Synthesis and characterization of composites based on Ce-MOF and CeO2 with carbonaceous matrices and evaluation of the potential application in electrochemical processes
Name: PATRÍCIA FIGUEIRÊDO SANTOS PIMENTA
Type: PhD thesis
Publication date: 18/12/2020
Advisor:
Name |
Role |
|---|---|
| PRISCILLA PAIVA LUZ | Advisor * |
Examining board:
| Name |
Role |
|---|---|
| JOSIMAR RIBEIRO | Internal Examiner * |
| JULIANA FONSECA DE LIMA | External Examiner * |
| MARCOS BENEDITO JOSÉ GERALDO DE FREITAS | Internal Examiner * |
| PEDRO MITSUO TAKAHASHI | External Examiner * |
| PRISCILLA PAIVA LUZ | Advisor * |
Summary: {[Ce(BTC)(H2O)]∙DMF}n (or Ce-BTC), a component of the MOF-76 family, was synthesized by the solvothermal reaction of hexahydrate cerium nitrate and trimethic acid, in dimethylformamide. A study was carried out to optimize the reaction parameters, time and temperature, based on a factorial design followed by statistical analysis of the reaction yield values. The morphology and reaction yield were more influenced by the temperature factor than by the time factor. The synthesis at 115 ºC for 24 h showed a reaction yield greater than 90%, being these parameters selected for the next syntheses. CeO2 was produced by calcining Ce-BTC at 250, 300 and 350 ºC, and the complete reduction occurs at 300 ºC for 2 h. Then, composites based on cerium and graphene oxide (GO) were also successfully produced. Ce-BTC was prepared with in situ GO, in different percentages (Ce-BTC-OGX%, X = 5, 10 and 30), and calcined to CeO2-OGX%. All the obtained materials were characterized by physical-chemical techniques. The presence of GO in the composites leads to less crystalline materials, improves thermal stability, changes morphology, and reduces the particle size compared to compounds without GO. All the synthesized materials were analyzed by cyclic voltammetry and some selected composites were evaluated by impedance spectroscopy and Tafel tests. The electron transfer of redox pair Ce3+/Ce4+ is observed at around 1.3 V for oxidation and 1.2 V for reduction. The current density was improved with the introduction of GO in the samples, improving the electron transfer and increasing the active area of electrode. The capacitive behavior is improved with the presence of GO, while resistances to the load transfer decrease. The slopes of the Tafel equation (84.3 mV dec-1 for HER, 60.5 and 127.9 mV dec-1 for OER) prove that Ce-BTC-OG30% is a promising alternative electrocatalyst for evolution reactions of H2 and O2. Similar works were carried out and presented as Annex 1: synthesis and characterization of Cu-BTC and Cu-BTC-OG30% to analyze the influence of GO on the structure, composition, and electrochemical behavior of the MOF. Cu-BTC-OG30% showed less crystallinity, smaller particle size and homogeneity than Cu-BTC. On the other hand, better electrochemical behavior than Cu-BTC, as was analyzed in studies of MOF-76.
