Mechanistic Study of Selective Catalytic Reduction of NOx with NH3

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Article x

Mechanistic Study of Selective Catalytic Reduction of NO with NH over Mn-TiO: A Combination of Experimental and DFT Study 3

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Huiling Zheng, Weiyu Song, Yan Zhou, Sicong Ma, Jianlin Deng, Yongheng Li, Jian Liu, and Zhen Zhao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06715 • Publication Date (Web): 21 Aug 2017 Downloaded from http://pubs.acs.org on August 23, 2017

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The Journal of Physical Chemistry

Mechanistic Study of Selective Catalytic Reduction of NOx with NH3 over Mn-TiO2: A Combination of Experimental and DFT study

Huiling Zheng,1 Weiyu Song,1,† Yan Zhou,1 Sicong Ma,1 Jianlin Deng,1 Yongheng Li,1 Jian Liu,1,* Zhen Zhao1,2,*

1

State Key Laboratory of Heavy Oil Processing, College of Science, China University of

Petroleum-Beijing, Chang Ping District, Beijing 102249, China

2

Institute of Catalysis for Energy and Environment, Shenyang Normal University,

Shenyang 110034, China

*

Corresponding Author.

E-mail:

[email protected]

(J.

Liu);

[email protected]

(Z.

Zhao);

[email protected] (Z. Zhao)

Tel: 86-10-89731586(o)

†

This author contributed equally as the first author.

1

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Abstract: Mn-TiO2 oxide catalyst has been studied intensively for selective catalytic reduction (SCR) of NO with NH3 due to its extraordinarily good low-temperature performance. However, the mechanism of SCR on Mn-TiO2 still remains unclear, especially with regard to the decomposition pathway of the NH2NO intermediate and the reason for the decreasing N2 selectivity with the increasing of temperature. In this work, we attempt to provide a molecular level understanding of these questions via a combination of DFT and experimental study. A complete catalytic cycle of the SCR reaction was proposed based on a model in which Mn is doped into the TiO2(101) surface by quantum-chemical DFT+U calculations. In situ DRIFTS experiments were performed to provide evidence to the important intermediates as proposed in the reaction mechanism. The doping Mn enhances NH3 adsorption and activation due to its lower conduction band. NH2NO can decompose into N2 and H2O fast via a concerted H migration step. The decreasing selectivity with rising temperature can be explained by the deep oxidation of NH3. This study provides atomic-scale insights into the catalytic cycle and the important role of doping Mn in NH3-SCR reaction on Mn-TiO2 catalysts, which is of significance for the design of high activity low-temperature SCR catalysts.

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Keywords: Mn-TiO2 mixed oxide; Selective catalytic reduction; DFT+U; Reaction mechanism; In situ DRIFTS; N2 selectivity

1. Introduction

Nitrogen oxides (NOx) mainly resulting from combustion processes of diesel engine and coal-fired power factories are the primary air pollutants, which can cause acid rain, photochemical smog and ozone depletion. The selective catalytic reduction (SCR) of NOx with ammonia is an efficient technique to remove NOx.1 The general reaction is as follows:2

4NO + 4NH3 + O2 → 4N2 + 6H2O

The key of the SCR technique is catalysts. V2O5/TiO2 catalysts with WO3 or MoO3 doped have been extensively employed in the SCR technology for its high SCR activity and sulfur-resistance capacity.3-4 But the toxicity and relatively low activity at low temperature of vanadia-based catalysts5 require other environmental-friendly SCR catalysts with high activity at low temperature. Catalysts based on manganese oxides have been reported to show high activity for SCR of NO with NH3 at low temperature.6-8 Mn shows various oxidation states including Mn2+, Mn3+ and Mn4+， which is beneficial to the redox cycle of

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SCR reaction. And Mn is generally regarded as a less toxic metal compared with V, Cu, Co and Ni commonly employed for the SCR catalysts.6 Mn-based catalysts usually require supports to improve its SCR activity and sulfur-resistance capacity. Most studied supports include TiO2,6-8 Al2O3,9-10 ACF,11 MWCNTs,12 USY,13 NaY,14 TiO2-ZrO2,15 CeO216 and so on. Among which, TiO2 has been shown to be a superior support due to its high specific surface area and great sulfur-resistance.17

The mechanism for SCR of NO with NH3 for Mn-based catalysts has been studied by many experimental research groups. Kapteijn et al. studied the reaction mechanism of NH3-SCR on alumina-supported manganese oxide catalysts using in situ IR and TPRD (temperature-programmed reaction and desorption).18 They proposed that adsorbing NH3 was first oxidized to NH2 species, which should be highly active to react with NO in the gas phase to produce N2 and H2O.18 Kijlstra et al. suggested that the adsorbed NH3 species can react with both gaseous NO (E-R mechanism) and adsorbed nitrite species (L-H mechanism) on MnOx/Al2O3 catalysts.19 Qi et al. proposed that the intermediate species NH2NO decomposes to get N2 and H2O in both E-R and L-H mechanism on MnOx/CeO2 catalysts.2 In addition, the N2 selectivity of Mn-based catalyst dramatically decreases with

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the increasing reaction temperature and Mn loadings, which might be attributed to the oxidation state or aggregation state of Mn or highly active surface oxygen.2,20

Some theoretical studies have also been performed by quantum-chemical method.21-23 The main proposed mechanism is as following: adsorbed NH3 on Lewis acid sites is activated by catalysts surface oxygen to form NH2, which is highly active to react with gas-phase NO to form NH2NO followed by decomposition to N2 and H2O. However, the calculated high barrier of NH2NO decomposition is not consistent with the experimental observed facile decomposition of NH2NO species under low temperatures.24 Additionally, the reoxidization of the reduced catalysts and the N2 selectivity issues have never been studied theoretically.

In this work, a series of Mn-Ti catalysts with different Mn loadings were prepared by hydrothermal synthesis method. Their physicochemical properties were characterized by means of XRD, Pyridine-IR and in situ DRIFTS. SCR performances are presented as NO conversion and N2 selectivity. A Mn-TiO2(101) model was employed to study the reaction mechanism by first principle methods. Two different decomposition ways of the NH2NO species were explored. After N2 formation the oxidation of the reduced catalyst surface has

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been investigated. N2 selectivity and N2O formation mechanism were also discussed. Electronic structure analysis was employed to interpret the different activity of various sites and species. The potential energy surface of these steps assisted in identifying the rate-limiting step and the preferable path for SCR of NO with NH3. To validate the proposed mechanism, vibrational frequencies for selected crucial intermediates were calculated and compared with experimental data.

