Potassium hydroxide influence on the structure and surface area development of brown coal under alkali activationстатья из журнала
Аннотация: This work was aimed on 1) evaluating alterations of spatial structure and paramagnetic properties of brown coal (BC) caused by KOH intercalated under alkali impregnation and 2) studying KOH influence on BC thermolysis and formation of porous carbons under alkali activation in argon (800 °C, 1 h ). The spatial structure of BC impregnated (BCI) was characterized by X-ray diffraction as parameters of coal crystallites: the interlayer distance d 002 , height L c , the average diameter of polyarene (graphene) layer L a , the volume V cr and the average number of layers N cr . The unpaired electron concentration [ е – ] was measured by ESR. Thermolysis of BC and BCI was described by differential thermogravimetry (4 deg/min) which gave the rates of volatile products formation ( ω t ) for the thermolysis temperature t = 300, 700, 800 °C. The relationships of BCI properties (d 002 , L c , L a , V кр N кр , [e], ω 300 , ω 700 , ω 800 ,) as well as yield (Y) and specific surface area (S BET ) of асtivated carbons versus the alkali/coal ratio R KOH ≤15 mmol/g were obtained. They identified two regions: I (R KOH ≤6 mmol/g) and II (R KOH > 6 mmol/g). In region I, the R KOH increase enhances the degree of substitution of acid groups protons by K + -ions (with full replacement at R KOH = 6 mmol/g), slightly changes the spatial structure, increases the concentration of [e‾] (from 2.7∙10 18 to 5.0∙10 18 spin/g), linearly decreases the rate ω 300 (from 0.12 to 0.03 mg/g∙s) due to alkali promotion of condensation reactions. They are revealed under alkaline activation (800 ° С ): the solids yield changes on a curve with a maximum at R KOH ~2 mmol/g (43 % → 49 % → 39 %), the S BET changes on a curve with a minimum (210 → 120 → 730 m 2 /g). In region II, the R KOH increase enhances the coal crystallite volume ( from 1.37 до 2.05 nm 3 ) due to KOH intercalation and formation of additional crystallite layers (4 vs 3), linearly raises the ω 300 rate (0.03 to 0.20 mg/g ∙s ) by increasing the contribution of C-O and C-C bonds heterolysis. The active carbons yield decreases (from 39 % to 33 %), the S BET surface grows linearly (from 730 to about 970 m 2 /g). The KOH capability to develop the surface was proposed to evaluate by the coefficient K EF =∆S BET /∆R KOH defining the S BET increment with increasing alkali quantities. The K EF values were determined to be significant (200–300 m 2 /mmol) within R KOH =2.5–4.0 mmol/g. Catalysis of KOH-initiated pore-forming reactions in this R KOH interval is presupposed to be a useful approach of porous structure development at low KOH/coal ratios that is technologically attractive due to decreasing volumes of reagents and alkaline waste water during activated carbon separation.
Год издания: 2017
Источник: Himia Fizika ta Tehnologia Poverhni
Ключевые слова: Supercapacitor Materials and Fabrication, Carbon Nanotubes in Composites, Fiber-reinforced polymer composites
Открытый доступ: bronze
Том: 8
Выпуск: 2
Страницы: 133–142