In situ preparation of N doped orthorhombic Nb2O5 nanoplates /rGO composites for photocatalytic hydrogen generation under sunlight

The synthesis of nitrogen doped orthorhombic niobium oxide nanoplates/reduced graphene oxide composites (NNb₂O₅/rGO) and their photocatalytic activity towards hydrogen generation from water and H₂S under natural sunlight has been demonstrated, uniquely. Nanostructured NNb₂O₅/rGO is synthesized by in situ wet chemical method using urea as a source of nitrogen and optimized by varying percentage of graphene oxide (GO). X?ray diffraction (XRD) study reveals that NNb₂O₅ have orthorhombic crystal structure with crystalline size, 35 nm. Further, X?ray photoelectron spectroscopy (XPS) confirm the presence of nitrogen and rGO in NNb₂O₅/rGO nanocomposite. Morphological features of (NNb₂O₅/rGO) were examined by FE?SEM and FE?TEM showed Nb₂O₅ nanoplates of diameter 25–40 nm anchored on 2D rGO. Diffuse reflectance spectra depicts the extended absorbance in the visible region with band gap of 2.2 eV. Considering the band gap in the visible region, the photocatalytic hydrogen generation from water and H₂S has been performed. The 1 wt % rGO hybridized NNb₂O₅ (S2) exhibited superior photocatalytic hydrogen generation (537 μmol/h) from water and (1385 μmol/h) from H₂S under sunlight. The improved photocatalytic activity is attributed due to an extended absorbance in the visible region, modified electronic structure upon doping and formation of well defined NNb₂O₅/rGO interface, provides large surface area, accelerates the supression of electron and hole pairs recombination rate. In our opinion, this works may provides facile route for energy efficient and economic approach for fabrication of NNb₂O₅/rGO nanocomposites as a visible light active photocatalyst.     

Numerical analysis of H2 formation during partial oxidation of H2SH2O upon activation of oxidizer by an electric discharge

The numerical analysis of H₂ production during partial oxidation of H₂SH₂O in a plug-flow reactor at atmospheric pressure and a rather low temperature (T₀ = 500 K) was conducted, when the oxidizer (oxygen or air) was preliminarily activated by an electrical discharge with different values of reduced electric field and input energy. It was shown that a significant hydrogen yield in flow reactor can be obtained only after ignition of the mixture. The ignition delay length depends on the reduced electric field E/N and input energy E_s in the discharge and is minimal at E/N～8–10 Td for the discharge in oxygen and at E/N～4–10 and 120–150 Td in air discharge, when O₂(a¹Δ_g) mole fraction in the discharge products is maximal. If the H₂SH₂OO₂(air) mixture ignites inside the flow reactor, the mole fraction of hydrogen and its relative yield do not depend on E/N. The relative hydrogen yield increases monotonically with an addition of water to H₂S. It was found, that the approach based on the partial oxidation of the H₂SH₂O mixture upon activation of oxygen by an electric discharge can ensure very low energy cost for H₂ production. The minimum specific energy requirement, obtained for the H₂SO₂ mixture, was found to be 0.83 eV/(molecule H₂) and 0.18 eV/(molecule H₂S) at atmospheric pressure and can be further decreased if the energy released during partial oxidation of H₂S is spent on heating the reagents. The use of air as an oxidizer requires higher energy costs and seems to be less promising.     

Hydrogen storage properties of nanocrystalline Mg2Ni prepared from compressed 2MgH2Ni powder

In the present work, nanocrystalline Mg₂Ni with an average size of 20–50 nm was prepared via ball milling of a 2MgH₂Ni powder followed by compression under a pressure of 280 MPa. The phase component, microstructure, and hydrogen sorption properties were characterized by using X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), pressure-composition-temperature (PCT) and synchronous thermal analyses (DSC/TG). Compared to the non-compressed 2MgH₂Ni powder, the compressed 2MgH₂Ni pellet shows lower dehydrogenation temperature (290 °C) and a single-phase Mg₂Ni is obtained after hydrogen desorption. PCT measurements show that the nanocrystalline Mg₂Ni obtained from dehydrogenated 2MgH₂Ni pellet has a single step hydrogen absorption and desorption with fairly low absorption (?57.47 kJ/mol H₂) and desorption (61.26 kJ/mol H₂) enthalpies. It has very fast hydrogen absorption kinetics at 375 °C with about 3.44 wt% hydrogen absorbed in less than 5 min. The results gathered in this study show that ball milling followed by compression is an efficient method to produce Mg-based ternary hydrides.     

