Insight into the NH3-assisted Selective Catalytic Reduction of NO on Î²

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Insight into the NH3-assisted Selective Catalytic Reduction of NO on #-MnO2(110): Reaction Mechanism, Activity Descriptor and Evolution from the Pristine State to Steady State Haiyang Yuan, Ningning Sun, Jian-Fu Chen, Jiamin Jin, Haifeng Wang, and Peijun Hu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02114 • Publication Date (Web): 30 Jul 2018 Downloaded from http://pubs.acs.org on July 30, 2018

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Insight into the NH3-assisted Selective Catalytic Reduction of NO on β-MnO2(110): Reaction Mechanism, Activity Descriptor and Evolution from the Pristine State to Steady State Haiyang Yuan,1 Ningning Sun,1 Jianfu Chen,1 Jiamin Jin,1 Haifeng Wang,1* Peijun Hu1,2 1

Key Laboratory for Advanced Materials, Centre for Computational Chemistry and Research Institute of Industrial Catalysis, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai, 200237, P. R. China. Email: [email protected] 2 School of Chemistry and Chemical Engineering, The Queen’s University of Belfast, Belfast BT9 5AG, U.K.

Abstract: To understand the molecular-level reaction mechanism and crucial activitylimiting factors of NH3-SCR process catalyzed by MnO2-based oxide to eliminate NO (4NH3+4NO+O2 →4N2+6H2O) at middle-low temperature, a systematic computational investigation is performed on β-MnO2(110) by first-principles calculations together with microkinetic analysis. Herein, the favored reaction pathways are unveiled: (i) NH3 tends to adsorb at the unsaturated Lewis-acid Mn5c site on MnO2(110), and then partially dissociates into NH2* (assisted by the surface lattice Obri) at the steady state, triggering the subsequent reactions; (ii) Interestingly, NO, either in the gas phase or at the adsorbed state, can readily react with NH2* into a key intermediate NH2NO, with the former one (i.e. the Eley-Rideal pathway) being slightly more kinetically preferred; (iii) NH2NO conversion is identified to proceed easily into N2 through the dehydrogenation/hydrogenation processes of NH2NO→NHNO→NHNOH →N2 +H2O; (iv) The removal of the accumulated surface H into H2O, assisted by O2, is relatively difficult, which preferentially occurs via the Mars-van Krevelen mechanism. Quantitative, a kinetic analysis is conducted to deal with such a complex reaction network, revealing that the rate-limiting steps are NH2* +NO(g)→NH2NO* and ObriH +O2# →OOH# +Obri. Moreover, the sensitivity analysis shows that the adsorption strengths of H on Obri and O2 in the Obri vacancy (Ovac) are two main activitydetermining factors for the overall NH3-SCR on MnO2(110); notably, it is further found that the Ovac formation energy correlates well with both factors and can thus serve as a unified activity descriptor. Also, the effects of catalyst surface environment under the reaction condition on the NH3-SCR activity and selectivity are discussed. Compared with the pristine state of MnO2(110), both the overall activity and N2 selectivity (versus N2O) would be interestingly enhanced when it arrives at the kinetically steady state that the surface Obri are largely covered by H. These results could provide a consolidated theoretical basis on understanding and optimizing MnO2 catalyst for NH3-SCR process. 1

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KEYWORDS: density functional theory, selective catalytic reduction, nitrogen oxides, MnO2, catalytic mechanism, activity descriptor

1.Introduction Nitrogen oxides, usually containing about 95% NO and 5% NO2, contribute to the formation of urban photochemical smog, acid rain, haze and ozone depletion,1-3 and their efficient removal has long drawn extensive attention. The selective catalytic reduction (SCR) of nitrogen oxides with NH3 in the presence of excess O2 over catalysts, e.g. the standard SCR reaction (4NO + 4NH3 + O2 → 4N2 + 6H2O), is one of the most effective methods in the actual industry.4 Currently, the widely used catalysts in industry is V2O5-WO3(MoO3)/TiO2 which operates with an optimum temperature range of 300400℃.5,6 However, because of the high operation temperature, V-based catalysts must be located upstream of the desulfurizer and would get deactivated by sulfur dioxide and particulate matter,7 and V element is less glamorous due to its toxicity. Therefore, developing the low temperature ( Mn5O8 > Mn2O3 > Mn3O4 > MnO.23 In addition to MnO2, Xie et al. proposed that the active ingredients also contain Mn3O4 and Mn2O3,45 and Kang et al. suggested that the amorphous Mn3O4 and Mn2O3 were mainly present in the active MnOx catalyst for NH3-SCR.46 Recently, we computationally examined the catalytic activity and basic structure-activity relations of Mn3O4(110) for NH3-SCR.47 Therefore, an unambiguous insight into the catalytic ability of MnO2 for NH3-SCR has been in pursuit as a comparison. In this work, we focus on β-MnO2, which is a common crystal phase (rutile type) of MnO2 showing high catalytic performance in some important systems (e.g. ORR, Li-air battery and supercapacitor electrode material),48, 49 as the object to explore the complicated NH3-SCR process by virtue of density functional theory (DFT) calculations, aiming to shed light on what pivotal factors that influence the NH3-SCR activity and how to further improve the activity of MnO2 catalyst. Specifically, here we have carried out the detailed mechanistic exploration on NH3-SCR on β-MnO2(110) surface, and conducted a kinetic analysis to estimate the rate of each elementary step 3


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and locate the rate-determining step in this system; meanwhile, a crucial factor has been proposed as a measure to assess the catalytic activity of MnO2 for NH3-SCR. Furthermore, as we know, for any catalytic system, the catalyst’s surface environment under the reaction condition, especially at the steady state, could usually change and accordingly tune the catalyst’s activity because of the coverage effect (at least). Here we also demonstrated the possible effect of β-MnO2(110) surface environment, evolved as the reaction proceeds, on the catalytic activity and selectivity of NH3-SCR.

