A facile synthesis of metal ferrites (MFe2O4, M = Co,Ni, Zn,Cu) as effective electrocatalysts toward electrochemical hydrogen evolution reaction

Developing low-cost, stable, and robust electrocatalysts for hydrogen evolution reaction (HER) is highly desirable for large-scale application. In this study, a highly efficient electrocatalysts of metal ferrites (MFe₂O₄, M = Co, Ni, Zn, Cu) with superior activity and durability are successfully fabricated on copper substrate through a facile co-precipitation method followed by doctor-blading deposition. The electrocatalytic performance of CoFe₂O₄, NiFe₂O₄, ZnFe₂O₄ and CuFe₂O₄ electrodes for hydrogen evolution reaction is studied in alkaline media using polarization curves and electrochemical impedance spectroscopy (EIS). Among them, CoFe₂O₄ presented the best electrocatalytic activities for HER with extremely low overpotentials of 270 mV (vs. RHE) at a current density of 10 mA cm^?2 in 1 M KOH. The electrocatalytic activity of MFe₂O₄ (M = Co, Ni, Zn, Cu) for HER to generate current density of 10 mA cm^?2 with low overpotential followed the order of CoFe₂O₄ > CuFe₂O₄ > NiFe₂O₄ > ZnFe₂O₄. The highly improved HER performance of CoFe₂O₄ is mainly due to a large number of exposed active sites, high electrical conductivity and low apparent activation energy, which are confirmed by a remarkable electrochemically active surface area (ECSA = 53.17 cm²), Nyquist plots analysis and Arrhenius plots measurement, respectively. Moreover, the CoFe₂O₄ electrode showed outstanding electrocatalytic stability even after 1000 cyclic voltametry tests. These results provide a promising avenue for developing cost-effective and high-efficiency electrocatalysts based on earth-abundant transition metal ferrite as advanced electrodes for large-scale energy conversion processes.     

Biomass-derived hierarchical porous CdS/M/TiO2 (M = Au,Ag, pt,pd) ternary heterojunctions for photocatalytic hydrogen evolution

We demonstrate a general method for the synthesis of biomass-derived hierarchical porous CdS/M/TiO₂ (M = Au, Ag, Pt, Pd) ternary heterojunctions for efficient photocatalytic hydrogen evolution. A typical biomass—wood are used as the raw sources while five species of wood (Fir, Ash, White Pine, Lauan and Shiraki) are chosen as templates for the synthesis of hierarchical porous TiO₂. The as-obtained products inherited the hierarchical porous features with pores ranging from micrometers to nanometers, with improved photocatalytic hydrogen evolution activity than non-templated counterparts. Noble metals M (M = Pt, Au, Ag, Pd) and CdS are loaded via a two-step photodeposition method to form core (metal)/shell (CdS) structures. The photocatalytic modules—CdS(shell)/metal (core)/TiO₂ heterostructures, have demonstrated to increase visible light harvesting significantly and to increase the photocatalytic hydrogen evolution activity. The H₂ evolution rates of CdS/Pd/TiO₂ ternary heterostructures are about 6.7 times of CdS/TiO₂ binary heterojunctions and 4 times higher than Pd/CdS/TiO₂ due to the vertical electron transfer process. The design of such system is beneficial for enhanced activity from morphology control and composition adjustment, which would provide some new pathways for the design of promising photocatalytic systems for enhanced performance.     

Metal oxide (MgO,CaO, and La2O3) promoted Ni-Ce0.8Zr0.2O2 catalysts for H2 and CO production from two major greenhouse gases

The metal oxide (MgO, CaO, and La₂O₃) promoted Ni-Ce_0.8Zr_0.2O₂ catalysts have been applied for carbon dioxide reforming of methane (CDR) reaction and investigated the coke formation and sintering phenomenon in used catalysts. The Ni-MgO-Ce_0.8Zr_0.2O₂ catalyst exhibits high activity and stability at a very high gas hourly space velocity of 480,000 h⁻¹, resulting from high resistance to coke formation and Ni sintering. This is mainly due to small Ni crystallite size, strong basicity of MgO, and an intimate interaction between Ni and MgO.     

