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Abstract(s)
O propósito central deste estudo consistiu na utilização de resíduos lenhocelulósicos como matéria prima primordial para a síntese de materiais de carbono, enriquecidos com dopantes de azoto. Quatro
precursores, carbonizados de casca de amêndoa (AC) e casca de noz (NC) pinha (PIC) e casca de noz não
tratada termicamente (NP) foram estudados usando três métodos diferentes para introduzir grupos
funcionais azotados. Os materiais precursores foram caracterizados e a ativação química foi usada para
desenvolver área superficial específica nos materiais. Foi estudada a influência da temperatura de ativação,
da concentração e da fonte de azoto, na área superficial especifica e no teor de azoto dos materiais
produzidos.
Foram testados três métodos para a preparação dos carvões ativados com azoto. No método 1
utilizou-se hidróxido de amónio como fonte de azoto e agente de ativação. Os métodos 2 e 3 usaram
hidróxido de potássio como agente de ativação. No método 2 foi utilizada melamina (MM) ou ureia (UR)
como fontes de azoto, enquanto no método 3, foi usado hidróxido de amónio. Os materiais preparados pelo
método 2 apresentaram um burn-off superior (média de 50,5%) e densidade aparente mais baixa quando
comparados com os obtidos pelos outros métodos de preparação, indicando o possível desenvolvimento de
estrutura porosa.
Os carvões ativados obtidos foram caracterizados texturalmente recorrendo à determinação das
isotérmicas de adsorção de azoto a 77 𝐾, verificando-se que os carvões preparados pelos métodos 1 e 3
apresentavam áreas superficiais especificas mais baixas. Os carvões ativados produzidos pelo método 2
demonstraram ser principalmente microporosos, com baixa área superficial externa. As áreas superficiais
especificas obtidas variaram com as condições de preparação e verificou-se que uma maior temperatura de
ativação aumentava a capacidade de adsorção do material, enquanto o aumento da concentração da fonte
de azoto era prejudicial no desenvolvimento de área superficial. Relativamente à fonte de azoto determinou se que esta não influenciava de forma notória a área superficial específica do material. Os materiais NC MM-KOH (900), PIC-MM-KOH (900) e PIC-UR-KOH (900) apresentaram as maiores áreas superficiais
especificas (1667, 1561 𝑒 1474 𝑚2
/𝑔). Por outro lado, os materiais com uma concentração de fonte de
azoto superior (PIC-MM2-KOH (700) e PIC-UR2-KOH (700)) obtiveram os valores mais baixos de todos
os materiais preparados pelo método 2 (373 𝑒 298 𝑚2
/𝑔). As áreas superficiais externas obtidas foram
baixas, sendo o maior valor obtido igual a 12,4 𝑚2
/𝑔 para o carvão ativado PIC-UR-KOH (900). A
percentagem média de volume de microporos relativamente ao volume total de poros foi de 96,6%, valor
indicativo que a maior parte da sua área superficial específica está localizada em microporos. Também foi
determinado o número de azul de metileno. Obtiveram-se índices superiores para os materiais com maiores
áreas superficiais especificas que apesar de serem materiais essencialmente microporosos demonstram uma
boa capacidade de adsorver moléculas de maiores dimensões.
Os carvões ativados preparados pelo método 2 foram caracterizados por análise elementar de forma
a determinar o teor de azoto. Todos os materiais apresentaram um teor de azoto e de carbono inferior ao
esperado. O teor de carbono mais baixo poderia estar associado a um elevado teor em cinzas que foi
comprovado por análise termogravimétrica. O teor de azoto variou de 1,07% para o carvão NC-MM-KOH
(700) a 0,01% para o material NC-UR-KOH (700). O aumento da temperatura de ativação para 900℃
conduziu a uma diminuição do teor de azoto. O aumento da concentração dos compostos fonte de azoto
resultou num maior teor de azoto. Entre a melamina e a ureia, o primeiro dopante produz materiais com
teor de azoto superior. Os carvões NC-MM-KOH (700), AC-MM-KOH (700) e PIC-MM-KOH (700)
apresentaram as maiores áreas superficiais especificas e os teores de azoto mais elevados simultaneamente
( 1314 𝑚2
/𝑔, 0,77%, 1141 𝑚2
/𝑔, 1,07% 𝑒 1068 𝑚2
/𝑔 , 0,60%, respetivamente), indicando que o método 2, que utiliza melamina como fonte de azoto, 𝐾𝑂𝐻 como agente de ativação e temperatura de ativação de 700°C, foi o mais eficaz no desenvolvimento de área superficial especifica e na introdução de
azoto nos carvões ativados.
