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Nanoscale oxide layer protected semiconductor photoelectrodes show enhanced stability and performance for solar fuels generation, although the mechanism for the performance enhancement remains unclear due to a lack of understanding of the microscopic interfacial field and its effects. Here, we directly probe the interfacial fields at p-GaP electrodes protected by n-TiO2 and its effect on charge carriers by transient reflectance spectroscopy. Increasing the TiO2 layer thickness from 0 to 35 nm increases the field in the GaP depletion region, enhancing the rate and efficiency of interfacial electron transfer from the GaP to TiO2 on the ps time scale as well as retarding interfacial recombination on the microsecond time scale. This study demonstrates a general method for providing a microscopic view of the photogenerated charge carrier's pathway and loss mechanisms from the bulk of the electrode to the long-lived separated charge at the interface that ultimately drives the photoelectrochemical reactions.

Interfacial electron transfer (IET) probabilities and rates between CdSe and CdS nanocrystals of diameters 2, 4, and 6 nm, and an anthraquinone dimer (AQ2H+), have been studied. To account for elec...

Bis(2-chloroethyl) sulfide or sulfur mustard (HD) is one of the highest-tonnage chemical warfare agents and one that is highly persistent in the environment. For decontamination, selective oxidation of HD to the substantially less toxic sulfoxide is crucial. We report here a solvent-free, solid, robust catalyst comprising hydrophobic salts of tribromide and nitrate, copper(II) nitrate hydrate, and a solid acid (NafionTM) for selective sulfoxidation using only ambient air at room temperature. This system rapidly removes HD as a neat liquid or a vapor. The mechanisms of these aerobic decontamination reactions are complex, and studies confirm reversible formation of a key intermediate, the bromosulfonium ion, and the role of Cu(II). The latter increases the rate four-fold by increasing the equilibrium concentration of bromosulfonium during turnover. Cu(II) also provides a colorimetric detection capability. Without HD, the solid is green, and with HD, it is brown. Bromine K-edge XANES and EXAFS studies confirm regeneration of tribromide under catalytic conditions. Diffuse reflectance infrared Fourier transform spectroscopy shows absorption of HD vapor and selective conversion to the desired sulfoxide, HDO, at the gas–solid interface. Decontamination of sulfur-containing chemical warfare agents can be achieved through selective, air-based oxidation. Here a solid, solvent-free catalyst for aerobic oxidative decontamination of sulfur mustard is reported.

The counterions of polyoxometalates (POMs) impact properties and applications of this growing class of inorganic clusters. Here, we used density functional theory (DFT) to elucidate the impact of fully hydrated alkali metal cations on the geometry, electronic structure, and chemical properties of the polyoxotungstate anion [PW12O40]3-. The calculations show that the HOMO of the free anion [PW12O40]3- is a linear combination of the 2p AOs of the bridging oxygens, and the first few LUMOs are the 5d orbitals of the tungsten atoms. The S0→ S1 electron excitation, near 3 eV, is associated with the O(2p) → W(5d) transition. Anion/cation complexation leads to formation of [PW12O40]3-[M+(H2O)16]3 ion-pair complexes, where with the increase of atomic number of M, the M+(H2O)16 cluster releases several water molecules and interacts strongly with the polyoxometalate anion. For M = Li, Na and K, [PW12O40]3-[M+(H2O)16]3 is characterized as a "hydrated" ion-pair complex. However, for M = Rb and Cs, it is a "contact" ion-pair complex, where the strong anion-cation interaction makes it a better electron acceptor than the "hydrated" ion-pair complexes. Remarkably, the electronic excitations in the visible part of the absorption spectrum of these complexes are predominantly solvent-to-POM charge transfer transitions (i.e. intermolecular CT). The ratio of the number of intermolecular charge transfer transitions to the number of O(2p)-to-W(5d) valence (i.e. intramolecular) transitions increases with the increasing atomic number of the alkali metals.

A series of heterogeneous catalysts based on phosphomolybdic salts with different metals in counter or substituent positions, LnPMo12O40 (L = Al3+, Fe3+, Cr3+, Ti4+, Zr4+ and Zn2+, abbreviated as LPMo12) and HxPMo11LO39 (L = Zn2+, Cr3+, Fe3+, Al3+, Ti4+, for Ti4+; the amount of O is 40, abbreviated as HPMo11L), were prepared using a simple calcination treatment and were evaluated in aerobic oxidation of glycerol. After calcination at about 250 °C for 4 h, homogeneous LnPMo12O40·nH2O and HxPMo11LO39·mH2O became heterogeneous. Their specific surface areas were also enhanced as well as their activity and reusability. Similar to the homogeneous catalysts, AlPMo12 treated at 400 °C (AlPMo12-400) was found to be the most active catalyst in glycerol oxidation to lactic acid, with 96.1% yield and 98.6% conversion at 60 °C for 5 h with 1 MPa of O2; it gave a turnover number (TON; TON = [LA]/[catalyst]) of 2.4 × 102, higher than that of AlPMo12-250 treated at 250 °C (TON = 2.0 × 102). AlPMo12 behaved as a heterogeneous catalyst in glycerol oxidation and could be reused at least 12 times. AlPMo12-400 also performed well in crude glycerol oxidation, affording lactic acid yields as high as 86.8%.

