Study of Pt electrode dissolution in H2O2-containing H2SO4 solution using an electrochemical quartz crystal microbalance

Pt electrode dissolution has been investigated using an electrochemical quartz crystal microbalance (EQCM) in H₂O₂-containing 0.5 mol dm⁻³ H₂SO₄. The Pt electrode weight-loss of ca. 0.4 μg cm⁻² is observed during nine potential sweeps between 0.01 and 1.36 V vs. RHE. In contrast, the Pt electrode weight-loss is negligible without H₂O₂ (<0.05 μg cm⁻²). To support the EQCM results, the weight-decrease amounts of a Pt disk electrode and amounts of Pt dissolved in the solutions were measured after similar successive potential cycles. As a result, these results agreed well with the EQCM results. Furthermore, the H₂O₂ concentration dependence of the Pt weight-decrease rate was assessed by successive potential steps. These EQCM data indicated that the increase in H₂O₂ accelerates the Pt dissolution. Based on these results, H₂O₂ is known to be a major factor contributing to the Pt dissolution.     

Electrospun Pt/TiO2 hybrid nanofibers for visible-light-driven H2 evolution

One-dimensional (1D) Pt/TiO₂ hybrid nanofibers (HNFs) with different concentrations of Pt were fabricated by a facile two-step synthesis route combining an electrospinning technique and calcination process. X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM) results showed that the Pt nanoparticles (NPs) with the size of 5–10 nm were well dispersed in the TiO₂ nanofibers (NFs). Further investigations from the UV–Vis diffuse reflectance (DR) and X-ray photoelectron spectroscopy (XPS) analysis revealed that some Pt ions were incorporated into the TiO₂ lattice as Pt⁴⁺ state, which contributed to the visible light absorption of TiO₂ NFs. Meanwhile, the Pt²⁺ ions existing on the surface of Pt NPs resulted in the formation of Pt–O–Ti bond at Pt NPs/TiO₂ NFs interfaces that might serve as an effective channel for improving the charge transfer. The as-electrospun Pt/TiO₂ HNFs exhibited remarkable activities for photocatalytic H₂ evolution under visible light irradiation in the presence of l-ascorbic acid as the sacrificial agent. In particular, the optimal HNFs containing 1.0 at% Pt showed the H₂ evolution rate of 2.91 μmol h⁻¹ and apparent quantum efficiency of 0.04% at 420 nm by using only 5 mg of photocatalysts. The higher photocatalytic activity could be ascribed to the appropriate amount of Pt ions doping and excellent electron-sink effect of Pt NPs co-catalysts.     

Simultaneous production of H2 and C2 hydrocarbons by using a novel configuration solid-electrolyte + fixed bed reactor

A novel combined single chamber solid electrolyte plus fixed bed reactor configuration was developed for the simultaneous production of H₂ and C₂ hydrocarbons from a humidified CH₄ atmosphere. Hence, a Pt/YSZ/Ag solid electrolyte cell was placed on the top of an active oxidative coupling catalyst powder bed Ce–Na₂WO₄/SiO₂. H₂ was produced via steam electrolysis in a Pt cathode of the solid electrolyte cell (H₂O + 2e⁻ → H₂ + O²⁻). Simultaneously, the produced O²⁻ ions were electrochemically pumped to the Ag anode, leading to the C_2s production, via oxidative coupling of CH₄ (4CH₄ + 3O²⁻ → C₂H₄ + C₂H₆ + 3H₂O + 6e⁻). Additionally, non-reacted O²⁻ molecules desorbed to the gas phase (2O²⁻ → O₂+4e⁻) and reacted with CH₄ in the catalyst bed leading to an increase of C_2s yield. The influence of different reaction parameters was investigated together with long-term reaction experiments, confirming the stability of this configuration for its practical application. The obtained results demonstrated that the addition of an active catalyst bed strongly enhances both: the efficiency of the single chamber steam electrolysis and the oxidative coupling process.     