2. Experimental and computational methods

2.1. Catalysts preparation

Mn-TiO2 catalysts were prepared by hydrothermal method. The procedures are as following: the appropriate amount of tetrabutyl titanate and acetylacetone were added to 30ml of ethanol as solution A, stirring for 0.5 h. And the appropriate amount of (NH4)2SO4, urea and Mn(NO3)2 were added to 20 ml of deioned water as solution B, stirring for 0.5 h. Then the solution A was dropped into the solution B. The slurry obtained was stirred for 2 h at room temperature and transferred to a Teflon-sealed autoclave and aged at 100 °C for 24 h. The obtained precipitate was filtered and washed with deioned water thoroughly after the

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autoclave was cooled down to the ambient temperature. The obtained solid was dried at 120 °C for 12 h and then calcined in air at 400 °C for 4 h. The same method was employed to prepare TiO2 (without the addition of Mn(NO3)2).

2.2. Catalyst characterization

Powder X-ray diffraction (XRD) was recorded on a powder X-ray diffractometer (Shimadzu XRD 6000) with Cu Kα (λ = 0.15406 nm) radiation at 40 kV and 10 mA in the 2θ range of 5-70° at a scanning rate of 4°/min. The IR spectra of pyridine adsorption over catalysts were recorded with a Nicolet 750 spectrometer. Before each experiment, the samples were pretreated at 400 °C for 1 h under evacuation in a pressure < 1 Pa. After cooling down to room temperature, the samples were exposed to a pyridine steam for 30 min, followed by pumping for 30 min to remove the weakly adsorbed pyridine, then the spectra were collected after heating to 200 °C at a rate of 10 °C/min. In situ DRIFTS were obtained on a Nicolet iS50 Fourier transform infrared equipment with a MCT detector. Diffuse reflectance signals were monitoring in situ in a high temperature cell equipped with a ZnSe window. Before each spectrum was collected, the sample was heated to 500 °C under N2 purging at a total flow rate of 100 mL/min for 60 min to remove any adsorbed impurities and

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then cooled down to room temperature. Background spectra were recorded in flowing N2 and subtracted from the sample spectrum for each measurement.

2.3. Catalytic activity test

The catalytic activity measurements for the reduction of NO by ammonia (NH3-SCR) were performed in a fixed bed quartz micro-reactor in steady flow mode with 0.4 g of sample (40-60 mesh) for each test. The typical reaction condition was as follows: 1000 ppm NO, 1000 ppm NH3, 3% O2 and balanced N2. The gas flow rate was 500 mL/min and the GHSV was 45,000 h-1. The data were collected at required temperature from 100 to 450 °C when the reaction reached a steady state. The NOx (NOx = NO + NO2) concentration in the inlet and outlet gas mixture was monitored by a NOx analyzer (Model-4000VM, SIGNAL International Ltd., UK), and the concentration of NH3, NO, NO2 and N2O were measured by a FT-IR spectrometer (Thermo Scientific Nicolet iS50, Thermo Fisher Scientific Inc., USA). The NO conversion and N2 selectivity in the SCR reaction were calculated by the following equations:25

NO Conversion=

[NO]inlet − [NO]outlet × 100% [NO]inlet

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  2[N 2O]outlet N 2 Selectivity =  1 −  × 100%  [NO x ]inlet + [NH 3 ]inlet − [NO x ]outlet − [NH 3 ]outlet  Where [NO]inlet and [NH3]inlet represent the NO concentration and NH3 concentration in the inlet (ppm) respectively, [NO]outlet and [NH3]outlet represent the NO concentration and NH3 concentration in the outlet (ppm) respectively.

2.4. Computational details

In this work, the Vienna ab initio simulation package (VASP)26,27 was used to calculate all

states

with

the

electron

exchange

correlation

effect

described

by

the

Perdew-Burke-Ernzerh of functional within the generalized gradient approximation (GGA-PBE).28 The calculations involved on-site Coulomb corrections29 (DFT+U, U = 4.2 eV for Ti 3d states30 and 4.5eV for Mn 3d states31). The spin-polarized calculations were performed. PAW pseudopotential was used to describe the core-valence electron interaction.32 Plane-wave basis set with an energy cutoff of 400 eV30 was used in this work. The climbing nudged elastic band method (CI-NEB)33,34 was employed to locate the transition states. Anatase TiO2(101) surface was presented by a six-layer slab model with a vacuum gap of 15 Å (Fig. 1). For all the surface calculations, the model was a periodic slab

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with a (2 × 2) surface unit cell. A Monkhorst pack 2 × 2 × 1 k-point mesh was used for the Brillouin zone integration.

During structural optimizations, all of the atoms except those in the bottom TiO2 two layers of the slab were allowed to relax until atom forces were smaller than 0.05 eV/Å. Adsorption energy has been calculated using the following expression:

Ead = Etot - Eslab - Ex

Where Etot is the total energy of the combined system with the adsorbate X bound to the slab, Eslab is the energy of the slab alone, and Ex is the energy of the adsorbate in the gas phase. According to this definition, exothermic adsorption results in a negative value of Ead.

3．．Results

3.1. Structure of the Mn-TiO2 catalyst

Fig. 2 shows the X-ray diffraction patterns of pure TiO2 and Mn-TiO2 catalysts with different Mn/(MnOx + TiO2) mass ratios. The strong characteristic peaks of titania which correspond to anatase phase can be observed in all the catalysts. But no peak for manganese oxide phase can be detected from the patterns of the Mn-TiO2 catalysts even at high Mn

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loadings, suggesting that manganese oxides might be in an amorphous or highly dispersed phase on the catalyst surface,6 and also insertion of manganese ions into the TiO2 lattice.35 Experimentally, it is difficult to unambiguously confirm the real state of Mn. However, a shift of peak at 37.87° of TiO2 (attributed to the (004) plane of anatase TiO2 as shown in Fig. 2) with the addition of Mn indicates that the most probable one might be the incorporation state into the TiO2 lattice.6

Accordingly, a doping model constructed via the substitution of one Ti atom of the TiO2(101) surface by a single Mn atom will be employed in our modeling. As Fig. 1 shows, there are two possible types of Ti atoms on the surface for substituting with Mn atom: five-coordinated Ti atom (denoted as Ti5c) and six-coordinated Ti atom (denoted as Ti6c). For both cases, the calculated spin magnetic moment of doping Mn is 3.15 µB, suggesting its valence state to be +4.36 The two substitution configurations show similar stability. However, no stable NH3 adsorption state was identified on the Mn6c sites due to its saturated coordination situation. This will rule out its potential rule in SCR reaction. Accordingly, only the Mn-TiO2(101) with Mn5c was employed for the following mechanistic study.

