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Garza, Alejandro J.

Publications and source records attributed to Garza, Alejandro J..

18 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Heterogenized Pyridine-Substituted Cobalt(II) Phthalocyanine Yields Reduction of CO2 by Tuning the Electron Affinity of the Co Center

Conversion of CO 2 to reduced products is a promising route to alleviate irreversible climate change. In this paper we report the synthesis of a Co-based phthalocyanine with pyridine moieties (CoPc-Pyr), which is supported on a carbon electrode and shows Faradaic efficiency ~90% for CO at 490 mV of overpotential (-0.6 V vs reversible hydrogen electrode (RHE)). In addition, its catalytic activity at -0.7 V versus RHE surpasses other Co-based molecular and metal-organic framework catalysts for CO 2 reduction at this bias. Density functional theory calculations show that pyridine moieties enhance CO 2 adsorption and electron affinity of the Co center by an inductive effect, thus lowering the overpotential necessary for CO 2 conversion. Our study shows that CoPc-Pyr reduces CO 2 at lower overpotential and with higher activity than noble metal electrodes, such as silver.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2019

Explaining the Incorporation of Oxygen Derived from Solvent Water into the Oxygenated Products of CO Reduction over Cu

The electrochemical reduction of CO and CO 2 over Cu makes a variety of multicarbon products. Interestingly, recent isotope experiments have suggested that the oxygen atoms contained in the multicarbon alcohols produced over Cu are derived from solvent water. This observation has brought into question many of the presented reaction mechanisms by which these multicarbon alcohols are produced over Cu. Yet, these surprising experimental observations are likely the result of isotopic scrambling between transiently produced carbonyl-containing intermediate reaction products, such as acetaldehyde, with solvent water and not another mechanism. The existence of such carbonyl-containing intermediate reaction products is supported by both experimental and theoretical studies. Moreover, theoretical calculations support the notion that the reversible hydration of these carbonyl-containing species is facile in the vicinity of the Cu surface.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Challenges in Modeling Electrochemical Reaction Energetics with Polarizable Continuum Models

A major challenge in the modeling of electrochemical phenomena is the accurate description of the interface between an electrolyte and a charged conductor. Polarizable continuum models (PCM) have been gaining popularity because they offer a computationally inexpensive method of modeling the electrolyte. In this Perspective, we discuss challenges from using one such model which treats the ions using a linearized Poisson–Boltzmann (LPB) distribution. From a physical perspective, this model places charge unphysically close to the surface and adsorbates, and it includes excessively steep ramping of the dielectric constant from the surface to the bulk solvent. Both of these issues can be somewhat mitigated by adjusting parameters built into the model, but in doing so, the resultant capacitance deviates from experimental values. Likewise, hybrid explicit-implicit approaches to the solvent may offer a more realistic description of hydrogen bonding and solvation to reaction intermediates, but the corresponding capacitances also deviate from experimental values. These deviations highlight the need for a careful adjustment of parameters in order to reproduce not only solvation energies but also other physical properties of solid–liquid interfaces. Continuum approaches alone also necessarily do not capture local variations in the electric field from cations at the interface, which can affect the energetics of intermediates with substantial dipoles or polarizability. Finally, since the double-layer charge can be varied continuously, LPB/PCM models provide a way to determine electrochemical barriers at constant potential. However, double-layer charging and the atomic motion associated with reaction events occur on significantly different timescales. As a result, we suggest that more detailed approaches, such as the modified Poisson–Boltzmann model and/or the addition of a Stern layer, may be able to mitigate some but not all of the challenges discussed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Reaction mechanism of the selective reduction of CO 2 to CO by a tetraaza [Co II N 4 H] 2+ complex in the presence of protons

The tetraaza [Co II N 4 H] 2+ complex ( 1 ) is remarkable for its ability to selectively reduce CO 2 to CO with 45% Faradaic efficiency and a CO to H 2 ratio of 3 : 2. We employ density functional theory (DFT) to determine the reasons behind the unusual catalytic properties of 1 and the most likely mechanism for CO 2 reduction. The selectivity for CO 2 over proton reduction is explained by analyzing the catalyst's affinity for the possible ligands present under typical reaction conditions: acetonitrile, water, CO 2 , and bicarbonate. After reduction of the catalyst by two electrons, formation of [Co I N 4 H] + –-CO 2 - is strongly favored. Based on thermodynamic and kinetic data, we establish that the only likely route for producing CO from here consists of a protonation step to yield [Co I N 4 H] + –CO 2 H, followed by reaction with CO 2 to form [Co II N 4 H] 2+ –CO and bicarbonate. This conclusion corroborates the idea of a direct role of CO 2 as a Lewis acid to assist in C–O bond dissociation, a conjecture put forward by other authors to explain recent experimental observations. The pathway to formic acid is predicted to be forbidden by high activation barriers, in accordance with the products that are known to be generated by 1 . Calculated physical observables such as standard reduction potentials and the turnover frequency for our proposed catalytic cycle are in agreement with available experimental data reported in the literature. The mechanism also makes a prediction that may be experimentally verified: that the rate of CO formation should increase linearly with the partial pressure of CO 2 .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Nonempirical Meta-Generalized Gradient Approximations for Modeling Chemisorption at Metal Surfaces