Preparation and photocatalytic properties of HLaNb2O7/(Pt,TiO2) perovskite intercalated nanomaterial

A novel perovskite intercalated nanomaterial HLaNb₂O₇/(Pt, TiO₂) is fabricated by successive intercalated reaction of HLaNb₂O₇ with [Pt(NH₃)₄]Cl₂ aqueous solution, n-C₆H₁₃NH₂/C₂H₅OH organic solution and acidic TiO₂ colloid solution, followed by ultraviolet light irradiation. The gallery height and the band gap energy of HLaNb₂O₇/(Pt, TiO₂) is less than 0.6 nm and 3.14 eV, respectively. The photocatalytic activity of HLaNb₂O₇/TiO₂ is superior to that of unsupported TiO₂ and is enhanced by the co-incorporation of Pt. The photocatalytic hydrogen evolution based on HLaNb₂O₇/(Pt, TiO₂) is 240 cm³ h⁻¹ g⁻¹ using methanol as a sacrificial agent under irradiation with wavelength more than 290 nm from a 100-W mercury lamp. High photocatalytic activity of HLaNb₂O₇/(Pt, TiO₂) may be due to the host with rare earth La element and perovskite structure, the quantum size effect of intercalated semiconductor and the coupling effect between host and guest.     

Large impact of heating time on physical properties and photocatalytic H2 production of g-C3N4 nanosheets synthesized through urea polymerization in Ar atmosphere

The effect of heating time on the polymerization processes of urea into g-C₃N₄ nanosheets was studied in Ar atmosphere. It was found that heating time had a great influence on the crystalline quality, specific surface area (S_BET) and photocatalytic H₂ production of the obtained g-C₃N₄ nanosheets. G-C₃N₄ nanosheets with some degree of disorders in crystal structure were formed within 1 h at 550 °C, and these structural disorders maintained after 4 h, however disorders disappeared after extended heating of 6 h. G-C₃N₄ nanosheets with similar disorders in the crystal structure hold similar S_BET and exhibit comparable H₂ production rates. The highest H₂ production rate of 1.4 mmol h⁻¹ g⁻¹ occurs after 8 h heating, corresponding to 2.6% quantum efficiency at 420 nm.     

Highly H2 permeable SAPO-34 membranes by steam-assisted conversion seeding

An effective steam-assisted conversion (SAC) seeding method was proposed for the growth of a thin and high-quality SAPO-34 membrane on a low-cost and coarse macroporous α-Al₂O₃ tubular support. This seeding technique was composed of depositing the seeds-containing paste on a support and transforming the paste into continuous and compact seed layer by the SAC process. The paste could serve as the binder to prevent small seeds penetrating into the support. With the aid of the perfect seed layer, a high-quality SAPO-34 membrane with the thickness about 4 μm was synthesized by secondary growth. The as-synthesized membrane exhibited a high H₂ permeance of 6.96 × 10⁻⁶ mol m⁻² s⁻¹ Pa⁻¹ at room temperature, with H₂/CO₂, H₂/N₂, H₂/CH₄, H₂/C₂H₆, H₂/C₃H₈, H₂/n-C₄H₁₀ and H₂/i-C₄H₁₀ ideal selectivities of 1.83, 7.58, 14.80, 18.24, 26.51, 40.15 and 53.02, respectively.     

Mesoporous CexMn1−xO2 composites as novel alternative carriers of supported Co3O4 catalysts for CO preferential oxidation in H2 stream

