Density Functional Theory Study of the Water Dissociation on Platinum

Article pubs.acs.org/JPCA

Density Functional Theory Study of the Water Dissociation on Platinum Surfaces: General Trends José L. C. Fajín,*,† M. Natália D. S. Cordeiro,† and José R. B. Gomes*,‡ †

REQUIMTE, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 687, P-4169-007 Porto, Portugal CICECO, Departamento de Química, Universidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal

‡

ABSTRACT: We report a comparative periodic density functional theory study of the reaction of water dissociation on five platinum surfaces, e.g., Pt(111) Pt(100), Pt(110), Pt(211), and Pt(321). These surfaces were chosen to study the surface structural effects in the reaction of water dissociation. It was found that water molecules adsorb stronger on surfaces presenting low coordinated atoms in the surface. In the cases of the stepped Pt(110) and kinked Pt(321) surfaces, the activation energy barriers are smaller than the adsorption energies for the water molecule on the corresponding surfaces. Therefore, the calculations suggest that the dissociation reaction will take place preferentially at corner or edge sites on platinum particles with the (110) orientation. The inclusion of the results obtained in this work in previous derived BEP relationships confirms that the adsorption energy of the reaction products arises as the most appropriate descriptor for water dissociation on transition metal surfaces.

1. INTRODUCTION The water dissociation is a vital step of the industrially relevant water gas shift reaction (WGS, CO + H2O → CO2 + H2). The WGS reaction plays an important role in several chemical processes, such as methanol synthesis, steam re-forming of hydrocarbons and CO elimination from the hydrogen-rich fuel stream used in fuel cells.1−3 The industrial catalysts for the WGS reaction at high temperature are based on Fe2O3−Cr2O3 whereas for the reaction at low temperature rely on copper nanoparticles dispersed onto a ZnO/Al2O3 support,4,5 but their characteristics are not ideal for fuel cell applications. Recent studies suggest that catalysts based on noble metals (e.g., Pt6,7 and Au7,8) supported on metal oxides are very promising for the low-temperature WGS reaction, as for other reactions, e.g., oxidation of CO or CH4.9,10 In the metal/metal oxide catalysts, the active phase of the catalyst is generally accepted to be the metal,11−13 but the support can strongly affect the catalyst performance.14−20 The catalytic activity is also influenced by the doping of the catalyst with other metals.21−24 Because there are several variables associated with the performance of the catalyst for the WGS reaction, a fundamental understanding of the effects caused by each variable in the catalyst performance is required. The reaction of water dissociation was suggested as the rate determining step in the WGS reaction catalyzed by copper surfaces.25,26 Having in mind that this step can be also the rate limiting step for the WGS reaction on other metal surfaces, we studied by density functional theory (DFT, PW91 functional) the energetics associated with the water dissociation on several flat, stepped, or kinked transition metal surfaces.27 The calculated energy barriers for the first O−H bond cleavage in © XXXX American Chemical Society

H2O on the different transition metal surfaces were found to be linearly correlated with several descriptors calculated for the corresponding surfaces, namely, reaction energy, adsorption energy of the reaction products, and also adsorption energy of an oxygen atom. Similar Brønsted−Evans−Polanyi (BEP) relationships were obtained for the breakage of the first O−H bond in the water molecule on transition metal surfaces doped with other transition metal atoms.28 Catalysts based on platinum considered for the WGS reaction are constituted by Pt nanoparticles dispersed onto a support such as Al2O3, CeO2, ZrO2, TiO2, MgO, La2O3, Fe2O3, SiO2, etc. (single or mixed).6,29−38 In the case of the ceria support, it was found that the catalytic activity for the WGS reaction depends on the size of the Pt particles, and that the nanoparticle−support interface plays an important role in catalysis.36 Recently, the effect of the Pt particle size on the adsorption and dissociation energies of the water molecule on a series of cuboctahedral Ptn nanoparticles (n = 13, 19, 38, 55, 79, and 140) were investigated by DFT (PW91 functional and PAW potentials) and compared with data calculated for the infinite Pt(111) surface.39 For all the particles, water prefers to bind low coordinated Pt atoms from top sites at the particle edges. In contrast to what has been found for adsorption of other molecules on metal nanoparticles,40 the adsorption energy calculated for water interacting with the Ptn particles Special Issue: Energetics and Dynamics of Molecules, Solids, and Surfaces - QUITEL 2012 Received: November 22, 2013 Revised: February 14, 2014

