Activity Trend for Low-Concentration NO Oxidation at Room

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Activity Trend for Low-Concentration NO Oxidation at Room Temperature on Rutile-Type Metal Oxides Haiyang Yuan, Jian-Fu Chen, Haifeng Wang, and Peijun Hu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03045 • Publication Date (Web): 11 Oct 2018 Downloaded from http://pubs.acs.org on October 11, 2018

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Activity Trend for Low-Concentration NO Oxidation at Room Temperature on RutileType Metal Oxides Haiyang Yuana, Jianfu Chena, Haifeng Wanga, *, Peijun Huab a

Key Laboratory for Advanced Materials, Research Institute of Industrial Catalysis and Centre for

Computational Chemistry, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai, 200237, P. R. China. b

School of Chemistry and Chemical Engineering, The Queen’s University of Belfast, Belfast, BT9 5AG, UK

Abstract: The catalytic oxidation of low concentration NO at room temperature has drawn increasing attention to eliminate NO in the large semi-closed spaces. However, the location of efficient catalysts is a challenging task. Herein, to rationalize the activity trend of NO oxidation and facilitate the catalyst screening/design, we computationally investigate the lowconcentration NO oxidation processes on an important rutile-type of metal oxides (MO2, M=Mn, Ru, Ir, Rh) at room temperature. Some key scaling relations for the elementary steps following either the Mars-van Krevelen (MvK) mechanism or Langmuir-Hinshelwood (LH) mechanism, are revealed as a function of Ef(Ovac) (the formation energy of Obri vacancy) or Eads(O@M5c) (the adsorption energy of O at the metallic M5c site), and a 3D activity map following the MvK mechanism at room temperature is quantitatively constructed by combining the DFT results with microkinetic analyses. First, we identified the active region in term of Ef(Ovac) and Eads(O@M5c) to obtain the optimum activity, which requires the bifunctional cooperation of the metallic M5c and lattice Obri site: M5c can efficiently adsorb NO and the Obri site can provide the reactive O species. MnO2 is close to the active region, accounting for its good catalytic activity. Second, Ef(Ovac) and Eads(O@M5c) show a linear-scaling limitation for the pure rutile-type oxides, yielding that their catalytic activity can be solely described by Ef(Ovac), i.e. giving a 2D volcano-typed curve, meaning that pure MnO2 cannot give rise to the optimum activity. To break this limitation, it requires to increase (decrease) Eads(NO@M5c) (Ef(Ovac)) for enhancing the catalytic activity of MnO2, which could be achieved by doping Ti into MnO2(110) from our calculation results. Third, we examined the activity with the LH 1

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mechanism for a comparison, which indicates an oxide-specific mechanism: from MnO2-based oxides to RhO2, the MvK mechanism is favored, but switches to the LH mechanism on the RuO2 and IrO2 surfaces as Ef(Ovac) increases. Equally importantly, the MvK mechanism is found to be favored comparing with the LH one on the whole, implying that the participation of lattice Obri is necessary for achieving room-temperature oxidation of low-concentration NO. This work could provide a significant insight into low-concentration NO oxidation at room temperature. Keyword: Room-temperature NO oxidation; Activity trend; Density functional theory calculation; Microkinetics; MnO2; Mars-van Krevelen mechanism

1. Introduction Nitrogen oxides (NOx, x = 1, 2) are harmful to humanity and environment, which could cause acid rain, photochemical smog and ozone depletion.1-3 For the increasing large-scale semi-closed spaces such as road tunnels and indoor parks, which usually correspond to low concentration NO and lack pre-installed NOx elimination equipment, the room temperature NO oxidation serves as one of the most important methods to eliminate the low concentration NO. However, few oxide materials have been applied in such a case. Recently, some metal oxides, such as Cr and Mn based oxides materials, have been applied, especially Mn-based metal oxides were demonstrated to be efficient for the low concentration NO oxidation at room temperature, where MnO2 could be an important active ingredient to catalyze this reaction.4-10 However, the identification of superior metal oxide catalysts is still a challenge. A theoretical framework is thus highly desired to understand the activity trend of low-concentration NO oxidation at room temperature. Regarding the NO oxidation mechanism, despite that many investigations including the Mars-van Krevelen and Langmuir-Hinshelwood mechanisms have been reported,11-23 which one is more favored to achieve the room-temperature oxidation of low concentration NO is still 2