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3.2. NH3-SCR activity and N2 selectivity

Fig. 3 shows the results of NOx conversion in NH3-SCR reactions over Mn-TiO2 catalysts with varying Mn/(MnOx + TiO2) mass ratios. With the increasing of manganese loadings, the low temperature activity first increases until 35wt%, and then further increase of Mn loading leads to decrease of the NO conversion. This trend has also been reported in previous studies.7,37,38 35wt%Mn-TiO2 catalyst exhibited the highest NO conversion and the widest temperature window of catalytic activity for the removal of NO. At the reaction temperature of 125 °C, the NO conversion of the 35wt%Mn-TiO2 reached 100%. Besides, the conversion rate of NO is above 90% in the temperature range of 100-350 °C, exhibiting a wider temperature window. Pure anatase TiO2 catalysts exhibit no SCR activity38,39 in the same temperature range. The good SCR performance of Mn-TiO2 catalyst at low temperature indicates a key role of the doping Mn in enhancing NH3-SCR, which will be illustrated in the following sections.

As shown in Fig. 4, N2 selectivity over 35wt%Mn-TiO2 decreases with the rising of temperature. In low temperature range of 100-250 °C, the N2 selectivity is above 70%. But when temperature increases to 450 °C, the N2 selectivity decreases to 5%. The reason that

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N2 selectivity decreases with increasing temperature will be illuminated later.

3.3. Adsorption and dissociation of NH3

The adsorption and activation of NH3 are the first step of SCR reaction.18,22 Two possible acid sites (Brønsted acid and Lewis acid sites) are available for NH3 adsorption. In the following, the Pyridine-IR technique was employed to study the relative intensity of the two types of acid sites. The NH3 adsorption and activation will then be discussed based on In situ DRIFTS and DFT technique.

3.3.1. Pyridine-IR

The pyridine-IR spectra of TiO2 and Mn-TiO2 are shown in Fig. 5, and they show similar bands. The bands at 1445, 1603 cm-1 on TiO2 and 1443, 1600 cm-1 on Mn-TiO2 are attributed to Lewis acid sites. The small bands at 1550, 1641 cm-1 on TiO2 and 1541, 1639 cm-1 on Mn-TiO2 are assigned to Brønsted acid sites. The bands at 1487 cm-1 on TiO2 and 1483 cm-1 on Mn-TiO2 are assigned to Brønsted and Lewis acid sites. The rest bands at 1575 cm-1 on TiO2 and 1574 cm-1 on Mn-TiO2 are attributed to physisorbed pyridine.40 The spectra show that Lewis acid sites are dominant acid sites on both TiO2 and Mn-TiO2. And

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more Lewis acid sites on Mn-TiO2 than that on TiO2 indicates Mn doping increases Lewis acid sites.

3.3.2. In situ DRIFTS

Fig. 6 shows the in situ DRIFT spectra of NH3 adsorption over 35wt%Mn-TiO2 catalyst at different temperatures. When NH3 was introduced into the DRIFTS cell at room temperature, several vibration bands can be detected in the range of 1100-1700 and 3000-3400 cm-1. The bands at 1603 and 1190 cm-1 could be attributed to coordinated NH3 bound to Lewis acid sites.41 The bands at 1461 cm-1 could be attributed to NH4+ species on Brønsted acid sites.41 In the N-H stretching region, bands are found at 3361, 3254, and 3170 cm-1, which can be ascribed to the N-H stretching vibration modes of the coordinated NH3.41 The bands at 1545 cm-1 might be assigned to the -NH2 species.19,42 The bands at 1303 cm-1 might be assigned to the nitro species43 which formed via deep oxidation of coordinated NH3 by lattice active oxygen. The bands of adsorbed NH4+ species bound to Brønsted acid sites decrease noticeably at higher temperature, while the bands of coordinated NH3 bound to Lewis acid sites still remain. It is consistent with the previous literature44 that the NH3 adsorbed on Lewis acid sites are relatively more stable than that on

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Brønsted acid sites at higher temperature. Therefore, NH3 might mainly adsorb on Mn and Ti ions over Mn-TiO2 catalysts. With the rising temperature, the bands of coordinated NH3 decrease and -NH2 increase, respectively. This may be due to the decomposition of NH3 to form -NH2 species. The increase of bands at 1303 cm-1 with rising temperature might be due to deep oxidation of coordinated NH3.43

As a comparison, Fig. 7 shows the in situ DRIFT spectra of NH3 adsorption over pure TiO2 catalyst at different temperatures. When NH3 was introduced into the DRIFTS cell at room temperature, several vibration bands can be detected in the range of 1100-1700 and 3000-3400 cm-1. Similar with that over Mn-TiO2, the bands at 1597 and 1180 cm-1 could be attributed to coordinated NH3 bound to Lewis acid sites, whereas the bands at 1468 cm-1 are assigned to NH4+ species on Brønsted acid sites.45 In the N-H stretching region, bands are found at 3371, 3264, and 3157 cm-1, which can be ascribed to the N-H stretching vibration modes of the coordinated NH3.41 The bands at 1536 cm-1 at low temperature (30-200 °C) and 1517 cm-1 at high temperature (300-500 °C) might be attributed to -NH2 species and the intermediate of oxidation of NH3.46 Same as DRIFTS on Mn-TiO2, the bands of coordinated NH3 and -NH2 decrease and increase respectively with the temperature rising.

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Similarly, this could be explained by the decomposition of NH3 to -NH2 species. Differently, the bands at 1603 cm-1 disappear at 100 °C on Mn-TiO2, while the bands at 1597 cm-1 still remain at 100 °C on pure TiO2. The bands attributed to -NH2 species increase slightly on pure TiO2, even at high temperature. And there’s no band attributed to nitro compounds from deep oxidation of NH3 on pure TiO2. These results indicate that it’s more difficult for pure TiO2 than Mn-TiO2 to decompose NH3 to -NH2 species and other oxidation intermediates. Quantum chemical calculations were performed to study the NH3 adsorption and activation based on TiO2(101) and Mn-TiO2(101) with the aim to provide a molecular level understanding to such difference.

