| Hydrogen is considered as an ideal energy vector owing to abundant resource, various source, environmental protection, regeneration, safety and advantageous storage. Therefore, it has greatly attracted worldwide attention. Hydrogen production is the fundamental step for hydrogen utilization. Consequently, developing hydrogen economy depends on large-scale and low-cost hydrogen production. Thermochemical water splitting cycle is a promising technique for hydrogen production featuring cleanness and sustainability. Hydrogen can be achieved by the decomposition of water through a series of related thermal reactions. Over one hundred kinds of thermochemical water splitting cycle have been reported to this day. Iodine sulfur thermochemical cycle is the most promising one due to its superiority with high thermal efficiency, low cost and easy large-scale industrialization.Iodine sulfur thermochemical cycle mainly consists of following three reactions:Bunsen reaction:xI2+SO2+2H2O=2HIx+H2SO4HI decomposition reaction:2HIx=xI2+H2H2SO4 decomposition reaction:H2SO4=SO2+H2O+1/2O2With regard to China, two key factors must be focused on. Pyrites as SO2 source are abundant and inexpensive. On the other hand, byproduct sulfuric acid is valuable and marketable. Therefore, the open-loop iodine sulfur cycle for hydrogen and sulfuric acid production has been proposed. The whole process is composed of following two reactions:Bunsen reaction:xI2+SO2+2H2O=2HIx+H2SO4HI decomposition reaction:2HIx=xI2+H2HI decomposition process is the unique procedure for generating hydrogen in the foregoing two cycles. And its conversion ratio directly influences systemic thermal efficiency and hydrogen production rate. Simulant and experimental results indicate the conversion of HI homogeneous decomposition is extremely low. Hence, the catalyst for improving HI decomposition is intensively applied. The catalysts of CeO2, Pt/Ce-Zr and Ni/Ce-Zr were investigated in the prior study on HI catalytic decomposition. Moreover, the catalytic reaction mechanism has been proposed by the detailed activity estimation and characterization study. The modified treatment including acid, oxidation, heat and metal loaded on activated carbon as HI decomposition catalysts are investigated to get more efficient and inexpensive.The activated carbon catalysts with different raw materials and vapor treatment have been tested to evaluate their effect on HI decomposition. AC-CS and AC-STONE showed the best catalytic performance. And AC-WOOD had the worst catalytic performance. AC-BAMBOO and AC-COAL represented the intermediate catalytic performance comparing with prior activated carbon catalysts. Catalytic characterization results indicated AC-CS and AC-STONE were similarly composed of high carbon content and low ash content. Meanwhile, their pore structures were both advanced. Regarding AC-BAMBOO and AC-COAL, the chemical composition contained high carbon content. Whereas, AC-BAMBOO's pore structure was less advanced, and ash content of AC-COAL was rather high. AC-WOOD contained the lowest carbon content and relatively high ash content.The wooden activated carbon catalysts with different preparation methods have been tested to evaluate their influence on HI decomposition. The results implied that AC-WH the highest catalytic performance, AC-WOOD the worst catalytic performance, and AC-WZ the intermediate catalytic performance. The characterization results showed AC-WH contained more carbon content and less oxygen-containing groups in comparison with AC-WZ. Both of them contained relatively low ash content and similar pore structure. AC-WOOD consisted of lowest carbon content and rather high ash content. Consequently, high carbon content, low ash content, appropriate pore structure and less oxygen-containing groups were in favor of HI catalytic decomposition.The coal based activated carbon catalysts with non-oxidative acid (HCl and HF) treatment have been tested to evaluate their influence on HI decomposition. It was found that COAL-CLF had the best catalytic performance. Characterization results demonstrated that crystallization, pore structure and superficial oxygen-containing groups didn't change significantly. Whereas, the sample ash content decreased obviously. Therefore, decreasing ash content could effectively improve the catalytic performance of activated carbon. The COAL-CLF catalyst with oxidative acid (HNO3) treatment has been tested to evaluate its catalytic performance. The results showed that COAL-CLF had the supreme catalytic performance. Characterization results indicated that superficial oxygen-containing groups and acidity increased by continuous oxidization. High temperature treatment easily resulted in specific surface area and pore volume decrease. Hence, oxidation treatment could depress the catalytic performance of activated carbon. Phosphoric acid activation wooden activated carbons with heat treatment at nitrogen or hydrogen atmosphere have been tested. Then samples with and without treatment were characterized and evaluated. The catalytic activity results showed that the catalytic performance improved as the heat treatment temperature increase. The characterization results demonstrated that crystallization and pore structure didn't change obviously due to high thermal stability. While superficial oxygen-containing groups decreased and alkalinity increased. Therefore, heat treatment enhanced the catalytic performance. Moreover, HI catalytic decomposition mechanism via activated carbon has been proposed.Five kinds of different physicochemical activated carbons as vector with Pd modified load have been tested. Activity and characterization results showed that Pd/C catalyst manifested the higher catalytic performance in HI decomposition reaction in comparison with activated carbon without Pd load. The influence of Pd dispersion on HI catalytic decomposition was greater than physicochemical characterization of activated carbon. The coal based activated carbons as vector with Pd load by HCl or HF non-oxidative acid treatment have been tested. Activity and characterization results demonstrated that Pd dispersion had the greater effect on HI catalytic decomposition in comparison with ash content. The coal based activated carbons as vector with Pd load by different temperatures and HNO3 oxidative acid treatment have been tested. The results indicated that the catalytic performance difference of activated carbons without Pd load was reduced by Pd dispersion variety. Moreover, HI catalytic decomposition mechanism via Pd/C catalyst has been proposed.Five kinds of different physicochemical activated carbons as vector with Ru modified load have been tested. Activity and characterization results showed that Ru/C catalyst manifested the higher catalytic performance in HI decomposition reaction in comparison with activated carbon without Ru load. Ru load manifested relatively high dispersion on each activated carbon. Therefore, activated carbons with Ru modified load had the outstanding anti-sintering property. The coal based activated carbons as vector with Ru load by HCl, HF or HNO3 treatment have been tested. Activity and characterization results demonstrated that Ru dispersion didn't change greatly by different treatments. Meanwhile, the influence of Ru dispersion on HI catalytic decomposition was less than physicochemical characterization variety of activated carbon. Five kinds of different physicochemical activated carbons as vector with Ni modified load have been tested. The results indicated that Ni sintering and agglomeration on activated carbon surface was extremely serious at high temperature. Then the agglomerated large particles blocked up partial pores. Therefore, Ni/C catalytic performance was not advanced than activated carbon's one. Catalytic performance of activated carbon with Ni load even slightly worse at low temperature comparing with activated carbon without Ni load. Moreover, HI catalytic decomposition mechanism via Ru/C catalyst has been proposed.
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