Remarkable Single Atom Catalyst of Transition Metal (Fe, Co & Ni) Doped on C2N Surface for Hydrogen Dissociation Reaction

Currently, hydrogen is recognized as the best alternative for fossil fuels because of its sustainable nature and environmentally friendly processing. In this study, hydrogen dissociation reaction is studied theoretically on the transition metal doped carbon nitride (C2N) surface through single atom catalysis. Each TMs@C2N complex is evaluated to obtain the most stable spin state for catalytic reaction. In addition, electronic properties (natural bond orbital NBO & frontier molecular orbital FMO) of the most stable spin state complex are further explored. During dissociation, hydrogen is primarily adsorbed on metal doped C2N surface and then dissociated heterolytically between metal and nitrogen atom of C2N surface. Results revealed that theFe@C2N surface is the most suitable catalyst for H2 dissociation reaction with activation barrier of 0.36 eV compared with Ni@C2N (0.40 eV) and Co@C2N (0.45 eV) complexes. The activation barrier for H2 dissociation reaction is quite low in case of Fe@C2N surface, which is comparatively better than already reported noble metal catalysts.


Introduction
Catalysts are the backbone of many commercially available energy conversion and industrial processes [1]. Currently, catalytic technology is managing the production of approximately more than ten trillion dollars of goods annually in power, petroleum, food, and chemicals industries [2]. Noble metals have displayed exceptionally outstanding catalytic properties for energy conversion and production and have surpassed all other catalysts. Despite their wide-range use, there are some challenges associated with noble metals catalysts, such as their limited availability and high prices [3]. From economical perspective, the cost and scarcity of many promising catalytic metals such as palladium and platinum reduces their extensive use [4,5]. To deal with this problem effectively, many other catalysts are being considered with the motive of obtaining commercially viable economical catalyst with uncompromised catalytic efficiency.
The key goal of alternative approaches being considered is to reduce the quantity of these expensive noble metals while possibly improving or at least maintaining performance. Thus, in recent times, single atom catalysis (SAC) approach has been proposed, so that catalysis can be accomplished via single metal atom on a support surface [6][7][8]. Single atom catalysts usually include uniformly distributed catalyst on supports which act to stabilize these catalysts. With the passage of time, many synthetic procedures have been developed to prepare these catalysts. Novel techniques that are used to synthesize and characterize single atom catalyst are wet chemistry [9], atomic layer deposition [10] and

Computational Methodology
In this study, M06-2X/6-31G(d,p) is used for the optimization of all the structures using Gaussian09 software. M06-2X is a long-range functional which is well reported to estimate non-covalent interaction energies and barrier heights [50]. Benchmark studies show that M06-2X functional perform better for interactions of stacked system. M06-2X also shows better performance when interactions of TMs are studied with other systems [51].
TMs show various spin states, therefore, each of these metal complexes were optimized at various spin states to obtain the most stable spins state. The electronic config-urations of Fe, Co and Ni are [Ar] 3d6 4 s 2 , [Ar] 3d6 7 s 2 and [Ar] 3d6 8 s 2 , respectively. In case of Fe and Ni doped C 2 N, singlet, triplet, quintet, and septet spin states were considered, and the most stable spin states are septet and triplet for Fe and Ni, respectively (see Table S1 of Supplementary Material). For Co doped C 2 N, doublet, quartet, sextet, and octet spin states were optimized in search of the most thermodynamically stable spin state of metal doped C 2 N. Among the optimized spin states, doublet is the most stable in case of Co@C 2 N complex. The interactions energies of these complexes were based on the stable spin states. Hydrogen dissociation reaction was also performed on the most stable spin state metals.
Interaction energies are calculated for the most stable geometries of studied complexes by using the following expression: Here, E T.M@C2N , E C2N and E T.M are interaction energies of TMs@C 2 N complexes, bare C 2 N surface and TMs, respectively.
For hydrogen dissociation, primary step is adsorption of hydrogen on metal doped C 2 N surface. The adsorption energy of hydrogen doped catalyst is calculated through following equation: Here, E H2@C2N-M , E M@C2N and E H2 represent the energies of the hybrid structures of hydrogen adsorbed complexes, metal@C 2 N, and H 2 , respectively.
For calculation of activation barrier and energy of reaction, Equations (3) and (4) were used, respectively.
In Equation (3), the E a represents activation barrier and E TS represents the energy of transition states. Whereas in Equation (4), the E R shows energy of reaction.

