Theoretical Study of Hydrogen Production from Ammonia Borane Catalyzed by Metal and Non-Metal Diatom-Doped Cobalt Phosphide

The decomposition of ammonia borane (NH3BH3) to produce hydrogen has developed a promising technology to alleviate the energy crisis. In this paper, metal and non-metal diatom-doped CoP as catalyst was applied to study hydrogen evolution from NH3BH3 by density functional theory (DFT) calculations. Herein, five catalysts were investigated in detail: pristine CoP, Ni- and N-doped CoP (CoPNi-N), Ga- and N-doped CoP (CoPGa-N), Ni- and S-doped CoP (CoPNi-S), and Zn- and S-doped CoP (CoPZn-S). Firstly, the stable adsorption structure and adsorption energy of NH3BH3 on each catalytic slab were obtained. Additionally, the charge density differences (CDD) between NH3BH3 and the five different catalysts were calculated, which revealed the interaction between the NH3BH3 and the catalytic slab. Then, four different reaction pathways were designed for the five catalysts to discuss the catalytic mechanism of hydrogen evolution. By calculating the activation energies of the control steps of the four reaction pathways, the optimal reaction pathways of each catalyst were found. For the five catalysts, the optimal reaction pathways and activation energies are different from each other. Compared with undoped CoP, it can be seen that CoPGa-N, CoPNi-S, and CoPZn-S can better contribute hydrogen evolution from NH3BH3. Finally, the band structures and density of states of the five catalysts were obtained, which manifests that CoPGa-N, CoPNi-S, and CoPZn-S have high-achieving catalytic activity and further verifies our conclusions. These results can provide theoretical references for the future study of highly active CoP catalytic materials.


Introduction
The energy crisis and environmental pollution are two major problems humans are faced with in society today. To deal with these severe problems, taking advantage of renewable energy to replace fossil energy is an important strategy for the international community.
As a renewable energy source, hydrogen energy is light in weight, high in heat, nontoxic, harmless, excellent in thermal conductivity, clean, and pollution-free, which makes it a representative of green energy [1][2][3][4]. To date, there are many common industrial hydrogen production methods [5][6][7][8], such as the methods of NH 3 BH 3 decomposition, water cracking, water and ethanol mixture pulse discharge, etc. However, how to release hydrogen safely and efficiently remains the main obstacle to the spread of hydrogen energy. NH 3 BH 3 is considered as one of the most ideal hydrogen storage materials because of its non-toxic, easy storage and transportation and reversible dehydrogenation reaction [9][10][11][12]. Catalytic decomposition of NH 3 BH 3 is accepted as a promising method for hydrogen production. Although traditional catalysts such as platinum-based or rhodium-based noble metal catalysts have high activity for NH 3 BH 3 decomposition, their application is limited due to high cost and short time [13][14][15]. Therefore, there is an upsurge in research to look for non-noble metal catalysts with high catalytic activity and service life [16][17][18][19]. At present, quantities of composite materials with high catalytic activity and high stability which can catalyze the evolution of hydrogen from ammonia borane have been synthesized [20][21][22][23][24].
Cobalt phosphide (CoP) has become a typical representative of inexpensive transition metal phosphides due to its advantages of low production cost, good stability, and catalytic activity [25][26][27]. CoP materials include CoP nanowire arrays [28][29][30], non-metallic-doped CoP materials [31,32], metal-doped CoP materials, etc. [33,34]. Various doped catalytic materials based on cobalt phosphide have been widely studied as excellent catalysts for NH 3 BH 3 decomposition to produce hydrogen [35][36][37], such as non-noble metal cobalt phosphide nanometer materials supported by layered porous carbon (CoPNPs), which were synthesized by step-by-step calcination and phosphating, using a cobalt-based organic framework (CO-MOF-74) as template [38], carbon-point-constrained CoP-CoO nanostructured materials with strong interfacial synergies, which trigger the strong hydrogen evolution performance of NH 3 BH 3 [39], nickel-loaded cobalt phosphide (Ni@CoP) materials, etc. [40,41]. These CoP-based modified catalysts improve the hydrogen evolution performance of NH 3 BH 3 . It was found that different types of doping have an important effect on the catalytic activity of the materials [42,43]. For example, silver and nitrogen diatom-doped zinc oxide has become a salient way to obtain high quality P-type zinc oxide [44]. The dehydrogenation of NH 3 BH 3 catalyzed by Co and Cu diatom-doped magnesium oxide is better than that of Co-and Cu-doped magnesium oxide alone [45]. The catalytic performance and efficiency of O and Mo diatom-doped cobalt phosphide lamellar nanomaterials as catalysts for water cracking has been significantly improved [46].
In view of CoP as an excellent catalyst for the dehydrogenation of NH 3 BH 3 and the significant improvement of catalyst performance by diatom-doped metal and non-metal co-doped CoP as a catalyst for hydrogen production from NH 3 BH 3 was studied in this paper, and the mechanism of its catalytic dehydrogenation is discussed. At the same time, the catalytic activity of different doped catalysts was studied. The study of the physical properties of doped CoP variants is expected to explain the correlation between the physical properties of the catalysts and the catalytic activity of NH 3 BH 3 dehydrogenation, which provides some theoretical references for the optimization and design of the catalysts for hydrogen production from NH 3 BH 3 .

