Water-Induced Oxidation and Dissociation of Small Cu Clusters on

Subscriber access provided by RYERSON UNIV

Article

Water-Induced Oxidation and Dissociation of Small Cu Clusters on ZnO(10-10) Matti Hellström, Daniel Spangberg, Peter Broqvist, and Kersti Hermansson J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 01 Dec 2014 Downloaded from http://pubs.acs.org on December 2, 2014

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Water-Induced Oxidation and Dissociation of Small ¯ Cu Clusters on ZnO(1010) Matti Hellström, Daniel Spångberg, Peter Broqvist, and Kersti Hermansson∗ Department of Chemistry Ångström Laboratory, Uppsala University, Box 538, SE-75121 Uppsala, Sweden E-mail: [email protected]

Phone: +46 18 4713767. Fax: KEYWORDS: catalysis, density functional theory, water gas shift reaction

Abstract The interaction between water molecules and small Cu clusters (up to a size of four atoms) adsorbed on the non-polar ZnO(10 10) surface has been studied using hybrid density functional theory. We nd that the water molecules can give rise to dierent scenarios: (i) In contrast to water adsorption on the ZnO(10 10) surface which occurs molecularly, the rst water molecule often preferentially dissociates upon adsorption on the Cu cluster, which may be a key step in the water-gas shift reaction. (ii) The adsorption of more than one water molecule on the (adsorbed) Cu clusters is always less favorable than the adsorption of only one water molecule. (iii) As a water molecule adsorbs on the (adsorbed) Cu atom, it induces charge transfer between the Cu and the ZnO, so that an electron from the Cu atom populates the ZnO conduction band (giving an oxidized Cu species). (iv) Water molecule adsorption on the (adsorbed) Cu trimer results in a spontaneous dissociation of the Cu trimer into an adsorbed dimer and an adsorbed atom, after which the water molecule adsorbs on the atom, again resulting in the Cu-ZnO charge transfer. We also show that the use of a hybrid density functional gives qualitatively dierent results compared to a semilocal density functional for this system, and we explain this in terms of the underestimation of the ZnO band gap obtained with the semilocal functional. ∗

To whom correspondence should be addressed 1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 Introduction Catalysts composed of Cu/ZnO are used for the catalysis of, for example, the water gas shift reaction (WGSR: CO + H2 O −→ CO2 + H2 ) at low temperatures. 1 This reaction nds its main use as a step before the Haber-Bosch process, where it is necessary to remove unwanted CO from the H2 feed gas. Metallic Cu is generally thought to be the active ingredient in this WGSR catalyst. 28 Campbell and Daube 2 performed post-reaction surface analyses (AES, XPS, LEED) on the Cu(111) surface and found the initial dissociation of the water molecule to be the ratedetermining step. Nakamura et al. 3 used a similar experimental technique for the less dense Cu(110) surface and found the same rate-determining step, although the reaction proceeded considerably faster than on the Cu(111) surface. Ovesen et al. 4,5 then created kinetic models based largely on the results of the previous studies and could reasonably well explain the observed water gas shift reaction rates by a surface redox mechanism , in which the water molecule dissociates completely into an adsorbed O atom and adsorbed H atoms, and the CO2 is formed from the O atom and a CO molecule. Wang and Nakamura 9 assumed such a surface redox mechanism and employed density functional theory (DFT) calculations to conrm the rate-limiting character of the inital water dissociation, and also found the barrier for water dissociation on Cu(110) to be smaller than that for Cu(111). Other DFT studies 10,11 have proposed an associative mechanism with a carboxyl intermediate on the Cu(111) surface. Also in these cases, initial water dissociation was found to be the rate-determining step. Yang et al. 12 used a multiscale modeling approach, combining DFT and kinetic Monte Carlo simulations to suggest that the WGSR occurs both over the edge and terrace sites of Cu nanoislands adsorbed on ZnO(000 1), with the surface redox and carboxyl intermediate reaction mechanisms competing with each other. Lately there has been an increased interest in the eect of the oxide support material on the catalytic activity for the WGSR. According to Spencer 7 and Saito et al., 8 there is little reason to believe that ZnO plays any signicant role in water gas shift catalysis, other than to disperse the Cu particles and to prevent catalyst deactivation through sulfur poisoning. Rodriguez et al. 13 combined XPS and DFT calculations to show that although Cu particles adsorbed on the ZnO(000 1) surface demonstrate higher catalytic activity than extended Cu surfaces, this is not necessarily related to the ZnO support but rather to the small size of the Cu particles, which provides a large number of catalytically active edge sites. In contrast to the generally accepted picture of an inactive ZnO support, Guo et al. 14 suggested that the Cu-ZnO interface participates directly in the WGS reaction, and Yao et al. 15 used DFT calculations to nd the dissociation of water at the Cu-ZnO interface to be practically barrierless. Sagata et al. 16 subsequently found that although the reaction rate was proportional to the Cu surface area, a much greater activity was found for a ZnO support compared to a CeO x support. The authors attributed this to the greater number of weak basic sites for ZnO compared to, for example, CeO x . Howver, in other studies, changing the support from ZnO to either CeO x 13 or TiO2 17 increased the catalytic activity. Recently, much focus has also been given to inverse catalysts, where the oxide is supported on the metal. For example, Mudiyanselage et al. 18 found that for the inverse CeO x /Cu(111) WGS catalyst, the metal-oxide/metal interface played a key role and lowered the activation barrier for water dissociation to such an extent that the rate-determining step was no longer 2


