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.     

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.     

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.     

Pt/zirconia catalyst for hydrogen generation from HI decomposition reaction of S–I cycle

Hydrogen energy is considered as one of the ideal solutions for the fulfilment of the ever increasing energy demand. It is mainly due to the following two reasons: firstly, it can be produced from a very abundant source, that is, water; and secondly, it does not leave any harmful effect on the environment. Thermochemical cycles are amongst the most promising ways to generate hydrogen from water in an environment‐friendly manner. Sulfur–iodine cycle is one of the most efficient thermochemical cycles. In this paper, we discuss synthesis of Pt/zirconia catalysts for HI decomposition reaction, which is one of the important steps of S–I thermochemical cycle. The catalysts were characterized by X‐ray diffraction, scanning electron microscopy (SEM), field emission gun‐SEM, transmission electron microscopy, N₂ adsorption and H₂ chemisorption. The catalytic activity and stability of these catalysts, for liquid phase decomposition of hydriodic acid was evaluated. Conversion is found to be dependent on the noble metal loading, with 18.7% conversion for 2% Pt/ZrO₂ catalyst as compared with 2.7% of without catalyst, although the specific activity is highest for 0.5% Pt/ZrO₂ catalyst. The catalyst was found to be stable under liquid phase HI decomposition reaction conditions. Copyright © 2014 John Wiley & Sons, Ltd.     

Detailed kinetic modeling of homogeneous HI decomposition for hydrogen production—Part II: Effect of I2 on HI decomposition

The kinetic modeling of homogeneous decomposition of hydrogen iodide (HI) and HI/H₂O vapors with the addition of diatomic iodine (I₂) using the mechanism proposed in the companion work (part I) in the sulfur–iodine cycle was investigated in this paper. Thermodynamic results calculated by FactSage and the kinetic experiment verified the applicability of the mechanism. The effect of temperature, residence time, pressure, HI/H₂O/I₂ molar ratio, HI/I₂ molar ratio, and sensitivity analysis on the HI conversion was observed in the modeling process. The addition of small amount of diatomic iodine greatly decreases the HI conversion, and the overall pressure could promote the HI decomposition rate in the kinetic process. Sensitivity analysis shows that hydrogen yield was most sensitive to reactions (4) HI + H = H₂ + I, (1) HI + HI = H₂ + I₂, (5) HI + I = I₂ + H, and (8) HI + OH = H₂O + I. The existence of diatomic iodine increases the reverse reaction of (1) and (5).     

Highly efficient Ru/TiO2‐NiAl mixed oxide catalysts for CO selective methanation in hydrogen‐rich gas

Selective CO methanation (CO‐SMET) is viewed as an effective H₂‐rich gas purification technique for proton exchange membrane fuel cells. In this work, improved composite‐supported Ru catalysts were developed for the CO‐SMET process. Mixed metal oxides (MMOs) obtained by calcination of layered double hydroxides precursor were used as an effective catalyst supports. After incorporation of TiO₂, the resulting TiO₂‐MMO composites were expected to have an enhanced catalytic performance. Therefore, a series of TiO₂‐NiAl layered double hydroxides was successfully prepared via 1‐pot deposition method. After calcination, the derived TiO₂‐NiAl MMO‐supported Ru catalysts obtained by impregnation method showed excellent catalytic performance for CO‐SMET reaction. The catalyst could deeply remove the CO outlet concentration (<10 ppm) with a high selectivity (>50%) over the wide low‐temperature window (175‐260°C). Furthermore, the catalyst also showed high stability with no deactivation during a long‐term durability test (120 h). Based on X‐ray diffraction, Fourier transform infrared, Raman, thermogravimetric differential scanning calorimetry, N₂ adsorption‐desorption, temperature‐programmed reduction, scanning electron microscopy, and transmission electron microscopy analyses, the enhanced catalytic performance of the TiO2‐NiAl MMO‐supported Ru catalyst was found to be related to the higher dispersion of Ru nanoparticles, partially reduced NiO species, and the increased specific surface area and structural stability of the support. The facile synthesis strategy proposed herein may open a new window for the efficient production of high‐quality H₂.     

A experimental study on the dehydrogenation performance of dodecahydro-N-ethylcarbazole on M/TiO2 catalysts

Liquid organic hydrogen carrier (LOHC) is considered as a promising candidate for large-scale hydrogen storage. In this work, we found that Pt/TiO₂ catalysts exhibited better catalytic activity and selectivity compared to Pd/TiO₂ and commercial Pd/Al₂O₃ catalysts in the dehydrogenation of dodecahydro-N-ethylcarbazole (12H-NECZ) at 453 K. The catalytic activity of the noble metal catalysts followed the trend of Pt/TiO₂ > Pd/TiO₂ > Rh/TiO₂ > Au/TiO₂ > Ru/TiO₂. Compared with the commercial Pd/Al₂O₃, Pt/TiO₂ greatly improved the selectivity and conversion rate, the reaction time was also shortened. In addition, kinetics calculation was carried out to obtain fundamental reaction parameters. It was found that the third step of 4H-NECZ dehydrogenation to NECZ was the rate-limiting step of the entire dehydrogenation reaction for all catalysts.     

