The Intrinsic Role of Excess Electrons in Surface Reactions on Rutile

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

The Intrinsic Role of Excess Electrons in Surface Reactions on Rutile TiO (110): Using Water and Oxygen as Probes 2

Hao Tian, Bin Xu, Jing Fan, and Hu Xu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12451 • Publication Date (Web): 29 Mar 2018 Downloaded from http://pubs.acs.org on March 31, 2018

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The Journal of Physical Chemistry

The Intrinsic Role of Excess Electrons in Surface Reactions on Rutile TiO2 (110): Using Water and Oxygen as Probes

Hao Tian,1,2 Bin Xu,3 Jing Fan,1 and Hu Xu1* 1

Department of Physics, Southern University of Science and Technology, Shenzhen 518055, China 2

3

Department of Physics, Hong Kong University, Hong Kong, China

Physics Department and Institute for Nanoscience and Engineering, University of Arkansas, Fayetteville, Arkansas 72701, USA

ABSTRACT Reactions on catalytically active surfaces often involve complex mechanisms with multiple interactions between adsorbates and various subsequently formed intermediates, and a variable number of excess electrons further complicates the involved mechanisms. Experimental techniques face challenges in precisely tuning or determining the number of excess electrons and in elucidating these complex reactions. In this work, the thermodynamic details and reaction pathways of interactions between the most prevalent and important molecular species, H2O and O2, on a prototypical rutile TiO2 (110) surface are investigated using density functional theory calculations on ten elementary reaction steps with the intention of gaining further insight into surface catalysis. The results suggest that the final product is independent of the reaction pathway when the number of excess electrons is sufficient. The intrinsic role of excess electrons at the reaction level is thus proposed to extend the understanding of the origin, distribution and transfer of excess electrons. Such understanding is beneficial to develop the high-performance catalysts.

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1. INTRODUCTION Titanium dioxide (TiO2) is used in a wide variety of technological areas, including energy conversion, environmental treatment, sensing and biocompatible materials.1-4 These applications are largely enabled by excess electrons, whose origin, distribution and transfer have been investigated extensively.5-11 The distribution and transfer of excess electrons, independent of their origin, are identical, indicating an intrinsic underlying characteristic. These findings suggest that excess electrons may also show similar properties at the surface-reaction level. To understand surface reactions, water and oxygen are frequently introduced as prototypical reactants, as they are the most prevalent and unavoidable chemicals in many applications. While the stoichiometric rutile (r)-TiO2 (110) surface shows poor chemical reactivity, the adsorption of O2 and H2O on a reduced surface has been widely studied.12-17 O2 naturally tends to fill one oxygen vacancy (OV) and leaves an O adatom (Oa) on the adjacent Ti5c site after dissociation.16, 18-19 If a H2O molecule is trapped in an OV, it forms a pair of bridge hydroxyl (OHb) groups.20-23 These two reactions are straightforward steps at the beginning of surface reactions involving H2O and O2, yet the following reaction mechanisms remain poorly understood. Various oxyhydroxides, i.e., H2O dimers, H2O2, OOH and terminal hydroxyl radicals (OHt), are proposed to be the final product or are at least experimentally observed.18, 24-27 These reports indicate that numerous established reaction equilibriums may exist among these investigated molecules and radicals, i.e., H2O, O2, Oa, OHb, H2O dimers, H2O2, OOH, and OHt. In addition, a change in the excess-electron concentration is predicted to reverse the direction of the reaction,28-29 making explicit insight into the above equilibriums even be more difficult to gain. It is thus an extensive and complicated situation involving intricate mechanisms that requires further research. The interaction between O2 and H2O on a reduced anatase TiO2 (101) surface was recently investigated through a combination of theory and experiment, which identified OHt as the final, stable reaction product, and the investigation provided important details of reactions that involve the intermediate HOO.29 The interaction 2


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between H2O and Oa16, 18 on r-TiO2 (110) was experimentally studied. In addition, the reaction between O2 and OHb25 was also experimentally investigated. These studies contributed the base knowledge of some elementary reaction steps. However, experimental techniques for directly determining excess electrons or for recording the detailed kinetics of complicated reactions remain limited. Therefore, results obtained under shielding conditions, such as excess electrons, can be difficult to compare. Conclusions from different experiments become isolated ‘fragments’ that cannot be generalized, which may explain why various oxyhydroxides were observed as the final product. Here, we intend to build a comprehensive and consistent picture of the interactions between H2O and O2 on r-TiO2 (110) by studying ten fundamental steps that cover many representative reactions. In other words, the reactions among H2O, O2 and related intermediates are included or could be reproduced from combinations of these steps. Conditions involving both sufficient excess electrons (SEC) and insufficient excess electrons (IsEC) are discussed. First, we evaluate the influence of excess electrons on the ten fundamental reactions, which reveals the competition between the adsorbates’ affinity and the excess-electron condition in selecting the product. Second, the different pathways to form the final product OHt under the SEC condition are revisited, where the low energy barriers suggest that the final product is independent of the pathway at room temperature. The catalytic role of water in the reactions is also discussed from two points of view, i.e., aggregating reactants and lowering the reaction barrier. These findings may contribute to an overall understanding of surface reaction mechanisms.

