First-Principles Study on O2 Adsorption and Dissociation Processes

Jul 21, 2017 - Through DFT calculations, a systematic description of O2 adsorption and dissociation processes over Rh(100) and Rh(111) surfaces has be...
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First-Principles Study on O2 Adsorption and Dissociation Processes over Rh(100) and Rh(111) Surfaces Lu Tan,† Liangliang Huang,‡ Qi Wang,*,† and Yingchun Liu*,† †

Department of Chemistry, Zhejiang University, Hangzhou 310027, People’s Republic of China School of Chemical, Biological & Materials Engineering, University of Oklahoma, Norman, Oklahoma 73019, United States

‡

ABSTRACT: Through DFT calculations, a systematic description of O2 adsorption and dissociation processes over Rh(100) and Rh(111) surfaces has been provided. The dominance of parallel orientation in molecularly adsorbed states and during the impinging processes has been identified, along with the explicit adsorption configurations and preferred impinging trajectories on both surfaces. The dissociation of O2 is found to occur either by precursor-mediated adsorption or by direct dissociation. O2 in its initial precursor state dissociates facilely on Rh(100), but this is a little harder on Rh(111) by going through a twostep process. The latter can be described as a preliminary rotation and subsequent dissociation, with the final locations of two O atoms disturbed easily by coadsorbed O atoms surrounding the dissociating O2 molecule due to the existence of a relatively flat potential energy surface stage along the way. The present work may provide the basis for kinetic modeling to investigate the catalytic properties on a realistic scale.

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INTRODUCTION Heterogeneous catalysis is considered to be a promising solution to environmental issues nowadays due to its high efficiency and controllable selectivity toward the removal of undesired side products.1−4 In the realm of the purification of automobile exhaust gases, the so-called three-way catalyst, which simultaneously promotes the reduction of nitrogen oxides (NOx) and the oxidation of CO and hydrocarbons, has been widely used for decades. In this catalyst, Pt, Rh, and Pd are always used together as active ingredients.1,5,6 Despite the current satisfactory performance, this commercial catalytic posttreatment system fails to meet the next-generation environmental regulations on exhaust gases. Lean-burn technology, which refers to the burning of fuel with excess of air, is a promising solution combining low fuel consumption and low CO and hydrocarbons emissions due to a more complete combustion.1,7 This makes the removal of NOx an outstanding issue. The challenge with the three-way exhaust catalyst is that Rh, the component responsible for NOx reduction, is inactive under typical lean-operating conditions.1,5,6 Thus, targeted NOx emission is not possible with current catalysts. Because of the existence of excessive oxygen under such lean-burn operating condition and its negative effect on the reduction of NOx, from a fundamental point of view, a molecular-level understanding of the O2/Rh system is required. The interaction between oxygen and transition-metal (TM) surfaces has been the subject of intense investigations.8−12 Among them, the behavior of the oxygen/Rh system has also drawn much attention, particularly the oxygen structure on the surface.13−18 From a theoretical point of view, density functional theory (DFT) has been used to study oxygen adsorption on a variety of TM surfaces. On the other hand, to © XXXX American Chemical Society

the best of our knowledge, the research on O2 adsorption and further dissociation processes on Rh surfaces is far from complete. Inderwildi et al.19 investigated the dissociation process of O2 on the Rh(111) surface and its dependence on oxygen coverage. Their results revealed an initial enhancement and a subsequent inhibition of O2 decomposition when the oxygen concentration becomes larger. It is worth pointing out that the authors considered only one possible precursor state in their research, where O2 adsorbed on a 3-fold fcc position perpendicular to the surface. Franz and Mittendorfer17 considered the oxygen desorption process in a preliminary calculation. Their results suggested that there exists a parallel O2 precursor state on the Rh(111) surface along with a relatively low energy barrier between the precursor and the dissociation state. Liu and Evans20 studied the O2 precursor states and their dissociation on the (100) surface for five kinds of TMs as part of their work, including Rh. They constructed some one-dimensional (1D) energy pathways for O 2 dissociation along high-symmetry directions, which provided useful information but could not correctly identify the transition states (TSs). In this work, we apply DFT calculations to study the adsorption and dissociation of molecular oxygen on two kinds of low-Miller-index Rh surfaces, namely, Rh(100) and Rh(111). While the latter is the most compact and thus thermodynamically most stable surface termination of the Rh crystal, the Special Issue: Tribute to Keith Gubbins, Pioneer in the Theory of Liquids Received: June 14, 2017 Revised: July 20, 2017 Published: July 21, 2017 A

