Synergistic Effect of FeNi Bimetallic Clusters for the Catalytic of Hydrogen Dissociation and Desorption: A Density Functional Theory Study

Cognition to the metal-metal interactions is of vital importance for the catalytic process and would help exploring novel catalysts. In the present study, the model system of FexNiy (x + y = 6) bimetallic clusters was utilized to study how metal-metal interactions influence the catalytic performance. The formation energies of different FeNi clusters, the hydrogen chemisorption energies together with the maximum hydrogen capacity and the saturated hydrogen atoms desorption energies were calculated. Bimetallic clusters exhibit a superior performance than pure clusters. Especially, Fe2Ni4 cluster has the highest hydrogen loading capacity, the most facile hydrogen molecule dissociation activation energy barrier and the lowest hydrogen atom desorption energy, suggesting that it is easier to dissociate H2 and release the H atoms. As a consequence, by adjusting appropriate metal/metal ratios, it is possible to design bimetallic catalysts with excellent catalytic performance.


1.Introduction
Transition metals are widely used in catalytic hydrogenation and dehydrogenation applications for their highly efficient catalytic performance [1−4]. VIII family metals, such as Pt, Pd, Ru and Rh, are most commonly used catalysts, displays remarkable activity in catalytic hydrogenation of unsaturated organic molecules [5−10]. Despite their high activity, the inherent defects such as the high cost and limited resources greatly limit their large-scale application. Therefore, non-noble metals are attracting more and more attentions. Bimetals, especially those based on non-noble, low-cost, and environmentalfriendly metals, are in massive need for different applications like catalysts [11][12][13], electrocatalysts [14][15][16], sensors [17][18][19] and so on in recent years.
The performance of a bimetallic catalyst is often determined by its metal-metal interaction. A metal can be combined with a variety of other metals with specific properties so as to obtain more active sites, such as Ni-Fe [20,21], Ni-Co [22,23], and Ni-Cu [11,15], the interactions between different metals make these bimetals showing significant improvement in activity. The alloy not only provides stable nanostructure for adsorption sites but also may give rise to the charge rearrangement of the catalyst, thus affect its catalytic performance. Corma et al. synthesized bimetallic nanoparticles of CoNi@C catalyst by one step hydrothermal treatment with glucose assisted, the result showed that the catalytic activity of the obtained bimetal are five times higher than the corresponding monometallic Co@C catalyst on the selective hydrogenation of nitroarenes to corresponding anilines [24]. Especially, they found that H2 chemisorption dissociation is the rate controlling step in the whole process. Another study on the NiFe@Cu hydrogen evolution catalyst showed that bimetal has a high activity in the catalytic conversion and selectivity [25]. Indeed, those studies have greatly promoted the development of morphology control of bimetallic nanoparticles, the theoretical study of the reaction mechanism is rare, and cluster level insight would further promote the catalytic performance, especially in the atomic catalysis.
The catalytic efficiency of hydrogenation reaction is extremely rely on how facile a hydrogen molecule dissociates and how quick a hydrogen atom escape from the catalyst surface. Ni clusters have been systematically studied, the results showed that Ni clusters have a superior H adsorption ability than other non-noble transition metal clusters [26][27][28]. These researches primarily focus on the structures stability, charge distribution [29][30][31], and the crystal growth process [32]. Yet, the reaction mechanism of hydrogen dissociation on Ni-based bimetallic catalysts with different components have not been systematically studied. Therefore, we combine Fe with Ni to form a series of FeNi clusters to perform DFT calculations on the changes in various important properties of the clusters. Although the microstructures and properties of a realistic FeNi nanoparticle are more complicated, the FeNi clusters are still set to a given sub-nano size. For certain studies have already proved that a cluster consist of more than six atoms may systematically demonstrate the catalytic reaction process sufficiently to offer helpful insights into metal-metal interactions [33][34][35].