2. Methods All the spin-polarized density functional theory (DFT) calculations were carried out in the Vienna ab initio simulation package (VASP),50 using the Perdew-BurkeErnzerhof (PBE) functional within the generalized gradient approximation (GGA).51 The project-augmented wave (PAW) method was used to represent the core-valence electron interaction.52 The valence electronic states were expanded in plane wave basis sets with an energy cutoff of 450 eV. The Broyden method was employed for geometry relaxation until the maximal forces on each relaxed atom was less than 0.05 eV/Å. As the most stable surface of rutile-typed β-MnO2 oxide, the (110) surface was selected to simulate the NH3-SCR process on it.53 The MnO2(110) surface is modeled with a p(3×1) periodic slab with four layers, and the vacuum between slabs is 12 Å; correspondingly, a 2×3×1 k-point mesh was used. During structural optimization, the bottom two layers of the slab were fixed at the bulk truncated position, and the top two layers and the adsorbates were fully relaxed. The transition states (TS) were searched using the constrained optimization scheme.54 To well describe the electronic and magnetic behavior of Mn oxides,55 here the DFT+U approach with the on-site Coulomb correction included is used to treat the electronic correlation in the localized d-orbital of Mn element. A Hubbard-like term Ueff (U=2.8 eV and J=1.2 eV) for Mn (3d) was adopted, as it has been reported that the electronic properties of MnO256, 57 and the unit cell volume58 can be reproduced in good agreement with experiment simultaneously. Also, it has been experimentally shown that β-MnO2 preferentially take the antiferromagnetic (AFM) state,59 and in our calculation an antiferromagnetic helical spin arrangement for β-MnO2 was thus imposed.60, 61 The adsorption energies of species X were calculated with Eads(X) = Ex/surf Esurf - Ex, where Ex, Esurf and Ex/surf are the total energies of adsorbates X in the gas phase, the clean surface, and the optimized adsorbate/surface system, respectively. The more negative Eads is, the more strongly the species X binds on the surface. Noteworthily, in the microkinetic analysis, the Gibbs free energy change (∆G) and the 4


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barrier (Ea) of the elementary step were estimated (∆G =∆H + ∆EZPE −T∆S), including the thermodynamic zero point energy (ZPE) correction and the entropy contribution derived from the standard partition functions62 or the experimental values63.

3. Result and discussion 3.1 Adsorption and Dissociation of NH3 over β-MnO2(110) As illustrated in Figure 1a, the β-MnO2(110) surface is mainly terminated by the rows of two-coordinated bridge oxygen (Obri) and the five-coordinated unsaturated Mn cations (Mn5c, the Lewis acid site) linked by the three-coordinated in-plane O3c, in which the Mn5c and Obri rows are in an alternating arrangement and could constitute basic adsorption sites for various species involved in the NH3-SCR process.

Figure 1. (a) The structure of stoichiometric β-MnO2(110) surface; (b-d) show the optimized adsorption configration of O2, NO and NH3 at the Lewis-acid Mn5c site, respectively, while (e) is for NH3 adsorption on the Brønsted acid site (the ObriH group). The purple, red and blue balls represent Mn, O and N atoms, respectively, which are used hereinafter.

Firstly, we examined the adsorption of the reactants O2, NO and NH3 on MnO2(110). On the Lewis-acid Mn5c site, it is found that O2 can only weakly adsorb with an adsorption energy as low as -0.13 eV, while NO and NH3 can efficiently adsorb with Eads being -0.78 eV and -1.48 eV, respectively; the optimized adsorption configurations are shown in Figure 1(b-d), respectively. By comparison, NH3 thus has a stronger adsorption ability competing for the Mn5c sites. Alternatively, on the possible Brønsted-acid site (provided by the ObriH+ group) on MnO2(110), NH3 adsorption as NH4+ gives an evidently weaker adsorption strength (Figure 1e, Eads = -0.52 eV) in comparison with that at Mn5c (-1.48 eV), complying with the weak Brønsted acid of ObriH+ (corresponding to the strong Obri-H bond, see discussion later).64 Therefore, NH3 tends to adsorb at Mn5c preliminarily, which is favorable to the so-called Lewis-acid 5


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pathway rather than the Brønsted-acid one in NH3-SCR on MnO2(110) surface. Secondly, it is identified that NH3* (* represents Mn5c site) at Mn5c can not directly couple with the gaseous or adsorbed NO to form the N-N bond owing to its bond saturation, and NH3 dissociation is hence necessary to trigger the subsequent reactions. Here, the surface Obri, Mn5c site, and the adsorbed O2* and NO* were initially tested as the H-acceptor to assist NH3 dehydrogenation, respectively; the corresponding reaction barriers (Ea) and enthalpy changes (ΔH) were calculated as listed in Table S1. Among these possibilities, Obri is the most reactive species to facilitate the N-H bond cleavage of NH3, which corresponds to the lowest barrier of 0.41 eV. Therefore, one may expect that Obri could serve as the main species to help NH3* dehydrogenation. The energy profile of Obri-assisted NH3* dissociation on clean MnO2(110) is shown in Figure 2 (see black line). Specifically, the initial dehydrogenation of NH3* into NH2* has a low barrier of 0.41 eV; in the optimized transition state (TS1, Figure 2), the N-H bond is elongated to 1.488 Å, and the forming Obri-H bond is 1.109 Å long. The further dissociations of NH2* give a barrier of 0.21 eV and 0.39 eV for the process NH2* +Obri →NH* +ObriH and NH* +Obri →N* +ObriH, respectively, with the transition states (TS2 and TS3, respectively) shown in Figure 2, and both processes are exothermic by -0.05 eV and -0.72 eV, respectively. From the overall energy profile, one can see that all the barriers of NH3* dissociation are not too high, indicating that Obri has enough power to dissociate NH3 into NH2, NH and even a N atom over MnO2(110), which accords with the experimental result that the atomic N* could be produced on MnO2 catalyst.65 Once formed, the N* species can easily react with NO* adsorbed at Mn5c into the undesired N2O byproduct at a low barrier of 0.03 eV, hence constituting one of the main reasons that decrease the N2 selectivity in NH3-SCR. Therefore, one may speculate that reducing the reactivity of Obri properly could be an effective way to improve the N2 selectivity on MnO2 catalysts (through hindering the further dehydrogenation of NH2).

6


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Figure 2. The energy profiles of NH3 dissociation assisted by the lattice Obri on clean MnO2(110) surface (black line) and MnO2(110) with H covered (red line). Inserted are the corresponding transition state structures (TS1, TS2 and TS3) for the process of NH3* +Obri →NH2* +ObriH, NH2* +Obri →NH* +ObriH and NH* +Obri →N* +ObriH , respectively.

