Pt/graphite catalyst for hydrogen generation by HI decomposition reaction in S–I thermochemical cycle

Hydrogen is an attractive energy carrier for future because of various reasons. Therefore its large scale production is the need of the hour. One of the ways to achieve this is sulfur iodine thermochemical cycle and HI decomposition reaction is one of the three reactions constituting the cycle. Pt/graphite catalysts with different loading of platinum were prepared by impregnating colloidal graphite with hexachloroplatinic acid solution followed by reduction under N₂ flow. The catalysts prepared have been characterized by X‐ray diffraction, Raman, scanning electron microscopy, X‐ray photoelectron spectroscopy and Brunauer–Emmett–Teller surface area. These catalysts have been employed for liquid phase HI decomposition under different conditions. To evaluate the stability of this catalyst against noble metal leaching under the reaction conditions, the eluent was analyzed by using ICP‐OES. Platinum loaded catalysts (0.5%, 1% and 2%) show 8.4%, 17.5% and 23.4% conversion respectively. From the present study we conclude that Pt/graphite is a suitable and stable catalyst for liquid phase HI decomposition reaction. Copyright © 2015 John Wiley & Sons, Ltd.     

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.     

Pt‐Ce0.9Cu0.1O2/activated carbon as highly active and stable HI decomposition catalyst

Effects of additives to Pt‐CeO₂/activated carbon (M563) catalysts on HI decomposition were studied. Among the additives studied, it was found that the addition of Cu to Pt‐CeO₂ is the most effective for increasing the catalytic activity to HI decomposition. On this catalyst, almost the equilibrium hydrogen iodide (HI) conversion was achieved at temperature higher than 573 K. Cu addition increased Pt dispersion by anchoring effects. Therefore, in spite of decreased Brunauer‐Emmett‐Teller surface area of the catalyst, dispersion of Pt was much increased by addition of Cu resulting in the increased HI decomposition activity and stability. Because formed I₂ adsorbed on the catalyst at initial ca 20 hours, HI conversion was higher than that of the equilibrium one; however, after 20 hours, stable HI decomposition conversion that was almost the same with equilibrium conversion was achieved in the examined temperature range.     

Platinum–ceria–zirconia catalysts for hydrogen production in sulfur-iodine cycle

In this study, Pt/Ce_1−xZr_x catalysts with different Zr mole concentration (x = 0, 0.2, 0.5, 0.8, 1) have been tested to evaluate their effects on hydrogen iodide (HI) decomposition for hydrogen production in the sulfur-iodine (SI or IS) cycle at various temperatures. The Pt/Ce_1−xZr_x catalysts strongly enhanced the HI conversion to H₂ by comparison with blank test, especially the Pt/Ce_0.8Zr_0.2 catalyst. BET, XRD, TEM, EDS, TPR were performed for catalysts characterization. It was found that, through introducing ZrO₂ into Pt/CeO₂, a synergistic effect between Pt and CeO₂-ZrO₂ solid solution was different from Pt and CeO₂ yield, such as improvement of the thermal stability and increase of Pt-O-Ce reducibility. Among the three samples containing Zr, the one with 20 mol% displayed the best activity for hydrogen production.     

Pt‐Rh/TiO2/activated carbon as highly active and stable HI decomposition catalyst for hydrogen production in sulfur‐iodine (SI) process

Pt‐TiO₂ loaded on activated carbon was studied as an active and stable catalyst to HI decomposition for H₂ formation in the sulfur‐iodine process. Although the activity of TiO₂‐loaded catalyst was slightly lower HI conversion than that of CeO₂ loaded one, the higher stability against HI decomposition reaction was achieved and almost equilibrium conversion was sustained over ~65 h examined. Moreover, effects of Rh or Ir addition on HI conversion were studied and it was found that Pt‐Rh bimetallic system was highly active and stable to HI decomposition. Scanning transmission electron micrograph observation suggested that the increased HI decomposition activity was assigned to the increased dispersion of Pt particles. High dispersion state of Pt was sustained after HI decomposition at 773 K by addition of Rh. Since the formation of PtI₄ was suggested by X‐ray photoelectron spectroscopy measurement during HI decomposition, increased stability by addition of Rh seems to be assigned to the high chemical stability of Rh against iodine. Almost the equilibrium HI conversion on Pt‐Rh‐TiO₂/M563 was sustained over 300 hours at 673 K.     

