Electronic Origin of the Surface Reactivity of Transition-Metal-Doped

Subscriber access provided by UNIV OF ALABAMA BIRMINGHAM

Article

The Electronic Origin of the Surface Reactivity of Transition Metal Doped TiO(110) 2

Mónica García-Mota, Aleksandra Vojvodic, Frank Abild-Pedersen, and Jens K. Norskov J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp310667r • Publication Date (Web): 11 Dec 2012 Downloaded from http://pubs.acs.org on December 12, 2012

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 24

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

The Electronic Origin of the Surface Reactivity of Transition Metal Doped TiO2(110) Mónica García-Mota,1 Aleksandra Vojvodic, *1 Frank Abild-Pedersen, 2 and Jens K. Nørskov1

1

SUNCAT Center for Interface Science and Catalysis, Department of Chemical Engineering,

Stanford University, Stanford, CA 94305, USA

2

SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, 2575 San Hill Road, Menlo Park, CA 94025, USA * [email protected]

ABSTRACT: We investigate the surface reactivity of doped rutile M-TiO2(110) (M=V, Cr, Mo, W, Mn, Fe, Ru, Co, Ir, and Ni) using density functional theory (DFT) and Hubbard-U corrected DFT calculations (DFT+U method). The oxygen adsorption bond, used as the surface reactivity measure, is stronger on the doped TiO2 surfaces as compared to that on the undoped TiO2 surface. We relate this increase in reactivity of the doped TiO2 surfaces to the presence of localized surface resonances and surface states in the vicinity of the Fermi level. We find that the center of these localized states on doped TiO2 is a good descriptor for the oxygen adsorption energy. The inclusion of the Hubbard-U correction to DFT barely modifies the oxygen adsorption energy on undoped TiO2 while it destabilizes the oxygen adsorption energies on 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

doped TiO2 when compared to results from standard DFT. Nevertheless, we find that the oxygen adsorption energy trends predicted by a standard GGA-DFT functional are reproduced when the Hubbard-U correction is applied.

Keywords: oxide surfaces, DFT, Hubbard correction, electronic structure, surface states and surface resonances, descriptor

2


Page 2 of 24

Page 3 of 24

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


Introduction

An understanding of the correlation between the electronic structure and reactivity of a surface enables tailoring of the surface, which ultimately can lead to design of novel catalysts for a broad range of reactions. For transition metal surfaces the d-band model provides a theoretical framework for understanding trends in reactivity from one surface to the next.1 When it comes to transition metal compounds, it has been shown that the reactivity of transition metal carbides can be understood in terms of an electronic structure descriptor based on the surface states and the surface resonances present on the surfaces of these compounds.2 In the present paper we address the problem of electronic structure descriptors of reactivity trends for transition metal oxide surfaces. We focus on surface properties of titanium oxides that are of importance both in fundamental research and in existing technological applications as well as in potential new applications. The TiO2 surfaces, especially the (110) surface of rutile TiO2, have become prototype model systems in the surface science of metal oxides.3 Both bulk and surface properties of rutile TiO2 have been studied extensively using density functional theory (DFT) calculations.4 In addition, titanium oxides are used in a wide variety of technological applications where surface properties play a crucial role. For example, TiO2 is widely used in heterogeneous catalysis as a corrosion-protective surface coating and as a photocatalyst.5-7 The photocatalytic capabilities of TiO2 are hampered due to the large band gap (3.0eV and 3.2eV for rutile and anatase, respectively) of the material.8,9 One promising technique for improving the activity of photoelectrolysis cells is doping. It has been suggested that substitutional dopant ions can induce an electronic coupling effect with the host atoms of the material which in turn gives rise to electronic states in the band gap of TiO2.10 Doping TiO2 with transition-metal and non-metal 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

elements results in modifications of the surface properties.11-19

Page 4 of 24

In particular, it has been

demonstrated that the presence of a well-chosen dopant improves the performance of rutile TiO2 in some heterogeneous catalytic reactions, such as CO oxidation,20 and acetone oligomerization.21 We have recently found an enhanced electrochemical activity for the oxygen evolution reaction of doped rutile TiO2 surfaces compared with the one of the undoped TiO2 surface.22

