Green hydrogen storage and delivery: Utilizing highly active homogeneous and heterogeneous catalysts for formic acid dehydrogenation

Significant technological revolution which allows the use of hydrogen as an efficient energy carrier and mitigates the use of fossil fuels is a crucial need of the day. Much attention has been paid to cope with the recent, reversible, and sustainable hydrogen storage technologies. Among recent strategies, formic acid (FA) has been marked as an efficient liquid chemical for hydrogen storage. The liquid organic hydrogen carriers are based on some vital characteristics that particularly reduce toxicity and consist of maximum hydrogen with efficient recyclability. This review summarizes the recent reports, studies, and investigations concerning the hydrogen production from dehydrogenation of FA, which is almost free of carbon monoxide and is considered as an excellent material in fuel cell (FC) technology. A comprehensive critical review is carried out to link the latest research progress for subsequent achievements and to investigate both the homogeneous and heterogeneous catalysts. Among these catalysts, the main focus is on precious and non-precious metals that have been progressively increased during the past decade. Furthermore, the recent developments in the generation of high-pressure hydrogen gas and its practical applications have been highlighted for better understanding.     

Nitrogen-phosphorus co-functionalized reduced graphene oxide supported NiCoPd-CeOx nanoparticles as a highly efficient and stable catalyst for formic acid dehydrogenation

Formic acid (HCOOH, FA), a common liquid hydrogen storage material, has attracted tremendous research interest. However, the development of efficient, low-cost and high-stable heterogeneous catalyst for selective dehydrogenation of FA remains a major challenge. In this paper, a simple co-reduction method is proposed to synthesize nitrogen-phosphorus co-functionalized rGO (NPG) supported ultrafine NiCoPd-CeO_x nanoparticles (NPs) with a mean size of 1.2 nm. Remarkably, the as-prepared Ni_0.2Co_0.2Pd_0.6-CeO_x/NPG shows outstanding catalytic activity for FA dehydrogenation, affording a high TOF value of 6506.8 mol H₂ mol Pd^?1 h^?1 at 303 K and a low activation energy of 17.7 kJ mol^?1, which is better than most of the reported heterogeneous catalysts, and can be ascribed to the combined effect of well-dispersed ultrafine NiCoPd-CeO_x NPs, modified Pd electronic structure, and abundant active sites. The reaction mechanism of dehydrogenation of FA is also discussed. Furthermore, the optimized Ni_0.2Co_0.2Pd_0.6-CeO_x/NPG shows excellent stability over 10^th run with 100% conversion and 100% H₂ selectivity, which may provide more possibilities for practical application of FA system on fuel cells.     

Amine-functionalized sepiolite: Toward highly efficient palladium nanocatalyst for dehydrogenation of additive-free formic acid

Formic acid has recently come to be considered as a promising material for chemical storage of hydrogen. To date, highly efficient and low-cost heterogeneous catalysts for formic acid dehydrogenation without additive under mild conditions are still limited. In this work, we employed amine-functionalized sepiolite, a typical natural clay mineral with fibrous structure, as support to synthesize a nanocatalyst by the simple impregnation-reduction method. The amine groups on amine functionalized sepiolite support are shown to significantly reduce the average size of Pd nanoparticles (from 7.60 to 2.80 nm) as well as provide a synergistic effect with the Pd nanoparticles, which boosts the rate of additive-free formic acid dehydrogenation (initial turnover frequency (TOF) values of 5587 h⁻¹ at 60 °C, equal to or even better than the most active heterogeneous catalysts). Compared to current catalyst systems, this catalyst can lead to a brighter prospect of cost-efficient heterogeneous catalysts for formic acid dehydrogenation.     

