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At least 199 records · Page 11

From tides to seasons: How cyclic tidal drivers and plant physiology interact to affect carbon cycling at the terrestrial-estuarine boundary (Final technical report)

Coastal ecosystems are among the most biologically and biogeochemically active and diverse systems on Earth. Because they act as important linkages between terrestrial ecosystems and the open ocean, their incorporation in Earth system models (ESMs) is critical to predict coastal and global responses to environmental changes. However, they vary greatly in the magnitude of tides and the volume and timing of freshwater input from land, making it challenging to model the major biogeochemical reactions that control productivity and greenhouse gas emissions across coastal terrestrial aquatic interfaces (TAIs). Our overall objective was to improve mechanistic process understanding and modeling of tidal wetland hydro-biogeochemistry in coastal TAIs. We established a new flux tower site (Ameriflux US-PLo) in the oligohaline part of the Parker River to continuously monitor ecosystem-scale carbon fluxes under temporally varying salinity conditions. The site is co-located with long-term monitoring plots of the Plum Island Ecosystems LTER project. We installed wells and redox sensors in the marsh interior and creek bank, established biomass monitoring plots and deployed novel optode sensors in both locations. We used this data to parameterize plant-mediated transport in PFLOTRAN and tested the impact of soil heterogeneity on porewater constituents and gas fluxes. We collected observations of root oxygen release with a novel planar optode system in the field. Flux data collected during the measurement period encompasses a large variation in salinity ranging from drought to record precipitation years. We developed a method to extract functional relationships from the flux data using artificial neural networks, identifying salinity thresholds for CH 4 fluxes. Finally, we are using the coupled ELM-PFLOTRAN model to test the impact of antecedent hydrological conditions on the salinity-CH 4 flux relationship. This grant contributed to the professional development of one postdoc, three research assistants and one graduate student. The sensor data has been shared with external collaborators.

54 ENVIRONMENTAL SCIENCES↗

Impact of Mineral Reactive Surface Area on Forecasting Geological Carbon Sequestration in a CO2-EOR Field

Mineral reactive surface area (RSA) is one of the key factors that control mineral reactions, as it describes how much mineral is accessible and can participate in reactions. This work aims to evaluate the impact of mineral RSA on numerical simulations for CO2 storage at depleted oil fields. The Farnsworth Unit (FWU) in northern Texas was chosen as a case study. A simplified model was used to screen representative cases from 87 RSA combinations to reduce the computational cost. Three selected cases with low, mid, and high RSA values were used for the FWU model. Results suggest that the impact of RSA values on CO2 mineral trapping is more complex than it is on individual reactions. While the low RSA case predicted negligible porosity change and an insignificant amount of CO2 mineral trapping for the FWU model, the mid and high RSA cases forecasted up to 1.19% and 5.04% of porosity reduction due to mineral reactions, and 2.46% and 9.44% of total CO2 trapped in minerals by the end of the 600-year simulation, respectively. The presence of hydrocarbons affects geochemical reactions and can lead to net CO2 mineral trapping, whereas mineral dissolution is forecasted when hydrocarbons are removed from the system.

58 GEOSCIENCES↗

Effect of Intermediate Semiconducting TiOx Thin Films on Nanoparticle-Mediated Electron Transfer: Electrooxidation of CO

The concept of nanoparticle-mediated electron transfer (eT) across insulating thin films was elucidated theoretically by Allongue and Chazalviel in 2011. In their model, metal nanoparticles (NPs) are immobilized atop passivating, self-assembled monolayers (SAMs). They found that under certain conditions, related to the thickness of the SAM and the size of the NPs, efficient faradaic oxidation and reduction reactions could proceed at the NP surface. In the absence of NPs, however, eT was suppressed by the insulating SAM thin films. Allongue and Chazalviel concluded that, within certain bounds, eT is mediated by fast tunneling between the conductive electrode and the metal NPs, while the kinetics of the redox reaction are controlled by the NPs. This understanding has been confirmed using a variety of experimental models. The theory is based on electron tunneling; therefore, the nature of the intervening medium (the insulator in prior studies) should not affect the eT rate. In the present manuscript, however, we show that the theory breaks down under certain electrochemical conditions when the medium between conductors is an n-type semiconductor. Specifically, we find that in the presence of either Au or Pt NPs immobilized on a thin film of TiOx, CO electrooxidation does not proceed. In contrast, the exact same systems lead to the efficient reduction of oxygen. At present, we are unable to explain this finding within the context of the model of Allongue and Chazalviel.

