Design Of Layered Double Hydroxides-based Catalysts And Their Applications In Photocatalytic CO₂ Reduction

Photocatalytic CO2 reduction can convert greenhouse gas CO2 into energy chemicals,which is expected to be one of the ways to solve environmental and energy problems.However,the design and preparation of efficient CO2 reduction photocatalysts still face challenges due to the high thermodynamic energy barrier of CO2 reduction reaction and the low carrier separation efficiency in photocatalysts.Layered double hydroxides(LDHs)are promising CO2 reduction photocatalysts because of their adjustable band gap and rich hydroxyl groups for CO2 adsorption.To address the lack of suitable models for revealing the role of anionic vacancy defects,the high cost of noble metal co-catalysts,and the complicated preparation steps of heterojunction systems in the current research on LDHs,this paper designs a mild reduction method to construct anionic vacancy defects,a selective etching method to construct cationic vacancy defects,a non-precious metal and noble metal alloying,and a one-step hydrothermal method to construct heterojunction strategies to regulate the photocatalytic CO2 reduction performance of LDHs materials.In order to help understand the conformational relationships of LDHs materials,to promote them to meet the requirements of practical applications,and to provide potential ideas for the design of other CO2 reduction photocatalysts.The main research of this dissertation are as follows.1.ZnCo-LDH rich in anionic vacancy defects realized enhanced CO2 photoreduction performance under visible light:The existing defect construction strategies are usually achieved by modulating the morphology,which leads to the increase of variables affecting the performance(e.g.,specific surface area),and the effect of defect concentration on the performance has not been studied yet.Therefore,there is still a need to develop research models to control the variables and reveal the effect of the introduction of defects in LDHs and the concentration of defects on the performance of photocatalytic CO2 reduction.To this end,this chapter designs an ascorbic acid reduction method to treat ZnCo-LDH,which successfully introduces hydroxyl vacancies on the nanosheet surface and maintains its morphology unchanged.In addition,the concentration of hydroxyl vacancies can be controllably adjusted by adjusting the concentration of ascorbic acid,thus constructing a suitable univariate study model for revealing the effect of the introduction of hydroxyl vacancies in LDHs and their concentration on the photocatalytic CO2 reduction performance.The results showed that the hydroxyl vacancies promoted the photocatalytic CO2 reduction activity of ZnCo-LDH,and the promotion effect increased with the increase of the concentration of hydroxyl vacancies.The hydroxyl vacancies increase the electronic states near the Fermi energy level of ZnCo-LDH and promote the separation and migration of carriers;meanwhile,they provide stronger adsorption sites and enhance the CO2 adsorption;in addition,the hydroxyl vacancy enhances the interaction of surrounding Zn atoms with COOH*and lowers the energy barrier of the rate-limiting step of the reaction.As a result,the productivity of H2 and CO in the best performing modified samples reached 110.2 μmol g-1 h-1 and 539.5 μmol g-1 h-1 under visible light,respectively.The yield of H2was 3.2 times higher than that of the unmodified sample,and the yield of CO was 2.4 times.2.Cationic vacancy defects in CoAl-LDH promoted photocatalytic CO2 reduction under visible light:Compared to the anionic vacancy defects,the role of cationic vacancy defects in regulating the photocatalytic CO2 reduction performance of LDHs materials is still not fully investigated.In addition,the introduction of cation vacancy defects is often accompanied by other types of anionic and cationic vacancy defects in the few existing studies,leading to the complexity of the influencing factors.To this end,the work in this chapter designs a NaOH etching method for the treatment of CoAl-LDH,and uses the selective removal of Al by NaOH to successfully restrict the defect type to a single type of cation vacancies,thus enabling the investigation of the mechanism by which cation vacancy defects affect the photocatalytic CO2 reduction performance.It is shown that Al vacancy defects can improve the photocatalytic CO2 reduction activity of CoAl-LDH.The mechanism of the improved performance can be elaborated as follows:The presence of Al vacancy defects leads to the generation of electronic states in the original forbidden band region,which induces better light absorption and carrier separation and transfer.In addition,the introduction of Al vacancy defects enhances the adsorption of H2O and CO2 by the catalyst.Ultimately,under visible light,CoAl-LDH-NaOH containing Al vacancy defects showed a H2 yield of 277.9 μmol g-1 h-1 and a CO yield of 1349.2 μmol g-1 h-1,which were about 3.9 and 2.4 times higher than those of CoAl-LDH.3.CoPd alloys modified NiAl-LDH for enhanced CO2 photoreduction to syngas under visible light:Current research on syngas production from LDHs focuses on the regulation of H2 and CO ratios,which is usually accompanied by a decrease in H2 or CO yields.It is still a great challenge to enhance the H2 and CO yields simultaneously and to ensure that the ratio of both meets the practical requirements.In this work,we designed a loading alloy strategy to load CoPd alloy co-catalyst on the NiAl-LDH surface by NaBH4 reduction method,which successfully achieved the simultaneous increase of H2 and CO yields while reducing the cost and meeting the practical requirements of the two ratios compared with the previous strategies such as loading a single noble metal Pd.The mechanism of the CoPd alloy for yield control is shown as follows:the CoPd alloy can form a Schottky junction with NiAl-LDH with a more suitable Schottky barrier height than the single metal Pd,which further promotes the carrier separation and transfer;in addition,the CoPd alloy can provide optimized adsorption sites for both CO2 and H2O.Ultimately,the H2 and CO yields of NiAl-LDHCoPd in visible light are about 14.5 and 2.1 times that of NiAl-LDH and 3.3 and 5.6 times that of NiAl-LDH-Pd,respectively,and the H2 and CO yields are close to 1:1 ratio.4.NiFe2O4/NiAl-LDH heterojunction enabled higher CH4 selectivity in CO2 photoreduction under visible light:Constructing heterojunctions is an effective strategy to enhance the activity of CH4 production,however,the existing heterojunction systems are complicated and time-consuming to prepare.In this work,we successfully developed a simple one-step hydrothermal method to construct a new type-II heterojunction composed of NiAl-LDH/NiFe2O4 by taking advantage of the similarity of the synthesis conditions between NiAl-LDH and NiFe2O4,which greatly reduces the steps and time consumption compared with the previous heterojunction construction methods and improves its practical value.In addition,under visible light,the CH4 yield of NiAl-LDH/NiFe2O4 was 34.0 μmol g-1 h-1,which was 6.4 times higher than that of NiAl-LDH,respectively,and the CH4 selectivity increased from 6.7%to 34.8%in NiAl-LDH,while the selectivity of H2 decreased from 11.6%in NiAl-LDH to 1.5%.The formation of type II heterojunction promotes the separation and migration of photogenerated carriers;NiAl-LDH in heterojunction enriches more electrons on the surface compared with single NiAl-LDH,which is favorable to the formation of CH4 that requires more electrons to participate in the reaction;in addition,the heterojunction has stronger CO adsorption capacity compared with NiAl-LDH,which is favorable to the further conversion of intermediate CO*to CH4.