Page 2 of 22

Page 3 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


water dissociation but the formation of a formyl intermediate. Similarly, Vecchietti et al. 19 recently proposed that the water dissociation step was not rate-determining for the Pt/CeO 2 WGS catalyst. The combined water/Cu/ZnO system (without any CO) has been studied with angleresolved photoemission spectroscopy (ARPES) experiments by Ozawa et al., 20 who showed that deposited Cu on the thermodynamically most stable ZnO(10 10) surface could be oxidized by preadsorbed water even at room temperature. In the present work, we use hybrid density functional theory calculations to model the interaction of water with small Cu clusters adsorbed on the ZnO(10 10) surface. We do not model the full catalytic cycle instead, we focus on the fundamental interactions between water, Cu, and ZnO, which in turn may well aect water gas shift catalysis. We will, among other things, show that the adsorption of water induces oxidation of the adsorbed Cu atom and of the trimer, such that an electron is removed from the Cu adsorbate and instead enters the ZnO conduction band, delocalizing over the ZnO substrate. However, this kind of oxidation is Cu cluster size-dependent, and, according to our calculations, does not occur for the dimer or the tetramer (unless the water pressure is high enough for more than one water molecule to adsorb on the clusters). We previously showed that Cu clusters (up to a size of nine atoms) adsorbed on ZnO(10 10) display an even-odd alternation in stability and charge transfer as a function of the number of atoms. 21 Odd-numbered clusters could either donate an electron to the ZnO conduction band (ending up positively charged/oxidized), or remain neutral, depending on the specic structure of the cluster. For odd sizes of n ≥ 5, the positively charged Cu clusters were at least as stable as the corresponding neutral clusters. Even-numbered clusters, in contrast, were always charge-neutral. It has previously been shown that the catalytic activity may well be aected by the electronic structure of the clusters. For example, the catalytic activity of Pd/TiO2 for CO oxidation was found to correlate with the electronic structure of the Pd clusters, which was cluster size-dependent. 22 The purpose of this paper is twofold: (i) we investigate the behavior of very small Cu clusters (with n ≤ 4) as water molecules adsorb on them, in order to investigate any oxidation of the Cu clusters, the stabilities of the Cu clusters, and the dissociation of the water molecules, and (ii) we illustrate the importance of using a hybrid DFT functional for the accurate representation of the complicated water/Cu/ZnO system, by comparing and explaining the dierent results obtained by a hybrid DFT functional and a non-hybrid DFT functional.

2 Method All calculations were carried out using the VASP 2326 code, with two dierent density functionals: (i) the semilocal PBE functional, 27 and (ii) the hybrid HSE06 0 functional (which is based on the HSE06 functional, 28,29 but with a fraction a = 0.375 of exact exchange and a screening parameter ω = 0.2 Å−1 for the short-range exchange). The PBE functional suers from the band-gap problem, which means that the band gaps of semiconductors, in general, are heavily underestimated with this functional. For example, the PBE-calculated band gap of ZnO is 0.7 eV, much less than the experimentally determined value of 3.4 eV. The HSE06 0 3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(also known as mod -HSE or HSE(a=0.375)) functional has previously been used by us 21,30 and others 31,32 to reproduce the experimental ZnO band gap and Zn 3d energies relative to the valence band maximum. The core-valence interactions were described by PAW potentials, 33,34 with 1, 6, 11, and 12 electrons treated explicitly for H, O, Cu, and Zn, respectively. The plane-wave basis set had an energy cuto of 500 eV, and a Gaussian broadening with σ = 0.05 eV was applied to the electronic states. All calculations were spin-polarized. The ZnO(10 10) surface was modeled as a periodic slab, with a vacuum gap of 15 Å along the surface normal ([10 10]) direction. The lateral supercell dimensions were 4 × 3 surface unit cells, or 13.2 Å × 15.9 Å. The slab consisted of four double-layers and was constructed from the PBE-optimized bulk wurtzite lattice parameters ( a = 3.288 Å, c = 5.305 Å, and u = 0.3789; u is the fractional displacement of Zn and O sublattices along the c direction). All adsorbates were adsorbed on both the upper and lower sides of the slab, in order to prevent the formation of large dipole moments in the surface normal direction and to facilitate the use of the band-lling correction (see more below). The geometry optimizations had a force convergence criterion of 0.05 eV/Å, and all atomic positions were relaxed. The PBE calculations were performed using a Γ-centered 3×3×1 k-point grid, while only the Γ-point was used for the HSE06 0 calculations. The PBE functional, because it is much less computationally demanding, was used to systematically screen a large number of water adsorption congurations on those adsorbed Cu clusters of sizes 1-4 atoms that we previously showed to be the most stable on the ZnO(10 10) surface. 21 All possible combinations of molecular/dissociated water adsorption were modeled for up to n water molecules adsorbed on the Cun cluster (with at most one water molecule adsorbed per Cu atom within the cluster). For the Cu atom adsorbate, we additionally modeled the adsorption of two water molecules. The most stable congurations found with PBE were subsequently re-optimized with HSE060 . The band-lling correction 30 was applied to the calculated total energies for any systems where the ZnO conduction band became populated through electron donation from adsorbed Cu. The band-lling correction is a method to extrapolate the total energy resulting from periodic calculations of the Cu + n /ZnO system using a standard supercell size, to the situation of an innite supercell (i.e., where the adsorbates are isolated with respect to each other). The correction adjusts for the lling-up of the ZnO conduction band (and the down-shift of the conduction band minimum (CBM), which is particularly prominent in the hybrid DFT calculations) arising from the use of a nite (small) supercell. We found that the zero-point energy (ZPE) contributions to the adsorption energy (i.e. the dierence between ZPE for a gas-phase water molecule and for a water molecule adsorbed on the ZnO surface), calculated using the harmonic approximation, were no larger than 3 kJ/mol for a few representative systems (molecular water adsorption on the bare ZnO surface as well as dissociative adsorption onto Cu 1 /ZnO). The small change in ZPE upon adsorption prompted us to neglect the ZPE for the remainder of the work (negligible ZPE contributions to the adsorption energy was also reported for the case of water adsorption on Pt 35 ).

