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.     

Comparison of various circuit designs in the HI decomposition section of the iodine–sulfur process

The hydriodic acid decomposition section (HI_x section) of the iodine–sulfur water splitting hydrogen production process (I–S process), to a large extent, determines the efficiency of the whole process. The flowsheet of the HI_x section includes several internal circuits, which can recover the lost HI due to the distillation of the azeotrope and the low one-pass conversion ratio of the HI decompostion reaction. The employment of the circuits is capable of increasing the utilization of the input HI. However, different designs of the circuits influence the mass and heat balance of the flowsheet, and furthermore result in distinctions of the hydrogen production efficiency and the required input rate. In order to examine the performance of the HI_x section flowsheets with diverse internal circuits and pursue high efficiency, the present research used simulation methods to invesigate the HI_x section flowsheet. The effects of different designs of internal circuits in the HI_x section on the required input flow rate and heat duty for producing 1 mol hydrogen were studied. Based on the results, the most efficient circuit design was proposed and compared with the work of other researchers. In addition, the heat recovery problems of the flowsheet were discussed.     

The stability of Pt–Ir/C bimetallic catalysts in HI decomposition of the iodine–sulfur hydrogen production process

Decomposition of HI is the key reaction of hydrogen production in the iodine–sulfur thermochemical water splitting cycle, so studies about the catalysts for HI decomposition have drawn increasing attention. In this study, a series of monometallic Pt/C((Pt/C-400, Pt/C-500, Pt/C-600, Pt/C-700 and Pt/C-800), Ir/C(Ir/C-400, Ir/C-500, Ir/C-600, Ir/C-700 and Ir/C-800) and bimetallic Pt–Ir catalysts supported on active carbon (Pt–Ir/C-400, Pt–Ir/C-500, Pt–Ir/C-600, Pt–Ir/C-700 and Pt–Ir/C-800 were prepared by the impregnation-reduction-calcination method. Their catalytic activities were evaluated for HI decomposition in a fixed bed reactor at 400 and 500 °C under atmospheric pressure. Their structures, metal particles size and distribution, and specific surface area were characterized by X-ray diffraction (XRD), Transmission electron microscopy (TEM) and Brunauer-Emmett-Teller (BET) surface area, respectively. The results showed that the bimetallic Pt–Ir catalyst had the excellent stability in terms of the anti-sintering structure and catalytic activity. Therefore, the bimetallic Pt–Ir catalysts are the good candidates to take the place of the traditional monometallic Pt/C catalyst for catalyzing the HI decomposition.     

Kinetic and thermodynamic studies of the Bunsen reaction in the sulfur–iodine thermochemical process

This work presents the kinetic and thermodynamic studies of the Bunsen reaction in the sulfur–iodine thermochemical cycle for hydrogen production by water splitting. A series of experimental runs have been carried out by feeding the gas mixture SO₂/N₂ in an I₂/H₂O medium in the temperature range of 336–358 K. The effects of the various operating parameters on the SO₂ conversion ratio have been evaluated. The results showed that the efficiency of SO₂ conversion into H₂SO₄ increased with the amount of I₂ or H₂O increase. The increasing reaction temperature impeded SO₂ conversion into H₂SO₄. A kinetic model has been developed to fit to the experimental data obtained in a semi-batch reactor. A good fitting can be observed for each experiment, which discloses the overall kinetic mechanism of the complex Bunsen reaction. The apparent activation energies were found to be 23.513 kJ mol⁻¹ and 9.212 kJ mol⁻¹ for the sequential reactions  and , respectively.     

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.     

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.     

Introduction of loop operating system to improve the stability of continuous hydrogen production for the thermochemical water-splitting iodine–sulfur process

The thermochemical water-splitting iodine–sulfur process facilitates hydrogen production. This study proposes a new loop operation by subdividing the process configuration into four sections before transferring the continuous operation. The loop operation should define the section affecting the fluctuations to easily stabilize the system. The proposed loop operation was validated by analyzing the material and heat balances using a process simulator. The calculated results showed that the material balances of the respective loop sections were closed without component discharge to outside sections. The loop operation would transfer to the continuous operation by connecting all sections. Considering the switching of operation modes, the material and heat balance showed no or little difference, indicating that two operation modes can only be changed by switching the pipelines. Thus, the loop sections can be operated individually to stabilize the IS process system, and the loop operation can be transferred smoothly to the continuous operation.     