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not clear. In addition, in the NO oxidation process catalyzed by metal oxides, the involved fundamental scaling relations for the elementary steps from one metal oxide catalyst to another, which relates with the basic properties of metal oxides and thus contributes to the catalyst design, are rarely reported. In other words, a specific activity descriptor for metal oxides to catalyze NO oxidation at room temperature does not exist unambiguously. To the best of our knowledge, the quantitative activity trend for the low concentration NO oxidation catalyzed by metal oxides at room temperature is lacking, thus limiting the determination of the optimum activity region. As an extension of MnO2 catalyst, usually rutile-type metal oxides are widely applied19, 2326

and worth being explored for NO oxidation at room temperature. The most stable (110)

surface of the typical rutile-type metal oxides is terminated by two kinds of active sites, i.e. the five-coordinated metallic M5c site and the two-coordinated surface lattice O (Obri), which correspond to two different oxidation mechanisms; the LH mechanism occurs only on metallic sites, while the MvK mechanism needs the participation of the lattice Obri (see Scheme 1). In this work, we examine both mechanisms on a series of extended rutile-type oxides including MnO2, RuO2, IrO2 and RhO2, aiming to shed light on the activity trend of low concentration NO oxidation and the rate-determining factor, and to identify the real mechanism that is appropriate for the low concentration NO oxidation at room temperature.

2. Methods The Vienna ab initio simulation package (VASP) was applied to carried out all the spinpolarized density functional theory (DFT) calculations in this work,27 and the Perdew-BurkeErnzerhof (PBE) functional28 within the generalized gradient approximation (GGA) was used. The core-valence electron interaction was described by the project-augmented wave (PAW) method.29 The valence electronic states were expanded in plane wave basis sets with an energy cutoff of 450 eV. The optimization of the structures would be stopped until the maximal forces 3


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on all atom was less than 0.05 eV/Å. The (110) surface of rutile-type metal oxides was selected to simulate the NO oxidation; the (110) surface is molded with a p(3×1) periodic slab with four layers, and the vacuum between slabs is 15 Å; correspondingly, a 2×3×1 k-point mesh was used. During structural optimization, we fixed the bottom two layers of the slab, and relaxed the top two layers and the adsorbates. The constrained optimization scheme was used to search the transition states (TS).30 Here the DFT+U approach was used to treat the electronic correlation in the localized d-orbital of Mn, Fe, Co and Ti elements. For Mn, Fe, Co and Ti, the values of U are 1.6 eV,26, 31 3.6 eV,32 2.0 eV,33 4.2 eV,34 respectively. β-MnO2 preferentially takes the antiferromagnetic (AFM) state,35 thus an antiferromagnetic helical spin arrangement for βMnO2 was imposed in our calculation.36, 37 The adsorption energies of species X on surface were calculated with the equation of Eads(X) = Ex/surf – Esurf – Ex, where Ex, Esurf and Ex/surf are the total energies of adsorbate X in the gas phase, the clean surface and the optimized surface with adsorbate, respectively. The more negative Eads indicates the more favorable adsorption of the species X on the surface. The formation energy of Obri vacancy was defined as Ef(Ovac) = Evac/surf + 0.5*EO2 – Esurf, in which Evac/surf, Esurf and EO2 are the total energies of the surface with an Obri vacancy, the pristine surface and the gaseous O2 molecule, respectively. Noteworthily, all the total energies contain the zero-point energy correction and in the microkinetic analysis, the Gibbs free energy change (∆G) of the elementary step was calculated with the equation of ∆G =∆H + ∆EZPE − T∆S. With respect to the zero-point energy (∆EZPE) and entropy effect term (T∆S), we calculated the vibrational frequencies of the surface intermediates and transition states within DFT calculation;26,38 for the gaseous molecules, the entropy contributions (T∆S) were obtained with the experimental values39 or calculated with the Gaussian 09 at the level of B3LYP/6-31G.

