Article pubs.acs.org/JPCC
Hydrogen Spillover to Nonreducible Supports R. Prins,*,† V. K. Palfi,‡ and M. Reiher*,‡ †
Institute for Chemical and Bio-Engineering and ‡Laboratory for Physical Chemistry, Eidgenössische Technische Hochschule, Zurich, Switzerland ABSTRACT: Density functional theory (DFT) calculations show that the interaction between a hydrogen atom and the surface of MgO is weak, the interaction between a hydrogen atom and the surface of SiO2 is repulsive, and the migration of H atoms from a metal particle to a nonreducible support is endothermic. As a consequence, transition-state theory estimates that the migration of a hydrogen atom from a metal particle to the surface of a nonreducible support is too slow to explain the observed hydrogenation of molecules adsorbed on the support by spillover of H atoms. On the other hand, H atoms bind strongly to the surface of WO3, because the H atoms decompose into electrons, which reduce the W6+ cations, and protons, which bind strongly to the oxygen anions. Hydrogen spillover to defect-free surfaces of nonreducible metal oxides cannot take place, but spillover to defects is possible, as is spillover from a metal particle to an oxygen-containing group on a carbon surface. Defects and carbonaceous deposits may therefore be responsible for observations that have been ascribed to hydrogen spillover.
1. INTRODUCTION WO3 can be reduced by H2 to WO3−x when it is brought into contact with a Pt catalyst.1 The reduction is caused by hydrogen atoms, which form by dissociative chemisorption of H2 molecules on the Pt particles, migrate from the Pt surface, and reduce the WO3 particles. The migration of the H atoms from Pt to metal oxide or support has been called spillover,2 which is more generally defined as “the transport of an active species, adsorbed or formed on a surface, to another surface, which does not adsorb or form this active species under the same conditions”.3 For hydrogen, it means that H atoms are produced by dissociative chemisorption of H2 molecules on the surface of a metal, metal oxide, or metal sulfide. The H atoms spill over from the catalyst particles to a metal oxide or support, where they are not usually produced, and react with the metal oxide or with molecules adsorbed on the support. Reverse spillover of hydrogen is also possible. It is defined as the migration of adsorbed H atoms from a receptor to a source where they recombine and desorb as molecules. It has, for instance, been invoked to explain an increase in the rate of dehydrogenation of alcohols when a metal was added to a support.3 After decomposition of the molecule on the support and diffusion of H atoms to the metal particles (reverse spillover), the metal allows the hydrogen atoms to combine to H2 and desorb. Thus the catalytic cycle is sustained. Many experimental results in several branches of chemistry and materials science have been ascribed to hydrogen spillover. For instance, when a supported metal catalyst is mechanically diluted with support, the specific activity of the catalyst in the hydrogenation of aromatics increases severalfold.4 Also, when a pure support such as Al2O3 or SiO2 is treated with H2 for a number of hours at elevated temperature by means of indirect contact with a metal-on-support catalyst, it can hydrogenate alkenes and aromatic molecules.3,5,6 Also, the reduction of © 2012 American Chemical Society
sensitivity of metal-supported catalysts to sulfur has been ascribed to spillover. Sulfur-containing molecules inhibit aromatic molecules from adsorption on metal particles, but hydrogen may still adsorb and be dissociated into H atoms. The resulting H atoms may then spill over to the support and hydrogenate the aromatic molecules that are adsorbed on the acid sites.7,8 A future use of spillover might be hydrogen storage for fuel cell applications. Storage of hydrogen atoms by chemisorption instead of storage of H2 molecules by physisorption would enable storage under ambient conditions.9 Metal particles could then be used as portholes for the formation of hydrogen atoms and hydrogen spillover, thus spreading the hydrogen atoms over the carbon surface and increasing storage capacity.10 Clear evidence of the spillover of H atoms from metals to reducible metal-oxide supports, such as WO3 and n-type semiconductors,11,12 inspired many scientists to investigate the analogous spillover from a metal to a nonreducible support such as alumina, silica, silica−alumina, magnesia, and zeolites, and many experimental results were explained by spillover.3,6 It has been proposed that hydrogen could migrate as atomic hydrogen atoms over the surface of the support or through the gas phase, as solvated protons, or as proton−electron pairs.13 Spillover of hydrogen atoms may occur when the difference in free enthalpy between the initial state, for example, H atoms on a metal surface and molecules on a support surface, and the final state, with hydrogenated molecules on the support surface, is negative. However, even a favorable free enthalpy difference does not automatically mean that spillover will occur. Before the H atoms reach the molecules that are to be hydrogenated, Received: December 20, 2011 Revised: June 20, 2012 Published: June 20, 2012 14274
