J. Phys. Chem. C 2009, 113, 16757–16765
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Mechanisms for H2 Reduction on the PdO{101} Surface and the Pd{100}-(5 × 5)R27°-O Surface Oxide M. Blanco-Rey,* D. J. Wales, and S. J. Jenkins Department of Chemistry, UniVersity of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K. ReceiVed: May 19, 2009; ReVised Manuscript ReceiVed: June 27, 2009
Surface hydroxylation is a major source of poisoning in Pd-based catalysts during CH4 combustion reactions. By means of density functional theory, the reactivity of the PdO{101} surface and the Pd{100}-(5 × 5)R27°-O surface oxide toward H2 is examined, as representative surfaces of the catalyst at different oxidation stages. We find that H2 can physisorb molecularly at undercoordinated Pd sites only on the former surface. On the latter surface, charge accumulation exerts a repulsive electrostatic force on H2. Water production on PdO{101} can eventually be achieved. The reaction barrier for this process is reduced from 1.94 to 0.69 eV in the presence of neighboring hydroxyls, which contribute to H-H bond destabilization. Introduction The chemistry of transition metal oxides (TMOs) has developed as a subject of increasing interest due to their potential use as catalysts in industrial processes. Surface science studies of these materials have the ability to describe, at a fundamental level and under a controlled environment (typically ultrahigh vacuum conditions and well ordered substrates), the underlying atomic mechanisms in a number of reactions.1 The growth of oxides on a metallic substrate is itself a complex process, still to be fully understood. Initial chemisorption of oxygen can induce reconstruction and facetting on the substrate, and this interaction can be strong enough to generate an epitaxial thin surface oxide. Characterization of the composition and the structure in these early stages of oxygen uptake is challenging. The surface oxide atomic structure can even differ from that of the corresponding bulk oxide.2,3 Further oxidation results eventually in the development of bulk oxide, which is expected to expose the face with lower surface energy. The transition metal (TM)-O bond character changes as oxygen is taken up due to increased mixing with oxygen p orbitals. While TMs such as Ru and Rh can accommodate up to 1 monolayer (ML) of on-surface oxygen, in Pd and Ag oxygen occupies subsurface positions at lower coverages of 0.75 and 0.33 ML respectively, values in good agreement with the coverage needed for oxide formation.2,4 We therefore anticipate that the evolution of surface properties as a function of oxygen coverage will have significant consequences for the surface reactivity of TM-based catalysts. CO combustion on Ru is a dramatic example of this phenomenon. While Ru{0001} is more inert than other late TMs (Pt, Pd, Rh) in UHV, CO2 production thrives in the presence of an O2-rich gas phase, where a RuO2{110} epitaxial film grows.5 Here, CO adsorbs at undercoordinated Ru sites and the reaction can follow a Mars van Krevelen mechanism, involving surface O atoms.6 The case of CH4 combustion on Pd is more subtle. Pd-based catalysts are commonly used in CH4 combustion at low temperatures, for reduction of CH4 emissions in diesel engines, and in the exploitation of natural gas as a combustible.7 Molecular beam experiments on Pd{110} show that adsorption * To whom correspondence should be addressed. E-mail: mb633@ cam.ac.uk.
sites for CH4 are blocked when oxygen is preadsorbed.8 However, it is commonly accepted that activity increases when a film of PdO is present9 (about four layers of oxide produce the same effects as the bulk oxide10). This behavior produces differences, both at the electronic and structural levels, between the chemisorbed oxygen and the oxide, which can be used diagnostically. Nevertheless, only a few surface studies of CH4 adsorption on PdO have been carried out, and there is still some controversy about the role of metallic Pd as a fundamental element to initiate the reaction. Some studies suggest that CH4 molecular adsorption at low-coordination Pd sites (O vacancies or metal nanoparticles) is needed11 as a precursor state prior to H extraction, while others suggest that CH4 can adsorb dissociatively in the absence of defects.12 However, the dissociation barrier for the latter process can be as high as 1.6 eV in defectfree surfaces because of strong electrostatic repulsion.13 The study of the interaction between hydrogen and TMOs is motivated by synthesis or dehydrogenation of alkanes, processes where hydrogen adsorption and diffusion are intermediate steps. In the CH4 case discussed above, hydroxylation of the surface is a major source of catalyst poisoning, and water desorption is indeed the reaction limiting step above 725 K.9 Dissociation of H2 on clean Pd{100} has been studied by quantum molecular dynamics.14 This is a nonactivated process, where the potential energy surface (PES) is barrierless at bridge sites15 and where vibrations play an important role.16 IR spectroscopy on PdO catalysts at high hydrogen coverage have suggested the existence of multiple adsorption sites and multicoordination for H, as well as H-bonds.17 Temperatureprogrammed desorption (TPD) and density functional theory (DFT) studies of H2O/PdO{101} report adsorption at the undercoordinated Pd site, which is further stabilized by formation of an OH-H2O complex at low coverage.18 In this paper, we focus our attention on the differences of reduction by hydrogen of the Pd{100}-(5 × 5)R27°-O surface oxide, hereafter denoted as the “5” structure, and the PdO{101} oxide surfaces. When in contact with gaseous O2, Pd{100} shows a rich phase diagram,19 as determined from surface X-ray diffraction at ambient pressures. Theoretical predictions, based on a combination of DFT and thermodynamic approaches,20 have succeeded in reproducing the basic qualitative features of such a diagram. The 5 structure is predicted
10.1021/jp904693t CCC: $40.75 2009 American Chemical Society Published on Web 08/31/2009
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Figure 1. Hard ball model of the 5 surface top view. The small balls correspond to O atoms, the large light balls are the Pd atoms in the oxide adlayer and the large dark balls are the Pd metal substrate. The unit cell and adsorption sites for H are also shown.