Performance of CoNiO spinel oxide coating on AISI 430 stainless steel as interconnect for intermediate temperature solid oxide fuel cell

In order to decrease oxide growth kinetics, maintain suitable conductivity and prevent Cr-volatilization of AISI 430 stainless steels (430 SS) as the interconnect for intermediate temperature solid oxide fuel cells (SOFCs), a CoNiO spinel oxide protective coating has been successfully fabricated on the 430 SS specimen using a simple and cheap process with two steps: 1) electroplation of CoNi alloy layer and 2) pre-oxidation treatment to convert the CoNi alloy into spinel oxide. The CoNiO spinel layer on the 430 SS (CoNiO 430 SS) is dense and uniform with 8–10 μm thickness. And the CoNiO spinel oxide protective coating consists of a main face-centered-cubic (fcc) NiCo₂O₄ spinel phase and a minor fcc NiO phase. Compared with bare 430 SS, the oxidation resistance and the conductivity of the CoNiO 430 SS have been improved remarkably under simulated typical SOFC operating cathode conditions (at 800 °C in air). After an isothermal oxidation test at 800 °C, the area specific resistance (ASR) of CoNiO 430 SS is much lower and stable (0.1 Ω cm² for 100 h and 0.9 Ω cm² for 600 h) than that of bare 430 SS (1.2 Ω cm² for 100 h and 2.4 Ω cm² for 600 h). These performances of CoNiO 430 SS imply that it can be a promising candidate interconnect for solid oxide fuel cell.     

Improved hydrogen storage properties of MgH2 with Ni-based compounds

The nanoscaled Ni-based compounds (Ni₃C, Ni₃N, NiO and Ni₂P) are synthesized by chemical methods. The MgH₂-X (X = Ni₃C, Ni₃N, NiO and Ni₂P) composites are prepared by mechanical ball-milling. The dehydrogenation properties of Mg-based composites are systematically studied using isothermal dehydrogenation apparatus, temperature-programmed desorption system and differential scanning calorimetry. It is experimentally confirmed that the dehydrogenation performance of the Mg-based materials ranks as following: MgH₂Ni₃C, MgH₂Ni₃N, MgH₂NiO and MgH₂Ni₂P. The onset dehydrogenation temperatures of MgH₂Ni₃C, MgH₂Ni₃N, MgH₂NiO and MgH₂Ni₂P are 160 °C, 180 °C, 205 °C and 248 °C, respectively. The four Mg-based composites respectively release 6.2, 4.9, 4.1 and 3.5 wt% H₂ within 20 min at 300 °C. The activation energies of MgH₂Ni₃C, MgH₂Ni₃N, MgH₂NiO and MgH₂Ni₂P are 97.8, 100.0, 119.7 and 132.5 kJ mol^?1, respectively. It' found that the MgH₂Ni₃C composites exhibit the best hydrogen storage properties. Moreover, the catalytic mechanism of the Ni-based compounds is also discussed. It is found that Ni binding with low electron-negativity element is favorable for the dehydrogenation of the Mg-based composites.     

Effective excitons separation on graphene supported ZrO2TiO2 heterojunction for enhanced H2 production under solar light

