ARTICLE pubs.acs.org/JPCC
Tuning the CO Dissociation Barriers by Low-Dimensional Surface Alloys A. Stroppa*,†,‡ and F. Mittendorfer‡,§ †
CNR-SPIN, L'Aquila, Italy Faculty of Physics, Universit€at Wien, and Center for Computational Materials Science, Sensengasse 8/12, A-1090 Wien, Austria § Institute of Applied Physics, TU Vienna, and Center for Computational Materials Science, Gusshausstrasse 25a, A-1050 Wien, Austria ‡
ABSTRACT: We explored the possibility of engineering the reactivity of a stepped rhodium surface for dissociation of carbon monoxide with the help of density functional theory calculations. Our results indicate that decorating the Rh step edges with late transition metals allows one to tune the local reaction barrier for nearly 2.5 eV. In the case of Ir@Rh(533), we predict at the same time a weaker adsorption in the final state and a lower dissociation barrier, which might help to overcome the poisoning problem in a realistic application.
C
onsiderable work has been carried out to understand catalytic reactions on transition metal surfaces15 such as adsorption and dissociation of CO.610 In the case of rhodium, it has been shown that CO dissociation is negligible on closepacked (111), (110), and (100) surfaces.11,12 Several mechanisms have been identified to increase the activity of the surfaces: (i) by introducing defects or stepped vicinal surfaces, providing highly undercoordinated adsorption sites,11,1315 and (ii) by alloy formation, i.e., exchanging specific surface atoms with another metal species.13,14 Combining both approaches offers a unique possibility to tailor the catalytic activity. In a previous study,1618 this concept has been demonstrated for well-ordered Ni step decorations on a stepped Rh (553) surface. By exchanging the alloying metal, i.e., replacing Ni by another transition metal, such as Mn,19 the reaction barrier can be lowered even further but also the adsorption energy of the final products is increased, resulting in a potential poisoning of the surface. In this letter, we present a systematic overview on the influence of Rh step edge decorations with the group VIIIX transition metals, namely, the 4d metals Ru, Pd, Ag and Os, Ir, Pt (5d) monatomic nanowires. Comparison of the adsorption energies and dissociation barriers of CO at the modified step edges to the bare Rh(111) and Rh(553) surfaces permits to separate the structural from the electronic contributions. In addition, comparison with the bare Rh (553) surface allows one to analyze the relation between the electronic modifications of the reaction centers and the catalytic properties and thus ultimately to selectively tune the reaction barriers. Thermodynamic and kinetic effects20 go beyond the purpose of the present study and are not considered here. r 2011 American Chemical Society
Our calculations suggest that with increasing band filling of the step-decorating metal, the dissociation at the steps becomes less exothermic with the transition state being shifted increasingly toward the final state. Furthermore, the dissociation energy can vary as much as 2.5 eV by changing the step-decoration atom along the 4d and 5d periodic row. In particular, the decoration with earlier transition metals favors CO dissociation. This is particularly attractive for the “computer laboratory design” of a bimetallic catalytic nanowire. Ideally, this approach does not only allow one to tailor the height of the reaction barrier, but the combination of strongly interacting, localized reaction centers on a weaker interacting substrate material allows to reduce the risk of poisoning the surface by increasing the adsorption energy too much. Calculations were performed with the Vienna ab-initio simulation package (VASP),21 the projector augmented wave (PAW) method22 in the implementation of Kresse and Joubert,23 and the PerdewBurkeErnzerhof parametrization for the exchangecorrelation energy functional.24 A cutoff energy of 400 eV and a 8 8 1 k-point mesh has been used. The Rh(553) surface consisted of five-atom-wide (111) terraces separated by monatomic 111-faceted steps approximately 10 Å apart. Further technical details as well as calculations to ensure convergence of the computational parameters and geometrical set ups can be found in refs 13 and 14. Figure 1ac shows a ball-and-stick model of the Rh(553) stepped surface. The Rh(553) surface Received: August 5, 2011 Revised: September 20, 2011 Published: September 28, 2011 21320
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Figure 2. DOS projected onto d states of the step (red) and terrace (blue) metal atom. Vertical lines correspond to the d-band center.