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displaced along all Cartesian directions by positive and negative displacement with a step size of 0.015 Å. The climbing image nudged elastic band (cNEB) method35,36 was applied to locate the TSs and generate the minimum-energy pathways. All TS structures were verified by vibrational frequency analysis as having a single mode of imaginary frequency. The visualization of the structures was done with the VESTA37 program.

former is slightly more open and thus is believed to have better catalytic performance. The adsorption states of both O atoms and O2 molecules have been calculated: from the adsorption of O2 molecules to the dissociation when O2 is close to the surfaces. The equilibrium images of O2 adsorption, the reaction pathways, and the TSs have been identified.

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COMPUTATIONAL DETAILS The periodic DFT calculations were performed by using the Vienna ab initio simulation package (VASP)21,22 to iteratively solve the Kohn−Sham equations in a plane-wave basis set. The projector augmented wave (PAW) method23,24 was used to treat core electrons and their interaction with valence electrons. The Rh 1s to 4p and O 1s electrons were treated as core states. The exchange-correlation energy was described by the generalized gradient approximation (GGA) using the Perdew−Burke−Ernzerhof (PBE) functional.25 Spin polarization was taken into account, and the kinetic energy cutoff for the plane-wave basis was 400 eV. The Brillouin zone was sampled by the Monkhorst−Pack scheme26 with the grid origin at the Γ point. The converge criteria for electronic self-consistency and force were 10−6 eV and 0.01 eV/Å, respectively. The bulk structure was optimized prior to building the surface systems. By fitting the computed total energies at different lattice parameters with the Murnaghan equation of state,27 the lattice parameter for bulk Rh was calculated to be 3.842 Å. This value agrees well with other computational results28−30 but is slightly larger than the experimental value of 3.80 Å,31 which is due to the use of GGA. A grid of 17 × 17 × 17 k-point sampling and the tetrahedron method with Blöchl corrections32 for partial occupancies were used in the bulk simulation. The lattice parameter obtained here was then used for the subsequent calculations. Both the Rh(100) and Rh(111) surfaces were modeled as periodically repeated five-layer slabs with an associated 15 Å vacuum region to ensure no interaction between the slabs. Atoms and molecules were adsorbed on only one side of the slab. For structure optimization on both surfaces, the bottom three atomic layers were fixed whereas the top two atomic layers were fully relaxed during all calculations. Adsorbates were allowed to relax in all or specific directions, depending on the purposes of calculations as discussed below. The calculations were performed with the (6 × 6 × 1) and (3 × 3 × 1) k-point grids for 2 × 2 and 4 × 4 unit cells, respectively. Partial occupancies for each orbital were calculated using the firstorder Methfessel−Paxton scheme33 with the width of the smearing at 0.1 eV. As for the isolated O2 molecule, the calculation was carried out by putting it inside a 14 Å × 15 Å × 16 Å noncubic cell, while the Γ point and Gaussian smearing33 with a width of 0.002 eV were used. The length of the O−O bond was calculated to be 1.234 Å, which approaches the experimental value of 1.207 Å.34 The energies shown in this work are all adsorption energies defined in the following expression: Ead = Eoxygen/slab − Eslab − N /2 × EO2 in which Eoxygen/slab is the total energy of the substrate with adsorbates, Eslab is the total energy of the clean surface, EO2 is the energy of gas-phase O2, and N stands for the number of O atoms in a unit cell. Vibrational frequencies of the adsorbed species were analyzed by applying a finite-difference method, where the substrate was kept frozen, and the adsorbed atoms were

RESULTS AND DISCUSSION Adsorption of Atomic and Molecular Oxygen. We first compute the adsorption of atomic oxygen in a 2 × 2 unit cell for both Rh(100) and Rh(111) surfaces. The possible highsymmetry sites for atom adsorption are illustrated in Figure 1:

Figure 1. Possible adsorption sites for atom on Rh(100) (a) and Rh(111) (b) surfaces. Large green spheres are Rh, and small red spheres are O. These notations also apply to subsequent figures.