4


Page 4 of 22

Page 5 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3 Results and Discussion In Table 1 we list the reactions considered in this work, together with the reaction energies ∆E calculated using both the HSE06 0 and PBE functionals. The reactions are divided into six dierent categories: (i) the adsorption of gasphase water molecules and Cu atoms onto the ZnO surface, (ii) the formation of adsorbed Cu 2 , Cu3 , and Cu4 from adsorbed Cu atoms, (iii)-(vi) the successive adsorption of more and more water molecules onto adsorbed Cu 1 , Cu2 , Cu3 , and Cu4 . Gas-phase species are suxed with (g), and adsorbed species with a subscript ad. Brackets are sometimes used to improve the readability, e.g. in [Cu 4 ]ad . Occasionally the calculated Cu cluster charges dier depending on which functional is used (HSE06 0 or PBE). This is indicated by putting the charge in parentheses, meaning that the positive charge is (+) obtained with PBE but not with HSE06 0 , e.g. in [Cu3 ]ad . If there is no parenthesis, this means that the same charge was obtained with both PBE and HSE06 0 . In all cases, any species listed in Table 1 refers to the most stable such species that we have found. For example, [Cu 3 (OH2 )3 ]+ ad refers to the most stable conguration of three water molecules adsorbed on the Cu trimer (in this particular case, one of the water molecules is dissociated and the other two are molecular).

3.1

Adsorption of water and Cu on clean ZnO(10 10)

The adsorption of a water molecule on ZnO(10 10) has been studied previously by for example 36 Meyer et al., who used the PBE functional and found the conguration depicted in Figure 1 to be the most stable with an adsorption energy of -91 kJ/mol. Raymand et al. 37 used the hybrid B3LYP functional with a local basis set and found the adsorption energy to be quite similar (-86 kJ/mol). In our case, the PBE functional yields an adsorption energy of -94 kJ/mol ( reaction 1, Table 1), in good agreement with the PBE-computed value by Meyer et al., while the hybrid HSE060 functional yields an adsorption energy of -106 kJ/mol. Thus, the adsorbed conguration is a little more stable with HSE06 0 compared to PBE. For the adsorption of a Cu atom ( reaction 2, Table 1), the dierences between the HSE060 and PBE functionals is larger. In fact, the most stable adsorption site for a Cu atom is dierent for the two dierent functionals. With HSE06 0 , the favored adsorption site (site a, see Figure 2a) is the position that a Zn atom would occupy had the ZnO grown by an extra layer, while with PBE, the favored adsorption site is above and between two surface O atoms (site b, Figure 2b). The relative stabilities of the two adsorption sites are dierent for the dierent functionals because of the resulting electronic structures after the Cu atom has adsorbed. At site b, the Cu atom donates an electron to the ZnO conduction band (see the DOS in Figure 2b), so that the Cu atom ends up positively charged (Cu + ad ). This electron transfer occurs for both the PBE and HSE06 0 functionals at site b, but not for site a. Because the calculated band gap is much underestimated with PBE (0.7 eV), the adsorption becomes too strong at site b with PBE. In general, PBE favors congurations that become positively charged through electron donation to the ZnO conduction band much more strongly than does HSE060 . At site a, the ZnO conduction band does not become populated (see the DOS in Figure 2a). 5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

In section 3.4, we will discuss the dierent results obtained with HSE06 0 and PBE in more detail. Until that point, we will focus our discussion on the HSE06 0 results, as the correct ZnO band gap is obtained with HSE06 0 but not with PBE.

3.2

Formation of Cu n /ZnO(10 10) from adsorbed Cu atoms

As we have reported previously, 21 the most stable adsorbed Cu dimer, trimer, and tetramer on ZnO(10 10) are all calculated to be neutral with HSE06 0 . As seen in Table 1 (reactions 3, 4, and 5), Cu atoms will exothermically form larger adsorbed Cu clusters. If the (HSE06 0 -calculated) reaction energies are normalized with respect to the number of Cu atoms, the resulting values become ∆E = -66 kJ/mol, -62 kJ/mol, and -75 kJ/mol for the formation of [Cu 2 ]ad , [Cu3 ]ad , and [Cu4 ]ad , respectively. These normalized reaction energies make clear that the adsorbed Cu trimer is, in fact, slightly less stable than both the dimer and the tetramer, which we have also reported previously. 21 The structures of the adsorbed dimer, trimer, and tetramer resemble those of the corresponding most stable clusters in the gasphase, 21 but the bonds become longer upon adsorption. For example, the bond length of the dimer increases from 2.23 Å to 2.32 Å, and the bond lengths in the trimer from 2.33 Å and 2.49 Å to 2.37 Å and 2.64 Å.

3.3

Successive water adsorption on Cu n /ZnO(10 10)

We now investigate the reaction energies as successively more water molecules are adsorbed on the Cu clusters. We always use an adsorbed water molecule (depicted in Figure 1) as one of the reactants, i.e., this is the reference state for the H 2 Oad energy. That way, the calculated reaction energies will immediately show whether the water molecule preferentially adsorbs on the Cu cluster (when ∆E < 0) or stays on the ZnO surface (when ∆E > 0). We reiterate that we here focus on the HSE06 0 -calculated results; dierent results obtained with HSE060 and PBE will be highlighted in section 3.4.

