Thermostable Rh Metal Nanoparticles Formed on Al₂O₃ by High-Temperature H₂ Reduction and Its Impact on Three-Way Catalysis

Abstract

The influence of high-temperature H2 reduction treatment on Rh and Pd catalysts supported on Al2O3 was studied in relation to thermal aging in air. After air-aging at ≥900 °C, the Rh/Al2O3 catalyst was more strongly deactivated compared with the Pd/Al2O3 catalyst. As has been widely recognized, the solid-state reactions between Rh oxide and Al2O3 decreased the active surface area and stabilized inactive Rh3+ species. The activity was restored by the postreduction treatment with 20% H2/He at 200 °C, whereas a striking enhancement of activity was achieved by the reduction at 800–1100 °C, where substantial deactivation occurred for Pd/Al2O3. A mechanistic interpretation is proposed based on local structural characterization, which explains these contrasting thermal behaviors. The high-temperature reduction treatment produced active and thermostable Rh metal nanoparticles, which were highly dispersed on Al2O3. The observed dispersion (as high as ∼20% after reduction at 1000 °C) is among the highest for supported Rh catalysts reported in the literature. This is in complete contrast to the rapid sintering of Pd and other precious metals (Ru and Pt) into large metal agglomerates greater than 50 nm. Because the thermal behavior observed for Rh/ZrO2 under both oxidizing and reducing atmospheres was similar to that of Pd/Al2O3, the stability of metal nanoparticles depended not only on metal species but also on the interactions with support materials. An important implication of this study is that Al2O3 is a very efficient support for anchoring Rh metal nanoparticles via interfacial Rh–O–Al bonding under strong reducing conditions, in contrast to the well-known incompatibility with Rh oxide under oxidizing conditions.

Influence of crystal structure of Y-doped ZrO₂ as support oxide on the three-way catalytic performance of supported Rh catalyst

“…Metallic state of Rh is known to act as catalytically active sites for TWC reactions. 37) In fact, Rh/3Y-ZrO 2 and Rh/3Y/ZrO 2 showed higher catalytic activity that Rh/ZrO 2 (Fig. 4).…”

Role of Ba in an Al2O3‐Supported Pd‐based Catalyst under Practical Three‐Way Catalysis Conditions

“…Al 2 O 3 is widely used as a support material in three‐way catalysts because of extensive advantages that include good thermal stability, mechanical strength, and high surface area [24] . Alkaline earth and lanthanide metals are often introduced into Al 2 O 3 as structural stabilizers that suppress the γ‐to‐α phase transformation [25–27] .…”

“…[22,23] Al 2 O 3 is widely used as a support material in three-way catalysts because of extensive advantages that include good thermal stability, mechanical strength, and high surface area. [24] Alkaline earth and lanthanide metals are often introduced into Al 2 O 3 as structural stabilizers that suppress the γ-to-α phase transformation. [25][26][27] Basic metals, such as La and Ba, are also known to stabilize Pd, which is the most commonly used active metal in modern commercial three-way catalysts [28][29][30][31][32][33] under TWC conditions, resulting in improved Pd dispersion even after catalyst exposure to harsh operating conditions.…”

Hollow Nano‐Mesosilica Spheres Containing Rhodium Nanoparticles Supported on Nitrogen‐Doped Carbon: An Efficient Catalyst for the Reduction of Nitroarenes under Mild Conditions

“…High dispersion Rh NPs possessed a high stability against sintering and they were more resistant to acid and alkali, so we chose the more stable metal-rhodium. [14] The outer mesoporous silica shell was hydrophilic and able to disperse well in water and avoid flammable solvents. On the basis of these merits, this catalyst showed good activity, selectivity, and recyclability for the hydrogenation of aromatic compounds under pure water, no base, low temperature (80°C) and atmospheric pressure (1 atm).…”

“…But Pd had a lower resistance to sintering than Rh under hydrogen atmosphere. High dispersion Rh NPs possessed a high stability against sintering and they were more resistant to acid and alkali, so we chose the more stable metal‐rhodium . The outer mesoporous silica shell was hydrophilic and able to disperse well in water and avoid flammable solvents.…”