Iron oxide-Palladium core-shell nanospheres for ferromagnetic resonance-based hydrogen gas sensing

Interfaces of ferromagnetic transition metals such as Iron, Cobalt, and Nickel with non-magnetic palladium are of interest due to their unique magnetic and spintronic properties. These interfaces enable ferromagnetic resonance (FMR) based sensing of hydrogen gas. In the present work, we synthesized Fe₃O₄–Pd core-shell nanospheres via a one-pot synthesis method using the thermal decomposition of Fe³⁺ acetylacetonate in the presence of a reducing agent to produce the Fe₃O₄ core, followed by the reduction of a Pd²⁺ precursor to form the pure Pd shell. We found that our in-situ synthesized core-shell nanostructure is magnetically active and shows excellent H₂ gas sensing properties. The effect of reversible hydrogen gas absorption on the magnetism of Fe₃O₄–Pd core-shell nanospheres was investigated. The hydrogen-induced ferromagnetic-resonance (FMR) peak shift amounted to 30% of the peak linewidth for the virgin state of the sample. In addition, in the presence of hydrogen gas, we observed a fully reversible decrease in the FMR peak linewidth by about two times. This was accompanied by a nearly doubling of the FMR peak height. Response and recovery times of about 2 and 50 s, respectively, were extracted from the measurements. All the data was collected using a mix of just 3% hydrogen in a nitrogen carrier gas.     

Low-temperature CO preferential oxidation in H2-rich stream over iron modified Pd–Cu/hydroxyapatite catalyst

The effect of FeCl₃ addition on the catalytic property of Pd–Cu/hydroxyapatite (Pd–Cu/HAP) for low-temperature CO preferential oxidation (CO-PROX) under H₂-rich condition has been investigated. It can be found that CO conversion of Pd–Cu/HAP rapidly decreases from 56% to 21% within 2 h at 30 °C in the presence of water, however, the Pd–Cu–Fe/HAP with the Fe/Cu atomic ratio of 1:1 presents a stable CO conversion of 40% and CO₂ selectivity of 100% under the same reaction conditions. The characterization results display that the addition of FeCl₃ to Pd–Cu/HAP causes the formation of Fe₂O₃ species, and the strong interaction presents between Fe₂O₃ species and Pd–Cu/HAP. Thus, the Pd⁰ species generated during CO-PROX over Pd–Cu–Fe/HAP can be more easily oxidized than that over Pd–Cu/HAP, which could avoid H₂ adsorption on Pd⁰ species and maintain CO adsorption and activation.     

Small Pd cluster adsorbed double vacancy defect graphene sheet for hydrogen storage: A first-principles study

Stability and electronic properties of small Pd_n clusters (n = 1–5), adsorbed on different types of double vacancy (DV) defect graphene sheets are thoroughly investigated by both density functional theory (DFT) and molecular dynamics (MD). Defect bridge sites of DV(555-777) defect graphene sheet are identified to be the most favorable for Pd₄ cluster adsorption. MD calculations, performed using a canonical ensemble, showed this system to be highly stable up to 800 K. Much better hybridization between C 2p and Pd 4d and 5s orbitals near Fermi level as well as higher charge transfer to graphene sheet was found to be the governing reason for enhanced stability of Pd₄ cluster on DV(555-777) defect site. Comparative analysis of H₂ storage on Pd₄ cluster adsorbed pristine and DV(555-777) defect graphene sheet showed, while adsorption energy/H₂ molecule for both cases lie well within desirable energy window for a hydrogen storage media, the later is much more efficient energetically as distorted in plane sp² hybridization reduces the saturations of C–C bonds in the defect regions, making more electron density available for bonding; which leads to higher net charge gain of Pd₄ cluster and higher charge sharing with H₂ molecule.     