Figure 1. Model of the Rh(553) stepped surface: (a) perspective, (b) side, and (c) top view. Green atoms highlight the step atoms. A, B, and C correspond to successive regions from the step toward the inner terrace region. Transition state geometry for (d) CO@Rh(111) and (e) CO@Rh(553). The oxygen atom is depicted in red and the carbon atom in blue.
displays monatomic steps in the [110] direction, forming (111)type microfacets and the five-atom-wide (111) terrace. For adsorption of CO, both high-symmetry sites (top, bridge, hcp hollow configurations) on the terrace and on the step edges have been considered. The adsorption energies have been referenced to the gas-phase molecule according to the following expression Eads ¼ Eslab=CO Eslab ECO
ð1Þ
where ECO is the energy of the isolated CO molecule in the equilibrium position, Eslab is the total energy of the relaxed slab, and Eslab/CO is the total energy of the optimized slab/adsorbate system. Equation 1 also holds for the transition state. In this case, the sign of a Ets indicates either predominantly CO dissociation (if negative) or desorption (if positive) of the CO molecule upon heating the surface. A comparison of the calculated (top-site) adsorption energies on the bare Rh(111), 1.86 eV, and Rh(553) surface, 2.02 eV, shows an increased adsorption energy in the vicinity of the steps (Figure 1A), while the adsorption energies on the terrace of the Rh(553) surface (Figure 1C) are close to the values on the flat surface.14 This confirms that steps generally provide more reactive sites with respect to the flat surfaces.2 The origin of this phenomenon can be understood in terms of the density of states (DOS) projected on d states of the step atom and the terrace atom farthest from the step edge as shown in Figure 2. Comparing the DOS for the step and the terrace we note that (i) the bandwidth does not change appreciably and (ii) there is a depletion of the DOS at low energy and an increase of the DOS at the Fermi level causing an upward shift of the DOS d-band center. The general dependence of the adsorption energy on d-band parameters as well as on other relevant parameters of the bare surface has been discussed in detail in ref 8. The increase
Table 1. Adsorption energy Eads and CO Bond Length dCO for Atop Adsorption, Transition State Energies (ETS), Bond Length for CO@X/Rh(553) Surfaces (where the X atom decorates the step edge; corresponding values for the stepped X surface are in parentheses), and Energies for the Final States after Dissociation Efs dCO (Å)
ETS (eV)
Ru 2.09(1.85)
1.17
0.60(0.55)
1.89
2.08(1.91)
Rh 2.02(2.02)
1.17
0.03(0.03)
2.03
1.07(1.07)
Pd 1.42(1.59)
1.16
1.00(0.83)
2.39
0.45(0.02)
Os 2.30 (2.12) Ir 2.30 (2.35)
1.17 1.17
0.64 (0.45) 0.04 (0.25)
1.83 1.94
2.36 (2.46) 1.00 (0.64)
1.81 (2.05)
1.16
0.81 (0.91)
2.18
0.28 (0.36)
X
Eads (eV)
dCO (Å)
Efs (eV)
4d
5d
Pt
of the adsorption energy is well rationalized within the d-band model:2 the upward shift of the d-band center implies a stronger π interaction and consequently an increase of the adsorption energy. A detailed analysis of the relation between the d-band filling and the adsorption energy of CO and an illustration of the role of the CO π* LUMO can be found in refs 2527. The strong modifications of the reactivity due to structural effects suggest that when combined with formation of a surface alloy it could be possible to tailor the adsorption strength of CO.16,17 We systematically screened this effect by exchanging the Rh step atoms by other late transition metals from groups VIIIX. Table 1 gives an overview over the adsorption energies of CO on the modified step edges. For a systematic comparison we also considered the adsorption energies of CO at the top site of the step edge regardless of the experimentally preferred adsorption mode. A comparison between the different metal decorations shows a common trend: (i) the adsorption energy of CO on the metal stripes increases as the d-band filling of the step-decorating metal atom decreases and it is larger for 5d than 4d step-decorating metal atoms; (ii) the decrease of the adsorption energy moving to the right along the periodic row (i.e., increasing the bandfilling) correlates with a decrease of the CO bond length. Consequently, decoration of the step edge with the group X transition metals (Pd and Pt) lowers the adsorption energy even below the values for the terrace, thus driving the CO away from 21321
