Molecular Structures and Energetics of the (ZrO2)n and (HfO2)n (n = 1−4) Clusters and Their Anions

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.

Ensemble representation of catalytic interfaces: soloists, orchestras, and everything in-between

Catalytic systems are complex and dynamic, exploring vast chemical spaces on multiple timescales. In this perspective, we discuss the dynamic behavior of fluxional, heterogeneous thermal and electrocatalysts and the ensembles of many isomers which govern their behavior. We develop a new paradigm in catalysis theory in which highly fluxional systems, namely sub-nano clusters, isomerize on a much shorter timescale than that of the catalyzed reaction, so macroscopic properties arise from the thermal ensemble of isomers, not just the ground state. Accurate chemical predictions can only be reached through a many-structure picture of the catalyst, and we explain the breakdown of conventional methods such as linear scaling relations and size-selected prevention of sintering. We capitalize on the forward-looking discussion of the means of controlling the size of these dynamic ensembles. This control, such that the most effective or selective isomers can dominate the system, is essential for the fluxional catalyst to be practicable, and their targeted synthesis to be possible. It will also provide a fundamental lever of catalyst design. Finally, we discuss computational tools and experimental methods for probing ensembles and the role of specific isomers. We hope that catalyst optimization using chemically informed descriptors of ensemble nature and size will become a new norm in the field of catalysis and have broad impacts in sustainable energy, efficient chemical production, and more.

Bonding properties of molecular cerium oxides tuned by the 4f-block from ab initio perspective

Probing chemical bonding in molecules containing lanthanide elements is of theoretical interest, yet it is computationally challenging because of the large valence space, relativistic effects, and considerable electron correlation. We report a high-level ab initio study that quantifies the many-body nature of Ce–O bonding with the coordination environment of the Ce center and particularly the roles of the 4 f orbitals. The growing significance of the overlap between Ce 4 f and O 2 p orbitals with the increasing coordination of Ce atoms enhances Ce–O bond covalency and in return directs the molecular geometry. Upon partial reduction from neutral to anionic ceria, the excessive electrons populate the Ce-centered localized 4 f orbital. The interplay between the admixture and localization of the 4 f-block dually modulates bonding patterns of cerium oxide molecules, underlying the importance of many-body interactions between ligands and various lanthanide elements.

Unprecedented neptunyl(V) cation-directed structural variations in Np2Ox compounds

Studies on transuranic oxides provide a particularly valuable insight into chemical bonding in actinide compounds, in which subtle differences between metal ions and oxygen atoms are of fundamental importance for the stability of these compounds as well as their existence. In the case of neptunium, it is still mainly limited to specific Np oxide compounds without periodicity in the formation of stable structures or different oxidation states. Here, we report a systematic global minimum search of Np₂O_x (x = 1-7) clusters and the computational study of their electronic structures and chemical bonding. These studies suggest that Np(V) ion could play the structure-directing role, and thus the mixed-valent Np(III/V) in Np₂O₄ is predicted accessible. In comparison with lower oxidation state Np analogues, significant 5f-orbital covalent interactions with Np(V)O bonding are observed, which shows that these model neptunium oxides can provide new understandings into the behavior of 5f-electrons in chemical bonding and structural design.

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.

Slow Photoelectron Velocity-Map Imaging of Cryogenically Cooled Anions

Slow photoelectron velocity-map imaging spectroscopy of cryogenically cooled anions (cryo-SEVI) is a powerful technique for elucidating the vibrational and electronic structure of neutral radicals, clusters, and reaction transition states. SEVI is a high-resolution variant of anion photoelectron spectroscopy based on photoelectron imaging that yields spectra with energy resolution as high as 1-2 cm^-1. The preparation of cryogenically cold anions largely eliminates hot bands and dramatically narrows the rotational envelopes of spectral features, enabling the acquisition of well-resolved photoelectron spectra for complex and spectroscopically challenging species. We review the basis and history of the SEVI method, including recent experimental developments that have improved its resolution and versatility. We then survey recent SEVI studies to demonstrate the utility of this technique in the spectroscopy of aromatic radicals, metal and metal oxide clusters, nonadiabatic interactions between excited states of small molecules, and transition states of benchmark bimolecular reactions.