hol, brg, and top for Rh(100) and fcc, hcp, brg, and top for Rh(111). The calculation suggests that atomic oxygen can be stably adsorbed on all three sites for Rh(100) and only the two 3-fold hollow sites for Rh(111), which has been confirmed by vibrational frequency analysis. The adsorption energies and vertical distances between O and the top Rh layer for each adsorption configuration are illustrated in Table 1. Our results for the Rh(111) surface agree Table 1. Adsorption Energy (Ead) and Height (h) from the Center of Mass of the Top Rh Layer to the O Atom for O Adsorbed on Rh(100) and Rh(111) site

Ead (eV)

h (Å)

hol brg top

−1.989 −1.837 −0.819

1.014 1.355 1.835

fcc hcp

−2.031 −1.952

1.233 1.228

Rh(100)

Rh(111)

well with those from previous DFT calculations.17 It is clear that the most stable adsorption sites are hol for Rh(100) with an adsorption energy of −1.989 eV and fcc for Rh(111) with an adsorption energy of −2.031 eV. On the other hand, although the adsorption of O on the top site over Rh(100) is confirmed as a stable configuration, the adsorption energy is relatively small. Therefore, we leave this configuration out of the following study. Furthermore, the diffusion of the O atom on both surfaces is identified via the cNEB method. On Rh(100), the activation B

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energy, the perpendicular configurations are less favorable than the parallel ones. For parallel adsorption, there is only one stable configuration labeled as tbt on Rh(100) and three stable configurations on Rh(111) labeled as tbt, tfb, thb. The two tbt configurations are flat, i.e., the two O atoms have the same height. As for the tfb and thb configurations on Rh(111), the O atom near the top site is about 0.2 Å higher than the other O atom. Comparing the three configurations of O2 on Rh(111), the tfb and thb configurations have a large geometric similarity and are energetically almost degenerate, with the thb configuration slightly favored. As for the tbt configuration, it has a slightly shorter O−O bond and a higher position relative to the surface, and the adsorption energy is about 0.1 eV smaller than that of the thb configuration. For all of the stable chemisorbed O2 molecules observed, only the tbt configuration on Rh(111) carries a magnetic moment of 0.73μB, whereas no magnetic moments are shown for all of the other adsorption configurations. It is noticeable that the three parallel adsorption configurations of O2 on Rh(111) are geometrically similar to those found theoretically on the Pt(111) surface38−40 but are adsorbed more strongly with a different energy ordering. tfb and tbt O2 on Pt(111) have almost the same adsorption energy of about −0.7 eV, and have been confirmed experimentally41−45 as peroxo-like (O22−, nonmagnetic) and superoxo-like (O2−, magnetic) species, respectively, while the thb one is relatively weakly adsorbed. On the other hand, the results show that the adsorption property of molecular O2 is quite different from that of NO and CO, which are mostly adsorbed perpendicularly and have a tendency to be adsorbed relatively more strongly on the site with a low coordination number.46−53 Adsorption Process of Molecular O2. In order to gain insight into the impingement of O2 molecule on the two Rh surfaces, we investigate the potential energy surface (PES) for the O2 approaching surface through constructing some 1D cuts. By calculating the total energy of the O2/Rh(100) or O2/ Rh(111) system as a function of the distance between the O2 molecule and the surface in the 2 × 2 unit cell, nine trajectories for each system are constructed. Besides the perpendicular and stable parallel adsorption configurations, five more parallel configurations for Rh(100) and two more for Rh(111), as depicted in Figure 3c,d, respectively, are also considered. During the approach calculation, the orientation of the O2 molecule is restricted to a certain configuration while the O−O bond length is allowed to change in each single structure optimization. The distance step size for each trajectory is 0.1 Å when the O2−surface distance is less than 3 Å and 0.5 Å otherwise. Such a distance change is sufficiently small to have an accurate calculation of the energy pathway. The results are shown in Figure 3a for Rh(100) and Figure 3b for Rh(111). As we can see, the PES exhibits significant anisotropy when the O2−surface distance is less than 3 Å. In general, the perpendicular trajectories are almost always energetically less favored than the parallel ones during the sticking stage; i.e., O2 sticking occurs predominantly with the molecular axis parallel to the surface due to the steering effect. For both O2/Rh(100) and O2/Rh(111) systems, the tbt trajectory is favored in comparison to all the others before reaching its chemisorbed state with a continuing decrease in energy. Beyond that equilibrated distance, the repulsive force will gradually become larger, and the dissociation would not happen directly through lengthening the O−O bond along the molecular axis. On the contrary, the O2 molecules going

