Mechanistic study of hydrazine decomposition on Ir(111)

Role of Hydrazine and Size-Tuning Parameter in Gold Nanoparticle Synthesis by Water-in-Oil Microemulsion: Experiment and Simulation

By tuning the drop size, self-assembled microemulsion drops are used to control the size of nanoparticles synthesized in it. However, nanoparticle size control is challenging, specifically when particles outgrow the initial drop diameter. This necessitates the search for a robust operating parameter to control the size of the nanoparticles in the microemulsion route. In this pursuit, gold nanoparticle (GNP) size is controlled here, via the molar ratio (P) of concentrations of reducing agent (hydrazine) to precursor (aurochloric acid). A kinetic Monte Carlo (kMC) simulation scheme was devised to investigate the underlying mechanism behind decreasing particle diameter on increasing P. Binary pairs of GNPs seen to be partially fused from TEM images confirmed the involvement of particle-particle coagulation as a key step. Coagulation was regulated in the presence of hydrazine, the latter stabilizing GNPs by chemisorbing on its surface. Addition of NaCl caused the Cl- ion to compress the diffuse double layer around hydrazine and form agglomerated GNPs. Finally, we found that at a fixed value of P = 12, even a 4-fold increase in precursor concentration does not affect the final diameter of GNPs, signifying P as a very important robust parameter. Therefore, the current proposed single parameter P reduces the experimental parametric space by eliminating the need to track individual concentrations of all reagents. It can be used to tune or predict the nanoparticle size even in the regime where the final particle diameter is bigger than the initial drop diameter.

Probing the Metal/Oxide Interface of IrCoCeOx in N2H4·H2O Decomposition: An Experimental and Computational Study

Understanding the structure of a functional catalyst is crucial to disclosing the complexity of heterogeneous processes and improving their efficiency. Herein, coprecipitated cobalt-ceria (CoCeOx) oxides doped with Ir (IrCoCeOx) were synthesized and used to assess the performances of metal/oxide interfaces in the N2H4·H2O decomposition performed in aqueous NaOH. Kinetic experiments in batch showed that CoO is the active phase of CoCeOx and that the copresence of Ir and Co (IrCoCeOx) enhanced H2 productivity. A comprehensive characterization (X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy, and in situ diffuse reflectance infrared Fourier transform spectroscopy) combined with robust computational modeling based on the density functional theory was employed to attribute the IrCoCeOx performance enhancement to the Ir/CoO metal/oxide interface, the active site of the reaction. On these sites, the improved H2 productivity in the presence of aqueous NaOH was studied operando through modulated excitation-attenuated total reflectance infrared coupled with phase sensitive detection. The formation of surface Co-hydroxyl and -imido groups at the Ir/CoO interface induced the preferential breakage of the N-H bond of N2H4·H2O, favoring the production of H2.

Effects of oxygen functionalities on hydrous hydrazine decomposition over carbonaceous materials

Metal-free heterogeneous catalysis is promising in the context of H2 generation. Therefore, establishing structure-activity relationships is a crucial issue to improve the development of more efficient catalysts. Herein, to evaluate the reactivity of the oxygen functionalities in carbonaceous materials, commercial functionalized pyrolytically stripped carbon nanofibers (CNFs) were used as catalysts in the liquid-phase hydrous hydrazine decomposition process and its activity was compared to that of a pristine CNF material. Different oxygenated groups were inserted by treating CNFs with hydrogen peroxide for 1 h (O1-H2O2) and HNO3 for 1 h (O1-HNO3) and 6 h (O6-HNO3). An increase in activity was observed as a function of the oxidizing agent strength (HNO3 > H2O2) and the functionalization time (6 h > 1 h). A thorough characterization of the catalysts demonstrated that the activity could be directly correlated with the oxygen content (O6-HNO3 > O1-HNO3 > O1-H2O2 > CNFs) and pointed out the active sites for the reaction at carbon-oxygen double bond groups (CO and COOH). Systematic DFT calculations supported rationalization of the experimental kinetic trends with respect to each oxygen group (CO, C-O-C, C-OH and COOH).

