Hydrogen generation from sodium hypophosphite catalyzed by metallic nanoparticles supported on graphitic carbon nitride

Sodium hypophosphite is a potential green source for the generation of clean elemental hydrogen without pollutants. The examination of the catalytic activity toward hydrogen production of different metallic nanoparticles supported on graphitic carbon nitride (GCN) was performed using sodium hypophosphite as hydrogen source. Four different metallic nanoparticles were assessed, namely, palladium, nickel, ruthenium, and rhodium. The Pd-based catalyst, Pd/GCN, was found to be the most active and stable of the four catalysts. The latter showed phenomenal catalytic activity of 100% conversion over five cycles hence makes this process sustainable. All four catalysts were characterized by PXRD, FTIR, XPS, TGA, TEM, and ICP-MS techniques. The blend of hypophosphite and Pd/GCN catalyst was weighed against the grouping of this catalyst with the traditional hydrogen source, potassium formate. We realized that the first combination is much more efficient and does not release byproducts, which makes this hydrogen source greener and more effectual.     

Co,Ni-based nanoparticles supported on graphitic carbon nitride nanosheets as catalysts for hydrogen generation from the hydrolysis of ammonia borane under broad-spectrum light irradiation

From the viewpoint of tailoring the atomic and nanoscale structures of semiconductors to enhance the solar-to-hydrogen energy conversion, we employed an in-situ gas template-assisted co-polymerization route, where melamine and 2,4,6-triaminopyrimidine were co-monomers and NH₄Cl was the in-situ gas template, to synthesize porous broad-spectrum light-responsive carbon nitride nanosheet (termed as CNN) species with increased π-electron availability. Then we developed CNN-supported Co and Ni nanoparticles (NPs) for catalytic hydrogen generation from aqueous ammonia borane (NH₃BH₃) under light irradiation (λ ≥ 420 nm) at room temperature. Though all the Co-based catalysts had the similar activities with total turnover frequency (TOF) values of 37.5–44.1 min⁻¹ in the dark, they exhibited significantly different and enhanced photocatalytic activities. Remarkably, the optimized catalyst had a total TOF value of 123.2 min⁻¹, exceeding the values of reported non-noble metal catalysts. Moreover, the porous CNN species possessed the C-substitution for N, tunable narrow bandgaps of 0.71–2.34 eV and efficient separation of photogenerated charge carriers. This resulted in the enriched electron density of metal NPs and the apparent quantum yield of 66.9% at 420 nm.     

Highly dispersed palladium nanoparticles anchored on polyethylenimine-modified carbon nanotubes for dehydrogenation of formic acid

Formic acid has been recognized as an excellent liquid hydrogen storage material. The development of catalysts with high performance for the dehydrogenation of formic acid is significant. Herein, we employed polyethylenimine-modified carbon nanotubes as a carrier for Pd nanoparticles to synthesize a novel catalyst by a wet chemical reduction method. It was found that the amino polymers on polyethylenimine-modified carbon nanotubes have great effects on reducing the size of Pd nanoparticles, changing the electronic state of Pd, and enhancing the hydrophilicity of catalyst. Therefore, the as-synthesized Pd/CNTs-A-PEI₁₈₀₀ catalyst showed superior activity for FA dehydrogenation in the absence of additives with an initial TOF value of 1506 h^?1 and a 100% selectivity of hydrogen.     

Highly dispersed Pd nanoparticles supported on 1,10-phenanthroline-functionalized multi-walled carbon nanotubes for electrooxidation of formic acid

Functionalization step is generally prerequisite to immobilize metal nanoparticles on multi-walled carbon nanotubes (MWCNTs) for production of a high efficient electrocatalyst. We herein report a novel method to functionalize MWCNTs with 1,10-phenanthroline (phen-MWCNTs) as a catalyst support for Pd nanoparticles. Raman spectroscopic analysis results reveal that this phen functionalization method can preserve the integrity and electronic structure of MWCNTs and provide the highly effective functional groups on the surface for Pd nanoparticles. According to the transmission electron microscopy (TEM) measurements, the as-prepared Pd nanop articles are evenly deposited on the surface of the phen-MWCNTs without obvious agglomeration, and the average particle size of the Pd nanoparticles is 2.3 nm. Electrochemical measurements demonstrate that the as-prepared Pd/phen-MWCNTs catalyst has a better electrocatalytic activity and stability for the oxidation of formic acid than Pd catalyst on acid-treated MWCNTs. It is concluded that the as-prepared Pd/phen-MWCNTs would be a potential candidate as an anode electrocatalyst in direct formic acid fuel cell (DFAFC).     