Protection from and removal of chemical warfare agents (CWAs) from the environment remains a global goal. Activated charcoal, metal oxides, metal organic frameworks (MOFs), polyoxometalates (POMs) and reactive polymers have all been investigated for CWA removal. Composite polymeric materials are rapidly gaining traction as versatile building blocks for personal protective equipment (PPE) and catalytic devices. Polymers are inexpensive to produce and easily engineered into a wide range of materials including films, electro-spun fibers, mixed-matrix membranes/reactors, and other forms. When containing reactive side-chains, hydrolysis catalysts, and/or oxidative catalysts polymeric devices are primed for CWA decontamination. In this review, recent advances in reactive polymeric materials for CWA removal are summarized. To aid in comparing the effectiveness of the different solid catalysts, particular attention is paid to the stoichiometric ratio of reactive species to toxic substrate (CWA or CWA simulant).

Materials that both sequester chemical warfare agents (CWAs) and then catalytically decontaminate the entrapped CWAs are highly sought. This article reports such a system for air-based catalytic removal of the sulfur mustard (HD) simulant, 2-chloroethyl ethyl sulfide (CEES). Hypercrosslinked polymers (HCPs) sequester CEES, and an HCP-embedded oxidation system comprising tribromide, nitrate, and acid (NOxBrxH+) simultaneously catalyzes the aerobic and selective, oxidative conversion of the entrapped CEES to the desired far less-toxic sulfoxide under ambient conditions (air and temperature). (NOxBrxH+) has been incorporated into three HCPs, a fluorobenzene HCP (HCP-F), a methylated HCP (HCP-M), and an HCP with acidic moieties (HCP-A). HCP-A acts as both an absorbing material and a catalytic component due to its acidic side chains. All three HCP/NOxBrxH+ systems work rapidly under these optimally mild conditions. No light or added oxidants are required. The HCP/NOxBrxH+ systems are recyclable.

Nanoscale oxide layer protected semiconductor photoelectrodes show enhanced stability and performance for solar fuels generation, although the mechanism for the performance enhancement remains unclear due to a lack of understanding of the microscopic interfacial field and its effects. Here, we directly probe the interfacial fields at p-GaP electrodes protected by n-TiO

Mixed 3d-metal oxides are some of the most promising water oxidation catalysts (WOCs), but it is very difficult to know the active site structures and thus structure-catalytic activity correlations at the molecular level in such insoluble materials. This study reports a molecular water oxidation catalyst, [Co2Ni2(PW9O34)2]10- (Co2Ni2P2), that constitutes a molecular model of the heterogeneous WOC, cobalt-nickel oxide. Both Co2Ni2P2 and its isostructural analogue, [Co4(PW9O34)2]10- (Co4P2), have the same CoO5(H2O) active sites but Co2Ni2P2 is an order of magnitude faster than Co4P2. Co2Ni2P2 is prepared by a new synthesis, and both the location and percent occupancy of Co and Ni in Co2Ni2P2 (Co outside and Ni inside the central belt are >97% for each) is confirmed by multiwavelength synchrotron X-radiation anomalous dispersion scattering (synchrotron XRAS), a technique applied for the first time to such complexes. Density functional theory (DFT) studies predicated and reveal that Co4P2 and Co2Ni2P2 have greatly altered frontier orbitals, while stopped-flow kinetic studies and DFT calculations indicate that water oxidation by both complexes follows analogous multi-step mechanisms, including Co-OOH formation, with the energetics of most steps being lower for Co2Ni2P2 than for Co4P2.

The demand for reform in energy production and reduction in carbon dioxide emission lays emphasis on the development and mechanistic study of photocatalytic water splitting photoelectrodes. Understanding the bottlenecks of the photoelectrochemical (PEC) conversion efficiency in photocatalytic water splitting systems is essential to impart guidance to material selection and photoelectrode design. Absorbed photon-to-current conversion efficiency (APCE), which represents the fraction of absorbed photons converted into electrons in photocurrents, is the key metric in the evaluation of PEC performance. A large portion of studies have reported an illumination fluence-independent APCE under continuous wave (CW) illumination with power densities around 10~100 mW/cm2, whereas few cases have reported the fluence-dependent APCE performance. In the present work, we observe a rarely reported fluence-dependent APCE performance in the p-GaAs/5 nm TiO2/2 nm Pt photocathode under a low-excitation intensity regime. The APCE reaches close to unity at 0.005 mW/cm2 and gradually decreases to 60% at 1 mW/cm2. However, a theoretical explanation for this fluence-dependent APCE reduction is not established. We develop a steady-state kinetic model with the least parameters to decipher such a phenomenon. In this model, the surface electrons in the GaAs/TiO2/Pt photocathode are consumed by three pathways: 1) transferred to redox active species in solution via the conduction band, 2) trapped into surface states that serve as the reaction sites, which competes with the electron-hole recombination at such surface states, and 3) directly recombining with the surface holes. The APCE decrease at higher illumination intensity is attributed to the increasing recombination between the GaAs holes and the TiO2 electrons caused by the saturation of surface sites. A detailed parametric survey shows that the interfacial electron transfer rate, ks, from the trap states to solution redox species is the only parameter that tunes the extent of the APCE reduction. With higher ks, APCE displays a smaller reduction in response to increasing fluences. Thus, the fluence-dependent feature of the APCE is an indicator that the surface reaction is the efficiency-determining step. Through this work, we establish a kinetic model for the semiconductor photoelectrode with catalyst layer and successfully explain the cause to the fluence-dependent APCE performance. These significant findings can be universally applied to address the function of catalytic layer (nanoparticle or molecular) and surface modifier in hybrid photoelectrode systems that exhibit fluence-dependent APCE.

