Coexistence of Square Pyramidal Structures of Oxo Vanadium (+5

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Coexistence of Square Pyramidal Structures of Oxo Vanadium (+5) and (+4) Species Over Low Coverage VOx/TiO2 (101) and (001) Anatase Catalysts Logi Arnarson, Søren B. Rasmussen, Hanne Falsig, Jeppe V. Lauritsen, and Poul Georg Moses J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b06132 • Publication Date (Web): 18 Sep 2015 Downloaded from http://pubs.acs.org on September 22, 2015

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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.

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The Journal of Physical Chemistry

Coexistence of Square Pyramidal Structures of Oxo Vanadium (+5) and (+4) Species Over Low Coverage VOx/TiO2 (101) and (001) Anatase Catalysts

ACS Paragon Plus Environment

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ABSTRACT We investigate the structural properties and coordination of VOx species supported on the facets of anatase TiO2 as the oxidation state changes during the catalytic cycle of selective catalytic reduction (SCR) of NOX. Density functional theory (DFT) calculations reveal how the molecular structures of hydrated, low coverage V sites depend on the surface facet of the TiO2 (anatase) support. Thus, the support effect is not limited to only disperse the active phase but it also dictates its shape and ultimately its reactivity. We find that hydrated VOX species, with V is present in oxidation state +5, are present in distorted tetrahedral configurations on the most stable TiO2(101) facet whereas on the less stable TiO2(001) facet they are present in distorted octahedral configurations. Furthermore we find the reduced states of the same species where V is present in oxidation state +4 also exhibit unchanged local coordination with the vanadyl unit V=O intact. We support the findings by electron paramagnetic resonance spectroscopy (EPR) data on a vanadia-titania catalyst partly reduced in situ by ammonia exposure. These results clarify the inconsistencies in the literature and interrelates the spectroscopic observations by Raman, IR with EPR and NMR.

KEYWORDS TiO2, anatase, vanadiumoxide, SCR catalysis, density functional theory, EPR


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INTRODUCTION Vanadium oxide dispersed on anatase (VOX/TiO2) is the industrial catalyst for reducing nitrogen oxides (NOX) by the selective catalytic reduction (SCR) process and a good catalyst for several selective oxidation reactions such as mercury oxidation, o-xylene to phthalic anhydride, and other oxidative dehydrogenation reactions of hydrocarbons.1–6 In SCR, NOX is effectively reduced by NH3 to yield N2 and H2O in the temperature range 200-550°C. It is essential to control the emission of NOX compounds, since they have negative effects on the environment, e.g. contributing to smog formation in large cities, acid rain formation and depletion of the ozone layer. The VOX/TiO2 system has been studied extensively, nonetheless the structural properties of the active sites in the system on the atomic level, i.e. the exact local coordination of each V atom, is still debated.7–10 Raman spectroscopy has been successfully applied for identification of different types of vibrational bonding e.g. vanadyl V=O, interface Ti-O-V and bridging V-O-V bonds.8,11–13 Infrared spectroscopy has provided insights into the distribution of Ti-OH, W-OH (on tungsta promoted catalysts) and V-OH present on the surface of the catalyst, and contributed significantly to mechanistic insights by studying the behaviour of adsorbed NH3 on Lewis acid sites and the formation of the NH4+ ion on Brønsted acid sites.14,15 A substantial amount of work has been done on this system by use of Nuclear Magnetic Resonance spectroscopy (NMR) where the structure of VOX species are described to be either in tetrahedral configuration or octahedral configuration depending on the degree of hydration due to the water content in the experiments.16–19 NMR is in principle an ideal technique, since it directly probes the coordination of supported V species, but the importance of studying catalytic systems such as VOX/TiO2 in real conditions still imposes a challenge for NMR.20–22 Also, EPR spectroscopy is very sensitive to changes in the oxide ligand field of V4+=O2+ unit, which is stable even at SCR conditions


