Optimized Synthesis Routes of MnOx-ZrO2 Hybrid Catalysts for Improved Toluene Combustion

Abstract

In this contribution, the three Mn-Zr catalysts with MnxZr1−xO2 hybrid phase were synthesized by two-step precipitation route (TP), conventional coprecipitation method (CP) and ball milling process (MP). The components, textural and redox properties of the Mn-Zr hybrid catalysts were studied via XRD, BET, XPS, HR-TEM, H2-TPR. Regarding the variation of synthesis routes, the TP and CP routes offer a more obvious advantage in the adjustment of the concentration of MnxZr1−xO2 solid solution compared to the MP process, which directly commands the content of Mn4+ and oxygen vacancy and lattice oxygen, and thereby leads to the enhanced mobility of reactive oxygen species and catalytic activity for toluene combustion. Moreover, the TP-Mn2Zr3 catalyst with the enriched exposure content of 51.4% for the defective (111) lattice plane of MnxZr1−xO2 exhibited higher catalytic activity and thermal stability for toluene oxidation than that of the CP-Mn2Zr3 sample with a value of 49.3%. This new observation will provide a new perspective on the design of bimetal catalysts with a higher VOCs combustion abatement.

VOCs Photothermo-Catalytic Removal on MnOx-ZrO2 Catalysts

“…The positive synergistic effect due to the addition of a small amount of zirconia on the MnO x and the solar multi-catalytic approach led to obtaining a low toluene T 90 , considering the absence of noble metals co-catalysts. The obtained value of T 90 with the MnO x -5% ZrO 2 sample (180 • C) is comparable or lower with respect to the other MnO xbased catalysts reported in the literature (in the range 200 • C-270 • C considering an initial toluene concentration of 1000 ppm [20,37]).…”

“…In both the mixed oxides, the signals related to ZrO2 are absent, probably due to the low amount of hosted oxide and/or to the good dispersion of zirconium oxide on manganese oxide. The ion radius of Zr 4+ (0.84 Å) is similar of Mn 2+ (0.83 Å), and this can favour the ionic exchange between these cations [20,23]. On the contrary, the smaller radius of Mn 3+ (0.64 Å) makes the Zr 4+ /Mn 3+ exchange more difficult.…”

“…Compared to the bare Mn3O4, the slight increase of the crystallite size of the mixed oxides determined a decrease of their surface area, which were about 85-86 m 2 /g for both the MnOx-ZrO2 samples (Table 1). The ion radius of Zr 4+ (0.84 Å) is similar of Mn 2+ (0.83 Å), and this can favour the ionic exchange between these cations [20,23]. On the contrary, the smaller radius of Mn 3+ (0.64 Å) makes the Zr 4+ /Mn 3+ exchange more difficult.…”

“…On the contrary, the smaller radius of Mn 3+ (0.64 Å) makes the Zr 4+ /Mn 3+ exchange more difficult. Probably, when the amount of ZrO 2 is low, the Zr ions partially replace the Mn 2+ promoting, in this way, the main presence of Mn 3+ , whereas a higher amount of ZrO 2 led to a preferential surface covering of the MnO x instead of a lattice incorporation of the zirconium ions in MnO x [20,23]. Thus, it can explain the major presence of Mn (III) on MnO x -5%ZrO 2 sample, and the coexistence of Mn II and III in the MnO x -10%ZrO 2 .…”

“…Furthermore, it is a large bandgap semiconductor (E g of about 5.0 eV [18] or lower depending to the zirconia synthesis). Therefore, its coupling with a lower bandgap semiconductor (as MnO x ) can be a performing and fascinating strategy to reduce the odds of charge recombination (a common reason for photocatalysts deactivation) and to synergistically exploit both the thermal stability and the redox properties of MnO x and ZrO 2 [19,20] together with their photocatalytic features.…”