Hydrogen generation rate enhancement by in situ Fe(0) and nitroarene substrates in Fe3O4@Pd catalyzed ammonia borane hydrolysis and nitroarene reduction tandem reaction

We report the catalytic enhancement of hydrogen generation by 1) in situ Fe (0) formed and 2) nitroarenes substrates during Fe₃O₄@Pd core-shell nanoparticles catalyzed tandem reaction. The active hydrogen species are generated in Pd shell, which either combine to form H₂ gas or take part in relatively faster nitroarene reduction reaction. The rate of hydrogen generation from ammonia borane is dependent on the nitroarene substrate and is higher when 4-nitrophenol is used. This is due to the difference in ammonia borane adsorption on the surface of the catalyst. During recyclability, the H₂ generation rate of 2 wt% Pd loaded samples is higher than other compositions. Such an enhancement has been attributed to the formation of Fe (0) via γ-FeOOH mediated by Pd species, presumably through Pd(OH)₂. The electronic connection between Fe and Pd interface is thus shown to play an important role in the catalytic enhancement of the tandem reaction.     

Theoretical study of hydrogen adsorption on nitrogen doped graphene decorated with palladium clusters

Density functional theory was used to study the adsorption of hydrogen on small palladium clusters (Pd_n, n = 1–4) supported on pyridine-like nitrogen doped graphene. Charge transfer and strong binding (up to four times higher than binding energy of Pd cluster supported on graphene) between graphene–nitrogen and palladium clusters prevent detachment of clusters and leads to three types of adsorption states of hydrogen. The first type is a molecular hydrogen physisorbed, the second one is an activated state of H₂ without adsorption barriers where H–H bond is relaxed and the third type is dissociated state. In dissociated cases, we found barriers below 0.56 eV. This means the process might occur spontaneously at room temperature. We also show that metal–N–vacancy complexes are very stable and involve nitrogen sp² and p_z, carbon p_z and palladium d orbitals. Besides, these systems exhibit an interesting magnetic behavior.     

Fast synthesis and electrocatalytic activity of M@Pt (M = Ru,Fe3O4, Pd) core–shell nanostructures for the oxidation of ethanol and methanol

We report the fast synthesis and electrochemical evaluation of M@Pt core–shell nanostructures (M = Ru, Fe₃O₄, Pd) as well as Pt-alone nanoparticles for the ethanol and methanol oxidation reactions (EOR and MOR, respectively) in H₂SO₄ solution. The cores were obtained in 60 s. The Pt shells were deposited afterward on the cores also in 60 s. The reducing agent was NaBH₄. XRD results showed that crystalline Pd, Fe₃O₄ and Pt materials were obtained from this rapid process, while Ru was formed as a quasi-amorphous structure. The average particle sizes of the core–shell nanostructures determined with the Scherrer equation, ranged from 7.35 to 9.29 nm. The electrochemical characterization revealed a catalytic activity of the novel Fe₃O₄@Pt anode as high as that of Ru@Pt for the EOR, and higher than the activity of Pt-alone. However, the Fe₃O₄@Pt catalysts showed a lower activity for the MOR than Ru@Pt and Pt-alone. Durability tests indicated a high electrochemical stability of the Fe₃O₄@Pt and Ru@Pt nanostructures.     

Pd nanoparticles anchoring to core-shell Fe3O4@SiO2-porous carbon catalysts for ammonia borane hydrolysis

A modified Stöber method is applied to synthesize the magnetic core-shell Fe₃O₄@SiO₂ particles, followed by compositing a series of porous glucose-derived carbon with ZnCl₂ as etchant. Then, ultrafine Pd nanoparticles (NPs) are successfully anchored to the resulting Fe₃O₄@SiO₂-PC composites with an in-situ reduction strategy. The particle sizes of Pd NPs are mainly centered in the range of 2.3–4.3 nm in the as-prepared Pd/Fe₃O₄@SiO₂-PC catalysts, owning a hierarchical porous structure with high specific surface area (S_BET = 626.0 m² g⁻¹) and large pore volume (V_p = 0.61 cm³ g⁻¹). Their catalytic behavior for the hydrogen generation from ammonia borane (AB) hydrolysis is investigated in details. The corresponding apparent activation energy is as low as 28.4 kJ mol⁻¹ and the reaction orders with AB and Pd concentrations are near zero and 1.10 under the present conditions, respectively. In addition, the magnetic catalysts, which could be easily separated out by a magnet, are still highly active even after nine runs, revealing their excellent reusability.     

