Fast and Non-equilibrium Uptake of Hydrogen by Pd Icosahedral Nanocrystals

We report for the first time that Pd nanocrystals can absorb H via a "single-phase pathway" when particles with a proper combination of shape and size are used. Specifically, when Pd icosahedral nanocrystals of 7- and 12-nm in size are exposed to H atoms, the H-saturated twin boundaries can divide each particle into 20 smaller single-crystal units in which the formation of phase boundaries is no longer favored. As such, absorption of H atoms is dominated by the single-phase pathway and one can readily obtain PdHx with anyx in the range of 0-0.7. When switched to Pd octahedral nanocrystals, the single-phase pathway is only observed for particles of 7 nm in size. We also establish that the H-absorption kinetics will be accelerated if there is a tensile strain in the nanocrystals due to the increase in lattice spacing. Besides the unique H-absorption behaviors, the PdHx (x=0-0.7) icosahedral nanocrystals show remarkable thermal and catalytic stability toward the formic acid oxidation due tothe decrease in chemical potential for H atoms in a Pd lattice under tensile strain.

TiO2 nanoparticle-supported Ni catalyst for the dehydrogenation of hydrazine hydrate

As one of the key factors that affect the application of hydrazine hydrate as a potential hydrogen source, efficient and cheap catalyst is particularly important. Nickel based catalysts have been widely studied because of their excellent catalytic performance for the decomposition of hydrazine hydrate to hydrogen. Herein, a Ni catalyst supported on anatase TiO₂ through reduction and impregnation methods was prepared. Structure of the catalyst was investigated by transmission electron microscopy (TEM), X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), and X-ray photoelectron spectroscopy (XPS). The effects of the amount of TiO₂ and the concentration of NaOH on the activity of the catalyst were investigated. The results showed that the catalyst prepared with a metal nickel content of 0.2 mmol using 100 mg of the nano-TiO₂ support had the best catalytic performance. Hydrazine hydrate could be completely decomposed at 343 K in 2.83 min, the hydrogen selectivity attained 100%, and the turnover frequency (TOF) value was 265.49 h^-1. In this catalyst, transition metal Ni was dispersed on the support surface in the form of amorphous elemental or oxide. Anatase TiO₂ support had the advantages of promoting the dispersion of metal Ni, exposing the active site, changing the electronic state of the active center, strengthening the strong metal-support interaction (SMSI), and improving the activity of the catalyst. After ten cycles of use, the performance of the catalyst stabilized and the hydrogen selectivity was still as high as 100%.

Selective decomposition of hydrazine over metal free carbonaceous materials

Herein we report a combined experimental and computational investigation unravelling the hydrazine hydrate decomposition reaction on metal-free catalysts. The study focuses on commercial graphite and two different carbon nanofibers, pyrolytically stripped (CNF-PS) and high heat-treated (CNF-HHT), respectively, treated at 700 and 3000 °C to increase their intrinsic defects. Raman spectroscopy demonstrated a correlation between the initial catalytic activity and the intrinsic defectiveness of carbonaceous materials. CNF-PS with higher defectivity (I_D/I_G = 1.54) was found to be the best performing metal-free catalyst, showing a hydrazine conversion of 94% after 6 hours of reaction and a selectivity to H₂ of 89%. In addition, to unveil the role of NaOH, CNF-PS was also tested in the absence of alkaline solution, showing a decrease in the reaction rate and selectivity to H₂. Density functional theory (DFT) demonstrated that the single vacancies (SV) present on the graphitic layer are the only active sites promoting hydrazine decomposition, whereas other defects such as double vacancy (DV) and Stone-Wales (SW) defects are unable to adsorb hydrazine fragments. Two symmetrical and one asymmetrical dehydrogenation pathways were found, in addition to an incomplete decomposition pathway forming N₂ and NH₃. On the most stable hydrogen production pathway, the effect of the alkaline medium was elucidated through calculations concerning the diffusion and recombination of atomic hydrogen. Indeed, the presence of NaOH helps the extraction of H species without additional energetic barriers, as opposed to the calculations performed in a polarizable continuum medium. Considering the initial hydrazine dissociative adsorption, the first step of the dehydrogenation pathway is more favourable than the scission of the N-N bond, which leads to NH₃ as the product. This first reaction step is crucial to define the reaction mechanisms and the computational results are in agreement with the experimental ones. Moreover, comparing two different hydrogen production pathways (with and without diffusion and recombination), we confirmed that the presence of sodium hydroxide in the experimental reaction environment can modify the energy gap between the two pathways, leading to an increased reaction rate and selectivity to H₂.

