Copper and iron co-doping effects on the structure, optical energy band gap, and catalytic behaviour of Co3O4 nanocrystals towards low-temperature total oxidation of toluene

Abstract

This study reports the effect of Cu and Fe addition on the structure, morphology, optical energy band gap and the catalytic performance of Co₃O₄ nanocrystals (NCs). Single-oxide (CuO, Fe₃O₄, and Co₃O₄) and trimetallic oxide (Cu_0.75Fe_0.25Co₂O₄) NCs were prepared by a co-precipitation technique. The physico-chemical properties of the as-prepared NCs were thoroughly investigated and their performance as catalysts was successfully evaluated. Compared with single oxides, the Cu_0.75Fe_0.25Co₂O₄ spinel with a dome top-like morphology exhibited the highest performance with almost 100% oxidation of C₇H₈ at 220 °C at a gas hourly space velocity (GHSV) of 22 500 mL g⁻¹ h⁻¹. The sequence of the catalytic performance was found to be proportional to the activation energy (E_a (kJ mol⁻¹)) and the performance order is as follows: Cu_0.75Fe_0.25Co₂O₄ (35.2) > Co₃O₄ (55.5) ∼ CuO (75.5) > Fe₃O₄ (83.4). The excellent performance of Cu_0.75Fe_0.25Co₂O₄ towards toluene total oxidation is suggested to be assigned to the Cu and Fe co-doping effect and behave as promoters in the matrix of the Co₃O₄ spinel. The promotion effect can be ascribed to: (a) the smallest crystallite grain size as responsible for the largest surface specific area, (b) the lowest-temperature reducibility responsible for the highest population of oxygen vacancies, (c) the highest mobility of oxygen species due to its good redox capability and (d) the highest ratio of active species (O_Lat/O_Ads, Co³⁺/Co²⁺, Cu²⁺/Cu⁺, and Fe³⁺/Fe²⁺) and the lowest E^opt_g. The overall findings suggest that toluene total oxidation over Cu_0.75Fe_0.25Co₂O₄ might follow a Mars van Krevelen process.