Galyamova, Aigerim↗

Isoconversional Kinetic Analysis of Oxygen Desorption for Sr 0.75 Ca 0.25 FeO 3-δ Perovskite in Dry and Steam-Based Environments

The desorption kinetics of Sr 0.75 Ca 0.25 FeO 3-δ perovskite material are examined in both dry and steam-based environments using redox gaseous products of a laboratory-scale fixed bed. First, desorption kinetics associated with the dry environment are proposed based on Friedman’s isoconversional method. It is found from the reconstructed reaction model that the desorption kinetics of this perovskite are controlled by a three-step mechanism, where hypothetically, the phase-boundary reaction rapidly prevails first, followed by diffusion and/or nucleation, and finally, unimolecular decay or random nucleation-controlled reaction. Subsequent analyses of the Arrhenius parameters indicated that both the early and later desorption stages, respectively, controlled by the phase boundary and unimolecular/random nucleation reactions, are associated with low energetic demand. In contrast, a high energetic penalty is required for the diffusion/nucleation reaction. Further, a satisfactory a priori verification of the proposed kinetics suggested that the three-step reaction mechanism reasonably describes the desorption of this perovskite in a dry environment. With the presence of steam in the desorption environment, oxygen production is substantially inhibited, but the hypothetical three-step mechanism is still found to control the desorption. Meanwhile, the primary effects of the steam on these mechanisms include the widening of the conversion extent associated with the diffusion/nucleation reaction, along with a higher energetic demand compared to the dry environment.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

Controlling selectivity for dechlorination of poly(vinyl chloride) with (xantphos)RhCl

Reaction of poly(vinyl chloride) (PVC) with 5 equiv. of triethyl silane in THF, in the presence of in situ generated (xantphos)RhCl catalyst, results in partial reduction of PVC via hydrodechlorination to yield poly(vinyl chloride-co-ethylene). Increasing catalyst loading or using N,N-dimethylacetamide (DMA) as a solvent both diminished selectivity for hydrodechlorination, promoting competitive dehydrochlorination reactions. Reaction of PVC with 2 equiv. of sodium formate in THF in the presence of (xantphos)RhCl affords excellent selectivity for hydrodechlorination along with complete PVC dechlorination, yielding polyethylene-like polymers. Higher catalyst loadings were necessary to activate PVC towards reduction in this case. In contrast, reaction of PVC with 1 equiv. of NaH in DMA, in the presence of (xantphos)RhCl, exhibited good selectivity for dehydrochlorination, as well as much higher reaction rates. These results combined shed light on the interplay between critical reaction parameters that control PVC's mode of reactivity.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

CO 2 hydrogenation over rhodium cluster catalyst nucleated within a manganese oxide framework

Rhodium-based manganese oxide frameworks were explored as a prototype for carbon dioxide reactive capture and conversion. Three-dimensional frameworks of MnOx were utilized as support structures to isolate Rh metal centers. V, Na, and Zn were introduced as counterions to stabilize the structure and for their beneficial effect as promoters. Here, with this multicomponent catalyst, Rh active centers with MnOxs and varied counterions, we were able to selectively tune the catalytic performance of the material via the choice of counterion and structure of the host material. With cryptomelane-type tunnel manganese oxides octahedral molecular sieve (OMS2), we found that Rh-V-OMS2 was highly stable even after 48 hours on stream with a reaction rate of around 1.5x10 -4 mol CO 2 /g Rh /s, surpassing the net reactivity of other initially more active combinations. Furthermore, during CO 2 hydrogenation, in situ XAFS showed that single Rh atoms nucleated into nanoparticles/ sub-nanometer clusters with a coordination number of 5.5 or less. Our finding of the correlation between the reaction rate and particle size offers the potential for enhanced control over the reaction rate by tuning particle size. Our activity study with control experiments demonstrates that the activities of the catalysts are proved due to the unique metal support interaction offered by the Rh-X-MnO.

