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Diazaphospholenes have emerged as a promising class of metal-free hydride donors and have been implemented as molecular catalysts in several reduction reactions. Recent studies have also verified their radical reactivity as hydrogen atom donors. Experimental quantification of the hydricities and electrochemical properties of this unique class of hydrides has been limited by their sensitivity towards oxidation in open air and moist environments. Here, we implement quantum chemical density functional theory calculations to analyze the electrochemical catalytic cycle of diazaphospholenes in acetonitrile. We report computed hydricities, reduction potentials, pKa values, and bond dissociation free energies (BDFEs) for 64 P-based hydridic catalysts generated by functionalizing 8 main structures with 8 different electron donating/withdrawing groups. Our results demonstrate that a wide range of hydricities (29–66 kcal mol−1) and BDFEs (58–81 kcal mol−1) are attainable by functionalizing diazaphospholenes. Compared to the more common carbon-based hydrides, diazaphospholenes are predicted to require less negative reduction potentials to electrochemically regenerate hydrides with an equivalent hydridic strength, indicating their higher energy efficiency in the tradeoff between thermodynamic ability and reduction potential. We show that the tradeoff between the reducing ability and the energetic cost of regeneration can be optimized by varying the BDFE and the reorganization energy associated with hydride transfer (λHT), where lower BDFE and λHT correspond to more efficient catalysts. Aromatic phosphorus hydrides with predicted BDFEs of ∼62 kcal mol−1 and λHT's of ∼20 kcal mol−1 are found to require less negative reduction potentials than dihydropyridines and benzimidazoles with predicted BDFEs of ∼68 and ∼84 kcal mol−1 and λHT's of ∼40 and ∼50 kcal mol−1, respectively.

We report a trinuclear copper(II) complex, [(DAM)Cu3(μ3-O)][Cl]4 (1, DAM = dodecaaza macrotetracycle), as a homogeneous electrocatalyst for water oxidation to dioxygen in phosphate-buffered solutions at pH 7.0, 8.1, and 11.5. Electrocatalytic water oxidation at pH 7 occurs at an overpotential of 550 mV with a turnover frequency of ∼19 s–1 at 1.5 V vs NHE. Controlled potential electrolysis (CPE) experiments at pH 11.5 over 3 h at 1.2 V and at pH 8.1 for 40 min at 1.37 V vs NHE confirm the evolution of dioxygen with Faradaic efficiencies of 81% and 45%, respectively. Rinse tests conducted after CPE studies provide evidence for the homogeneous nature of the catalysis. The linear dependence of the current density on the catalyst concentration indicates a likely first-order dependence on the Cu precatalyst 1, while kinetic isotope studies (H2O versus D2O) point to involvement of a proton in or preceding the rate-determining step. Rotating ring-disk electrode measurements at pH 8.1 and 11.2 show no evidence of H2O2 formation and support selectivity to form dioxygen. Freeze-quench electron paramagnetic resonance studies during electrolysis provide evidence for the formation of a molecular copper intermediate. Experimental and computational studies support a key role of the phosphate as an acceptor base. Moreover, density functional theory calculations highlight the importance of second-sphere interactions and the role of the nitrogen-based ligands to facilitate proton transfer processes.

The Chevrel phase (CP) is a class of molybdenum chalcogenides that exhibit compelling properties for next-generation battery materials, electrocatalysts, and other energy applications. Despite their promise, CPs are underexplored, with only ∼100 compounds synthesized to date due to the challenge of identifying synthesizable phases. We present an interpretable machine-learned descriptor (Hδ) that rapidly and accurately estimates decomposition enthalpy (ΔHd) to assess CP stability. To develop Hδ, we first used density functional theory to compute ΔHd for 438 CP compositions. We then generated >560 000 descriptors with the new machine learning method SIFT, which provides an easy-to-use approach for developing accurate and interpretable chemical models. From a set of >200 000 compositions, we identified 48 501 CPs that Hδ predicts are synthesizable based on the criterion that ΔHd < 65 meV/atom, which was obtained as a statistical boundary from 67 experimentally synthesized CPs. The set of candidate CPs includes 2307 CP tellurides, an underexplored CP subset with a predicted preference for channel site occupation by cation intercalants that is rare among CPs. We successfully synthesized five of five novel CP tellurides attempted from this set and confirmed their preference for channel site occupation. Our joint computational and experimental approach for developing and validating screening tools that enable the rapid identification of synthesizable materials within a sparse class is likely transferable to other materials families to accelerate their discovery.

Objectives This study demonstrates a spontaneous redox polymerization process located at the adhesive-composite interface that precedes photocure of the composite with the intent to improve bonding. Methods An aromatic amine and benzoyl peroxide redox initiator system was partitioned between BAPO-photoinitiated BisGMA/HEMA adhesive and BisGMA/TEGDMA resin-composites. The composite was placed on the photocured adhesive layer with a brief delay before photopolymerization of the composite layer. Micro-tensile bond strength between the adhesive and composite was assessed in comparison with the non-redox active control materials. Results The presence of amine or peroxide in these resins without the redox initiation contribution enhanced both the rate and the final conversion of the BAPO-based photopolymerizations. Control formulations using redox-only initiation showed active polymer formation starting at approximately 30 s when physical mixing of the redox components was involved; however, simply by waiting 60 s between composite placement and photocure provided adequate time for passive interfacial diffusion of benzoyl peroxide from the pre-cured adhesive into the overlaid aromatic amine-containing composite such that a sufficient degree of redox initiated interfacial polymerization occurred prior to the composite photocure. The result was a significant increase in the adhesive to composite micro-tensile bond strength with the failure site moved away from the mainly interfacial failure noted for the control. Significance The stress-free autonomous pre-conversion of a redox-initiated thin film of composite that then provides a compositionally homogeneous interface for composite photopolymerization offers a means to enhance at least short-term bond strength between the adhesive and composite phases during restorative placement.

