Water Adsorption and Dissociation Processes on Small Mn-Doped

7 Jan 2014 - Metal oxide complexes have high catalytic potential in many fields, such as oxidation, dehydrogenation, dehydration, reductive coupling, ...
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Water Adsorption and Dissociation Processes on Small Mn-Doped TiO2 Complexes Choongkeun Lee and Christine M. Aikens* Department of Chemistry, Kansas State University, Manhattan, Kansas 66506, United States ABSTRACT: Metal oxide complexes have high catalytic potential in many fields, such as oxidation, dehydrogenation, dehydration, reductive coupling, etc. The adsorption of molecules is a fundamental process in catalytic reactions on the metal oxide complex. In this study, water adsorption and dissociation processes on small Mn-doped TiO2 complexes are investigated at the density functional theory (DFT) level of theory. Water adsorption at terminal Mn atoms is typically found to have an energy around −0.7 eV, which is smaller than the −1.2 eV observed at terminal Ti atoms. Dissociation energies at Mn atoms are determined to be about −0.6 eV, which are also smaller than the approximately −1.2 eV dissociation energies at Ti atoms. Molecular adsorption without dissociation is favorable in energy after water adsorbs at each metal atom. Mn doping reduces the reaction energy; the reaction energy of the doped system is not similar to that of the pure manganese oxide complex.

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INTRODUCTION Metal oxides have been used as catalysts in many fields for processes such as oxidation, dehydrogenation, dehydration, reductive coupling, etc., because of their wide range of surface reactivity.1 They are also of interest as catalysts for photooxidations and as chemical sensors. The adsorption of molecules on the metal oxides is a fundamental process that affects their ability to work as catalysts. In addition, the physicochemical properties of adsorbates may be changed on the metal oxide surface. Many studies have been undertaken to understand the fundamental interactions between adsorbates and metal oxides.2−8 Photocatalysts using metal oxide complexes have been a strong area of focus after Fujishima and Honda announced electrochemical photolysis of water on TiO2.9 The adsorption and dissociation processes of water molecules on metal oxide complexes have been investigated to understand the water oxidation process by photocatalysis.3,5,6,10−15 On TiO2, water usually adsorbs at a missing oxygen defect site, which is called bridge bonded oxygen vacancy.16 The coordination number of the Ti atom at the defect site is lower than that at a normal site. Two forms of adsorptions, dissociative and molecular, are possible when water adsorbs at a defect site on anatase TiO2. The adsorption strongly depends on the surface of anatase. On the (101) surface, the molecular water adsorption energy (about −0.7 eV) is much larger than that of the dissociated case (about −0.4 eV), but dissociative water adsorption on the (100) surface is more stable than the molecular adsorption structure and water molecules are found to dissociate spontaneously.17 Molecular or dissociative water adsorption is influenced by temperature. Dissociative water adsorption only occurs above 230 K on the anatase TiO2(001) surface, but © 2014 American Chemical Society

below 230 K both dissociative and molecular adsorptions are observed.19 Only molecular water adsorption is possible below 160 K on the rutile TiO2(110) surface because of the dissociation energy barrier height.18,19 Mn is a common doping material to improve the photocatalytic properties of pure TiO2.20 Mn is known to generate a vacant level in the band gap of TiO2.21,22 Our previous work described the Mn doping effect on the HOMO− LUMO band gap and on structural changes of various TiO2 complexes. The band gap was significantly decreased by Mn doping, but it did not decrease proportionally with the number of doped Mn unlike the bulk system.22 The aim of this research is to study how water adsorption and dissociation is affected by Mn doping in small TiO2 small complexes. Water adsorption and dissociation processes on MnxTin−xO2n (n = 2, 3; x = 0, 1) complexes are investigated in this study.