The central purpose of this study was to use lignocellulosic waste as the primary raw material for synthesising carbon materials enriched with nitrogen dopants. Four precursors, carbonised almond (AC), walnut shells (NC), pine (PIC) and non-heat-treated walnut shells (NP) were studied using three different methods to introduce nitrogen functional groups. The precursor materials were characterised and chemical activation was used to develop the specific surface area of the materials. The influence of the activation temperature, concentration and nitrogen source on the surface area and nitrogen content of the materials produced was studied. Three methods were tested for the preparation of N-doped activated carbons. Method 1 used ammonium hydroxide as the nitrogen source and activation agent. Methods 2 and 3 used potassium hydroxide as the activation agent. Method 2 used melamine (MM) or urea (UR) as nitrogen sources, while method 3 used ammonium hydroxide. The materials prepared by method 2 showed a higher burn-off (an average of 50,5%) and lower apparent density when compared to those obtained by the other preparation methods, indicating the possible development of a porous structure. The activated carbons obtained were texturally characterised by determining the nitrogen adsorption isotherms at 77 K. It was found that the carbons prepared by methods 1 and 3 had lower specific surface areas. The activated carbons produced by method 2 proved to be mainly microporous, with a low external surface area. The specific surface areas obtained varied with the preparation conditions and it was found that a higher activation temperature increased the material's adsorption capacity, while increasing the concentration of the nitrogen source was detrimental to surface area development. With regard to the nitrogen source, it was determined that it had no notable influence on the specific surface area of the material. Materials like NC-MM-KOH (900), PIC-MM-KOH (900) and PIC-UR-KOH (900) had the highest specific surface áreas (1667, 1561 𝑒 1474 𝑚2 /𝑔). On the other hand, the materials with a higher nitrogen source concentration (PIC-MM2-KOH (700) and PIC-UR2-KOH (700)) obtained the lowest values of all the materials prepared by the second method (373 𝑒 298 𝑚2 /𝑔). The external surface areas obtained were low, with the highest value being for 12,4 𝑚2 /𝑔 PIC-UR-KOH (900). The average percentage of micropore volume in relation to total pore volume was 96.6%, indicating that most of its specific surface area is located in micropores. The methylene blue number was also determined. Higher methylene blue numbers were obtained for materials with larger specific surface areas which, despite being essentially microporous materials, demonstrate a good ability to adsorb larger molecules. The activated carbons prepared by method 2 were characterised by elemental analysis in order to determine the nitrogen content. All the materials had a lower nitrogen and carbon content than expected. The lower carbon content could be associated with a high ash content, which was confirmed by thermogravimetric analysis. The nitrogen content ranged from 1.07% for the NC-MM-KOH (700) to 0.01% for the NC-UR-KOH (700). Increasing the activation temperature to 900℃ led to a decrease in the nitrogen content. Increasing the concentration of nitrogen source compounds resulted in a higher nitrogen content. Between melamine and urea, the first dopant produces materials with a higher nitrogen content. NC-MM-KOH (700), AC-MM-KOH (700) and PIC-MM-KOH (700) materials had the highest specific surface areas and the highest nitrogen contents simultaneously ( 1314 𝑚2 /𝑔, 0,77%, 1141 𝑚2 / 𝑔, 1,07% 𝑒 1068 𝑚2 /𝑔 , 0,60%, respectively), indicating that method 2, using melamine as the nitrogen source, 𝐾𝑂𝐻 as the activation agent and an activation temperature of 700°𝐶, was the most effective in developing specific surface area and introducing nitrogen into the activated carbons.
