Rational Design And Synthesis Of Catalysts Towards Hydrogenation Reaction

Nowadays,the majority of hydrogenation reactions in chemical engineering are catalyzed by metals,especially by noble metals.Typically,vast reactions are metal-catalyzed,including selective hydrogenation of nitro compounds,selective hydrogenation of oxygen-containing compound,and hydrogenation of carbon monoxide.However,so far these catalysts are all high-cost,low-selectivity,and short-lifetime,causing a tremendous waste of resources and energy,while bringing in pollutions,all of which restrict the further development of metal catalysts.Therefore,the desperate issue is to develop catalysts with high activity,selectivity,and stability,thus lowering cost.In this paper,we report here the controllable synthesis and the use in hydrogenation reaction of different systems of nanoparticles.The material systems include nanocubes of Rh-doped PdRh alloy,nanowires of high-index facets of Pt-Fe,and nanocubes and octahedrons of core-shell Fe3O4@?-Fe5C2 Based on the performance of these catalysts in hydrogenation of small organic molecules and carbon monoxide,we studied the influence of composition,the role of high-index facets,as well as the structure-activity relations of different facets.The main contents of this paper are as follows:1.We achieved the optimization of both activity and selectivity towards selective hydrogenation reaction of 3-nitrostyrene over Rh-doped Pd nanocubes.The doping of Rh into Pd nanocubes enhanced both the activity and selectivity for C=C group relative to Pd nanocubes.Moreover,Rh-doped Pd nanocubes also exhibited high activity and selectivity for C=C groups in selective hydrogenation of divers<e compounds containing C=C and NO2 groups.Mechanistic studies revealed that the introduction of Rh dopant into Pd nanocubes strengthened the adsorption of 3-nitrostyrene relative Pd nanocubes,accounting for the enhanced activity.Moreover,the ensemble composed of Rh dopant and nearby Pd atoms in Rh-doped Pd nanocubes induced the adsorption configuration of 3-nitrostyrene favorable for the activation of C=C groups,responsible for the high selectivity.2.We developed Pt-Fe nanowires with abundant high-index facets which created steric effect for improved selectivity without occupying or passivating active sites.During the selective hydrogenation of acetophenone,the turnover frequency number of Pt-Fe nanowires was 97.7 min-1 at 70?,5.7 times higher than that(17.2 min-1)of cubic counterparts.The selectivity for C=O?groups over Pt-Fe nanowires reached 91.7%at a conversion of 99.3%,whereas that over nanocubes was only 60.2%at a conversion of 98.4%.The high C=O selectivity of Pt-Fe nanowires was attributed to the steric effect,where the stepped surfaces composed of high-index facets selectively adsorbed C=O bonds and inhibited the adsorption of aromatic rings.3.We constructed a well-defined Fe-based catalyst composed of a Fe3O4 cubic core and a X-Fe5C2 shell exposing(020)facets(denoted as Fe3O4@X-Fe5C2 nanocubes)to achieve the high activity and selectivity simultaneously towards Fischer-Tropsch to olefins(FTO).Fe3O4@X-Fe5C2 nanocubes on SiC exhibited the turnover frequency(TOF)number as high as 1.32 s-1,outperforming the most active FTO catalysts ever reported.Meanwhile,the selectivity for lower olefins reached 63.0 C%.To the best of our knowledge,Fe3O4@X-Fe5C2 nanocubes on SiC were the only catalyst which has simultaneously enabled the TOF number above 1.0 s-1 and the lower-olefin selectivity above 60.0 C%.According to mechanistic studies,Fe3O4@X-Fe5C2 nanocubes followed hydrogen-assisted CO dissociation which lowered the activation energy to activate CO,affording high FTO activity.Moreover,the weak adsorption of lower olefins on Fe3O4@X-Fe5C2 nanocubes suppressed both the secondary hydrogenation into paraffins and C-C coupling into Cs+ products,rendering high selectivity for lower olefins.4.We developed a highly selective catalyst for diesel production by constructing the(020)facet of X-Fe5C2 via in-situ reconstruction of Fe3O4 nanocubes under syngas(denoted as Fe3O4@X-Fe5C2 nanocubes/Zn2SiO4).During Fischer-Tropsch synthesis(FTS),Fe3O4@X-Fe5C2 nanocubes/Zn2SiO4 achieved a record-high selectivity of 82.7 C%for diesel production at a CO conversion of 40.3%.The deviation of product distribution from the ASF model was attributed to the hydrocracking of C21+hydrocarbons on Bronsted acid sites.Specifically,the(020)facet of X-Fe5C2 optimized the dissociation of H2 into H atoms which spilt over onto Zn2SiO4 and migrated to Lewis acid sites.At Lewis acid sites where H atoms released electrons to become protons,Br(?)nsted acid sites with proper acidity and density were formed,thereby optimally hydrocracking C2+hydrocarbons into diesel.