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CALCULATION METHOD Geometry optimizations are performed with the Amsterdam density functional (ADF) package.23 All calculations are performed using the BP86 functional and the ATZP (augmented polarized triple-ζ) basis set.24−29 Initial Ti2O4 and Ti3O6 structures are obtained from ref 30 and the Mndoped structures, MnTiO4 and MnTi2O6, are obtained from ref 22 (Figure 1). Out of six different structural isomers for Ti3O6,22,30 linear Ti3O6 is chosen for this study because the linear form is the most stable of the isomers and its Mn-doped Received: October 9, 2013 Revised: December 18, 2013 Published: January 7, 2014 598

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Figure 1. Initial structures of pure Ti2O4, Mn-doped TiO2, and Mn2O4 complexes.

form is also the most stable.22 The oxidation state of Mn atom is set at 4+, because the oxidation state of Ti is also 4+, to maintain the neutral charge of the Mn-doped TiO2 complex; the multiplicity of the singly Mn-doped system is 4. Unrestricted calculations31 are performed for the optimization of intermediates and transition states for water molecule adsorption and dissociation on Mn-doped TiO2 complexes. Initial structures for transition states were obtained from linear least motion paths with the MacMolPlt graphical user interface program.32 The transition state structures were confirmed by Hessian calculations which showed one imaginary frequency. All structures are shown in Figures 2−14 and are labeled by bold-faced numbers. The reaction energies are calculated as follows: ΔEads = E(MO·OH 2) − E(MO) − E(H 2O)

(1)

ΔE‡ = E(MO·H ·OH) − E(MO·OH 2)

(2)

ΔErxn = E(MOH ·OH) − E(MO·OH 2)

(3)

Figure 3. Third water adsorption and dissociation on the MnTiO4 complex. Relative energies are in electronvolts. The black arrows represent the reactions at Mn, and the red arrows represent the reactions at the Ti atom. The solid line means the water dissociation process, and the dashed line means the water adsorption reaction.

Figure 4. Fourth, fifth, and sixth water adsorption and dissociation on the MnTiO4 complex. Relative energies are in electronvolts. The black arrows represent the reactions at Mn, and the red arrows represent the reactions at the Ti atom. The solid line means the water dissociation process, and the dashed line means the water adsorption reaction.

‡

where ΔEads, ΔE , and ΔErxn are water adsorption, activation, and dissociation energies, respectively, and MO represents the metal oxide cluster.

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RESULTS AND DISCUSSION Water adsorption and dissociation processes on singly Mndoped (TiO2)n (n = 2,3) are described in detail in this section. 1. MnTiO4. Water adsorption and dissociation processes on the MnTiO4 complex (denoted 1) are shown in Figures 2−4. a. First Water Molecule Adsorption and Dissociation. Water can adsorb to either Mn or Ti as shown in Figure 2. The structure in which the first water molecule adsorbs at Ti (denoted 2) is more stable than that in which the water adsorbs

Scheme 1. Hydrogen Dissociation of First Adsorbed Water Molecule on MnxTi2‑xO4 (x = 0, 1, 2): (a) First Water Adsorption Structure; (b), (c) Transition States of the First and Second Pathways, Respectively

at Mn (denoted 3), by about 0.51 eV. As shown in Scheme 1a, hydrogen dissociation can proceed in two ways: dissociation to a terminal oxygen or to a bridging oxygen with transition states as shown in Scheme 1b,c, respectively. Dissociation to the terminal oxygen yields a more stable isomer than dissociation to the bridging oxygen. Reaction energies for dissociation to a bridging oxygen are −0.29 eV regardless of the metal atom involved. For the two structures with dissociation to the terminal oxygen, 4 is more stable than 5 by about 1.12 eV, but the activation energy for water dissociation at Mn is smaller than that at the Ti site by about 0.10 eV. The activation energies for these two reactions are 0.29 and 0.39 eV, respectively, so they are reasonably accessible at room temperature. b. Second Water Molecule Adsorption and Dissociation. As shown in Scheme 2, the second water can adsorb in two positions. In pathway A, the second water molecule adsorbs at

Figure 2. First and second water adsorption and dissociation processes on the MnTiO4 complex. Relative energies are in electronvolts. The black arrows represent the reactions at Mn, and the red arrows represent the reactions at the Ti atom. The solid line means the water dissociation process, and the dashed line means the water adsorption reaction. 599

