The Role of CeO₂ as a Gateway for Oxygen Storage over CeO₂‑Grafted Fe₂O₃ Composite Materials

The surface grafting of CeO₂ onto Fe₂O₃ with a 1:5 molar ratio produced a thermally stable composite material with greater and faster oxygen storage/release than its separate constituents. In the composite, CeO₂ and Fe₂O₃ were intimately contacted by interfacial Ce–O–Fe bonding, and no solid solutions or mixed Ce and Fe oxides were formed after heating at 900 °C. The oxygen storage capacity and initial rate of oxygen release/storage were both increased in the composite structure by virtue of the Fe₂O₃ and CeO₂, respectively. The reduction–oxidation cycles in which Fe₂O₃ is reduced via Fe₃O₄ to Fe metal by CO or H₂ and then reoxidized by O₂ were stabilized by surface-grafting Fe₂O₃ with CeO₂. In situ Raman spectra demonstrated that the surface-grafted CeO₂ acts as an oxygen gateway, activating the dissociation of O₂ into oxide ions or the recombination of oxide ions into O₂ and transferring oxide ions to/from Fe₂O₃. Meanwhile, Fe₂O₃ acts as an oxygen reservoir that expands the O₂ storage capacity. The composite material was tested in a simulated exhaust gas stream with lean/rich perturbations (which occur in automotive three-way catalysts). The synergistic effect of the surface grafting effectively buffered the system against air-to-fuel ratio fluctuations

Catalytic SO₃ Decomposition Activity and Stability of Pt Supported on Anatase TiO₂ for Solar Thermochemical Water-Splitting Cycles

Pt-loaded anatase TiO₂ (Pt/TiO₂-A) was found to be a highly active and stable catalyst for SO₃ decomposition at moderate temperatures (∼600 °C), which will prove to be the key for solar thermochemical water-splitting processes used to produce H₂. The catalytic activity of Pt/TiO₂-A was found to be markedly superior to that of a Pt catalyst supported on rutile TiO₂ (Pt/TiO₂-R), which has been extensively studied at a higher reaction temperature range (≥800 °C); this superior activity was found despite the two being tested with similar surface areas and metal dispersions after the catalytic reactions. The higher activity of Pt on anatase is in accordance with the abundance of metallic Pt (Pt⁰) found for this catalyst, which favors the dissociative adsorption of SO₃ and the fast removal of the products (SO₂ and O₂) from the surface. Conversely, Pt was easily oxidized to the much less active PtO₂ (Pt⁴⁺), with the strong interactions between the oxide and rutile TiO₂ forming a fully coherent interface that limited the active sites. A long-term stability test of Pt/TiO₂-A conducted for 1000 h at 600 °C demonstrated that there was no indication of noticeable deactivation (activity loss ≤ 4%) over the time period; this was because the phase transformation from anatase to rutile was completely prevented. The small amount of deactivation that occurred was due to the sintering of Pt and TiO₂ and the loss of Pt under the harsh reaction atmosphere

Macroporous Supported Cu–V Oxide as a Promising Substitute of the Pt Catalyst for Sulfuric Acid Decomposition in Solar Thermochemical Hydrogen Production

The macroporous supported Cu–V oxide prepared by a novel dissolution–reprecipitation process was found to be the first example of a promising substitute of Pt catalysts for sulfuric acid decomposition at moderate temperatures (∼600 °C), which is required in solar thermochemical hydrogen production. Stepwise impregnation of Cu­(NO₃)₂ and NH₄VO₃ onto 3-D ordered mesoporous SiO₂, and subsequent heating at 650 °C yielded the deposition of copper pyrovanadate (Cu₂V₂O₇, melting point: 780 °C) not only in mesopores but also on the external surface. Thermal aging at 800 °C caused the congruent melting of Cu₂V₂O₇ followed by smooth penetration of the melt into mesopores and homogeneous covering of cavity walls. Because of the solubility of SiO₂ into the molten vanadate, dissolution–reprecipitation should be equilibrated to allow substantial structural conversion from mesoporous to macroporous SiO₂ frameworks. The resulting macroporous catalyst consisting of highly dispersed thin layers of active Cu₂V₂O₇ is considered efficient for catalytic reactions and the mass transfer of reactants and products in the presence of high-concentration vapors

Platinum Supported on Ta₂O₅ as a Stable SO₃ Decomposition Catalyst for Solar Thermochemical Water Splitting Cycles