energy for the hop from the hol site to the nearest-neighbor brg site is 0.163 eV forward and 0.011 eV backward. While on Rh(111), the activation energy for the hop from the fcc site to the nearest-neighbor hcp site is 0.518 eV forward and 0.439 eV backward. The results reveal a higher mobility of the O atom on the Rh(100) surface than on the Rh(111) surface. The adsorption of the O2 molecule is also explored in a 2 × 2 unit cell. We mainly focus on two kinds of molecular orientation with respect to the surface, parallel and perpendicular. Various initial configurations distinguished by the position of the O2 center and the direction of the O−O axis are tested in order to determine stable adsorption configurations. The configurations of molecularly adsorbed O2 are illustrated in Figure 2. Those images have been examined by

Figure 2. Schematic illustration on O2 adsorption configurations on Rh(100) (a) and Rh(111) (b) surfaces.

vibrational frequency analysis to make sure that there is no imaginary frequency. The notation for those images is defined as the indices used for atom adsorption indicating perpendicularly adsorbed O2 at a certain site and other two- or threeletter indices indicating parallel configurations. For the latter situation, the middle letter (if it exists) defines the position of the O2 center, and the other two are the positions of O atom orientation, with the letter indicating the same site as the site for atom adsorption having an identical first letter. For example, the notation “tbt” means that the center of O2 is over the brg site and the two O atoms are pointing to the nearby top sites. The adsorption energy and structural properties of molecularly adsorbed O2 are listed in Table 2. The results show that for perpendicularly adsorbed O2, sites with high coordination numbers are preferred, which is similar to the adsorption of O atoms. However, because of the ∼1 eV higher Table 2. Adsorption Energy (Ead), Length of the O−O Bond (dO−O), and Height from the Center of Mass of the Top Rh Layer to Each O Atoma for O2 Adsorbed on Rh(100) and Rh(111) configuration

Ead (eV)

dO−O (Å)

h1 (Å)

h2 (Å)

tbt hol

−1.681 −0.640

1.372 1.324

1.896 1.278

1.896 2.602

tbt tfb thb fcc hcp

−1.340 −1.434 −1.438 −0.530 −0.424

1.373 1.415 1.421 1.312 1.308

1.951 1.650 1.643 1.463 1.490

1.951 1.895 1.879 2.775 2.798

Rh(100)

Rh(111)

a

The smaller one is labeled h1, and the other is labeled h2. C

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trajectories, but the exact TS and the corresponding activation energy need to be explored more carefully, as we will discuss in the next section. On the basis of the series of 1D PES, we conclude that the dissociation of O2 on Rh(100) and Rh(111) surfaces can occur either by precursor-mediated adsorption or by direct dissociation, depending upon the initial conditions. When the O2 molecule impinges on the surface with low kinetic energy, it will be efficiently reoriented into the favorable geometry along the minimum-energy trajectory, i.e., the tbt trajectory for both Rh(100) and Rh(111) surfaces, and rest at the molecularly adsorbed state. Such a precursor state is able to trap the O2 molecule for a while and thus increase the probability of dissociation. With the increase in O2 kinetic energy, the time for steering could be reduced. Therefore, the incident O2 molecule can follow energetically less favored trajectories, through which the trapping may be replaced by bouncing back to the gas phase or direct dissociation. This results in a decrease in the probability of dissociating through the precursormediated adsorption mechanism. Meanwhile, higher incident energy can help O2 to overcome a larger range of activation barriers along trajectories leading to stable dissociated states, which also contributes to the increasing dominance of the direct dissociation mechanism. In accordance with the previous analysis, we can qualitatively predict the kinetic energy dependence of the O2 initial sticking coefficient, i.e., an initial decline with a subsequent increase. The turning point is lower for Rh(100) than for Rh(111) because of the existence of some relatively more preferred trajectories for direct dissociation. Such a trend has also been observed in experiments for the O2/Pt54−56 and O2/Pd57,58 systems. To gain more insight into the statistical property of O2 sticking on the Rh(100) and Rh(111) surfaces, the construction of a more sophisticated PES along with molecular dynamics simulation afterward can be considered, as has been done for the O2/Pt(111) system.59,60 Dissociation of the Molecular O2 Precursor. Because of the high existence possibility of molecular O2 precursors in a parallel configuration on both surfaces, especially under low kinetic energy, the dissociation of such O2 molecules is investigated thoroughly in this section. We first explore the dissociation process on the Rh(100) surface using the cNEB approach in the 2 × 2 unit cell. With the tbt configuration being the initial state, two elementary dissociation pathways leading to different final states have been identified, as shown in Figure 4. The pathway which ends up with the O atoms adsorbed at two next-nearest-neighbor brg sites (bb-2n) is energetically more preferred with a small activation energy of 0.032 eV. In this process, the O2 molecule first translates to the bhb configuration with the O−O bond only slightly lengthened. The TS is located in this step. The dissociation occurs afterward by two O atoms moving toward opposite brg sites and finally reaching the bb-2n geometry. The other pathway with a higher activation energy of 0.421 eV results in a final state where O atoms are adsorbed at two thirdnearest-neighbor brg sites (bb-3n). For this path, two O atoms depart away from each other at the beginning, forming a TS with the O−O bond length increased by about 40%. After that, the O atoms travel around the nearby top sites and move simultaneously to the brg sites in opposite directions and eventually take their final positions. Considering the relatively low oxygen diffusion barrier on Rh(100), we conclude that after the initial dissociation the O atoms located at the brg sites can