We first study the reactivity of a set of doped TiO2 surfaces by adsorbing oxygen on these surfaces. Then we investigate the predictions of standard DFT. This is followed by a comparison to results from DFT+U calculations in which a Hubbard-type term is added to account for the on-site Coulomb interactions of the localized d orbitals.23-25 We consider several different combinations of U values. Finally, we explain the oxygen adsorption energy trends by analyzing the electronic structure, in particular the d-projected density of states (PDOS), of the surface atoms of the doped TiO2 systems. We identify a descriptor, namely the center of mass of states in the vicinity of the Fermi level, which resembles the descriptor for adsorption on carbides suggested in Ref. [2].

Methods

We study the adsorption of oxygen on transition metal doped TiO2(110) surfaces by firstprinciples density functional theory (DFT)26 calculations. All calculations are performed using the GPAW code27 and the ASE simulation package.28 In the simulations we use the RPBE exchange-correlation functional,29 PAW pseudopotentials,30 a uniform real-spaced grid with a spacing of 0.2Å for the representation of the electronic wavefunctions, and a 3x3x1 MonkhorstPack k-point sampling.31 The surfaces are modeled by (1x2) supercell slabs that are 4


Page 5 of 24

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


separated by more than 16Å of vacuum. We consider O adsorption on top of the stoichiometric TiO2(110) slab. This is the equilibrium state of the surface under the extremely oxidizing conditions relevant for the electrochemical oxygen evolution. The oxygen adsorbate (O*) and the two top Ti-layers are allowed to relax while the lowest layers are kept frozen at their bulk positions. We carry out spin-polarized calculations and include the dipole correction.32

We simulate the doped rutile TiO2(110) surfaces by a substitutional model with a 6.25% of transition metal impurities relative to the host Ti atoms in the slab, that is, M-Ti15O32. The following transition metals: M=V, Cr, Mo, W, Mn, Fe, Ru, Co, Ir, and Ni are considered as dopants in a five-coordinated (5c-M) substitutional site in the top-most surface layer of the TiO2(110) surface, see Figure 1b. Many monovalent transition metal oxides consisting of M metal atoms are stable in the rutile phase.33 We find that the considered doped TiO2 surfaces do not show any substantial surface reconstruction. Oxygen is adsorbed on either the five-fold coordinated dopant M atom or the five-fold coordinated Ti atom, and it pulls the surface atom outwards. Using standard DFT, we find the strongest oxygen-surface bond in the case when oxygen is adsorbed on-top of the five-fold coordinated M dopant atom. Therefore, we consider the five-fold coordinated site as the active site and will focus on this site in the rest of the analysis.

It is known that DFT fails to obtain correct electronic structure for strongly correlated systems due to errors associated with the on-site Coulomb and exchange interactions.34 To investigate these effects, we apply a Hubbard-U correction (DFT+U method) as implemented in GPAW 35 to improve the description of localized d-electrons of Ti and M in the M-doped TiO2 systems. An alternative would be to use hybrid DFT, however, it is computationally heavy for large 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

systems.36 There is no universally accepted method for the choice of the value of U. Ceder and co-workers carried out a systematic study of the oxidation energies of transition metal oxides and suggested that the choice of U should be such that the formation energies of different oxides are described accurately.23 A similar approach has been used in recent studies, where it was suggested that the U value should be taken to fit the reaction energy of the different oxide phases relevant for the catalytic reaction under study.24,25 The reaction enthalpy of TiO2 can be obtained from the reaction energy for the oxidation of TiO2 to Ti2O3 according to the equation:

2TiO2 + H2 → Ti2O3 + H2O

(1)