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Scheme 2. Second Water Molecule Adsorption on MnTiO4a

a

process (leading to structure 16), one hydrogen atom in the water behind the Ti atom dissociates to a bridging oxygen atom (second pathway in Scheme 1a). During the dissociation process, one hydrogen atom in the water adsorbed on top of the Mn atom (in Figure 2) transfers to the next hydroxyl group, which is on the top of the Ti atom in Figure 2, and the resulting hydroxyl group makes a new bridge between Mn and Ti atoms. In structure 16, three bridging hydroxyl groups exist and two metal atoms have hexacoordinated structure due to the new bridging hydroxyl group. f. Sixth Water Molecule Adsorption and Dissociation. The sixth water molecule can adsorb at the Ti atom of structure 15, because both of the metal atoms already have octahedral coordination in structure 16, as shown in Figure 4. This adsorption process leads to structure 17. The reaction energy is −1.36 eV, which is larger than any other adsorption energy in previous adsorptions. The dissociation energy (leading to structure 18) is −0.55 eV. The activation energy of this dissociation process is 0.73 eV, which is larger than the previous one. g. Water Adsorption on MnTiO4 without Dissociation. As mentioned above, the water dissociation process is not an exothermic process after the third water molecule adsorption, although the reaction leading to structure 11 is only slightly endothermic at this level of theory. In this section, additional water adsorption without dissociation is described, as shown in Figure 5. There are two isomers when three water molecules

M1 is Mn and M2 is Ti, or M1 is Ti and M2 is Mn.

the same metal atom as the first water adsorption; in pathway B, the water adsorbs at the other metal atom. Pathway B is favored over pathway A. The adsorption energy of the water on Mn (6) is −0.69 eV, which is close to the first water adsorption energy at Mn (3) of −0.71 eV; similarly, adsorption of the second water on Ti (7) is exothermic by −1.25 eV, which is quite close to the first adsorption energy of −1.22 eV on Ti (2). Structure 6 is more stable than structure 7 by about 0.53 eV. Dissociation to a terminal oxygen as shown in Figure 2 is again preferred over dissociation to bridging oxygens; reaction energies to bridging oxygens are predicted to be −0.02 eV (dissociation from Mn−H2O to bridging oxygen) and −0.44 eV (dissociation from Ti−H2O to bridging oxygen). The dissociation process to a terminal oxygen from either 6 or 7 leads to the same isomer, 8. The activation energies for these dissociation reactions are 0.22 and 0.34 eV, respectively, which are little bit smaller than those of the activation energies for the dissociation of the first adsorbed water. c. Third Water Molecule Adsorption and Dissociation. The third water molecule can adsorb at the Ti site of 8 (leading to isomer 9) or at the Mn site (isomer 10), as shown in Figure 3. Adsorption energies do not differ greatly between the two paths. 10 is slightly more stable (by 0.03 eV) than 9. The adsorption energy of the third water is significantly smaller than those of previous water adsorption steps, as shown in Figure 2, although it is still about −0.5 eV. The third water molecule can only dissociate to a bridging oxygen because all terminal oxygens are already occupied by hydrogen. Dissociation at the Mn site (isomer 11) is more stable than that at Ti site (isomer 12); the former is endothermic by only 0.03 eV, whereas the latter is endothermic by 0.33 eV. The activation energy for the dissociation at Mn site (11) has also a lower barrier than that at the Ti site (12), by 0.16 eV. d. Fourth Water Molecule Adsorption and Dissociation. Further water adsorption has been examined on the lower energy structure, 11. A fourth water molecule can adsorb at Mn (leading to structure 13 shown in Figure 4) or to Ti (not shown). The former is more stable than the latter by about 0.3 eV. This may be because this leads to a hexacoordinated Mn. Dissociation of the fourth water molecule (leading to structure 14) is again endothermic with a reaction energy of 0.33 eV and an activation energy of 0.85 eV, so it is not likely to occur. e. Fifth Water Molecule Adsorption and Dissociation. A fifth water is likely to absorb on structure 13 rather than the fully dissociated structure 14, because structure 14 is less stable than 13 and the activation energy for dissociation is high, as shown in Figure 4. The fifth water molecule can adsorb at the Ti atom in structure 13 (leading to structure 15), because Mn already has an octahedral structure; this adsorption process is exothermic by −0.96 eV. The dissociation and the activation energies are −0.29 and 0.12 eV, respectively. In the dissociation

