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 degrees 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 (Pt-0), which favors the dissociative adsorption of SO3 and the smooth desorption of the products (SO2 and O-2). This feature is in accordance with a lower activation energy and a less negative partial order with respect to O-2. 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.