Density functional and microkinetic study of CO2 hydrogenation to methanol on subnanometer Pd cluster doped by transition metal (M= Cu,Ni, Pt,Rh)

We study the CO₂ hydrogenation to methanol on subnanometer Pd₇ and transition metal doped Pd₆M (M = Cu, Ni, Pt, and Rh) clusters using a combination of density functional theory and microkinetic calculations. We find that, in general, the inclusion of transition metal dopants could decrease the activation energy of several important elementary reactions. This condition results in a significant improvement in the activity of the catalyst, especially for the Pd₆Ni cluster. We find that the Pd₆M clusters are more selective toward the formate pathway than the RWGS + CO hydrogenation pathway. We also compare the turnover frequency profiles of the clusters with that of the Cu(111) surface, representing the standard industrial catalyst. We find that the Pd₆Ni cluster can successfully overcome the TOF of Cu(111) surface, even at the low-pressure condition.     

Potential Co-catalyst application of novel M3P2 (M=Zn,Cd) in photocatalytic hydrogen evolution reaction from water splitting

New noble-metal-free co-catalysts based on transition metal phosphides, Zn₃P₂ and Cd₃P₂, were fabricated and loaded on hetero-structure of BiFeO₃–CdWO₄ with the aim of promoting hydrogen evolution from water splitting without using sacrificial agent. The opto-electrochemical properties of the photocatalysts were investigated by various characterization techniques and it was observed that the co-catalyst loaded BiFeO₃–CdWO₄ exhibited better light harvestability, charge separation efficiency and charge mobility. The photocatalytic hydrogen productivity reached up to 572.81 μmol h⁻¹.g_cat⁻¹ h by loading 12 wt% Zn₃P₂ and 488.25 μmol h⁻¹.g_cat⁻¹ by loading 9 wt% Cd₃P₂ over BiFeO₃–CdWO₄. Zn₃P₂ and Cd₃P₂ have shown light absorption in visible to near IR region, thus they may also have the additional role of a photocatalyst other than being active sites for the photocatalytic reduction half-reaction. Loading the co-catalysts also resulted in multiplication of specific surface area which means an increase in the number of surface active sites. We observed a higher hydrogen productivity with lower photocatalytic rate by loaded Zn₃P₂ in compared with Cd₃P₂. This is attributed to the different photoresponsivity and band edge energy of the co-catalysts. The details of charge transfer mechanism between the host hetero-structure photocatalyst and loaded co-catalysts has been discussed.     

Pd loading,Mn+ (n=1, 2, 3) metal ions doped TiO2 nanosheets for enhanced photocatalytic H2 production and reaction mechanism

Pd nanoparticles (NPs) loading, main group metal ions doped TiO₂ nanosheets were prepared by a hydrothermal method, followed by photo-deposition of Pd. The samples were characterized, and their photocatalytic hydrogen production activities were tested in a methanol aqueous solution. The effects of cationic charge, radius and concentration of the doping ions (Na⁺, K⁺, Mg²⁺, Al³⁺) on the photocatalytic activities were investigated systematically. The photocatalytic reaction mechanism was discussed by considering the three aspects: specific surface area, light absorption and charge transfer/separation. The results show that the cation dopings significantly increased the photocatalytic activities of the TiO₂ nanosheets, which may be attributed to the enhanced UV-vis light absorption and accelerated charge transfer/separation of the catalysts. Particularly, the Pd/0.2%K⁺-TiO₂ possesses the highest photocatalytic H₂ production activity (76.6 μmol h^?1), which is more than twofold higher than that of the undoped Pd/TiO₂. The apparent quantum efficiency of hydrogen evolution system reaches 3.0% at 365 nm. The high activity of the Pd/K⁺-TiO₂ may be attributed to the lower electronegativity of K⁺, caused by the lower cationic charge or the larger cationic radius, compared to Na⁺, Mg²⁺ and Al³⁺. The doping metal cations with higher electronegativity may compete electrons with H⁺, which eventually partly depressed the reduction of H⁺ to H₂.     

