Quantum mechanical modelling of engineering nanostructures for CO2 adsorption and dissociation

Abstract

Carbon dioxide (CO2) emission from anthropogenic sources has become the major source of greenhouse gases that have caused global climate change in recent times. Hence, much attention has been given to developing highly effective capture materials that can overcome the cost implications of existing technologies. This doctoral project centers on the development of two-dimensional (2D) nanostructured materials, which are extremely selective, stable, and sensitive to CO2 capture and improve catalytic efficiency to achieve viable transformation of CO2 on an industrial scale. The 2D nanostructure materials studied include graphene and molybdenum disulphide (MoS2) monolayers, graphene-MoS2 confinements, and a series of defect-free monolayer MoS2 as well as MoS2 surface with different types of vacancies (single and double vacancies) with and without N-doping.
A comprehensive analysis of CO2 adsorption energies (EAE) at various interlayer spacing of different multilayer structures comprising graphene/graphene (GrapheneB) and MoS2/MoS2 (MoS2B) bilayers as well as graphene/MoS2 (GMoS2) and MoS2/graphene (MoS2G) hybrids is performed to obtain the most stable adsorption configurations. It was found that 7.5 Å and 8.5 Å interlayer spacing is the most stable conformation for CO2 adsorption on the bilayer and hybrid structures, respectively. Adsorption energies of the multilayer structures decreased in the following trend: MoS2B > GrapheneB > MoS2G > GMoS2. By incorporating van der Waals (vdW) interactions between the CO2 molecule and the surfaces, we find that CO2 binds more strongly on these multilayer structures. Bader charge analysis shows that the interaction between CO2 and these surfaces causes charge transfer and redistributions. By contrast, the density of state (DOS) plots shows that CO2 physisorption does not have a substantial effect on the electronic properties of graphene and MoS2.
A DFT study was also conducted to analyze CO2 adsorption on defective and non-defective MoS2 surfaces with or without nitrogen doping. Results showed the dissociative chemisorption of CO2 on the MoS2_1Vs and a significantly enhanced physisorption of CO2 on the MoS2_1VMo_3NS, which displays adsorption energy of -1.818 eV compared with -0.139 eV of the pristine MoS2 surface. Meanwhile, the MoS2_1Vs exhibits excellent selective adsorption of CO2 over N2 and H2O, with the highest adsorption ratio of 5.1 and 3.5, respectively. Partial dissociation of CO2 to CO over the MoS2_1Vs is also observed and attributed to increased covalent attractions at the vacant site, while the improved CO2 physisorption over the MoS2_1VMo_3NS is attributed to the enhanced electrostatic interactions at the vacancy site due to N doping. These are confirmed by the computed vibrational frequencies of CO2 bound on these surfaces.
Furthermore, the adsorption and dissociation of CO2 and H2O on MoS2 monolayers with defects and N-doped vacancy sites are investigated. The calculations reveal that the MoS2_1VMo_3NS are the most catalytically active sites. The interactions with CO2 and H2O are enhanced by the larger electron distribution with N dopants and neighbouring S atoms. Climbing image nudged elastic band (Cl-NEB) and ab initio molecular dynamics (AIMD) analyses indicate that the interactions are exothermic and result in spontaneous molecular dissociation. Here, CO2 dissociates into CO٭ and O٭ on two N atoms with no barrier, while H2O dissociates via two mechanisms: 1) into adsorbed OH٭ and H٭ species (Ea = 0.21 eV), and 2) into adsorbed O, H, and H atoms ((activation energy (Ea) = 0.10 eV). The computed Ea values are significantly lower than the threshold energy barrier for chemical reactions at room temperature (0.8 eV), which also indicates that CO2 and H2O dissociation is spontaneous at ambient temperature. Given the ease of formation of CO٭, O٭, OH٭and H٭ radicals.
Finally, the adsorption of different gas molecules (CO2, CH4, N2, H2 and H2O) on the MoS2_1VS surface is investigated using first-principles calculations and Grand Canonical Monte Carlo (GCMC) simulations. DFT and GCMC simulation results demonstrated that MoS2_1VS enhances the adsorption of CO2, and H2O at 1 bar, 298 K, and H2 at 1 bar, 77 K relative to that of the pure surface. CO2 loadings of 2.49 wt.% and 0.55 wt.%, H2O loadings of 55.27 wt.% and 44.25 wt.%, and H2 loadings of 0.29 wt.% and 0.20 wt.%, for the MoS2_1VS and PMoS2 surfaces, respectively, were observed. In addition, optimized MoS2_1VS configuration stipulates that adsorption of CO2, H2 and H2O was via dissociative chemisorption, in contrast to optimized PMoS2 configuration, for which physisorption was the only adsorption mechanism. CI-NEB analysis in agreement with DFT results from geometry optimization shows that the partial CO2 and H2O dissociation and complete H2 splitting processes exhibited energy barriers of 1.11 eV, 0.65 eV and 0.18 eV. The computed free energy of activation (∆Ga) for the partial and complete dissociation of CO2 and H2O, and H2 are 0.35 eV, 0.30 eV, and -0.66 eV. These values are lower than the threshold energy barrier for chemical reactions at room temperature (0.8 eV), which reveals that the dissociation is spontaneous at ambient temperature.

Date of Award	Jul 2022
Original language	English
Awarding Institution	University of Nottingham
Supervisor	Hainam Do (Supervisor), Mengxia Xu (Supervisor), Tao Wu (Supervisor) & Mike George (Supervisor)