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they must migrate over the support surface. H atoms bind strongly to most metal surfaces and, if they bind weakly to the support surface, spillover cannot occur. If, on the other hand, the binding of H atoms to the support is substantial, then the hydrogenation of the molecule at the support surface may become a problem. Therefore, we calculated the interaction energy between a hydrogen atom and the surfaces of MgO and SiO2. These oxides are taken as examples of nonreducible metal oxides that are used as supports in catalysis. MgO is a model for a more ionic metal oxide and SiO2 for a more covalent metal oxide. The density functional theory (DFT) calculations show that the binding energy of a hydrogen atom is small for MgO and even repulsive for SiO2, while bonding of an H atom to a metal atom of a Pt cluster situated on the support is strong. As a consequence, the change in free enthalpy is strongly positive when H atoms migrate to the surface of a nonreducible support. Because of this large activation energy, transition-state theory predicts a spillover rate that is much lower than observed for reactions that have been ascribed to spillover.3,6 Spillover from a metal particle to a defect-free nonreducible support cannot explain phenomena that have been ascribed in literature to spillover. To explain those experimental phenomena, we analyze the role of defects and carbonaceous deposits on the surface of the support and conclude that carbonaceous deposits may explain some of these phenomena.
2. COMPUTATIONAL METHODOLOGY Quantum chemical calculations of the interaction between a hydrogen atom and the surface of MgO, SiO2, and WO3 were carried out with the Turbomole 5.10 package,14 employing the BP8615,16 and PBE17 density functionals and a SVP basis set.18 To evaluate the results calculated with the SVP basis set, a locally dense basis set was also employed, in which a TZVPP basis set19 was placed on the approaching hydrogen atom and on the oxygen atom to which it binds. This mixed basis set is denoted SVP/TZVPP[H,O] in the following. A comparison of BP86/SVP and PBE/SVP single-point energy calculations showed that the dependence of the binding energy between a hydrogen atom and the surface of metal oxides on the functional is up to 15 kJ/mol. Such deviations should be taken into account when chemical conclusions are drawn based on the BP86 data, as it is an indication for the method-inherent error that stems from the approximate nature of the density functional chosen. Model structures of the metal oxides were cut out from infinite periodic crystal structures taken from the Crystallography Open Database.20 The MgO model cluster consists of an equal number of Mg and O atoms with an alternating number of 12 and 13 Mg and O atoms, respectively, from layer to layer in the four-layer model cluster (Figure 1). The SiO2 and WO3 structures contained three layers of Si and W atoms, respectively, with connecting oxygen atoms. Atoms at the borders of the model structures were saturated with hydrogen atoms (Figure 1). While we have frozen most of the atoms of each cluster structure in order to preserve the bulk structure of our model, we allowed all hydrogen atoms (the absorbed ones as well as those used to saturate free oxygen valences) to relax in a structure optimization. Additionally, selected atoms were allowed to relax in order to model the structural response of the surface. One should note that structural relaxation has a pronounced effect on the energetics obtained, as will be discussed under Results.
Figure 1. Model clusters considered as supports in the quantum chemical calculations. (Top) MgO. (Middle) SiO2 with monovalent oxygen atoms saturated by hydrogen atoms. (Bottom) WO3 with monovalent oxygen atoms saturated by hydrogen atoms.
We determined reaction energies for all model structures at 0 K without vibrational corrections. Temperature and entropy effects can hardly be modeled properly in our approach and are qualitatively discussed, which turns out to be sufficient for our purposes. Atomic partial charges were calculated with Turbomole, based on results of Mulliken population analysis. We also investigated an empirical dispersion correction as applied by Grimme,21 but such interactions turned out to be negligible and have therefore not been considered any further. In a second set of cluster models, we placed a five-atom Pt cluster on the MgO and WO3 surfaces and let a H2 molecule interact with the Pt5 cluster (Figure 2). The H2 molecule was split into two H atoms on the Pt cluster, giving 2H-Pt5/MgO and 2H-Pt5/WO3, and finally the H atoms were positioned on the MgO and WO3 supports, giving Pt5/MgO-2H and Pt5/ WO3-2H, respectively.
