Performance of Activated-Carbon-Supported Ni, Co, and Ni-Co Catalysts for Hydrogen Iodide Decomposition in a Thermochemical Water-Splitting Sulfur-Iodine Cyclet

Bimetallic Ni-Co/activated carbon (Ni-Co/AC) and monometallic Ni/AC and Co/AC catalysts were prepared to investigate their catalytic activity for hydrogen-iodide decomposition in the sulfur-iodine (SI) cycle. Transmission electron microscopy (TEM) revealed an average size of approximately 3 nm particles for Ni-Co/AC. Ni-Co/AC possesses a higher I-D/I-G intensity ratio in Raman spectroscopy than the monometallic catalysts and support, which is an indication of high degree of defects. Hydrogen-iodide decomposition was performed on a fixed vertical bed quartz reactor at a weight hourly space velocity (WHSV) of 12.9 h(-1) and different temperatures (400-550 degrees C). Bimetallic catalysts exhibited better activity and stability than the monometallic catalysts. The composition of Ni/Co in the bimetallic Ni-Co catalyst played the key role in dictating the activity of catalyst. It was observed that the loading ratio of 3:1 for Ni/Co achieved the maximal hydrogen-iodide conversion value. Bimetallic Ni(3%)-Co(1%)/AC showed excellent time-on-stream stability of 70h for the hydrogen-iodide decomposition reaction. The post-reaction characterization studies (X-ray diffraction and Brunauer-Emmett-Teller surface area measurements) confirmed that the bimetallic Ni-Co/AC catalyst has a stable structure and shows high corrosion resistance against the corrosive hydrogen iodide environment. Also, it was observed that the apparent activation energy of the bimetallic Ni-Co/AC catalyst was smaller than the monometallic Ni and Co catalysts. The effect of iodine on hydrogen-iodide conversion was also studied.