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Density Functional Theory Modeling on the Positive Effect of H2O in Hydrogen Oxidation Reaction on Perovskite Oxide Anodes for Solid-Oxide Fuel Cells
Presented by LRST at the MS&T2020, 11/4
Dispersion of sulfur creates a valuable new growth medium formulation that enables earlier sulfur oxidation in relation to iron oxidation in Acidithiobacillus ferrooxidans cultures
Abstract Acidithiobacillus ferrooxidans is an acidophilic chemolithoautotroph that is commonly reported to exhibit diauxic population growth behavior where ferrous iron is oxidized before elemental sulfur when both are available, despite the higher energy content of sulfur. We have discovered sulfur dispersion formulations that enables sulfur oxidation before ferrous iron oxidation. The oxidation of dispersed sulfur can lower the culture pH within days below the range where aerobic ferrous iron oxidation can occur. Thus, ferric iron reduction can be observed quickly which had previously been reported over extended incubation periods with untreated sulfur. Therefore, we demonstrate that this substrate utilization pattern is strongly dependent on the cell loading in relation to sulfur concentration, sulfur surface hydrophobicity, and the pH of the culture. Our dispersed sulfur formulation, lig‐sulfur, can be used to support the rapid antibiotic selection of plasmid‐transformed cells, which is not possible in liquid cultures where ferrous iron is the main source of energy for these acidophiles. Furthermore, we find that media containing lig‐sulfur supports higher production of green fluorescent protein compared to media containing ferrous iron. The use of dispersed sulfur is a valuable new tool for the development of engineered A. ferrooxidans strains and it provides a new method to control iron and sulfur oxidation behaviors.
A redox model for NO oxidation, NH 3 oxidation and high temperature standard SCR over Cu-SSZ-13
A kinetic model is developed to predict the influence of temperature and hydrothermal aging on the redox of active Cu sites under standard SCR, NO oxidation and NH 3 oxidation conditions over a practically relevant fully-formulated Cu-SSZ-13 catalyst. NO 2 /N 2 production during NO/NH 3 titration of Cu II sites is utilized to identify rate parameters associated with NO-only RHC (reduction half cycle) and NH 3 -only RHC respectively. Integral N 2 formation during subsequent NO + NH 3 titration is consistent with the production of one NO 2 per two CuII sites reduced during NO-only RHC and one N 2 per six Cu II sites reduced during NH 3 -only RHC. Decreased reduction of Cu II sites by NO/NH 3 upon hydrothermal aging, along with the production of one NO 2 per two CuII sites during NO-only RHC, is accordant with the involvement of proximal ZCuOH and oxygen-bridged dimeric Cu II sites. Oxidation of partially solvated and framework coordinated ZCu (Cu I ) sites occurs in presence of O 2 , does not produce N 2 and can lead to the consumption of Brønsted acid sites. A global OHC kinetic model is developed to predict transient and integral N 2 formation during exposure of CuI sites to a mixture of NO and O 2 . The resulting redox kinetic model quantitatively predicts NO and NH 3 consumption during isothermal transient response Cu redox (TRCR) protocols, along with temperature and age dependent steady-state standard SCR and oxidation conditions. The redox model presented in this work synthesizes recent kinetic, spectroscopic and computational findings to provide a foundational description of active site redox during standard SCR, NO oxidation and NH3 oxidation over Cu-SSZ-13.
High surface area mesoporous tungsten oxide for fast, green oxidation of organosulfur compounds in crude oil
Mesoporous, multi-valent tungsten oxide with high surface area (209 m 2 /g) is synthesized via a novel metal dissolution coupled with reverse micelle formation approach. Mesoporous tungsten oxide with highest surface area reported is obtained by this method. The material is used as an excellent heterogeneous catalyst for the oxidation of organosulfur compounds present in crude oil. The complete oxidation of dibenzothiophene was observed with excellent yield (>99%) within 15 min at room temperature (20 °C). The catalyst is reusable up to 5 reaction cycles without losing significant activity. A green oxidant, H 2 O 2 is used and the reaction mechanism involves the activation of peroxo species by W 6+ . The oxidation activity decreases in the order of DBT > 4,6-DMDBT > 1-BT due to the changes in the electronic environment of sulfur and steric effects.
High temperature oxidation of corrosion resistant alloys from machine learning
Parabolic rate constants, k p , were collected from published reports and calculated from corrosion product data (sample mass gain or corrosion product thickness) and tabulated for 75 alloys exposed to temperatures between ~800 and 2000 K (~500–1700 °C; 900–3000°F). Data were collected for environments including lab air, ambient and supercritical carbon dioxide, supercritical water, and steam. Materials studied include low- and high-Cr ferritic and austenitic steels, nickel superalloys, and aluminide materials. A combination of Arrhenius analysis, simple linear regression, supervised and unsupervised machine learning methods were used to investigate the relations between composition and oxidation kinetics. The supervised machine learning techniques produced the lowest mean standard errors. The most significant elements controlling oxidation kinetics were Ni, Cr, Al, and Fe, with Mo and Co composition also found to be significant features. The activation energies produced from the machine learning analysis were in the correct distributions for the diffusion constants for the oxide scales expected to dominate in each class.