We assess the accuracy of popular nonempirical GGAs (PBE, PBEsol, RPBE) and meta-GGAs (TPSS, revTPSS, and SCAN) for describing chemisorption reactions at metal surfaces. Except for RPBE, all the functionals tend to overbind the adsorbate significantly. We then propose a nonempirical meta-GGA, denoted as RTPSS, that is based on RPBE in the same way that TPSS is based on PBE. The RTPSS functional remedies the overbinding problem and improves the description of chemisorption energies. As an example of an application of RTPSS, we study the adsorption of CO on Cu surfaces (a notably difficult problem for semilocal functionals) and find that RTPSS is the only tested functional that predicts accurate chemisorption energies and the preferred adsorption site of CO. Although RTPSS gives an accurate description of chemisorption, nonlocal correlation may be necessary to describe physisorption if long-range van der Waals interactions are involved (however, this is true for semilocal functionals in general). We suggest that RTPSS can be a useful meta-GGA for studying chemisorption processes and mechanisms of heterogeneous catalysis.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Is Subsurface Oxygen Necessary for the Electrochemical Reduction of CO 2 on Copper?

It has recently been proposed that subsurface oxygen is crucial for the adsorption and subsequent electroreduction of CO 2 on copper. Using density functional theory, we have studied the stability and diffusion of subsurface oxygen in single crystals of copper exposing (111) and (100) facets. Oxygen is at least 1.5 eV more stable on the surface than beneath it for both crystal orientations; interstitial sites are too small to accommodate oxygen. Here, the rate of atomic oxygen diffusion from one layer below a Cu(111) surface to the surface is 5 × 10 3 s –1 . Oxygen can survive longer in deeper layers, but it does not promote CO 2 adsorption there. Diffusion of subsurface oxygen is easier to the less-dense Cu(100) surface, even from lower layers (rate ≈ 1 × 107 s–1). Finally, once the applied voltage and dispersion forces are properly modeled, we find that subsurface oxygen is unnecessary for CO 2 adsorption on copper.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Mechanism of CO 2 Reduction at Copper Surfaces: Pathways to C 2 Products

On the basis of constraints from reported experimental observations and density functional theory simulations, in this paper we propose a mechanism for the reduction of CO 2 to C 2 products on copper electrodes. To model the effects of an applied potential bias on the reactions, calculations are carried out with a variable, fractional number of electrons on the unit cell, which is optimized so that the Fermi level matches the actual chemical potential of electrons (i.e., the applied bias); an implicit electrolyte model allows for compensation of the surface charge so that neutrality is maintained in the overall simulation cell. Our mechanism explains the presence of the seven C 2 species that have been detected in the reaction, as well as other notable experimental observations. Furthermore, our results shed light on the difference in activities toward C 2 products between the (100) and (111) facets of copper. Finally, we compare our methodologies and findings with those in other recent mechanistic studies of the copper-catalyzed CO 2 reduction reaction.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Electronic Structure and Properties of Berkelium Iodates

The reaction of 249 Bk(OH) 4 with iodate under hydrothermal conditions results in the formation of Bk(IO 3 ) 3 as the major product with trace amounts of Bk(IO 3 ) 4 also crystallizing from the reaction mixture. The structure of Bk(IO 3 ) 3 consists of nine-coordinate Bk III cations that are bridged by iodate anions to yield layers that are isomorphous with those found for Am III , Cf III , and with lanthanides that possess similar ionic radii. Bk(IO 3 ) 4 was expected to adopt the same structure as M(IO 3 ) 4 (M = Ce, Np, Pu), but instead parallels the structural chemistry of the smaller Zr IV cation. Bk III –O and Bk IV –O bond lengths are shorter than anticipated and provide further support for a postcurium break in the actinide series. Photoluminescence and absorption spectra collected from single crystals of Bk(IO 3 ) 4 show evidence for doping with Bk III in these crystals. In addition to luminescence from Bk III in the Bk(IO 3 ) 4 crystals, a broad-band absorption feature is initially present that is similar to features observed in systems with intervalence charge transfer. However, the high-specific activity of 249 Bk ( t 1/2 = 320 d) causes oxidation of Bk III and only Bk IV is present after a few days with concomitant loss of both the BkIII luminescence and the broadband feature. The electronic structure of Bk(IO 3 ) 3 and Bk(IO 3 ) 4 were examined using a range of computational methods that include density functional theory both on clusters and on periodic structures, relativistic ab initio wave function calculations that incorporate spin–orbit coupling (CASSCF), and by a full-model Hamiltonian with spin–orbit coupling and Slater–Condon parameters (CONDON). Some of these methods provide evidence for an asymmetric ground state present in Bk IV that does not strictly adhere to Russel–Saunders coupling and Hund’s Rule even though it possesses a half-filled 5 f 7 shell. Multiple factors contribute to the asymmetry that include 5 f electrons being present in microstates that are not solely spin up, spin–orbit coupling induced mixing of low-lying excited states with the ground state, and covalency in the Bk IV –O bonds that distributes the 5f electrons onto the ligands. These factors are absent or diminished in other f 7 ions such as Gd III or Cm III .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Predicting Band Gaps with Hybrid Density Functionals

