1H NMR Study of the HCa2Nb3O10 Photocatalyst with Different Hydration Levels.

Marina G Shelyapina,Elizaveta A Andronova,Anastasiia O Antonenko,Denis Y Nefedov,Irina A Zvereva,Oleg I Silyukov,Alexander Missyul,Sergei A Kurnosenko

doi:10.3390/molecules26195943

Marina G Shelyapina, Elizaveta A Andronova + Show 6 more

Open Access

https://doi.org/10.3390/molecules26195943

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Abstract

The photocatalytic activity of layered perovskite-like oxides in water splitting reaction is dependent on the hydration level and species located in the interlayer slab: simple or complex cations as well as hydrogen-bonded or non-hydrogen-bonded H2O. To study proton localization and dynamics in the HCa2Nb3O10·yH2O photocatalyst with different hydration levels (hydrated—α-form, dehydrated—γ-form, and intermediate—β-form), complementary Nuclear Magnetic Resonance (NMR) techniques were applied. 1H Magic Angle Spinning NMR evidences the presence of different proton containing species in the interlayer slab depending on the hydration level. For α-form, HCa2Nb3O10·1.6H2O, 1H MAS NMR spectra reveal H3O+. Its molecular motion parameters were determined from 1H spin-lattice relaxation time in the rotating frame (T1ρ) using the Kohlrausch-Williams-Watts (KWW) correlation function with stretching exponent β = 0.28: eV, s. For the β-form, HCa2Nb3O10·0.8H2O, the only 1H NMR line is the result of an exchange between lattice and non-hydrogen-bonded water protons. T1ρ(1/T) indicates the presence of two characteristic points (224 and 176 K), at which proton dynamics change. The γ-form, HCa2Nb3O10·0.1H2O, contains bulk water and interlayer H+ in regular sites. 1H NMR spectra suggest two inequivalent cation positions. The parameters of the proton motion, found within the KWW model, are as follows: eV, s.

Highlights

In recent years, layered perovskite-like oxides have attracted much attention because of their outstanding physical and chemical properties, including high-temperature superconductivity [1,2], colossal magnetoresistance [3], the capability of photocatalytic water decomposition under sunlight irradiation for further hydrogen storage [4,5], and ionic conductivity due to high mobility of interlayer cations [6,7]
For the most hydrated α-form, HCa2Nb3O10·1.6H2O, 1H MAS Nuclear Magnetic Resonance (NMR) spectra reveal the presence of different proton-containing species: H3O+, which comprises all the lattice protons, and water molecules that are localized in different sites of the interlayer slab and supposedly participate in the formation of charged complexes like H+ . . . 2H2O
The signal from the water proton is so broad that it is undetectable; the only detectable signal is from H3O+

Summary

Introduction

In recent years, layered perovskite-like oxides have attracted much attention because of their outstanding physical and chemical properties, including high-temperature superconductivity [1,2], colossal magnetoresistance [3], the capability of photocatalytic water decomposition under sunlight irradiation for further hydrogen storage [4,5], and ionic conductivity due to high mobility of interlayer cations [6,7]. Water content and its state and localization should affect both the pathway and efficiency of chemical or photocatalytic reactions From this perspective, an identification of proton-containing species and a comprehensive study of their motion in the interlayer slab is required. We report on the results of the proton NMR spectroscopy and relaxation studies of the layered perovskite-like niobate HCa2Nb3O10 with different hydration levels: hydrated—α-form, dehydrated—γ-form, and intermediate—β-form.

X-ray and TG Analysis

Materials and Methods

Conclusions