Bond Activation and Hydrogen Evolution from Water through Reactions with M3S4 (M = Mo, W) and W3S3 Anionic Clusters

Transition metal sulfides (TMS) are being investigated with increased frequency because of their ability to efficiently catalyze the hydrogen evolution reaction. We have studied the trimetallic TMS cluster ions, Mo₃S₄^-, W₃S₄^-, and W₃S₃^-, and probed their efficiency for bond activation and hydrogen evolution from water. These clusters have geometries that are related to the edge sites on bulk MoS₂ surfaces that are known to play a role in hydrogen evolution. Using density functional theory, the electronic structures of these clusters and their chemical reactivity with water have been investigated. The reaction mechanism involves the initial formation of hydroxyl and thiol groups, hydrogen migration to form an intermediate with a metal hydride bond, and finally, combination of a hydride and a proton to eliminate H₂. Using this mechanism, free energy profiles of the reactions of the three metal clusters with water have been constructed. Unlike previous reactivity studies of other related cluster systems, there is no overall energy barrier in the reactions involving the M₃S₄ systems. The energy required for the rate-determining step of the reaction (the initial addition of the cluster by water) is lower than the separated reactants (-0.8 kcal/mol for Mo and -5.1 kcal/mol for W). They confirm the M₃S₄^- cluster's ability to efficiently activate the chemical bonds in water to release H_2. Though the W₃S₃^- cluster is not as efficient at bond activation, it provides insights into the factors that contribute to the success of the M₃S₄ anionic systems in hydrogen evolution.

Prediction of Bond Dissociation Energies/Heats of Formation for Diatomic Transition Metal Compounds: CCSD(T) Works

It was recently reported ( J. Chem. Theory Comput. 2015 , 11 , 2036 - 2052 ) that the coupled cluster singles and doubles with perturbative triples method, CCSD(T), should not be used as a benchmark tool for the prediction of dissociation energies (heats of formation) for the first row transition metal diatomics based on a comparison with the experimental thermodynamic values for a set of 20 diatomics. In the present work the bond dissociation energies as well as the heats of formation for those diatomics have been calculated by the Feller-Peterson-Dixon approach at the CCSD(T)/complete basis set (CBS) level of theory including scalar relativistic corrections and correlation of the outer shell of core electrons in addition to the valence electrons. Revised experimental values for the hydrides are presented that are based on new heterolytic R-H bond dissociation energies, which are needed for analysis of the mass spectrometry experiments. The agreement between the calculated bond dissociation energies and the revised experimental values of the hydrides is good. Good agreement of the calculated bond dissociation energies/heats of formation is also found for most of the chlorides, oxides, and sulfides given the experimental error bars from experiment and those of the transition metal atoms in the gas phase. Thus, reliable results can be achieved by the CCSD(T) method at the CBS limit. The use of PW91 orbitals for the CCSD(T) calculations improves the predictions for some compounds with large T₁ diagnostics at the HF-CCSD(T) level. The optimized bond distances and calculated vibrational frequencies for the diatomics also agree well with the available experimental values.

Light-induced water splitting by titanium-tetrahydroxide: a computational study

Light induced water splitting by Ti(OH)₄ following the hydroxyl radical generation mechanism. Subsequent reactions lead to O₂ and H₂ production.

Reactivity of metal oxide clusters with hydrogen peroxide and water--a DFT study evaluating the performance of different exchange-correlation functionals

We have performed a density functional theory (DFT) investigation of the interactions of H2O2, H2O and HO radicals with clusters of ZrO2, TiO2 and Y2O3. Different modes of H2O adsorption onto the clusters were studied. In almost all the cases the dissociative adsorption is more exothermic than molecular adsorption. At the surfaces where H2O has undergone dissociative adsorption, the adsorption of H2O2 and the transition state for its decomposition are mediated by hydrogen bonding with the surface HO groups. Using the functionals B3LYP, B3LYP-D and M06 with clusters of 26 and 8 units of ZrO2, the M06 functional performed better than B3LYP in describing the reaction of decomposition of H2O2 and the adsorption of H2O. Additionally, we investigated clusters of the type (ZrO2)2, (TiO2)2 and (Y2O3) and the performance of the functionals B3LYP, B3LYP-D, B3LYP*, M06, M06-L, PBE0, PBE and PWPW91 in describing H2O2, H2O and HO˙ adsorption and the energy barrier for decomposition of H2O2. The trends obtained for HO˙ adsorption onto the clusters are discussed in terms of the ionization energy of the metal cation present in the oxide. In order to correctly account for the existence of an energy barrier for the decomposition of H2O2, the functional used must include Hartree-Fock exchange. Using minimal cluster models, the best performance in describing the energy barrier for H2O2 decomposition was obtained with the M06 and PBE0 functionals - the average absolute deviations from experiments are 6 kJ mol(-1) and 5 kJ mol(-1) respectively. With the M06 functional and a larger monoclinic (ZrO2)8 cluster model, the performance is in excellent agreement with experimental data. For the different oxides, PBE0 was found to be the most effective functional in terms of performance and computational time cost.