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(while a perfect tetrahedral V4+ species would be visible only at low temperatures).23 However, in situ EPR data consistently exhibit several contributions from monomeric vanadyl species superimposed on a broad contribution probably from polymeric vanadia.24 Nonetheless, the possible existence of variations of monomeric structures are most often disregarded when spectroscopic experiments are interpreted and theoretical calculations are performed. Instead, the active V site is consequently described as a tetrahedral species, which perhaps can become tetragonal (or octahedral) by exposure to water in the reaction environment.4,25–28 Catalysis over vanadium species depends in part on the reduction and oxidation properties of the surface species, which we here investigate by studying the structure of oxidized (oxidation state +5) and (partially) reduced V (oxidation state +4) species. In this letter we illuminate the relationship between the surface structure of different TiO2 facets and the structure of V species at low coverages (< 2 V/nm2). We focus on the two most stable surface terminations of TiO2 anatase which is the (101) facet and the less stable (001) facet29,30 and reveal that each surface facilitates different local coordination of VOX species in consensus with EPR data. COMPUTATIONAL/EXPERIMENTAL METHODS All calculations were done using the grid-based Projected Augmented Wave method as implemented in the GPAW software package ASE interface

33

31,32

through the

The exchange-correlation interaction are modelled with the BEEF-vdW functional34

on the GGA approximation level including the long range van der Waals (vdW) interactions. Electrons are described using the projector augmented wave (PAW) in the frozen core approximation. The crystal structure was optimized using the stress tensor method available in ASE to be a = 3.80Å and c = 9.67 Å which deviates from the experimental values by 0.54% and 1.8% respectively from

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. The unit cell

used in the calculations was periodic in the xy-plane with a minimum of 10 Å of vacuum between successive slabs in the z-direction. The specific coverage values correspond to different sizes of the


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supercell with a single vanadium atom used in the calculations: 0.8 V/nm is 1 3 and 3 3 supercell on (101) and (001) respectively, 1.2 V/nm is 1 2 supercell on (101) and 1.7 V/nm is

2 2 supercell on (001). In Figure 1 we show the two surface facets along with superimposed supercells used to define the coverages.

Figure 1: The two surface facets of TiO2 and the supercells used in the calculations a) (101), the surface unit cell (black), (blue) and (purple). b) (001), the surface unit cell (black), (blue) and (purple). The periodic boundary conditions combined with a single vanadium atom in each cell allow us to study exclusively structures containing single vanadium atoms while at the same time establishing if the VOx species are more stable than dissociationed VOX species on the surface. . The GPAW program supports three different operating modes for running DFT calculations: LCAO mode where the wave functions are represented with atomic orbital basis set, PW where the wave functions are expanded as plane waves and at last grid-based mode where on a uniform real space orthorhombic grids. This is utilized in this paper by performing a two step structure optimization. First the LCAO mode with dzp basis set is used since it is faster and therefore larger parts of the potential energy surface (PES) can be scanned. On top of the LCAO calculations we run the more accurate but slower grid-based mode with


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grid spacing of h≈0.20Å in each direction. In the former method we use a 4 electron PAW setup for Ti atoms which considers the 3d and 4s electrons in the valence shell where as in the latter we include the 3s and 3p electrons in a 12 electron PAW setup. To obtain the most stable structures of the VOX species on the two TiO2 facets we use a recently developed genetic algorithm (GA).36 The GA is initiated by randomly generating a population of 20 structures of the VOX on the TiO2 surface slab. Each structure is relaxed and optimized both with respect to the total energy and geometry. To generate new structures two structures from the start population are paired with a certain operator that preserves the stoichiometry of the species. If the new structure has lower energy than any in the start populations, it is included and the highest energy structure is dismissed. To ensure diversity in the population and to avoid getting stuck in a local minima in the PES the GA has four mutations (Rattle, Twist, Permutation and Mirror) with probability of 20% to be expressed.36 The criteria for a converged GA run is when identical structures with small energy variation start dominating the population. The two step optimization scheme is used when running the GA to explore as large as possible area of the PES. All different structures found by the GA with energies up to 1 eV above the energy of the most stable structure are relaxed with the grid-based method to get accurate formation energies. A Bruker EMX with a 12 kW 10′′ magnet was used for in situ EPR measurements as described earlier by Eriksen et.al 37,38 with a ER ER4102ST cavity. A cut piece of a monotlithic (20 x 2 x 1mm) catalyst sample containing 3%V2O5 over TiO2 with a S(BET)=70 m2/g was fitted inside a flow through in situ EPR reactor consisting of two concentric quartz tubes (i.d. 1.8 and 3 mm, respectively, see ref. 38 for more details) and subjected to 20 Nml/min of 20%O2/N2 followed by 1000 ppm NH3/N2 at 250°C. The temperature was measured by a 1/16″ chromel-constantan thermocouple placed on top of the catalyst


