Study On Preparation And The Catalytic Performance Of Co-based Catalysts For Catalytic Oxidation Of NO

Nitrogen oxide?NOx?emissions is one of the causes of the formation of acid rain,haze,etc.The handy and inexpensive way to remove NOx from industrial furnaces is that catalytic oxidation of NO to NO₂ by using the remaining O₂ in fule gas,then adsorbing the mixed gas by alkaline liquor.On the base of NO catalytic oxidation,Co-based catalysts were prepared and their performance was studied.And the catalysts were characterized by various techniques such as X-ray diffraction?XRD?,Raman spectra?Raman?,X-ray photoelectron spectroscopy?XPS?,in-situ infrared spectroscopy?DRIFTS?.The mechanism on catalytic oxidation of NO over Co-based catalysts was investigated.Firstly,Co₃O₄/Ce_xZr_1-xO₂ catalysts were synthesized by two routes:a facile one-pot method using citrate as complexing agent?CC-series?and a stepwise method?impregnation method,IM-series?,and their performance was investigated with respect to their behaviors in catalytic oxidation of NO to NO₂.The results showed that CC-series exhibited higher catalytic activity than IM-series.The NO oxidation efficiency reached above 60% at 300 ?under the condition of 0.04% NO,8% O₂ and 30000 h^-1 GHSV.From the characterization results,it was found that CC-series possessed easier reducibility and lower crystallite size compared with IM-series.Moreover,there was higher relative amount of Ce³⁺ on the surface of CC-series,which promoted the oxygen mobility.All these features in turn led to their higher catalytic activity compared with IM-series.Secondly,Zr_0.4Ce_0.6-xCo_xO₂ solid solutions?ZCCox? were successfully synthesized by sol-gel method.The catalytic activity firstly increased with the increasing loading of cobalt,then decreased.The ZCCo0.36?x=0.36? catalyst exhibited more than 50% NO oxidation efficiency in the temperature range from 250 to 350 ?.The chemico-physical properties of catalysts were characterized by XRD,Raman spectroscopy,XPS,et al.The results indicated that cobalt was introduced into the lattice of CeO₂,forming Co-Ce-Zr solid solution.There existed strong interaction between Co and Ce,which improved NO adsorption capacity and the amount of Ce³⁺ on the surface of the catalyst.The features may be responsible for the improving of catalytic activity of Co-Ce-Zr metal-oxides for NO oxidation.At the same time,the effects of different operation conditions on the catalytic activity of ZCCo0.36 was studied.The results showed that decreasing GHSV and NO initial concentration or increasing O₂ concentration led to the increase of NO oxidation efficiency.Moreover,it found that when the reaction gas contained H₂O?g?,the NO oxidation efficiency was inhibited due to the competitive adsorption from H₂O?g?.However,this inhibition was reversible.When cut off the H₂O?g?,the NO oxidation efficiency was restored to the original level.The stability tests indicated that it was feasible to employ ZCCo0.36 as an effective catalyst for practical application.Thirdly,the reaction mechanism of NO catalytic oxidation over ZCCo0.36 was investigated by the characterization of in-situ DRIFTS and kinetic experiments.Nitrates were found to be the main intermediates for NO oxidation.NO catalytic oxidation reaction followed a Langmuir-Hinshelwood mechanism,that is,the reactant NO and O₂ was firstly adsorbed on the catalyst,then the adsorbed NO reacted with adsorbed O₂ to form NO₂.The kinetic fitting results indicated that the reaction order with respect to NO was 0.53,while the reaction order with respect to O₂ was 0.44.Finally,Co-Ce-Ti mixed oxide catalysts were synthesized by a facile sol-gel method and evaluated through NO oxidation.The catalytic oxidation experiments results showed that when Co and Ce coexisted in catalyst,the catalytic activity was high.Ce0.2Co0.2Ti exhibited high catalytic activity of NO oxidation with maximum oxidation efficiency of 76% at 300 ? under the condition of 0.04% NO₂ 8% O₂ and 30000 h^-1 GHSV.By the XPS analysis result,it can deduce that the catalytic oxidation of NO conducted via the redox cycle,Ce³⁺ + Co³⁺???Ce⁴⁺ + Co²⁺,which was supposed to be the source of the synergistic effect on the catalytic properties.