10 SYNTHETIC FUELS↗

Spin-Correlated Radical Pairs as Magnetic Switches for Controlling Emissive Triplet States via Triplet–Triplet Energy Transfer

Magnetic fields offer a powerful means to control molecular emission, enabling quantum sensing and spin-level control of chemical reactions. Here, we demonstrate a strategy to magnetically control red to near-infrared phosphorescence via triplet–triplet energy transfer (TTET) from donor–chiral bridge–acceptor (D−χ–A) molecules that generate spin-correlated radical pairs (SCRPs) upon photoexcitation. These SCRPs yield non-emissive triplet excited states whose formation is sensitive to magnetic fields. Here, by transferring this energy to emissive Pt- and Pd-based π-extended porphyrins, we enable magnetic control over phosphorescence that would otherwise be unresponsive to weak magnetic fields (<1 T). This approach establishes a platform for quantifying magnetic field effects on silent triplet states while extending magnetically responsive emission into the near-infrared. Coupling SCRP-based molecular magnetic switches to long-wavelength emissive acceptors offers a new way for probing and modulating spin-dependent processes and triplet-state populations in molecular systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

Deciphering Reaction Mechanisms of Molecular Proton Reduction Catalysts with Cyclic Voltammetry: Kinetic vs Thermodynamic Control

The kinetics and thermodynamics of elementary reaction steps involved in the catalytic reduction of protons to hydrogen define the reaction landscape for catalysis. The mechanisms can differ in the order of the elementary proton transfer, electron transfer, and bond-forming steps and can be further differentiated by the sites at which protons and electrons localize. Access to fully elucidated mechanistic, kinetic, and thermochemical details of molecular catalysts is crucial to facilitate the development of new catalysts that operate with optimal efficiency, selectivity, and durability. The mechanism by which a catalyst operates, as well as the kinetics and thermodynamics associated with the individual steps, can often be accessed through electroanalytical studies. Here, this Account details the application of cyclic voltammetry to interrogate reaction mechanisms and quantify the kinetics and thermodynamics of elementary reaction steps for a series of molecular catalysts that mediate electrochemical proton reduction. I distinguish the limiting scenarios wherein a catalyst operates under kinetic control vs thermodynamic control, with a focus on detecting how cyclic voltammetry features shift with proton source strength and concentration, as well as scan rate. For systems that operate under kinetic control, catalytic currents are observed at, or slightly positive toward, the formal potential for the redox process that triggers catalysis. Under thermodynamic control, catalytic responses shift as a function of the proton source pKa and effective pH of the solution. After drawing this distinction, we introduce the appropriate voltammetry experiments and accompanying analytical expressions for extracting key metrics from the data. To illustrate analytical strategies to quantify elementary reaction steps of catalysts operating under kinetic control, I describe our studies of proton reduction catalysts Co(dmgBF 2 ) 2 (CH 3 CN) 2 (dmgBF 2 = difluoroboryl-dimethylglyoxime) and [Ni(P 2 Ph N 2 Ph ) 2 ] 2+ (P 2 Ph N 2 Ph = 1,5-phenyl-3,7-phenyl-1,5-diaza-3,7-diphosphacyclooctane). Here, peak shift analysis, foot-of-the-wave analysis, and plateau current analysis are applied to data sets wherein voltammetric response are recorded as a function of catalyst concentration, proton source concentration, proton source strength, and scan rate to quantify rate constants for elementary proton transfer and bond-forming steps in a catalytic cycle. Further, the case study of [Ni(P 2 Ph N 2 Ph ) 2 ] 2+ illustrates how complementary spectroscopic methods can bolster the mechanistic assignment. Collectively, these two studies showcase how detailed mechanistic studies inform on rate-limiting elementary steps in catalysis and other key processes underpinning catalysis. Second, I present analytical strategies to interrogate catalysts operating under thermodynamic control, centered on the case study of [Ni II (P 2 Ph N 2 Bn ) 2 ] 2+ (P 2 Ph N 2 Bn = 1,5-dibenzyl-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane). Here, the application of nonaqueous Pourbaix theory to extract thermodynamic information is introduced, and the construction of a coupled Pourbaix diagram is detailed. This study identifies ligand-based protonation as the key process that places catalysis under thermodynamic control and influences the reaction mechanism. Together, the work detailed in this Account showcases the utility of electroanalytical methods to disentangle complex reaction mechanisms and extract key thermochemical and kinetic parameters for elementary steps of catalysis. Through detailed presentation of the key analytical expressions that underpin these analyses, this Account seeks to facilitate the adoption of cyclic voltammetry by the community to fully extract kinetic, thermochemical, and mechanistic information on electrochemical small-molecule activation.