In-situ nitrogen and Cr2O3 co-doped MOF-derived porous carbon supported palladium nanoparticles: A highly effective catalyst towards formic acid dehydrogenation

Fast and high selective dehydrogenation of formic acid (FA) is regarded as one of the most promising pathways to obtain the clean energy carrier: hydrogen. In this work, a nitrogen and Cr₂O₃ co-doped hierarchical carbon material has been successfully synthesized by in-situ pyrolysis of NH₂-MIL-101(Cr) in one step, followed by boiling in hot NaOH solution for surface etching. The activated carbon material is used to anchor the ultrafine Pd nanoparticles (2.18 nm) for formic acid dehydrogenation (FAD). As a result, the 5 wt% Pd@Cr₂O₃-NPCB-850 exhibits an excellent catalytic activity towards FAD: the turnover frequency (TOF) value is as high as 11 241 h⁻¹ at 333 K, and the selectivity of H₂ is up to 100%. The excellent catalytic performance is mainly attributed to the existence of N species and Cr₂O₃, which plays an important role of electron transfer and anti-aggregation. Our studies open a new methodology for convenient and fast syntheses of nitrogen and metal oxide co-doped activated carbon material, which also provides potential access for producing more highly effective catalysts for other catalytic reactions.     

Pd/NH2-KIE-6 catalysts with exceptional catalytic activity for additive-free formic acid dehydrogenation at room temperature: Controlling Pd nanoparticle size by stirring time and types of Pd precursors

Pd nanoparticle size is one of important factors to determine the catalytic activity of formic acid dehydrogenation catalysts. Thus various approaches to minimization of Pd nanoparticles have been attempted. In this study, we first tried to decrease Pd nanoparticles size and increase Pd dispersion of Pd/NH₂-mesoporous silica (Pd/NH₂-KIE-6) catalysts by controlling only stirring time and types of Pd precursors. It was demonstrated that the stirring time and types of Pd precursors significantly affect the performance of the catalysts. As a result, the Pd/NH₂-KIE-6 exhibited the highest catalytic activity (TOF: 8185 mol H₂ mol catalyst^?1 h^?1) ever reported for additive-free formic acid dehydrogenation at room temperature. In addition, the Pd/NH₂-KIE-6 provided higher TOF even than the case with additives such as sodium formate. Considering that the catalytic activity of Pd-based catalysts for formic acid dehydrogenation was previously controlled by promoter, support type and surface chemistry of supports, controlling the stirring time and types of Pd precursors is novel and very intriguing solutions to go beyond the current kinetic limitation for formic acid dehydrogenation.     

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.     

Fast hydrogen evolution by formic acid decomposition over AuPd/TiO2-NC with enhanced stability

Well-dispersed AuPd nanoparticles were immobilized on TiO₂-NC supports derived from NH₂-MIL-125(Ti) and used as highly active, stable catalysts for hydrogen production from formic acid under mild conditions. The highest total turnover frequency, i.e., 3207 h⁻¹, for formic acid dehydrogenation was achieved with Au₂Pd₈/TiO₂-NC-800 as the catalyst at 60 °C; this is 1.4 times that achieved with Au₂Pd₈/TiO₂–C-800 under the same conditions. The excellent performance of the Au₂Pd₈/TiO₂-NC-800 catalyst originates from the high anatase TiO₂ content, pyridinic N and oxygen vacancies in the support, the small size and alloying effect of the AuPd nanoparticles, and the metal–support synergistic effect. Doping the support with N improves the catalyst stability because N prevents metal particle aggregation to some extent. These results provide guidelines for the future development and applications of catalysts based on TiO₂ and metal–organic-framework-derived carbon-based materials.     

PdCoNi nanoparticles supported on nitrogen-doped porous carbon nanosheets for room temperature dehydrogenation of formic acid

The development of cost-effective heterogeneous catalysts for the dehydrogenation of formic acid (FA) is the key challenge for the commercialization of FA as a hydrogen-storage medium. Herein, PdCoNi nanoparticles (NPs) with different element ratios supported on N-doped carbon nanosheets (N-CN) were designed, which exhibit excellent catalytic dehydrogenation performance for FA. Compared with PdCoNi NPs loaded on the carbon nanosheets (CN), the introduction of pyrrolic N to CN induces the formation of ultrafine, monodispersed and amorphous Pd_0.6Co_0.2Ni_0.2 NPs with a size of 1.60 nm, which significantly increases the number of active sites and the instant contact between FA and catalysts. The as-prepared Pd_0.6Co_0.2Ni_0.2/N-CN catalyst shows more than 99% conversion and 100% H₂ selectivity at room temperature, with a record-high initial turnover frequency (TOF_initial) of 1249.0 h⁻¹ among non-noble containing Pd-based catalysts, which demonstrates the high potential of Pd_0.6Co_0.2Ni_0.2/N-CN as a practical catalyst for the hydrogen generation from FA.     