To calculate the reaction enthalpies, we fully optimize the titanium structures for each of the tested U values applying the U ontop of RPBE. We avoid the well-known errors associated with the DFT-calculated energy of O2 by using the H2O reference.37 The experimental reaction enthalpy for reaction (1) at room temperature is 1.30eV. The value calculated using the RPBE functional is 1.51eV, which is close to the experimental value (see Figure 1). The difference between the experimental and theoretical values is similar to the one found for transition metal surfaces and can be attributed to any number of shortcomings of semi-local DFT and accuracy of the experimental method. If we choose to associate the difference to the effect of an on-site coulomb repulsion, we find that the calculated reaction enthalpy is equal to the experimental one at a value of U=2.0eV, see Figure 1a. This value is in reasonable agreement with previous studies.24 Alternatively, the U parameter can be obtained by fitting the one electron band gap to the experimental one. By using this method we find a value of U ~ 5.0eV, also in agreement with previous studies.38 Calculations by Hammer35 and co-workers show that a value of U=2.5eV is needed for the description of the defect states in bulk TiO2 to be in good agreement with 6


Page 6 of 24

Page 7 of 24

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


experimental data. In the case of a reduced TiO2(110) surface, Watson et al.39 demonstrated that a value of U can be chosen to get a reasonable localization of the O vacancy state. However, they also show that it is not possible to correctly describe both the conduction band – valence band gap and the localization nature of the defect state with a single value of U. In addition, it has been argued that one should not expect to be able to open up the band gap to its experimental value using any “reasonable” value of U.35,40 A too high U value can result in a wrong description of the adsorbate-surface bond when it comes to adsorption properties.41 We note that using U=5.0eV, which gives the correct band gap, would result in a poor description of the reaction energy for Eq. (1), see Figure 1a.

When it comes to doped systems, it is not clear which U value to use for the M dopant, especially since different U values may be needed to properly describe the redox reactions and band gaps of the different MxOy oxides. We perform a series of calculations applying a constant Hubbard-U value on Ti (UTi) of 2.0eV and different U values on the dopant (UM=0, 1, 2eV). Our approach can be considered as a first approximation to monitor the sensitivity of the oxygen adsorption energy trends on the choice of U for the doped TiO2 surfaces.

Results and Discussion

Oxygen Adsorption Energetics We probe the surface reactivity by studying oxygen adsorption on a set of doped TiO2 surfaces. Figure 2 shows the calculated adsorption energies on the five-fold coordinated M dopant site on the considered doped surfaces. First, we calculate the oxygen adsorption energy (ΔEO) on MTi15O32(110) (M=V, Cr, Mo, W, Mn, Fe, Ru, Co, Ir, and Ni) by using a standard GGA 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

functional, RPBE to be precise. We find that the oxygen interaction with all of the doped TiO2 surfaces is stronger (smaller ΔEO) than the one with undoped TiO2. In addition, we find that the oxygen-surface bond gets weaker with increasing group number of the M dopant. The oxygen adsorption energies range from -0.46 on W-Ti15O32 to +4.42eV on Ni-Ti15O32 which should be compared to +4.47eV on the undoped TiO2(110) surface. An interesting observation is that the variations in oxygen adsorption energies are larger within a group than within a period as opposed to what is found for metals. In addition, the oxygen-surface bond gets stronger as the dopant is changed from 3d to 4d to 5d within a group, which also differs from the trend found for metals.

Next, we apply a Hubbard-U correction of UTi=2eV on Ti and either UM = 0, 1 or 2eV on the M dopant. Our calculations show that the Hubbard-U correction barely affects the adsorption energy of oxygen on the undoped TiO2(110) surface when compared to result from standard GGA, see dashed horizontal lines in Figure 2. For UTi=2eV and UM= 0 the adsorption energies are not modified as compared with DFT. However, we find that the inclusion of UM=1 or 2eV destabilizes O* on all considered doped TiO2 surfaces (except Ni-Ti15O32), see Figure 2. Nevertheless, we observe that the oxygen adsorption energy trends predicted by standard DFT are reproduced when the Hubbard-U correction is applied. For the UTi=UM=2eV case, the oxygen adsorption energy ranges from -0.14 on W-Ti15O32 to +4.66eV on Ni-Ti15O32, which correspond to a weaker oxygen-surface bond as compared to corresponding results from RPBE. We notice that the oxygen adsorption energy scales linearly with the Hubbard-U correction. Hence, our result together with the findings of Helali41 et al., who studied adsorption of transition metal atoms on TiO2(110), show that the introduction of a reasonable Hubbard-U correction preserves 8


Page 8 of 24

Page 9 of 24

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 atomic adsorption energy trends.

