Theoretical Study On Selective Hydrogenation Of Aromatic Nitro Compounds Over Au,Pd Based Catalysts

Aniline and chloronitrobenzene are the important chemicals and intermediates in the industries of dyeing,pesticides,pharmaceuticals and so on.The selectively hydrogenation of aromatic nitro-compounds to the corresponding aniline attracted people's widespread attention with its environmentally friendly,catalyst recycling and product easier purification.But because of the complexity of the reactions,we need to choose a proper catalyst,which can maintain high conversion efficiency and inhibit dechlorination reaction occurred at the same time.Previous experimental results indicated that Au,Pd based catalysts shows excellent catalytic performance in the selective hydrogenation of aromatic nitro-compounds.In this paper,we studied the effect of Au cluster size and alloying on hydrogenation of p-chloronitrobenzene.In addition,we systematically investigated the adsorption and hydrogenation mechanism of nitrobenzene to aniline on Pd3/Pt(111)bimetallic surface based on the generalized gradient approximation density functional theory.The main results are as follows:(1)The effect of the different Au cluster sizes(Au13,Au20,Au55)for the the selective hydrogenation of p-chloronitrobenzene was studied.The calculations show that Au20 is the most stable cluster with the largest HOMO-LUMO energy gap.The smaller size of clusters,the stronger adsorption ability for the p-CNB,p-CAN and intermediates.The hydrogenation and dechlorination on different size clusters are all strong exothermic reactions.The Au13 cluster with small size has the better hydrogenation activity,and the relatively large size of Au55 cluster is more advantageous to inhibit dechlorination.(2)The effect of Pd-doped Au20 clusters for the selective hydrogenation of p-chloronitrobenzene was studied.It is observe that Au19Pd-S has the most stable structure.The species are preferably absorbed on Pd sites and the calculatedadsorption energy increased as Au19Pd-S < Au19Pd-E < Au19Pd-V.Compared with Au20 cluster,Pd on edge and surface of Au19 Pd clusters facilitate p-CNB hydrogenation and enhance the inhibition of C-Cl bond cleavage,but the Pd on vertex site reduce the catalytic performance for hydrogenation of p-CNB.(3)We systematically investigate the adsorption and hydrogenation mechanism of nitrobenzene to aniline on Pd3/Pt(111)bimetallic surface.The results show that the adsorption at the Pd-top-top site through O atom is the most stable configuration when the nitrobenzene is perpendicular to the Pd3/Pt(111)bimetallic surface.The hydrogenation processes are almost exothermic and the mechanism of nitrobenzene on Pd3/Pt(111)bimetallic surface preferentially follows the direct hydrogenation route and best fit for the Jackson reaction mechanism.