Shortlisting of catalyst oxides for the sulphur trioxide splitting (STS) reaction in a sulphur-based, efficient thermochemical cycle for long-term solar energy storage
Loading...
Date
2025-07-04
Authors
Μπαλτανίδης, Ηλίας
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
The urgent need to reduce greenhouse gas emissions and limit the effects of climate change has intensified the global pursuit of decarbonization strategies, particularly through the development of sustainable and large-scale energy storage systems. Among emerging solutions, solar thermo-chemical energy storage (STES) offers a promising pathway for long-duration renewable energy storage without CO₂ emissions. The SULPHURREAL project proposes a novel solar-driven closed thermochemical cycle based on sulphur chemistry, where sulphuric acid is decomposed and regener-ated to enable the efficient capture and release of solar energy. A key step in this cycle is the hetero-geneous catalytic decomposition of sulphur trioxide (SO₃) into sulphur dioxide (SO₂) and oxy-gen (O₂), an endothermic reaction with significant energy requirements. The present thesis focuses on the experimental evaluation of catalysts for the SO₃ splitting reaction (STS), aiming to identify the most efficient formulations for future integration into the SULPHURREAL cycle.
A total of 31 promising catalytic samples were evaluated under dry conditions in a fixed-bed reactor. These samples were selected from an initial set of 60 based on prior physicochemical charac-terization, which was conducted in collaboration with CERTH. The performance of each catalyst was assessed using reaction rate measurements and Arrhenius-based analysis to determine activation en-ergies. Both medium-temperature (MT) and high-temperature (HT) catalysts were studied across temperature ranges relevant to the STS process.
Among the MT catalysts where V2O5, Cu-V and Ce-V mixed oxides were tested, Cu-V mixed ox-ides supported on kaolinite or SiO₂ showed the highest reaction rates, with kaolinite-supported sam-ples exhibiting superior performance. These catalysts achieved apparent rates of 100–600 NmL(SO₃)/(gcat·min) in majority and demonstrated a favorable balance between activity and ther-mal stability.
For HT applications, catalysts containing CuO, Fe₂O₃, and Al₂O₃, both supported and unsupport-ed, were examined. Notably, one CuO-based catalyst supported on SiO₂ achieved the highest recorded rate of 6000 NmL(SO₃)/(gcat·min). Following this, Fe-Cu-Al mixed oxide catalysts also demonstrated very high efficiency, ranking among the most promising in the HT category. Turnover frequency (TOF) comparisons further supported these trends.
The experimental work was conducted using a custom-built dual-reactor system designed to generate SO₃ in house and quantify conversions using online UV-Vis spectrophotometry. The reac-tor system was fully commissioned, tested for leaks, and optimized for thermal and flow stability. Ab-sorbance measurements were used to calculate SO₃ conversion yields and derive kinetic parameters.
Overall, this thesis presents a comprehensive kinetic evaluation of a wide range of metal oxide-based catalysts for SO₃ decomposition, identifying the most promising formulations for application in solar thermochemical storage cycles. The findings provide valuable insight into the structure–activity relationship of STS catalysts and offer a pathway toward the development of efficient and scalable catalytic systems for renewable energy integration.
Description
Keywords
Sulphur trioxide splitting, Sulphurreal, Catalyst shortlisting, Sulfur-based thermochemical cycles, Long-term solar energy storage