High Energy Density Fuels Research Articles

Thermal decomposition behaviors of adamantane and 1-methyladamantane in oxygen atmosphere: ReaxFF molecular dynamics simulation

High energy density fuels (HEDFs) have garnered considerable interest in the aerospace applications due to their high density and effective heat dissipation capabilities. However, there is still no clear understanding of the pyrolysis and oxidation mechanisms of HEDFs. In this work, a computational strategy based on ReaxFF molecular dynamics reveals dehydrogenation and ring-opening reaction events for adamantane and 1-methyladamantane. A comparative analysis of the initial decomposition behavior, intermediate reaction mechanism, and final product distribution demonstrates that the decay of the reactants is dependent on temperature and oxygen concentration. The main decomposition pathways of adamantane and 1-methyladamantane involve multi-step dehydrogenation and ring opening by the following reaction: i. C10H16 → C10H15 → C7H10 + C3H5, ii. C10H16 → C10H12, iii. C11H18 → C11H16, iv. C11H18 → C11H15 → C2H4 + C9H11. C7H10, C3H5, and C9H11 undergo further bond-breaking reactions to form linear hydrocarbon chains. At low temperatures, C10H16 and C11H18 exhibit a preference for reacting with OH and HO2, accounting for 15 %–35 % of the reactions. At high temperatures, the preferred reaction for C10H16 and C11H18 is with O2, accounting for 33 %–64 % of the reactions. The bridge carbon atom acts as the initial dehydrogenation site for both adamantane and 1-methyladamantane, as well as the active site for ring opening. The decomposition energy barrier for 1-methyladamantane (108.16–130.39 kJ/mol) is lower than that for adamantane (130.72–142.61 kJ/mol). Our findings shed light on the complicated interplay of oxidation of adamantane and 1-methyladamantane.

Accurate Density Prediction of Sesquiterpenoid HEDFs and the Multiproperty Computing Server SesquiterPre.

Accurate and rapid evaluation of density is crucial for evaluating the packing and combustion characteristics of high-energy-density fuels (HEDFs). This parameter is pivotal in the selection of high-performance HEDFs. Our study leveraged a polycyclic compound density data set and quantum chemical (QC) descriptors to establish a correlation with the target properties using the XGBoost algorithm. We utilized a recursive feature elimination method to simplify the model and developed a concise and interpretable density prediction model incorporating only six QC descriptors. The model demonstrated robust performance, achieving coefficients of determination (R 2) of 0.967 and 0.971 for internal and external test sets, respectively, and root-mean-square errors (RMSE) of 0.031 and 0.027 g/cm3, respectively. Compared to the other two mainstream methods, the marginal discrepancy between the predicted and actual molecular densities underscores the model's superior predictive ability and more usefulness for energy density calculation. Furthermore, we developed a web server (SesquiterPre, https://sespre.cmdrg.com/#/) that can simultaneously calculate the density, enthalpy of combustion, and energy density of sesquiterpenoid HEDFs, which greatly facilitates the use of researchers and is of great significance for accelerating the design and screening of novel sesquiterpenoid HEDFs.

Optimization effect of propylene oxide (PO) on evaporation, combustion, and pollutant emissions of high-energy–density JP-10 fuel

JP-10 is a high-energy–density fuel that is widely used in aerospace propulsion systems due to its wide range of synthetic raw materials and low production costs. However, it faces challenges related to difficult ignition, low combustion efficiency and high pollutant emissions. To address this, blending JP-10 with propylene oxide (PO), a highly volatile substance, provides a practical solution. This study investigates the propagation and evolution characteristics of JP-10/PO binary fuel in large horizontal pipelines. The results demonstrate a synergistic enhancement effect achieved by the JP-10/PO hybrid fuel. PO, with its lower boiling point, quickly evaporates from the mixed droplets, creating a continuous primary flame. As the droplet temperature increases, JP-10, known for its high combustion heat, starts to evaporate, actively participating in the explosion process and forming a secondary flame that spreads in two directions. The combustion of the JP-10/PO mixed droplets involves both physical and chemical processes, including droplet evaporation and gaseous combustion. A binary droplet evaporation model is developed to quantitatively assess the impact of PO fuel ratio on spray evaporation. Additionally, the study examines steam explosion parameters and reaction product characteristics under different PO fuel ratios. It is observed that the evaporation of JP-10/PO mixed droplets occurs in three stages: transient heating, binary equilibrium evaporation, and single component equilibrium evaporation. Increasing the PO fuel ratio leads to a higher temperature rise rate of the droplets and an increased evaporation reaction rate constant. Moreover, the ignition delay time of JP-10/PO gas-phase mixture fuel decreases, while the adiabatic flame temperature gradually reduces. Furthermore, higher PO fuel ratios help reduce the emissions of unstable products in both mixed fuels due to the inherent oxygen content of PO molecules. The current research results can provide scientific references for the improvement of high-energy density fuels and the development of new propellants.