In this work, we try to understand how metal-metal interactions influence the catalytic activity of a bimetal. FexNiy bimetallic clusters (x + y = 6) were used to represent the model system. DFT were utilized to systematically simulate the whole catalytic process of hydrogen on FexNiy bimetallic clusters. The vital properties, including the average formation energies, the hydrogen chemisorption energies, the activation energies of H2 dissociation on different clusters, hydrogen atoms desorption energies, as well as the H capacity of the clusters are systematically studied. Our findings provide an useful insight into the interactions between different metals for the promoted catalytic behavior on the dissociation and desorption of hydrogen and shed light on the design of noble metal free bimetallic catalysts.

Surface model and computational methods
All simulations were carried out by utilizing DFT/GGA with the PBE exchange-correlation functional as performed in the DMol 3 package [36,37]. All the electronic structure of different FeNi clusters were simulated by utilizing a spin-polarization pattern to address the open shell systems. In order to illustrate the valence electrons and the core electrons, the DNP basis set and an effective core potential were chosen [38,39]. A previous study proved that the basis set of this type will lead to ignorable superposition effects [40]. All configurations were totally optimized, the charge distribution and transfer were carried out by employing the Mulliken population analysis [40]. The Tkatchenko and Scheffler method was used to evaluate the effect of dispersion correction for the adsorption and desorption of hydrogen atoms on the clusters, and the results showed that it is ignorable [41]. To obtain perception into the kinetics barriers of hydrogen molecule chemisorption dissociation, the transition state (TS) search for hydrogen molecule on Fe6, Ni6 and Fe2Ni4 cluster were conducted using LST/QST method [42]. Normal mode analysis with an imaginary frequency was used to confirm the transition state structure.
The equation of the average formation energy (ΔEFE) for FexNiy cluster is computed by utilizing where E(Fe), E(Ni) and E(FexNiy) are the energies of the Fe, Ni and the FexNiy clusters, respectively. The different chemisorption energies of hydrogen molecule on FexNiy clusters were calculated by the following equation where E(FexNiyHn), E(H2) and n represent the energy of FexNiy cluster absorbed with hydrogen, the energy of an hydrogen molecule and the number of hydrogen atoms absorbed on the cluster, respectively. The realistic catalytic hydrogenation procedure is always completed under the condition of a certain hydrogen pressure where H coverage reach saturation or oversaturation on the catalyst surface. Hence, it is necessary to calculate the desorption energy of H atom from the catalyst with hydrogen saturated adsorption to assess its catalytic activity. It is noted that hydrogen atoms prefer to desorb consecutively. Here, the desorption energies of the FexNiy clusters with hydrogen atoms fully covered were calculated by employing the equation below where E(H) represents the total energy of the hydrogen atom and m represents the hydrogen atoms number that is in saturated adsorption. Besides, AIMD calculations under 298.15K were carried out on the FexNiy clusters with hydrogen atoms fully covered for 3 ps in a NVT canonical ensemble utilizing the Nose-Hoover thermostat [43,44] for temperature regulate. When running the Molecular Dynamics, superfluous hydrogen atoms absorbed on the cluster will recombine to generate hydrogen molecules, which lead to a weak association with the cluster [45].

3.Results and discussions
The purpose of this study is to understand how metal-metal interaction affect the catalytic performance of a bimetallic catalyst. Previous studies on pure Ni, Pd and Pt clusters showed that the variability of average formation energy is relatively small after the number of atoms in a cluster exceeds six [26]. Hence, the atom number of the model system FexNiy cluster was set to six. Considering the various isomeric structures for different FexNiy clusters for a given cluster size. In order to confirm the most stable configuration of a FexNiy cluster, massive structural optimizations were carried out to acquire the structure with lowest energy. The Mulliken charge analysis and the average formation energies of different FexNiy clusters with the most stable structures are shown in Figure 1. The results suggest that the average formation energies of FexNiy bimetallic clusters rises with the number of Ni atom