Kinetic study of oxygen reduction reaction and PEM fuel cell performance of Pt/TiO2-C electrocatalyst

The electrochemical activity and thermal stability of the Pt/TiO₂-C were evaluated in the oxygen reduction reaction (ORR) in acid medium at different temperatures. The platinum was selectively deposited onto the TiO₂ (E_bg = 2.3 eV) by the photo-irradiation of platinum precursor (Pt⁴⁺→Pt⁰). The Pt/TiO₂-C electrocatalyst prepared was characterized by XRD, TEM/EDS, cyclic and lineal voltammetry techniques. TEM images indicated that platinum nanoparticles (<5 nm) were deposited in agglomerates form around the oxide sites. EDS and XRD results confirm the composition and crystalline structure of Pt/TiO₂-C. The thermal stability and electrochemical activity of the Pt/TiO₂-C for ORR at different temperatures (298–343 K) is higher than Pt/C commercial sample (Pt-Etek). A more favorable apparent enthalpy of activation for Pt/TiO₂-C was greatly influenced by addition of oxide in the catalyst compare to Pt-Etek. Single H₂/O₂ fuel cell performance results of Pt/TiO₂-C show an improvement of the power density with the increase of the temperature.     

Improved catalytic stability of Pt/TiO2 catalysts for methylcyclohexane dehydrogenation via selenium addition

To improve the catalytic activity of Pt catalysts for methylcyclohexane (MCH) dehydrogenation, which is utilized for hydrogen transportation, the effects of the addition of Se on the performance of Pt/TiO₂ catalysts were investigated. In Se/Pt/TiO₂ catalysts, even a small amount of Se addition (Se/Pt = 0.01) improved the catalyst stability. Se was highly dispersed on the Pt/TiO₂ surface, without volatilizing in a reducing atmosphere at temperatures below 450 °C, and did not form an alloy with Pt. The analysis of adsorption-desorption characteristics revealed that the addition of Se promoted the desorption of products, including the main product, toluene. Moreover, an electron donation effect from Se to Pt was observed by FT-IR measurement after the reduction. The desorption characteristic caused by the electron donation effect suppressed the deterioration of the catalyst and allowed stable catalytic activity toward the MCH dehydrogenation reaction.     

A high‐performance CeO2@Pt‐Beta yolk‐shell catalyst used in low‐temperature ethanol steam reforming for high‐purity hydrogen production

A novel Pt‐based Beta encapsulated CeO₂ yolk‐shell catalyst was successfully synthesized via a RF layer in the synthetic process. The CeO₂@Pt‐Beta catalyst showed high catalytic activity and stability toward the LT‐ESR with respect to the reference Pt‐Beta, CeO₂‐Pt‐Beta, which benefited from the special yolk‐shell structure and the synergistic effect between the CeO₂ movable core and Pt metal. The confinement effect of the yolk‐shell architecture contributed to the high dispersion of Pt nanoparticles as well as to the accumulation of reactant molecules in the enclosed void space, which ensured the reactants reacted with CeO₂ and Pt to achieve a complete reaction.     

Photo-induced reforming of alcohols with improved hydrogen apparent quantum yield on TiO2 nanotubes loaded with ultra-small Pt nanoparticles

Photo-induced reforming of methanol, ethanol, glycerol and phenol at room temperature for hydrogen production was investigated with the use of ultra-small Pt nanoparticles (NPs) loaded on TiO₂ nanotubes (NTs). The Pt NPs with diameters between 1.1 and 1.3 nm were deposited on TiO₂ NTs by DC-magnetron sputtering (DC-MS) technique. The photocatalytic hydrogen rate achieved an optimum value for a loading of about 1 wt% of Pt. Apparent quantum yield for hydrogen generation was measured for methanol and ethanol water solutions reaching a maximum of 16% under irradiation with a wavelength of 313 nm in methanol/water solution (1/8 v/v). Pt NPs loaded on TiO₂ NTs represented also a true water splitting catalyst under UV irradiation and pure distilled water. DC-MS method appears to be a technologically simple, ecologically benign and potentially low-cost process for production of an efficient photocatalyst loaded with ultra-small NPs with precise size control.     

