Chemisorption of NH3 on Monomeric Vanadium Oxide Supported on

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 3

Chemisorption of NH on Monomeric Vanadium Oxide Supported on Anatase TiO: A Combined DRIFT and DFT Study 2

Inhak Song, Jaeha Lee, Geonhee Lee, Jeong Woo Han, and DoHeui Kim J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02291 • Publication Date (Web): 09 Jul 2018 Downloaded from http://pubs.acs.org on July 10, 2018

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The Journal of Physical Chemistry

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Chemisorption of NH3 on Monomeric Vanadium

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Oxide Supported on Anatase TiO2: A Combined

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DRIFT and DFT Study

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Inhak Song†, Jaeha Lee†, Geonhee Lee‡, Jeong Woo Han*,§, Do Heui Kim*,†

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†School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul

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National University, Seoul 08826, Republic of Korea

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‡Department of Chemical Engineering, University of Seoul, Seoul 02504, Republic of Korea

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§Department of Chemical Engineering, Pohang University of Science and Technology

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(POSTECH), Pohang 37673, Republic of Korea

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Corresponding Author

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*Corresponding author: [email protected], [email protected]

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ACS Paragon Plus Environment

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

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ABSTRACT

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V/TiO2 catalysts are used in various reactions including oxidative dehydrogenation, partial

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oxidation of ethanol, and selective catalytic reduction of NOx with NH3. In this work, we

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investigated the effect of supported monomeric vanadium oxide (VO3) on the acidity of anatase

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TiO2(101) surface by using density functional theory (DFT) calculations combined with in situ

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DRIFT experiments. The hydrogenation of TiO2 to form hydroxyl groups on the surface was

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energetically more favorable in the presence of the supported monomeric vanadium oxide.

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Charge transfer between TiO2 support and VO3 was considered as an origin of –OH stabilization,

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which made Brønsted acid sites more abundant on V/TiO2 surface than on TiO2. Also, it was

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observed that cationic vanadium center in VO3 can act as much weaker Lewis acid sites than

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Titanium center in TiO2. Such weakened acidity of Lewis acid sites in the presence of

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monomeric vanadium oxide was consistently observed in in situ DRIFT results, which could

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explain the higher reactivity of NH3 adsorbed on Lewis acid sites of V/TiO2 than those of TiO2

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in the NH3-SCR reaction.

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1. Introduction

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Vanadium oxide supported on anatase titania catalyst (VOx/TiO2) is active for various

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reactions including selective oxidation or reduction.1-5 It has been well known as a conventional

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catalyst used in the selective catalytic reduction (SCR) process, which selectively reduce harmful

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NOx with NH3 in lean conditions.6-9 Many researches have been perfomed to investigate

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structure-activity relationship in VOx/TiO2 catalyst, where it is widely accepted that the degree of

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polymerization of vanadia on TiO2 is a key factor to determine the activity or selectivity in the

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reaction.10-11

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There are, however, relatively few papers addressing the acidic properties of VOx/TiO2

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catalyst. Supported vanadium oxide catalysts are known to have two kinds of acidic sites on the

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surface, Lewis acid sites and Brønsted acid sites.12 The standard SCR reaction usually occur with

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Eley-Rideal mechanism, which means that the NH3 adsorbed on the acidic sites of catalysts can

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reduce gaseous NO molecule.13-14 Hence, the site at which NH3 molecules are adsorbed is an

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important factor in determining the activity or selectivity of the SCR reaction.15 In fact, it has

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been long controversy as to which NH3 molecules are involved in the reaction more dominantly.

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For example, Ramis et al. claimed that NH3 molecule can be chemisorbed and activated on

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Lewis acid sites, which subsequently reacts with NO to form the intermediate species.16-17 Their

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key observation from DRIFT study was that NH3 adsorbed on Lewis acid sites can react more

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actively than that on Brønsted acid sites. Meanwhile, Topsøe et al. found correlation between the

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amount of Brønsted acid sites and the SCR activity.18-19 By varying vanadium loading on TiO2,

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they found that increasing V loading can raise the number of Brønsted acid sites, which leads to

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the enhanced activity. It is obvious that NH3 on Lewis acid sites initially participates in the

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reaction with NO, while NH3 on Brønsted acid sites also seem to play a certain role in the


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reaction, rather than exist as a spectator. In the case of oxidative dehydrogenation with

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VOx/TiO2, it was reported that the selectivity of reaction is a function of acid-base properties of

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catalyst because the acid sites play an important role in adsorbing alkane molecule and its

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activated form on the surface.20-21 Thus, understanding the acidic nature of VOx/TiO2 catalyst is

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quite important to precisely analyze the reaction.

