Enhancing the Combustion of Magnesium Nanoparticles via Low-Temperature Plasma-Induced Hydrogenation

Energetic Characteristics and Reaction Mechanism of Hydrogenated Magnesium Nanoparticles: The Role of Condensed-Phase Reaction

Understanding the reaction mechanism of energetic composites is crucial for tuning their reactivity and energy release. Although magnesium hydride nanoparticles (NP) have shown tremendous potential as high-performance reactive materials due to their high combustion enthalpy, the fundamental energy release mechanism and kinetics are yet to be explored. In this work, nonthermal plasma processing is implemented to hydrogenate magnesium nanoparticles, which are prepared via in-flight gas condensation of Mg vapor. Nanoparticle-based metals face multiple challenges, such as loss of nanostructure or sintering at high temperatures before combustion and the presence of a native oxide layer, which acts as the kinetic barrier to reaction. Magnesium has the advantage of high vapor pressure, allowing it to resist sintering; however, Mg must still diffuse out through the oxide layer, which is the rate-limiting step for ignition to take place. Our experiments revealed that upon the desorption of hydrogen, magnesium hydride leaves behind a fresh metallic magnesium surface, which undergoes a solid-state reaction, unlike Mg NPs, for which ignition initiation depends on the outward diffusion of Mg released from the core. The ignition temperature is significantly lowered from 690 °C for Mg nanoparticles to 480 °C for hydrogenated Mg nanoparticles with ∼9-fold reactivity enhancement.

Effect of Ti-EG-Ni Dual-Metal Organic Crystal-Derived TiO2/C/Ni on the Hydrogen Storage Performance of MgH2

To effectively address the kinetic sluggishness associated with MgH2, this study utilized Ti-EG-Ni dual-metal organic crystal as precursors and employed carburization to prepare the unique rod-shaped structure TiO2/C/Ni. The catalyst was incorporated into MgH2 by ball milling, demonstrating excellent hydrogen storage performance. The composite of MgH2-8 wt % TiO2/C/Ni exhibited a lower initial dehydrogenation temperature of 185 °C and a marked dehydrogenation activation energy of 60.537 kJ/mol. At 300 and 150 °C, it only required 300 s to release 6.17 wt % H2 and absorb 5.72 wt % H2 within 20 s, respectively. Additionally, the composites demonstrated excellent cycling stability, maintaining 94% reversible capacity after 50 cycles. Theoretical computations suggested that the in situ-generated metal Mg2Ni and semiconductor TiO2 created a Schottky heterojunction, which stimulated an internal electric field between Ni and TiO2, accelerating electron transfer. The strong electronic interaction between the catalyst and MgH2 weakened the Mg-H bond energy and elongated the Mg-H bond, promoting hydrogen dissociation. During hydrogen absorption and desorption, the composite material exhibited excellent hydrogen storage performance due to the uniform distribution of elements, the in situ-generated catalytic active sites (multivalent Ti and Mg2Ni/Mg2NiH4), and the support provided by carbon to the nanostructures. Our findings provide a deeper understanding of how highly active catalysts of metal oxides/C/Ni enhance the hydrogen storage performance of MgH2.

Using Surface-Enhanced Raman Spectroscopy to Probe Surface-Localized Nonthermal Plasma Activation

Low-temperature, nonthermal plasmas generate a complex environment even when operated in nonreactive gases. Plasma-produced species impinge on exposed surfaces, and their thermalization is highly localized at the surface. Here we present a Raman thermometry approach to quantifying the resulting degree of surface heating. A nanostructured silver substrate is used to enhance the Raman signal and make it easily distinguishable from the background radiation from the plasma. Phenyl phosphonic acid is used as a molecular probe. Even under moderate plasma power and density, we measure a significant degree of vibrational excitation for the phenyl group, corresponding to an increase in surface temperature of ∼80 °C at a plasma density of 2 × 1010 cm-3. This work confirms that surface-localized thermal effects can be quantified in low-temperature plasma processes. Their characterization is needed to improve our understanding of the plasma-induced activation of surface reactions, which is highly relevant for a broad range of plasma-driven processes.