The central purpose of this study was to use lignocellulosic waste as the primary raw material for synthesising carbon materials enriched with nitrogen dopants. Four precursors, carbonised almond (AC), walnut shells (NC), pine (PIC) and non-heat-treated walnut shells (NP) were studied using three different methods to introduce nitrogen functional groups. The precursor materials were characterised and chemical activation was used to develop the specific surface area of the materials. The influence of the activation temperature, concentration and nitrogen source on the surface area and nitrogen content of the materials produced was studied. Three methods were tested for the preparation of N-doped activated carbons. Method 1 used ammonium hydroxide as the nitrogen source and activation agent. Methods 2 and 3 used potassium hydroxide as the activation agent. Method 2 used melamine (MM) or urea (UR) as nitrogen sources, while method 3 used ammonium hydroxide. The materials prepared by method 2 showed a higher burn-off (an average of 50,5%) and lower apparent density when compared to those obtained by the other preparation methods, indicating the possible development of a porous structure. The activated carbons obtained were texturally characterised by determining the nitrogen adsorption isotherms at 77 K. It was found that the carbons prepared by methods 1 and 3 had lower specific surface areas. The activated carbons produced by method 2 proved to be mainly microporous, with a low external surface area. The specific surface areas obtained varied with the preparation conditions and it was found that a higher activation temperature increased the material's adsorption capacity, while increasing the concentration of the nitrogen source was detrimental to surface area development. With regard to the nitrogen source, it was determined that it had no notable influence on the specific surface area of the material. Materials like NC-MM-KOH (900), PIC-MM-KOH (900) and PIC-UR-KOH (900) had the highest specific surface áreas (1667, 1561 𝑒 1474 𝑚2 /𝑔). On the other hand, the materials with a higher nitrogen source concentration (PIC-MM2-KOH (700) and PIC-UR2-KOH (700)) obtained the lowest values of all the materials prepared by the second method (373 𝑒 298 𝑚2 /𝑔). The external surface areas obtained were low, with the highest value being for 12,4 𝑚2 /𝑔 PIC-UR-KOH (900). The average percentage of micropore volume in relation to total pore volume was 96.6%, indicating that most of its specific surface area is located in micropores. The methylene blue number was also determined. Higher methylene blue numbers were obtained for materials with larger specific surface areas which, despite being essentially microporous materials, demonstrate a good ability to adsorb larger molecules. The activated carbons prepared by method 2 were characterised by elemental analysis in order to determine the nitrogen content. All the materials had a lower nitrogen and carbon content than expected. The lower carbon content could be associated with a high ash content, which was confirmed by thermogravimetric analysis. The nitrogen content ranged from 1.07% for the NC-MM-KOH (700) to 0.01% for the NC-UR-KOH (700). Increasing the activation temperature to 900℃ led to a decrease in the nitrogen content. Increasing the concentration of nitrogen source compounds resulted in a higher nitrogen content. Between melamine and urea, the first dopant produces materials with a higher nitrogen content. NC-MM-KOH (700), AC-MM-KOH (700) and PIC-MM-KOH (700) materials had the highest specific surface areas and the highest nitrogen contents simultaneously ( 1314 𝑚2 /𝑔, 0,77%, 1141 𝑚2 / 𝑔, 1,07% 𝑒 1068 𝑚2 /𝑔 , 0,60%, respectively), indicating that method 2, using melamine as the nitrogen source, 𝐾𝑂𝐻 as the activation agent and an activation temperature of 700°𝐶, was the most effective in developing specific surface area and introducing nitrogen into the activated carbons.
Description
Keywords
Lignocellulosic waste activated carbon nitrogen doping adsorption