3.3.1

Water adsorption on Cu 1 /ZnO(10 10)

Reaction 6 in Table 1 is that between an H 2 O molecule adsorbed on the ZnO surface

(Figure 1) and an adsorbed Cu atom (Figure 2a for the HSE06 0 -calculated reaction). The reaction is highly exothermic ( ∆E = -74 kJ/mol), and results in the conguration depicted in Figure 3a, where the water molecule is dissociated. In addition, the water molecule induces electron transfer between the Cu and ZnO. The end result is a positively charged (oxidized) [Cu(OH2 )]+ ad species (and a negatively charged (reduced) ZnO substrate). The adsorption of a second water molecule onto the [Cu(OH 2 )]+ ad ] complex (reaction 7) is highly endothermic ( ∆E = +86 kJ/mol, compared to having the second water molecule adsorbed on the ZnO surface instead of the Cu cluster), and the most stable such [Cu(OH 2 )2 ]+ ad conguration is shown in Figure 3b, where one water molecule is dissociated and one is molecular. This conguration is only 3 kJ/mol more stable than one where both water molecules are dissociated (not shown). Although there now are two water molecules bound to the Cu atom, no further electron transfer between the Cu and ZnO occurs (see the DOS in Figure 3b). 6


Page 6 of 22

Page 7 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.3.2


Figure 4a displays the most stable adsorbed Cu dimer on ZnO(10 10). Figure 4b shows the most stable adsorbed conguration for the Cu dimer with one adsorbed water molecule, [Cu2 (OH2 )]ad , and the water molecule is seen to be dissociated. The adsorption is exothermic (reaction 8, ∆E = -45 kJ/mol) and, in contrast to the case of the single atom, does not induce any charge transfer between Cu and ZnO (see the DOS in Figure 4b). If another water molecule adsorbs on the dimer, the most stable conguration, depicted in Figure 4c, consists of a more or less dissociated Cu dimer (the Cu-Cu distance is 3.08 Å), with the two water molecules forming a hydrogen bond. The structure, which we denote + [Cu2 (OH2 )2 ]2+ ad , can be thought of as two adjacent [Cu(OH 2 )]ad species (c.f. Figure 3a). + The [Cu2 (OH2 )2 ]2+ ad complex is actually less stable than two separate [Cu(OH 2 )]ad complexes (compare reactions 9a and 9b), which we attribute to the repulsion between the pair of positively charged Cu atoms and the repulsion between the pair of negatively charged OH species adsorbed on the Cu (this repulsion is, apparently, greater than the water-water hydrogen-bond strength). + However, the formation of both the [Cu 2 (OH2 )2 ]2+ ad (reaction 9a) and 2 [Cu(OH 2 )]ad (reaction 9b) species are unfavorable compared to the adsorption of a water molecule on the bare ZnO surface in combination with the [Cu 2 (OH2 )]ad species, since ∆E > 0 for both reactions 9a and 9b. Thus, the second water molecule will preferentially adsorb on the ZnO surface, and no charge transfer between Cu and ZnO is induced.

3.3.3


The most stable Cu trimer adsorbed on ZnO(10 10) is shown in Figure 5a. The adsorption of a single water molecule on the Cu trimer ( reaction 10a and Figure 5b) is molecular and induces electron transfer between Cu 3 and ZnO, giving a [Cu 3 (OH2 )]+ ad species. This behavior is very similar to that of the single water molecule adsorption onto the adsorbed Cu atom, where the adsorption of the water molecule also induced charge transfer between Cu and ZnO (section 3.3.1). However, the adsorption of the water molecule on the trimer is endothermic (∆E = +15 kJ/mol), unlike the water adsorption on the atom, which was exothermic. Although the water conguration depicted in Figure 5b is molecular, we note that the corresponding dissociated conguration is only 1 kJ/mol less stable than the molecular conguration. The water adsorption on the lowermost Cu atom in the top view in Figure 5b is 30 kJ/mol more stable than the adsorption of either of the other two Cu atoms. Although the direct adsorption of a water molecule on the trimer is endothermic, the adsorption-induced dissociation of the trimer is exothermic ( reaction 10b, ∆E = -20 kJ/mol). Thus, an adsorbed water molecule will react with the adsorbed trimer, and induce the dissociation of the trimer into a dimer and an atom. The water molecule will then preferentially adsorb on the atom (since ∆E for reaction 6 is more negative than ∆E for reaction 8). The water molecule adsorption on Cu 3 thus induces both charge transfer between Cu and ZnO and dissociation of the trimer into a dimer and an atom. For completeness, we also list the successive adsorption energies for water molecules on the trimer (reactions 11 and 12, Figures 5c and 5d). These are endothermic, and a 7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

quick look at reactions 10a, 11 and 12 reveal that the adsorption becomes progressively weaker the more water molecules are adsorbed on the Cu trimer. Interestingly, although the most stable [Cu 3 (OH2 )]+ ad complex had molecularly adsorbed water, the corresponding water molecule dissociates when the second water molecule adsorbs (compare Figures 5b and 5c). No further charge transfer between the Cu and ZnO occurs as a result of the additional water adsorption.

3.3.4


Finally, we discuss water adsorption on the Cu tetramer (Cu 4 ), depicted in Figure 6a. The rst water molecule adsorbs exothermically ( reaction 13: ∆E = -71 kJ/mol, Figure 6b) and becomes dissociated. No charge transfer between Cu and ZnO is induced (this is similar to the single water molecule adsorption on Cu 2 ). The adsorption of the second water molecule on the tetramer ( reaction 14a, Figure 6c) does induce electron transfer from Cu to ZnO, although the adsorption is strongly endothermic (∆E = +69 kJ/mol) and thus not likely to occur. A more favorable, but still endothermic, reaction is reaction 14b where the adsorbed water molecule reacts with [Cu 4 (OH2 )]ad to form 2 [Cu2 (OH2 )]ad (∆E = +13 kJ/mol). Interestingly, the adsorption of the third water molecule on the tetramer ( reaction 15: ∆E = +49 kJ/mol, Figure 6d) is less endothermic than the adsorption of the second water molecule (reaction 14a, ∆E = +69 kJ/mol). This is because the adsorption of the third water molecule is not associated with any charge transfer between Cu and ZnO. The adsorption of the fourth water molecule is more endothermic still ( reaction 16, ∆E = +82 kJ/mol, Figure 6e).