Covalency competition dominates the water oxidation structure–activity relationship on spinel oxides
Spinel oxides have attracted growing interest over the years for catalysing the oxygen evolution reaction (OER) due to their efficiency and cost-effectiveness, but fundamental understanding of their structure–property relationships remains elusive. Here we demonstrate that the OER activity on spinel oxides is intrinsically dominated by the covalency competition between tetrahedral and octahedral sites. The competition fabricates an asymmetric M$_T-O-M_O$ backbone where the bond with weaker metal–oxygen covalency determines the exposure of cation sites and therefore the activity. Driven by this finding, a dataset with more than 300 spinel oxides is computed and used to train a machine-learning model for screening the covalency competition in spinel oxides, with a mean absolute error of 0.05 eV. [Mn]$_T$[Al$_{0.5}$Mn$_{1.5}$]OO$_4$ is predicted to be a highly active OER catalyst and subsequent experimental results confirm its superior activity. This work sets mechanistic principles of spinel oxides for water oxidation, which may be extendable to other applications.
Chemisorbed oxygen or surface oxides steer the selectivity in Pd electrocatalytic propene oxidation observed by operando Pd L-edge X-ray absorption spectroscopy
Controlled electrochemical oxidation of hydrocarbons to desired products is an attractive approach in catalysis. Here we study the electrochemical propene oxidation under operando conditions using Pd L-edge X-ray absorption spectroscopy (XAS) as a sensitive probe to elucidate surface processes occurring during catalysis. Together with ab initio multiple-scattering calculations, our XAS results enable assignment of characteristic changes of the Pd L-edge intensity and energy position in terms of a mechanistic understanding of the selective oxidation of propene. The results, supported by electrochemical density functional theory DFT simulations, show that in the potential range of 0.8–1.0 V vs. the reversible hydrogen electrode (RHE), selective oxidation of propene to acrolein and acrylic acid occurs on the metallic Pd surface. Here, these reactions are proposed to proceed via the Langmuir–Hinshelwood mechanism. In contrast, for the potential range of 1.1–1.3 V vs. RHE, selective oxidation of propene to propylene glycol takes place on a Pd oxide surface.
Understanding methanol dissociative adsorption and oxidation on amorphous oxide films
Interactions between a transition metal (oxide) catalyst and a support can tailor the number and nature of active sites, for instance in the methanol oxidation reaction.In this report we use ambient pressure X-ray photoelectron spectroscopy (AP-XPS) to identify and compare the surface adsorbates that form on amorphous metal oxide films that maximize such interactions. Considering Al (1−x) M x O y (M = Fe or Mn) films at a range of methanol : oxygen gas ratios and temperatures, we find that the redox-active transition metal site (characterized by methoxy formation) dominates dissociative methanol adsorption, while basic oxygen sites (characterized by carbonate formation) play a lesser role. Product detection, however, indicates complete oxidation to carbon dioxide and water with partial oxidation products (dimethyl ether) comprising a minor species. Comparing the intensity of methoxy and hydroxyl features at a fixed XPS chemical shift suggests methanol deprotonation during adsorption in oxygen rich conditions for high transition metal content. However, increasing methanol partial pressure and lower metal site density may promote oxygen vacancy formation and the dehydroxylation pathway, supported by a nominal reduction in the oxidation state of iron sites. These findings illustrate that AP-XPS and mass spectrometry together are powerful tools in understanding metal–support interactions, quantifying and probing the nature of catalytic active sites, and considering the link between electronic structure of materials and their catalytic activity.
A computational fluid dynamics model to estimate local quantities in firebrand char oxidation
Firebrand burning is a complex phenomenon that is influenced by several parameters which are difficult to fully explore experimentally. Computational fluid dynamics models capable of predicting local quantities are essential for accurate prediction of char oxidation in firebrands. This article presents a computational fluid dynamics model to estimate firebrand mass loss, diameter change, and surface temperature during char oxidation. The model was validated using previously conducted wind tunnel experiments. These experiments were conducted for firebrands of two different aspect ratios, which were arranged in three different configurations (single, horizontal array, and vertical array), and for four different wind speeds (0.5, 1, 1.5, and 2 m/s). The computational fluid dynamics results were compared with a previous 1 D model. In all the test cases, the computational fluid dynamics model predicted the physical phenomena with significantly improved accuracy compared to a 1 D model. The char oxidation model presented in this article can be coupled with other models to study firebrand generation and trajectory, biomass pyrolysis, fluidized bed reactors, and coal combustion.
