70 Search Results

Microwave absorbing catalysts have the potential to electrify high-temperature thermal reactions such as the dry reforming of methane process (DRM: CO₂ + CH₄ → 2CO + 2 H₂). However, microwave catalysts present unique challenges due to their dual requirements of maintaining microwave absorption in both oxidative and reductive environments and stability across a range of temperatures in inherently non-isothermal reactors. Here, catalyst candidates from the La_0.8Sr_0.2CoO₃-La_0.8Sr_0.2NiO₃-La_0.8Sr_0.2MnO₃ perovskite systems were screened (28 total) to identify promising microwave catalysts free of noble metals for dry reforming methane. The best performing candidates met two main criteria. First, they occurred at crystal phase boundaries,more » giving rise to a pseudocubic perovskite structure. The combined use of Goldschmidt tolerance factor and octahedral tolerance factors appeared to be suitable for predicting pseudocubic perovskites. Second, they provided a balance of reducible metal sites with an irreducible metal oxide support. The best performing catalyst was found to exsolve Ni-Co alloy particles as active sites for the DRM reaction which offered superior resistance to coking for excellent reforming efficiency and stability.« less

Materials that exhibit negative thermal expansion (NTE) are of fundamental interest due to their rarity and counterintuitive behavior. While research in this area has been directed toward the discovery of materials that display NTE and explaining its origin, there has been less attention to describing the complexities of a secondary phase or other guests that could influence the magnitude and mechanism of NTE. We report herein that zeolite-templated carbon (ZTC), a soft carbonaceous framework solid with ordered microporosity, exhibits a large and widely tunable thermal expansion in the presence of adsorbed guests. For ZTC in the presence of CO₂ atmore » 1 bar, the largest coefficient of isotropic NTE ever observed (−8.4 × 10^–4 K^–1) is measured between 200 and 220 K. These results comprise a tunable mechanism of thermal expansion based on the interaction between two independent, positively expanding phases that together give rise to an anomalous guest-induced NTE under certain conditions.« less

Arctic permafrost soils are increasingly subject to thermokarst that is, abrupt ground subsidence caused by thaw. Wetlands can form within these depressions, leading to changes in organic matter decomposition and gas fluxes (CO₂, CH₄, N₂O, NH₃). Thermokarst wetlands tend to be dominated by graminoids, while surrounding upland tussock tundra tends to be dominated by mixed communities of shrubs and graminoids. Here, to investigate how thermokarst alters the land-atmosphere exchange of C and N gases in Arctic tundra, we analyzed soil, porewater, above- and belowground biomass, and measured gas fluxes across dominant plant functional types (PFTs) within a lowland thermokarst wetlandmore » and adjacent upland tussock tundra. Both locations were overall sinks of CO₂, sources of CH₄, and sources of both N₂O and NH₃. We found that thermokarst wetlands emitted enough CH₄ to generate a positive radiative forcing in CO₂ equivalents (+1.2 μmol m⁻² s⁻¹ CO₂-eq), counteracting the high CO₂ uptake. In contrast, the upland tussock tundra had a net negative radiative forcing (−1.2 μmol m⁻² s⁻¹ CO₂-eq). Differences in gas flux and soil chemistry between upland and lowland are primarily driven by flooded conditions present in thermokarst wetland. Additionally, root biomass from graminoids across both lowlands and uplands significantly correlated with CH₄ fluxes, supporting previous observations of plant-mediated transport of CH₄. Graminoid cover was correlated with increases in low molecular weight dissolved organic carbon, possibly associated with root exudates that fuel methanogenesis. Forb cover in the upland tussock tundra was significantly correlated with nine soil chemical variables, indicating that forbs may influence local soil chemistry or conversely, that soil chemistry controls where forbs grow. Overall, our findings indicate the variability in gas fluxes in the upland tussock tundra is partially controlled by PFT cover, while thermokarst wetlands emit enough CH₄ to counteract CO₂ uptake, with implications for carbon budget changes in Arctic systems.« less