Fortunately, under the realistic reaction condition, there would be some H atoms accumulated at Obri accompanying NH3*dissociation (see kinetic analysis later), which could tune the surface properties of MnO2(110) and weaken the deep dehydrogenation of NH3*. Herein, the Obri-assisted NH3 dissociation on MnO2(110) in the presence of H atoms occupying the nearby Obri (taking θ(H@Obri)=3/4 ML as example) was examined, whose energy profile is shown in Figure 2 (red line). Relative to the clean MnO2(110), the adsorption of NH3* on H-covered MnO2(110) is weaker by 0.48 eV(Eads= -0.99 eV) , and the dissociation barrier of NH3* into NH2* becomes also higher by 0.15 eV, indicating that the self-generated surface H would inhibit the proceeding of NH3 dissociation itself to some extent. Moreover, the dissociation of NH2* into NH* has an evidently increased barrier, and becomes endothermic in contrast to that on the clean surface (see Figure 2). Comparing these two energy profiles with and without H existed at Obri, one can see that the effective barrier of the overall process (NH3* + 3Obri→N* +3ObriH) substantially increases (indicative of the higher position of TS3’ relative to TS3), and the corresponding enthalpy (ΔH) becomes endothermic (0.98 eV). Therefore, on the H-covered MnO2(110) in reality, the deep dehydrogenation of NH3 would be restrained, which is beneficial to the formation of NH2* species. 3.2 Coupling of NH2* with NO into NH2NO As the intermediate species NH2* forms, it could react with NO toward the direction of N2 formation. First, it is interestingly found that the NO in the gas phase can readily couple with NH2*into a NH2NO species (the Eley-Rideal (E-R) mechanism), following an almost barrierless process in terms of total energy; even putting a NO 7


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molecule above NH2* at a long distance of 3.200 Å (see Figure 3a), the NO gradually descends and bonds with NH2* spontaneously upon a typical structural optimization, forming a monodentate NH2NO configuration (dN-N =1.508 Å) with the N-end in –NH2 group anchored at Mn5c (see Figure 3b); such an adsorption configuration of NH2NO tends to further transform into a thermodynamically more stable configuration with the N-end from –NO bonded with Mn5c (denoted as NH2N*O, see Figure 3e). This whole process (NH2* + NO(g) →NH2N*O) is strongly exothermic (ΔH= -2.58 eV). To understand the origin of this barrierless process, the electronic state of NH2* at Mn5c was analyzed. Figure 3f shows the charge density difference (CDD) of NH2 adsorption, and one can see that the Mn5c-NH2 bond exhibits some ionic bond property owing to the charge transfer, but the amount of charge transfer from Mn5c to NH2 is limited with only 0.015 e (Bader charge), which implies that the adsorbed NH2* keeps a radical character of free NH2. Also, NO molecule, being an open shell radical, carries an unpaired electron, and thus the easy coupling between NH2* and NO can be expected in the light of radical chemistry.

Figure 3. (a) The initial structure of gas-phase NO attacking NH2* at Mn5c, in which NO was put above NH2* at a long distance, and (b) the sturcture of formed monodentate NH2NO through NO coupling with NH2* directly; (c) The initial structure of NO co-adsorption with NH2* along the Mn5c-row site, and (d) the spontaneously formed NH2NO structure (with ON-NH2 linked) upon typical optimiation; (e) The stable adsorption configuration of NH2NO with the N-end from –NO bonded with Mn5c. (f) and (g) The 3D contour plots of charge density difference of NH2 and NO adsorption at Mn5c, respectively, according to the equation Δρ = ρ(M/surf) – ρ(surf) – ρ(M) (M=NH2 or NO, isovalue = 0.003), in which yellow indicates the electron depletion and green for depletion accumulation.

Second, in addition to the E-R pathway, NH2* can alternatively react with the adsorbed NO* at Mn5c easily (the Langmuir-Hinshelwood (L-H) mechanism). When optimizing the initial coadsorption configuration of NO* and NH2* on two adjacent 8


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Mn5c sites (corresponding to a long dN-N = 2.909 Å, see Figure 3c), it was found that these two species would also bond with each other spontaneously to form NH2NO in a bidentate adsorption configuration with dN-N = 1.619 Å (see Figure 3d), which can then transform into the more stable monodentate NH2N*O configuration (see Figure 3e). Similarly, the charge analysis for the adsorbed NO* was also made (see CDD in Figure 3g) to rationalize this barrierless process, which shows that NO loses little electron (0.205 e) upon adsorption, according with the long Mn5c-NO bond (2.279 Å). Hence, the adsorbed NO* could keep a majority of radical property of free NO, and the reaction between NH2* and NO* may be also a radical-like reaction resulting in a barrerless process. As a result, NO, either in the gas phase or adsorbed at Mn5c site, can easily react with NH2* to form an intermediate NH2NO over MnO2(110) ascribed to the possible radical-like reaction. However, it is worth noting that it is difficult to distinguish which route is more favorable only relying on energetically comparing the E-R process (NH2* +NO →NH2N*O) and L-H process (NH2* +NO* →NH2N*O). It inherently requires also comparing the relative concentration of the surface adsorbed NO* and gaseous NO by taking into consideration the systematic kinetic balance at the reaction steady state, and the possible coverage effect on the adsorption of NO and NH3 and other related elementary steps, which will be discussed in section 3.5 later. 3.3 Conversion of the intermediate NH2NO The conversion of NH2NO into N2 in principle involves the cleavage of N-H and N-O eventually, which is a rather complex process containing a series of H migrations and isomerization steps, and the possible decomposition pathways of NH2NO on MnO2(110) are illustrated in Scheme 1. As shown in Scheme 1, three pathways were taken into account for the initial conversion of NH2NO, including (i) the intramolecular H migration to form NHNOH (NH2NO→NHNOH), (ii) the N-O bond cleavage into NNH2 on the Mn5c row, and (iii) dehydrogenation assisted by Obri to form NHNO, respectively. In pathway (i), the intramolecular H migration in NH2NO requires a barrier as high as 1.70 eV (see TS in Figure S1a), indicating the difficulty for the direct formation of NHNOH. Owing to the large reaction enthalpy change (ΔH= 2.96 eV), pathway (ii) is hard to occur either with an expected higher barrier (Ea >2.96 eV, see TS in Figure S1b). By comparison, the pathway (iii) leading to the formation of NHNO (see Figure S2a) is found to be much easier without an evident barrier, and also being exothermic by 0.24 eV; in the optimized TS structure (TS1 in Figure 4), the breaking N-H bond and forming Obri-H 9


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bond is elongated and shortened to 1.260 Å and 1.207 Å, respectively.