Density functional study of water-gas shift reaction on M3O(3x)/Cu(111)

Density functional theory (DFT) was employed to study the water dissociation and water-gas shift (WGS) reaction on a series of inverse model catalysts, M(3)O(3x)/Cu(111) (M = Mg, Ti, Zr, Mo, W; x = 1, 2, 3). It has been found that the WGS reaction on Cu can be facilitated by introducing various oxides to lower the barrier of water dissociation. Accordingly, the calculated reaction energy for water dissociation was used as a scaling descriptor to screen the WGS activity of oxide-Cu model catalysts. Our calculations show that the activity towards water dissociation decreases in a sequence: Mg(3)O(3)/Cu(111) > Zr(3)O(6)/Cu(111) > Ti(3)O(6)/Cu(111) > W(3)O(9)/Cu(111), Mo(3)O(9)/Cu(111). It seems that Mg(3)O(3)/Cu(111) is the best WGS catalyst among the systems studied here, being able to dissociate water with no barrier. During the process, both Cu and oxides participate in the reaction directly. The strong M(3)O(3x)-Cu interaction is able to tune the electronic structure of M(3)O(3x) and therefore the activity towards water dissociation. Further studies of the overall WGS reaction on Mg(3)O(3)/Cu(111) show that water dissociation may not be the key step to control the WGS reaction on Mg(3)O(3)/Cu(111) and the removal of H from Mg(3)O(3) can be problematic. The strong interaction between H and O from Mg(3)O(3) blocks the O sites for further water dissociation and therefore the WGS reaction. Our study observes a very different behavior of oxide clusters in such small size from the bigger ones supported on Cu(111) and provides new insight into the rational design of the WGS catalysts.

Toward accurate theoretical thermochemistry of first row transition metal complexes

The recently developed correlation consistent Composite Approach for transition metals (ccCA-TM) was utilized to compute the thermochemical properties for a collection of 225 inorganic molecules containing first row (3d) transition metals, ranging from the monohydrides to larger organometallics such as Sc(C(5)H(5))(3) and clusters such as (CrO(3))(3). Ostentatiously large deviations of ccCA-TM predictions stem mainly from aging and unreliable experimental data. For a subset of 70 molecules with reported experimental uncertainties less than or equal to 2.0 kcal mol(-1), regardless of the presence of moderate multireference character in some molecules, ccCA-TM achieves transition metal chemical accuracy of ±3.0 kcal mol(-1) as defined in our earlier work [J. Phys. Chem. A2007, 111, 11269-11277] by giving a mean absolute deviation of 2.90 kcal mol(-1) and a root-mean-square deviation of 3.91 kcal mol(-1). As subsets are constructed with decreasing upper limits of reported experimental uncertainties (5.0, 4.0, 3.0, 2.0, and 1.0 kcal mol(-1)), the ccCA-TM mean absolute deviations were observed to monotonically drop off from 4.35 to 2.37 kcal mol(-1). In contrast, such a trend is missing for DFT methods as exemplified by B3LYP and M06 with mean absolute deviations in the range 12.9-14.1 and 10.5-11.0 kcal mol(-1), respectively. Salient multireference character, as demonstrated by the T(1)/D(1) diagnostics and the weights (C(0)(2)) of leading electron configuration in the complete active self-consistent field wave function, was found in a significant amount of molecules, which can still be accurately described by the single reference ccCA-TM. The ccCA-TM algorithm has been demonstrated as an accurate, robust, and widely applicable model chemistry for 3d transition metal-containing species with versatile bonding features.