3. RESULTS In agreement with ab initio MO calculations by Karna and coworkers,22,23 we found that the approach of the hydrogen atom to the SiO2 structure is repulsive. Two series of calculations 14275
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Figure 2. Two hydrogen atoms adsorbed to the Pt5 subclusters (left) and to one of the surfaces of the MgO (top) and WO3 (bottom) support clusters. For better visibility, the two hydrogen atoms are marked by circles and arrows.
allowed relaxation of the two oxygen atoms to which the two hydrogen atoms bind, each H atom had a binding energy of {−115} or −97 kJ/mol with the MgO cluster and the binding energy of H2 decreased to {+222} or +258 kJ/mol. When the next-neighbor Mg cations were also allowed to relax (Figure 3), each H atom had a binding energy of −176 kJ/mol and the BP86/SVP/TZVPP[H,O] binding energy of H2 as two separate oxygen-bonded hydrogen atoms became +99 kJ/mol (Table 2). Hence, we conclude that the binding energy of a single H atom to our MgO cluster is on the order of −176 kJ/mol (Tables 1 and 2), indicating a covalent interaction with an oxygen atom of the (locally relaxed) MgO cluster. Note that the energetical reference is then a single H atom at infinite distance to the MgO model cluster and thus does not include the energy required for breaking the H−H bond of H2. In the second series of calculations, we attached five Pt atoms in a pyramidal structure to one MgO surface of the original MgO cluster and partially relaxed the Pt5 cluster. In order to avoid energetical artifacts (due to the small size of this cluster) upon adsorption energies, we then kept this Pt5 cluster structure fixed in all calculations. Hence, for the adsorption of two hydrogen atoms at this Pt5 cluster, relaxation of the Pt atoms to which they bind was not considered (apart from a test calculation, which demonstrated that the energy gain upon relaxation is less than about 5 kJ/mol). The BP86/SVP adsorption energy for molecular hydrogen to the Pt5 cluster that is situated on the MgO support cluster was {−89} kJ/mol, which comprises the energy for splitting the H−H bond and the adsorption energy of the two H atoms at Pt5 (Figure 4). Movement of the two H atoms from the Pt5 cluster to the face of the MgO support opposite the one on which the Pt5 cluster was situated increased the total energy by {+425} kJ/
were performed for MgO. First we calculated the binding energies of H atoms to the MgO cluster with the BP86/SVP and BP86/SVP/TZVPP[H,O] combinations of density functional and basis set, where the latter uses a locally dense basis set with the large TZVPP basis for the adsorbed H atoms as well as for the O atoms that bind these two H atoms. Energies obtained with BP86/SVP will be indicated in braces {}, and those obtained with BP86/SVP/TZVPP[H,O] without enclosures. The energies obtained for the model structures MgO + 2H [bound on the (100) surface of MgO, cf. Figure 1] and MgO + H2 (gas phase) showed that the adsorption of H2 as two H atoms at the MgO surface was endothermic by {+318} or +292 kJ/mol if the hydrogen atoms were bound to the surface without the surface being allowed to relax. The endothermicity was lowest when the hydrogen atoms were above oxygen atoms. With a calculated (electronic) dissociation energy (at 0 K) of H2 of {458} or 452 kJ/mol (the experimental value is 436 kJ/mol), this means that each H atom has a binding energy of {−70} or −80 kJ/mol at the MgO cluster (Table 1). Surface relaxation increased the binding energy of each H atom to the surface and decreased the (endothermic) adsorption energy of H2. For instance, when we Table 1. Binding of an H Atom to the Surface of MgO and WO3a
a
relaxed neighboring atoms
MgO-H (kJ/mol)
WO3-H (kJ/mol)
O O, M
−80 −97 −176
−136 −223 −254
Calculated with BP86/SVP/TZVPP[H,O]. 14276