3.3.3. Adsorption and dissociation of NH3 on TiO2(101) surface

There are two different adsorption sites of NH3 on the TiO2(101) surface as shown in Fig. 1. The most stable adsorption site is the Ti5c, namely Lewis acid site with the Ead of -1.16 eV, similar with the previous computational results.21 The calculated Bader charge of NH3 ad-molecule is -0.15 e, indicating electron transfer from NH3 to Ti. Then one H atom will be dissociated from the coordinated NH3 to the catalyst surface. There are five different dissociation sites on the TiO2(101) surface including two three-coordinated O sites (O3c) and

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three two-coordinated O sites (O2c) as shown in Fig. 8a. However, the dissociation energy of H to the O3c sites is as high as 3.70 eV, ruling out the possibility under the reaction conditions. For all three O2c sites, similar energy barrier of about 1.30 eV was observed. However, the reaction energy differs with respect to the configuration of -NH2, with the -NH2 plane perpendicular to the Ti5c-O2c-1 line more stable by 0.24 eV than that parallel to this line. The stability difference could be understood from the hydrogen interaction between -NH2 and its neighboring O2c atoms. This interaction can be visualized via the local charge density figure shown in Fig. 8b & 8c.

3.3.4. Adsorption and dissociation of NH3 on Mn-TiO2(101) surface

As shown in section 3.1, the Mn-TiO2(101) model consists of a single Mn atom doped into Ti lattice site of the TiO2(101) surface. Ti5c and Mn5c are the two Lewis acid sites for NH3 adsorption, with adsorption energy of -1.27 eV and -1.38 eV, respectively. For adsorption on Mn5c, the Bader charge of NH3 is -0.24 e, slightly larger than that on TiO2(101) surface, indicating more electron transfer from NH3 to Mn. This is consistent with the higher adsorption energy. The difference can be understood by the lower conduction band energy level of Mn 3d orbital than Ti 3d orbital (Fig. 9a). The essence of NH3 adsorption is the

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hybridization of the HOMO orbital of NH3 with the conduction band of the support.47 Accordingly, the lower the energy level of conduction band will lead to a stronger interaction.

For the dissociation of NH3 on Mn-TiO2(101), same as on TiO2(101) surface, the H goes to the O2c site and the remaining -NH2 species stays perpendicular to Mn5c-O2c-1 line. Of all three O2c sites, the one directly binding to Mn is the most active sites for H abstraction for its lowest reaction energy. This can be understood by the lower conduction band energy level of O 2p orbital (Fig. 9b), to which the electron will be donated from H. The activation barrier and reaction energy of NH3 dissociation on Mn-TiO2(101) is 0.97 eV and 0.66 eV, respectively (Fig. 8d). Both decreased as compared with those on TiO2(101) surface (1.31 and 0.99 eV, respectively). The doping of Mn activates the lattice O atom, which shows higher capability for H abstraction as shown in Fig. 9c.

3.4. Formation and decomposition of NH2NO species

The -NH2 species generated from NH3 dissociation reacts with NO from gas phase spontaneously on the Mn-TiO2 surface (Fig. 10). The enthalpy change for this spontaneous

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process is -2.18 eV. And the spin magnetic moment of Mn increases from 3.15 µB to 3.89 µB, indicating valence state change of Mn from +4 to +3,36 which is due to the transferred electron from the dissociated H. In contrast to the spontaneous reaction of NO with -NH2 on Mn-TiO2, the optimization of NO molecule initially close to -NH2 species on TiO2 leads to the desorption of NO into gas phase. This suggests that an energy barrier needs to be overcome. The NH2NO formation on TiO2(101) surface shows an enthalpy change of -1.04 eV, which is much lower than that on Mn-TiO2(101) (-2.18 eV). The difference can be understood from the orbital interaction point of view. The reaction of NO with -NH2 involves the interaction of NO HOMO with -NH2 LUMO band. Therefore, the lower LUMO band energy level (Fig. 9d) induces a higher activity of -NH2 species on Mn-TiO2(101) towards NO.

The decomposition of NH2NO species to form N2 and H2O is vital for the NH3-SCR process. Experimentally, a fast transformation of NH2NO to N2 and H2O was observed, indicating this would be a facile process.24 However, theoretically, it is a very complex process and difficult to be investigated. Some possible paths have been investigated. A popular path is the “inner-hydrogen transfer” path, namely one H atom of the NH2NO species

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transfers to the O atom of NH2NO successively to form HONNH species.21 Same decomposition path in the gas phase was also studied.22 In present contribution, a barrier of 1.44 eV for the first hydrogen transfer step on Mn-TiO2(101) surface has been gotten (Fig. S1 in the Supplementary Material), which is similar to previous studies.21,22 So the most serious problem regarding these proposed paths is the high activation barrier, which is inconsistent with the experimental observed fast decomposition of NH2NO.24 Obviously, more reaction paths need to be explored to explain the experimental results. In the following, two candidates are proposed.

3.4.1. Path1

Fig. 11 presents potential energy diagram with structures of each state of path1. The NH2NO dissociates H to the surface O, forming a NHNO species and surface OH species (state iii, Fig. 11) with a low energy barrier of 0.54 eV. The dissociation step is slightly endothermic by 0.34 eV. The spin magnetic moment of Mn changes from 3.89 µB to 4.61 µB, indicating valence state change of Mn from +3 to +2.36 This is due to the electron transfer from dissociated H. The second H dissociation proceeds with a negligible energy barrier and an exothermal energy of -1.07 eV (state iv, Fig. 11). The generating N2O molecule weakly

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bonds to the catalyst surface with Ead of -0.03 eV. The dissociation of N2O to N2 requires a surface oxygen vacancy. Two adjoining OH species on Mn-TiO2(101) surface (state v, Fig. 11) can combine to form H2O. One oxygen vacancy at O2c-3 site was generated (state vi, Fig. 11). This reaction can occur easily with an energy barrier of 0.53 eV. It takes 1.23 eV to desorb H2O into gas phase. The high binding energy of water would limit the oxygen vacancy formation process, thus inhibiting SCR reaction. This might be one reason for the water poisoning effect as observed experimentally.48,49 Then the N2O in the gas phase can adsorb on the oxygen vacancy (state viii, Fig. 11) and be dissociated to form N2. The dissociated O atom heals the oxygen vacancy on the catalyst surface (state ix, Fig. 11). N2O dissociation step is barrierless, as proven by previous molecular dynamic study.50 Finally, N2 desorbs with a small desorption energy of 0.08 eV, closing the reaction.