Density of States Analysis

To be able to understand the trends in oxygen adsorption energies we analyze the electronic structure, namely the metal projected density of states (PDOS), of the surface atoms of the clean undoped and doped TiO2(110) surfaces as well as the PDOS of the adsorbed oxygen. We start by analyzing the PDOS’s of the undoped TiO2 surface followed by the doped ones. The analysis is based on DFT calculations and calculations with UTi=UM=2eV.

When it comes to the electronic structure of the undoped TiO2(110) surface, we find that RPBE predicts a gap for TiO2, however, as expected it is smaller than the experimental one.8 The valence band-edge is 1.4eV below the Fermi level and the conduction band-edge is 0.7eV above the Fermi level, see Figure 3a. The effect of U on the PDOS’s is a small redistribution and broadening of the states in the valence band, and a conduction band that is slightly pushed toward higher energies resulting in a marginally larger band gap. While the band gap is indeed underestimated with standard GGA-DFT, that does not affect the state distribution and occupancy significantly. This is reflected in the oxygen adsorption energy which is almost the same from DFT and DFT+U calculations. The adsorbed O 2p, positioned in the TiO2 band gap, is only slightly overlapping with the TiO2 valence band, see Figure 4a. Hence, the oxygensurface interaction is weak. We find that the oxygen adsorption on the five-fold coordinated Ti site is properly described with standard DFT. This result is in agreement with recent work by Valdes et al. showing that the inclusion of self-interaction correction to DFT does not cause any significant difference in the oxygen adsorption energy on a five-fold coordinated Ti site of the 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

Page 10 of 24

TiO2(110), however, it does change the oxygen vacancy formation energy.42

To illustrate the effect of an M dopant on the electronic structure of the TiO2(110) surface, we have chosen to show the d-projected density of states of Cr of Cr-Ti15O32 as a representative of the doped surfaces, see Figure 3b, although the whole series has been studied, see Figure 1 in Supporting Information. The d-PDOS of the Cr dopant atom of the clean Cr-Ti15O32(110) surface shows more features than the d-PDOS of Ti of the clean TiO2(110) surface. We observe that occupied states throughout the upper part of the valence band and unoccupied states in the band gap have emerged as compared to the PDOS of undoped TiO2(110). We will refer to these states as surface resonance states (SR) if they overlap with states of undoped TiO2 and surface states (SS) otherwise. In the literature the SS are sometimes referred to as gap states. Identification of SR and SS on doped TiO2 surfaces resembles previous experimental and theoretical observations showing that doping transition-metals into the TiO2 lattice narrows its band gap.13-15 We find that the Cr d states overlap with Ti d states (compare black and dashed lines in Figure 3b) indicating that there is a hybridization between the nearest Cr atom and the Ti in the Cr-Ti15O32. The effect of U on the d-PDOS of Cr reveals a reduction of the number of lower lying SS, a significant redistribution of the number of states in the valence band, and a shift up of the conduction band of ~0.5eV as compared with d-PDOS based on a DFT calculation. However, the position and intensity of the SS at ~0.5e are only changed slightly when treated with the DFT+U method. The fact that the SS is almost unaffected suggests that the PDOS is converged with respect to U and that the band gap is wide enough.

The interaction between the SR and SS localized on the Cr dopant and the O 2p adsorbate levels should give rise to bonding and

antibonding 10


O

states.

