Aaron M. Holder

Mechanism of homogeneous reduction of CO2 by pyridine: proton relay in aqueous solvent and aromatic stabilization.

We employ quantum chemical calculations to investigate the mechanism of homogeneous CO(2) reduction by pyridine (Py) in the Py/p-GaP system. We find that CO(2) reduction by Py commences with PyCOOH(0) formation where: (a) protonated Py (PyH(+)) is reduced to PyH(0), (b) PyH(0) then reduces CO(2) by one electron transfer (ET) via nucleophilic attack by its N lone pair on the C of CO(2), and finally (c) proton transfer (PT) from PyH(0) to CO(2) produces PyCOOH(0). The predicted enthalpic barrier for this proton-coupled ET (PCET) reaction is 45.7 kcal/mol for direct PT from PyH(0) to CO(2). However, when PT is mediated by one to three water molecules acting as a proton relay, the barrier decreases to 29.5, 20.4, and 18.5 kcal/mol, respectively. The water proton relay reduces strain in the transition state (TS) and facilitates more complete ET. For PT mediated by a three water molecule proton relay, adding water molecules to explicitly solvate the core reaction system reduces the barrier to 13.6-16.5 kcal/mol, depending on the number and configuration of the solvating waters. This agrees with the experimentally determined barrier of 16.5 ± 2.4 kcal/mol. We calculate a pK(a) for PyH(0) of 31 indicating that PT preceding ET is highly unfavorable. Moreover, we demonstrate that ET precedes PT in PyCOOH(0) formation, confirming PyH(0)s pK(a) as irrelevant for predicting PT from PyH(0) to CO(2). Furthermore, we calculate adiabatic electron affinities in aqueous solvent for CO(2), Py, and Py·CO(2) of 47.4, 37.9, and 66.3 kcal/mol respectively, indicating that the anionic complex PyCOO(-) stabilizes the anionic radicals CO(2)(-) and Py(-) to facilitate low barrier ET. As the reduction of CO(2) proceeds through ET and then PT, the pyridine ring becomes aromatic, and thus Py catalyzes CO(2) reduction by stabilizing the PCET TS and the PyCOOH(0) product through aromatic resonance stabilization. Our results suggest that Py catalyzes the homogeneous reductions of formic acid and formaldehyde en route to formation of CH(3)OH through a series of one-electron reductions analogous to the PCET reduction of CO(2) examined here, where the electrode only acts to reduce PyH(+) to PyH(0).

Reduction of CO2 to methanol catalyzed by a biomimetic organo-hydride produced from pyridine.

We use quantum chemical calculations to elucidate a viable mechanism for pyridine-catalyzed reduction of CO2 to methanol involving homogeneous catalytic steps. The first phase of the catalytic cycle involves generation of the key catalytic agent, 1,2-dihydropyridine (PyH2). First, pyridine (Py) undergoes a H(+) transfer (PT) to form pyridinium (PyH(+)), followed by an e(-) transfer (ET) to produce pyridinium radical (PyH(0)). Examples of systems to effect this ET to populate PyH(+)s LUMO (E(0)(calc) ∼ -1.3 V vs SCE) to form the solution phase PyH(0) via highly reducing electrons include the photoelectrochemical p-GaP system (E(CBM) ∼ -1.5 V vs SCE at pH 5) and the photochemical [Ru(phen)3](2+)/ascorbate system. We predict that PyH(0) undergoes further PT-ET steps to form the key closed-shell, dearomatized (PyH2) species (with the PT capable of being assisted by a negatively biased cathode). Our proposed sequential PT-ET-PT-ET mechanism for transforming Py into PyH2 is analogous to that described in the formation of related dihydropyridines. Because it is driven by its proclivity to regain aromaticity, PyH2 is a potent recyclable organo-hydride donor that mimics important aspects of the role of NADPH in the formation of C-H bonds in the photosynthetic CO2 reduction process. In particular, in the second phase of the catalytic cycle, which involves three separate reduction steps, we predict that the PyH2/Py redox couple is kinetically and thermodynamically competent in catalytically effecting hydride and proton transfers (the latter often mediated by a proton relay chain) to CO2 and its two succeeding intermediates, namely, formic acid and formaldehyde, to ultimately form CH3OH. The hydride and proton transfers for the first of these reduction steps, the homogeneous reduction of CO2, are sequential in nature (in which the formate to formic acid protonation can be assisted by a negatively biased cathode). In contrast, these transfers are coupled in each of the two subsequent homogeneous hydride and proton transfer steps to reduce formic acid and formaldehyde.We use quantum chemical calculations to elucidate a viable homogeneous mechanism for pyridine-catalyzed reduction of CO2 to methanol. In the first component of the catalytic cycle, pyridine (Py) undergoes a H+ transfer (PT) to form pyridinium (PyH+) followed by an e- transfer (ET) to produce pyridinium radical (PyH0). Examples of systems to effect this ET to populate the LUMO of PyH+(E0calc ~ -1.3V vs. SCE) to form the solution phase PyH0 via highly reducing electrons include the photo-electrochemical p-GaP system (ECBM ~ -1.5V vs. SCE at pH= 5) and the photochemical [Ru(phen)3]2+/ascorbate system. We predict that PyH0 undergoes further PT-ET steps to form the key closed-shell, dearomatized 1,2-dihydropyridine (PyH2) species. Our proposed sequential PT-ET-PT-ET mechanism transforming Py into PyH2 is consistent with the mechanism described in the formation of related dihydropyridines. Because it is driven by its proclivity to regain aromaticity, PyH2 is a potent recyclable organo-hydride donor that mimics the role of NADPH in the formation of C-H bonds in the photosynthetic CO2 reduction process. In particular, in the second component of the catalytic cycle, we predict that the PyH2/Py redox couple is kinetically and thermodynamically competent in catalytically effecting hydride and proton transfers (the latter often mediated by a proton relay chain) to CO2 and its two succeeding intermediates, namely formic acid and formaldehyde, to ultimately form CH3OH. The hydride and proton transfers for the first reduction step, i.e. reduction of CO2, are sequential in nature; by contrast, they are coupled in each of the two subsequent hydride and proton transfers to reduce formic acid and formaldehyde.

