Solar driven CO2 hydrogenation on transition metal doped Zn12O12 cluster

Low-temperature oxidation of methane mediated by Al-doped ZnO cluster and nanowire: a first-principles investigation

CONTEXT

First-principles calculations are performed to investigate the catalytic oxidation of methane by using N2O as an oxidizing agent over aluminum (Al)-doped Zn12O12 cluster and (Zn12O12)2 nanowire. The impact of Al impurity on the geometry, electronic structure, and surface reactivity of Zn12O12 and (Zn12O12)2 is thoroughly studied. Our study demonstrates that Al-doped ZnO systems have a better adsorption ability than the corresponding pristine counterparts. It is found that N2O molecule is initially decomposed on the Al site to provide the N2 molecule, and an Al-O intermediate which is an active species for the CH4 oxidation. The conversion of CH4 into CH3OH over AlZn11O12 and (AlZn11O12)2 requires an activation energy of 0.45 and 0.29 eV, respectively, indicating it can be easily performed at normal temperatures. Besides, the overoxidation of methanol into formaldehyde cannot take place over the AlZn11O12 and (AlZn11O12)2, due to the high energy barrier needed to dissociate C-H bond of the CH3O intermediate.

METHOD

Dispersion-corrected density functional theory calculations were performed through GGA-PBE exchange-correlation functional combined with a numerical double-ζ plus polarization (DNP) basis set as implemented in DMol3. To include the relativistic effects of core electrons of Zn atoms, DFT-semicore pseudopotentials were adopted. The DFT + D scheme proposed by Grimme was used to involve weak dispersion interactions within the DFT calculations. The reaction energy paths were generated by the minimum energy path calculations using the NEB method.

Sc-functionalized porphyrin-like porous fullerene for CO2 storage and separation: A first-principles evaluation

In recent years, there has been a lot of interest in capturing and storing carbon dioxide (CO₂) on porous materials as an efficient method for decreasing the adverse effects of this greenhouse gas on the environment and climate change. The current work introduces a Sc-decorated porphyrin-like porous fullerene (Sc₆@C₂₄N₂₄) as an efficient material for CO₂ capture, storage, and separation using density functional theory calculations. While CO₂ is physisorbed over pristine C₂₄N₂₄, the addition of Sc atoms on the N₄ sites of C₂₄N₂₄ greatly enhances CO₂ adsorption energy. Each Sc atom in Sc₆@C₂₄N₂₄ may adsorb up to three CO₂ molecules, resulting in a gravimetric density of 48%. Moreover, temperature may be used to modulate CO₂ adsorption/desorption over the substrate. The Sc-decorated C₂₄N₂₄ fullerene exhibits a lower affinity for adsorbing N₂, CH₄, and H₂ molecules than CO₂. As a consequence, this material might be considered for purifying CO₂ molecules from CO₂/N₂, CO₂/CH₄, and CO₂/H₂ mixtures. This study also sheds light on the nature of the Sc-CO₂ interaction as well as the underlying mechanism of selective CO₂ adsorption on Sc decorated C₂₄N₂₄.

Role of Dispersion Interactions in Endohedral TM@(ZnS)12 Structures

II-VI semiconducting materials are gaining attention due to their optoelectronic properties. Moreover, the addition of transition metals, TMs, might give them magnetic properties. The location and distance of the TM are crucial in determining such magnetic properties. In this work, we focus on small hollow (ZnS)₁₂ nanoclusters doped with TMs. Because (ZnS)₁₂ is a cage-like spheroid, the cavity inside the structure allows for the design of endohedral compounds resembling those of C₆₀. Previous studies theoretically predicted that the first-row TM(ZnS)₁₂ endohedral compounds were thermodynamically unstable compared to the surface compounds, where the TM atom is located at the surface of the cluster. The transition states connecting both structure families were calculated, and the estimated lifetimes of these compounds were predicted to be markedly small. However, in such works dispersion effects were not taken into account. Here, in order to check for the influence of dispersion on the possible stabilization of the desired TM(ZnS)₁₂ endohedrally doped clusters, several functionals are tested and compare to MP2. It is found that the dispersion effects play a very important role in determining the location of the metals, especially in those TMs with the 4s3d shell half-filled or completely filled. In addition, a complete family of TM doped (ZnS)₁₂ nanoclusters is explored using ab initio molecular dynamics simulations and local minima optimizations that could guide the experimental synthesis of such compounds. From the magnetic point of view, the Cr(⁷S)@(ZnS)₁₂ compound is the most interesting case, since the endohedral isomer is predicted to be the global minimum. Moreover, molecular dynamics simulations show that when the Cr atom is located at the surface of the cluster, it spontaneously migrates toward the center of the cavity at room temperature.