Control of Explosive Chemical Reactions by Optical Excitations: Defect-Induced Decomposition of Trinitrotoluene at Metal Oxide Surfaces.

Interfaces formed by high energy density materials and metal oxides present intriguing new opportunities for a large set of novel applications that depend on the control of the energy release and initiation of explosive chemical reactions. We studied the role of structural defects at a MgO surface in the modification of electronic and optical properties of the energetic material TNT (2-methyl-1,3,5-trinitrobenzene, also known as trinitrotoluene, C7H5N3O6) deposited at the surface. Using density functional theory (DFT)-based solid-state periodic calculations with hybrid density functionals, we show how the control of chemical explosive reactions can be achieved by tuning the electronic structure of energetic compound at an interface with oxides. The presence of defects at the oxide surface, such as steps, kinks, corners, and oxygen vacancies, significantly affects interfacial properties and modifies electronic spectra and charge transfer dynamics between the oxide surface and adsorbed energetic material. As a result, the electronic and optical properties of trinitrotoluene, mixed with an inorganic material (thus forming a composite), can be manipulated with high precision by interactions between TNT and the inorganic material at composite interfaces, namely, by charge transfer and band alignment. Also, the electron charge transfer between TNT and MgO surface reduces the decomposition barriers of the energetic material. In particular, it is shown that surface structural defects are critically important in the photodecomposition processes. These results open new possibilities for the rather precise control over the decomposition initiation mechanisms in energetic materials by optical excitations.

Achieving tunable chemical reactivity through photo-initiation of energetic materials at metal oxide surfaces.

Known applications of high energy density materials are impressively vast. Despite this, we argue that energetic materials are still underutilized for common energy purposes due to our inability to control explosive chemical reactions releasing energy from these materials. The situation appears paradoxical as energetic materials (EM) possess massive amounts of energy and, hence, should be most appropriate for applications in many energy-intensive processes. Here, we discover how chemical decomposition reactions can be stimulated with laser excitation and therefore, highly controlled by selectively designing energetic material - metal oxide interfaces with an example of pentaerythritol tetranitrate (PETN)-MgO and trinitrotoluene (TNT)-MgO composite samples. Density functional theory and embedded cluster method calculations were combined with measurements of the optical absorption spectra and laser initiation experiments. We found that the first (1064 nm, 1.17 eV), second (532 nm, 2.33 eV), and third (355 nm, 3.49 eV) laser harmonics, to all of which pure energetic materials are transparent, can be effectively used to trigger explosive reactions in the PETN-MgO samples. We propose a consistent electronic mechanism that explains how specific sub-band optical transitions initiate decomposition chemistry. Also, this selectivity reveals a fundamental difference between materials chemistry at interfaces as we show on examples of PETN and TNT energetic materials.

Adsorption and decomposition of dimethyl methylphosphonate on size-selected (MoO3)3 clusters.

The adsorption and decomposition of dimethyl methylphosphonate (DMMP), a chemical warfare agent (CWA) simulant, on size-selected molybdenum oxide trimer clusters, i.e. (MoO3)3, was studied both experimentally and theoretically. X-ray photoelectron spectroscopy (XPS), temperature programmed reaction (TPR), and density functional theory (DFT)-based simulations were utilized in this study. The XPS and TPR results showed both, desorption of intact DMMP, and decomposition of DMMP through the elimination of methanol at elevated temperatures on (MoO3)3 clusters. Theoretical investigation of DMMP on (MoO3)3 clusters suggested that, in addition to pure (MoO3)3 clusters, reduced molybdenum oxide clusters and hydroxylated molybdenum oxide clusters also play an important role in decomposing DMMP via a "reverse Mars-van Krevelen mechanism". The present study, which focused on oxide clusters, underlines the importance of surface defects, e.g., the oxygen vacancies and surface hydroxyls, in determining the reaction pathway of DMMP, in agreement with previous studies on thin films. In addition, the structural fluxionality and the Lewis acidity of molybdenum oxide clusters, i.e. (MoO3)3, may make them good candidates for adsorption and decomposition of chemical warfare agents with similar structures to DMMP.