The water oxidation reaction is considered as the bottleneck and rate-determining step in photoelectrochemical water-splitting processes. To achieve efficient water oxidation on the photoanode, sufficient built-in potential on semiconductors under light illumination needs to be maintained. Herein, we introduced the in-situ Electric Field Induced Second Harmonic Generation (EFISH) technique to probe the change of built-in potential under the water oxidation reaction process of a single-crystal rutile TiO2 photoanode. Under the dark condition, parabolic relation between the second harmonic generation signal and applied potential is observed in TiO2 depletion region. Under UV illumination, the EFISH signal decreased compared to dark at the photocurrent saturation potential region. Change in the signal is attributed to the decrease of built-in potential in TiO2. The amount of built-in potential change increased with light illumination and saturated at 520 mV under 15mW/cm2 360 nm UV illumination in an unbuffered solution. This light-induced band edge unpinning effect can be explained by the accumulation of holes at surface states and surface protons at Helmholtz layer that are generated under illumination. Kinetic isotope EFISH experiments suggest that proton-coupled electron transfer (PCET) is the rate-determining step in water oxidation process. Screening of built-in potential under illumination can be mitigated by adding buffer ions, suggesting that the rate-determining step can be altered by changing the solution microenvironment.

Developing novel and more efficient filters for chemical warfare agent (CWA) decomposition remains an important challenge for modern technology due to the continuous threat those weapons present in...

Glycerol is a co-product in the manufacture of biodiesel from vegetable oil and animal fat. A significant increase in biodiesel production results in adramatic overproduction of glycerol. The oxidation of glycerol to lactic acid (LA) by O2 is considered to be a promising technology to increase the value of glycerol. Studies on the fundamental reaction mechanism vary for different catalytic systems. In this work we have chosen Keggin polyoxometalates (POMs) as an exemplary family of water-soluble hydrolytically and oxidatively-stable, fully-inorganic complexes with transition-metal and non-transition-metal counter-cations (M in the formula, MPMo12O40, where M = H+, K+, Zn2+, Cu2+, Al3+, Cr3+, Fe3+ in appropriate number to counterbalance the 3- charge of PMo12O403−). Reactions involving hydrogen peroxide, an intermediate during glycerol oxidation, produce free radicals. A kinetic analysis reveals that the radical chain length under typical conditions is about 20–30. After minimal optimization, the yield of LA was 88% at glycerol conversion > 97% (1.0 M aqueous solution of glycerol, 4.0 mM of the Al salt of PMo12O403−, henceforth “AlPMo”, 10 bar O2 at 60 °C, reaction time 6.0 h).

We report computational studies of (O,O-dimethyl)-(O-4-nitrophenyl)-phosphate (DMNP) and (O,O-dimethyl)-(O-phenyl)-phosphate (DMPP) decomposition by the Zr-substituted Polyoxometalate {α-PW11O39Zr(μ-OH)(H2O)}4−, which has been recently shown to be a catalytic active species in the reaction of (Et2NH2)8[{α-PW11O39Zr(μ-OH)(H2O)}2]·7H2O with nerve agents. We studied two possible mechanisms of this reaction described as “hydrolysis first” and “OH-transfer first”. Both reaction pathways are initiated from the same pre-reaction complex (H2O)-(OH)-POM-(nerve agent). The “hydrolysis first” pathway starts by the concerted dissociation of the adsorbed water molecule and nucleophilic addition of the resulting OH group to the nerve agent. Conversely, the “OH-transfer first” pathway starts by nucleophilic addition of the Zr-coordinated OH ligand to the phosphorus of the nerve agent simulant. Calculations show that the “OH-transfer first” pathway exhibits a lower energy barrier for the decomposition of DMPP by ZrPOM. Thus, the presence of a hydroxo ligand in the coordination sphere of Zr(IV) introduces a mechanism switch from “hydrolysis first” [which was recently reported for the Sarin (GB) decomposition mechanism by the hexaniobate POM Cs8Nb6O19] to “OH-transfer first”. These findings imply that the pH of the catalytic solution could play a critical role and potentially control the mechanism of nerve agent and simulant decomposition by polyoxometalates. We also predict and corroborate that the presence of a strong electron-withdrawing para-substituent in the substrate phenyl group accelerates this reaction: DMNP decomposition by ZrPOM occurs with a smaller rate-limiting energy barrier. The calculations reveal several factors of the DMNP decomposition by the Zr(IV)-substituted polyoxometalates that provide design elements of Zr-based materials (including MOFs and POMs) for catalytic CWA decomposition under ambient conditions.

