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J. Phys. Chem. C 2008, 112, 14010–14014
Coadsorption of Gold with Hydrogen or Potassium on TiO2(110) Surface Se´bastien Fernandez, Alexis Markovits, and Christian Minot* Laboratoire de Chimie The´orique, UniVersite´ Pierre & Marie Curie-Paris 6, CNRS, UMR7616, case 137, 4 place Jussieu, Paris, F-75252 Cedex France ReceiVed: January 24, 2008; ReVised Manuscript ReceiVed: July 2, 2008
We performed periodic DFT calculations to study the influence of H or K on the gold adsorption on TiO2(110) rutile at low coverage. On the clean surface, using a p(3 × 1) cell, we find that Au interacts preferentially with surface oxygen ions. The most favorable adsorption site is the 3-fold site with Au interacting with one bridging oxygen, O2c, and two tricoordinated oxygen, O3c followed by on-top site of O2c. Next, we have considered Au adsorption on the surface in presence of coadsorbed hydrogen or potassium using a p(3 × 1) and p(4 × 1) unit cell, respectively. The coadsorbed species adsorb as on the clean surface: H is on-top O2c and K bridges two O2c. On the contrary, on both surfaces, Au prefers to interact with surface titanium cation. This is the consequence of a formal change in the oxidation state of the two coadsorbed species. A redox process occurs between the two coadsorbed species that allows electron pairing: the supersystem is closed shell. Formally, Au-I adsorbs on the surface acidic site, Ti+IV, while H+ (or K+) does on the surface basic sites, O-II. This analysis is confirmed by the density of states. Our results confirm that K coadsorption should have a significant effect on the binding of Au particles on TiO2 surfaces and hence on the growth. Introduction Nondefective TiO2 crystal is semiconducting with a band gap of 3.05 eV. A gap is also found by calculations for regular slabs with a (110) orientation preserving the stoichiometry. The adsorption on the 110 surface is then controlled by the electron count.1 Lewis acid-base relationship between adsorbate and substrate is the rule for stoichiometric surfaces. Lewis bases bind to cationic active sites and Lewis acids to the anionic ones. Thus, when the adsorbate is characterized by electron pairs, the stoichiometric oxide remains in a low spin state upon adsorption. Electron transfer (redox mechanism) is necessary when adsorbates have unpaired electrons. This is the case when a radical is adsorbed since the number of electrons count is odd. It is also the case when the surface is reduced (hydrogenated or O defective) or oxidized. Then, an appropriate electron exchange between the adsorbate and the surface empties the levels in the energy gap in the supersystem. The adsorption of a radical may induce surface oxidation (radical acceptor as chlorine)2 or surface reduction (radical donor as potassium or hydrogen).3 This leads to an ideal count when the surface is already reduced (radical acceptor as chlorine) or oxidized (radical donor as potassium or hydrogen) and the adsorption energy is large. On a regular naked system, the electron count for a single radical adsorption is never ideal and results in a high spin system, the adsorption energy being small. For instance, the H adsorption leads to a reduction of a Ti4+. Coupling the adsorption of an acceptor radical with that of a donor one is very energetically favorable, leading to a low spin state. The adsorption of these two radicals indeed offers the possibility of avoiding levels in the gap by transferring an electron from one radical to the other. In other words, referring to adsorption of an AB molecule whose homolytic cleavage would generate the radicals A and B, the possibility of a * To whom correspondence should be addressed. E-mail: minot@lct. jussieu.fr. Tel.: +33(0)144272682. Fax: +33(0)144274117.
