Thermochemical Analysis of Molybdenum Thin Films on Porous Alumina

Molybdenum (Mo) thin films (thickness <100 nm) were physically deposited by e-beam evaporation on a porous alumina substrate and were analyzed for their stability and reactivity under various thermal and gas conditions. The Mo thin-film composites were stable below 300 °C but had no reactivity toward gases. Mo thin films showed nitrogen incorporation on the surface as well as in the subsurface at 450 °C, as confirmed by X-ray photoelectron spectroscopy. The reactivity toward nitrogen was diminished in the presence of CO₂, although no carbon species were detected either on the surface or in the subsurface. The Mo thin films have a very stable native oxide layer, which may further oxidize to higher oxidation states above 500 °C due to the reaction with the porous anodized alumina substrate. The oxidation of Mo thin films was accelerated in the presence of oxidizing gases. At 600 °C in N₂, the Mo thin film on anodized alumina was completely oxidized and may also have been volatilized. The results imply that choosing thermally stable and inactive porous supports and operating in nonoxidizing conditions below 500 °C will likely maintain the stability of the Mo composite. This study provides key information about the chemical and structural stability of a Mo thin film on a porous substrate for future membrane applications and offers further insights into the integrity of thin-film composites when exposed to harsh conditions

Theoretical and experimental investigations of mercury adsorption on hematite surfaces

<p>One of the biggest environmental concerns caused by coal-fired power plants is the emission of mercury (Hg), which is toxic metal. To control the emission of Hg from coal-derived flue gas, it is important to understand the behavior and speciation of Hg as well as the interaction between Hg and solid materials in the flue gas stream. In this study, atomic-scale theoretical investigations using density functional theory (DFT) were carried out in conjunction with laboratory-scale experimental studies to investigate the adsorption behavior of Hg on hematite (α-Fe₂O₃). According to the DFT simulation, the adsorption energy calculation proposes that Hg physisorbs to the α-Fe₂O₃(0001) surface with an adsorption energy of −0.278 eV, and the subsequent Bader charge analysis confirms that Hg is slightly oxidized. In addition, Cl introduced to the Hg-adsorbed surface strengthens the Hg stability on the α-Fe₂O₃(0001) surface, as evidenced by a shortened Hg-surface equilibrium distance. The projected density of states (PDOS) analysis also suggests that Cl enhances the chemical bonding between the surface and the adsorbate, thereby increasing the adsorption strength. In summary, α-Fe₂O₃ has the ability to adsorb and oxidize Hg, and this reactivity is enhanced in the presence of Cl. For the laboratory-scale experiments, three types of α-Fe₂O₃ nanoparticles were prepared using the precursors Fe(NO₃)₃, Fe(ClO₄)₃, and FeCl₃, respectively. The particle shapes varied from diamond to irregular stepped and subrounded, and particle size ranged from 20 to 500 nm depending on the precursor used. The nanoparticles had the highest surface area (84.5 m²/g) due to their highly stepped surface morphology. Packed-bed reactor Hg exposure experiments resulted in this nanoparticles adsorbing more than 300 μg Hg/g. The Hg L_III-edge extended X-ray absorption fine structure spectroscopy also indicated that HgCl₂ physisorbed onto the α-Fe₂O₃ nanoparticles.</p> <p><i>Implications</i>: Atomic-scale theoretical simulations proposes that Hg physisorbs to the α-Fe₂O₃(0001) surface with an adsorption energy of −0.278 eV, and the subsequent Bader charge analysis confirms that Hg is slightly oxidized. In addition, Cl introduced to the Hg-adsorbed surface strengthens the Hg stability on the α-Fe₂O₃(0001) surface, as evidenced by a shortened Hg-surface equilibrium distance. The PDOS analysis also suggests that Cl enhances the chemical bonding between the surface and the adsorbate, thereby increasing the adsorption strength. Following laboratory-scale experiment of Hg sorption also shows that HgCl₂ physisorbs onto α-Fe₂O₃ nanoparticles which have highly stepped structure.</p

Vanadium As a Potential Membrane Material for Carbon Capture: Effects of Minor Flue Gas Species

Vanadium and its surface oxides were studied as a potential nitrogen-selective membrane material for indirect carbon capture from coal or natural gas power plants. The effects of minor flue gas components (SO₂, NO, NO₂, H₂O, and O₂) on vanadium at 500–600 °C were investigated by thermochemical exposure in combination with X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and in situ X-ray diffraction (XRD). The results showed that SO₂, NO, and NO₂ are unlikely to have adsorbed on the surface vanadium oxides at 600 °C after exposure for up to 10 h, although NO and NO₂ may have exhibited oxidizing effects (e.g., exposure to 250 ppmv NO/N₂ resulted in an 2.4 times increase in surface V₂O₅ compared to exposure to just N₂). We hypothesize that decomposition of surface vanadium oxides and diffusion of surface oxygen into the metal bulk are both important mechanisms affecting the composition and morphology of the vanadium membrane. The results and hypothesis suggest that the carbon capture performance of the vanadium membrane can potentially be strengthened by material and process improvements such as alloying, operating temperature reduction, and flue gas treatment