A novel photocatalyst comprises of ZrO₂TiO₂ immobilized on reduced graphene oxide (rGO) – a ternary heterojunction (ZrO₂TiO₂/rGO) was synthesized by using facile chemical method. The nanocomposite was prepared with a strategy to achieve better utilization of excitons for catalytic reactions by channelizing from metal oxide surfaces to rGO support. TEM and XRD analysis results revealed the heterojunction formed between ZrO₂ and single crystalline anatase TiO₂. The mesoporous structure of ZrO₂TiO₂ was confirmed using BET analysis. The red shift in absorption edge position of ZrO₂TiO₂/rGO photocatalyst was characterized by using diffuse reflectance UV–Visible spectra. ZrO₂TiO₂/rGO showed greater interfacial charge transfer efficiency than ZrO₂TiO₂, which was evidenced by well suppressed PL intensity and high photocurrent of ZrO₂TiO₂/rGO. The suitable band gap of 1.0 wt% ZrO₂TiO₂/rGO facilitated the utilization of solar light in a wide range by responding to the light of energy equal to as well as greater than 2.95 eV by the additional formation of excited high-energy electrons (HEEs). ZrO₂TiO₂/rGO showed the enhanced H₂ production than TiO₂/rGO, which revealed the role of ZrO₂ for the effective charge separation at the heterojunction and the solar light response. The optimum loading of 1.0 wt% of ZrO₂ and rGO on TiO₂ showed the highest photocatalytic performance (7773 μmolh^?1$g_{cat}^{?1}$) for hydrogen (H₂) production under direct solar light irradiation.     

Experimental and first principle studies on hydrogen desorption behavior of graphene nanofibre catalyzed MgH2

With the combination of experiment and first-principles theory, we have evaluated and explored the catalytic effects of graphitic nanofibres for hydrogen desorption behaviour in magnesium hydride. Helical form of graphene nanofibres (HGNF) have larger surface area, curved configuration and high density of graphene layers resulting in large quantity of exposed carbon sheet edges. Therefore they are found to considerably improve hydrogen desorption from MgH₂ at lower temperatures compared to graphene (onset desorption temperature of MgH₂ catalyzed by HGNF is 45 °C lower as compared to MgH₂ catalyzed by graphene). Using density functional theory, we find that graphene sheet edges, both the zigzag and armchair type, can weaken MgH bonds in magnesium hydride. When the MgH₂ is catalyzed with higher electronegative and reactive graphene edge of graphene, the electron transfer occurs from Mg to carbon, due to which MgH₂ is dissociated into hydrogen and MgH component. The Mg gets bonded with the graphene edge carbon atoms in the form of CMgH and CH bonds. In the as formed CMgH, the graphene edges “grab” more electronic charge as compared to the normal charge donation of Mg to H. This leads to the weakening of the MgH bond, causing hydrogen to desorbs at lower temperatures.     

Hollow nanoporous NiPd catalysts with enhanced performance for ethanol electro-oxidation

In this work, bimetallic NiPd hollow nanoporous (HNiPd) catalysts are prepared by in-situ deposition of Pd nanoparticles (Pd NPs) on hollow Ni (HNi) microspheres. Scanning electron microscope (SEM) and transmission electron microscopy (TEM) reveal the hollow nanoporous essence of HNiPd catalysts. Meanwhile, using high-angle annular dark-field scanning TEM (HAADF-STEM) and elemental mapping, it is found that tiny dendritic-like NiPd nanocomposites attach on the exterior of microspheres. The content of Pd is easily tailored to constitute HNiPd catalysts with different Ni/Pd atomic ratios. Further electrochemical evaluation vindicates that the as-prepared HNiPd catalysts have a good catalytic activity and stability toward ethanol oxidation reaction (EOR) in alkaline medium. Notably, the peak current density of HNi_3.1Pd catalyst and the chronoamperometric current density of HNi_4.6Pd catalyst are 4 and 2 times of Pd/C (JM) catalyst, respectively, which show that HNiPd catalysts hold great potential in application of alkaline direct ethanol fuel cells (DEFCs).     

Promoted activity of porous silica coated Ni/CeO2ZrO2 catalyst for steam reforming of acetic acid

Porous silica coated Ni/CeO₂ZrO₂ catalysts were synthesized for steam reforming of acetic acid. The silica coated Ni/CeO₂ZrO₂ catalyst showed a significantly enhanced activity (95% acetic acid conversion) than the Ni/CeO₂ZrO₂ catalysts (62% acetic acid conversion) at a low temperature (550 °C). Interaction between Ni/CeO₂ZrO₂ and silica layer was proved to be a crucial role on enhancing of catalytic activities. Further characterization (XPS, H₂-TPR) indicates this interaction facilitates the steam reforming reaction and raises the selectivity of CO by modifying the surface Ni electronic structure. In addition, the coated catalyst also exhibited a good stability and no obvious deactivation was detected at 550 °C and 600 °C within 30 h.     