Probing the enhanced methanol electrooxidation mechanism by promoted CO tolerance on the Pd catalyst modified with TaN: A combined experimental and theoretical study

The low-palladium Pd/TaN–C catalyst is synthesized by a surfactant-free solvothermal approach and exhibits high activity (2613.18 mA mg_Pd⁻¹), durability and CO tolerance for MOR (methanol oxidation reaction) in alkaline media, 12.4 folds that of the commercial Pd/C. XPS and electrochemical results indicate that the interfacial Pd–TaNO bond is generated. This also brings the enhancement of OH_ad adsorption responsible for anti-CO poisoning ability. Density Functional Theory (DFT) calculations indicate that the reaction pathway and the rate-determining step are changed for methanol decomposition to CO on the Pd₄/TaN(001) surface compared with Pd (111). The preferred pathway can be described as: CH₃OH→CH₃O→CH₂O→CHO→CO. Furthermore, the results indicate that the adsorption of OH is enhanced and the energy barrier of COOH formation from CO + OH is reduced with the high concentration of hydroxyl on the Pd₄/TaN(001) surface, further confirming the bi-functional effect of hydroxyl on the CO tolerance.     

Sm2O3 promoted Pd/rGO electrocatalyst for formic acid oxidation

Herein, we report the synthesis of Sm₂O₃ incorporated palladium based electrocatalyst, supported over reduced graphene oxide (rGO). Pd_xSm_y nanoparticles (20 wt %) are distributed over rGO surface by using sodium borohydride reduction technique. Different physicochemical analyses are used to characterize the Pd_xSm_y/rGO electro-catalysts (x = 2,4,6 and y = 8,6,4). The synthesized materials (Pd_xSm_y/rGOs) are investigated for their catalytic capabilities toward electro-oxidation of formic acid. The results show that adding Sm₂O₃ to the Pd/rGO electrocatalyst boosts electrochemical activities of the materials toward formic acid oxidation. The optimized catalyst (Pd₆Sm₄/rGO) shows excellent activity towards formic acid oxidation with current density of 46.70 mA/cm² compared to reference catalysts i.e., Pd/rGO (28.78 mA/cm²) and Pd/C (22.22 mA/cm²). The optimized catalyst also demonstrates high CO tolerance and great stability during formic acid oxidation reaction. The enhanced activity and stability are attributed to the synergistic interaction between Sm₂O_3, and palladium nanoparticles supported over reduced graphene oxide.     

Defect and interstitial B synergistically promote the stability and H2 dissociation of Pd6 supported by graphene

As a potential hydrogenation catalyst, palladium nanoparticles supported by graphene encounter three major problems: transition metal agglomeration, interstitial H atom, and the competition between desorption of H₂ and Pd_n-H_x complex from graphene sheet. In this paper, defects and interstitial B are used to promote the stability and H₂ dissociation of Pd₆ supported by graphene. The introduction of defects increases the binding energy of Pd, Pd₆ and Pd₆B to graphene by a factor of 5–7 and 7–9 before and after hydrogen adsorption, respectively. It indicates that defects can effectively avoid the desorption competition between Pd_nH_x and hydrogen molecules. Moreover, the energy barrier of dissociation for the first hydrogen molecule on Pd₆B/C₄₉ is 0.49 eV, which is lower than 0.75 eV on Pd₆/C₄₉ and 0.69 eV on Ti₆/C₄₉.     

Effect of preoxidation on the reduction of ilmenite concentrate powder by hydrogen

In this study, hydrogen reduction of raw and preoxidized ilmenite concentrates powder, with particle size less than 74 μm, is investigated using a thermal analyzer at 1123, 1173, and 1223 K in a 30% H₂ + 70% Ar atmosphere. The reduction rate by hydrogen was found to be accelerated due to preoxidation treatment of the raw ilmenite concentrate. The reduction of both raw and preoxidized ilmenite concentrates can be divided into two stages: Fe³⁺→Fe²⁺ and Fe²⁺→Fe. The Fe₃O₄→FeO stage overlaps with the Fe₂O₃→Fe₃O₄ stage during reduction process of the preoxidized ilmenite concentrate. Moreover, the preoxidation treatment can effectively decrease the reduction activation energy. The scanning electron microscopy (SEM) and the X-ray powder diffraction (XRD) techniques were used to characterize the micromorphology and phase of the products. The promotion mechanism of reduction of ilmenite by hydrogen through preoxidation treatments is also discussed.     