Geometries and Electronic Properties
The optimized structure of C 2 N consists of hexagonal unit cell (see Figure 1). The C-N bond length is 1.32 Å whereas for C-C bond length is 1.46 Å in the benzene ring and 1.42 Å in the pyrazine ring. The observed C-N-C bond angle is 116.73 • . All of these bonding parameters are comparable with already reported in the literature [52]. For each TM being considered, we have studied various spin states in order to obtain thermodynamically the stable spin state. The most stable spin state geometries are reported in the main manuscript (see Figure 2), while least stable M@C2N complexes are  For each TM being considered, we have studied various spin states in order to obtain thermodynamically the stable spin state. The most stable spin state geometries are reported in the main manuscript (see Figure 2), while least stable M@C 2 N complexes are given in Supplementary Information (Figure S1). While the interaction energy values for studied TM@C 2 N clusters are reported in Table 1. For each TM being considered, we have studied various spin states in order to obtain thermodynamically the stable spin state. The most stable spin state geometries are reported in the main manuscript (see Figure 2), while least stable M@C2N complexes are given in supplementary information ( Figure S1). While the interaction energy values for studied TM@C2N clusters are reported in Table 1.    Optimized structure of Fe@C 2 N cluster is presented in Figure 2a. Fe atom binds with neighboring nitrogen atoms. The stabilization or adsorption energy in case of Fe@C 2 N cluster is −3.19 eV. The geometry of Co@C 2 N surface is given in Figure 2b, Co shows interaction with neighboring nitrogen atoms and the calculated stabilization energy is −1.42 eV, whereas adsorption of Ni atom over C 2 N surface resulted in the interaction energy of −2.51 eV (Figure 2c). The highest interaction energy value is observed in case of Fe@C 2 N among all studied M@C 2 N clusters, which is attributed to least interaction distance between Fe and N atoms of C 2 N surface. The bonding distance between Fe and nitrogen atoms of C 2 N is 2.53 Å, as compared to 2.64 Å and 2.66 Å for Ni@C 2 N and Co@C 2 N, respectively. Furthermore, it is observed in studied complexes that as the number of unpaired electrons (d-orbital of TMs) decreases, decrease in their interaction energy is observed. Moreover, no distortion is observed in M@C 2 N clusters upon adsorption of TMs (Fe, Ni & Co) due to fused rings of benzene and pyrazine. In addition, no change in bond lengths of C-N and C-C are observed upon adsorption of TMs on C 2 N surface.

HOMO-LUMO and DOS Analysis
HOMO-LUMO analysis and DOS spectra have been investigated to fully understand the corresponding changes in electronic properties. The charge transfer results and the HOMO-LUMO energy gap of metal doped C 2 N complexes are reported in Table 1. Density of states graphs are presented in Figure 3, which show the formation of new states causes the change in H-L gap. In the DOS graph of Ni doped on C 2 N, the formation of new HOMO states also confirms the change in H-L energy gap.
Upon adsorption of metal atoms, the energy gap (E H-L ) is significantly reduced. Least reduction in energy gap is observed in case of Co@C 2 N complex which is from 5.61 eV to 5.11 eV. However, a significant decrease in energy gap is observed for Fe@C 2 N and Ni@C 2 N complexes as compared to bare C 2 N surface i.e., E H-L values are 2.56 eV and 1.50 eV, respectively. The change in electronic parameters is confirmed through DOS analysis which clearly shows the formation of new states. Thus, it also explains the significant lowering of HOMO-LUMO energy gap in Fe@C 2 N and Ni@C 2 N complexes.
In case of Ni@C 2 N complex, potential decrease in H-L gap is observed due to increase in HOMO and decrease in LUMO energies as compared to bare C 2 N surface. Same type of observations is observed from TDOS spectra of Ni@C 2 N due to increase in HOMO energy and decrease in LUMO energy.  Upon adsorption of metal atoms, the energy gap (EH-L) is significantly reduced. Le reduction in energy gap is observed in case of Co@C2N complex which is from 5.61 eV 5.11 eV. However, a significant decrease in energy gap is observed for Fe@C2N a Ni@C2N complexes as compared to bare C2N surface i.e., EH-L values are 2.56 eV and 1 eV, respectively. The change in electronic parameters is confirmed through DOS analy which clearly shows the formation of new states. Thus, it also explains the significant lo ering of HOMO-LUMO energy gap in Fe@C2N and Ni@C2N complexes.
In case of Ni@C2N complex, potential decrease in H-L gap is observed due to incre in HOMO and decrease in LUMO energies as compared to bare C2N surface. Same t of observations is observed from TDOS spectra of Ni@C2N due to increase in HOMO ergy and decrease in LUMO energy.

