Catalytic Non-thermal Plasma Reduction Of Carbon Dioxide With Pd/ZnO:the Study Of Performance And Reaction Mechanism

The continuous consumption of fossil fuels and the massive emission of carbon dioxide?CO₂?are the major challenges for survival and development faced by human.The emission reduction and utilization of CO₂ are major demands of China and even the world.As an important carbon source and the reaction basis of carbon chemistry,CO₂ demonstrates great application potential in the field of energy.Due to the inactive chemical properties of CO₂,traditional catalytic transformation of CO₂ needs to be carried out under both high temperature and high pressure to acquire the ideal CO₂ conversion and the yield of products.However,the harsh reaction conditions also put forward a higher requirement for the stability of the catalyst.Non-thermal plasma?NTP?has been widely studied in CO₂ catalytic reduction due to the characteristics of self-sustaining,high efficiency,non-thermal equilibrium and easy to be scaled up.Dielectric barrier discharge?DBD?is a typical generation mode of NTP,and the vibration excitation generated by DBD is an important way to efficiently dissociate CO₂.Meanwhile,the introduction of catalyst can both reduce the reaction activation energy and change the discharge characteristics of DBD to improve the energy efficiency.Accordingly,coupling of the DBD with catalyst is considered as an important way for the highly efficient conversion of CO₂.Therefore,this work takes DBD-Pd/ZnO system as the object to study the performance and the reaction mechanism of CO₂ reduction in catalytic non-thermal plasma?plasma-catalytic?system.The main contents and conclusions are as follows:?1?Pd/ZnO catalysts with different loading of Pd were prepared by co-precipitation method and filled into DBD reactor to investigate the catalytic performance of different catalysts.It was found that CO₂ conversion increased with the increasing of the contents of Pd.The Pd-ZnO interface formed by the partially-reduced ZnO was thought to play a significant role for the plasma-excited CO₂ adsorption and activation.Additionally,the increased content of Pd was found to simultaneously enhance the amount of medium CO₂ adsorption and decrease desorption temperature,which can promote the desorption of the products,and therefore,leading to a higher performance for plasma-catalytic CO₂ reduction.?2?Different reaction pathways in DBD-ZnO and DBD-Pd/ZnO systems were revealed by comparing the product distribution characterized by online mass spectrometry?MS?.Catalysts'adsorption and activation properties of CO₂ and H₂ were characterized by temperature-programmed desorption of CO₂?CO₂-TPD?and H₂?H₂-TPD?.Comparing with ZnO,Pd/ZnO demonstrated much higher activation property for both CO₂ and H₂,promoting plasma-catalytic CO₂ conversion by surface reactions.Then,the surface reactions and gas phase reactions in plasma-catalytic process were characterized by in situ Fourier transform infrared spectroscopy?in situ FTIR?and optical emission spectroscopy?OES?,with the combination of kinetic analysis.According to the results,there are more carbonate and formate species formed through surface hydrogenation process over Pd/ZnO due to its much higher CO₂ and H₂ activation property.As for ZnO,the weaker CO₂ and H₂ activation property cannot provide sufficient surface reactive species for surface hydrogenation reactions,which may lead CO produced by the gas phase interaction and the splitting of surface adsorbed CO₂.?3?Characterizations of high-resolution transmission electron microscopy?HRTEM?,scanning transmission electron microscopy-high angle annular dark field?STEM-HAADF?,scanning transmission electron microscopy-X-ray energy dispersive spectroscopy?STEM-XEDS?and quasi in situ X-ray photoelectron spectroscopy?quasi in situ XPS?revealed the formation of ZnO_x@Pd Zn at Pd-ZnO interface,which could provide more active sites for medium CO₂ adsorption.And in situ Raman spectroscopy studies proved that the ZnO_x@Pd Zn remained largely intact during the reaction process.Taking above results into consideration,mechanisms of plasma-assisted CO₂ activation and reduction over Pd/ZnO are proposed as following:?1?in the presence of Pd,H₂ is dissociated into H atoms and then transferred to the surfaces of ZnO;?2?the reduction of ZnO leads to the formation of Pd Zn alloy,resulting in a negative partial charge of Pd favoring the weakening of C-O-bond and CO desorption;?3?the SMSI effects between Pd and ZnO lead to the formation of ZnO_x@Pd Zn under plasma excitation,producing moderate basic sites at Pd-ZnO interface for CO production;The comprehensive study thus unveiled that Pd-ZnO interface with the presence of ZnO_x@Pd Zn enhanced the CO₂ activation property on Pd/ZnO catalyst and eventually led to a high plasma-catalytic performance in CO₂ reduction to CO.?4?Plasma-catalytic performances of catalysts with different Pd-ZnO interfaces were investigated.The effects of the ratio of Zn/Pd on the performance of DBD-Pd/ZnO system were revealed based on the previous study of the reaction mechanism over Pd-ZnO interface.Pd-ZnO interface was modified by changing the ratio of Zn/Pd.The catalysts with different Zn/Pd were loaded over Zr O₂ to acquire Pd_xZn_y/Zr O₂ catalysts,which were used for plasma-catalytic CO₂ reduction to optimize the ratio of Zn/Pd.Then,the surface chemical properties,redox properties and CO₂ adsorption and activation properties of Pd_xZn_y/Zr O₂ catalysts were characterized.Based on the results of interface optimization,the synergetic effect between plasma and catalyst was enhanced by adjusting discharge power,gas flow rate and the ratio of reaction gases to improve the energy efficiency,and finally,obtaining a plasma-catalytic system with high CO₂ conversion,high selectivity and high energy efficiency.Based on the construction of highly efficient plasma-catalytic system,the reaction pathways and mechanisms of plasma-catalytic CO₂ reduction were revealed by coupling several in situ characterizations and dynamic analysis.This study provides some scientific basis and certain practical significance for the development of highly efficient plasma-catalytic system.