Photocatalytic hydrogen production using Fe2O3-based core shell nano particles with ZnS and CdS

Stability and efficiency of photocatalysts are important to realize the practical applications of them for photocatalytic hydrogen production from industrial sulfide effluent. Novel, magnetically separable core–shell nano photocatalysts viz., CdS/Fe₂O₃, ZnS/Fe₂O₃ and (CdS + ZnS)/Fe₂O₃ were prepared and their hydrogen evolution activity under visible light was examined. The XRD result shows that CdS and ZnS were very well coated on the surface of the iron oxide core shell particles. The HR-TEM result also confirms the core shell formation. (CdS + ZnS)/Fe₂O₃ evolved higher volume of hydrogen than the other catalysts. It is ascribed to rapid migration of excited electrons from (CdS + ZnS) toward Fe₂O₃ suppressing electron hole annihilation compared to other catalysts. The catalysts can be easily recovered from the reaction medium using external magnetic bar and so the photocatalyst can be reused without any mass loss. Hence, it can be a potential catalyst for recovery of hydrogen from industrial sulfide containing waste streams.     

Design of PdxIr/g-C3N4 modified FTO to facilitate electricity generation and hydrogen evolution in alkaline media

In current work, the performance of Pd_xIr hybridized with g-C₃N₄ (Pd_xIr/g-C₃N₄) onto a fluorine-doped tin oxide (FTO) glass was investigated for alcohols oxidation reaction and hydrogen evolution reaction in alkaline media. The nanostructures were synthesized with convenient one-step solvothermal method and characterized with ICP-AES, EDX, XRD and TEM techniques. For comparison, Pd/g-C₃N₄ and Pt/C catalyst-coated FTO were also investigated. Higher current densities of 250 for methanol oxidation and 2570 mA mg⁻¹ for glycerol oxidation and better stability in the presence of Pd₃Ir/g-C₃N₄ compared to the other catalysts were proven by cyclic voltammetry and chronoamperometry. Electro-oxidation mechanism was investigated using linear sweep voltammetry method. Also, Pd₃Ir/g-C₃N₄ showed the Tafel slope of 72 mV dec⁻¹ and good stability in alkaline media which is comparable to that of other catalysts for hydrogen evolution reaction.     

Tuning the alloy degree for Pd-M/Al2O3 (M=Co/ Ni /Cu) bimetallic catalysts to enhance the activity and selectivity of dodecahydro-N-ethylcarbazole dehydrogenation

Hydrogen is a promising candidate to substitute the fossil fuels. However, the efficient hydrogen storage technologies restrict the commercial applications. Developing new catalysts with high activity and selectivity is important for the dehydrogenation reaction in N-ethylcarbazole/dodecahydro-N-ethylcarbazole (NECZ/12H-NECZ) hydrogen storage system. In this work, a series of Pd-M/Al₂O₃ (M = Co, Ni and Cu) bimetallic catalysts are synthesized successfully and show good performance in the dehydrogenation reaction of 12H-NECZ than the commercial Pd/Al₂O₃ catalyst. The Pd₁Co₁/Al₂O₃ catalyst (Practical Pd content = 2.4136 wt%) showed the highest catalytic performance with 95.34% H₂ release amount, TOF of 230.5 min⁻¹ and 85.4% selectivity of NECZ. Combined with the characterization analysis, it can be proposed that the dehydrogenation performance of 12H-NECZ is dependent on the alloy phases, reasonable electronic structures and nanoparticle size of catalysts. The fine-tuned alloy degree and appropriate nanoparticle size of Pd₁Co₁/Al₂O₃ bring the 17.7% increase of H₂ release amount and 99.5% increase of NECZ selectivity than those of Pd/Al₂O₃. For the bimetallic catalysts, the enhancement of selectivity of NECZ is mainly from the increase of the kinetic constant of rate-limiting step.     

