Nucleation and Growth of Pd-Ag Bimetallic Catalyst On Anatase TiO2(101) Surface: A First-Principles DFT Study

Abstract

Supported bimetallic nanoparticles are important catalytic materials in chemical industry because of their superior chemical and physical properties in comparison to single metal clusters. Especially, alloying group VIII metals with a group IB metal is generally known to enhance the selectivity for various industrial processes. For example, in the reaction of selective hydrogenation of acetylene in ethylene-rich feedstock (which has a significant impact on ethylene plant economics), TiO₂-supported Pd-Ag bimetallic catalyst are widely accepted as a replacement of pure Pd/Al₂O₃ catalyst for its high ethylene selectivity and reduction of green oil formation. Because the catalytic performance has generally been attributed to geometric or electronic effects of the catalysts, a detailed knowledge of the energetics and local atomic and electronic structures of anatase TiO₂-supported Pd-Ag bimetallic clusters is fundamental to understand clearly the catalytic performance but also to design the better catalysts.

In this work, first-principles density functional theory studies have been carried out to investigate the structural stabilities and electronic properties of supported Ag_n(n=1-5) and Pd_mAg_n(m+n = 2-5) bimetallic clusters as well as their growth mechanism on both perfect and defective anatase TiO₂(101) surface. When Ag_n(n=1-5) deposited at the TiO₂(101) surface, the most stable configurations are similar to that of Pd [Zhang et al. J. Phys. Chem. C, 2008, 112, 19506]. But their growth mechanism is different. Compared with Pd, the adsorbed Ag is easier to move on the TiO₂ support and Ag clusters would be aggregated to large particles at high temperatures. In addition, the adsorbed Ag atoms and clusters transferred electrons not only to the perfect TiO₂(101) surface but also to the defective TiO₂(101) surface, which may be caused by the low electronegativity of Ag element. Because the interaction of Pd with TiO₂(101) surface is much stronger than that of Ag, Pd atoms take the advantage to occupy the most active sites when the Pd-Ag bimetallic cluster deposited at both perfect and defective TiO₂ surface, indicating Pd is the nucleation core for Pd-Ag nanoparticle growth on the TiO₂ support. As the Ag% increasing, the stability of the adsorbed Pd-Ag cluster at the TiO₂ surface reduces and the supported Pd-Ag clusters prefer to aggregate because of the mobility of Ag. The effect of Pd/Ag ratio on the electronic properties and activities of Pd-Ag/TiO₂ catalyst in the reaction of acetylene selective hydrogenation was also discussed.