C2 route, evidenced by the absence of the DEMS signals for molecules with one carbon atom such as CH4 and CO2. Additionally, this material was active and stable for H2 electro-oxidation even in the acetaldehyde presence, what turns it into a potential electrocatalyst for application in the anode of indirect hydrogen fuel cells. In the second part of this work, we investigated conditions and electrocatalysts selective to the C1 route. The ethanol electro-oxidation and its reaction products were investigated by on-line DEMS at room and intermediate temperature. At room, and intermediate temperature (245oC), the electrolytes were aqueous sulfuric acid and solid-state acid (CsH2PO4), respectively. The catalysts investigated were SnOxRuOx-Pt/C and Pt/C. The results of potentiodynamic polarizations at room temperature showed much higher electrocatalytic activity for the SnOxRuOx-Pt/C material, with current efficiency for CO2 formation of 15.6% against 15.2% for Pt/C under stagnant conditions. The reaction residues stripping after the ethanol electro-oxidation, under continuous flow conditions, showed the accumulation of species containing 1 carbon atom (CO and CHx), which are oxidized just at high potentials (ca. 1.0 V) and they cause the obstruction of the active sites. On the other hand, the polarization curves at 245oC showed higher values of current efficiencies (45% for Pt/C for both potentials 0.5 V and 0.8 V against 36% and 50% to SnOxRuOx-Pt/C at 0.5 V and 0.8 V respectively) for the CO2 formation than at ambient condition, however, with similar activities for SnOxRuOx-Pt/C and Pt/C. For both electrocatalysts, in parallel with the electrochemical pathways, heterogeneous chemical catalysis of ethanol decomposition also takes place, producing H2 and CO2, as major products. (AU)

• FAPESP's process: 12/17581-3 - Electrochemical Oxidation of Ethanol on Polymer Electrolyte Fuel Cell. A Quantitative Study of the Reaction Pathways by On-line Mass Spectrometry
• Grantee: Adriana Coêlho Queiroz
• Support Opportunities: Scholarships in Brazil - Doctorate