This

is

indeed

Page 11 of 24

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


confirmed by analyzing the O 2p PDOS after adsorption and comparing it to the PDOS of the clean Cr-Ti15O32 surface, see Figure 4b. The O 2p bonding peak is positioned in the lower part of the Cr valence band. In addition, there is a distribution of O 2p states throughout the upper part of the valence band and there is a peak located just below and a peak just above the Cr-localized SS ~0.5eV present at the clean surface. The existence of clear bonding and antibonding peaks below and above the SS in the band gap suggests that the adsorption energy of O on the CrTi15O32(110) surface is due to strong interaction between the adsorbate and the surface states in the vicinity of the Fermi level. Based on this conclusion we can understand why the O adsorption energy obtained from DFT+U is weaker as compared with the one from DFT. The DFT+U PDOS of the clean Cr-Ti15O32(100) has a lower number of states in the gap compared with the DFT PDOS, see Figure 3b. Hence, the number of interacting states is reduced and gives a weaker adsorption energy for the DFT+U case. We observe a similar effect for the other doped TiO2 surfaces considered in this study, see Figure 2 in the Supporting Information.

Identification of an Electronic Structure Based Descriptor To be able to predict the adsorption energy of a species on a given surface or design a surface that will give a certain adsorption energy, it is quite cumbersome to perform such a detailed electronic structure analysis as described above. A descriptor that is able to capture the most important electronic characteristics that can describe the trends is desirable. The interaction observed between oxygen and surface localized states of the doped TiO2 resembles the one between an adsorbate and the localized surface states of transition metal carbide surfaces,2 and between an adsorbate and a perovskite surface.

43,44

From our analysis of the density of states, we

observe that not all the d-states play the same role in the bonding. We attribute the main 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

contribution to the bonding to the states located in the vicinity of the Fermi Level. We calculate the center of mass of the extra occupied and unoccupied surface states (SS) localized on the dopant atom as the average of the integration of the d-PDOS in a certain range of energy weighted by their energy (εdSS). This is done in analogy with the εCCM descriptor for surface resonances on carbide surfaces2 and with the d-band center in the d-band model.1

Figure 5 shows a linear correlation between ΔEO and εdSS, which is calculated based on the states in the energy region between -3.0eV to +0.8eV relative to the Fermi level. In this way we include the surface states in the TiO2 gap as well as the surface resonance states that emerge at the upper shoulder of the valence band of the TiO2. This correlation exists independently if the εdSS descriptor is calculated based on d-PDOS from standard DFT or from DFT+U. We find that the εdSS based on the d-PDOS from DFT+U has a lower value than the one calculated from the DFT d-PDOS. Therefore, the DFT+U adsorption energy of oxygen is weaker as compared with the DFT one. Even though the interaction between the adsorbate and the oxide surface is complex, our findings imply that εdSS is a good enough descriptor (measure of the electronic structure of the surface) to account for variations in the adsorption energy trends. This finding leaves us with a simple calculation scheme for prediction of the reactivity of doped TiO2 since the SS can be identified based on the PDOS of the clean surface.

Conclusions

We have achieved an understanding of how one can tailor the surface reactivity of TiO2 by transition metal substitution based on the knowledge of the surface electronic structure. This knowledge can serve as a guideline in the design

of novel catalysts for a broad range of 12


Page 12 of 24

Page 13 of 24

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


reactions. We observe that the oxygen-surface interaction on doped TiO2 surfaces is significantly stronger than on the undoped TiO2 surface. We relate this fact to the presence of surface states and surface resonances localized on the dopant. The inclusion of the Hubbard-U correction barely affects the stability of oxygen on undoped TiO2 when compared to result from standard DFT. However, that correction destabilizes the oxygen adsorbate on doped TiO2 by 0.4-0.8eV. Nevertheless, oxygen adsorption trends predicted by standard DFT are reproduced when the Hubbard-U correction is applied. We explain oxygen adsorption energy trends by analyzing the d-projected density of states (d-PDOS) of the surface atoms of the doped TiO2 systems. In addition, we show that the center of the dopant derived surface states (εdSS) of M-Ti15O32 is a descriptor for the oxygen adsorption energy similar to the descriptor found for adsorption on carbide surfaces2. We have described a method for rational design by doping TiO2 for reactions involving oxygen adsorption which most probably can be used for similar oxide systems.