Blanco-Rey et al. RuO2{110} with a reported binding energy of ∼0.32 eV/H2 from both TPD and DFT,24 or 0.36 eV/H2 from DFT,25 where the H-H bond is stretched from 0.75 to 0.89 Å.24 H2 can adsorb dissociatively on RuO2{110} at surface bridging O-sites, O(b), forming either dihydride (i.e., waterlike) metastable species with adsorption energy 0.5624 or 0.68 eV/H2,25 or hydroxy is, with 0.89 eV/H, the latter being the most stable configuration.24,26,27 Water can bind at Ru(u) sites as well, and this is in fact the last step in the pathway for water production on the surface, achievable from diffusion of O, where H atoms are contributed by the gas phase or H2 at Ru(u).25,28 H-bonds contribute to stabilization throughout. This model explains why water can be formed even at low temperatures (200 K) upon H2 dosing on RuO2{110}. Dihydrogen chemisorption is a relatively unusual event.29 Interestingly, in Pd{210} dihydrogen coexists with adsorbed atomic H, even at nonsaturation regimes30 (H2 dissociates on the clean surface), with adsorption energies J0.20 eV. In these examples, the H2-TM bonding occurs through a donation/ backdonation mechanism, where the molecule is polarized. There is a bonding interaction between the TM dz2 and H2 σ orbitals, and weak antibonding between TM dxz,yz, and H2 σ*. The Pd(u) atom present in the PdO{101} and 5 surfaces is a promising candidate for observing dihydrogen chemisorption. In the present paper, these cases are studied by means of DFT, and reduction pathways are characterized that lead to water production. As H2 polarization is necessary to achieve nondissociative chemisorption, and given that H2 is a weakly polarizable molecule, we anticipate that minor differences in the electronic structure of the substrate will have major consequences for H2 adsorption. In fact, despite the 5O-2Pd-O trilayer and the PdO{101} outer layers having similar atomic structure, we find that dihydrogen chemisorbs only for the latter surface, where the charge distribution exerts an overall electrostatic attraction on the polarized molecule. The effect of previous hydroxylation of PdO{101} on the reduction mechanisms is also considered. Calculations
Figure 2. Ball-and-stick model for top and side views of the PdO{101} surface. The small(large) balls correspond to O(Pd) atoms. The (1 × 2) supercell used in the calculation and the adsorption sites are also shown.