The mesoporous Co₃O₄ supported catalysts on Ce–M–O (M = Mn, Zr, Sn, Fe and Ti) composites were prepared by surfactant-assisted co-precipitation with subsequent incipient wetness impregnation (SACP–IWI) method. The catalysts were employed to eliminate trace CO from H₂-rich gases through CO preferential oxidation (CO PROX) reaction. Effects of M type in Ce–M–O support, atomic ratio of Ce/(Ce + Mn), Co₃O₄ loading and the presence of H₂O and CO₂ in feed were investigated. Among the studied Ce–M–O composites, the Ce–Mn–O is a superior carrier to the others for supported Co₃O₄ catalysts in CO PROX reaction. Co₃O₄/Ce_0.9Mn_0.1O₂ with 25 wt.% loading exhibits excellent catalytic properties and the 100% CO conversion can be achieved at 125–200 °C. Even with 10 vol.% H₂O and 10 vol.% CO₂ in feed, the complete CO transformation can still be maintained at a wide temperature range of 190–225 °C. Characterization techniques containing N₂ adsorption/desorption, X-ray diffraction (XRD), H₂ temperature-programmed reduction (H₂-TPR) and scanning electron microscopy (SEM) were employed to reveal the relationship between the nature and catalytic performance of the developed catalysts. Results show that the specific surface area doesn’t obviously affect the catalytic performance of the supported cobalt catalysts, but the right M type in carrier with appropriate amount effectively improves the Co₃O₄ dispersibility and the redox behavior of the catalysts. The large reducible Co³⁺ amount and the high tolerance to reduction atmosphere resulted from the interfacial interaction between Co₃O₄ and Ce–Mn support may significantly contribute to the high catalytic performance for CO PROX reaction, even in the simulated syngas.     

The size and valence state effect of Pt on photocatalytic H2 evolution over platinized TiO2 photocatalyst

Photocatalytic hydrogen production from water or organic compounds is a promising way to resolve our energy crisis and environmental problems in the near future. Over the past decades, many photocatalysts have been developed for solar water splitting. However, most of these photocatalysts require cocatalyst to facilitate H₂ evolution reaction and noble metals as key cocatalysts are widely used. Consequently, the condition of noble metal cocatalyst including the size and valence state etc plays the key role in such photocatalytic system. Here, the size and valence state effect of Pt on photocatalytic H₂ evolution over platinized TiO₂ photocatalyst were studied for the first time. Surprisingly, it was found that Pt particle size does not affect the photoreaction rate with the size range of several nanometers in this work, while it is mainly depended on the valence state of Pt particles. Typically, TOFs of TiO₂ photodeposited with 0.1–0.2 wt% Pt can exceed 3000 h⁻¹.     

Y2O3-promoted NiO/SBA-15 catalysts highly active for CO2/CH4 reforming

A series of Y₂O₃-promoted NiO/SBA-15 (9 wt% Ni) catalysts (Ni:Y weight ratio = 9:0, 3:1, 3:2, 1:1) were prepared using a sol–gel method. The fresh as well as the catalysts used in CO₂ reforming of methane were characterized using N₂-physisorption, XRD, FT-IR, XPS, UV, HRTEM, H₂-TPR, O₂-TPD and TG techniques. The results indicate that upon Y₂O₃ promotion, the Ni nanoparticles are highly dispersed on the mesoporous walls of SBA-15 via strong interaction between metal ions and the HO–Si-groups of SBA-15. The catalytic performance of the catalysts were evaluated at 700 °C during CH₄/CO₂ reforming at a gas hourly space velocity of 24 L g_cat⁻¹ h⁻¹(at 25 ^°C and 1 atm) and CH₄/CO₂molar ratio of 1. The presence of Y₂O₃ in NiO/SBA-15 results in enhancement of initial catalytic activity. It was observed that the 9 wt% Y–NiO/SBA-15 catalyst performs the best, exhibiting excellent catalytic activity, superior stability and low carbon deposition in a time on stream of 50 h.     

Photocatalytic water splitting with In-doped H2LaNb2O7 composite oxide semiconductors

The In-doped HLaNb₂O₇ oxide semiconductors synthesized by solid-state reaction followed by an ion-exchange reaction were found to be a novel composite photocatalyst system with enhanced activity for water splitting. Pt was incorporated in the interlayer of In-doped HLaNb₂O₇ by the stepwise intercalation reaction. The In-doped HLaNb₂O₇ powder samples were characterized with X-ray diffraction (XRD) and UV-vis diffuse reflectance spectrometry. The photocatalytic activities of Pt-loaded In-doped HLaNb₂O₇ and individual precursor materials were evaluated by H₂ evolution from aqueous CH₃OH solution under UV light irradiation. It was found that the composite In-doped HLaNb₂O₇ showed a higher H₂ evolution rate in comparison with individual materials. The hydrogen production activity of In-doped HLaNb₂O₇ was greatly enhanced by Pt co-incorporation. The In content in the In-doped HLaNb₂O₇ system was discussed in relation to the photophysical and photocatalytic properties. As In content equal 5 mol%, the HLaNb₂O₇:In/Pt showed a photocatalytic activity of 354 cm³ g⁻¹ hydrogen evolution in 10 vol% methanol solution under irradiation from a 100 W mercury lamp at 333 K for 3 h.     

