Ni-induced structural modification and decreased catalytic activity of α-MnO₂ in toluene oxidation

Abstract

Ni-modified α-MnO₂ catalysts containing 5 and 10 mol.% Ni were synthesized by a redox precipitation method using KMnO₄ and ethanol, followed by calcination at 400 °C. The influence of Ni addition on the structural, textural, and redox properties of α-MnO₂ and its catalytic performance in toluene oxidation was investigated. X-ray diffraction analysis confirmed the formation of cryptomelane-type K₂Mn₈O₁₆ as the main phase. At 5 mol.% Ni, a slight shift of the characteristic α-MnO₂ reflection suggested partial Ni incorporation, whereas at 10 mol.% Ni, a secondary NiMnO₃ phase was observed. SEM observations revealed significant morphological changes from interconnected fibrous structures to fragmented rod-like particles. Nitrogen adsorption measurements showed that Ni addition reduced the specific surface area from 61 to 27–42 m² g⁻¹ and the pore volume from 0.37 to approximately 0.12 cm³ g⁻¹. H₂-TPR analysis showed that the main reduction peak shifted from approximately 284 °C to 317–321 °C after Ni addition, indicating lower low-temperature reducibility. Pristine α-MnO₂ exhibited the highest catalytic activity, with a T₅₀ of 274 °C and a CO₂ yield of 97.6% at approximately 390 °C. The corresponding T₅₀ values of 5Ni–α-MnO₂ and 10Ni–α-MnO₂ were 316 and 291 °C, with CO₂ yields of 75% and 86%, respectively. The lower catalytic performance of the Ni-modified catalysts was associated with the combined effects of reduced porosity, decreased reducibility, and Ni-induced structural changes.