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Recently, many attempts have been made to investigate VOx/TiO2 system by various

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theoretical methods. Du et al. investigated the structure and the stability of monomeric HVOx

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species on anatase TiO2 support using periodic density functional theory.22 Cheng et al. studied

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the VOx sites on anatase TiO2 (001) surface to understand propane oxidative dehydrogenation.23

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The study for size-dependent catalytic activity of vanadia catalysts in the oxidative

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dehydrogenation reaction was also reported.24 Recently, Arnarson et al. investigated the

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supported VOx species on anatase, and also tried to understand the complete reaction mechanism

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for SCR reaction.25-27 Yuan et al. also studied the Lewis or Brønsted acid, and nitrite mechanism

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in SCR reaction over the optimized structure of the V6O20H10 cluster.28 Recent studies by

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Kristoffersen et al. about the interaction between supported monomeric vanadium oxide species

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and titania provides great insight into the origin of the stability of vanadia clusters.29-30 However,

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there are relatively few papers investigating the acidic properties of VOx/TiO2 catalyst with

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computational method, and there are also few attempts to correlate the experimental results with

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calculated ones precisely.

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In this study, we have optimized the configuration of hydrogenated monomeric

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vanadium oxides on anatase TiO2(101) using density functional theory (DFT) calculations to

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explore which atoms act as the acidic sites where NH3 molecules are adsorbed. Considering that

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a small amount of vanadium, about 0.5 ~ 1 wt%, is used in conventional vanadia catalyst for


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SCR reaction, it can be suggested that monomeric vanadium oxide may be abundant in the real

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catalyst. Several NH3 adsorption models were also calculated and compared with in situ DRIFT

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results.

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2. Experimental

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2.1. Computational methods

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All the periodic density functional theory (DFT) calculations were performed with a

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Vienna ab initio simulation package (VASP).31-32 Exchange-correlation effects were considered

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by the Perdew-Burke-Ernzerhof (PBE) functional based on generalized gradient approximation

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(GGA).33 The spin-polarized DFT +U calculations were applied to the both Ti 3d state and V 3d

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state with a same value of Ueff = 3.5 eV to treat the electron localization in d-orbitals more

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accurately. These Ueff values were accepted to be reasonable to describe the VOx/TiO2 system in

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the previous reports.34-37 The plane wave energy cutoff in the calculations was set to 400 eV for

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all atoms. The Gaussian smearing method with a width of 0.2 eV was applied to accelerate the

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convergence of electronic relaxation. Structures were optimized using a conjugate gradient

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algorithm until the forces on unconstrained atoms were less than 0.05 eV/Å.

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A slab model of anatase TiO2(101) surface was used in this work, which is the most

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stable surface among the low index surfaces of anatase TiO2.38 A (1 × 2) surface unit cell with

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four stoichiometric TiO2 layers and a vacuum spacing of 15 Å in a direction perpendicular to the

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surface were constructed. The surface area of the slab was 10.38 × 7.65 Å2, and the Monkhorst-

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Pack grid of 1 × 2 × 1 k-points were used. The bottom two stoichiometric layers were fixed in

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their bulk positions while the rest of top two layers were fully relaxed. The geometries of the gas


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phase molecules were optimized in the large periodically repeated cubic boxes of 10 Å on each

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side. The bcc structure is used to calculate the vanadium solid for reference. (No Hubbard U term

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is used when calculating the bulk vanadium) The formal charges of the atoms on the system were

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calculated by Bader charge method.39-40

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2.2. Experimental methods

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Commercial TiO2 (P25, Degussa) powder was used in this work. VOx/TiO2 samples were

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prepared with conventional wet impregnation method.41 Ammonium metavanadate (99 %, Sigma

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Aldrich) with the amount of targeted V surface concentration was dissolved in 0.05 M oxalic

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acid solution to make the vanadium precursor solution. The mixture of TiO2 powder and the

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vanadium precursor solution was stirred for 30 min, and subsequently vaporized in a rotary

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evaporator. The obtained solid was dried overnight in an oven at 105 oC and calcined at 400 oC

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for 3 h in static air condition.

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N2 adsorption–desorption experiments were carried out at liquid nitrogen temperature (-

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196 oC) by using ASAP 2010 apparatus of Micromeritics to measure the BET surface area of

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each sample. About 0.1 g of the sample was degassed at 300 oC for more than 6 h before

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measurement to remove any impurities on the surface. Powder XRD pattern were taken using

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Ultra X18 (Rigaku) with a Cu Kα radiation. The diffraction patterns were recorded in the range

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of 10 - 80 o (2θ) with the step size of 0.02 o at the counting rate of 0.24 s per step.