The deposition of alumina ALD films on Li ion battery cathode particles is known to enhance the cycling stability of lithium ion batteries fabricated from those coated particles. It is commonly ass...

Metal oxide-supported Au catalysts, particularly those with small Au nanoparticles, catalyze a variety of reactions including low-temperature CO oxidation and selective hydrogenation of alkynes. Ho...

Solar thermochemical hydrogen production from water is a path towards a carbon-free sustainable hydrogen economy. Here, isothermal on-sun hydrogen production is demonstrated using active iron aluminate (hercynite) particles contained in dual fluidized bed reactors. The two fluidized beds were held in a single cavity solar receiver that was heated with a 10 kW high flux solar furnace. During 8 h of on-sun testing, 5.3 L of H2 were generated with an average productivity of 597 µmol H2/g using an intermittent process with optimized redox cycle times. Redox cycling was performed isothermally and continuously with equivalent oxidation and reduction times producing 547 µmol H2/g over two cycles. These results show excellent agreement with the H2 production measured in an 800X scaled-down electrically heated laboratory stagnation flow reactor and surpass on-sun H2 production of current benchmark materials. The effect of environmental variability on the incident solar radiation and hydrogen production is evaluated during on-sun testing. Finally, a discussion of the important factors for scalability of the process to commercial applications is provided, highlighting the importance of robust containment and active materials. This work links current materials research and development with commercial implementation in an on-sun process and demonstrates the viability of solar thermochemical hydrogen production that leverages continuous isothermal redox cycling.

Free-radical polymerizations are used for a wide range of applications but are detrimentally impacted by the presence of oxygen. Zinc phthalocyanines have been previously used as singlet oxygen generators to excite radical-consuming ground-state triplet oxygen into its less reactive singlet state prior to photoexcitation of a photoinitiator. We report for the first time that polymerization can be achieved via irradiation of UV band of phthalocyanine and that photosensitization and photoinitiation can be independently achieved via irradiation of its two distinct absorption bands to reduce oxygen inhibition and initiate polymerization without the need for additional treatment. We propose a mechanism for this unique photoinitiation phenomenon and verify its feasibility via computational and experimental approaches. This new class of dual-photosensitive molecules shows promising utility in applications that are adversely impacted by the presence of oxygen, such as coatings and stereolithography.

Ultra-thin, oxidation-resistant, pinhole-free alumina coatings were deposited on silicon carbide (SiC) particles using atomic layer deposition (ALD), and investigated as environmental barrier coatings (EBCs) for high-temperature steam oxidation. The uncoated and alumina coated SiC particles were exposed to high-temperature steam for 20 h at temperatures ranging from 1050 °C to 1150 °C to assess oxidation rates. The kinetic triplet (activation energy, rate constant, and pre-exponential factor) for each sample was determined using the D4 kinetic model (representing 3-dimensional diffusion) which was demonstrated to be accurate via the isothermal isoconversional method. Activation energies for oxidation of 247.4 ± 0.1 kJ/mol, 250.6 ± 0.5 kJ/mol, and 253.0 ± 1.0 kJ/mol were calculated for uncoated SiC, SiC coated with a 5 nm alumina film, and SiC coated with a 10 nm alumina film, respectively. Reaction rate data for the oxidation reaction of uncoated SiC and SiC coated with a 10 nm film indicate that the ALD coating reduces the rate of oxidation of SiC by up to a factor of 5 for steam oxidation at temperatures between 1050 °C and 1150 °C. This is the first investigation of steam oxidation kinetics for SiC coated with an ALD deposited EBC. The results of this study indicate that ultra-thin alumina films deposited by ALD act as effective EBCs with oxidation resistance comparable to CVD films that are orders of magnitude thicker.

Despite the powerful nature of the aza-Michael reaction for generating C–N linkages and bioactive moieties, the bis-Michael addition of 1° amines remains ineffective for the synthesis of functional, step-growth polymers due to the drastic reduction in reactivity of the resulting 2° amine mono-addition adduct. In this study, a wide range of commercial hydrazides are shown to effectively undergo the bis-Michael reaction with divinyl sulfone (DVS) and 1,6-hexanediol diacrylate (HDA) under catalyst-free, thermal conditions to afford moderate to high molecular weight polymers with M(n) = 3.8–34.5 kg mol(-1). The hydrazide-Michael reactions exhibit two distinctive, conversion-dependent kinetic regimes that are 2(nd)-order overall, in contrast to the 3(rd)-order nature of amines previously reported. The mono-addition rate constant was found to be 37-fold greater than that of the bis-addition at 80 °C for the reaction between benzhydrazide and DVS. A significant majority (12 of 15) of the hydrazide derivatives used here show excellent bis-Michael reactivity and achieve >97% conversions after 5 days. This behavior is consistent with calculations that show minimal variance of electron density on the N-nucleophile among the derivatives studied. Reactivity differences between hydrazides and hexylamine are also explored. Overall, the difference in reactivity between hydrazides and amines is attributed to the adjacent nitrogen atom in hydrazides that acts as an efficient hydrogen-bond donor that facilitates intramolecular proton-transfer following the formation of the zwitterion intermediate. This effect not only activates the Michael acceptor but also coordinates with additional Michael acceptors to form an intermolecular reactant complex.