Natural Bond Orbital (NBO) Analysis
NBO analysis was performed on studied TM doped C2N clusters to investigate transfer of charge between C2N surface and TM atoms. The values of NBO charges reported in Table 1. The adsorption of Fe on C2N surface resulted in a net charge of 1.4 on Fe atom. The appearance of positive charge (1.43e − ) on the Fe atom upon adsorpt over C2N represents the electron recipient character of C2N and electropositive natur Fe in the most stable geometry of Fe@C2N complex. Similarly, in case of Co and Ni pants, the NBO charges observed are 0.92e -and 0.74e -, respectively. In both Co@C2N a Ni@C2N complexes, the positive sign of charge transfer indicates that charge is shift towards C2N surface from TMs, revealing the electropositive character of studied T Highest charge transfer is observed in case of Fe@C2N complex, which reveals the stro interaction among Fe atom and C2N support through a charge transfer from Fe atom C2N surface [53].
NBO analysis reveals that TMs adsorbed on C2N surface showed electroposit character due to their metallic behavior and electron rich C2N surface. Highest cha transfer is observed in case of Fe, which verify its high interaction energy with C2N s face. However, in case of Co@C2N and Ni@C2N complexes NBO charges observed 0.92e − and 0.74e − , respectively.

Hydrogen Dissociation Reaction on Iron Doped C2N Surface
The reaction started with adsorption of H2 molecule on C2N surface (Figure 4). T hydrogen molecule is adsorbed at iron with adsorption energy of −1.35 eV. The m

Natural Bond Orbital (NBO) Analysis
NBO analysis was performed on studied TM doped C 2 N clusters to investigate the transfer of charge between C 2 N surface and TM atoms. The values of NBO charges are reported in Table 1. The adsorption of Fe on C 2 N surface resulted in a net charge of 1.43e − on Fe atom. The appearance of positive charge (1.43e − ) on the Fe atom upon adsorption over C 2 N represents the electron recipient character of C 2 N and electropositive nature of Fe in the most stable geometry of Fe@C 2 N complex. Similarly, in case of Co and Ni dopants, the NBO charges observed are 0.92e − and 0.74e − , respectively. In both Co@C 2 N and Ni@C 2 N complexes, the positive sign of charge transfer indicates that charge is shifting towards C 2 N surface from TMs, revealing the electropositive character of studied TMs. Highest charge transfer is observed in case of Fe@C 2 N complex, which reveals the strong interaction among Fe atom and C 2 N support through a charge transfer from Fe atom to C 2 N surface [53].
NBO analysis reveals that TMs adsorbed on C 2 N surface showed electropositive character due to their metallic behavior and electron rich C 2 N surface. Highest charge transfer is observed in case of Fe, which verify its high interaction energy with C 2 N surface. However, in case of Co@C 2 N and Ni@C 2 N complexes NBO charges observed are 0.92e − and 0.74e − , respectively.

Hydrogen Dissociation Reaction on Iron Doped C 2 N Surface
The reaction started with adsorption of H 2 molecule on C 2 N surface ( Figure 4). The hydrogen molecule is adsorbed at iron with adsorption energy of −1.35 eV. The metal atom (Fe1) shows interaction with both hydrogen atoms marked as H2 and H3 with interaction distances of 2.06 Å and 2.07 Å, respectively. Initially H-H bond length of isolated H 2 is 0.75Å. After adsorption, the hydrogen dissociation proceeds. In the transition state, H-H bond length increases from 0.75Å to 0.93Å and the Metal-Hydrogen bond length decreases to 1.78Å in transition state. Single imaginary frequency confirms that the transition state is located on Fe@C 2 N (see Table S2 for more details). In the product, the Fe-H bond length is 1.67Å and N-H bond length is 1.03Å. The activation barrier for this hydrogen dissociation reaction occurring on Fe@C 2 N is 0.36 eV, while the enthalpy of reaction is −0.05 eV, as mentioned in Table 2. H2 is 0.75Å. After adsorption, the hydrogen dissociation proceeds. In the transition state, H-H bond length increases from 0.75Å to 0.93Å and the Metal-Hydrogen bond length decreases to 1.78Å in transition state. Single imaginary frequency confirms that the transition state is located on Fe@C2N (see Table S2 for more details). In the product, the Fe-H bond length is 1.67Å and N-H bond length is 1.03Å. The activation barrier for this hydrogen dissociation reaction occurring on Fe@C2N is 0.36 eV, while the enthalpy of reaction is −0.05 eV, as mentioned in Table 2.    In case of Fe doped C 2 N catalyst, the activation barrier reduced significantly, which show higher catalytic activity of Fe doped C 2 N catalyst. Iron possesses greater number of unpaired electrons (d-orbital), which are responsible for higher catalytic efficiency of Fe@C 2 N complex.