Amine-functionalized carbon nanotubes supported NiAuPd nanoparticles as an efficient in-situ prepared catalyst for formic acid dehydrogenation

Hydrogen (H₂) generation from formic acid (FA) decomposition is a promising route in practical application of hydrogen energy. As a promising H₂ supplier, besides the advantage of high H₂ content and excellent stability, FA can also be used as a mild reducing agent. Herein, an in-situ prepared NiAuPd nanoparticles (NPs) supported on amine-functionalized carbon nanotubes (NiAuPd/NH₂-CNTs) with FA as the reducing agent is successfully developed at room temperature. The as-prepared NiAuPd/NH₂-CNTs are directly used for the catalytic decomposition of FA, exhibiting excellent activity and 100% H₂ selectivity with the initial turnover frequency (TOF_initial) value of 699.1 and 3006.1 mol H₂ mol Pd^-1 h^-1 at 303 and 333 K, respectively. Moreover, the additive sodium formate (SF) can further facilitate the reduction process and enhance the catalytic performance, with the TOF_initial value of 4391.1 mol H₂ mol Pd^-1 h^-1 at 333 K, which are comparable to most of the reported heterogeneous catalysts with the complicated post-treatment. The excellent catalytic performance of NiAuPd/NH₂-CNTs is mainly attributed to the high dispersion of NPs and the boost effect of -NH₂ group on O–H cleavage. This work provides a feasible strategy to design in-situ prepared catalysts for the efficient high-quality H₂ generation from FA for fuel cells application.     

NiCoP nanoparticles anchored on CdS nanorods for enhanced hydrogen production by visible light-driven formic acid dehydrogenation

Photocatalytic dehydrogenation of formic acid (FA), HCOOH→H₂+CO₂, is a promising strategy for hydrogen production. Although tremendous efforts have been made, developing efficient and robust system driven by visible light without noble metal still remains a huge challenge. Herein, we report for the first time the use of NiCoP nanoparticles anchored on CdS nanorods (NiCoP@CdS NRs) as a highly efficient and robust catalyst for photocatalytic FA dehydrogenation. NiCoP nanoparticles as cocatalyst can effectively separate the electron-hole pairs generated by CdS NRs. The H₂ production rate of the NiCoP@CdS nanorods reached ~354 μmol mg⁻¹ h⁻¹ under visible light irradiation (λ > 420 nm) and the apparent quantum yield (AQY) was ~45.5 % at 420 nm which are among the best values ever reported in photocatalytic FA dehydrogenation systems. This work provides a prospective strategy for developing noble-metal-free photocatalytic FA dehydrogenation systems and hydrogen-based energy applications.     

Ultrafine palladium nanoparticles anchored on NH2-functionalized reduced graphene oxide as efficient catalyst towards formic acid dehydrogenation

The development of active and stable catalyst is of significance for hydrogen generation from formic acid. Herein, a novel palladium catalyst with ultrafine metallic nanoparticles anchored on NH₂-functionalized reduced graphene oxide (NH₂-rGO) was synthesized by a facile wet chemical reduction process using sodium borohydride as the reducing agent. The TEM and XPS characterization results confirmed the successful functionalization of rGO with 3-aminopropyltriethoxysilane (APTES), which plays a very important role in evenly dispersing ultrafine Pd nanoparticles with a small average size of about 2.3 nm. As a result, the as-prepared Pd/NH₂-rGO catalyst exhibited excellent activity with a high initial turnover frequency of 767 h⁻¹ and 100% hydrogen selectivity, which was predominant among the currently available pure Pd catalysts towards formic acid dehydrogenation under room temperature.     

