CeO_2、CaO助剂对甲烷部分氧化制合成气Ni/MgO-Al_2O_3催化剂结构和性能的影响

Effects of CeO_2 and CaO on properties of Ni/MgO-Al_2O_3 catalysts for partial oxidation of methane to syngas

采用BET、H2-TPR、XRD、TEM和活性评价等表征手段,考察了CeO2、CaO助剂对Ni/MgO-Al2O3催化剂物化性质和甲烷部分氧化制合成气反应性能的影响。实验结果表明,单独加入CeO2或CaO助剂可以改善Ni/MgO-Al2O3催化剂中镍物种的还原性能,以CaO尤为明显;CaO作为结构助剂可以降低还原态催化剂中的镍晶粒尺寸,使改性的催化剂具有较好的活性,而CeO2对催化剂的活性未产生显著影响。当CeO2与CaO两种助剂同时对Ni/MgO-Al2O3进行改性时,虽然催化剂中镍物种的还原性能没有发生明显变化,但仍具有很好的反应性能,这与CeO2与CaO能够形成CaO-CeO2固溶体有关。CaO-CeO2固溶体不仅与镍物种间存在相互作用,提高了镍物种的分散度、减小了镍晶粒尺寸,还可以提高催化剂的储氧能力和晶格氧的流动性,从而有利于改善其甲烷部分氧化反应性能。

The effects of CeO2 and/or CaO as promoters on the physico-chemical properties and the catalytic performance over Ni/MgO-Al2O3 catalyst for partial oxidation of methane to syngas were studied by means of BET, XRD, HE-TPR, TEM and reaction activity evaluation. Compared with the catalyst without promoters, the catalyst promoted by CeO2 or CaO alone exhibited better reducibility of nickel species, and the one promoted by CaO showed better catalytic activity. When both of CeO2 and CaO were added to modify the Ni/MgO-Al2O3 catalyst, an obvious synergetic effect between CeO2 and CaO were detected. It was deduced that the formation of CaO-CeO2 solid solution could result in a better dispersion of active nickel species and promote the conversion of Ce^4＋ to Ce^3＋ species, thus reduced the size of nickel crystallites efficiently. Better oxygen adsorption ability, more oxygen-vacant sites and more migration of oxygen anions could benefit oxygen exchange between NiO and CeO2. The above effects were contributed to higher CH4 conversion, CO selectivity and HE selectivity over the catalyst promoted with CeO2 and CaO in the partial oxidation of methane.