不同价态金属掺杂对La_(2)O_(3)(001)表面上甲烷活化影响的研究

Effect of different valence metals doping on methane activation over La_(2)O_(3)(001)surface

La_(2)O_(3)因具有优异的稳定性和较高的C_(2)烃选择性,因此,常被用于催化甲烷氧化偶联反应,而较差的甲烷解离活性却限制了其广泛应用。为了提高镧基催化剂活化甲烷的性能,将不同价态的金属掺杂在La_(2)O_(3)(001)表面,并采用密度泛函理论方法对CH_(4)在催化剂表面的活化行为进行了研究。结果表明,低价态金属(Li、Na、K、Mg、Ca、Sr和Ba)和等价态金属(Al、Ga、In)的掺杂可以显著提高La_(2)O_(3)(001)表面的CH4解离活性。其中,CH4在Li-La_(2)O_(3)(001)表面解离的活化能最低,仅为13.0 kJ/mol。而高价态金属(Zr、Nb、Re和W)掺杂不能提高La_(2)O_(3)(001)表面的CH4解离活性。此外,通过研究催化剂表面氧空位形成能、酸碱性与CH4解离活化能之间的关系发现,随着掺杂金属价态的增加,氧空位形成能也逐渐增大,而CH4的解离活性呈现出降低趋势;碱金属和碱土金属的掺杂增大了催化剂表面的碱性,且碱金属掺杂的碱性强于碱土金属,同时,较强的碱性也表现出较高的CH4转化活性。本研究对提高La_(2)O_(3)催化剂的甲烷转化活性具有指导意义。

La_(2)O_(3)as a catalyst is used for oxidative coupling of methane(OCM)reactions due to its excellent stability and high C_(2)selectivity,but poor activity on methane dissociation limits its wide application.Different valence metals are doped on the La_(2)O_(3)(001)surface to improve the methane conversion activity,and the activation of methane on metal-doped La_(2)O_(3)(001)surfaces has been investigated via the density functional theory(DFT)calculations.The relationship between the valence states of doped metals and the methane conversion activities shows that doping low valence metals(Li,Na,K,Mg,Ca,Sr and Ba)and equivalent metals(Al,Ga,In)can significantly improve the conversion activity of methane.Among them,the activation energy of methane on the Li-La_(2)O_(3)(001)surface is the lowest,which is only 13.0 kJ/mol.However,doping of high valence metals(Zr,Nb,Re and W)cannot improve the CH4 dissociation activity.Furthermore,the relationships between surface oxygen vacancy formation energies,acid-base properties and the activation energies of CH4 have also been investigated.The results show that with the increase of metal valence state,the oxygen vacancy formation energy increases,while the dissociation activity of CH_(4) decreases.The introduction of alkali and alkaline earth metals increases the alkalinity of La_(2)O_(3)(001)surface,and the alkalinity of La_(2)O_(3)(001)doped with the alkali metal is stronger than that with the alkaline earth metal,exhibiting higher dissociation activity of CH4.Our research may provide a guide for improving methane conversion activity on La_(2)O_(3)catalysts.