The Synthesis And Nitrobenzene Hydrogenation Performance Of Pd-supported N-doped CNTs

Heteroatom doping can produce defects in carbon materials and effectively modify the electronic structure,morphology and physical and chemical properties.Nitrogen-doped carbon nanotubes(NCNTs)exhibit excellent catalytic performance either as metal-free catalysts or as catalyst supports to achieve high dispersion of noble metals.The metal-support interaction(MSI)of N-doped carbon material supported metal catalysts is crucial for the catalytic performance,which is not understood well yet.In this work,we focus on the MSI of Pd supported on N-doped carbon nanotubes.It was found that the trace amount of impurity metals in supports from the residual growth catalysts affect the catalytic performance of Pd/NCNT for the hydrogenation of nitrobenzene.Ruling out the effect of metal impurities,the MSI between Pd and different types of nitrogen sites,i.e.prydinic and graphitic nitrogen,was explored.The main contents are as follows:(1)It was found that the metal impurities,mainly Fe,in the residual growth catalysts of NCNTs have great negative impact on the catalytic performance of supported Pd NPs for the hydrogenation of NB.As a result,the NCNTs-supported Pd catalyst may perform much worse than conventional CNTs-supported one.XPS analysis revealed that the chemical state of the Pd almost maintain no change after iron doping.A DFT calculation suggested that the trace amount of iron on Pd behaves as blocker of active sites through a geometrical effect.Moreover,we also demonstrated that a wide spectrum of metallic impurities,including Fe,Co,Ni,Mn,Cr,Cu,Zn,Mo,Al and Mg,have similar effect on the Pd catalyst for NB hydrogenation.(2)The results in this work uncover the significant effect of trace metallic impurities in carbon materials.To exclude the effect of residual catalyst in NCNTs,we proposed the coaxial cable N-doped carbon@CNTs as supporting materials of Pd NPs.By this means,the high activity of Pd NPs for NB hydrogenation can be achieved taking advantage of unique SMI between Pd and N-doped carbon materials without the interference of impurities.(3)The interaction between Pd nanoparticles(NPs)and specific nitrogen functionalities of NCNTs has been systematically investigated.According to the XPS,the Pd NPs supported on N@CNTs shows the characteristic of electron deficiency compared to Pd/CNTs.Strong interaction between Pd NPs and pyridinic nitrogen was observed,which lead to the preferential anchoring of Pd NPs on pyridinic nitrogen sites.In the meantime,DFT calculations suggested that Pd NPs will become electron-deficient adsorbing on either pyridinic N or graphitic N,while the former displays the stronger electron-withdrawing ability.These results may help for the rational design of Pd catalysts supported on N-doped carbon materials with desired electronic properties.