3. Result and discussion In the MvK mechanism, which has been demonstrated on MnO2(110) at room temperature 4


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as shown in Scheme 1,40 NO adsorbs on the metallic M5c site first (NO*, * represents M5c site), and then NO* is oxidized by the lattice Obri, forming a NO2 species (NO2#, # represents the lattice O vacancy, Ovac) which can desorb directly to form an Ovac. At the Ovac site, the adsorbed O2 (O2#) can oxidize NO* to yield an intermediate ONOO, which can then break its O-O bond to release NO2, finishing the whole catalytic cycle. NO could also adsorb at the Ovac site, and then is oxidized by Obri to form NO2# (see the dash line in Scheme 1). With respect to the LH mechanism,19, 23, 40 as shown by Scheme 1, two routes could occur: (i) the O2 adsorbs on the metallic M5c site (O2*) and then dissociates into O atoms (O*), which then oxidizes NO* to form NO2; (ii) the molecular O2* directly couples with NO* to form the intermediate ONOO**, which then breaks the O-O bond and transforms into NO2* and an O* atom.

Scheme 1. The pathways of NO oxidation on rutile-type metal oxides (110) following the MvK mechanism (left) and LH mechanism (right), respectively. Green, purple, white, red and blue balls represent the metallic M5c site, the lattice metals linking with Obri, the Obri vacancy, O atoms and N atoms, respectively. Notably, for the conversion of ONOO species, our calculations show that it tends to firstly generate a surface-bound 1-N-nitro NO2* intermediate when the O-O bond breaks, which could then release the gaseous NO2, or transform into the energetically more favored bidentate 2-O,O’-nitrito adsorption configuration, giving a two-step elementary process: (1) ONOO → NO2* + Obri; (2) NO2* → NO2(g) +*. 5


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We calculated the barrier (Ea) and enthalpy change (ΔH) of each elementary step, as well as the adsorption energies of all the intermediates (Eads(i@j), where i and j represent the adsorbed species i and active site j, respectively), and examined their possible correlations. A range of linear relationships are found. Notably, Eads(O@M5c) and Ef(Ovac) are selected as descriptors, corresponding to describing the reactivity of the metallic site and lattice Obri site on rutile-type metal oxides, respectively.

Figure 1. The linear relationships of the adsorption/desorption energies of gaseous molecules on the metallic site (M5c) or the Ovac site as a function of Eads(O@M5c) or Ef(Ovac). (a) Calculated adsorption energies of NO (black) and O2 (blue) on the metallic site and desorption energy of NO2 (red) from the metallic site as a function of Eads(O@M5c); (b) Calculated adsorption energies of NO (red) and O2 (blue) on the Ovac site and desorption energy of NO2 (black) from the Ovac site as a function of Ef(Ovac). The points from left to right represent MnO2, RhO2, RuO2 and IrO2, respectively.

First of all, the scaling relations involved in the MvK mechanism (see left in Scheme 1) on the rutile-type metal oxides are discussed. As shown in Figure 1a, Eads(NO@M5c) linearly increases as a function of Eads(O@M5c). For the step of NO* oxidization with Obri to form NO2# (NO* +O# →NO2# +*), which involves both the metallic and Obri sites, Ea and ΔH have good linear relations with Ef(Ovac)–Eads(O@M5c), i.e. Ea=0.35*(Ef(Ovac)–Eads(O@M5c)) + 0.16 and ΔH=0.50*(Ef(Ovac)–Eads(O@M5c)) – 0.57, corresponding to Figure 2a and 2b, respectively. For the case of Ea, the positive slope indicates that the reactivity of Obri in oxidizing NO* would decrease as Ef(Ovac) increases or Eads(O@M5c) becomes more negative. Regarding the 6