The Detection of Monoclinic Zirconia and Non-Uniform 3D Crystallographic Strain in a Re-Oxidized Ni-YSZ Solid Oxide Fuel Cell Anode
The solid oxide fuel cell (SOFC) anode is often composed of nickel (Ni) and yttria-stabilized zirconia (YSZ). The yttria is added in small quantities (e.g., 8 mol %) to maintain the crystallographic structure throughout the operating temperatures (e.g., room-temperature to >800 °C). The YSZ skeleton provides a constraining structural support that inhibits degradation mechanisms such as Ni agglomeration and thermal expansion miss-match between the anode and electrolyte layers. Within this structure, the Ni is deposited in the oxide form and then reduced during start-up; however, exposure to oxygen (e.g., during gasket failure) readily re-oxidizes the Ni back to NiO, impeding electrochemical performance and introducing complex structural stresses. In this work, we correlate lab-based X-ray computed tomography using zone plate focusing optics, with X-ray synchrotron diffraction computed tomography to explore the crystal structure of a partially re-oxidized Ni/NiO-YSZ electrode. These state-of-the-art techniques expose several novel findings: non-isotropic YSZ lattice distributions; the presence of monoclinic zirconia around the oxidation boundary; and metallic strain complications in the presence of variable yttria content. This work provides evidence that the reduction–oxidation processes may destabilize the YSZ structure, producing monoclinic zirconia and microscopic YSZ strain, which has implications upon the electrode’s mechanical integrity and thus lifetime of the SOFC.
Elucidating the role of surface species in CO oxidation catalyzed by boron nitride nanotube supported transition metal oxides
Boron nitride nanotube (BNNT) is considered a highly promising catalyst support due to its outstanding thermal stability and chemical inertness. These characteristics make BNNT an attractive alternative for high-temperature applications. However, most studies to date have focused on incorporating platinum group metals (PGMs) to achieve high activity. Although BNNT-supported PGM catalysts are highly effective, their scarcity and high cost hinder widespread use in industrial processes. In this study, BNNT-supported transition metal oxides (TMO x /BNNT; TM = Fe, Co, Ni, and Cu) catalysts were investigated, and CO oxidation was applied as a model reaction to evaluate their catalytic performance. Several characterization techniques, including SEM-EDX, TEM, SXRD, H 2 -TPR, and XPS, were employed to examine their physicochemical properties. Notably, the particle size of the metal oxides differed significantly depending on the metal type. This variation is primarily attributed to the inherent metal–support interactions and the thermodynamic stability of each oxide during synthesis. These properties also affected catalytic activity, and various parameters, such as oxygen mobility and redox behavior, played important roles in determining performance. Finally, in situ DRIFTS, CO-TPSR, reaction-order analysis, and 18 O 2 isotope-labeling experiment were used to investigate the reaction mechanism. In conclusion, the findings provide insights into the design of cost-effective BNNT-supported catalysts and highlight their potential applicability in oxidation reactions.
Engineering surface segregation of perovskite oxide through wet exsolution for CO catalytic oxidation
Cation segregation occurring near the surface or interfaces of solid catalysts plays an important role in catalytic reactions. Unfortunately, the native surface of perovskite oxides is dominated by passivated A-site segregation, which severely hampers the catalytic activity and durability of the system. To address this issue, we present a wet exsolution method to reconstruct surface segregation in perovskite cobalt oxide. Here, under reduction etching treatment of glycol solution, inert surface Sr segregation was transformed into active Co 3 O 4 segregation. By varying the reaction time, we achieved differing coverage of the active Co 3 O 4 segregation on the La 0.5 Sr 0.5 CoO 3-δ (LSCO) perovskite oxide surface. This study reveals that CO oxidation activity exhibits a volcano-shaped dependence on the coverage of Co 3 O 4 segregation at the surface of a perovskite cobalt oxide. Furthermore, we find that a suitable coverage of Co 3 O 4 segregation can dramatically improve the catalytic activity of the perovskite catalyst by enhancing interface interactions. Co K-edge, Co L-edge, and O K-edge X-ray absorption spectra confirm that the synergistic effect optimizes the covalence of the metal-oxygen bond at the surface and interface. This work not only contributes to the design of perovskite-type catalysts, but also provides important insight into the relationship between surface segregation and catalytic activity.