to be stable at a wide range of O2 pressures (10-6-103 mbar) for T j 650 K. It consists of an O-2Pd-O trilayer of strained PdO{101} in registry with the Pd{100} substrate, with two Pd and two O atoms per unit cell, as shown in Figure 1.21 That model has been refined by introducing a lateral shift in the trilayer.22 This geometry has been verified by high-resolution core-level spectroscopy (HRCL), scanning tunneling microscopy (STM), quantitative low-energy electron diffraction (LEED) and DFT, so it can be taken as reliable. Above 10 mbar and T J 650 K, PdO is formed, although it is {100} that is the most stable face.23 The PdO{101} surface (see Figure 2) provides an interesting case study, since it possesses an undercoordinated Pd atom, Pd(u), which is expected to show the reactivity enhancement commonly associated with surface defects on PdO, or a similar behavior to the undercoordinated (4-fold) Ru atom, Ru(u), of RuO2{110}, despite the different crystallography of these oxides. Interestingly, H2 can weakly adsorb nondissociatively at Ru(u) sites on
The DFT calculations were performed within the generalized gradient approximation (GGA) for the exchange and correlation functional31 with the CASTEP program,32 which uses a plane wave basis set to describe the electronic wave functions and ultrasoft pseudopotentials to describe the ion cores.33 The reciprocal space was sampled using Monkhorst-Pack (MP) meshes,34 and the cutoff energy for the basis set was 340 eV.35 Details of the gas-phase molecules and Pd and PdO bulk calculations can be found in ref 13. Adsorption of hydrogen on PdO{101} was studied for the (1 × 2) supercell shown in Figure 2 containing three O-2Pd-O trilayers. The total energy was minimized allowing geometry relaxation in the topmost trilayer. In the 5 structure, one surface unit cell was used to model adsorption in a slab consisting of five Pd layers plus the O-2Pd-O trilayer. The trilayer and the topmost Pd substrate atoms were allowed to relax. In both cases, the vacuum region extended for about ∼14 Å and a 4 × 4 × 1 MP mesh was used. The convergence criteria for these geometry optimizations were 0.05 eV/Å for the forces and 0.001 Å for the atomic displacements. The linear synchronous and quadratic synchronous transit (LSTQST) method with conjugate gradient refinements36 was used to determine preliminary transition state (TS) candidates, which were converged to a tolerance of 0.1 eV/Å in the rootmean-square (rms) gradient, or smaller when possible. In most
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TABLE 1: Comparative Mulliken Charge Analysis of the PdO Atoms in the Oxide Surface and the Film in |e| Unitsa atom
PdO{101}
5
(h)
–0.47 –0.48 0.49 0.37
–0.45, –0.46 –0.45, –0.46 0.56, 0.54 0.16, 0.22
O O(l) Pd(c) Pd(u)
a Bulk charges are 0.5|e| for Pd and -0.5|e| for O. The 5 structure contains two Pd and two O inequivalent atoms per unit cell, ordered as i ) 1, 2 in the table.
Figure 3. Stability regions of PdO and the 5 surface oxide as a function of gas-phase composition, given by the chemical potentials of O2 and H2. The top and right axes show the partial pressures of those two gases at T ) 700 K.
cases, the residual forces in the resulting structures were still high, especially on the H atoms. Tighter convergence was then applied for the electronic calculations and accurately converged TSs were obtained using a gradient only formulation of hybrid eigenvector-following,37–39 reducing the rms gradient to 0.002 eV/Å. This method has previously been applied in surface science to the study of dehydrogenation processes on Pt surfaces.40–42 The Rayleigh-Ritz formulation of hybrid eigenvector-following involves a two-sided numerical derivative to estimate the curvature,37–39 which requires tight convergence of the electronic structure. A tolerance of 10-8 eV/atom for the energy convergence was found to provide reliable values for the unique negative Hessian eigenvalue and the correspinding eigenvector in the Rayleigh-Ritz phase. Substrates. PdO{101} can actually show three terminations. The one considered here, ending in an O-2Pd-O trilayer (see Figure 2), is the most stable one.23 The (1 × 1) surface unit cell contains two distinct O and two Pd atoms. The lower O atom (O(l)) is 4-fold coordinated, and the higher one (O(h)) is 3-fold. The surface Pd atoms are coplanar. One of them (Pd(c)) is fully (i.e., 4-fold) coordinated, while the other one (Pd(u)) is only 3-fold coordinated. In the 5 unit cell (see Figure 1) the homologous atoms are present in pairs. The atomic sites mentioned above have inequivalent counterparts that will be denoted by O(l),i, O(h),i, Pd(c),i, and Pd(u),i, with i ) 1, 2. The rigid lateral displacement in the trilayer proposed in ref 22 produces an additional stabilization of 0.07 eV. The final relaxed structure is in good agreement (positions of the relaxed atoms differ by j0.04 Å) with the theoretical geometry reported there. A Mulliken analysis of the charge (see Table 1) shows that Pd(u) in PdO{101}, where it is bound to three O atoms, gains electron density with respect to bulk PdO (it has a net charge of 0.37|e| instead of 0.50|e| in the bulk). This effect is more pronounced on the 5 surface, where both Pd(u),i (i ) 1, 2)
TABLE 2: Atomic H Adsorption Energies at Different Sites of the Surfaces under Study with Respect to an Isolated H Atoma atom
PdO{101}
5
(h)
–3.06(–3.26) –1.57(–1.72) –1.53(–1.60) –2.45(–2.45) –2.31(–2.32)
–3.04, –3.09 –2.87, –2.35 –1.63, –1.66 –1.98, –1.85 –2.58, –2.61
O O(l) Pd(c) Pd(u) Pd(b)
a Units are eV. The 5 structure contains two Pd and two O inequivalent atoms per unit cell, ordered as i ) 1, 2 in the table. In the oxide, spin polarization has also been considered. The corresponding values are shown in parentheses. For Pd(u) only, the spin polarization vanishes.