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dx.doi.org/10.1021/jp411500j | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

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Figure 1. Adsorption positions at the (a) Pt(111), (b) Pt(100), (c) Pt(110), (d) Pt(211), and (e) Pt(321) surfaces.

with low coordinated sites is energetically more favorable than on terraces, which agrees well with experimental evidence from temperature-programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS) results.43 The calculated adsorption energies on the extended stepped and kinked surfaces, e.g., in the range −0.46 to −0.55 eV, are also close to the values reported for the Pt nanoparticles. The effect of intermolecular hydrogen bonding obtained from the adsorption studies of dimers and trimers on the stepped Pt(221) and Pt(322) surfaces was found to be less important than on the planar Pt(111) surface, i.e., water−surface interaction in the former surfaces is more important than water−water stabilization in the latter. Similar conclusions were obtained from another computational study also devoted to water adsorption on Pt(221).44 Donadio et al. found that linear water clusters or chains at the steps, stabilized by cooperative effects of chemical bonds with the substrate and hydrogen bonds are the preferential configurations on Pt(221). Interestingly, on the basis of the results from the calculations carried out with the PBE functional and USPP, they concluded that at steps water molecules dissociate, forming mixed hydroxyl/water structures, through a sort of autocatalytic mechanism promoted by H-bonding; i.e., water adsorption was suggested to be partially dissociative when coverage is enough to produce trimers or larger clusters.44 Water adsorption and dissociation on low and high Miller index surfaces seem to be quite different, i.e., adsorption is weaker and dissociation is more difficult in the former than in the latter surfaces. Accordingly, effects of high Miller index surfaces in the water adsorption and dissociation processes have to be carefully understood for aiding the interpretation of experimental findings.45,46 In the present work, density functional theory calculations on periodic slab models of planar, stepped, and kinked platinum surfaces were performed to take out the effect of the variation of the surface Miller indices in the catalysis of the water dissociation reaction. This work is organized as follows. The computational methods are described in detail in section 2, whereas the calculated results

is not influenced significantly on going from the smaller to the larger particle. Adsorption energies corrected with the zeropoint vibrational energy (ZPVE) are, in electronvolts, −0.54, −0.47, −0.49, −0.59, −0.50, and −0.55 for nuclearities 13, 19, 38, 55, 79, and 140, respectively. The main reason for the distinct behavior is probably due to the rather weak adsorption of water when compared with other adsorbates, e.g., CO and NO.40 Interestingly, the adsorption energy for water on the largest particle is far from the calculated value on the extended Pt(111) surface, i.e., −0.27 eV, also on top sites. The large difference between the two types of systems is due to lowcoordinated atoms in the particles, which are absent in Pt(111). The calculated energy barriers show an oscillating behavior with the particle size but convergence with size to the Pt(111) limit was found for this property. In general, the activation energy barriers increase with nanoparticles size, from 0.30 eV, in the case of Pt19, to 0.70 eV, in the case of Pt140, which is close to the barrier computed for the reaction occurring on Pt(111), i.e., 0.78 eV. The barrier calculated for the planar surface is similar to that reported by Phatak et al., 0.75 eV,41 based also on the PW91 functional but employing ultrasoft pseudopotentials (USPP). Thus, it was found that water dissociation is easier on very small particles than on particles with moderate sizes and that the activity of larger nanoparticles (those constituting the real catalysts) tends slowly toward the activity of extended surfaces. Water adsorption on planar, Pt(111), stepped, Pt(221) and Pt(322), and kinked, Pt(763) and Pt(854), surfaces was recently investigated by Á rnadóttir and co-workers.42 These authors performed periodic slab model calculations, at the DFT-PW91 level of theory, to analyze the interaction of water monomers, dimers, and trimers with the different platinum surfaces. The interaction on the Pt(111) surface is preferential at top sites for isolated water and for water clusters, and the formation of intermolecular hydrogen bonds in the latter leads to an important stabilization effect. The adsorption energy per water molecule increases from −0.30 eV in the case of monomeric adsorption to ∼−0.45 eV in the case of water clusters. As found by Fajı ́n et al.,39 adsorption on the regions B



