A carregar...
Publicação
Implicações mecanísticas do processo de precipitação electroless de metais nobres em polianilina
| Nome: | Descrição: | Tamanho: | Formato: | |
|---|---|---|---|---|
| 2.35 MB | Adobe PDF |
Resumo(s)
A Polianilina (PAni), tal como alguns polímeros condutores, tem a capacidade de se reduzir e oxidar espontaneamente em meio ácido sem aplicação de potencial. Está estabelecido que quando existem iões metálicos em solução, estes podem actuar como espécie oxidante para o polímero, em particular para os que têm na sua constituição átomos de azoto. A precipitação electroless consiste na redução espontânea de iões metálicos numa superfície polimérica de electroactividade específica. Uma vez que as transições de estado da PAni correspondentes a esta electroactividade ocorrem entre 0,3 V e 0,8 V vs. SCE, os metais que podem ser afectados por este processo deverão ter o potencial de redução nesta janela de potencial, limitando este processo aos metais nobres. Justificado pelo interesse na área da extracção metalúrgica o presente trabalho aborda as alterações estruturais que o polímero sofre durante a precipitação electroless, ainda não esclarecidas apesar do processo já ter sido descrito há mais de duas décadas. Neste trabalho as alterações químicas e electroquímicas provocadas pela redução espontânea do metal na matriz polimérica e degradação do polímero foram avaliadas, e assim a precipitação electroless foi detalhadamente caracterizada. A electrosíntese do polímero foi acompanhada por medidas de Microbalança Electroquímica de Cristal de Quartzo (Electrochemical Quartz Cristal Microbalance, EQCM), tendo sido observadas as fases de formação de radicais catião, de nucleação e finalmente o crescimento do polímero. Posteriormente, os eléctrodos modificados foram caracterizados mais uma vez por EQCM, utilizando voltametria cíclica, evidenciando a entrada e saída de solvente da matriz polimérica, bem como as transições conhecidas de estado de oxidação da PAni aos 0,3 V e 0,8 V vs. SCE (varrimento anódico). A PAni foi ainda caracterizada pela medição de potencial de circuito aberto (Open Circuit Potential, OCP) que demonstrou atingir um patamar de potencial a 0,59 V vs. SCE, ao fim de 950 segundos de permanência em 0,2 M H2SO4. A polianilina foi extensivamente estudada por métodos espectroscópicos de radiação UV-Vis, infravermelha e raios X, e comparada com a literatura, tendo-se determinado que as principais estruturas poliméricas presentes nos eléctrodos modificados são quinóides e benzílicas com uma enorme contribuição do elemento de azoto e, consoante o grau de pureza, oxigénio. Foram ainda calculados potenciais formais a diferentes valores de pH para construir um diagrama do tipo Pourbaix para o polímero em estudo. O processo de precipitação electroless é descrito em forma de diagrama potencial/pH que depois leva a que seja detalhadamente exposta a redução espontânea de prata, mercúrio e ouro. É ainda inequivocamente mostrado que a precipitação electroless é responsável pela redução espontânea dos metais em estudo. É também demonstrado que o método de integração do pico de oxidação da prata e do mercúrio é preciso para a quantificação de metal incorporado na matriz polimérica com uma incerteza associada inferior a 10%. A degradação do polímero é também investigada dando origem a uma revisão do mecanismo tomado como aceite até à data, demonstrando-se a formação de 2,5-diaminobenzeno-1,4-diol e de 2,5-Diamino-p-benzoquinona e não só de p-hidroquinonas e p-benzoquinonas. Esta análise conduz à proposta de um mecanismo e estabelece as reais implicações mecanísticas da precipitação electroless evidenciando a inevitável degradação do polímero para a promoção da redução espontânea de iões metálicos.