There are several issues related to this path. First, the weak adsorption of N2O on oxygen vacancy indicates low N2 yield. This is consistent with the experimental observation that N2O is a by-product in SCR reaction.43 Second, the reaction cycle leaves one H on the catalyst surface, reducing Mn from +4 to +3. The higher conduction band energy level of Mn3+ than Mn4+ (Fig. S2 in the Supplementary Material) decreases the adsorption energy of

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NH3 to a value of -0.75 eV, even lower than that on Ti sites (-1.16 eV). And the NH3 dissociation energy to O2c-2 site is 1.32 eV (Fig. S3 in the Supplementary Material), as high as pure TiO2, due to the occupation of the active O2c-1 site. To summarize, the doping Mn will not catalyze the reaction effectively any more.

3.4.2. Path2

Based on the above considerations, another path for the decomposition of NH2NO species has been investigated. NH2NO first undergoes a configuration change (state i to ii, Fig. 12). Now, the NH2NO species (state ii, Fig. 12) is perpendicular to the catalyst surface. The NH2NO plane, O2c-1-H and O2c-2 are in one plane. There are two hydrogen bond interactions and a stronger Mn-N bond (bond length: 2.38 Å) between adsorbed NH2NO species and the Mn-TiO2(101) surface. Accordingly, the system is stabilized by 0.33 eV. Then a concerted process, in which one N-H bond dissociates and the surface hydroxyl group dissociates the H to the ONNH (state iii to iv, Fig.12), takes place. The activation barrier and reaction energy is 0.53 eV and 0.40 eV, respectively. The formed HONNH species then rotates to a configuration parallel to the catalyst surface with a small endothermic energy of 0.29 eV (state v, Fig. 12). The second N-H bond can be easily decomposed, accompanied

22


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with the N-O bond dissociated. Three OH groups and N2 form (state vi, Fig. 12). This step is spontaneous with a high exothermic energy of -1.85 eV. The OH groups coordinated to Mn is very active to abstract H atom connected O2c-2 to form H2O molecule (state vii, Fig.12) with an exothermic energy of -0.82 eV. Then the H2O desorbs from the catalyst surface with an endothermic energy of 0.71 eV, leaving a H atom bound to O2c-3. From state iv, the HO-NNH species can also change to the configuration with the -OH group close to the surface hydroxyl. With the dissociation of N-H bond, the -OH group spontaneously dissociates to react with surface hydroxyl forming water. Meanwhile N2 is formed. From NH3 adsorption to N2 formation, NH3 dissociation presents the highest energy barrier, proving it’s the rate-determining step. Therefore, the value of its energy barrier is very important to affect the SCR activity.

In contrast to path1, the leaving H is connected with O2c-3 instead of active O2c-1. So NH3 adsorption and dissociation could also be possible, as shown in Fig. S4 in the Supplementary Material. The Ead of NH3 adsorption on Mn3+ is -0.59 eV, lower than that on Mn4+ (-1.38 eV), which is due to the higher conduction energy level of Mn3+ (Fig. S2 in the Supplementary Material). The energy barrier of NH3 dissociation is 0.96 eV and the reaction

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energy is 0.76 eV, almost same as NH3 dissociation on Mn4+. The -NH2 then react with NO molecule spontaneously to form NH2NO species which decomposes to N2 and H2O (PES in Fig. 13) via same path as discussed above. Finally, one more H atom left on the catalyst surface to occupy the active O2c-1 site inhibits the following NH3 adsorption and dissociation. The two H atoms on the catalyst surface transfer two electrons to change valence state of Mn into +2. The valence electron configuration of Mn2+ is 3d5, namely the 3d orbital of Mn2+ is half-filled, which is a stable electron configuration. DOS in Fig. S2 in the Supplementary Material also shows no LUMO band near Fermi level. These both suggest that it will be difficult for Mn2+ to accept additional electron. So a reoxidation process is necessary to recover the catalyst.

3.5. Reoxidation of the reduced catalyst

The essence to recover the catalyst is the removal of H atoms via O2 molecule of reaction mixture. The path is via the water formation from two H atoms and desorption. The generated oxygen vacancy then will be healed by O2. The detailed path will be presented in the following.

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To recombine with each other, the generated two H atoms first need a diffusion step to be neighboring with each other. The H atom connected with O2c-1 moves to O3c-1 with a high energy barrier of 1.34 eV (state i to iii, Fig. 14). This can be understood by a coordination situation of two different O atoms. O3c is less reactive than O2c. In contrast, the migration to O2c-2 (state iii, Fig. 14) takes a small energy barrier of 0.34 eV (state iv, Fig. 14). The two adjacent OH species on the catalyst surface (state v, Fig. 14) recombine to form a H2O molecule and an oxygen vacancy (state vi, Fig. 14) with an endothermic energy of 0.89 eV. Due to the strong interaction between adsorbed H2O and oxygen vacancy, the desorption of the H2O molecule shows a high endothermic energy of 1.18 eV. Then the oxygen molecule in the gas phase should be dissociated to oxygen atom on the catalyst surface, then the oxygen atom goes to heal the oxygen vacancy. And the catalyst is reoxidized to recover its initial configuration. The valence state of Mn recovers to +4.

3.6. Vibrational frequencies and in situ DRFIT analyses

IR spectroscopy is an important tool to probe surface intermediates in heterogeneous catalysis. Theoretical vibrational frequencies for surface intermediates were calculated to compare with experimental data to validate the proposed reaction mechanism. Table 1 lists

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the vibrational frequencies of stretching modes (ν) and deformation modes (Σ) of ammonia species adsorbed on Mn-TiO2(101). As for NH3 adsorbed on Lewis acid sites, three N-H stretching modes from 3300 cm-1 to 3500 cm-1 and three deformation modes at 1602, 1595 and 1198 cm-1 are shown. The NH2 adsorbate from the decomposition of NH3 results in two N-H stretching modes at 3299 and 3388 cm-1 and one deformation mode at 1518 cm-1. For NH3 species adsorbed on Brønsted acid sites, the calculated vibrational frequencies show three N-H stretching modes in the range of 3100-3500 cm-1 and one deformation mode at 1475 cm-1. These data are consistent with the in situ DRIFT spectra in Fig. 6. What’s different, the calculated bands at 1602 and 1595 cm-1 might overlap to one band at 1603 cm-1 shown in DRIFT spectra.