MxOy (M = Mg,Zr, La,Ce) modified Ni/CaO dual functional materials for combined CO2 capture and hydrogenation

The integrated CO₂ capture and utilization has recently attracted attention as a promising approach to reduce CO₂ emissions as well as produce value-added chemicals and fuels. Herein, metal oxides (M_xO_y, M = Mg, Zr, La, and Ce) modified Ni/CaO dual functional materials (M-Ni/Ca DFMs) were synthesized and applied to the combined CO₂ capture and hydrogenation using a single reactor at one temperature. The La–Ni/Ca showed the highest CO₂ adsorption capacity (13.8 mmol/g), CO₂ conversion (64.3%) and CO yield (8.7 mmol/g). Results indicated that the addition of metal oxides increased the number of basic sites which played important role in efficient CO₂ capture. The high activities of M-Ni/Ca were attributed to the formation of highly dispersed small-sized Ni particles. Furthermore, the La–Ni/Ca exhibited excellent cyclic stability after 20 cycles due to the La₂O₃ as a physical barrier and a support for inhibiting the growth and sintering of CaO and Ni particles.     

Catalytic activity trends from pure Pd nanoclusters to M@PdPt (M = Co,Ni, and Cu) core-shell nanoclusters for the oxygen reduction reaction: A first-principles analysis

The trends of the catalytic activity toward the oxygen reaction reduction (ORR) from Pd₄₄ nanoclusters to M₆@Pd₃₀Pt₈ (M = Co, Ni, and Cu) core-shell nanoclusters was investigated using auxiliary density functional theory. The adsorption energies of O and OH were computed as predictors of the catalytic activity toward the ORR and the following tendency of the electrocatalytic activity was computed: Pt₄₄ ≈ M₆@Pd₃₀Pt₈ > M₆@Pd₃₈ > Pd₄₄. In addition, the adsorption of O₂ on the Ni₆@Pd₃₀Pt₈ and Pt₄₄ nanoclusters were investigated, finding an elongation of the O–O bond length when O₂ is adsorbed on the Ni₆@Pd₃₀Pt₈ and Pt₄₄ nanoclusters, suggesting that the O₂ is activated. Finally, the stabilities of the M₆@Pd₃₈ and M₆@Pd₃₀Pt₈ core-shell nanoclusters were analyzed both in vacuum and in oxidative environment. From the calculated segregation energies for the bimetallic and trimetallic nanoclusters in vacuum, it can be clearly observed that the M atoms prefer to be in the center of the M₆@Pd₃₈ and M₆@Pd₃₀Pt₈ nanoclusters. Nevertheless, it is observed that the segregation energies of M atoms for the M₆@Pd₃₈ nanoclusters with an oxidizing environment tend to decrease compared with their M₆@Pd₃₈ nanoclusters counterparts in vacuum, which suggests that in an oxidative environment, M atoms may tend to segregate to the surface of the M₆@Pd₃₈ nanoclusters.     

Oxygen vacancies mediated in-situ growth of noble-metal (Ag,Au, Pt) nanoparticles on 3D TiO2 hierarchical spheres for efficient photocatalytic hydrogen evolution from water splitting

