Design And Structure Dependence Of Palladium Nanocatalysts For Organic Synthesis

As we all know,organic synthesis provides a solid foundation for our society and life.However,excessive consumption has also led us to face the crisis of resource and energy shortage.In order to solve these problems,the development of catalytic science provides a broad platform for the realization of efficient,high-atom economic and low-energy consuming conversion process for organic synthesis.As an important catalyst,palladium plays a key role in the organic reactions.Thanks to the advancement of nanotechnology,we have been able to effectively control the preparation of a series of metal nanocrystals including palladium,which laid the foundation for palladium nanocrystals as a new member of the palladium catalyst family.Palladium nanocrystals are typically applied to organic reactions in the form of heterogeneous catalysts,while the limited ratio of surface atoms results a very small fraction of the atoms being able to contact with the reactants,which limits their catalytic efficiency.On the other hand,some organic reactions require the palladium catalysts to behave adequate activity to increase their selectivity to the specific products.Therefore,research on the structure-activity relationship of palladium nanocatalysts will provide a guidance for designing catalysts with high efficiency and selectivity.1.The crystal plane of nanocrystals determines the arrangement of their surface atoms and lattice spacing,which would have critical impacts on the adsorption and activation processes of reactant molecules.The palladium nanocrystals with relatively uniform morphology and size were taken as the model catalysts.The activity of palladium nanocrystals with different surface crystal faces and sizes was systematically investigated in catalyzing Heck coupling reaction.We found that the atoms on the crystal facets are the main catalytic sites,and the atoms on the {111} plane are more active than the atoms on the {100} plane.This difference reflects the important role of the structure in determining the catalytic performance of palladium nanocrystals.2.On the basis of the previous work,we found that the surface atoms on the palladium nanocube whose surface is dominated by {100} facets are less active in catalyzing Heck reaction than the counterpart on the palladium octahedron whose surface is dominated by {111} facets.The catalytic activity of the former will gradually increase along with the the storage duration in H2O within a certain time range,while this phenomenon does not appeare in the case of palladium octahedron.Combining the characterizations of the morphology,phase,surface potential and element valence state,we found that the oxidation process of the surface palladium atoms altered the performance of the palladium nanocrystals in catalyzing Heck reaction,while the different surface oxidation rates on the {100} and {111} crystal facets that occured during the preservation process played a critical role in the change in catalytic performance.Based on these observations,we proposed a regulation strategy for the oxidation process of palladium nanocrystals under storage conditions(solvent,time,temperature),which would provide a feasible idea for the long-term preservation of the catalyst.3.To tackle the lack of activity for palladium nanocrystals in catalytic organic synthesis,the structure that combined palladium nanocrystals with oxides was designed.The palladium nanocrystals supported on different types of oxide supports were systematically studied under various illumination conditions.Under these conditions,the activity of the palladium nanocrystals for Heck reaction was investigated.We eliminated the influence of the hot electrons,which were generated by the surface plasmon resonance effect of the palladium nanocrystals themselves,on the catalytic reaction.As a result,we confirmed the injection of photogenerated carriers towards the palladium nanocatalyst under illumination by controlling the surface electron density as well as their catalytic performance.In combination with the DRIFTS characterization,we found that the C-I bond in the iodine-aromatic substrate readily cleaved on the palladium nanocatalyst under irradiation,indicating that the illumination boosted the oxidative addition step of the substrate on the surface of the catalyst.On this basis,we successfully extended the the enhanced performance of palladium nanocrystals supported on TiO2 under irradiation to Sonogashira reaction and phenylacetylene hydrogenation.This reaction mechanism under illumination greatly improved the catalytic performance of palladium nanocrystals.4.To improve the limited selectivity of palladium nanocrystals in catalyzing the hydrogenation of phenylacetylene,we designed a strategy to introduce Ru atoms into the lattice of palladium nanocrystals to regulate their lattice atomic composition and electronic state.The controlled synthesis of PdRu nanocrystals with regular octahedral morphology was achieved by controlling the preperation conditions of PdRu nanocrystals via the solvothermal synthesis.By investigating the effect of Pd:Ru ratio on the activity and selectivity of PdRu nanocrystals for the hydrogenation of phenylacetylene,we found that the introduction of Ru reduced the rate of catalytic reaction and increased the selectivity of styrene.The suitable proportion of Ru can avoid the rapid hydrogenation of styrene-the semihydrogenation product of phenylacetylene-to form the overhydrogenation product,thereby kinetically controlling the selectivity of PdRu nanocrystals to catalyze the semihydrogenation of phenylacetylene to styrene.