The Mn-TiO2 with 35wt%Mn loading was first purged with NH3 for 30 min followed by N2 purging for 30 min at 150 °C. Then NO + O2 were introduced, and the IR spectra were recorded as a function of time. As shown in Fig. 15, before NO + O2 introduced, the bands attributed to ammonia species are same as Fig. 6. One more band at 1236 cm-1 might be assigned to weakly adsorbed NH3 or gas-phase NH3.41 When NO + O2 were introduced, the bands attributed to coordinated NH3 on Lewis acid sites, -NH2 and weakly adsorbed

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NH3 diminished rapidly, indicating quick consumption of coordinated NH3 by NO. As time went by, the bands attributed to surface NOx species including bidentate nitrates (1573 cm-1),41 monodentate nitrites (1367 cm-1)42 and monodentate nitrates (1280 cm-1)41 appear. The thermally stable NH4NO3 can be reduced by NO at low temperature because of the catalyst’s acidity.51 And the bands assigned to NH4+ on Brønsted acid sites (1436 cm-1)44 remain even after 30 min of introducing NO + O2, suggesting NH3 adsorption on Brønsted acid sites isn’t active sites for SCR reaction.

The two in situ DRIFT spectra results and the calculated vibrational frequencies give a firm evidence to our theoretically proposed mechanism.

4. Discussion

The Mn-based catalyst shows a very good low temperature SCR performance. A molecular level understanding to the role of Mn doping will promote further strategies to improve the activity. Figure 16. shows a reaction scheme of the overall SCR reaction cycle. The red arrows indicate the “inner-hydrogen transfer” path with a high barrier of 1.44 eV, which is regarded as an unfavorable path. The black arrows from NH2NO species indicate

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the two different paths of NH2NO decomposition. The blue arrows indicate the reoxidation path, among which the blue dotted arrow means undergoing the second NH2NO decomposition cycle. Based on the proposed mechanism, we will try to discuss from the following four points: NH3 adsorption, NH3 dissociation, NH2NO formation and decomposition.

The absolute value of NH3 adsorption energy on Mn-TiO2 is 0.22 eV higher than on TiO2, indicating greater ability to adsorb NH3. The difference of adsorption energy can be interpreted by DOS in Fig. 9a that the LUMO of Mn 3d lies much lower than Ti 3d. So the first rule of Mn doping is due to the lower conduction band of Mn 3d as compared with Ti 3d.

NH3 dissociation is a key step in SCR reaction. The value of its energy barrier is very important to affect the SCR activity. According to DFT calculation, the barrier of NH3 dissociation on Mn-TiO2 is 0.34 eV lower than on TiO2, suggesting easier dissociation of NH3 on Mn-TiO2 than on TiO2 due to lower LUMO of O 2p with Mn doping (Fig. 9c). The same conclusion can be made from in situ DRIFT spectra (Fig. 6). The bands attributed to -NH2 species start to emerge at 30 °C and increase obviously as temperature rising,

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indicating a facile decomposition of NH3 on Mn-TiO2. In contrast, the bands attributed -NH2 species increase slightly on pure TiO2, even at high temperature shown in Fig. 7. The second rule of Mn doping is activation of neighboring O to Mn, which promotes the activation of NH3 decomposition.

The formation of NH2NO species and its key rule in SCR reaction have been well established. Our calculations show that the Mn doping induces a spontaneous reaction between -NH2 and NO in gas phase. In contrast, on pure TiO2, the optimization of NO molecule initially close to -NH2 species leads to the desorption of NO into gas phase. The dramatic different performance is due to the lower LUMO of -NH2 species on Mn-TiO2. As for path2 of NH2NO decomposition, the readsorption of the NH2NO species on Mn help hydrogen transfer with two hydrogen bonds existing. Then the HNNOH species decomposes to N2 and OH species easily. The OH adsorbed on Mn is very active to abstract one H atom on surface to form water barrierlessly. As compared, H2O formation in path1 shows an energy barrier of 0.53 eV without Mn assistance.

The N2 selectivity is a very important factor for the industrialization of a SCR catalyst. Mn-based catalyst shows a good low temperature catalytic performance. However, the

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selectivity dramatically decreases with the increasing temperature. Regarding the reason for such phenomenon, some studies suggested that increasing temperature leading to high N2O selectivity,37,52 while others attributed it to highly active oxygen.19,53 As shown in Fig. 6, -NH2 species and its deep oxidation products can form with temperature rising. The NH3 might be abstracted one hydrogen atom by surface oxygen to form -NH2 first, then the -NH2 can be abstracted one hydrogen atom by surface oxygen atom to form -NH species. The reaction path from -NH shown in Fig. 17 (also green arrows in Fig. 16), -NH2 species overcome an energy barrier of 1.33 eV to form -NH species, which is 0.36 eV higher than NH3 dissociation. Then NO in the gas phase can react with the -NH species spontaneously to form NHNO species with an exothermal energy of 3.04 eV. The NHNO species dissociates the H atom to form N2O molecule with a small energy barrier of 0.19 eV. To summarize, the oxidation of NH3 is the preliminary condition for the SCR reaction. The formed -NH2 species will lead to the formation of N2 and H2O. However, a deep oxidation to -NH species will lead to the side production of N2O. On the one hand, Mn doping activates surface oxygen, which will oxidize NH3 more efficiently and increase SCR activity. At the same time, it will also lead to the deep oxidation of -NH2 species,

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decreasing the N2 selectivity. On the other hand, N2O is formed by deep oxidation of NH3 to NH. It would overcome more energy barrier to form NH than to NH2. As temperature rising, the higher energy barrier could be overcome. Meanwhile more NH can be formed to obtain N2O. Therefore, more N2O will be formed with the increasing of temperature. This conclusion is consistent with the experimental result that N2 selectivity would decrease with the increasing of temperature, thus N2O is mainly formed at high temperature.

Based on the present investigation, the SCR reaction of NO by NH3 on the Mn-TiO2 catalyst most probably takes place according to the following steps (as well as Fig. 16). (1)~(3) show NH3 activation, (4)~(14) show three different paths of NH2NO decomposition ((4)~(8) show path1, (9)~(12) show path2 and (13)~(14) show high-barrier path)), (15)~(19) show reoxidation path and (20)~(22) show N2O formation path (g denotes gas phase, a denotes adsorbed species, s denotes surface species of catalyst).