2. COMPUTATIONAL DETAILS The adsorption geometry, adsorption energy, and reaction barrier of the co-adsorption of O2 and H2O on a reduced r-TiO2 (110) surface were investigated using density functional theory (DFT) calculations. The electron exchange–correlation was treated using the Perdew and Wang (PW91) form30 of the generalized gradient approximation (GGA). The projected augmented wave method31 was employed as implemented in 3



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the Vienna Ab-initio Simulation Package.32-33 The Ti 3p, 3d, and 4s electrons were treated as valence electrons. To describe the localized electronic states of the excess electrons originating from defects, on-site Coulomb corrections34 with fixed values of UTi=4.1 eV, widely adopted in prior worksDeskins, et al. 35, were applied to the Ti 3d electrons. As both DFT and DFT+U results give the similar energy differences (see Table 1), so we adopted DFT calculations in this work. The energy barriers along the reaction pathways were calculated using the climbing image nudged elastic band (CI-NEB) method.36 A p(4×2) r-TiO2 (110) supercell with six TiO2 triple layers (i.e., 18 atomic layers) and a vacuum region of ~12 Å was used to build the surface model. The Monkhorst-Pack37 samplings of (2×2×1) for geometry optimization and (1×1×1) for CI-NEB were used throughout the calculations. A cutoff energy of 450 eV and the conjugate gradient method were adopted. The geometries were fully relaxed until the forces acting on each atom were less than 0.02 eV/Å. All of these parameters were fully tested to ensure the convergence of the calculations.

3. RESULTS AND DISCUSSION The representative r-TiO2 (110) structure and all radicals investigated in this work are shown in Fig. 1a. The charge distributions of typical SEC and IsEC models using DFT+U calculations (see Fig. 1b and Fig. S1) indicate that roughly two excess electrons are separately localized on Ti6c in the IsEC model and that six excess electrons appear in the SEC model, corresponding to prior results.28,

38

Reactions

between oxygen and water on the r-TiO2 (110) surface are likely to involve intricate mechanisms. In this work, we introduce ten fundamental steps shown in Fig. 2a that cover many representative reactions. The geometric details of these investigated reactions are illustrated in Fig. 3. Reactions 4-10 have been experimentally observed,18, 25, 39 while reactions 1-3 have not been experimentally observed. We first evaluate the effects of excess electrons on the ten fundamental steps. DFT and DFT+U calculations were employed to study the reaction energy of each step, and the corresponding results in which both SEC and IsEC conditions are considered are listed in Table 1. The reaction energy is defined as the energy difference between the 4


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precursor and product configurations. A negative reaction energy indicates an exothermic process.

3.1 Competition between excess electrons and the affinity of the adsorbates. As shown in Fig. 2a, the reaction energies vary with the concentration of excess electrons, which suggests that excess electrons determine selectivity toward the adsorption geometry in almost all steps. In particular, excess electrons play a dominant role in reactions 2, 5, 6 and 8. Depending on the availability of excess electrons, the difference in the reaction energy can exceed 4 eV in the formation of terminal HOO (HOOt) in reactions 5 and 8. For reaction 2, the product configuration could not stably exist after optimization under the IsEC condition, while OHb transfers from a bridge O site to the nearby Ti6c, releasing an energy of 3.67 eV when the concentration of excess electrons is sufficient. In addition, the direct dissociation of O2 on Ti5c is also not supported under the IsEC condition. In other words, these reactions (2, 5, 6 and 8) can be reversible in different excess-electron environments. Returning to the overall view, the 10 fundamental reactions can be classified into two categories, i.e., those involving proton transfer (reactions 1, 4, 7, 9 and 10) and those involving O=O scission (reactions 2, 3, 5, 6 and 8). For convenience, we shall hereafter refer to these two reaction categories as P-reactions and O-reactions, respectively. As shown in Fig. 2a, considerable energy can be released from only O-reactions (reactions 2, 3, 5, 6 and 8), and these reactions occur only under the SEC condition, typically producing Oa and OHt. To explain such dependences, we note two important factors, the electron affinity of adsorbates and the availability of excess electrons.

