SNIC
SUPR
SNIC SUPR
Assembling and Disassembling of Polyoxometalates
Dnr:

SNIC 2018/3-422

Type:

SNAC Medium

Principal Investigator:

Magdalena Pascual Borras

Affiliation:

Umeå universitet

Start Date:

2018-09-03

End Date:

2019-10-01

Primary Classification:

10404: Inorganic Chemistry

Secondary Classification:

10402: Physical Chemistry

Tertiary Classification:

10403: Materials Chemistry

Allocation

Abstract

The behavior of metal oxides and molecular metal-oxo cluster at an aqueous interface or in solution is dictated by numerous complex interrelated. The most important solution characteristics affecting metal-oxo cluster speciation include pH, concentration, temperature, electrochemical potential and electrolytes. The effect of some of these characteristics are well understood and predictable while others are confounding, interdependent and variable. Polyoxometalates (POMs) are group 5/6 d0 polynuclear clusters that are surface stabilized by multiple bonded oxo-ligands. It well-known that POMS of Mo, V and W assemble via bottom-up route. Acidifying aqueous mononuclear oxoanions MO42/3- (M=W, Mo, V) drives hydrolysis and condensation. On the other hand, mechanisms of Nb and Ta POMs are not understood, and best described as a top-down approach. Monomer oxoanions of Ta and Nb are not isolable, because the larger size and lower charge densities of Ta/Nb+5, compared to V+5 and W/Mo+6. Instead, dissolution of Ta and Nb oxide in highly alkaline solution yields only Lindqvist anion [M6O19]8-, M=Ta and W. Thus, there are a lack of knowledge of Nb(Ta) POM formation mechanisms. Decaniobate ([Nb10O28]6-) is a simple POM-related to [Nb6O19]8-. Decaniobate is stable and soluble at neutral pH, but converts to [Nb6O19]8- with pH increase. The conversion of [Nb10O28]6- to [Nb6O19]8- proceeds via ‘heptaniobate’ indeterminate. Then, the study of [Nb7O22]9- could give us clues of how these POMs are formed. Herein, we will explore the formation of [Nb6O19]8- from [Nb7O22]9- and [Nb10O28]6- by adding water molecules by leading to structure opening pathways by using Density Functional Theory. On the other hand, Raman spectroscopy has been shown to be a suitable tool for investigating Tantalum and Niobium speciation, including ogliomerization and its dependence in concentration, ionic strength and pH as well as interactions with different cations species. Combining experimental and computation data, we will examine how polyoxotantalates and polyoxoniobates ions reacts with hydrogen peroxide and develop an understanding of their solutions behaviors by comparison both data sets. We will compute the Raman frequencies of all possible products and try to rationalize with experimental data. The results of this project will be particular relevant to the use of niobates and tantalates as a water-splitting catalysts, since the terminal niobium/tantalum-bound oxygen atoms will be replaced by peroxy groups, a product of water oxidation.