Scheme 1. The conversion diagram of the intermediate NH2NO.

Then, four possible routes exist for the NHNO conversion: (i) NHNO + Mn5c → NHN + Mn5cO, (ii) NHNO →NNOH, (iii) NHNO +Obri →N2O +ObriH, and (iv) NHNO +ObriH →NHNOH + Obri. The former two pathways (i, ii), corresponding to the N-O bond rupture and the intramolecular H migration in NHNO, are not feasible for the high barriers of 1.75 eV and 1.71 eV, respectively (see TS in Figure S1c and S1d). For pathway (iii), NHNO dehydrogenation assisted by Obri into N2O has a not high barrier of 0.55 eV (see TS in Figure S1e), while the pathway (iv), i.e. NHNO re-hydrogenation into NHNOH (see Figure S2b) with the H sitting on Obri, gives a lower barrier of 0.16 eV. The optimized transition state structure of pathway (iv) is shown by TS2 in Figure 4, characteristic of the NHNO-H and Obri-H bond lengths being 1.115 Å and 1.340 Å, respectively. In comparison, one can thus see that the formation of NHNOH is energetically preferred to occur among these possible pathways. In other word, such a conversion process of NH2NO interestingly follows the so-called push-pull mechanism (NH2NO→NHNO→NHNOH) on MnO2(110), also identified in the zeolite system.31 Further, NHNOH conversion could proceed via the following three routes: (i) NHNOH →N2 +H2O (intermolecular dehydration); (ii) NHNOH +Mn5c →NHN + Mn5cOH; and (iii) NHNOH +Obri →NNOH +ObriH. Owing to the high barriers (2.07 eV and 1.39 eV, respectively), the former two routes were identified to be less feasible (see TSs in Figure S1f and S1g). Fortunately, the route (iii), i.e. the Obri-assisted N-H bond cleavage of NHNOH into NNOH (see Figure S2c), is feasible with a barrier as 10


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low as 0.10 eV; the optimized transition state (TS3) is shown in Figure 4, in which the N-H bond is elongated to 1.246 Å from initial 1.064 Å, and the forming Obri-H bond length is 1.259 Å. Moreover, it is found that NNOH has a very weak N-O bond and can easily break on Mn5c row upon simple structural optimization, leading to N2 and a hydroxyl (OH*) at Mn5c; lots of tests have been done to locate the transition state but failed, indicating it being possibly a barrierless (low-barrier at least) step. Noteworthily, here we also examined two other possibilities for NNOH conversion, including the dehydrogenation into N2O or hydrogenation to form N2/H2O, respectively, which give a barrier of 0.18 eV and 0.42 eV (see TSs in Figure S1h and S1i), respectively, and are thus not preferential relative to direct N-O cleavage process. Now, an optimal conversion pathway for NH2NO can be figured out as shown in Figure 4, corresponding to NH2NO→NHNO→NHNOH→NNOH→N2+OH. As seen from the calculated energy profile in Figure 4, the whole process is strongly exothermic and has no evident barrier, unraveling the NH2NO conversion into N2 being very easy.

Figure 4. The energy profile and the transition states for the optimal conversion pathway of NH2NO. TS1, TS2 and TS3 show the optimized transition states of the breaking of N-H bond in NH2NO, NHNO re-hydrogenation into NHNOH and the breaking of N-H bond in NHNOH, respectively.

3.4 Removal of H Atoms on MnO2(110) Accompanying NH3 dissociation and NH2NO decomposition (i.e. NH3 +NO +Obri → N2 +H2O +ObriH), the released H atoms would continually occupy the surface active site Obri. Hence, to recover Obri and finish the whole cycle of NH3-SCR, it is necessary to explore how the H atoms are converted and removed. Initially, we conceived the removal of H atoms by adsorbed O2. However, as mentioned above, O2 has a low adsorption energy (only -0.13 eV) at Mn5c and can hardly dissociate with a high barrier 11


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of 1.97 eV, implying an extremely low coverage of O2* or O* at Mn5c, which can thus not efficiently remove H atom. Accordingly, we resorted to other possible pathways as shown in Scheme 2.

Scheme 2. The O2-assisted H removal pathways.

Firstly, two adjacent surface ObriH could react through the H-transfer and generate a H2O molecule (2OH# → H2O# + Obri, # represent the Obri vacancy site (Ovac)), which corresponds to a barrier of 1.48 eV and is endothermic by 1.02 eV; in the optimized transition state (TS1 in Figure 5), the breaking Obri-H bond is elongated to 1.183 Å and the forming HObri-H bond is 1.235 Å long. The desorption of H2O# with an Ovac left costs an energy of 0.97 eV, implying that this process is relatively not easy. Secondly, in the Ovac site, O2 can moderately adsorb with its O-O bond parallel to the surface (see Figure 5) and gives an adsorption energy (Eads(O2#)) of -0.50 eV. Then, regarding the conversion of O2# species, two possible channels were considered as shown in Scheme 2: (i) O2# dissociation with one O* atom adsorbing at Mn5c and the other healing Ovac at the same time (O2# + *(Mn5c) →O# +O*); (ii) O2# hydrogenation into OOH# assisted by ObriH group. For the channel (i) (see magenta line in Figure 5), the formed O* at Mn5c is reactive and can easily capture H from adjacent ObriH groups to form OH* with a low barrier of 0.19 eV and even H2O with a barrier of 0.33 eV (see related TS5 and TS7 in Figure S3b and S3c). However, the dissociation of O2# to generate O* is relatively difficult with a barrier of 1.30 eV (see Figure S3a) and limits the channel (i), as illustrated by the high position of TS4 in the energy profile in Figure 5. Alternatively, O2# could seize a H atom from its adjacent ObriH group directly, leading to the formation of OOH# at Ovac (see Figure 5). It corresponds to an energy barrier of 0.69 eV (see TS2 in Figure 5) which is much lower than that in the O2# dissociation (1.30 eV), despite being a little endothermic by 0.29 eV. Subsequently, OOH# could break its O-O bond (i.e. OOH# + *(Mn5c) →Obri +OH*) or alternatively capture another H from the adjacent ObriH to form H2O directly (OOH# + OH# → 2Obri +H2O); they give a barrier of 1.01 eV and 0.87 eV, respectively, indicating that the latter pathway being energetically more practical (see TS6 versus TS3 in Figure 5). 12


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Figure 5. The energy profile for the H removal process on MnO2(110) and some key adsorption and transition state configurations in this process.