Acknowledgments MGM, AV and JKN acknowledge support from Center of Nanostructuring for Efficient Energy Conversion (CNEEC) at Stanford University, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Basic Energy Sciences under Award Number DESC0001060. FAB acknowledges support from the U.S. Department of energy (DOE) under the contract number DE-AC02-76SF00515. .

Supporting Information See supporting information for the d-projected density of states of the M surface atom of the 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

clean M-Ti15O32(110) surfaces (M=V, Cr, Mo, W, Mn, Fe, Ru, Co, Ir, and Ni) obtained with standard DFT and DFT+U (UTi=2 and UM=2eV) calculations (figure 1). Also included are the DFT+U calculated projected DOS of the systems where oxygen is adsorbed on-top of an M surface atom of the M-Ti15O32 (M=V, Cr, Mo, W, Mn, Fe, Ru, Co, Ir, and Ni) surface (figure 2). Both the O 2p PDOS and the d-PDOS of an M and Ti surface atoms are presented (figure 2). This material is available free of charge via the Internet at http://pubs.acs.org.

14


Page 14 of 24

Page 15 of 24

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


REFERENCES

1 Hammer, B; Nørskov, J. K. Adv. Catal. 2000, 45, 71-129. 2 Vojvodic, A.; Hellman, A.; Ruberto, C.; Lundqvist, B. I. Phys. Rev. Lett. 2009, 103, 146103. 3 Diebold, U. Surf. Sci. Rep. 2003, 48, 53-229. 4 Gong, X.-Q.; Khorshidi, N.; Stierle, A.; Vonk, V.; Ellinger, C.; Dosch, H.; Cheng, H.; Selloni, A.; He, Y.; Dulub, O.; Diebold, U. Surf. Sci. 2009, 603, 138-144. 5 Fujishima, A.; Honda, K. Nature 1972, 238, 37-38. 6 Linsebigler, A.L.; Lu, G.; Yates, Jr.J.T Chem. Rev. 1995, 95, 735-758. 7 Grätzel, M. Nature 2001, 414, 338-344. 8 Arntz, F.; Yacoby, Y. Phys. Rev. Lett. 1966, 17, 857-860. 9 Tang, H.; Lévy, F.; Berger, H.; Schmid, P.E. Phys. Rev. B 1995, 52, 7771-7774. 10 Matsumoto, Y.; Murakami, M.; Shono, T.; Hasegawa, T.; Fukumura, T.; Kawasaki, M.; Ahmet, P.; Chikyow, T.; Koshihara, S.; Koinuma, H. Science 2001, 291, 854-856. 11 Hu, Z.; Li, B.; Sun, X.Y.; Metiu, H. J. Phys. Chem. C 2011, 115, 3065-3074. 12 Metiu, H.; Chrétien, S.; Hu, Z.; Li, B.; Sun, X.Y. J. Phys. Chem. C 2012, 116, 1043910450. 13 Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269-271. 15