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sample. The SIM simulation software written by Weihe was used to extract numerical values of spin Hamiltonian parameters from the experimental EPR spectra.39 RESULTS/DISCUSSION The structure optimization of VOX species on the stoichiometric anatase TiO2(101) and (001) facets was performed in the low coverage limit (here considered < 2.0 V/nm2) where monomeric species are known to dominate the population.40 The search for the most stable structure of an adsorbate on a substrate is an important but challenging task within atomic scale modeling. In particular if both adsorbate and surface are complex metal oxides, a manual search in the vast energy landscape of the combined system becomes formidable. We have therefore used a Genetic Algorithm (GA)36, described in the previous section, which provides an unbiased approach to the energy minimization problem of finding the most stable VOX species on the TiO2 surfaces. Specifically the GA was used to optimize the VOX structures at 0.8 V/nm on (101) and 1.7 V/nm on (001) and then the most stable VOX structures were transferred to different coverages and relaxed with respect to energy and geometry in order to calculate the formation energies.

The formation energies for each species are calculated with respect to V2O5(b), O2(g) and H2O(g) for hydrated species according to the chemical reaction + + +∗ → ∗ (1) The formation energy change at 0K is calculated as ! = !#$%∗ − !#' $( ) − !*' $ + − !$' + (2)


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and in order to include energy change due to the effect of changed entropy as molecules are either adsorbed or desorbed from/to the reference states we calculate the Gibbs free energy of formation as , = ,#$%∗ − ,#' $( ) − ,*' $ + − ,$' + (3) = #$%∗ − -.#$%∗ − #' $( ) − -.#'$( ) − *' $ + − -.*' $ + − $' + − -.$' + (4) ≃ !#$%∗ − !#' $( ) − !*' $ + − -.*' $ + − !$' + − -.$' + (5) Where S for the gas phase molecules is calculated from NIST chemistry webbook. 41

In Eq. (1) – (5) a = ½ for monomeric species but b and c vary for different VOX species, * denotes the surface site. We have considered three different monomeric VOX species: VO3H, VO3H2 and VO2 i.e. one fully oxidized structure (+5) and two reduced structures (+4), where the formation energies are calculated according to the following equations: 6

1234

6

!#$0 * = !#$0 */∗ − !∗ − 5 !#' $( ) + 7 (6) 1234

6

6

1234

= !#$' /∗ − !∗ − 5 !#' $( ) − 9 7 (8)

!#$0 *' = !#$0*' /∗ − !∗ − 8 !#' $( ) + − 9 : (7) !#$'

6

6

and the Gibbs free energy of formation likewise according to Eq. (3) – (6). where a negative value indicates an exothermic process. These are plotted in Figure 2a) for the two TiO2 facets (squares for (101) and circles for (001)). Before we discuss the resulting VOX structures in detail, one immediately sees in Figure 2a) that the TiO2(001) facet shows much stronger adhesion of VOX species on the surface compared to the TiO2(101) facet as all, except one formation energy on that