catalysts↗

Catalytic resonance theory for the kinetics of photon-promoted catalysis

The illumination of catalytic surfaces with a continuous or pulsed stream of photons dynamically modulates surface chemistry for faster rates, non-equilibrium conversion, or product selectivity control. To establish fundamental principles of dynamic photon-modulated catalysis, the photocatalytic conversion of a generic surface reaction was simulated using the kinetic Monte Carlo method to understand the kinetic implications of an independent stream of photons that promotes surface product desorption. The time-averaged photocatalytic rate at differential conditions for varying photon flux and temperatures indicated three kinetic regimes described by product thermal desorption control, surface reaction control, and an intermediate kinetic regime with a zero slope Arrhenius plot, consistent with a degree of rate control dominated by the photon arrival frequency (i.e., per-site photon flux). Here, the maximum photocatalytic rate occurred orders of magnitude above the Sabatier limit at the resonance frequency, identified as the photon arrival frequency matching the surface reaction rate constant.

Canavan, Jesse R. [University of Minnesota, Minnea↗

Importance of the Heat Treatment Scheme on Self-Reducing Reactive Silver Inks

Self-reducing reactive silver inks can print high-quality silver at reasonable temperatures. While numerous studies have explored the impact of processing temperature on electrical properties, the role of the heat treatment scheme has not yet been studied. Common heat treatment schemes include printing and drying inks at room temperature before heat treatment, performing heat treatments while the inks are still wet, and directly printing the inks onto heated substrates. Each scheme generates distinct heat transfer and mass transport kinetics that can affect the silver morphology and electrical properties. However, to date, the impact of different schemes has not been systematically investigated. To address this knowledge gap, this work investigates how different heat treatment schemes impact metal formation, sintering, and the resultant electrical properties of printed self-reducing reactive silver inks. Heat treatment on room-temperature dried inks and wet inks, with top-down (oven) and bottom-up (hot plate) heating sources, were compared. Inks dried at room temperature resulted in extremely porous films that required high heat treatment temperatures (>100 °C) for silver densification. However, these dried films could only densify locally, and their high initial porosities resulted in voids and high resistances (~0.6 Ω mm –1 ) even after heat treatments above 300 °C. Inks that were wet during heat treatment showed moderate improvements in electrical properties with resistances on the order of 0.4 Ω mm –1 at heat treatments of 250 °C. Printing reactive inks directly onto heated substrates yielded the best low-temperature results, with 0.46 Ω mm –1 (5.5 × bulk silver) line resistances achieved at only 90 °C. Overall, this work’s experimental results provide detailed insights into why printing onto a heated substrate results in superior electrical performance compared to more common heat treatment schemes. Additionally, controlling where the precipitation reaction occurs is critical to controlling the film morphology and properties. As a result, this work shows that even an ammonia-based silver reactive ink can achieve good, low-temperature electrical properties with the proper heat treatment scheme.