3.3.5

Water dissociation and hydrogen bonds

We nd that the water molecules, when they adsorb on the Cu clusters, can dissociate in two distinct ways. The rst type of dissociation involves a hydrogen bond between the water O (O w ), and the dissociated hydrogen atom, which is bound to a surface O (O s ), i.e. HOw · · · HOs . This kind of dissociation occurs, for example, for the dissociated water molecule in [Cu 3 (OH2 )2 ]+ ad (Figure 5c), and one of the water molecules in [Cu 4 (OH2 )2 ]+ (Figure 6c). We nd that this ad type of dissociation is preferred when the water oxygen atom is bound only to Cu (and not to Zn). The second type of dissociation occurs when the water O is bound to both Cu and Zn, and in this case no hydrogen bond is formed (the dissociated H, bound to a surface O, distinctly points away from the water OH fragment): HO w + HOs . This can be seen, for example, for [Cu(OH2 )]+ ad (Figure 3a), [Cu 2 (OH2 )]ad (Figure 4b), and [Cu 4 (OH2 )]ad (Figure 6b). Finally, we note that any molecularly adsorbed water molecule always forms a hydrogen bond with one of surface O atoms: HO w H · · · Os .

3.3.6

Summary of this section

The odd-numbered (Cu 1 , Cu3 ) and even-numbered (Cu 2 , Cu4 ) clusters in this study behave dierently as water molecules move from the ZnO surface to the clusters (which themselves 8


Page 8 of 22

Page 9 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


are adsorbed on the ZnO surface). When a water molecule adsorbs on an odd -numbered cluster, the cluster becomes positively charged (oxidized) and donates an electron to the ZnO conduction band. In contrast, such oxidation occurs for an even -numbered cluster only if two or more water molecules adsorb, which is always energetically unfavorable. In addition, it is thermodynamically favorable for the Cu trimer to dissociate into an atom and a dimer when the water molecule adsorbs on the cluster ( water adsorption-induced dissociation of Cu3 ). In general, a water molecule dissociates when it adsorbs on a Cu cluster (unlike when it adsorbs on the ZnO surface), although there are some exceptions to this rule, e.g. the adsorption of the rst water molecule on Cu 3 . There are two dierent kinds of dissociation, one where a hydrogen bond is formed (for water O bound to Cu and not to Zn), and one where no hydrogen bond is formed (for water O bound to both Cu and Zn).

3.4

Discussion of the dierent results obtained with HSE06 PBE

0

and

In this section, we discuss some advantages and disadvantages of using the HSE06 0 and PBE functionals. Although the HSE06 0 functional is computationally very demanding, we propose that its use is necessary for the accurate modeling of the combined water/Cu/ZnO system. Results obtained at the PBE level of theory can be both quantitatively and qualitatively misleading. PBE severely underestimates the ZnO band gap (calculated value 0.7 eV, experimental value 3.4 eV), while HSE06 0 reproduces the experimental band gap. This has the implication that the adsorbed Cu species may become positively charged with PBE, but not with HSE06 0 . For example, the most stable Cu trimer becomes positively charged with PBE ([Cu 3 ]+ ad ) but not with HSE060 ([Cu3 ]ad ), despite the optimized structures being very similar with the two methods. The underestimated band gap (or, more precisely, the erroneous relative positions of the electronic levels on Cu and ZnO) may also adversely aect the calculated reaction energies for the cases where electron transfer to/from the ZnO substrate occurs. Cu species or complexes that become positively charged through electron donation to the ZnO become far too stable when calculated with PBE. For example, in section 3.2 we noted that the adsorbed Cu trimer is less stable than both the dimer and tetramer, when computed with HSE06 0 . The opposite result is obtained with PBE, where the positively charged trimer is more stable than both the (neutral) dimer and (neutral) tetramer ( ∆E normalized per number of Cu atoms for reactions 3, 4, and 5 are -30 kJ/mol, -53 kJ/mol, and -51 kJ/mol, respectively). It is instructive to compare the HSE06 0 and PBE-calculated reaction energies for some of the reactions in Table 1. For example, reaction 6 (the adsorption of a water molecule onto a Cu atom) is less exothermic with HSE06 0 than with PBE because of the charge transfer induced by the water molecule in the HSE06 0 case (with PBE, the atom is positively charged even without the water molecule). In contrast, reactions 7 and 8 involve no charge transfer and consequently the reaction energies computed with HSE06 0 and PBE are very similar. Reactions 9a and 9b again involve charge transfer, the result being that the reactions are endothermic with HSE06 0 but exothermic with PBE. 9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Although the above discussion highlights some of the aws of the PBE functional, the PBE functional is quite reliable and useful to compare relative energies of systems that are not too dierent and that do not dier with respect to the number of electrons in the ZnO conduction band. For example, the PBE functional may be used to gauge the relative stabilities of dierent adsorption congurations of a water molecule on a Cu atom, where the water molecule always induces charge transfer no matter if it adsorbs in the most stable position (discussed in the previous section), or other, less stable, positions. To illustrate this, Figure 7 shows the relative water adsorption energies of three dierent congurations (the value 0 is taken to be the most stable [Cu(OH 2 )]+ ad complex for each of the two methods). 0 Although PBE and HSE06 do not give perfectly equivalent results (as the black and red lines in Figure 7 do not perfectly overlap), it is clear that the relative adsorption energies match (both lines have positive slope). This justies our approach of rst screening many dierent congurations and positions of the adsorbed water molecule using the PBE functional, and subsequently selecting the best such positions for further evaluation at the HSE06 0 level. Finally, we note that the optimized structures with PBE and HSE06 0 often are remarkably similar. The only case where the structures of any of the species listed in Table 1 dier (+) considerably between the two methods, is Cu ad , where the most stable congurations are shown in Figure 2a and Figure 2b for HSE06 0 and PBE, respectively.