Hydrogen Dissociation Reaction on Cobalt Doped C 2 N Surface
In the first step, H 2 molecule is adsorbed over the C 2 N surface ( Figure 5). The stabilization energy observed for the adsorption of hydrogen molecule at cobalt site is −1.93 eV, which is higher than the value observed for adsorption at Fe site (−1.35 eV). Optimized geometry of H 2 molecules over Co@C 2 N surface reveals that H 2 is bit tilted. Initially, the interaction distances of Co atom with H2 and H3 atoms of reactant molecule are 1.83 Å and 1.92 Å, respectively (see Figure 5). Then, hydrogen dissociation proceeds through a transition state, where H-H bond length increases from 0.76 Å to 0.85 Å and the Co-H bond length is decreased to 1.76 Å. At final step, the Co-H and N-H bond lengths observed are 1.60 Å and 1.03 Å, respectively. The activation barrier for hydrogen dissociation reaction occurring on Co@C 2 N surface is 0.45 eV, and the enthalpy of reaction is −0.08 eV (see Table 2). In case of Fe doped C2N catalyst, the activation barrier reduced significantly, which show higher catalytic activity of Fe doped C2N catalyst. Iron possesses greater number of unpaired electrons (d-orbital), which are responsible for higher catalytic efficiency of Fe@C2N complex.

Hydrogen Dissociation Reaction on Cobalt Doped C2N Surface
In the first step, H2 molecule is adsorbed over the C2N surface ( Figure 5). The stabilization energy observed for the adsorption of hydrogen molecule at cobalt site is −1.93 eV, which is higher than the value observed for adsorption at Fe site (−1.35 eV). Optimized geometry of H2 molecules over Co@C2N surface reveals that H2 is bit tilted. Initially, the interaction distances of Co atom with H2 and H3 atoms of reactant molecule are 1.83 Å and 1.92 Å, respectively (see Figure 5). Then, hydrogen dissociation proceeds through a transition state, where H-H bond length increases from 0.76 Å to 0.85 Å and the Co-H bond length is decreased to 1.76 Å. At final step, the Co-H and N-H bond lengths observed are 1.60 Å and 1.03 Å, respectively. The activation barrier for hydrogen dissociation reaction occurring on Co@C2N surface is 0.45 eV, and the enthalpy of reaction is −0.08 eV (see Table 2). In case of Co@C2N complex, the activation barrier of 0.45 eV is observed for hydrogen dissociation, which is comparatively higher as compared to Fe@C2N complex (0.36 eV). The higher potential barrier for Co@C2N catalyst is due to less unpaired electrons (d-orbital) in TM (Co). In case of Co@C 2 N complex, the activation barrier of 0.45 eV is observed for hydrogen dissociation, which is comparatively higher as compared to Fe@C 2 N complex (0.36 eV). The higher potential barrier for Co@C 2 N catalyst is due to less unpaired electrons (d-orbital) in TM (Co).