Figure 3. 1D PES cuts of O2 approaching Rh(100) (a) and Rh(111) (b) surfaces. The distance to the surface for perpendicular O2 is calculated with respect to the near-surface O atom. The dashed line indicates the dissociation of O2 when the distance is smaller than a threshold value. Five parallel configurations other than the molecularly adsorbed image on the Rh(100) surface (c) and two other configurations on the Rh(111) surface (d) are also illustrated.

through some other energetically less preferred trajectories, such as bhb, tht, and hbh for Rh(100) and hh and ff for Rh(111), will dissociate almost barrierlessly when they are close enough to the surface by a distance of around 1.8 Å. It is also worth mentioning that the energy evolutions along the tfb and thb trajectories for Rh(111) are similar to each other, and both lead to a stable molecular adsorption state. The two trajectories are both slightly unfavored compared to the tbt trajectory before approximately reaching the chemisorbed state, but the preference reverses at shorter O2−surface distances. There is an energy barrier for the dissociation through these two D

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Figure 4. Dissociation pathways of the molecular O2 precursors on the Rh(100) surface. Both of them start from the tbt configuration, with the final states being O atoms located at two next-nearest-neighbor brg sites (bb-2n) or two third-nearest-neighbor brg sites (bb-3n). Corresponding configurations along the two pathways are also illustrated.

Figure 5. Dissociation pathways of the molecular O2 precursors on the Rh(111) surface. One starts from a tfb configuration and ends in two nearest-neighbor hcp sites (hh). The other starts from a thb configuration and ends in two nearest-neighbor fcc sites (ff). Corresponding configurations along the two pathways are also illustrated.

further migrate to the hol sites and form a more stable configuration. The dissociation processes on the Rh(111) surface are then investigated similarly. Though many different reaction pathways have been tested, the minimum-energy pathway for the dissociation starting from the tbt configuration always reveals an intermediate minimum-energy state of either the tfb or thb configuration. Such rotation from the tbt configuration to the tfb or thb configuration happens as one O atom moves toward the nearby brg site while the other O atom remains almost unchanged, accompanied by a little elongation of the O−O bond. The small change in the bond length confirms that the O2 molecule remains intact during the rotation process. The rotation energy barrier is computed to be about 0.03 eV. The shallow well in the potential energy profile also suggests that the tbt configuration may convert to the tfb or thb configuration facilely. The same search is carried out for O2 dissociation pathways starting from both the tfb and thb configurations. Take the study of the tfb configuration as an example. Three possible dissociation products are considered, namely, two nearestneighbor fcc sites (ff), two nearest-neighbor hcp sites (hh), and next-nearest-neighbor fcc and hcp sites (fh). The results show that only the pathway leading to the hh final state is an elementary step. All other studied cases go through this dissociated state as an intermediate state (taking the underlying periodicity into consideration). The process, with tfb and hh configurations as the initial and final states, respectively, is shown in Figure 5. First, the O−O bond stretches along the O2