Relationship between molecular structure and pyrolysis performance for high-energy-density fuels

High-energy-density endothermic hydrocarbon fuels (HDEHFs) are the ideal on-board coolants for the thermal management of advanced aircrafts. However, the pyrolysis performace of high-energy-density (HED) fuels with different molecular structures remains ambiguous. Herein, we investigated the pyrolysis performance of five HED fuels with different cyclic structures, i.e., bicyclohexyl (BCH), perhydrofluorene (PHF), decahydronaphthalene (DHN), exo-tetrahydrodicyclopentadiene (exo-THD) and cyclopropyl-tetrahydrodicyclopentadiene (CTHD), to reveal the molecular structure-reactivity relationship. The pyrolysis conversion (650 °C, 4 MPa) of the tested fuels is in the order of CTHD > PHF > BCH > DHN > exo-THD, which is highly associated with their C + H atom numbers, nH+C (except for CTHD with high-tension ring). With the analysis of product distribution as well as DFT calculation, the decomposition reaction pathways of the fuels were proposed. The heat sink of the fuels follows the order of BCH > PHF > DHN > exo-THD > CTHD, which is closely correlated with nH/C6.9 × nH+C3.8. Meanwhile, the fitting results between molecular structures and physicochemical properties of the fuels were also demonstrated. This study could provide a guidance on the design and development of HDEHFs fuels for the high-performance aerospace vehicles with long flight distance and hypersonic speed.

Insights into the enhanced performance of ignition and self-sustained combustion of n-decane over Co3O4-based catalysts

Aiming to achieve efficient utilization of high-energy–density fuels in the promising portable power systems, self-sustained combustion of n-decane over Co3O4-based catalysts in a narrow quartz reactor was investigated. Chemiluminescence, gas chromatography and various physicochemical characterization methods were used in this study. The evidences suggest that Co3O4-based catalysts can significantly reduce the ignition temperature and broaden the flammability limits of n-decane. The heat release stored in the catalyst bed can help improve the flame stability, delay the flame extinction, and achieve auto-ignition without forced heating. Besides, great promotion of the catalytic performance of Co3O4 spinel is achieved by doping with 6 wt% PdO. PdO/Co3O4 catalysts exhibit higher activity and enhanced ability in effectual resistance toward deactivate than pure Co3O4. Furthermore, the coupling of heterogeneous and homogeneous reactions of n-decane has been analyzed. It can be revealed that both the adsorbed and gaseous OH* radicals play greatly different roles when using different catalysts. These findings promote the use of heavy hydrocarbon fuels for energy supply and contribute to designing efficient Co3O4-based catalysts rationally.

Synthesis and Performance of Cyclopentadiene-Based Spirocyclopropane High-Energy-Density Fuels

High-energy–density fuels play a vital role in the development of the aviation industry because they can provide more propulsion energy than conventional liquid fuels. Constructing more five- or six-membered rings in fuel molecules is an efficient way to increase fuel density. However, as the number of rings increases, the hydrogen content of fuel molecules gradually decreases, resulting in a decrease in the mass calorific value, which limits the further improvement of the volumetric calorific value, meanwhile, the low-temperature characteristics of the fuel become worsen. Considering the high strain energy stored in cyclopropane moiety and the good cryogenic properties of spiro fuel, in this work, two spirocyclopropane high-energy–density fuels have been synthesized from cyclopentadiene by phase transfer catalytic cyclopropanation and palladium catalytic cyclopropanation, and the reaction conditions have been optimized. The synthesized fuels possess heat value and specific impulse higher than the widely used high-energy–density fuel JP-10. Additionally, their kinematic viscosity at low temperatures is only about one-third that of JP-10. The outstanding properties of the synthesized strained fuels suggest their good potential as high-energy–density fuels.