Synthesis of (CuIn)xCd2(1−x)S2 photocatalysts for H2 evolution under visible light by using a low-temperature hydrothermal method

A new series visible-light driven photocatalysts (CuIn)_xCd_2(1−x)S₂ was successfully synthesized by a simple and facile, low-temperature hydrothermal method. The synthesized materials were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) surface area measurement, X-ray photoelectron spectroscopy (XPS) and ultraviolet-visible spectroscopy (UV–Vis DRS). The results show that the morphology of the photocatalysts changes with the increase of x from 0.01 to 0.3 and their band gap can be correspondingly tuned from 2.37 eV to 2.30 eV. The (CuIn)_xCd_2(1−x)S₂ nanocomposite show highly photocatalytic activities for H₂ evolution from aqueous solutions containing sacrificial reagents, SO₃²⁻ and S²⁻ under visible light. Substantially, (CuIn)_0.05Cd_1.9S₂ with the band gap of 2.36 eV exhibits the highest photocatalytic activity even without a Pt cocatalyst (649.9 μmol/(g h)). Theoretical calculations about electronic property of the (CuIn)_xCd_2(1−x)S₂ indicate that Cu 3d and In 5s5p states should be responsible for the photocatalytic activity. Moreover, the deposition of Pt on the doping sample results in a substantial improvement in H₂ evolution than the Pt-loaded pure CdS and the amount of H₂ produced (2456 μmol/(g h)) in the Pt-loaded doping system is much higher than that of the latter (40.2 μmol/(g h)). The (CuIn)_0.05Cd_1.9S₂ nanocomposite can keep the activity for a long time due to its stability in the photocatalytic process. Therefore, the doping of CuInS₂ not only facilitates the photocatalytic activity of CdS for H₂ evolution, but also improves its stability in photocatalytic process.     

Enhancement of the H2 desorption properties of LiAlH4 doping with NiCo2O4 nanorods

Lithium aluminum hydride (LiAlH₄) is considered as an attractive candidate for hydrogen storage owing to its favorable thermodynamics and high hydrogen storage capacity. However, its reaction kinetics and thermodynamics have to be improved for the practical application. In our present work, we have systematically investigated the effect of NiCo₂O₄ (NCO) additive on the dehydrogenation properties and microstructure refinement in LiAlH₄. The dehydrogenation kinetics of LiAlH₄ can be significantly increased with the increase of NiCo₂O₄ content and dehydrogenation temperature. The 2 mol% NiCo₂O₄-doped LiAlH₄ (2% NCO–LiAlH₄) exhibits the superior dehydrogenation performances, which releases 4.95 wt% H₂ at 130 °C and 6.47 wt% H₂ at 150 °C within 150 min. In contrast, the undoped LiAlH₄ sample just releases <1 wt% H₂ after 150 min. About 3.7 wt.% of hydrogen can be released from 2% NCO–LiAlH₄ at 90 °C, where total 7.10 wt% of hydrogen is released at 150 °C. Moreover, 2% NCO–LiAlH₄ displayed remarkably reduced activation energy for the dehydrogenation of LiAlH₄.     

High catalytic kinetic performance of amorphous CoPt NPs induced on CeOx for H2 generation from hydrous hydrazine

CeO_x-induced amorphization of CoPt nanoparticles (NPs) is achieved by a facile co-reduction method using sodium borohydride (NaBH₄) as the reducing agent at room temperature (298 K) under ambient atmosphere. The investigation results indicate that CeO_x plays a critical role in transferring the crystalline CoPt nanoalloy into the amorphous one. To our surprise, the resultant Co_0.65Pt_0.30(CeO_x)_0.05 NPs exhibit high catalytic kinetic performance with 72.1% hydrogen (H₂) selectivity for the H₂ generation from hydrous hydrazine (N₂H₄) within only a few minute at 298 K. Although complete conversion is not achieved, but the initial turnover frequency value of 194.8 h⁻¹ for the present amorphous catalyst is much higher than that of crystalline one. Moreover, such a highly rapid catalyst may greatly encourage the practical application of hydrous N₂H₄ as a hydrogen storage material.     