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

14 Radecka, M.; Wierzbicka, M.; Komornicki, S.; Rekas, M. Physica B 2004, 348, 160. 15 Devi, L.G.; Murthy, B.N. Catal. Lett. 2008, 125, 320-330. 16 Kubacka, A.; Colón, G.; Fernández-García, M. Catal. Today 2009, 143, 286-292. 17 Yin, W.-J.; Tang, H.; Wei, S.-H.; Al-Jassim, M.M.; Turner, J.; Yan, Y. Phys. Rev. B 82, 045106 (2010). 18 a) Dholam, R.; Patel, N.; Adami, M.; Miotello, A. Int. J. Hydrogen Energy 2009, 34, 53375346; b) Long, R.; Dai, Y.; Meng, G.; Huang, B. Phys. Chem. Chem. Phys. 2009, 11, 8165-8172; c) Lin, H.; Rumaiz, A.K.; Schulz, M.; Huang, C.P.; Shah, S.I. J. Appl. Phys. 2010, 107, 124305. 19 Gai, Y.; Li, J.; Li, S.S.; Xia, J.B.; Wei, S.H. Phys. Rev. Lett. 2009, 102, 36402. 20 Chrétien, S.; Metiu, H.; Catal. Lett. 2006, 107, 143-147. 21 Zamora, M.; López, T.; Gómez, R.; Asomoza, M.; Melendrez, R. Catal. Today 2005, 107108, 289-293. 22 García-Mota, M.; Vojvodic, A.; Metiu, H.; Man, I.C.; Su, H.Y.; Rossmeisl, J.; Nørskov, J.K. ChemCatChem 2011, 3, 1607-1611. 23 L. Wang, T. Maxisch, and G. Ceder, Phys. Rev. B 73, 195107 (2006). 24 Hu, Z.; Metiu, H. J. Phys. Chem. C 2011, 115, 5841-5845. 25 Lutfalla, S.; Shapovalov, V.; Bell, A.T. J. Chem. Theory Comput. 2011, 7, 2218-2223.

16


Page 16 of 24

Page 17 of 24

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


26 a) Anisimov, V.I.; Aryasetiawan, F.; Liechtenstein, A.I. J. Phys. Condens. Matter 1997, 9, 767-808. ; b) Anisimov, V. I.; Zaanen, J.; Andersen, O.K. Phys. Rev. B 1991, 44, 943-954. 27 Enkovaara, J.; Rostgaard, C.; Mortensen, J.J.; Chen, J.; Dulak, M.; Ferrighi, L.; Gavnholt, J.; Glinsvad, C.; Haikola, V.; Hansen, H.A. et al. J. Phys. - Cond. Mat. 2010, 22, 253202. 28 Bahn, S.R.; Jacobsen, K.W. Comp. Sci. Eng. 2012, 4, 56-66. 29 Hammer, B.; Hansen, L. B.; Nørskov, J. K. Phys. Rev. B 1999, 59, 7413-7421. 30 Kresse, G.; Joubert D. Phys. Rev. B 1999, 59, 1758-1775. 31 Monkhorst, H.J.; Pack, J.D. Phys. Rev. B 1976, 13, 5188-5192. 32 Bengtsson, L. Phys. Rev. B 1999, 59, 12301-12304. 33 Martínez, J.I.; Hansen, H.A.; Rossmeisl, J.; Nørskov, J.K. Phys. Rev. B 2009, 79, 045120. 34 Morgan, D.; Wang, B.; Ceder, G.; van de Walle, A. Phys. Rev. B 2003, 67, 134404. 35 Stausholm-Møller, J.; Kristoffersen, H.H.; Hinnemann, B.; Madsen, G.K.H.; Hammer, B. J. Chem. Phys. 2010, 133, 144708. 36 Marsman, M.; Paier, J.; Stroppa, A.; Kresse G. J. Phys.: Condens. Matter 2008, 20, 064201. 37 Nørskov, J.K.; Rossmeisl J.; Logadottir, A; Lindqvist, L.; Kitchin, J.R.; Bligaard, T.; Jónsson, H. J. Phys. Chem. B 2004, 108, 17886-17892. 38 Yang, K.; Dai, Y.; Huang, B.; Whangbo, M. H. Chem. Mater. 2008, 20 6528-6534. 39 Morgan, B. J.; Watson, G. W, Surf. Sci. 2007, 601, 5034-5041. 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

40 Solovyev, I.; Hamada, N.; Terakura, K. Phys. Rev. B 1996, 53, 7158-7170. 41 Helali, Z.; Markovits, A.; Minot, C.; Abderrabba, M. Struc. Chem. 2011, 86, 41-58. 42 Valdés, Á.; Brillet, J.; Grätzel, M.; Gudmundsdóttir, H.; Hansen, H.A.; Jónsson, H.; Klüpfel, P.; Kroes, G.J.; Le Formal, F.; Man, I.C. et al M. Phys. Chem. Chem. Phys. 2011, 14, 49-70. 43 Suntivich, J.; May, K.J.; Gasteiger, H.A.; Goodenough, J.B.; Shao-Horn, Y. Science 2011, 334, 1383-1385. 44 Vojvodic, A.; Nørskov, J.K. Science 2011, 334, 1355-1356.

