Vibrational Frequencies of Water Adsorbed on (111) and (221) Nickel Surfaces from First Principle Calculations

Abstract

Transition metal surfaces play a crucial role for many reactions in heterogeneous catalysis. Their catalytic functionality can be affected by a variety of factors, such as the morphology of the surface, defects, or poisoning. The most prominent example of poisoning is the adsorption of CO on platinum and similar surfaces. [1][2][3] Another important issue is the co-adsorption of several species, which may have an important influence on dissociation processes. [3,4] More generally, the adsorption of small molecules from the environment can significantly modify the catalytic efficiency of such metal surfaces. One particular case, in this scenario, is the adsorption of water. There are many theoretical and experimental studies of the structure and properties of water layers on metal surfaces in the literature. [5][6][7][8][9][10] Normally, the presence of a full layer is assumed in these investigations. However, the process of wetting, which is initiated by the adsorption of a single water molecule or small water clusters, is still poorly understood. From the view of an adsorbing water molecule, the surface has to compete thermodynamically with larger water clusters or simply the gas phase. Both phases provide a significantly larger entropic contribution to the free energy, which has to be compensated by a corresponding energy difference. Therefore, the theoretical investigation of the structural and energetic properties of the initial adsorption process on realistically modeled surfaces deserves particular attention.In particular, the crucial role of surface defects on adsorption processes is not always considered, especially in theoretical studies. Recently, we showed that a simple step defect on the nickel surface has the potential to enhance the adsorption energy of the initial water molecule by as much as 40 %. [11] Also the incremental adsorption energy of an additional second water molecule is higher than at a defect-free adsorption site. In principle, these adsorption energies can be determined experimentally, but spectroscopic parameters are often easier to obtain. First-principle calculations of experimentally accessible spectra are very scarce because of the relatively high computational cost involved in realistic and accurate calculations.Herein we want to bridge the gap between experiment and theory by providing ab initio calculations of IR peaks as a function of adsorption sites and cluster sizes, enabling for the first time a direct comparison of measurements and calculations. The initial steps of water adsorption on different nickel surfaces by means of their harmonic frequencies are characterized. The modification of the vibrational modes and frequencies of water clusters (monomer, dimer and trimer) upon adsorption are illustrated and these new vibrational modes involving the nickel-oxygen bond are described. Recent theoretical and experimental studies have shown that both the antisymmetric and the symmetric stretch vibrations can promote catalytic processes such as the chemisorption of methane on n...

First-Principles Modeling of Direct versus Oxygen-Assisted Water Dissociation on Fe(100) Surfaces

“…Noble metals such as Ru [9,10], Rh [10,11], Pd [10], Ir [10][11][12][13], and Pt [10,14] are active for steam and oxidative reforming but are not preferred in conventional industrial reformers due to their high costs. This motivates studies on non-precious metal catalysts, such as Ni- [15][16][17], Cu- [18,19], Co- [20,21], and Fe- [22][23][24][25][26][27] based materials. Due to extremely low cost and high catalytic activity, Fe-based catalysts have caught great attentions.…”

Gas Chromatographic, Ab Initio and Molecular Statistical Study of the Cluster Adsorption of Water and Methanol Molecules onto Lyophilic Centres on a Generally Lyophobic Adsorbent Surface

“…In this case, the free valencies closest to the adsorption centre were saturated by methyl groups while more distant free valencies were saturated by protons [Figure 2(c) and (d)]. The quantum chemical ab initio simulations of the adsorption systems consisting either of 'bare' centres or of centres with one to four adsorbed water or methanol molecules were performed using the PC Gamess software (Shmidt et al 1993;Nemukhin et al 2004) in the adsorption sub-monolayer of water at the surface of β-AgI monocrystals was demonstrated by Shevkunov (2006Shevkunov ( , 2007, while the presence of cyclic structures in bulk water was recently reported by Novakovskaya (2007). The formation of such structures is energetically preferable if they comprise up to six 'oxygen links': six water molecules in bulk water; five adsorbed molecules and the -OH group onto which the structure is adsorbed; and four adsorbed molecules and the -COOH group which acts as a 'twin' adsorption centre with the formation of one proton-donor and one proton-acceptor bond with adjacent adsorbed molecules [see Figure 3 .…”

“…This constituent is inversely proportional to the number of adsorbed molecules in the cluster. In principle, it is possible to perform quantum chemical calculations of these constituents of the increment: see, for example, Murakhtina et al (2006) where the DFT method was used to calculate the vibrational modes of water monomers, dimers and trimers adsorbed at the (111) and (221) faces of an Ni crystal. However, for the systems considered here, such calculations would present not only essentially technical difficulties (caused by the fact that the system, in addition to the portion of the graphene layer and the adsorbed molecules, also involves the lyophilic group), but also fundamental difficulties related to the fact that the actual detailed structure of the adsorption complex can be rather different from the idealised model.…”

“…The energy loss caused by such a loss of one degree of freedom at 350 K is approximately equal to RT/2 ~ 2.5 kJ/mol, which can provide a quantitative explanation of the difference between the experimental (fitted) and calculated (quantum chemical) values of the increments listed in Table 1. It should also be noted that the differences in the adsorption energies related to the changes in the frequencies of the vibrational modes caused by the increase in the number of adsorbed molecules by one [such frequencies for water molecules adsorbed onto Ni were calculated by Murakhtina et al (2006)] are of the order of 1 kJ/mol. Note that, with increasing numbers of adsorbed molecules in the cluster, this difference decreases rapidly -see equation (12); this fact is also in agreement with the data presented in Table 1.…”

Force correcting atom centred potentials for generalised gradient approximated density functional theory: Approaching hybrid functional accuracy for geometries and harmonic frequencies in small chlorofluorocarbons

“…And even for small molecules adsorbed on surfaces, the computational pediction of infra † as of summer 2013 red spectra has already been evinced, e.g. water adsorbed on Ni (111) and (211) 10,11 , or organic molecules adsorbed on Si (111) 12 .…”