The

latter

is

discussed

above

and

is

responsible

for

the

excess-electron-related dependence, while the product-related dependence could be ascribed to electron affinities. Deskins, et al. 9 theoretically reported that more charge transfer occurs when the affinity of the adsorbate is high, thus building a stable configuration. Our results lend support to this idea. O-reactions are intensively exothermic under the SEC condition because the electron affinities of products Oa (1.5 5



eV)9 and OHt (1.8 eV)9 are obviously higher than those of OOH (1.1 eV),9 H (0.8 eV),40 O2 (0.5 eV)3 and H2O (0.1 eV).41 In contrast, the reaction energy is low in P-reactions (reactions 1, 4, 7, 9 and 10), where breaking one O-H bond is always accompanied by the formation of another, implying that less charge transfer occurs between the surface and the adsorbates. This characteristic, on the one hand, could explain the low reaction energy (less than 0.24 eV for reactions 1, 4, 7 and 10) and, on the other hand, results in an insignificant reaction energy difference under varying excess-electron conditions (less than 0.22 eV). The insignificant reaction energy difference indicates the weak influence of excess electrons on P-reactions. Moreover, the energy barrier of typical proton transfer reactions is predicted to be less than 0.15 eV (see Fig. S2). In view of the low reaction energy, the insignificant reaction energy difference and the facile reaction barrier, the speculation that protons exchange freely among surface-bonded oxyhydroxides and O2 may be generalized to reactions under both SEC and IsEC conditions. One important understanding of proton exchange is that it can realize oxyhydroxide diffusion on the surface through single- or multi-step reversible proton transfer among H2O, Oa, Ob or OHt, of which Fig. S2 shows the selected pathways. Such diffusion could aggregate the reactants, thus accelerating the reactions. This mechanism supplements early experimental observations of the catalytic behavior of protons or water16, 25, 42 and provides more details of the catalytic mechanism. Here, we propose that H2O, the origin of the protons, may play a catalytic role in surface reactions involving proton transfer. Usually, P-reactions are generally excess-electron independent. For reaction 9, the energy difference exceeds 1 eV due to that Oa prefers different adsorption locations under different concentrations of excess electrons.19 For P-reactions, it only involves proton transfer, indicating that the breaking of one O-H bond is always accompanied by forming another O-H bond. The slightly lower energy may suggest that the formation of OHt terminates P-reactions occurring under SEC conditions and results in OHt as the stable product. Next, we raise questions as to why OHt is stable under the SEC condition and as to when other intermediates will remain. The answers are not straightforward at first 6


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glance. While various factors (intrinsic defects, photoexcitation, and/or foreign atoms)43 determine the amount of excess electrons on the surface that can be transferred, the electron affinity of the adsorbates describes their ability to accept excess electrons. For an adsorption system, larger charge transfer generally indicates a stronger interaction and thus a more stable configuration. Therefore, there is naturally a competition between excess electrons and the affinity of the adsorbates in selecting the final product, except in the extreme situations of sufficient excess electrons and no excess electrons. The latter is well understood because O2 only physically binds to perfect r-TiO2 (110) surfaces, even at low temperatures, resulting in no reaction. The precondition of sufficient excess electrons allows us to focus on only the adsorbates’ affinity when considering the final product. At this point, products with higher affinity are expected to be more stable, and so there is no competition either. When the excess-electron concentration is between the two stated extremes, the reaction mechanism can be very complex, and every single intermediate may be observed. Based on the above understanding, we summarize the details of Fig. 2a into Fig. 2b as a schematic of the kinetic pathway. This schematic demonstrates that adsorbates could undergo dissociation (involving O=O scission) from molecular adsorption and perhaps finally form the product OHt, depending on the excess-electron concentration. The surface acts as a Lewis base that donates electrons to the adsorbates, and O2 plays an electron-scavenging role through O=O scission. The surface drives O=O scission, and proton transfer plays a catalytic role. This is most likely how multi-stage equilibrium is established on the surface.

