Investigating the product distribution behaviour of CO_2 methanation through thermodynamic optimized experimental approach using micro/nano structured titania catalyst

摘要

Catalytic CO2 hydrogenation yields a wide range of products, making products selectivity a major challenge. This study provides a comprehensive investigation on CO2 hydrogenation to CH4 by involving thermodynamic optimized experimental approach. First, thermodynamic analysis was carried out using equilibrium rate constants and Gibbs free and insights on the behaviour and product distribution of CO2 methanation were investigated. A low temperature (150-450 degrees C), elevated pressure and H-2/CO2 feed ratio above 4 are essential for CH4 production. More importantly, as temperature rises, the product formation shifts from CH4 to CO, C2H6 and finally solid carbon. Upon thermodynamic optimization, an experimental approach was conducted using micro/nano structured conductive catalysts. CO2 methanation over Ni-dispersed conductive TiO2 microparticles (MPs)/nanowires (NWs) catalysts involves various complex reactions such as the adsorption, desorption and activation of molecules. An ideal Ni loading of 15 wt, with GHSV of 6,300 mL.g(-1)h(-1) is beneficial for the CH4 production. Furthermore, TiO2 NWs provided a 1.32-fold enhancement in activity and CH4 production comparing to MPs, elucidating the enhancement effect of a one-dimensional (1D) nanowire structure over zero-dimensional (0D) spherical microparticle structure. Y-CH4, S-CH4 and X-CO2 over 15 Ni/TiO2 NWs achieved 88.9, 99.1 and 89.8, respectively. Despite a good agreement between the thermodynamic and experimental results, there was a slight difference in trend because the theoretical values can be obtained by just considering the feasibility of reactions in terms of Delta G and ln (K). Besides, catalytic experimental runs involves complex reactions through adsorption and desorption, leading to CH4 formation via different pathways. Due to the conductive characteristics and 1D structure of TiO2, the products distribution was close to thermodynamics value and exhibited higher stability that would be beneficial for further investigation in catalytic reactions.