Enhanced phenanthrene hydrogenation saturation performance of Ni-Co/NiAlOX catalysts and its catalytic mechanism

Hydrogenation saturation of phenanthrene was an effective method to acquire high energy density fuels. It is essential to design highly active and stable hydrogenation saturated catalyst to realize the above reaction. Herein, series of Ni-Co/NiAlOx catalysts were synthesized by reducing at different temperatures. Activity evaluation showed that the catalysts reduced at 560 °C (Cat-560) possessed a phenanthrene conversion amount of 8.0 mmol·g-1 active component·h−1 and 98% perhydrophenanthrene selectivity after running 6 h under 300 °C, 5 MPa. Furthermore, it was found that Cat-560 presented superior specific activity (4.3 mmol·kg−1·s−1) and TOF (41.1 × 10−3 s−1) for symmetric octahydrophenanthrene hydrogenation (the rate determining reaction), and its specific activity was 3 times as high as that of the catalysts reduced at 520 °C The excellent hydrogenation performance of Cat-560 could be attributed to its special geometric and electronic structure. Characterization indicated that the Ni2+ in Ni-Co/NiAlOX catalyst was mainly located in the nickel aluminum spinel octahedral sites, which was beneficial to the reduction of Ni and the formation of smaller Ni particles. Moreover, it was explored that the Ni d charge density of Cat-560 catalyst was −0.12 e, which was suitable for the adsorption and activation of phenanthrene and symmetric octahydrophenanthrene.

Study on the initial pyrolysis kinetics of strained polycyclic hydrocarbons

Strained polycyclic hydrocarbons are one of the most important classes of high-energy density fuels. In this work, the thermodynamics and initial pyrolysis kinetics of five strained polycyclic hydrocarbons, which are isomers of JP-10 and are linearly aggregates of two cyclopropyl rings and one cyclobutyl ring, were investigated using quantum chemical computations. The initial pyrolysis reactions include ring bond breaking and bridge bond breaking. The ring bond breaking reactions include a diradical pathway which is the direct C-C bond breaking to produce a diradical and an isomerization pathway involving concert bond breaking and H-atom transfer, and the bridge bond breaking reactions lead to two radicals. Our potential energy calculation shows that the ring bond breaking reactions are much more energetically favorable than the bridge bond breaking reactions, which can be neglected in the initial pyrolysis process of strained polycyclic hydrocarbons. The rate constants of the ring bond breaking reactions were computed using variational transition state theory (VTST) or transition state theory (TST). Our calculated results demonstrated that ring bond breaking reactions of the three-membered ring are competitive with that of the four-membered ring. The studied reactions are divided into classes depending on the size of the rings (three- or four-membered ring) and the types of the carbon atoms of the breaking C-C bond (Cs-Cs, Cs-Ct, Ct-Ct and Cq-Cs). Reaction rate rules are established for each class, which are useful for the mechanism generation for analogous strained polycyclic hydrocarbons. Thermodynamic properties including net calorific values per mole, gravimetric net heat of combustion and impulse time of our studied strained polycyclic hydrocarbons are higher than those of JP-10. A comparison of the initial pyrolysis kinetics of our studied strained polycyclic hydrocarbons with the isomer JP-10 indicates that these compounds may be easier to pyrolyze.