The energy profiles of all the above H removal processes are compared in Figure 5, and an energetically optimal pathway can thus be obtained (see black line in Figure 5) to follow the Mars-van Krevelen mechanism: (i) H2O formation through two ObriH groups coupling and desorption with an Ovac left; (ii) O2 adsorbing at Ovac site and then continuously capturing two H atoms from the ObriH groups, which refills Ovac site and produces another H2O. Notably, from the obtained energy profile, the H removal process (4ObriH + O2 → 2H2O + 4Obri) can be anticipated to be relatively difficult, as implied by the large effective barrier and unfavorable thermodynamics. 3.5 Kinetic Analysis of NH3-SCR on clean MnO2 Based on the above discussions, the NH3-SCR on MnO2(110) presents a complicated reaction network, including four key reaction stages, i.e. (i) NH3 adsorption and dissociation, (ii-iii) NH2NO formation and decomposition into N2/H2O and (iv) O2-assisted H removal (see Figure 6). To quantitatively unveil the rate-limiting steps (or factors) and rationally improve the NH3-SCR activity on MnO2(110), a systematic microkinetic analysis is performed with the obtained energetics for each elementary step. The involved important reaction steps are summarized in Table 1, where the rate constants, k, of the surface reactions were calculated by Transition State Theory (TST), while the collision theory was used to deal with those of the adsorption/desorption process; particularly, to describe the rate constant of R5 reaction (NH2*+NO → NH2NO*) at the given temperature, i.e. the gas-phase NO coupling with the surface NH2* species which has no evident barrier by virtue of DFT, the collision theory was also utilized (see kinetic details in Supporting Information). Taking the 13


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typical experimental condition,23 T = 450 K, PNH3 = 5.5×10-4 atm, PNO = 5.5×10-4 atm, PO2 = 0.02 atm, PN2 = 0.8 atm and PH2O = 0.05 atm, and with the entropy effect of gasphase NH3, NO, O2, N2 and H2O included, the rate constant (k+), the rate (ri = ri+ - ri-; see Table S3/S4 for the forward (ri+) and reverse (ri-) rates), the reversibility (Zi), and the degree of rate control (XRC,i, see definition in SI or ref. 66) of each step i in NH3SCR over MnO2(110) were calculated at the steady state, which are shown in Table 1. The coverage (θ(j)) of each species j in the Ovac site (#) or at the Mn5c site (*) is listed in Table 2.

Figure 6. The energy profile for the complete NH3-SCR process on MnO2(110).

From Table 1, one can see the following features. Firstly, the NH3 adsorption and dehydrogenation into NH2* and ObriH (stage i) arrive at equilibrium (Zi = 1, i =1 to 3), revealing them being relatively fast processes. However, the coupling reactions of NH2* with NO into NH2NO (R4 and R5, stage ii) are extremely irreversible, indicating R4 and R5 being possibly the critically rate-limiting steps. Secondly, the rate of step R5 (NH2* + NO(g) →NH2NO*) is slower than that of R4 (NH2* + NO* →NH2NO* + *). These indicate that the formation of NH2NO is preferred via the L-H mechanism rather than the E-R mechanism on the clean MnO2(110) (without considering the realistic H coverage effect). Thirdly, the decomposition of NH2NO into N2 and H2O on MnO2(110) (stage iii, corresponding to an overall reaction: NH2NO →N2 + H2O) is straightforward and ended with the final irreversible step R9 (NNOH* →N2(g) + OH*), as a result of its strong exothermicity (see Figure 6). Fourthly, for the H removal process (stage iv), the R13 (O2# +OH# →OOH# +O#) and R14 (OOH# +OH# →H2O +2O#) are irreversible and constitute the apparent rate-limiting steps, rationalized from the increasingly raised energy profile along the whole stage iv. In order to further distinguish the relative importance of each elementary step in the proposed mechanism for controlling the net rate, the degree of rate control, XRC,i, was calculated in Table 1. 14


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ACS Catalysis

The bigger XRC,i the elementary step i has, the greater impact it usually has on the whole reaction. By comparing XRC,i in Table 1, the step R13 shows the largest XRC of 4.94×101

, suggesting that R13, i.e. the removal of H from the Obri sites by adsorbed O2 in Ovac,

can most sensitively increase (decrease) the overall NH3-SCR activity by decreasing (increasing) its barrier. Table 1. Kinetic data, ri, Zi and XRC,i, for each elementary reaction step in the NH3-SCR over clean MnO2(110) and H-Covered MnO2(110). * and # represent Mn5c and Ovac site, respectively. Clean MnO2(110) Step i

-1

ri /S

H-Covered MnO2(110)