Hydrogen Dissociation Reaction on Nickel Doped C 2 N Surface
H 2 molecule is also adsorbed on Nickel of Ni@C 2 N surface with the adsorption energy of −2.02 eV (see Figure 6). In optimized geometry, reactant hydrogen molecule is oriented almost parallel over the C 2 N surface. The bond distances between nickel atom of Ni@C 2 N and, H2 and H3 atoms of molecule are 1.90 Å and 1.89 Å, respectively, whereas the H-H bond length is 0.76 Å. At transition state, H-H bond length increases from 0.76 Å to 0.84 Å and the Ni-H bond length is decreased from 1.89 Å to 1.75 Å. Finally at product side, the Ni-H bond length gets further reduced to 1.56 Å, whereas N-H bond length is 1.03 Å. In case of Ni@C 2 N cluster, the activation barrier for hydrogen dissociation reaction is 0.40 eV (Table 2), while the enthalpy of reaction is 0.23 eV.
H2 molecule is also adsorbed on Nickel of Ni@C2N surface with the adsorption energy of −2.02 eV (see Figure 6). In optimized geometry, reactant hydrogen molecule is oriented almost parallel over the C2N surface. The bond distances between nickel atom of Ni@C2N and, H2 and H3 atoms of molecule are 1.90 Å and 1.89 Å, respectively, whereas the H-H bond length is 0.76 Å. At transition state, H-H bond length increases from 0.76 Å to 0.84 Å and the Ni-H bond length is decreased from 1.89 Å to 1.75 Å. Finally at product side, the Ni-H bond length gets further reduced to 1.56 Å, whereas N-H bond length is 1.03 Å. In case of Ni@C2N cluster, the activation barrier for hydrogen dissociation reaction is 0.40 eV (Table 2), while the enthalpy of reaction is 0.23 eV. Figure 6. Potential energy surface diagram of H2 dissociation on Ni@C2N for reactant, transition state and product. Where, grey color is for carbon, white for hydrogen, blue for nitrogen and cobalt blue for nickel atom.
The hydrogen dissociation barrier in Ni@C2N complex is 0.40 eV. The observed value of activation barrier in this case is lower than Co@C2N catalyst and greater than the Fe@C2N catalyst.
Overall, the order activation barrier observed for studied catalysts is Fe@C2N < Ni@C2N < Co@C2N. The observed trend is quite similar with the trend of TMs doped Al2O3 reported by Yang et al. [54] for the oxidation of CO by single atom catalysis.
For comparison, the activation barrier of hydrogen dissociation in our work and some other surfaces are reported in Table 3. Our results show good agreement with already reported values of dissociation barrier using noble TMs. In our case, the lowest hydrogen dissociation barrier is observed for Fe@C2N catalyst (0.36 eV), and the value is The hydrogen dissociation barrier in Ni@C 2 N complex is 0.40 eV. The observed value of activation barrier in this case is lower than Co@C 2 N catalyst and greater than the Fe@C 2 N catalyst.
Overall, the order activation barrier observed for studied catalysts is Fe@C 2 N < Ni@C 2 N < Co@C 2 N. The observed trend is quite similar with the trend of TMs doped Al 2 O 3 reported by Yang et al. [54] for the oxidation of CO by single atom catalysis.
For comparison, the activation barrier of hydrogen dissociation in our work and some other surfaces are reported in Table 3. Our results show good agreement with already reported values of dissociation barrier using noble TMs. In our case, the lowest hydrogen dissociation barrier is observed for Fe@C 2 N catalyst (0.36 eV), and the value is much better than the reported value of Au/TiO 2 complex (0.54 eV). Our results are in accordance with the already reported values of dissociation barriers obtained on different surfaces doped with noble TMs. In our case, Fe-incorporated C 2 N surface displays the smallest activated barrier (0.36 eV), which is due to the presence of strong interaction between the metal d orbitals and molecular orbital of H 2 .

Conclusions
Herein, we have theoretically investigated the hydrogen dissociation reaction on TMs doped C 2 N surface through single atom catalysis. Single atom catalysis provides better efficiency and stability in heterogeneous catalysis. Stable spin states of TMs@C 2 N complexes evaluated for catalytic hydrogen dissociation reaction. Electronic properties (NBO, FMO) of the most stable spin state of TMs@C 2 N complexes are further explored. NBO analysis reveals the electropositive character of TMs, thus, significant charge transfer is observed between TMs and C 2 N surface. Hydrogen molecule, primarily adsorbed on metal doped C 2 N surface during dissociation and then heterolytically dissociated between metal and nitrogen atom of C 2 N surface. The mechanistic pathway of hydrogen dissociation reaction shows that Fe@C 2 N complex is the most suitable catalyst for hydrogen dissociation reaction with activation barrier of 0.36 eV compared to Ni@C 2 N (0.40 eV) and Co@C 2 N (0.45 eV) complexes. Our results indicate that the studied TMs@C 2 N complexes significantly decrease in the activation barrier, which speaks volumes about their success. However, the highest reduction in activation barrier is observed in the case of Fe@C 2 N complex, thus can act as a promising catalyst for hydrogen dissociation reaction in single atom catalysis.
Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/nano13010029/s1, Figure S1: Optimized geometries of studied M@C 2 N complexes at various possible spin states at M06-2X/6-31G(d,p) level of theory. Where grey color is for carbon, light grey for hydrogen, blue for nitrogen, orange for nickel, purple for iron and cobalt blue for cobalt atom; Table S1: Relative stabilities of studied M@C 2 N complexes at various possible spin states (all values are in eV). Table S2: Energies and lowest vibrational frequencies of reactants, products, and transition states over designed M@C 2 N catalysts.