Ni掺杂对Ni/MgO体系中电荷分配影响

Effect of Ni Doping on Electron Transfer in Ni/MgO Catalysts

采用密度泛函理论(DFT)计算方法,研究了载体Ni掺杂对Ni/MgO催化剂的电子结构及其对CH_4/CO_2重整反应的影响。结果显示,随着载体Ni:Mg比的增加,CH_4解离吸附和CH_x氧化过程的反应能垒均会随之升高,CH也更容易产生热解C,从而导致催化剂的活性和稳定性的下降。通过分析重整过程的Hirshfeld电荷分配,发现金属载体之间电子转移的方向会随着载体中Ni含量的上升由载体向Ni金属转移变为由Ni金属向载体转移。当Ni金属富电子时,反应物活化时电子的转移主要发生在表层Ni原子与吸附物种之间,参与氧化的CH_x物种为CH_2;当Ni金属缺电子时,反应物活化时Ni簇的电子结构基本保持稳定,电子主要在载体表层的Ni原子与反应物之间转移,CH是主要参与氧化的CH_x物种。

First-principles calculations based on density functional theory （DFT） have been used to investigate how Ni doping in the supporter impacts CH4/CO2 reforming over Ni/MgO catalysts. Ni8 clusters supported on different NixMgyO27（100） slabs （x＋y= 27） have been selected to model Ni/MgO catalysts with different Ni ： Mg ratio. The CH4/CO2 reforming mechanisms on different Ni8/NixMgyO27（100） slabs indicate that the energy barriers of CH4 dissociated adsorption and CHx oxidation both increase with an increase in the Ni ： Mg ratio of the supporter. CH would be easily generated from the pyrolytic carbon. The electron structure analysis shows that the direction of electron transfer changes to “between metal and supporter” with increasing Ni ： Mg ratio of the supporter, but not to “from supporter to metal” on pure Ni/MgO. When the Ni cluster is negative, electron transfer occurs between adsorbed species and Ni atoms in the surface layer of the Ni cluster, and CH2 is the main species for CHx oxidation. When the Ni cluster is positive, its Hirshfeld is stable, electron transfer occurs between adsorbed species and Ni atoms in the surface layer of the NiO-MgO solid solution, and CH is the main oxidative species. The electron deficiency of the Ni cluster is the reason for the poor catalytic performance of Ni/MgO with a high Ni doping ratio.