Calculation Methods
In this study, all the structure optimization, band structures, and density of states (DOS) were calculated using Dmol3 in the Material Studio 8.0 program developed by Accelrys, Inc. The generalized gradient approximation Perdew-Burke-Ernzerhof (PBE) exchangecorrelation functional was adopted and the nuclear electron was described by effective core potential (ECP) [47]. To expand the electronic wave function, the double numerical plus polarization (DNP) basis set was used [48]. A 2 × 2 × 1 k-points was sampled using the Monkhorst-Pack method. On this basis, the energy convergence criterion of the selfconsistent iterative process was set to 2 × 10 −5 Ha, the force convergence accuracy was set to 0.004 Ha/Å, and the maximum displacement was set to 0.005 Å. Meanwhile, The LST/QST method was used to search for reaction transition states whose structures were further confirmed by frequency analysis [49]. A 2 × 3 CoP (101) slab model with six-layer-atom was built to represent CoP catalysts, which was consistent with that of Deniel et al. [50] and Cao et al. [51,52]. This model contained 72 Co and 72 P atoms. In order to prevent the interactions between periodic images of the slabs, a 15 Å vacuum layer is added in Z direction. In doped CoP, one of the P atoms on pristine CoP (101) surface were replaced by non-metallic N or S atom, and one of the Co atoms on the pristine CoP (101) surface was replaced by Ni, Ga, or Zn atoms, respectively, as shown in Figure 1. Then, metal and non-metal diatom-doped CoP catalyst models were obtained. Herein, we focus on four different types of doped CoP: Ni and N diatom-doped CoP, as denoted CoP Ni-N ; Ga and N diatom-doped CoP, denoted CoP Ga-N ; Ni and S diatom-doped CoP, denoted CoP Ni-S ; and Zn and S diatom-doped CoP, denoted CoP Zn-S .
Co atoms on the pristine CoP (101) surface was replaced by Ni, Ga, or Zn atoms, respectively, as shown in Figure 1. Then, metal and non-metal diatom-doped CoP catalyst models were obtained. Herein, we focus on four different types of doped CoP: Ni and N diatom-doped CoP, as denoted CoPNi-N; Ga and N diatom-doped CoP, denoted CoPGa-N; Ni and S diatom-doped CoP, denoted CoPNi-S; and Zn and S diatom-doped CoP, denoted CoPZn-S.