Influence of oxidizing and reducing pretreatment on the catalytic performance of CeO 2 for CO oxidation
Cerium oxide (CeO 2 ) and ceria-based materials have been extensively investigated as catalyst and support materials for various catalytic reactions, due to higher oxygen storage capacity and excellent redox properties. In the current work, we investigated the impact of pretreatment conditions (e.g., oxidation and reduction) on the physical properties of bulk CeO 2 and catalytic activity for CO oxidation as a model reaction. To understand the physical properties of pretreated CeO 2 catalysts, a suite of complementary characterization techniques, including X-ray diffraction (XRD), surface area analysis (BET), X-ray photoemission spectroscopy (XPS), and Raman spectroscopy, were applied. The results showed that a higher pretreatment temperature led to a decreased specific surface area (SSA), a decrease in oxygen vacancy/defect sites, and increased crystallite size, while surface Ce 3+ /Ce 4+ ratio did not show a specific relationship to the treatment conditions. The 700 °C treated CeO 2 samples under oxidizing and reducing conditions showed higher specific oxidation rate (μmolCO/s/m 2 ) compared to other samples at 280 and 300 °C (or < 15% CO conversion). The CO conversion per total mass of catalysts, however, decreased with increasing temperatures, especially at 700 °C under reducing condition, indicating that the catalytic performance was affected by the physical properties (SSA, oxygen vacancy/defect sites, and crystallite size).
Low-temperature oxidation of methane and methanol on iridium oxides
Iridium oxides (IrO 2 ) are of significant interest for low-temperature oxidation of small molecules such as CH 4 and CH 3 OH, although the physical origin of their high activity remains under debate. Here, we demonstrate that the enhanced activity of IrO 2 arises from the formation of coordinatively unsaturated (CUS) oxygen species. By combining ambient-pressure X-ray spectroscopy and density functional theory calculations, we present evidence for the formation of CUS oxygen during CH 4 and CH 3 OH oxidation. Such surface speciation correlates with the conversion of methane to carbon dioxide and methanol to methyl formate on rutile IrO 2 and hydrous IrO 2 powder catalysts in a plug-flow reactor at room temperature. These findings extend the understanding of the physical origin of the higher activity of iridium oxide thin-film catalysts to powder catalysts and provide insights into the tuneability of iridium-oxide-containing catalysts for low-temperature C–H and O–H bond activation.
Selective Oxidation and Cr Segregation in High-Entropy Oxide Thin Films
High-entropy oxides (HEOs) offer exceptional compositional flexibility and structural stability, making them promising materials for energy and catalytic applications. Here, in this study, we investigate Sr doping effects on B-site cation oxidation states, local composition, and structure in epitaxial La 1–x Sr x (Cr 0.2 Mn 0.2 Fe 0.2 Co 0.2 Ni 0.2 )O 3 thin films. X-ray spectroscopies reveal that Sr doping preferentially promotes Cr oxidation from Cr 3+ to Cr 6+ , partially oxidizes Co and Ni, while leaving Mn 4+ and Fe 3+ unchanged. Atomic-resolution scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy shows pronounced Cr segregation, with depletion at the interface and enrichment at the surface, along with partial amorphization in heavily Sr-doped samples. This segregation is likely driven by oxidation-induced migration of smaller, high-valence Cr cations during growth. These findings highlight the critical interplay between charge compensation, local strain, and compositional fluctuations in HEOs, indicating that precise control over growth conditions is critical for tuning their surface composition and electronic structure toward more robust electrocatalyst design.
Electrochemical Oxidation of Metal–Catechol Complexes as a New Synthesis Route to the High-Quality Ternary Photoelectrodes: A Case Study of Fe 2 TiO 5 Photoanodes
A new electrochemical, solution-based synthesis method to prepare uniform multinary oxide photoelectrodes was developed. This method involves solubilizing multiple metal ions as metal-catechol complexes in a pH condition where they are otherwise insoluble. When some of the catechol ligands are electrochemically oxidized, the remaining metal complexes become insoluble and are deposited as metal-catechol films on the working electrode. The resulting films are then annealed to form crystalline multinary oxide electrodes. Since catechol can serve as a complexing agent for a variety of metal ions, the newly developed method can be used to prepare a variety of multinary oxide films. In the present study, we used this method to prepare n-type Fe 2 TiO 5 photoanodes and investigated their photoelectrochemical properties for use in a photoelectrochemical water splitting cell. We also performed a computational investigation with two goals. The first goal was to investigate small electron polaron formation in Fe 2 TiO 5 . Charge transport in most oxide photoelectrodes involves small polaron hopping, but small polaron formation in Fe 2 TiO 5 has not been examined prior to this work. The second goal was to investigate the effect of substitutional Sn doping at the Fe site on the electronic band structure and the carrier concentration of Fe 2 TiO 5 . Here, the combined experimental and theoretical results presented in this study greatly improve our understanding of Fe 2 TiO 5 for use as a photoanode.