Activation Mechanism of Water Dissociation for Catalytic Hydrogen Production: Insights from First-Principles Molecular Dynamics Simulation

1. Introduction

Hydrogen, a clean and renewable energy source, has the potential to address global warming, environmental pollution, and the fossil energy crisis [1,2,3]. The overall reaction for hydrogen production by water splitting is an “uphill reaction” with an unfavorable positive free Gibbs energy of ΔG_o = 237.13 kJ/mol^-1, and it usually cannot occur spontaneously due to a barrier. Catalysts with electronic and optical properties that can be tailored to specific dimensions, shapes, and compositions can reduce the energy barriers associated with chemical reactions, thereby facilitating their occurrence [4]. Since the initial report in 1972 demonstrating photoelectrochemical water splitting under UV irradiation with an n-type TiO₂ photoanode and a Pt cathode for H2 evolution (the Honda-Fujishima effect) [5], there has been a vast quantity of research articles and reviews focusing on various materials for photocatalytic water splitting [6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21].

The dissociation of water can be achieved by electronic excitations [22,23] or by vibrational excitation [24,25]. The water molecule exhibits three distinct vibrational normal modes: bending (v₂), symmetric (v₁), and antisymmetric (v₃) stretching modes. The two stretching modes are strongly coupled (Darling–Dennison coupling) due to the close vibrational frequencies and anharmonicity, while the coupling between the bending mode and the stretching motion is weak [26,27]. Polanyi’s rule posits that a “late” barrier is more readily surmounted by vibrational excitation, whereas an “earlier” barrier is more readily overcome by translational energy. This is based on the observation that the “promoting coordinate” coincides with the reaction coordinate near the transition state in each case [28]. The dissociative chemisorption of H₂O has a late barrier, where the bond cleavage occurs after the molecule adsorbs on the surface. Consequently, vibrational excitation of H₂O may have a significant impact on reactivity. Vibrationally excited molecular hydrogen has been observed in the dense photodominated regions [29]. Experimentalists have previously reported that in the presence of a laser field, vibrational excitation increases the reactivity of unimolecular and bimolecular reactions more than translational and rotational excitation [30,31]. Theoretical studies of water dissociative chemisorption have predicted that water on Cu(111) and Ni(111) exhibits strong mode specificity, bond selectivity, and steric effects [32,33,34,35]. In a pioneering study, Zhang et al. employed full-dimensional quantum dynamics to investigate the dissociative chemisorption of H₂O on rigid Cu(111). Their findings revealed that the excitations in vibrational modes of H₂O play a more pivotal role than increasing the translational energy in promoting the reaction. Moreover, they demonstrated that the enhancement of the excitation in asymmetric stretching is the most pronounced [36]. The influence of alloying on mode-selectivity in H₂O dissociation on Ni(100), Ni(110), Ni(111) and Cu/Ni bimetallic surfaces has been investigated by Tiwari et al. The authors employed a fully quantum approach based on the reaction path Hamiltonian, which revealed that mode specificity of H₂O dissociation was observed on all the surfaces. This invariably suggested that the vibrational excitation resulted in a significant enhancement in reactivity for all systems [37,38]. These conclusions are consistent with those of quantum state-resolved molecular beam experiments on the dissociation of heavy water on Ni (111) [39]. The vibrational enhancement of reactivity can be understood by the sudden vector projection (SVP) model, and the enhancement of reactivity by Polanyi’s rule mode reflects its coupling with the reaction coordinate at the transition state [40,41]. Indeed, Kim et al. have successfully manipulated the state-selective dissociation of a single water molecule using inelastic tunneling electrons with a low-temperature scanning tunnelling microscope. This process results in the dissociation of water into hydroxyl by the excitation of the vibrational state of the stretching modes and into atomic oxygen by the excitation of the electronic state [42]. Additionally, vibrationally excited hydrolysis dissociation experiments are an effective means of measuring hydrolysis dissociation thresholds. For instance, Maksyutenko et al. accurately determined the value of the first dissociation threshold of water, D₀ = 41145.94 ± 0.15 cm⁻¹, from the onset of the dissociative continuum in triple-resonance spectra [43].

