J. Phys. Chem. C 2009, 113, 7269–7276
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Density Functional Theory Comparison of Water Dissociation Steps on Cu, Au, Ni, Pd, and Pt Abhijit A. Phatak,†,‡ W. Nicholas Delgass,† Fabio H. Ribeiro,† and William F. Schneider*,‡,§ School of Chemical Engineering, Purdue UniVersity, 480 Stadium Mall DriVe, West Lafayette, Indiana 47907, Department of Chemical and Biomolecular Engineering, and Department of Chemistry and Biochemistry, UniVersity of Notre Dame, 182 Fitzpatrick Hall, Notre Dame, Indiana 46556 ReceiVed: NoVember 20, 2008; ReVised Manuscript ReceiVed: March 6, 2009
We report a plane-wave, supercell density functional theory (DFT) investigation of the adsorption and dissociation of water on (111) surfaces of Cu, Au, Pt, Pd, and Ni, five metals of potential interest in the context of water gas shift (WGS) catalysis. Binding energies, preferred adsorption sites, and configurations for H2O and its dissociation products (OH, H, and O) are determined on all five metals, as are the minimum energy paths (MEPs) and activation energies for H-OH and O-H dissociation. Results are compared with the diverse computational literature on H2O dissociation over these metals. Both dissociation steps are found here to be highly endothermic on Au(111) and slightly endothermic on Pt(111) and Pd(111). The first H abstraction from adsorbed H2O is exothermic on Cu(111) and Ni(111), but subsequent OH dissociation is endothermic on Cu(111) and slightly exothermic on Ni(111). Using a simple Langmuir equilibrium model, we show that under the high H2 background pressures typical of low-temperature WGS, the surface coverage of OH is expected to be several orders of magnitude higher than that of O, consistent with a significant role of OH in surface CO oxidation. 1. Introduction The interaction of water with solid surfaces is of central importance to many chemical, materials science, and biological phenomena including environmental chemistry, electrochemistry, corrosion, and biophysics. Therefore, the adsorption of water and its structure at the solid interface continues to be a subject of great interest.1,2 H2O-metal interactions play a particularly prominent role in heterogeneous catalysis. In fact, the observation of the water formation reaction on a platinum surface marked the birth of the term “catalysis”.3 Many important industrial catalytic processes (e.g., steam reforming of natural gas, water gas shift), as well as reactions in fuel cells or other electrocatalytic devices, involve adsorption/desorption and dissociation/formation of water molecules at metal surfaces. Thus, an atomic level understanding of these elementary processes is necessary to improve our knowledge of heterogeneous catalytic reaction mechanisms. Water adsorption and dissociation has been extensively studied over metal surfaces using various surface science experiments, as reviewed in detail by Thiel and Madey1 and Henderson.2 At sub-100 K temperatures, water is observed using scanning tunneling microscopy (STM) to adsorb molecularly and to readily aggregate on close-packed metal surfaces as dimers or larger structures.4-10 Michaelides et al.11 have reported density functional theory (DFT) calculations to characterize molecular water monomer adsorption on most of the latetransition metals. Surface science studies show that the irreversible water dissociation occurs at higher temperatures under ultrahigh vacuum (UHV) conditions on Cu(111), Ni(100), and * To whom correspondence should be addressed. E-mail: wschneider@ nd.edu. Phone: (574) 631-8754. † Purdue University. ‡ Department of Chemical and Biomolecular Engineering, University of Notre Dame. § Department of Chemistry and Biochemistry, University of Notre Dame.