Tafel analysis of electrocatalysts is essential in their characterization. This paper analyzes the application of Tafel-like analysis to the four-electron nonelectrochemical oxidation of water by the stoichiometric homogeneous 1-electron oxidant [Ru(bpy)3]3+ to dioxygen catalyzed by homogeneous catalysts, [Ru4O4(OH)2(H2O)4(&gamma, SiW10O36)2]10&minus, (Ru4POM) and [Co4(H2O)2(PW9O34)2]10&ndash, (Co4POM). These complexes have slow electron exchange rates with electrodes due to the Frumkin effect, which precludes the use of known electrochemical methods to obtain Tafel plots at ionic strengths lower than 0.5 M. The application of an electron transfer catalyst, [Ru(bpy)3]3+/2+, increases the rates between the Ru4POM and electrode, but a traditional Tafel analysis of such a complex system is precluded due to a lack of appropriate theoretical models for 4-electron processes. Here, we develop a theoretical framework and experimental procedures for a Tafel-like analysis of Ru4POM and Co4POM, using a stoichiometric molecular oxidant [Ru(bpy)3]3+. The dependence of turnover frequency (TOF) as a function of electrochemical solution potential created by the [Ru(bpy)3]3+/[Ru(bpy)3]2+ redox couple (an analog of the Tafel plot) was obtained from kinetics data and interpreted based on the suggested reaction mechanism.

The threat of chemical warfare agents (CWAs), assured by their ease of synthesis and effectiveness as a terrorizing weapon, will persist long after the once-tremendous stockpiles in the U.S. and elsewhere are finally destroyed. As such, soldier and civilian protection, battlefield decontamination, and environmental remediation from CWAs remain top national security priorities. New chemical approaches for the fast and complete destruction of CWAs have been an active field of research for many decades, and new technologies have generated immense interest. In particular, our research team and others have shown metal-organic frameworks (MOFs) and polyoxometalates (POMs) to be active for sequestering CWAs and even catalyzing the rapid hydrolysis of agents. In this Forum Article, we highlight recent advancements made in the understanding and evaluation of POMs and Zr-based MOFs as CWA decontamination materials. Specifically, our aim is to bridge the gap between controlled, solution-phase laboratory studies and real-world or battlefield-like conditions by examining agent-material interactions at the gas-solid interface utilizing a multimodal experimental and computational approach. Herein, we report our progress in addressing the following research goals: (1) elucidating molecular-level mechanisms of the adsorption, diffusion, and reaction of CWA and CWA simulants within a series of Zr-based MOFs, such as UiO-66, MOF-808, and NU-1000, and POMs, including Cs8Nb6O19 and (Et2NH2)8[(α-PW11O39Zr(μ-OH)(H2O))2]·7H2O, (2) probing the effects that common ambient gases, such as CO2, SO2, and NO2, have on the efficacy of the MOF and POM materials for CWA destruction, and (3) using CWA simulant results to develop hypotheses for live agent chemistry. Key hypotheses are then tested with targeted live agent studies. Overall, our collaborative effort has provided insight into the fundamental aspects of agent-material interactions and revealed strategies for new catalyst development.

This document was prepared by the MicroBooNE Collaboration using the resources of the Fermi National Accelerator Laboratory (Fermilab), a U.S. Department of Energy, Office of Science, HEP User Facility. Fermilab is managed by Fermi Research Alliance, LLC (FRA), acting under Contract No. DE-AC02-07CH11359. MicroBooNE is supported by the following: the U.S. Department of Energy, Office of Science, Offices of High Energy Physics and Nuclear Physics; the U.S. National Science Foundation; the Swiss National Science Foundation; the Science and Technology Facilities Council (STFC), part of the United Kingdom Research and Innovation; and The Royal Society (United Kingdom). Additional support for the laser calibration system and cosmic ray tagger was provided by the Albert Einstein Center for Fundamental Physics, Bern, Switzerland., We present upper limits on the production of heavy neutral leptons (HNLs) decaying to μπ pairs using data collected with the MicroBooNE liquid-argon time projection chamber (TPC) operating at Fermilab. This search is the first of its kind performed in a liquid-argon TPC. We use data collected in 2017 and 2018 corresponding to an exposure of 2.0×1020 protons on target from the Fermilab Booster Neutrino Beam, which produces mainly muon neutrinos with an average energy of ≈800 MeV. HNLs with higher mass are expected to have a longer time of flight to the liquid-argon TPC than Standard Model neutrinos. The data are therefore recorded with a dedicated trigger configured to detect HNL decays that occur after the neutrino spill reaches the detector. We set upper limits at the 90% confidence level on the element |Uμ4|2 of the extended PMNS mixing matrix in the range |Uμ4|2, Fermilab is managed by Fermi Research Alliance, LLC (FRA), acting under Contract No. DE-AC02-07CH11359

Our group reported that the polyoxometalate Na10[Co4V2W18O68]·26H2O (Co4V2) is an active water oxidation catalyst and provided characterization of this system (J. Am. Chem. Soc. 2014, 136 (26), 9268). Two recent publications called into question the stability of Co4V2, one noting the miss-assignment of a 51V NMR peak (Inorg. Chem. 2016, 55 (11), 5343) and another providing additional stability studies (ACS Catal., 2017, 7 (1), 7). We report here solution studies that further clarify stability limitations in this system by locating the correct 51V NMR resonance of Co4V2 and the other V-containing species present. Furthermore, we demonstrate that the observed catalytic activity cannot be explained simply by Co(II)aq, but arises from multiple active WOC species in solution. Key points about investigating such complex equilibrating aqueous catalyst systems are addressed.

The generation of five types of Ln(III)-containing tungstotellurates(VI), dimeric (DMAH)12Na2[H10(WO2){Ln(H2O)5(TeW18O65)}2]·nH2O (abbreviated as {Ln2Te2W37}; Ln = Eu, Gd, or Tb; DMAH = dimethylamm...

