SO3 decomposition over CuO–CeO2 based catalysts in the sulfur–iodine cycle for hydrogen production

The effect of several catalyst supports with large specific surface area (such as SiC, Al₂O₃, SiC–Al₂O₃–ball, and SiC–Al₂O₃) on catalytic activity was evaluated in this study. CuO–CeO₂ supported on SiC–Al₂O₃ exhibited high stability and activity, which was considerably close to the thermodynamic equilibrium curve at 625 °C during the stability test for 50 h. The SO₃ decomposition temperature decreased from 750 °C to 625 °C. SiC–Al₂O₃contained numerous micropores and mesopores and had a large specific area, indicating strong adsorption, as determined by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and nitrogen adsorption measurement. X-ray photoelectron spectroscopy (XPS) revealed that the surface of SiC–Al₂O₃consisted of Al₂O₃, SiC, and SiO₂ and that the cerium oxide surface had the largest number of defects. Temperature-programmed reduction (H₂-TPR) results indicated that the cerium–copper oxides on the surface of powdered SiC–Al₂O₃ had the strongest redox potential and that CuO had the lowest reduction temperature.     

Experimental study and development of an improved sulfur–iodine cycle integrated with HI electrolysis for hydrogen production

The sulfur–iodine (SI or IS) thermochemical cycle assembled with solar or nuclear energy has been proposed as a large-scale, clean and renewable hydrogen production method. In present work, an improved SI cycle integrated with HI electrolysis for hydrogen production was developed according to experiments and simulation. The mathematical models of HI electrolysis using proton exchange membrane (PEM) electrolytic cell was developed, and then the user-defined module of HI electrolysis was set up through Aspen Plus and verified by experimental data. After designing and simulating the new flowsheet of the SI cycle based on HI electrolysis, 10 L/h of H₂ and 5 L/h of O₂ were obtained. The theoretic thermal efficiency of flowsheet reached 25–42% in terms of the utilization of waste heat. An ideal thermal efficiency of 33.3% through the proper internal heat exchange in the flowsheet was determined. Sensitivity analyses of parameters in the system were conducted. Increasing proton transfer number of PEM electrolytic cell in HI section improved the thermal efficiency of SI cycle. The ratio of distillate to feed rate and the plate number of distillation column in H₂SO₄ section were the most sensitive factors to the heat duty of overall SI cycle. The proposed new flowsheet for SI cycle is competitive to the flowsheets previously proposed in the field of flowsheet simplification.     

Ceria as a catalyst for hydrogen iodide decomposition in sulfur–iodine cycle for hydrogen production

In this work, ceria (CeO₂) prepared with different methods and at various calcination temperatures have been tested to evaluate their effect on hydrogen iodide (HI) decomposition in sulfur–iodine (SI) cycle at various temperatures. The CeO₂ catalysts' strongly enhance the HI decomposition by comparison with blank test, especially gel CeO₂ 300. TG–FTIR, BET, XRD, TEM and TPR were performed for catalysts' characterization. The results show that the CeO₂ catalyst synthesized by citric-aided sol–gel method and calcined at low temperature (<500 °C) shows more lattice defects, smaller crystallites, larger surface area and better reducibility. Oxygen can promote the significantly rapid surface reaction, but simultaneously consume hydrogen species (H) contained in HI. Lattice defects, especially the reduced surface sites, i.e., Ce³⁺ and oxygen vacancy, play the dominant role in surface reactions of HI decomposition. A new reaction mechanism for HI catalytic decomposition over CeO₂ catalyst is proposed.     

Influence of the structural and surface characteristics of activated carbon on the catalytic decomposition of hydrogen iodide in the sulfur–iodine cycle for hydrogen production

Coal-based activated carbon (AC-COAL) catalysts subjected to acid treatment were tested to evaluate their performance on hydrogen-iodide (HI) decomposition for hydrogen production in sulfur-iodine (SI or IS) cycle. The effects of acid treatment on catalysts and the relations between sample properties and catalytic activities were discussed. The AC-COAL obtained by non-oxidative acid treatments had the best catalytic activity. However, the catalytic activity of AC-COAL decreased after the treatment of nitric acid. Higher surface area, higher carbon contents, lower ash contents and fewer surface oxidation groups contributed to the catalytic activity of ACs. HI decomposition on the AC surface itself may be due to high densities of unpaired electrons associated with structural defects and edge plane sites with similar structural ordering. Moreover, the oxygen-containing groups reduced the electron transfer capability associated with the basal plane sites.     

