Preparation Of Pd-Pt Bimetallic Catalysts And Their Dry/wet Methane Oxidation Properties

Natural gas vehicles have become the fastest growing and most used alternative energy vehicles due to their low pollutant emissions and high fuel economy.However,because the unburned methane in natural gas vehicles can cause a more serious greenhouse effect than carbon dioxide.The catalytic oxidation treatment of methane,which has a stable regular tetrahedral structure and whose C-H bonds are difficult to be activated and broken,has become a natural gas vehicle exhaust emission control.Precious metal catalysts have excellent methane catalytic activity and are the mainstream catalytic materials for the purification of natural gas vehicle exhaust,especially Pd-based catalysts.However,their activity in the atmosphere of water-containing natural gas vehicle exhaust(also known as‘wet methane catalytic activity')will be significantly inhibited due to the presence of large amounts of water vapor.A lot of research work has been carried out at home and abroad.Among them,the Pd-Pt bimetallic catalyst prepared by adding Pt has the best catalytic activity for wet methane,but the specific influencing factors and structure-activity relationship are rarely reported.For in-depth exploration,our research group has formulated a systematic research plan,the work of this thesis is part of it.In this thesis,the effects of preparation methods,calcination temperature and the molar ratio of Pd and Pt on the catalytic activity of Pd-Pt bimetallic catalysts for dry/wet methane oxidation were studied,aiming to study the relationship between'preparation-structure-performance'of Pd-Pt bimetallic catalysts,and lay the foundation for the development of high-performance natural gas vehicle exhaust purification catalysts with water vapor resistance.The main research work and results are as follows:(1)Effect of preparation methods on dry/wet methane oxidation performance of Pd-Pt bimetallic catalyst.The structure,surface properties and dry/wet methane oxidation performance of fresh and hydrothermal aged Pd-Pt bimetallic catalysts prepared by PVA-protected co-reduction method,step-by-step high pressure organic solvent method,colloid chemistry method and mechanical mixing method were characterized and tested.It was found that the crystal structure,microstructure,redox ability,chemical state of precious metals and the dry/wet methane oxidation performance of catalysts were affected by preparation methods.The independent-site fresh and hydrothermally aged Pd-Pt bimetallic catalysts prepared by colloidal chemistry method showed excellent dry/wet methane catalytic oxidation performance and strong water resistance,which was attributed to the high dispersion of precious metal particles,the strong redox ability of catalysts and the high Pd²⁺content.(2)Effect of calcination temperature on dry/wet methane oxidation performance of Pd-Pt bimetallic catalyst.The structure,surface properties and dry/wet methane catalytic oxidation performance of fresh and hydrothermal aged Pd-Pt bimetallic catalysts prepared by the same preparation method(colloid chemistry method)and calcined at 450°C,550°C,600°C and 750°C respectively were characterized and tested.It was found that the specific surface area,crystal structure,redox ability and chemical state of precious metals of fresh catalysts by affecting the growth or sintering degree of Pd and Pt nanoparticles and the interaction between Pd and Pt were affected by the calcination temperature.However,the effect of calcination temperature on the texture parameters and crystal structure of hydrothermal aged catalysts treated under the same hydrothermal aged condition is not obvious,which only slightly affects the redox ability of hydrothermal aging catalysts and the chemical state of precious metals.Controlling the appropriate calcination temperature plays an important role in the preparation of highly active Pd-Pt bimetallic catalysts,which will not only reduce energy consumption but also obtain highly efficient catalysts for dry/wet methane oxidation of natural gas vehicles.(3)Effect of different Pd/Pt molar ratios on dry/wet methane oxidation performance of catalysts.The structure,surface properties and dry/wet methane catalytic oxidation performance of fresh and hydrothermal aged Pd-Pt bimetallic catalysts prepared with four different ratios of Pd and Pt(the molar ratio of Pd and Pt is 1:3,1:1,3:1 and 5:1)and single Pd,single Pt catalysts were characterized and tested.It was found that the texture parameters,crystal structure,redox ability,chemical state of precious metals and the dry/wet methane oxidation activity were affected by different molar ratios of Pd and Pt.Due to the obvious synergistic effect between the two noble metals in the Pd-Pt bimetallic catalyst with an appropriate Pd/Pt ratio(e.g.,the molar ratio of Pd to Pt was 5:1),the sintering resistance of the single Pd and single Pt catalysts was effectively improved,the high temperature thermal stability was improved,the redox ability was enhanced,and the concentration of Pd²⁺in the active component of the catalyst was increased,thereby enhancing the dry/wet methane oxidation activity of the single Pd and single Pt catalysts.Based on the research of this thesis,the following important conclusions are drawn:(1)The independent site-type Pd-Pt bimetallic catalyst prepared by colloidal chemistry and calcined at 600?with a ratio of Pd and Pt of 5:1 has outstanding dry/wet methane catalytic activity;(2)Oxidized palladium species(Pd²⁺)play an important role in ensuring high catalytic activity for methane oxidation.Therefore,it is necessary to adopt appropriate preparation methods,appropriate calcination temperature and appropriate metal-metal ratio to stimulate and promote the formation of Pd O species,the increase of Pd O content and the high dispersion of Pd O particles,and maximize its catalytic effect under the synergistic effect of two or more metals.This conclusion has important guiding significance for the further research on the basic theory of wet methane oxidation and the development of higher performance natural gas vehicle exhaust purification catalysts.