Zi

XRC,i

ri /S-1

Zi

XRC,i

R1 NH3(g) + * ⇆ NH3*

1.12×10-5

1.00

-1.36×10-7

1.60×10-5

1.00

1.48×10-9

R2 NO(g) + * ⇆ ON*

1.06×10-5

1.00

1.35×10-7

7.23×10-6

1.00

1.42×10-10

R3 NH3* + O#⇆ NH2* +OH#

1.12×10-5

1.00

2.84×10-8

1.60×10-5

1.00

5.21×10-9

R4 NH2* + ON* ⇆ NH2NO* + *

1.06×10-5

9.38×10-17

4.78×10-1

7.23×10-6

1.32×10-17

3.33×10-1

R5 NH2*+ NO(g) ⇆ NH2NO*

5.21×10-7

2.82×10-16

2.34×10-2

8.76×10-6

5.58×10-18

4.03×10-1

R6 NH2NO* +O#⇆ NHNO*+OH#

1.12×10-5

2.87×10-1

2.22×10-12

1.60×10-5

2.86×10-1

9.54×10-18

R7 NHNO* + OH# ⇆ NHNOH* +O#

1.12×10-5

8.71×10-1

1.15×10-13

1.60×10-5

8.71×10-1

3.29×10-17

R8 NHNOH* + O# ⇆ NNOH* +OH#

1.12×10-5

1.26×10-2

3.43×10-11

1.60×10-5

6.42×10-3

2.92×10-16

R9 NNOH* ⇆ N2(g) +HO*

1.12×10-5

2.09×10-26

4.38×10-13

1.60×10-5

7.16×10-28 3.71×10-18

R10 OH# + OH# ⇆ H2O# +O#

2.79×10-6

1.00

1.67×10-4

4.00×10-6

1.00

4.77×10-8

R11 H2O# ⇆ H2O(g) + #

2.79×10-6

1.00

3.51×10-9

4.00×10-6

1.00

1.13×10-9

R12 O2(g) + # ⇆ O2#

2.79×10-6

1.00

1.33×10-8

4.00×10-6

1.00

4.08×10-9

R13 O2# +OH# ⇆ OOH# +O#

2.79×10-6

8.18×10-3

4.94×10-1

4.00×10-6

8.12×10-1

4.97×10-2

R14 OOH# +OH# ⇆ O# +O# +H2O(g)

2.79×10-6

4.33×10-10

4.07×10-3

4.00×10-6

2.49×10-6

2.14×10-1

R15 OH# + OH* ⇆ H2O* + #

1.12×10-5

1.00

5.22×10-15

1.60×10-5

1.00

1.98×10-16

R16 H2O* ⇆ H2O(g) + *

1.12×10-5

1.00

9.59×10-13

1.60×10-5

1.00

1.96×10-18

Further, on observing the rate-determining step R13 (r13 =k13+*θ(OH#)*θ(O2#) *(1-Z13), θ(OH#)=9.99×10-1, Z13 = 8.18×10-3), one can see that to increase k13+ and θ(O2#)(1.10×10-11) would apparently enhance the rate, which could be respectively achieved by (i) weakening the H adsorption strength on Obri (decrease Eads(H@Obri)), which would inherently facilitate the H transfer from ObriH to the O2 sitting in Ovac and thus decrease the corresponding barrier (i.e. increase k13+) and (ii) increasing O2 adsorption in Ovac (Eads(O2@Ovac)), which corresponds to a larger θ(O2#). Therefore, we speculated that Eads(H@Obri) and Eads(O2@Ovac) are two important factors on modulating the catalytic activity of β-MnO2 for NH3-SCR. Interestingly, we identified that both Eads(H@Obri) and Eads(O2@Ovac) are essentially correlated with the formation energy of Obri vacancy (Ef(Ovac)) on examining other rutile-type metal oxides (RuO2, 15


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MoO2 and NbO2), as shown in Figure 7; Eads(H@Obri) (Eads(O2@Ovac)) linearly decreases (increases) with the increase of Ef(Ovac). Therefore, Ef(Ovac) could serve as an activity descriptor to assess the activity of MnO2-based catalysts for NH3-SCR.

Figure 7. Blue line represents the scaling relation between the adsorption energy of O2 on Ovac (Eads(O2@Ovac)) and Ef(Ovac); Red line represents the scaling relation between the adsorption energy of H on the Obri site (Eads(H@Obri)) and Ef(Ovac).

3.6 Further Remarks on Reaction Kinetics and Material Optimization The above quantitative result confirms that the formation of surface H occupying the Obri sites from NH3 dissociation (NH3* +O# →NH2* +OH#) is relatively fast in energetics, while the removal of produced H on the Obri sites by O2 through the Marsvan Krevelen mechanism (4ObriH +O2 → 2H2O + 4Obri) can not efficiently proceed. As a result, the surface Obri site is rich of H atoms (θ(OH#) = 9.99×10-1, see Table 2), which accordingly leads to an extremely low coverage of Obri site (θ(O#) = 6.81×10-7); also, it is worth noting that the resulting coverage of Ovac is not high, which thus could not affect much the stability of MnO2(110) surface. Therefore, such excess occupation of H atoms on Obri could be inferred as one reason limiting the catalytic activity of MnO2(110), deviating from the usually expected one for the optimum catalyst.67, 68 For example, as a result of low θ(O#), the efficient dissociation of NH3 (NH3* + O# →NH2* +OH#) would be inhibited in turn and thus decrease the coverage of NH2* at the steady state (θ(NH2*) = 5.43×10-11, see Table 2), which yields that the NH2NO formation is also a kinetically critical step (R4: XRC = 4.78×10-1; R5: XRC = 2.34×10-2). Furthermore, we calculated the degree of thermodynamic rate control (XTRC,n , see definition in SI or ref. 66) of each intermediate n involved in the reaction, in order to quantitatively investigate the importance of the adsorption strength of intermediates in modulating the overall activity. From Table 2, we can find that the adsorbed H species on Obri site (OH#) has indeed the most negative XTRC of about -9.96×10-1, which means 16


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ACS Catalysis

that OH# has the most important effect and decreasing the adsorption ability of H atom on Obri (Eads(H@Obri)) is beneficial to decrease θ(OH#) and increase the overall reaction rate. Considering the proposed correlation between Eads(H@Obri) and Ef(Ovac), one can expect that increasing Ef(Ovac), i.e. deactivating the surface lattice Obri, would be beneficial for the whole NH3-SCR rate over β-MnO2(110). Additionally, it is worth mentioning that NH3* species has the most negative XTRC of about -9.75×10-1 among those adsorbed at Mn5c sites; hence, decreasing the adsorption energy of NH3 would improve the NH3-SCR activity on clean MnO2(110) through facilitating the L-H pathway of NH2* +NO*→NH2NO* +*, due to the less competitive adsorption for NO at Mn5c and higher surface coverage of NO*. Table 2. The coverage (θ(j)) of each intermediates on Mn5c site or Ovac and the degree of thermodynamic rate control (XTRC，j) of each intermediate j. Species

Clean MnO2(110) θ(j)

H-Covered MnO2(110)

XTRC,j

θ(j)