The Zr-containing polyoxometalates (POMs), including (Et2NH2)8{[α-PW11O39Zr(μ–OH)(H2O)]2}·7H2O (1), effectively catalyze the hydrolysis of nerve agent simulants at near-neutral pH. Analogous Zr-containing heterogeneous systems are much-studied and effective nerve-agent hydrolysis catalysts, but due to their heterogeneous nature, it is very challenging to know the exact structure of the catalytic sites during turnover and to clarify at the molecular level the elementary mechanistic processes. Here, under homogeneous conditions, hydrolysis rates of the nerve-agent simulant methyl paraoxon catalyzed by 1 are examined as a function of pH, ionic strength, catalyst, and substrate concentrations. In addition, the specific effect of three commonly used buffers is examined, revealing that acetate functions as a co-catalyst, phosphate inhibits hydrolytic activity, and 2-(N-morpholino)ethanesulfonic acid (MES) has no effect on the hydrolysis rate. Spectroscopic (31P nuclear magnetic resonance) and computational stud...

Two isostructural cobalt containing polyoxometalate water oxidation catalysts, [Co4(H2O)2(α-PW9O34)2]10− (Co4P2) and [Co4(H2O)2(α-VW9O34)2]10− (Co4V2), exhibit large differences in their catalytic performance. The substitution of phosphorus centers in Co4P2 with redox-active vanadium centers in Co4V2 leads to electronic structure modifications. Evidence for the significance of the vanadium centers to catalysis, predicted by theory, was found from soft X-ray absorption (XAS) and resonant inelastic X-ray scattering (RIXS). The XAS and RIXS spectra determine the electronic structure of the cobalt and vanadium sites in the pre-reaction state of both Co4V2 and Co4P2. High-energy resolution RIXS results reveal that Co4V2 possesses a smaller ligand field within the tetra-cobalt core and a cobalt-to-vanadium charge transfer band. The differences in electronic structures offer insights into the enhanced catalysis of Co4V2.

The impact of ambient gas molecules (X), NO2, CO2 and SO2 on the structure, stability and decontamination activity of Cs8Nb6O19 polyoxometalate was studied computationally and experimentally. It was found that Cs8Nb6O19 absorbs these molecules more strongly than it adsorbs water and Sarin (GB) and that these interactions hinder nerve agent decontamination. The impacts of diamagnetic CO2 and SO2 molecules on polyoxoniobate Cs8Nb6O19 were fundamentally different from that of NO2 radical. At ambient temperatures, weak coordination of the first NO2 radical to Cs8Nb6O19 conferred partial radical character on the polyoxoniobate and promoted stronger coordination of the second NO2 adsorbent to form a stable diamagnetic Cs8Nb6O19/(NO2)2 species. Moreover, at low temperatures, NO2 radicals formed stable dinitrogen tetraoxide (N2O4) that weakly interacted with Cs8Nb6O19. It was found that both in the absence and presence of ambient gas molecules, GB decontamination by the Cs8Nb6O19 species proceeds via general base hydrolysis involving: (a) the adsorption of water and the nerve agent on Cs8Nb6O19/(X), (b) concerted hydrolysis of a water molecule on a basic oxygen atom of the polyoxoniobate and nucleophilic addition of the nascent OH group to the phosphorus center of Sarin, and (c) rapid reorganization of the formed pentacoordinated-phosphorus intermediate, followed by dissociation of either HF or isopropanol and formation of POM-bound isopropyl methyl phosphonic acid (i-MPA) or methyl phosphonofluoridic acid (MPFA), respectively. The presence of the ambient gas molecules increases the energy of the intermediate stationary points relative to the asymptote of the reactants and slightly increases the hydrolysis barrier. These changes closely correlate with the Cs8Nb6O19–X complexation energy. The most energetically stable intermediates of the GB hydrolysis and decontamination reaction were found to be Cs8Nb6O19/X-MPFA-(i-POH) and Cs8Nb6O19/X-(i-MPA)-HF both in the absence and presence of ambient gas molecules. The high stability of these intermediates is due to, in part, the strong hydrogen bonding between the adsorbates and the protonated [Cs8Nb6O19/X/H]+-core. Desorption of HF or/and (i-POH) and regeneration of the catalyst required deprotonation of the [Cs8Nb6O19/X/H]+-core and protonation of the phosphonic acids i-MPA and MPFA. This catalyst regeneration is shown to be a highly endothermic process, which is the rate-limiting step of the GB hydrolysis and decontamination reaction both in the absence and presence of ambient gas molecules.

Polyoxometalates (POMs) and in particularly Keggin heteropolytungstates are much studied and commercially important catalysts for dioxygen-based oxidation processes. The rate-limiting step in many POM-catalyzed O2-based oxidations is reoxidation of the reduced POM by O2. We report here that this reoxidation process, as represented by the one-electron-reduced Keggin complexes POMred (α-PW12O404– and α-SiVW11O406–) reacting with O2, is efficiently catalyzed by a combination of copper and iron complexes. The reaction kinetics and mechanism have been comprehensively studied in sulfate and phosphate buffer at pH 1.8. The catalytic pathway includes a reversible reaction between Cu(II) and Fe(II), followed by a fast oxidation of POMred by Fe(III) and Cu(I) by O2 to regenerate Fe(II) and Cu(II). The proposed reaction mechanism quantitatively describes the experimental kinetic curves over a wide range of experimental conditions. Since the oxidized forms, α-PW12O403– and α-SiVW11O405–, are far better oxidants of or...

