Rhodium Nanoparticle Anchoring on AlPO₄ for Efficient Catalyst Sintering Suppression

Abstract

Rhodium catalysts exhibited higher dispersion with tridymite-type AlPO 4 supports than with Al 2 O 3 during thermal aging at 900 °C under an oxidizing atmosphere. The local structural analysis via X-ray photoelectron spectroscopy, transmission electron microscopy, X-ray absorption fine structure, and infrared spectroscopy suggested that the sintering of AlPO 4supported Rh nanoparticles was significantly suppressed because of anchoring via a Rh−O−P linkage at the interface between the metal and support. Most of the AlPO 4 surface was terminated by phosphate P−OH groups, which were converted into a Rh−O−P linkage when Rh oxide (RhO x ) was loaded. This interaction enables the thin planar RhO x nanoparticles to establish close and stable contact with the AlPO 4 surface. It differs from Rh−O−Al bonding in the oxide-supported catalyst Rh/Al 2 O 3 , which causes undesired solid reactions that yield deactivated phases. The Rh−O−P interfacial linkage was preserved under oxidizing and reducing atmospheres, which contrasts with conventional metal oxide supports that only present the anchoring effect under an oxidizing atmosphere. These experimental results agree with a density functional theory optimized coherent interface RhO x / AlPO 4 model.

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

“…This result is consistent with the experimental suggestion that Rh particles adsorb at the PO site. 15,17 Table 1 also shows that the deformation energy of the surface is considerably high in all adsorption structures, whereas the deformation energy of the Rh 2 is extremely small in all cases. Although the interaction energy between the Rh 2 and the AlO hy site is considerably large, the deformation energy of the surface is also extremely large at this site, which is the reason for the small binding energy of the Rh 2 adsorption at the AlO hy site.…”

“…31 In addition, Machida's group reported that the partial dehydration occurs by increasing the temperature. 17 In the partially dehydrated surface, three types of adsorption structure are possible, depending on the manner of partial dehydration. In one structure, the Rh dimer interacts with the dehydrated PO site, which is near the dehydrated Al uns atom, as shown in Figure 2B.…”

“…In this regard, Machida and co-workers experimentally and theoretically investigated the Rh 2 O 3 −AlPO 4 system and found that the Rh atom of Rh 2 O 3 binds with the O atom of AlPO 4 to form an Rh−O−P linkage, which plays an important role in the anchoring effect to suppress sintering, where first-principles DFT calculations were conducted with the slab model of Rh 2 O 3 −AlPO 4 . 17 The adsorption of Pt 18−22 and Ir clusters 23,24 on γ-Al 2 O 3 surface and that of Cu, Ag, and Au clusters on α-Al 2 O 3 surface 25−30 were investigated theoretically using first-principles DFT calculations. In those studies, the nucleation, growth, and adsorption positions have been discussed.…”

“…However, the adsorption of Rh clusters on the AlPO 4 surface has not been investigated theoretically at all. Only the interaction between Rh 2 O 3 and the AlPO 4 surface has been investigated experimentally and theoretically by Machida and co-workers, 17 as described above. To suppress the sintering, this linkage must be preserved in both oxidizing and reducing atmospheres.…”

Lean NOx reduction over Rh/ZrP2O7 catalyst under steady-state and perturbation conditions

“…We have previously studied various metal phosphates, such as AlPO 4 , ZrP 2 O 7 , and LaPO 4 , as thermally stable supports for Rh catalysts [11][12][13][14][15][16][17][18][19]. These materials produce optimum interactions with the Rh species via Rh O P bonding and enable high dispersion of Rh anchored onto the phosphate unit on the surface [13,14,17]. The interface Rh O P bonding also affects the electronic states and redox behaviors of Rh; Rh oxide (Rh 2 O 3 )…”