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local minimum. The TS structures were characterized as firstorder saddle points by vibration analyses, i.e., computation of a single imaginary frequency characterized by an associated movement driving toward products. The adsorption energies, E°ads, were calculated from the difference between the calculated energy for the adsorbate-slab system, Eadsorbate‑slab ° , and those of the separated adsorbate, Eadsorbate ° , and slab, Eslab ° , systems,

are reported and discussed in section 3. Finally, the most important conclusions are summarized in section 4.

2. CATALYST SURFACES MODELS AND COMPUTATIONAL DETAILS The catalyst models were obtained by cutting a slab of adequate size from bulk platinum, with the lattice parameter optimized in a previous work,27 along appropriate planes given by the Miller indices. As common practice in the field, a vacuum region of ∼10 Å thick was introduced between repeated slab models in one of the slab directions. Surface models with low Miller indices (111) and (100) are models of flat surfaces, without low coordinated atoms, whereas the surface with the (110) index presents low coordinated atoms in the surface ridges. Low coordinated atoms are also found on the stepped Pt(211) and on the kinked Pt(321) surfaces. The slabs used to model the Pt(111), Pt(100), and Pt(110) surfaces are based on 2 × 2 unit cells with four atomic layers, whereas 2 × 1 and 1 × 1 cells, also with four atomic layers, were employed to represent the Pt(211) and Pt(321) surfaces. The atomic positions in the two upmost metallic layers were fully relaxed during the calculations, whereas those in the remaining metal layers were hold fixed. The adsorption positions considered on each surface are given in Figure 1; four adsorption sites, e.g., top, bridge, hollow fcc, and hollow hcp were considered on Pt(111), three positions, e.g. top, hollow, and bridge, on Pt(100), and five sites, e.g., top, short bridge, long bridge, hollow-3, and hollow-4, on Pt(110). A much larger number of different positions (top, bridge, and hollow sites) may be found in the cases of the Pt(211) and Pt(321) surfaces and are also identified in Figure 1. All the DFT calculations considered were performed with the VASP 4.6.3 computer code47−49 and relied on the PW91 exchange−correlation potential50 based on the generalized gradient approach (GGA). The PW91 functional was used in the study of water dissociation on several metallic surfaces26,27 and was found to provide essentially the same results as the PBE functional, with PW91 and PBE performing much better than the RevPBE functional for the adsorption and dissociation energies of water on the Cu(111) surface.51 The projected augmented-wave (PAW) method as implemented in VASP52,53 was considered to take into account the effect of core electrons in the valence electron density, the cutoff used for the plane wave expansion was 415 eV, and the Monkhorst−Pack54 grid of special k-points for the numerical integration in the reciprocal space was 7 × 7 × 1. Spin polarization was found to have negligible effects on the calculated geometries and energies for the type of adsorbate−substrate systems considered in this work and, therefore, were not considered.55 Note that spin polarization was used for the gaseous species as recommended.55 Additionally, dipole corrections were also found to have a negligible effect in calculated geometries and energies. The most favorable configurations for adsorbed water and OH + H pairs on the slab models were obtained through energy minimization with respect to the geometry. These configurations were taken, respectively, as initial (IS) and final (FS) states for the water dissociation reaction (H2O* + * → OH* + H*). The transition state (TS) structures for the first O−H bond cleavage in water were obtained with the dimer approach.56 Very strict convergence criteria, e.g., 10−6 eV for the total energy change and 10−3 eV/Å for the forces acting on the ions, were used to avoid that the algorithm converges to a

