Ultrasonic-assisted Preparation Of Hydrotalcite-supported Palladium-based Nanocatalysts For The Degradation Of P-nitrophenol

p-Nitrophenol(4-NP)is an important chemical raw material,which causes great pollution to the water environment.The reduction of 4-NP to p-aminophenol(4-AP)which is less toxic and easier to degrade is an effective degrading method for pollutants.The reduction of 4-NP by Na BH₄ is an important reaction of chemical reduction of4-NP,and the key technology is the development of efficient catalysts.The supported ultrafine nano metal Pd-based catalysts have a great effect on the 4-NP degradation reaction.In order to ensure the stability of nano-metal particles,chemical reducing agents and stabilizers are used in the catalyst preparation process,and wastewater will be generated during elution.In this paper,NiFe-Layered double hydroxides(NiFe-LDHs)are prepared by hydrothermal co-precipitation method.The hydrogen radicals(H·)with strong reducing activity obtained by ultrasonic excitation of the hydroxyl groups on the surface of LDHs are used to reduce metal ions to metal nanoparticles.Therefore,a series of supported Pd-based nanocatalysts can be prepared easily by ultrasonic assisted reduction method without chemical reducing and stabilizing agents.The size,crystal structure,morphology and valence state of each element of the catalyst nanoparticles are characterised by means of TEM,XRD,FT-IR,SEM and XPS.The effects of ultrasonic power,ultrasonic time,loading amount,metal ratio and addition order on the influence rule of catalytic 4-NP degradation performance are investigated.The NiFe-LDHs supported ultrafine Pd nano-catalyst is prepared by ultrasonic-assisted in-site reduction method with NiFe-LDHs as the carrier and Na₂PdCl₄as the palladium source.When the ultrasonic time and power is 1h and 400W respectively,the Pd-LDHs prepared with a Pd loading of 5 wt%has the best catalytic performance for the degradation of 4-NP.The particle size of Pd NPs detected by TEM ranges from 0.77 nm to 2.06 nm,with an average particle size of 1.43 nm.The dissociation of H·from Ni-OH and Fe-OH under the action of ultrasound can reduce Pd²⁺to Pd NPs,and the coordination of Pd NPs with Ni-O and Fe-O further enhances the stability of Pd-LDHs.The 4-NP can be completely degraded within 5 min at room temperature under the conditions of n(Pd)/n(4-NP)=2%,n(Na BH₄)/n(4-NP)=6.67 and p H=9.4.And the TOF value is 597.66 h^-1,which is 16.7 times as that of the commercial Pd/C catalyst.The 4-NP conversion rate is still up to 98.75%after continuous use of Pd-LDHs for 10 times.In addition,the adaptability of Pd-LDHs to the degradation reactions of methyl orange,methylene blue and congo red is tested,all of them can be degraded completely within 10 min.The supported palladium-copper bimetallic catalyst(Pd Cu-LDHs)is prepared by ultrasonic assisted method with Na₂PdCl₄as the palladium source,Cu Cl₂as the copper source and NiFe-LDHs as the reducing agent and carrier.Compared with that of Pd-LDHs,the characterization analysis shows that the binding energy position of Pd~03d3/2 shifts to the direction of high binding energy by 0.07 e V,while the corresponding characteristic peak of Ni²⁺and Fe³⁺shifts to the direction of high binding energy by0.86 e V and 0.7 e V respectively.And there is no obvious characteristic peak in the XPS image of Cu,which may be covered by Pd,and there may be an electron transfer between Cu and Pd,which enhances the interaction between Pd and NiFe-LDHs and makes the catalyst have a high stability.In the catalytic degradation probe reaction of4-NP,4-NP can be completely reduced in 7 min under the action of this catalyst,and the TOF value is 713.57 h^-1,which is 19.9 times as that of the commercial Pd/C catalyst.In addition,the catalyst also has good adaptability to the catalytic degradation reaction of MO,MB and CR dyes.The supported palladium-nickel bimetallic catalyst(Pd Ni-LDHs)is prepared by ultrasonic assisted method with Na₂PdCl₄ as the palladium source,Ni(NO₃)₂ as the nickel source and NiFe-LDHs as the reducing agent and carrier.According to characterization analysis,compared with that of Pd-LDHs,the binding energy positions of Pd~0 3d3/2 and Pd~0 3d5/2 shift to the direction of high binding energy by 0.13 e V and0.12 e V respectively,while the corresponding characteristic peak of Ni²⁺and Fe³⁺shifts to the direction of high binding energy by 0.8 e V and 0.54 e V respectively.It can be seen that the presence of Ni promotes a stronger interaction between Pd and NiFe-LDHs.and there may be an electron transfer between Ni and Pd,which makes the catalyst have a high stability.In the catalytic degradation probe reaction of 4-NP,4-NP can be completely reduced in 11 min under the action of this catalyst,and the TOF value is679.22 h^-1,which is 18.9 times as that of the commercial Pd/C catalyst.Pd Ni-LDHs has high catalytic activity for the degradation of MO,MB and CR dyes.The results show that the ultrasonic-assisted in-situ reduction method can prepare the hydrotalcite-supported Pd-based nanocatalyst with high catalytic activity and good stability,and provides a green and efficient preparation method of the supported palladium-based catalyst.