4 Conclusions The adsorption of water on Cu/ZnO(10 10) can induce electron transfer from Cu to ZnO (water adsorption-induced oxidation of Cu). For the Cu atomic adsorbate, water adsorption on the Cu atom is preferred (as opposed to water adsorption on the ZnO surface), and oxidation of the Cu atom occurs. For the Cu dimer, the water molecule also prefers to adsorb on the dimer, but in this case, no oxidation is induced. The Cu trimer is subject to water adsorption-induced dissociation ; when a water molecule adsorbs, the Cu trimer will rst oxidize and then spontaneously dissociate into a dimer and an atom, where the water molecule adsorbs on the atom (which results in oxidation of the atom). The Cu tetramer behaves similarly to the Cu dimer; one water molecule favorably adsorbs on the cluster, but no oxidation (nor dissociation) of the cluster occurs. Thus, the odd-numbered clusters (the atom and the trimer) will be oxidized (and potentially dissociated) upon water adsorption, while the even-numbered clusters remain neutral and intact. This is reminiscent of our previous work, 21 where we showed that odd-numbered clusters with at least ve atoms could oxidize spontaneously even without any water adsorbate. Here, we have shown that the oxidation of odd-numbered clusters occurs even for the atom and the trimer in the presence of adsorbed water. In general, the rst water molecule adsorbs dissociatively on these clusters a possible key reaction step in the water gas shift reaction. One exception is the (intact) Cu trimer, where molecular adsorption is preferred but the dissociative adsorption is only 1 kJ/mol less stable. The adsorption of more than one water molecule on the clusters is always endothermic (compared to water adsorption on the ZnO surface). If the water pressure is high enough for more than one water molecule to adsorb on the Cu clusters, also the even-numbered (dimer and tetramer) Cu clusters are subject to water adsorption-induced oxidation . 10


Page 10 of 22

Page 11 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In all cases where the Cu cluster ends up oxidized, the ZnO substrate ends up reduced an electron enters the ZnO conduction band. For this reason, it was necessary to use a DFT functional which describes the ZnO band gap correctly. Here, we used the HSE06 0 functional, which gives the experimental ZnO band gap, and compared its results with those of the PBE functional, which underestimates the band gap. The results diered both quantitatively and qualitatively, highlighting the importance of using the HSE06 0 functional for this kind of system where the ZnO conduction band can become populated.

Acknowledgement This work was supported by the Swedish Research Council (VR) and the Swedish strategic e-science research programme eSSENCE. The calculations were performed on resources provided by the Swedish National Infrastructure for Computing (SNIC) at UPPMAX and NSC and by the Matter computer consortium. We would also like to acknowledge the European Union through the COST Action CM1104 (Reducible oxide chemistry, structure and functions).

References (1) Ratnasamy, C.; Wagner, J. Water Gas Shift Catalysis. Cat. Rev. 2009, 51, 325440. (2) Campbell, C. T.; Daube, K. A Surface Science Investigation of the Water-Gas Shift Reaction on Cu(111). J. Catal. 1987, 104, 109 119. (3) Nakamura, J.; Campbell, J. M.; Campbell, C. T. Kinetics and Mechanism of the WaterGas Shift Reaction Catalysed by the Clean and Cs-Promoted Cu(110) Surface: A Comparison with Cu(111). J. Chem. Soc., Faraday Trans. 1990, 86, 27252734. (4) Ovesen, C.; Stoltze, P.; Nørskov, J.; Campbell, C. A Kinetic Model of the Water Gas Shift Reaction. J. Catal. 1992, 134, 445 468. (5) Ovesen, C.; Clausen, B.; Hammershøi, B.; Steensen, G.; Askgaard, T.; Chorkendor, I.; Nørskov, J.; Rasmussen, P.; Stoltze, P.; Taylor, P. A Microkinetic Analysis of the Water-Gas Shift Reaction under Industrial Conditions. J. Catal. 1996, 158, 170 180. (6) Ginés, M.; Amadeo, N.; Laborde, M.; Apesteguía, C. Activity and Structure-Sensitivity of the Water-Gas Shift Reaction over CuZnAl Mixed Oxide Catalysts. Appl. Catal. A 1995, 131, 283 296. (7) Spencer, M. The Role of Zinc Oxide in Cu/ZnO Catalysts for Methanol Synthesis and the Water-Gas Shift Reaction. Top. Catal. 1999, 8, 259266. (8) Saito, M.; Wu, J.; Tomoda, K.; Takahara, I.; Murata, K. Eects of ZnO Contained in Supported Cu-Based Catalysts on Their Activities for Several Reactions. Catal. Lett. 2002, 83, 14.

