Elsevier

Applied Surface Science

Volume 258, Issue 4, 1 December 2011, Pages 1429-1436
Applied Surface Science

Theoretical investigation of CO adsorption on TM-doped (MgO)12 (TM = Ni, Pd, Pt) nanotubes

https://doi.org/10.1016/j.apsusc.2011.09.097Get rights and content

Abstract

CO adsorption on TM-doped magnesia nanotubes (TM = Ni, Pd and Pt) have been studied by using density functional theory. Our calculation results show that CO favors adsorption on TM-doped magnesia nanotubes in the form of C atom bonding with TM atom. Fukui indices analysis clearly exhibits that doping of impurity TM atom allows for a noticeably enhancement of nucleophilic reactivity ability of magnesia nanotube. The adsorption energies demonstrate that CO molecule is more strongly bound on the 3-fold TM atoms than the 4-fold TM atoms. This finding is well confirmed by TM–C bond length, charge transfer and C–O vibrational frequency. The high adsorption energy of 2.55 eV is found when CO adsorbs on 3-fold Pt in Pt-doped magnesia nanotubes, implying the kind of the doping TM atom has a significant influence on the chemical reactivity.

Highlights

Adsorption behavior of CO on TM-doped (MgO)12 nanotubes has investigated. ► CO is more strongly bound to the 3-fold transition metal atoms in TM-1 models. ► TM–C bonding can be described in 5σ forward-donation and 2π* back-donation mode.

Introduction

Detection of toxic gas molecules, such as carbon monoxide (CO), is of critical importance in the environment and human beings. In practice, metal oxides, as main gas sensors, are widely used for monitoring this task. Among the metal oxides, MgO plays an important role owing to its simple crystalline structure and easy preparation in experiment [1]. Recently, CO adsorption on MgO surfaces has been extensively investigated both theoretically [2], [3], [4], [5], [6], [7], [8], [9], [10], [11] and experimentally [12], [13], [14], [15], [16], [17], [18], [19], [20]. On the theoretical side, Pacchioni et al. [2], [3], [4] have treated the MgO surface by a cluster approach. He [8] and Staemmler [11] have reported that none (or at most only one) of the about 20 previous theoretical treatments of this presumably simplest adsorption system came close to the experimental value of the adsorption energy, 0.14 eV [17]. Nygren and Pettersson [5] obtained a CO adsorption energy of 0.08 eV with ab initio model potential (AIMP) embedding and extensive treatment of dynamic electron correlation. Illas and co-workers [7] using the three functional of M06-HF family (which contain a nonzero percentage of Hartree-Fock exchange-M06, M06-2x, and M06-HF) studied the adsorption of CO an MgO(0 0 1). It was shown that the results given by both standard and newly developed exchange-correlation functional are not completely satisfactory. By adopting a B3LYP+MP2 mixed scheme within a periodic ONIOM-like approach, Ugliengo and Damin [10] have found a binding energy of 0.13 eV and a quite relevant dispersive interactions (0.07 eV) for CO interacting with MgO surface. From the experimental point of view, the adsorption energies of the system of CO/MgO are measured just from 0.09 eV to 0.45 eV, which is a typical physisorption. MgO surfaces cannot detect CO well since the CO adsorbed weakly on the MgO surfaces. The neutral surface of the insulating MgO is no doubt one of the main reasons.