Enhanced stability of sub-nanometric iridium decorated graphitic carbon nitride for H2 production upon hydrous hydrazine decomposition

Stabilizing metal nanoparticles is vital for large scale implementations of supported metal catalysts, particularly for a sustainable transition to clean energy, e.g., H₂ production. In this work, iridium sub-nanometric particles were deposited on commercial graphite and on graphitic carbon nitride by a wet impregnation method to investigate the metal-support interaction during the hydrous hydrazine decomposition reaction. To establish a structure-activity relationship, samples were characterized by transmission electron microscopy and X-ray photoelectron spectroscopy. The catalytic performance of the synthesized materials was evaluated under mild reaction conditions, i.e. 323 K and ambient pressure. The results showed that graphitic carbon nitride (GCN) enhances the stability of Ir nanoparticles compared to graphite, while maintaining remarkable activity and selectivity. Simulation techniques including Genetic Algorithm geometry screening and electronic structure analyses were employed to provide a valuable atomic level understanding of the metal-support interactions. N anchoring sites of GCN were found to minimise the thermodynamic driving force of coalescence, thus improving the catalyst stability, as well as to lead charge redistributions in the cluster improving the resistance to poisoning by decomposition intermediates.

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.

Thermal and Catalytic Decomposition of 2-Hydroxyethylhydrazine and 2-Hydroxyethylhydrazinium Nitrate Ionic Liquid

To develop chemical kinetics models for the combustion of ionic liquid-based monopropellants, identification of the elementary steps in the thermal and catalytic decomposition of components such as 2-hydroxyethylhydrazinium nitrate (HEHN) is needed but is currently not well understood. The first decomposition step in protic ionic liquids such as HEHN is typically the proton transfer from the cation to the anion, resulting in the formation of 2-hydroxyethylhydrazine (HEH) and HNO₃. In the first part of this investigation, the high-temperature thermal decomposition of HEH is probed with flash pyrolysis (<1400 K) and vacuum ultraviolet (10.45 eV) photoionization time-of-flight mass spectrometry (VUV-PI-TOFMS). Next, the investigation into the thermal and catalytic decomposition of HEHN includes two mass spectrometric techniques: (1) tunable VUV-PI-TOFMS (7.4-15 eV) and (2) ambient ionization mass spectrometry utilizing both plasma and laser ionization techniques whereby HEHN is introduced onto a heated inert or iridium catalytic surface and the products are probed. The products can be identified by their masses, their ionization energies, and their collision-induced fragmentation patterns. Formation of product species indicates that catalytic surface recombination is an important reaction process in the decomposition mechanism of HEHN. The products and their possible elementary reaction mechanisms are discussed.

Electrocatalytically active cuprous oxide nanocubes anchored onto macroporous carbon composite for hydrazine detection

Cuprous oxide (Cu₂O) is a p-type semiconductor with excellent catalytic activity and stability that has gained much attention because it is non-toxic, abundant, and inexpensive. Porous carbon materials have large specific surface areas, which offer abundant electroactive sites, enhance the electrical conductivity of materials, and prevent the aggregation of Cu₂O nanocubes. In this study, a composite with high electrocatalytic activity was prepared based on Cu₂O nanocubes anchored onto three-dimensional macroporous carbon (MPC) by a simple, eco-friendly, and cheap method for hydrazine detection. Due to the synergistic effect of MPC and Cu₂O, the sensor exhibited high electrocatalytic activity, sensitivity, better selectivity, and low limit of detection. The resulting sensor could be a sensitive and effective platform for detecting hydrazine and promising practical applications.

Excessive consumption mechanism of hydrazine in the reaction with ReO4−: Re species evolution and ReO2·nH2O-catalyzed decomposition

ReO4− is slowly reduced to ReO42− and Re(iv) species by hydrazine, and ReO2·nH2O catalyzes hydrazine decomposition.