° = Eadsorbate ° ° − Eadsorbate ° Eads ‐ slab − Eslab

(1)

In eq 1, energies were corrected with the corresponding zero point vibrational energy (ZPVE) corrections, obtained from the consideration of the vibrational frequencies for selected configurations and the harmonic oscillator approximation, and Eadsorbate ° stands for the energy of the adsorbate in the gas phase. Calculated adsorption energies are negative if the corresponding interactions with the Pt slabs are favorable. Activation energy barriers were obtained from subtraction of the energy of the initial state to that of the transition state. Additionally, rate constants (k) at 463 K (common temperature for low temperature WGS)57 were estimated with the following equation:58 ⎛ k T ⎞⎛ q ⧧ ⎞ k = ⎜ B ⎟⎜⎜ ⎟⎟e−Eact° / kBT ⎝ h ⎠⎝ q ⎠

(2)

where kB and h are the Boltzmann and Planck constants, respectively, T is the absolute temperature, Eact ° is the ZPVEcorrected activation energy barrier, and q⧧ and q are the vibrational partition functions for the TS and initial states, respectively, which were approximated from the harmonic vibrational frequencies. Free Gibbs energies for water dissociation at T = 463 K and pressure (p) = 1 bar were calculated as27,41 ° + Δ(pV ) − T ΔS ΔG = Ereact

(3)

where E°react is the ZPVE-corrected reaction energy, Δ(pV) is the pressure−volume work, and ΔS the entropy change associated with the reaction. The Δ(pV) term is 0, and the entropy term was calculated by assuming that adsorbates lose all translational degrees of freedom via the following equation: Si* = Si,g − Si,trans

(4)

where Si,g are the entropies of the gaseous phase species (i = H2O, HO, H) calculated with the Shomate equation: Si,g = A × ln(T /1000) + B × T /1000 + C × (T /1000)2 /2 + D × (T /1000)3 /3 − E /(2 × (T /1000)2 ) + G

(5)

and with parameters A, B, C, D, E, and G taken from the NIST chemistry web book.59 The translational entropy was calculated using the Sackur−Tetrode equation: ⎡ ⎛⎛ 2πmk T ⎞3/2 k T ⎞⎤ 5 B B ⎟⎥ ⎟ Si,trans = R ⎢ + ln⎜⎜⎜ 2 ⎠ ⎢⎣ 2 p ⎟⎠⎥⎦ ⎝⎝ h

(6)

where m is the mass of molecule i and R is the universal gas constant, and other terms have the same meaning as above. C



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3. RESULTS 3.1. Adsorption of Reactants and Products. The results obtained for water adsorption on the several platinum surfaces considered in the present work are compiled in Table 1. Views

water molecule is found to be parallel to the surface whereas on Pt(110), Pt(211), and Pt(321) surfaces, one of the hydrogen atoms is pointing toward a neighboring metallic atom. The adsorbate to surface distances decrease in the order Pt(111) > Pt(100) > Pt(211) ≈ Pt(110) > Pt(321), which follows the ordering of the coordination numbers of the platinum atoms in the surface to which the water molecule is directly bonded, i.e., 9 > 8 > 7 = 7 > 6, respectively. The interaction is stronger on Pt(321) and weaker on Pt(111); adsorption energies, in absolute value, decrease in the order Pt(321) > Pt(110) > Pt(100) ≈ Pt(211) > Pt(111). Note the change in the relative position of the adsorption energies for water on Pt(100) and Pt(211); i.e., adsorption on the planar Pt(100) surface is slightly more stable than on the stepped Pt(211) surface. Nevertheless, interesting correlations exist between the calculated adsorption energies and the water-surface bond length or the coordination number of the atoms in the surface to which the water molecule binds (Figure 3). These correlations show that stronger water adsorption is correlated with smaller water−surface distances or with smaller coordination number of the surface atoms. Similar correlations were identified in the work of Tang and Chen on the water interaction with planar and stepped copper surfaces.60