Influence of the initial HI on the multiphase Bunsen reaction in the sulfur–iodine thermochemical cycle

In the sulfur–iodine cycle flowsheet, HI may exist in the feeds of Bunsen reaction. The effects of the initial HI and the operating temperature on the kinetic process and thermodynamic equilibrium of the multiphase Bunsen reaction were investigated. Increasing initial HI concentration (HI/H₂O = 0–1/18) or temperature (303 K–358 K) amplified the reaction kinetic rate, and led to the earlier appearance of liquid–liquid separation and less time to reach the thermodynamic equilibrium. But the separation became difficult for further increase of the initial HI content. The liquid–liquid equilibrium (LLE) phase separation was enhanced with rising temperature. An increase in the initial HI content slightly weakened the LLE phase separation at a lower temperature, while at 345 K and 358 K, the LLE phase separation characteristics showed little variation in the HI/H₂O molar ratio range of 0–1/18. A hyper-azeotropic HI concentration in the HIx phase was obtained with feeding HI. The conversion of SO₂ lowered as the initial HI content and the temperature increased.     

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.     

Process simulations of HI decomposition via reactive distillation in the sulfur–iodine cycle for hydrogen manufacture

Process simulations of HI decomposition via reactive distillation in the Sulfur–Iodine (S–I) cycle have been performed using heat pumps for energy recovery and a recently developed thermodynamic properties model. Several differences from previous flow sheets have been found through manual optimization of reflux ratio, number of stripping and rectifying stages, and pressure of the distillation column for typical inlet conditions to the HIx Section III. In particular, the RD column should have a minimal stripping section, can have as few as 10 total stages, an operating pressure of 12 bar, and a reflux ratio of 0.75, while achieving the production requirements. Though this design has limited improvement in energy requirements because the General Atomics energy recovery system is extremely effective, these results mean there should be a significant reduction in capital costs from prior estimates. In addition, as the inlet flow rate is increased, the input energy requirements decrease because of an increased ratio of H₂O to I₂ in the reboiler, lowering its temperature, and reducing the temperature differences for heat pump operations. The optimal inlet flow is between 126 and 140 mol/mol H₂, with a Section energy requirement of 367 kJ/mol H₂, and an overall process thermal efficiency estimated to be 41.5% relative to the higher heating value of hydrogen. These findings suggest there may be greater flexibility in conditions for the Bunsen reaction section as well as other possibilities for further energy efficiency improvement.     

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.     

Total and partial pressure measurements for the sulphur–iodine thermochemical cycle

First results on the measurement of total and partial pressures over the ternary system HI–_I2–_H2O$\mathrm{HI}–{\mathrm{I}}_2–{\mathrm{H}}_2\mathrm{O}$ are reported. Using original optical online measurements, data on the gas phase speciation are obtained which will help to scale and optimize the reactive distillation column we promote for the HI section of the sulphur–iodine cycle.     

Effects of the second metals on the active carbon supported Pt catalysts for HI decomposition in the iodine–sulfur cycle

The active carbon supported monometallic Pt and Pt-based bimetallic catalysts (including Pt–Ni, Pt–Pd and Pt–Rh) were prepared by the impregnation-reduction method. Their catalytic activities were compared for HI decomposition in a fixed bed reactor. The fresh and used monometallic Pt and Pt-based bimetallic catalysts were characterized by BET, XRD and TEM in order to investigate their changes in surface area, structure, and morphology, respectively. The results showed that the Pt-based bimetallic catalysts had better activity and higher stability than the monometallic Pt catalyst in HI decomposition.     

Activity and stability of Pt catalysts supported on carbon nanotubes,active carbon and γ-Al2O3 for HI decomposition in iodine–sulfur thermochemical cycle

Pt catalysts supported on carbon nanotubes (CNT), activated carbon and γ-Al₂O₃ were prepared by the electroless plating method. For comparison, the CNT supported Pt was also prepared by the traditional impregnation–reduction method. The physical properties, structure, morphology and Pt loadings of the different catalysts were characterized by BET, XRD, TEM and ICP, respectively. The catalytic activity for HI decomposition was investigated in a fixed bed reactor under atmospheric pressure. The results of XRD and the activity evaluation indicated that the Pt/CNT prepared by the electroless plating method had better catalytic performance than that prepared by the impregnation–reduction method. Among the three kinds of supported Pt catalysts by the electroless plating method, the CNT supported Pt catalyst not only showed the highest activity for HI decomposition, but also had the best stability in specific surface area, structure and morphology.     