Platinum supported on Ta₂O₅ was found to be a very active and stable catalyst for SO₃ decomposition, which is a key reaction in solar thermochemical water splitting processes. During continuous reaction testing at 600 °C for 1,800 h, the Pt/Ta₂O₅ catalyst showed no noticeable deactivation (activity loss ≤ 1.5% per 1,000 h). This observed stability is superior to that of the Pt catalyst supported on anatase TiO₂ developed in our previous study and to those of Pt catalysts supported on other SO₃-resistant metal oxides Nb₂O₅ and WO₃. The higher stability of Pt/Ta₂O₅ is due to the abundance of metallic Pt (Pt⁰), which favors the dissociative adsorption of SO₃ and the smooth desorption of the products (SO₂ and O₂). This feature is in accordance with a lower activation energy and a less negative partial order with respect to O₂. Pt sintering under the harsh reaction environment was also suppressed to a significant extent compared to that observed with the use of other support materials. Although a small fraction of the Pt particles were observed to have grown to more than several tens of nanometers in size, nanoparticles smaller than 5 nm were largely preserved and were found to play a key role in stable SO₃ decomposition

Local Structures and Catalytic Ammonia Combustion Properties of Copper Oxides and Silver Supported on Aluminum Oxides

The local structures and catalytic NH₃ combustion properties of copper oxides (CuO_<i>x</i>) and silver (Ag) catalysts supported on aluminum oxides (Al₂O₃) were studied. In order to achieve high catalytic NH₃ combustion activity and high N₂ (low N₂O/NO) selectivity, the preparation conditions for impregnated binary catalysts were optimized. In comparison with the single CuO_<i>x</i>/Al₂O₃ and Ag/Al₂O₃, binary CuO_<i>x</i>–Ag supported on Al₂O₃ showed high performance for catalytic NH₃ combustion. Among the binary catalysts, sequentially impregnated CuO_<i>x</i>/Ag/Al₂O₃ exhibited the highest activity and N₂ selectivity. Because the combustion activity is closely associated with the Ag–Ag coordination number estimated from Ag K-edge XAFS, highly dispersed Ag nanoparticles supported on Al₂O₃ are considered to play a key role in the low-temperature light-off of NH₃. CuO_<i>x</i>/Ag/Al₂O₃ also showed higher N₂ (lower NO) selectivity for temperatures at which NH₃ conversion reached approximately 100%, indicating that the N₂ is directly produced from the NH₃ combustion reaction over CuO_<i>x</i>/Ag/Al₂O₃. Based on several analyses, a reaction mechanism for catalytic NH₃ combustion over CuO_<i>x</i>/Ag/Al₂O₃ was finally suggested

Copper Oxides Supported on Aluminum Oxide Borates for Catalytic Ammonia Combustion

The catalytic NH₃ combustion properties and local structures of copper oxides (CuO_<i>x</i>) supported on aluminum oxide borates (Al₂₀B₄O₃₆, 10Al₂O₃·2B₂O₃: 10A2B) were studied by means of high-angle annular dark-field scanning transmission electron microscopy, energy dispersive X-ray mapping, X-ray absorption fine structure, X-ray photoelectron spectroscopy, gas adsorption techniques, etc. Among the CuO_<i>x</i> supported on various metal oxide materials, CuO_<i>x</i>/10A2B exhibited high catalytic NH₃ combustion activity, highest N₂ (lowest N₂O·NO) selectivity, and high thermal stability. Because the combustion activity is closely associated with the reducibility and dispersion of CuO_<i>x</i>, highly dispersed CuO_<i>x</i> nanoparticles on supports are considered to play a key role in the low temperature light-off of NH₃. For NO and N₂O selectivities, the oxidation state of CuO_<i>x</i> and the dissociative species of adsorbed NH₃ are suggested to be important catalytic combustion properties, respectively. On the basis of these discussions, the reaction mechanism of catalytic NH₃ combustion over CuO_<i>x</i>/10A2B is described

Structures and Catalytic Properties of Cr–Cu Embedded CeO₂ Surfaces with Different Cr/Cu Ratios

