Computational Study of the Hydrolysis Reactions of Small MO₂ (M = Zr and Hf) Nanoclusters with Water

Abstract

Density functional theory (DFT) has been used to study the hydrolysis reaction of (MO2) n (M = Zr, Hf, n = 1–4) nanoclusters in the ground singlet and first triplet states. The reactions for singlet n = 1 were benchmarked at the CCSD(T) level of theory. The reactions of H2O with the metal site having an MO bond and/or M–O bonds as well as H transfer to both terminal O atoms and bridge −O atoms have been studied. The partial charge on M increases as the MO bonds are replaced with M–OH bonds. The first H2O adsorption (physisorption) energies for these MO2 nanoclusters are calculated to be −20 to −30 kcal/mol for the singlet state and −15 to −48 kcal/mol for the triplet state. These physisorption energies depend on the cluster size and the adsorption site, consistent with existing experimental and computational studies. The first hydrolysis (dissociative chemisorption) reaction energies of the MO2 nanoclusters are calculated to have a much broader range, −30 to −80 kcal/mol for the singlet states and −30 to −100 kcal/mol for the triplet states. Steric effects play an important role in determining the physisorption and chemisorption energies, especially for the trimers and tetramers. The potential energy surfaces for hydrolysis in both the singlet and triplet states are calculated. The calculated Lewis acidities (fluoride affinities) correlate with the hydrolysis properties of the nanoclusters. Our calculations show that H2O readily reacts with both the singlet and triplet states of the MO2 nanoclusters to form the hydroxides. The reaction barriers are generally less than 10 kcal/mol for the singlet states, and because the H2O physisorption energies are large, the barriers occur below the (MO2) n asymptote.

Gas Phase Hydrolysis and Oxo‐Exchange of Actinide Dioxide Cations: Elucidating Intrinsic Chemistry from Protactinium to Einsteinium

“…The computed initial complexation energy, Δ H (hydrate), ranges from −30 to −41 kcal mol −1 , an energy range consistent with a Lewis acid‐base interaction of H 2 O with a positively charged metal centre . The Δ H (hydrate) are close to −37 kcal mol −1 (to within <3 kcal mol −1 ) for PaO 2 + through CmO 2 + ; such near constancy is expected for such a Lewis acid‐base interaction.…”

Unveiling the Coexistence of Cis and Trans Isomers in the Hydrolysis of ZrO2: A Coupled DFT and High-Resolution Photoelectron Spectroscopy Study

“…Experimentally determined parameters are reported in Tables 1 and 2 Dixon and co-workers 91 and similar to our previous report on TiO(OH) 2 , 64 a dissociative adduct (1-1a ) with a cis-OH geometry was found to be the lowest energy neutral structure.…”

“…Bare (ZrO 2 ) 0/n clusters have been extensively studied using PES, [81][82][83] matrix-IR spec-troscopy, 84 Fourier-transform microwave spectroscopy, 85 laser-induced fluorescence, 86 resonant multi-photon ionization, 86 dispersed fluorescence, 86 and computational methods. 81,84,[87][88][89][90] Despite this growing body of work, there is no experimental data on the reactions of these clusters with a water molecule, though computational studies by Dixon et al 91 on the (ZrO 2 ) n + H 2 O (n=1-4) reaction find these clusters are capable of splitting water. That work shows the dissociative adduct to be more stable than the molecularly adsorbed species by roughly 200 kJ/mol, at both the DFT and coupled cluster levels of theory, in agreement with experimental observations on bulk ziroconia.…”

Reliable Potential Energy Surfaces for the Reactions of H2O with ThO2, PaO2+, UO22+, and UO2+

“…17 We have been exploring the reactions of H 2 O with related transition metal oxide molecules and nanoclusters and have found similar types of potential energy surfaces. 18,19,20 In the current work, we have expanded our study to the early actinides and have studied the addition of H 2 O to ThO 2 , PaO 2 + , and UO 2 2+ to understand how the trends observed previously for the +5 actinyls depend on the oxidation state changing from +IV to +VI. We also studied the hydrolysis reaction of UO 2 + as well.…”