Hydrous Hydrazine Decomposition for Hydrogen Production Using of Ir/CeO2: Effect of Reaction Parameters on the Activity

In the present work, an Ir/CeO₂ catalyst was prepared by the deposition-precipitation method and tested in the decomposition of hydrazine hydrate to hydrogen, which is very important in the development of hydrogen storage materials for fuel cells. The catalyst was characterised using different techniques, i.e., X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), scanning electron microscopy (SEM) equipped with X-ray detector (EDX) and inductively coupled plasma-mass spectroscopy (ICP-MS). The effect of reaction conditions on the activity and selectivity of the material was evaluated in this study, modifying parameters such as temperature, the mass of the catalyst, stirring speed and concentration of base in order to find the optimal conditions of reaction, which allow performing the test in a kinetically limited regime.

Kinetic and mechanistic analysis of NH3 decomposition on Ru(0001), Ru(111) and Ir(111) surfaces

We investigated the catalytic NH3 decomposition on Ru and Ir metal surfaces using density functional theory. The reaction mechanisms were unraveled on both metals, considering that, on the nano-scale, Ru particles may also present an fcc structure, hence, leading to three energy profiles. We implemented thermodynamic and kinetic parameters obtained from DFT into microkinetic simulations. Batch reactor simulations suggest that hydrogen generation starts at 400 K, 425 K and 600 K on Ru(111), Ru(0001) and Ir(111) surfaces, respectively, in excellent agreement with experiments. During the reaction, the main surface species on Ru are NH, N and H, whereas on Ir(111), it is mainly NH. The rate-determining step for all surfaces is the formation of molecular nitrogen. We also performed temperature-programmed reaction simulations and inspected the desorption spectra of N2 and H2 as a function of temperature, which highlighted the importance of N coverage on the desorption rate.

Role of defects in carbon materials during metal-free formic acid dehydrogenation

Commercial graphite (GP), graphite oxide (GO), and two carbon nanofibers (CNF-PR24-PS and CNF-PR24-LHT) were used as catalysts for the metal-free dehydrogenation reaction of formic acid (FA) in the liquid phase. Raman and XPS spectroscopy demonstrated that the activity is directly correlated with the defectiveness of the carbon material (GO > CNF-PR24-PS > CNF-PR24-LHT > GP). Strong deactivation phenomena were observed for all the catalysts after 5 minutes of reaction. Density functional theory (DFT) calculations demonstrated that the single vacancies present on the graphitic layers are the only active sites for FA dehydrogenation, while other defects, such as double vacancies and Stone-Wales (SW) defects, rarely adsorb FA molecules. Two different reaction pathways were found, one passing through a carboxyl species and the other through a hydroxymethylene intermediate. In both mechanisms, the active sites were poisoned by an intermediate species such as CO and atomic hydrogen, explaining the catalyst deactivation observed in the experimental results.

Single transition metal atoms anchored on a C2N monolayer as efficient catalysts for hydrazine electrooxidation

Searching for highly-efficient and low-cost electrocatalysts for the hydrazine oxidation reaction (HzOR) is a key issue in the development of direct hydrazine fuel cells for hydrogen production, which is a promising energy-efficient conversion technology to replace the sluggish oxygen evolution reaction in water splitting. Herein, the potential of a series of single transition metal atoms anchored on nitrogenated holey graphene (TM@C₂N, TM = Ti, Mn, Fe, Co, Ni, Cu, Mo, Rh, Ru, Pd, Pt, Au, Ag, and W) as catalysts for the HzOR was systematically explored by means of comprehensive density functional theory (DFT) computations. Our results revealed that these TM atoms anchored on a C₂N monolayer exhibit high stability due to their strong interactions with the N atoms on the C₂N monolayer. Furthermore, on the basis of the computed free energy profiles, Ru@C₂N, Mo@C₂N, Ti@C₂N, Co@C₂N, and Fe@C₂N were shown to display high HzOR catalytic activity due to their lower (or comparable) limiting potential compared to the well-established Fe-doped CoS₂ nanosheet. In particular, Ru@C₂N is identified as the best catalyst with the lowest limiting potential of -0.24 V due to its optimum difference between the adsorption strength of N₂H₃* and N₂H₂* species. More interestingly, we found that single Mo and Ti atoms also exhibit excellent catalytic performance for the hydrogen evolution reaction, suggesting their bifunctional activity towards hydrazine splitting for H₂ production. Our findings provide a new avenue to develop an efficient single-atom electrocatalyst for experimental validation to convert hydrazine into hydrogen.

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.