Synthesis Of Pd-based Catalysts Toward Selective Hydrogenation Of Alkyne And The Influence Of Layered Double Hydroxide Support

The selective hydrogenation of alkyne,such as acetylene,and phenylacetylene,to alkene is of much importance to the industrial manufacturing and scientific research.Nowadays,supported Pd-based catalysts have been widely used in such reaction because of their high activity and selectivity toward hydrogenation.In previous work,methods of poisoning and alloying to the Pd active sites are used to enhance the selectivity to ethylene.However,such methods often reduce the activity at the same time.Therefore,it is of much significance to prepare Pd-based catalysts with high selectivity and activity toward hydrogenation.In this thesis,semiconducting ZnCr-layered double hydroxide(LDH)is prepared at first.Then,single atom Pd and nano-Pd supported ZnCr-LDH are synthesized with photodeposition method.X-ray diffraction,X-ray photoelectron spectroscopy,TG-DTA,high resolution transition electron microscope,and HADDF-STEM are applied to characterize the structure,morphology,oxidation state of these two Pd-based catalysts.Also,the influence of reduction temperature of H2 to the structure of single atom Pd and nano-Pd is investigated.It is found that the interaction between ZnCr-LDH support and Pd active site is strong enough to generate PdZn alloy.Then,these two catalysts are used in the selective hydrogenation of phenylacetylene.Not only the efficiency is tested,but also the structure-property correlation is discussed.The major contents and results are as follows:(1)In this thesis,Pd is photodeposited on the surface of ZnCr-LDH.When the loading amount is 0.14 wt%,Pd exists in the form of single atom.When the loading amount is increased to 1 wt%,Pd exists in the form of nanoparticle with a size of 5 nm.It is found that after the H2 reduction at 500?,single atom Pd does not aggregate while ZnCr-LDH converts into ZnO/ZnCr2O4 complex.Single atom Pd catalyst shows higher activity and selectivity than that of nano-Pd catalyst.The turnover frequency of single atom Pd toward hydrogenation of phenylacetylene is 31.2 s-1,while that of nano-Pd is 6.1 s-1,in other words,the turnover frequency of single atom Pd is five times of that for nano-Pd.This is resulted from a lower activation barrier of single atom Pd.When conversion rate reaches 100%,the selectivity of single atom Pd is 90%while that of nano-Pd is 48%.No subsurface hydrogen is contained in single Pd catalyst,which increases the selectivity toward styrene.After a H2 reduction at 500?,the activity and selectivity of single atom Pd supported on ZnO/ZnCr2O4 complex are both increased.The turnover frequency is further increased to 64.5 s-1.The selectivity is further increased to 92%.This is derived from the strong interaction between Pd and the support.The electron transfer between Zn and Pd,reduces the activation barrier of H2 and the adsorption energy of styrene.(2)The influence of reduction temperature to the structure and property of nano-Pd catalyst is studied.The phase of ZnCr-LDH changes with the increasing temperature.Before 300?,the interlayer water and carbonate are removed.ZnCr-LDH turns into mixed metal oxides.ZnO occurs at 300 ? and ZnO/ZnCr2O4 complex occurs at 500 ?.And PdZn alloy is generated during the reduction.It is found that the Pd catalyst reduced at 300 ? has high activity(turnover frequency of 13.2 s-1)and selectivity(90%).This is resulted from the generation of PdZn alloy with small size.When the reduction temperature reaches 500?,the size of PdZn alloy increases to 10 nm,which leads to the decrease of selectivity.