18


Page 18 of 24

Page 19 of 24

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


Figure 1. a) Enthalpy of formation (ΔHf) of the “redox” reaction: 2TiO2+H2 →Ti2O3+H2O as a function of the Hubbard correction (U) applied. The horizontal dotted line indicates the experimental reaction enthalpy for the redox reaction at room temperature (1.30eV) and the vertical dotted line indicates the value of U at which the calculated reaction enthalpy equals the experimental one (U=2.1eV). The value of U to open up the band gap to its experimental value is also indicated with a vertical dotted line (U~5eV). b) Side view of the model M-Ti15O32 surface used in the DFT and DFT+U calculations. The dopant M is substituting a five-fold coordinated Ti atom (5c-M) in the top-most surface layer. The bridging oxygen atoms and the six-fold coordinated titanium atoms (6c-Ti) are indicated in the figure.

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

Page 20 of 24

Figure 2. Adsorption energy of oxygen (ΔEO) on M-Ti15O32(110) (M=V, Cr, Mo, W, Mn, Fe, Ru, Co, Ir, Ni) calculated by standard DFT calculations (red points) and when the Hubbard-U correction is included. Green (black) points represent results from a Hubbard correction of UTi=2 and UM=1eV(UTi=2eV and UM=2eV) for Ti and for M, respectively. The dashed red and black lines mark ΔEO on undoped TiO2 calculated by DFT and DFT+U (UTi=2eV), respectively. ΔEO is calculated relative to H2O and H2 in the gas phase.

20


Page 21 of 24

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


Figure 3. a) The d-projected density of states (d-PDOS) of a five-fold coordinated Ti surface atom of the pure TiO2 surface. b) d-PDOS of a five-fold coordinated Cr surface atom of the CrTi15O32 surface. The d-PDOS of a Ti atom in the surface of Cr-Ti15O32 calculated with UTi=2 and UCr=2eV has also been plotted (dashed black line). Red and black lines represent results for standard DFT and for DFT+U (UTi=2 and UCr=2eV) calculations, respectively.

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

Figure 4. a) The d-projected density of states (d-PDOS) of a five-fold coordinated Ti surface atom of the pure TiO2 surface (black line) and the 2p-PDOS of an oxygen atom adsorbed on-top Ti of the pure TiO2 surface (red area). b) d-PDOS of Cr and Ti of the Cr-Ti15O32 surface (black solid and dashed lines, respectively) and 2p-PDOS of an oxygen atom adsorbed on-top Cr of the Cr-Ti15O32 surface. The d-PDOS are based on DFT+U (UTi=2 and UCr=2eV) calculations.

22


Page 22 of 24

Page 23 of 24

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


Figure 5. Correlation between the oxygen adsorption energy on doped TiO2 (ΔEO) and the center of mass of the d-projected surface and resonance states (εdSS) in the selected energy region EF3eV< E< EF+0.8eV of the surface atom where the adsorption takes place. The linear relationship based on DFT calculations follows ΔEO= -2.59*εdSS +0.83, with a coefficient regression factor of 0.68. The linear relationship based on DFT+U calculations follows ΔEO= -2.91*εdSS – 0.45, with a coefficient regression factor of 0.74. Red and black points represent results from standard DFT and DFT+U with UTi=UM=2eV, respectively. The dashed red and black lines mark ΔEO on undoped TiO2 calculated with DFT and DFT+U (UTi=2eV), respectively. ΔEO is calculated relative to H2O and H2 in the gas phase.

23



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

24


Page 24 of 24

Electronic Origin of the Surface Reactivity of Transition-Metal-Doped

Recommend Documents