Photo-catalytic hydrogen production over Fe2O3 based catalysts

The hydrogen photo-evolution was successfully achieved in aqueous (Fe_1−xCr_x)₂O₃ suspensions (0 ≤ x ≤ 1). The solid solution has been prepared by incipient wetness impregnation and characterized by X-ray diffraction, BET, transport properties and photo-electrochemistry. The oxides crystallize in the corundum structure, they exhibit n-type conductivity with activation energy of ∼0.1 eV and the conduction occurs via adiabatic polaron hops. The characterization of the band edges has been studied by the Mott Schottky plots. The onset potential of the photo-current is ∼0.2 V cathodic with respect to the flat band potential, implying a small existence of surface states within the gap region. The absorption of visible light promotes electrons into (Fe_1−xCr_x)₂O₃-CB with a potential (∼−0.5 V_SCE) sufficient to reduce water into hydrogen. As expected, the quantum yield increases with decreasing the electro affinity through the substitution of iron by the more electropositive chromium which increases the band bending at the interface and favours the charge separation. The generated photo-voltage was sufficient to promote simultaneously H₂O reduction and SO₃²⁻ oxidation in the energetically downhill reaction (H₂O + SO₃²⁻ → H₂ + SO₄²⁻, ΔG = −17.68 kJ mol⁻¹). The best activity occurs over Fe_1.2Cr_0.8O₃ in SO₃²⁻ (0.1 M) solution with H₂ liberation rate of 21.7 μmol g⁻¹ min⁻¹ and a quantum yield 0.06% under polychromatic light. Over time, a pronounced deceleration occurs, due to the competitive reduction of the end product S₂O₆²⁻.     

The structural characterization of (NH4)2B10H10 and thermal decomposition studies of (NH4)2B10H10 and (NH4)2B12H12

The structure of (NH₄)₂B₁₀H₁₀ (1) was determined through powder XRD analysis. The thermal decomposition of 1 and (NH₄)₂B₁₂H₁₂ (2) was examined between 20 and 1000 °C using STMBMS methods. Between 200 and 400 °C a mixture of NH₃ and H₂ evolves from both compounds; above 400 °C only H₂ evolves. The dihydrogen bonding interaction in 1 is much stronger than that in 2. The stronger dihydrogen bond in 1 resulted in a significant reduction by up to 60 °C, but with a corresponding 25% decrease in the yield of H₂ in the lower temperature region and a doubling of the yield of NH₃. The decomposition of 1 follows a lower temperature exothermic reaction pathway that yields substantially more NH₃ than the higher temperature endothermic pathway of 2. Heating of 1 at 250 °C resulted in partial conversion of B₁₀H₁₀²⁻ to B₁₂H₁₂²⁻. Both 1 and 2 form an insoluble polymeric material after decomposition. The elements of the reaction network that control the release of H₂ from the B₁₀H₁₀²⁻ can be altered by conducting the experiment under conditions in which pressures of NH₃ and H₂ are either near, or away from, their equilibrium values.     

A study on H2S permeability of CsHSO4 membranes

The gas permeability of H₂S gas at 150 °C through ultra-thin cesium hydrogen sulfate (CsHSO₄) membranes has been investigated. Gas chromatography–mass spectrometry analyses indicate that CsHSO₄ membrane is impermeable to H₂S gas under test conditions. The apparent micropore diameter of the membrane averaged between 9.5 and 11.5 Å with a maximum permeance of 0.09 Barrer (6.75 × 10⁻¹⁹ m² s⁻¹ Pa⁻¹). Atomic force microscope and X-ray diffraction analyses show respectively that the surface morphology and crystal structure of the membranes are preserved, with no adverse effect from prolonged exposure to H₂S gas. Electrochemical impedance spectroscopy analysis confirm over a 30% decrease in membrane resistance via an 80% reduction in membrane thickness.     