Regulate the mesopores and acidic structure of Pt/USY zeolite catalyst for improved performance in the hydroisomerization of phenanthrene to alkyl-adamantane

Adamantane-based high energy density fuels (HEDFs) play a key role in military and aerospace applications due to their high volumetric calorific value and high density. In the preparation of adamantane-based HEDFs by hydroisomerization of polycyclic aromatic hydrocarbons (PAHs) in coal tar, there is competition between cracking and ring-opening reactions. Therefore, USY zeolites were etched with NaOH solution to expand the mesoporous and regulate the acidity, inhibit the cracking reaction, and promote the ring-shift isomerization of perhydrophenanthrene (PHP) to alkyl-adamantane (AlA), increasing the yield of AlA from 19% to 32%. There are two processes in the conversion of PAHs to AlA, hydrogenation and ring-shift isomerization. Phenanthrene was first hydrogenated to PHP, then isomerized to alkyl-adamantane, the product distribution of alkyl-adamantanes with different side chains was discovered, part of the C14 alkyl-adamantanes cracked, and the hydroisomerization reaction of PHP occurred only on the outer surface or pore mouth of USY zeolite. It provides a new perspective for the hydroconversion of PAHs to HEDFs.

One-pot conversion of α-pinene into biomass high energy density fuel over a bifunctional zeolite catalyst

A shape-selective bifunctional catalyst (Rux/SOT-Hβ) was furnished by loading Ru onto the Hβ zeolite prepared via seed organic template (SOT) method using the seeds obtained via steam-assisted conversion (SAC) method. The Rux/SOT-Hβ catalyst exhibited both favorable acid-catalytic performance and hydrogenation performance in the two-stage one-pot conversion of α-pinene, enabling an energy-efficient process for producing biomass blended high energy density fuels (HEDF). The effect of preparation conditions of Hβ zeolite carrier, metal type, loading amount, and loading method on the catalytic performance of the bifunctional catalyst was investigated. After the optimization of the catalytic process over Ru1.2/SOT-Hβ using pinane as the reaction medium, the resulting liquid reaction mixture without undergoing further distillation and other operations was found to possess good biomass HEDF properties. Feasible recyclability of Ru1.2/SOT-Hβ catalyst in α-pinene one-pot conversion was also demonstrated by a 3-run recycling test.

Deep generative fuel design in low data regimes via multi-objective imitation

Commercial fuel discovery faces a constantly decreasing return of investment due to due to increasingly tight environmental criteria and reducing potential uses for each new fuel. In this paper, a deep generative model, termed Latent Interspace Generative Adversarial Network with a Domain of Stacking (LIGANDS), has been established to screen desired fuel molecules in the large chemical space without setting design rules manually. A variational autoencoder, a generative adversarial network and a stacking model are well integrated in LIGANDS through model convergence. Given only the structures of 255 typical high-energy–density fuels in low data regimes, LIGANDS generated 3461 new fuel molecules with similar property distribution and improved energy performance as the qualified candidates of next-generation fuels. To expand and enrich the fuel-relevant chemical space with innovative molecular entities on demand, in-depth multi-objective imitation on the key properties of target fuel is realized by LIGANDS through optimizing generative molecular structures and their distribution.

Hydrodeoxygenation of lignin biophenolics to cyclohexanes over sub-nanometric Ru multifunctional catalyst

Hydrodeoxygenation of lignin-derived phenolics into cyclohexanes is recognized as an attractive philosophy for producing high energy density fuels in a sustainable manner. Here, we report a catalyst made of low-loading, sub-nanoscaled Ru species and multifarious acidic centers on amorphous-silica-alumina, which shows excellent activity, selectivity, and stability in hydrodeoxygenation of 4-propylguaiacol to propylcyclohexane. Catalytic performance exhibits a linear relationship to the total acidities of this family catalysts, demonstrating a synergistic interaction between the metallic and acidic components. The superior performance of this catalyst allows the conversion of various lignin-derived monophenols, dimers, more significantly, a biophenolic stream from miscanthus lignin into cyclohexanes with high efficiency and yield. This work highlights the importance of the sub-nanometric size of Ru species in the advancement of efficient and affordable approaches towards lignin-based fuel production.