PtRh alloys on hybrid TiO2 – Carbon support as high efficiency catalyst for ethanol oxidation

High cost and poor stability of catalysts remain major obstacles for the commercialization of direct ethanol fuel cells (DEFCs). In this work, a Pt₉Rh/TiO₂C nanostructured catalyst is synthesized via an impregnation-reduction method followed by thermal annealing in N₂ at ambient pressure. X-ray powder diffraction (XRD) and scanning transmission electron microscopy (STEM) are used to characterize the corresponding physico-chemical properties of the as-prepared catalysts. The results reveal that PtRh nanoparticles are uniformly distributed on the TiO₂C hybrid support material. Cyclic voltammetry, linear scan voltammetry, CO-stripping voltammograms, chronoamperometry and chronopotentiometry methods are employed to investigate their catalytic performance for ethanol oxidation. The results show that the Pt₉Rh/TiO₂C produced a current density of 1039.5 mA mg_Pt^?1, which are 3.98, 8.31 and 2.43 times higher than Pt/TiO₂C, Pt/C and Pt₉Rh/C, respectively. Furthermore, the Pt₉Rh/TiO₂C also has greater resistance to CO-poisoning and displays better stability for ethanol oxidation than other catalysts. Pt₉Rh/TiO₂C therefore provides a promising material for ethanol oxidation in direct ethanol fuel cells.     

Fatigue limit of carbon and CrMo steels as a small fatigue crack threshold in high-pressure hydrogen gas

The fatigue limit properties of a carbon steel and a low-alloy CrMo steel were investigated via fully-reversed tension-compression tests, using smooth specimens in air and in 115-MPa hydrogen gas. With respect to the CrMo steel, specimens with sharp notches were also tested in order to investigate the threshold behavior of small cracks. The obtained SN data inferred that the fatigue limit was not negatively affected by hydrogen in either of the steels. Observation of fatigue cracks in the unbroken specimens revealed that non-propagating cracks can exist even in 115-MPa hydrogen gas, and that the crack growth threshold is not degraded by hydrogen. The experimental results provide justification for the fatigue limit design of components that are to be exposed to high-pressure hydrogen gas.     

Hydrogen-iodide decomposition over PdCeO2 nanocatalyst for hydrogen production in sulfur-iodine thermochemical cycle

A highly active and stable catalyst for hydrogen-iodide decomposition reaction in sulfur-iodine (SI) cycle has been prepared in the form of PdCeO₂ nanocatalyst by sol-gel method with different calcination temperatures (300 °C, 500 °C, and 700 °C). XRD and TEM confirmed a size around 6–8 nm for PdCeO₂ particles calcined at 300 °C. Raman study revealed large number oxygen vacancies in PdCeO₂-300 when compared to PdCeO₂-500 and PdCeO₂-700. With increase in calcination temperature, the average particle size increased whereas the specific surface area and number of oxygen vacancies decreased. Hydrogen-iodide catalytic-decomposition was carried out in the temperature range of 400°C–550 °C in a quartz-tube, vertical, fixed-bed reactor with 55 wt % aqueous hydrogen-iodide feed over PdCeO₂ catalyst using nitrogen as a carrier gas. PdCeO₂-300 showed hydrogen-iodide conversion of 23.3%, which is close to the theoretical equilibrium conversion of 24%, at 550 °C. It also showed a reasonable stability with a time-on-stream of 5 h.     

Grain refinement and Al-water reactivity of AlGaInSn alloys

AlGaInSn alloys were prepared by using traditional casting metallurgy method with different additions of Al5Ti1B grain refiner. Their microstructures were investigated by means of X-ray diffraction (XRD) and scanning electron microscope (SEM) with energy dispersed X-ray (EDX). The Al grains of alloys are refined significantly from 129 μm to 57 μm with increasing Ti content from 0.03 wt% to 0.24 wt%. Many thin dendrites that are a few micrometers thick are observed within Al grains.Al-water reactivities were performed under different water temperatures. The alloy with Ti content of 0.12 wt% shows the maximum H₂ generation rate under different water temperatures, which is above 5 times of Ti-free alloy. The H₂ yields of alloys drop from 87% to 30% with rising Ti content from 0.03 wt% to 0.24 wt% at the water temperature of 30 °C, but they rise to about 90% when the water temperature is above 50 °C.The growth mechanism of alloys and the effect of grain refinement on Al-water reactivities are discussed.     

