Density Functional Theory Studies On Methanation Reaction Of Carbon Monoxide Over Ni-based And Ni-Fe Bimetallic Catalysts

Carbon monoxide methanation（CO + 3H2 = CH4 + H2O） is an important process in the chemical industry. It is used to remove a small amount of carbon monoxide of the feed gas in ammonia and hydrogen plants and it is also the main reaction that converts syngas into SNG. Ni/γ-Al2O3 is the most widely used catalyst in CO mehanation. However, experimental study confimed that under the low temperature, the efficiency of Ni-based catalysts catalyzing CO mehanation reactions is not high. Adding Fe promoter into Ni-based catalysts and forming bimetallic catalysts can improve the activity and selective of CO methanation, but it is rarely studied by theory research. Therefore, in this paper, we theoretically expound the reaction mechanism of mechane synthesis from syngas on Ni/γ-Al2O3 and Ni-Fe/γ-Al2O3 catalysts, respectively, and find the cause that Ni-Fe bimetallic catalysts can improve the performance of CO methanation.With density functional theory method, we firstly investigate the s tability and nucleation of Nin（n =1–6） clusters on three different γ-Al2O3 surfaces. Through the comparion of stability and nucleation about different size of Ni clusters on different supports, we find out the best group of Ni/ γ-Al2O3 catalyst. Then we explore the optimum reaction pathway and reaction mechanism of CO methanation on Ni4/γ-Al2O3 catalyst and understand the key step for the reaction. Finally, we discuss the reaction mechanism of CO methanation on Ni3Fe/γ- Al2O3 catalyst under the low temperature（0oC） and compare the mechanism with that of CO methanation on single Ni catalyst, in order to obtain the cause that Ni-Fe bimetallic catalysts can improve the performance of CO methanation. The main conlusions are as follows:（1） On dehydrated γ-Al2O3（110） surface, Ni clusters present the best stability and nucleation and metal-suppore interaction is the strongest. As to the supported Nin clusters, structures of Ni2 to Ni4 clusters remain stable and as only one dimensional structure, Ni4 cluster is more conducive to the chemical reaction. Therefore, in this article, we adopt the model of Ni4 clusters supported on dehydrated γ-Al2O3（110） surface to catalyze CO methanation reation.（2） Direct CO dissociation is not energetically favorable, while hydrogen–assisted CO dissociation is identified as the dominating CO methanation pathway on Ni4/γ-Al2O3 catalyst. the optimal pathway of CO methanation is CO + 6H â†' CHO + 5H â†' CH2 O + 4H â†' CH3 O + 3H â†' CH3 + H + H2 O â†' CH4 + H2 O. The rate-controlling step for CH4 formation is C–O bond cleavage of CH3 O, which must overcome an activation barrier of 1.75 eV. Meanwhile, another product CH3 OH can be produced by the process of CO + 4H â†' CHO + 3H â†' CH2 O + 2H â†' CH3 O + H â†' CH3 OH. One H atom attacking C atom of CH3 O to form CH3 OH species is the rate-controlling step for CH3 OH formation with an an activation barrier of 1.47 e V. Furthermore, the highest barrier for the conversion of CO to CH4 is 2.54 eV, whereas that for CH3 OH formation is 2.26 eV, indicating that the formation of CH3 OH becomes more favorable. Therefore, CH3 OH formation will greatly reduce the productivity and selectivity of CO hydrogenation into CH4 products.（3） For Ni₃Fe/γ-Al₂O₃ catalyst, direct CO dissociation is not energetically favorable, while hydrogen–assisted CO dissociation is also the dominating CO methanation pathway. The feasible pathway of CO hydrogenation is CO + 6H â†' CHO + 5H â†' CH + 3H + H2 O â†' CH2 + 2H + H2 O â†' CH3 + H + H2 O â†' CH4 + H2 O. The rate-controlling step for CH4 formation in this reaction process is CO hydrogenation into CHO species with an activation barrier of 1.37 eV. The highest barrier for the conversion of CO to CH4 is 2.14 eV, which is lower than that on Ni4/γ-Al2O3, indicating that CH4 formation is more active on Ni₃Fe/γ-Al₂O₃. In addition, since there is no by-product such as CH3 OH, it further illustrates that Ni-Fe bimetallic catalyst has better selective for methane production.