11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(9) Wang, G.-C.; Nakamura, J. Structure Sensitivity for Forward and Reverse Water-Gas Shift Reactions on Copper Surfaces: A DFT Study. J. Phys. Chem. Lett. 2010, 1, 30533057. (10) Gokhale, A. A.; Dumesic, J. A.; Mavrikakis, M. On the Mechanism of Low-Temperature Water Gas Shift Reaction on Copper. J. Am. Chem. Soc. 2008, 130, 14021414. (11) Tang, Q.-L.; Chen, Z.-X.; He, X. A Theoretical Study of the Water Gas Shift Reaction Mechanism on Cu(111) Model System. Surf. Sci. 2009, 603, 2138 2144. (12) Yang, L.; Karim, A.; Muckerman, J. T. Density Functional Kinetic Monte Carlo Simulation of Water-Gas Shift Reaction on Cu/ZnO. J. Phys. Chem. C 2013, 117, 34143425. (13) Rodriguez, J. A.; Liu, P.; Hrbek, J.; Evans, J.; Pérez, M. Water Gas Shift Reaction on Cu and Au Nanoparticles Supported on CeO2(111) and ZnO(000 ¯ 1): Intrinsic Activity and Importance of Support Interactions. Angew. Chem. Int. Edit. 2007, 46, 13291332. (14) Guo, P.-J.; Chen, L.-F.; Yu, G.-B.; Zhu, Y.; Qiao, M.-H.; Xu, H.-L.; Fan, K.-N. Cu/ZnO-Based Water-Gas Shift Catalysts in Shut-Down/Start-Up Operation. Catal. Commun. 2009, 10, 1252 1256. (15) Yao, K.; Wang, S.-S.; Gu, X.-K.; Su, H.-Y.; Li, W.-X. First-Principles Study of Water Activation on Cu-ZnO Catalysts. Chin. J. Catal. 2013, 34, 1705 1711. (16) Sagata, K.; Imazu, N.; Yahiro, H. Study on Factors Controlling Catalytic Activity for Low-Temperature Water-Gas-Shift Reaction on Cu-Based Catalysts. Catal. Today 2013, 201, 145 150. (17) Rodriguez, J. A.; Evans, J.; Graciani, J.; Park, J.-B.; Liu, P.; Hrbek, J.; Sanz, J. F. High Water-Gas Shift Activity in TiO2(110) Supported Cu and Au Nanoparticles: Role of the Oxide and Metal Particle Size. J. Phys. Chem. C 2009, 113, 73647370. (18) Mudiyanselage, K.; Senanayake, S. D.; Feria, L.; Kundu, S.; Baber, A. E.; Graciani, J.; Vidal, A. B.; Agnoli, S.; Evans, J.; Chang, R. et al. Importance of the Metal-Oxide Interface in Catalysis: In Situ Studies of the Water-Gas Shift Reaction by AmbientPressure X-Ray Photoelectron Spectroscopy. Angew. Chem. Int. Edit. 2013, 52, 5101 5105. (19) Vecchietti, J.; Bonivardi, A.; Xu, W.; Stacchiola, D.; Delgado, J. J.; Calatayud, M.; Collins, S. E. Understanding the Role of Oxygen Vacancies in the Water Gas Shift Reaction on Ceria-Supported Platinum Catalysts. ACS Catal. 2014, 4, 20882096. (20) Ozawa, K.; Oba, Y.; Edamoto, K. Oxidation of Copper Clusters on ZnO(10 10): Eect of Temperature and Preadsorbed Water. Surf. Sci. 2007, 601, 3125 3132. (21) Hellström, M.; Spångberg, D.; Hermansson, K.; Broqvist, P. Small Cu Clusters Adsorbed on ZnO(10 10) Show Even-Odd Alternations in Stability and Charge Transfer. J. Phys. Chem. C 2014, 118, 64806490. 12


Page 12 of 22

Page 13 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(22) Kaden, W. E.; Wu, T.; Kunkel, W. A.; Anderson, S. L. Electronic Structure Controls Reactivity of Size-Selected Pd Clusters Adsorbed on TiO2 Surfaces. Science 2009, 326, 826829. (23) Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for Liquid Metals. Phys. Rev. B 1993, 47, 558. (24) Kresse, G.; Furthmüller, J. Eciency of Ab-Initio Total Energy Calculations for Metals and Semiconductors Using a Plane-Wave Basis Set. Comput. Mater. Sci. 1996, 6, 15. (25) Kresse, G.; Furthmüller, J. Ecient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169. (26) Kresse, G.; Hafner, J. Ab Initio Molecular-Dynamics Simulation of the Liquid-Metal Amorphous-Semiconductor Transition in Germanium. Phys. Rev. B 1994, 49, 14251. (27) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 38653868. (28) Heyd, J.; Scuseria, G. E.; Ernzerhof, M. Hybrid Functionals Based on a Screened Coulomb Potential. J. Chem. Phys. 2003, 118, 82078215. (29) Krukau, A. V.; Vydrov, O. A.; Izmaylov, A. F.; Scuseria, G. E. Inuence of the Exchange Screening Parameter on the Performance of Screened Hybrid Functionals. J. Chem. Phys. 2006, 125, 224106. (30) Hellström, M.; Spångberg, D.; Hermansson, K.; Broqvist, P. Band-Filling Correction Method for Accurate Adsorption Energy Calculations: A Cu/ZnO Case Study. J. Chem. Theory Comput. 2013, 9, 46734678. (31) Oba, F.; Togo, A.; Tanaka, I.; Paier, J.; Kresse, G. Defect Energetics in ZnO: A Hybrid Hartree-Fock Density Functional Study. Phys. Rev. B 2008, 77, 245202. (32) Wang, Z.; Zhao, M.; Wang, X.; Xi, Y.; He, X.; Liu, X.; Yan, S. Hybrid Density Functional Study of Band Alignment in ZnO-GaN and ZnO-(Ga 1−x Znx )(N1−x Ox )-GaN Heterostructures. Phys. Chem. Chem. Phys. 2012, 14, 1569315698. (33) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 1795317979. (34) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector AugmentedWave Method. Phys. Rev. B 1999, 59, 1758. (35) Árnadóttir, L.; Stuve, E. M.; Jónsson, H. Adsorption of Water Monomer and Clusters on Platinum(111) Terrace and Related Steps and Kinks: I. Congurations, Energies, and Hydrogen Bonding. Surf. Sci. 2010, 604, 1978 1986. (36) Meyer, B.; Rabaa, H.; Marx, D. Water Adsorption on ZnO(10 10): from Single Molecules to Partially Dissociated Monolayers. Phys. Chem. Chem. Phys. 2006, 8, 15131520.

13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(37) Raymand, D.; van Duin, A. C. T.; Goddard III, W. A.; Hermansson, K.; Spångberg, D. Water Adsorption on Stepped ZnO Surfaces from MD Simulation. Surf. Sci. 2010, 604, 741 752.

14


Page 14 of 22

Page 15 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1: Reactions (where NCu Cu atoms and NH2 O water molecules participate) and reaction energies ∆E (in kJ/mol) calculated with HSE06 0 and PBE. Species that become positive with PBE but not with HSE06 0 are labelled with (+). If parentheses are not used to indicate the charge, the same charge is obtained with both methods. In cases where reactions are not balanced with respect to charge, any charge missing is transferred to/from the ZnO conduction band. The gures where the products of the reactions are shown are also given. Reaction no.

NCu

NH2 O

1. 2.

0 1

1 0

3. 4. 5.

