Publications

Abstract not provided.

Ga₂O₃ films were deposited on (100) MgAl₂O₄ spinel substrates at 550, 650, 750, and 850 °C using metal-organic chemical vapor deposition and investigated using x-ray diffraction and transmission electron microscopy. A phase-pure γ-Ga₂O₃-based material having an inverse spinel structure was formed at 850 °C; a mixture of the γ-phase and β-Ga₂O₃ was detected in films grown at 750 °C. Only β-Ga₂O₃ was determined in the films deposited at 650 and 550 °C. A β- to γ-phase transition occurred from the substrate/film interface during growth at 750 °C. The growth and stabilization of the γ-phase at the outset of film growth at 850 °C was affected by the substantial Mg and Al chemical interdiffusion from the MgAl₂O₄ substrate observed in the energy-dispersive x-ray spectrum. Further, atomic-scale investigations via scanning transmission electron microscopy of the films grown at 750 and 850 °C revealed a strong tetrahedral site preference for Ga and an octahedral site preference for Mg and Al. It is postulated that the occupation of these atoms in these particular sites drives the β-Ga₂O₃ to γ-phase transition and markedly enhances the thermal stability of the latter phase at elevated temperatures.

Abstract not provided.

Reactive separations of CO/CO₂ mixtures are a promising pathway to lower the energy requirement of CO₂ hydrogenation to chemicals and fuels, with applications in the U.S. Navy’s seawater-to-fuel process. With the CO/CO₂ feedstock, a challenge is activating CO to produce heavier hydrocarbons while preventing CO₂ methanation, requiring low-temperature Fischer-Tropsch synthesis (FTS) catalysts. In this work, we demonstrate that a Ru–Co single atom alloy (SAA) catalyst produces C₅₊ hydrocarbons at a rate of 11.7 μmol/s/g-cobalt (hexane basis) in a 50/50 CO/CO₂ stream with ≤1% CO₂ conversion. The reaction operates at a relatively low temperature (200 °C) and high gas hourly space velocity (GHSV: 84,000 mL/g/h) that is compatible with the upstream reverse water-gas shift reaction. Detailed experiments, catalyst characterizations, and density functional theory (DFT) calculations have been conducted to understand the active phase, the role of the Ru dopant, and catalyst restructuring that occurs at elevated temperatures (>200 °C). Ru dopants are found to promote the reduction of Co species, enabling catalytic activity for CO hydrogenation without pre-reduction, but may not enhance the FTS activity or desired C₅₊ hydrocarbon selectivity.