Figure 5. Water adsorption without dissociation on the MnTiO4 complex. The black arrows represent the reactions at Mn, and the red arrows represent the reactions at the Ti atom. All reaction energies are adsorption energy, ΔEads, in electronvolts.

adsorb, denoted 9 and 10. Three isomers exist when the fourth water adsorbs, denoted 19, 20, and 21. The fourth water adsorption energies are all about −0.5 eV, but when water adsorbs at the metal atom not involved in the adsorption of the third water molecule, which is similar to pathway B of the second water adsorption, the energy is slightly more negative: −0.52 and −0.51 eV compared to −0.48 and −0.49 eV, similar to pathway A in Scheme 2. Of these three structures, 21 is the most stable. This may be due to two reasons. In 21, Mn achieves a coordination number of 6; in addition, there is hydrogen bonding between adsorbed water molecules. There are two structural isomers, 22 and 23, for the complex with five adsorbed water molecules. The latter is more stable than the former. These two isomers form two geometric isomers when the sixth water adsorbs. Two metal atoms are located in the center of a distorted octahedral structure, but the geometry of the adsorbed water molecules at Mn is different. The cis form (24) is more stable than the trans form (25) by about 0.23 eV. This may be due to the number of hydrogen 600

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bonds. There are three hydrogen bonds in the cis form, but there is only one hydrogen bond in the trans form, as shown in Figure 5. It should be noted that the cis form (24) is less stable than structure 18 by 1.60 eV, so water dissociation is favorable overall. 2. MnTi2O6. Two geometric isomers for MnTi2O6 are possible: in one, the Mn substitutes for the Ti at the central position, and in the other, it substitutes at the terminal position. The latter is more stable than the former, as described in our previous work,22 so it is considered in this study. Water adsorption and dissociation processes on the MnTi2O6 complex are displayed in Figures 6−9. Figure 8. Fourth water adsorption and dissociation on the MnTi2O6 complex. Relative energies are in electronvolts. The black arrows represent the reactions at Mn, the red arrows represent the reactions at the terminal Ti atom, and the blue arrows represent the reactions at the central Ti atom. The solid line means the water dissociation process, and the dashed line means the water adsorption reaction.

Figure 6. First and second water adsorption and dissociation on the MnTi2O6 complex. Relative energies are in electronvolts. The black arrows represent the reactions at Mn, the red arrows represent the reactions at the terminal Ti atom, and the blue arrows represent the reactions at the central Ti atom. The solid line means the water dissociation process, and the dashed line means the water adsorption reaction.

Figure 9. Fifth and sixth water adsorption and dissociation on the MnTi2O6 complex. Relative energies are in electronvolts. The black arrows represent the reactions at Mn, the red arrows represent the reactions at the terminal Ti atom, and the blue arrows represent the reactions at the central Ti atom. The solid line means the water dissociation process, and the dashed line means the water adsorption reaction.

with water dissociated at the terminal Ti atom (30) is more stable than that at the terminal Mn atom (31) by about 1.06 eV. The dissociation and activation energies of the first adsorbed water to terminal oxygen are −1.18 and 0.40 eV for 30 and −0.59 and 0.27 eV for 31, respectively. As in MnTi2O6, the activation energy for water dissociation at Mn is slightly smaller than at Ti, and the calculated barrier heights are similar to those predicted for MnTiO4 (about 0.3 and 0.4 eV, respectively). b. Second Water Molecule Adsorption and Dissociation. The adsorption of a second water molecule is preferred at the opposite terminal metal atom from the metal to which the first water molecule adsorbed, as shown in Figure 6. Adsorption energies at the terminal Mn (32) and terminal Ti (33) are predicted to be −0.78 and −1.23 eV, respectively; these values appear to be essentially independent of the order in which the water molecules adsorb as well as of whether or not intervening Ti atoms are present. 32 is more stable than 33 by about 0.6 eV. As before, structures in which the second water adsorbs to the central Ti (not shown) are less stable than the other isomers by about 0.2 eV (from 30) and 0.8 eV (from 31) compared to energies of 32 and 33, respectively. Water dissociation to a terminal oxygen is again more stable than dissociation to bridging oxygen atoms. The dissociation and