Adsorption of NH3BH3 on the Surface of CoP and Its Doped Catalysts
The stable adsorption structure of NH3BH3 on CoP(101) slabs was obtained by optimizing the model of CoP, as shown in Figure 2. In the optimized adsorption configuration, the H(1) atom on the NH3BH3 was adsorbed on the Co(3) atom on the CoP(101) surface, and the distance between the H(1) atom and the Co(3) atom was shortened from 1.79 Å to 1.65 Å. However, the bond length of B-H(1) in NH3BH3 increased from 1.26 Å to 1.28 Å. The electron density map of the adsorption configuration of NH3BH3 on CoP(101) surface is also shown in Figure 2. It can be seen from Figure 2 that the overlap of electron cloud occurs between H(1) atom of NH3BH3 and Co(3) atom of CoP(101) surface, indicating that electron interaction occurs between the H(1) atom of NH3BH3 and Co(3) atom on the CoP(101) surface. In the process of adsorption, a part of the energy is released due to the reduction of molecular motion velocity, and this part of the energy is called adsorption energy (Eads), which can be calculated as: In this formula, Etotal, Eslab, and EAB are potential energies of AB adsorbed on the slab model, the slab model, and AB molecule. The adsorption process of NH3BH3 on CoP (101) surface is an activation process, and its adsorption energy is −1.19 eV. The other stable adsorption structures of NH3BH3 absorbed on the surface of four different kinds of diatom-doped CoP catalysts (CoPNi-N, CoPGa-N, CoPNi-S, and CoPZn-S) are shown in Figure S1 The electron density maps of the adsorption structure are also shown in Figure S1. The adsorption energies of NH3BH3 on the four types of doped catalysts are −1.22 (CoPNi-N), −1.29 (CoPGa-N), −1.21 (CoPNi-S), and −1.25 eV (CoPZn-S) , which indicates that NH3BH3 can be stably adsorbed on the surface of the four kinds of doped catalysts, and the adsorption process of NH3BH3 on the surface of the four kinds of diatom-doped catalysts is also an activation process.

Adsorption of NH 3 BH 3 on the Surface of CoP and Its Doped Catalysts
The stable adsorption structure of NH 3 BH 3 on CoP(101) slabs was obtained by optimizing the model of CoP, as shown in Figure 2. In the optimized adsorption configuration, the H(1) atom on the NH 3 BH 3 was adsorbed on the Co(3) atom on the CoP(101) surface, and the distance between the H(1) atom and the Co(3) atom was shortened from 1.79 Å to 1.65 Å. However, the bond length of B-H(1) in NH 3 BH 3 increased from 1.26 Å to 1.28 Å. The electron density map of the adsorption configuration of NH 3 BH 3 on CoP(101) surface is also shown in Figure 2. It can be seen from Figure 2 that the overlap of electron cloud occurs between H(1) atom of NH 3 BH 3 and Co(3) atom of CoP(101) surface, indicating that electron interaction occurs between the H(1) atom of NH 3 BH 3 and Co(3) atom on the CoP(101) surface. In the process of adsorption, a part of the energy is released due to the reduction of molecular motion velocity, and this part of the energy is called adsorption energy (Eads), which can be calculated as: In this formula, E total , E slab , and E AB are potential energies of AB adsorbed on the slab model, the slab model, and AB molecule. The adsorption process of NH 3 BH 3 on CoP (101) surface is an activation process, and its adsorption energy is −1.19 eV. The other stable adsorption structures of NH 3 BH 3 absorbed on the surface of four different kinds of diatom-doped CoP catalysts (CoP Ni-N , CoP Ga-N , CoP Ni-S, and CoP Zn-S ) are shown in Figure S1. The electron density maps of the adsorption structure are also shown in Figure S1. The adsorption energies of NH 3 BH 3 on the four types of doped catalysts are −1.22 (CoP Ni-N ), −1.29 (CoP Ga-N ), −1.21 (CoP Ni-S ), and −1.25 eV (CoP Zn-S ), which indicates that NH 3 BH 3 can be stably adsorbed on the surface of the four kinds of doped catalysts, and the adsorption process of NH 3 BH 3 on the surface of the four kinds of diatomdoped catalysts is also an activation process.

Hydrogen Evolution Mechanism of NH3BH3 on the Surface of the Catalyst
According to our investigation, there are four potential pathways available for the hydrogen evolution reaction of NH3BH3 on the catalyst surfaces, as shown in Figure 3.

Hydrogen Evolution Mechanism of NH 3 BH 3 on the Surface of the Catalyst
According to our investigation, there are four potential pathways available for the hydrogen evolution reaction of NH 3 BH 3 on the catalyst surfaces, as shown in Figure 3.

Figure 2.
The left shows its adsorption configuration and the right shows the electron density diagram of the adsorption configuration of ammonia borane on the CoP(101) plane.