increases, and Fe1Ni5 has the highest average formation energy of 2.33 eV. Clearly, FeNi clusters tend to be more stable than Fe6 and Ni6 pure clusters. The Mulliken charge analyses show that Fe atoms have lower electron density than that of Ni atoms due to the charge transfer from Fe atoms to adjacent Ni atoms, which is well agreed with the truth that Ni (1.91) has a relatively higher electronegativity than Fe (1.83). All the Ni atoms display negative charge in the FeNi bimetallic clusters. As the number of Ni atom increases, the charges each Ni atom can share undergoing a trend from rising to decreasing. The Fe atoms, on the contrary, continuously losing charges with the charge value increases from 0 to positive 0.308. Therefore, we can conclude that the charge redistribution on the clusters and the increased stability of FexNiy clusters may have an important impact on the interaction between hydrogen and FexNiy clusters. The structures of FexNiy clusters with the largest formation energy were selected among their isomers to study the chemisorption process of hydrogen. Initially, a H2 molecule moves to the top position of a FexNiy cluster. In general, there are two adsorption sites (the top sites of Fe and Ni atom) that can hold a H2 molecule. Figure 2 shows the chemisorption structures with the lowest energy and dissociation chemisorption energies of H2 on FexNiy clusters. The H-H bond is activated on the adsorption site in each cluster, this makes it become longer. Udrea et al. [46] reported that the formation of FeNi alloy could favor the activation of H2 and facilitate the generation of new active surface species. According to the calculation results, we can learn that the dissociation chemisorption energies vary with the change in the components ratio. Obviously, the chemisorption energies of FexNiy clusters are higher than their corresponding pure Fe and Ni clusters, indicating that hydrogen molecules prefer to adsorb on FexNiy bimetallic clusters and can be readily dissociated into hydrogen atoms. to two opposite octahedral edges with an reaction energy of minus 1.10 eV. Meanwhile, the same process on Fe6 cluster needs surpass a higher energy barrier of 1.01 eV, and result in the two H atoms spread to two opposite edges with an reaction energy of minus 1.15 eV. Apparently, the H2 dissociation becomes easier on Fe2Ni4 cluster, for the energy barrier tend to be more moderate for Fe2Ni4-N (0.36 eV) and Fe2Ni4-F (0.40 eV). Notably, this result is similar to our previous work [47] on the interaction between Pd cluster and support, that the charge redistribution will give rise to the decrease of H2 dissociation activation energy and the increase of the catalytic activity. Except for the H2 dissociation chemisorption energy, the desorption energy of a hydrogen atom on a cluster with hydrogen fully covered is also an extremely important indicator for assessing the catalytic activity. Hence, we conducted the successive loading of hydrogen atoms to see the hydrogen capacity of the FeNi clusters. Generally, during the hydrogen dissociative chemisorption process, hydrogen atoms prefer to adsorb on the edge sites than the hollow sties. As the H loading amount increases, some active top sites of the clusters are also occupied by the extra hydrogen atoms. Subsequently, AIMD runs were performed at 298.15 K to make sure that all the hydrogen atoms are in the state of chemisorption. The radial distribution of H-H bond length of Fe2Ni4-H18 and Fe2Ni4-H16 clusters is shown in Figure 4. The curve of Fe2Ni4-H18 cluster displays a intensive peak close to 0.7 Å, suggesting that at least one hydrogen molecule is formed due to the oversaturation of H atoms. On the other side, a strong peak of Fe2Ni4-H16 is emerged near 2.3 Å, implying that all the hydrogen atoms are well separated. Apparently, the maximum saturated adsorption of hydrogen atoms is 16 for Fe2Ni4 cluster. Considering the contributions of entropic and to further confirm the AIMD results, the Gibbs free energies for the formation of Fe2Ni4-H18 (Fe2Ni4 + 9H2 → Fe2Ni4-H18) and Fe2Ni4-H16 (Fe2Ni4 + 8H2 → Fe2Ni4-H16) clusters at 298.15 K were also calculated. It is found that the value of ΔG is -18.3 kcal/mol for Fe2Ni4−H16 and it increases to 6.9 kcal/mol after two extra hydrogen atoms are loaded, confirming that the adsorption of 16 hydrogen atoms is the maximum capacity for the Fe2Ni4 cluster. Figure 5 shows the fully optimized structures of FeNi clusters with hydrogen adsorption saturated and the corresponding desorption energies. Compare to Fe6, Ni6, and other FexNiy clusters which can hold only 14 hydrogen atoms, Fe2Ni4 has the highest hydrogen capacity of 16. https://doi.org/10.37358/RC. 21.3.8434 After the dissociative hydrogen atoms occupied all the 12 edges of the octahedral Fe2Ni4 clusters, four more opposite top sites of Ni will be available for extra four hydrogen atoms. The desorption energies ΔE(DE) of FexNiy clusters in the hydrogen adsorption saturated state vary in a range of 1.99 eV to 2.25 eV. Apparently, the combination of Fe and Ni as well as the rearrangement of charge with an appropriate ratio would help raising the hydrogen atoms adsorption amount and make the hydrogen atom become easier to escape from the FexNiy clusters.    