Regulating The Micro-structures Of Supported Palladium Catalysts For Enhancing The Semihydrogenation Of Alkynes

Selective hydrogenation is considered to be one of the most common catalytic transformations in chemical industry,among which alkynes semi-hydrogenation has always been a focus of attention in the academic and industrial field due to its relevance in the chemical manufacturing of polymers and fine chemicals.Supported Pd-based nanocatalysts are prevalently utilized for the selective hydrogenation of alkynes in industry.However,the current engineering strategies for heterogenous hydrogenation catalysts inevitably restrict the catalytic performance,which are confronted with the paradox between high reactivity and alkene selectivity.The use of various additives and the complex interface structure make it much challenging for researchers to explicitly resolve the structure-property relationship of such catalytic systems.Based on the above-mentioned research context and existing problems,this thesis takes the supported Pd nanocatalysts as the research core,which has concentrated on the precise regulation of the micro-structure and coordination environment of the active metal phase for enhancing the selective hydrogenation of alkynes,concurrently pursuing the reduced employment of precious metal Pd.Additionally,by the combination of experimental analysis and theoretical calculation,this paper has generalized the structure-property relationship between the active site structures and the semihydrogenation performance of the series Pd catalysts,which is capable to provide reference and guidance for the further engineering of highly-efficient hydrogenation catalysts.The detailed studying contents are as follows.（1）Firstly,a dynamic organic ligands modification strategy is developed by preferentially grafting flexible alkylamine（DETA and TETA,etc.）to the NHPC support surface for dynamically modifying the Pd-based nanocatalysts.The DETA-modified Pd catalyst has demonstrated exceptional reactivity(14412 h^-1)and chemoselectivity as high as 95%for the partial hydrogenation of 2-methyl-3-butyn-2-ol（MBY）under mild reaction conditions,presenting a 36-fold higher activity than the commercial Lindlar catalyst,which also manifests outstanding cycling stability even up to 20 cycles.Furthermore,the MBE yield exceeds 98.2%under complete conversion and solvent-free conditions.Systematic experiments and DFT calculation have revealed that the flexible alkylamine takes effect in a cyclical“breathing pattern”for the adsorption discrimination by creating dynamic metal-support interaction（DMSI）,enabling selective repellence of alkenes from the surface of Pd active phase.Different from the conventional organic ligand modification strategy,DETA is capable to dynamically modulate the adsorption patterns of reactants and effectively avoid the permanent poisoning of Pd active sites,concurrently achieving the unprecedented catalytic reactivity and chemoselectivity.This work has illuminated a novel and practical method for the rational engineering of heterogeneous nanocatalysts by constructing dynamic metal-support interactions.（2）Secondly,by leveraging the thermally-induced surface restructuring strategy,the micro-structure and coordination environment of the active sites over Pd/Ce O₂-T_calc and Pd nanoparticle catalysts have the capacity to be precisely regulated.The structure-activity relationship between the reconstituted Pd active phase and the catalytic performance of selective hydrogenation of MBY has also been deeply investigated.The correlation between the diameter ratios（W values）of the restructuring active phases and hydrogenation performance has also been summarized.Systematic characterizations and molecular dynamics stimulation illustrate that the surface restructuring of the active phase is accompanied by the transformation of the electronic structure and coordination environment,which leads to the formation of uniformly dispersed flat Pd structures with lower diameter ratios and more interfacial Pd-O-Ce linkage,thus significantly enhancing the metal-support interaction.Meanwhile,the kinetic analysis and DFT calculations further confirm that the structural evolution effectively modulates the adsorption pattern of MBY on the surface of the active site,which substantially relieves the self-induced reactant poisoning and thus resulting in the extraordinary promotion in catalytic performance for selective hydrogenation.The N₂ and Ar aging experiments have displayed the synergistic effect of the calcination atmosphere and Ce O₂ lattice oxygen on the structural evolution of Pd/Ce O₂,further demonstrating the diameter ratios of Pd species could be utilized as the descriptor to reflect the geometric and electronic structure changes of the reconstituted active phase,which is also confirmed to present a linear correlation with the catalytic performance of MBY semihydrogenation.Moreover,the structural characterizations and catalytic performance evaluation of Pd _NP/Ce O₂ with different sizes has further manifested that the extent of surface restructuring of the active phase triggered by the air calcination treatment gradually decreases with the increase of the average size of Pd nanoparticles.This work systematically explores the structure-activity relationship between the structural evolution of reconstituted Pd active phase and the catalytic performance of selective hydrogenation of MBY,which has provided useful reference for the engineering of surface restructuring catalysts.（3）Thirdly,atomically dispersed 0.1Pd₁/Mo S₂ nanocatalyst is successfully prepared for efficiently selective hydrogenation of acetylene in excess ethylene stream by regulating the active metal dispersion and coordination environment.Spherical aberration-corrected TEM,EXAFS and quasi-in situ XPS characterizations have indicated that Pd single atoms are mainly distributed at the edge sites of Mo S₂nanosheets,forming a unique Pd₁-S₄ structure.The optimal 0.1Pd₁/Mo S₂ single-atom catalyst has exhibited excellent catalytic performance for the selective hydrogenation of acetylene in ethylene-rich streams,achieving complete conversion of acetylene at140°C and ethylene selectivity up to 97%,significantly outperforming the Pd nanoparticle catalysts.More importantly,the 0.1Pd₁/Mo S₂catalyst has presented outstanding long-range stability with imperceptible decay in the catalytic activity and selectivity in the continuous hydrogenation reaction for more than 1200 h,implying the potential for the industrial application.Systematic experiments combined with DFT theoretical calculations definitely reveal that the Pd single-atom active site anchored at the edge of the Mo S₂ support has changed the adsorption conformation of the reactants and effectively reduces the binding strength of C₂H₄,thus distinctly enhancing the catalytic performance for semihydrogenation of acetylene.This work provides new insights into the engineering of highly efficient and durable sulfide-supported single atom catalysts.