Hydrogen Evolution Mechanism of NH3BH3 on the Surface of the Catalyst
According to our investigation, there are four potential pathways available for the hydrogen evolution reaction of NH3BH3 on the catalyst surfaces, as shown in Figure 3.  Firstly, NH 3 BH 3 is adsorbed on the surface of the catalyst to form a stable adsorption reactant denoted as M1. Then, NH 3 BH 3 is dehydrogenated through four different reaction pathways to obtain the product P1. In the reaction pathway I, the reactant M1 generates the intermediate M2 via the transition state TS1, and then M2 through the transition state TS2 generates the product P1. In this process, one of the B-H bonds of the NH 3  The structural changes involved in the process of NH 3 BH 3 dehydrogenation catalyzed by the five catalysts are shown in Figure 4 (CoP Zn-S ), Supplementary Materials Figure S2 (CoP), Figure S3 (CoP Ni-N ), Figure S4 (CoP Ga-N ), and Figure S5 (CoP Ni-S ). Since each reaction pathway is similar for different catalysts, herein only CoP Zn-S is discussed. The relevant structural parameters of the hydrogen evolution reaction catalyzed by CoP Zn-S are listed in Table 1, and the relative energies and activation energies in each step are listed in Table 2 and as shown in Figure 4. The structural changes involved in the process of NH3BH3 dehydrogenation catalyzed by the five catalysts are shown in Figure 4 Figure S4 (CoPGa-N), and Figure S5 (CoPNi-S). Since each reaction pathway is similar for different catalysts, herein only CoPZn-S is discussed. The relevant structural parameters of the hydrogen evolution reaction catalyzed by CoPZn-S are listed in Table 1, and the relative energies and activation energies in each step are listed in Table 2 and as shown in Figure 4.  For reaction path I, NH 3 BH 3 is adsorbed on the catalyst surface to form M1 Zn-S , and the adsorption site of NH 3 BH 3 is above the Co (3)  In the process of M2 Zn-S →TS2 Zn-S →P1 Zn-S , the distance between H(2) and N atoms changes from 1.05 Å to 1.92 Å and finally to 3.66 Å.
For reaction pathway II, firstly, the reactant NH 3 BH 3 is adsorbed on the surface of the catalyst to form M1 Zn-S , and the M1 Zn-S, through the transition state TS1 Zn-S, forms the intermediate M2 Zn-S , which is the same as the process of M1 Zn-S →M2 Zn-S in pathway I. Subsequently, in the intermediate M2 Zn-S , the H(2) atom on the N atom of NH 3 BH 3 is gradually detached from the NH 3 BH 3 and adsorbed between the Co(3) and Zn on the CoP(101) surface of the catalyst. With the movement of the H(2) atom, the H(1) atom is adsorbed to the upper left of the Co(3) atom to form the intermediate M3 Zn-S , which is the transition state TS3 Zn-S with an activation energy of 42.04 kcal/mol. In this process, the distance between the H(2) atom and the N atom of NH 3 BH 3 increases from 1.05 Å to 1.51 Å and finally to 2.55 Å in M3 Zn-S . The distance between the H(1) and Co(3) atoms varies from 1.58 Å for M2 Zn-S to 1.54 Å for TS3 Zn-S and finally to 1.65 Å for M3 Zn-S . The distances between H(2) and Co(3), H(2), and Zn vary from 3.31 Å and 2.96 Å in M2 Zn-S to 3.15 Å and 2.52 Å in TS3 Zn-S and finally to 1.59 Å and 3.05 Å in M3 Zn-S . In the intermediate M3 Zn-S , the two H atoms adsorbed on the surface of the catalyst are close to each other via the transition state TS4 Zn-S to form product P1. The activation energy of the transition state TS4 Zn-S is 9.73 kcal/mol.
For reaction path III, the reactant NH 3 BH 3 is first adsorbed on the catalyst surface to form M1 Zn-S . Then, the H(1) and H(2) atoms, respectively cleaved from the B and N atoms of NH 3 BH 3 , were adsorbed on the surface of the catalyst above the Zn atom of Co(3) atom to form the intermediate M3 Zn-S through the transition state TS5 Zn-S with an activation energy of 28.80 kcal/mol. In this process, the distance between the B atom and H(1) atom increases from 1.28 Å to 0.178 Å, and the distance between the H(2) atom and N atom increases from 1.03 Å to 1.50 Å. The intermediate M3 Zn-S through the transition state TS4 Zn-S forms the product P1 Zn-S , which is consistent with the process of M3 Zn-S →P1 Zn-S in path II.
For reaction path IV, in M1 Zn-S , the H atom on the B atom of NH 3 BH 3 and the H atom on the N atom of NH 3 BH 3 directly generated the H 2 via the transition state TS6 Zn-S with the activation energy of 22.15 kcal/mol, in which the distance between the H(1) and H(2) atoms decreases from 2.53 Å to 1.99 Å in TS6 Zn-S . Table 1. The bond length (Å) parameters of reaction sites in CoP Zn-S -catalyzed NH 3 BH 3 hydrogen evolution process. (Å).

