Understanding the Behavior of TiO2(110)-Supported Pd7 Cluster: A Density Functional Study

Abstract

TiO2(110)-supported small Pd (≤25 atoms) nanoparticles have been found capable of catalyzing CO oxidation. In the present article, the structure and activity toward CO of a small Pd7 cluster supported on TiO2(110) support were examined using density functional theory (DFT). Our results show that using TiO2(110) as a support makes a big difference for such small metal cluster compared to the gas-phase. For the gas-phase Pd7, the most stable structure is the pentagonal bipyramid in three dimensions (3D), and the transformation from the 3D to the centered hexagon structure in two dimensions (2D) is highly activated. In contrast, TiO2(110) allows the 3D → 2D transformation of the supported Pd7 with less energy cost and much lower barrier. The difference between the gas-phase Pd7 and Pd7/TiO2(110) is also observed on CO exposure. The CO adsorption at high coverage (≥3 CO molecules per Pd7 cluster) destabilizes the Pd7-3D structure on TiO2(110) and makes the 3D → 2D transformation thermodynamically favorable; while, in the case of Pd7, the Pd7-3D stays during CO saturation. Depending on the binding energy, the CO molecules adsorbed on Pd7/TiO2(110) can be divided into two groups: relatively strongly bound species at high symmetric hollow and bridge sites of Pd with the C–O bond mostly perpendicular to the surface and relatively weakly bound species at the top sites of peripheral Pd with the C–O bond parallel to the surface. Our DFT results explain well the experimental observations on such model system, which have been previously reported. Our results suggest that special attentions have to be paid to use such small cluster in catalysis. Significantly different behaviors from the gas-phase can be expected by using supports and under reaction conditions.