heterolytic cleavage is energetically better since it is leading only to electron pairs. The acceptor is bound as an anionic species on the surface cations while the donor binds as cation to the oxygen. Formally the redox transfer takes place between the two adsorbates. For MgO(100), since Mg2+ is not reducible, this situation occurs even for two equivalent radicals. This is, for instance, what happens when two hydrogen atoms are adsorbed.3,4 In this case, one electron is transferred from one H to the other, allowing the adsorption of one of them as a proton on the surface oxygen anion while the other adsorbs as a hydride on the surface metal cation. No new levels appear in the gap under adsorption: neither oxidation nor reduction occurs at the surface. For TiO2(110), it only appears when the electronegativity of the radicals is sufficiently different. Gold has an unpaired number of valence electrons and is a radical species. As a metal, it is generally thought of as an electron donor; however, its high electron affinity, 2.31 eV,5 exceptional among transition metals,6 makes it a possible electron acceptor. In the case of coadsorption with H or K, it appears to be more electronegative than the coadsorbate (compare 2.4 for Au with 2.1 and 0.8 for H and K according to the Pauling scale), and it should become a possible electron acceptor. In the case of Au + H or Au + K, we should therefore think in terms of Au- + H+ and Au- + K+. The question of the coadsorption of potassium and gold is important for heterogeneous catalysis. Alkali metals are wellknown promoters.7 A recent experimental work8 carried out with scanning tunneling microscopy and Auger-electron spectroscopy shows that K has a major effect on the morphology of gold nanoparticles deposited on the TiO2(110) rutile surface. Indeed, K decreases the average size of gold particles from 4.3 (clean TiO2(110) surface) to 2.5 nm at a temperature of 320 K, which is crucial for the catalytic activity. For a long time, gold has been thought to be inert. Gold’s electronic structure makes it “the least reactive metal towards atoms or molecules”.9 However, the inertia of gold has recently
10.1021/jp800708u CCC: $40.75 2008 American Chemical Society Published on Web 08/14/2008
Coadsorption of Au with H or K on TiO2 Surface
J. Phys. Chem. C, Vol. 112, No. 36, 2008 14011
been questioned: gold-catalyzed reactions have become a hot issue in both homogeneous and heterogeneous catalysis.10 One of the most striking examples of this catalytic activity is the CO oxidation.11-24 Although discovered almost 20 years ago,11 the reason for this activity remains unclear. For instance, the role of the support is still a matter of controversy. Does it only permit spread of the metal?19 Does the reducible nature of the support play a key role, the electronic effects between the metal oxide support and the adsorbed metal being determining?17 In this case, the efficiency of reducible supports may be attributed to defect sites.25 We are concerned in this study by the influence of an electron-donor coadsorbed species on the gold adsorption in terms of a redox process. For simplicity, we have considered the well-known (110) surface of rutile titanium dioxide, which allows understanding of the redox processes. Rutile (110) is the most stable and most reactive titanium dioxide surface. Several studies already dealt with the adsorption of a single gold atom on such a surface. These studies were achieved with different modeling (clusters26 or slabs 27-32) and density functional theory (DFT) functionals (B3LYP,26 PW91,28-30 RPBE 27,32). The two main conclusions of these papers are that the adsorption site for gold involves the interaction with the surface oxygen atoms and that gold is a poor electron donor, leading to small charge transfers. A controversy exists about the nature of the adsorption site. Our calculations without coadsorption confirm the previous conclusions and conclude a predominant binding to O atoms. We further proceeded to the coadsorption of gold with hydrogen or potassium and show that in both cases gold’s most favorable adsorption site is different from that on the naked surface. A similar behavior was explained by a steric effect between K and Au.31 According to our analysis, the coadsorption should be analyzed in the light of electronic structure. Spin disappears both on Au and on the coadsorbed species. Then, the interaction is explained by Au- and K+ adsorbed on the surface acidic site (metal) and basic site (oxygen), respectively. Computational Details We performed spin polarized calculations based on DFT in the generalized gradient approximation (using the PW91 exchange-correlation functional) as implemented in the VASP code,33-36 which uses a plane wave basis set (with a kinetic energy cutoff at 400 eV). The electron-ion interactions were described by projector augmented wave pseudopotentials.37 Relativistic effects were partially taken into account through the use of relativistic scalar pseudopotentials. The effect of spin-orbit coupling was ignored. The relaxation of the atomic positions in the supercell takes place until the subsequent energy steps are smaller than 0.001 eV. The calculations were performed sampling the Brillouin zone in a 5 × 5 × 1 Monkhorst-Pack set for the p(3 × 1) cell and a 5 × 3 × 1 grid was used for the p(4 × 1) cell. The densities of states were computed with the same grids. The energies were computed within the tetrahedron method with Blöchl correction. Monopole, dipole, and quadrupole corrections to the energy were included along the direction perpendicular to the slab. To model the TiO2(110) surface, we used a three TiO2 layers (nine atomic layers) slab, which we believe to be thick enough for our purpose. However, in order to get more accurate values,38 we have repeated some calculations using a five TiO2 layers (15 atomic layers) slab. The calculations being periodic in three dimensions, we imposed between two successive slabs a distance large enough (>9 Å) to prevent any noticeable interaction. The
Figure 1. Top view of TiO2(110) rutile surface. Five different adsorption modes are shown, labeled from A to E.
