Hetero-bimetallic paddlewheel complexes for enhanced CO2 reduction selectivity in MOFs: a first principles study

The reduction of carbon dioxide (CO2) into value-added feedstock materials, fine chemicals, and fuels represents a crucial approach for meeting contemporary chemical demands while reducing dependence on petrochemical sources. Optimizing catalysts for the CO2 reduction reaction (CO2RR) can entail employing first principles methodology to identify catalysts possessing desirable attributes, including the ability to form diverse products or selectively produce a limited set of products, or exhibit favorable reaction kinetics. In this study, we investigate CO2RR on bimetallic Cu-based paddlewheel complexes, aiming to understand the impact metal substitution with Mn(II), Co(II), or Ni(II) has on bimetallic paddlewheel metal-organic frameworks. Substituting one of the Cu sites of the paddlewheel complex with Mn results in a more catalytically active Cu center, poised to produce substantial quantities of formic acid (HCOOH) and smaller quantities of methane (CH4) with a suppressed production of C2 products such as ethanol (CH3CH2OH) or ethylene (C2H4). Moreover, the presence of Mn significantly reduces the limiting potential for CO2 reduction from 2.22 eV on the homo-bimetallic Cu paddlewheel complex to 1.19 eV, thereby necessitating a smaller applied potential. Conversely, within the Co-substituted paddlewheel complex, the Co site emerges as the primary catalytic center, selectively yielding CH4 as the sole reduced CO2 product, with a limiting potential of 1.22 eV. Notably, the Co site faces significant competition from H2 production due to a lower limiting potential of 0.81 eV for hydrogen reduction. Our examination of the Cu-Ni paddlewheel complex, featuring a Ni substituent site, reveals two catalytically active centers, each promoting distinct reductive processes. Both the Ni and Cu sites exhibit a propensity for HCOOH formation, with the Ni site favoring further reduction to CH4, whereas the Cu site directs the reaction towards methanol (CH3OH) production. This study holds significance in informing and streamlining future experimental efforts for synthesizing and evaluating novel catalysts with superior capabilities for CO2 reduction.

High Accuracy Ab Initio Potential Energy Curves and Dipole Moment Functions for the X1Σ+ and a3Π Spin States of the CF+ Diatomic Molecule

Potential energy curves and dipole moment functions constructed using high-accuracy ab initio methods allow for an in-depth examination of the electronic structure of diatomic molecules. Ab initio computations serve as a valuable complement to experimental data, offering insights into the nature of short-lived molecules such as those encountered within the interstellar medium (ISM). While laboratory experiments provide critical groundwork, the ISM's conditions often permit longer lifetimes for lower stability molecules, enabling unique observations. The CF+ diatomic molecule is one such molecule that has been observed spectroscopically in the ISM. Previous experimental and theoretical work have examined different spectroscopic aspects of the CF+ molecule, but the development of newer, more complete potential energy curves and dipole moment functions allows for even greater insight. We constructed both potential energy curves and dipole moment functions for the ground X1Σ+ and first excited a3Π states of CF+ for both the 12C and 13C isotopologues. The potential energy curves were constructed using coupled cluster with single, double, and perturbative triple excitations (CCSD(T)) at the complete basis set limit with corrections from full triple, quadruple, quintuple, and hextuple excitations within a finite-basis coupled cluster wave function as well as corrections from full configuration interaction and relativistic effects. Rovibrational wave functions were calculated using a vibrational Hamiltonian matrix, which moves beyond the harmonic oscillator approximation. The equilibrium bond length, vibrational constant, and rotational constant were reproduced to within 0.00013 Å, 0.28 cm-1, and 0.00045 cm-1, respectively, of experimental values. Experimental transition energies from rovibrational spectra were reproduced with an error of no larger than 0.63 cm-1. The triplet excited state (a3Π) was found to have a longer equilibrium bond length at 1.21069 Å, a vibrational constant of 1611.29 cm-1, and a rotational constant of 1.56376 cm-1. Rovibrational line lists for the 12C and 13C isotopologues for both the X1Σ+ and the excited a3Π states were generated.

Catalyst Engineering for the Selective Reduction of CO2 to CH4 : A First-Principles Study on X-MOF-74 (X=Mg, Mn, Fe, Co, Ni, Cu, Zn)

The conversion of carbon dioxide (CO2 ) into more valuable chemical compounds represents a critical objective for addressing environmental challenges and advancing sustainable energy sources. The CO2 reduction reaction (CO2 RR) holds promise for transforming CO2 into versatile feedstock materials and fuels. Leveraging first-principles methodologies provides a robust approach to evaluate catalysts and steer experimental efforts. In this study, we examine the CO2 RR process using a diverse array of representative cluster models derived from X-MOF-74 (where X encompasses Mg, Mn, Fe, Co, Ni, Cu, or Zn) through first-principles methods. Notably, our investigation highlights the Fe-MOF-74 cluster's unique attributes, including favorable CO2 binding and the lowest limiting potential of the studied clusters for converting CO2 to methane (CH4 ) at 0.32 eV. Our analysis identified critical factors driving the selective CO2 RR pathway, enabling the formation CH4 on the Fe-MOF-74 cluster. These factors involve less favorable reduction of hydrogen to H2 and strong binding affinities between the Fe open-metal site and reduction intermediates, effectively curtailing desorption processes of closed-shell intermediates such as formic acid (HCOOH), formaldehyde (CH2 O), and methanol (CH3 OH), to lead to selective CH4 formation.

