Adsorption of hydrogen atoms on Pd (211), (311) and (511) stepped defective surfaces
HOU Lu-bing(侯路斌), DENG Hui-qiu(邓辉球), HU Wang-yu(胡望宇)
Department of Applied Physics, Hunan University, Changsha 410082, China
Received 10 April 2006; accepted 25 April 2006
Abstract: Using embedded-atom-method potential for Pd and MORSE potential for the interaction between H and Pd atoms, the adsorption properties of H atoms on Pd (211), (311) and (511) stepped defective surfaces were calculated systematically. For Pd (311) surface, it is found that the four-fold hollow sites H4 are the preferable sites for H atoms being adsorbed on these Pd defective surfaces. The sites H4 are the most stable adsorbed sites and the three-fold hollow sites Hf and Hh are metastable ones. The calculated results are in reasonable agreement with the HREELS experiment results. For the (211) and (511) stepped defective surfaces of Pd, our calculation shows that the most stable adsorption sites are H5 and H2 respectively, both of them are four fold hollow sites.
Key words: surface adsorption; palladium; hydrogen; embedded-atom-method
1 Introduction
It is well known that palladium has a special tendency to adsorb hydrogen and the hydrogen- palladium system is one of the extensively studied systems due to its importance in a variety of technological application, such as the catalysis of hydrogenation reactions and the storage of hydrogen [1, 2]. Considerable effort has been devoted to the study of the interaction of hydrogen with transition metal surfaces during the last decades[1]. Lots of experiments on Pd low index surfaces were investigated and it is found that hydrogen molecules adsorb dissociatively on Pd low index surfaces and the most favorable adsorption site for atomic hydrogen is that with a high coordination number[3-5]. For example, H atoms adsorb stably on fcc hollow sites on Pd (111) surface[3], while H atoms occupy the four-fold hollow sites of Pd(100) surface[4], and pseudo three fold hollow of Pd(110) surface[5].
Stepped surface defective sites exhibit special chemisorption and catalytic properties than the sites on perfectly flat parts of the surfaces and have been known to play an important role in determining both the dynamics of the gas-surface interactions as well as further understanding the catalytic mechanisms[6-13]. Experimental and theoretical studies of chemisorption on stepped surfaces are of great current interest due to close connection to real catalytic substrates. GEE et al[6] used the molecular beam experiments to research the influence of step sites on the dissociation of hydrogen and found that the step sites are clearly more reactive towards hydrogen dissociation than terrace sites. Recently experiments have identified three atomic and two molecular adsorption states below 100 K on Pd (210)[13]. A novel H2 molecular adsorption state on metal surfaces has been detected by temperature programmed desorption and electron energy loss spectroscopy experiments, and it turns out that this state is stabilized by the presence of atomic hydrogen on the Pd(210) surface[9]. FARIAS et al[11] investigated H/Pd(311) system by performing preliminary high resolution electron energy loss spectroscopy(HREELS) experiment and found hydrogen atoms will adsorb on the fourfold hollows when the coverage is lower than 0.5 ML, whereas both threefold and fourfold coordinated sites are occupied at coverage above 0.5 ML. Recently WANG et al[12] calculated the H/Pd(311) system in detail using 5-MP method, and both surface and subsurface adsorption sites were found[12].
Both 3-fold and 4-fold coexist on the Pd(211), (311) and (511) stepped defective surfaces, which may leads to an interesting competition in the case of adsorbates such as hydrogen and oxygen. As far as we know, the systems of H/Pd (211) and H/Pd (511) have not been investigated theoretically. In the study, we theoretically study the adsorption properties of H atoms on Pd (211), (311) and (511) stepped defective surfaces with semiempirical interaction potentials for H and Pd.
2 Calculation method
A surface slab model with periodical and nonperiodical boundary conditions is used to describe the palladium surface clusters in the present paper. The z direction is perpendicular to the surface and there is nonperiodical along this direction. The x and y directions are with periodical boundary conditions. The surface slabs are at least ten layers thick and each layer has 10
10 atoms when we simulate the Pd (100), (110), (111) clusters, thus the whole clusters have no less than 1 000 platinum atoms. Since the smaller surface density of Pd atoms on Pd(211), Pd (311) and Pd(511) surface slab clusters, the number of the atomic layers increases to 20 layers, and each layer contains 128, 166 and 204 Pd atoms, as shown in Fig.1, respectively. The cell sizes of palladium used here are tested carefully, it is found that they are large enough and have no effect on the calculation results. During all the calculations the palladium slab clusters are frozen and the surface relaxations are not permitted.
There are many empirical and semiempirical many-body interaction potential energy functions being used in atomistic computer simulations now. It is very important to construct suitable interaction potentials among the atoms for the simulation. The embedded atom method(EAM) originally provided by DAW and BASKES is one of the most famous semiempirical many-atom potentials for computing the total energy[14]. In the present paper the interaction for Pd-Pd atoms is described with our modified analytic embedded-atom method(MAEAM) potentials, which has been used to simulate the surface vibrations, surface segregation of binary alloys, melting and evolution of nanocrystallines and nanoclusters[15-17]. The potential function for H atom is the model of ANGELO and BASKES[18, 19]. The interaction between H and Pd atoms is described with a MORSE-type potential as follows:
where ai is the fitting parameter and can be obtained by fitting the data of adsorption energy and bond length of experiments.
