【作者】 张群；

【导师】 徐洪波；

【作者基本信息】 哈尔滨工业大学 ， 化学工艺（专业学位）， 2017， 硕士

【摘要】 随着经济和科技的不断发展,对各类水体污染物和染料的降解再利用研究日益深入。对硝基苯酚(4-NP)作为农药、医药、染料等精细化学品的中间体,因其用量大、毒性高、刺激性强,迫切需要合适的方法实现快速大量处理。目前可实现对硝基苯酚降解的方式主要有微生物降解法和还原法等,由于微生物降解的时效性较差,常用的方法主要是还原法。因此,选择合适的反应体系和催化剂尤为重要,贵金属纳米粒子具有高的比表面积,活泼的表面键态、电子态,表面电子配位不全等特点,赋予了贵金属纳米粒子独特的催化活性。其中,Au纳米粒子是理想催化还原对硝基苯酚(4-NP)的催化剂,但是由于其成本高,不易回收,易团聚失活等缺点,限制了它的实际使用性能。为了提高Au纳米粒子的稳定性、降低成本、便于回收,科学工作者将多孔材料包覆Au纳米粒子表面。上述方法虽然实现了低成本、高稳定以及易回收催化剂的制备,但是负载量较低,严重降低了反应速度。为了克服上述问题,本文从三个方面制备高金纳米粒子负载量的介孔二氧化硅结构催化剂,具体工作如下:1.通过水热法制备稳定的金纳米粒子,作为合成负载催化剂的前驱体,加入PVP(聚乙烯吡咯烷酮)防止金纳米粒子团聚。制备的Au NPs在520 nm左右有较强的吸收峰,并且可以高效催化4-NP的还原反应,但其耐溶剂性和酸碱性较差,极易团聚失活,无法重复利用。为了增加金纳米粒子催化剂的机械强度和循环稳定性,以制备的金纳米粒子为核,使用CTAB(十六烷基三甲基溴化铵)修饰离子表面,包覆介孔二氧化硅,再通过煅烧去除CTAB,获得标准MCM-41结构。包覆后的核壳结构粒径在100 nm左右,介孔结构较为均匀,保持了金纳米粒子10 nm左右粒径和化学环境,耐溶剂性和酸碱性明显增强,可以稳定催化4-NP的还原反应。但是,该方法制备的催化剂的中金纳米粒子负载量较低,无法提高催化剂的催化效率。2.通过一步法获得高Au纳米粒子负载量的催化剂。为了合成具有高负载量金纳米粒子的催化剂,利用甲醛还原HAu Cl4水溶液,一步法合成了Au@MSNs,并探究HAu Cl4添加量对于金纳米粒子尺寸及催化性能的影响。制备的结构在HAu Cl4添加量为0.2-0.6 mL和0.8-3.0 m L添加量样品中,由于金核的尺寸不同,介孔Si O2的结构和孔径均有所差异。所制得的催化剂在2.0 m L的HAu Cl4水溶液添加量条件下催化效率非常高,并且具有良好的重复使用能力。制备的催化剂在亚甲基蓝、罗丹明B的还原反应以及TMB的氧化反应中有明显的催化效果。但在2.0 mL的HAu Cl4水溶液添加量条件下金纳米粒子的尺寸较大,导致催化剂的TOF(转化效率)较低。3.通过二次包覆降低负载Au纳米粒子的尺寸,提高催化活性。为了改善催化剂样品的机械性能并降低金纳米粒子的尺寸,在碱性条件下对样品进行Si O2的二次包覆,二次包覆后样品颗粒颜色均变为粉红色。该过程使金核的粒径变小,并增加了一层Si O2的覆盖层,使得金纳米粒子被包埋的更深了。进行二次包覆后的催化剂样品保持了其催化性能,在5次循环之后仍能够催化该反应加速进行。虽然未经过二次包覆的催化剂反应转化率4组均在90%以上,而经过二次包覆的催化剂反应转化率要低一些,但二次包覆后催化剂的转化效率明显高于未进行二次包覆的Au@MSNs,显示出更好的催化效果。在100°C加热的条件下,催化反应的速率有了明显提升,证明催化剂在高温条件下仍然保持了催化稳定性。更多还原