Defect engineering is effective to extend the light absorption range of TiO₂. However, the oxygen vacancy defects in TiO₂ may serve as recombination centers, hampering the separation and transfer of photo-generated charges. Here, we present a strategy of in-situ depositing noble-metal (M = Ag, Au or Pt) nanoparticles (NPs) on defective 3D TiO₂ hierarchical spheres (THS) with large surface area through the redox reaction between metal ions in solution and the electrons trapped at oxygen vacancies in THS. The oxygen vacancies at the THS surface are consumed, resulting in direct contact between TiO₂ and noble-metal NPs, while the other oxygen vacancies in the bulk are retained to promote visible light absorption. The noble-metal NPs with well-controlled size and distribution throughout the porous hierarchical structure not only facilitate the generation of electron-hole pairs in THS due to the effect of surface plasmon-induced resonance energy transfer (SPRET) from noble-metal NPs to TiO₂, but also expediate the electron transfer from TiO₂ to noble-metal NPs due to the Schottky junction at the TiO₂/M interface. Therefore, THS-M shows improved photocatalytic performance in water splitting compared to THS. The optimum performance is achieved on THS-Pt (13.16 mmol h⁻¹g⁻¹) under full-spectrum (UV–Vis) irradiation but on THS-Au (1.49 mmol h⁻¹g⁻¹) under visible-light irradiation. The underlying mechanisms are proposed from the surface plasmon resonance of noble-metal NPs as well as the Schottky junction at the TiO₂/M interface.     

Manipulating photocatalytic pathway and activity of ternary Cu2O/(001)TiO2@Ti3C2Tx catalysts for H2 evolution: Effect of surface coverage

A Cu₂O/(001)TiO₂@Ti₃C₂T_x photocatalyst was synthesized via a wet-chemistry reduction method by N, N-dimethylformamide (DMF). By scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD), it was revealed that the surface coverage of photocatalyst increased with the loading amount of Cu₂O, while the particle size of Cu₂O did not change significantly. The photocatalytic activity and mechanism of ternary Cu₂O/(001)TiO₂@Ti₃C₂T_x photocatalyst heavily depend on the surface coverage of copper species. When the surface coverage of photocatalyst by Cu₂O was low, the Ti₃C₂T_x acted as hole reservoir. Cu₂O was firstly reduced in situ to metallic copper by excited electrons. Then the reverse movement of carriers enabled the spatial separation of photogenerated electron-hole pairs, and afforded relatively high hydrogen evolution (more than 1100 μmol h⁻¹ (g CuO_x TiO₂)⁻¹). When the coverage of Cu₂O on (001)TiO₂@Ti₃C₂T_x was too high at high loading amounts, Ti₃C₂T_x failed to play the role of hole trapping. Under that circumstance, the photocatalytic reaction follows p-n junction mechanism, leading to low hydrogen productivity. The results here shed light on the relationship between structure and activity of Cu₂O/(001)TiO₂@Ti₃C₂T_x, which was conducive to the development of the MXene-based photocatalysts.     

Integration of 2D printing technologies for AV2O6 (A=Ca,Sr, Ba)-MO (M=Cu,Ni, Zn) photocatalyst manufacturing to solar fuels production using seawater

The non-polluting nature of photocatalytic H₂ production makes of interest the study of semiconductors for this process. Scale-up of the photocatalytic hydrogen process to a pilot plant requires the photocatalyst's immobilization to enhance the charge transfer and facilitate its recovery. In this work, screen-printed films from the AV₂O₆ (A = Ca, Sr, Ba) semiconductor family were fabricated and evaluated in photocatalytic water splitting for H₂ production in distilled water and seawater under UVA light. The films exhibited ∼3.1 eV band gaps, high crystallinity, and heterogeneous morphologies. BaV₂O₆ film exhibited the highest H₂ production in distilled water (691 μmol/g), related to the synergistic effect between a higher crystallinity and traces of V⁺⁴ species that decrease the recombination of the photogenerated charges. Also, to take advantage of the dissolved species in seawater that could act as sacrificial agents, the BaV₂O₆ film was evaluated in seawater, in which H₂ production was up to 6 times higher (4374 μmol/g) than in distilled water. The BaV₂O₆ film was decorated with simple oxides (CuO, NiO, and ZnO) by the ink-jet printing technology to increase its photocatalytic performance for H₂ production. The highest efficiency with distilled water was obtained with the BaV₂O₆-CuO film, which reached an H₂ production up to 30 times higher than the bare BaV₂O₆, own to the n-p heterostructure formation that enhances the charge transport in the photocatalytic process. When the BaV₂O₆-CuO film was evaluated in seawater, a more constant H₂ production was observed; moreover, the efficiency was similar compared to the production in distilled water (20,563 μmol/g). To elucidate the seawater compounds that most influence the H₂ production, a two levels Plackett–Burman experiments design was carried out in simulated seawater. The analysis revealed that the SO₄²⁻ ions from the CaSO₄ could be decreasing the H₂ production by acting as Lewis's acid sites that trap the photogenerated e⁻ competing for its usage with the H⁺. Additionally, the Cl⁻ ions and the HCO₃⁻ reduction improved the HCOOH production from simulated seawater, reaching 26 times a higher production (23,333 μmol/g) than in distilled water.     