Selective reduction of CO

Electrochemical routes provide an attractive alternative to the Haber-Bosch process for cheaper and more efficient ammonia (NH

Selective reduction of CO2 to formate represents an ongoing challenge in photoelectrocatalysis. To provide mechanistic insights, we investigate the kinetics of hydride transfer (HT) from a series of metal-free hydride donors to CO2. The observed dependence of experimental and calculated HT barriers on the thermodynamic driving force was modeled using the Marcus hydride transfer formalism to obtain the insights into the effect of reorganization energies on the reaction kinetics. Our results indicate that, even if the most ideal hydride donor were discovered, the HT to CO2 would exhibit sluggish kinetics (less than 100 turnovers at 0.1 eV driving force), indicating that the conventional HT may not be an appropriate mechanism for Solar conversion of CO2 to formate. We propose that the conventional HT mechanism should not be considered for CO2 reduction catalysis and argue that the orthogonal HT mechanism, previously proposed to address thermodynamic limitations of this reaction, may also lead to lower kinetic barriers for CO2 reduction to formate.

Selective reduction of CO2 to formate represents an ongoing challenge in photoelectrocatalysis. To provide mechanistic insights, we investigate the kinetics of hydride transfer (HT) from a series of metal-free hydride donors to CO2. The observed dependence of experimental and calculated HT barriers on the thermodynamic driving force was modeled using the Marcus hydride transfer formalism to obtain the insights into the effect of reorganization energies on the reaction kinetics. Our results indicate that, even if the most ideal hydride donor were discovered, the HT to CO2 would exhibit sluggish kinetics (less than 100 turnovers at 0.1 eV driving force), indicating that the conventional HT may not be an appropriate mechanism for Solar conversion of CO2 to formate. We propose that the conventional HT mechanism should not be considered for CO2 reduction catalysis and argue that the orthogonal HT mechanism, previously proposed to address thermodynamic limitations of this reaction, may also lead to lower kinetic barriers for CO2 reduction to formate.

Amine–peroxide redox polymerization (APRP) is the prevalent method for producing radical-based polymers in the many industrial and medical applications where light or heat activation is impractical. We recently developed a detailed description of the APRP initiation process through a combined computational and experimental effort to show that APRP proceeds through S(N)2 attack by the amine on the peroxide, followed by the rate-determining homolysis of the resulting intermediate. Using this new mechanistic understanding, a variety of peroxides were computationally predicted to initiate APRP with fast kinetics. In particular, the rate of APRP initiation can be improved by radical and anion stabilization through increased π-electron conjugation or by increasing the electrophilicity of the peroxy bond through the addition of electron-withdrawing groups. On the other hand, the addition of electron-donating groups lowered the initiation rate. These design principles enabled the computational prediction of several new peroxides that exhibited improved initiation rates over the commonly used benzoyl peroxide. For example, the addition of nitro groups (NO(2)) to the para positions of benzoyl peroxide resulted in a theoretical radical generation rate of 1.9 × 10(−9) s(−1), which is ~150 times faster than the 1.3 × 10(−11) s(−1) radical generation rate observed with unsubstituted benzoyl peroxide. These accelerated kinetics enabled the development of a redox-based direct-writing process that exploited the extremely rapid reactivity of an optimized redox pair with a custom inkjet printer, capable of printing custom shapes from polymerizing resins without heat or light. Furthermore, the application of more rapid APRP kinetics could enable the acceleration of existing industrial processes, make new industrial manufacturing methods possible, and improve APRP compatibility with biomedical applications through reduced initiator concentrations that still produce rapid polymerization rates.

In this work, we implement a facile microwave-assisted synthesis method to yield three binary Chevrel-Phase chalcogenides (Mo

Chemical looping is a promising approach for improving the energy efficiency of many industrial chemical processes. However, a major limitation of modern chemical looping technologies is the lack of suitable active materials to mediate the involved subreactions. Identification of suitable materials has been historically limited by the scarcity of high-temperature (> 600 °C) thermochemical data to evaluate candidate materials. An accurate thermodynamic approach is demonstrated here to rapidly identify active materials which is applicable to a wide variety of chemical looping chemistries. Application of this analysis to chemical looping combustion correctly classifies 17/17 experimentally studied redox materials by their viability and identifies over 1,300 promising yet previously unstudied active materials. This approach is further demonstrated by analyzing redox pairs for mediating a novel chemical looping process for producing pure SO2 from raw sulfur and air which could provide a more efficient and lower emission route to sulfuric acid. 12 promising redox materials for this process are identified, two of which are supported by previous experimental studies of their individual oxidation and reduction reactions. This approach provides the necessary foundation for connecting process design with high-throughput materials discovery to accelerate the innovation and development of a wide-range of chemical looping technologies.

This work examines the relative reactivities of Re

Highly dispersed cobalt atoms were deposited on porous alumina particles using atomic layer deposition (ALD) with a CoCp

Metastable inorganic materials with unique properties are important in many practical applications, but their synthesis is often challenging. In physics, epitaxial stabilization, also known as pseudomorphic growth, is used to synthesize metastable polymorphs, but usually only as very thin films and on expensive single-crystal substrates. In chemistry, the templated growth of inorganic solid-state materials on self-assembled monolayers of organic molecules is reported. Bridging these two fields, here, we show that the synthesis of metastable polymorphs is possible up to large film thickness on amorphous substrates covered with thin inorganic seed layers that serve as templates. The stabilization of a 500-nm-thick metastable wurtzite (WZ) $\mathrm{Mn}\mathrm{Te}$ film by a 5-nm-thick $\mathrm{Zn}\mathrm{Te}$ seed layer sputtered on amorphous glass substrates is experimentally demonstrated. Theoretical calculations explain this experimental observation by the small WZ polymorph energy relative to that of the ground-state nickeline (NC) structure of $\mathrm{Mn}\mathrm{Te}$ and a large lattice constant difference of the two. The resulting metastable WZ-$\mathrm{Mn}\mathrm{Te}$ polymorph exhibits a wide band gap of 2.7 eV and a low hole density of ${10}^{12}\phantom{\rule{0.25em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$, which is relevant to optoelectronic applications. These properties are in sharp contrast to those of the narrow-band-gap highly doped NC-$\mathrm{Mn}\mathrm{Te}$ with 1.3 eV band gap and ${10}^{19}\phantom{\rule{0.25em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ hole density. The difference in hole density is due to the calculated difference in the formation energy of manganese vacancy acceptor defects. Overall, these results suggest that templated growth on amorphous substrates with seed layers can be used to synthesize metastable polymorphs of other materials, without the need for expensive single-crystal substrates.

