Catalytic conversion of carbon dioxide to hydrocarbons

• Main Author: Mohan, Ojus
• Other Authors: Xu Rong
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Chemical engineering; Engineering::Materials
• Online Access: https://hdl.handle.net/10356/146544
• Institution: Nanyang Technological University

CO2 methanation and dry reforming of methane (DRM) are promising routes for CO2 utilization. Ni-based catalysts are widely employed for these reactions. However, these are less active and are prone to deactivation due to carbon deposition. Gaining mechanistic insights is pivotal in developing more active and coke resistant Ni-based catalysts. This work combines Density Functional Theory (DFT) calculations with microkinetic modeling (MKM), to provide mechanistic insights into CO2 methanation (on Ni and Ru) and DRM reactions (on Ni and boron-doped Ni (NiB)). Based on the computed energetics, this work predicts a novel catalyst (Mn-NiB single atom alloy (SAA)) for these reactions. The choice of correct DFT functional is crucial to accurately predict the reaction energetics. Hence, a benchmarking study was performed for the first time to identify a DFT functional for studying CO2 conversion reactions. A functional screening was performed based on CO2 and CO adsorption energies, DFT-XPS, and density of state calculations. rPBE-vdW functional with a correction of 28 kJ/mol for gas-phase CO2 energy, was proposed to be the best available functional for studying CO2 conversion reactions. The inability of a functional to accurately predict energetics was typically attributed to the wrong description of adsorbate-surface interactions. However, for rPBE-vdW functional, we found that this is due to the erroneous treatment of C=O double bonds in gas phase CO2. Employing the benchmarked functional, CO2 methanation (46 elementary reactions) and DRM (38 elementary reactions) reactions were studied by combined DFT and MKM. CO2 methanation reaction mechanism is debated and the reaction mechanism elucidation is crucial in developing efficient catalysts based on a bottom-up approach. The most debated step is the activation routes of CO2 and CO and whether the reaction proceeds with/without forming CO* intermediate. The current study resolved the discrepancy in the CO2 methanation reaction mechanism on Ru (most active noble metal catalyst) and Ni surfaces. We identified that the dominant reaction pathways are CO2*→CO*→HCO*→CH*→ CH2*→CH3*→CH4 and CO2*→CO*→COH*→C*→HC*→CH2*→CH3*→CH4 on Ni (111) and Ru (001) respectively. On comparing Ni (111) and Ru (001), Ni (111) was more selective towards methane formation. Therefore, CO2 methanation proceeds via CO* intermediate and this study resolves the contradictions in CO2 methanation reaction mechanisms on Ni and Ru surfaces. For DRM reaction, the deactivation of Ni-based catalyst is a key challenge, and this impedes commercializing the DRM process. Doping boron at the sub-surface interstitial sites in Ni prevents the diffusion of carbon. Hence, we investigated NiB as a potential catalyst. The effect of doping is evaluated by comparing the dominant pathway with that on Ni (111). The dominant reaction pathway is CO2*→CO*+O*; CH4→CH3*→CH2*→C*→CO* and CO2*→CO*+O*; CH4→CH3*→ CH2*→CH2O*→CHO*→CO* on Ni (111) and NiB surfaces respectively. We reveal that boron doping alters the dominant reaction pathway (via CH2* oxidation route) to kinetically hinder carbon formation (no C* intermediate). In contrast to Ni, the barriers in CH4 activation routes and Boudouard reaction are significantly lower on NiB but the CO2 activation barrier (124 kJ/mol) is high resulting in reduced conversion. The strategy is to reduce the CO2 activation barrier selectively to improve the activity of NiB without compromising the stability. A computational screening was performed to identify thermodynamically stable NiB-based SAA that selectively reduces the CO2 activation barrier. The thermodynamic stability was evaluated against clustering and Mn-NiB SAA was identified as the only candidate on which there is a significant reduction in the CO2 activation barrier (68 kJ/mol). Subsequently, CO2 methanation and DRM reactions were studied on Mn-NiB SAA. The barriers for key reactions (CO2 and CO activation, CH* hydrogenation) are low on Mn-NiB SAA compared to Ni and Ru making it a suitable catalyst for CO2 methanation reaction. For the DRM reaction, Mn-NiB SAA has a significantly lower barrier for CH4 (compared to Ni) and CO2 (compared to NiB) activation. Additionally, the high endergonicity for CH4 stepwise dehydrogenation to form C* combined with a low barrier for Boudouard reaction prevents the catalyst deactivation. Employing an appropriate DFT functional, the combined DFT and MKM approach adopted in this investigation predicted the CO2 methanation and DRM reaction mechanisms. The gained insights from SAA would serve as guidelines for the development of alloy catalysts which can prevent carbon deposition without compromising the catalytic activity.