Particle Regulation And Selective Hydrogenation Performance Of S-doped Carbon Materials Supported Pd

The shape,size and dispersion of metal nanoparticles could have an impact on various reactions,especially selective catalytic hydrogenation reactions which have a wide range industrial applications in pharmaceutical production and fine chemicals.Thus,regulation of metal nanoparticles has long been one of the hotspots of researchers.Recent decades,researchers found that using elements such as B,N,S,P,et al.to modify carbon materials could not only effectively improve the physical and chemical properties of the materials,but also significantly enhance its catalytic performance in hydrogenation reactions.Lots of literatures have demonstrated that elements like N could effectively influence the size and distribution of metal particles.However,researches on using S-doping to regulate metal size have rarely been reported.In this paper,S was successfully introduced into the activated carbon via a high-temperature calcination strategy under an innert atmosphere.Subsequently,the loading of Pd was finished through a simple wet impregnation method.Experiments indicated that the particle size and distribution of Pd nanoparticles were closely related to the S content introduced in the activated carbon.The size of Pd nanoparticles and the S content showed a“volcanic-type”relationship.The size of the Pd nanoparticles first increased and then decreased as the S content gradually increased.What's more,the distribution of Pd nanoparticles on the surface of the activated carbon could be regulated by adjusting the S content.Subsequently,the prepared catalysts were then applied in selective hydrogenation of o-CNB,which is sensitive to the Pd nanosize,in order to investigate the effect of metal size on the performance of the catalysts in this reaction.It was found that Pd/CS_x exhibited superior selectivity compared to normal Pd/AC,dechlorination was almost completely inhibited.It was worth noting that an abnormal relationship between Pd nanoparticle size and catalytic performance in selective hydrogenation of o-CNB was observed,which was contrary to previous studies.It was reported that smaller Pd size would lead to higher activtity and poorer selectivity.However,our work found that catalysts with smaller Pd size performed lower activity and higher selectivity.Besides,kinetic experiments and characterizations such as XRF,TEM,XRD and XPS were conducted to further inverstigate the influence of S-doping on the catalytic performance of Pd/CS_x in selective hydrogenation of o-CNB.Experiments indicated that electronic effects between S and Pd rather than geometric effects of metal particles played a dominant role in this reaction,contributing to the abnormal phenomenon that was contrary to literature.Due to the electron-withdrawing effect of S,the electron density around Pd decreased and the valence state of Pd increased,leading to the decreased activity and enhanced selectivity.In addition,performance of catalysts modified by different S-containing precursors in selective hydrogenation of o-CNB was also studied,which confirmed that the electron transfer between Pd and S could lead to the change of selectivity of the catalysts in this reaction.