Facile synthesis of well dispersed Pd nanoparticles on reduced graphene oxide for electrocatalytic oxidation of formic acid

In present study, we report a facile synthesis of crystalline, small size Pd nanoparticles (NPs) on reduced graphene oxide (RGO) abbreviated as Pd/RGO for electrocatalytic oxidation of formic acid (FA). Here, first graphene oxide (GO) was reduced by the green method using l-ascorbic acid and citric acid and further Pd NPs were decorated on RGO by a facile method without using any reducing agents. The reduction of GO to RGO and synthesis of Pd NPs was confirmed by the X-ray diffraction (XRD) and X-ray photoelectrons (XPS) techniques. Surface morphology of Pd/RGO nanocomposite was evaluated by the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. The electrocatalytic behavior of Pd/RGO nanocomposite was tested by using of cyclic voltammetric (CV) technique for electro-oxidation of FA in mixed solution of 0.5 M HCOOH + 0.5 H₂SO₄ at RT. Results shows that the higher electrocatalytic activity of Pd/RGO nanocomposite compare to Pd NPs.     

Hydrogen production from formic acid in fluidized bed made out of Ni-cenosphere catalyst

This research presents a method of hydrogen production from formic acid with the use of fluidized bed technology. The core-shell catalyst was developed by applying the Ni layer on cenospheres via a technique of gaseous deposition. The efficiency of the decomposition of formic acid was tested continuously in the range of 200–500 °C. An analytical method, based on infrared spectroscopy, allowing the continuous monitoring of the concentration of products in the gas phase has been developed. A 67% yield of hydrogen was achieved at 233 °C. The proposed solution in a fluidized bed has been compared with other methods of obtaining hydrogen from formic acid. The most important advantages of the proposed solutions are on-demand hydrogen generation; the use of an energy carrier that can be obtained from biomass or CO₂; simplicity of the process including easy control of the process temperature; repeatability; ease of scaling the fluidized process; the possibility of continuous monitoring of the products of the process; high efficiency of hydrogen generation per unit volume of the reactor.     

Hydrogen generation from formic acid decomposition at room temperature using a NiAuPd alloy nanocatalyst

Formic acid has been widely regarded as a safe and sustainable hydrogen storage material. Despite tremendous efforts, developing low-noble-metal-loading material with high activity for the dehydrogenation of formic acid remains a great challenge. Here, carbon supported highly homogeneous trimetallic NiAuPd alloy nanoparticles are prepared and employed as catalyst for the selective dehydrogenation of formic acid. Unexpectedly, at Ni molar contents as high as 40%, the resultant Ni_0.40Au_0.15Pd_0.45/C exhibits high activity and 100% hydrogen selectivity for hydrogen generation from formic acid aqueous solution without any additives even at 298 K. Such a low-noble-metal-loading catalyst with high activity may greatly encourage the practical application of formic acid as a hydrogen storage material.     

Au3Pd1 intermetallic compound as single atom catalyst for formic acid decomposition with highly hydrogen selectivity

Developing efficient catalysts for formic acid decomposition has been studied extensively. Herein, the Au₃Pd₁ intermetallic compound is designed as a single atom catalyst for the dehydrogenation of formic acid. By using density functional theory calculations, the thermodynamic stability, electronic structure, and reaction mechanism for the Au₃Pd₁ catalyst are systematically investigated, and the surface charge polarization and atom-ordered arrangement were confirmed to play an important role in the efficient formic acid dehydrogenation. The special positively charged Pd single atom on the Au₃Pd₁ surface becomes the adsorption site of HCOO⁻ and the reaction site for formic acid decomposition. The nearby Au sites suppress the C–O bond cleavage due to their weak interaction with CO1 and OH1. As a result, the HCOO⁻ dehydrogenation pathway is predominant on the Pd single atomic sites and the CO formation is well inhibited. This intermetallic-based catalyst can be extended to other systems and provided general guidance for efficient catalyst design.     

