Sulphur based thermochemical cycles: Development and assessment of key components of the process

et al. 2016

Ind. Eng. Chem. Res.

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.

Section: Introductionmentioning

confidence: 99%

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

Kawada

Sueyoshi

et al. 2016

Ind. Eng. Chem. Res.

“…Even for solar heat, several research groups have been studying SO 3 decomposition at a higher temperature (≥800 °C), which can be generated by solar-tower and solar-dish collectors. , Although higher temperatures tend to favor greater efficiency, the practical operation temperature should be restricted below ≤650 °C due to material limitations of thermochemical reactors and heat fluid. In order for reaction to proceed with the use of a heated fluid supplied by solar-trough collectors (∼600 °C), active and stable SO 3 decomposition catalysts that work at much more moderate temperatures are necessary. − It should also be noted that lower temperature leads to degradation of the SO 3 conversion and thermal efficiency of the cycle; the equilibrium conversion of SO 3 to SO 2 is below 40% at 600 °C, and thus an equilibrium-shift reactor is indispensable. Catalytic-membrane reactors are potential candidates for such advanced reactors, and they enable O 2 to be separated from a catalyst bed so that the forward SO 3 decomposition reaction can be favored and the reaction efficiency improved.…”

Section: Introductionmentioning

confidence: 99%

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

Nur

ACS Appl. Energy Mater.

Funada

et al. 2018

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.

“…It has been reported that two classes of materials are candidates for active SO 3 decomposition catalysts that work at around 600 °C. One is precious metals, the following sequence of their activities was reported: Pt > Pd > Rh > Ir > Ru. ,− Because metallic Pt promotes the dissociation of adsorbed SO 3 into SO 2 and O and the subsequent desorption these decomposition products most efficiently, supported Pt catalysts are promising for the required temperature range. ,− Stable Pt catalysts can be prepared by depositing Pt onto SO 3 -resistant support materials, including TiO 2 , ,,, SiO 2 , , SiC, − and Ta 2 O 5 . Although these supported Pt catalysts demonstrate excellent catalytic activities, their widespread use should be limited due to the high cost and rarity.…”

Section: Introductionmentioning

confidence: 99%

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

Nur

ACS Appl. Energy Mater.

Ikematsu

et al. 2018

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.