Table 1. Adsorption Energies and Distances for Water Adsorption on Several Platinum Surfaces surface

sitea

E°ads(/eV

dO‑surfb/Å

dO−Hc/Å

Pt(111)d Pt(100) Pt(110) Pt(211) Pt(321)

top top top top “1” top “1”

−0.27 −0.43 −0.50 −0.41 −0.69

2.51 2.37 2.26 2.29 2.24

0.98/0.98 0.98/0.98 0.98/1.00 0.98/0.99 0.98/0.99

a

For labeling of adsorption positions please refer to Figure 1. Distance between water oxygen atom and nearest Pt atom in the surface. cO−H bond length. dResults taken from ref 27.

b

of most stable configurations for water on each surface are shown in Figure 2 (panels on the left). It was found that adsorption is more favorable on top positions, with the water molecule approaching the platinum surfaces with its O atom. In the case of adsorption on flat surfaces, the HOH plane of the

Figure 2. Optimized structures for the initial (IS, leftmost panels), transition (TS, central panels), and final (FS, rightmost panels) states of the most favorable path for the H2O → OH + H reaction on (a) Pt(111), (b) Pt(100), (c) Pt(110), (d) Pt(211), and (e) Pt(321). The length of the cleaved O−H bond is given in angstroms. D



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Table 2. Co-adsorption Energies and Distances for OH + H Pairs on Several Platinum Surfaces site (OH/H)a

surface d

Pt(111) Pt(100) Pt(110) Pt(211) Pt(321)

bridge/hollow fcc bridge/bridge short bridge/short bridge bridge step/bridge step b2−1/top “1”

Ecoads ° / eV

dO‑surfb/Å

dH‑surfc/Å

0.39 −0.60 −0.44

2.18; 2.18 2.10; 2.10 2.12; 2.12

1.89; 1.89; 1.79 1.76; 1.76 1.76; 1.76

−0.61

2.11; 2.11

1.74; 1.74

−0.42

2.10; 2.14

1.57

a

For labeling of adsorption positions please refer to Figure 1. b Distance between hydroxyl oxygen atom and nearest Pt atoms in the surface. cDistance between hydrogen adatom and nearest Pt atoms in the surface. dResults taken from ref 27.

with bridge positions on all the surfaces considered with the H adatom binding different positions, i.e., bridge, top, or hollow sites, depending on the surface. Co-adsorption is found to be more favorable on the Pt(211) and Pt(100) surfaces, with energies of about −0.6 eV, followed by coadsorption on the Pt(110) and Pt(321) surfaces, with energies of ∼−0.4 eV. The interaction with the Pt(111) surface is significantly less favorable and a positive coadsorption energy with respect to water in the gas phase is calculated. The coadsorption energies calculated for the extended surfaces, with the exception of Pt(111), are not far from the values reported in a previous work for the interaction of the same species with Pt79 and Pt140 particles, respectively, −0.54 and −0.55 eV. 3.2. Water Dissociation. The energies of the initial, transition, and final states were employed to calculate the activation energies (E°act), rate constants (k), reaction energies (Ereact ° ), and free Gibbs energies (ΔG) associated with the reaction of water dissociation on platinum. All of these data are given in Table 3. Representations of the initial, transition, and

Figure 3. Variation of the water adsorption energy (Eads ° ) with (a) the water-surface bond length (d) or (b) with the coordination number (C.N.) of the surface atom.