Inverse oxide/metal catalysts have proved to be excellent systems for the generation of methanol by CO₂ hydrogenation or the partial oxidation of methane. These systems can exhibit unique structural and chemical properties due to the nano size of the oxide component and strong oxide–metal interactions. Recent studies for ZnO/Cu, CeO₂/Cu, TiO_2/Cu, MgO/Cu, In₂O₃/Cu, and In₂O₃/Au catalysts have shown large variations in the composition and morphology of the oxide overlayer as a function of temperature and chemical environment. These oxide–metal interfaces are able to react with CO₂ and CH₄ at room temperature, and both reactants have a strong influence on themore » physical and chemical properties of the catalysts. Under reaction conditions, switches between oxide–metal and metal–metal interfaces can take in the catalyst surface. A dynamic behavior that can be linked to a high selectivity for methanol production over systems such as ZnO/Cu, CeO₂/Cu and In₂O₃/Au. Finally, this type of correlation deserves additional systematic studies since it could be a powerful tool for designing highly efficient catalysts for methanol synthesis.« less

Reservoir operations influence emissions via multiple causal pathways. In this paper, we quantify indicators of carbon alteration (ICAs) focused on methane. ICAs were chosen to reflect the potential for methane emission along four causal pathways: 1) water column mixing, 2) wet-dry cycles in sediment, 3) sediment redistribution, and 4) vegetation. We developed algorithms to calculate ICAs for three reservoirs along a longitudinal gradient in the Tennessee River basin of the southeast US. The ICAs revealed interesting longitudinal patterns. Indicators of both methane production and destruction increased downstream. The potential for ebullitive methane emissions driven by sub-daily water level fluctuations andmore » emissions mediated by vegetation were higher in downstream mainstem reservoirs than in the upstream tributary reservoir. Along the remaining two pathways, longitudinal patterns were equivocal (sediment pathway) or suggested decreased emissions downstream (water-column mixing). We also observed seasonal patterns and, by combining ICAs, inferred times when ramping could be achieved with lower risk of emissions. The ICAs demonstrated here are the first step in quantifying mechanistic relationships between reservoir operation and methane emissions. In future, they may lead to improved operations in reservoir cascades and regional-scale estimates of emissions that account for differences among reservoirs.« less

Inaccuracy in our knowledge of the transport properties of relevant mixtures under planetary interior conditions is a roadblock in predicting the observable properties of planets in our solar system and beyond. In this work, we investigate methane–hydrogen mixtures using data sets obtained from density functional theory calculations for the electronic structure, combined with molecular dynamics simulations for the ions for a wide range of pressure and temperature. Hydrogen concentration significantly affects the equation of state (EOS) but has little influence on transport properties. We provide an analytical expression to model thermal EOS and transport properties as a function of hydrogenmore » content, with the maximum deviation observed at low P–T conditions. These insights are particularly relevant to improve the planetary models and enhance our ability to predict the properties of “ice” giants and beyond.« less

The direct conversion of methane-to-methanol remains a critical challenge in methane valorization. In this study, we unveil the crucial role of PdAu/CeO₂ catalysts in enabling selective methane transformation under mild conditions, using only water as the sole oxidant. Through a combination of experimental techniques, including XPS and catalytic testing, alongside density functional theory (DFT) calculations, we demonstrate that a Pd_0.3Au_0.7/CeO₂ catalyst, which predominantly exposes isolated Pd atoms, achieves remarkable methanol selectivity (∼80%) at 500 K with a 1:1 methane-to-water ratio. While Pd/CeO₂ efficiently activates methane, its tendency for overreaction leads to complete methanol decomposition, thereby limiting selectivity. Alloying Pd withmore » Au on ceria mitigates this over-reactivity, preventing methanol degradation while maintaining sufficient catalytic activity. The PdAu/CeO₂ composite exhibits a synergistic effect: Pd in contact with the ceria support facilitates methane activation and water dissociation, while Au fine-tunes reactivity to promote methanol formation. DFT calculations confirm that isolated Pd sites at the PdAu/CeO₂ interface play a key role in balancing activity and selectivity. This work underscores the importance of alloy/oxide interfaces in controlling selective methane conversion with water and offers valuable insights for designing highly efficient catalysts for methanol synthesis.« less

