Characterization of AlPO₄(110) Surface in Adsorption of Rh Dimer and Its Comparison with γ‑Al₂O₃(100) Surface: A Theoretical Study

Adsorption of Rh dimer on AlPO₄(110) and γ-Al₂O₃(100) surfaces was theoretically investigated by periodic DFT calculation with a slab model to elucidate characteristic features of the AlPO₄ surface in comparison with the γ-Al₂O₃ surface. The adsorption at the PO site is the most favorable in both nonhydrated and hydrated AlPO₄ surfaces, which is consistent with the experimental finding. The adsorption at the AlO site is the least favorable. The adsorption energy at the PO site of the AlPO₄ surface is considerably larger than that at the γ-Al₂O₃ surface. One important reason is that the deformation energy of the γ-Al₂O₃ surface is much larger than that of the AlPO₄ surface. Bader charge analysis, difference electron density map, and projected density of states (p-DOS) clearly disclose that the charge transfer (CT) occurs from the Rh dimer to the AlPO₄ surface. This CT is stronger than in the adsorption on the γ-Al₂O₃ surface. The lowest unoccupied band (LU band in conduction band) plays a crucial role as an electron-acceptor orbital in this CT interaction. The LU band of the AlPO₄ exists at a lower energy than that of γ-Al₂O₃. Therefore, the CT from the Rh dimer to the AlPO₄ surface is considerably larger than that to the γ-Al₂O₃ surface. These results show that the presence of the isolated LU band at a low energy and the flexible AlPO₄ structure are important factors for the anchoring effect, which achieves outstanding thermal stability of the supported Rh nanoparticles on the AlPO₄ surface and therefore enables a reduction in quantity of Rh in the three-way catalyst using AlPO<sub>4