Role of surface oxygen-to-metal ratio on the wettability of rare-earth oxides

Hydrophobic surfaces that are robust can have widespread applications in drop-wise condensation, anti-corrosion, and anti-icing. Recently, it was shown that the class of ceramics comprising the lanthanide series rare-earth oxides (REOs) is intrinsically hydrophobic. The unique electronic structure of the rare-earth metal atom inhibits hydrogen bonding with interfacial water molecules resulting in a hydrophobic hydration structure where the surface oxygen atoms are the only hydrogen bonding sites. Hence, the presence of excess surface oxygen can lead to increased hydrogen bonding and thereby reduce hydrophobicity of REOs. Herein, we demonstrate how surface stoichiometry and surface relaxations can impact wetting properties of REOs. Using X-ray Photoelectron Spectroscopy and wetting measurements, we show that freshly sputtered ceria is hydrophilic due to excess surface oxygen (shown to have an O/Ce ratio of ∼3 and a water contact angle of ∼15°), which when relaxed in a clean, ultra-high vacuum environment isolated from airborne contaminants reaches close to stoichiometric O/Ce ratio (∼2.2) and becomes hydrophobic (contact angle of ∼104°). Further, we show that airborne hydrocarbon contaminants do not exclusively impact the wetting properties of REOs, and that relaxed REOs are intrinsically hydrophobic. This study provides insight into the role of surface relaxation on the wettability of REOs.National Science Foundation (U.S.) (Career Award 0952564)Center for Clean Water and Clean Energy at MIT and KFUPM (Project MIT-KFUPM-R16)Hydro Research Foundation (Graduate Award

Reactivity of Perovskites with Water: Role of Hydroxylation in Wetting and Implications for Oxygen Electrocatalysis

Oxides are instrumental to applications such as catalysis, sensing, and wetting, where the reactivity with water can greatly influence their functionalities. We find that the coverage of hydroxyls (*OH) measured at fixed relative humidity trends with the electron-donor (basic) character of wetted perovskite oxide surfaces. Using ambient pressure X-ray photoelectron spectroscopy, we report that the affinity toward hydroxylation, coincident with strong adsorption energies calculated for dissociated water and hydroxyl groups, leads to strong H bonding that is favorable for wetting while detrimental to catalysis of the oxygen reduction reaction (ORR). Our findings provide novel insights into the coupling between wetting and catalytic activity and suggest that catalyst hydrophobicity should be considered in aqueous oxygen electrocatalysis.National Science Foundation (U.S.). Materials Research Science and Engineering Centers (Program) (Award DMR-0819762)National Science Foundation (U.S.). Graduate Research Fellowship (Grant DGE-1122374)National Science Foundation (U.S.) (Career Award (0952564

E-logpO2 diagrams for ironmaking by molten oxide electrolysis

Molten oxide electrolysis is a promising approach to sustainable iron extraction, where direct electrolytic decomposition of iron ore proceeds to yield liquid metallic iron and pure oxygen gas. Here, through fundamental investigations, we constructed thermodynamic E-logpO2 diagrams for systems containing iron and its oxides to elucidate the chemistry of the electrolysis cell at various electric potentials and oxygen partial pressures. Two isotherms, 1473 and 1873K, were investigated, representing the conditions of the frozen electrolyte sidewall and molten oxide electrolyte in the electrolysis cell, respectively. Stability regions of solid and liquid oxides were determined and the effect of electric potential and oxygen partial pressure on their stoichiometry was explored. The results would enable further development of the electrolysis cell through providing a means for improving the design of the electrolyte to maximize current efficiency.Natural Sciences and Engineering Research Council of Canada (Grant number:498382

Technospheric Mining of Rare Earth Elements from Bauxite Residue (Red Mud): Process Optimization, Kinetic Investigation, and Microwave Pretreatment

Abstract Some rare earth elements (REEs) are classified under critical materials, i.e., essential in use and subject to supply risk, due to their increasing demand, monopolistic supply, and environmentally unsustainable and expensive mining practices. To tackle the REE supply challenge, new initiatives have been started focusing on their extraction from alternative secondary resources. This study puts the emphasis on technospheric mining of REEs from bauxite residue (red mud) produced by the aluminum industry. Characterization results showed the bauxite residue sample contains about 0.03 wt% REEs. Systematic leaching experiments showed that concentrated HNO3 is the most effective lixiviant. However, because of the process complexities, H2SO4 was selected as the lixiviant. To further enhance the leaching efficiency, a novel process based on microwave pretreatment was employed. Results indicated that microwave pretreatment creates cracks and pores in the particles, enabling the lixiviant to diffuse further into the particles, bringing more REEs into solution, yielding of 64.2% and 78.7% for Sc and Nd, respectively, which are higher than the maximum obtained when HNO3 was used. This novel process of “H2SO4 leaching-coupled with-microwave pretreatment” proves to be a promising technique that can help realize the technological potential of REE recovery from secondary resources, particularly bauxite residue

Supercritical Fluid Extraction of Rare Earth Elements from Nickel Metal Hydride Battery

Today’s world relies upon critical green technologies that are made of elements with unique properties irreplaceable by other materials. Such elements are classified under strategic materials; examples include rare earth elements that are in increasingly high demand but facing supply uncertainty and near zero recycling. For tackling the sustainability challenges associated with rare earth elements supply, new strategies have been initiated to mine these elements from secondary sources. Waste electrical and electronic equipment contain considerable amounts of rare earth elements; however, the current level of their recycling is less than 1%. Current recycling practices use either pyrometallurgy, which is energy intensive, or hydrometallurgy that rely on large volumes of acids and organic solvents, generating large volumes of environmentally unsafe residues. This study put emphasis on developing an innovative and sustainable process for the urban mining of rare earth elements from waste electrical and electronic equipment, in particular, a nickel metal hydride battery. The developed process relies on supercritical fluid extraction utilizing CO₂ as the solvent, which is inert, safe, and abundant. This process is very efficient in the sense that it is safe, runs at low temperature, and does not produce hazardous waste while recovering ∼90% of rare earth elements. Furthermore, we propose a mechanism for the supercritical fluid extraction of rare earth elements, where we considered a trivalent rare earth element state bonded with three tri-<i>n</i>-butyl phosphate molecules and three nitrates model for the extracted rare earth tri-<i>n</i>-butyl phosphate complex. The supercritical fluid extraction process has the double advantage of waste valorization without utilizing hazardous reagents, thus minimizing the negative impacts of process tailings

Reactivity of Perovskites with Water: Role of Hydroxylation in Wetting and Implications for Oxygen Electrocatalysis

Oxides are instrumental to applications such as catalysis, sensing, and wetting, where the reactivity with water can greatly influence their functionalities. We find that the coverage of hydroxyls (*OH) measured at fixed relative humidity trends with the electron-donor (basic) character of wetted perovskite oxide surfaces. Using ambient pressure X-ray photoelectron spectroscopy, we report that the affinity toward hydroxylation, coincident with strong adsorption energies calculated for dissociated water and hydroxyl groups, leads to strong H bonding that is favorable for wetting while detrimental to catalysis of the oxygen reduction reaction (ORR). Our findings provide novel insights into the coupling between wetting and catalytic activity and suggest that catalyst hydrophobicity should be considered in aqueous oxygen electrocatalysis.National Science Foundation (U.S.). Materials Research Science and Engineering Centers (Program) (Award DMR-0819762)National Science Foundation (U.S.). Graduate Research Fellowship (Grant DGE-1122374)National Science Foundation (U.S.) (Career Award (0952564