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the position of the H atoms, which is a consequence of our cluster model that is too limited in size to resolve such deviations. By inspection of the resulting cluster structures, it became evident that the two H atoms were distorted toward the Pt5 cluster, which indicates an attractive interaction (cf. Figure 2 top right). Hence, we may regard the adsorption to the face opposite the Pt5 cluster as a more suitable reference for modeling spillover to the support, and 381 kJ/mol should be taken when partial relaxation of the surface upon hydrogen binding is allowed to occur. In the following, we will therefore use 381 kJ/mol as an endothermic reference value for the energy difference between 2H-Pt5/MgO, with two H atoms on the Pt5 cluster, and Pt5/MgO-2H, with two H atoms on the MgO support. As a result, the energy of Pt5/MgO-2H is −89 + 381 = {+292} kJ/mol higher than that of Pt5/MgO + H2 (gas phase) (Figure 4). Note that we can recalculate the binding energy of a single H atom to a single (relaxed) oxygen atom of the Pt5/MgO model cluster and obtain (−458 + 292)/2 = {−83} kJ/mol, which is to be compared with the −97 kJ/mol obtained above for the metal-free MgO cluster. The difference of 14 kJ/mol indicates the limitations of our model cluster. The binding energy depends slightly on the oxygen atom to which H is bound and on the optimized model-cluster structure. In the case of Pt5/ MgO, two H atoms are always bound, so that only one of them could be bound to the central O atom of the MgO surface. These small deviations should be taken into account in the qualitative discussion of our results because they slightly depend on the cluster models studied; that is, if we adsorb two hydrogen atoms to any of the oxide surfaces, one of the hydrogen atoms will be close to the cluster boundary, and structural relaxation of the neighboring atoms can have an effect on the energies on the order of 10−15 kJ/mol. However, our adsorption studies with two hydrogen atoms have the advantage that we can rely on closed-shell restricted Kohn− Sham calculations, while we had to adopt a doublet state in all models when we studied the binding of a single hydrogen atom to the center of an oxide surface. In the case of WO3, each H atom had a BP86/SVP/ TZVPP[H,O] binding energy of −136 kJ/mol with the WO3 cluster (Table 1), and the adsorption energy for molecular hydrogen to our WO3 model cluster was endothermic by +180 kJ/mol when the cluster structure was kept fixed. The resulting H atoms were bonded to two bridging oxygen atoms of the WO3 cluster. If the hydrogen-accepting oxygen bridges were allowed to structurally relax, each H atom had a binding energy of −223 kJ/mol with the WO3 cluster and the binding energy of H2 became +6 kJ/mol. When the next-neighbor W cations were also allowed to relax, each H atom had a binding energy of −254 kJ/mol (Table 1) and the binding energy of H2 was −56 kJ/mol (Table 2). Hence, by contrast to the nonreducible support MgO, binding of H2 as two H atoms bound to WO3 is now exothermic by at least −56 kJ/mol (at 0 K and without inclusion of temperature and entropic effects). We have also considered additional relaxation by allowing more atoms in our cluster model to relax (in such a way that the relaxed structure is still embedded in the rigid framework given by the bulk and modeled by frozen positions of Mg and O atoms, or W and O atoms, at the cluster boundaries). This additional relaxation decreased the endothermic binding energy of H2 to the MgO cluster from +99 to +86 kJ/mol and the exothermic binding energy of H2 to the WO3 cluster from −56 to −62 kJ/mol. These changes are small and can be neglected
Figure 3. Superimposed structures of a single H atom at a rigid MgO cluster and of an H atom at MgO after structural relaxation of the neighboring atoms. Arrows point to slightly displaced Mg atoms, while the circle highlights the O−H moiety that is pushed out of the blue plane. Although the structural changes are not very large, their contribution to the exothermic binding energy of the H atom is up to −100 kJ/mol (see text for further discussion).
Table 2. Binding of an H Atom to the Relaxed Surfaces of MgO and WO3 and to the Surface of Pt5 Clusters on These Supportsa binding (kJ/mol) H + MgO → MgO-H H + WO3 → WO3-H H2 + MgO → MgO-2H H2 + WO3 → WO3-2H H2 + Pt5/MgO → 2H-Pt5/MgO H2 + Pt5/WO3 → 2H-Pt5/WO3 a
−176 −254 +99 −56 −84 to −114 −83
Calculated with BP86/SVP/TZVPP[H,O].
Figure 4. Energy scheme for exothermic chemisorption of H2 into two H atoms on a Pt5 cluster supported on MgO (2H-Pt5/MgO) and for endothermic chemisorption on the MgO support itself (Pt5/MgO2H).
