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

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Abstract
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.

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