Effect of Doping Metals on OMS-2/γ-Al2O3 Catalysts for Plasma-Catalytic Removal of o-Xylene

A series of Pd-, Ce-, Co-, and Ti- doped OMS-2 catalysts were prepared by a one-step hydrothermal method. The removal of o-xylene by a combination of metal-doped OMS-2/gamma-Al2O3 catalysts and nonthermal plasma was systematically investigated. Catalysts were characterized by inductively coupled plasma optical emission spectrometry (ICP-OES), X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), extended X-ray absorption fine structure (EXAFS), X-ray photoelectron spectroscopy (XPS), and H-2 temperature-programmed reduction (H-2-TPR). Among the metal-doped OMS-2/gamma-Al2O3 catalysts used in the plasma-catalytic process, Pd-0.07-OMS-2/gamma-Al2O3 and Ce-0.07-OMS-2/gamma-Al2O3 exhibited excellent catalytic activity for o-xylene removal with 100% o-xylene conversion, above 60% CO2 selectivity, and above 80% (CO+CO2) (i.e., COx) yield at a low specific energy density of 18 J/L. These two catalysts had high redox ability, abundant surface defects for absorbing and activating oxygen, and more active surface oxygen species than Co-0.07-OMS-2/gamma-Al2O3 and Ti-0.07-OMS-2/gamma-Al2O3 catalysts, therefore resulting in much higher catalytic activity in the oxidation of o-xylene. The concentration of by products produced by nonthermal plasma such as O-3 and NOx was greatly reduced when using these metal -doped OMS-2/gamma-Al2O3 catalysts. pd(0.07)-OMS-2/gamma-Al2O3 had the highest suppression ability for O-3 and NOx which were reduced to near zero. Moreover, Fourier transform infrared spectroscopy (FT-IR) results suggested that introduction of metal-doped OMS-2/gamma-Al2O3 catalysts could oxidize o-xylene to CH3CHO and/or CH3COOH and further oxidize CH3CHO and/or CH3COOH to CO or CO2.