Study of the activity of the binary RuO2-Ta2O5 system prepared for two methods of thermal decomposition in the oxidation of the Ethanol

Author(s)

Josimar Ribeiro

• Document type: Doctoral Thesis
• Press: Ribeirão Preto.
• Institution: Universidade de São Paulo (USP). Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto (PCARP/BC)
• Defense date: 2006-02-22

Examining board members

Adalgisa Rodrigues de Andrade; Luiz Henrique Dall'Antonia; Paulo Olivi; Germano Tremiliosi Filho; Hebe de Las Mercedes Villullas

• Advisor: Adalgisa Rodrigues de Andrade

Abstract

In this work, the influence of the preparation method on the structural and electrochemical properties of the RuO2/Ta2O5 system was investigated. A systematic investigation of the structural, morphological, electrochemical, and electrocatalytic properties for O2 evolution from acidic solutions was carried out using anodes of nominal composition Ti/RuO2-Ta2O5 (Ru:Ta - 10 - 90 % Ru step 10 atom %), prepared by the polymeric precursor method (PPM), in which the solutions were prepared by mixing citric acid (Merck) in Ethylene glycol (Merck) with RuCl3.xH2O (Aldrich) and Ta(OC2H5)5 (ACROS). Anodes of nominal composition Ti/RuO2-Ta2O5 (Ru:Ta = 30:70; 50:50 and 80:20 atom %) were also prepared by the standard method (SM) from the appropriate molar ratios of the precursors mixtures (RuCl3.xH2O and TaCl5 - Aldrich) in alcoholic solutions (isopropanol - Aldrich). For both methods, thermal decomposition was carried out under at 450 oC for 1 h - O2-flux of 5 dm3 min-1. Analyses of the composition and the morphology of the oxide layer were done before and after the accelerated life tests (ALT), through EDX (Energy dispersive X-rays) and SEM (Scanning electron microscopy), respectively. SEM results gave evidence of typical mud-flat-cracking morphology for the majority of the films. EDX analyses even after total deactivation showed that there is reduction in the content of Ru in the coating. Structure data were obtained by XRD (X-rays diffraction), and they showed the rutile-type struture for RuO2 and the orthorhombic one for Ta2O5. Cyclic voltammograms of this system where registered in acidic solutions (0.5 mol dm-3 of H2SO4). The observed peak was attributed to the solid-state redox transition of Ru(III)/Ru(IV). At low overpotentials, the Tafel slope (b) displayed average values of 40 mV. The electrocatalytic activity showed a strong dependence on the content of RuO2 in the film. The stability of the electrodes was evaluated by ALT, performed at 0.5 mol dm-3 in aqueous H2SO4 (750 mA cm-2 and T = 80 oC). Results showed a performance that is 100 % better than that obtained with the electrodes prepared by SM. In the presence of an organic compound (Ethanol and Acetaldehyde), the VC and Tafel results showed that electrodes prepared by PPM are electrocatalytically better than those obtained by SM. The studies on Ethanol and Acetaldehyde oxidation showed high reactivity for Acid Acetic and CO2 formation at low Ethanol concentration (5 mmol dm-3). On the other hand, the increase in Ethanol concentration (15 - 1000 mmol dm-3) favors Acetaldehyde formation (1 - 3 %) and, therefore, the formation of Acid Acetic and CO2 is lower. (AU)

• FAPESP's process: 02/06465-0 - Study of decomposing binary oxide electrodes RuO2+Ta205: investigation on the oxidation of ethanol and acetic acid in acid medium as model reactions of the C-C bond breaking
• Grantee: Josimar Ribeiro
• Support Opportunities: Scholarships in Brazil - Doctorate