Experimental study on the purification of HIx phase in the iodine–sulfur thermochemical hydrogen production process

Purification of hydriodic phase (HIx) plays an important role in avoiding the undesirable side reactions between HI and H₂SO₄ in the sulfur–iodine thermochemical cycle. In this paper, a series of experiments on HIx phase purification were conducted by means of a stirred reactor using N₂ as stripping gas. The effects of the iodine concentration, reaction temperature, and the striping gas flow rate on HIx phase purification were investigated systematically in terms of the conversion of H₂SO₄ and the reaction types during purification. It was observed that the iodine concentration played a significant role in dictating the reactions during purification. The quantitative analysis of the compositions of the initial and purified HIx phases showed that not only the conversion of H₂SO₄ was enhanced but also the side reactions were effectively impeded by increasing the iodine concentration, temperature and the stripping gas flow rate. Based on the experimental data, the suitable operating conditions for HIx phase purification were proposed.     

Numerical analysis of the electrochemical Bunsen reaction for hydrogen production from sulfur–iodine cycle

The sulfur–iodine (S-I) water-splitting cycle is one of the most promising hydrogen production methods. The Bunsen reaction in the cycle affects the flowsheet complexity and thermal efficiency, but an electrochemical technique has recently been applied to make the S-I cycle more simplified and energy efficient. However, the performance of the electrochemical Bunsen reaction, especially the electrode reactions inside the electrolytic cell (EC) are not clear at present. In this work, a two-dimensional numerical model of EC was developed. The detailed reaction process was numerically calculated with considering the coupling of mass transfer and electrochemical reactions, and was verified using experimental data. The effects of various operating parameters on the reactions were investigated. The results showed that the increase of current density significantly improves the conversion rates of reactants. A higher temperature is unfavorable for concentrating H₂SO₄ and HI. Increase in the inlet flow rate reduces the conversion rates of reactants, but the impact declines with further rising flow rate. An optimal operating condition is also proposed. The theoretical simulation study will provide guidance for the improvement of experimental work.     

The HI catalytic decomposition for the lab-scale H2 producing apparatus of the iodine–sulfur thermochemical cycle

The HI decomposition is the key reaction to produce hydrogen in the iodine–sulfur thermochemical cycle. In this paper, the HI catalytic decomposition for the lab-scale H₂ producing apparatus of IS-10 (The H₂ production rate is 10 L/h) in INET (Institute of Nuclear and New Energy Technology, Tsinghua University) was studied. The effects of the different supports (carbon nanotubes, active carbon, carbon molecular sieve, graphite and Al₂O₃), mass of catalyst and reaction temperature on the decomposition of HI were investigated. Also, the fresh and used active carbon supported platinum catalysts were characterized by XRD, BET and TEM. The experiment results showed that the active carbon and carbon molecular sieve had the higher catalytic activity for HI decomposition than other supports. The active carbon was selected to support platinum as the catalyst to catalyze the HI decomposition in the IS-10. In the closed cycle operation, the conversion of HI over the active carbon supported platinum catalyst was more than 20% which was near the thermodynamic equilibrium value. The results of the characterization about the fresh and used active carbon supported platinum catalysts indicated that the specific surface area decreased and the Pt particles size increased, which showed the stability of the catalyst should be improved.     

Hydrogen iodide decomposition over nickel–ceria catalysts for hydrogen production in the sulfur–iodine cycle

In this work, pure CeO₂ and three nickel–ceria catalysts prepared by different methods have been tested to evaluate their effect on hydrogen iodide (HI) decomposition in the sulfur–iodine (SI or IS) cycle at various temperatures. BET, XRD, HRTEM and TPR were performed for catalysts characterization. Indeed, the pure CeO₂ also strongly enhance the decomposition of HI to H₂ by comparison with blank yield. Nickel–ceria catalysts show better catalytic activity, especially Ni-doping-G sample. It is found that, through the sol-gel method, the Ni²⁺ ions have dissolved into the ceria lattice instead of the Ce⁴⁺ ions during the synthesis process of Ni-doping-G sample. Oxygen vacancies are formed because of the charge imbalance and lattice distortion in CeO₂. The presence of Ni during the CeO₂ synthesis process of Ni-doping-G also causes smaller average particle size, larger surface area, better thermal stability and better Ni dispersion than the Ni-loading samples. These provide nickel–ceria catalyst with a potential to be used in the SI cycle for HI decomposition.     

