Effect of TiO2 support on the structural and electronic properties of PdmAgn clusters: a first-principles study.

First-principles density functional theory studies have been carried out to investigate the effects of perfect and defective anatase TiO2 supports on the structural stabilities and electronic properties of PdmAgn(m + n = 2-5) bimetallic clusters. Our results showed that the structures of supported Pd-Ag bimetallic clusters are distorted compared to their structures in the gas phase, which is caused by the balance of the cluster inner-interaction and the metal-support interfacial interaction. In particular, Pd1Ag3 and Pd1Ag4 clusters prefer to form three-dimensional structures on both perfect and defective anatase TiO2 support while their most stable structures in the gas phase are planar. In the most stable structures of supported PdmAgn bimetallic clusters, Pd atoms always occupy the most active sites of TiO2(101) surface, which induced Pd enriched at the interface of TiO2 support and Ag atoms exposed at the surface of the bimetallic cluster. As Ag% increases, the perfect TiO2 support gets more electrons from the Pd-Ag bimetallic cluster, which reduced the stability of the supported Pd-Ag cluster. The Mulliken population and electron density difference analysis demonstrated that the co-deposition of Ag induced the charge of adsorbed Pd on the perfect TiO2 support from positive to negative as a result of charge transfer from the half-filled s-orbital of Ag(5s(1)) to the d-orbit of Pd, and the negative charges of Pd on the defective TiO2 support were also increased by Pd-Ag charge polarization. Therefore, the selectivity of acetylene hydrogenation is enhanced by anatase-TiO2 supported Pd-Ag bimetallic catalyst as it serves as an electron donor.