Figure 7. Third water adsorption and dissociation on the MnTi2O6 complex. Relative energies are in electronvolts. The black arrows represent the reactions at Mn, the red arrows represent the reactions at the terminal Ti atom, and the blue arrows represent the reactions at the central Ti atom. The solid line means the water dissociation process, and the dashed line means the water adsorption reaction.

a. First Water Molecule Adsorption and Dissociation. The first water adsorption prefers the terminal Ti site (27) similarly to the MnTiO4 complex. The isomer with water adsorbed at the central Ti atom (28) is less stable than the others, as shown in Figure 6. The adsorption energies are −1.24 eV for 27, −0.46 eV for 28, and −0.77 eV for 29. Again, dissociation of the first adsorbed water molecule at a terminal metal atom can occur to either a bridging or a terminal oxygen. Like the MnTiO4 system, dissociation to a terminal oxygen is more stable than dissociation to a bridging oxygen. The structure 601

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activation energies to terminal oxygen atoms are −0.59 and +0.23 eV from 32 and −1.20 and +0.38 eV from 33, respectively. Both pathways lead to the same structure, 34. c. Third Water Adsorption and Dissociation. Adsorption energies of the third water to Mn, Ti (central), and Ti (terminal) of 34 are −0.57, −0.56, and −0.52 eV, respectively, as shown in Figure 7. These adsorption energies are on the order of −0.5 and −0.6 eV, similar to the third water adsorption on MnTiO4. 35 is slightly more stable than 36. Upon water dissociation, the dissociated structure 38 is slightly more stable than 39, but it has a higher energy barrier leading to it of 0.51 eV. An energy barrier of 0.28 eV for dissociation from 36 is the lowest barrier height. Although structures 35 and 38 are more stable than structures 36 and 39, because the energy difference is very small (less than 0.03 eV) and the energy barrier is much higher than the process leading to 39, the reaction leading to 38 is less likely to occur. Two dissociated isomers are available for structure 36: structure 39 in which the hydrogen attaches to an oxygen bridging two Ti atoms, and structure 40 in which it dissociates to a bridging oxygen atom between Ti and Mn. The former is more stable than the latter by 0.21 eV, and the former pathway has a lower energy barrier by about 0.25 eV. The dissociation energy of 37 to 41 is −0.07 eV with an activation energy of 0.46 eV, so it is also less likely to occur than the pathway leading to 39. d. Fourth Water Adsorption and Dissociation. We consider the fourth water adsorption process starting from 39 because this is the most probable isomer from the first three water adsorption and dissociation processes. The adsorption energies of the third water at Mn (42), central Ti (43), and terminal Ti (44) are −0.68, −0.59, and −0.35 eV, respectively. In 43, the water is bound not by metal oxygen bonds but by hydrogen bonding, as shown in Figure 8. The dissociation energies of 42 (leading to 45) and 44 (leading to 46) are −0.13 and −0.06 eV, respectively, which are small compared to those of previous dissociation steps. The energy barrier of the process leading to 45 is higher than that of the process leading to 46, by about 0.15 eV, but the latter pathway is less likely to occur than the former process because structure 44 is much less stable than structure 42. The energy barriers of two pathways (leading to 45 and 46) are much higher than those of previous steps, by about 0.30 eV. e. Fifth Water Adsorption and Dissociation. When the fifth water adsorbs at the Mn in structure 45 (leading to 47) as shown in Figure 9, the reaction energy is −1.25 eV. Two more isomers exist in which the water adsorbs at Ti atoms (middle and terminal, not shown) which are less stable than 47 by 0.53 and 0.56 eV. A hydrogen bond between the adsorbed water and a bridging oxygen, as shown in Figure 4, occurs in the most stable structure 47. The structure is bent by the hydrogen bond. In its related dissociated form (48), the hydrogen bond is broken and the structure returns to linear. During the process, an OH group at Mn makes a bond to the Ti atom at the central position. The dissociation energy is −0.39 eV and the energy barrier is 0.44 eV. f. Sixth Water Adsorption and Dissociation. The adsorption energy is −0.95 eV when the sixth water adsorbs at the terminal Ti atom (49), as shown in Figure 9. The adsorption reaction breaks one hydroxo bridging bond to Ti (central positon) which is formed at the previous step in structure 48 and then the hydroxyl group makes a hydrogen bond to OH at the top of the central Ti atom. The dissociation energy (leading to 50) is −0.22 eV. In structure 50, the OH,