Depositing additional materials on the well-defined MgO surfaces has been shown to be suitable models for heterogeneous environmental catalysis [21], [22], [23], [24], [25], [26], where the active additional materials are known to be single atoms, size-selected metal atoms and clusters. Experimentally, small Au aggregates on clean MgO(1 0 0) surface and CO adsorption on this Au/MgO(1 0 0) system have been researched by Freund [22] through scanning tunneling spectroscopy (STS) experiment. Theoretically, using the hybrid B3LYP functional within DFT calculation, Pacchioni et al. [23] have studied the thermally stable and chemically active gold nanoclusters on MgO surface. Next, his co-works [24] reported the adsorption properties of CO adsorbed on Au clusters supported on MgO/Ag(0 0 1) thin films with GGA-PW91 method in VASP program. They found that CO only binds to the low-coordinated Au atoms and a red shift of about 50–60 cm−1 occurs. Grönbeck et al. [25] using GGA-PBE method studied CO molecule adsorption on the modified MgO(1 0 0) surface by supported Pd, Ag, Pt, and Au in the form of atoms. They found that there is a strong binding energy between CO and the supported metal atoms, and CO adsorption has increased the barrier of diffusion for metal atoms. Ferullo et al. [26] investigated the interaction of CO with Au atoms adsorbed on terrace and low-coordinates sites (edge and corner) of MgO(1 0 0) surface by using DFT-B3LYP method. They pointed out that CO adsorbs strongly on atomic Au deposited on anionic (O2−) sites with binding energies changing from 0.51 to 0.69 eV.

In this paper, the main aim is to perform a systematic study on inducing CO in the modified (MgO)12 nanotubes by doping with transition metal atoms (TM) of Ni, Pd and Pt using DFT. For (MgO)12, previous theoretical studies indicated that the tubelike ground state is about 0.12–0.66 eV higher in energy than the cage and cubic isomers [27], [28]. In experiment, observations from mass spectra [29], [30], [31] revealed that small MgO clusters prefer the nanotube geometry over the bulklike structure. Thus (MgO)12 nanotube can be taken as a good model for the investigation of CO adsorption.

The rest of the paper is organized as follows. In Section 2, the models and computational methods are introduced. In Section 3, we first focus our attention on the geometries, the stability, the Mulliken atomic charges, the HOMO–LUMO gaps and the Fukui functions of the TM-doped (MgO)12 nanotubes (TM = Ni, Pd and Pt). The detailed results are summarized in Section 3.1. More importantly, in Section 3.2, the properties of the CO molecule adsorption on these TM-doped (MgO)12 nanotubes are systematically investigated. The interaction between CO and those TM-doped (MgO)12 nanotubes is analyzed in terms of adsorption energies, Mayer bond orders, electron transfer, density of states (DOS) and C–O vibrational frequency. In Section 4, the conclusion of the study is given.

Section snippets

Models

The pristine (MgO)12 nanotube is a barrel-like structure consisting of four stacked hexagonal (MgO)3 sub-units as shown in Fig. S1 in the Supporting Information. It displays a high symmetry of D3h with all ions on the surface. The Mg–O bond lengths in the outmost layers are optimized at 1.915 Å, while the average Mg–O bond lengths in the middle layers are about 2.064 Å. For the inter-layers, the average Mg–O distances are optimized at 1.989 Å. These results are in good agreement with previous (MgO)

Geometric structures and stabilities

In Fig. 1, the equilibrium geometries of TM-doped (MgO)12 nanotubes are presented. The corresponding bond lengths of these nanotubes are summarized in Fig. S2 in the Supporting Information. For Ni-1 nanotube, there are two shortened Ni-O distance of 1.850 Å in the outermost layer, compared to the pristine Mg–O distance of 1.919 Å. These relatively shorter Ni–O distances might stem from the strong covalent bonding between the Ni and O atoms (their Mayer bond orders are around 0.85 while the Mg–O

Conclusion

We have investigated the adsorption behavior of CO on TM-doped magnesia nanotubes (TM = Ni, Pd and Pt) with DFT and GGA. Our calculations show that doping the (MgO)12 nanotube with impurity transition metal atoms almost have not change the stability of the prestine structure. Fukui indices analysis exhibit that there is a noticeably improve in the nucleophilic reactivity ability (nucleophiles, such as CO molecule) with a impurity TM atom in the magnesia nanotube. In the optimized adsorption

Acknowledgments

This work is supported by the National Natural Science Foundation of China (20876005 and 21076007) and the National Basic Research Program of China (2010CB732301). This project or paper is supported by “Chemical Grid Project” of Beijing University of Chemical Technology.

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