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following NO2# desorption step and O2 adsorption step at the Ovac site (NO2# →NO2(g) + #, O2(g) + # →O2#), two linear correlations are established as a function of Ef(Ovac), i.e. Edes(NO2@Ovac) = 0.67*Ef(Ovac) + 0.55 and Eads(O2@Ovac) = -0.96*Ef(Ovac) + 0.11, as shown by Figure 1b, which show that the larger Ef(Ovac) makes NO2 desorption from Ovac more difficult and O2 adsorption on Ovac more favorable. For the oxidation step of O2# (O2# + NO* → ONOO#*) related to both the metallic site and lattice Obri, it is clear from Figure 2 that as Ef(Ovac)–Eads(O@M5c) increases, Ea and ΔH linearly increase with slopes of 0.10 and 0.39, respectively. Hence, O2# would be more difficult to participate in the following oxidation process as Ef(Ovac) increases or Eads(O@M5c) becomes more negative. Furthermore, in the OO bond cleavage of the intermediate ONOO (ONOO#* → NO2* + O#), we find that the O-O bond becomes weaker and tends to break more easily as Ef(Ovac) increases or Eads(O@M5c) becomes negative, as implied by the linear correlation with a negative slope (the blue line in Figure 2). It is worth discussing that, when Ef(Ovac)–Eads(O@M5c) increases to 3.28 eV, Ea would decrease to zero following the linear correlation, indicating that the O-O bond breaking in ONOO would become a barrierless process. Hence, in the latter kinetic model, the barrier of this step is treated as 0 eV as Ef(Ovac)–Eads(O@M5c) > 3.28 eV. Finally, for the step of NO adsorption on Ovac (NO + # →NO#), Eads(NO@Ovac) shows a similar linear relation in term of Ef(Ovac) (see the red line in Figure 1b) as Eads(O2@Ovac) does, and Eads(NO@Ovac) is always larger than Eads(O2@Ovac), indicating that the NO adsorption is more favorable than the O2 adsorption on the Ovac site. For the possible NO# oxidation step (NO# + O# → NO2# + #), it is clear from Figure 2 (the magenta line) that it would become more difficult to occur with a higher barrier as Ef(Ovac) increases.

7


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Figure 2. The linear relationships of (a) the energy barrier (Ea) and (b) the reaction enthalpy change (ΔH) of each elementary step in MvK mechanism as a function of Ef(Ovac) – Eads(O@M5c) or Ef(Ovac). The black, red, blue and magenta lines correspond to the step of NO* + O# → NO2# + *, NO* + O2#→ ONOO#*, ONOO#* → NO2* +O# and NO# + O# → NO2# + #, respectively. Notably, the step of NO# + O# → NO2# + # (magenta) is as a function of Ef(Ovac), while the other three steps are as a function of Ef(Ovac) – Eads(O@M5c). The points from left to right represent MnO2, RhO2, RuO2 and IrO2, respectively.

We also investigated the scaling relations for the elementary steps following the LH mechanism (see right in Scheme 1) occurring at the M5c site as a function of Eads(O@M5c) on rutile-type metal oxides, and the following features are found. First, in the adsorption and dissociation steps of O2 at the M5c sites (O2 + * →O2*; O2* + * →2O*), as Eads(O@M5c) becomes more negative on different rutile-type metal oxides, the adsorption strength of O2 molecule increases (see the blue line in Figure 1) and the O2* dissociation barriers become smaller gradually (see the black line in Figure 3), indicating the easier generation of O* atoms. With respect to the NO* oxidation step by O* (NO* + O* →NO2* + *), it would become more difficult both kinetically and thermodynamically on the whole as the adsorption strength of O* on M5c increases; e.g. there exists Ea= -0.21*Eads(O@M5c) + 0.22 with a negative slope (see the red line in Figure 3). Noteworthily, here the linear correlation for such an association reaction (NO* +O* →NO2* +*) is, however, poor,41, 42 and in the latter microkinetic model, the reverse dissociation reaction (NO2* +*→NO* +O*) showing a better linear BEP relation in term of Eads(O@M5c) (see Figure S2) was used to more accurately express the barrier of NO* coupling 8


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with O*. Second, for the step of O2* coupling with NO* (O2* + NO* → ONOO**), similar trend exists with the coupling reaction of O* with NO*; the more negative Eads(O@M5c) is, the more difficult the intermediate ONOO would be formed (with a larger barrier and energy cost, see the blue lines in Figure 3). By comparison, the O-O bond of the intermediate ONOO is easier to break to form NO2 as Eads(O@M5c) become more negative (ONOO**→ NO2* + O*; see the magenta line in Figure 3).

