Rational Design And Structure-Performance Relationship Of High-Efficiency Carbon-Based Energy Molecules Conversion Catalysts

In the long history,fossil energy has always occupied a dominant position in the energy consumption structure of various countries in the world.While the utilization of fossil fuels has brought us an era of unprecedented prosperity,it has also brought significant greenhouse gas emissions such as carbon dioxide(CO2)due to its inefficient utilization,which has a negative impact on the global climate.Therefore,more efficient conversion of carbon-based energy molecules into high-value-added fuels and feedstocks plays an increasingly important role in the current energy and chemical supply.However,the relatively inert nature of C-H and C-O bond in carbon-based energy molecules makes it difficult to achieve selective activation and oriented conversion to high-value-added products.In addition to reams of studies on the conversion of carbon-based energy molecules at high temperatures(such as thermal cracking),the catalytic conversion under mild conditions has received much attention due to better selectivity,lower energy consumption and equipment maintenance costs.However,highly active catalysts are required to reduce the reaction energy barrier at such circumstance.Therefore,how to establish an understanding of the catalytic mechanism and the structure-performance relationship through in-depth understanding of the catalytic process and reaction mechanism,so as to improve the catalytic activity and selectivity to target products is still a major scientific problem to be solved.In response to these problems,the catalytic mechanism and the structureperformance relationship of different types of catalysts for the conversion of carbon-based energy molecules,including propyne semi-hydrogenation(PSH),direct propane dehydrogenation(PDH)and electrocatalytic CO2 reduction(ECR)were studied in this paper.The theoretical basis for the potential applications of these catalysts with specific structures in the conversion of carbon-based energy molecules were given.The main work contents are as follows:In Chapters 3 and 4,the structure-performance relationship in PSH was discussed.Through systematically DFT studies,we propose that different from the existing debates where the formation of Pd-C species or specific facets of Pd nanoparticles(NPs)are critical,the apexes of Pd(111)octahedrons are the active sites for highly-performance PSH.The propylene selectivity on Pd octahedrons can be ascribed to site-selective propyne adsorption on the apexes prior to reactions and subsequent difficult to access intermediate states toward over-hydrogenation.Experimentally,the Pd octahedrons enclosed by(111)facets can selectively hydrogenate trace amounts of propyne in propylene gas stream at room temperature,eliminating the propyne molecule that poisons propylene polymerization catalysts,which is critical for polypropylene production.Inspired by the characteristics of active sites revealed in Chapter 3,combined with the experimentally synthesized monodisperse PdxGayNPs,we successfully constructed the structural model through theoretical calculations.Calculations demonstrate that the strong metal-support interactions(SMSI)within PdxGay NPs can change its surface properties dramatically,providing the Pd-Ga alloy structure and defect-rich Ga2O3.Such geometric effects isolate the Pd atoms and weaken the adsorption ability of H2,lead to variations of reaction paths and enhancement of propylene selectivity.This work may not only deepen the basic understanding of SMSI,but also shed some light on the design and application of composite NPs,and promote the design and application of heterogeneous catalysts.In Chapter 5,the structure-performance relationship in PDH was discussed.The performance dependency of local chemical environments of SACs to PDH has not yet been addressed.Through theoretical calculations,we report for the first time that compared with the in-plane Fe-N4-C site,the N and P dual-coordinated single Fe(FeN3P-C)site exhibits outstanding performance for PDH at industrial PDH temperatures.The catalytic activity and selectivity of Fe-N3P-C resulted from P substitution can be attributed to the geometric effect,where the out-of-plane structure of Fe-N3P-C can break conjugation-like electronic states near Fermi level and exhibits suitable H affinity,which not only effectively promotes the C-H bond scission but also provides the appropriate H diffusion rate that ensures both the high selectivity of propylene and the regeneration of catalysts.The current work reveals a novel mechanism of triggering the activity of PDH by tuning the local environment with atomic precision.In Chapters 6 and 7,the structure-performance relationship in ECR was discussed.Metal SACs supported on two-dimensional(2D)materials are promising ECR catalysts.The electronic structure of 2D-SACs can change the catalytic reaction pathway and improve the selectivity of products.Utilizing the experimental synthesized 2D-IrO2 catalyst,through theoretical calculations,we first report a novel Mn-SAC(Mn-Ov-IrO2)with outstanding ECR activity and selectivity,where the distribution of products can be controlled by applying an appropriate applied voltage.The difference of free energies between reaction pathways can be attributed to the difference of bonding characteristics between intermediates and Mn.Finally,the competitive HER can be effectively suppressed due to the weak adsorption of H on Mn-Ov-IrO2.We believe that the superior performance exhibited by Mn-Ov-IrO2 can inspire the rational design of heterogeneous catalysts for ECR.Recent studies have shown the decisive influences of different metal centers on the catalytic performance.By screening experimental synthesized 2D N-P bicoordinated graphene nanosheets as ECR catalysts,we demonstrate that the selection of metal atoms can significantly affect the ECR performance of N3P-C.Fe-N3P-C and MnN3P-C with more Bader charge and higher d-band center have better CO2 activation ability.Ultimately,formic acid is the product on Fe-N3P-C,while methanol is the product on Mn-N3P-C.In addition,due to high HER energy barriers,the most important competitive side-reaction in ECR will be inhibited.This work contributes to the understanding of the ECR mechanism on N3P-C monolayers and can provide theoretical guidance for the design of high-performance ECR catalysts.Through DFT theoretical calculations,the thesis offered deep understanding of the catalytic process and reaction mechanism of carbon-based energy molecule conversion,and the origin of catalytic performance,that is,the relationship between catalyst structure and performance.It not only demonstrates the application potential of catalysts with different structures in the field of carbon-based energy molecule conversion,provides theoretical guidance for the design of novel catalysts,but also provides a fresh idea for the rational design of active centers at the atomic level.