mol when the MgO surface was not allowed to relax and by {+381} kJ/mol when the two O atoms to which the two H atoms bind were allowed to relax. However, movement of the two H atoms to the same face of the MgO cluster increased the total energy by only { +316} kJ/mol and to {+258} kJ/mol for two O atoms relaxed. This indicates that the energy depends on 14277
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between the H atoms reduces the binding energy of H2 on Pt and thus facilitates hydrogen spillover from the Pt particle to WO3. To investigate this coverage effect at our model cluster, we conducted two exploratory calculations. Since the two Pt5 clusters on MgO and WO3 turned out to behave similarly in terms of binding energies (see above), we selected the Pt5 pyramidlike cluster on MgO for these exploratory calculations, as it allows us to more easily cover the whole pyramid with hydrogen atoms. We covered the Pt5 cluster with more than 20 hydrogen atoms, most of which desorbed as H2 molecules upon structure optimization. With the locally dense basis set, we found bridging hydrogen atoms between the Pt atom at the top of the pyramid and the four Pt atoms at the basis of the pyramid. In addition, single hydrogen atoms were bonded to individual Pt atoms (two at the Pt pyramid top). In BP86/ SVP/TZVPP[H,O] single-point calculations, we obtained binding energies of the H atoms from −57 kJ/mol for the singly bonded H atoms to −42 kJ/mol for the bridging hydrogen atoms (Table 2). Hence, we may assume a reduction in binding energy by about 25% when the Pt particle is saturated by hydrogen atoms. This is in accordance with the reduction in experimental adsorption enthalpy at a Pt surface from 75 to 50 kJ/mol.24 Population analysis of the H attachment to the WO3 cluster showed that the charge of the approaching H atom is +0.31 e. This charge is donated to the oxygen atom to which it is bound as well as to the two W atoms to which the O atom is bound. However, accompanying structural changes make a specific charge assignment difficult as the W−O(H) distance increases by about 20 pm upon H binding to O if this O atom and the two W atoms binding this O atom are allowed to relax. As a consequence, the distance of the two W atoms to the oxygen atoms in trans position to the H-binding O atom is shortened by about 5−9 pm (as no point-group symmetry is imposed in the calculations). In the case of the MgO cluster, the attached hydrogen atom has only a charge of +0.13 e and thus a smaller charge fraction is transferred onto the crystal model in case of MgO when compared to WO3, as one would expect for a nonreducible support.
in light of the approximations made in terms of methodology and model cluster setup (see section on Computational Methodology). Further BP86 calculations with the locally dense SVP/ TZVPP[H,O] basis set revealed that binding of two H atoms to two Pt atoms of a structurally relaxed Pt5 cluster adsorbed on the WO3 model structure is energetically favored by −83 kJ/ mol if Pt5/WO3 + H2 is chosen as energy reference (i.e., the H−H bond energy needed to be overcome) (Figure 5).
Figure 5. Energy scheme for the exothermic chemisorption of H2 into two H atoms on a Pt5 cluster supported on WO3 (2H-Pt5/WO3) and on the WO3 support itself (Pt5/WO3-2H).
Although the structure of our two Pt5 model clusters turned out to be different when optimized on the MgO and WO3 support clusters, their energy difference was only about 10 kJ/mol when taken isolated, that is, when compared without the support clusters. Also their reactivity with respect to H2 was similar (−83 versus {−84} or −114 kJ/mol) (Table 2), which we confirmed by calculating the reaction energy of H2 with both isolated Pt5 cluster structures. When taking the same cluster structure and binding two hydrogen atoms to oxygen atoms on the side of the WO3 that is opposite the side that binds the Pt5 cluster (which we proved to be a good model for binding of H atoms on a support in our small model cluster, that is, the Pt cluster is sufficiently far away from the binding site so that no electronic effects are exerted on the hydrogen-accepting oxygen atoms), we obtained a binding energy for H2 of −28 kJ/mol. This means that if the two hydrogen atoms are transferred from the Pt cluster to the support in this model (i.e., the H−H bond does not need to be broken as it has already been upon adsorption to the Pt cluster), the energy for this spillover of two H atoms is 83 − 28 = +55 kJ/mol and thus endothermic (Figure 5). Note that in these calculations we have considered structural relaxation of the hydrogen-accepting oxygen atoms as well as of the four tungsten atoms that bind these two oxygen atoms, while all other atoms have been kept fixed. While the endothermic electronic energy change for spillover of +55 kJ/mol calculated at zero temperature can be regarded a good estimate for the enthalpy, the process is associated with an entropy change that can compensate for this comparatively small endothermic energy such that the overall free energy becomes negative in accordance with experiment.1 We can thus estimate an entropy change of up to 185 J/(mol·K) at room temperature. When an uncertainty of at least 10 kJ/mol is considered with respect to the model-cluster setup and the approximate nature of the density functional chosen, this increase in entropy could be as little as 151 J/(mol·K) at room temperature in order to produce an overall negative Gibbs free energy change for spillover on WO3. Furthermore, it should be realized that at increasing hydrogen pressure the surface of the Pt particles becomes extensively covered by H atoms. The resulting repulsion