【Abstract】 In recent years,environmental protection and resource utilization have become a new concern.P-nitrophenol(4-NP)is often used as an intermediate for fine chemical products such as pesticides,pharmaceuticals and dyes.Because of its large dosage,high toxicity,strong irritation and difficulty of natural degradation,there is urgent need for the right way to achieve rapid degradation of 4-NP.At present,the feasible methods are microbial degradation method and reduction method.Due to poor timeliness of microbial degradation,common method is reduction method.Therefore,it is particularly important to select the appropriate reaction system and catalyst.Owing to the high specific surface area,active surface bonding state,electronic state,surface electron coordination and other characteristics,precious metal nanoparticles have unique catalytic activity.Gold,which has historically been considered to be an inert metal,has promising catalytic properties for the many reactions including the 4-NP decomposition reaction after the formation of Au nanoparticles(Au NPs)which is widely used in catalyst preparation.Although the Au NPS have many advantages as catalysts,but the high cost,difficulty of recovery,easiness of reunion inactivation and other shortcomings limit its practical performance.In order to improve the stability of Au nanoparticles,reduce costs and simplify recovery,scientists have covered Au nanoparticles with porous materials.Although the low cost,high stability and easy recovery catalyst can be prepared by the above method,but the loading capacity of nanoparticles is low,which seriously reduces the reaction rate.Herein,this paper prepared three mesoporous silica structure catalysts with high loading capacity of nanoparticles.1.Au NPs with different particle sizes were prepared by hydrothermal method.As a precursor of synthetic catalyst,polyvinylpyrrolidone(PVP)was added to prevent the agglomeration of Au NPs during the preparation.The prepared Au NPs have strong absorption at the characteristic absorption peak of about 520 nm,and can catalyze the reduction reaction of 4-NP efficiently,but its solvent and acid resistance are poor,at the same time it is easy to inactivation.In order to increase the mechanical strength and cyclic stability of the gold nanoparticle catalyst,the prepared Au NPs were used as precursor and the surface ions of the Au NPs was modified with CTAB(cetyltrimethylammonium bromide),then the Au NPs was coated with mesoporous silica,and the CTAB was removed by calcination to obtain the standard MCM-41 structure.The core-shell structure,with about 100 nm particle size and the relatively uniform mesoporous structure,maintain the particle size and chemical environment of the Au NPs.Moreover,the solvent and acid resistance are obviously enhanced and the catalytic performance is stable.However,the loading of the Au NPs in the catalyst prepared by this method is low,and the catalytic efficiency of the catalyst can not be improved efficiently.2.The loading catalyst of Au nanoparticles with high Au nanoparticles was obtained by one-pot method.In order to synthesize a catalyst with a higher gold nanoparticle loading,Au@MSNs were synthesized by one-pot method using formaldehyde to reduce HAu Cl4 aqueous solution.The samples were added 0.2 m L,0.4 m L,0.6 m L,0.8 m L,1.0 mL,2.0 mL,3.0 mL HAu Cl4 aqueous solution(0.05 mol/L),respectively.In the samples with additions of 0.2-0.6 m L and 0.8-3.0 mL,the structure of mesoporous Si O2 were different,due to the different sizes of gold cores.The catalyst has a very high catalytic efficiency under the condition of 2.0 m L aqueous solution of HAu Cl4 and has good reproducibility.The catalyst has obvious catalytic effect in the reduction reaction of methylene blue and Rhodamine B and the oxidation reaction of TMB.However,the size of Au NPs under the condition of 2.0 m Laqueous solution of HAu Cl4 is relatively large,which leads to the low TOF(conversion efficiency)of the catalyst.3.The size of complex Au nanoparticles was reduced by recoating to improve the catalytic activity.In order to improve the mechanical properties of the sample,the samples were subjected to recoating of Si O2 under alkaline conditions,and the color of the samples after the recoating became pink.The process minimizes the grain size of the gold core and adds a layer of Si O2 to cover the Au NPs so that the Au NPs are buried deeper.The catalyst sample after the recoating has maintained its catalytic performance,and the reaction can be accelerated after 5 cycles.The conversion efficiency of the samples subjected to recoating was more than 90%,which was higher than that of the recoating.However,the conversion efficiency of the catalyst after recoating was significantly higher and has better catalytic effect.Under the condition of heating at 100 °C,the rate of catalytic reaction has been improved obviously,which proves that the catalyst still maintains the catalytic stability under high temperature condition.更多还原