The atomic-scale insights into the interactions of water molecules with catalysts have long been recognized as essential, given the fundamental role of these interactions in the elementary process of hydrogen generation. The theoretical calculations based on density functional theory (DFT) have been employed to elucidate the mechanism of water interaction with metal surfaces at the molecular level [33]. Despite substantial efforts in both experimental and theoretical studies, to date only a few works have been published on the role of chemical shifts in the infrared spectra of water molecules when adsorbed on catalysts in characterizing the reactivity of water molecules. Since vibrational spectroscopy can provide information on the structure and the bonding of the molecules, the correlation between reactivity and chemical shift in the infrared vibrational spectra of H₂O on the Cu_(10-n)Pt_n and Cu_(9-n)Pd_n alloy catalysts has been studied [44,45]. In these studies, we introduced the concept of the center of the v₁ and v₃ modes (the center of the v₁ and v₃ modes = (v₁ ×intensity of v₁ + v₃×intensity of v₃)/ (intensity of v₁ + intensity of v₃)) is used to characterize the reactivity of water molecules due to the strong coupling of the two stretching modes of H₂O with the reaction coordinate. The results indicated that the dissociation barrier of H₂O is closely dependent on the red shift of the center of the v₁ and v₃ modes of H₂O adsorbed. The larger the red shift of the center of the v₁ and v₃ modes is, the lower the corresponding barrier is. It is therefore hypothesized that the red shift of the center due to the adsorption of water molecules on the catalyst can be used to characterize the activation properties of the catalysts. This study examines the chemical shifts observed in the infrared vibrational spectra of H₂O adsorbed on 88 alloy clusters, with the objective of testing this hypothesis and developing a predictive understanding of the activity of the catalysts.

2. Computational Details

In this study, we optimized the structures of catalysts using the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional [46] and a density functional theory (DFT)-based relativistic semicore pseudo-potential (DSPP) [47]. Subsequently, infrared spectra were calculated. In order to investigate the effect of the heat of adsorption reaction of water molecules, we selected the NVE ensemble and conducted a first-principles molecular dynamics simulation of the catalytic dissociation of H₂O molecules at temperatures ranging from 0 to 1500 K, with a temperature step of 100 K. The time step and the total simulation time were 1 fs and 5 ps, respectively. All pertinent details have been provided in the supplementary materials.

3. Results and Discussions

3.1. Identification and Construction of Research Subjects

The adsorption behavior of H₂O on the first 75 single atoms in the periodic table was initially investigated. Three cases of being positively charged, uncharged, and negatively charged were considered separately, as illustrated in Figure 1. The red shifts of the center of the v₁ and v₃ modes of H₂O adsorbed on single atoms were then plotted in relation to that of the isolated H₂O in an electrically neutral state. It is evident that the red shift of the center of the stretching vibration modes of H₂O adsorbed on the single atom carrying a negative charge is considerably larger than that of the other two cases. This phenomenon exhibits a certain periodic behavior that is closely dependent on the nature of the elements. The reason why inert elements exhibit high activity in negative charge is that their high stability makes it easier for electrons to transfer to water molecules, thereby leaving the water molecules in a more active state. Consequently, the extent of the red shift of the center of the v₁ and v₃ modes of H₂O adsorbed on inert elements is comparable to that observed when the water molecule carries a direct negative charge.

The catalytic dissociation of water is typically achieved through the use of Cu and Pt metal surfaces [9,38]. The red shift of the center of the stretching (v₁ and v₃) modes of H₂O adsorbed on the negatively charged Cu atom is 615 1/cm, as illustrated by the green dash-dotted line in Figure 1. The construction of 24 diatomic alloy clusters, 30 triatomic alloy clusters, and 34 tetraatomic alloy clusters, based mainly on Pt, was carried out using sodium elements representing alkali metals and elements with redshift scales exceeding that of copper, as shown in Table 1. Subsequently, the catalytic dissociation of H₂O molecules over the aforementioned catalysts was simulated within a temperature range of 50–1500 K.