4. DISCUSSION 4.1. Interaction of an H Atom with a Metal Oxide Surface. We shall first compare our results with those obtained by other groups. Our DFT calculations demonstrate that hydrogen atoms interact weakly with a rigid surface of MgO and are even repelled by the rigid surface of SiO2, both of which consist of atoms/ions with closed-shell electron configurations. When structural relaxation of the MgO surface is permitted, the binding energy is increased. Still, it is small compared to the binding energies obtained for WO3 [compare −80 and −136 kJ/mol for rigid MgO and WO3, respectively, and −176 and −254 kJ/mol for locally relaxed surfaces of MgO and WO3, respectively (Table 1)] and insufficient to stabilize H atoms relative to H2 molecules at the MgO surface. Two groups have reported exothermic binding energies of H atoms to the surface of MgO. Both groups applied effective core potentials, an approximation that we have omitted in our all-electron calculations. Bartczak and Stawowska25 performed DFT calculations employing BP86 as well as the hybrid functionals B3P86, B3LYP, and B3PW91 on a single slab Mg8O8 cluster with MgO (100) surface geometry embedded in a point-charge field to model the bulk and obtained a binding energy of −269 kJ/mol of the H atom and a fractional charge 14278
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oxygen atom decreases the energy of the final state, and this leads to low activation energy for the transition of the H atom from the metal atom to the support (Figure 6). For instance,
on the H atom of 0.36 e. This means that 0.64 electron transfer to the MgO cluster had taken place and might explain why the binding energy is approaching that of the OH− anion. It was, however, not explained how an H atom can reduce the MgO cluster. Considering this and the fact that −269 kJ/mol is significantly more exothermic than the −176 kJ/mol that we find, the question arises what the effect of the point-charge embedding of the very small, two-dimensional Mg8O8 cluster on the binding energy is. The small cluster could be overpolarized and artifical charge leakage could result. This issue would be important to resolve as Bartczak and Stawowska25 only considered a single layer of Mg and O atoms and did not take relaxation into account. Certainly, additional DFT calculations with much larger MgO model clusters embedded in a field of point charges would be necessary to resolve this discrepancy. DFT results were also reported by Wu et al.26 They performed DFT calculations of the interaction of an H atom with the MgO (100) surface with the Vienna ab initio simulation program (VASP), employing a different approximation for the exchange−correlation density functional, namely, the local-density approximation (LDA; the authors used the 1992 Perdew−Wang expression for the correlation energy). In contrast to the calculations discussed so far, these calculations were carried out with periodic boundary conditions. They used a six-layer structure with a 2 × 2 surface unit cell and allowed the atoms in the three upper layers to relax, while the atoms in the lower layers were fixed at the optimized bulk positions. The authors calculated a binding energy for an H atom on top of an oxygen atom of −26 kJ/mol relative to 0.5 H2. This energy is to be compared to the +49 kJ/ mol that we found. While our results point to an endothermic binding energy when compared to molecular hydrogen, Wu et al.26 found exothermic binding. However, it is well-known that LDA strongly overestimates binding. This has been observed not only for molecules and bulk structures but also for adsorbates (see, for example, ref 27 and references cited therein). The exothermic binding found by Wu et al.26 is therefore most likely an artifact of the LDA density functional. Hence, although a single hydrogen atom is exothermically bonded to the MgO surface, the hydrogen binding at MgO surfaces is endothermic when molecular hydrogen is the energy reference. The magnitude of the exothermicity for a single H atom is increased by structural relaxation of the surface, but it does not reach the binding energy of a hydrogen atom to a reducible support. Our results are in agreement with a calculation that showed that H2 molecules do not dissociate at a defect-free (100) MgO surface and that only defects can bind H2 exothermally,28 and with a molecular dynamics simulation of the interaction of H2 with a Pt/Al2O3 catalyst.29 In this simulation an H2 molecule split into H atoms on the Pt surface, but when the H atoms were allowed to migrate to the support they recombined to H2. This confirms that H atoms are not stable relative to H2 molecules on the defect-free surface of a nonreducible support. We want to emphasize the qualitative difference in binding behavior of H atoms toward reducible and nonreducible supports. Although H atoms do not bind to surface atoms with saturated bonds, protons have strong chemical bonds to oxygen atoms and anions.22 H atoms can become protons by donating an electron to the support if the support contains reducible cations. In that case the H atom becomes a proton and a metal cation is reduced. The interaction between the proton and the
Figure 6. Energy scheme for spillover of a H atom in the form of a proton−electron pair from a metal particle to a support with reducible cations. The qualitative energy scheme is based on observations discussed in the text.
