Influence of calcination temperature on catalytic activity of nanoceria

Abstract

Nanoceria was synthesized by reaction of cerium nitrate deposition in an aqueous medium without stabilizers at room temperature. Nano-sized cerium oxide was dried at 20 °C and calcinated in air for 1 hour at 120, 300, 500, 800 °C. SEM images of samples demonstrated that the morphology of the obtained cerium oxide does not significantly change with the increase of the temperature of heat treatment. Electron microscopy showed that the average diameter of CeO₂ particles varies in the range of 12.4–15.9 nm. Sample element content was determined by the energy-dispersive X-ray spectrometry method. The Oxygen:Cerium elements ratio in the samples is in the range 1.7–2.1. X-Ray Diffraction method was used to determine the structural characteristics of materials. It was demonstrated that with increasing annealing temperature, the average crystallite diameter increases from 10 to 23 nm, and the degree of crystallinity changes from 60 % for Ce-20 to 100 % for Ce-800. The characteristics of the porous structure were determined based on low-temperature nitrogen adsorption/desorption isotherms. The specific surface area (BET) of the samples heated to 500 °C varies within 46–61 m²/g. The total pore volume varies from 0.19 to 0.22 cm³/g. After heating to 800 °C, the specific surface area and total pore volume decrease to 17 m²/g and 0.13 cm³/g, respectively. By TGA method was found that 5 % mass loss between 20 and 300 °C is attributed to adsorbed water, while 9 % starting at around 500 °C refers to release from the surface of chemically bonded water molecules. The I_{UVS Ce}⁴⁺/I_{UVS Ce}³⁺ ratio in samples was found from UV-spectra of diffuse reflectance; it varied in the range of 1.60 to 2.08. Calcination of nanoceria samples at temperatures above 500 °C leads to the oxidation of Ce³⁺ to Ce⁴⁺ and reduction of nanoceriа surface defects. The catalytic activity of the synthesized materials was evaluated by the determination of the reaction rate constant (k) of the H₂O₂ decomposition reaction at the different concentrations (1–10 %) at room temperature and within pH 8.0–11.0. Materials calcinated at different temperatures demonstrate maximum catalytic activity at pH 10.0, which is determined by the increase in the content of deprotonated ceranol groups on the surface of cerium oxide with an increase in the pH from 8.0 to 10.0, and the formation of insoluble Cerium compounds with a further increase in pH. The activation energy (E_a) of the reaction of hydrogen peroxide decomposition by nanoceria in the temperature range of 20–40 °C at pH 10 was determined by kinetic data. The E_a for un-annealing sample Ce-20 is 127 kJ/mol. Increasing temperature to 120 °C does not change the E_a. It was shown that the smallest value of activation energy is 77 kJ/mol for the sample calcinated at 300 °C. Heating the samples at temperatures 500 and 800 °C causes growth of E_a – to 94 and 95 kJ/mol, respectively.

We did not find correlation between degree of crystallinity, specific surface area, total pore volume, crystallite size and calcinated samples catalytic activity. The dependence of the rate constant (activity) on the calcination temperature is extreme with a maximum at 300 °C was found. A sample Ce-300, which has the highest O:Ce ratio (2.08), the largest O content (67.5 %), the lowest Ce⁴⁺/Ce³⁺ ratio (0.15) among the calcinated samples, and therefore the largest number of surface defects exhibits the highest catalytic activity and has the lowest activation energy for the hydrogen peroxide decomposition reaction. Presumably, when heating CeO₂ samples, some parallel processes occur. Changes in the values of structural parameters either have no effect or are insignificant and also do not affect the catalytic activity of nanoceria. Desorption of physically adsorbed water, which inactivates the catalytic centers, leads to an increase in catalytic activity of the material. In addition, destruction of ceranol groups on the surface of cerium oxide with loss of oxygen and oxidation of Ce³⁺ to Ce⁴⁺ reduces catalytic activity.