atoms gain electrons (ending up with positive charges of 0.16|e| and 0.22|e|, for i ) 1 and 2, respectively), while Pd (c),i atoms are weakly depleted of electrons. Here, Pd (u),i atoms are bound to two O atoms and to four and two Pd{100} surface atoms for i ) 1 and 2, respectively (see Figure 1). Most of the charge transfer is made through these Pd(trilayer)-Pd{100} bonds. Alternatively, the Mulliken charges can be compared to those of a “gas phase” O-2Pd-O trilayer. Thus, Pd(u) loses -0.07|e| upon adsorption on PdO{101}, but it gains -0.12|e| (-0.08|e|) when adsorbed at the Pd(u),1 (Pd(u),2) site on the 5 surface. A constrained thermodynamics analysis has been used to establish the pressure and temperature, (p, T), conditions for 5 and PdO stability in a combined atmosphere of O2 and H2, following the methodology of refs 20 and 43. The solid is considered to be in thermodynamic equilibrium with each of the gas phases, independently. The phase diagram is shown in Figure 3 as a function of the chemical potentials for O and H, where
1 ∆µO(p, T) ) (µO2(p, T) - EOtot2) 2
(1)
1 ∆µH(p, T) ) (µH2(p, T) - EHtot2) 2
(2)
EOtot2 and EHtot2 are the gas-phase total energies for O2 and H2, respectively. PdO (and its surfaces, the {101} face being among the most stable ones23) and the 5 structure are therefore achievable under certain ranges of (p, T). Figure 3 shows as an example the partial pressures compatible with stable phases at T ) 700 K, where µO2 and µH2 have been calculated from tabulated empirical thermodynamic values.44 The growth of the 521,22,45 and the PdO{101}18,46 surfaces is well established in UHV surface science experiments, although growth of PdO in a high pressure O2 atmosphere may result in noncrystalline structures.19,47 Atomic Hydrogen Adsorption. The atomic H adsorption energies for equivalent sites in the two structures are summarized in Table 2, with respect to a gas-phase atomic H, for both spin polarized and unpolarised calculations. Adsorption energies at Pd(c) and O(h) atoms are similar in the two structures, indicating that the chemistry of these high-lying atoms is relatively uncoupled from the metallic substrate in the 5 structure. H adsorption at the O(l) atom is more favorable on the 5 surface, since hydroxyl formation at that site involves a significant lattice distortion, which is less impeded in the surface oxide. The Pd(u) case deserves particular attention, and will be discussed below in more detail.
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On the 5 surface, H atoms at bridge positions Pd(b),i relax inward, ending up in a subsurface hollowlike position surrounded by three Pd atoms. However, on PdO{101} the corresponding bridge position remains stable. This result may indicate that subsurface diffusion into metallic Pd will not be hindered by the surface oxide. For the PdO{101} surface, the barrier found for H diffusion from Pd(u) to O(h) is 0.85 eV (1.66 eV is the reverse barrier) if calculated without spin polarization. The barrier corresponding to the spin polarized case is 0.83 eV, just 0.02 eV smaller (the reverse barrier is 1.44 eV). For the 5 surface, the barrier for a similar process takes a much smaller value of 0.26 eV (1.33 eV for the reverse barrier). PdO is a semiconductor with a small direct band gap of around 0.7-1.0 eV according to the experimental literature.48–50 The band gap opening in PdO is a consequence of the tetragonal crystal field splitting of the Pd 4d orbitals.51 Theoretically, within the local density approximation (LDA) for the exchange and correlation functional52,53 the reported band gap is 0.1 eV, or even no gap at all.51 The GGA calculations performed in the present work were also unable to reproduce the gap. Band gap underestimation in semiconductors is a well known limitation of local functionals. For the case of PtO, with similar semiconducting properties to PdO, GGA-based functionals also fail to yield a gap, but the use of more accurate hybrid functionals (i.e., containing contributions from the exact exchange) opens a 0.86 eV gap.54 However, this approach is too computationally expensive for large systems at the present time. The influence of different functionals on the adsorption and semiconducting properties of PdO surfaces remains an interesting open question.55 The shortcoming