A DFT+U study of acetylene selective hydrogenation on oxygen defective anatase (101) and rutile (110) TiO_2 supported Pd_4 cluster

摘要

The reaction mechanisms for selective acetylene hydrogenation on three different supports, Pd_4 cluster, oxygen defective anatase (101), and rutile (110) titania supported Pd_4, cluster are studied using the density functional theory calculations with a Hubbard U correction (DFTU). The present calculations show that the defect anatase support binds Pd_4 cluster more strongly than that of rutile titania due to the existence of Ti~3 in anatase titania. Consequently, the binding energies of adsorbed species such as acetylene and ethylene on Pd_4 cluster become weaker on anatase supported catalysts compared to the rutile supported Pd_4 cluster. Anatase catalyst has higher selectivity of acetylene hydrogenation than rutile catalyst. On the one hand, the activation energies of ethylene formation are similar on the two catalysts, while they vary a lot on ethyl formation. The rutile supported Pd catalyst with lower activation energy is preferable for further hydrogenation. On the other hand, the relatively weak adsorption energy of ethylene is gained on anatase surface, which means it is easier for ethylene desorption, hence getting higher selectivity. For further understanding, the energy decomposition method and micro-kinetic analysis are also introduced.