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 AlPO4.本研究采用平板模型（slab model）结合周期性密度泛函理论（periodic DFT）计算，从理论层面探究了铑二聚体（Rh dimer）在磷酸铝（AlPO₄）(110)与γ-氧化铝（γ-Al₂O₃）(100)表面的吸附行为，旨在对比阐明磷酸铝表面相较于γ-氧化铝表面的特征性质。无论是未水化还是水化的磷酸铝表面，在PO位点的吸附均为最稳定构型，这与实验观测结果一致；而在AlO位点的吸附则最不稳定。磷酸铝表面PO位点的吸附能显著高于γ-氧化铝表面的对应吸附能，其中一个重要原因是γ-氧化铝表面的形变能量远大于磷酸铝表面。通过巴德电荷分析（Bader charge analysis）、差分电子密度图（difference electron density map）以及投影态密度（projected density of states, p-DOS），本研究清晰揭示了电荷转移（charge transfer, CT）过程由铑二聚体向磷酸铝表面发生，且该电荷转移强度相较于吸附于γ-氧化铝表面时更强。导带中的最低未占据能带（lowest unoccupied band, LU band）在该电荷转移相互作用中作为电子受体轨道发挥关键作用，而磷酸铝的最低未占据能带能级低于γ-氧化铝。因此，铑二聚体向磷酸铝表面的电荷转移量远大于向γ-氧化铝表面的转移量。上述结果表明，低能级下孤立的最低未占据能带与柔性的磷酸铝结构，是实现锚定效应的重要因素，该效应赋予了磷酸铝负载铑纳米颗粒优异的热稳定性，进而可减少使用磷酸铝基三效催化剂（three-way catalyst）中铑的使用量。