FIGURE 5 Optimized configurations of different catalysts: (a) Ni@N-C SAC. (b) Ni@NC. (c) the H2 dissociation of energy of Ni@N-C SAC and Ni@NC.
The selective hydrogenolysis begins with the adsorption and activation of H248, and then the activated hydrogen species react with PPE molecules resulting in the cleavage of the C-O bond. The calculated Ni adsorption energies were -3.17 eV for Ni@N-C SAC and -1.21 eV for Ni@NC, respectively (Figure 5a and 5b). The lower adsorption energies of Ni in the Ni@N-C SAC model suggests that Ni atoms prefer to chelate with the N atoms of the N-doped carbon support to form single-atom sites (Ni@N-C SAC) rather than aggregate with each other to form Ni clusters (Ni@NC). Moreover, the dissociation energy of molecular hydrogen on the surface of Ni@N-C SAC and Ni@NC are 1.21 eV and 1.69 eV, respectively (Figure 5c), indicating that Ni@N-C SAC contributes to more active hydrogen species, and hence exhibits higher catalytic activity.
The time course of the product distribution of PPE hydrogenolysis was monitored (Figure S9). Results shows that PPE is almost quantitatively converted to phenol, ethylbenzene and phenethoxybenzene within 150 min. Phenethoxybenzene increases initially but decreases with the prolonging of time. Therefore, phenethoxybenzene should be an intermediate for the transformation, which might be produced from the dehydration of PPE on the OH group of Cα, and it can be further hydrogenated to produce phenol and ethylbenzene by the cleavage of the aliphatic C-O bond (Figure 6a, route A). Despite there is another possible dehydration path between Cα-OH and Cβ-H, the vinyl ether intermediate was not detected yet even at low temperature, we thus exclude this dehydration pathway. Phenylethyl alcohol was a minor product of the PPE hydrogenolysis, which was probably produced through the cleavage of the aliphatic C-O bond (Figure 6a, route B). The yield of phenol and ethylbenzene increases with the prolonging of the reaction time, which indicates that route A is the main reaction path of PPE hydrogenolysis.
According to the hydrogenolysis reaction pathway A, the DFT calculation of the mechanism with free energy profile was also shown in Figure 6. The reaction energy barrier diagram indicates that the two dissociated hydrogen atoms combine with the oxygen atom on Cα-OH (energy barrier: 0.979 eV) and the Cα (energy barrier: 0.354 eV), respectively, to produce phenethoxybenzene, which is further cleaved to generate phenol and ethylbenzene by overcoming the energy barriers of 0.493 eV and 0.264 eV. For Ni@NC, it was found that the energy barriers of the four transition states (TS1, TS2, TS3, and TS4) are lower than those in the Ni@N-C SAC-catalyzed PPE hydrolysis. Therefore, Ni@N-C SAC exhibits higher catalytic activity than Ni@NC in the cleavage of C-O bonds.