Different modified multi-walled carbon nanotube–based anodes to improve the performance of microbial fuel cells

To clearly illustrate the activity effect of multi-walled carbon nanotubes (MWCNTs) and their functionality on anodic exoelectrogen in microbial fuel cells (MFCs), the growth of E. coli and anode biofilm on MWCNT-, MWCNTCOOH and MWCNTNH₂ modified anodes were compared with a bare carbon cloth anode. The activity effect was characterized by the amount of colony-forming units (CFUs), activity biomass, morphology of biofilms and cyclic voltammetric (CV). The results showed that MWCNTs, MWCNT-COOH and MWCNT-NH₂ exhibited good biocompatibility on exoelectrogenic bacteria. The performance of MFCs were improved through the introduction of MWCNT-modified anodes, especially in the presence of COOH/NH₂ groups. The MFCs with the MWCNTCOOHmodified anode achieved a maximum power density of 560.40 mW/m², which was 49% higher than that obtained with pure carbon cloth. In conclusion, the positive effects of MWCNTs and their functionality were evaluated for promoting biofilm formation, biodegradation and electron transfer on anodes. Specifically, the MWCNTCOOHmodified anode demonstrated the largest application potential for the development of MFCs.     

Effect of combining Al,Mg, Ce or La oxides to extracted rice husk nanosilica on the catalytic performance of NiO during COx-free hydrogen production via methane decomposition

Amorphous nanosilica powder was extracted from rice husk and used as a catalyst support as well as a starting material for the preparation of different binary oxides, i.e., SiO₂Al₂O₃, SiO₂MgO, SiO₂CeO₂ and SiO₂La₂O₃. A series of supported nickel catalysts with the metal loading of 50 wt % were prepared by wet impregnation method and evaluated in methane decomposition to “CO_x-free” hydrogen production. The fresh and spent catalysts were extensively characterized by different techniques. Among the evaluated catalysts, both Ni/SiO₂Al₂O₃ and Ni/SiO₂La₂O₃ catalysts were the most active with an over-all H₂ yield of ca. 80% at the initial period of the reaction. This distinguishable higher catalytic activity is mainly referred to the presence of free mobile surface NiO and/or that NiO fraction weakly interacted with the support easily reducible at low temperatures. The Ni/SiO₂CeO₂ catalyst has proven a great potential for application in the hydrogen production in terms of its catalytic stability. The formation of Mg_xNi_(1?x)O solid solution caused the Ni/SiO₂MgO catalyst to lose its activity and stability at a long reaction time. Various types of carbon materials were formed on the catalyst surface depending on the type of support used. TEM images of as-deposited carbon showed that multi-walled carbon nanotubes (MWCNTs) and graphene platelets were formed on Ni/SiO₂, while only MWCNTs were deposited on all binary oxide supported Ni catalysts.     

Achieving the dehydriding reversibility and elevating the equilibrium pressure of YFe2 alloy by partial Y substitution with Zr

To overcome the hydrogen-induced amorphization and phase disproportionation in the fast de-/hydrogenation of YFe₂, the alloying of partial substituting Y with Zr was carried out to obtain Y_1?xZr_xFe₂ (x = 0.1, 0.2, 0.3, 0.5) alloys. All YZrFe alloys remained single C15 Laves phase structure at states of as-annealed, hydrogenated and dehydrogenated. With the increasing of Zr content, the YZrFe alloys showed the decrease in the lattice constants and hydrogenation capacity, but the increase in the dehydrogenation capacity and dehydriding equilibrium pressure. The alloy Y_0.9Zr_0.1Fe₂ showed maximum initial hydrogenation capacity of 1.87 wt% H, while the alloy Y_0.5Zr_0.5Fe₂ showed highest desorption capacity of 1.26 wt% with obvious dehydriding plateau. Based on experiment analysis and first principle calculation of binding energy, the great improvement in the dehydriding thermodynamics for YZrFe alloys is attributed to the change in the unit cell volume, electron concentration and stability of hydrides due to the Zr substitution.     