Advances and Prospects in Metal-Organic Frameworks as Key Nexus for Chemocatalytic Hydrogen Production

Hydrogen is a clean and sustainable energy carrier, which is considered a promising alternative for fossil fuels to solve the global energy crisis and respond to climate change. Social concerns on its safe storage promote continuous exploration of alternatives to traditional storage methods. In this case, chemical hydrogen storage materials initiate plentiful research with special attention to the design of heterogeneous catalysts that can enhance efficient and highly selective hydrogen production. Metal-organic frameworks (MOFs), a kind of unique crystalline porous materials featuring highly ordered porosities and tailorable structures, can provide various active sites (i.e., metal nodes, functional linkers, and defects) for heterogeneous catalysis. Furthermore, the easy construction of active sites in highly ordered MOFs, which can work as plate for the delicate active site engineering, make them ideal candidates for a variety of heterogeneous catalysts including chemocatalytic hydrogen production. This review concentrates on the application of MOFs as heterogeneous catalysts or catalyst supports in chemocatalytic hydrogen production. Recent progresses of MOFs as catalysts for chemocatalytic hydrogen production are comprehensively summarized. The research methods, mechanism analyses, and prospects of MOFs in this field are discussed. The challenges in future industrial applications of MOFs as catalysts for hydrogen production are proposed.

Nitrogen-functionalized carbon nanotube-supported bimetallic PtNi nanoparticles for hydrogen generation from hydrous hydrazine

Herein, ultrafine PtNi nanoparticles (NPs) supported on N-functionalized multi-walled carbon nanotubes (N-MWCNTs) were facilely obtained. The N species on N-MWCNTs not only benefit the formation of PtNi NPs but also accelerate the dehydrogenation of hydrous hydrazine, affording a high turnover frequency of 1595 h^-1 with 100% H₂ selectivity. In addition, the N species can be used as an "anchor" to stabilize the bimetallic NPs and efficiently restrain the aggregation, giving robust stability of the catalysts for decomposition of hydrous hydrazine over PtNi/N-MWCNTs.

Distance Effect of Ni-Pt Dual Sites for Active Hydrogen Transfer in Tandem Reaction

Unveiling the distance effect between different sites in multifunctional catalysts remains a major challenge. Herein, we investigate the distance effect by constructing a dual-site distance-controlled tandem catalyst with a five-layered TiO₂/Pt/TiO₂/Ni/TiO₂ tubular nanostructure by template-assisted atomic layer deposition. In this catalyst, the Ni and Pt sites are separated by a porous TiO₂ interlayer, and the distance between them can be precisely controlled on the subnanometer scale by altering the thickness of the interlayer, while the inner and outer porous TiO₂ layers are designed for structural stability. The catalyst exhibits superior performance for the tandem hydrazine hydrate decomposition to hydrogen and subsequent nitrobenzene hydrogenation when the Ni and Pt site distance is on the subnanometer level. The performance increases with the decrease of the distance and is better than the catalyst without the TiO₂ interlayer. Isotopic and kinetic experiments reveal that the distance effect controls the transfer of active hydrogen, which is the rate-determining step of the tandem reaction in a water solvent. Reduced Ti species with oxygen vacancies on the TiO₂ interlayer provide the active sites for hydrogen transfer with -Ti-OH surface intermediates via the continuous chemisorption/desorption of water. A smaller distance induces the generation of more active sites for hydrogen transfer and thus higher efficiency in the synergy of Ni and Pt sites. Our work provides new insight for the distance effect of different active sites and the mechanism of intermediate transfer in tandem reactions.