stepped Ni(111), whereas dissociation of isolated water is not observed on Au(111), Pd(111), and Pt(111) (ref 2 and references therein). At higher coverages, water is observed and calculated to form mixed dissociated and molecular states.12-18 The adsorption and stepwise dissociation of an isolated water molecule on the late-transition metals has been examined in a number of supercell DFT simulations:19-24
H2O* + * f OH* + H*
(1)
OH* + * f O* + H*
(2)
Although these studies give generally consistent pictures of relative reaction and activation energies across metals, neither the methodologies nor the results are entirely self-consistent. Further, most of these studies employed 2 × 2 surface unit cells, small enough to possibly include unphysical interactions between neighbor cells. Thus, a fully consistent first-principles investigation of water adsorption and dissociation at low coverage, as relevant under catalytic conditions, on latetransition metals has not been presented so far in literature. This fact motivated our study of water adsorption and dissociation at low coverage on Au(111), Cu(111), Pt(111), Pd(111), and Ni(111) surfaces. A particularly important example of water dissociation reactions is in the mechanism of water gas shift (WGS):20,25-27
CO + H2O a CO2 + H2
(3)
Using a simple kinetic model for low-temperature (LT)-WGS parametrized against DFT calculations, Schumacher et al.28 showed that under typical reaction conditions the (111) surfaces of most Pt-group metals will be saturated with CO, whereas (111) surfaces of Cu and Au will be almost clean. Similarly, Phatak et al.25 showed that supported Pt particles are saturated with CO under typical LT-WGS conditions. Recently, Gokhale et al.20 showed that hydrogen abstraction from water is rate-
10.1021/jp810216b CCC: $40.75 2009 American Chemical Society Published on Web 04/08/2009
7270 J. Phys. Chem. C, Vol. 113, No. 17, 2009 limiting in LT-WGS over Cu(111). The other part of the WGS reaction mechanism involves oxidation of CO. In the commonly accepted redox mechanism, adsorbed CO is oxidized by adsorbed O atoms generated either by complete water dissociation or by the disproportionation of OH. Alternatively, in the carboxyl mechanism,20,25,29 CO is directly oxidized by OH. Typical feed conditions for a LT-WGS reactor have significant concentration of H2, ca. 7% CO, 8% CO2, 22% H2O, 37% H2 and remaining inert gas at 200-300 °C and 1 atm total pressure.25,27 It is important to determine whether OH or O is the preferred CO oxidant under these conditions. In this study, we perform a periodic slab self-consistent DFT investigation of water adsorption and dissociation on Au(111), Cu(111), Ni(111), Pd(111), and Pt(111) surfaces in the lowcoverage regime. We determine binding energies, preferred adsorption sites, and configurations for H2O and its dissociation products (OH, H, and O). Furthermore, we report the thermochemistry and activation energy barriers for reactions 1 and 2, contrasting our calculated values with those reported in literature. We find that both dissociation steps are endothermic on (111) surfaces of Au, Pt, and Pd. The first H abstraction from adsorbed H2O is exothermic on Cu(111) and Ni(111), whereas the second H abstraction or OH dissociation is endothermic on Cu(111) and slightly exothermic on Ni(111). Using a Langmuir equilibrium model, we show that, under typical WGS conditions, the surface coverage of OH is several orders of magnitude higher than that of O on these metals. We also discuss the implications of this result for the WGS mechanism.
Phatak et al.
Figure 1. Top and side views of the structure of a H2O monomer adsorbed on a close-packed metal surface.
TABLE 1: GGA-Calculated Binding Energies and Optimized Structural Parameters of Top-Site H2O Monomer surface
BE (eV)
h (Å)a
d (Å)a
Θ (deg)a
R (deg)a
∆metal (Å)b
∆Oxy (Å)c
Cu(111) Au(111) Ni(111) Pd(111) Pt(111)
0.21 0.14 0.29 0.30 0.33
2.34 2.63 2.19 2.37 2.31
0.98 0.98 0.98 0.98 0.98
105 105 106 105 106
7 10 13 8 8
0.08 -0.05 0.11 0.06 0.03
0.28 0.06 0.01 0.08 0.12
a
Geometric parameters defined in Figure 1. b Vertical displacement of the top-site atom from the surface plane. c Lateral displacement of O from the top site.
using vibrational frequency analysis, confirming a unique normal mode eigenvector corresponding to negative curvature at the saddle point.
