锶掺杂对氧化镧催化甲烷氧化偶联反应的影响

甲烷氧化偶联反应(OCM)是天然气直接转化利用的重要途径之一.该反应通过甲烷和氧气在催化剂作用下一步将甲烷直接转化为乙烯等具有高附加值的产品,避免了涉及高能耗过程的合成气间接路径,不仅有可能减少中间副产物的生成,还有可能大大提升整个过程的能源利用效率.因此,研究OCM反应具有十分重要的实际意义.目前氧化镧基催化剂具有良好的催化活性、产物选择性和热稳定性,但在OCM反应中产品收率仍未能达到工业应用的要求,因而近几十年来高效OCM催化剂的研发一直是研究热点.实验发现,锶掺杂氧化镧催化剂具有更为优异的催化性能,主要表现在具有比纯氧化镧催化剂更高的催化活性和产物选择性,但对于锶掺杂的影响机制仍然缺乏系统的理论研究.目前普遍认为,甲烷活化是OCM反应的第一步,也是决速步,这主要是由于C?H键活化需要越过很高的能垒,因此往往需要很高的温度.本文主要采用团簇模型,通过密度泛函理论计算来研究OCM反应中锶掺杂对氧化镧催化剂上甲烷活化性能的影响及其作用原理.本文构建了八种锶掺杂的氧化镧团簇作为该催化剂模型,可分为没有自由基性质的团簇(LaSrO2(OH),La2SrO4,La3SrO5(OH),La5SrO8(OH))和具有自由基性质的团簇(LaSrO3,La2SrO4(OH),La3SrO6,La5SrO9).我们计算了甲烷在这些锶掺杂氧化镧团簇上Sr?O和La?O酸碱对位点以及氧自由基活性位点上的活化机制,以研究锶掺杂对OCM反应活性的影响,并与我们前期计算的纯氧化镧团簇上甲烷活化性能进行了对比.通过计算甲烷在不同锶掺杂氧化镧团簇上的物理和化学吸附能、活化能垒以及甲基自由基的脱附能,发现锶掺杂氧化镧团簇上的甲烷活化在热力学和动力学上都要比纯氧化镧团簇上更为有利.对于具有相同金属原子数目的团簇,甲烷在La?O上活化的能垒大小为:化学计量比的La?Sr?O团簇<非化学计量比的La?Sr?O团簇<化学计量比的La?O团簇;而甲烷在Sr?O上活化的能垒大小依次是:化学计量比的La?Sr?O团簇<非化学计量比的La?Sr?O团簇.给定一个锶掺杂氧化镧团簇,甲烷在不同活化位点上的活化能垒大小通常是:O·<

Sr-doping effects on La2O3 catalyst for oxidative coupling of methane

Density-functional theory calculations were carried out to study the strontium (Sr)-doping effect on methane activation over a lanthanum-oxide (La2O3) catalyst for the oxidative coupling of methane (OCM) using the cluster model. Eight Sr-doped La2O3 cluster models were built from pure La2O3 clusters that were used previously to model the La2O3 catalyst. These form two distinct categories, namely, those without a radical character (LaSrO2(OH), La2SrO4, La3SrO5(OH), and La5SrO8(OH)) and those with a radical character (LaSrO3, La2SrO4(OH), La3SrO6, and La5SrO9). The potential-energy surface for CH4 activation to form a CH3 radical at different Sr–O and La–O pair sites on these Sr-doped La2O3 clusters was calculated to study the Sr-doping effect on the OCM catalytic activity. CH4 physisorption and chemisorption energies, and activation barriers, and CH3 desorption energies were predicted. Compared with the pure La2O3 clusters, in general, the Sr-doped La2O3 clusters are thermodynamically and kinetically more reactive with CH4. For the Sr-doped La2O3 clusters without the radical character, the Sr–O pair site is more reactive with CH4 than the La–O pair site, although a direct release of the CH3 radical is also highly endothermic as in the case of the pure La2O3 clusters. In contrast, for the Sr-doped La2O3 clusters with a radical character, the activation of CH4 at the oxygen radical site and the release of the CH3 radical are much easier. Thus, our calculations suggest that the Sr dopant prompts the OCM catalytic activity of the La2O3 catalyst by providing a highly active oxygen-radical site and by strengthening the basicity of the M–O pair site, which leads to lower CH4 activation energies and lower CH3 desorption energies.