Figure 3. The linear relationships of (a) barrier (Ea) and (b) reaction enthalpy change (ΔH) of each elementary step in the LH mechanisms as a function of Eads(O@M5c). The black, red, blue and magenta lines correspond to the step of O2* +* →O* +O*, NO* + O*→NO2* +*, NO* + O2*→ONOO** and ONOO** →NO2* +O*, respectively. The points from left to right represent MnO2, RhO2, RuO2 and IrO2, respectively. Note: for the reaction step of O2* + NO* →ONOO** (the blue line in (a)), our calculations show that it would become nearly barrierless when Eads(O@M5c) > 0.24 eV, and thus Ea is treated as 0 eV therein.

Having obtained the above basic scaling relations of the MvK and LH mechanisms on the (110) surfaces of different rutile-type metal oxides, we are in the position to discuss the activity descriptors in the systems: One can see that the binding ability of the metallic M5c site (represented by Eads(O@M5c) and the reactivity of lattice Obri (represented by Ef(Ovac)) can correlate well with the thermodynamic/kinetic properties of all the elementary steps. Consequently, we carried out a microkinetic analysis to quantitatively understand the overall activity trends of low concentration NO oxidation at room temperature (see details in the supporting information), where the logarithms of TOFs were plotted as a function of Ef(Ovac) 9


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and Eads(O@M5c). A 3D activity map following the MvK mechanism is obtained (Figure 4), considering the typical experimental conditions:9 T = 300 K, PNO = 5.0×10-5 atm (50 ppm), PNO2 = 4.0×10-5 atm (40 ppm) and PO2 = 0.21 atm. As shown in Figure 4, the indicated optimum Ef(Ovac) of highly active catalysts is in range of about 0.40~0.55 eV, implying that a reasonably high reactivity of Obri is necessary for achieving efficient conversion of low concentration NO oxidation at room temperature. For the Eads(O@M5c) descriptor corresponding to the metallic site, it should be controlled in the range of 0.35~0.85 eV. Also, it can be seen from Figure 4 that RuO2, IrO2 and RhO2 could not provide enough activity for low-concentration NO oxidation at room temperature, while MnO2 is close to the optimum active region, explaining that MnO2 could be a good basic material for low concentration NO oxidation at room temperature. This is in good agreement with the experimental result.9

Figure 4. Activity map of NO oxidation as a function of Ef(Ovac) and Eads(O@M5c) following the MvK mechanism. The inserted black line indicates the linear constraint between Ef(Ovac) and Eads(O@M5c) for pure MnO2, RhO2, RuO2 and IrO2 oxides. The blue points show MnO2 doped with Fe, Co and Ti, respectively.

Moreover, increasing the adsorption ability of M5c toward O* (i.e. making Eads(O@M5c) more negative) or decreasing Ef(Ovac) of MnO2 could further approach the optimum activity peak. However, it is interesting to note that Eads(O@M5c) correlates close with Ef(Ovac) on pure MO2, showing a good linear relation of Eads(O@M5c) = -1.06*Ef(Ovac) + 1.58 (see the black line in Figure 4). As Ef(Ovac) increases, the adsorption ability of the metallic M5c site also increases. 10


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In other words, there is a constraint between these two descriptors, which accordingly yields that the pure rutile-type metal oxides cannot reach the optimum activity peak, as shown in Figure 4. Significantly, these two activity descriptors correspond to two different kinds of active sites on the (110) surfaces of the rutile-type metal oxide, i.e. the metallic M5c site and the lattice Obri site. There is a possibility to tune their properties separately through locally substituting the M5c or M6c bonded with Obri with other metals, which is different from the usual transition metal catalysts with the single-type active sites exposed.42, 43 To break this constraint and achieve higher catalytic activity based on the MnO2(110) catalyst, i.e. increasing the NO adsorption strength (corresponding to make Eads(O@M5c) more negative) while weakening Ef(Ovac) slightly, we examined a few dopants such as Fe, Co and Ti into MnO2, and found that doping Ti could push MnO2 significantly to approach the optimum activity point as shown by Figure 4. In other words, Ti-doped MnO2(110) may achieve the efficient oxidation conversion of low concentration NO at room temperature. Since a good linear relationship exists between Eads(O@M5c) and Ef(Ovac), 2D volcano curve of activity trend following the MvK mechanism as a function of Ef(Ovac) can be obtained, as shown in the red curve in Figure 5a, where the coverage changes of some species are illustrated in Figure 5b. It can be observed from the figure that the volcano curve consists of several sections. Starting from the very small Ef(Ovac), the curve rises significantly at the beginning and gradually reaches an inflection point at about Ef(Ovac) = 0.40 eV. This increase is benefited from the more negative Eads(NO@M5c) and Eads(O2@Ovac) with the increase of Ef(Ovac), which leads to the rapid increase of θ(NO*) and θ(O2#) (see the red and olive curves in Figure 5b). At this inflection point, θ(O2#) also reaches the maximum, and θ(NO#) is almost 1 ML, indicating clearly that the adsorption of NO on Ovac is an important factor influencing NO oxidation due to the competitive adsorption. After Ef(Ovac) = 0.40 eV, the volcano curve becomes flat and gradually reaches the maximum at about Ef(Ovac) = 0.70 eV. From the coverage changes in Figure 5b, we can find that although θ(O2#) reaches the maximum and remains 11