Table 3. Calculated Parameters for Water Dissociation on Several Platinum Surfaces surface

The adsorption energies shown in Table 1 for planar, stepped, and kinked platinum surfaces are in qualitative agreement with the values reported by Á rnadóttir et al.42 for same surface types; i.e., adsorption is stronger on kinked and weaker on planar surfaces. Quantitative differences arise from the consideration of a different computational setup (ultrasoft pseudopotentials, different cutoff for the plane waves, and different Monkhorst−Pack k-points grid) and surface coverage in ref 42. These trends agree well with experimental observations where adsorption on steps was suggested to be more favorable than on terraces.43,45 However, the calculated energy for Pt(100) is close to data reported for the stepped Pt(221) and Pt(322) surfaces, −0.46 or −0.47 eV, respectively, whereas the adsorption energy calculated in this work for Pt(321) is more negative than the values reported for the kinked Pt(763) and Pt(854) surfaces, −0.51 or −0.55 eV, respectively, and also more negative than the values calculated for water on several Pt nanoparticles with energies between −0.47 and −0.59 eV.39,42 Calculated results for coadsorption of the water dissociation products (OH + H pair) on the platinum surfaces are given in Table 2. Views of the most favorable coadsorption configurations are provided in the rightmost panels of Figure 2. When coadsorbed with H adatoms, OH species interact preferably

Pt(111) Pt(100) Pt(110) Pt(211) Pt(321)

d

O···Ha/Å

Eact ° /eV

Ereact ° /eV

ΔGb/eV

1.78 1.92 1.68 1.62 1.63

0.78 0.63 0.44 0.66 0.63

0.66 −0.17 0.05 −0.20 0.26

1.07 0.24 0.46 0.21 0.68

kc/s−1 5.1 5.0 2.3 1.1 1.9

× × × × ×

103 105 107 105 105

a

Length in the TS structure of the bond being broken. bGibbs free energy at 463 K and 1 bar. cReaction rate constant at 463 K. dResults taken from ref 27.

final states for the most favorable reaction paths are shown in Figure 2. As can be seen, water dissociation is more favorable on the Pt(110) surface, with E°act equal to 0.44 eV. The highest barrier was computed for Pt(111), 0.78 eV with a rate constant 4 orders of magnitude smaller than that for reaction on Pt(110), whereas barriers of 0.63−0.66 eV with rate constants that are 2 orders of magnitude smaller than on Pt(110) were calculated for the remaining surfaces. Thus, the results suggest that not only the presence of low coordinated atoms on the platinum catalyst is important but the type of surface rippling plays also a decisive role. In previous works devoted to the study of the dissociation reactions of water on Cu and Ni,27 and of molecular oxygen on Ag,61 the (110) facet was found also to be more active than the (321) facet. Also, in the case of E