CO2 methanation over nanocrystalline Ni catalysts supported on mechanochemically synthesized Cr2O3-M (M=Fe,Co, La,and Mn) carriers

In the present study, a series of Cr₂O₃ powders modified by different promoters such as Fe, Co, La, and Mn were synthesized using a facile and solvent-free mechanochemical method and the prepared powders were used as a catalyst carrier for the preparation of 20 wt%Ni catalysts in CO₂ methanation. The results indicated that among all catalysts, the nickel catalyst supported on the Mn-promoted Cr₂O₃ exhibited the best catalytic performance. The results showed that there was an optimum for the Mn content and the increment in Mn content up to 15 wt% improved the catalytic performance due to its positive influence on increasing nickel dispersion and catalyst reducibility. The 20 wt%Ni/15 wt%Mn–Cr₂O₃ catalyst possessed a CO₂ conversion of 72.12% and CH₄ selectivity of 100% at 350 °C (H₂/CO₂ = 3 M ratio, GHSV = 18,000 ml/g_cat.h) with high stability during 12 h on stream. The obtained results showed that the increment in H₂/CO₂ molar, and the decrement in GHSV value and calcination temperature improved the catalytic performance.     

Hydrogen production through autothermal reforming of CH4: Efficiency and action mode of noble (M = Pt,Pd) and non-noble (M = Re,Mo, Sn) metal additives in the composition of Ni-M/Ce0.5Zr0.5O2/Al2O3 catalysts

Hydrogen production through autothermal reforming of methane (ATR of CH₄) over promoted Ni catalysts was studied. The control of the ability to self-activation and activity of Ni-M/Ce_0.5Zr_0.5O₂/Al₂O₃ catalysts was achieved by tuning their reducibility through the application of different types (M = Pt, Pd, Re, Mo or Sn) and content (molar ratio M/Ni = 0.003, 0.01 or 0.03) of additive. The comparison of the efficiency and action mode of noble (M = Pt, Pd) and non-noble (M = Re, Mo, Sn) metal additives in the composition of Ni-M/Ce_0.5Zr_0.5O₂/Al₂O₃ catalysts was performed using X-ray fluorescence analysis, N₂ adsorption, X-ray diffraction, high-resolution transmission electron microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, temperature-programmed reduction with hydrogen, and thermal analysis. The composition-characteristics-activity correlations were determined. It was shown that the introduction of a promoter does not affect the textural and structural properties of catalysts but influences their reducibility and performance in ATR of CH₄. At the similar dispersion of NiO active component (11 ± 2 nm), the Ni²⁺ reduction is intensified in the following order of additives: Mo < Sn < Re ≤ Pd < Pt. It was found that for the activation of Ni and Ni–Sn catalysts before ATR of CH₄ tests, the pre-reduction is required. On the contrary, the introduction of Pt, Pd and Re additives leads to the self-activation of catalysts under the reaction conditions and an increase of the H₂ yield due to the enhanced reducibility of Ni²⁺. The efficient and stable catalyst for hydrogen production has been developed: in ATR of CH₄ at 850 °C over an optimum 10Ni-0.9Re/Ce_0.5Zr_0.5O₂/Al₂O₃ catalyst the H₂ yield of 70% is attained. The designed catalyst has enhanced stability against oxidation and sintering of Ni active component as well as high resistance to coking.     

