La/Ni(111)表面CO甲烷化:助剂La对活性和选择性的影响

CO Methanation on La/Ni(111)Surface:Effect of La Electron Delocalization on Activity and Selectivity

针对Ni(111)表面上因副产物CH_3OH形成而导致CH_4选择性低的问题,采用量子化学密度泛函理论(density functional theory,DFT)计算的方法,通过添加富电子的助剂La调节表面Ni原子的电子状态,增大Ni的d电子平均能以增加La/Ni(111)表面的反应性,实现La与Ni的协同催化,从而提高CO甲烷化活性和CH_4生成选择性。同时,基于DFT结果,以Microkinetic modeling模拟实验条件下CH_4和CH_3OH的生成速率,结果表明,反应速率r随着温度升高而增大;在同一温度下,CH_4生成速率r(CH_4)远大于CH_3OH的生成速率r(CH_3OH),且S_(CH4)在反应温度550~750 K内高达100%,表明在La/Ni(111)表面上的CO甲烷化过程中没有副产物CH_3OH的生成。究其原因,在电子水平上通过对CH_4形成路径Path1、Path4、Path9和Path10所对应的关键中间体CO、HCO、CH_2O和COH进行Bader电荷和pDOS分析,发现C—O键明显弱化,C—O断键能垒显著降低,因而无CH_3OH生成的微观机理是La→Ni电子离域和La与O强相互作用而产生的"给电子诱导"效应。进一步通过La与邻近Ni原子的三维差分电荷密度分析得知,La原子的电荷损耗是沿着"La→Ni"方向,这就给出了助剂La与Ni协同催化CO甲烷化并高活性高选择性地生成CH_4的微观解释。

In attempting to promote the activity and selectivity of CO conversion to CH 4 and simultaneously suppress CH 3OH formation,density functional theory(DFT)calculation has been employed to insight into the reaction mechanism and the effect of the promoter La on CO conversion to CH 4 on La/Ni(111).Our results indicate that the promoter La could enrich the outer layer valence electron density of Ni,make the d-band center of La/Ni(111)upward,and thereby lead to a significant increase of the reactivity.Accordingly,the enhanced activity and selectivity to CH 4 as well as CH 3OH resistance are mainly originated from the electronic effect of the promoter La on La/Ni(111),where the synergistic effect between La and Ni plays an important role.Meanwhile,the microkinetic modeling is used to estimate the production rates of CH 4 and CH 3OH under the experimental conditions,and the result shows that r(CH 4)is larger than r(CH 3OH)at the same temperature,and the relative selectivity of CH 4 reaches almost as high as 100%in the temperature range of 550 to 750 K,and thereby no CH 3OH is formed when La is doped.Further,to clarify the effect of La promoter on CH 4 formation at electron level,Bader charges and the projected density of states(PDOS)have been examined for CO,HCO,COH and CH 2O,which are the key intermediates of Path1,Path2,Path3,and Path4 for CH 4 formation,respectively.The results indicate that it is electron transfer from La to Ni and the strong interaction between La and O that weaken the C—O bond and promote the cleavage of C—O bond,and thereby lead to no CH 3OH yield,which controls the selectivity to CH 4.Through analyzing the differential charge density of La atom and its surrounding Ni atoms over La/Ni(111),the result of the direction along La→Ni of charge transfer,has been shed light on furtherly.Conclusively,La/Ni(111)shows a significant increase in the activity and selectivity to CH 4 compared to Ni(111),which is mainly originated from the synergistic effect between La and Ni.