Snythesis Of Platinum-based Nanocatalysts And Their Catalytic Performance For Hydrogenation Of Nitrobenzene And Its Derivatives

The noble metal-based catalysts have important applications in heterogeneous catalysis,especially in the hydrogenation of nitrobenzene and its derivatives,with arylamine compounds,one of its catalytic hydrogenation products,being important organic intermediates in the fields of dyes,medicine,pesticides and etc.But as precious metal resources are scarce and expensive.It is crucial to improve the utilization rate of precious metals while taking into account the high selectivity,high activity and good stability of the catalysts.In addition,the hydrogenation of nitrobenzene and its derivatives is often used as a probe reaction for fundamental theoretical research on catalysis.This study used the hydrogenation of nitrobenzene and its derivatives as a probe reaction and carbon black?BP-2000?as a carrier,to synthesize Pt/C by the method of impregnation,synthesize Ni/C,Co/C and NiCo/C by hydrazine hydrate reduction and prepare PtNi/C,PtCo/C and PtNiCo/C multi-metallic nanocatalysts by chemical substitution method.Thanks to the synergetic effect among the precious metal,transition metal and transition metal hydroxide,the content of precious metal consumed can be reduced with the catalytic performance?activity,selectivity and stability?of the catalysts improved.In this study,the relationship between loadings different Pt in PtNiCo/C and theircatalytic hydrogenation performance was investigated.By changing the heat treatment temperature of 1.6%PtNiCo/C catalysts in 90%N₂+10%H₂?1.6%PtNiCo/C^X catalysts?,different nanostructures of catalysts were obtained,and the relationship between different nanostructures and their catalytic hydrogenation performance was further explored.T o be specific,the main contents of this paper are as follows:1.The catalytic performance of monometallic-,bimetallic-and tri-metallic nanocatalysts under mild reaction conditions was determined and compared by calculating the conversion rate?X?of the reactants,the selectivity?S?of the product and the conversion frequency?TOF?of the catalyst.The catalysts activity order is found to be 1.6%PtNiCo/C>1.6%PtNi/C?1.6%PtCo/C?>1.6%Pt/C,which proves that the tri-metallic nanocatalysts showed the most excellent catalytic performance.For example,under the reaction conditions of 40°C,20 min and 1.0 MPa H₂,1.6%PtNiCo/C tri-metallic nanocatalysts exhibited the best catalytic performance in the hydrogenation of 4-nitrobenzonitrile(X=93.5%,TOF=364.9 h^-1,S_{4-aminobenzonitrile}=100%).The excellent performance of 1.6%PtNiCo/C tri-metallic nanocatalysts is ascribed to the synergetic effect among Pt,Ni,Co,Ni?OH?₂,Co?OH?₂ species.The synergetic mechanism is that H₂ is adsorbed and activated by the precious metal Pt,the active sites of Ni?OH?₂-Co?OH?₂ adsorb and activate the substrate.The active sites of Ni and Co as a"bridge"?hydrogen overflow?in the catalyst transfer the activated hydrogen species H*to Ni?OH?₂-Co?OH?₂,followed by H*reacting with the activated substrate via hydrogenation reaction to produce the target product.2.The effect of Pt loadings?1.6%,0.8%,0.04%?on catalyst performance under mild reaction conditions was investigated.The experimental results showed that the reaction conversion rate will increase and the TOF will reduce as the Pt loading of the catalysts increases.The TOF sequence for catalytic hydrogenation of tri-metallic nanocatalysts is:0.04%PtNiCo/C>0.8%PtNiCo/C>1.6%PtNiCo/C.The order of the catalytic activity of the monometallic and bimetallic nanocatalysts with different Pt loadings is similar to the tri-metallic nanocatalysts.For example,0.04%PtNiCo/C showed the best catalytic performance in hydrogenation of 4-nitrobenzonitrile with different Pt loadings catalysts under the reaction conditions of 40°C,20 min and 1.0 MPa H₂(X=93.6%,TOF=14612.8 h^-1,S_{4-aminobenzonitrile}=100%).It is found that when the loading of precious metal Pt is reduced,Pt obtains better dispersion on the catalyst surface in the form of single atoms,and the Pt content in the oxidation state is higher than metallic Pt,so it can exhibit better catalytic hydrogenation activity and nitro addition hydrogen selectivity.3.The catalytic performance of nitrobenzene and its derivatives over 1.6%PtNiCo/C with different heat treatment temperatures?1.6%PtNiCo/C^X,X=200°C,300°C,400°C and 500°C?was compared under mild reaction conditions.The experimental results showed that the 1.6%PtNiCo/C²00 catalyst exhibited the best catalytic performance.For example,among 1.6%PtNiCo/C catalysts with different heat treatment temperatures catalyzing the hydrogenation of 4-nitroacetophenone,the 1.6%PtNiCo/C²00 exhibited good catalytic performance under the conditions of 40°C,1 h and 1.0 MPa H₂(X=100%,TOF=130.1 h^-1,S_{4-aminoacetophenone}=100%).It is found that when the calcination temperature is 200°C,Pt in nanocatalyst loaded on NiCo alloy nanoparticles?Pt/NiCo?was in the form of nanoclusters or single atoms in which the synergetic effect of multiple active sitesimproves the catalytic performance of the catalyst.While after calcined at 400°C and 500°C,PtNiCo alloy structure formed in 1.6%PtNiCo/C catalyst,so the synergetic effect becomes weaker,and thus the catalytic performance reduces.In summary,the synergetic effect among Pt,Ni and Co related species in the PtNiCo/C,increasing the dispersion of Pt on the catalyst surface,and setting the heat treatment for catalysts at 200°C,could all effectively improve the catalytic performance of the catalysts.This research will provide a theoretical basis for the design and preparation of multi-metallic nanocatalyst with high precious metal utilization,high activity,high selectivity and robust stability.