The relative scaling factor SH of the electron density of hydrogen is specified for the hydrogen-palladium system as the same form of WEN’s model, which is
Fig.1 Surface sites are illustrated on Pd(211), (311) and (511), dark gray ball: topmost layer; gray ball: second layer; light gray: third layer; dark dot: adsorption sites
successfully applied to screw dislocations and hydrogen embrittlement [20, 21].
3 Calculation results and discussion
The relative scaling factor SH and the MORSE potential parameters ai concerning the interaction between hydrogen and palladium are listed in Table 1. Using the interactions between hydrogen and palladium, the adsorption energies of hydrogen atoms on the three low-index palladium surfaces and the H-Pd bonding length are listed in Table 2. Here H is the hollow site, LB is the long bridge site and H3 the three-follow site in the Table. The present results are in good agreement with the experimental data[22-28] which shows the potential used here is reasonable.
Table 1 MORSE potential parameters for H-Pd
Table 2 Adsorption energies and bonding length of H-Pd system
Fig.1(a) shows the surface cluster model and adsorption sites on the Pd (211) stepped defective surface. Pd(211) is a regularly stepped vicinal surface that consists of Pd(100) and Pd(111) facets with Pd(100) as step and Pd(111) as the terrace, which can be regarded as defect (111) surface. Our calculation results are listed in Table 2. For the sake of obtaining a more straightforward figure, we scanned the potential energy surface (PES) of H atom adsorption and diffusion on Pd(211) stepped surface along a-a and b-b sections of Fig.1. The PES is shown in Figs.3(a) and (b) respectively. It is clear that H5 is the most stable adsorption state, while H4 is completely annihilated since it is near to H5. In other words, H4 is not a stable adsorption site. The same phenomenon was observed on O/Cu(211) system[29]. From Fig.2, it can be seen that there are also 3-fold sites H1, H2 and H3 farther from the step, which are more similar to 3-fold hollow H site of Pd(111) plane in binding energy and adsorption geometry. On Pd(211) surface, the step edge H5 site locates on the outmost, thus we deduce that it will be the preferable site for hydrogen atom adsorption.
Fig.1(b) shows the cluster model and adsorption sites on the Pd (311) stepped defective surface. Due to the existence of (111) and (100) microfacets, 3-fold and 4-fold hollow sites are distributed with equal density in the Pd(311) surface. Calculation results indicate that the lowest adsorption energy -3.01 eV lies in the pseudo-4- fold hollow site H4, and hydrogen atom exists a little above the surface. Using 5-MP potentials WANG et al[12] give the same most stable adsorption site H4. Preliminary HREELS found that H4 is the only occupied site for H on Pd(311) when coverage is lower than 0.5ML[11]. There are two 3-fold hollow sites, i.e. Hf and Hh ones, as shown in Table 3. When H atom adsorbs on Hf, the adsorption energy of -2.91 eV and adsorption height of 0.039 nm are obtained. Hh has a binding energy of -2.86 eV and the adsorption height is 0.102 nm above the surface. Note there is an adsorption energy of -2.89 eV on B’ site, although the adsorption energy is lower than Hh, it is actually not a stable adsorption site since it is near to H4, as shown clearly in Fig.3. We scanned the potential energy surface of the H atom adsorption and diffusion on the Pd (311) surface along the a-a section at a height of 0.05-0.20 nm above the surface. Vertical coordination is the height of the adsorbed H atom from the Pd (311) surface; the abscissa shows the location of adsorption states, as shown in Fig.3. There are three adsorption sites for H adsorption on Pd (311). It is clear that H4 is the most stable adsorption state, and the other two minimum energy sites are the Hh and Hf sites. Therefore it is reasonable that H4 will be the preferable site for H atom to occupy when the hydrogen exposure is low, and the
Fig.3 PES of H atom adsorption and diffusion on Pd(311) stepped surface scanned along a-a section of Fig.1
Table 3 Critical characteristics of H-Pd (211), (311) and (511) stepped surface sysems
other two metastable surface sites Hh and Hf will be occupied by the hydrogen atoms when the hydrogen exposure is high, which is supported by the HREELS experimental results[11].
Fig.1(c) shows the cluster model and adsorption sites on the Pd (511) stepped defective surface. Pd(511) is a stepped vicinal surface that consists of Pd(100) and Pd(111) facets with Pd(111) as step and Pd(100) as the terrace. From Table 3 and Fig.4, we know that both H2 and H1 are possibly the most stable sites for their adsorption energies and bonding length are almost the same. There are two 3-fold hollow sites H3 and H4, although similar to 3-fold hollow site of Pd (111), the bonding energy is lower. The adsorption height is -0.036 nm on the H2 site while that of H2 site is 0.024 nm. Thus, it is not easy for the H atoms accessing to H2 sites when the hydrogen exposure is low. Based on the above fact, it can be concluded that H1 is the favorite site for hydrogen adsorption in the Pd (511) surface.
Fig.4 PES of H atom adsorption and diffusion on Pd(511) stepped surface scanned along a-a section of Fig.1
Acknowledgment
The authors thank the High Performance Computing Center of Hunan University for providing the computer resource.
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(Edited by PENG Chao-qun)
Foundation item: Project(20403004) supported by the National Nature Science Foundation of China
Corresponding author: DENG Hui-qiu; Tel/Fax: +86-731-8823971; E-mail: hqdeng@hnu.cn