Overview of nuclear hydrogen production research through iodine sulfur process at INET

Thermochemical water-splitting cycle is a promising process to produce hydrogen using solar or nuclear energy. R&D on hydrogen production through iodine sulfur (IS) thermochemical cycle was initiated in 2005 at INET. Fundamental studies on the three reactions of IS cycle, i.e., Bunsen reaction, HI decomposition reaction, sulfuric acid decomposition reaction, and related techniques, such as separation, concentration and purification, were carried through. In Bunsen section, the reaction kinetics and separation characteristics of H₂SO₄ and HIx phases were studied. In HI section, Pt catalysts were loaded on different supporters by various methods and used for HI decomposition; and electro-electrodialysis(EED) was developed for concentration of HI acid. In sulfuric acid section, non-Pt catalysts were developed for SO₃ decomposition. Based on fundamental researches, a closed-loop test apparatus of 10 NL/h H₂ was designed and established. The current status of IS process research is summarized in this paper. In addition, R&D plan of IS process at INET is presented.     

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.     

Pt/ZrO2 catalyst for a single-stage water-gas shift reaction: Ti addition effect

Ti modified Pt/ZrO₂ catalysts were prepared to improve the catalytic activity of Pt/ZrO₂ catalyst for a single-stage WGS reaction and the Ti addition effect on ZrO₂ was discussed based on its characterization and WGS reaction test. Ti impregnation into ZrO₂ increased the surface area of the support and the Pt dispersion. The reducibility of the catalyst was enhanced in the controlled Ti impregnation (∼20 wt.%) over Pt/ZrO₂ by the Pt-catalysed reduction of supports, particularly, at the interface between ZrO₂ and TiO₂. The significant CO₂ gas band in the DRIFTS results of Pt/Ti[20]/ZrO₂ indicated that the Ti addition made the formate decomposition rate faster than the Pt/ZrO₂ catalyst, linked with the enhanced Pt dispersion and reducibility of the catalyst. Consequently, Ti impregnation over the ZrO₂ support led to a remarkably enhanced CO conversion and the reaction rate of Pt/Ti[20]/ZrO₂ increased by a factor of about 3 from the bare Pt/ZrO₂ catalyst.     

Decomposition of hydrogen iodide over Pt–Ir/C bimetallic catalyst

Decomposition of hydrogen iodide (HI) is one of the key reactions in the sulfur–iodine (S–I) thermochemical water splitting promising for the massive hydrogen production. Much effort has been made to explore the preparation of high performance catalyst for this hydrogen-producing reaction. Although platinum has long been found to be an efficient metallic catalyst, it was prone to agglomerate at elevated temperature resulting in a decrease in the hydrogen yield. A series of bimetallic Pt–Ir/C catalysts were prepared by electroless plating to investigate the effect of Ir/Pt molar ratio on the HI conversion compared with Pt/C and Ir/C catalysts. The physical properties and morphology of the catalysts were characterized by BET, XRD, TEM and ICP-AES. The synergistic effect of platinum and iridium with respect to HI decomposition was confirmed by the fact that the bimetallic Pt–Ir/C-0.77 catalyst with 1 wt% Pt loading and 0.77 wt% Ir loading showed much higher catalytic activity and thermostability compared with Pt/C and Ir/C catalyst. Based on the experimental results obtained, it may be concluded that the bimetallic Pt–Ir/C catalyst was supposed to be a cost-effective and high performance catalyst promising to be employed for the hydrogen production via the S–I thermochemical water splitting cycle.     