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facet, are negative. On the TiO2(101) facet all species have positive formation energies and the corresponding reactions are therefore endothermic, except VO3H. The relative difference in reactivity towards forming VOX species on the two different facets can be understood in terms of the coordination of the Ti atoms in the uppermost layer of the TiO2 surfaces (See the local coordination of each surface on Figure 3 A and C). The less stable (001) facet exposes only undercoordinated 5-folded, Ti(5f), atoms and is therefore stabilized by allowing half of the Ti(5f) atoms in the unit cell to become fully coordinated and form the preferred bulk-like octahedral coordination of the surface Ti. Secondly, very little coverage dependence of formation energies on the (101) facet is observed whereas the (001) surface exhibits a clear coverage dependence. On the (001) facet VOX species become more stable at lower coverages which is attributed to the surface relaxations that are allowed at low coverage while at higher coverages such relaxation of the TiO2 surface are less prominent (The Ti-Ti distance between neighbouring Ti atoms linked on the clean surface via a O(2f) bridging oxygen is increased ~22% at coverage 0.8 V/nm and ~9% at coverage 1.7 V/nm ). The lack of coverage dependence of VOX formation energies on the (101) facet for the two hydrated species VO3H and VO3H2 is attributed to the fact that the surface is stable enough that VOX species interact weakly with the surface and hence no relaxations are induced. Hydrated VOX structures (VO3H) are the most stable structures on both facets where V is in oxidation state +5. Reduction of V in VO3H to oxidation state as +4 can be accomplished in several ways e.g. by effective adsorption of a hydrogen atom to form VO3H2 or by desorption of a hydroxyl group to form VO2. The H adsorbed process is endothermic by 1.2 eV and the latter by 1.8 eV on (101) (Figure 2a)). On the (001) facet the two processes are endothermic by respectively 0.8 eV and 2.6 eV. In Figure 2b) we show the Gibbs free energy of formation for the three VOX species at coverage 0.8 V/nm on the (101) facet (upper panel) and at coverage 1.7 V/nm on the (001) facet


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(lower panel), where we have included the entropy change due to adsorbed/desorbed molecules according to equations (2)-(4). This causes the equilibrium to shift towards VO2 to be favored rather than VO3H2 at all temperatures above 350K on (101) where as VO3H2 is more stable than VO2 on (001) at all catalytically relevant temperatures or up to 800K. In-situ IR data under SCR conditions find that the partial reduction of vanadia from +5 to +4 is associated with formation of new OH groups bonded to more reduced V sites 42. A direct comparison with in-situ IR spectroscopy requires a microkinetical analysis to determine the kinetically controlled surface composition, and might of course also be pertubed by slightly different speciation on the sample pellets studied experimentally. However, the present results i.e. that VO3H is the most stable species independent of facet and VO3H2 is more stable than VO2 on the (001) facet, indicate that the OH groups appearing under SCR conditions are associated with the (001) facet. In order to quantify the relative stability of each species with respect to a VO3H species on each facet (points are marked with a black border on Figure 2a)) we have used the Bayesian ensemble error estimate approach

included in the BEEF-vdW functional used for the

calculations.34,43 It provides a sensitivity measure of the DFT results on the choice of exchangecorrelation functional by calculating each energy for an ensemble containing 2000 differentfunctionals. This approach provides an quantitative and computational inexpensive approach to obtain the sensitivity on the choice of XC functional which could previously only be obtained by extensive and expensive calculation using several functionals of the GGA and vdW34 type. Thereby making it possible to determine if the conclusions are indepent on the choice of XC-functional. Given these uncertainties we have validated the order of formation energies at a given coverage within our model.


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Figure 2: a) Formation energy as a function of coverage for different monomeric VOX species on TiO2(101) and TiO2(001).

Negative (positive) values correspond to exothermic (endothermic)

reactions. The error bar on each data point indicate a Bayesian error estimate of the computational error on the formation energies relative to the structure marked with thick black border on that surface. b) Gibbs free energy of formation at standard pressures as a function of temperature in the interval from 300K to 1000K for VOX/TiO2(101) at 0.8 V/nm (upper panel) and VOX/TiO2(001) at 1.7 V/ nm (lower panel). Figure 3 illustrates the resulting structure and bonding details of the VO3H species resulting from the GA search. The two (A,B) and four (C,D,E,F) in Figure 3 reflect the distinctive lowest lying energy


