Preparation Of Molecular Sieve-Suppotered Noble Metal Nanocatalysts And Their Catalytic Performance For Toluene Oxidation

Volatile organic compounds（VOCs）are important precursors of fine particulate matter in the atmosphere,it not only brings serious damage to atmospheric environment,but also damages human health to a great extent.Catalytic oxidation is considered to be the most effective way to eliminate VOCs,and the key is to study high-performance catalysts.Supported noble metal catalysts have attracted much attention which has good low-temperature catalytic activity and poison resistance.Zeolite have good hydrothermal stability,abundant pores and acid sites,and are widely used in industrial catalysis.In this dissertation,Silicalite-1（S-1）support was prepared by the sol-gel method.The x Pd/S-1（x=0.26-0.28 wt%）catalysts were prepared by in situ synthesis method.Physicochemical properties of the samples were characterized by means of numerous analytical techniques,and the catalytic activity of the catalyst was evaluated using toluene as a reaction probe,and the reaction mechanism was explored.The main research results obtained are as follows:（1）Silicalite-1 zeolite have been preparedunder direct hydrothermal conditions using[Pd（NH₂CH₂CH₂NH₂）₂]Cl₂as precursor.It is found that the as-prepared catalysts displayed a hexagonal prism morphology with good microporous structure and high specific surface area（390-440 m²/g）,and precious metal nanoparticles are uniformly dispersed on the surface and inside of S-1.（2）The 0.28Pd/S-1-H catalyst exhibited the best performance for toluene oxidation,with the temperatures required for achieving 50 and 90%toluene conversions being 180 and 189 ^oC at space velocity=40,000 m L/（g h）,respectively,and the apparent activation energy of toluene oxidation over this catalyst was the lowest（41 k J/mol）.（3）The 0.28Pd/S-1-H catalyst has good water vapor and carbon dioxide resistance.When water vapor（2.0 or 5.0 vol%）and carbon dioxide（5.0 vol%）were introduced into the reaction system,the toluene conversion rate decreased slightly.After the water vapor and carbon dioxide were cut off,the activity returned to the original level.while 50 ppm SO₂ caused irreversible deactivation of the catalyst.（4）In situ DRIFTS and GC-MS results of toluene oxidation demonstrate that p-methylbenzoquinone,maleic anhydride,benzoic acid,benzaldehyde,acetone and acetic acid are the main intermediates in the toluene oxidation process.Based on the characterization results,we concluded that the good catalytic performance of0.28Pd/S-1-H was associated with its high adsorbed oxygen species concentration,large toluene adsorption capacity,enhanced acidity,and high Pd⁰/Pd²⁺molar ratio.