Synthesis and catalytic evaluation of ni supported on fibrous SBA-15 for CO2 methanation / Syahida Nasuha Mohd Bukhari

Faculty of Chemical and Natural Resources Engineering

Thesis (Master of Science) -- Universiti Malaysia Pahang – 2019

Includes bibliographical references

The utilization of CO2 has become an important topic in recent years. Due to energy crisis and environmental issues that related to CO2 emission in the atmosphere, there is a growing interest in its use as a feedstock in chemical processes. The conversion of CO2 into CH4 has been investigated extensively, using a variety of supported metal catalysts including Ni/SBA-15. However, Ni/SBA-15 rapidly deactivate due to the carbon deposition and metal sintering, arose from the weak metal-support interaction. Transformation of SBA-15 into F-SBA-15 could promote better metal dispersion and strengthen the metal-support interaction. Thus, this study focuses on the synthesis and catalytic evaluation of Ni/F-SBA-15 for CO2 methanation. Ni/F-SBA-15 catalysts were prepared by modifying SBA-15 into F-SBA-15 using microemulsion system coupled with SBA-15 crystal-seed crystallization method, followed by impregnation of Ni to the synthesized F-SBA-15. The properties of Ni/F-SBA-15 were determined by XRD, BET, FTIR, TEM, FESEM-EDX, and H2-TPR. The CO2 methanation was carried out in a stainless steel fixed-bed reactor, employing GHSV of 24,900 mL gcat-1 h-1, stoichiometric H2/CO2 ratio of 4/1 and temperature of 523-723 K, while the proposed mechanistic path of CO2 methanation was clarified using in-situ FTIR pyrrole and in-situ FTIR (H2 and CO2). The results showed that a superior performance towards CH4 production was achieved by Ni/F-SBA-15 whereby it possessed higher performance at temperature of 673 K (CO2 conversion = 99.7% and CH4 yield = 98.2%) compared with Ni/SBA-15 (CO2 conversion = 91.1% and CH4 yield = 87.5%). This phenomenon was due to the fascinating characteristics of F-SBA-15 that allowed a homogenous and better Ni insertion onto the support with a smaller Ni crystallites size, and thus strengthen the metal-support interaction. This study also explored the effect of Ni loadings (1, 3, 5, and 10 %) onto F-SBA-15 towards an efficient CH4 production from H2 and CO2. Regarding the obtained results, it showed that increasing Ni loadings onto F-SBA-15 promoted excellent performance towards CO2 methanation with a sequence of 1%Ni/F-SBA-15 < 3%Ni/F-SBA-15 < 5%Ni/F-SBA-15 ≈ 10%Ni/F-SBA-15, revealing the highest activity of 5%Ni/F-SBA-15. It proved that the fibrous support enhanced the quantity of Si-O-Ni bond, triggered better Ni dispersion, strengthen metal-support interaction, and increased the basicity. However, higher Ni loadings (10 wt.%) onto F-SBA-15 slightly declined the performance of CO2 methanation due to the limited spaces for substitution of Ni species with the silanol groups of F-SBA-15 upon the bulk Ni phase, poorer Ni dispersion, weaker metal-support interaction, and lower basicity. In addition, 5%Ni/F-SBA-15 also showed the most stable activity with no deactivation sign up to 120 h time-on-stream at 673 K, which closely related to high resistance of metal sintering and carbon deposition. The in-situ FTIR studies of adsorbed H2, CO2, and H2 + CO2 confirmed the CO2 methanation of Ni/SBA-15 and Ni/F-SBA-15 proceeded by CO dissociative reaction pathway. Ni as the metal sites was responsible in the dissociation of CO2 and H2, while the basic sites influenced the CO2 adsorption ability of the catalyst. The main adsorption species for Ni/F-SBA-15 were unidentate carbonates, bidentate carbonates, and linear carbonyl. Meanwhile, for Ni/SBA-15, only bidentate carbonates were observed. The new findings of combination between F-SBA-15 with an optimum Ni loading contributed towards an excellent performance and thus could be applied in various applications.