Bamboo-charcoal-loaded graphitic carbon nitride for photocatalytic hydrogen evolution

Crystalline graphitic carbon nitride is an excellent photocatalyst for hydrogen production due to its non-toxicity, stability, elemental abundance, and visible-light response. Herein, we present a new type of composite photocatalysts, eco-friendly bamboo-charcoal-loaded graphitic carbon nitrides to accelerate the separation of electron-hole pairs. The suitable loading of bamboo charcoal on graphitic carbon nitrides shows an increased specific surface area from 85 to 120 m² g^?1, and excellent visible-light photocatalytic hydrogen production activity of 4.1 mmol g^?1 h^?1, which is 2.3 times higher than that of pristine carbon nitride (1.8 mmol g^?1 h^?1). Under irradiation, the photogenerated electrons fast migrate from graphitic carbon nitride to bamboo charcoal through an ohmic contact between them, reducing the recombination of electron-hole pairs. This study highlights the effect of carbonaceous material loading on photocatalytic activity of carbon nitrides and opens an avenue to design efficient loaded photocatalysts with natural abundant materials.     

Nano-structured palladium impregnate graphitic carbon nitride composite for efficient hydrogen gas sensing

Nanoparticles of palladium (Pd) were incorporated into graphitic carbon nitride (g-C₃N₄) matrix with a view to improving hydrogen sensing efficiency of g-C₃N₄, by a fairly new chemical process that uses ammonium tetrachloropalladate as a Pd metal nanoparticle source along with an appropriate reducing agent. Researchers have explored g-C₃N₄ for various applications such as a catalyst for water splitting, photoluminescence, storage because of its relatively low cost, easy synthesis, and ready availability. For the synthesis of g-C₃N₄, urea was used as a precursor at 550 °C and at atmospheric pressure under a muffle furnace without add-on support. The final solution of the Pd/g-C₃N₄ nanocomposite was then centrifuged and dried for use as a hydrogen-sensing material. g-C₃N₄ and Pd/g-C₃N₄ were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), UV-VIS-NIR spectroscopy, Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), and energy dispersive X-ray spectroscopy (EDS). Pd-dispersed graphitic carbon nitride film was deposited on an inter digited carbon electrode by using a screen printing technique. From the qualitative analysis by I–V measurement, a significant change in the resistance was observed during the presence and absence of the hydrogen gas. The results show Pd/g-C₃N₄ nanocomposite as an efficient hydrogen sensing material.     

First-principles study on hydrogen storage by graphitic carbon nitride nanotubes

First-principles calculations based on density functional theory were carried out to investigate the hydrogen storage capacity of graphitic carbon nitride nanotubes. Graphitic carbon nitride nanotubes could be attractive hydrogen sorbent for two reasons: firstly, its porous structure allows easy access of hydrogen into the interior of the nanotubes; and secondly, the doubly bonded nitrogen at its pore edges provides active sites for either the adsorption of hydrogen (chemically and physically), or functionalization with metal catalysts. Our calculations show that an isolated nanotube can uptake up to 4.66 wt. % hydrogen, with an average overall hydrogen adsorption energy of about −0.22 eV per H atom. In the form of a bulk bundle, the hydrogen storage capacity is enhanced due to the increased availability of space among the tubes. We predict that the hydrogen storage capacity in the bundle is at least 5.45 wt. %. Importantly, hydrogen molecules can easily access the tube’s interior due to the low energy barrier (∼0.54 eV) for their passage through the pores, indicating a fast uptake rate at relatively low pressure and temperature. Our findings show that graphitic carbon nitride nanotubes should be applicable to practical hydrogen storage because of the high gravimetric capacity and fast uptake rate.     

Trimetallic nanoparticles: Synthesis,characterization and catalytic degradation of formic acid for hydrogen generation

PdAgFe, FePdAg and FeAgPd trimetallic nanoparticles were synthesized by seedless and step-wise simultaneous chemical reduction of Fe³⁺, Ag⁺ and Pd²⁺ by using hydrazine in presence of cetyltrimethylammonium bromide and used as a catalyst for the degradation of formic acid. The effects of nanoparticle composition, presence of sodium format (promoter), [catalyst], [formic acid] and temperature play key roles in the hydrogen generation. The Ba(OH)₂ trap experiment and water displacement technique were used to determine the generation of CO₂ and H₂, respectively. The decomposition of formic acid followed complex-order kinetics with respect to [formic acid]. It was found that FeAgPd showed a maximum catalytic activity (turn over frequency) of 75 mol H₂ per mol catalyst per h. The activation energy (E_a = 51.6 kJ/mol), activation enthalpy (ΔH = 48.9 kJ/mol) and activation entropy (ΔS = −151.0 JK^-1 mol⁻¹) were determined and discussed for the catalytic reaction. The reusability of the FeAgPd at 50 °C shows an efficient degree of activity for six consecutive catalytic cycles.     