described above is related to the spin polarization observed in our calculations upon H adsorption in some of the cases of Table 2. For example, the spin polarized surface is favored by 0.20 eV compared to the spin-paired one when atomic H adsorbs at O(h) sites. Here, the spin density accumulates at surface-exposed Pd(u) atoms (a Mulliken spin population analysis gives 0.2 µB for this atom). However, adsorption of H at Pd(u) sites yields a nonmagnetic surface. Figure 4 shows the projected density of states (PDOS) on the Pd(u), O(h), and H atoms in these two configurations, in addition to the corresponding values for the clean PdO{101} surface for reference. The clean surface PDOS curves [panel (a)] have a clear dip at EF, which is a hint of the ideal gap location. Pd(u) has an unoccupied states peak at 0.4 eV above EF, which is shifted down in energy upon H adsorption. When adsorption occurs at Pd(u) sites [panel (b)], the H 1s orbital hybridizes with Pd(u) d levels homogeneously across the 4d band, and the PDOS is depleted of empty states immediately above EF. Upon adsorption at O(h), a large density of states is present at EF for the Pd(u) atom if the spin parity constraint is imposed, which might induce weak localized ferromagnetic behavior. In fact, on removing this constraint the narrow d-band peak splits and undergoes opposite shifts of ∼0.35 eV for spin up and down states, achieving an overall energy stabilization. Therefore, we consider this residual spin polarization to be a spurious consequence of the inability of GGA to account for the PdO semiconductivity properly. The spin effect on the TS for diffusion described above is, however, negligible. Molecular H2 Adsorption. Molecular H2 chemisorbs on PdO{101} at the Pd(u) site when the molecule lies parallel to the surface, with energy -0.48(-0.56) eV when parallel to the OX(OY) axis. The H-H bond length is elongated upon adsorption from 0.75 Å in the gas phase to 0.81(0.84) Å. The
Blanco-Rey et al.
Figure 4. PDOS on relevant atoms of clean PdO{101} surface (a) and atomic H adsorption (b-d). Atom labeling is shown in Figure 2. Units are number of states per eV. In panel (d), positive and negative PDOS correspond to spin up and down states, respectively.
distance from the molecule center of mass to the Pd(u) atom is 1.73(1.70) Å (see Figure 5(a) for the OX parallel orientation). Molecular adsorption at other sites and orientations has not been observed. For RuO2{110}, smaller adsorption energies of ∼0.3224 or 0.36 eV25 have been found, where the H2 bond is also stretched to 0.81 Å. Figure 6(a) shows the charge density difference (CDD)56 upon H2 adsorption, which represents the charge accumulation or depletion regions when the molecule-surface bond is created. The bonding results from the same donation/backdonation mechanism observed in H2/RuO2{110}.24 Further evidence is found in the projected density of states (PDOS) on the atoms involved in the bond, shown in Figure 7(a). The peak below the d band (∼8 eV in binding energy), strongly localized in the H2 and Pd(u) atoms, corresponds to the bonding interaction, while the antibonding peak is displaced ∼5 eV above the Fermi level. The main Pd(u) peak is displaced by about 1 eV to lower energies upon bond formation, increasing stability. H2 molecular adsorption is not stable, however, for the 5 structure (OX, OY, and upright molecule orientations, an O(h),1, Pd(l),1, Pd(u),1, and Pd(b),1 sites were studied). We examine the origin of the instability in the nonequilibrium structure of Figure 6(b), which shows the repulsive forces acting on the molecule when it is constrained to have an adsorption geometry equivalent to that of the oxide, i.e., when it is at the Pd(u),1 atop site with
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Figure 5. Side view of the H2 dissociation on PdO{101} and water formation sequence, for the molecule initially lying parallel to the OX axis. Binding energies are measured with respect to gas phase H2 and a clean PdO{101} substrate with no spin polarization. (a), (c), and (e) are stable configurations and (b) and (d) are the transition states between them. In (e), the waterlike moiety lies parallel to the surface plane, with the H atoms pointing in (OY directions. The relevant PDOS curves in steps (c), (d), and (e) are shown below.