Roles of the Lewis Acid and Base in the Chemical Reduction of CO2 Catalyzed by Frustrated Lewis Pairs

We employ quantum chemical calculations to discover how frustrated Lewis pairs (FLP) catalyze the reduction of CO2 by ammonia borane (AB); specifically, we examine how the Lewis acid (LA) and Lewis base (LB) of an FLP activate CO2 for reduction. We find that the LA (trichloroaluminum, AlCl3) alone catalyzes hydride transfer (HT) to CO2 while the LB (trimesitylenephosphine, PMes3) actually hinders HT; inclusion of the LB increases the HT barrier by ∼8 kcal/mol relative to the reaction catalyzed by LAs only. The LB hinders HT by donating its lone pair to the LUMO of CO2, increasing the electron density on the C atom and thus lowering its hydride affinity. Although the LB hinders HT, it nonetheless plays a crucial role by stabilizing the active FLP·CO2 complex relative to the LA dimer, free CO2, and free LB. This greatly increases the concentration of the reactive complex in the form FLP·CO2 and thus increases the rate of reaction. We expect that the principles we describe will aid in understanding other catalytic CO2 reductions.

Mechanisms of LiCoO2 Cathode Degradation by Reaction with HF and Protection by Thin Oxide Coatings

Reactions of HF with uncoated and Al and Zn oxide-coated surfaces of LiCoO2 cathodes were studied using density functional theory. Cathode degradation caused by reaction of HF with the hydroxylated (101̅4) LiCoO2 surface is dominated by formation of H2O and a LiF precipitate via a barrierless reaction that is exothermic by 1.53 eV. We present a detailed mechanism where HF reacts at the alumina coating to create a partially fluorinated alumina surface rather than forming AlF3 and H2O and thus alumina films reduce cathode degradation by scavenging HF and avoiding H2O formation. In contrast, we find that HF etches monolayer zinc oxide coatings, which thus fail to prevent capacity fading. However, thicker zinc oxide films mitigate capacity loss by reacting with HF to form a partially fluorinated zinc oxide surface. Metal oxide coatings that react with HF to form hydroxyl groups over H2O, like the alumina monolayer, will significantly reduce cathode degradation.

Intrinsic Material Properties Dictating Oxygen Vacancy Formation Energetics in Metal Oxides

Oxygen vacancies (V(O)) in oxides are extensively used to manipulate vital material properties. Although methods to predict defect formation energies have advanced significantly, an understanding of the intrinsic material properties that govern defect energetics lags. We use first-principles calculations to study the connection between intrinsic (bulk) material properties and the energy to form a single, charge neutral oxygen vacancy (E(V)). We investigate 45 binary and ternary oxides and find that a simple model which combines (i) the oxide enthalpy of formation (ΔH(f)), (ii) the midgap energy relative to the O 2p band center (E(O 2p) + (1/2)E(g)), and (iii) atomic electronegativities reproduces calculated E(V) within ∼0.2 eV. This result provides both valuable insights into the key properties influencing E(V) and a direct method to predict E(V). We then predict the E(V) of ∼1800 oxides and validate the predictive nature of our approach against direct defect calculations for a subset of 18 randomly selected materials.

Catalytic Reduction of CO2 by Renewable Organohydrides

Dihydropyridines are renewable organohydride reducing agents for the catalytic reduction of CO2 to MeOH. Here we discuss various aspects of this important reduction. A centerpiece, which illustrates various general principles, is our theoretical catalytic mechanism for CO2 reduction by successive hydride transfers (HTs) and proton transfers (PTs) from the dihydropyridine PyH2 obtained by 1H(+)/1e(-)/1H(+)/1e(-) reductions of pyridine. The Py/PyH2 redox couple is analogous to NADP(+)/NADPH in that both are driven to effect HTs by rearomatization. High-energy radical intermediates and their associated high barriers/overpotentials are avoided because HT involves a 2e(-) reduction. A HT-PT sequence dictates that the reduced intermediates be protonated prior to further reduction for ultimate MeOH formation; these protonations are aided by biased cathodes that significantly lower the local pH. In contrast, cathodes that efficiently reduce H(+) such as Pt and Pd produce H2 and create a high interfacial pH, both obstructing dihydropyridine production and formate protonation and thus ultimately CO2 reduction by HTPTs. The role of water molecule proton relays is discussed. Finally, we suggest future CO2 reduction strategies by organic (photo)catalysts.