XTRC,j

NH3*

9.96×10

NH2*

5.43×10-11

-1.46×10-9

6.76×10-10

-2.89×10-9

NO*

2.09×10-8

-2.05×10-8

3.22×10-10

-1.57×10-10

NH2NO*

3.17×10-12

-3.11×10-12

6.28×10-18

-6.71×10-18

NHNO*

8.30×10-17

-8.13×10-17

2.34×10-16

-2.50×10-4

NHNOH*

3.46×10-11

-3.39×10-11

6.80×10-17

-7.26×10-17

NNOH*

1.19×10-18

-1.17×10-18

1.71×10-18

-1.82×10-18

HO*

2.03×10-8

-1.99×10-8

8.61×10-12

-9.21×10-12

H2O*

3.32×10-4

-3.25×10-4

1.39×10-10

-1.49×10-10

*

3.91×10-3

-3.83×10-3

9.99×10-1

-1.07×100

H2O#

3.79×10-5

-3.78×10-5

4.24×10-9

1.57×10-9

HO#

9.99×10-1

-9.96×10-1

5.05×10-1

-2.66×10-1

HOO#

1.13×10-10

-1.54×10-10

1.55×10-13

8.17×10-10

O2#

1.10×10-11

-3.16×10-9

2.81×10-11

8.24×10-10

O#

6.81×10-7

-6.75×10-7

4.93×10-1

-2.60×10-1

#

4.64×10-4

-4.62×10-4

2.34×10-3

-1.23×10-3

-1

-9.75×10

-1

1.33×10

-4

-1.42×10-4

3.6 H-induced activity variation of NH3-SCR on MnO2(110) in realistic condition As mentioned above (and also in SI, see Table S5/S6 and some additional discussion of the coverage effect of NH3 at Mn5c site on the overall kinetics), as the NH3-SCR arrives at the steady state, amounts of self-evolved H atoms accumulate along the Obri rows on MnO2(110) surface. On such a hydrogenated surface, a reduced surface 17


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Page 18 of 26

state ([H]Obri +Mn4+ →H+Obri +Mn3+) appears, and the elementary steps may be affected via the electronic effect (Mn4+ versus Mn3+) or the possible Coulomb repulsion (especially for that among the surface ObriH). For example, we have demonstrated that the NH3 adsorption and dissociation would be energetically hindered owing to the presence of surface H (see Figure 2). To evaluate the catalytic activity under realistic steady state condition and unravel the coverage effect of H, we also examined the kinetically crucial elementary steps on H covered MnO2(110) (in which a large H coverage of θ(H@Obri)=3/4 ML was used), and the calculated results are summarized in Table 3. One can see the following features in comparison to those on the pristine MnO2(110). (i) Energetically, the NH3 adsorption and dissociation assisted by Obri becomes relatively harder. (ii) However, the H detachment steps from ObriH, such as R10 (2OH# →H2O# +O#), R11 (H2O desorption), R14 (OOH# +OH# →2O# +H2O(g)) and R15 (OH# +OH* →O# +H2O*) become easier, with a decreased barrier and reaction enthalpy change, while the adsorption of O2 in Ovac and hydrogenation from Obri change little. It implies that the presence of surface H could overall enhance the H removal process energetically. Quantitatively, these differences induced activity change was estimated via the microkinetic calculation for the whole NH3-SCR process, and the results are summarized in Table 1. It is interesting that the overall activity can be improved by 1.45 fold. Essentially, such an activity enhancement could be rationalized from the weakened H adsorption energy at Obri owing to the repulsion effect in the high H coverage condition, which also accords with the trend estimation given by the kinetic analysis for the activity descriptor (decreasing the H adsorption strength at Obri is beneficial for the overall activity of initially clean MnO2(110)). Table 3. Some Thermodynamic data on the clean surface and surface with high H coverage. Clean MnO2(110)

Step NH3(g) + * ⇆ NH3* NO(g) + * ⇆ NO* NH3* + O# ⇆ NH2* + OH# NH2* + O# ⇆ NH* + HO# NH* + O# ⇆ N* + HO# OH# + OH# ⇆ H2O# + O# H2O# ⇆ H2O(g) + # O2(g) + # ⇆ O2# O2# + OH# ⇆ OOH# + O# OOH# + OH# ⇆ O# + O#+ H2O(g)

H-Covered MnO2(110)

Ea /eV

ΔH /eV

Ea /eV

ΔH /eV

\ \ 0.41 0.21 0.39 1.48 \ \ 0.69 0.87

-1.48 -0.78 0.38 -0.05 -0.72 1.02 0.97 -0.50 0.30 0.51

\ \ 0.56 0.65 0.74 1.17 \ \ 0.71 0.60

-0.99 -0.43 0.54 0.61 -0.19 0.86 0.59 -0.45 0.28 -0.44

18


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ACS Catalysis

OH# + OH* ⇆ H2O* + O#

0.33

0.13

0.06

-0.03

In addition, it is worth noting that, the rate of the elementary step R5 (NH2* +NO(g) →NH2NO*) would become more reactive compared with R4 (NH2* +NO* →NH2NO* +*) in the H-covered condition, as shown in Table 1. These indicate that the formation of NH2NO is more preferred via the E-R mechanism rather than the L-H mechanism on MnO2(110) if considering the realistic H coverage effect. Such a variation is mainly resulting from the weakened NO adsorption at Mn5c (i.e. the lower coverage of NO*). 4. Conclusion In summary, NH3-SCR over β-MnO2(110) has been systematically investigated by DFT calculations together with microkinetic analysis, aiming to understand the reaction mechanism and crucial activity-limiting factors. The main mechanismstic insights and their interplay with the catalyst surface environment are as follows. (i) NH3 prefers to adsorb at the Lewis acid site (Mn5c) rather than on the Brönsted acid site as NH4+, largely leading to a more preferred LA mechanism. The adsorbed NH3 can dissociate into NH2*, NH* and even a N* atom on initially clean MnO2(110), while it can only partially dehydrogenate into NH2* when MnO2(110) is largely covered by H, efficiently guaranteeing the N2 selectivity superior to N2O; (ii) NO, either being gaseous or adsorbed at Mn5c, can readily couple with NH2* into NH2NO with the N-N bond formed, following a (almost) barrierless process in term of total energy, with the latter being kinetically preferable on the pristine MnO2(110), while the former is for the H-covered MnO2(110); (iii) NH2NO conversion is found to proceed relatively easily and the most favorable pathway is identified: it tends to dehydrogenate into NHNO and then hydrogenate as NHNOH, while NHNOH can break N-H bond, yielding an unstable NNOH that would readily decompose into N2. (iv) An optimal Mars-van Krevelen pathway to remove H atoms assisted by O2 on MnO2(110) is given. To deal with such a complex four-state reaction network, a detailed kinetic estimation and sensitivity analysis is made and reveals the H removal process and NH2NO formation being the kinetically difficult steps, while Ef(Ovac) (i.e. the reactivity of the lattice Obri) could be a unified descriptor for evaluating the NH3-SCR activity on MnO2-based catalyst; making the lattice Obri inert to some extent is beneficial to increase the overall NH3-SCR activity on MnO2(110). Moreover, we demonstrate that the catalytic activity of NH3-SCR can be interestingly self-promoted at the realistic 19