Membrane electrode assemblies doped with H3PO4 for high temperature proton exchange membrane fuel cells

H₃PO₄ content plays a critical role in high temperature proton exchange membrane fuel cells (HT-PEMFC), as it is responsible for the majority of the conductivity of the key components under high temperature operation. The conductivities of commercial AB-PBI membranes doped by immersing in 85 wt.% H₃PO₄ for different times and temperatures are investigated. The effect of H₃PO₄ loading in electrodes, including the AB-PBI polymer and a Pt/C catalyst, is also studied. The as-prepared electrodes and membranes are combined to fabricate a membrane electrode assembly for HT-PEMFCs. The results reveal that AB-PBI membranes doped with 85 wt.% H₃PO₄ at 90 °C for 9 h display a maximum conductivity of 33 mS cm⁻¹. This membrane was selected and combined with electrodes including 15 wt.% AB-PBI and 0.75 mg cm⁻² Pt with different H₃PO₄ loadings. A maximum current density of 260 mA cm⁻² was achieved in the as-prepared MEA (with 5 mg cm⁻² H₃PO₄ in electrodes) operating at 0.6 V and 160 °C, using oxygen and hydrogen.     

Studies on B sites in Fe-doped LaNiO3 perovskite for SCR of NOx with H2

A series of LaNi_1−xFe_xO₃ (x = 0.0, 0.2, 0.4, 0.7, and 1.0) perovskites were synthesized and characterized by X-ray diffraction (XRD), N₂ physisorption, scanning electron microscopy (SEM), H₂-temperature-programmed reduction (H₂-TPR), and X-ray photoelectron spectroscopy (XPS). The perovskites were investigated for selective catalytic reduction of NO_x by hydrogen (H₂-SCR). It is shown that Fe addition into LaNiO₃ leads to a promoted efficiency of NO_x removal, as well as a high stability of perovskite structure. Moreover, easy reduction of Ni³⁺ to Ni²⁺ with the aid of appropriate Fe component mainly accounts for the enhanced activity. Meanwhile, deactivation of the sulfated catalysts is due to that sulfates mainly deposit on active Ni component while doping of Fe can protect Ni to some extent at the expense of partial sulfation.     

The effect of low concentrations of CO on H2 adsorption and activation on Pt/C. Part 1: In the absence of humidity

The presence of CO in the H₂-rich gas used as fuel for hydrogen fuel cells has a detrimental effect on PEMFC performance and durability at conventional operating conditions. This paper reports on an investigation of the effect of CO on H₂ activation on a fuel cell Pt/C catalyst close to typical PEMFC operating conditions using H₂-D₂ exchange as a probe reaction and to measure hydrogen surface coverage. While normally limited by equilibrium in the absence of impurities on Pt at typical fuel cell operating temperatures, the presence of ppm concentrations of CO increased the apparent activation energy (E_a) of H₂-D₂ exchange reaction (representing H₂ activation) from approximately 4.5-5.3 kcal mole⁻¹ (Bernasek and Somorjai (1975) [24], Montano et al. (2006) [25]) (in the absence of CO) to 19.3-19.7 kcal mole⁻¹ (in the presence of 10-70 ppm CO), similar to those reported by Montano et al. (2006) [25]. Calculations based on measurements indicate a CO surface coverage of approximately 0.55 ML at 80 °C in H₂ with 70 ppm CO, which coincide very well with surface science results reported by Longwitz et al. (2004) [5]. In addition, surface coverages of hydrogen in the presence of CO suggest a limiting effect on hydrogen spillover by CO. Regeneration of Pt/C at 80 °C in H₂ after CO exposure showed only a partial recovery of Pt sites. However, enough CO-free Pt sites existed to easily achieve equilibrium conversion for H₂-D₂ exchange. This paper establishes the baseline and methodology for a series of future studies where the additional effects of Nafion and humidity will be investigated.