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In situ DRIFT spectra were obtained in a FT-IR spectroscopy (Nicolet 6700, Thermo

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Scientific) equipped with high-temperature DRIFT cell having ZnSe windows. The samples were

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finely ground in a porcelain mortar, and about 0.025 g of powder was rigidly packed in a


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circular-type cell to prevent the collapse of samples during pretreatment. The IR spectra were

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recorded by accumulating 32 scans at a resolution of 1 cm−1 in all cases. For NH3 adsorption

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experiments, the samples packed in DRIFT cell were pretreated in 5 % O2 balanced with N2 (200

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mL/min) at 400 oC for 1 h to remove impurities including adsorbed water on the catalyst surface.

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After the pretreatment, the cell was cooled to 100 oC with N2 purging and the background

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spectrum was obtained in this step. Then, samples were exposed to 0.38 Torr of NH3 at 100 oC.

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During NH3 adsorption, spectra were acquired per minute to track the variation of surface

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species.

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3. Results and Discussion

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3.1. Stabilization of hydroxyl group on monomeric vanadium oxide on TiO2 system

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It has been known that anatase phase TiO2 crystals are dominantly composed of

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thermodynamically stable (101) surface, which occupies 94 percent in the Wulff construction of

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anatase TiO2 crystal.42 The stoichiometric anatase TiO2(101) facet was optimized as shown in

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Figure 1. Anatase TiO2(101) surface consists of four kinds of atoms including fully-coordinated

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Ti6C (six-fold coordinated Ti) and O3C (three-fold coordinated O), and under-coordinated Ti5C

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(five-fold coordinated Ti) and O2C (two-fold coordinated O). Ti6C and Ti5C are linked by bridging

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O2C while two Ti5C atoms and one Ti6C atom are linked by O3C atom. Thus, linearly arranged O2C

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atoms alternate with aligned O3C atoms along the TiO2(101) surface. The obtained surface energy

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was 0.61 J/m2, which agreed well with the data from other research groups.43


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Figure 1. (a) Side view and (b) Top view of anatase TiO2(101) surface optimized in the calculations. The unit cell of surface (1×2) was shown as a black rectangle. Blue and red balls represent Ti and O atoms, respectively.

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Also, the most stable structure of monomeric VO3 unit with tetrahedral coordination on

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anatase TiO2 surface was determined (Figure 2). The structures of three different tetrahedral

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vanadium oxide monomers on TiO2 are suggested, for which under-coordinated atoms bound to

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the V atom are controlled. Two Ti5C (in the form of Ti5C-O-V) and one O2C are used to bind VO3

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in monomer-A, one Ti5C and two O2C are used to bind VO2 in monomer-B, and only two O2C are

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used to bind VO2 in monomer-C to investigate which structure is the most stable. The formation

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energy of VO4 unit on TiO2 ( ) is calculated, which is defined as

/

2


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where / , ,

, and are energies of the monomeric vanadium oxide supported

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on anatase TiO2(101) system, the clean anatase TiO2(101) surface, the bulk vanadium metal of

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bcc structure (per one vanadium atom), and the gas phase O2, respectively.

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Figure 2. The optimized structures of monomeric vanadium oxide species on anatase TiO2(101) surface are proposed. Side (upper panel) and Top views (middel panel) and the local configuration of VO4 unit (lower panel) of (a) monomer-A, (b) monomer-B, and (c) monomer-C. Green ball represents the vanadium atom.

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The negative formation energy stands for the stabilization of vanadium oxide monomer on

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TiO2(101) surface. The formation energy was – 7.25 eV for monomer-A, – 6.67 eV for

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monomer-B, and – 5.82 eV for monomer-C, respectively, which means that the monomer-A is


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the most stable VO4 unit structure on the surface. The local configuration of VO4 unit shows that

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V=O double bond, which has the short bond length of 1.62 ~ 1.65 Å, was formed when the

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oxygen bound to vanadium atom is not linked to the other atoms. Transformation of monomer-C

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to monomer-B suggests that the bond formation between Ti5C and vanadyl group (V=O)

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stabilizes the system by 0.85 eV. The additional Ti5C-O-V bond formation in monomer-A further

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stabilizes the system by 0.58 eV than monomer-B though one V-O2C bond is broken. Thus, in the

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calcination step of V/TiO2 catalyst, the impregnated [VO]2+ ion will be stabilized by making a

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bond preferentially with Ti5C on TiO2(101) surface.

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Figure 3. Formation of hydroxyl group upon the adsorption of hydrogen atom (white) on (a) O2 and (b) O5 site on VO3/TiO2(101) surface, and (c) O4 and (d) O5 site on TiO2(101) surface. (e) The energy axis represents the adsorption energies of H atom on the different O species summarizing Table 1. (f) DRIFT spectra obtained at 100 oC after the pretreatment of samples at 400 oC for 1 h in N2 gas.