We report a novel metal-free chemical reduction of CO2 by a recyclable benzimidazole-based organo-hydride, whose choice was guided by quantum chemical calculations. Notably, benzimidazole-based hydride donors rival the hydride-donating abilities of noble-metal-based hydrides such as [Ru(tpy)(bpy)H]+ and [Pt(depe)2H]+. Chemical CO2 reduction to the formate anion (HCOO-) was carried out in the absence of biological enzymes, a sacrificial Lewis acid, or a base to activate the substrate or reductant. 13CO2 experiments confirmed the formation of H13COO- by CO2 reduction with the formate product characterized by 1H NMR and 13C NMR spectroscopy and ESI-MS. The highest formate yield of 66% was obtained in the presence of potassium tetrafluoroborate under mild conditions. The likely role of exogenous salt additives in this reaction is to stabilize and shift the equilibrium toward the ionic products. After CO2 reduction, the benzimidazole-based hydride donor was quantitatively oxidized to its aromatic benzimidazolium cation, establishing its recyclability. In addition, we electrochemically reduced the benzimidazolium cation to its organo-hydride form in quantitative yield, demonstrating its potential for electrocatalytic CO2 reduction. These results serve as a proof of concept for the electrocatalytic reduction of CO2 by sustainable, recyclable, and metal-free organo-hydrides.

Increasing atmospheric CO2 concentration and dwindling fossil fuel supply necessitate the search for efficient methods for CO2 conversion to fuels. Assorted studies have shown pyridine and its derivatives capable of (photo)electrochemically reducing CO2 to methanol, and some mechanistic interpretations have been proposed. Here, we analyze the thermodynamic and kinetic aspects of the efficacy of pyridines as hydride-donating catalytic reagents that transfer hydrides via their dihydropyridinic form. We investigate both the effects of functionalizing pyridinic derivatives with electron-donating and electron-withdrawing groups on hydride-transfer catalyst strength, assessed via their hydricity (thermodynamic ability) and nucleophilicity (kinetic ability), and catalyst recyclability, assessed via their reduction potential. We find that pyridines substituted with electron-donating groups have stronger hydride-donating ability (having lower hydricity and larger nucleophilicity values), but are less efficiently recycled (having more negative reduction potentials). In contrast, pyridines substituted with electron-withdrawing groups are more efficiently recycled, but are weaker hydride donors. Functional group modification favorably tunes hydride strength or efficiency, but not both. We attribute this problematic coupling between the strength and recyclability of pyridinic hydrides to their aromatic nature and suggest several avenues for overcoming this difficulty.

Classical materials readily switch phases (solid to fluid or fluid to gas) upon changes in pressure or heat; however, subsequent reversion of the stimulus returns the material to their original phase. Covalently cross-linked polymer networks, which are solids that do not flow when strained, do not change phase even upon changes in temperature and pressure. However, upon the addition of dynamic cross-links, they become stimuli responsive, capable of switching phase from solid to fluid, but quickly returning to the solid state once the stimulus is removed. Reported here is the first material capable of a bistable switching of phase. A permanent solid to fluid transition or vice versa is demonstrated at room temperature, with inherent, spatiotemporal control over this switch in either direction triggered by exposure to light., Polymers cross-linked with dynamic bonds can switch the phase from solid to fluid upon stimulus but return quickly to the solid state once the stimulus is removed. Here the authors report a light triggered permanent solid to fluid transition at room temperature with inherent spatiotemporal control in either direction

The effect of amines on the kinetics and efficacy of radical-mediated thiol-ene coupling (TEC) reactions was investigated. By varying the thiol reactant and amine additive, it was shown that amines retard thiyl radical-mediated reactions when the amine is adequately basic enough to deprotonate the thiol affording the thiolate anion, e.g., when the weakly basic amine tetramethylethylenediamine was incorporated in the TEC reaction between butyl 2-mercaptoacetate and an allyl ether at 5 mol %, the final conversion was reduced from quantitative to

We propose a method to approximate the kinetic properties of hydride donor species by relating the nucleophilicity (N) of a hydride to the activation free energy ΔG⧧ of its corresponding hydride transfer reaction. N is a kinetic parameter related to the hydride transfer rate constant that quantifies a nucleophilic hydridic species’ tendency to donate. Our method estimates N using quantum chemical calculations to compute ΔG⧧ for hydride transfers from hydride donors to CO2 in solution. A linear correlation for each class of hydrides is then established between experimentally determined N values and the computationally predicted ΔG⧧; this relationship can then be used to predict nucleophilicity for different hydride donors within each class. This approach is employed to determine N for four different classes of hydride donors: two organic (carbon-based and benzimidazole-based) and two inorganic (boron and silicon) hydride classes. We argue that silicon and boron hydrides are driven by the formation of the m...