3.2 Path-independent characteristic. The above discussion confirms that the final product is the result of competition between the excess-electron environment and the affinity of the adsorbate, which further suggests a path-independent characteristic of surface reactions. This speculation is supported by the following transition state calculations. In addition to contributing excess electrons, OV also acts as a reactive center and thus cannot be ignored. Therefore, we select two typical pathways, pathway-O and pathway-W, from the kinetics schematic in Fig. 2b. A water molecule 7



dissociates at OV during the beginning of pathway-W, while an oxygen molecule initially fills the OV in pathway-O. For the final state of each pathway, we discuss the various possible adsorption configurations shown in Fig. S3, and paired OHt groups are the most energetically favorable. The pathways and atomic configurations are presented in Fig. 4 and Fig. 5, respectively. Both pathways are predicated to possess low energy barriers. The initial structure of pathway-W (configuration 1 in Fig. 4) has a pair of OHb groups from the known interaction between H2O and OV. The adjacent adsorbed O2 first overcomes a low energy barrier of 0.17 eV (configuration 2) to capture a H atom from OHb (configuration 3). This is P-reaction R7, and then it forms OOHt. In the following, O=O scission occurs by overcoming a barrier of 0.17 eV (configuration 5). It corresponds to O-reaction R8, leaving one OHt group and one Oa (configuration 6). Next, another proton transfers (through R10 and R9) indirectly from OHb to Oa (configurations 7, 8) to form the second OHt (configuration 9). Finally, the system reaches its most stable state, that of paired OHt groups. In pathway-O (see Fig. 5), a H2O molecule anchors to the nearest Ti5c bonded to the trapped O2, establishing a hydrogen bond (configuration a). After surmounting a low energy barrier of 0.33 eV (configuration b), a H atom of H2O is transferred to O2, reaching configuration c shown in Fig. 5 and forming one OOHb and one OHt. One can find that the energy requirement to split H2O is in accord with R1. The O=O bond subsequently breaks by overcoming an energy barrier of 0.05 eV (configuration d), and one OHin bonds to the nearest Ti6c (configuration e). The generation process of OHin is O-reaction R2. Finally, 0.60 eV is required for OHin to migrate from the Ti6c site to the Ti5c site (configuration h) by the combination of reactions R2 and R3, resulting in the formation of the most stable configuration, a OHt pair (configuration i). The presence of OHin has not been experimentally confirmed. Our theoretical results suggest that OHin may exist briefly at low temperatures because of the small barrier (less than 0.4 eV) to its formation from configuration a to configuration e. By contrast, a barrier of 0.60 eV blocks its migration to the nearest Ti5c site. We thus believe that OHin may be observed experimentally as a meta-stable state. 8


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As expected, O=O scission (marked in Figs. 4 and 5) is accompanied by a release of considerable energy (2.76-2.97 eV) from the interaction between the adsorbed water and oxygen. The pathways in Figs. 4 and 5 demonstrate two different manners of O=O scission, that is, with and without the assistance of bridging OV. Under SEC conditions, the O=O bond can readily break with or without this assistance because it has an energy barrier smaller than 0.17 eV. If the excess-electron concentration is not sufficient, however, such a reaction is hindered by reaction barriers in the range of 1.0-1.4 eV, as shown in Fig. S4. The importance of excess electrons in the surface reaction is again highlighted. O=O scission without protons under SEC conditions is also considered, and the corresponding result is shown in Fig. S5. The calculated energy barrier is 0.56 eV, which is significantly higher than that with protons. A second catalytic role of proton transfer is therefore revealed, i.e., lowering the O=O scission barrier. As pathway-O and pathway-W can be decomposed into fundamental steps listed in section 3.1, it is reasonable to expect that intricate reaction equilibriums among H2O, O2 and subsequently formed intermediates shall create numerous reaction pathways that are far more complex than we can consider, and different pathways should follow fundamental steps. Considering the low barrier to forming the final product OHt at room temperature, we propose a path-independent interaction between O2 and H2O that reflects the intrinsic role of the excess electrons. The validity of this path-independent feature, however, is accompanied by two conditions: i) sufficient excess electrons and ii) water as the origin of protons for catalysis. Such viewpoint of excess electrons would be beneficial for catalysis design. If the target product in a photocatalysis system is OHt, then attention must be paid to the concentration of effective photoexcitation electrons. This would thus invite effort to reduce hole-electron recombination, which is one of the key problems in photo-catalysis. Surface-bonded OHt may further react to produce •OH by desorption if one photogenerated hole is captured.44

3.3 Validity of the model. Both water and oxygen are abundant in the ambient environment, while the concentrations of Tiint and OV cannot be quantitatively 9



determined. Our model, involving only one H2O, O2 and OV, is likely to be the primary one. However, two facts are commonly accepted. First, one OV (donating two electrons) and one Tiint (donating no fewer than two electrons3, 45) can provide a sufficient number of electrons to stabilize the adsorbate in both the dissociative and molecular states. Deskins, et al.