14 SOLAR ENERGY↗

Mechanism and kinetic study of rare earth extraction from allanite by direct acid leaching

Direct acid leaching was conducted to recover rare earth elements (REEs) from an allanite ore assaying 9,861.4 ppm of REEs. The effects of operational variables such as acid type, sulfuric acid (H 2 SO 4 ) concentration, temperature, solid/liquid (S/L) ratio, and particle size on the total REE (TREE) recovery were systematically examined. The REE mineralogy and the involved leaching mechanisms were investigated by microscopic characterization and residue re-leaching tests. It was found that approximately 80% of REEs was extracted using 1 M H 2 SO 4 at 75 °C for 2 h. Increasing temperature significantly improved the REE extraction, while changing acid type, H 2 SO 4 concentration, and particle size did not. The TREE recovery also gradually decreased at higher S/L ratios. The low fraction of Si leached (<4.0%) suggested that the extraction did not significantly alter the crystal structure of the dominant mineral. The leaching kinetics of REEs were fast within the first 10 min, followed by a much slower stage (i.e., 10-120 min). The kinetic modeling results showed that each stage may be controlled by both chemical reaction and diffusion (i.e., mix control). The activation energies for REE leaching using 1 M H 2 SO 4 in the 0-10 min and 10-120 min were determined to be 20.3 and 10.8 kJ/mol, respectively. Finally, the residue re-leaching results suggested that the fast leaching kinetics at the beginning may be explained by the preferential decomposition of metamict allanite rather than the formation of silica gel, while the following slower leaching stage was due to the dissolution of difficult-to-leach REE-bearing minerals such as well-crystallized allanite.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

Advanced Electrochemical Separations of Actinide/Fission Products via the Control of Nucleation and Growth of Electrodeposits. Final Report

This is the final report to the NEUP regarding the “Advanced Electrochemical Separations of Actinide/Fission products via the Control of Nucleation and Growth of Electrodeposits” project. This report describes the project activities conducted during the period October 01, 2016 through September 30, 2019. The report includes a brief statement of the project objectives and approach, followed by the details of the performed research activities and suggestion for the future work. It is well known that according to current knowledge the pyroprocessing of spent nuclear fuels invariably leads to the dendritic form of electrodeposits, which from technological point of view is undesirable for many reasons. The aim of this research was to study and preclude the formation of electrodeposits with dendritic morphology (of essential importance to FC-1 Program). This project is characterized with four novel study categories: (1) exploring electrodeposit morphology control by controlling the electrochemical deposition reaction rate, (2) the use of rotating disk electrode technique to distinguish the electrodeposition control modes (reaction rate vs. diffusion), (3) morphology studies of lanthanides (La) and actinides (U) in characteristic stages of nucleation and growth, (4) exploring the need for separation of electrochemical cell in compartments for cathodic and anodic reactions. All these study categories were applied on two distinct electrolyte systems (a) high temperature ionic liquids, i.e. molten LiCl-KCl salts, and (b) room temperature ionic liquids (compounds exclusively made of organic cations, and organic/inorganic anions). Because the quality (dense and smooth) of electrodeposits is a direct function of the electrochemical reduction rate (slower the better), the major expectation was that addition of organic molecules would provide the desired slowing down of electroreduction effect, avoiding the undesired dependence on mass transfer. What was learned is summarized in seven key areas.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗

Elucidating the Influence of Metal Surface Composition on Organic Adsorbate Binding Using Active Particle Dynamics

The adsorption strengths of organic compounds on metal surfaces are sensitive to the metal composition, and they play a central role in many catalytic reactions, helping to control the coverage of the reactant and altering the overall reaction rate. While adsorption energies are straightforward to measure and calculate in vacuum and gas-phase environments, adsorption energetics can be dramatically altered by the presence of solvent in liquid-phase reactions. However, the effects of metal composition on binding strengths in a liquid environment are less well understood, primarily due to the difficulty of accurate in situ measurements of organic binding on metal surfaces in the liquid phase. Here, we utilize the motion of active particles in water to probe the adsorption energies of an organic adsorbate (furfural) on a range of metal surfaces (pure Pd, pure Pt, and four PdAu alloy compositions) to elucidate the effect of metal composition. Janus particles with catalytic caps of particular metal compositions all exhibited active motion resulting from consumption of H 2 O 2 ; adsorbate binding was inferred through the decrease in the velocity of active motion and was modeled by a Langmuir adsorption isotherm. The measured adsorption affinities were used to extract the adsorption enthalpy of furfural on the different metals. The Pd surface was found to bind furfural more strongly than the Pt surface by some 10 kJ/mol. Furthermore, the adsorption of furfural on the alloys was found to increase monotonically in magnitude with Pd content. As a result, the data reported herein aid the development of accurate understanding of organic adsorption in the presence of solvent and the role of the metal surface in tuning adsorption strengths to optimize catalytic processes in the liquid phase.