H2 reduces WO3 that is in contact with metal particles by reaction of the H atom on the metal to form a proton and an electron in the conduction band of the metal.30 At the border of the metal particle and the reducible supports, the H+ cation protonates an oxygen anion and the electron reduces a metal cation of the support.31 Spillover may remain localized, around the metal particles where the hydrogen atoms are created, or it may spread completely over the reducible metal-oxide support surface and even into the bulk of the metal oxide. The spreading occurs by coupled motion of the proton and electron. The distance that the spilled-over H atoms can travel, and thus the extent of the reduction of the metal oxide, depends, of course, on the activation energy of the hydrogen transfer. For Pt/WO3, spillover leads to the formation of bulk HxWO3, and for Pt/TiO2, spillover leads to a hydrated surface with Ti3+ cations only around the Pt particles.32 Recent DFT calculations of the spillover of hydrogen in Pt/MoO3 (analogous to Pt/ WO3) confirmed that protons and electrons rather than H atoms migrate.33−35 As a consequence of spillover, molybdenum bronze HxMoO3 forms and the high mobility of protons in this material is due to hydrogen bonding. Charge analysis indicated that the electron of the H atom moved to a Mo atom next to the O−H bond and that the H atom became part of an OH group and thus became a proton. A hydrogen atom has in general a weak or repulsive H−O2− interaction with a negatively charged oxygen anion with saturated valences, but when it transforms into a proton and an electron on the MoO3, an attractive H+−O2− (proton−oxygen anion) interaction develops. A hydrogen atom has an ionization energy of 1312 kJ/mol, and the interaction between a proton and an oxygen anion is about 800 kJ/mol.22 The electron affinity of (the cations of) the metal oxide support thus has to be at least on the order of 500 kJ/mol (5 eV) for the H atom to transfer to the support, and this is only attained with reducible cations. In our DFT calculation we did not consider this energetically clearly feasible mechanism of the transformation of an H atom on a reducible support into a proton and an electron. Instead, we considered only the initial and the final states, from which we could conclude that the process is thermodynamically possible and that indeed charge must have been transferred from each bound H atom to the support. 4.2. H Spillover to a Nonreducible Support Is Very Slow. Spillover of hydrogen means that a hydrogen atom moves from the metal surface to the support surface and migrates over the surface until it finds a molecule that can be hydrogenated. The decrease in enthalpy between initial state, 14279
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with an H atom on the Pt particle, and final state, with the H atom attached to the molecule, is the thermodynamic driving force of spillover. But before an H atom has formed a hydrogenated molecule, three intermediate steps must be made. To reach the molecule that is to be hydrogenated, the H atom must first migrate from the metal particle onto the support surface. Second, the H atom must migrate over the support surface. Third, even if the hydrogenated molecule would have a lower enthalpy than the original molecule, activation energy has to be overcome for the hydrogenation reaction to occur. To the first point, the migration of the H atom from metal to support, we can say the following. H atoms are bonded strongly to the metal surface and only moderately to a support such as MgO or not at all to SiO2, as we have seen in the foregoing. This means that there is a substantial positive enthalpy difference between an H atom on a metal atom and an H atom on a support atom. On the other hand, the translational freedom of the H atom on the support constitutes a positive change in entropy and this would lower the difference in free enthalpy. Diffusion on the metal is limited by high H coverage at temperatures below about 500 K. If we assume that the H atoms are immobile on the metal surface and have twodimensional freedom on the support, the entropy change is about TΔS = 300 × 2/3 × 0.109 = 22 kJ/mol at 300 K [the entropy of an H atom with three degrees of translational freedom is 0.109 kJ/(mol·K) at 300 K].36 At higher temperature TΔS is much smaller, because even though T increases, ΔS decreases strongly due to the almost unrestricted diffusion of H atoms on the metal surface when the surface coverage becomes low. Thus, the change in enthalpy of (89 + 99)/2 = 94 kJ/mol is a minimum value for the activation energy for spillover of an H atom from a Pt atom to a neighboring O atom of MgO at the perimeter of the Pt particle. An estimate of the true activation energy can be obtained from the work of Chen et al.37 They calculated the activation energy for movement of an H atom from a Pt particle to the surface of graphene and found that the activation energy was 26 kJ/mol larger than the difference in energy between the initial and final state. Assuming a similar extra activation energy, we arrive at an activation energy of 120 kJ/mol for the movement of an H atom from a Pt particle to the surface of MgO. The second step that an H atom must do is to migrate over the support surface. The activation barrier (without quantum tunneling) for movement of an H atom between two neighboring O sites of MgO was calculated to be 36 kJ/ mol.26 However, this value was calculated with the LDA density functional, which is known to