Nano cobalt oxides for photocatalytic hydrogen production

Nano structured metal oxides including TiO_2, Co₃O₄ and Fe₃O₄ have been synthesized and evaluated for their photocatalytic activity for hydrogen generation. The photocatalytic activity of nano cobalt oxide was then compared with two other nano structured metal oxides namely TiO₂ and Fe₃O₄. The synthesized nano cobalt oxide was characterized thoroughly with respect to EDX and TEM. The yield of hydrogen was observed to be 900, 2000 and 8275 μmol h⁻¹ g⁻¹ of photocatalyst for TiO₂, Co₃O₄ and Fe₃O₄ respectively under visible light. It was observed that the hydrogen yield in case of nano cobalt oxide was more than twice to that of TiO₂ and the hydrogen yield of nano Fe₃O₄ was nearly four times as compared to nano Co₃O₄. The influence of various operating parameters in hydrogen generation by nano cobalt oxide was then studied in detail.     

Controllable electrodeposition synthesis of Pd/TMxOy-rGO/CFP composite electrode for highly efficient methanol electro-oxidation

A novel and high-efficiency Pd/TM_xO_y-rGO/CFP (TM_xO_y = Co₃O₄, Mn₃O₄, Ni(OH)₂) electrocatalyst for directly integrated membrane electrode was synthesized by controllable cyclic voltammetry electrodeposition combined with hydrothermal process. The results showed excellent performance towards methanol oxidation reduction. The Pd/Co₃O₄-rGO/CFP as-prepared catalyst has the best electrocatalytic activity, and mass activity is 5181 mA·mg⁻¹_Pd, which is about 40 times and 4.3 times that of the commercial Pd/C and Pt/C catalyst (JM). It can be attributed that the small size of Pd nanoparticle, uniformity of distribution, and the synergistic interaction between transition metal oxide on the support surface and Pd nanoparticles. The prepared Pd/TM_xO_y-rGO/CFP composite electrode is a promising catalyst for integrated membrane electrode assembly of proton exchange membrane fuel cells in the future.     

Cu-based tri-metallic nanoparticles with noble metals (Ag,Pd, and Ir) and their catalytic activities for hydrogen generation

The growth of simple and cost effective heterogeneous catalysts for the release of hydrogen is the key technological challenge for the fuel-cell based hydrogen economy. Stepwise metal displacement plating method was used for the fabrication of Cu⁰-based nanoparticles, Cu–Ag–Ir, Cu–Pd–Ir, and Cu–Ag–Pd to generate hydrogen from hydrazine hydrogen storage material. The preliminary indications of CuNPs production were the appearance of red-chocolate color with NaBH₄, sodium dodecyl sulfate (SDS) under light emitting diode (LED) irradiation. Ag and Pd were deposited on the surface of Cu⁰ by rapid reduction with NaBH₄ in SDS. Catalytic activity of trimetallic (Cu–Pd–Ir, Cu–Ag–Ir, Cu–Ag–Pd)) were higher than that of bimetallic (Cu–Ag, Cu–Ir, and Cu–Pd) due to the synergistic effect and electron interactions between the three metals. The catalytic performance of these materials depends on the structure of outer, and middle metal layers over a Cu inner metal. The nessler's reagent solution was employed to trap ammonia formation along with evolution of hydrogen generation. Cu₂₅–Pd₂₅–Ir₅₀ exhibited the superior catalytic activity, with rate constant of 5.6 × 10⁻⁴ s⁻¹ at 303 K, activation energy of 32 kJ/mol, activation enthalpy of 30 kJ/mol, and turn over frequency of 350 h⁻¹.     