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structures of fully oxidized V specie VO3H on (101) and (001) at coverages 0.8 V/nm and 1.7 V/nm respectively. On the (101) facet the most stable structure (A) consists of V in a tetrahedral (4-fold) configuration with three interface Ti-O-V oxygens where one has the H atom linked to it. One of these three oxygens is provided by the surface. V in a distorted octahedral (6-fold) configuration (B) is found by the GA on the (101) facet with four interface Ti-O-V oxygens, where two surface oxygens participate in the formation. This structure is 0.65 eV ± 0.40 eV less stable and is therefore unlikely to exist on the surface. Interestingly, the local coordination is very different on the (001) facet where V is found exclusively in octahedral (6-fold) coordination and never in tetrahedral coordination as was the case for the (101) surface. Here the most stable structure (C) consists of four interface Ti-O-V oxygens whereas two O atoms is provided by the surface and a single vanadyl V=O oxygen. The sixth oxygen on the octahedron is a O(3f) provided by the surface. The GA finds other configurations (D,E,F) of VO3H on the (001) facet which are 0.54 eV ± 0.04 eV, 0.60 eV ± 0.10 eV and 0.71 eV ± 0.11 eV more unstable respectively. Structures C, D and E differ only by the location of the H atom. In D it is linked to the vanadyl V=O oxygen and in E it is not on the monomeric structure but on the surface oxygen atom. At this coverage it is found to destabilize the system but at a lower coverages 0.8 V/nm it is favourable by 0.3 eV to separate the H atom from the VO3 entity. The effect is due to the fact that at such a low coverages the surface is more exposed and the formation of hydroxyl groups results in lower formation energies.27,44 This is not the case at coverage of 1.7 V/nm where having a surface hydroxyl group is less stable compared to having the hydroxyl group on the monomer. Structure F is the least stable and consists effectively of a VO2 and a OH group linked to a Ti. The octahedral VO3H structure C should therefore strongly dominate the population on the (001) surface whereas the (101) surface is predicted to contain predominantly tetrahedral VO3H species. Structures A and C on Figure 3


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are found to be the most stable structures independent of the choice of the XC functional verified by using the BEEF error estimation method (see error bars on the energy axis at the top of Figure 3).

Figure 3: Monomeric VO3H species on TiO2(101) (A,B) (shown in top view) and (001) (C-F) (shown in side view). A and C are the two most stable species on the two facets. The other structures are higher


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in energy as depicted on the energy axis at the top of the figure. Red balls (sticks) are O atoms (bonds) from the TiO2, yellow balls (sticks) are O atoms from V2O5(b), grey balls (sticks) are Ti atoms (bonds), cyan balls (sticks) are V atoms (bonds) and white balls (sticks) are H atoms (bonds). The energy axis at top shows the formation energies of each species along with the baysiean error estimate relative to the most stable structure on each surface (marked with border).

The SCR process is a redox process i.e. it involves electrons being exchanged between the active phase and the adsorbates, and during the catalytic cycle the V changes oxidation state by accommodating electrons and is consequently reduced. The structures of the most stable VO3H species on both facets are shown in Figure 4 along with the structure of the two types of reduced VOX structures derived from the initial state, VO3H2 and VO2 where V is in oxidation state +4 if an H atom is adsorbed or if an OH group desorbs, respectively. The former structure would be favoured under SCR reaction conditions wheras the latter would be favoured in extremely dry environment or at elevated temperatures on the (101) facet. Interestingly we find on both facets unchanged local coordination of the reduced V atom relative to the initial state independent of the reduction path taken. In the upper panel of Figure 4 these structures are shown on the (101) surface where the most stable VO3H structure is in tetrahedral configuration. VO3H2 exhibits the same local configuration as VO3H except an extra H is attached to an interface Ti-O-V oxygen. The VO2 is likewise in a tetrahedral configuration despite having one less oxygen to form a bond to the surface, instead it shifts the adsorption site on the (101) surface and uses two oxygens from the support to form the tetrahedron. In the lower panel of Figure 4 the analogous sequence of VOX structures are presented on the (001) surface. In the fully oxidized state the VO3H is in a distorted octahedral configuration with the angle between the short V=O bond and the long V-O


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bond equal 179.2°. VO3H reduced with an extra H atom keeps the octahedral coordination but now has two H atoms bonded to the interface Ti-O-V oxygens and becomes slightly more distorted than VO3H with an angle of 173.2° between the V=O and V-O bonds that form the top and the bottom of the octahedron. The dry version of a monomeric species on the (001) is VO2 and in the same manner as VO3H it is accommodated in an octahedral configuration but more distorted. The angle between the two axial bonds of the octahedron is 165.3° and an extra oxygen from the support is used in the square plane of the octahedron with bond length of ~0.4Å longer than the other bonds making it drastically more distorted than the other two structures.


