| Summary: | Carbon dioxide (CO2) hydrogenation into methane (CH4) is a promising technique in environment conservation while producing sustainable fuel to fulfil the energy demand. In this study, mesoporous silica KAUST Catalytic Centre number 1 (KCC-1) with a unique fibrous structure was successfully synthesized and compared with other silica-based catalysts to study the influence of catalyst support morphology on CO2 methanation. Different transition metals loaded on KCC-1 were prepared by impregnation method to investigate the enhancement of catalytic activity and different mechanism pathways of CO2 methanation. In addition, the bimetallic promoted KCC- 1 catalysts prepared via a co-impregnation method were studied with different promoter loading (2.5-10 wt.%). The catalysts were characterized using X-ray diffraction, nitrogen physisorption, field emission scanning electron microscopy, transmission electron microscopy, Fourier transform infrared (FTIR), pyrrole adsorbed FTIR, CO2 adsorbed FTIR, nuclear magnetic resonance, hydrogen temperature programmed reduction and electronic spin resonance. Catalytic activity was conducted at 423-723 K under atmospheric pressure. KCC-1 has significantly a higher number of basicity and oxygen vacancy than those of mobil composition of matter number 41 (MCM-41) and commercial silica (SiO2) which is directly correlated with the CO2 adsorption and catalytic performance. At 723 K, the CO2 conversion for KCC-1, MCM-41 and SiO2 was 9.2, 8.6 and 5.9 %, respectively. For different transition metal loaded on KCC-1, at 673 K, the catalytic activity follows the trend of nickel (Ni/KCC-1) > cobalt (Co/KCC-1) > zinc (Zn/KCC-1) with CH4 yield of 90.6, 71.6 and 10.8 % respectively. It was discovered that Ni/KCC-1 and Co/KCC-1 follows a dissociative mechanism pathway in which CO2 molecule was dissociated on the surface of metal before migrating onto KCC-1 surface. The Zn/KCC-1 on the other hand follows an associative mechanism pathway where H2 plays a role in CO2 dissociation which primarily occurs on KCC-1 surface. Vanadium (V) outperformed other second metals promoted Ni/KCC-1 such as chromium, manganese, iron, copper and zinc. The reaction light off temperature was lower on V-Ni/KCC-1 (423 K), compared to Ni/KCC-1 (473 K). At 623 K, the CH4 yield of 7.5V-Ni/KCC-1 reaches 94.4 % while 77.6 % for Ni/KCC-1. This could be attributed to the presence of V mitigated the agglomeration of Ni metal, thus the highly dispersed and exposed Ni active sites. Moreover, the amphoteric properties of V provide additional adsorption sites of CO2 and in turn influencing the catalyst activity. The optimum CH4 yield predicted by Response Surface Methodology was 98.6 % at reaction temperature of 641.3 K, GHSV of 10816.13 mL g-1 h-1 and H2:CO2 ratio of 5.8. The experiment carried out at these parameters yielded 95.8 % CH4 with an error of 1.44 %. The presence of O2 was found to inhibit the activity of the catalyst due to competitive adsorption of gases.This study reported for the first time, the utilization of unique silica morphology KCC-1 as a catalyst support and highlighted the contribution of promoted V-Ni/KCC-1 in the CO2 methanation research, particularly in the utilization of CO2 towards greener environment.
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