Correlating CC, CO, and CN Hydrogenation Activity with Hydrogen Binding Energies on Ni–Fe Bimetallic Catalysts

Ni–Fe bimetallic catalysts have been found active in various hydrogenation reactions. Previous reports have hypothesized many explanations to demonstrate the promotion effect of Fe. However, these theories cannot predict the falling part of the volcanic curve of the hydrogenation activity with the Fe/(Ni + Fe) ratio. Here, we studied the hydrogenation pathways over Ni monometallic and Ni–Fe bimetallic sites with density functional theory (DFT) calculations. Ethylene, furfural, and methanimine were chosen as the representative reactants with CC, CO, and CN, respectively. The energy profiles on Ni3, Ni2Fe, and NiFe2 sites were calculated. The results showed that the activation energy of the rate-determining step follows the trend Ni2Fe < NiFe2 < Ni3 for all three hydrogenation reactions. Therefore, the Ni3Fe1 catalyst was predicted to be the most active because it has the highest density of Ni2Fe sites. Furthermore, the H binding energy was proved to be an efficient descriptor for the activation energy and hydrogenation activity. With a decrease in the H binding energy, the activation energy of hydrogenation reactions increased, indicating lower reaction activity. To verify our calculation results, the Ni monometallic and Ni–Fe bimetallic catalysts were synthesized with the co-impregnation method. The catalysts were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM) and were evaluated in ethylene hydrogenation reactions. As predicted by DFT calculations, the activity showed a volcanic relationship with the Fe/(Ni + Fe) ratio with Ni3Fe1 located at the top.