dx.doi.org/10.1021/jp207498u |J. Phys. Chem. C 2011, 115, 21320–21323
The Journal of Physical Chemistry C the step edge. In contrast, a step decoration with the earlier transition metals like Ir and Os even strengthens the interaction with the CO molecule. A similar observation holds when we consider the reaction barriers for dissociation of CO. For the bare Rh surfaces, the calculated reaction barriers (with respect to the desorption energy) are ETS = 1.05 and 0.03 eV on Rh(111) and Rh(553), respectively. In both cases, the transition state is rather late with a bond length changing from from dCO =1.89 Å on the flat surface to dCO = 2.03 Å on Rh(553). This observation is related to the higher coordination of the molecule at the transition state (see Figure 1d and 1e). In the latter case, the oxygen atom is located at the bridge site of the step edge while the carbon atom is located in the hollow site of the terrace near the step edge. Consequently, we confirm previous studies12 showing that dissociation on the Rh(111) surface is highly unfavorable, i.e., the molecule will preferentially desorb when the surface is heated up. On the other hand, the step decorations show a pronounced effect on the reactivity of the surface. Table 1 shows the reaction barriers and corresponding transition states (TS) for dissociation of CO at the decorated step edges of Rh(553). The steps greatly reduce the dissociation barrier. Although even the presence of the step edges on the bare Rh(553) surface already makes dissociation a competitive process to desorption,13,14 only decoration with other transition metals lowers the reaction barrier enough for a dominant dissociation of the CO molecule. A comparsion of the transition state energies ETS for the step edges decorated with the 4d and 5d transition metals displays a similar relation as for adsorption of CO (Table 1). Yet taking a closer look at the correlation between Eads and ETS at the step edge of the Rh(553) for different step decorations (Figure 3a) we note that although both quantities follow the same trend no linear relation can be observed. The basic trend in ETS and Eads can be explained by a simple and well-known interaction scheme: the π*d interaction (back-donation term) destabilizes the internal CO bond. It is expected to increase when increasing the d-band filling of the step-decorating atom, and consequently it drives the molecule toward dissociation. At the same time, the higher filling of the d band favors an upward shift of the d-band center.2527 Indeed, plotting both Eads and ETS as a function of the center of mass of the d band projected into the step atom (Figure 3b) displays a linear correlation of Eads as well as ETS with the εd in accordance with the d-band model. Therefore, decoration with late transition metals, such as Pt (Pd), leads to a transition state energy of +1.0 eV (0.8 eV) above the desorption energy. On the other hand, the reaction barriers of step edges decorated with Ru or Os are significantly lowered, but also the products of the reaction are stronger bound, leading to a potential poisoning of the surface. The noteworthy exception from this rule is the decoration of the Rh step edges with Ir: the transition state is slightly below the desorption energy (0.04 eV), but it also has a lower adsorption energy of the final state products compared to the bare Rh(553) surface. This is important to note as the CO molecule will rather dissociate on the Ir decorated edge than desorbe, while the reaction products can still be removed. This effect is a pronounced result of the localized reaction centers. While the material dependence of the dissociation barriers of CO on flat, pure transition metal surfaces has been analyzed before,12 this effect goes beyond a simple chemical exchange: Table 1 shows a comparison between dissociation on the metallic M(553) surface (values in brackets) of the active
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Figure 3. (Top) Trends for Eads(top) and ETS (in eV) as a function of the step-decorating atom. (Bottom) Variation of the Eads (top) and ETS as a function of the d-band center.