Enhanced formic acid dehydrogenation by the synergistic alloying effect of PdCo catalysts supported on graphitic carbon nitride

The catalytic ability of graphitic carbon nitride (g-C₃N₄)-supported composition-controlled PdCo catalysts towards H₂ generation from the formic acid dehydrogenation reaction was assessed in this study and a noticeable composition dependence was evidenced. It was seen that the alloying effect combined with the nitrogen functionalities present on g-C₃N₄ assisted the formation of small and well-distributed nanoparticles. This fact, combined with the electronic promotion of Pd species via charge transfer from Co and basic features of the support, resulted in enhanced catalytic activities compared to that displayed by the counterpart Pd/g-C₃N₄, reaching a TOF value of 1193 h⁻¹ for the most active catalyst among investigated (PdCo/g-C₃N₄ (1/0.7)). Furthermore, the present catalytic system showed high selectivity towards formic acid dehydrogenation, suppressing the generation of undesired CO via formic acid dehydration, which makes it a suitable candidate for practical application in fuel cells.     

Strong organic acid-assistant synthesis of holey graphitic carbon nitride for efficient visible light photocatalytic H2 generation

A facile and cost-effective method was developed for the synthesis of holey N-deficient graphitic carbon nitride nanosheets (FCN) using trifluoroacetic-acid-treated urea as a precursor. The role of trifluoroacetic acid on the composition, structure and photocatalytic performance of the prepared catalysts was carefully investigated. The obtained samples displayed laminated porous morphology with nitrogen defects, larger specific surface areas, extended range of spectral response and enhanced electron mobility of charge carriers. Consequently, the optimized catalyst FCN-400 exhibited superb photocatalytic performance and excellent cycling stability for hydrogen evolution. The hydrogen evolution rate over FCN-400 reached 309.3 μmol/h under visible light irradiation, which is 11.3-fold of that of urea-derived graphitic carbon nitride (27.3 μmol/h).     

ZnCr LDH nanosheets modified graphitic carbon nitride for enhanced photocatalytic hydrogen production

ZnCr layered double hydroxides (ZnCr LDH) nanosheets modified graphitic carbon nitride (g-C₃N₄) nanohybrids were fabricated via a self-assembly procedure through electrostatic interaction between these two components. Such 2D-2D inorganic-organic hybrid material was employed for photocatalytic hydrogen production under visible light for the first time. The physical and photophysical properties of the hybrid nanocomposites were investigated to reveal the effect of ZnCr LDH nanosheets on the photocatalytic activities of g-C₃N₄. It was found that 1 wt% ZnCr LDH nanosheets modified g-C₃N₄ was optimal for the formation of intimate interfacial contact. The visible light photocatalytic H₂ production activity over g-C₃N₄ was enhanced about 2.8 times after ZnCr LDH nanosheets modification. The significant enhancement in photocatalytic performance for ZnCr LDH/g-C₃N₄ heterojunction should be attributed to the promoted charge transfer and separation efficiency, resulting from the intimate interfacial contact and Type II band alignment between ZnCr LDH and g-C₃N₄.     

Potassium doped and nitrogen defect modified graphitic carbon nitride for boosted photocatalytic hydrogen production

Controlling the structure of semiconductors to tailor are physicochemical and photoelectronic structure features. Graphitic carbon nitride has triggered a new impetus in the field of photocatalysis. However, the rapid recombination of charge carriers limited its photocatalytic activity. Herein, we demonstrate that potassium doped and nitrogen defects into graphitic carbon nitride (KCN_x) framework are favorable for visible light harvesting, charge separation and have highly efficient photocatalytic behavior for water splitting. It exhibits a high hydrogen evolution activity of 59.9 mmol·g^?1·h^?1 (66.6 times much higher than that of pristine g-C₃N₄), and remarkable apparent quantum efficiency of 57.17% at 420 nm. The superior photocatalytic performance of the KCNx sample was attributed to the less recombination rate of photogenerated electron and hole, and enhanced conductivity, which was proven by photoelectrochemical and PL. This work reveals the synergistic mechanism of introducing foreign elements and defects into the framework of graphitic carbon nitride to improve its photocatalytic activity.     

