Intercalation And Doping Strategy In Layered Double Hydroxides Nanosheets And Their Visible Light Reduction Of CO₂

The pressure exerted on available fossil fuel resources renders photocatalytic reduction of CO2with water oxidation as the most economically viable approach to sustainable energy.This process,otherwise referred to as artificial photosynthesis,usesCO2 as a feedstock molecule for the production ofhighly value-added chemicals while mitigating greenhouse gases as well.Its obvious potential has inspired extensive research.Catalytic reduction of CO2 faces stiff competition from overall water splitting,due to similar redox potentials and high thermodynamic stability.Furthermore,the efficiency of the photocatalytic process is often restricted by the low charge transport,faster recombination of photo-induced charge carriers,and the limited visible light harvesting capacity.Moreover,photoreduction of CO2 is a multiple electron process involving activation of water molecule.An efficient photocatalyst is therefore highly desirable to meet the challenge.Among the reported photocatalysts,layered double hydroxides(LDHs),a brucite(Mg(OH)2)-like synthetic two dimensional(2D)materials with a highly tunable compositionand interlayer space,have been regarded as a platform for desirable photocatalysis.Therefore,LDHsoffer an avenue for the design of efficient photocatalysts for reduction of CO2 by finely tuning of its layer and interlayer compositions to enhance their light absorption and electronic properties for potential photocatalysis.This thesis is engrained on visible light heterogeneous reduction of CO2 by usingLDHs nanosheets through controlling the interlayer species and the composition of the layers.In this thesis,a review of fundamentals of CO2properties and reduction processes,the LDHsproperties,structure,syntheses,characterization and applications are overviewed in chapter 1.Chapter 2 deals withthe interlayer modification of LDHs by presenting a series of nitrate and carbonate intercalated NiAl-LDH,CoAl-LDH and MgAl-LDH.The influence of the metal compositions on CO2 reduction performance follows a standard pattern according to nature of the layers.Because of intercalation of N03-anion in LDHs architecture,their corresponding photocatalytic performance is enhanced as compared to the CO32-anion intercalated LDHs.The NiAl-NO3exhibitsa selectivity of 6.07%to CH4 and 83%to CO against a selectivity of 0.49%and 66%for CH4 and CO for the CO32-anion intercalated counterpart,respectively.The NiAl-NO3 offers enhanced defect sites(oxygen and metal vacancies)in the LDHs structure,which highly exposed surface hydroxyl groups and unsaturated coordination active sites as well as enhanced absorption of visible light as evidenced by X-ray absorption Spectroscopy(XAS)and Ultraviolet-visible light diffuse reflectance spectroscopy(UV-Vis DRS)measurements,respectively.Other characterizations like electrochemical impedance spectroscopy(EIS)also justified the superiority of NO3-anion intercalated LDHs in photocatalytic reductions.This work demonstrates that LDHs interlayer modulation can be used to boost desirable properties for CO2PR photocatalysts.Chapter 3 explores the effect of doping rare earth elements(REEs)into LDHs lattices.REEs(like Ce)are renowned especially for filled f-orbitals and strong coordination capabilities.The selectivity of Ce doped MgAl-LDH to CO is increased from 11.5%to 41.5%by using a catalyst with an optimum Ce doping content of 7.5%of the total tri valent cation content(Mg6Al1.85Ce0.15-LDH denoted as Ce-0.15 LDH).The Ce-doping of LDHs enhances photo-induced charge transport efficiency as evidenced by EIS.This is due to existence of Ce ions with inherent robust electronic properties from highly delocalized electrons.This property is imparted on the Ce-doped LDHs.The Ce dopant in optimum amounts not only promotes absorption of light in the visible region,but also enhances the charge transport and simultaneously suppresses the recombination rates.The above factors synergistically enhance the photocatalytic efficiency.The doping of REEs with special electronic properties provide a potential alternative for tunable band structure and photo-induced charge transport to explore highly efficient LDHs photocatalysts.This work therefore serves as a spotlight and a stimulator to develop low-cost efficient catalysts for CO2PR.