3.2. Dependence of Dissociation Temperature on Infrared Vibrational Spectra

The dependence of the catalytic dissociation temperature of water on the red shift of the center of the v₁ and v₃ modes of H₂O on different clusters is shown in Figure 2. For comparison, the red shift of the center of the v₁ and v₃ modes of H₂O is relative to the negatively charged isolated H₂O (∆IR). For catalysts with a water dissociation temperature above 1500K, the dissociation temperatures are all marked as 1500K for ease of plotting. It is evident that clusters with less than 300 1/cm of ∆IR have high catalytic water dissociation activity, corresponding to a dissociation temperature of no more than 200K. For ∆IR is between 300 and 700 1/cm, except for Sr₃Pt and CuPt, these clusters still showed high catalytic activity with dissociation temperatures not higher than 600K. For the red shifts in the 700-1016 1/cm range, different catalysts showed significant variations in catalytic activity. Other catalysts corresponding to higher ∆IR showed no visible catalytic activity. The results here show that the red shift of the center of the v₁ and v₃ modes of H₂O, which takes into account the strong coupling of the two stretching modes [26,27], allows a good characterization of the catalytic reactivity of water molecules. Given that the dissociation of water on these catalysts occurs mainly through a late-barrier reaction, as shown in Figure S1, it is reasonable to expect the above results from the Polanyi’s law and the Sudden Variation Vector Projection (SVP) model [39,40] as well. It is also consistent with the results observed by Tiwari’s group in their work on vibrationally excited dissociation of water molecules on Ni surfaces, where the softening of symmetric stretching and bending modes suggests that the excitation of these modes would lead to a significant enhancement in reaction probabilities [37].

As illustrated in Figure 2, while the red shift of the center of the stretching modes is an effective descriptor of catalytic reactivity in water, the origin of some exceptions remains unknown. A multitude of factors, including molecular orbitals in proximity to the Fermi level, tunneling, resonances, the zero-point energy (ZPE) effect, and electron spin polarization, among others, can influence the reactivity of water [36,37,38,48,49,54]. The following section will provide a detailed examination of the mechanism of catalytic water activation.

3.3. Resonant Absorption of Heat of Adsorption Versus Dissociation Temperature

Faradzhev et al. found that water dissociation on Ru(0 0 0 1) can be achieved by thermal or by electronic excitation [50]. Therefore, charge transfer, adsorption energy, bond length, frontier orbitals, and density of states were considered to elucidate the underlying mechanisms of catalytic water activation. In order to examine the relationship between the heat of reaction of adsorption of water molecules and the excitation of the electronic state, we combine the LUMO-HOMO gap (E_g) and the adsorption energy of H₂O (E_a). The adsorption energies of the water molecule on catalysts with high catalytic activity and several inactive catalysts, the LUMO-HOMO gaps of the catalysts adsorbing H₂O, and the corresponding dissociation temperature are listed in Table 2. From these data, it can be seen that lower dissociation temperatures always tend to correspond to systems where the sum of LUMO-HOMO gap and the adsorption energy of H₂O (E_g+E_a) is close to zero or the adsorption energy slightly higher than the LUMO-HOMO gap. If the exceptions are excluded, as in the cases of SrRe₃ and FPt, the dissociation temperature and the absolute values of E_g+E_a are strongly correlated in Table 2 with a Pearson correlation coefficient of 0.90. This means that if the heat released by the adsorption of H₂O is equal to or slightly greater than the energy required to excite an electron from the HOMO to the LUMO orbital, the catalytic water dissociation reaction will tend to occur. On the contrary, when the absolute value of (E_g+E_a) is greater than 0.25 eV, the reaction is generally difficult to occur. Accordingly, electronic state excitation triggered by resonant absorption of the heat of adsorption reaction is one of the important reasons for promoting the catalytic dissociation process of water molecules. This result is readily understandable when one considers that some scientists have observed that catalytic dissociation of water can be achieved on some metal substrates and oxide monolayers by providing energy corresponding to the lowest unoccupied molecular orbital (LUMO) energy of water [41,51,52,53]. Although the adsorption energy of water on Sr₂Re is slightly 0.05 eV lower than that of the LUMO-HOMO gap, the quantum tunneling effect may be contributing factors to the catalytic water dissociation reaction.

For the case of SrRe₃, it has the absolute value of (E_g+E_a) of 0.3 eV, but a low dissociation temperature of 100 K, which attributes to the Frontier molecular orbitals and will be discussed in Section 3.4. For FPt, the corresponding (E_g+E_a) value is 2.11 eV, which are considerably far from the resonance region, but with a dissociation temperature of 600 K. This is mainly due to the fact that the products of dissociation of water molecules on FPt are an H adsorbed on the Pt atom and a free HO group, which leads to the entropy from the intermediate state to the product is increased, that is, the entropy-driven effect plays an important role in this reaction. This is opposite to the entropy change in the dissociation reaction of water on other catalysts, as shown in Figure S1.

