The Selectivity Of CO₂ Photoreduction By Tuning The Defects And Electronic Structure Of Layered Double Hydroxides

CO2 photoreduction(CO2PR)plays an important role in overcoming evnvironmental pollution and alleviating energy crisis.The designing of high efficiency catalysts remains great challenge in improving the selectivity of high-valuable products,suppressing the by-product H2 evolution,promoting the ultilization of wider absorption range and investigating the photocatalytic mechanism.Among the reported photocatalysts,layered double hydroxides(LDHs)is a typical 2D layered clay materials.Ascribe to cheap raw materials and simple synthesis process,LDHs is applied as a model photocatalyst with tunable particle size and thickness,adjustable element composition and proportion in the host layer,adjustable guest anions in the interlayer and controllable morphology.Based on the rich tunable properties of LDHs,this thesis focused on the designing of active sites by controlling the thickness and size,tuning the intercalated anions or adjusting the metal element composition of LDHs,thus achieving the regulating of defects and electronic structure in catalyst.The as-prepared catalysts performed high activity,unique selectivity of products and promoted the ultilization of wider absorption range in CO2PR.Moreover,the combination of in situ characterizations(such as,in situ DRIFTS and in situ XAS et al.)and DFT calculations were applied to explore the photocatalytic mechanism.This thesis achieved the tunable selectivity in CO2PR by regulating the defects and electronic structure in catalyst,thus providing a guideline for the design of producing CH4 with high selectivity under long-wavelength irradiation.The main contents are shown as follows:1.Based on the tunable particle size and thickness of LDHs,a series of NiAl-LDHs with different thickness and particle size were synthesized,containing b-NiAl-LDH with multiple layers,f-NiAl-LDH with few layers and m-NiAl-LDH with monolayer.With decreasing the thickness and particle size of NiAl-LDHs,the corresponding selectivity of CH4 increased and the selectivity of by-product H2 decreased in CO2PR under irradiation with ?>400 nm.The monolayer NiAl-LDH achieved 70.3%selectivity of CH4 and completely suppressed the evolution of by-product H2 under irradiation with ?>600 nm.XAFS,XAS and PAS proved the abundant metal and hydroxyl defects in monolayer NiAl-LDH.Meanwhile,DFT calculations were applied to explore the relationship between structure of catalysts and photocatalytic performance in CO2PR.The results illustrated that the defect sites in monolayer NiAl-LDH played as the active sites in CO2PR.Most intriguingly,under irradiation with ?>600 nm,the driving force of m-NiAl-LDH(VNi&OH)(0.313 eV)can overcome the Gibbs free energy barrier of CO2PR to CH4(0.127 eV),instead of producing H2(0.425 eV),thus effectively suppressing the evolution of by-product H2.2.Ascribe to the adjustable guest anions in the interlayer of LDHs,a series of lattice-strained curved morphology LDHs(denoted as LDH-Cx)were developed via intercalating surfactant anion.Compared with traditional hexagonal LDH-F,HRTEM,XAS,EXAFS and XRD results found that LDH-C2 contained compressed M(OH)6 and M-O-M bonds,meanwhile,abundant metal and hydroxyl defects presented in the lamellar.WT-XAFS further demonstrated that VNi was the main metal defects in LDH-C2.The selectivity of CH4 was 22.9%on LDH-C2 under irradiation with ?>400 nm.More intriguingly,under irradiation with ?=600 nm,the selectivity of CH4 was 83.07%by LDH-C2 and the by-product H2 evolution was efficiently suppressed with the selectivity of 3.84%.Optoelectronic characterizations revealed that the LDH-C2 enhanced the photocurrent and performed efficient photogenerated electrons-hole transfer and separation ability than LDH-F,thus promoted the activity of CO2PR.In situ DRIFTS results further proved that the lattice-strained LDH-C2 with abundant defects strengthen the interactions between intermediates(such as,OCOH*,CHx*et al.)and catalysts,promoting the evolution of CH4.Meanwhile,the protons from water splitting was limited to couple on LDH-C2,thus suppressed the formation of by-product H2.3?Inspired by the controllable metal element composition in the host layer of LDHs,a series of monolayer Ni3X-LDHs(denoted as m-Ni3X-LDH,X=Cr,Mn,Fe,Co)were reported.These as-prepared monolayer Ni3X-LDHs(X=Cr,Mn,Fe,Co)displayed a volcano-like trend of CO2PR to CH4 under irradiation with X>400 nm.Meanwhile,the monolayer Ni3Mn-LDH achieved nearly 99%selectivity of CH4 with totally suppressed H2 evolution under irradiation with X=600 nm.XAFS demonstrated the existence of abundant metal and hydroxyl defects in monolayer Ni3Mn-LDH and the lowest valence state of Ni in monolayer Ni3Mn-LDH was elucidated by XPS.Optoelectronic characterizations revealed that monolayer Ni3Mn-LDH performed the most efficiency of photogenerated electrons-hole pairs generation,transfer and separation.Furthermore,in-situ DRIFTS and DFT+U calculations illustrated that the defect state in monolayer Ni3Mn-LDH supplied appropriate driving force to CO2PR and monolayer Ni3Mn-LDH performed strongest absorption of intermediate product CO*,promoting the further hydrogenation of CO to CH4,thus improve the selectivity of CH4.