Studies of the sulfur-iodine thermochemical water-splitting cycle

Literature thermodynamic values were experimentally confirmed for the Bunsen reaction producing H₂SO₄- and HI-rich phases. The sulfur-iodine water-splitting cycle, which uses the Bunsen reaction, has been improved by enriching the H₂SO₄ solution to 57% in a system involving the H₂SO₄ product and counter-current liquid I₂ flow. The system was saturated with SO₂. The decomposition of H₂SO₄ was investigated. Pt/SiO₂, Pt/ZrO₂, Pt/TiO₂ and Pt/BaSO₄ were all good catalysts for H₂SO₄ vapor decomposition to SO₂ at high temperatures. Pt/Al₂O₃ was found to fail due to substrate sulfation. The importance of pressure to sulfation temperature is presented. A summary of catalyst studies for H₂SO₄ vapor decomposition compares catalyst effectiveness.     

ZrO2 anchored core-shell Pt–Co alloy particles through direct pyrolysis of mixed Pt–Co–Zr salts for improving activity and durability in proton exchange membrane fuel cells

Highly active and durable Pt-based catalysts for oxygen reduction reaction (ORR) are very important and necessary for the proton exchange membrane fuel cells (PEMFCs). In this paper, we report the preparation and performance study of ORR catalysts composed of core-shell Pt–Co alloy nanoparticles (NPs) on multi-walled carbon nanotubes (MWCNTs) anchored with ZrO₂ NPs (denoted as Pt–Co–ZrO₂/MWCNTs). Thanks to the unique three-phase structure, the mass activity of Pt–Co–ZrO₂/MWCNTs for ORR at 0.9 V versus reversible hydrogen electrode (RHE) is1577 mA mg_Pt^?1, which is ～6.6-fold higher than that of the commercial Pt/C (238 mA mg_Pt^?1). After 50,000 cycles for durability test, the mass activity of Pt–Co–ZrO₂/MWCNTs for ORR remained 88% of its initial value. In stark contrast, that of Pt/C kept only about 56.3% of its initial value. More importantly, its catalytic performance was fully observed/verified in a H₂-air PEMFC single cell test. When the Pt loading of Pt–Co–ZrO₂/MWCNTs loaded cathode was one fourth of that with commercial Pt/C as the cathode catalyst, comparable cell performance was achieved. More impressively, the MEA with Pt–Co–ZrO₂/MWCNTs underwent only 24.5% degradation in maximum power density after 30,000 accelerated durability tests (ADTs). However, the MEA with Pt/C after 30,000 ADTs exhibited 39.6% performance loss in maximum power density. The enhanced mass activity and catalytic durability of Pt–Co–ZrO₂/MWCNTs could be attributed to the core-shell Pt–Co alloy NPs with Pt-rich surface and the interface effect between Pt–Co alloy NPs and oxygen vacancy-rich ZrO₂ NPs. In addition, this research also provided a solution to the durability issue of cathodes without sacrificing ORR mass activity, which would promote practical application of PEMFCs.     

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.     

H2SO4 poisoning of Ru-based and Ni-based catalysts for HI decomposition in SulfurIodine cycle for hydrogen production

The sulfur–iodine (SI) cycle is deemed to be one of the most promising alternative methods for large-scale hydrogen production by water splitting, free of CO₂ emissions. Decomposition of hydrogen iodide is a pivotal reaction that produces hydrogen. The homogeneous conversion of hydrogen iodide is only 2.2% even at 773 K [1]. A suitable catalyst should be selected to reduce the decomposition temperature of HI and attain reaction yields approaching to the thermodynamic equilibrium conversion. However, residual H₂SO₄ could not be avoided in the SI cycle because of incomplete purification. The H₂SO₄ present in the HI feeding stream may lead to the poisoning of HI decomposition catalysts. In this study, the activity and sulfur poisoning of Ru and Ni catalysts loaded on carbon and alumina, respectively, were investigated at 773 K. HI conversion efficiency markedly decreased from 21% to 10% with H₂SO₄ (3000 ppm) present, which was reversible when H₂SO₄ was withdrawn in the case of Ru/C. In the case of Ru/C and Ni/Al₂O₃, catalyst deactivation depends on the concentration of H₂SO₄; the higher the concentration of H₂SO_4, the greater the severity of deactivation. Catalysts before and after sulfur poisoning were characterized by transmission electron microscopy (TEM), energy-dispersive X-Ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). Experimental results and characterization of poisoned and fresh catalysts indicate that the catalyst deactivation could be ascribed to the competitive adsorption of sulfur species and change in its surface properties.     