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Figure 3a-e demonstrates the adsorption configuration and energy of hydrogen on the various

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oxygen atoms of optimized VO3/TiO2 and TiO2 structure, when hydroxyl groups are formed. For

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hydrogenation, 5 positions (O1~O5) of VO3/TiO2 and 2 positions (O4, O5) of TiO2 were


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considered. (Hydrogen adsorption on O1, O3, and O4 sites of VO3/TiO2 was shown in Figure

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S1.) The calculation results are summarized in Table 1 where the hydrogen adsorption energy

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( ) is defined as 1 / /

2

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where / , / , and are total energies of the H adsorbed VO3/TiO2 system, the

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initial VO3/TiO2 system, and the gas phase H2, respectively. The negative hydrogen adsorption

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energy indicates the stabilization of H on VO3/TiO2 system.

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Table 1. The hydrogen adsorption energy (EH) on the different O species of anatase TiO2(101) surface and vanadium oxide on anatase TiO2(101) surface and the resulting Bader charges of H and VO3 species in e- on each model. Structure

H-VO3/TiO2

H-TiO2 a

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a

O site H on O1 H on O2 H on O3 H on O4 H on O5 H on O4 H on O5

Position VO3 TiO2 TiO2

EH (eV) -2.19 -2.71 -1.12 -2.60 -2.67 +0.16 -0.09

VO3/TiO2

Bader charges H VO3 +0.66 -0.76 +0.67 -0.81 +0.67 -0.57 +0.67 -0.57 +0.68 -0.44 +0.66 +0.66 -0.35

For comparison, the Bader charges of VO3/TiO2 structure (Figure 2a) was calculated.

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The most stable hydroxyl group is located on O2 site bridging V and Ti5C whose

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adsorption energy is – 2.71 eV. Figure 3e clearly indicates that the presence of VO3 group on

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TiO2 can lower the hydrogen adsorption energy (EH) on the surface. The hydrogen adsorption

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energy on TiO2(101) surface is nearly zero in the absence of VO3 group, which means that it is

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hard to produce hydroxyl groups on bare TiO2 surface. On the other hand, it was confirmed that

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the same site, O4 and O5, can be easily hydrogenated in the presence of anchoring VO3. It seems


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that VO3 monomer can stabilize the formation of neighboring hydroxyl group on TiO2 surface. A

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Bader charge analysis on TiO2 support, adsorbed hydrogen, and VO3 monomer was performed to

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investigate charge transfer between these species as presented in Table 1. The previous reports

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about V/TiO2 system, with the various Ueff value of 2.5, 3.5, and 4.5 eV, showed that the

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formation energy of vanadium oxide clusters on rutile TiO2(110) surface depended on the Ueff

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value, while the relative stability was not affected by the Ueff value.29 Hence, the Ueff of 3.5 eV in

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this study is good enough to reflect the electron localization effect. The characteristic of

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adsorbate on TiO2 support determines the direction of charge transfer of system. While hydrogen

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atom donates electron to TiO2 support with the formation of –OH group on pure TiO2 surface,

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VO3 group can attract electron from the support, which seems to have a correlation with different

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adsorption energies of H and VO3. After hydrogenation (H-VO3/TiO2), there is a trend that the

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additional charge is localized on VO3 group compared with VO3/TiO2 system as shown in Table

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1. Thus, our model suggests that the role of VO3 group on TiO2 is localizing additional electrons

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from hydrogenation, thereby stabilizing hydroxyl groups on the surface. It was also confirmed

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that several –OH groups can simultaneously exist on VO3/TiO2 system (Figure S2). Sequential

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hydrogen adsorption on vanadium oxide monomer on TiO2 showed that the second hydrogen

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adsorption stabilized the system more significantly. Table 2. Surface V concentration of prepared V/TiO2 samples in this study

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α

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α

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b

Sample

V content (wt%)

BET surface area (m2/g)

TiO2 1.26V/TiO2 2.51V/TiO2

0.53 1.06

58 56 53

V surface concentration was calculated by this equation:

V surface concentration (/nm2) 1.24 2.60

b

Theoretical coverage 0.16 0.33

!"!" # "#$% &' (! ()"(

Theoretical monolayer coverage of V/TiO2 was assumed as 8 V/nm2.

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For the purpose of experimental verification, a series of V/TiO2 catalysts were prepared

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as summarized in Table 2. XRD patterns of these samples only represent anatase and rutile phase

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indicating that vanadia species are highly dispersed on the TiO2 surface without formation of

4

bulk V2O5 (Figure S3). It has been known that the samples with the full theoretical monolayer

5

coverage can inevitably have a small amount of crystalline VxOy cluster, which were not

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considered in DFT calculation.44 In fact, it was also reported that V-O-V species exist even at

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extremely low surface coverages (

Chemisorption of NH3 on Monomeric Vanadium Oxide Supported on

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