The exchange of thiolates and thiols has long been held as a nearly ideal reaction in dynamic covalent chemistry. The ability for the reaction to proceed smoothly in neutral aqueous media has propelled its widespread use in biochemistry, however, far fewer applications and studies have been directed towards its use in material science which primarily is performed in organic media. Herein, we present the exploration of this dynamic exchange in both small molecule and polymer settings with a wide sampling of thiols, thioesters, organic bases, and nucleophilic catalysts in various organic solvents. Effects of the character of the thiol and thioester, pKa or nucleophilicity of the catalyst, and heat on the reaction were investigated. The mechanism regarding the previously unexplored effectiveness of nucelophilic catalysts, such as quinuclidine or DABCO, to affect the thiol-thioester exchange was also explored. Finally, the use of the thiol-thioester exchange in a network polymer to reduce applied stresses or change shape of the material following polymerization was shown and the ability of basic and nucleophilic catalysts to promote these effects were benchmarked. The influence of polarity in these networks was also explored, with the rate of exchange shown to be easily tuned by the addition of diluents with varying polarities. Presented here is a so-called “user's guide” to the thiol-thioester exchange; we hope that this guide is instructive to practitioners in the field of material science which seek to utilize the thiol-thioester exchange in both linear and network polymers.

Metal-free hydrides are of increasing research interest due to their roles in recent scientific advances in catalysis, such as hydrogen activation with frustrated Lewis pairs and electrocatalytic CO2 reduction with pyridinium and other aromatic cations. The structural design of hydrides for specific applications necessitates the correct description of their thermodynamic and kinetic prowess using reliable parameters - thermodynamic hydricity (ΔGH-) and nucleophilicity (N). This review summarizes reported experimental and calculated hydricity values for more than 200 metal-free hydride donors, including carbon-, boron-, nitrogen- and silicon-based hydrides. We describe different experimental and computational methods used to obtain these thermodynamic and kinetic parameters. Furthermore, tabulated data on metal-free hydrides are discussed in terms of structure-property relationships, relevance to catalysis and contemporary limitations for replacing transition-metal hydride catalysts. Finally, several selected applications of metal-free hydrides in catalysis are described, including photosynthetic CO2 reduction and hydrogen activation with frustrated Lewis pairs.

Hydrogen environmental barrier coatings reduce hydrogen diffusion and concomitant hydrogen embrittlement of materials such as those used in nuclear and fuel cell applications where hydrogen is used as a fuel source. In this work, atomic layer deposition (ALD) was used to coat substrates with boron nitride (BN) films of approximately 6, 8, and 15 nm thicknesses. Differential thermal analysis of the coated samples in hydrogen gas showed resistance to reaction with hydrogen to at least 1713 K. Diffusion of atomic hydrogen into the hexagonal BN (0 0 1) surface and between sheets as well as material stability were computationally studied using density functional theory. A high activation energy of 3.25 eV was calculated for atomic hydrogen diffusion into the (0 0 1) hexagonal BN surface through a sheet. However, lower activation energies of 1.35 eV, 1.11 eV, and 0.12 eV were computed for unique hydrogen diffusion pathways between sheets, suggesting that sheet orientation parallel to the substrate surface is vital for attaining desirable barrier film properties. A predicted positive nitrogen vacancy formation energy of 4.3 eV at 2773 K suggests that hexagonal BN is stable at nuclear thermal propulsion operating temperatures, and stability was confirmed experimentally up to 1773 K.

The attachment of molecular catalysts to conductive supports for the preparation of solid-state anodes is important for the development of devices for electrocatalytic water oxidation. The preparation and characterization of three molecular cyclopentadienyl iridium(III) complexes, Cp*Ir(1-pyrenyl(2-pyridyl)ethanolate-κO,κN)Cl (1) (Cp* = pentamethylcyclopentadienyl), Cp*Ir(diphenyl(2-pyridyl)methanolate-κO,κN)Cl (2), and [Cp*Ir(4-(1-pyrenyl)-2,2′-bipyridine)Cl]Cl (3), as precursors for electrochemical water oxidation catalysts, are reported. These complexes contain aromatic groups that can be attached via noncovalent π-stacking to ordered mesoporous carbon (OMC). The resulting iridium-based OMC materials (Ir-1, Ir-2, and Ir-3) were tested for electrocatalytic water oxidation leading to turnover frequencies (TOFs) of 0.9–1.6 s−1 at an overpotential of 300 mV under acidic conditions. The stability of the materials is demonstrated by electrochemical cycling and X-ray absorption spectroscopy analysis before and after catalysis. Theoretical studies on the interactions between the molecular complexes and the OMC support provide insight onto the noncovalent binding and are in agreement with the experimental loadings.

Reactivity of molecular catalysts can be controlled by organic ligands that regulate the steric and electronic properties of catalyst sites. This level of control has generally been unavailable for heterogeneous catalysts. We show that self-assembled monolayers (SAMs) on titania with tunable electronic properties provided fine control over surface reactivity. Controlling the identity of substituents on benzylphosphonic acid SAMs modulated the near-surface electrostatics, enabling regulation of the dehydration activity of 1-propanol and 1-butanol over a wide range, with activities and selectivities of the optimal catalyst far exceeding those of uncoated TiO2. The dipole moment of the adsorbed phosphonate was strongly correlated to the dehydration activity; kinetic measurements and computational modeling indicated that the interfacial electric field altered the transition-state structure and energy. Coating catalysts with SAMs having controllable charge distributions may provide a general approach to hetero...