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unchanged, θ(NO*) still increases rapidly, which leads to this slow activity rise in the range of 0.40 eV < Ef(Ovac) < 0.70 eV. From the black curve in Figure 5b, it can be seen clearly that the coverage of the final product (NO2) on the metallic site (θ(NO2*)) reaches maximum at about Ef(Ovac) = 0.70 eV, showing that at this point the rutile-type metal oxide reaches the highest activity for low concentration NO oxidation at room temperature. Furthermore, as Ef(Ovac) continues to increase from 0.70 eV to 1.05 eV, the activity begins to decline slowly as described by Figure 5a; in this range, as Ef(Ovac) increases, θ(NO*) and θ(NO#) reach almost 1 ML, while θ(O2#) keeps unhanged (see the olive curve in Figure 5b), leading to the slow decrease of activity. After Ef(Ovac) >1.05 eV, the shapely decreased θ(O2#) results in the rapid decrease of the activity.

Figure 5. (a) The calculated volcano-typed activity trend for the low concentration NO oxidation on (110) surface of rutile-type metal oxides (MO2) at room temperature following the MvK mechanism (red curve) or LH mechanism (black dashed curve), in which the logarithms of the turnover frequencies (TOFs) were plotted as Ef(Ovac). (b) The coverage of the involved key species in the MvK mechanism varied as a function of Ef(Ovac). The black and red curves represent that of NO2 and NO on the metallic site, i.e. θ(NO2*) and θ(NO*), respectively, while the olive and magenta curves are for O2 and NO at the Ovac site, corresponding to θ(O2#) θ(NO#), respectively.

To further investigate the activity trend of rutile-type metal oxides, we also explored the possible LH mechanism, and the corresponding volcano-typed activity curve is obtained as a 12


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function of Ef(Ovac) (see the black dashed curve in Figure 5a). Comparing the two curves in Figure 5a, we can find obviously that the MvK mechanism is more favorable than the LH mechanism for the low concentration NO oxidation at room temperature; the LH mechanism may work when Ef(Ovac) > 1.35 eV. For example, IrO2 and RuO2 catalysts tend to follow the LH mechanism instead of the MvK mechanism. However, the maximum point of activity curve following the LH mechanism is lower than that with the MvK mechanism (10-5 s-1 versus 10-3 s-1). Therefore, for the low concentration NO oxidation at room temperature, the MvK mechanism is the main pathway on rutile-type metal oxides, while the LH mechanism only becomes favored after Ef(Ovac) increases to a certain extent. 4. Conclusion In summary, we have obtained basic scaling relations involved in two different mechanisms (the MvK and LH mechanisms) for NO oxidation, and Eads(O@M5c) and Ef(Ovac) can serve as two descriptors for depicting the reactivity of metallic M5c site and lattice Obri, respectively. With the increase of Ef(Ovac), Obri becomes inert, leading to that the oxidation of NO* with Obri/O2# become difficult with larger barriers, while the cleavage of O-O bond in ONOO becomes easier. As a result, a 3D activity map following the MvK mechanism has been obtained, and the optimum activity region for low concentration NO oxidation at room temperature has been located, where Ef(Ovac) and Eads(O@M5c) should be in rang of 0.40~0.55 eV and 0.35~0.85 eV, respectively. It has been found that MnO2 is close to this optimum region, accounting for its superior catalytic activity. However, there is a reactivity constraint between the M5c and lattice Obri sites on the pure rutile oxides (MO2) and it eventually converts the 3D map into a 2D volcano-typed curve, which prevents the pure MO2 from being the optimum catalyst. We have proposed that Ti-doping could efficiently strengthen Eads(O@M5c) and decrease Ef(Ovac) simultaneously and thus improve activity of MnO2. Finally, comparing the activity trend of the MvK and LH mechanism on metal oxides, we have found that the MvK 13


