Thermal Decomposition Mechanism of Ammonium Nitrate on the Main Crystal Surface of Ferric Oxide: Experimental and Theoretical Studies

Understanding the decomposition process of ammonium nitrate (AN) on catalyst surfaces is crucial for the development of practical and efficient catalysts in AN-based propellants. In this study, two types of nano-Fe2O3 catalysts were synthesized: spherical particles with high-exposure (104) facets and flaky particles with high-exposure (110) facets. Through thermal analysis and particle size analysis, it was found that the nanosheet-Fe2O3 catalyst achieved more complete AN decomposition despite having a larger average particle size compared to nanosphere-Fe2O3. Subsequently, the effects of AN pyrolysis on the (110) and (104) facets were investigated by theoretical simulations. Through studying the interaction between AN and crystal facets, it was determined that the electron transfer efficiency on the (110) facet is stronger compared to that on the (104) facet. Additionally, the free-energy step diagrams for the reaction of the AN molecule on the two facets were calculated with the DFT + U method. Comparative analysis led us to conclude that the (110) facet of α-Fe2O3 is more favorable for AN pyrolysis compared to the (104) facet. Our study seeks to deepen the understanding of the mechanism underlying AN pyrolysis and present new ideas for the development of effective catalysts in AN pyrolysis.

Analysis of Intermediates and Products from the Dehydrogenation of Mg(BH4)2

The thermodynamic properties of key compounds Mg(B₃H₈)₂, MgB₂H₆, MgB₁₀H₁₀, Mg(B₁₁H₁₄)₂, Mg₃(B₃H₆)₂, and MgB₁₂H₁₂, proposed to be formed in the release of hydrogen from magnesium borohydride Mg(BH₄)₂ and the uptake of hydrogen by MgB₂, have been investigated using solid-state density functional theory (DFT) calculations. More accurate tretment of the cell-size effects with respect to the entropies was also investigated in order to improve the accuracy of the thermodynamic properties of complex borohydrides. We find that the zero-point energy corrections can lower the electronic energies of reaction by 20-30 kJ/(mol H₂) for these intermediates, while adding the thermal and entropy contibutions results in a total decrease of up to ∼50 kJ/(mol H₂). Although our treatment lowers the calculated formation energy of Mg(B₃H₈)₂, it is still too high to explain the experimental observation of B₃H₈^-. We discuss possible reasons for this disparity and propose that the formation of B₃H₈^- and H^- in a disordered amorphous phase has a large energy difference compared to the phase-separated Mg(B₃H₈)₂ and MgH₂ considered in calculations. A comparison of the experimental and NMR chemical shifts calculated within a DFT approach for known species Mg(BH₄)₂, Mg(B₃H₈)₂, Mg(B₁₁H₁₄)₂, MgB₁₀H₁₀, and MgB₁₂H₁₂ provides validation for predicting the chemical shifts of the other compounds which are yet to be confirmed experimentally. These include MgB₂H₆ and the proposed trianion species Mg₃(B₃H₆)₂ that both have favorable thermodynamics for reversible hydrogen storage in Mg(BH₄)₂ without the formation of MgH₂ as a coproduct which could phase separate and inhibit rehydrogenation.

Powered by DFT: Screening methods that accelerate materials development for hydrogen in metals applications

CONSPECTUS: Not only is hydrogen critical for current chemical and refining processes, it is also projected to be an important energy carrier for future green energy systems such as fuel cell vehicles. Scientists have examined light metal hydrides for this purpose, which need to have both good thermodynamic properties and fast charging/discharging kinetics. The properties of hydrogen in metals are also important in the development of membranes for hydrogen purification. In this Account, we highlight our recent work aimed at the large scale screening of metal-based systems with either favorable hydrogen capacities and thermodynamics for hydrogen storage in metal hydrides for use in onboard fuel cell vehicles or promising hydrogen permeabilities relative to pure Pd for hydrogen separation from high temperature mixed gas streams using dense metal membranes. Previously, chemists have found that the metal hydrides need to hit a stability sweet spot: if the compound is too stable, it will not release enough hydrogen under low temperatures; if the compound is too unstable, the reaction may not be reversible under practical conditions. Fortunately, we can use DFT-based methods to assess this stability via prediction of thermodynamic properties, equilibrium reaction pathways, and phase diagrams for candidate metal hydride systems with reasonable accuracy using only proposed crystal structures and compositions as inputs. We have efficiently screened millions of mixtures of pure metals, metal hydrides, and alloys to identify promising reaction schemes via the grand canonical linear programming method. Pure Pd and Pd-based membranes have ideal hydrogen selectivities over other gases but suffer shortcomings such as sensitivity to sulfur poisoning and hydrogen embrittlement. Using a combination of detailed DFT, Monte Carlo techniques, and simplified models, we are able to accurately predict hydrogen permeabilities of metal membranes and screen large libraries of candidate alloys, selections of which are described in this Account. To further increase the number of membrane materials that can be studied with DFT, computational costs need to be reduced either through methods development to break bottlenecks in the performance prediction algorithm, particularly related to transition state identification, or through screening techniques that take advantage of correlations to bypass constraints.