Preparation And Catalytic Performance Of Pt/Oxide Catalysts For Ammonia Oxidation

Ostwald ammonia oxidation has always been important process in the field of nitro chemical,but the platinum based catalysts used in catalytic reaction of ammonia oxidation are high cost,poor selectivity and stability.How to improve the catalytic selectivity,activity and stability of the platinum based catalyst and seek alternatives to platinum precious metal catalyst has been a key science and engineering problems in the ammonia oxidation field.As a result,the development of high selectivity,high stability,high temperature resistance,long life platinum or non-platinum(oxide catalyst)catalysts is becoming a hot spot of catalytic reaction of ammonia oxidation.The preparation,structure characterization,performance testing and catalytic reaction simulation and theory of catalyst for ammonia oxidation have been conducted in this thesis.The main results are as follows:First of all,the Pt-Rh-Pd alloying method has been used to modify the platinum based catalysts.The effect of concentration gradient of Rh and Pd on selectivity and stability of Pt catalyst was investigated.Through thermodynamic calculation combined with the experimental results,the formation mechanism of Pt-Rh-Pd alloy solid solution and the surface reconstruction model of Pt-Rh-Pd alloy in catalytic reaction of ammonia oxidation are studied.The product was applied to large nitric acid unit,which make ammonia oxidation conversion efficiency reached 97%and 1%higher than that of traditional platinum net.As a result,the running cost was reduced by 33%.Secondly,the direct decomposition method,precipitation method,sol–gel method and hydrothermal were used in preparation of CeO₂ catalyst,systematically studies the microstructure and redox properties of CeO₂ catalyst were systematically studied.The experimental results show that the CeO₂catalyst prepared by sol–gel method has smaller grain size,higher specific surface and strong ability of redox reaction.Using the A series of TiO₂ catalyst was prepared by sol-gel and precipitation method.The microstructure and catalytic performance of TiO₂ were systematically investigated.The experimental results show that TiO₂catalysts prepared by sol–gel method have higher specific surface,which is facilitated to load the Pt catalyst on TiO₂substrate and is helpful to boost catalytic performance of ammonia oxidation.In addition,Ti_xCe_1-xO₂ catalyst was prepared by sol–gel method,the influence of Ce/Ti ratio and microstructure of Ti_xCe_1-xO₂ on catalytic reaction of ammonia oxidation has been systematically investigated.The experimental results show that TiO₂–CeO₂ solid solution was formed by Ti⁴⁺diffusion into the CeO₂ lattice,which is benefit to boost catalytic performance of the ammonia oxidation.When the Ce/Ti ratio is 1:1,the catalytic performance of Ti_0.5Ce_0.5O₂ is best.On this basis,Pt_0.005%/Ti_xCe_1-xO₂ and Pt_y/Ti_0.5Ce_0.5O₂ catalyst were prepared by reduction of acetyl acetone platinum.The influence of Ce/Ti ratio and loading amount of Pt on microstructure and catalytic performance of ammonia oxidation were systematically studied.The experimental results show that the highest selectivity of Pt_0.005%/Ti_xCe_1-xO₂ reach 97.7%with x=0.5,and the highest selectivity of Pt_y/Ti_0.5Ce_0.5O₂ reach 97.84%with y=0.5%.Finally,CeCaTi_0.95Pt_0.05O₃ catalyst was prepared by alkoxide method.In this alloy,Pt atoms occupy 5%B site of perovskite,amount of Ce is according to CeO₂ equivalent accounted for 4%of overall weight mixture.The microstructure and catalytic performance of Ce Ca Ti_0.95Pt_0.05O₃ catalyst were systematically investigated.The results show that the Ce Ca Ti_0.95Pt_0.05O₃ catalyst is mixture of perovskite and pyrochlore phase.The catalytic selectivity of Ce Ca Ti_0.95Pt_0.05O₃ is up to 98.45%.Ce and Pt atoms uniformly are doped in the perovskite structure and partly replace the Ti⁴⁺,which changes the lattice constant and improves catalytic reaction rate due to the increase of these defects.The surface catalytic reaction model of Ce Ca Ti_0.95Pt_0.05O₃ammoxidation catalyst was established by Mars–van Krevelen,Langmuir–Hinshelwood and Marcus theory,which explains the synergy of the platinum and oxide catalyst and self regeneration mechanism of Ce Ca Ti_0.95Pt_0.05O₃ catalyst.Based on first principles density functional theory,the formation energy,the density states and NH₃adsorption of Ca Ti_0.9Pt_0.05Ce_0.05O₃ were calculated.The results show that the NH₃ adsorption performance of Ca Ti_0.9Pt_0.05Ce_0.05O₃ is closed to that of Pt.In addition,Ca Ti_0.9Pt_0.05Ce_0.05O₃ catalyst shows high selectivity and oxidization in ammonia oxidation process,which is consistent with the experimental results.