Multivariate Flexible Framework with High Usable Hydrogen Capacity in a Reduced Pressure Swing Process

Step-shaped adsorption-desorption of gaseous payloads by flexible metal-organic frameworks can facilitate the delivery of large usable capacities with significantly reduced energetic penalties. This is desirable for the storage, transport, and delivery of H₂, as prototypical adsorbents require large swings in pressure and temperature to achieve usable capacities approaching their total capacities. However, the weak physisorption of H₂ typically necessitates undesirably high pressures to induce the framework phase change. As de novo design of flexible frameworks is exceedingly challenging, the ability to intuitively adapt known frameworks is required. We demonstrate that the multivariate linker approach is a powerful tool for tuning the phase change behavior of flexible frameworks. In this work, 2-methyl-5,6-difluorobenzimidazolate was solvothermally incorporated into the known framework CdIF-13 (sod-Cd(benzimidazolate)₂), resulting in the multivariate framework sod-Cd(benzimidazolate)_1.87(2-methyl-5,6-difluorobenzimidazolate)_0.13 (ratio = 14:1), which exhibited a considerably reduced stepped adsorption threshold pressure while maintaining the desirable adsorption-desorption profile and capacity of CdIF-13. At 77 K, the multivariate framework exhibits stepped H₂ adsorption with saturation below 50 bar and minimal desorption hysteresis at 5 bar. At 87 K, saturation of step-shaped adsorption occurs by 90 bar, with hysteresis closing at 30 bar. These adsorption-desorption profiles enable usable capacities in a mild pressure swing process above 1 mass %, representing 85-92% of the total capacities. This work demonstrates that the desirable performance of flexible frameworks can be readily adapted through the multivariate approach to enable efficient storage and delivery of weakly physisorbing species.

High accuracy ab initio potential energy surface for the H2O-H van der Waals dimer

A representation of the three-dimensional potential energy surface (PES) of the H₂O-H van der Waals dimer is presented. The H₂O molecule is treated as a rigid body held at its experimentally determined equilibrium geometry, with the OH bond length set to 1.809 650 34 a₀ and the HOH bond angle set to 1.824 044 93 radians. Ab initio calculations are carried out at the coupled-cluster single, double, and perturbative triple level, with scalar relativistic effects included using the second-order Douglas-Kroll-Hess approximation. The ab initio calculations employ the aug-cc-pVnZ-DK series of basis sets (n = D, T, Q), which are recontracted versions of the aug-cc-pVnZ basis sets that are appropriate for relativistic calculations. The counterpoise method is used to reduce the basis set superposition error; in addition, results obtained using the aug-cc-pVTZ-DK and aug-cc-pVQZ-DK basis sets were extrapolated to the complete basis set (CBS) limit. The PES is based on calculations carried out at 1054 symmetry-unique H₂O-H geometries for which the distance R between the H-atom and the H₂O center of mass ranges from R = 2.5-9.0 Å. The reproduction of the PES along the orientational degrees of freedom was performed using Lebedev quadrature and an expansion in spherical harmonics. The mean absolute error of the reproduced PES is <0.02 cm^-1 for R ≥ 3.0 Å and <0.21 cm^-1 for R between 2.5 and 3.0 Å. The global minimum for the CBS PES is a coplanar H₂O-H geometry, with R = 3.41 Å, in which the angle formed between the H₂O C₂ symmetry axis and the H-atom is 122.25°; the CBS binding energy for this geometry is 61.297 cm^-1. In addition, by utilizing the symmetry of the H₂O molecule, the spherical harmonic expansion was simplified with no loss in accuracy and a speedup of ∼1.8 was achieved. The reproduced PES can be used in future molecular dynamics simulations.

Electrocatalytic Dechlorination of Dichloromethane in Water Using a Heterogenized Molecular Copper Complex

The remediation of organohalides from water is a challenging process in environment protection and water treatment. Herein, we report a molecular copper(I) complex with two triazole units, CuT2, in a heterogeneous aqueous system that is capable of dechlorinating dichloromethane (CH₂Cl₂) to afford hydrocarbons (methane, ethane, and ethylene). The catalytic performance is evaluated in water and presented high Faradaic efficiency (average 70% CH₄) across a range of potentials (-1.1 to -1.6 V vs Ag/AgCl) and high activity (maximum -25.1 mA/cm² at -1.6 V vs Ag/AgCl) with a turnover number of 2.0 × 10⁷. The CuT2 catalyst also showed excellent stability for 14 h of constant exposure to CH₂Cl₂ and 10 h of CH₂Cl₂ exposure cycling. The control compound, a copper-free triazole unit (T1), was also investigated under the same condition and showed inferior catalytic activity, indicating the importance of the copper center. Plausible catalytic mechanisms are proposed for the formation of C₁ and C₂ products via radical intermediates. Computational studies provided additional insight into the reaction mechanism and the selectivity toward the CH₄ formation. The findings in this study demonstrate that complex CuT2 is an efficient and stable catalyst for the dehalogenation of CH₂Cl₂ and could potentially be used for the exploration of the removal of halogenated species from aqueous systems.