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Figure 4: Upper (lower) panel: VO3H, VO3H2 and VO2 on TiO2(101) (TiO2(001)) along with the local configuration of V of each species respectively showing bonding lengths and angles. NMR has been used extensively to study the coordination of TiO2-supported vanadia catalysts. Vanadia monomers have been reported to be present in both octahedral (6-fold) and tetrahedral (4-fold) structures. The presence of octahedral (6-fold) structures have been attributed to vanadia desolved in a surface water layer upon drying dehydration.16 However, a key finding of the present study is that the type of V species and their coordination depends strongly on the surface facet of TiO2 where tetrahedra are present on the (101) facet and octrahedra are present on the (001) facet. The relative distribution of


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these species must therefore also depend on the relative exposure of the predominant facets on of the anatase support crystallites (i.e their shape) Secondly, we find that the local V coordination is preserved independent of oxidation state (limited to the catalytic relevant 5+ and 4+ oxidation states.) even when the structures are reduced by dehydration (removal of an OH group). Furthermore, our experimental EPR data given in Figure 5A shows in situ spectra of a model catalyst in air and after partial reduction with NH3. As can be seen, the formation of a complicated spectrum, well described in the literature24,45 interpreted as the sum of a pair of monomeric VO2+ spectra superimposed on a broad ill-defined contribution from polymeric VO2+ is observed. A tentative assignment of the different EPR lines based on simulations of the individual octahderal and tetrahedral species is given in Figure 5B, where it is reasonable to propose that two monomers indeed exists. This is also in accord with extensively treated (ex situ) data by Paganini et al

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and Brückner and co-workers

45

. This

experimentally supports our calculational finding that VO3H2 exists in two different conformations depending on the exposed facet. The implications of these EPR and earlier observations of the existence of V(IV) species during NH3-SCR with clear vanadylic structure (maintenance of the (V=O)2+ unit) has until now not been accuentated strongly enough. The combination with the DFT calculations makes it now clear that vanadium is present on the catalyst during SCR and with the vanadium in valence four having its vanadyl unit intact and thus not present as a e.g. tetrahedral species in its pure symmetric form. Rather, we see structures that are distorted in between octahedral and tetrahedral arrangements, which once again suggests that the vanadyl unit, clearly observed for V5+ with Raman and FTIR and for V4+ with EPR is present in the active catalyst, but it is not the reactive functional group of the vanadium entities.


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Figure 5: In situ EPR spectra at 250°C. A) Comparison of oxidized and reduced spectra. B) Difference spectra between oxidized and reduced spectra (red) and simulated contributions from two monomeric VO2+ species with different Spin Hamiltionians: distorted octahedral (blue) and distorted tetrahedral (green).

CONCLUSION In summary we have combined theoretical calculations based on DFT with EPR spectroscopy in order to investigate the structural properties of low coverage VOX species on TiO2 anatase. The two most stable surface terminations (101) and (001) accommodate different types of fully oxidized, hydrated monomeric species which are in tetrahedral and octahedral configuration, respectively. Importantly, the coordination is retained in the reduced state (oxidation state +4) as these species exhibit the same local configuration independent of the reduction path i.e. by effective adsorption of H (SCR condition) or desorption of OH (extreme dry condition). EPR identifies two different monomeric structures in their reduced state which fits with the theoretical predictions. The


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calculations thus offer a new and more detailed view on the nature of VOX species in observations done by Raman, IR, EPR and NMR. The results provide clues on the structure of the active species in the VOX/TiO2 catalysts under working conditions.

ASSOCIATED CONTENT Supporting Information. Supporting Information Available: List of formation energies and xyz coordinates of most stable structures. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding author: Poul Georg Moses Telephone number: +45 22754015 e-mail address: [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT


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The Danish Council for Strategic Research (grant Cat-C) is gratefully acknowledged for financial support.

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Figure 6: Table of content graphic. Local coordination of oxidized (+5) and reduced (+4) Vanadia species on the two most stables surface facets of TiO2.


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Coexistence of Square Pyramidal Structures of Oxo Vanadium (+5

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