Electron Spectroscopy and Computational Studies of Dimethyl Methylphosphonate.

Dimethyl methylphosphonate (DMMP) is one of the most widely used molecules to simulate chemical warfare agents in adsorption experiments. However, the details of the electronic structure of the isolated molecule have not yet been reported. We have directly probed the occupied valence and core levels using gas phase photoelectron spectroscopy and the unoccupied states using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Density functional theory (DFT) calculations were used to study the electronic structure, assign the spectral features, and visualize the molecular orbitals. Comparison with parent molecules shows that valence and core-level binding energies of DMMP follow trends of functional group substitution on the P center. The photoelectron and NEXAFS spectra of the isolated molecule will serve as a reference in studies of DMMP adsorbed on surfaces.

Molecular Theory of Detonation Initiation: Insight from First Principles Modeling of the Decomposition Mechanisms of Organic Nitro Energetic Materials.

This review presents a concept, which assumes that thermal decomposition processes play a major role in defining the sensitivity of organic energetic materials to detonation initiation. As a science and engineering community we are still far away from having a comprehensive molecular detonation initiation theory in a widely agreed upon form. However, recent advances in experimental and theoretical methods allow for a constructive and rigorous approach to design and test the theory or at least some of its fundamental building blocks. In this review, we analyzed a set of select experimental and theoretical articles, which were augmented by our own first principles modeling and simulations, to reveal new trends in energetic materials and to refine known existing correlations between their structures, properties, and functions. Our consideration is intentionally limited to the processes of thermally stimulated chemical reactions at the earliest stage of decomposition of molecules and materials containing defects.

Photochemistry of the α-Al2O3-PETN Interface.

Optical absorption measurements are combined with electronic structure calculations to explore photochemistry of an α-Al2O3-PETN interface formed by a nitroester (pentaerythritol tetranitrate, PETN, C5H8N4O12) and a wide band gap aluminum oxide (α-Al2O3) substrate. The first principles modeling is used to deconstruct and interpret the α-Al2O3-PETN absorption spectrum that has distinct peaks attributed to surface F°-centers and surface-PETN transitions. We predict the low energy α-Al2O3 F°-center-PETN transition, producing the excited triplet state, and α-Al2O3 F°-center-PETN charge transfer, generating the PETN anion radical. This implies that irradiation by commonly used lasers can easily initiate photodecomposition of both excited and charged PETN at the interface. The feasible mechanism of the photodecomposition is proposed.

Effect of polar surfaces on decomposition of molecular materials.

We report polar instability in molecular materials. Polarization-induced explosive decomposition in molecular crystals is explored with an illustrative example of two crystalline polymorphs of HMX, an important energetic material. We establish that the presence of a polar surface in δ-HMX has fundamental implications for material stability and overall chemical behavior. A comparative quantum-chemical analysis of major decomposition mechanisms in polar δ-HMX and nonpolar ß-HMX discovered a dramatic difference in dominating dissociation reactions, activation barriers, and reaction rates. The presence of charge on the polar δ-HMX surface alters chemical mechanisms and effectively triggers decomposition simultaneously through several channels with significantly reduced activation barriers. This results in much faster decomposition chemistry and in higher chemical reactivity of δ-HMX phase relatively to ß-HMX phase. We predict decomposition mechanisms and their activation barriers in condensed δ-HMX phase, sensitivity of which happens to be comparable to primary explosives. We suggest that the observed trend among polymorphs is a manifestation of polar instability phenomena, and hence similar processes are likely to take place in all polar molecular crystals.

Decomposition mechanisms and kinetics of novel energetic molecules BNFF-1 and ANFF-1: quantum-chemical modeling.