Shape control of exposed planes in ceria-zirconia based electrocatalysts for methanol oxidation

Nano CeO₂–ZrO₂ composites with different shapes are synthesized by using hydrothermal and deposition-precipitation methods, and used to obtain Pt/CeO₂–ZrO₂/RGO catalysts, where reduced graphene oxide(RGO) acts as a carrier. X-ray Diffrattometry (XRD), Raman spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy (XPS) detection methods are used to characterize morphology, phase composition, exposed planes and oxygen defects of the catalyst. The obtained CeO₂–ZrO₂ supports are in the form of nanorods, nanospheres, and nanocubes, which mainly expose (110), (111), and (100) planes, respectively. Electrocatalytic methanol oxidation tests onto the Pt/CeO₂–ZrO₂/RGO catalysts show that the electrochemically active surface area and the peak current density of nanorods are the largest ones among the three catalysts, reaching 61.4 m²/g and 230 A/g. In addition, the long-term discharge stability and CO poisoning resistance of the nanorod-shaped Pt/CeO₂–ZrO₂/RGO catalyst are the highest ones among the examined catalysts.     

HI decomposition over PtNi/C bimetallic catalysts prepared by electroless plating

The catalytic decomposition of hydrogen iodide (HI) has drawn increasing attention because it is the key reaction for hydrogen production in the Iodine–sulfur (IS) thermochemical water splitting cycle, which is considered one of the most promising alternative methods for massive hydrogen production with high efficiency and without CO₂ emissions. Because it is very difficult for HI to decompose without the catalysts even at 500 °C, some catalysts have to be used to catalyze this reaction. In this study, four kinds of PtNi bimetallic catalysts supported on activated carbon (PtNi/C) were prepared by electroless plating and their catalytic activities were compared for HI decomposition in a fixed bed reactor at 400 and 500 °C under atmospheric pressure. Their differences in structures, surface areas, and morphology were characterized by XRD, BET and TEM, respectively. The used catalysts were also analyzed by TEM characterization in order to investigate the stability of catalysts. The results showed that the PtNi/C bimetallic catalysts are promising catalysts for HI decomposition because of their high activity and good stability, especially at high reaction temperature.     

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.     

H2 production from a single stage water–gas shift reaction over Pt/CeO2, Pt/ZrO2, and Pt/Ce(1−x)Zr(x)O2 catalysts

To develop a single stage water–gas shift reaction (WGS) catalyst for compact reformers, Pt/CeO₂, Pt/ZrO₂, and Pt/Ce_(1−x)Zr_(x)O₂ catalysts have been applied for the target reaction. The CeO₂/ZrO₂ ratio was systematically varied to optimize Pt/Ce_(1−x)Zr_(x)O₂ catalysts. Pt/CeO₂ showed the highest turnover frequency (TOF) and the lowest activation energy (E_a) among the catalysts tested in this study. It has been found that the reduction property of the catalyst is more important than the dispersion for a single stage WGS. Pt/CeO₂ catalyst also showed stable catalytic performance. Thus, Pt/CeO₂ can be a promising catalyst for a single stage WGS for compact reformers.     