A mechanism of water oxidation catalyzed by the carbon-free tetra-Co containing polyoxometalates [Co4(H2O)2(PW9O34)2]10− (PCo4) and [Co4(H2O)2(VW9O34)2]10− (VCo4) is elucidated by DFT calculations. Computational analysis for PCo4 suggests that a first PCET step may proceed via a sequential electron-then-proton transfer (ET + PT) pathway and leads to one electron oxidize species S1 (POM-CoIII OH). In contrast, the second PCET, which controls the potential required to form POM-CoIII O active species S2 is clearly a concerted process. The overall S0 → S2 transformation is estimated to require less than 1.48 V and 1.62 V applied potential at pH = 8 for PCo4 and VCo4 anions, respectively. At operando conditions, with the presence of a buffer and with an applied potential above the threshold potential the two H-atom removal could take place via concerted pathways. These steps represent rapid pre-equilibria before the rate determining step, which corresponds to the O O bond formation. The key chemical step occurs via nucleophilic attack of an external water molecule to intermediate S2. We assume that this step governs the kinetics of the reaction. Comparison of the calculated energetics and electronic structures of intermediate species in the PCo4 and VCo4 catalyzed water oxidation cycle shows that coupling of d orbitals of V and Co atoms in VCo4 increases the oxidation potential of the Co-center. The orbital coupling is also responsible for the higher catalytic activity of VCo4 because it increases the electrophilicity of CoIII O moiety in the key S2 species.

The mechanism and kinetics of interactions between dimethyl methylphosphonate (DMMP), a key chemical warfare agent (CWA) simulant, and Zr6-based metal organic frameworks (MOFs) have been investigated with in situ infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), powder X-ray diffraction (PXRD), and DFT calculations. DMMP was found to adsorb molecularly to UiO-66 through the formation of hydrogen bonds between the phosphoryl oxygen and the free hydroxyl groups associated with Zr6 nodes on the surface of crystallites and not within the bulk MOF structure. Unlike UiO-66, the infrared spectra for UiO-67 and MOF-808, recorded during DMMP exposure, suggest that uptake occurs through both physisorption and chemisorption. The XPS spectra of MOF-808 zirconium 3d electrons reveal a charge redistribution following exposure to DMMP. In addition, analysis of the phosphorus 2p electrons following exposure and thermal annealing to 600 K indicates that two types of stable phosphorus-containing species e...

Hydrocarbon diffusion and binding within porous molecular networks are critical to catalysis, separations, and purification technologies. Fundamental insight into n-butane uptake and mobility within a new class of materials for separations, metal–organic frameworks (MOFs), has been gained through in situ infrared spectroscopy. These ultrahigh vacuum (UHV) based measurements revealed that adsorption of n-butane within UiO-66 proceeds through the formation of hydrogen bonds between the alkane molecules and hydroxyl groups located at the inorganic node of UiO-66. Modeling the gas transport of n-butane with Fick’s second law yielded diffusion coefficients at several temperatures. The Arrhenius parameter for the activation energy of diffusion was found to be 21.0 ± 1.2 kJ/mol. These studies have further shown that the rate-determining step for diffusion is likely the dissociation of n-butane from a binding site located within the tetrahedral pores of UiO-66.

A novel double-anion complex, H13[(CH3)4N]12[PNb12O40(VVO)2⋅(VIV4O12)2]⋅22 H2O (1), based on bicapped polyoxoniobate and tetranuclear polyoxovanadate was synthesized, characterized by routine techniques and used in the catalytic decontamination of chemical warfare agents. Under mild conditions, 1 catalyzes both hydrolysis of the nerve agent simulant, diethyl cyanophosphonate (DECP) and selective oxidation of the sulfur mustard simulant, 2-chloroethyl ethyl sulfide (CEES). In the oxidative decontamination system 100 % CEES was transformed selectively to nontoxic 2-chloroethyl ethyl sulfoxide and vinyl ethyl sulfoxide using nearly stoichiometric 3 % aqueous H2O2 with a turnover frequency (TOF) of 16 000 h−1. Importantly, the catalytic activity is maintained even after ten recycles and CEES is completely decontaminated in 3 mins without formation of the highly toxic sulfone by-product. A three-step oxidative mechanism is proposed.

Zr-based metal organic frameworks (MOFs) have been recently shown to be among the fastest catalysts of nerve-agent hydrolysis in solution. We report a detailed study of the adsorption and decomposition of a nerve-agent simulant, dimethyl methylphosphonate (DMMP), on UiO-66, UiO-67, MOF-808, and NU-1000 using synchrotron-based X-ray powder diffraction, X-ray absorption, and infrared spectroscopy, which reveals key aspects of the reaction mechanism. The diffraction measurements indicate that all four MOFs adsorb DMMP (introduced at atmospheric pressures through a flow of helium or air) within the pore space. In addition, the combination of X-ray absorption and infrared spectra suggests direct coordination of DMMP to the Zr6 cores of all MOFs, which ultimately leads to decomposition to phosphonate products. These experimental probes into the mechanism of adsorption and decomposition of chemical warfare agent simulants on Zr-based MOFs open new opportunities in rational design of new and superior decontaminat...