Conflicting experimental results for the electrocatalytic reduction of CO2 to CH3OH on a glassy carbon electrode by the 6,7-dimethyl-4-hydroxy-2-mercaptopteridine have been recently reported [J. Am. Chem. Soc. 2014, 136, 14007−14010, J. Am. Chem. Soc. 2016, 138, 1017–1021]. In this connection, we have used computational chemistry to examine the issue of this molecule’s ability to act as a hydride donor to reduce CO2. We first determined that the most thermodynamically stable tautomer of this aqueous compound is its oxothione form, termed here PTE. It is argued that this species electrochemically undergoes concerted 2H+/2e– transfers to first form the kinetic product 5,8-dihydropteridine, followed by acid-catalyzed tautomerization to the thermodynamically more stable 7,8-dihydropteridine PTEH2. While the overall conversion of CO2 to CH3OH by three successive hydride and proton transfers from this most stable tautomer is computed to be exergonic by 5.1 kcal/mol, we predict high activation free energies (ΔG‡...

The nature of intramolecular charge transfer (CT) of N,N-diaryl dihydrophenazine photocatalysts (PCs) in different solvents is explored in context of their performance in organocatalyzed atom transfer radical polymerization (O-ATRP). PCs having a computationally predicted lowest energy excited state exhibiting CT character can operate a highly controlled O-ATRP in a wide range of solvent polarities, from non-polar hexanes to highly polar N,N-dimethylacetamide. For PCs having a computationally predicted lowest energy excited state not possessing CT character, their ability to operate a controlled O-ATRP is decreased. This study confirms the importance of CT character in the excited state for N,N-diaryl dihydrophenazine PCs, and a deeper understanding of the activity of CT PCs has enabled the synthesis of polymers of low dispersity (

Titanium oxide is often decorated with metal nano-particles and either serves as a catalyst support or enables photocatalytic activity. The activity of these systems degrades over time due to catalytic particle agglomeration and growth by Ostwald ripening where adatoms dissociate from metal particles, diffuse across the surface and add to other metal particles. In this work, we use density functional theory calculations to study the diffusion mechanisms of select group VIII and 1B late-transition metal adatoms commonly used in catalysis and photocatalysis (Au, Ag, Cu, Pt, Rh, Ni, Co and Fe) on the anatase TiO2(101) surface. All metal adatoms preferentially occupy the bridge site between two 2-fold-coordinated oxygen anions (O2c). Surface migration was investigated by calculating the minimum energy pathway from one bridge site to another along three pathways: two in the [010] direction along a row of surface O2c anions and one in the [10] direction between two rows of surface O2c anions. For all adatoms, migration along the [010] direction is favored over migration along the [10] direction due to closer packing of the atoms in the [010] direction and therefore stronger adatom–surface interactions. As the adatom hops along the [010] direction, it preferentially moves through a metastable OTiO structure in which the adatom partially embeds itself within the surface, with the exception of Au, which remains above the surface. The adatoms migrate with relative activation energies of: Au (0.24 eV) < Ag (0.48 eV) < Rh (0.60 eV) < Co (0.78 eV) < Pt (0.84 eV) < Ni (0.86 eV) < Cu (1.23 eV) < Fe (1.79 eV) along the favored pathway. This preference arises from the strength of adatom–surface bonding and the electronegativity difference between the metal adatom and the TiO2 surface. We found a linear correlation between the binding energy/electronegativity and the activation energy for hopping where stronger binding energies and more oxidized adatoms have higher activation energies for adatom migration. The linear correlation developed in this work enables rapid estimations of the hopping rates of other transition metal adatoms across the TiO2 surface.

Metal oxide-supported Au catalysts, particularly those with small Au nanoparticles, catalyze a variety of reactions including low-temperature CO oxidation and selective hydrogenation of alkynes. However, the facile nature of Au particle growth at even moderate temperatures poses significant challenges to maintaining catalyst activity under reaction conditions. Here, we present a method to reduce the rate of sintering and coke formation in TiO

Electrochemical regeneration of organic hydrides is often hindered by the rapid dimerization of organic radicals produced as the first intermediates of these electrochemical transformations. In this work, we utilize proton-coupled electron transfer to outcompete the undesired dimerization and achieve successful hydride regenerations of two groups of organic hydrides – acridines and benzimidazoles. This work provides an analysis of the critical factors that control the regeneration pathways of organic hydrides.

Solar thermochemical ammonia (NH3) synthesis (STAS) is a potential route to produce NH3 from air, water, and concentrated sunlight. This process involves the chemical looping of an active redox pair that cycles between a metal nitride and its complementary metal oxide to yield NH3. To identify promising candidates for STAS cycles, we performed a high-throughput thermodynamic screening of 1,148 metal nitride/metal oxide pairs. This data-driven screening was based on Gibbs energies of crystalline metal oxides and nitrides at elevated temperatures, G(T), calculated using a recently introduced statistically learned descriptor and 0 K DFT formation energies tabulated in the Materials Project database. Using these predicted G(T) values, we assessed the viability of each of the STAS reactions—hydrolysis of the metal nitride, reduction of the metal oxide, and nitrogen fixation to reform the metal nitride—and analyzed a revised cycle that directly converts between metal oxides and nitrides, which alters the thermo...

Amine-peroxide redox polymerization (APRP) has been highly prevalent in industrial and medical applications since the 1950s, yet the initiation mechanism of this radical polymerization process is poorly understood so that innovations in the field are largely empirically driven and incremental. Through a combination of computational prediction and experimental analysis, we elucidate the mechanism of this important redox reaction between amines and benzoyl peroxide for the ambient production of initiating radicals. Our calculations show that APRP proceeds through SN2 attack by the amine on the peroxide but that homolysis of the resulting intermediate is the rate-determining step. We demonstrate a correlation between the computationally predicted initiating rate and the experimentally measured polymerization rate with an R2 = 0.80. The new mechanistic understanding was then applied to computationally predict amine reductant initiators with faster initiating kinetics. This led to our discovery of N-(4-methoxyphenyl)pyrrolidine (MPP) as amine reductant, which we confirmed significantly outperforms current state-of-the-art tertiary aromatic amines by ∼20-fold, making it the most efficient amine-peroxide redox initiator to date. The application of amines with superior kinetics such as MPP in APRP could greatly accelerate existing industrial processes, facilitate new industrial manufacturing methods, and improve biocompatibility in biomedical applications conducted with reduced initiator concentrations yet higher overall efficiency.