which is broken in 49, creates a bond to the central Ti atom again. The energy barrier for the dissociation process is 1.11 eV, which is much higher than that for the previous processes. 3. Water Adsorption and Dissociation on Pure TiO2 Complexes. Adsorption and dissociation processes at the terminal metal atom of pure TiO2 are studied to compare to the processes on Mn-doped TiO2. As shown in Figures 10 and 11,

Figure 10. First, second, and third water adsorption and dissociation on the Ti2O4 complex. Relative energies are in electronvolts. The solid line means the water dissociation process, and the dashed line means the water adsorption reaction.

Figure 11. First and second water adsorption and dissociation on the Ti3O6 complex. Relative energies are in electronvolts. The solid line means the water dissociation process, and the dashed line means the water adsorption reaction.

the reaction energies of water adsorption and dissociation on TiO2 are nearly identical regardless of the order in which the molecules adsorb or whether the cluster has two or three metal atoms. The first and second water adsorption energies are −1.15 and −1.18 eV for Ti2O4 and −1.20 and −1.21 eV for Ti3O6, respectively. Their dissociation energies are also similar to the adsorption energy: −1.18 and −1.21 eV for Ti2O4 and −1.22 and −1.22 eV for Ti3O6, respectively. The activation energies for dissociation are 0.34 eV except for the second dissociation on Ti3O6, which is 0.35 eV. The adsorption energy for the third water (leading to 51) is −0.47 eV, which is much smaller compared to the first two water adsorption energies of about −1.2 eV. In this process, one hydrogen atom could dissociate to a bridging oxygen from the adsorbed water (not shown). This process is an endothermic reaction requiring 0.83 eV, similar to the case of the MnTiO4 complex, and the energy barrier is 1.25 eV. The energy barrier is much higher than the dissociation energy barriers in the previous two dissociation steps to terminal oxygen by about 0.3 eV. So, this dissociation reaction does not likely occur. Structures for the adsorption of additional waters to the Ti2O4 complex are shown in Figure 12. When a fourth water adsorbs at the undissociated structure 51, there are three 602

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adsorbs at the other Mn atom with an energy of −0.37 eV. The dissociation occurs to a terminal oxygen that does not have a hydrogen atom, like the other complexes. The dissociation and activation energies are −0.46 and 0.32 eV, respectively. The third water adsorption produces structure 56. The reaction energy is −0.76 eV, which is larger than that of the third water adsorption in previous complexes. One hydrogen atom dissociates to a bridging oxygen, leading to structure 57, with a slightly exothermic reaction energy of −0.05 eV, as shown in Figure 14. This exothermic reaction of the dissociation on third

Figure 12. Fourth, fifth, and sixth water adsorption on the Ti2O4 complex. Relative energies are in electronvolts. The solid line means the water dissociation process, and the dashed line means the water adsorption reaction.