The effect of impregnation strategy on methane dry reforming activity of Ce promoted Pt/ZrO2

Dry reforming of methane has been studied over Pt/ZrO₂ catalysts promoted with Ce for different temperatures and feed compositions. The influence of the impregnation strategy and the cerium amount on the activity and stability of the catalysts were investigated. The results have shown that introduction of 1 wt.% Ce to the Pt/ZrO₂ catalyst via coimpregnation method led to the highest catalytic activity and stability. 1 wt.%Ce–1 wt.%Pt/ZrO₂ catalyst prepared by sequential impregnation displayed inferior CH₄ and CO₂ conversion performances with lowest H₂/CO production ratios. 1 wt.%Ce–1 wt.%Pt/ZrO₂ catalyst prepared by coimpregnation showed the highest activity even for the feed with high CH₄/CO₂ ratio. The reason for high activity was explained by the intensive interaction between Pt and Ce phases for coimpregnated sample, which had been verified by X-ray photoelectron spectroscopy and Energy Dispersive X-Ray analyses. Strong and extensive Pt–Ce surface interaction results in an increase in the number of Ce³⁺ sites and enhances the dispersion of Pt.     

Pt encapsulated hollow mesoporous SiO2 sphere catalyst for sulfuric acid decomposition reaction in SI cycle

Sulfuric acid (SA) decomposition is one of three key reactions in sulfur Iodine (SI) cycle to produce hydrogen. The catalysts for the decomposition should be active and stable in a wide temperature range of 550–900 °C. Pt based catalysts have been explored for the application, but suffered from the Pt loss in high temperature (∼850 °C). TiO₂ and Al₂O₃ are used as a support. They can stabilize Pt metal at higher temperature, but are degraded at the temperature lower than 700 °C. SiO₂ supports with a high surface area are relatively stable in a sulfuric acid vapor stream, but the lower interaction with Pt results in high Pt sintering and Pt loss. Both Pt loss and Pt sintering at the high temperature are originated from Pt vaporization. Here, Pt metallic components are placed at the inner wall of hollow mesoporous SiO₂ spheres (Pt-HMSS) to preserve Pt components even at 850 °C. PtOx vapor vaporized during the SA decomposition can be re-dispersed on the inner wall of mesoporous SiO₂ shell, which can suppress the Pt loss; (1) temperature at outer wall is higher than temperature at inner wall during the endothermic reaction on Pt at the inner wall, (2) the mesoporous shell afford the long path to suppress the diffusion of PtOx vapor at the inner wall to the outer wall. Pt catalyst at the outer walls of hollow mesoporous SiO₂ spheres (HMSS-Pt) is prepared and tested for clarifying the hypothesis. Additionally, TiO₂-Pt catalyst, one of highly stable catalytic systems for the SA decomposition, is also prepared and compared with the Pt-HMSS catalyst.     

Partial oxidation of ethanol on supported Pt catalysts

This work studied the effect of the nature of the support on the performance of Pt/Al₂O₃, Pt/ZrO₂, Pt/CeO₂ and Pt/Ce_0.50Zr_0.50O₂ catalysts on partial oxidation of ethanol. The reducibility and oxygen transfer capacity were evaluated by temperature-programmed reduction (TPR) and oxygen storage capacity (OSC) experiments. The results showed that the support plays an important role on the products distribution of the partial oxidation of ethanol. Acetic acid was the main product on Pt/Al₂O₃ catalyst whereas methane and acetaldehyde were the only products detected on Pt/ZrO₂, Pt/CeO₂ and Pt/Ce_0.50Zr_0.50O₂ catalysts.The products distribution obtained on Pt/ZrO₂, Pt/CeO₂ and Pt/Ce_0.50Zr_0.50O₂ catalysts was related to their redox properties. The OSC experiments showed that the oxygen exchange capacity was higher on Pt/CeO₂ and Pt/Ce_0.50Zr_0.50O₂ catalysts. A high oxygen storage capacity favored the formation of acetate species, which could be decomposed to CH₄ and/or oxidized to CO₂ via carbonate species. On the other hand, the lower oxygen exchange capacity of Pt/ZrO₂ catalyst led to a higher ethoxy species formation. These species can be dehydrogenated and desorb as acetaldehyde. Then, the higher selectivity to acetaldehyde observed on Pt/ZrO₂ catalyst could be assigned to its low oxygen storage/release capacity.In the case of Pt/Al₂O₃ catalyst, the production of acetic acid could be related to its acidic properties, since this material did not show redox properties, as revealed by OSC analysis.