Engineering of the N-doped carbon support for improved performance of supported Pd catalysts in hydrogen production from gas-phase formic acid

Development of N-doped Pd/C catalysts for hydrogen production from gas-phase formic acid is a challenge. To elucidate the efficient routes of nitrogen insertion on the surface of a mesoporous carbon support, the latter was treated with melamine (Mel), dicyandiamide or NH₃ at 673 and 823 K. Pyrolysis of the melamine/carbon mixture taken in a 1:2 ratio provides an increase in the reaction rate by a factor of 5. The inserted N-sites strongly interact with Pd leading to the formation of highly dispersed Pd nanoparticles (∼1.6 nm) and active atomically dispersed Pd²⁺ species. With a further increase of the Mel/C ratio, the number of surface N-sites decreases due to occupation of carbon support pores with a g–C₃N₄–type residue. This provides a decrease in the Pd dispersion leading to lower reaction rates. Therefore, melamine is an efficient N precursor. The considered synthesis of N-doped catalysts could be scaled.     

Magnetite nanoparticles enhanced glucose anaerobic fermentation for bio-hydrogen production using an expanded granular sludge bed (EGSB) reactor

The feasibility and efficiency of magnetite nanoparticles (Fe₃O₄NPs) enhanced bio-hydrogen production from glucose anaerobic fermentation were evaluated in this study. The results demonstrated that the maximum hydrogen yield (HY) of 12.97 mL H₂/g-VSS was obtained with 50 mg/L and 40–60 nm of Fe₃O₄NPs in batch experiments. Moreover, the optimum dosage of Fe₃O₄NPs produced hydrogen production (HP) of 4.95 L H₂/d in an expanded granular sludge bed (EGSB) reactor. Fe₃O₄NPs involved could promote ethanol and acetic acid accumulation. Fe²⁺ as by-product of iron corrosion could effectively promote the activity of key coenzymes and soluble microbial products (SMPs). Importantly, Fe₃O₄NPs addition resulted in the formation of electronic conductor chains to enhance the electron transport efficiency in the granular sludge. Microbial community analysis revealed that the relative abundance of butyrate-hydrogen-producing bacteria (Clostridium) decreased from 40.55% to 11.45%, while the relative abundance of ethanol-hydrogen-producing bacteria (Acetanaerobacterium and Ethanoligenens) increased from 19.62% to 35.35% with Fe₃O₄NPs involved, confirming that the fermentation type was transformed from butyrate-type to ethanol-type, which finally facilitated more hydrogen production.     

Chemical looping gasification of biomass char for hydrogen-rich syngas production via Mn-doped Fe2O3 oxygen carrier

Because of its low cost, an iron-based oxygen carrier is a promising candidate for hydrogen-rich syngas production from the chemical looping gasification of biomass. However, it needs modification from a reactivity point of view. In this study effect of Mn doping on Fe₂O₃ has been investigated for hydrogen-rich syngas production from biomass char at different temperatures (700–900 °C) and steam flow rates (60–100 μL/min). Several techniques (XRD, XPS, BET, and TPR-H₂) have been utilized to characterize fresh and spent oxygen carriers. The result demonstrated Mn-doing boosted the redox activity and the amount of oxygen vacancies, which increased hydrogen gas generation. Hydrogen production displayed different behavior across temperatures due to detecting Fe₂O₃ and MnFeO₃ phases for spent oxygen carriers. For the Fe₂O₃ oxygen carrier hydrogen gas yield is 1.67 Nm³/kg which is due to reduction of Fe₂O₃ phase to Fe₃O₄. However, the MnFe₂O₄ spinel phase detected in the spent MnFeO₃ oxygen carrier is a reason for improving hydrogen gas yield to 1.84 Nm³/kg. Change reaction temperature from 900 °C to 850 °C reduced hydrogen gas yield from 1.84 Nm³/kg to 1.83 Nm³/kg for with MnFeO₃ oxygen carrier. Regarding different steam flows, the proper flow rates that can maintain the formed phases and obtained best hydrogen gas yield are 80 and 90 μL/min, respectively. Meanwhile, the best hydrogen gas yield (2.21Nm³/kg) are obtained with MnFeO₃ oxygen carrier at optimum conditions (850 °C and 90 μL/min).