adsorbates and the first layer (the three first layers) of the 3 L-slab (5 L-slab) were relaxed whereas the two bottom layers were kept at bulk positions. The adsorption energy of gold was evaluated on a p(3 × 1) cell. We used the same cell to study the coadsorption with hydrogen. K being bigger, in order to avoid steric interactions with gold, we used a p(4 × 1) cell. Adsorption energies were calculated as follows:
Eads)Eref+Eatom-Esystem
(1)
Eref is the energy of the reference system. Eatom is the spin polarized energy of the isolated atom (Au, H, or K). Esystem is the total energy of the whole system (adsorbate/substrate). Thus, positive adsorption energy corresponds to an exothermic process. Results and Discussion Adsorption of Au on the Naked Surface. We considered different adsorption sites (Figure 1) and report the adsorption energies in Table 1. We found that the most favorable modes correspond to the interaction between Au and the surface
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Fernandez et al.
TABLE 1: Adsorption Energies (eV) of Au on TiO2(110)a A. on- B. bridging C. ontop O2c O2c top Ti5c this work 3 L this work 5 L ref28 5 L ref30 5 L
0.80 0.45 0.51 0.36
0.58 0.44 0.33 0.21
0.48 0.41 0.44 0.42
D. Four E. Three fold fold hollow hollow 0.36 0.37 0.34
0.79 0.59 0.60 0.55
a The labels are those from Figure 1. The number of TiO2 layers is indicated (3 L or 5 L).
Figure 3. Spin density maps of Au/TiO2(110). Au is adsorbed on-top a bridging oxygen atom, O2c. The vertical plane with protruding oxygen anions on the left, the second vertical plane with surface titanium cations on the right. The warmer the color, the larger the density. Thus, spin is delocalized over the gold atom and over the second vertical plane exhibiting surface titanium cations. Figure 2. Density of states of Au on-top a bridging TiO2 oxygen atom, O2c. Black lines are the total DOS, and the blue shapes are the projected DOS on gold. The vertical line represents the Fermi level. All energies are referred to the vacuum.
oxygen atoms: the on-top O2c site, labeled A (see labels in Figure 1) and the 3-fold hollow site E (between an O2c and two O3c). The B mode for which the Au atom is bridging two O atoms is less favorable. The two other modes, C-D, for which Au also interacts with the Ti atoms of the surface, are less energetically favorable. The calculation using 5 TiO2 layers also leads to E and A modes as most favorable ones, with a preference for E. The on-top O2c site, A, is often reported as the most stable one.26,27 Lopez et al.27 even reported large adsorption energy of 1.55 eV. Three other studies,28,30,39 using thicker slabs, mentioned the 3-fold hollow site, E, to be the most stable and a third one31 conclude to site B rather than E and A (the energy difference between B, E, and A is smaller than 0.3 eV while it is 0.7-0.8 eV for C-D).31 For the coadsorption study, we have considered the A mode rather than the E mode, since it also characterizes the interaction with a surface anion and involves less geometrical parameters than the E mode. When gold is adsorbed on-top the oxygen (A), the system remains in a high spin state. Although gold remains nearly neutral (the Bader analysis gives 10.6 valence electrons on the gold atom), we found that the spin is delocalized between the gold and the titanium atoms (the spin remaining on the gold atom is 0.20 e). In Figure 2, we show the densities of states of A. From the projected densities of state, it is apparent that most of the spin is transferred to the support. Looking at spin densities, presented in Figure 3, we also see that the spin is delocalized, chiefly to the subsurface plane. Such a spin dilution is due to GGA as explained elsewhere.3,40 The main conclusion is that Au binds to surface oxygen as H does. Noble metals are isolobal to hydrogen, and the adsorption of gold on TiO2 is similar to that of hydrogen.3 Considering oxidation numbers, Au and H should be Au+
and H+ binding to O2-. Since Ti4+ is reducible, the most favorable adsorption mode is that of a cation on the surface oxygen, the extra electron serving for the reduction of a Ti4+ to Ti3+. The difference lies in the fact that, unlike hydrogen, gold is known to be a poor electron donor (ionization potential of ∼9 eV) and only a partial electron transfer occurs. Considering gold on-top a Ti5c (C), no electron transfer is observed (the Bader analysis gives 10.9 valence electrons on the gold atom). No noticeable spin delocalization is observed; the spin remains on the gold atom; from the Bader analysis, the spin remaining on the gold atom is 0.81 e. Coadsorption of Au with H or K. The adsorption mode changes dramatically in the presence of a coadsorbed species (hydrogen or potassium). The analogy between noble metals and hydrogen is again instructive. The coadsorption of Au and H or K then resembles that of two hydrogen atoms on MgO.3,4 The adsorption is formally that of ion pairs and leads to