Microstructure Evolution And Structure-performance Relationship Investigation Of Acetylene Selective Hydrogenation Catalysts

Establishment of structure-catalytic performance relationship is crucial to catalyst design and synthesis.This dissertation was focused on acetylene selective hydrogenation catalyst,by means of transmission electron microscopy as the main method,to systematically investigate the reaction condition-catalyst structure-performance relationship,catalyst design and controllable synthesis.1.Palladium-based catalysts are widely recognized in acetylene selective hydrogenation reaction,and Pd(100)facet was considered as most selective one compared with other facets.This paper was started from the stability study of Pd(100)facets under reaction conditions,to explore the Pd(100)facets evolution under continuous interaction with reaction gases.The(100)facets of cubic Pd/Gr catalyst exhibited good ethylene selectivity initially.After a long time reaction,the acetylene conversion was maintained but the selectivity was largely decreased,accompanied with the surface structure evolution of cubic Pd without obvious coalescence and shape changing.The varied ethylene selectivity was ascribed to the surface microstructure changing of the well-defined(100)facets,which is induced by the generous acetylene/ethylene adsorption and desorption to get lower surface energy of rough(100)facets with steps and corners compared with that of flat(100)facets during reaction.2.PdZn intermetallic compound displays excellent catalytic performance in acetylene selective hydrogenation reaction due to "active sites isolation effect".This paper presents the visualization of the phase transition process from Pd to PdZn on the ZnO nanorod support.The XRD results provide structural information of PdO/ZnO after various treatments.Combined with the H2-TPR,in-situ XRD and in-situ TEM studies,it is reasonable to conclude that H atoms are dissolved into the Pd NP to generate PdHx phase under hydrogen atmosphere.Then H in PdHx is coupled with O in ZnO to be released as H2O with the PdZn phase growing at above 250 ?.The microstructure evolution from Pd to PdHx under hydrogen atmosphere and the phase transition from PdHx to PdZn as the temperature increased were monitored by in-situ TEM.The Pd NPs exist as PdHx form under hydrogen atmosphere and the PdHx structure acts as an intermediate during the phase transition process.The alloying process is initialized at the interface between PdHx NP and ZnO support.Then it grew along PdHx<111>direction.The driving force for PdZn is owing to the large enthalpy from the Pd-Zn bond generation and the production of water.These results reveal the formation of PdZn intermetallics from Pd/ZnO under hydrogen atmosphere at atomic-scale for the first time.3.Establishment of reaction condition-catalyst structure-performance relationship in Pd-ZnO system.This work correlates the evolved structures at various SMSI states of Pd-ZnO system to the catalytic performance in acetylene selective hydrogenation reaction.The crystalline and electronic structure was gradually changed as the interaction fortified between Pd and ZnO supports on increasing temperatures.The produced Pd1-xZnx and PdZn structure with Zn atoms incorporation into the Pd NP were both exhibited good selectivity towards ethylene.But the enhanced selectivity toward ethylene was govern by electronic effect and active sites isolation effect in turn.The altered electronic structure case in Pd1-xZnx structure was demonstrated to afford the superior selectivity and 15-35 folds activity enhancement compared with active sites isolation PdZn case.This work evidenced that tunable electronic structure and unchanged Pd geometric surface structure could gain better catalytic performance in acetylene selective hydrogenation reaction and provided ideas though subtle adjustable SMSI state to design catalysts and control activity/selectivity.4.According to the generated PdC,structure and structure-catalytic performance relationship of NiZn catalysts,an original strategy was presented to synthesize the Ni3ZnC0.7 structure by introduce the carbon atoms to the octahedral interstitial site of Ni3Zn from gaseous phase carbon source to get an improved catalytic performance in acetylene selective hydrogenation reaction.The Ni3Zn/oCNT catalyst was easily prepared by a typical impregnation method and the Ni3ZnC0.7 structure was obtained by the following acetyelne treatment at 200 ?.The phase transtion process was clearly elucidated by in-situ XRD and TPSR,and the detail structure information were systematically investigated by TEM.The Ni3ZnC0.7/oCNT exhibited excellent selectivity in acetylene selective hydrogeantion reaction owning to the electronic effect and expanded atom distance by the carbon introduction.The stability was also improved by reducing the butenes selectivity,which is the precusor of weight oligomers,and supposed to the weak bonding of acetylene.As mentioned above,the Ni3ZnC0.7 could be a promising replacement of Pd based catalysts in acetylene selective hydrogenation reaction for its low-cost and high selectivity.And the strategy for tuning the structure of alloy is expected to be a effective way to tune the selectivity of hydrogenation reaction of transition metals and provide new opportunites for catalyst design,synthesis and utilization.