Hydrogen storage on graphitic carbon nitride and its palladium nanocomposites: A multiscale computational approach

Hydrogen storage capacity (HSC) of multilayer graphitic carbon nitride, d-g-C₃N₄ (d is interlayer spacing), and its palladium nanocomposite, d-Pd@g-C₃N₄, were investigated using multiscale computational techniques including quantum mechanics calculations and grand canonical Monte Carlo (GCMC) simulation. According to the results, the volumetric HSC of 8-g-C₃N₄ and 8-Pd@g-C₃N₄ can reach to DOE target of 30 gH₂/L at 177 K, 5.7 MPa, and 177 K, 4.0 MPa, respectively. The gravimetric HSC of 10-g-C₃N₄ and 12-Pd@g-C₃N₄ meet the DOE target of 4.5 wt% at 150 K, 3.5 MPa, and 125 K, 4.0 MPa, respectively. The incorporation of Pd atoms enhances the delivery volumetric HSC of 6-, 8-, 10-, and 12-g-C₃N₄ by 49, 55, 129, and 146%, respectively at 177 K and 0.5 MPa. On the other hand, the incorporation of Pd atoms has a negative effect on the delivery gravimetric HSC of 6- and 8-g-C₃N₄ and positive effect for 10- and 12-g-C₃N₄. The estimated isostric heat, Q_st, of adsorption is 5.5–8.5 kJ/mol. The maximum value of Q_st for both nanoadsorbents belong to those with d = 8 Å. The structure of adsorbates and possibility of multilayer adsorption occurrence were also investigated using pair correlation functions and density profiles.     

Pd nanoparticles supported on copper phthalocyanine functionalized carbon nanotubes for enhanced formic acid electrooxidation

The hydrothermal synthesis of a novel Pd electrocatalyst using copper phthalocyanine-3,4′,4″,4′″-tetrasulfonic acid tetrasodium salt (TSCuPc) functionalized multi-walled carbon nanotubes (MWCNTs) composite as catalyst support for Pd nanoparticles is reported. The prepared nanocomposites were characterized by UV–vis absorption spectroscopy, Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, thermogravimetric analysis (TGA), X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and electrochemical tests. It is found that Pd nanoparticles are uniformly deposited on the surface of TSCuPc-MWCNTs, and their dispersion and electrochemical active surface area (ECSA) are significantly improved. Studies of cyclic voltammetry and chronoamperometry demonstrate that the Pd/TSCuPc-MWCNTs exhibits much higher electrocatalytic activity and stability than the Pd/AO-MWCNTs catalyst for formic acid oxidation. This study implies that the as-prepared Pd/TSCuPc-MWCNTs will be a promising candidate as an anode electrocatalyst in direct formic acid fuel cell (DFAFC).     

Pd nanoparticles immobilized on aniline-functionalized MXene as an effective catalyst for hydrogen production from formic acid

A simple wet chemical method was used to prepare two-dimensional transition metal carbides (MXene); PDA-MXene was prepared by alkalization of p-phenylenediamine (PDA) on MXene. And further, Pd metal nanoparticles (NPs) were conveniently loaded on the surface to catalyze the dehydrogenation of formic acid. The as-prepared Pd/PDA-MXene catalyst for the formic acid dehydrogenation was characterized by XRD, IR, TEM, and XPS. Pd-NPs with a size of about 4 nm were formed upon the PDA-MXene support surface and were well dispersed. The Pd/PDA-MXene exhibited good catalytic activity in the formic acid decomposition process without any additives, and the turnover frequency value at 50 °C was 924.4 h⁻¹, which is comparable to most of the reported noble metal catalysts under similar conditions. It is worth mentioning that the prepared catalyst maintained good catalytic activity in five consecutive catalytic cycles of the formic acid dehydrogenation experiment.     

Effect of precursor on the hydrogen evolution activity and recyclability of Pd-Supported graphitic carbon nitride

The examination of graphitic carbon nitride (GCN) synthesis and its catalytic activity in hydrogen production from potassium formate was done as a function of the precursor selection. Four different precursors were assessed, namely urea, dicyandiamide, melamine and thiourea. The catalytic activity of the catalysts fabricated from different GCN precursors and palladium (Pd) was compared. The catalyst prepared from dicyandiamide, Pd-GCN(D), was found to be the most active of the four precursors tested during the first reaction cycle. Nonetheless, the catalyst prepared from urea, Pd-GCN(U), has been attributed by us as the preferred catalyst due to its excellent catalytic activity as well as its phenomenal stability over multiple cycles, which was not observed for the other three catalysts. The better catalytic activity of Pd-GCN(U) is correlated to the high surface area and pore volume of the material. Both the GCN and Pd-GCN samples were characterized by Fourier transform infrared spectroscopy, thermogravimetric analysis, powder X-ray diffraction, Brunauer-Emmett-Teller methods, scanning tunneling electron microscopy, and X-ray photoelectron spectroscopy.     