Decomposition mechanisms, activation barriers, Arrhenius parameters, and reaction kinetics of the novel explosive compounds, 3,4-bis(4-nitro-1,2,5-oxadiazol-3-yl)-1,2,5-oxadiazole (BNFF-1), and 3-(4-amino-1,2,5-oxadiazol-3-yl)-4-(4-nitro-1,2,5-oxadiazol-3-yl)-1,2,5-oxadiazole (ANFF-1) were explored by means of density functional theory with a range of functionals combined with variational transition state theory. BNFF-1 and ANFF-1 were recently suggested to be good candidates for insensitive high energy density materials. Our modeling reveals that the decomposition initiation in both BNFF-1 and ANFF-1 molecules is triggered by ring cleavage reactions while the further process is defined by a competition between two major pathways, the fast C-NO2 homolysis and slow nitro-nitrite isomerization releasing NO. We discuss insights on design of new energetic materials with targeted properties gained from our modeling.

Bimolecular Reaction Mechanism in the Amido Complex-Based Atomic Layer Deposition of HfO2.

The surface chemistry of the initial growth during the first or first few precursor cycles in atomic layer deposition is decisive for how the growth proceeds later on and thus for the quality of the thin films grown. Yet, although general schemes of the surface chemistry of atomic layer deposition have been developed for many processes and precursors, in many cases, knowledge of this surface chemistry remains far from complete. For the particular case of HfO2 atomic layer deposition on a SiO2 surface from an alkylamido-hafnium precursor and water, we address this lack by carrying out an operando atomic layer deposition experiment during the first cycle of atomic layer deposition. Ambient-pressure X-ray photoelectron spectroscopy and density functional theory together show that the decomposition of the metal precursor on the stoichiometric SiO2 surface in the first half-cycle of atomic layer deposition proceeds via a bimolecular reaction mechanism. The reaction leads to the formation of Hf-bonded methyl methylene imine and free dimethylamine. In addition, ligand exchange takes place involving the surface hydroxyls adsorbed at defect sites of the SiO2 surface.

Recruiting Perovskites to Degrade Toxic Trinitrotoluene.

Everybody knows TNT, the most widely used explosive material and a universal measure of the destructiveness of explosions. A long history of use and extensive manufacture of toxic TNT leads to the accumulation of these materials in soil and groundwater, which is a significant concern for environmental safety and sustainability. Reliable and cost-efficient technologies for removing or detoxifying TNT from the environment are lacking. Despite the extreme urgency, this remains an outstanding challenge that often goes unnoticed. We report here that highly controlled energy release from explosive molecules can be accomplished rather easily by preparing TNT-perovskite mixtures with a tailored perovskite surface morphology at ambient conditions. These results offer new insight into understanding the sensitivity of high explosives to detonation initiation and enable many novel applications, such as new concepts in harvesting and converting chemical energy, the design of new, improved energetics with tunable characteristics, the development of powerful fuels and miniaturized detonators, and new ways for eliminating toxins from land and water.

Degradation of Fatal Toxic Nerve Agents on Dry TiO2.

Despite a recent dramatically increased risk of using chemical warfare agents in chemical attacks and assassinations, fundamental interactions of toxic chemicals with other materials are poorly understood, and micromechanisms of their chemical degradation are yet to be established. This represents an outstanding challenge in both fundamental science and practical applications in combat against chemical weapons. One of the most versatile and multifunctional oxides, TiO2, has been suggested as a promising material to quickly adsorb and effectively destroy toxins. In this paper, we explore how sarin (also known as GB) adsorbs and decomposes on dry nanoparticles of TiO2 anatase and rutile phases. We found that both anatase and rutile readily adsorb sarin gas molecules because of a strong electrostatic attraction between the phosphoryl oxygen and surface titanium atoms. The sarin decomposition most likely proceeds via a propene elimination; however, the reaction is exothermic on the rutile (110) surface and endothermic on the anatase (101) surface. High energy barriers suggest that sarin would hardly decompose on pristine dry surfaces of TiO2, and degradation reactions can be triggered by defects or contaminants under realistic operational conditions.