comparable bondlengths and tilt angle, as shown in Figures 5(a) and 6(a). Interestingly, strong repulsive forces act on the neighboring O atoms as well, suggesting an electrostatic origin for this behavior, since these are the moieties with associated negative charge. The forces on the Pd atoms are almost negligible, even for those in the trilayer. The CDD [Figure 6(b)] and the PDOS [Figure 7(b)] features, however, resemble those of PdO{101}, suggesting that the nature of the bonding should be similar in both systems. Table 1 shows a Mulliken analysis of the charge distribution in the clean substrates. Pd(u) in PdO{101} has a positive charge of 0.37|e|, while Pd(u),1 in 5 has a value of only 0.16|e| prior to H2 adsorption. As shown in Table 3, these atoms have positive charges of 0.60|e| and 0.43|e| after H2 adsorption, respectively. In PdO{101}, the group of surface atoms O(l), O(h), Pd(c), and Pd(u) have a net positive charge of 0.11|e|. Thus, they exert an overall electrostatic attractive force on the polarized H2 molecule (which has a net negative charge of -0.17|e| at the adsorption site), stabilizing the bond to the Pd(u) atom. For the 5 surface, considering only O(l),1, O(h),1, Pd(c),1, and Pd(u),1 (which lie near to the H2), the net charge contribution is almost negligible. This effect causes the repulsive forces depicted in Figure 6(b), despite similar amounts of charge being transferred in the two systems. Therefore, it is not the surface structure, but the nature of the
metal-film bond that influences the surface reactivity by altering the electrostatic properties of the O-2Pd-O trilayer. In fact, the H2 σ* peak is removed from the PDOS above EF in the 5 model (see Figure 7), meaning that more back-donated electrons enter into that orbital when the H2 comes close to the surface, i.e., the back-donation mechanism enters in the repulsiVe regime at shorter overlapping distances in the 5 case, since the Pd(u),1 atom has more electrons from the start. In PdO{101} the overlap between Pd(u) d states and H2 {σ,σ*} orbitals is small. Bonding and antibonding states are concentrated at binding energies around -9 and 5 eV, respectively, as shown in Figure 7, and therefore, back-donation is weak. Pd has smaller s-d overlap and more d electrons than Ru, and it should therefore interact less strongly with H2.57 The above behavior can be extended to other nonpolar molecules. For example, our calculations show that CH4 can weakly adsorb at Pd(u) sites on PdO{101} but not on the 5 surface. In the former case, the most favorable adsorption energy, -0.20 eV per CH4 molecule, is found when two C-H bonds lie pointing toward the surface and are perpendicular to the OX axis. The distance to the substrate is d(C-Pd(u)) ) 2.45 Å. The PDOS curves (Figure 8) show that, as in the case of H2, backdonated electrons occupy the CH4 antibonding orbital if the molecule is brought close to the 5 surface.
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Figure 6. Side views of the charge density difference plot of H2 adsorption on surfaces (a) PdO{101} and (b) 5. The latter structure is unstable. The arrows indicate the direction of the forces on the atoms. These amount to ∼0.7 eV/Å on the H atoms, ∼0.6 eV/Å on O(l), and ∼1 eV/Å on O(h). Red (blue) shade indicates charge density accumulation (depletion) regions. The units are |e| Å-3.
Zero point motion (ZPM) contributions are typically nonnegligible when working with hydrogen adsorption and diffusion. An accurate ZPM correction to the total energy should include all the possible vibration modes in the system. When working with metallic substrates, it is usually acceptable to uncouple (and eventually neglect) surface phonons from the molecular modes. The stretching mode frequency in gas-phase H2 is 4370 cm-1 (the experimental value44 is 4404 cm-1). Upon adsorption on PdO{101} in the geometry shown in Figure 5(a), and freezing out the substrate, this mode is reduced to 3515 cm-1, so 1/2h∆νstretch ) -0.053 eV can be taken as a crude estimate of the error in the adsorption energy due to ZPM effects. Reduction in the number of degrees of translational and rotational freedom upon adsorption generates extra modes. It is not trivial to identify each of these modes with its gas-phase counterpart, since sometimes they do not have a pure vibrational, translational, or rotational nature. In the example above, extra modes appear at 469 and 871 cm-1 with a clear frustrated translational character. A 1500 cm-1 mode also exists with a strong rotational component (with axis parallel to OY), but with some degree of H-H bond stretching as well. Therefore, it should also be considered in the correction, yielding a value of 0.04 eV. We note that neglecting the motion of the relatively light oxygen atoms represents a further approximation. H2 Dissociative Adsorption and H2 Dissociation. On the PdO{101} surface, dissociation of the H2 adsorbed at the Pd(u) site is facile, resulting in atomic H adsorbed at contiguous O(h) and Pd(u) sites. The barrier from the reactant is only 0.28 eV.
Blanco-Rey et al.
Figure 7. PDOS of the relevant atoms involved in molecular adsorption of H2 on the surfaces (a) PdO{101} and (b) 5. The latter is a nonequilibrium configuration. The corresponding geometries are shown in Figure 6.