2. Methods
3. Results
Electronic structure calculations are performed within the framework of supercell DFT as implemented in the Vienna ab initio simulation package (VASP).30,31 A three-layer metal slab with a 3 × 3 (corresponding to 1/9 ML coverage) surface unit cell and about 15 Å of vacuum between any two successive metal slabs is used. The bottom two atomic layers are fixed at the bulk interatomic distance, and the third atomic layer is allowed to relax. This slab thickness is commonly used and presents a compromise between accuracy and computational efficiency; test calculations indicate that adsorbate binding energies can vary by 0.2 eV from a thicker slab. These adsorption energy errors are expected to largely cancel in calculated surface reaction energies. The first Brillouin zone is sampled with a Γ-centered 3 × 3 × 1 Monkhorst-Pack k-point mesh. Ionic cores are described by ultrasoft pseudopotentials32 (USPP) and the Kohn-Sham one-electron valence states33 are expanded in a basis of plane waves with kinetic energy up to 400 eV. Electron exchange and correlation are described within the PW91 generalized gradient approximation (GGA).34,35 All metals and adsorbates can be treated non-spin-polarized save for ferromagnetic Ni; on Ni, inclusion of spin polarization decreases H, O, and OH adsorbate absolute binding energies up to 0.4 eV, and the spinpolarizedvaluesarereportedhere.Second-orderMethfessel-Paxton smearing36 of states near the Fermi level is applied with kBT ) 0.2 eV and finally extrapolated to kBT ) 0 eV. Total electronic energies are converged to 0.0001 eV and forces to 0.02 eV/Å. The calculated lattice constants for bulk Au, Cu, Pt, Pd, and Ni are 4.188 (4.078), 3.646 (3.615), 3.996 (3.924), 3.960 (3.89), and 3.538 (3.524) Å, respectively, and are within 2.8% of the corresponding experimental values (in parentheses). The climbing-image nudged elastic band (CI-NEB)37 method is used to determine the minimum energy paths for H2O and OH dissociation steps. The transition states (TS) are verified
In this section we describe the adsorption of water and its dissociation products (O, H, OH) on the five metals studied including the binding energies, site preferences, and binding geometries, surface relaxation caused by adsorption, and estimated diffusion barriers, and compare these results with available literature. After the adsorption results, we describe the two-step water dissociation pathway for each metal. We calculated adsorption at four high-symmetry sites on the (111) surface of each metal. Adsorbate binding energies (BE) are reported as -(Eadsorbate+slab - Eslab - Eadsorbate), where Eadsorbate+slab is the adsorbed configuration total energy, Eslab is the clean slab total energy, and Eadsorbate is the gas-phase adsorbate energy. For atomic species, the BE is defined with respect to 1/2 of the corresponding molecule in the gas phase, e.g., BE of atomic O (BEO) ) -(EO+slab - Eslab - 0.5EO2(g)). The absolute accuracy of well-converged DFT binding energies are reported to be of the order of 0.2 eV;38 error cancelation often improves the reliability of relative binding energies. 3.1. Adsorption of Water and Dissociation Products. Figure 1 shows the calculated structures of adsorbed H2O monomers. A water monomer is found to preferentially adsorb with oxygen slightly displaced from the top of a metal atom and to orient almost parallel to the surface. The binding energy and geometrical parameters on each metal are summarized in Table 1 and agree well with previous DFT results for this system.11 We find binding energies in a small window between 0.14 and 0.33 eV, which is on the order of the energy of a hydrogen bond between H2O molecules (ca. 0.25 eV). The absolute binding energies increase in the order Au < Cu < Ni < Pd < Pt. The intramolecular structure of the adsorbed water monomer is only slightly perturbed from that in the gas phase. The tilt angle (R) between the molecular plane and the surface is ca. 9°, with a minimum value of 7° on Cu(111), and a maximum value of 13° on Ni(111). The top-site metal atom is
Water Dissociation Steps on Cu, Au, Ni, Pd, and Pt
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TABLE 2: GGA-Calculated Atomic H Binding Energies (eV) at High-Symmetry Sites, Referenced to 1/2H2(g)a fcc hcp top bridge
Cu(111)
Au(111)
Ni(111)
Pd(111)
Pt(111)
0.34 0.33 -0.27 0.20b
-0.02 -0.07 -0.29 -0.10
0.49 0.47 -0.06 0.35b
0.59 0.56 0.07 0.45
0.54 0.49 0.52 0.51
a The preferred sites are indicated in boldface. laxed along surface normal.
b
Adsorbate re-
TABLE 3: GGA-Calculated Atomic O Binding Energies (eV) at High-Symmetry Sites, Referenced to 1/2O2(g)a fcc hcp top bridge a
Cu(111)
Au(111)
Ni(111)
Pd(111)
Pt(111)
1.94 1.84 0.22 1.57b
0.36 0.11 -1.02 -0.01b
2.14 2.00 0.42 1.65b
1.28 1.01 -0.36 0.77b
1.30 0.79 -0.00 0.80b
Preferred sites are indicated in boldface. along surface normal.
b
Adsorbate relaxed
vertically displaced out of the surface plane by