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dissociation reaction of NH2 to yield NH and H species,62 the activation energy barrier on the Pt(100) surface was found to be 50% higher than on Pt(111), which contrasts with the results calculated here for H2O where the barrier for the former surface is 20% smaller than when the reaction occurs on Pt(111). The calculated reaction energies show that the dissociation reaction is exothermic on Pt(100) and Pt(211) surfaces, slightly endothermic on Pt(110), and endothermic on Pt(321) and Pt(111). At the temperature of 463 K and pressure of 1 bar, estimated free Gibbs energies vary between 0.21 and 1.07 eV; i.e., the water dissociation reaction is slightly endothermic on Pt(211) and Pt(100), moderately endothermic on Pt(110) and Pt(321), and highly endothermic on Pt(111). Whether water will desorb or will dissociate on the catalyst surface can be ascertained from comparison of the activation energy barriers for water dissociation on the five platinum surfaces considered in this work (Table 3) and of the water adsorption energies on the corresponding surfaces (Table 1). The adsorption energy of the water molecule on Pt(111) and Pt(100) surfaces is clearly smaller, in absolute value, than the energy required for water dissociation and, hence, dissociation is not expected on these planar platinum surfaces without the presence of promoters, e.g., OH and O species, on the surface. Experimentally, water was found to adsorb, and then to desorb intact at T ∼ 160 K, on Pt(111).63,64 In the case of the stepped Pt(211) surface, the water adsorption and dissociation energies calculated at the step-edge are very similar to those reported for the planar Pt(100), and therefore, upon annealing it is expected that water will desorb intact without proceeding to OH + H species deposited on the surface. In the case of the Pt(211) surface, low coordinated atoms at the edges of the (111) terraces originate an increase of the adsorption energy, as suggested experimentally,45,63 and lead to a decrease of the barrier for the O−H bond cleavage in the water molecule but insufficient to contribute to the formation of the coadsorbed OH and H species. A different picture is found for the adsorption and dissociation of water on the Pt(321) and Pt(110) surfaces. In these cases, the absolute value of the adsorption energy is slightly larger, i.e., by 0.06 eV in both cases, than the energy required to dissociate the molecule into the OH and H fragments; hence, water dissociation is suggested to occur on platinum surfaces with these Miller indices. Importantly, the small energetic differences between the adsorption and dissociation energies on these surfaces is in agreement with results from experimental works where it has been concluded that the water dissociation and WGS reactions were suggested to be structure sensitive on platinum.36,63 Thus, dissociative chemisorption of water on platinum catalysts may occur at peculiar surface configurations containing low coordinated platinum atoms; note that most atoms in stepped surfaces are found at the (111) terraces and the fraction of low coordinated atoms is usually small. The results obtained for the Pt(111) and Pt(110) surfaces and the suggestions made above are in line with experimental observations for water adsorption on equivalent Ni surfaces. In the case of the Ni(111) surface, the water monolayer was found to desorb at T ∼ 165 K when heated, without evidence of water dissociation products remaining on the surface.63,65 Please note that the desorption temperature is similar to that reported for the Pt(111) surface, T ∼ 160 K,63,64 which suggests similar adsorbate−surface interactions on both metals. On Ni(110),

water monolayer remains almost intact on the surface up to temperatures of about 200 K and, above this value, dissociation is suggested to take place.63 Quite interesting, Benndorf and Madey,66 estimated an energy barrier for O−H bond breakage on clean Ni(110) of ∼0.5 eV which is also close to the barrier calculated in this work for dissociation on the Pt(110) surface. A considerable computational effort is needed to fully explore a single elementary catalytic step because model systems having many atoms and many electrons are usually taken into consideration. This is an important reason for the proposal of descriptors and BEP-like relationships for reactions at surfaces such as those introduced before for the dissociation of water on extended surfaces or nanoparticles.27,28,39 In these previous studies, the reaction energy and the adsorption energies for the reaction products or for an oxygen atom were considered as valid descriptors. Note that the d-band center is a valid descriptor for understanding the trends on different surface facets of the same element but does not hold when surfaces of different elements are compared,27 even if the facets are the same; therefore, this descriptor was not considered in the present work. The calculated data arising from the present work were combined with previous results and new representations are provided in Figure 4. Adsorption energies for an oxygen adatom on the most favorable locations on the different platinum surfaces are reported in Table 4. As can be seen, there is a linear correlation between the activation energy barriers calculated for the reaction of water dissociation and the three different descriptors. Data collected for the platinum surfaces fit well in the global trendline, but the quality of the correlation for the relationship between the energy barrier and the adsorption energy of an oxygen atom is less meaningful after the incorporation of the new data. Additionally, because the determination of the reaction energy requires the knowledge of the transition state structure connecting the initial and the final states, hence, of the reaction path, the adsorption energy of the reaction products arises as the most interesting descriptor. Analyses of the atomic charges in the water molecule along the reaction path shows no clear correlation with any of the calculated energetic quantities.67 In fact, either Bader68 or REPEAT69 total charges in the adsorbate are similar on the five platinum surfaces considered; Bader charges in the adsorbate are ∼0.1, ∼−0.25, and ∼−0.5 au for the initial (H2O), transition (HO···H), and final (HO + H) states, respectively, whereas REPEAT charges for the same states are ∼−0.25, ∼−0.5, and ∼−0.6 au, respectively. The REPEAT charges in the hydrogen atoms when far from the metallic surface are positive but become negative upon interaction with the surface, e.g. transition and final states.

