Based on all these characterization results, the following views can be obtained: the most active catalyst, Ni@N/C-1, is characterized by the formation of specific metallic Ni nanoparticles, which are encapsulated in a thin graphene shell. With the successful development of graphene thin layer encapsulated N-doped catalysts (Ni@N/C-x), we were interested in their general application for the semi-hydrogenation of challenging terminal alkynes substrates. Herein, we choose the hydrogenation of phenylacetylene as the standard reaction for the catalysts' activity and reaction conditions' evaluations. As shown in Figure , all the prepared N-doped catalysts were evaluated in the standard reaction conditions. It is clear to see that they gave different catalytic reactivity and product selectivity for the hydrogenation of phenylacetylene. The Ni@N/C-1 gave the best results among the catalysts with 53% of conversion and 94% of 1a selectivity, and the other overhydrogenated product selectivity of 6%. The Ni@N/C-4 and Ni@N/C-6 were not active for the semi-hydrogenation of phenylacetylene under the selected reaction conditions. With the optimized catalyst Ni@N/C-1 in hand, we next evaluated the reaction temperature. As illustrated in Figure , the conversion increased from 79% to 99% when the reaction temperature increased from 70 °C to 100 °C, while in the meantime, the 1a selectivity showed a decreasing trend accordingly. At 80 °C, the Ni@N/C-1 catalyzed semi-hydrogenation of phenylacetylene gave the best results of 92 % conversion and 87 % of 1a selectivity. Then under this reaction temperature, we shorten and prolonged the reaction time to see the catalytic activity and product distributions. As shown in Figure , when the reaction time was prolonged from 6 h to 9 h, the conversion increased from 86 % to 99%, but the product selectivity decreased from 85 % to 50 %. Although the cyclohexane and CH3CN gave excellent selectivity, the catalyst showed low activity in these solvents. Furthermore, to showcase the generality and selectivity of this novel N-doped graphene layer encapsulated Ni-based nano-catalyst, its reactivity was compared with other commercially available catalysts, the Lindlar catalyst, and some representative recently reported catalysts . As illustrated in Figure , the commercially noble catalysts Pd/C, Rh/C, Raney Ni, and Raney Co, which were usually used as hydrogenation catalysts due to their excellent hydrogenation ability, gave the full conversion of the semi-hydrogenation of phenylacetylene but with the over-hydrogenated alkane product. The less active Ru/C catalyst gave 40% conversion and 85 % 1a selectivity. The NiO was not active for the phenylacetylene semi-hydrogenation during the selected reaction condition. With a slight modification of reaction conditions (10 bar H2, 70 °C, 8 h), the Ni@N/C-1 exhibited excellent reactivity of 99% conversion and comparable excellent selectivity of 91%. The prepared Pd/Al2O3 had excellent activity but with a low selectivity (47.8%) under mild reaction conditions (1 bar H2, 40 °C, 1.3 h). Although the commercially widely used catalyst Lindlar catalyst shows a high selectivity (90.2%) and super activity (99% con.), the reaction needs a long reaction time of up to 30 hours. Although the other two Ni-based catalysts Ni/Al2O3 and Ni/SiO2 showed excellent selectivity (93% and 92%) under 1 bar of H2 pressure and mild reaction temperature of 40 °C, they had lower activity, with conversions of only 23.5% and 29.8%, respectively, even prolonging the reaction time of up to 15 hours. The reported bi-metal NiZn3 /AlSBA-15 (40), and NiCu Nanoalloy catalysts gave the similar 1a selectivity of 90% with 99% and 96% conversion respectively at the modified reaction conditions separately. The in situ prepared Ni-NPs catalyst could also give super reactivity of 100% conversion and moderate 1a selectivity of 79% with the help of ionic liquid under mild reaction conditions. Under slightly harsh reaction conditions (10 bar H2, 110 °C, 5 h), the Ni-fructose@SiO2-800 catalyst can semi-hydrogenate phenylacetylene with 99% conversion and 88% 1a selectivity. After having the optimized reaction conditions in hand, we then evaluated the substrate scope of Ni@N/C-1 catalyzed semi-hydrogenation reaction. As shown in Table , using the Ni@N/C-1 under the optimized reaction conditions of 20 bar H2, 80 ℃, 7h, most of the aromatic alkynes substrates could be transformed to the corresponding target alkene products with excellent conversion and selectivity (exceeds 99%), which indicating that the graphene encapsulated Ni@N/C-1 catalyst's superior application in the field of non-noble heterogeneous catalyzed semi-hydrogenation reactions. The standard substrate phenylacetylene gave 99% conversion and 75% 1a product selectivity could be obtained under the given standard reaction condition. The 1a selectivity could be increased to 91% under slight modification of the reaction conditions. Then we first investigated the electronic effect of the substituent group on the meta position of aromatic alkynes. For substrate 2 with the electron-withdrawing group -NH2 on the meta position, full conversion of >99% and 87% selectiv-ity were obtained. The substrate 3 bearing the strongest electron-withdrawing group -F at the meta position was a bit difficult to semi-hydrogenate and only gave 61% conversion and 86% 3a selectivity. While for the other two less electronwithdrawing groups -Br and -Cl, no matter where the substituent group was located on the meta or ortho position, the catalyst showed excellent catalytic reactivity and product selectivity. We then further examined the effect of changing the substituent group at the para position of the aromatic alkynes. Interestingly, substrate 6 with the strongest electron-withdrawing group NO2 could produce the chemo semi-hydrogenation product 6a in excellent conversion and good selectivity. The substrate having the strong electron-donating group NMe2 also gave an excellent conversion of 94% and excellent selectivity of 99%. The substrates with weak electron-donating group t-butyl, N-propyl, only gave moderated conversion but without any over-hydrogenation products. The graphene encapsulated Ni@N/C-1 also showed excellent catalytic reactivity of 93% conversion and 74% selectivity for the E isomer product(12a). Here it is known that the electronwithdrawing and electron-donating groups can both delocalize the electron negativities of the aromatic ring, which can further affect the terminal alkynes' electron negativity. Here, the substrates have electron-withdrawing groups NH2, Br, Cl, NO2, NMe2 gave higher catalytic reactivity and selectivity of the desired semi-hydrogenated product than the substrates with electron-donating groups of t-butyl, N-propyl. The Ni@N/C-1 catalyst's stability and recycling test were further carried out in both batch reactor and advanced flow reactor. Surprisingly, the graphene encapsulated Ni@N/C-1 catalyzed semi-hydrogenation of phenylacetylene did not loss significant reactivity and product selectivity after 10 times' recycling test (Table ). And also it needs to mention, the stable catalyst with magnetic properties can be applied in the industrially applicable flow reactor, and reaction conditions can be optimized easily by switching the control valve. The catalyst showed excellent reactivity and selectivity for the semi-hydrogenation of phenylacetylene in the flow reactor and the catalyst can be recovered easily (Table , Scheme S2). The graphene encapsulated Ni-based catalysts semi-hydrogenation in flow reactor process was high environmental friendlier with producing less byproduct and easily recycling use of H2. This can provide a feasible reference method for industrialized selective production of ethylene.