The performance of density functional theory approximations for predicting materials thermodynamics is typically assessed by comparing calculated and experimentally determined enthalpies of formation from elemental phases, ΔHf. However, a compound competes thermodynamically with both other compounds and their constituent elemental forms, and thus, the enthalpies of the decomposition reactions to these competing phases, ΔHd, determine thermodynamic stability. We evaluated the phase diagrams for 56,791 compounds to classify decomposition reactions into three types: 1. those that produce elemental phases, 2. those that produce compounds, and 3. those that produce both. This analysis shows that the decomposition into elemental forms is rarely the competing reaction that determines compound stability and that approximately two-thirds of decomposition reactions involve no elemental phases. Using experimentally reported formation enthalpies for 1012 solid compounds, we assess the accuracy of the generalized gradient approximation (GGA) (PBE) and meta-GGA (SCAN) density functionals for predicting compound stability. For 646 decomposition reactions that are not trivially the formation reaction, PBE (mean absolute difference between theory and experiment (MAD) = 70 meV/atom) and SCAN (MAD = 59 meV/atom) perform similarly, and commonly employed correction schemes using fitted elemental reference energies make only a negligible improvement (~2 meV/atom). Furthermore, for 231 reactions involving only compounds (Type 2), the agreement between SCAN, PBE, and experiment is within ~35 meV/atom and is thus comparable to the magnitude of experimental uncertainty.

Simple and interpretable data-driven descriptor accurately predicts the synthesizability of single and double perovskites., Predicting the stability of the perovskite structure remains a long-standing challenge for the discovery of new functional materials for many applications including photovoltaics and electrocatalysts. We developed an accurate, physically interpretable, and one-dimensional tolerance factor, τ, that correctly predicts 92% of compounds as perovskite or nonperovskite for an experimental dataset of 576 ABX3 materials (X = O2−, F−, Cl−, Br−, I−) using a novel data analytics approach based on SISSO (sure independence screening and sparsifying operator). τ is shown to generalize outside the training set for 1034 experimentally realized single and double perovskites (91% accuracy) and is applied to identify 23,314 new double perovskites (A2BB′X6) ranked by their probability of being stable as perovskite. This work guides experimentalists and theorists toward which perovskites are most likely to be successfully synthesized and demonstrates an approach to descriptor identification that can be extended to arbitrary applications beyond perovskite stability predictions.

The development of an economically viable solar thermochemical fuel production process relies largely on identifying redox active materials with optimized thermodynamic and kinetic properties. Iron aluminate (FeAl2O4, hercynite) and cobalt-iron aluminate (CoxFe1-xAl2O4) have both been demonstrated as viable redox-active materials for this process. However, doping with cobalt creates a tradeoff between the thermodynamics and kinetics of H2 production mediated by hercynite in which the kinetics are improved at the expense of lowering the yield. In this work, we evaluate four spinel aluminate materials with varying cobalt contents (FeAl2O4, Co0.05Fe0.95Al2O4, Co0.25Fe0.75Al2O4, and Co0.40Fe0.60Al2O4) to better understand the role of cobalt in the redox mediating properties of these materials and to quantify its effect on the thermodynamic and kinetic properties for CO2 reduction. A solid-state kinetic analysis was performed on each sample to model its CO2 reduction kinetics at temperatures ranging from 1200 °C to 1350 °C. An F1 model representative of first-order reaction kinetics was found to most accurately represent the experimental data for all materials evaluated. The computed rate constants, activation energies, and pre-exponential factors all increase with increasing cobalt content. High temperature in-situ XPS was utilized to characterize the spinel surfaces and indicated the presence of metallic states of the reduced cobalt-iron spinel, which are not present in un-doped hercynite. These species provide a new site for the CO2 reduction reaction and enhance its rate through an increased pre-exponential factor.

High-performance lithium-ion batteries require electrolytes that are stable over wide operating voltages. We used density functional theory to investigate the degradation of ethylene carbonate (EC) electrolytes activated by interactions with LiCoO2 cathode surfaces and PF5 species in the electrolyte. We report detailed mechanisms for the activation of EC ring-opening reactions by Lewis acids to form CO2, organics, or organofluorines. We find that Lewis acid–base complexation between EC and either PF5 or LiCoO2 weakens the C–O bonds of the EC ring and consequently lowers the barrier to and energy of EC ring-opening reactions. Our results predict that ring opening activated by the LiCoO2 cathode surface forms a cathode–electrolyte interphase primarily composed of an organic and organofluorine film. Simultaneous degradation of an EC molecule and PF6– forms PF5 and a surface organofluorine with an activation barrier of 1.28 eV and reaction energy of −0.26 eV. Ring opening of EC activated by the cathode to for...

