Design of Boron and Transition Metal Embedded Two-Dimensional Porous Carbon Nitride for Electrocatalytic Synthesis of Urea

Electrocatalytic coupling of CO and N2 to synthesize urea under ambient conditions is considered a promising strategy to replace traditional industrial technology. It is crucial to find efficient electrocatalysts that can adsorb and activate N2 and promote the C–N coupling reaction. Herein, a new two-dimensional porous carbon nitride material with multiactive sites is designed, in which boron and transition metal are embedded. Through a series of screening, B2Cr2, B2Mn2, and B2Os2 are predicted to be potential electrocatalysts for urea synthesis. Mechanistic studies are performed on bidentate metal–metal and metal–boron sites, and both NCON and CO mechanisms are explored. The electronic structure analysis shows that there is a strong N2 chemical adsorption within the bidentate site and that the N≡N bond is significantly activated. A new mechanism where free CO is inserted for C–N coupling within the two-dimensional porous structure is proposed.

Table S1.The dM-M represents the distance between two adjacent metal atoms around the pore.The dB-M represents the distance between two relatively close boron and metal atoms around the pore.The dB-B represents the distance between two adjacent boron atoms around the pore.

Figure S2 .
Figure S2.The relative position of transition metals and B atoms: (a) Optimized structures of two B atoms substituted with metals at the para position.(b) Optimized structures of two B atoms substituted with metals at the adjacent positions.

Figure S3 .
Figure S3.Possible pathways for the experimental synthesis of B2M2 structures.

Figure S4 .
Figure S4.Top view of optimized structures of fourteen candidate catalysts.

Figure S5 .
Figure S5.The band structure and the total density of states of B2Cr2.The Fermi level is assigned at zero.

Figure S6 .
Figure S6.The band structure and the total density of states of B2Mn2.The Fermi level is assigned at zero.

Figure S7 .
Figure S7.The band structure and the total density of states of B2Os2.The Fermi level is assigned at zero.

Figure
Figure S10.ICOHP of the bonding between N2 and each metal, and the corresponding N2 adsorption energy on bimetallic sites.

Figure S12 .
Figure S12.Charge difference density, pDOS and COHP of absorbed N2 with side-on pattern on B and metal sites on (a-c) B2Cr2, (d-f) B2Mn2 and (g-i) B2Os2 catalysts.In the pDOS diagram, "N" represents the nitrogen atom of N2.The contour level is set to 0.005 e/Å 3 .

Figure
Figure S15.(a) Pathway diagram for urea synthesis via the NCON mechanism on Cr and Cr sites of B2Cr2 catalyst and (b) intermediate configurations.

Figure
Figure S16.(a) Pathway diagram for urea synthesis via the NCON mechanism between Os and Os sites of B2Os2 system and (b) intermediate configurations.

Figure S17 .
Figure S17.The computed reaction pathway for the reaction of N≡N bond dissociation on bimetallic sites on (a)B2Cr2 and (b)B2Mn2 catalysts.

Figure S18 .
Figure S18.The computed reaction pathway for the C-N coupling of CO and *N2 on bimetallic sites on (a)B2Cr2 and (b)B2Os2 catalysts.

Figure
Figure S19.(a) Pathway diagram for urea synthesis via the CO mechanism on Cr and Cr sites of B2Cr2 catalyst and (b) intermediate configurations.

Figure
Figure S20.(a) Pathway diagram for urea synthesis via the CO mechanism on Os and Os sites of B2Os2 catalyst and (b) intermediate configurations.

Figure S21 .
Figure S21.The computed reaction pathway for the C-N coupling of CO and H2*NN*H2 on bimetallic sites on (a) B2Cr2 and (b) B2Os2 catalysts.

Figure
Figure S22.(a) The diagram of free energy changes for urea synthesis via the NCON mechanism on B and Cr sites on B2Cr2 catalyst.The connecting lines between elements only represent bonding, and do not indicate bond order.The brown and yellow paths represent adding H to the N atom on the boron side and adding H to the N atom on the metal side, respectively.The purple path represents the alternating mechanism of adding H to the N atom on the B side, the blue path represents the alternating mechanism of adding hydrogen to the N atom on the metal side, the red path represents the distal mechanism of adding hydrogen to the N atom on the B side, and the green path represents the distal mechanism of adding hydrogen to the N atom on the metal side.(b) One of the optimal pathways for urea synthesis via the NCON mechanism on B and Cr sites on B2Cr2 catalyst.

Figure S23 .
Figure S23.The diagram of free energy changes for urea synthesis via the NCON mechanism on B and Mn sites on B2Mn2 catalyst.The connecting lines between elements only represent bonding, and do not indicate bond order.The brown and yellow paths represent adding H to the N atom on the boron side and adding H to the N atom on the metal side, respectively.The purple path represents the alternating mechanism of adding H to the N atom on the B side, the blue path represents the alternating mechanism of adding hydrogen to the N atom on the metal side, the red path represents the distal mechanism of adding hydrogen to the N atom on the B side, and the green path represents the distal mechanism of adding hydrogen to the N atom on the metal side.