Figure 6 displays the curves of the average Fe-H and Ni-H bonds length in different FexNiy clusters with the capacity of 12 H atoms. The specific data is shown in Table 1. Zhou et al [26] reported that compare to the Pd-H bond lengths in Pd-based cluster (1.70-1.80 Å), the Ni-H bond lengths in Ni-based cluster are slightly shorter (1.60 -1.70 Å). In the present study, the average Fe-H bond lengths in FexNiy clusters are ranging from 1.69-1.76 Å, whereas the average Ni-H bond lengths are within a relatively narrower range of 1.60-1.65 Å. All the bond lengths undergo a change from decreasing to increasing. Especially, the average Ni-H bond length of Fe4Ni2 cluster is close to the lowest value of 1.60 Å, indicating that H atoms on this cluster may be not easy to escape. To compare variety of the calculated properties, Figure 7 shows the average formation energies of FexNiy clusters, the hydrogen chemisorption energies, the maximum hydrogen capacity as well as the hydrogen desorption energies on FexNiy clusters. The average formation energies of FexNiy clusters tend to be higher than that the corresponding Fe6 and Ni6 clusters (Figure 7a), suggesting that the configurations of bimetals are more stable than monometals during the hydrogen adsorption/desorption process. The hydrogen chemisorption energies (Figure 7b) of both Fe and Ni top sites vary within a small range of 0.27 -0.58 eV, in which, Fe2Ni4 cluster has the highest value of 0.58 eV. Besides, the maximum hydrogen capacity also appears on Fe2Ni4 cluster (Figure 7c), indicating that it can load extra two hydrogen atoms compare to the other FexNiy clusters. The curve of hydrogen desorption energy exhibits a wave like tendency (Figure 7d), in which, Fe2Ni4 cluster possesses the lowest value of 1.98 eV, while the Fe4Ni2 cluster shows a relatively higher E(DE) of 2.23 eV. This result is in good agreement with the deduction obtained in Figure 6. Clearly, although bimetallization is helpful to the desorption of hydrogen atoms, it is still necessary to choose appropriate components to obtain an optimal catalytic performance. This is well agreed with a previous report [48] that a maximum activity for 25Fe75Ni alloy on the catalytic CO hydrogenation. Based on these analyses, we can make a conclusion that a bimetal with an appropriate metal/metal ratio (e.g., Fe2Ni4) would help promoting the stability of its structure, increasing the hydrogen chemisorption energy, improving the capacity of hydrogen atoms and reducing the desorption energy of hydrogen, which could finally lead to the boost of its catalytic performance. On the other hand, an experimental study also proposed a possible mechanism for the promoted stability and efficient for the FeNi alloy, that adjacent nickel sites prevents the surface passivation of iron and in turn the iron sites donor electrons to the Ni sites to improve the activity [49].

Conclusions
By performing massive minimum energy simulations, we explored the chemical properties of subnano FexNiy (x + y = 6) bimetallic clusters. It is found that the interaction between Fe and Ni as well as the charge redistribution give rise to the structures of bimetallic clusters become more stable than pure Fe and Ni clusters. FexNiy bimetallic clusters have higher hydrogen chemisorption energies, this makes them more active in adsorbing and dissociating hydrogen. The reduced hydrogen desorption energies result in the readily release of hydrogen atoms from the cluster surface. The superior activity of Fe2Ni4 cluster implying that a bimetallic cluster with an appropriate metal/metal atomic ratio and charge distribution may obtain a remarkable promotion in the catalytic performance. We believe that the findings in this study could provide useful insights in understanding the interactions between different metals and adjust the constituents for the design of bimetals or even multi-metals, thereby shed light on the development of non-precious, environment friendly and highly effective nanoscale catalysts.