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steady state when MnO2(110) is much covered by H at Obri sites, compared with the initial clean state of MnO2(110). Overall, the obtained mechanistic insights could help to understand more deeply the complex NH3-SCR on MnO2-based oxides and give some guidance for the further material optimization. ASSOCIATED CONTENT *Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. It contains the barriers of NH3 reacting with Mn5c and kinds of oxygen species, the supplementary TS structures involved in the NH2NO conversion and H2O formation, as well as the microkinetic details and some related data for NH3SCR on MnO2(110). AUTHOR INFORMATION Corresponding Authors E-mail for *H.W.: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENT This project was supported by the NSFC of China (21333003, 21622305), National Ten Thousand Talent Program for Young Top-notch Talents in China, The Shanghai Shuguang scholar program (17SG30), and the Fundamental Research Funds for the Central Universities (WJ1616007). REFERENCES (1) Schneider, H.; Tschudin, S.; Schneider, M.; Wokaun, A.; Baiker, A. In Situ Diffuse Reflectance FTIR Study of the Selective Catalytic Reduction of NO by NH3 over Vanadia-Titania Aerogels. J. Catal. 1994, 147, 5-14. (2) Tsekov, R.; Smirniotis, P. G. Theoretical Models for the Rate of NO Decomposition over Copper-Exchanged Zeolites. J. Phys. Chem. B. 1998, 102, 9525-9531. (3) Wang, H. F.; Guo, Y. L.; Lu, G.; Hu, P. NO Oxidation on Platinum Group Metals Oxides: First Principles Calculations Combined with Microkinetic Analysis. J. Phys. Chem. C. 2009, 113, 18746-18752. (4) Sanchez-Escribano, V.; Montanari, T.; Busca, G. Low Temperature Selective Catalytic Reduction of NOx by Ammonia over H-ZSM-5: An IR Study. Appl. Catal. BEnviron. 2005, 58, 19-23. 20


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(5) Dumesic, J. A.; Topsøe, N. Y.; Topsøe, H.; Chen, Y.; Slabiak, T. Kinetics of Selective Catalytic Reduction of Nitric Oxide by Ammonia over Vanadia/Titania. J. Catal. 1996, 163, 409-417. (6) Chen, L.; Li, J.; Ge, M. Promotional Effect of Ce-doped V2O5-WO3/TiO2 with Low Vanadium Loadings for Selective Catalytic Reduction of NOx by NH3. J. Phys. Chem. C. 2009, 113, 21177-21184. (7) Liu, Y.; Gu, T.; Weng, X.; Wang, Y.; Wu, Z.; Wang, H. DRIFT Studies on the Selectivity Promotion Mechanism of Ca-Modified Ce-Mn/TiO2 Catalysts for LowTemperature NO Reduction with NH3. J. Phys. Chem. C. 2012, 116, 16582-16592. (8) Smirniotis, P. G.; Pena, D. A.; Uphade, B. S. Low-Temperature Selective Catalytic Reduction (SCR) of NO with NH3 by Using Mn, Cr, and Cu Oxides Supported on Hombikat TiO2. Angew. Chem. Int. Ed. 2001, 40, 2479-2482. (9) Liu, C.; Yang, S.; Ma, L.; Peng, Y.; Hamidreza, A.; Chang, H.; Li, J. Comparison on the Performance of α-Fe2O3 and γ-Fe2O3 for Selective Catalytic Reduction of Nitrogen Oxides with Ammonia. J. Catal. Lett. 2013, 143, 697-704. (10) Mou, X.; Zhang, B.; Li, Y.; Yao, L.; Wei, X.; Su, D. S.; Shen, W. Rod-Shaped Fe2O3 as an Efficient Catalyst for the Selective Reduction of Nitrogen Oxide by Ammonia. Angew. Chem. Int. Ed. 2012, 51, 2989-2993. (11) Shan, W.; Liu, F.; He, H.; Shi, X.; Zhang, C. Novel Cerium-Tungsten Mixed Oxide Catalyst for the Selective Catalytic Reduction of NOx with NH3. Chem. Commun. 2011, 47, 8046-8048. (12) Peng, Y.; Li, K.; Li, J. Identification of the Active Sites on CeO2-WO3 Catalysts for SCR of NOx with NH3: An in Situ IR and Raman Spectroscopy Study. Appl. Catal. B-Environ. 2013, 140, 483-492. (13) Chen, L.; Li, J.; Ge, M.; Zhu, R. Enhanced Activity of Tungsten Modified CeO2/TiO2 for Selective Catalytic Reduction of NOx with Ammonia. Catal. Today 2010, 153, 77-83. (14) Liu, B.; Liu, J.; Ma, S.; Zhao, Z.; Chen, Y.; Gong, X. Q.; Song, W.; Duan, A.; Jiang, G. Mechanistic Study of Selective Catalytic Reduction of NO with NH3 on W-doped CeO2 Catalysts: Unraveling the Catalytic Cycle and the Role of Oxygen Vacancy. J. Phys. Chem. C. 2016, 120, 2271-2283. (15) Xin, Y.; Li, H.; Zhang, N.; Li, Q.; Zhang, Z.; Cao, X. M.; Hu, P.; Zheng, L.; Anderson, J. A. Molecular-Level Insight into Selective Catalytic Reduction of NOx with NH3 to N2 over Highly Efficient Bifunctional Va-MnOx Catalyst at Low Temperature. ACS Catal. 2018, 8, 4937-4949. 21


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Insight into the NH3-assisted Selective Catalytic Reduction of NO on Î²

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