Evidence for hydrogen two-level systems in atomic layer deposition oxides

Two-level system (TLS) defects in dielectrics are known to limit the performance of electronic devices. We study TLS using millikelvin microwave (6.4 GHz) loss measurements of three atomic layer deposited (ALD) oxide films–crystalline BeO (c-BeO), amorphous Al2O3 (a–Al2O3), and amorphous LaAlO3 (a–LaAlO3)–and interpret them with room temperature characterization measurements. We find that the bulk loss tangent in the crystalline film is 6 times higher than in the amorphous films. In addition, its power saturation agrees with an amorphous distribution of TLS. Secondary ion mass spectrometry (SIMS) impurity analysis of the c-BeO film showed excess surface carbon (C) impurities and a uniform hydrogen (H) impurity distribution, which coupled with the analysis of loss tangent strongly suggests H limited loss. Impurity analysis of the amorphous films reveals that they have excess H impurities at the ambient-exposed surface, and we extract the associated H-based surface loss tangent. We compare two a–Al2O3 films...

Novel phase diagram behavior and materials design in heterostructural semiconductor alloys

Theoretically predicted metastable phases are realized in thin-film synthesis of Mn1−xZnxO and Sn1−xCaxS alloys. Structure and composition control the behavior of materials. Isostructural alloying is historically an extremely successful approach for tuning materials properties, but it is often limited by binodal and spinodal decomposition, which correspond to the thermodynamic solubility limit and the stability against composition fluctuations, respectively. We show that heterostructural alloys can exhibit a markedly increased range of metastable alloy compositions between the binodal and spinodal lines, thereby opening up a vast phase space for novel homogeneous single-phase alloys. We distinguish two types of heterostructural alloys, that is, those between commensurate and incommensurate phases. Because of the structural transition around the critical composition, the properties change in a highly nonlinear or even discontinuous fashion, providing a mechanism for materials design that does not exist in conventional isostructural alloys. The novel phase diagram behavior follows from standard alloy models using mixing enthalpies from first-principles calculations. Thin-film deposition demonstrates the viability of the synthesis of these metastable single-phase domains and validates the computationally predicted phase separation mechanism above the upper temperature bound of the nonequilibrium single-phase region.

Synthesis of a mixed-valent tin nitride and considerations of its possible crystal structures

Recent advances in theoretical structure prediction methods and high-throughput computational techniques are revolutionizing experimental discovery of the thermodynamically stable inorganic materials. Metastable materials represent a new frontier for these studies, since even simple binary non-ground state compounds of common elements may be awaiting discovery. However, there are significant research challenges related to non-equilibrium thin film synthesis and crystal structure predictions, such as small strained crystals in the experimental samples and energy minimization based theoretical algorithms. Here, we report on experimental synthesis and characterization, as well as theoretical first-principles calculations of a previously unreported mixed-valent binary tin nitride. Thin film experiments indicate that this novel material is N-deficient SnN with tin in the mixed ii/iv valence state and a small low-symmetry unit cell. Theoretical calculations suggest that the most likely crystal structure has the space group 2 (SG2) related to the distorted delafossite (SG166), which is nearly 0.1 eV/atom above the ground state SnN polymorph. This observation is rationalized by the structural similarity of the SnN distorted delafossite to the chemically related Sn3N4 spinel compound, which provides a fresh scientific insight into the reasons for growth of polymorphs of metastable materials. In addition to reporting on the discovery of the simple binary SnN compound, this paper illustrates a possible way of combining a wide range of advanced characterization techniques with the first-principle property calculation methods, to elucidate the most likely crystal structure of the previously unreported metastable materials.

Using heterostructural alloying to tune the structure and properties of the thermoelectric Sn1−xCaxSe

We grow and kinetically stabilize the isotropic rocksalt phase of SnSe thin films by alloying SnSe with CaSe. Thin polycrystalline films of the metastable heterostructural alloy Sn1−xCaxSe are synthesized by pulsed laser deposition on amorphous SiO2 over the entire composition range 0 < x < 1. We observe the theoretically-predicted, composition-driven change from a layered, orthorhombic structure to an isotropic, cubic structure near x = 0.18, in reasonable agreement with the theoretical value of x = 0.13 calculated from first principles. The optical band gap is highly non-linear in x and the trend agrees with theory predictions. Compared to the layered end-member SnSe, the isotropic alloy near the orthorhombic-to-rocksalt transition has a p-type electrical resistivity three orders of magnitude lower, and a thermoelectric power factor at least ten times larger. Thus manipulation of the structure of a functional material like SnSe via alloying may provide a new path to enhanced functionality, in this case, improved thermoelectric performance.