Controlled Preparation Of PdAg Alloy Nanocatalyst And Selective Hydrogenation Of Acetylene Performance Study

As one of the most significant raw materials in the petrochemical industry,the production of ethylene reflects the overall power of a country’s petrochemical industry.Ethylene is mainly generated by cracking naphtha,and a small amount of acetylene is produced along with this process.The presence of acetylene will affect the subsequent polymerization reaction,resulting in substandard performance of polymer-grade ethylene products.Therefore,the acetylene content must be reduced to less than 5 ppm.The selective catalytic hydrogenation of acetylene to obtain ethylene is considered one of the most effective methods to reduce acetylene impurities in ethylene-rich streams.Most of the catalysts currently used in industrial ethylene plants are PdAg catalysts,using a carrier ofα-Al₂O₃.Due to the complex composition of the feed gas in the pre-hydrogenation process,the performance of the PdAg catalyst is strictly required,and improving the performance of the PdAg catalyst is of great significance.In this graduation thesis,a series of PdAg catalysts were prepared by equal volume impregnation using Al₂O₃commonly used in industry as a carrier.The effects on the performance of PdAg catalysts were explored by different PdAg ratios,alkali metal doping modifications and different carrier treatments,and analyzed and explained by a series of characterization methods.The main studies are as follows:（1）A series of catalysts with different PdAg ratios were prepared using an isovolume impregnation method.On this basis,the catalysts with different PdAg ratios were reduced in both gas phase and liquid phase to investigate the influence of the reduction methods on their properties.It was concluded from the catalyst performance evaluation that the activity decreases continuously with increasing Ag/Pd ratio,while when the Ag content is too low,it is not conducive to improving ethylene selectivity.And whether it is gas-phase reduction or liquid-phase reduction,it is concluded that when the ratio of PdAg is 1:3,the catalytic performance is the best.Combining various performance indicators,it is found that gas-phase reduction of PdAg₃is more suitable for long-term operation.Combined with TEM and NH₃-TPD analysis,compared with gas-phase reduction,the catalyst metal particles of liquid-phase reduction have larger average particle size and stronger surface acidity,which are the main reasons affecting their performance.（2）The performance of PdAg catalysts was improved by doping alkali metals K and Cs and adjusting the loading content by an equal volume impregnation method.From the performance evaluation,it was concluded that the performance of alkali metal K and Cs loading of 0.3 wt%was better,with the best performance of alkali metal K（0.3 wt%）doped PdAg catalyst.With the increase of alkali metal loading content,the metal particle size does not always decrease,indicating that too little or too much alkali metal doping content is not conducive to the dispersion of metals.According to EDS element distribution and line scan characterization,it is found that alkali metal K is more easily doped into PdAg alloy structure than Cs,which in turn affects the properties of PdAg.Combined with XPS characterization,it is concluded that K and Cs have different effects on the electronic effect between PdAg,and K modification has a significant effect on the electron transfer between PdAg.（3）The influence of carrier Al₂O₃treatment on catalyst properties was explored by different roasting temperatures and hydrothermal treatment temperatures after modification of carrier Al₂O₃with equal volume impregnation loaded PdAg fraction.Firstly,the influence of Al₂O₃roasting temperature on the performance of PdAg catalyst was firstly investigated,and1100℃was found to be the optimum roasting temperature.Based on this roasting temperature,the impact of hydrothermal treatment temperature on the carrier Al₂O₃was continued to investigate how to improve the performance of PdAg catalyst.The best performance of the catalyst was selected by combining the performance indexes of the catalysts and the hydrothermal treatment temperature of 160℃.By the above modification method,the physical properties of the carrier Al₂O₃were mainly changed and its surface acidity was affected.The surface acidity of the carrier has a significant effect on the performance of the catalyst.Appropriate acidity strength is beneficial to improve the catalytic activity,but too strong acidity will aggravate the occurrence of side reactions.In addition,after the carriers were treated with different hydrothermal temperatures,the electron transfer between PdAg was promoted and the interaction between metals was enhanced.