Simple and high-precision DFT-QSPR prediction of enthalpy of combustion for sesquiterpenoid high-energy–density fuels

Sesquiterpenoids that are renewable and have high energy density and low freezing point are promising biomass high-energy–density fuels (HEDFs). The enthalpy of combustion (ΔcH) is the key characteristic to measure the heat energy content of HEDFs. Its accurate theoretical prediction and evaluation is essential for designing novel sesquiterpenoid HEDFs. Combining the screened and optimized density functional theory (DFT) method with triple computational correction, we presented a standardized calculation scheme for ΔcH with an average absolute error of only 2.6 % by 295 structurally diverse sesquiterpenoid HEDFs. Then, the extreme gradient boosting (XGBoost) algorithm was used to determine the correlation between the calculated ΔcH and 54 quantum chemical (QC) descriptors and the shapley additive explanation (SHAP) method to elucidate the key factors affecting ΔcH. Lastly, a quantitative structure–property relationship (QSPR) prediction model with only four QC features was constructed by using the SHAP results via the multiple linear regression (MLR) algorithms. Notably, the model has excellent prediction performance, decreased complexity, and improved applicability. The coefficient of determination (R2) of the internal training set and external test set of the model are 0.957 and 0.956, while the root mean square error (RMSE) are 8.626 kcal/mol and 9.012 kcal/mol, respectively. The proposed DFT-QSPR (the ingenious combination of DFT and QSPR) scheme can effectively guide the design of ΔcH of novel sesquiterpenoid HDEFs.

Tandem Electrochemical Conversion of CO2 to Liquid Fuels and Chemical Feedstocks

Increasing demand for energy has led to a high dependence on fossil fuels, which are limited and have been closely associated with major environmental issues such as groundwater pollution and imbalances in the carbon cycle. A more sustainable and cleaner method of producing chemicals and fuels is the electrochemical CO2 reduction reaction (CO2RR). When coupled with renewable electricity, CO2 can be converted to high energy density fuels and commodity chemicals, such as ethanol, propanol, and ethylene, in an environmentally friendly way.Recently, the use of CO feedstocks, instead of CO2, was shown to enhance the selectivity toward multi-carbon products on copper-based electrocatalysts. Technoeconomic analyses have also demonstrated that CO can be produced from CO2RR cost-effectively. This has led to increased efforts in developing tandem electrocatalytic systems. However, state-of-the-art CO2-to-CO electrocatalysts are based on expensive noble metals such as Ag and Au, while earth-abundant Zn displays relatively poorer selectivity and activity. Herein, we show that oxide-derived Zn with high surface area can reduce CO2 to CO with a Faradaic efficiency of 86% and a partial current density (j CO) of −201 mA cm-2. While oxygen vacancies were previously implicated for CO2RR to CO, we pinpointed by detailed experiments and density functional theory calculations that highly undercoordinated Zn sites provide even higher activity, in view of their nearly optimal *COOH adsorption energies. These findings indicate that suitably engineered ZnO-derived materials can potentially be an alternative to the more costly Ag and Au electrocatalysts, and that the main guideline for their design is to increase the undercoordination of the catalytic sites.We also investigate the electrochemical CO reduction reaction (CORR) to liquid fuels and industrial feedstocks on copper-based and mixed copper-silver catalysts. To further enhance the CORR activity and product selectivities, a systematic optimization of the experimental environment, such as the use of various supports, catalyst binder and electrolytes, was done. We develop a set of experimental conditions for optimal CORR performance to value-added products.

Influence of micron-sized aluminum particles on micro-explosion and combustion characteristics of ethanol-biodiesel droplets under co-heat flow