Pd nanoparticles supported on functionalized multi-walled carbon nanotubes (MWCNTs) and electrooxidation for formic acid

To improve the utilization and activity of anodic catalysts for formic acid electrooxidation, palladium (Pd) particles were loaded on the MWCNTs, which were functionalized in a mixture of 96% sulfuric acid and 4-aminobenzenesulfonic acid, using sodium nitrite to produce intermediate diazonium salts from substituted anilines. The composition, particle size, and crystallinity of the Pd/f-MWCNTs catalysts were characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and energy dispersive spectroscopy (EDS) measurements. The electrocatalytic properties of the Pd/f-MWCNTs catalysts for formic acid oxidation were investigated by cyclic voltammetry (CV) and linear sweep voltammetry (LSV) in 0.5 mol L⁻¹ H₂SO₄ solution. The results demonstrated that the catalytic activity was greatly enhanced due to the improved water-solubility and dispersion of the f-MWCNTs, which were facile to make the small particle size (3.8 nm) and uniform dispersion of Pd particles loading on the surface of the MWCNTs. In addition, the functionalized MWCNTs with benzenesulfonic group can provide benzenesulfonic anions in aqueous solution, which may combine with hydrogen cation and then promote the oxidation of formic acid reactive intermediates. So the Pd/f-MWCNTs composites showed excellent electrocatalytic activity for formic acid oxidation.     

Mesoporous carbon/graphitic carbon nitride spheres for photocatalytic H2 evolution under solar light irradiation

Herein, two different photocatalytic composites based on ordered (OCS) and disordered (DCS) mesoporous hollow carbon spheres and graphitic carbon nitride (gCN) have been successfully fabricated through facile acid treatment. The influence of carbon shell morphology of the spheres on gCN loading and photocatalytic H₂ production under simulated solar light irradiation has been revealed. The amount of evolved H₂ was ~6.2 (OCS/gCN) and ~5.3 (DCS/gCN) times higher in comparison to pristine gCN. It was found that graphitic carbon nitride was much more homogenously supported onto ordered mesoporous carbon spheres than disordered ones. The deposition of gCN onto ordered carbon spheres was found to be more efficient to increase carrier concentration, enhance photogenerated charge carrier transport and separation. It is assigned to the formation of the graphitic carbon nitride/carbon heterojunction facilitating the contact surface between the two phases of hybrid. Therefore, via tuning of the morphology of carbon shell being a host for gCN it was possible to find more promising candidate as a photocatalyst in H₂ production under solar light irradiation.     

Steam exfoliation of graphitic carbon nitride as efficient route toward metal-free electrode materials for hydrogen production

This study demonstrates the potential of obtaining nanostructured materials based on g-C₃N₄ with a high specific surface area for use as efficient electrode materials for hydrogen production. The study uses a novel method of g-C₃N₄ exfoliation that increases the specific surface area of the starting material by a factor of three. Nanocrystalline g-C₃N₄ is obtained through the thermolysis of urea and treated with steam in a specified temperature range. The resulting series is analyzed using a range of physicochemical methods to determine the optimal temperature for steam exfoliation. Catalytic electrochemical tests are carried out in the electrolytic reforming of ethanol. It has been demonstrated that steam exfoliation can boost the rate of electrocatalytic reforming by 1.3 times while decreasing the amount of hydrogen evolution overpotential. The results of this study demonstrate the potential for the use of steam exfoliation as an effective method for obtaining high-performing electrode materials for hydrogen production.     

An efficient exfoliation method to obtain graphitic carbon nitride nanosheets with superior visible-light photocatalytic activity

Graphitic carbon nitride (g-C₃N₄) has a promising application in the photocatalytic field due to its large aspect ratio and the favorable band gap energy. Herein, g-C₃N₄ nanosheets (g-C₃N₄ NS) with high photoactivity are obtained with the aid of isopropanol (IPA) in the synthesis process. The introduced IPA causes a more intense oxidation in the exfoliation process and the obtained g-C₃N₄ NS owns its unique properties of a broaden absorption range of visible light, an enlarged surface area and the irregular surface. As a result, the g-C₃N₄ NS has good photocatalytic activity in the degradation of organic pollutant. Moreover, the photocatalytic hydrogen evolution rate of g-C₃N₄ NS is three times as that of g-C₃N₄ NS* synthesized without IPA using the same method.