Insights into morphology-dependent Au/TiO2catalyst in glycerol aqueous solutions towards photothermal reforming hydrogen production

Probing the effect of spatial morphology of catalyst on its photothermal catalytic performance is crucial for solar-driven renewable catalytic reforming of hydrogen production. In this study, Au nanoparticles loaded on various morphologies of TiO₂ nanoparticles were synthesized and characterized. The experimental results indicated that decorating TiO₂ with Au nanoparticles could dramatically increase its photocatalytic activities by 20–40 times. The photothermal conversion efficiency of Au/TiO₂ (12.74%–25.54%) was higher than those of TiO₂ due to the introduction of LSPR of Au nanoparticles could effectively improve the utilization of solar spectrum. Titania nanoflower (TNF) nanoparticles with high light absorption capacity, better colloidal dispersion stability, porous properties and narrow band gap represented the highest H₂ productivity (144.13 μmol·g⁻¹·h⁻¹). The coarse surface structure was also conducive to the dispersion of gold particles on the surface of the carrier and the growth rate of Au/TNF hydrogen production (40 times) which was higher than that of other morphology within 2 h. The results of glycerol photothermal hydrogen generation highlighted the effect of temperature on colloidal dispersion stability and hydrogen production capability of nanoparticle suspension. It demonstrated that the photothermal effect aroused a temperature rise that would deteriorate the dispersion stability of the suspension although a local entropy increase in the catalyst nanoparticles might occur. At the same time, the temperature rise caused by the photothermal effect efficiently produces hydrogen in the reaction temperature range. Therefore, an ideal temperature setting for maximal hydrogen generation could be validated and improved the photothermal synergistic impact on biomass-reformed hydrogen generation.     

Excellent dispersion and electrocatalytic properties of Pt nanoparticles supported on novel porous anatase TiO2 nanorods

We synthesize, for the first time, a new Pt based catalyst for direct methanol fuel cells using homemade novel porous anatase TiO₂ nanorods as a new catalyst support. Pt nanoparticles are prepared by an improved ethylene glycol reduction method and supported on the surface of TiO₂ with excellent dispersion and without any aggregates. The structure and elemental composition of the TiO₂ and Pt/TiO₂ catalyst are characterized by transmission electron micrography (TEM), nitrogen sorption, energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). The electrocatalytic properties of the Pt/TiO₂ catalyst for methanol and carbon monoxide electro-oxidation reactions are investigated by cyclic voltammetry (CV) in an acidic medium. Apparent electrocatalytic activity for methanol electro-oxidation reaction, high carbon monoxide tolerance and good stability are all observed for the Pt/TiO₂ catalyst. These may be attributed to the excellent dispersion of the Pt nanoparticles and the special properties of the TiO₂ support. These results imply that this Pt/TiO₂ catalyst has promising potential applications in direct methanol fuel cells.     

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.     

Enhanced hydrogen evolution reaction on highly stable titania‐supported PdO and Eu2O3 nanocomposites in a strong alkaline solution

In this work, PdO/TiO₂ and Eu₂O₃/TiO₂ nanocomposites (NCs) were synthesized using a new facile, template‐free, and one‐step solvothermal approach and characterized by several instrumentation techniques. X‐ray photoelectron spectroscopy studies revealed the presence of oxidized form of the Pd and Eu nanoparticles within the NC materials (PdO and Eu₂O₃). The two catalysts exhibited remarkable activity for the hydrogen evaluation reaction (HER) in a strong alkaline solution (4.0 M NaOH) with PdO/TiO₂ catalyst being the best, which recorded an exchange current density (j_o) of 0.26 mA cm^?2 and a Tafel slope (β_c) of 125 mV dec^?1. Such parameters are not far from those recorded for a commercial Pt/C catalyst (0.71 mA cm^?2 and 120 mV dec^?1) performed here under the same operating conditions. Eu₂O₃/TiO₂ catalyst recorded j_o and β_c values of 0.05 mA cm^?2 and 135 mV dec^?1. The Tafel slopes 125 and 135 mV dec^?1 calculated on the PdO/TiO₂ and Eu₂O₃/TiO₂ catalysts suggest a HER kinetics controlled by the Volmer step. PdO/TiO₂ catalyzed the HER with a high turnover frequency of 2.3 H₂/s at 0.2 V versus the reversible hydrogen electrode, while Eu₂O₃/TiO₂ catalyst only measured a turnover frequency value of 1.25 H₂/s at the same overpotential. The two catalysts exhibited excellent stability and durability after 10 000 cycles and 72 hours of controlled potential electrolysis at a high cathodic overpotential, reflecting their practical applicability. Scanning electron microscope and X‐ray photoelectron spectroscopy examinations revealed that the morphology and chemistry of both catalysts were not altered as a result of the performed long‐term stability and durability tests.     

