Sulfuric acid decomposition on the Pt/n-SiC catalyst for SI cycle to produce hydrogen

The sulfuric acid decomposition should be performed in the wide temperature ranges from 550 °C to 950 °C to absorb the sensible heat of He in SI process. Therefore, the catalysts for the reaction should be stable even in the very corrosive reaction condition of 650 °C. Here, the Pt/n-SiC catalyst was prepared for the purpose and compared with the Pt/SiC catalyst. The both catalysts showed the high stability in the temperature ranges from 650 to 850 °C. The n-SiC with the surface area of 187.1 m²/g was prepared using nano-sized SiO₂, which resulted in amorphous SiC phase. The SiC support with the surface area of 19.2 m²/g for the comparison showed the well crystalline structure. In spite of the large surface area differences between the n-SiC and SiC support, the Pt particle sizes of the Pt/n-SiC (average Pt size: 26.4 nm) catalyst were not so much different from those of the Pt/SiC (average Pt size: 26.1 nm) catalyst after the calcination at 1000 °C for 3 h. However, the catalytic activity of the Pt/n-SiC was much higher than that of the Pt/SiC. XRD analysis indicated that the Pt particles on the Pt/n-SiC was more stable than those of the Pt/SiC in the sulfuric acid decomposition and XPS analysis showed that the Pt valence state on the Pt/n-SiC was higher than that on the Pt/SiC. The surface analysis showed that the surface of the n-SiC particles was covered by SiO₂ and Si₄C_4−xO₄. These experimental results indicate that the Pt metal particles on n-SiC were stabilized on the oxidized Si surface. Therefore, it is suggested that the Pt particles stabilized on the oxidized Si surface can be a reason for the higher activity of the Pt/n-SiC catalyst as compared with the Pt/SiC 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.     

Development of Pt-Carbon catalysts using MCM-41 template for HI decomposition reaction in S–I thermochemical cycle

Different Pt-Carbon catalysts have been synthesized by hard templating route and have been employed for production of hydrogen from liquid phase HI decomposition at 160 °C temperature. The physical properties and catalytic activities of these catalysts are compared with that of the platinum on activated carbon catalysts. These catalysts have been characterised by X-Ray diffraction, Raman, SEM and BET surface area. Eluant analysis has been carried out using ICP-OES for evaluation of the extent of noble metal leaching under the catalytic activity test conditions. From the present study we have concluded that MCM-41 based Pt/carbon has higher catalytic activity and stability than other Pt/carbon catalysts.     

Decoration of carbon-supported Pt catalysts with Sn to promote electro-oxidation of ethanol

Carbon-supported Pt/Sn catalysts were prepared by decorating carbon-supported Pt with Sn, by decorating carbon-supported Sn with Pt, and by co-deposition of Pt and Sn on carbon. All of the Pt/Sn catalysts exhibited greatly enhanced activities for ethanol oxidation relative to Pt alone, with the Sn decorated Pt catalyst showing the best performance. The latter catalyst was shown by XPS to contain no metallic Sn, emphasizing the importance of surface Sn oxide species in the activation of Pt for ethanol oxidation. Decoration of Sn with Pt is shown to be a feasible way to increase Pt utilization.     

Spillover enhanced hydrogen uptake of Pt/Pd doped corncob-derived activated carbon with ultra-high surface area at high pressure

Corncob-derived activated carbon (CAC) was prepared by potassium hydroxide activation. The Pt/Pd-doped CAC samples were prepared by two-step reduction method (ethylene glycol reduction plus hydrogen reduction). The as-obtained samples were characterized by N₂-sorption, TEM and XRD. The results show the texture of CAC is varied after doping Pt/Pd. The Pd particles are easier to grow up than Pt particles on the surface of activated carbon. For containing Pt samples, the pore size distributions are different from original sample and Pd loaded sample. The hydrogen uptake results show excess hydrogen uptake capacity on the Pt/Pd-doped CAC samples are higher than pure CAC at 298 K, which should be attributed to hydrogen spillover effects. The 2.5%Pt and 2.5%Pd hybrid doped CAC sample shows the highest hydrogen uptake capacity (1.65 wt%) at 298 K and 180 bar, The particle size and distribution of Pt/Pd catalysts could play a crucial role on hydrogen uptake by spillover. The total hydrogen storage capacity analysis show that total H₂ storage capacities for all samples are similar, and spillover enhanced H₂ uptakes of metal-doped samples could not well support total H₂ storage capacity. The total pore volume of porous materials also is a key factor to affect total hydrogen storage capacity.     

Sulfonation of carbon-nanotube supported platinum catalysts for polymer electrolyte fuel cells

Sulfonic acid groups were grafted onto the surface of carbon-nanotube supported platinum (Pt/CNT) catalysts to increase platinum utilization in polymer electrolyte fuel cells (PEFCs) by both thermal decomposition of ammonium sulfate and in situ radical polymerization of 4-styrenesulfonate. The resultant sulfonated Pt/CNT catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectrometry, thermal gravimetric analysis (TGA) and electrochemical methods. The electrodes with the Pt/CNT catalysts sulfonated by the in situ radical polymerization of 4-styrenesulfonate exhibited better performance than did those with the unsulfonated counterparts, mainly because of the easier access with protons and well dispersed distribution of the sulfonated Pt/CNT catalysts, indicating that sulfonation is an efficient approach to improve performance and reduce cost for the Pt/CNT-based PEFCs. The electrodes with the Pt/CNT catalysts sulfonated by the thermal decomposition of ammonium sulfate, however, did not yield the expected performance as in the case of carbon black supported platinum (Pt/C) catalysts, probably due to the significant agglomeration of platinum particles on the CNT surface at high temperatures, indicating that the Pt/CNT catalysts are more sensitive to temperature than the Pt/C catalysts.     

