Direct Observation of Rhodium Ex-Solution from a Ceria Nanodomain and Its Use for Hydrogen Production via Propane Steam Reforming.

The ex-solution phenomenon has received attention as a promising technique to prepare highly durable heterogeneous catalysts. Perovskite materials have been mainly used as host oxides for ex-solution, but their small surface areas have limited their practical use. Here, Rh was ex-solved by reducing Rh-doped ceria solid solution, and nanosized Rh catalysts with a high surface area of 70.7 m2/g were prepared. The Rh nanoparticles ex-solved from the ceria nanodomains were directly monitored by in situ transmission electron microscopy. The Rh nanoparticles whose sizes are 2-3 nm were not coarsened during the propane steam reforming process carried out at 700 °C for 65 h, leading to high resistance against sintering and coke formation. On the contrary, the Rh catalyst simply deposited on CeO2 was significantly sintered after the reaction, and the size of Rh nanoparticles increased to 25 nm, resulting in severe coke formation. Our work shows that ex-solution from a ceria-based nanodomain can be a good way to prepare metal nanoparticle catalysts with a large surface area and excellent durability for gas-phase reactions at high temperatures.

The role of surface hydroxyl groups on a single-atomic Rh1/ZrO2 catalyst for direct methane oxidation.

A single atomic Rh catalyst immobilized on zirconia (Rh1/ZrO2) was modified by hydrothermal treatment to have surface hydroxyl groups and used for direct methane oxidation. Both O2 and H2O2 were used as oxidants. The amount of H2O2 could be reduced with enhanced methanol productivity in the presence of the surface hydroxyl groups. The formation of the surface hydroxyl groups upon hydrothermal treatment and their disappearance upon reaction were confirmed with diffuse reflectance infrared Fourier transform spectroscopy, indicating that the surface hydroxyl groups participate in the surface reaction.

Oxidative Methane Conversion to Ethane on Highly Oxidized Pd/CeO2 Catalysts Below 400 °C.

Methane upgrading into more valuable chemicals has received much attention. Herein, we report oxidative methane conversion to ethane using gaseous O2 at low temperatures (<400 °C) and atmospheric pressure in a continuous reactor. A highly oxidized Pd deposited on ceria could produce ethane with a productivity as high as 0.84 mmol gcat -1 h-1 . The Pd-O-Pd sites, not Pd-O-Ce, were the active sites for the selective ethane production at low temperatures. Density functional theory calculations confirmed that the Pd-O-Pd site is energetically more advantageous for C-C coupling, whereas Pd-O-Ce promotes CH4 dehydrogenation. The ceria helped Pd maintain a highly oxidic state despite reductive CH4 flow. This work can provide new insight for methane upgrading into C2 species.

Selective Activation of Methane on Single-Atom Catalyst of Rhodium Dispersed on Zirconia for Direct Conversion.

Direct methane conversion into value-added products has become increasingly important. Because of inertness of methane, cleaving the first C-H bond has been very difficult, requiring high reaction temperature on the heterogeneous catalysts. Once the first C-H bond becomes activated, the remaining C-H bonds are successively dissociated on the metal surface, hindering the direct methane conversion into chemicals. Here, a single-atom Rh catalyst dispersed on ZrO2 surface has been synthesized and used for selective activation of methane. The Rh single atomic nature was confirmed by extended X-ray fine structure analysis, electron microscopy images, and diffuse reflectance infrared Fourier transform spectroscopy. A model of the single-atom Rh/ZrO2 catalyst was constructed by density functional theory calculations, and it was shown that CH3 intermediates can be energetically stabilized on the single-atom catalyst. The direct conversion of methane was performed using H2O2 in the aqueous solution or using O2 in gas phase as oxidants. Whereas Rh nanoparticles produced CO2 only, the single-atom Rh catalyst produced methanol in aqueous phase or ethane in gas phase.