Hydrogen peroxide decomposition by nanocomposites halloysite nanotubes/cerium oxide

A number of nanomaterials based on halloysite nanotubes (HNT) decorated with CeO2 were synthesized by the precipitation method of aqueous cerium nitrate at room temperature without stabilizers in the presence of HNT. The amount of nanoceria deposited in nanomaterials ranges from 0.99 to 19.15 % wt. Electron microscopy has shown that the size of CeO2 particles varies in the range of 2.6–17.5 nm. The cubic structure of cerium dioxide has been shown by X-ray diffraction analysis of the samples. The characteristics of the porous structure were determined based on low-temperature nitrogen adsorption/desorption isotherms. The specific surface area ranged from 31 to 54 m2/g. IR spectroscopy determined the type of interaction between the modifier and the matrix. The IUVS Ce4+/IUVS Ce3+ ratio in nanocomposites was estimated from UV-spectra of diffuse reflectance; it varied in the range of 0.25 to 2.55. The catalytic activity of the synthesized materials and halloysite nanotubes was determined by calculating the affinity constant from kinetic data of the model reaction of hydrogen peroxide decomposition in the pH range of 8.0 to 11.0 and compared to the catalase enzyme and commercial nanosized CeO2. The catalytic activity of pristine HNT decreased with increasing pH. We can assume that the activity of the synthesized nanocomposites at pH range 9.0–11.0 is determined by the presence and properties of a decorator – nanoceria. The pH dependence of the activity of the nanocomposites was shown to be extreme with the maximum in the pH range of 9.5–10.5. An extreme dependence of the activity on the content of the decorator with a maximum for the sample of 3.19 % wt. CeO2 was found. Recalculation of the catalytic capability of nanocomposites to 100 % of the content of the decorator in them makes it possible to analyze the factors that determine the activity of nanoceria. Under these conditions, the best catalytic activity demonstrates by sample HNT-1CeO2, which contains 0.99 % wt. of the modifier and has a ratio of Ce4+/Ce3+ 0.25. It was found that the activity of nanoceria in nanocomposites decreases with: increasing Ce4+/Ce3+ ratio, i.e. with decreasing the number of oxygen vacancies; with increasing cerium oxide content - due to aggregation of nanocrystallites; and with increasing particle diameter due to the reduction of the surface. The activation energy (Ea) of the reaction of hydrogen peroxide decomposition by the nanocomposites and pristine HNT in the temperature range of 20–40 °C at pH 10 was determined by kinetic data. The Ea for pristine HNT is 170 kJ/mol. It has been shown that the increasing content of nanoceria in nanocomposites leads to a decrease in the activation energy of the reaction of hydrogen peroxide decomposition from 154 kJ/mol for the material with the lowest modifier content to 112 kJ/mol for the nanocomposite with the highest amount of a modifier.