Identifying the roles of MFe2O4 (M=Cu,Ba, Ni,and Co) in the chemical looping reforming of char,pyrolysis gas and tar resulting from biomass pyrolysis

Biomass chemical looping gasification (BCLG), which employs oxygen carriers (OCs) as the gasification agent, is drawing more attention for its low cost and environmental friendliness. However, the complex products of biomass pyrolysis and the reactions between OCs and the pyrolysis products constrain its development. In this study, MFe₂O₄ (M = Cu, Ba, Ni and Co) ferrites synthesized via the sol-gel method were investigated as OCs in BCLG for hydrogen-rich syngas production. The properties of the as-prepared and spent OCs were characterized by X-ray diffraction (XRD), H₂-temperature programmed reduction (TPR), scanning electron microscopy (SEM), and automatic surface area porosimetry (BET). The three-phase products (char, pyrolysis gas and toluene) derived from biomass pyrolysis were employed as the reactants to investigate the reactivity of the ferrites. Then, BCLG experiments using biomass were conducted on the four ferrites to further determine their performance. The characterization results suggested that the four ferrites are all attractive for the chemical looping process, exhibiting good oxygen transferability and wide distributions of metal cations because of their metal synergistic effects in the spine structure. Reactions with pyrolysis gas and biomass char indicated that BaFe₂O₄ has a higher reactivity via a solid-solid reaction but a lower reactivity with pyrolysis gas, which make it very favorable for the production of hydrogen-rich syngas. Furthermore, BaFe₂O₄ showed excellent performance for toluene catalytic cracking with small amounts of carbon deposition. The synergetic effects between Ba and Fe metals considerably enhanced selective oxidation to produce 26.72% more H₂ than CoFe₂O₄ and 13.79% more H₂ than NiFe₂O₄ and CuFe₂O₄ for biomass gasification. The hydrogen yield produced by BaFe₂O₄ with the assistance of steam for biomass gasification can reach 41.8 mol/kg of biomass.     

Investigations on the effect of Cu/Cu redox couples and oxygen vacancies on photocatalytic activity of treated LaNi1−xCuxO3 (x=0.1, 0.4, 0.5)

Perovskite-like oxides LaNi_1−xCu_xO₃ (x = 0.1, 0.4, 0.5) were prepared by means of the citric acid complexing method. TPR revealed the incorporation of Cu into the perovskite lattice increased the reducibility of the catalyst. After LaNi_1−xCu_xO₃ were pretreated in H₂ for 2 h at certain low temperature, the material still retained its perovskite structure and oxygen vacancies were generated in the lattice. DRS showed that narrowing of band-gap of reduced LaNi_1−xCu_xO₃ was governed by the crystalline structure and the defect in the catalyst. In the photocatalytic water splitting experiment, 200 and 250°C-reduced LaNi_0.6Cu_0.4O₃, 200°C-reduced LaNi_0.5Cu_0.5O₃ possessed the high and colse catalytic activity. XPS showed that the molar ratio of Cu²⁺/Cu¹≈1 and lattice oxygen/adsorb oxygen ≈ 0.2 in the catalysts had high catalytic activity. According to the outcome of our experiments, we conclude that there is a balance relation either between oxygen vacancies and catalytic activity or between Cu²⁺/Cu¹⁺ redox couples and catalytic performance of these materials for hydrogen production from photocatalytic water splitting. Enhancement of hydrogen yield can be attributed to the small band-gap and the lowering the recombination probability for electron-hole pairs.