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

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

Role of Oxygen Vacancies in Catalytic SO₃ Decomposition over Cu₂V₂O₇ in Solar Thermochemical Water Splitting Cycles

We report the structure–activity relationship of copper pyrovanadate (Cu₂V₂O₇) as an efficient catalyst for SO₃ decomposition in solar thermochemical water splitting cycles. Of the α, β, and γ polymorphs of Cu₂V₂O₇, the α-phase, which has a blossite-type structure, was stable under the catalytic reaction conditions. Spontaneous oxygen desorption accompanied by charge compensation through the reduction of Cu²⁺ to Cu⁺ produced an oxygen deficiency corresponding to Cu₁₆V₁₆O₅₅ at 600 °C. Density functional theory calculations based on these results showed that oxygen vacancy formation is more favorable on the Cu–O–V bridging sites than on the V–O–V site in the pyrovanadate unit. The oxygen vacancy formation energy of the (100) surface is considerably less than that of bulk Cu₁₆V₁₆O₅₆. The reaction, Cu₁₆V₁₆O₅₅ + SO₃ → Cu₁₆V₁₆O₅₆ + SO₂, is exothermic, suggesting that oxygen vacancies play a key role in catalytic SO₃ decomposition over a Cu₂V₂O₇ catalyst

Surface Properties of Rh/AlPO₄ Catalyst Providing High Resistance to Sulfur and Phosphorus Poisoning

A rhodium catalyst supported on AlPO₄ exhibited a much higher resistance to sulfur and phosphorus poisoning compared with a reference catalyst (Rh/Al₂O₃). The acidic surface of AlPO₄ was effective in preventing the adsorption of sulfur oxides (SO₂), whereas Lewis acid/base sites on Al₂O₃ favored SO₂ adsorption followed by the formation of sulfite, leading to deterioration of the activity of Rh/Al₂O₃ for the model NO–CO–C₃H₆–O₂ reaction. Similarly, the AlPO₄ support suppressed the extent of phosphorus poisoning caused by dimethylphosphite (DMP) (CH₃O)₂POH, which was used as a model phosphorus source. A greater amount of inactive phosphate overlayers were deposited from the gas feed containing DMP and O₂ on Rh/Al₂O₃ than Rh/AlPO₄ because of the reaction between P₂O₅ vapors and Al₂O₃. Consequently, the active Rh surface was covered to a greater extent for Rh/Al₂O₃ than Rh/AlPO₄

Stability of Molten-Phase Cs–V–O Catalysts for SO₃ Decomposition in Solar Thermochemical Water Splitting

Supported molten cesium vanadate catalysts (Cs–V–O/SiO₂) showed activities comparable to that of a reference Pt catalyst (1 wt % Pt/TiO₂) for SO₃ decomposition at moderate temperatures (∼600 °C), which is essential as an O₂ evolution reaction in solar thermochemical water splitting cycles. Stability testing of the catalyst over a 1000 h continuous reaction at 600 °C resulted in deactivation by ∼20% of the initial activity. Kinetic analysis of the activity versus time-on-stream indicated that the observed deactivation behavior can be divided into an induction period (≤100 h) and an acceleration period (>100 h). The deactivation is mainly caused by the vaporization loss of active components (Cs and V) from the molten phase. At the earliest stage, most vapor is generated in the upstream section of the catalyst bed and then redeposits therebelow. Upon repeating these vaporization and deposition cycles, Cs and V move gradually downstream. During this induction period, the deactivation is not obvious because the total Cs and V content of the catalyst bed remains almost unchanged. After this period, however, detachment of Cs and V from the downstream end of the catalyst bed induces accelerated deactivation. The vaporization loss was found to be significantly suppressed by inverting the catalyst bed every 100 h during the stability test. Consequently, this operation reduced the extent of catalyst deactivation from 20% to less than 10% of the initial activity

Selective Formation of Cu Active Sites with Different Coordination States on Pseudospinel CuAl₂O₄ and Their NO Reduction Catalysis

In the spinel framework, copper (Cu) in two distinct coordination states exhibits catalytic activity for NO reduction through different mechanisms. However, detailed exploration of their respective catalytic properties, such as the redox behavior of Cu and substrate molecule adsorption, has been challenging due to difficulties in their separate formation. In this study, we present the controlled formation of pseudospinel CuAl2O4, containing exclusively tetrahedrally or octahedrally coordinated Cu, achieved by manipulating aging temperature and O2 concentration. Through these materials, we observed that in the CO–NO reaction, the step primarily determining the rate differs: NO reduction dominates with octahedrally coordinated Cu, whereas carbon monoxide (CO) oxidation is prominent with tetrahedrally coordinated Cu. The lower coordination number of Cu significantly benefits NO reduction but negatively impacts the CO–NO reaction, albeit positively influencing NO reduction in three-way catalytic reactions

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

Catalytic SO₃ Decomposition Activity and Stability of A–V–O/SiO₂ (A = Na, K, Rb, and Cs) for Solar Thermochemical Water-Splitting Cycles

SiO₂-supported molten alkaline metal oxides (A–V–O/SiO₂) were studied as SO₃ decomposition catalysts for solar thermochemical water splitting. Their catalytic activities at moderate temperatures (≤600 °C), which were superior to those of Cu–V–O/SiO₂ catalysts, were dependent on A, exhibiting the following sequence: Cs > Rb > K > Na. These activities increased with the A/V ratio. This result is in accordance with the basicity, which favors the adsorption of SO₃ to form sulfate. Another important effect of A is to form molten liquid phases, which dissolve the sulfate and facilitate its decomposition to SO₂/O₂. However, the molten phase with high A/V ratios led to the collapse of the porous SiO₂ structure by a corrosion effect. Consequently, the highest catalytic activity was achieved at a composition of A/V ≈ 1.0 for A = K and Cs. The long-term stability test of K–V–O/SiO₂ at 550 °C demonstrated no indication of noticeable deactivation during the first 100 h, whereas 20% deactivation occurred during the following 400 h. The deactivation mechanism involves the vaporization loss of active components from the molten phase, which is accelerated in the presence of SO₃

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