Effect of interaction between Pd and Fe in modified red mud on catalytic decomposition of toluene

As an industrial solid waste produced by alumina industry, red mud was modified as support of Pd catalysts for toluene catalytic oxidation in this paper. The xPd/MRM catalysts had high activity for toluene catalytic oxidation, and the 0.3Pd/MRM catalyst showed the best catalytic performance (T₅₀ = 175 °C and T₁₀₀ = 200 °C). The results indicated that the prepared 0.3Pd/MRM catalyst had more ratio of surface-adsorbed oxygen and Fe³⁺, rather than MRM and RM, which benefitted to the toluene oxidation. The excessive Pd species and the growth of the PdO nanoparticles negatively affected the catalytic efficiency of toluene. 0.4Pd/MRM activity decreased because of PdO aggregation in the catalyst, which could be confirmed by TEM analysis. The results of XPS, H₂-TPR, FT-IR, O₂-TPD, and Raman examination revealed that the formation of Pd-O-Fe under the interaction between Fe in MRM and Pd (Pd²⁺ + Fe ²⁺ → Pd⁰ + Fe³⁺) increased the electron transfer and raised the mobility of surface-adsorbed oxygen. Furthermore, in situ DRIFTS and GC-MS were used to detect intermediate products of catalytic reactions, and the reaction mechanism of catalysts was also studied. The catalytic oxidation of toluene on 0.3Pd/MRM catalyst might have two reaction paths simultaneously. The first reaction path would be toluene → species benzyl → benzaldehyde → benzoic acid → long-chain aldehydes or carboxylic acids → CO₂ and H₂O. The second reaction path would be toluene → benzene → phenol → long-chain aldehydes or carboxylic acids → CO₂ and H₂O.