As a high-energy metal fuel, aluminum powder particles (AP) have long been used as fuel propellants and combustion-supporting additives in explosives to meet the design requirements of a new generation of high-energy–density fuels. In this experiment, the micro-explosion and combustion characteristics of ethanol-biodiesel mixed droplets with different aluminum powder particle contents and particle sizes were studied under high temperature and pure oxygen environment. In order to better control the experimental variables, a more accurate single-droplet suspension method was adopted in this experiment. The entire evaporation and combustion process of the mixed fuel was completely recorded and analyzed by using high-speed photography technology, binarization imaging technology and droplet optimization algorithms. The experimental results show that when the content of aluminum powder particles remains unchanged, increasing the particle size of aluminum powder in the mixed droplets will promote the micro-explosion phenomenon. When the particle size of the aluminum powder contained in the mixed fuel is different, the increase of the particle content will have a completely different effect on the heating time of the droplet. When the particle size of aluminum powder particles is small, the combustion duration of the mixed droplets increases with the increase of aluminum powder content, while the opposite results are shown when the aluminum powder particles are larger. Increasing the amount of aluminum powder also extends the full life of the droplets. In addition, this study introduces the microburst intensity (MI), which provides a new idea and method for laterally comparing the microburst phenomena of fuel droplets with different initial diameters. For mixed fuel containing micron-sized aluminum particles, the particle content that produces the best micro-explosion intensity is about 20%.

Ensemble learning directed classification and regression of hydrocarbon fuels

Predicting the properties of fuels based on their structures is essential to develop next generation fuels for various applications, e.g., aircraft, rocket, and missile. In this study, an ensemble learning assisted method has been developed for accurate and quick prediction of fuel properties based on their structures. Two descriptors of hydrocarbon molecules, continuous operable molecular entry specification (COMES) and Coulomb matrix (CM), are compared as the input of the learning model. The optimized stacking of various base learners, including Boosting, Bagging, NeuralNetwork, Trees, Voting, Linear, Neighbors and Kernel, have been achieved for efficient screening of potential high energy density fuels (HEDFs) and accurate prediction of fuel properties. Hydrocarbon molecules are efficiently classified into HEDFs and non-HEDFs based on their structures by the stacking model of ensemble learning, which exhibits superior generalization performance to the single learner. The properties of these potential HEDFs are accurately predicted by another stacking model, including density, heat value, freezing point, boiling point, flash point, and specific impulse. Both underfitting and overfitting have been effectively restricted by the COMES descriptor and the ensemble-learning model. Based on efficient classification and regression realized by our method, the discovery of new hydrocarbon fuels with specific criteria will be accelerated.

The dynamics and mechanism of JP-10 thermal oxidative deposition

High-energy–density (HED) fuels (like JP-10) have drawn more and more attention for volume-limited aerospace vehicles to extend the flight range and/or increase the payload. However, their thermal oxidation and deposition performance are still ambiguous. In this work, the dynamics and mechanism of JP-10 thermal oxidative deposition were investigated by the jet fuel thermal oxidation stability tester (JFTOT), ellipsometric tube rater (ETR) and comprehensive two-dimensional gas chromatography coupled with mass spectrometry and flame ionization detector (GC × GC–MS/FID). The results reveal that both the peak deposit thickness and total deposits volume of JP-10 increase with increasing temperature with the apparent deposition activation energy of approximately 71.2 kJ/mol. The deposits on the tube surface have globular structure, and they undergo particle formation, growth, adherence, agglomeration and size reduction during thermal stress. The fine characterizations of liquid products after JFTOT test confirm that the thermal oxidative deposition of JP-10 occurs via a series of radical reactions, such as oxidative decomposition and radical addition, which will be significantly accelerated at higher temperature. Importantly, the stability of alkoxy radical seems to be a significant factor affecting the thermal oxidation stability of HED fuels. This work may shed light on the possible deposition mechanism of HED fuels and will guide the rational design of HED fuels with high thermal oxidation stability.

Mechanistic study on oxidative degradation and deposition of exo-tetrahydrodicyclopentadiene

The advancement of propulsion systems accelerates the exploitation and application of various high-energy–density (HED) fuels, and yet severe oxidative-fouling hinders the adoption of such fuels in thermal-stressed conditions. In this work, we investigated the overall thermal-oxidative degradation and deposition process of a typical HED fuel, exo-tetrahydrodicyclopentadiene (exo-TCD), known as JP-10 (NHOC = 39.6 MJ/L), with the presence of oxygen. The oxidation mechanism of exo-TCD at relatively low temperatures is illustrated on the basis of experimental and computational results. For the first time, the chirality of oxidation products has been discussed, revealing the potential impact of radical configuration on the oxidation process of exo-TCD. Besides, an in-depth investigation on the deposition formation process was carried out by analyzing the structure and composition of polymeric macromolecules with the assistance of TG-IR-GC/MS technique. The oxidative deposition of exo-TCD is illustrated with a three-step mechanism of “oxidation”, “polymerization”, and “deposition”. The solubility of macromolecular species formed upon oxidation is decisive for the deposition propensity of fuels. This work aims to provide a theoretical and methodological basis for the future study on the oxidative stability of HED fuels.