a low spin system. Since Au is more electronegative than H and K, Au becomes Au- and binds to the surface cation. The coadsorbed species, H or K, binds as positive ion H+ or K+ to a surface oxygen ion. Formally, a redox process takes place between the two adsorbates. We have therefore compared two sites of adsorption of gold atom, A, on-top O2c and, C, on-top of Ti5c. The hydrogen was adsorbed on top of an O2c atom; potassium atom was adsorbed bridging two O2c, which has been found the most favorable adsorption site for K on TiO2(110) rutile surface.41 The adsorption energies of gold on these systems are displayed in Table 2. They are calculated with respect to the surface already covered by the coadsorbed species (H/TiO2 or K/TiO2) and the spin polarized Au atom in gas phase. If no electronic effect were present, we should expect a decrease of adsorption energy due to an increase of coverage. The very first remark is a modification of the site preference. Gold preferentially adsorbs on the titanium site. This is what is expected for Au-I anion, which binds to the surface cation. The second remark is that
Coadsorption of Au with H or K on TiO2 Surface
J. Phys. Chem. C, Vol. 112, No. 36, 2008 14013
TABLE 2: Adsorption Energies (eV) of Au Coadsorbed with X (X ) H or X ) K) on TiO2(110)a X Au Eads 3 L Eads 5 L
X)H
X)K
A. on-top O2c
B. bridging O2c
A. on-top O2c C. on-top Ti5c A. on-top O2c C. on-top Ti5c -0.14
0.64
0.80 0.53
1.42 1.16
a The labels are those from Figure 1. The reference is the metal surface with X (X/TiO2(110)) and the spin polarized Au atom in gas phase. The number of TiO2 layers in the model is indicated (3 L or 5 L).
Conclusion We have studied the adsorption of gold on TiO2(110) rutile surface and explained the variation of adsorption site in terms of electron count and redox process. Gold adsorbs as an electron donor or as an electron acceptor depending on the presence of preadsorbed K (or H). On a clean surface, Au binds preferentially to surface oxygen atoms with modification of its oxidation state from 0 to +I. The projected density of states shows a partial spin transfer from Au to titanium cations of the support. We have next studied the effect of the coadsorption of two electron donor species on the adsorption of gold, H, and K. Gold adsorbs as an electron acceptor. The adsorption takes place above a titanium cation and is very strong. A formal redox process occurs between the two coadsorbed species: Au is reduced to Au-I while H (or K) is oxidized to H+ (or K+), maintaining a band gap. The K precovered surface is more reactive toward Au than the bare metal oxide surface. Our results confirm also that K may have a large influence on gold particles’ growth and that it may induce spillover. Acknowledgment. The authors thank IDRIS, CINES, and CCRE for computing facilities. References and Notes
Figure 4. Density of states of Au on top a TiO2 titanium cation, Ti5c, coadsorbed with a hydrogen atom. It is a closed shell system. Black line is the total DOS, and the blue shape is the projected DOS on gold. The vertical line represents the Fermi level. All energies are referred to the vacuum.
the system has no spin; this is expected considering the redox process occurring between Au and H (or K), which results in coupling all the electrons. The last remark is that, in the case of coadsorption with K, the binding energy for the C mode, 1.42 eV, is much larger than that for the clean surface. The effect is much pronounced for K than for H because of the large difference in electronegativity between Au and K. Nevertheless, the increase of the adsorption energy for the C mode represents 1/3 in the case of H coadsorption. These increases are clearly the consequence of restoring a band gap (Figure 4). Mutombo31 also found that potassium displaced gold from an adsorption site favoring interaction with O atoms (the bridge site) and caused its migration to the top of the 5-fold titanium atom. Graciani and Sanz have found similar results on another reduced TiO2 surface.42 Referring to the surface already covered by H or K, we can also analyze the gold adsorption as that of an electron acceptor adsorbate on a reduced substrate. Then, a redox process reoxidizes the surface, restoring an ideal electron count. In Table 2, we also report the values using a five TiO2 layers slab; qualitative results are unchanged; even the difference in the adsorption energies for both sites is not affected, and since these values are smaller, the relative effect appears more striking. To summarize, the presence of K modifies gold’s site adsorption and strongly enhances its binding energy. Finally, we must underline that the unit cell which has been used (p(4 × 1)) is large enough to prevent any noticeable interaction between coadsorbed species. An explanation for Au displacement by steric repulsive interaction with K does not hold in our modeling.
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JP800708U