Effects of the composition on the active carbon supported Pd–Pt bimetallic catalysts for HI decomposition in the iodine–sulfur cycle

A series of binary Pd–Pt catalysts supported on active carbon were prepared by the co-impregnation and reduction method. For comparison, active carbon supported monometallic Pt and Pd catalysts were also prepared by the impregnation–reduction method. Their structure, morphology and surface area were investigated by means of X-ray diffraction (XRD), Transmission electron microscopy (TEM) and Brunauer–Emmett–Teller (BET) surface area, respectively. Their catalytic activities were evaluated for the decomposition of hydrogen iodide (HI). Furthermore, their thermal stabilities were also investigated. The results of activity tests showed that the composition of Pd–Pt binary catalysts played the important role in dictating the catalyst activity. Among the Pt, Pd and binary Pd–Pt catalysts, the 2.5%Pd–2.5%Pt/C showed the best catalytic performance for the decomposition of HI. The results of thermal stability tests showed that the binary Pd–Pt catalyst had the higher stability than the monometallic Pt and Pd catalysts.     

Apparent kinetics of the Bunsen reaction in I2/HI solution for the iodine–sulfur hydrogen production process

Massive hydrogen production featuring high efficiency, CO₂ free, and cost effectiveness is a crucial challenge for the hydrogen economy. Nuclear hydrogen production through thermochemical iodine–sulfur (IS) process is a potential candidate for this purpose. Chemical reaction kinetics data are indispensable for developing a high-performance reactor as well as the scaling up of the process. The apparent kinetics of the reaction under simulated recycling conditions of IS closed cycle operation was studied by initial rate method. The effects of key parameters, including agitation speed, SO₂ partial pressure, I₂ concentration, and reaction temperature, on reaction rate, were systematically investigated by measuring the variation in SO₂ pressure with reaction time. Initial rate analysis method indicated that the Bunsen reaction rates were 0.23 ± 0.01 and 0.77 ± 0.01 order with respect to SO₂ pressure and I₂ concentration. The apparent activation energy was 5.86 ± 0.21 kJ/mol. Based on these results, an exponential rate expression of the Bunsen reaction was established. In addition, a simplified method for calculation of kinetics parameters was proposed and compared with conventional techniques. Experimental results provide theoretical basis for design and development of Bunsen reactors and for elucidating the reaction process.     

Influence of iridium content on the performance and stability of Pd–Ir/C catalysts for the decomposition of hydrogen iodide in the iodine–sulfur cycle

In this paper, a series of bimetallic palladium–iridium catalysts supported on active carbon (Pd–Ir/C) were prepared by the impregnation-reduction method. Effects of the composition on the performance and stability of bimetallic catalysts in the decomposition of HI were investigated. X-ray diffraction (XRD), Transmission electron microscopy (TEM) and Brunauer–Emmett–Teller (BET) surface area were employed to characterize their structure, morphology and surface area, respectively. The results of activity tests indicated that all the bimetallic Pd–Ir catalysts were more active than the monometallic 5% Pd/C and 5% Ir/C catalysts at both 400 °C and 500 °C. Among Pd, Ir and binary Pd–Ir catalysts, the 4% Pd–1% Ir/C was the most active for HI catalytic decomposition at 400 °C. Due to the low cost, high activity and good stability, the carbon supported bimetallic Pd–Ir catalysts are more potential candidates to replace Pt/C for HI decomposition in the IS thermochemical cycle.     

Corrosion resistance of structural materials in high-temperature aqueous sulfuric acids in thermochemical water-splitting iodine–sulfur process

Very harsh environments exist in the iodine–sulfur process for hydrogen production. Structural materials for sulfuric acid vaporizers and concentrators are exposed to high-temperature corrosive environments. Immersion tests were carried out to evaluate the corrosion resistance of ceramics and to evaluate corrosion-resistant metals exposed to environments of aqueous sulfuric acids at temperatures of 320, 380, and 460 °C, and pressure of 2 MPa. The aqueous sulfuric acid concentrations for the temperatures were 75, 85, and 95 wt%, respectively. Ceramic specimens of silicon carbides (SiC), silicon-impregnated silicon carbides (Si–SiC), and silicon nitrides (Si₃N₄) showed excellent corrosion resistance from weight loss measurements after exposure to 75, 85, and 95 wt% sulfuric acid. High-silicon irons with silicon content of 20 wt% showed a fair measure of corrosion resistance. However, evidence of crack formation was detected via microscopy. Silicon enriched steels severely suffered from uniform corrosion with a corrosion rate in 95 wt% sulfuric acid of approximately 1 g m⁻² h⁻¹. Among the tested materials, the ceramics SiC, Si–SiC, and Si₃N₄ were found to be suitable candidates for structural materials in direct contact with the considered environments.     

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.