Nanoscale spinel lithium manganese oxide is of interest as a high-rate cathode material for advanced battery technologies among other electrochemical applications. In this work, the synthesis of ultrathin films of spinel lithium manganese oxide (LiMn2O4) between 20 and 200 nm in thickness by room-temperature electrochemical conversion of MnO grown by atomic layer deposition (ALD) is demonstrated. The charge storage properties of LiMn2O4 thin films in electrolytes containing Li+, Na+, K+, and Mg2+ are investigated. A unified electrochemical band-diagram (UEB) analysis of LiMn2O4 informed by screened hybrid density functional theory calculations is also employed to expand on existing understanding of the underpinnings of charge storage and stability in LiMn2O4. It is shown that the incorporation of Li+ or other cations into the host manganese dioxide spinel structure (λ-MnO2) stabilizes electronic states from the conduction band which align with the known redox potentials of LiMn2O4. Furthermore, the cyclic voltammetry experiments demonstrate that up to 30% of the capacity of LiMn2O4 arises from bulk electronic charge-switching which does not require compensating cation mass transport. The hybrid ALD-electrochemical synthesis, UEB analysis, and unique charge storage mechanism described here provide a fundamental framework to guide the development of future nanoscale electrode materials for ion-incorporation charge storage.

This work investigates the use of ozone as a post-treatment of ALD-grown MnO and as a coreactant with bis(ethylcyclopentadienyl)manganese (Mn(EtCp)2) in ALD-like film growth. In situ quartz crystal microbalance measurements are used to monitor the mass changes during growth, which are coupled with ex situ materials characterization following deposition to evaluate the resulting film composition and structure. We determined that during O3 post-treatment of ALD-grown MnO, O3 oxidizes the near-surface region corresponding to a conversion of 22 Å of the MnO film to MnO2. Following oxidation by O3, exposure of Mn(EtCp)2 results in mass gains of over 300 ng/cm(2), which exceeds the expected mass gain for reaction of the Mn(EtCp)2 precursor with surface hydroxyls by over four times. We attribute this high mass gain to adsorbed Mn(EtCp)2 shedding its EtCp ligands at the surface and releasing Mn(II) ions which subsequently diffuse into the bulk film and partially reduce the oxidized film back to MnO. These Mn(EtCp)2 and O3 reactions are combined in sequential steps with (a) Mn(EtCp)2 reacting at the surface of an O-rich layer, shedding its two EtCp ligands and freeing Mn(II) to diffuse into the film followed by (b) O3 oxidizing the film surface and withdrawing Mn from the subsurface to create an O-rich layer. This deposition process results in self-limiting multilayer deposition of crystalline Mn5O8 films with a density of 4.7 g/cm(3) and an anomalously high growth rate of 5.7 Å/cycle. Mn5O8 is a metastable phase of manganese oxide which possesses an intermediate composition between the alternating MnO and MnO2 compositions of the near-surface during the Mn(EtCp)2 and O3 exposures.

Aluminum nitride (AlN) is used extensively in the semiconductor industry as a high-thermal-conductivity insulator, but its manufacture is encumbered by a tendency to degrade in the presence of water. The propensity for AlN to hydrolyze has led to its consideration as a redox material for solar thermochemical ammonia (NH3) synthesis applications where AlN would be intentionally hydrolyzed to produce NH3 and aluminum oxide (Al2O3), which could be subsequently reduced in nitrogen (N2) to reform AlN and reinitiate the NH3 synthesis cycle. No quantitative, atomistic mechanism by which AlN, and more generally, metal nitrides react with water to become oxidized and generate NH3 yet exists. In this work, we used density-functional theory (DFT) to examine the reaction mechanisms of the initial stages of AlN hydrolysis, which include: water adsorption, hydroxyl-mediated proton diffusion to form NH3, and NH3 desorption. We found activation barriers (Ea) for hydrolysis of 330 and 359 kJ/mol for the cases of minimal adsorbed water and additional adsorbed water, respectively, corroborating the high observed temperatures for the onset of steam AlN hydrolysis. We predict AlN hydrolysis to be kinetically limited by the dissociation of strong Al-N bonds required to accumulate protons on surface N atoms to form NH3. The hydrolysis mechanism we elucidate is enabled by the diffusion of protons across the AlN surface by a hydroxyl-mediated Grotthuss mechanism. A comparison between intrinsic (Ea = 331 kJ/mol) and mediated proton diffusion (Ea = 89 kJ/mol) shows that hydroxyl-mediated proton diffusion is the predominant mechanism in AlN hydrolysis. The large activation barrier for NH3 generation from AlN (Ea = 330 or 359 kJ/mol, depending on water coverage) suggests that in the design of materials for solar thermochemical ammonia synthesis, emphasis should be placed on metal nitrides with less covalent metal-nitrogen bonds and, thus, more-facile NH3 liberation.

Precise control from a metal-free catalyst Polymerization can be a rather dangerous free for all, with molecules joining randomly in chains at a chaotic pace. One of modern chemistry's great accomplishments has been the development of methods to assemble polymers in steady, orderly steps. However, order comes at a price, and often it's the need for metal catalysts that are hard to remove from the plastic product. Theriot et al. used theory to guide the design of a metal-free light-activated catalyst that offers precise control in atom transfer radical polymerization, alleviating concerns about residual metal contamination (see the Perspective by Shanmugam and Boyer). Science , this issue p. 1082 ; see also p. 1053

Ferrite spinels are metal oxides used in a wide variety of applications, many of which are controlled by the diffusion of metal cations through the metal oxide lattice. In this work, we used density functional theory (DFT) to examine the diffusion of Fe, Co, and Ni cations through the Fe3O4, CoFe2O4, and NiFe2O4 ferrite spinels. We apply DFT and crystal field theory to uncover the principles that govern cation diffusion in ferrite spinels. We found that a migrating cation hops from its initial octahedral site to a neighboring octahedral vacancy via a tetrahedral metastable intermediate separated from octahedral sites by a trigonal planar transition state (TS). The cations hop with relative activation energies of Co ≈< Fe < Ni; the ordering of the diffusion barriers is controlled by the crystal field splitting of the diffusing cation. Specifically, the barriers depend on the orbital splitting and number of electrons which must be promoted into the higher energy t2g orbitals of the tetrahedral metastable in...