Publication: Efficient co2 photoreduction into methane over electrochemically exfoliated graphene – g-c3n4 photocatalyst under visible light irradiation
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Date
2023-04-01
Authors
Maher Talib Abdul-Zahra Al-Shamkhani
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Abstract
Among different renewable energy conversion systems, CO2 heterogeneous photocatalysis conversion to hydrocarbon fuels is the most promising one. Recently, graphene-based photocatalysts received great attention for CO2 photoreduction. Among top-down graphene exfoliation methods, the electrochemical exfoliation method has been reported as a cost-effective process for the large-scale synthesis of high-quality graphene. However, this method suffers from the accumulation of intercalation ions between graphite layers when the constant voltage (cV) is used. Therefore, an electrochemical exfoliation cell of graphene was fabricated using the switching voltage (sV) instead of cV and molten salts (NaOH: KOH) as an electrolyte. Commercial graphite pencil cores type (HB) were extracted and used as graphite precursors. Results indicated that the sV technique was efficient to achieve better exfoliation efficiency compared to the constant voltage (cV) and tackled the issue of the accumulated ions. The highest achieved graphene yield rates were 0.50 and 0.43 g min-1 when the anode-to-cathode voltage application time ratio was 1.33, and the cathode-to-anode voltage application time ratio was 0.75, respectively. The resultants graphene possessed low ID/IG ratios of 0.65 and 0.45, and good electrical conductivities of 75.28 and 182.62 S m-1 on both the anode and cathode, respectively. Different loading ratios of the electrochemically exfoliated graphene (EG) were incorporated with a metal-free semiconductor (g-C3N4) to develop an efficient photocatalyst for continuous CO2 photoreduction to methane using a simple impregnation-calcination method. The as-synthesized photocatalysts were extensively characterized by TEM, HRTEM, SEM, XRD, XPS, FTIR, UV-Vis (DRS), TPR, and PL-spectra to discuss the relationship between the physicochemical attributes of the photocatalysts and their photocatalytic activity. Results confirmed that the optimized 0.075 EG-CN photocatalyst achieved the highest CH4 evolution of 21.32 μmol gcatalyst-1 and high stability after four successive cycles that manifested a significant 7.35 fold enhancement in CH4 production compared to the pure CN after 6 h of light irradiation. Langmuir-Hinshelwood and Sips isotherm models were used to predict the kinetic parameters for uniform and nonuniform surfaces, respectively. The fitting results showed that the reaction mechanism followed the Sip isotherm with a reaction rate constant of 93.3 μmol gcatalyst-1 h-1 with a degree of precision at an R-squared value of 0.95. Box-Behnken experimental design (BBED) was used to investigate the interaction effects between parameters. The results obtained from BBED showed a significant total CH4 evolution of 33.5 μmol gcatalyst-1 when the light intensity, PCO2, and PH2O were 100.5 W m-2, 63 kPa, and 35.51 kPa, respectively. The desirability study was conducted with the objective to maximize the total CH4 evolution. Among five suggested optimized conditions, the total CH4 evolution achieved was 35.51 μmol gcatalyst-1 when the light intensity, PCO2, and PH2O values were 80 W m-2, 100 kPa, and 29 kPa, respectively. Finally, The mechanism of photoactivity by the developed catalyst was proposed, and the possible reaction pathway was discussed. Since a large amount of EG was produced by the sV technique and was applicable in photocatalytic applications, a large amount of EG-CN photocatalyst nanoarchitecture could be prepared with less hazardous and abundant materials towards realizing a large-scale CH4 production via CO2 photoreduction.