We report the synthesis and characterization of a new class of organic/inorganic hybrid polymers composed of covalently-bound 1,3,5-benzenetricarboxamide linkers and anionic polyoxovanadate clusters with varying counter-cations. These materials form gels within seconds upon contact with polar aprotic organic liquids and catalyze the degradation of odorants and toxic molecules under mild conditions including aerobic oxidation of thiols, hydrogen peroxide-catalyzed oxidation of sulfides, and hydrolysis of organophosphate chemical warfare agent analogues.

Cosmic ray (CR) interactions can be a challenging source of background for neutrino oscillation and cross-section measurements in surface detectors. We present methods for CR rejection in measurements of charged-current quasielastic-like (CCQE-like) neutrino interactions, with a muon and a proton in the final state, measured using liquid argon time projection chambers (LArTPCs). Using a sample of cosmic data collected with the MicroBooNE detector, mixed with simulated neutrino scattering events, a set of event selection criteria is developed that produces an event sample with minimal contribution from CR background. Depending on the selection criteria used a purity between 50 and 80% can be achieved with a signal selection efficiency between 50 and 25%, with higher purity coming at the expense of lower efficiency. While using a specific dataset and selection criteria values optimized for the MicroBooNE detector, the concepts presented here are generic and can be adapted for various studies of exclusive $$\nu _{\mu }$$ ν μ CCQE interactions in LArTPCs.

Peroxidation of glycerol has been carried out in a polyoxometalate (POM)-based microfluidic reactor, which was fabricated on a capillary by using a layer-by-layer strategy. Lactic acid (LA) is produced selectively in high yield with a TOF as high as 20 000 h-1 , compared to a TOF of 200 h-1 in batch mode. This POM microfluidic reactor is readily prepared, scalable, highly stable, reusable, and also potentially applicable to selective oxidation of other bio-wastes.

In this work, we use wavefunction engineering by varying the size of Quantum Dots (QDs) and tuning the delocalization (or diffuseness) of frontier orbitals of an acceptor molecule to modulate charge transfer dynamics at the QD/molecule interface. For this purpose, we apply our recently developed bulk-adjusted linear combination of atomic orbitals (BA-LCAO) approach for nanostructures and a density functional theory (DFT) for the acceptor molecules. These electronic structure calculations, combined with extensive molecular dynamics simulations using a fragmented molecular mechanics (FraMM) force field, reveal intimate details of charge transfer across the QD/Acceptor interface. For the spherical wurtzite-(CdSe)201 and (CdSe)693 nanostructures, as model QDs with respective 2.8 and 4.1 nm diameters, and anthraquinone-2,3-dicarboxylic acid and its derivatives with the 7-OH, 7-OF, 10-BH, and 10-CH2 substituents, as model molecular acceptors, we find that (1) both the electron donating and withdrawing groups greatly enhance hole transfer by means of diffusing the acceptor HOMO; (2) electron transfer is affected only by the electron donating groups; (3) solvent effects are largely negligible for the orbital overlaps, and (4) consistent with spatial confinement theories, the electron density of the smaller QD penetrates farther into the vacuum than the corresponding density of the larger QD leading to stronger coupling with the acceptor. These findings suggest that (a) one can effectively control charge transfer across the QD/molecule interface by either changing the size of the QD or by tuning diffuseness of frontier orbitals of the acceptor molecule and (b) the combination of the recently developed BA-LCAO approach for QDs with a DFT for the acceptor molecules, facilitated by the use of the FraMM force field and extensive molecular dynamics simulations, provide qualitatively accurate description of charge transfer dynamics at the QD/acceptor interface.

The LArIAT liquid argon time projection chamber, placed in a tertiary beam of charged particles at the Fermilab Test Beam Facility, has collected large samples of pions, muons, electrons, protons, and kaons in the momentum range 0∼30-0140 MeV/c. This paper describes the main aspects of the detector and beamline, and also reports on calibrations performed for the detector and beamline components.

Two novel polyoxovanadate (POV)-containing fluorosurfactants, each with two hydrophobic fluorinated "tails" and one nanosized, hydrophilic, rigid POV "head group", are synthesized for the first time. They self-assemble into spherical, bilayer vesicles in acetonitrile/water mixed solvents, as evidenced by systemic studies using laser light scattering (LLS) and electron microscopy techniques. The vesicle sizes demonstrate dynamic change over different solvent compositions mainly as a result of the solvent swelling of the fluorocarbon chains, although the charge number on the POVs changes over the solvent polarity as well.

We present a detailed mechanism for the hydrolysis of Sarin catalyzed by Cs8Nb6O19 obtained using electronic structure calculations. The initial steps of the reaction involve the adsorption of water and Sarin on the hexaniobate catalyst via nonbonding interactions. Dissociation of the coordinated water molecule generates a hydroxide ion that adds nucleophilically to the coadsorbed Sarin molecule in a concerted manner, following a general base catalysis mechanism. The addition of OH– to the nerve agent generates a trigonal bipyramidal pentacoordinated phosphorus intermediate that subsequently undergoes facile dissociation forming either HF or isopropanol and a corresponding phosphonic acid. The rate-determining step of the overall reaction is found to be the dissociation of water on the catalyst in concert with the nucleophilic addition of the nascent OH– to the nerve agent. The calculated barrier for this step is considerably smaller than that measured for bulk base hydrolysis. This work represents a blue...