axis and reaches a TS with one O atom at a brg site and the other at a near-top position. The corresponding energy barrier is 0.202 eV. Second, the O−O bond lengthens even more as the brg O slides down to the hcp site and the near-top O climbs up to the top site. As confirmed by atomic oxygen adsorption, the oxygen atom staying at the hcp site is stable whereas that located at the top site is unfavored. Thus, as the O atom trapped at the hcp site moves little thereafter, the other O atom at the top site continues to fall into another hcp site to complete the dissociation. The situation is quite similar for the dissociation starting from the thb configuration and ending in the ff state, as depicted in Figure 5. The activation energy is a little lower at 0.159 eV. Despite the fact that the tfb and thb configurations have almost the same adsorption energy, the difference in the energy barrier is about 43 meV. Another notable difference is that the near-top O atom migrates around the top site rather than nearly directly passing over it in the process. This indicates a weaker repulsion force from the fcc O atom than from the hcp O atom. For comparison, the activation energies for different dissociation pathways along with the structural information on TSs are summarized in Table 3. The existence of a lowbarrier dissociation pathway suggests that the molecular O2 precursor may dissociate more easily on Rh(100) than on Rh(111). This is in accordance with the experiments in which the Rh(100) surface has relatively better catalytic capability. Meanwhile, it is worth noting that the dissociation processes on the Rh(111) surface are qualitatively analogous to those on the E

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Langmuir Table 3. Activation Energy (Ea) and Length of O−O Bond (dO−O), Height from the Center of Mass of the Top Rh Layer to Each O Atoma in the Transition State for Different Molecular O2 Precursor Dissociation Pathwaysb on Rh(100) and Rh(111) IS

FS

Ea (eV)

dO−O (Å)

h1 (Å)

h2 (Å)

tbt tbt

bb-2n bb-3n

0.032 0.421

1.383 1.902

1.817 1.826

1.817 1.826

tbt tbt tfb thb

tfb thb hh ff

0.034 0.030 0.202 0.159

1.386 1.386 1.743 1.723

1.859 1.853 1.516 1.527

1.928 1.924 1.821 1.805

Rh(100)

Rh(111)

a

The smaller one is labeled h1, and the other one is labeled h2. bThe initial state is labeled IS, and the final state is labeled FS.

Pt(111) surface. On the Pt(111) surface, the tbt configuration also needs to rotate to the tfb or thb configuration first, and the dissociation from the tfb or thb configuration follows a pathway similar to that described earlier. The theoretical values for the dissociation energy barrier are 0.56 eV for the tfb state and 0.38 eV for the thb state when the coverage of O2 is the same as in our study, 0.25 ML.38 The relatively low energy barriers on the Rh surface reflect that the molecular O2 precursor is more unstable on the Rh surface than on the Pt surface. Considering that the dissociation of the molecular O2 precursor is observed above 150 K on the Pt(111) surface,40 it can be predicted that detecting the molecular O2 precursor on the Rh surface is not easy in experiments. By analyzing the dissociation process on Rh(111), we notice that the O atom near the top in the tfb and thb configurations can move along various directions once the O atom has arrived at the top site. However, because of the small size of the 2 × 2 unit cell used, only one of them has been considered. We thus expand the cell to a larger 4 × 4 unit cell to explore the evolution of the O atom near the top. We first carry out a calculation of PES for the dropping process of the top O atom by calculating the total energy of the 2O/Rh(111) system as a function of the projected position of the top O atom in the x−y plane. One O atom is fixed at the final 3-fold hollow site, while the other O atom is moved around the next-nearest-neighbor top site as suggested by the aforementioned calculations. Four trajectories leading to different nearest-neighbor 3-fold hollow sites of the top site and three trajectories along the bisectors between each two of them are used to construct half of the PES. The other half is constructed according to the underlying symmetry. The distance step size for each trajectory is 0.196 Å, i.e., one-eighth of the distance between the top site and the nearest-neighbor 3fold hollow site. For each single calculation, the x and y coordinates of the top O atom are fixed, but its z coordinate and the other O atom and the top two substrate layers are allowed to relax. We present the PES projections with the schematic surface structures in Figure 6. It is obvious that the potential energy profile is flat when the O atom is moving to a dissociated state but within a 0.6 Å radius of the top site. The energy change during this stage is less than 0.15 eV, and the largest energy difference at the same distance away from the top site is about 0.05 eV starting from the tfb precursor state and 0.08 eV starting from the thb precursor state. A quick energy decrease