Decomposition of hydriodic acid by electrolysis in the thermochemical water sulfur–iodine splitting cycle

Although several technologies, such as reactive distillation and catalytic membrane reactor, have been proposed to improve HI conversion efficiency, they still experience several challenges for the application in HI section. In this study, an electrochemical cell was employed for hydriodic acid decomposition under the presence of iodine. Several commercial proton-exchange membranes (PEMs), namely, Nafion 117 and Nafion 115, were used as separators for the electrochemical cell. Anodization of iodide anion occurred at the graphite electrode in the anode compartment. Hydrogen was generated by the reduction reaction of hydrogen cations, which migrated from anolyte to catholyte. In electrolysis experiments, PEM showed good performance in terms of high transport number of proton and low iodine permeation. Several parameters, such as operating temperature, HI molarity, and I₂ molarity in anolyte, which affected current efficiency, iodine permeance, and electric resistance of test cell, were investigated. High operating temperature and I₂ molarity in anolyte enhanced the permeability of iodine, which had several negative influences on electrochemical cell performance. Although current efficiency was negatively affected by increasing temperature and I₂ molarity, it still remained above 0.85 in the range of 30 °C–75 °C. Ohmic resistance, which is a component of cell resistance, offered by PEM was investigated with Nafion 117 and 115. Apart from graphite plates, activated carbon papers were adopted as electrodes to reduce the overpotentials due to their high specific surface characteristic.     

DDR zeolite membrane reactor for enhanced HI decomposition in IS thermochemical process

The third section of closed loop Iodine Sulphur (IS) thermochemical cycle, dealing with HI_x processing, suffers from low equilibrium decomposition of HI to hydrogen with a conversion value of only ～22% at 700 K. Here, we report a significant enhancement in conversion of HI into hydrogen (up to ～95%) using a zeolite membrane reactor for the first time. The all silica DDR (deca dodecasil rhombohedral) zeolite membrane with dense, interlocked structure was synthesized on the seeded clay alumina substrate by sonication mediated hydrothermal process. The synthesized membranes along with seed crystals were characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM) and energy dispersive X-ray spectroscopy (EDX). Corrosion studies were carried out by exposing the membrane samples to simulated HI decomposition reaction environment (at 450 °C) for different durations of time upto 200 h. The FESEM, EDX and XRD analyses indicated that no significant changes occurred in the morphology, composition and structure of the membranes. Iodine adsorption on to the membrane surface was observed which got increased with the exposure duration as confirmed by secondary ion mass spectrometry studies. A packed bed membrane reactor (PBMR) assembly was fabricated with integration of in-house synthesized zeolite membrane and Pt-alumina catalyst for carrying out HI decomposition studies. The tube side was chosen as reaction zone and the shell side as the permeation zone. The HI decomposition experiments were carried out for different values of temperature and feed flow rates. DDR zeolite based PBMR was found to enhance the single-pass conversion of HI up to ～95%. The results indicate that for achieving optimal performance of PBMR, it should be operated with space velocities of 0.2–0.3 s^?1 and temperature in the range of 650 K–700 K with permeate side vacuum of 0.12 kg/cm². It is believed that the in-house developed zeolite PBMR shall be a potential technology augmentation in making the IS thermochemical cycle energy efficient.     

Preparation of Pt/C electrode with double catalyst layers by electrophoresis deposition method for PEMFC

Pt/C double catalyst layer (DCL) electrodes are prepared by pulsed electrophoresis deposition (PED) method from a Pt colloidal solution as a plating bath. The PED is optimized by varying the deposition time in a galvanostatic mode. The catalyst layers of the electrodes prepared by this method are structurally characterized by EDX and SEM studies. The catalytic activities of Pt/C DCL electrodes are evaluated by cyclic voltammetry technique. The loading amount of the Pt catalyst is controlled by varying the deposition time. With the same Pt catalyst loading, the DCL electrode has enhanced catalytic activity than single catalyst layer (SCL) electrode.     

Commercial activated carbon for the catalytic production of hydrogen via the sulfur-Iodine thermochemical water splitting cycle

Eight commercial activated carbon catalysts were examined for their catalytic activity to decompose hydroiodic acid (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. No statistically significant correlation was found between the measured catalyst sample properties and catalytic activity. Four of the eight samples were examined for one week of continuous operation at 723 K. All samples appeared to be stable over the period of examination.     

The short-channel function of hollow carbon nanoparticles as support in the dehydrogenation of cyclohexane

The structure function of hollow carbon nanoparticles (HCNP) as support of Pt particles in the dehydrogenation of cyclohexane was studied. The catalysts and supports were characterized by the XRD, TEM, TG-MS and N₂ adsorption/desorption technologies. The results indicated that the short-channel of HCNP can improve the activity and stability of Pt/HCNP, which was confirmed and explained by the temperature programmed desorption (TPD) experiments. The extent of coke deposited on HCNP was much less than that on MCNT, which can be attributed to fast diffusion and quick removal of benzene from active sites through the short path of HCNP. The HCNP with short channel is very important for improving catalyst performance in the storage system of liquid organic hydrides.