9

showed that extra defects do not affect the

adsorption geometry or formation energy when the excess-electron concentration is sufficient.9 Second, as shown in Fig. 2a, further reactions between OHt and oxygen, between OHt and water, or between OHt and any other intermediates are not energetically favorable when the number of excess electrons is sufficient. Specifically, extra adsorbate rarely changes the final product. We are therefore confident that our model is applicable and that a general understanding of surface reactions from the present model is reasonable.

4. CONCLUSIONS In summary, we studied the interactions between O2 and H2O on a r-TiO2 (110) surface under SEC and IsEC conditions. We find that the stability of the adsorbates highly depends on their affinities and the excess-electron environment. Under the SEC conditions, OHt is the most stable, final product, and other intermediates may be stable depending on the concentration of excess electrons. The pathway to form the final product mainly involves two processes, O=O scission and proton transfer. O=O scission is strongly exothermic and therefore supports the formation of the product OHt, whose electron affinity is high. Proton transfer plays a catalytic role in these interactions in two ways, i.e., by aggregating precursors and lowering the O=O scission energy barrier. We further propose a path-independent characteristic of the surface reaction when the excess-electron concentration is sufficient. Our findings enrich the understanding of the nature of surface reactions and may facilitate the design of catalytic systems.

■ ASSOCIATED CONTENT 10


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Supporting Information The configurations and adsorption energies of H2O and O2, potential energy profiles for proton transfer and the formation of paired OHt, and potential energy profiles for O2 dissociation without proton transfer under SEC conditions. ■ AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Nos. 11674148, 11334004, and 11704177), the Guangdong Natural Science Funds for Distinguished Young Scholars (No. 2017B030306008), and the Basic Research Program of Science, Technology and Innovation Commission of Shenzhen Municipality (Nos. JCYJ20160531190054083 and JCYJ20170412154426330).

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Table 1. PW91 and PW91+U results of the energy difference (unit in eV) between the initial and final states of various adsorbates (as shown in Fig. 1) under IsEC and SEC conditions. The adsorbate is denoted N/A when it does not stably exist on the r-TiO2(110) surface. PW91 Reaction label R1 R2 R3 R4 R5 R6 R7 R8 R9 R10

IsEC 0.24 N/A -1.61 0.35 0.75 N/A 0.12 1.97 0.98 0.12

SEC 0.24 -2.83 -3.96 -0.39 -3.32 -1.46 -0.10 -2.57 -0.10 0.08

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PW91+U IsEC SEC 0.04 0.04 N/A -2.50 -1.70 -4.24 0.33 -0.42 1.17 -3.48 N/A -0.94 0.07 -0.07 2.55 -2.27 0.86 -0.26 -0.08 -0.12

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Figure 1. (a) A ball-and-stick model of the r-TiO2 (110) surface. 5-fold coordinated Ti5c, 6-fold coordinated Ti6c, bridging oxygen vacancy OV, interstitial titanium Tiint, bridging oxygen Ob, oxygen adatom (O bonded to Ti5c) Oa, bridging hydroxyl (hydrogen bonded to Ob) OHb, terminal hydroxyl (OH on top of Ti5c) OHt, hydroxyl bonded to an in-plane surface (OH on Ti6c) OHin, bridging hydroperoxyl (hydroxyl bonded to Ob) OOHb, and terminal hydroperoxyl (OOH on top of Ti5c) OOHt are labelled. (b) The side views of the illustrated SEC (with one Tiint and one OV) and IsEC (with one OV) models and the corresponding spin densities with an isovalue of 0.007 e/bohr3. The red, green and white spheres represent Ti, O, and H atoms, respectively. These labels are used throughout this paper.

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Figure 2. Fundamental steps of the interactions between H2O, O2 and other intermediates under SEC (with one interstitial Ti) and IsEC (without interstitial Ti) conditions. (a) Typical reactions and their corresponding energy changes. (b) Schematic evolution of surface adsorbates under SEC. Pathway-O and pathway-W are two typical pathways and are initiated with O2 and H2O, respectively.

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Figure 3. Initial and final configurations of the reactions listed in Fig. 2a.

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Figure 4. Potential energy profiles of pathway-W under SEC conditions (with one Tiint and one OV). The reaction starts with the splitting of water into the paired OHb groups by filling the O vacancy, and O2 subsequently binds the nearest Ti5c site. The O=O scission process is marked. The reverse reaction R10 is denoted by R10*.

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Figure 5. Potential energy profiles of pathway-O under SEC conditions (with one Tiint and one OV). The reaction starts with the pre-adsorption of O2 at the OV site, and H2O subsequently binds the nearest Ti5c site. The O=O scission process is marked. The combination of R2 and R3 is denoted by R2-R3#.

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Table of Contents Graphic

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The Intrinsic Role of Excess Electrons in Surface Reactions on Rutile

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