Figure 6. Projection charts of PES obtained from fixing one O atom at an hcp (a) or fcc (b) site while moving the other around the nextnearest-neighbor top site, corresponding to the dissociation process from the tfb precursor state or the thb precursor state, respectively. The origin is set to where the fixed O atom is located. The schematic surface structures for the O atom fixed at the hcp (c) or fcc (d) site are also illustrated, where the notation is hcp1 and fcc1 for nearestneighbor hcp and fcc sites, hcp2 and fcc2 for next-nearest-neighbor hcp and fcc sites, and hcp3 and fcc3 for third-nearest-neighbor hcp and fcc sites.

along with significant anisotropy shows up once beyond this platform. Finally, the system reaches a local minimum at the dissociated state with two O atoms at two 3-fold hollow sites. With these results, it can be concluded that the near-top O F

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adsorption and dissociation behavior of the O2 molecule on Rh(100) and Rh(111) surfaces. These data can provide fundamental information on kinetic modeling such as the kinetic Monte Carlo simulation that we are currently developing to investigate the catalytic properties on a realistic scale.

atom in the tfb or thb configuration can move into 3-fold hollow sites other than the nearest-neighbor same-type site through a single dissociation step. The thermal hopping is not necessary for such separation. It is likely that the final state reached is highly sensitive to the local configuration of coadsorbed O atoms surrounding the dissociating O2 molecule because the fluctuation of PES around the top site can be disturbed easily by the background O atoms due to the repulsive interactions, and the most favorable trajectory may flexibly change accordingly. To obtain an estimation of the coverage dependence of the activation energy on the Rh(111) surface, we also perform the cNEB calculations in the 4 × 4 unit cell which corresponds to a coverage of 0.06 ML. For the dissociation from the tfb precursor state, three trajectories leading to different final states as revealed in the PES are examined. The one resulting in the near-top O atom adsorbed at the next-nearest-neighbor hcp site converges with an energy barrier of 0.168 eV. As for the thb precursor state, there are also three trajectories tested. Two of them converge and lead to the near-top O atom finally adsorbed at the next-nearest-neighbor fcc site or third-nearestneighbor hcp site, respectively. Both trajectories have the same TS with the activation energy being 0.136 eV. The difference in convergence for two initial configurations agrees with our aforementioned result that the hcp O atom has a stronger repulsion force than does the fcc O atom. On the other hand, compared to the results for the 0.25 ML coverage case obtained from the 2 × 2 unit cell, the dissociation barrier decreased by 34 meV for the tfb precursor state and 23 meV for the thb precursor state, which shows a positive correlation between the activation energy and the O2 coverage.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Liangliang Huang: 0000-0003-2358-9375 Qi Wang: 0000-0002-2246-1401 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Program for the National Natural Science Foundation of China (grants 21676232 and 21673206).

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REFERENCES

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CONCLUSIONS We applied the DFT method to systematically investigate the adsorption and dissociation of the O2 molecule over Rh(100) and Rh(111) surfaces. The results suggest that the parallel orientation of O2, i.e., the O−O bond being parallel to the surface, is the dominating configuration both in the molecularly adsorbed states and during the impinging processes. There is one stable parallel configuration for molecular adsorption labeled as tbt on Rh(100) and three configurations labeled as tbt, tfb, and thb respectively on Rh(111). Impinging through the tbt trajectory is the most favorable path, which leads to the corresponding molecular adsorption precursor state barrierlessly for both surfaces. The dissociation of O2 can occur either by precursor-mediated adsorption or by direct dissociation, depending upon the initial kinetic energy of the incident molecule. For the dissociation via the precursor, the results show that O2 in the tbt precursor state can dissociate facilely on Rh(100) but a little harder on Rh(111) as a two-step process. The latter can be described as a preliminary rotation to the tfb or thb precursor state, followed by a further dissociation with the final location of the near-top O atom disturbed easily by coadsorbed O atoms, which is confirmed by the existence of a relatively flat potential energy surface stage along the way. Such dissociation potential agrees with the fact that the Rh(100) surface is catalytically more reactive. It is worth noting that the adsorption and dissociation behaviors on the Rh(111) surface are qualitatively similar to those on the Pt(111) surface whereas the explicit energy evolution varies, which indicates a greater instability of the molecular O2 precursor on the Rh surfaces and thus a harder experimental detection of it. Also, our calculations shed light on the more detailed interpretation for the G

DOI: 10.1021/acs.langmuir.7b02030 Langmuir XXXX, XXX, XXX−XXX

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Langmuir

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DOI: 10.1021/acs.langmuir.7b02030 Langmuir XXXX, XXX, XXX−XXX