Understanding Dimethyl Methylphosphonate Adsorption and Decomposition on Mesoporous CeO2.

The increased risk of chemical warfare agent usage around the world has intensified the search for high-surface-area materials that can strongly adsorb and actively decompose chemical warfare agents. Dimethyl methylphosphonate (DMMP) is a widely used simulant molecule in laboratory studies for the investigation of the adsorption and decomposition behavior of sarin (GB) gas. In this paper, we explore how DMMP interacts with the as-synthesized mesoporous CeO2. Our mass spectroscopy and in situ diffuse reflectance infrared Fourier transform spectroscopy measurements indicate that DMMP can dissociate on mesoporous CeO2 at room temperature. Two DMMP dissociation pathways are observed. Based on our characterization of the as-synthesized material, we built the pristine and hydroxylated (110) and (111) CeO2 surfaces and simulated the DMMP interaction on these surfaces with density functional theory modeling. Our calculations reveal an extremely low activation energy barrier for DMMP dissociation on the (111) pristine CeO2 surface, which very likely leads to the high activity of mesoporous CeO2 for DMMP decomposition at room temperature. The two reaction pathways are possibly due to the DMMP dissociation on the pristine and hydroxylated CeO2 surfaces. The significantly higher activation energy barrier for DMMP to decompose on the hydroxylated CeO2 surface implies that such a reaction on the hydroxylated CeO2 surface may occur at higher temperatures or proceed after the pristine CeO2 surfaces are saturated.

Dimethyl methylphosphonate adsorption and decomposition on MoO2 as studied by ambient pressure x-ray photoelectron spectroscopy and DFT calculations.

Organophosphonates range in their toxicity and are used as pesticides, herbicides, and chemical warfare agents (CWAs). Few laboratories are equipped to handle the most toxic molecules, thus simulants such as dimethyl methylphosphonate (DMMP), are used as a first step in studying adsorption and reactivity on materials. Benchmarked by combined experimental and theoretical studies of simulants, calculations offer an opportunity to understand how molecular interactions with a surface changes upon using a CWA. However, most calculations of DMMP and CWAs on surfaces are limited to adsorption studies on clusters of atoms, which may differ markedly from the behavior on bulk solid-state materials with extended surfaces. We have benchmarked our solid-state periodic calculations of DMMP adsorption and reactivity on MoO2 with ambient pressure x-ray photoelectron spectroscopy studies (APXPS). DMMP is found to interact strongly with a MoO2 film, a model system for the MoO x component in the ASZM-TEDA© gas filtration material. Density functional theory modeling of several adsorption and decomposition mechanisms assist the assignment of APXPS peaks. Our results show that some of the adsorbed DMMP decomposes, with all the products remaining on the surface. The rigorous calculations benchmarked with experiments pave a path to reliable and predictive theoretical studies of CWA interactions with surfaces.

Surface-Accelerated Decomposition of δ-HMX.

Despite extensive efforts to study the explosive decomposition of HMX, a cyclic nitramine widely used as a solid fuel, explosive, and propellant, an understanding of the physicochemical processes, governing the sensitivity of condensed HMX to detonation initiation is not yet achieved. Experimental and theoretical explorations of the initiation of chemistry are equally challenging because of many complex parallel processes, including the ß-δ phase transition and the decomposition from both phases. Among four known polymorphs, HMX is produced in the most stable ß-phase, which transforms into the most reactive δ-phase under heat or pressure. In this study, the homolytic NO2 loss and HONO elimination precursor reactions of the gas-phase, ideal crystal, and the (100) surface of δ-HMX are explored by first principles modeling. Our calculations revealed that the high sensitivity of δ-HMX is attributed to interactions of surfaces and molecular dipole moments. While both decomposition reactions coexist, the exothermic HONO-isomer formation catalyzes the N-NO2 homolysis, leading to fast violent explosions.