Copper-based complexes have been largely neglected as potential water reduction catalysts. This article reports the synthesis and characterization of a tetra-copper-containing polyoxotungstate, Na3K7[Cu4(H2O)2(B-α-PW9O34)2]·30H2O (Na3K7-Cu4P2). Cu4P2 is a water-compatible catalyst for efficient visible-light-driven hydrogen evolution when coupled to (4,4'-di-tert-butyl-2,2'-dipyridyl)-bis(2-phenylpyridine(1H))-iridium(III) hexafluorophosphate ([Ir(ppy)2(dtbbpy)][PF6]) as a light absorber and triethanolamine (TEOA) as sacrificial electron donor. Under minimally optimized conditions, a turnover number (TON) of ∼1270 per Cu4P2 catalyst is obtained after 5 h of irradiation (light-emitting diode; λ = 455 nm; 20 mW); a photochemical quantum efficiency of as high as 15.9% is achieved. Both oxidative and reductive quenching pathways are observed by measuring the luminescence intensity of excited state [Ir(ppy)2(dtbbpy)](+*) in the presence of Cu4P2 or TEOA, respectively. Many stability studies (e.g., UV-vis absorption, FT-IR, dynamic light scattering, transmission electron microscopy, and scanning electron microscopy/energy-dispersive X-ray spectroscopy) show that catalyst Cu4P2 undergoes slow decomposition under turnover conditions; however, both the starting Cu4P2 as well as its molecular decomposition products are the dominant catalytically active species for H2 evolution not Cu or CuOx particles. Considering the high abundance and low cost of copper, the present work provides considerations for the design and synthesis of efficient, molecular, water-compatible Cu-based water reduction catalysts.

A series of hetero-bimetallic transition metal-substituted polyoxometalates (TMSPs) were synthesized based on the Co(II)-centered ligand [Co(II)W11O39](10-). The eight complex series, [Co(II)(M(x)OHy)W11O39]((12-x-y)-) (M(x)OHy = V(IV)O, Cr(III)(OH2), Mn(II)(OH2), Fe(III)(OH2), Co(II)(OH2), Ni(II)(OH2), Cu(II)(OH2), Zn(II)(OH2)), of which six are reported for the first time, was synthesized starting from [Co(III)W11O39](9-) and studied using spectroscopic, electrochemical, and computational techniques to evaluate the influence of substituted transition metals on the photodynamics of the metal-to-polyoxometalate charge transfer (MPCT) transition. The bimetallic complexes all show higher visible light absorption than the plenary [Co(II)W12O40](6-) and demonstrate the same MPCT transition as the plenary complex, but they have shorter excited-state lifetimes (sub-300 ps in aqueous media). The decreased lifetimes are rationalized on the basis of nonradiative relaxation due to coordinating aqua ligands, increased interaction with cations due to increased negative charge, and the energy gap law, with the strongest single factor appearing to be the charge on the anion. The most promising results are from the Cr- and Fe-substituted systems, which retain excited-state lifetimes at least 50% of that of [Co(II)W12O40](6-) while more than tripling the absorbance at 400 nm.

A wide range of chemical warfare agents and their simulants are catalytically decontaminated by a new one-dimensional polymeric polyniobate (P-PONb), K12 [Ti2 O2 ][GeNb12 O40 ]⋅19 H2 O (KGeNb) under mild conditions and in the dark. Uniquely, KGeNb facilitates hydrolysis of nerve agents Sarin (GB) and Soman (GD) (and their less reactive simulants, dimethyl methylphosphonate (DMMP)) as well as mustard (HD) in both liquid and gas phases at ambient temperature and in the absence of neutralizing bases or illumination. Three lines of evidence establish that KGeNb removes DMMP, and thus likely GB/GD, by general base catalysis: a) the k(H2 O)/k(D2 O) solvent isotope effect is 1.4; b) the rate law (hydrolysis at the same pH depends on the amount of P-PONb present); and c) hydroxide is far less active against the above simulants at the same pH than the P-PONbs themselves, a critical control experiment.

Highly efficient electrocatalytic oxidation of ethanol and methanol has been achieved using the ruthenium-containing polyoxometalate molecular catalyst, [{Ru4O4(OH)2(H2O)4}(γ-SiW10O36)2](10-) ([1(γ-SiW10O36)2](10-)). Voltammetric studies with dissolved and surface-confined forms of [1(γ-SiW10O36)2](10-) suggest that the oxidized forms of 1 can act as active catalysts for alcohol oxidation in both aqueous (over a wide pH range covering acidic, neutral, and alkaline) and alcohol media. Under these conditions, the initial form of 1 also exhibits considerable reactivity, especially in neutral solution containing 1.0 M NaNO3. To identify the oxidation products, preparative scale bulk electrolysis experiments were undertaken. The products detected by NMR, gas chromatography (GC), and GC-mass spectrometry from oxidation of ethanol are 1,1-diethoxyethane and ethyl acetate formed from condensation of acetaldehyde or acetic acid with excess ethanol. Similarly, the oxidation of methanol generates formaldehyde and formic acid which then condense with methanol to form dimethoxymethane and methyl formate, respectively. These results demonstrate that electrocatalytic oxidation of ethanol and methanol occurs via two- and four-electron oxidation processes to yield aldehydes and acids. The total faradaic efficiencies of electrocatalytic oxidation of both alcohols exceed 94%. The numbers of aldehyde and acid products per catalyst were also calculated and compared with the literature reported values. The results suggest that 1 is one of the most active molecular electrocatalysts for methanol and ethanol oxidation.