NH3(g) → NH3(a)

(1)

NH3(a) + O(s) → NH2(a) + OH(s)

(2)

NH2(a) + NO(g) → NH2NO(a)

(3)

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NH2NO(a) + O(s) → NHNO(a) + OH(s)

(4)

NHNO(a) + O(s) → N2O(g) + OH(s)

(5)

OH(s) + OH(s) → H2O(a) + O(s) +VO

(6)

H2O(a) → H2O(g)

(7)

N2O(g) + VO → N2(g) + O(s)

(8)

NH2-NO(a) +OH(s) → NH-NOH(a) + OH(s)

(9)

NH-NOH(a) + O(s) + Mn(s)→ N2(g) +Mn-OH(s) + OH(s)

(10)

Mn-OH(s) + OH(s) → Mn(s) + H2O(a) +O(s)

(11)

H2O(a) →H2O(g)

(12)

NH2-NO(a) → NH-NOH(g)

(13)

NH-NOH(g) → N2(g) + H2O(g)

(14)

OH(s) + OH(s) →H2O(a) + O(s) +VO

(15)

H2O(a) → H2O(g)

(16)

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O2(g) → O2(a)

(17)

O2(a) → O(a) + O(a)

(18)

O(a) + VO → O(s)

(19)

NH2(a) + O(s) → NH(a) + OH (s)

(20)

NH(a) + NO(g) →NHNO(a)

(21)

NHNO(a) + O(s) → N2O(g) + OH(s)

(22)

5. Conclusions

The mechanism of selective catalytic reduction of NO by NH3 catalyzed by Mn-TiO2 has been studied via a combination of experimental and theoretical modeling approach. The reaction starts with the adsorption of NH3 on the Mn Lewis acid site, followed by dissociation to form -NH2. NO from gas phase can spontaneously reacts with -NH2 to form NH2NO species. The decomposition of NH2NO follows a concerted H exchange step with support to form HNNOH with a low energy barrier, which can spontaneously dissociated to N2 and H2O. Then the reduced catalyst was recovered via an oxidation step by O2. Mn

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doping enhances NH3 adsorption due to the lower conduction band energy of the Mn 3d orbital states. Mn reduces the NH3 activation barrier for activating neighboring O2c atom to abstract the H from adsorbed NH3 more easily. The N2 selectivity reduction with increasing temperature can be attributed to the deep oxidation of NH3 to form N2O on the Mn-TiO2. All above are in good agreement with the in situ DRIFT results.

Acknowledgments This work was supported by National Natural Science Foundation of China (21503273, 21477164, 21673290, U1662103), Scientific Research Foundation of China University of Petroleum Beijing (2462015YJRC005).

Supporting Information The pathway of NH2NO dissociation by inner-hydrogen transfer path; DOS for Mn of different oxidation state in Mn-TiO2(101); the optimized structure of the NH3 adsorption and dissociation on Mn-TiO2(101) with a extra H; potential energy surface for the dissociation of NH3 on the surface of Mn-TiO2 with a extra H; DRIFT spectra of NO and O2 co-adsorption on Mn-TiO2.

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(36) Wang, L.; Maxisch, T.; Ceder, G. Oxidation energies of transition metal oxides within the GGA + U framework. Phys. Rev. B. 2006, 73, 195107. (37) Putluru, S. S. R.; Schill, L.; Jensen, A. D.; Siret, B.; Tabaries, F.; Fehrmann, R. Mn/TiO2 and Mn-Fe/TiO2 catalysts synthesized by deposition precipitation—promising for selective catalytic reduction of NO with NH3 at low temperatures. Appl. Catal. B Environ. 2015, 165, 628-635. (38) Qi, G.; Yang, R. T. Low-temperature selective catalytic reduction of NO with NH3 over iron and manganese oxides supported on titania. Appl. Catal. B Environ. 2003, 44, 217-225. (39) Ramis, G.; Busca, G.; Lorenzelli, V.; Forzatti, P. Fourier transform infrared study of the adsorption and coadsorption of nitric oxide, nitrogen dioxide and ammonia on TiO2 anatase. Appl. Catal. 1990, 64, 243-257. (40) Busch, O. M.; Brijoux, W.; Thomson, S.; Schüth, F. Spatially resolving infrared spectroscopy for parallelized characterization of acid sites of catalysts via pyridine sorption: Possibilities and limitations. J. Catal. 2004, 222, 174-179. (41) Wu, Z.; Jiang, B.; Liu, Y.; Wang, H.; Jin, R. DRIFT study of manganese/titania-based

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catalysts for low-temperature selective catalytic reduction of NO with NH3. Environ. Sci. Technol. 2007, 41, 5812-5817. (42) Hu, H.; Cai, S.; Li, H.; Huang, L.; Shi, L.; Zhang, D. In situ DRIFTs investigation of the low-temperature reaction mechanism over Mn-doped Co3O4 for the selective catalytic reduction of NOx with NH3. J. Phys. Chem. C. 2015, 119, 22924-22933. (43) Li, Y.; Wan, Y.; Li, Y.; Zhan, S.; Guan, Q.; Tian, Y. Low-temperature selective catalytic reduction of NO with NH3 over Mn2O3-doped Fe2O3 hexagonal microsheets. ACS Appl. Mater. Interfaces. 2016, 8, 5224-5233. (44) Qiu, L.; Pang, D.; Zhang, C.; Meng, J.; Zhu, R.; Ouyang, F. In situ IR studies of Co and Ce doped Mn/TiO2 catalyst for low-temperature selective catalytic reduction of NO with NH3. Appl. Surf. Sci. 2015, 357, 189-196. (45) Liu, C.; Chen, L.; Li, J.; Ma, L.; Arandiyan, H.; Du, Y.; Xu, J.; Hao, J. Enhancement of activity and sulfur resistance of CeO2 supported on TiO2-SiO2 for the selective catalytic reduction of NO by NH3. Environ. Sci. Technol. 2012, 46, 6182-6189. (46) Chen, L.; Li, J.; Ge, M. DRIFT study on cerium-tungsten/titiania catalyst for selective catalytic reduction of NOx with NH3. Environ. Sci. Technol. 2010, 44, 9590-9596.

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(53) Liu, C.; Shi, J. W.; Gao, C.; Niu, C. Manganese oxide-based catalysts for low-temperature selective catalytic reduction of NOx with NH3: A review. Appl. Catal. A Gen. 2016, 522, 54-69.

