Binding of SO3 to Group 4 Transition Metal Oxide Nanoclusters

Transition metal oxide (TMO) clusters are being studied for their ability to absorb acid gases generated by energy production processes. The interaction of SO3, a byproduct of common industrial processes, with group 4 metal (Ti, Zr, and Hf) oxide nanoclusters, has been predicted using electronic structure methods. The calculations were done at the density functional theory (DFT) and correlated molecular orbital coupled cluster singles and doubles CCSD(T) theory levels. There is a reasonable agreement between the DFT/ωB97x-D energies with the CCSD(T) results. SO3 is predicted to strongly chemisorb to these clusters, as do NO2 and CO2. For SO3, these chemisorption processes favor binding to TMO clusters as SO42- sulfate in both the terminal and bridging configurations. It is predicted that SO3 fully extracts the bridging oxygen from the TMO lattice to form bridging SO42-. This is favorable because of the lower S-O bond dissociation energy of SO3, whereas other acid gases add across the bridging oxygen because of their higher A-O bond dissociation energy. SO3 is capable of physisorption as long as an exposed metal center is present in the lattice. If a metal center has a terminal oxo-group, then SO3 will prefer the SO42- configuration. An approximately linear relationship exists between the physisorption energy and proton affinity for rows 2 and 3 elements.

Dinuclear Complexes of Uranyl, Neptunyl, and Plutonyl: Structures and Oxidation States Revealed by Experiment and Theory

Dinuclear perchlorate complexes of uranium, neptunium, and plutonium were characterized by reactivity and DFT, with results revealing structures containing pentavalent, hexavalent, and heptavalent actinyls, and actinyl-actinyl interactions (AAIs). Electrospray ionization produced native complexes [(AnO₂)₂(ClO₄)₃]^- for An:An = U:U, Np:Np, Pu:Pu, and Np:Pu, which are intuitively formulated as actinyl(V) perchlorates. However, DFT identified lower-energy structures [(AnO₂)(AnO₃)(ClO₄)₂(ClO₃)]^- comprising a perchlorate fragmented to ClO₃, actinyl(VI) cation An^VIO₂²⁺, and neutral AnO₃. For U:U and Np:Np, and Np in Np:Pu, the coordinated AnO₃ is calculated as actinyl(VI) with an equatorial oxo, [O_yl═An^VI═O_yl][═O_eq], whereas for Pu:Pu, it is plutonyl(V) oxyl, [O_yl═Pu^V═O_yl][-O_eq^•]. The implied lower stability of Pu^VI versus Np^VI indicates weaker Pu═O_eq versus Np═O_eq bonding. Adsorption of O₂ by the U:U complex suggests oxidation of U^V to U^VI, corroborating the assignment of perchlorate [(An^VO₂)₂(ClO₄)₃]^-. DFT predicts the O₂ adducts are [(An^VIO₂)(O₂)(An^VIO₂)(ClO₄)₃]^- with actinyls oxidized from +V to +VI by bridging peroxide, O₂^2-. In accordance with reactivity, O^2- addition is computed as substantially exothermic for U:U and least favorable for Pu:Pu. Collision-induced dissociation of native complexes eliminated ClO₂ to yield [(AnO₂)(O)₂(AnO₂)(ClO₄)₂]^-, in which fragmented O atoms bridge as oxyl O^-• and oxo O^2- to yield uranyl(VI) and plutonyl(VI), or as oxos O^2- to yield neptunyl(VII), [O_yl═Np^VII═O_yl]³⁺.

Predicting the Mechanism and Products of CO2 Capture by Amines in the Presence of H2O

An extensive correlated molecular orbital theory study of the reactions of CO₂ with a range of substituted amines and H₂O in the gas phase and aqueous solution was performed at the G3(MP2) level with a self-consistent reaction field approach. The G3(MP2) calculations were benchmarked at the CCSD(T)/CBS level for NH₃ reactions. A catalytic NH₃ reduces the energy barrier more than a catalytic H₂O for the formation of H₂NCOOH and H₂CO₃. In aqueous solution, the barriers to form both H₂NCOOH and H₂CO₃ are reduced, with HCO₃^- formation possible with one amine present and H₂NCOO^- formation possible only with two amines. Further reactions of H₂NCOOH to form HNCO and urea via the Bazarov reaction have high barriers and are unlikely in both the gas phase and aqueous solution. Reaction coordinates for CH₃NH₂, CH₃CH₂NH₂, (CH₃)₂NH, CH₃CH₂CH₂NH₂, (CH₃)₃N, and DMAP were also calculated. The barrier for proton transfer correlates with amine basicity for alkylammonium carbamate (ΔG^‡_aq < 15 kcal/mol) and alkylammonium bicarbonate (ΔG^‡_aq < 30 kcal/mol) formation. In aqueous solution, carbamic acids, carbamates, and bicarbonates can all form in small amounts with ammonium carbamates dominating for primary and secondary alkylamines. These results have implications for CO₂ capture by amines in both the gas phase and aqueous solution as well as in the solid state, if enough water is present.

Reaction of NO2 with Groups IV and VI Transition Metal Oxide Clusters

The addition of NO₂ to Group IV (MO₂)_n and Group VI (MO₃)_n (n = 1-3) nanoclusters was studied using both density functional theory (DFT) and coupled cluster theory (CCSD(T)). The structures and overall binding energetics were predicted for Lewis acid-base addition without transfer of spin (a physisorption-type process) and the formation of either cluster-ONO (HONO-like or bidentate bonding) or NO₃^- formation where for both the spin is transferred to the metal oxide clusters (a chemisorption-type process). Only chemisorption of NO₂ is predicted to be thermodynamically allowed at temperatures ≥298 K for Group IV (MO₂)_n clusters with the formation of surface chemisorbed NO₂ being by far the most energetically favorable. The ligand binding energies (LBEs) for physisorption and chemisorption on the TiO₂ nanoclusters are consistent with computational studies of the bulk solids. Chemisorption is only predicted to occur for (CrO₃)_n clusters in the form of a terminal nitrate containing species whereas the larger chemisorbed nitrate structures for (MoO₃)_n and (WO₃)_n were found to be metastable and unlikely to form in any appreciable amount at temperatures of 298 K and higher. NO₂ is predicted to only be capable of physisorbing to (MoO₃)_n and (WO₃)_n at lower temperatures and therefore unlikely to bind NO₂ at temperatures ≥298 K. Correlations between the (MO₃)_nNO₂ ligand bond energies and the chemical properties of the parent (MO₃)_n clusters (Lewis acidity, ionization potentials, excitation energies, and M = O/M-O bond strengths) are described.