TABLE 3: Comparative Mulliken Charge Analysis of the PdO Atoms in the Oxide Surface (Clean and Partially Hydroxylated) and the 5 Film in |e| Units, for the H2 Adsorption Structures of Figure 6a atom
PdO{101}
H-PdO{101}
5
(h)
–0.49 –0.48 0.48 0.60 –0.17
–0.64 –0.46 0.49 0.53 –0.20
–0.47, –0.47 –0.45, –0.46 0.52, 0.53 0.43, 0.19 –0.20
O O(l) Pd(c) Pd(u) H2
The 5 structure contains two Pd and two O inequivalent atoms per unit cell, ordered as i ) 1, 2 in the table. a
The dissociative adsorption energy for this final state with respect to gas phase H2 is -1.41 eV. Figure 5(a-c) shows the corresponding pathway. In the equivalent structure on the 5 surface, i.e., forming H-Pd(u,1) and H-O(h,1) bonds, the dissociative adsorption energy takes a less stable -0.54 eV value. There is no molecular H2 precursor on the surface, so dissociation must happen directly from the gas phase, if at all. We explored H2 adsorption geometries in different orientations (parallel and perpendicular to the surface) at different sites on the 5 structure (Figure 1). However, we cannot locate a transition state for dissociation of H2 approaching the surface from the gas phase, since the configuration space is too large to be explored in practical terms. A molecular dynamics study might be better suited for investigating this effect, similar to those performed to describe the interaction of H2 and Pd{100}.14 Water Formation. In the PdO{101} surface, the O(h) site is found to stabilize a waterlike dihydride [Figure 5(e)] with
H2 Reduction of PdO Surfaces
Figure 8. PDOS for the CH4 molecule adsorbed at Pd(u) site in the surfaces (a) PdO{101} and (b) nonequilibrium 5.
adsorption energy -0.27 eV with respect to gas phase H2. This waterlike moiety lies parallel to the surface and the O(h)-Pd(c) bonds are broken. The H-O(h)-H bond lengths (0.99 Å) and angle (105°) are similar to those of gas-phase water. The water molecule could dissociate from the surface, leaving an O(h) vacancy behind, with an energy increase of only 0.08 eV. While dihydride produces major distortion in the PdO lattice bonds, it does not make O(b) atoms on RuO2{110} lose their bridging sites.24,25 There is a pathway for dihydride formation from recombination of H adatoms at O(h) and Pd(u) sites, shown in Figure 5(c-e). However, the barrier is quite large (1.94 eV), which makes this an unlikely route for water production on PdO{101}. In the 5 structure, no stable dihydride structures are found at the O(h) atoms, suggesting that water may desorb as soon as it is formed on the surface. The dissociative adsorption of H at a surface O(h)-Pd(u) pair is also less stable than in PdO{101}, and the H diffusion barriers are also smaller, as mentioned above. Considering that the waterlike species produces a dramatic distortion in the PdO{101} surface, and since the O-2Pd-O trilayer is not tightly bound to the Pd{100} substrate (except by the low-lying O(l) atoms), it may well be that the surface oxide is highly unstable in the presence of hydrogen. Hydroxyl-Assisted Water Production. For RuO2 molecular H2 adsorption can coexist with surface hydroxyls, even in the high hydrogen coverage (1 × 1) phase.24 The presence of hydroxyl increases back-donation into H2 σ* orbitals. This backdonation weakens the molecular bond and reduces the distance between Ru(u) and H2, but the change in adsorption energies is small.24 An alternative mechanism for water formation on PdO{101} can be found that requires the presence of a surface hydroxyl.