Pathway B-H(1) Co(3)-H(1) N-H(2) Co(3)-H(2) Zn-H(2) H(1)-H(2)
I M1 The reaction mechanisms of pristine CoP, CoP Ni-N , CoP Ga-N , or CoP Ni-S catalyzed NH 3 BH 3 are similar to the CoP Zn-S catalyst. The details of the configuration changes and configuration parameters of the reaction process are shown in Supplementary Materials Figure Figures S2 and S3-S5, can also indicate that the reaction mechanism of CoP and the other diatom-doped CoP catalysts has small differences.
The conclusion can be draw from Table 2 that in the reaction of NH 3 BH 3 dehydrogenation catalyzed by CoP Zn-S , the control steps of each reaction pathway are different, (CoP, CoP Ni-N , CoP Ga-N , CoP Ni-S , as shown in Tables S5-S8) which are M2 Zn-S →TS2 Zn-S (pathway I), M2 Zn-S →TS3 Zn-S (pathway II), M1 Zn-S →TS5 Zn-S (pathway III), and M1 Zn-S →TS6 Zn-S (pathway IV), respectively. The energy barrier values of each control step are 39.56 kcal/mol (pathway I), 42.04 kcal/mol (pathway II), 28.08 (pathway III) kcal/mol and 22.15 kcal/mol (pathway IV), respectively. According to the comparison of activation energy of each reaction path control step, the optimal pathway of the NH 3 BH 3 dehydrogenation reaction is reaction pathway IV, and the energy barrier of the control step is 22.15 kcal/mol. Considering the energy changes in the five catalysts, the activation energies of CoP, CoP Ni-N , CoP Ga-N , CoP Ni-S, and CoP Zn-S catalyzing the decomposition of NH 3 BH 3 to hydrogen at each step are listed in Table 3. In CoP-catalyzed NH 3 BH 3 dehydrogenation, as shown in Table S5, the activation energy of the optimal control step of the four reaction pathways is 31.35 kcal/mol. The activation energies of CoP Ni-N , CoP Ga-N, and CoP Ni-S are 27.11 kcal/mol, 23.18 kcal/mol, and 20.67 kcal/mol, respectively. The energy level changes of the five catalysts in the reaction process are shown in Figure S6 (CoP), Figure S7 (CoP Ni-N ), Figure S8 (CoP Ga-N ), Figure S9 (CoP Ni-S ), and Figure S10 (CoP Zn-S ). By comparing the catalytic activities of CoP, CoP Ni-N , CoP Ga-N , CoP Ni-S, and CoP Zn-S , it is found that the simultaneous doping of metal and non-metal with CoP is beneficial to the improvement of NH 3 BH 3 hydrogen evolution activity. A large number of studies on cobalt-phosphidemodified materials can prove that the dopant of N, S, Ni, Zn, and Ga can improve the catalytic performance of CoP, which is consistent with our theoretical calculation results. For instance, Chen et al. [53] reported that Ni-doped CoP could accelerate the process of hydrogen evolution both in acid and alkaline media, showing excellent electrochemical stability and durability. Li et al. [54] found N and Mo co-doped heteroatoms can optimize the morphology and surface structure of CoP. Anjum et al. [55] studied sulfur-doped cobalt phosphide electrocatalysts and concluded that their performance is better than all-noblemetal electrocatalysts in alkaline electrolyzers for overall water splitting. Yang et al. [56] synthesized Zn-doped CoP nanowire arrays for boosting hydrogen generation, and they found the overpotential of Zn-doped CoP was two times lower than undoped CoP. Zhang et al. [57] also reported that Ga dopant could enhance the activity of CoP.