Three-dimensional electrochemical Bunsen reaction characteristics in the sulfur–iodine water splitting cycle for hydrogen production

Hydrogen production from the sulfur–iodine water splitting cycle integrated with solar or nuclear energy has been proposed as a promising technique. Bunsen reaction is one of the three main steps in the cycle and electrochemical method has been applied to this reaction. In present work, a three-dimensional numerical study of the electrochemical Bunsen reaction was conducted. A three-dimensional, steady state, laminar and isothermal mathematical model of electrolytic cell was developed and verified by experiments. The spatial maldistribution of species concentration was found between electrodes and proton exchange membrane (PEM). The electric power drives most H₂SO₄ and I₂ to the anode and cathode surface, respectively, while the proton attraction contributes to HI enrichment on the surface of PEM. At the high inlet H₂SO₄ concentration of 50 wt%, the transformation of flow channel from single serpentine to single entry & double serpentine with the same inlet flow rate cannot solve the insufficient problem of SO₂. But the increase of the overall inlet flow rate in the double entry & double serpentine flow channel make SO₂ sufficient for anode reaction. Further decreasing the inlet H₂SO₄ concentration to 40 wt% and 30 wt% make the initial SO₂ sufficient for overall reactions. The single serpentine channel gives the highest SO₂ conversion rate, followed by the single entry & double serpentine and double entry & double serpentine flow channels. The single serpentine flow channel at the H₂SO₄ inlet concentration of 40 wt% is found optimal for achieving a high electrochemical Bunsen reaction performance.     

Stability of nickel catalyst supported by mesoporous alumina for hydrogen iodide decomposition and hybrid decomposer development in sulfur–iodine hydrogen production cycle

We propose a hybrid HI decomposer, which combines a high-temperature catalytic decomposition reactor with a dual bed temperature-swing decomposer. For the high temperature step, we screened and identified a catalyst that is stable above 700 °C by preparing nickel catalysts supported on mesoporous alumina and evaluating their activity toward the high-temperature catalytic decomposition reaction. The catalysts achieved HI decomposition yields up to 23% at 650 °C and maintained over 20% yield after 100 h of operation. For the temperature-swing process, we investigated the adsorption, desorption, and regeneration efficiency, and the optimal regeneration temperature of nickel catalysts supported on silica/alumina adsorbents. The optimal regeneration temperature was 400 °C. Based on these results, we propose hybrid HI decomposers in two configurations: 1) residual HI decomposer and 2) HI concentrator. The residual HI decomposer improves the overall conversion efficiency to levels above the thermodynamic limit, while the HI concentrator increases the concentration of HI by simple temperature control, even at below-azeotropic HI concentrations.     

Comparison of sulfur–iodine and copper–chlorine thermochemical hydrogen production cycles

Sulfur–iodine and copper–chlorine water splitting cycles are promising methods of thermochemical hydrogen production. In this paper, these two cycles are compared from the perspectives of heat quantity, heat grade, thermal efficiency, related engineering challenges, and hydrogen production cost. The heat quantity and grade required by each step of the cycles are evaluated and the thermal efficiencies are approximated from the heat requirements. It is found that the overall heat requirements of the two cycles do not have significant differences and the overall efficiencies of the two cycles are similar, between 37 and 54%, depending on the portion of heat recovery. The copper–chlorine cycle has the advantage of a lower maximum temperature of 803 K, which is 300 K lower than the maximum temperature of 1123 K in the sulfur–iodine cycle. This indicates that the copper–chlorine cycle can link more readily with various heat sources, such as grade Generation IV nuclear and fossil fuel power stations. It is also reported that the copper–chlorine cycle can have fewer challenges of equipment materials and product separation. A cost analysis shows that the copper–chlorine and sulfur–iodine cycles have similar hydrogen production costs, which are lower than steam-methane reforming, and conventional and high temperature electrolysis, due to less use of electricity, no carbon related charges and no methane requirement in the thermochemical cycles.     

