Design And Performance Of Palladium-Based Nanocatalysts For Formic Acid Decomposition

Increasing energy demand and environmental pollution issues call for higher demands on the design and research of catalysts.It is well known that the heterogeneous catalytic process mainly occurs on the surface of catalyst.The catalytic activity depends on the interaction between the surface structure of the catalyst and the substrate molecules so that the structure and performance of the catalyst are directly correlated.The surface structure of the catalyst plays an important role in controlling the activity and selectivity of the catalytic reaction,while the size and morphology of the catalyst affect the specific surface area and surface structure of the catalyst.Moreover,the surface charge state of catalysts is an important physical parameter to the catalyst surface structure.By tailoring the surface charge states,the performance of the catalyst can be tuned.In this dissertation,palladium-based nanocatalysts with well-defined structures were designed and synthesized.Based on the experimental results and related characterization,the differences in the activity of catalytic decomposition of formic acid were systematically explored,and the mechanisms for the influence of its structure on the activity of the catalyst were investigated.The main results can be summarized as follows:1.The controlled synthesis of nanocrystals provides a simple model for studying the active sites of catalytic reactions.We designed an approach to synthesize two types of palladium tetrahedral and octahedral nanocrystals with the same {111} facets by thermodynamics control.Due to the same {111} facets,the active sites of the two nanocrystals should be consistent for the same reaction.The different specific surface areas brought by their different geometries have become the reason for determining the difference in catalytic activity of the two nanocrystals.We also briefly compared the activity of two nanocrystals having {111} facets with that of nanocube having {100}facets in catalytic hydrogenation of styrene,and explored the facet-dependent selectivity of the reaction.2.Based on the above synthesis results,we loaded Pd tetrahedral and octahedral nanocrystals with the same {111} facets on the surface of TiO2.The electronic and steric effects on the decomposition reaction activity of HCOOH were then investigated.It revealed that the Pd tetrahedrons-TiO2 hybrid structure whose Pd{111}-TiO2 interface possessed a larger angle showed higher catalytic activity,owing to the reduced steric effect as compared to Pd octahedrons-TiO2.3.Surface modification can be utilized as an efficient strategy for controlling the surface electronic state of catalysts.Pd tetrahedrons-TiO2 nanostructures have been selected as a model catalyst.The deposition of other metallic atoms was achieved through tightly controlling the reduction rates on the surface of Pd tetrahedrons.The surface polarization resulting from the difference in work functions,together with the Schottky junction between TiO2 and metals,affected the surface electronic state of Pd.4.Since Pd nanocrystals possess good catalytic activity for both dehydrogenation of HCOOH and hydrogenation of organic materials,they can be used as a catalyst for hydrogen transfer reaction using HCOOH as a hydrogen source.We identified specific active sites for HCOOH decomposition and alkene hydrogenation by employing six different Pd nanocrystals in tunable sizes and shapes.Based on the experimental results,we also proposed a mechanism for the tandem reaction that involves HCOOH decomposition and styrene hydrogenation.Guided by the information on the limiting step,we further boosted the overall performance of tandem reaction by optimizing the structure of the catalyst.