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Table/Figure captions Table 1. Vibrational Frequencies of Ammonia Species Adsorbed on Mn-TiO2(101). Fig. 1. The optimized structure and two types of metal sites of anatase TiO2(101) (color scheme: red - O; grey - Ti). Fig. 2. XRD patterns of Mn-TiO2 with different Mn loadings. Fig. 3. NO conversion as a function of reaction temperature over the catalysts with different Mn loadings. Fig. 4. N2 selectivity as a function of reaction temperature over 35wt%Mn-TiO2. Fig. 5. Pyridine-IR spectra on the surface of TiO2 (black) and 5wt%Mn-TiO2 (red) at 200 °C. Fig. 6. DRIFT spectra of NH3 adsorption on Mn-TiO2 with 35wt%Mn loading at 30-250 °C. Fig. 7. DRIFT spectra of NH3 adsorption on pure TiO2 at 30-500 °C. Fig. 8. (a) The adsorption geometries of NH3 on TiO2(101) surface and the possible dissociation sites of NH3. Potential energy surface for the dissociation of NH3, which H is dissociated to O2c-1 on the surface of (b) TiO2(101) and (d) Mn-TiO2(101) (the energy of the

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initial state is taken as zero). (c) The other optimized structure and local charge density figure of the H is dissociated to O2c-1 on TiO2(101) surface (color scheme: purple - Mn; blue - N; white - H; yellow - local charge density). Fig. 9. DOS for Ti 3d on TiO2(101) surface (black) and Mn 3d on Mn-TiO2(101) surface (red). (b) DOS for O 2p of three O2c atoms on Mn-TiO2(101) surface. (c) DOS for O 2p of O2c-1 on TiO2(101) surface (black) and Mn-TiO2(101) surface (red). (d) DOS for NH2 on TiO2(101) surface (black) and Mn-TiO2(101) surface (red). The dashed lines represent the Fermi level. Fig. 10. (a) Initial states with a NO molecule right above the NH2 group with a N-N distance of 3 Å. (b) Spontaneous formation of NH2NO complex. Fig. 11. Potential energy surface for the decomposition of the NH2NO species of path1, where the energy of the reactant is taken as zero. Fig. 12. Potential energy surface for the decomposition of the NH2NO species of path2, where the energy of the reactant is taken as zero. Fig. 13. Potential energy surface for the second cycle of decomposition of the NH2NO species (path2) on Mn-TiO2(101), where the energy of the reactant is taken as zero.

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Fig. 14. Potential energy surface for reoxidation of the reduced catalyst Mn2+-TiO2(101) by H-diffusion path, where the energy of the reactant is taken as zero. Fig. 15. Dynamic changes of the in situ DRIFT spectra over Mn-TiO2 with 35wt%Mn loading as a function of time in a flow of NO + O2 after the catalysts was pre-exposed to a flow of NH3 for the 30 min followed by N2 purging for 30 min at 150 °C. Fig. 16. The scheme of the overall SCR reaction cycle (the red arrows indicate the high-barrier path, the blue arrows indicate the reoxidation path, the green arrows indicate the N2O formation via deep oxidation of NH3). Fig. 17. Potential energy surface for the further oxidation of NH2 to N2O on Mn-TiO2(101), where the energy of the reactant is taken as zero.

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Table 1. Vibrational Frequencies of Ammonia Species Adsorbed on Mn-TiO2(101). species

mode

NH3(L acid site)

NH2(L acid site) Mn-TiO2

υ(N-H)

3499，3443，3314

Σ

1602，1595，1198

υ(N-H) Σ

NH3(B acid site)

wavenumbers (cm-1)

3388，3299 1518

υ(N-H)

3516，3461，3144

υ(O-H)

1475

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Fig. 1. The optimized structure and two types of metal sites of anatase TiO2(101) (color scheme: red - O; grey - Ti).

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Fig. 2. XRD patterns of Mn-TiO2 with different Mn loadings.

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Fig. 3. NO conversion as a function of reaction temperature over the catalysts with different Mn loadings.

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Fig. 4. N2 selectivity as a function of reaction temperature over 35wt%Mn-TiO2.

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Fig. 5. Pyridine-IR spectra on the surface of TiO2 (black) and 5wt%Mn-TiO2 (red) at 200 °C.

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Fig. 6. DRIFT spectra of NH3 adsorption on Mn-TiO2 with 35wt%Mn loading at 30-250 °C.

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Fig. 7. DRIFT spectra of NH3 adsorption on pure TiO2 at 30-500 °C.

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Fig. 8. (a) The adsorption geometries of NH3 on TiO2(101) surface and the possible dissociation sites of NH3. Potential energy surface for the dissociation of NH3, which H is dissociated to O2c-1 on the surface of (b) TiO2(101) and (d) Mn-TiO2(101) (the energy of the initial state is taken as zero). (c) The other optimized structure and local charge density figure of the H is dissociated to O2c-1 on TiO2(101) surface (color scheme: purple - Mn; blue - N; white - H; yellow - local charge density).

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Fig. 9. DOS for Ti 3d on TiO2(101) surface (black) and Mn 3d on Mn-TiO2(101) surface (red). (b) DOS for O 2p of three O2c atoms on Mn-TiO2(101) surface. (c) DOS for O 2p of O2c-1 on TiO2(101) surface (black) and Mn-TiO2(101) surface (red). (d) DOS for NH2 on TiO2(101) surface (black) and Mn-TiO2(101) surface (red). The dashed lines represent the Fermi level.

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Fig. 10. (a) Initial states with a NO molecule right above the NH2 group with a N-N distance of 3 Å. (b) Spontaneous formation of NH2NO complex.

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Fig. 11. Potential energy surface for the decomposition of the NH2NO species of path1, where the energy of the reactant is taken as zero.

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Fig. 12. Potential energy surface for the decomposition of the NH2NO species of path2, where the energy of the reactant is taken as zero.

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Fig.13. Potential energy surface for the second cycle of decomposition of the NH2NO species (path2) on Mn-TiO2(101), where the energy of the reactant is taken as zero.

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Fig. 14. Potential energy surface for reoxidation of the reduced catalyst Mn2+-TiO2(101) by H-diffusion path, where the energy of the reactant is taken as zero.

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Fig. 15. Dynamic changes of the in situ DRIFT spectra over Mn-TiO2 with 35wt%Mn loading as a function of time in a flow of NO + O2 after the catalysts was pre-exposed to a flow of NH3 for the 30 min followed by N2 purging for 30 min at 150 °C.

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Fig. 16. The scheme of the overall SCR reaction cycle (the red arrows indicate the high-barrier path, the blue arrows indicate the reoxidation path, the green arrows indicate the N2O formation via deep oxidation of NH3).

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Fig. 17. Potential energy surface for the further oxidation of NH2 to N2O on Mn-TiO2(101), where the energy of the reactant is taken as zero.

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TOC graphic

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Mechanistic Study of Selective Catalytic Reduction of NOx with NH3

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