Tracking the phase changes in micelle-based NiGa nanocatalysts for methanol synthesis under activation and working conditions

The hydrogenation of CO2 into high energy density fuels such as methanol, where the required H2 is obtained from renewable sources, is of utmost importance for a sustainable society. In recent years, NiGa alloys have attracted attention as promising catalyst material systems for the hydrogenation of CO2 into methanol at ambient pressures. They thus represent an energy-saving alternative to the Cu-based catalysts employed in today’s catalytic industry that require high pressures for the CO2 hydrogenation. However, the underlying reaction mechanisms for the NiGa system are still under debate. One of the challenges here is to unravel the evolution and coexistence of the different species in the heterogeneous NiGa catalyst system under activation and reaction conditions. To shed light on their evolution under activation in H2 and their catalytic roles under CO2 hydrogenation working conditions on well-defined Ni3Ga1 and Ni5Ga3 nanoparticle (NP) catalysts, we employed a multi-probe approach. It included advanced machine learning-based analysis of operando X-ray absorption spectroscopy data combined with operando powder X-ray diffraction and near ambient pressure X-ray photoelectron spectroscopy measurements, as well as reactivity studies using bed-packed mass flow reactors. In addition, we employed atomic force microscopy and scanning transmission electron microscopy for structural characterization.Under H2 activation at 1 bar total pressure, we concluded the formation of metallic Ni, starting for Ni3Ga1 at 300 °C, and for Ni5Ga3 at 400 °C. At higher temperatures, the formation of NiGa alloys follows. The α’-Ni3Ga1 alloy phase is predominantly formed for the Ni3Ga1 NPs, while the coexistence of α’-Ni3Ga1, δ-Ni5Ga3 and Ga2O3 phases is observed for the Ni5Ga3 NPs after the H2 activation. The formation of the Ga2O3 phase also results in the presence of excess metallic Ni. Under CO2 hydrogenation reaction conditions, Ga partially oxidizes again to form a Ga2O3-rich particle shell for both NP compositions, yet, to a larger extent for the Ni3Ga1 NPs, which, in turn, feature a higher amount of excess Ni. We reveal that metallic Ni is responsible for the high selectivity of the Ni3Ga1 NPs towards the production of methane in our catalytic tests. In contrast, the Ni5Ga3 NPs display a strong selectivity toward methanol production (>92%), more than one order of magnitude higher than that for the Ni3Ga1 NPs, which we ascribe to the presence of the δ-Ni5Ga3 phase.

Ignition performance of TiO2 coated boron particles using a shock tube

This study coated the surface of irregularly shaped 5-μm boron particles with TiO2 nanoparticles to improve the ignition performance of the boron. A simple and inexpensive chemical method was used to coat the surface of boron with TiO2. Five different samples of boron coated with TiO2 nanoparticles were obtained by varying the concentration of Ti precursor. Surface structures were analyzed using different characterization techniques, which showed the formation of nanocrystalline TiO2 nanoparticles over the boron surface. The nanoparticles of TiO2 were well dispersed over the boron surface, and exhibited strong interfacial contact with the boron. The oxidation of boron and boron coated with TiO2 was analyzed by thermogravimetric technique in an air atmosphere from room temperature to 1000 °C. Results revealed that the oxidation of boron started at a temperature approximately 162 °C lower after coating with TiO2. The ignition behavior of the boron and boron coated with TiO2 particles was studied using a shock tube. The results of the shock tube experiments demonstrated the TiO2 coated boron had a shorter ignition delay time than the bare boron. An approximate 35% reduction was observed in the ignition delay time of boron after coating with TiO2 nanoparticles, showing its potential value in high energy density fuels.