Sulfur trioxide electrolysis studies: Implications for the sulfur–iodine thermochemical cycle for hydrogen production

In this paper we describe our efforts to develop a sulfur trioxide (SO₃) electrolyzer that could lower the temperature of the SO₃ decomposition step in the sulfur–iodine and hybrid sulfur thermochemical cycles. The objective is to develop an alternative to the standard process of converting SO₃ to SO₂, which is thermal decomposition at 830 °C and above. Thermodynamic calculations show that high SO₃ conversions can be obtained at 590 °C if oxygen is removed during the SO₃ decomposition stage. One way of achieving oxygen removal during SO₃ decomposition is electrolysis, if suitable electrode and electrolyte materials can be found. Active oxygen electrode materials are already developed and we have demonstrated suitability of a thin doped-zirconia electrolyte in this study. The main difficulty came in the development of an active and stable SO₃ electrode. Using Ga–V–O/NbB₂/Au electrodes we demonstrated high catalytic activity, but could not achieve acceptable electrochemical performance.     

Activated carbon catalysts for the production of hydrogen via the sulfur–iodine thermochemical water splitting cycle

Seven activated carbon catalysts obtained from a variety of raw material sources and preparation methods were examined for their catalytic activity to decompose hydrogen iodide (HI) to produce hydrogen, a key reaction in the sulfur–iodine (S–I) thermochemical water splitting cycle. Activity was examined under a temperature ramp from 473 to 773 K. Within the group of lignocellulosic steam-activated carbon catalysts, activity increased with surface area. However, both a mineral-based steam-activated carbon and a lignocellulosic chemically activated carbon displayed activities lower than expected based on their higher surface areas. In general, ash content was detrimental to catalytic activity while total acid sites, as determined by Boehm's titrations, seemed to favor higher catalytic activity within the group of steam-activated carbons. These results suggest that activated carbon raw materials and preparation methods may have played a significant role in the development of surface characteristics that eventually dictated catalyst activity and stability as well.     

Preliminary process analysis for the closed cycle operation of the iodine–sulfur thermochemical hydrogen production process

In the iodine–sulfur thermochemical hydrogen production process, a separation characteristic of 2-liquid phase (H₂SO₄ phase and HI_x phase) in the separator at 0°C was measured. Two-phase separation began to occur at about 0.32 of I₂ molar fraction and over. The separation characteristic became better with the increase in iodine concentration in the solution. The effect of flow rate variations of HI solution and I₂ solution from the HI_x distillation column on the process was evaluated. The flow rate increase in HI solution from the distillation column did not have a large effect on the flow rate of HI solution fed to the distillation column from the separator. The decreasing flow rate of I₂ solution from the distillation column decreased the flow rate of I₂ solution fed to the distillation column from the separator. The variation of I₂ molar fraction in the H₂SO₄ phase in the separator was sensitive to the variation in flow rate of both solutions from the distillation column. The tolerance level of the variation was investigated by considering I₂ solubility, 2-liquid phase disappearance and SO₂ reaction amount.     

Preparation and characterization of bimetallic Ni–Ir/C catalysts for HI decomposition in the thermochemical water-splitting iodine–sulfur process for hydrogen production

A series of bimetallic 10%Ni-xIr/C (x = 0.5, 1.0, 1.5, 2.0 wt%) and monometallic 10%Ni/C and 2%Ir/C catalysts were prepared through the impregnation–reduction method modified by adding the ionic surfactant hexadecyltrimethylammonium bromide (CTAB) as the stabilizing agent during the impregnation. Their catalytic performance was tested by HI decomposition under atmospheric pressure at 400 °C and 500 °C. X-ray diffraction, Brunauer–Emmett–Teller surface area, transmission electron microscopy, and X-ray photoelectron spectroscopy were adopted to characterize the structure, specific surface area, morphology, and surface chemical state, respectively. Results showed that the addition of Ir metal and the use of CTAB played important roles in enhancing the activity and stability of the Ni-based bimetallic catalysts. Among all the catalysts tested, the bimetallic 10%Ni-1.5%Ir/C catalyst presented excellent activity and stability for HI decomposition.     