The correlation between dissociation temperature and both (E_g+E_a) and ∆IR has been illustrated in Figure 3, thereby facilitating a more straightforward comprehension of this relationship. One can see that when the ∆IR of water molecules after adsorption is lower and the value of (E_g+E_a) within the range of 0 to -0.30 eV (between the two blue dashed lines), the corresponding catalytic dissociation temperature of water molecules is usually lower, as shown in Figure 3. It is in alignment with the conclusions presented in both Section 3.2 and Section 3.3.

3.4. The Function of the Frontier Molecular Orbitals (FMOs)

The concept of locality in physical space is of paramount importance in the understanding of the underlying mechanisms of chemical reactivity. Frontier molecular orbitals (FMOs) serve to identify the locality of chemical bonds that are chemically reactive, due to the associated orbital energies, and have thus far proven to be an invaluable tool in the description of chemical reactivity [55,56]. In order to gain a more comprehensive understanding of the reaction kinetics of water dissociation, a detailed analysis was performed of the frontier orbitals of the catalysts with adsorbed H₂O molecules and the frontier orbitals of the intermediate state in the catalytic dissociation reaction, as illustrated in Figure S1. By comparing the frontier orbitals of the reaction precursors and the intermediate states, it is possible to identify which orbitals play a role in the dissociation process. The configuration of the frontier orbitals (HOMO-1, HOMO, and LUMO) of precursors is essentially identical to that of the intermediate states in the reaction, which indicates that the frontier orbitals of reaction precursors play a pivotal role in the catalytic reaction process. In a few instances, the order of frontier orbits underwent a slight alteration. For instance, as illustrated in Figure S1, the energy hierarchy of the HOMO and LUMO of SrReH₂O (precursor) underwent a reversal, with the energy levels of HOMO-1 and HOMO of Sr₂ReH₂O (precursor) shifting upwards to become the HOMO and LUMO of the intermediate state, respectively. A comparison of the frontier orbitals (HOMO-1, HOMO, and LUMO) of the precursor and intermediate state in the dissociation process reveals that the number of orbitals involved in the reaction process (orbitals from catalysts with increased overlap with H atoms during H₂O dissociation) is inversely proportional to the corresponding H₂O dissociation temperature. This is a logical conclusion, as the larger the overlap integral, the stronger the interaction between H and catalytic atoms will be. This phenomenon has been observed by Kawai and colleagues in their study of the role of molecular orbitals near the Fermi level in the excitation of vibrational modes of a single water molecule. In this study, a molecule is chemisorbed onto the metal surface, resulting in the formation of a molecule-metal bond and the rehybridization of the molecule’s orbitals [57]. In Figure 4, we utilize CuPt, SrPt, and Sr₂Re as illustrative examples to elucidate the influence of frontier orbitals on catalytic water dissociation reactions. For CuPt, a comparison of the two sets of frontier orbitals on the left in Figure 4 reveals that only the LUMO orbitals are involved in the catalytic separation of H from H₂O molecules, without the assistance of resonance absorption. Consequently, this material is unable to effectively catalyze the water dissociation reaction at temperatures below 1500K. From an energetic standpoint, the HOMO-LUMO gap (E_g) and the heat generated by the adsorption of water molecules are nearly identical, which evidently satisfies the prerequisite for the resonance absorption in the case of SrPt. Furthermore, the incorporation of both the HOMO and LUMO orbitals into the process of H separation can be observed in the frontier orbitals of the intermediate state. Both of these processes result in lower dissociation temperatures on SrPt. In the case of Sr₂Re, as illustrated in the two sets of images on the right in Figure 4, the HOMO-1 and LUMO orbitals of the intermediate state participate in the catalytic process of H₂O decomposition through changing the level order, thus lowering the corresponding dissociation temperature compared to that on CuPt. Ditto for the dissociation of water molecules on SrRe₃ as shown in Figure S1(e), where the homo-1, homo and LUMO orbitals are all involved in the catalytic water dissociation reaction, and thus the corresponding dissociation temperatures are as low as 100 K under conditions away from the energy resonance.

4. Conclusions

In conclusion, the redshift of the center of the v₁ and v₃ modes of H₂O (∆IR) can reflect the catalytic activation of water molecules to a certain extent. However, a more profound comprehension of the activation mechanism of water molecules necessitates a comprehensive examination of the degree of involvement of the frontier orbitals and entropy changes. It is our conviction that these findings will stimulate further experimental and theoretical research in this domain.