Co/multi-walled carbon nanotubes as highly efficient catalytic nanoreactor for hydrogen production from formic acid

Formic acid is well-recognized as safe and convenient hydrogen carrier. Development of active and cost-effective catalysts for formic acid to hydrogen conversion is important problem of hydrogen energy field. Herein, we report on new Co catalysts supported on oxidized multi-walled carbon nanotubes (MWCNTs), which demonstrate high efficiency in the gas-phase formic acid decomposition affording molecular hydrogen. Various parameters of the catalysts, Co loading, MWCNTs structure, and nanotubes treatment conditions, have been investigated in terms of their influence on the catalytic properties. The catalysts morphology has been characterized with a set of physicochemical methods. It is found that the catalytic activity of Co particles depends on their electronic state and location on the support. Co species located inside the MWCNTs channels are less active than Co species stabilized on the outer surface. An increase in the content of Co nanoparticles on the MWCNT outer surface leads to a higher catalytic activity.     

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.     

Nitrogen/oxygen co-doped graphene bonded nickel particles composite for dehydrogenation of formic acid at near room temperature

Low-cost non-noble metal catalyst for dehydrogenation of formic acid to hydrogen at near room temperature is considered as a key to promoting commercial technology for clean energy. We have constructed reduced graphene oxide (RGO) self-assembly bonded nickel particles for synthesis of graphene nanosheets embedded with nickel nanoparticles architecture Ni@xRGO. Nitrogen and oxygen co-doped graphene facilitates the adsorption of hydrogen protons from formic acid. Electron transfer ability of Ni@xRGO with active sites is enhanced via Ni–C bond in the interface between the RGO nanosheets and nickel particles, which promoted the C–H bond breaking for dehydrogenation of formic acid. The Ni@0.20RGO has excellent catalytic performance for hydrogen production from formic acid at near room temperature (the yield of hydrogen, 240.0 mL g^?1 h^?1 at 50 °C), comparable to the most active non-noble metal catalysts.     

Effect of MoO3 on Pd nanoparticles for efficient formic acid electrooxidation

Palladium is a promising formic acid electro-oxidation (FAO) catalyst due to its higher initial activity than platinum. However, suffering from the adsorption of hydrogen and CO-like species, the activity and stability of Pd are still unsatisfied. Herein, palladium nanoparticles deposited on carbon supported molybdenum trioxide (Pd–MoO₃/C) is prepared with MoO₃ as the promoter for FAO. X-ray photoelectron spectroscopy analysis proves the close contact between Pd and MoO₃, which generates the hydrogen spillover effect and forms the Pd–Mo structure. The hydrogen spillover effect enhances the desorption of hydrogen from Pd and facilitates the FAO activity. Both the spillover effect and Pd–Mo structure contribute to the removal of CO_ad and facilitate the durability and anti-CO poisoning ability of the catalyst. With the optimized ratio of MoO₃ to carbon black, the Pd–MoO₃/C-20 catalyst owns the best specific activity of 5.86 mA cm⁻², which is 1.86 times of Pd/C.     

Solvent effects on high-pressure hydrogen gas generation by dehydrogenation of formic acid using ruthenium complexes

High-pressure H₂ was produced by the selective dehydrogenation of formic acid (DFA) using ruthenium complexes at mild temperatures in various organic solvents and water. Among the solvents studied, 1,4-dioxane was the best candidate for this reaction to generate high gas pressure of 20 MPa at 80 °C using the Ru complex having a dearomatized pyridine-based pincer PN³P* ligand. This complex shows reusability for the high-pressure DFA in 1,4-dioxiane while maintaining the catalytic performance, however, deactivation occurred in other solvents. In dimethyl sulfoxide, its decomposition products may cause catalytic deactivation. The gas pressure generated in 1,4-dioxane was lower than that in water due to the high dissolution of 1,4-dioxane into CO₂ according the vapor-liquid equilibrium calculations. The role of solvent is crucial since it affected the catalytic performance and also the generated gas pressure (H₂ and CO₂) from FA.