Ni/CeO2/ZSM-5 catalysts for the production of hydrogen from the pyrolysis–gasification of polypropylene

The production of hydrogen from the two-stage pyrolysis–gasification of polypropylene using a Ni/CeO₂/ZSM-5 catalyst has been investigated. Experiments were conducted on CeO₂ loading, calcination temperature and Ni loading of the Ni/CeO₂/ZSM-5 catalyst in relation to hydrogen production. The results indicated that with increasing CeO₂ loading from 5 to 30 wt.% for the 10 wt.% Ni/CeO₂/ZSM-5 catalyst calcined at 750 °C, hydrogen concentration in the gas product and the theoretical potential hydrogen production were decreased from 63.0 to 49.8 vol.% and 50.4 to 21.6 wt.%, respectively. In addition, the amount of coke deposited on the catalyst was reduced from 9.5 to 6.2 wt.%. The calcination temperature had little influence on hydrogen production for the catalyst containing 5 wt.% of CeO₂. However, for the 10 wt.% Ni/CeO₂/ZSM-5 catalyst with a CeO₂ content of 10 or 30 wt.%, the catalytic activities reduced when the calcination temperature was increased from 500 to 750 °C. The SEM results showed that large amounts of filamentous carbons were formed on the surface of the catalysts. The investigation of different Ni content indicates that the Ni/CeO₂/ZSM-5 ((2-10)-5-500) catalyst containing 2 wt.% Ni showed poor catalytic activity in relation to the pyrolysis–gasification of polypropylene according to the theoretical potential H₂ production (7.2 wt.%). Increasing the Ni loading to 5 or 10 wt.% in the Ni/CeO₂/ZSM-5 ((2-10)-5-500) catalyst, high potential hydrogen production was obtained.     

Construction materials development in sulfur–iodine thermochemical water-splitting process for hydrogen production

The sulfur iodine (SI) water splitting cycle for hydrogen production consists of three coupled chemical reactions, which includes the generation and decomposition of HI. The _HIx${\mathrm{HI}}_x$ environment is extremely corrosive and the severity increases with temperature. Immersion coupon corrosion screening tests were performed on materials selected from four classes of corrosion resistant materials: refractory metal, reactive metal, superalloys and ceramics. Of the materials tested, only Ta and Nb-based refractory metals and ceramic mullite can tolerate the extreme _HIx${\mathrm{HI}}_x$ environment. Severe pitting and dissolution was observed in two different reactive metal zirconium. A nickel based superalloy, C-276, also showed severe dissolution in _HIx${\mathrm{HI}}_x$ solution. The materials which showed good corrosion behavior will undergo further long-term immersion testing to assess performance. In addition, C-ring, U-bend and DCB test samples fabricated from qualified materials will be tested under stress corrosion conditions to investigate their crack initiation and growth properties.     

Energy and exergy analyses of a novel sulfur–iodine cycle assembled with HI–I2–H2O electrolysis for hydrogen production

A novel sulfur–iodine (SI or IS) cycle integrated with HI–I₂–H₂O electrolysis for hydrogen production was developed and thermodynamically analyzed in this work. HI–I₂–H₂O electrolysis was used to replace the conventional concentration, distillation, and decomposition processes of HI, so as to simplify the flowsheet of SI cycle. And then the new cycle was divided into Bunsen reaction, H₂SO₄ decomposition and HI–I₂–H₂O electrolysis sections. Through incorporating the user-defined module of HI–I₂–H₂O electrolysis with Aspen Plus, the cycle was simulated and 0.448 mol/h (10 L/h) of H₂ was produced. The overall energy and exergy efficiencies of the novel SI system were estimated to be 15.3–31.0% and 32.8%, respectively. Most exergy destruction occurred in the H₂SO₄ decomposer and condenser for H₂SO₄ decomposition and Bunsen reaction sections, which accounted for 93.0% and 63.4%, respectively. A high exergy efficiency of 92.4% for HI–I₂–H₂O electrolysis section with less exergy destruction was determined, mostly due to the transformation of the overall electricity in electrolytic cell to exergy. Appropriate internal heat exchange and waste heat recovery will favor improving the energy and exergy efficiencies.