36 MATERIALS SCIENCE↗

Controlled Synthesis of SnO 2 Nanocrystals with Tunable Band Gaps

Tin(IV) oxide nanocrystals (SnO 2 NCs) have significant potential in various applications, with their performance closely related to their band gap. The band gap is influenced by the size and shape of the NCs, which can be precisely controlled by adjusting reaction conditions. In this study, we present deliberately designed synthesis protocols to produce high-quality SnO 2 NCs with tunable band gaps using different methods. Key factors affecting the synthesis include control of the oxidizing agent, reaction temperature, solvent selection, and reaction time optimization. The resulting NCs were characterized by using TEM, XRD, XPS, and optical spectroscopy. Notably, SnO 2 NCs synthesized by controlling the oxidizing agent (air injection) in a hot organic solution were smaller in size and exhibited abundant oxygen vacancies. In contrast, extending the reaction time or using ethanol as a solvent in hydrothermal systems facilitated larger spherical or rod-like SnO 2 NCs with fewer oxygen vacancies. Further analysis of the band gap and valence band maximum energy revealed that the abundant vacancies in SnO 2 NCs synthesized with the air-controlled hot organic solution method resulted in a narrower band gap and an upshifted valence band. These synthetic strategies illustrate the potential for deliberately designing SnO 2 NCs with optimized electronic structures for various applications.

band gap↗

Analysis and prediction of reaction kinetics using the degree of rate control

“Degree of rate control” (DRC) analysis provides a quantitative approach for analysing the kinetics of multi-step reaction mechanisms that has been widely applied to both heterogeneous and homogeneous catalysis research, as well as electrocatalysis. The DRC of any given transition state or intermediate is defined as a partial derivative such that it approximately equals the fractional increase in net rate to the product of interest per differential decrease in its standard-state free energy for that species (÷RT), holding constant the standard-state free energies of all other transition states and intermediates. Even very complex mechanisms usually have only a few species with non-zero DRCs and are thus the species whose interactions with the catalyst most strongly affect the net rate. These key DRC values thus offer a simple and intuitive route to optimize catalyst materials, especially with the assistance of computational methods like density functional theory (DFT). These high-DRC species are also the species whose energetics must be most accurately measured or calculated to achieve an accurate kinetic model for any reaction mechanism. In simple cases with a single “rate-determining step”, the DRC for its transition state (TS) is + 1. Catalyst-bound intermediates, on the other hand, often have negative DRCs equal to a small integer times their fractional occupancy of catalyst sites. The apparent activation energy equals a weighted average of the standard-state enthalpies (relative to reactants) of all the species (intermediates, transition states and products) in the reaction mechanism, each weighted by its DRC (+RT). It has been shown that the apparent transfer coefficient in electrocatalysis, an inverted form of the Tafel slope, is a weighted average of the number of electrons transferred to generate each intermediate or product species in the mechanism, weighted again by the DRC. Quantitative analysis of kinetic isotope effects (KIEs, or the ratio of net rates for different reactant isotopes) in complex mechanisms has shown that the logarithm of the KIE equals the weighted average over all species in the mechanism of the difference between the two isotopes in their standard-state free energies (÷RT), again weighted by the DRC. The reaction orders with respect to fluid-phase concentrations of reactants, products and intermediates have also been proven to be directly related to DRCs. Thus, there are numerous experimental observables which equate to short linear combinations of DRCs, so that combinations of experimental measurements might provide access to DRC values. Since its invention for transition states in 1994 and its generalization to include intermediates in 2009, the DRC has thus far mainly been calculated for microkinetic models based either entirely on DFT or on DFT with the key energies (i.e., those for high-DRC species) being fine-tuned to match experiments. The relationships summarized in this work provide new opportunities for using experiments earlier in the development and optimization of microkinetic models that require input from computational catalysis.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