Synthesis Of Palladium Nanoparticles By Biomaterial Cellulose And Study On The Performance Of Porous Catalysts

Water resource is closely related to human life.However,the discharged industrial wastewater often contains nitrophenols and organic dyes,which are harmful to nature and human beings.It has been proved that palladium nanoparticles(PdNPs)can effectively catalyze the degradation of these harmful organic pollutants.However,in the process of synthesis of PdNPs,self-aggregation always occurs,which could reduce their catalytic activity.Therefore,carriers often need to be introduced during the preparation of PdNPs.Cellulose is one of the most abundant and widely distributed polysaccharides in nature.Cellulose is an excellent carrier for PdNPs due to its low cost,easy access,environmental protection and degradability.Therefore,in this thesis,cellulose was dissolved in Na OH/urea system to expose its hydroxyl groups and exert its reducibility to synthesize PdNPs in situ.Secondly,according to the content of solid impurities in wastewater,a three-dimensional porous cellulose-PdNPs microspheres catalyst and a bio-inspired(natural wood xylem)hierarchical porous filter catalyst with micron-scale unidirectional channels and nanopores were prepared respectively.Finally,the catalytic capacity of the two catalysts,kinetics and reusability for methylene blue(MB)Congo red(CR)and 4-nitrophenol(4-NP)were studied.The specific research results are as follows:1.First of all,this study developed a simple and green method for synthesis of PdNPs in-situ by using the exposed hydroxyl group of cellulose after the dissolution in Na OH/urea system both as a reducing agent and a carrier under water-bath and microwave heating conditions,respectively.HRTEM characterization proved that ultrafine PdNPs(3.1±1.4 nm)with uniform distribution and no self-aggregation were successfully prepared by microwave method,which is smaller than that prepared by water-bath heating(51.4±21.7 nm).2.Secondly,cellulose-PdNPs sol obtained by microwave heating method was prepared into three-dimensional porous microsphere catalyst with particle size of 30?m and high specific surface area(212.14±1.24 m²/g)by sol-gel transformation method and reverse suspension polymerization reaction.This microsphere catalyst only needed 4.5 min and 8 min respectively to complete the reduction of MB and CR dyes,and the conversion rate was up to99.9%.The kinetic reaction rate constants(k)of the two organic dyes were 1.0725 and 0.3886min^-1,respectively,which are higher than most reports at present.In addition,this microsphere catalyst also showed excellent reuse ability.After five cycles,the conversion rate of MB and CR could still reach 92.8%and 94.9%.3.At last,a bio-inspired hierarchical porous filter was prepared by chemical crosslinking and unidirectional freezing methods.This filter was transferred to a dynamic filter device for the continuous fluid catalytic reduction of 4-NP,MB and CR.This dynamic filtration device and the hierarchical porous structure could facilitate mass transfer and accelerate the contact between the reactants and the catalyst,thus giving the catalyst extremely high catalytic activity.It only needed 3 min for the catalytic reduction of 4-NP by this unidirectional filter with the kinetic reaction rate constant(k)of 2.42 min^-1,which is higher than most reports so far.Moreover,it needed only 0.5 min for the catalytic reduction of MB and CR and the solution became colorless instantaneously after contact with the filter catalyst.In addition,the conversion rate of 4-NP and MB could still remain 99.9%in recycle experiments.Finally,due to the unique unidirectional channels of the filter,the pressure(0.89 k Pa)in fluid catalysis was similar to that of the natural wood filter(0.74 k Pa),which was only 1.7%of the pressure of the material with random channel(51.99 k Pa).Therefore,the cellulose-PdNPs porous catalysts constructed in this thesis realized the reduction and loading of PdNPs and effectively prevents their self-aggregation.At the same time,the porous catalysts showed high catalytic ability and stable reusable ability for organic dyes and nitrophenols.Therefore,the two prepared cellulose-PdNPs catalysts have potential application value in the field of environment.In addition,the method proposed in this study also provides some new ideas for the development and utilization of biomass cellulose resources.