Polyaniline (PAni), like other conductive polymers, spontaneously reduces and oxidizes in acidic medium at open circuit potential. It is established that when metallic ions are present in solution, they act as an oxidizing agent to polymers, especially those containing nitrogen atoms. electroless precipitation is the sustained reduction of metal ions onto a polymeric surface of specific electroactivity. Knowing that state transitions of polyaniline occur at 0,3 V and 0,8 V vs. SCE the metal ions undergoing electroless precipitation should have a reduction potential in this potential range, limiting this reduction process to noble metals. Justified by the possibility of being used in extractive metallurgy, the present work approaches the structural transformations that the polymer undergoes at open circuit potential, since although being known for more than two decades this has not been explained until date. In this work the chemical and electrochemical changes due to the sustained reduction of metal ions onto the polymer’s matrix and its degradation are evaluated, and so allowing to fully characterize the electroless precipitation process. The polymers are prepared by electrochemical techniques using EQCM for a detailed study of the polymerization mechanism, it is observed that there are three stages of polymerization namely the radical cations formation, the nucleation and finally the newly formed film growth. After which the electroactivity of said modified electrodes is presented by the mass and electrochemical changes shining light on the flux of solvent, maintaining the medium neutrality. The several oxidation states of PAni are also reported being at 0,3 V leucoemeraldine (L)/ protonated emeraldine (E) and 0,8 V vs. SCE protonated emeraldine / pernigraniline (P), anodic sense. PAni was also analysed by the measuring of open circuit potential, that shows a stable level at 950 seconds of 0,59 V vs. SCE in 0,2 M H2SO4 medium. Polyaniline is then extensively characterised by spectroscopic methods that involve UV-Vis, infrared and X-ray radiation, all together point to a mainly benzenic and quinoid structure with a big contribution of nitrogen atoms and, depending on the purity of the polymer oxygen. The formal potential associated with state transitions of the studied polymer are calculated giving rise to a Pourbaix type diagram. A Pourbaix type diagram is used to describe the electroless precipitation process which leads to the detailed explanation of sustained reduction of silver, mercury and gold. It is unequivocally showed that the increase of mass on the electrode’s surface although due to some galvanic displacement is indeed due to the electroless precipitation of metal ions. It is also shown that by integration of the oxidation peak we arrive at the precise value of incorporated metal (silver or mercury) with an uncertainty of less than 10%. The polymer’s degradation is explored as well giving rise to a review on the accepted mechanism until date, demonstrating the involvement of 2,5-diaminobenzene-1,4-diol and 2,5-Diamino-p-benzoquinone and not only the formation of p-hydroquinones and p-benzoquinones. This review leads to the determination of the mechanism and implications of the electroless precipitation process showing that the degradation of the polymer is necessary for the sustained reduction taking place.
Polyaniline (PAni), like other conductive polymers, spontaneously reduces and oxidizes in acidic medium at open circuit potential. It is established that when metallic ions are present in solution, they act as an oxidizing agent to polymers, especially those containing nitrogen atoms. electroless precipitation is the sustained reduction of metal ions onto a polymeric surface of specific electroactivity. Knowing that state transitions of polyaniline occur at 0,3 V and 0,8 V vs. SCE the metal ions undergoing electroless precipitation should have a reduction potential in this potential range, limiting this reduction process to noble metals. Justified by the possibility of being used in extractive metallurgy, the present work approaches the structural transformations that the polymer undergoes at open circuit potential, since although being known for more than two decades this has not been explained until date. In this work the chemical and electrochemical changes due to the sustained reduction of metal ions onto the polymer’s matrix and its degradation are evaluated, and so allowing to fully characterize the electroless precipitation process. The polymers are prepared by electrochemical techniques using EQCM for a detailed study of the polymerization mechanism, it is observed that there are three stages of polymerization namely the radical cations formation, the nucleation and finally the newly formed film growth. After which the electroactivity of said modified electrodes is presented by the mass and electrochemical changes shining light on the flux of solvent, maintaining the medium neutrality. The several oxidation states of PAni are also reported being at 0,3 V leucoemeraldine (L)/ protonated emeraldine (E) and 0,8 V vs. SCE protonated emeraldine / pernigraniline (P), anodic sense. PAni was also analysed by the measuring of open circuit potential, that shows a stable level at 950 seconds of 0,59 V vs. SCE in 0,2 M H2SO4 medium. Polyaniline is then extensively characterised by spectroscopic methods that involve UV-Vis, infrared and X-ray radiation, all together point to a mainly benzenic and quinoid structure with a big contribution of nitrogen atoms and, depending on the purity of the polymer oxygen. The formal potential associated with state transitions of the studied polymer are calculated giving rise to a Pourbaix type diagram. A Pourbaix type diagram is used to describe the electroless precipitation process which leads to the detailed explanation of sustained reduction of silver, mercury and gold. It is unequivocally showed that the increase of mass on the electrode’s surface although due to some galvanic displacement is indeed due to the electroless precipitation of metal ions. It is also shown that by integration of the oxidation peak we arrive at the precise value of incorporated metal (silver or mercury) with an uncertainty of less than 10%. The polymer’s degradation is explored as well giving rise to a review on the accepted mechanism until date, demonstrating the involvement of 2,5-diaminobenzene-1,4-diol and 2,5-Diamino-p-benzoquinone and not only the formation of p-hydroquinones and p-benzoquinones. This review leads to the determination of the mechanism and implications of the electroless precipitation process showing that the degradation of the polymer is necessary for the sustained reduction taking place.
Descrição
Tese de mestrado em Química, apresentada à Universidade de Lisboa, através da Faculdade de Ciências, em 2018
Palavras-chave
Polianilina PAni Precipitação electroless Mecanismo processual Metais nobres Teses de mestrado - 2018