4. CONCLUSIONS The reaction of water dissociation was studied on planar, stepped and kinked platinum surfaces by means of density functional theory together with the periodic slab approach. The (111) and (100) Miller indices were chosen as examples of flat surfaces, (110) and (211) were chosen as cases of stepped surfaces and the (321) index was chosen as a model of a kinked (and stepped) surface. As found experimentally, water adsorption is preferential at regions nearby the steps and adsorption energies are found to correlate well with the coordination numbers of the nearest neighbor Pt atoms in the surface. The adsorbate to surface distance is found to decrease with the decrease of the coordination of the Pt atoms on the F



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Table 4. Adsorption Energies and Distances for Adsorption of Atomic Oxygen on Several Platinum Surfaces sitea

Eads ° /eV

hollow fcc hollow hollow-3 hollow fcc hollow “a”

−4.36 −4.26 −4.26 −4.30 −4.46

surface Pt(111) Pt(100) Pt(110) Pt(211) Pt(321)

c

dO‑surfb/Å 2.04; 2.10; 2.00; 2.05; 2.03;

2.04; 2.10; 2.00; 2.05; 2.06;

2.04 2.01 2.13 2.07 2.08

a

For labeling of adsorption positions please refer to Figure 1. Distance between oxygen adatom and nearest Pt atoms in the surface. c Results taken from ref 27. b

Inspection of reaction paths for the water dissociation on these surfaces showed that the Pt(110) has the lowest activation energy barrier for the dissociation reaction whereas the highest one was calculated for Pt(111). The comparison of water dissociation energy barriers and water adsorption energies on the five platinum surfaces considered suggest that dissociation is possible on Pt(110) and Pt(321) surfaces whereas on the other surfaces, water will desorb intact upon annealing. However, due to the quite small difference between the activation and adsorption energies on Pt(110) and on Pt(321), water dissociation will only occur under special circumstances, which is in line with some experimental evidence. Finally, inclusion of the energetic results obtained in this work in the Brønsted−Evans−Polanyi relationships derived in a previous work confirms that the adsorption energy of the reaction products is a good descriptor for the water dissociation on metallic surfaces.

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AUTHOR INFORMATION

Corresponding Authors

́ e-mail, [email protected]; phone, +351 220 402 *J. L. C. Fajin: 552; fax, +351 220 402 659. *J. R. B. Gomes: e-mail, [email protected]; phone, +351 234 401 423; fax, +351 234 401 470. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Thanks are due to Fundaçaõ para a Ciência e Tecnologia (FCT), Lisbon, Portugal, and to FEDER for financial support to REQUIMTE (projects Pest-C/EQB/LA0006/2013 and NORTE-07-0124-FEDER-000067-NANOCHEMISTRY) and to CICECO (project Pest-C/CTM/LA0011/2013) and for Programa Investigador FCT. This work has been supported also by FCT through project PTDC/QUI-QUI/117439/2010 (FCOMP-01-0124-FEDER-020977) cofinanced by Programa COMPETE. J.L.C.F. acknowledges FCT for the grant SFRH/ BPD/64566/2009 cofinanced by the Fundo Social Europeu (FSE).

Figure 4. BEP relationships between the activation energy barriers ° ) for the reaction of water dissociation on metallic surfaces and the (Eact (a) reaction energy (E°react), (b) adsorption energy of coadsorbed HO ° ), and (c) adsorption energy of atomic and H species (Eads(OH+H) oxygen (E°ads(o)). Yellow and red diamonds (green and black text) refer to data from this work or from ref 27, respectively.

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Density Functional Theory Study of the Water Dissociation on Platinum

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