We look at the ability of four popular hybrid density functionals (B3LYP, B3PW91, HSE, and PBE0) for predicting band gaps of semiconductors and insulators over a large benchmark set using a consistent methodology. We observe no significant statistical difference in their overall performance, although the screened hybrid HSE is more accurate for typical semiconductors. HSE can improve its accuracy for large band gap materials—without affecting that of semiconductors—by including a larger portion of Hartree–Fock exchange in its short-range. Given that screened hybrids are computationally much less expensive than their global counterparts, we conclude that they are a better option for the black box prediction of band gaps.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Actinide chemistry using singlet-paired coupled cluster and its combinations with density functionals

Singlet-paired coupled cluster doubles (CCD0) is a simplification of CCD that relinquishes a fraction of dynamic correlation in order to be able to describe static correlation. Combinations of CCD0 with density functionals that recover specifically the dynamic correlation missing in the former have also been developed recently. Here, we assess the accuracy of CCD0 and CCD0+DFT (and variants of these using Brueckner orbitals) as compared to well-established quantum chemical methods for describing ground-state properties of singlet actinide molecules. Finally, the f 0 actinyl series (UO$2+\atop{2}$, NpO$3+\atop{2}$, PuO$4+\atop{2}$), the isoelectronic NUN, and thorium (ThO, ThO 2+ ) and nobelium (NoO, NoO 2 ) oxides are studied.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

On the equivalence of LIST and DIIS methods for convergence acceleration

Self-consistent field extrapolation methods play a pivotal role in quantum chemistry and electronic structure theory. We, here, demonstrate the mathematical equivalence between the recently proposed family of LIST methods [Wang et al., J. Chem. Phys. 134, 241103 (2011); Y. K. Chen and Y. A. Wang, J. Chem. Theory Comput. 7, 3045 (2011)] and the general form of Pulay’s DIIS [Chem. Phys. Lett. 73, 393 (1980); J. Comput. Chem. 3, 556 (1982)] with specific error vectors. Our results also explain the differences in performance among the various LIST methods.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Synergy between pair coupled cluster doubles and pair density functional theory

Pair coupled cluster doubles (pCCD) has been recently studied as a method capable of accounting for static correlation with low polynomial cost. We present three combinations of pCCD with Kohn–Sham functionals of the density and on-top pair density (the probability of finding two electrons on top of each other) to add dynamic correlation to pCCD without double counting. With a negligible increase in computational cost, these pCCD+DFT blends greatly improve upon pCCD in the description of typical problems where static and dynamic correlations are both important. We argue that—as a black-box method with low scaling, size-extensivity, size-consistency, and a simple quasidiagonal two-particle density matrix—pCCD is an excellent match for pair density functionals in this type of fusion of multireference wavefunctions with DFT.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Electronic correlation without double counting via a combination of spin projected Hartree-Fock and density functional theories

Several schemes to avoid the double counting of correlations in methods that merge multireference wavefunctions with density functional theory (DFT) are studied and here adapted to a combination of spin-projected Hartree-Fock (SUHF) and DFT. The advantages and limitations of the new method, denoted SUHF+f{sub c}DFT, are explored through calculations on benchmark sets in which the accounting of correlations is challenging for pure SUHF or DFT. It is shown that SUHF+f{sub c}DFT can greatly improve the description of certain molecular properties (e.g., singlet-triplet energy gaps) which are not improved by simple addition of DFT dynamical correlation to SUHF. However, SUHF+f{sub c}DFT is also shown to have difficulties dissociating certain types of bonds and describing highly charged ions with static correlation. Possible improvements to the current SUHF+f{sub c}DFT scheme are discussed in light of these results.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