possible pathways: the water adsorbs at same Ti atom that is already occupied by the third adsorbed water (not shown), at the cis position at the next Ti atom (not shown), and at the trans position (leading to 52). The reaction energies are −0.32, −0.42, and −0.44 eV, respectively, which are similar to the energy of third water adsorption. Structure 52 is the most stable of the three isomers. Their dissociation processes are also endothermic reactions with reaction energies about 0.03 eV and energy barriers of about 0.53 eV (not shown). The energy barrier is not too high, but the dissociation process breaks the main backbone structure, in which least one metal to bridging oxygen bond is broken. Further water molecules can adsorb at the undissociated structure 52, to maintain the main backbone structure. The fifth water molecule adsorption to the three isomers leads to the same structure 53. The adsorption energy from isomer 52 is −0.34 eV. In the dissociated structure (not shown) of 53, one water molecule is bound not at a metal atom but by hydrogen bonding, as in structure 43. The adsorption energy for the sixth water (leading to 54) is −0.38 eV, which is similar to that of the fifth water. Its dissociation process is also endothermic, 0.39 eV, and the energy barrier is 0.96 eV. 4. Water Adsorption and Dissociation on Pure Mn2O4 Complex. The water adsorption and dissociation processes on a pure Mn2O4 complex are studied to compare to the reactions on the Mn-doped and pure TiO2 complexes. The lowest energy structures are shown in Figure 13. The initial structure 55 was

Figure 14. Third water dissociation and the fourth, fifth, and sixth water adsorption on the Mn2O4 complex. Relative energies are in electronvolts. The solid line means water the dissociation process, and the dashed line means the water adsorption reaction.

water is quite different from that of the other systems with two metal atoms, because for those the third dissociation process is an endothermic reaction. The activation energy is slightly high at 0.55 eV. For further water adsorption, the dissociation process is endothermic like the others. Fourth, fifth, and sixth water adsorption reactions lead to structures 58, 59, and 60, of which the reaction energies are −1.32, −0.88, and −4.47 eV, respectively. The sixth adsorption energy is the highest in this study and is much larger compared to the others. The reason is not entirely clear but is likely due to the completion of octahedral coordination around the Mn atom. Often, when adsorption leads to octahedral coordination, as shown in Figures 2, 4, and 6, the adsorption energy is larger compared to energies for other structures. However, this does not always occur; in Figure 3, the adsorption energy for completing the octahedral coordination is not much different from the others. 5. Comparison of Water Adsorption Reactions among MnTiO4, MnTi2O6, Ti2O4, Ti3O6, and Mn2O4 Complexes. Water adsorption and dissociation reactions can be classified in two ways on Mn-doped TiO2 complexes: these reactions may occur at the Ti atom or at the Mn atom. The reaction energies at the Ti atom are similar to those on pure TiO2 (Ti2O4 and Ti3O6), but those at the Mn atom are significantly different. For MnTiO4, the water adsorption energies at the Ti atom are −1.22 and −1.25 eV for the first and second water, respectively, and the values at the Ti atom for MnTi2O6 are −1.24 and −1.23 eV for the first and second water, respectively. Those values are a little bit larger than the average value, −1.19 eV, on pure TiO2, but the values at the Mn are −0.71 and −0.66 eV for MnTiO4 and −0.77 and −0.78 eV for MnTi2O6 for the first and second water, respectively, which are much smaller than the average value on pure TiO2. In the Mn2O4 complex, the first and second adsorption energies are −0.62 and −0.37 eV,

Figure 13. First, second, and third water adsorption and dissociation on the Mn2O4 complex. Relative energies are in electronvolts. The solid line means the water dissociation process, and the dashed line means the water adsorption reaction.

obtained in our previous work.22 The first water adsorption energy is −0.62 eV, which is smaller than the adsorption energies of −0.7 eV at the Mn atom of Mn-doped complexes and significantly less than adsorption energies of −1.2 eV at the Ti atoms of pure and doped systems. Subsequently, one hydrogen atom dissociates to a terminal oxygen. The reaction energy for this process is −1.01 eV, and the activation energy is 0.50 eV. The activation energy is higher than in the other systems described above. The dissociation energy is smaller than that of the pure TiO2 complex, but higher than that at the Mn atom in Mn-doped TiO2 complexes. The second water 603