Fabrication of Pt-doped carbon aerogels for hydrogen storage by radiation method

A radiation method was investigated to fabricate Pt-doped carbon aerogels (CAsPt). The physicochemical properties of the pristine CAs and CAsPt were systematically characterized by X-ray diffraction, scanning electron microscope, transmission electron microscopy, and nitrogen adsorption measurements. The results showed that not only a great number of Pt nanospheres but also many Pt nanoparticles presented in the network of CAs after radiation. The influence of Pt doping on the hydrogen uptake capacity of CAs was studied. In comparison with the pristine CAs, it was remarkable that the hydrogen uptake capacity of the CAsPt had been significantly enhanced, which was contributed by the hydrogen spillover of Pt.     

The catalytic effect of an inert additive (SrTiO3) on the hydrogen storage properties of 4MgH2Na3AlH6

Investigations on the catalytic effects of a non-reactive and stable additive, SrTiO₃, on the hydrogen storage properties of the 4MgH₂Na₃AlH₆ destabilized system were carried out for the first time. The Na₃AlH₆ compound and the destabilized systems used in the investigations are prepared using ball milling method. The doped system, 4MgH₂Na₃AlH₆SrTiO₃, had an initial dehydrogenation temperature of 145 °C, which 25 °C lower as compared to the un-doped system. The isothermal absorption and desorption capacity at 320 °C has increased by 1.2 wt% and 1.6 wt% with the addition of SrTiO₃ as compared to the 4MgH₂Na₃AlH₆ destabilized system. The decomposition activation energy of the doped system is estimated to be 117.1 kJ/mol. As for the XRD analyses at different decomposition stages, SrTiO₃ is found to be stable and inert. In addition to SrTiO₃, similar phases are found in the doped and the un-doped system during the decomposition and dehydrogenation processes. Therefore, the catalytic effect of the SrTiO₃ is speculated owing to its ability to modify the physical structure of the 4MgH₂Na₃AlH₆ particles through pulverization effect.     

Mechanistic insights into tandem amine-borane dehydrogenation and alkene hydrogenation catalyzed by [Pd(NHC)(PCy3)]

The mechanism of tandem dimethylamine-borane (NHMe₂BH₃, DMAB) dehydrogenation and alkene hydrogenation catalyzed by [Pd(NHC)(PMe₃)] are investigated by density functional theory (DFT) calculations [NHC = N,N′-bis(2,6-diisopropylphenyl) imidazole-2-ylidene]. Four possible DMAB dehydrogenation mechanisms have been carefully investigated involving concerted BH/NH activation, sequential BH/NH activation, sequential NH/BH activation, and proton transfer mechanism. DFT studies show that the NH proton transfers to ligated carbene carbon and sequential CH/BH activation is the most kinetically favorable pathway with the lowest activation barrier of 23.8 kcal/mol. For hydrogenation, it was found that a trans-dihydride Pd(II) complex, [Pd(H)₂(NHC)(PMe₃)], formed in the dehydrogenation process, serves as an effective catalyst for reduction of trans-stilbene.     

One-step synthesis of CuCo alloy/Mn2O3Al2O3 composites and their application in higher alcohol synthesis from syngas

CuCo alloy/Mn₂O₃Al₂O₃ composites were synthesized by a facile one-step sol–gel method using citric acid as a chelating agent. The catalytic performance of the as-prepared catalysts was investigated in reaction of CO hydrogenation to higher alcohols. According to the characterization data obtained by TG-DSC, XRD, TPR, BET, ICP, SEM, TEM and XPS, a stronger interaction between the Cu and Co ions in the CuCo₂O₄ particles led to the formation of CuCo alloy in the reduced catalysts, and the Mn/Al molar ratio significantly influenced the performance of the catalysts. Mn₂O₃Al₂O₃ composites reduce the unwanted CoAl₂O₄ spinel phase, offer tunable pore sizes and surface areas, and also appear to act as barriers to hinder the CuCo alloy particles sintering. The results suggest that the metal nanoparticles of CuCo alloy together with Mn₂O₃Al₂O₃ contributed to the high selectivity of higher alcohols as well as the good stability. A Mn/Al molar ratio of 5/3 was found to be most suitable for the catalyst properties in terms of activity and product distribution.