2 3 4

0 0 0

Reaction Figure ∆E (HSE060 ) Adsorption of gas-phase Cu atom and water molecule H2 O(g) −−→ H2 Oad 1 -108 (+) Cu(g) −−→ Cuad 2a,b -116 Formation of Cu clusters from Cu atoms on the ZnO surface (+) 2 Cuad −−→ [Cu2 ]ad 4a -132 (+) (+) 3 Cuad −−→ [Cu3 ]ad 5a -185 (+) 4 Cuad −−→ [Cu4 ]ad 6a -298

∆E (PBE) -94 -183 -61 -158 -204

(+)

6. 7.

1 1

1 2

Successive water adsorption onto Cuad (+) H2 Oad + Cuad −−→ [Cu(OH2 )]+ 3a ad + + H2 Oad + [Cu(OH2 )]ad −−→ [Cu(OH2 )2 ]ad 3b

8. 9a. 9b.

2 2 2

1 2 2

Successive water adsorption onto [Cu2 ]ad H2 Oad + [Cu2 ]ad −−→ [Cu2 (OH2 )]ad 4b 2+ H2 Oad + [Cu2 (OH2 )]ad −−→ [Cu2 (OH2 )2 ]ad 4c H2 Oad + [Cu2 (OH2 )]ad −−→ 2 [Cu(OH2 )]+ 3a ad

1 1 2 3

Successive water adsorption onto [Cu3 ]ad (+) H2 Oad + [Cu3 ]ad −−→ [Cu3 (OH2 )]+ 5b ad (+) + H2 Oad + [Cu3 ]ad −−→ [Cu(OH2 )]ad + [Cu2 ]ad 3a,4a H2 Oad + [Cu3 (OH2 )]+ −→ [Cu3 (OH2 )2 ]+ 5c ad − ad + H2 Oad + [Cu3 (OH2 )2 ]ad −−→ [Cu3 (OH2 )3 ]+ 5d ad

+15 -20 +21 +62

-37 -5 +13 +43

1 2 2 3 4

Successive water adsorption onto [Cu4 ]ad H2 Oad + [Cu4 ]ad −−→ [Cu4 (OH2 )]ad 6b H2 Oad + [Cu4 (OH2 )]ad −−→ [Cu4 (OH2 )2 ]+ 6c ad H2 Oad + [Cu4 (OH2 )]ad −−→ 2 [Cu2 (OH2 )]ad 4b + + H2 Oad + [Cu4 (OH2 )2 ]ad −−→ [Cu4 (OH2 )3 ]ad 6d + H2 Oad + [Cu4 (OH2 )3 ]+ − − → [Cu (OH ) ] 6e ad 4 2 4 ad

-71 +69 +13 +49 +82

-82 -38 +60 +23 +102

-74 +86

-102 +83

-45 +54 +29

-51 -77 -91

(+)

10a. 10b. 11. 12. 13. 14a. 14b. 15. 16.

3 3 3 3 4 4 4 4 4

15



DOS α

β

}

conduction band (DOS scaled 6 times)

CBM

H water O surface Zn

3.40 eV band gap

Ef

surface O subsurface Zn

VBM

subsurface O

}

energy

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 22

valence band

Figure 1: Structural depictions and spin-polarized HSE06 0 -calculated electronic densities of states (DOS) for the most stable adsorption conguration for a water molecule on ZnO(10 10) (H2 Oad ). In the DOS, the lower and upper dashed red lines mark the positions of the clean slab VBM and CBM, respectively, and they are separated by a 3.40 eV energy dierence (energy runs along the vertical axis). The blue line marks the position of the Fermi level Ef (all states below the blue line are occupied). Because of the small values of the DOS in the conduction band, the DOS in the conduction band has been scaled 6 times.

16


Page 17 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a)

(b)

Figure 2: Structural depictions and spin-polarized HSE06 0 -calculated DOS for (a) Cu ad (adsorption site a ), and (b) Cu+ ad (adsorption site b ). Cu is shown in green. The DOS is explained in the caption to Figure 1. The DOS for the mid-gap states have been scaled 6 times together with the CB DOS for clarity.

(a)

(b)

Figure 3: Structural depictions and HSE06 0 -calculated DOS for (a) [Cu(OH 2 )]+ ad , and (b) [Cu(OH2 )2 ]+ . The DOS is explained in the captions to Figures 1 and 2. ad

17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a)

Page 18 of 22

(c)

(b)

Figure 4: Structural depictions and HSE06 0 -calculated DOS for (a) [Cu 2 ]ad , (b) [Cu2 (OH2 )]ad , and (c) [Cu2 (OH2 )2 ]2+ ad . The DOS is explained in the captions to Figures 1 and 2.

18


Page 19 of 22

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(b)

(a)

(d)

(c)

Figure 5: Structural depictions and HSE06 0 -calculated DOS for (a) [Cu 3 ]ad , (b) + + [Cu3 (OH2 )]ad , (c) [Cu3 (OH2 )2 ]+ ad , and (d) [Cu3 (OH2 )3 ]ad . The DOS is explained in the captions to Figures 1 and 2.

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a)

Page 20 of 22

(c)

(b)

(e)

(d)

Figure 6: Structural depictions and HSE06 0 -calculated DOS for (a) [Cu 4 ]ad , (b) + + [Cu4 (OH2 )]ad , (c) [Cu4 (OH2 )2 ]+ ad , (d) [Cu4 (OH2 )3 ]ad , and (e) [Cu4 (OH2 )4 ]ad . The DOS is explained in the captions to Figures 1 and 2.

20


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Relative water adsorption energy (kJ/mol)

Page 21 of 22

PBE HSE06’

30 25 20 15 10 5 0

Figure 7: Relative adsorption energies for some [Cu(OH 2 )]+ ad congurations. Top views of 0 the congurations optimized with HSE06 are shown (the PBE-optimized structures are very similar).

21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

H2O dissociates Cu dissociates Cu gets oxidized

ΔE = -20 kJ/mol

22

e-


Page 22 of 22

Water-Induced Oxidation and Dissociation of Small Cu Clusters on

Recommend Documents