strongly underestimate activation energies.38 The true activation energy for this diffusion step will be substantially higher and may be of the same magnitude as the energy difference of 94 kJ/mol that we calculated for the migration of the H atom from the Pt particle to the MgO support. Because the activation energies of the movement of the H atom from Pt to support and of the movement over the support are similar, it suffices to concentrate on one movement, for instance, the first one from Pt to support. Transition-state theory can be used to estimate the rate of this movement. In analogy to the desorption of a molecule from a surface, we can write for the migration of a H atom from the metal surface r = ν exp(−ΔE/RT), with ν = e(kT/h)(q#/q),39 where q# is the partition function of the transition state without the reaction coordinate, q is the partition function of the initial state, and ΔE is the energy difference between initial state and
transition state. With the standard value for the pre-exponential frequency factor of e(kT/h)(q#/q) ≈ 1013, transition-state theory predicts a rate of 4 × 10−4 s−1 at 300 K for the migration of an H atom from a Pt atom to the support when ΔE = 94 kJ/ mol, the energy difference between the H atom on the Pt surface and on the MgO surface. Because the activation energy will be larger than the energy difference between initial and final state, the rate of hydrogen spillover will be even smaller than 4 × 10−4 s−1 at 300 K. If we would use a more realistic value for the barrier, that is, our estimate of 120 kJ/mol, we would obtain a rate of 10−8 s−1. Both rates are orders of magnitude less than the turnover frequency measured for the hydrogenation of ethene on a Pt catalyst at 300 K40 and for other reactions that have been ascribed to spillover.3,6 Transport of a hydrogen atom from a metal particle to the support surface will take place only when a bond of substantial energy forms between the H atom and the support. For nonreducible supports with a low or even repulsive interaction between an H atom and the surface, this means that spillover would have a high activation energy (Figure 7) and, thus, cannot explain phenomena ascribed to spillover.
Figure 7. Spillover of a D atom from a metal particle to the support followed by H−D exchange on the support. The qualitative energy scheme is based on observations discussed in the text.
Also from another perspective it can be concluded that spillover of H atoms does not play an important role in the hydrogenation of molecules on the support. Spillover is a sequential reaction of an H atom that migrates from the metal to the support, migrates over the support surface, and then hydrogenates a molecule on the support surface. It can be compared with the sequential reaction of a molecule on a catalyst with two catalytic functions, in which the first reaction is reversible and the second reaction is irreversible, A ↔ B → C. If the two reactions take place on sites that are separated in space on the catalyst surface, the intermediate molecule B must diffuse from the site on which it is created to the second site on which it reacts further to C. As extensively discussed by Weisz,41 when the A ↔ B equilibrium is strongly on the side of A, the conversion of A to C depends on the distance between the two types of catalytic sites. If the distance is very small, the irreversible reaction shifts the pre-equilibrium and all A molecules will react to C, even though at all times the concentration of B molecules is extremely low. On the other hand, if the distance is very large, then the conversion is determined by the A ↔ B equilibrium and is low. Also in spillover there is a pre-equilibrium, between an H atom on the metal and an H atom on the support, which is strongly on the side of the initial state. Then the H atom diffuses over the support surface until it encounters a reducible molecule A and reacts with it to HA. This gives the sequence HM ↔ HS → HA. Full hydrogenation can be obtained if the distance between the metal site and the reducible molecule is small. If the distance is 14280
dx.doi.org/10.1021/jp212274y | J. Phys. Chem. C 2012, 116, 14274−14283
The Journal of Physical Chemistry C
Article
large, then the conversion is limited by the equilibrium reaction HM ↔ HS and will be very low. For a reaction with equilibrium at the reactant side followed by an irreversible second reaction, the first change in free enthalpy is positive and the overall change is negative. From results published by Weisz,41 one can calculate that, for E(HS) − E(HM) > 120 kJ/mol, the distance between the HM ↔ HS site (metal particle) and HS → HA site (hydrogenation site) must be smaller than 2 nm, otherwise the conversion is very small. That means that molecules that are to be hydrogenated must be in close contact with the H atoms on the metal surface and that spillover of H atoms does not play a role in hydrogenation. Finally, the activation energy for the hydrogenation reaction between the H atom on the support surface and the molecule to be hydrogenated needs attention. On a metal catalyst surface, chemisorption splits H2 into H atoms and weakens bonds in the molecule, thus preparing the reactants for reaction. Bonding is much weaker on the support surface, however, and activation energies will be much higher. For the hydrogenation of an alkene this may be less of a problem, because in the gas phase both radical reactions of an H atom with ethene and an ethyl radical have very low activation energy (