Figure
Figure S24.(a) The diagram of free energy changes for urea synthesis via the NCON mechanism on B and Os sites on B2Os2 catalyst.The connecting lines between elements only represent bonding, and do not indicate bond order.The brown and yellow paths represent adding H to the N atom on the boron side and adding H to the N atom on the metal side, respectively.The purple path represents the alternating mechanism of adding H to the N atom on the B side, the blue path represents the alternating mechanism of adding hydrogen to the N atom on the metal side, the red path represents the distal mechanism of adding hydrogen to the N atom on the B side, and the green path represents the distal mechanism of adding hydrogen to the N atom on the metal side.(b) One of the optimal pathways for urea synthesis via the NCON mechanism on B and Os sites on B2Os2 catalyst.

Figure S25 .
Figure S25.The computed reaction pathway for the C-N coupling of CO and *N2 on B and metal sites on B2Cr2 and B2Os2 catalysts.

Figure
Figure S26.(a) The diagram of free energy changes for urea synthesis via the CO mechanism on B and Cr sites on B2Cr2 catalyst.The connecting lines between elements only represent bonding, and do not indicate bond order.The brown and yellow paths represent adding H to the N atom on the boron side and adding H to the N atom on the metal side, respectively.The purple path represents the alternating mechanism of adding H to the N atom on the B side, the blue path represents the alternating mechanism of adding hydrogen to the N atom on the metal side, the red path represents the distal mechanism of adding hydrogen to the N atom on the B side, and the green path represents the distal mechanism of adding hydrogen to the N atom on the metal side.(b) One of the optimal pathways for urea synthesis via the CO mechanism on B and Cr sites on B2Cr2 catalyst.

Figure
Figure S27.(a) The diagram of free energy changes for urea synthesis via the CO mechanism on B and Mn sites on B2Mn2 catalyst.The connecting lines between elements only represent bonding, and do not indicate bond order.The brown and yellow paths represent adding H to the N atom on the boron side and adding H to the N atom on the metal side, respectively.The purple path represents the alternating mechanism of adding H to the N atom on the B side, the blue path represents the alternating mechanism of adding hydrogen to the N atom on the metal side, the red path represents the distal mechanism of adding hydrogen to the N atom on the B side, and the green path represents the distal mechanism of adding hydrogen to the N atom on the metal side.(b)One of the optimal pathways for urea synthesis via the CO mechanism on B and Mn sites on B2Mn2 catalyst.

Figure
Figure S28.(a) The diagram of free energy changes for urea synthesis via CO mechanism on B and Os sites on B2Os2 catalyst.The connecting lines between elements only represent bonding, and do not indicate bond order.The brown and yellow paths represent adding H to the N atom on the boron side and adding H to the N atom on the metal side, respectively.The purple path represents the alternating mechanism of adding H to the N atom on the B side, the blue path represents the alternating mechanism of adding hydrogen to the N atom on the metal side, the red path represents the distal mechanism of adding hydrogen to the N atom on the B side, and the green path represents the distal mechanism of adding hydrogen to the N atom on the metal side.(b) One of the optimal pathways for urea synthesis via the CO mechanism on B and Os sites on B2Os2 catalyst.

Figure S29 .
Figure S29.The computed reaction pathway for the C-N coupling of CO and H2*NN*H2 on B and metal sites on (a) B2Cr2 and (b) B2Mn2 and (c) B2Os2 catalysts.

Figure
Figure S30.(a) Free energy change diagram for NRR on Cr and Cr sites on B2Cr2 catalyst.(b) Free energy change diagram for NRR on B and Cr sites on B2Cr2 catalyst.

Figure
Figure S31.(a) Free energy change diagram for NRR on Mn and Mn sites on B2Mn2 catalyst.(b) Free energy change diagram for NRR on B and Mn sites on B2Mn2 catalyst.

Figure
Figure S32.(a) Free energy change diagram for NRR on Os and Os sites on B2Os2 catalyst.(b) Free energy change diagram for NRR on B and Os sites on B2Os2 catalyst.

Figure
Figure S33.(a) Free energy change diagram for CORR on Cr and Cr sites on B2Cr2 catalyst.(b) Free energy change diagram for CORR on B and Cr sites on B2Cr2 catalyst.

Figure
Figure S34.(a) Free energy change diagram for CORR on Mn and Mn sites on B2Mn2 catalyst.(b) Free energy change diagram for CORR on B and Mn sites on B2Mn2 catalyst.

Figure
Figure S35.(a) Free energy change diagram for CORR on Os and Os sites on B2Os2 catalyst.(b) Free energy change diagram for CORR on B and Os sites on B2Os2 catalyst.

Table S4 .
Free energy changes for each step in urea synthesis via the CO mechanism on bimetal sites on B2Cr2,

Table S5 .
Free energy change for each step in urea synthesis via the NCON mechanism on B and metal sites on B2Cr2, B2Mn2 and B2Os2 catalysts.The black represents the N atom on the B side that is preferentially hydrogenated, while the green represents the N atom on the metal side that is preferentially hydrogenated.

Table S6 .
Free energy change for each step in urea synthesis via the CO mechanism on B and metal sites on B2Cr2, B2Mn2, and B2Os2 catalysts.The black represents the N atom on the B side that is preferentially hydrogenated, while the green represents the N atom on the metal side that is preferentially hydrogenated.