Synthesis and characterization of molybdenum carbide from oil palm frond based activated carbon for carbon dioxide reduction

Abstract

Energy-efficient technique is one of the vital keys to sustainable development in manufacturing processes. Microwave technique is an attractive approach in manufacturing as this technique offers several advantages such as rapid process and low energy requirement. In this study, microwave induced technique was applied to the preparation of activated carbons and molybdenum carbides. The activated carbons were prepared using oil palm fronds and activated using potassium hydroxide and phosphoric acid solutions at different concentrations. The products were then characterized using Fourier transform infrared spectroscopy (FTIR), nitrogen gas adsorption analysis and scanning electron microscopy (SEM). The results indicated that the highest surface area obtained by activation using phosphoric acid was 638.14 m2/g. However, the activated carbons prepared using a potassium hydroxide showed low surface areas for all samples. In the production of molybdenum carbide and potassium doped molybdenum carbide via microwave induced alloying, the prepared activated carbons were mixed with a 1 M molybdenum solution followed by the microwave-induced technique. All the prepared samples were chemically characterized using X-ray diffraction analysis (XRD), nitrogen gas adsorption analysis, SEM, energy dispersive X-ray (EDX), ammonia-temperatureprogrammed desorption analysis (TPD-NH3), acidic titration method and electron spin resonance (ESR) spectroscopy. XRD analysis indicated that the perfect ratio for the preparation of molybdenum carbide is 2 Mo: 7 C. Nitrogen gas analysis showed that molybdenum carbide and potassium doped molybdenum carbide had surface areas of 95.0537 and 61.9302 m2/g, respectively. The ESR spectroscopy indicated the presence of a singlet peak at g-value of 1.9723 in the ESR spectra of potassium doped molybdenum carbide; this suggested that an electron was donated from potassium to the surface of molybdenum carbide. TPD-NH3 indicated the presence of weak, moderate and strong acid sites for the molybdenum carbide and potassium doped molybdenum carbide. However, the addition of potassium decreased the amount of acid sites for the molybdenum carbide. The prepared carbides were evaluated as catalysts in the reduction reactions of carbon dioxide that were conducted using in-house modified reactor. The product was then characterized using FTIR. In the catalytic application, the molybdenum carbide showed a higher activity compared to the potassium doped molybdenum carbide. This was probably caused by the potassium moiety which poisoned the reaction while blocking the active sites. To better understand the reaction, kinetic analysis was carried out for the hydrogenation of carbon dioxide via molybdenum carbide. Results suggested an optimum temperature of 450°C, second order reaction, and the activation energy of 34.27 kJ/mol. In addition, thermodynamic parameters were also calculated using the Eyring equation and it was revealed that the values of ?H‡, ?S‡ and ?G‡ were +40.04 kJ/mol, -1478.4 J/mol K and +1108.7 kJ/mol, respectively. These parameters proved that this reaction is not spontaneous and it is endothermic in nature with associative reaction mechanism.