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The distance effect is an interesting and important topic in catalysis

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The distance of Ni-Pt dual sites is precisely controlled in subnanometer scale on a TiO₂/Pt/TiO₂/Ni/TiO₂ five-layer catalyst by ALD

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The distance controls the water-assisted hydrogen transfer, determining the overall efficiency of the tandem reaction

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A close distance in subnanometer induces more active sites for hydrogen transfer and efficient synergy of Ni and Pt sites

Pt-Ni Nanoalloys for H2 Generation from Hydrous Hydrazine

Hydrous hydrazine (N2H4∙H2O) is a candidate for a hydrogen carrier for storage and transportation due to low material cost, high hydrogen content of 8.0%, and liquid stability at room temperature. Pt and Pt nanoalloy catalysts have been welcomed by researchers for the dehydrogenation of hydrous hydrazine recently. Therefore, in this review, we give a summary of Pt nanoalloy catalysts for the dehydrogenation of hydrous hydrazine and briefly introduce the decomposition mechanism of hydrous hydrazine to prove the design principle of the catalyst. The chemical characteristics of hydrous hydrazine and the mechanism of dehydrogenation reaction are briefly introduced. The catalytic activity of hydrous hydrazine on different supports and the factors affecting the selectivity of hydrogen catalyzed by Ni-Pt are analyzed. It is expected to provide a new way for the development of high-activity catalysts for the dehydrogenation of hydrous hydrazine to produce hydrogen.

Progress and Prospective of Nitrogen-Based Alternative Fuels

Alternative fuels are essential to enable the transition to a sustainable and environmentally friendly energy supply. Synthetic fuels derived from renewable energies can act as energy storage media, thus mitigating the effects of fossil fuels on environment and health. Their economic viability, environmental impact, and compatibility with current infrastructure and technologies are fuel and power source specific. Nitrogen-based fuels pose one possible synthetic fuel pathway. In this review, we discuss the progress and current research on utilization of nitrogen-based fuels in power applications, covering the complete fuel cycle. We cover the production, distribution, and storage of nitrogen-based fuels. We assess much of the existing literature on the reactions involved in the ammonia to nitrogen atom pathway in nitrogen-based fuel combustion. Furthermore, we discuss nitrogen-based fuel applications ranging from combustion engines to gas turbines, as well as their exploitation by suggested end-uses. Thereby, we evaluate the potential opportunities and challenges of expanding the role of nitrogen-based molecules in the energy sector, outlining their use as energy carriers in relevant fields.

Noble-metal-free nanocatalysts for hydrogen generation from boron- and nitrogen-based hydrides

We focus on the recent advances in non-noble metal catalyst design, synthesis and applications in dehydrogenation of chemical hydrides (e.g. NaBH₄, NH₃BH₃, NH₃, N₂H₄, N₂H₄BH₃) due to their high hydrogen contents and CO-free H₂ production.

Direct conversion of phenols into primary anilines with hydrazine catalyzed by palladium

Primary anilines are essential building blocks to synthesize various pharmaceuticals, agrochemicals, pigments, electronic materials, and others. To date, the syntheses of primary anilines mostly rely on the reduction of nitroarenes or the transition-metal-catalyzed Ullmann, Buchwald-Hartwig and Chan-Lam cross-coupling reactions with ammonia, in which non-renewable petroleum-based chemicals are typically used as feedstocks via multiple step syntheses. A long-standing scientific challenge is to synthesize various primary anilines directly from renewable sources. Herein, we report a general method to directly convert a broad range of phenols into the corresponding primary anilines with the cheap and widely available hydrazine as both amine and hydride sources with simple Pd/C as the catalyst.

MoOx-modified bimetallic alloy nanoparticles for highly efficient hydrogen production from hydrous hydrazine

MoO_x-doped NiM (M = Pt, Rh, Ir, Ru, Au, and Ag) catalysts exhibit higher catalytic performances toward the decomposition of hydrazine in aqueous solution as compared to those of pure NiM NPs.

NiFePd/UiO-66 nanocomposites as highly efficient catalysts to accelerate hydrogen evolution from hydrous hydrazine

NiFe nano-catalysts modified by the dopant of Pd were reducted on UiO-66 by NaBH₄, which have been used as a low cost and high efficient catalyst with outstanding activity and durability for hydrogen generation from the decomposition of hydrazine hydrate.

Anchoring ultrafine RhNi nanoparticles on titanium carbides/manganese oxide as an efficient catalyst for hydrogen generation from hydrous hydrazine

RhNi NPs of 2.8 nm are successfully monodispersed on the bi-support MnO_x/MXene surface.