No abstract

Pt-loaded anatase TiO 2 (Pt/TiO 2 -A) was found to be a highly active and stable catalyst for SO 3 decomposition at moderate temperatures (∼600 °C), which will prove to be the key for solar thermochemical water-splitting processes used to produce H 2 . The catalytic activity of Pt/TiO 2 -A was found to be markedly superior to that of a Pt catalyst supported on rutile TiO 2 (Pt/TiO 2 -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 (Pt 0 ) found for this catalyst, which favors the dissociative adsorption of SO 3 and the fast removal of the products (SO 2 and O 2 ) from the surface. Conversely, Pt was easily oxidized to the much less active PtO 2 (Pt 4+ ), with the strong interactions between the oxide and rutile TiO 2 forming a fully coherent interface that limited the active sites. A long-term stability test of Pt/TiO 2 -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 TiO 2 and the loss of Pt under the harsh reaction atmosphere.

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.

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.

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.