Fabricating PdCe/OMS-2 catalysts with boosted low-temperature activity for toluene deep degradation

Abstract

Porous cryptomelane-type octahedral molecular sieve (OMS-2) with mixed Mn valence and abundant lattice oxygen species has attracted much attention in volatile organic compounds (VOC) catalytic elimination. However, complete conversion of arene over OMS-2 catalysts at relatively low temperature is still a challenge due to its limited crystal structure and inferior stability. Here, a series of PdCe/OMS-2 catalysts with different Pd/Ce molar ratios was fabricated by a facile impregnation method and the physicochemical properties of which were extensively characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM), Brunauer–Emmett–Teller (BET) method, X-ray fluorescence (XRF), X-ray photoelectron spectroscopy (XPS), temperature programmed reduction of H2 (H2-TPR), Raman, In situ diffused reflectance infrared Fourier transform spectra (DRIFTS), and density functional theory (DFT) calculations. Results show that the total conversion of toluene can be achieved at 207 °C over PdCe2 with apparent activation energy as low as 62.6 kJ/mol. The strong synergistic effect between Pd and Ce remarkably boosts the catalytic activity of OMS-2, attributed to the abundant Mn3+–O bands and active surface oxygen species. DFT results reveal that oxygen vacancy can be formed over PdCe2 much easily than that of Pd/OMS-2 and Ce/OMS-2 with the oxygen vacancy formation energy of 2.42, 2.83 and 2.68 eV, respectively. Simply increasing the Pd content cannot promote the catalytic activity although PdO is a critical active center in toluene oxidation. Oxygen vacancy attributed to the integrative effect of Pd, Ce and Mn species plays a prominent role over prepared catalysts in toluene activation process. The findings reported in this work showed new insights into the designing of highly efficient OMS-2 catalysts for VOC deep oxidation by tuning oxygen vacancy concentration.