Performance Calculation of Catalysts
The stable catalyst models of CoP, CoP Ni-N , CoP Ga-N , CoP Ni-S, and CoP Zn-S were optimized and obtained. The band structure and density of states (DOS) of the stable catalyst were calculated, as shown in Figure S11. The diagram of band structure is marked by 1 on the left, and the map of density of states is marked by 2 on the right, in which a1, b1, c1, d, and e1 represent the band structure diagram of CoP, CoP Ni-N , CoP Ga-N , CoP Ni-S, and CoP Zn-S, respectively. Meanwhile, a2, b2, c2, d2, and e2 represent the map of density of states of CoP, CoP Ni-N , CoP Ga-N , CoP Ni-S, and CoP Zn-S . The red dashed line in the figure represents the Fermi level.
The DOS map, which refers to the number of states in a unit frequency interval, is used to characterize the distribution of electron cloud density near the Fermi level. The Fermi level is a parameter used to measure the catalytic activity, and its value is the average of the sum of the highest energy occupied orbital and the lowest energy occupied orbital. The greater the density of the electron cloud near the Fermi level, the stronger the catalytic activity of the material. We calculated the total DOS of the five catalysts at the Fermi level as follows: 37 [58].
In the end, to research the effect of interfacial adsorption on catalytic activity for the five doped catalysts, the charge density difference (CDD) between NH 3 BH 3 and the five different catalysts were calculated. CDD is one of the important methods to study electronic structure. The electron flow direction after the interaction of each segment can be intuitively obtained, or the change of electron density during the formation of atoms into molecules, and the nature of chemical bonds can be explored. As shown in Figure 5, blue is the electron accumulation, while red represents the electron depletion. From Figure 5, we can realize the charge transfer characteristics of NH 3 BH 3 and the five catalytic adsorption processes. Compared with undoped CoP, the electronic interaction between doped catalysts and NH 3 BH 3 is enhanced, which indicates the strong electronic interaction between the catalyst and NH 3 BH 3, determining the catalytic activity in the adsorption process.

Conclusions
In this paper, metal and non-metal diatom-doped CoP as catalyst was applied to study hydrogen evolution from NH3BH3 by DFT calculations. The doped catalysts involved in CoPNi-N, CoPGa-N, CoPNi-S, and CoPZn-S were formed by replacing Co atoms with Ni, Ga, or Zn, and P atoms with S or N on the surface of the CoP(101), respectively. First of all, the adsorption process of NH3BH3 on each catalyst was explored, and the adsorption energy and electron density maps were obtained. From the values of adsorption energies and electron density maps, the conclusion can be drawn that each doped type of catalyst has a strong adsorption effect on NH3BH3, which is activated on the surface of the catalyst. Then, we further studied the reaction mechanism of the decomposition of NH3BH3 into H2 and NH2BH2 catalyzed by five catalysts (CoP, CoPNi-N, CoPGa-N, CoPNi-S, and CoPZn-S). In this investigation, four pathways were designed, and the best reaction pathways for each catalyst were found. By analyzing the activation energy of the control step, it can be seen clearly that the energy barrier values of the control step for the five catalysts are Ea (CoP) > Ea

