Molecular Structures, Acid-Base Properties, and Formation of Group 6 Transition Metal Hydroxides

Journal Article · Thu Apr 28 00:00:00 EDT 2011 · Journal of Physical Chemistry. C

DOI:https://doi.org/10.1021/jp111031x· OSTI ID:1023183

Li, Shenggang; Guenther, Courtney L.; Kelley, Matthew S.; Dixon, David A.

Density functional theory (DFT) and coupled cluster theory (CCSD(T)) were used to study the group 6 metal (M = Cr, Mo, W) hydroxides: MO3-m(OH)2m (m = 1-3), M2O6-m(OH)2m (m = 1-5), M3O9-m(OH)2m (m = 1, 2), and M4O11(OH)2. The calculations were done up to the complete basis set (CBS) limit for the CCSD(T) method. Molecular structures of many low-energy conformers/isomers were located. Brønsted acidities in the gas phase and pKa values in aqueous solution were predicted for MO3-m(OH)2m (m = 1-3) and MnO3n-1(OH)2 (n = 2-4). In addition, Brønsted basicities and Lewis acidities (fluoride affinities) were predicted for MO3-m(OH)2m (m = 1-3) as well as the metal oxide clusters MnO3n (n = 1-3). The metal hydroxides were predicted to be strong Brønsted acids and weak to modest Brønsted bases and Lewis acids. The pKa values can have values as negative as -31. Potential energy surfaces for the hydrolysis of the MnO3n (n = 1-4) clusters were calculated. Heats of formation of the metal hydroxides were predicted from the calculated reaction energies, and the agreement with the limited available experimental data is good. The first hydrolysis step leading to the formation of MnO3n-1(OH)2 was predicted to be exothermic, with the exothermicity becoming less negative as n increases and essentially converged at n = 3. Reaction rate constants for the hydrogen transfer steps were calculated using transition state theory and RRKM theory. Further hydrolysis of MnO3n-1(OH)2 tends to be endothermic especially for M = Cr. Fifty-five DFT exchange-correlation functionals were benchmarked for the calculations of the reaction energies, complexation energies, and reaction barriers by comparing to our CCSD(T) results. Overall, the DFT results for the potential energy surfaces are semiquantitatively correct, but no single functional works for all processes and all three metals. Among the functionals benchmarked, the wB97, wB97X, B1B95, B97-1, mPW1LYP, and X3LYP functionals have the best performance. Linear correlations between the calculated reaction barrier and reaction energy for hydrogen transfer reactions to the terminal =O atom and to the bridge O atom were found to be quite different, indicating their different reaction properties. The calculated Lewis acidity (fluoride affinity) was found to best correlate with the calculated adsorption energy, the dissociative adsorption energy, and the reaction barrier for hydrogen transfer reactions to the terminal =O atom.

• Research Organization: Pacific Northwest National Lab. (PNNL), Richland, WA (United States). Environmental Molecular Sciences Lab. (EMSL)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 1023183
• Journal Information: Journal of Physical Chemistry. C, Vol. 115, Issue 16; ISSN 1932-7447
• Publisher: American Chemical Society
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
ADSORPTION
AFFINITY
AQUEOUS SOLUTIONS
ATOMS
BROENSTED ACIDS
DENSITY FUNCTIONAL METHOD
FUNCTIONALS
HYDROGEN TRANSFER
HYDROLYSIS
HYDROXIDES
LEWIS ACIDS
MOLECULAR STRUCTURE
OXIDES
PERFORMANCE
PH VALUE
POTENTIAL ENERGY
REACTION KINETICS
TRANSITION ELEMENTS
Environmental Molecular Sciences Laboratory