The selective conversion of methane (CH4) under mild conditions remains challenging due to strong C-H bonds and catalyst coking. We systematically investigated sub-monolayer MgO nanostructures on Au(111), where two-dimensional (2D) MgO islands with stable Mg-O-Au interfaces catalyze low-temperature CH4 activation and C-C coupling. Upon CH4 exposure at 300 K, surface-bound CHx and C2Hx intermediates formed and persisted post-evacuation, indicating robust CHx-O-Mg linkages. Temperature-programmed studies revealed that C-H activation and C-C coupling intensify with heat: the CHx signal grew continuously while the C2Hx signal reached a plateau at 400-500 K. O 1s and Mg 2p attenuation confirmed adsorption of the hydrocarbonsmore » on MgO. Catalytic tests at 500 K yielded C2H6 (70%) and C2H4 (30%) without coking, underscoring MgO's role as an active catalyst. These results offer new design principles for developing coke-resistant and low-temperature methane upgrading catalysts.« less

As with most aquatic ecosystems, reservoirs play an important role in the global carbon (C) cycle and emit greenhouse gases (GHG) as carbon dioxide (CO₂) and methane (CH₄). However, GHG emissions from reservoirs are poorly quantified, especially in temperate systems, resulting in high uncertainty. We compared reservoir C emission estimates and uncertainty of diffusive, ebullitive, and degassing pathways in six hydropower reservoirs in the southeastern United States among four data sources: two field-based surveys and two models (including the GHG Reservoir “G-res” Tool). We found that CH₄ diffusion was most similar across data sources (modeled minus observed, bias = -more » 21 g CO_2-eq m^-2 y^-1) and had low relative uncertainty (coefficient of variation, CV = 0.98). On the other hand, CO₂ diffusion was least consistent across data sources (bias = - 518 g CO_2-eq m^-2 y^-1). Both field surveys indicated strong negative CO₂ diffusion (i.e., CO₂ uptake) at all reservoirs, while G-res estimated positive CO₂ diffusion. By extension, total C emissions showed similar discrepancies, leading to high uncertainty in upscaling and interpreting reservoir source-sink dynamics. Finally, CH₄ ebullition had the highest relative uncertainty (CV = 2.77) due to high variability across sites. We discuss limitations of field surveys and these models, including temperature-based annualization methods, varying definitions of ebullition zones, low sampling resolution, and lack of dynamism. Future field efforts focused on capturing variability in CO₂ diffusion and CH₄ ebullition will be especially valuable in reducing uncertainty and improving models to advance our understanding reservoir GHG emissions.« less

In living cells, alpha-ketoacid decarboxylases (KDCs, EC 4.1.1.-) are a class of enzymes that convert alpha-ketoacids into aldehydes through decarboxylation. These aldehydes serve as either drop-in chemicals or precursors for the biosynthesis of alcohols, carboxylic acids, esters, and alkanes. These compounds play crucial roles in cellular metabolism and fitness and the bioeconomy, facilitating the sustainable and renewable biomanufacturing of platform chemicals and fuels. This review explores the diversity and classification of KDCs, detailing their structures, mechanisms, and functions. We highlight recent advancements in repurposing KDCs to enhance their efficiency and robustness for biomanufacturing. Additionally, we present modular KDC-dependent metabolic pathwaysmore » for the microbial biosynthesis of aldehydes, alcohols, carboxylic acids, esters, and alkanes. Lastly, we discuss recent developments in the modular cell engineering technology that can potentially be applied to harness the diversity of KDC-dependent pathways for biomanufacturing platform chemicals and fuels.« less