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overcoming a moderately high energy barrier height. When hydrogen bonding occurs between adsorbed waters, the complex is more stable. Mn doping in TiO2 complexes reduces the reaction, adsorption, and dissociation energies, but the effects on the activation energy vary with the configuration, such as dissociation to terminal or bridging oxygen atoms, hydrogen bonding environment, etc.

respectively, which are smaller than those at the Mn atom in Mn-doped TiO2 complex and on TiO2. The activation energy of water dissociation also shows a similar trend. In the MnTiO4 complex, the water dissociation energies at the Ti atom are 0.39 and 0.34 eV for the first and second water, respectively, but 0.29 and 0.19 eV at the Mn atom. Those at the Ti atom are similar to those on the pure TiO2 complex, but those at the Mn atom are much smaller. The activation energies at the Ti atom are 0.40 and 0.38 eV for the first and second water, respectively, which are larger than those on the pure TiO2 complex, but the values at the Mn atom in MnTi2O6 are 0.27 and 0.23 eV, which are smaller than those on TiO2. However, the activation energies are increased for the pure MnO2 complex: these values are 0.50 and 0.32 eV for the first and second water dissociation, respectively. Overall, Mn doping reduces the activation energy as well as overall reaction energy for Mn-doped TiO2 systems, but the activation energy for water dissociation on Mn2O4 is higher than that on Mndoped TiO2 and on TiO2 complexes. Water adsorption and dissociation processes at the terminal metal atom of Mn-doped TiO2 complexes are exothermic reactions, but after all terminal oxygens have a hydrogen atom in MnTiO4, the dissociation reaction is not exothermic anymore. In water adsorption without dissociation of third and fourth water on MnTiO4 complex, the adsorption reaction energy is about −0.5 eV, but after that the energy is significantly increased because of hydrogen bonding. The adsorption energy is significantly decreased and the dissociation process is usually not favorable after a water molecule adsorbs at each metal atom. Further dissociation processes are often endothermic reactions with somewhat high energy barriers in many cases. However, in MnTi2O6 the dissociation process is exothermic until it is saturated and the energy barrier does not change much as additional waters are added.

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AUTHOR INFORMATION

Corresponding Author

*C. M. Aikens: e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation under Grant No. CHE-0955515. C.M.A. also thanks the Alfred P. Sloan Foundation for a Sloan Research Fellowship (2011−2013) and the Camille and Henry Dreyfus Foundation for a Camille Dreyfus Teacher-Scholar Award (2011−2016).

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REFERENCES

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CONCLUSIONS Water adsorption and dissociation on small metal oxide complexes has been investigated. On Mn-doped TiO 2 complexes, the first water molecule can adsorb at either a Mn or Ti atom. The adsorption energy at Ti is larger than that at Mn. The second water molecule prefers to adsorb to a different metal atom than the first water molecule. When adsorbing on a complex with three metal atoms, water can adsorb at the terminal or central metal atoms; adsorption at the terminal atom is preferred energetically to the central atom. Water can dissociate to either a terminal or bridging oxygen atom. Dissociation to a terminal oxygen atom leads to a more stable structure than dissociation to a bridging oxygen atom. In Mn-doped TiO2 complexes, the adsorption and dissociation energies at Ti atoms are similar to those on pure TiO2 complexes, but the energies at Mn atoms are different from those on a pure MnO2 complex. However, the reaction energies on Mn atoms in Mn-doped TiO2 complexes are remarkably constant regardless of number of waters adsorbed and the cluster size (MnTiO4 or MnTi2O6), which suggests that larger clusters may also have similar adsorption and dissociation energies for water molecules on similarly coordinated Mn atoms. The adsorption and dissociation process significantly changes after water adsorbs at each metal atom. After two water molecules have adsorbed, molecular adsorption is usually favored in energy over dissociative adsorption. The dissociation of adsorbed water is typically endothermic and requires 604

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