Stability evaluation of ethanol dry reforming on Lanthania‐doped cobalt‐based catalysts for hydrogen‐rich syngas generation

Catalytic stability with time‐on‐stream is an important aspect in ethanol dry reforming (EDR) since catalysts could encounter undesirable deterioration arising from deposited carbon. This work examined the promotional effect of La on 10%Co/Al₂O₃ in terms of activity, stability, and characteristics. Catalysts were characterized by X‐ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), Raman, and X‐ray photoelectron spectroscopy (XPS) measurements whilst catalytic EDR performance of La‐promoted and unpromoted 10%Co/Al₂O₃ prepared via wet impregnation technique was investigated at 973 K for 72 h using a stoichiometric feed ratio (C₂H₅OH/CO₂ = 1/1). La promoter substantially enhanced both metal dispersion and metal surface area from 0.11% to 0.64% and 0.08 to 0.43 m² g^?1, respectively. Ethanol and CO₂ conversions appeared to be stable within 50 to 72 h after experiencing an initial activity drop. The conversion of C₂H₅OH and CO₂ for La‐promoted catalyst was about 1.65 and 1.34 times greater than unpromoted counterpart in this order. The carbonaceous deposition was considerably decreased from 55.6% to 36.8% with La promotion due to La₂O₂CO₃ intermediate formation. Additionally, 3%La‐10%Co/Al₂O₃ possessed greater oxygen vacancies acting as active sites for CO₂ adsorption and hence increasing carbon gasification. Even though graphitic and filamentous carbons were formed on used catalyst surface, La‐addition diminished graphite formation and increased the reactiveness of amorphous carbon.     

A highly active Pt nanocatalysts supported on RuO2 modified TiO2-NTs for methanol electrooxidation with excellent CO tolerance

Poisoning devitalization of Pt catalyst caused by the absorption of carbon monoxide is an important issue in direct Methanol Fuel Cell (DMFC). To solve this problem, this work introduced a novel nano-structured Pt catalytic electrode, in which RuO₂ modified TiO₂ nanotube arrays (TiO₂-NTs) was used as a carrier for the load of Pt nanocatalysts. Specifically, RuCl₃ sol was filled into the voids of TiO₂-NTs under vacuum condition, followed by thermal decomposition to form RuO₂/TiO₂-NTs support, and then Pt particles were loaded on the RuO₂/TiO₂-NTs support by pulse potential electrodeposition from H₂PtCl₆ aqueous solution. The electrochemical results show that the methanol oxidation current on Pt/RuO₂/TiO₂-NTs is much higher than that on Pt/TiO₂-NTs. In addition, the current attenuation on Pt/RuO₂/TiO₂-NTs with the increased scan cycle is also decreased. The Pt/RuO₂/TiO₂-NTs electrode with 8 g m⁻² RuO₂ exhibits the most stable performance, indicating a strong effects of anti CO poisoning endowed by RuO₂. In Nyquist diagrams, one capacitance arc representing the action of deprivation of H atom appears in the first quadrant and one inductance arc representing the action of deprivation of CO appears in the fourth quadrant. From the fitting results, both the reaction resistance R_ct and the inductance L decrease with the argument of RuO₂ content under bias potential of 600 mV, and in this case CO oxidation is the rate controlling step.     

Steam reforming of ethanol over bimetallic RhPt/La2O3: Long-term stability under favorable reaction conditions

Recently, the steam reforming of biofuels has been presented as a potential hydrogen source for fuel cells. Because this scenario represents an interesting opportunity for Colombia (South America), which produces large amounts of bioethanol, the steam reforming of ethanol was studied over a bimetallic RhPt/La₂O₃ catalyst under bulk mass transfer conditions. The effect of temperature and the initial concentrations of ethanol and water were evaluated at space velocities above 55,000 h⁻¹ to determine the conditions that maximize the H₂/CO ratio and reduce CH₄ production while maintaining 100% conversion of ethanol. These requirements were accomplished when 21 mol% H₂O and 3 mol% C₂H₅OH (steam/ethanol molar ratio = 7) were reacted at 600 °C. The catalyst stability was assessed under these reaction conditions during 120 h on stream, obtaining ethanol conversions above 99% during the entire test. The effect of both H₂ and air flows as catalyst regeneration treatments were evaluated after 44 and 67 h on stream, respectively. The results showed that H₂ treatment accelerated catalyst deactivation, and air regeneration increased both the catalyst stability and the H₂ selectivity while decreasing CH₄ generation. Fresh and spent catalyst samples were characterized by TEM/EDX, XPS, TPR, and TGA. Although the Rh and Pt in the fresh catalyst were completely reduced, the spent samples showed a partial oxidation of Rh and small amounts of carbonaceous residue. A possible Rh–Pt–Rh₂O₃ structure was proposed as the active site on the catalyst, which was regenerated by air treatment.     

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.