group VIIIX metals and step decorations. Clearly, only the combined effect of the step decorations allows one to overcome the linear relation between the adsorption energies and the corresponding transition states. In summay, we demonstrated that quasi-1D structures assembled on the vicinal surface can be used for nanocatalysis engineering. We predict a twofold advantage of the metallic step decorations: on one hand, combination of the structural properties of the nanostructures and the electronic properties of the selected materials offers the means to tune the interaction strength of the relevant reaction intermediates. Consequently, the appropriate choice of the step decoration allows to tailor the reaction barriers of a selected reaction. On the other hand, the strictly localized reaction center allows one to partially circumvent a common problem, which is often depicted by volcano plots, namely, poisoning of the catalysts by an overbinding of the reactants. By choosing a less reactive substrate the general adsorption energy can be reduced, allowing for easier removal of the final products. In the case of an Ir step decoration, we find at the same time a decreased reaction barrier and also a weaker adsorption of the reaction products. This aspect has not been covered in this paper, but completion of the catalytic cycle by removal of the adsorbates will be addressed in a forthcoming paper. 21322
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’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by the Austrian Science Fund (FWF) through the National Research Network “Nanoscience on Surfaces” (S9008). ’ REFERENCES (1) Bond, G. C.; Thompson, D. T. Catalysis by gold. Cat. Rev.-Sci. Eng. 1999, 41, 319–388. (2) Hammer, B.; Norskov, J. K. Theoretical surface science and catalysis-Calculations and concepts. Adv. Catal. 2000, 45, 71–129. (3) Henry, C. R. Surface studies of supported model catalysts. Surf. Sci. Rep. 1998, 31, 231–325. (4) Meyer, R.; Lemire, C.; Shaikhutdinov, S. K.; Freund, H.-J. Surface chemistry of catalysis by gold. Gold Bull. 2004, 37, 72–124. (5) Biloen, P.; Sachtler, W. M. H. Mechanism of hydrocarbon synthesis over FischerTropsch catalysts. Adv. Catal. 1981, 30, 165–216. (6) Hammer, B.; Hansen, L. B.; Norskov, J. K. Improved adsorption energetics within density-functional theory using revised Perdew-BurkeErnzerhof functionals. Phys. Rev. B 1999, 59, 7413–7421. (7) Ruban, A.; Hammer, B.; Stoltze, P.; Skriver, H. L.; Norskov, J. K. Surface electronic structure and reactivity of transition and noble metals. J. Mol. Catal. 1997, 115, 421–429. (8) Stroppa, A.; Kresse, G. The shortcomings of semi-local and hybrid functionals: what we can learn from surface science studies. New J. Phys. 2008, 10, 0630201–06302017. (9) Stroppa, A.; Termentzidis, K.; Paier, J.; Kresse, G.; Hafner, J. CO adsorption on metal surfaces: A hybrid functional study with plane-wave basis set. Phys. Rev. B 2007, 76, 1954401–19544012. (10) Marsman, M.; Paier, J.; Stroppa, A.; Kresse, G. Hybrid functionals applied to extended systems. J. Phys-Condes. Matter 2008, 20, 0642011–06420119. (11) Mavrikakis, M.; Baumer, M.; Freund, H. J.; Norskov, J. K. Structure sensitivity of CO dissociation on Rh surfaces. Catal. Lett. 2002, 81, 153–156. (12) Liu, Z. P.; Hu, P. General trends in CO dissociation on transition metal surfaces. J. Chem. Phys. 2001, 114, 8244–8247. (13) Stroppa, A.; Mittendorfer, F.; Andersen, J. N.; Parteder, G.; Allegretti, F.; Surnev, S.; Netzer, F. P. Adsorption and Dissociation of CO on Bare and Ni-Decorated Stepped Rh(553) Surfaces. J. Phys. Chem. C 2009, 113, 942–949. (14) Koch, H. P.; Singnurkar, P.; Schennach, R.; Stroppa, A.; Mittendorfer, F. A RAIRS, TPD, and DFT study of carbon monoxide adsorption on stepped rh(553). J. Phys. Chem. C 2008, 112, 806–812. (15) Streber, R.; Papp, C; Lorenz, M. P. A.; Bayer, A.; Denecke, R.; Steinr€uck, H.-P. Angew. Chem., Int. Ed. 2009, 48, 9743–9746. (16) Schoiswohl, J.; Mittendorfer, F.; Surnev, S.; Ramsey, M. G.; Andersen, J. N.; Netzer, F. P. Chemical reactivity of Ni-Rh nanowires. Phys. Rev. Lett. 2006, 97, 126102–11261024. (17) Schoiswohl, J.; Mittendorfer, F.; Surnev, S.; Ramsey, M. G.; Andersen, J. N.; Netzer, F. P. The self-assembly of metallic nanowires. Surf. Sci. Lett. 2011, 600, L274–L280. (18) Surnev, S.; Alegretti, F.; Pareteder, G.; Franz, T.; Mittendorfer, F.; Andersen, J. N.; Netzer, F. P. One-dimensional oxide-metal hybrid structures: site-specific enhanced reactivity for CO oxidation. Chem. Phys. Chem. 2011, 11, 2506–2509. (19) Ma, X. F.; Su, H. Y.; Li, W.-X. Carbon Monoxide Adsorption and Dissociation on Mn decorated Rh(111) and Rh(553) Surfaces: A first-principles study. Catal. Today 2011, 160, 228–233. (20) Kuhnke, K.; Kern, K. Vicinal metal surfaces as nanotemplates for the growth of low-dimensional structures. J. Phys.-Condens. Matter 2003, 15 (47), S3311–S3335.
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