J. Phys. Chem. C, Vol. 113, No. 38, 2009 16763 This pathway is shown in Figure 9 for a (1 × 2) cell with one hydroxyl moiety per cell. On hydroxylated PdO{101}, the adsorption energy of H2 is reduced to -0.12 eV/H2 (measured with respect to gas phase H2 and the (1 × 2) hydroxylated substrate). The H-H distance is 0.80 Å, and the Pd(u)-H2 distance is 1.79 Å. Therefore, the bonding geometry is only slightly altered by the presence of the OH. The extra charge transfer into the molecule is also negligible, as shown in Table 3. The barrier to produce a waterlike species through this mechanism is 0.69 eV, considerably smaller than from recombination from H-O(h) and H-Pd(u). There is practically no reverse barrier, but desorption of water, leaving a H atom at the Pd(u) site and a O(h) vacancy behind, is energetically favored by 0.56 eV, and it is therefore expected to occur immediately after water formation on the surface. In a realistic combustion environment, the vacancy will be rapidly replenished in an O2 atmosphere or even by mass transport from the bulk.9 Creating an oxygen vacancy requires 2.37 eV in PdO{101}, similar to the 2.15 eV value found for PdO{100}.13 Both energies are in good agreement with ref 23 and lie well above the defect-free PdO energies in the stability regime -1.10 e ∆µO (eV) e 0. Hence, vacancy replenishment is exothermic. This waterformation mechanism can therefore be regarded as autocatalytic, since the H atom can diffuse from Pd(u) to O(h) by surmounting a relatively small barrier of 0.85 eV, leaving the Pd(u) site empty again for a H2 molecule to restart the process. The effect that the presence of a hydroxyl has in the surface reduction energetics can be understood in terms of the PDOS. Figures 5(c-e) and 9 show the PDOS corresponding to each step in both mechanisms. The hydroxyl peaks are localized in both cases. In the initial step of Figure 5(c), the states corresponding to the H-Pd(u) bond are spread in the d band. However, in the initial state of Figure 9(a) the H-states are localized and the H2 σ and σ* orbitals can be identified at the bottom of the d band and above EF, respectively. The presence of the hydroxyl enhances the hybridization between the σ* unoccupied levels and the Pd(u) d band, further destabilizing the H-H bond. The Pd(u) narrow peak shifts to higher energies with respect to the clean surface peak in the Figure 5 mechanism, while this shift is less noticeable in the Figure 9 when the OH is present, making the TS more accessible. The mobile H moiety PDOS remains strongly localized at the TS [Figure 9(b)], where the water levels are already identifiable. The O(h) does not seem to have an important influence in the mechanism, although in the final step the waterlike species is more strongly bound in Figure 9, where the water peaks have smaller binding energies. Conclusions Reduction of the oxide surface PdO{101} and the surface oxide Pd{100}-(5 × 5)R27°-O by H2 follows different mechanisms, despite both surfaces sharing the same O-2Pd-O trilayer termination. These processes are key to developing Pdbased catalysts, as surface hydroxylation is an intermediate step during CH4 combustion and a major source of their loss in catalytic activity. The PdO{101} surface chemistry is predominantly dictated by an undercoordinated Pd atom, Pd(u), where H2 can adsorb molecularly. This is not the case for the 5 surface oxide Pd(u) atoms, where electron density accumulates, enhancing electrostatic repulsion, whereas in PdO{101} the charge is distributed among additional subsurface Pd-O bonds. This behavior seems to be quite general, as it is also found for CH4. It may explain
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Figure 9. Side view of the H2 dissociation (when it approaches parallel to the OX axis) on PdO{101} and water formation sequence in the presence of a hydroxyl at O(h). Initial (a), transition (b), and final (c) states. Binding energies are measured with respect to gas phase H2 and a hydroxylated PdO{101} substrate. In (c), the waterlike moiety lies parallel to the surface plane, with the H atoms pointing in (OY directions. The relevant non-spin-polarized PDOS curves in each step are shown below.
why a sudden enhancement in CH4 conversion is found when a few oxide monolayers are grown on the surface of a Pd catalyst,10 since this physisorbed state may act as a precursor state for the Mars van Krevelen process. It also highlights the relevance of the presence of defects on the surface oxide for the CH4 dissociation to occur. Dissociation of the physisorbed H2 is facile on PdO{101}, but recombination with lattice oxygen to form water has a large energy barrier. This barrier is dramatically reduced from 1.94 to 0.69 eV in the presence of neighboring surface hydroxyl moieties, suggesting that saturation of surface oxygen atoms must happen for the catalyst to be successfully reduced. This mechanism can be regarded as autocatalytic as long as oxygen vacancy replenishment is guaranteed. Surface waterlike species are not stable on the 5 surface oxide, suggesting that oxygen removal (by water formation) may be barrierless there.
DFT studies of adsorption on narrow band gap oxides is challenging from the methodological point of view, as the exchange and correlation functional description plays a key role in accounting for semiconducting properties. PdO surfaces appear as promising candidates to explore the performance of hybrid functionals and their consequences for the adsorption energetics. We have shown here that the use of GGA functionals leads to spurious spin polarization of the surfaces. Acknowledgment. M.B.R. and S.J.J. gratefully acknowledge financial support from MICINN (Spain) and The Royal Society (UK), respectively, and the Cambridge High Performance Computing Cluster Darwin for computing time. References and Notes (1) Henrich, V. E.; Cox, P. A. The surface science of metal oxides; Cambridge University Press: Cambridge, 1994.
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