Conclusions
In this paper, metal and non-metal diatom-doped CoP as catalyst was applied to study hydrogen evolution from NH 3 BH 3 by DFT calculations. The doped catalysts involved in CoP Ni-N , CoP Ga-N , CoP Ni-S, and CoP Zn-S were formed by replacing Co atoms with Ni, Ga, or Zn, and P atoms with S or N on the surface of the CoP(101), respectively. First of all, the adsorption process of NH 3 BH 3 on each catalyst was explored, and the adsorption energy and electron density maps were obtained. From the values of adsorption energies and electron density maps, the conclusion can be drawn that each doped type of catalyst has a strong adsorption effect on NH 3 BH 3 , which is activated on the surface of the catalyst. Then, we further studied the reaction mechanism of the decomposition of NH 3 BH 3 into H 2 and NH 2 BH 2 catalyzed by five catalysts (CoP, CoP Ni-N , CoP Ga-N , CoP Ni-S, and CoP Zn-S ). In this investigation, four pathways were designed, and the best reaction pathways for each catalyst were found. By analyzing the activation energy of the control step, it can be seen clearly that the energy barrier values of the control step for the five catalysts are Ea (CoP) > Ea (CoP Ni-N ) > Ea (CoP Ga-N ) > Ea (CoP Zn-S ) > Ea (CoP Ni-S ). According to the energy barrier results, the activity of the five catalysts should be CoP Ni-S > CoP Zn-S > CoP Ga-N > CoP Ni-N . Finally, the structural performance of the catalyst was investigated, and the band structure and DOS of the CoP Ni-N , CoP Ga-N , CoP Ni-S , and CoP Zn-S catalysts were calculated. The total DOS of the five catalysts at the Fermi level are 37.4 (CoP), 41.1 (CoP Ni-N ), 42.8 (CoP Ga-N ), 43.8 (CoP Ni-S ), and 42.1 (CoP Zn-S ). The study results we obtained have revealed the relationship between the physical properties of doped CoP materials and their catalytic activities, which provides theoretical support for a large number of high-activity cobalt phosphide materials doped with non-metals (N, S) and metals (Ni, Ga, Zn) and references for the future study of highly active CoP catalytic materials.
Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/molecules27238206/s1, Figure S1: Stable structures and electron density maps of NH3BH3 absorbed on the surface of four different kinds of diatom-doped CoP catalysts (CoP Ni-N , CoP Ga-N , CoP Ni-S and CoP Zn-S ); Figure S2: General flow chart of NH3BH3 hydrogen production reaction on CoP (101) surface; Figure S3: General flow chart of NH 3 BH 3 hydrogen production reaction on CoP Ni-N (101) surface; Figure S4: General flow chart of NH 3 BH 3 hydrogen production reaction on CoP Ga-N (101) surface; Figure S5: General flow chart of NH 3 BH 3 hydrogen production reaction on CoP Ga-N (101) surface; Figure S6: The energy profiles of NH 3 BH 3 dehydrogenation reaction catalyzed by CoP; Figure S7: The energy profiles of NH 3 BH 3 dehydrogenation reaction catalyzed by CoP Ni-N ; Figure S8: The energy profiles of NH 3 BH 3 dehydrogenation reaction catalyzed by CoP Ga-N ; Figure S9: The energy profiles of NH 3 BH 3 dehydrogenation reaction catalyzed by CoP Ga-N ; Figure S10: The energy profiles of NH 3 BH 3 dehydrogenation reaction catalyzed by CoP Ga-N ; Figure S11: Schematic diagram of energy band structure(EBS) and density of states(DOS). (subscript 1 is EBS, subscript 2 is DOS, (a), (b), (c), (d), (e) refer to CoP, CoP Ni-N , CoP Ga-N , CoP Ni-S , CoP Zn-S respectively); Table S1: Bond lengths (nm) of various sites in the reaction of NH 3 BH 3 catalyzed by CoP; Table S2: Bond lengths (nm) of various sites in the reaction of NH 3 BH 3 catalyzed by CoP Ni-N ; Table S3: Bond lengths (nm) of various sites in the reaction of NH 3 BH 3 catalyzed by CoP Ga-N ; Table S4: Bond lengths (nm) of various sites in the reaction of NH 3 BH 3 catalyzed by CoP Ni-S ; Table S5: The each position energies (E), relative energies (E rel ) and activation energies (E a ) of ammoborane reaction catalyzed by CoP; Table S6: The each position energies (E), relative energies (E rel ) and activation energies (E a ) of ammoborane reaction catalyzed by CoP Ni-N ; Table S7: The each position energies (E), relative energies (E rel ) and activation energies (E a ) of ammoborane reaction catalyzed by CoP Ga-N ; Table S8: The each position energies (E), relative energies (E rel ) and activation energies (E a ) of ammoborane reaction catalyzed by CoP Ni-S .

Data Availability Statement:
The data presented in this study are available on reasonable request from the corresponding author.