Ru3@Mo2CO2 MXene single-cluster catalyst for highly efficient N2-to-NH3 conversion

ABSTRACT Single-cluster catalysts (SCCs) representing structurally well-defined metal clusters anchored on support tend to exhibit tunable catalytic performance for complex redox reactions in heterogeneous catalysis. Here we report a theoretical study on an SCC of Ru3@Mo2CO2 MXene for N2-to-NH3 thermal conversion. Our results show that Ru3@Mo2CO2 can effectively activate N2 and promotes its conversion to NH3 through an association mechanism, in which the rate-determining step of NH2* + H* → NH3* has a low energy barrier of 1.29 eV. Notably, with the assistance of Mo2CO2 support, the positively charged Ru3 cluster active site can effectively adsorb and activate N2, leading to 0.74 |e| charge transfer from Ru3@Mo2CO2 to the adsorbed N2. The supported Ru3 also acts as an electron reservoir to regulate the charge transfer for various intermediate steps of ammonia synthesis. Microkinetic analysis shows that the turnover frequency of the N2-to-NH3 conversion on Ru3@Mo2CO2 is as high as 1.45 × 10−2 s−1 site−1 at a selected thermodynamic condition of 48 bar and 700 K, the performance of which even surpasses that of the Ru B5 site and Fe3/θ-Al2O3(010) reported before. Our work provides a theoretical understanding of the high stability and catalytic mechanism of Ru3@Mo2CO2 and guidance for further designing and fabricating MXene-based metal SCCs for ammonia synthesis under mild conditions.


Computational details
For the reaction rate constant and free energy correlation, we calculated the reaction rates of the rate-determining steps (b15 → b16) in the distal I reaction mechanism for N 2 on Ru 3 @Mo 2 CO 2 .The Eyring-Polanyi equation based on the transition state theory (TST) 1 is used to calculate the reaction rate constant, which can be written as Eq. ( 1): where: σ is the transmission coefficient; T is the absolute temperature; h is the Planck's constant; R is the gas constant;  0 is the standard atmospheric pressure; And ∆ 0,≠ is the free energy of activation, where: ∆ 0,≠ () = ∆  0 () − ∆  0 () (2) The rate-determining step in distal I is the formation of NH 3 species.To ensure the reaction rate constant, we firstly calculate the free energy of activation.The vibrational partition function is written as Eq. ( 3): Where,   is the vibrational energies.
The internal energy is written as: And the entropy, , is calculated via: Since the zero-point vibrational energy (ZPVE) has already been considered in the reaction energy profiles, the thermal correction to G(T) in the present work at 298.15 K and P = 1 atm is defined as Eq. ( 6): (298.15)=   (298.15)+ 298.15 *   (298.15)(6) Thus, the free energy of activation ∆ 0,≠ for the rate-determining step of pathway III is 1.29 eV calculated from VASPkit postprocessing tool 2 .Accordingly, the calculated reaction rate constant is 2.41×10 -8 /(s mol L -1 ).

References:
1            S5.The relevant elementary reactions steps of ammonia synthesis reaction on Ru 3 @Mo 2 CO 2 in the optimal associative alternative pathway I and dissociation pathways.for microkinetic analysis using CatMAP software package. Step

Figure S2 .
Figure S2.The total energy (eV), the average bond length (Å) of Ru-Ru in Ru 3 cluster and Ru-O fluctuation on the Ru 3 @Mo 2 CO 2 during AIMD simulations at 673 K.

Figure S3 .
Figure S3.The predicted profile of reaction pathway for the transformation of dinitrogen configurations.

Figure S4 .
Figure S4.The stability and electronic properties of H 2 adsorbed on Ru 3 @Mo 2 CO 2 .(a) The optimized geometry and (b) the calculated electron density difference of Ru 3 @Mo 2 CO 2 with H 2 and N 2 co-adsorption; (c) the PDOS and (d) the -COHP of Ru-H in Ru 3 @Mo 2 CO 2 .

Figure S5 .
Figure S5.The predicted profile of dissociation pathway and the corresponding optimized structures for NH 3 synthesis.

Figure S6 .
Figure S6.The predicted profile of the associative alternative pathway I and distal pathway I and the corresponding optimized structures for NH 3 synthesis.

Figure S7 .
Figure S7.The predicted profile of the alternative pathways II and III and the corresponding optimized structures for NH 3 synthesis.

Figure S8 .
Figure S8.The energy profile of the optimal associative alternative pathway I at 700 K.

Table S3 .
The N-N bond length change of dinitrogen during the NH 3 synthesis.

Table S4 .
Calculated Bader charges (|e|) for Mo 2 CO 2 , N x H y , Ru 3 , N a and N b in the structures of alternative pathway I.