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

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

Phase-Dependent Formation of Coherent Interface Structure between PtO₂ and TiO₂ and Its Impact on Thermal Decomposition Behavior

This study investigated the thermal decomposition behaviors of platinum oxide (PtO₂) nanoparticles deposited on polycrystalline TiO₂ in different crystal phases. The dissociation of PtO₂ to metallic platinum in air occurred at 400 °C on anatase TiO₂ (Pt/TiO₂-A), but required 650 °C or higher on rutile TiO₂ (Pt/TiO₂-R). The higher thermal stability of PtO₂ on rutile TiO₂ is caused by thermodynamic effect and rather than kinetic effect. In contrast to the thermodynamic prediction, metallic Pt (Pt⁰) on TiO₂-R was reversibly oxidized to PtO₂ (Pt⁴⁺) at 650 °C. This behavior was attributed to the coherent interface structure formed by strong interactions between PtO₂ and rutile TiO₂, as revealed by combined extended X-ray adsorption spectroscopy (EXAFS) and density functional theory (DFT) studies. At the optimized interface structure, between the (100) planes of α-PtO₂ and rutile TiO₂, the interface formation energy was −17.04 kJ mol^–1 Å^–2 versus −9.84 kJ mol^–1 Å^–2 in the anatase TiO₂ model. The larger interface formation energy provides a stabilizing effect against PtO₂ dissociation. Therefore, the widely used Pt-loaded rutile TiO₂ typifies the interfacial interactions under an oxidizing atmosphere, which differ from the strong metal–support interactions prevailing under a reducing atmosphere