The effects of the Cr/Cu ratios of Cr–Cu-embedded CeO₂ surfaces (0.065 wt % Cu loading) on their local structures and catalytic activities were studied using experimental and theoretical approaches. The sample with a weight ratio of Cr/Cu = 1, which was prepared by wet impregnation followed by subsequent thermal aging at 900 °C for 25 h, showed catalytic activity higher than that of the Cu/CeO₂ sample in both CO–O₂ and CO–NO reactions. The activity of the catalyst was enhanced by increasing the Cr/Cu ratio. The highest activity occurred for a Cr/Cu ratio of around 3, and after it had been thermally aged, its activity was superior to that of Rh/CeO₂. Having more Cr than Cu increases the surface concentration of the Cu⁺ sites, which promotes CO adsorption and its reaction with surface O atoms. As-formed surface O vacancies are filled by the dissociative adsorption of O₂ and/or NO. At the optimum composition, almost all of the Ce sites on the outermost layer are replaced by Cr and Cu, and oxidative chemisorption of CO and NO as carbonate and nitrate/nitrite, respectively, on the CeO₂ surface becomes difficult. This situation enables more efficient dissociation of adsorbed NO and faster desorption of CO₂, thereby leading to higher catalytic activity. Isocyanate species (NCO) that form on the Cu⁺ sites are a possible reaction intermediate for the CO–NO reaction

Redox Dynamics of Rh Supported on ZrP₂O₇ and ZrO₂ Analyzed by Time-Resolved In Situ Optical Spectroscopy

In situ time-resolved diffuse reflectance spectroscopy was first applied to supported Rh catalysts (0.4 wt % Rh/ZrO₂ and Rh/ZrP₂O₇) under dynamic three-way catalysis conditions fluctuating between fuel-lean and fuel-rich gas atmospheres. The optical absorption at 650 nm was found to decrease upon lean-to-rich switching of the gas feed, which led to the reduction of Rh oxide (Rh³⁺) to metallic Rh (Rh⁰), followed by a reversible increase upon back switching rich-to-lean. The kinetic analysis suggested that the reduction of Rh³⁺ to Rh⁰ was faster than the reoxidation over Rh/ZrP₂O₇, whereas the reduction was comparable with or slower than the reoxidation over Rh/ZrO₂. The activation energy of Rh/ZrP₂O₇ for the reduction, 13.6 kJ mol^–1, was smaller than that for the oxidation, 48.7 kJ mol^–1, which contrasted with those of Rh/ZrO₂ (21.4 and 34.1 kJ mol^–1, respectively). These results were closely associated with the higher NO reduction activity of Rh/ZrP₂O₇ than Rh/ZrO₂ under a lean-gas atmosphere because Rh was more active in the metallic state than in the oxide state. Applying fast lean–rich perturbation of the gas feed with 1 s intervals led to an immediate and significant drop of the optical absorption intensity, suggesting that the reduction of Rh substantially penetrated to deeper layers under the surface. This study provided the first in situ evidence for the formation of active metallic Rh species under high-frequency lean–rich oscillations

Unusual Redox Behavior of Rh/AlPO₄ and Its Impact on Three-Way Catalysis

The influence of the redox behavior of Rh/AlPO₄ on automotive three-way catalysis (TWC) was studied to correlate catalytic activity with thermal stability and metal–support interactions. Compared with a reference Rh/Al₂O₃ catalyst, Rh/AlPO₄ exhibited a much higher stability against thermal aging under an oxidizing atmosphere; further deactivation was induced by a high-temperature reduction treatment. In situ X-ray absorption fine structure experiments revealed a higher reducibility of Rh oxide (RhO_<i>x</i>) to Rh, and the metal showed a higher tolerance to reoxidation when supported on AlPO₄ compared with Al₂O₃. This unusual redox behavior is associated with an Rh–O–P interfacial linkage, which is preserved under oxidizing and reducing atmospheres. Another effect of the Rh–O–P interfacial linkage was observed for the metallic Rh with an electron-deficient character. This leads to the decreasing back-donation from Rh <i>d</i>-orbitals to the antibonding π* orbital of chemisorbed CO or NO, which is a possible reason for the deactivation by high-temperature reduction treatments. On the other hand, surface acid sites on AlPO₄ promoted oxidative adsorption of C₃H₆ as aldehyde, which showed a higher reactivity toward O₂, as well as NO, compared with carboxylate adsorbed on Al₂O₃. A precise control of the acid–base character of the metal phosphate supports is therefore a key to enhance the catalytic performance of supported Rh catalysts for TWC applications

Rhodium Nanoparticle Anchoring on AlPO₄ for Efficient Catalyst Sintering Suppression

Rhodium catalysts exhibited higher dispersion with tridymite-type AlPO₄ supports than with Al₂O₃ 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 AlPO₄-supported 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 AlPO₄ surface was terminated by phosphate P–OH groups, which were converted into a Rh–O–P linkage when Rh oxide (RhO_<i>x</i>) was loaded. This interaction enables the thin planar RhO_<i>x</i> nanoparticles to establish close and stable contact with the AlPO₄ surface. It differs from Rh–O–Al bonding in the oxide-supported catalyst Rh/Al₂O₃, 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_<i>x</i>/AlPO₄ model