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mechanism is responsible for the efficient oxidation of low concentration NO at room temperature, and the LH mechanism is effective on rutile-type oxides with high Ef(Ovac). Therefore, the reactive lattice Obri is a necessary species for the low concentration NO oxidation at room temperature. ASSOCIATED CONTENT *Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. It contains the adsorption energies of key species on rutile-type metal oxides (110), the optimized structures of the surface intermediates and transition states involved in MvK and LH mechanisms on MnO2(110), the linear relation for the NO2* dissociation and the calculated zero-point energy and entropic contributions for each elementary step; also, the computational details on microkinetic analysis are given. 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 NSFC (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). REFERENCE (1) Liu, Z.; Ihl Woo, S. Recent Advances in Catalytic DeNOx Science and Technology. Catal. Rev. 2006, 48, 43-89. (2) Roy, S.; Baiker, A. NOx Storage-Reduction Catalysis: From Mechanism and Materials 14


Page 14 of 18

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

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Properties to Storage-Reducation Performance. Chem. Rev. 2009, 109, 4054-4091. (3) McEwen, J. S.; Anggara, T.; Schneider, W. F.; Kispersky, V. F.; Miller, J. T.; Delgass, W. N.; Ribeiro, F. H. Integrated operando X-ray absorption and DFT characterization of Cu-SSZ-13 exchange sites during the selective catalytic reduction of NOx with NH3. Catal. Today 2012, 184, 129-144. (4) Liu, S.; Zhao, K.; Yang, W.; Shi, J.; He, D. Effects of Precipitant and Calcination Temperature on the Catalytic Activities of Chromic Oxide Catalysts for NO Oxidation at Ambient Temperature. React. Kinet. Mech. Catal. 2017, 122, 839-852. (5) Wang, A.; Guo, Y.; Gao, F.; Peden, C. H. Ambient-Temperature NO Oxidation over Amorphous CrOx-ZrO2 Mixed Oxide Catalysts: Significant Promoting Effect of ZrO2. Appl. Catal. B-Environ. 2017, 202, 706-714. (6) Huang, H. Y.; Yang, R. T. Removal of NO by Reversible Adsorption on Fe-Mn Based Transition Metal Oxides. Langmuir 2001, 17, 4997-5003. (7) Shu, Z.; Huang, W.; Hua, Z.; Zhang, L.; Cui, X.; Chen, Y.; Chen, H.; Wei, C.; Wang, Y.; Fan, X.; Yao, H.; He, D.; Shi, J. Template-Free Synthesis of Mesoporous X-Mn (X= Co, Ni, Zn) Bimetal Oxides and Catalytic Application in the Room Temperature Removal of LowConcentration NO. J. Mater. Chem. A 2013, 1, 10218-10227. (8) Shu, Z.; Chen, Y.; Huang, W.; Cui, X.; Zhang, L.; Chen, H.; Zhang, G.; Fan, X.; Wang, Y.; Tao, G.; He, D.; Shi, J. Room-Temperature Catalytic Removal of Low-Concentration NO over Mesoporous Fe-Mn Binary Oxide Synthesized Using a Template-Free Approach. Appl. Catal. B-Environ. 2013, 140, 42-50. (9) Wang, M.; Liu, H.; Huang, Z. H.; Kang, F. Activated Carbon Fibers Loaded with MnO2 for Removing NO at Room Temperature. Chem. Eng. J. 2014, 256, 101-106. (10) Zhang, M.; Li, C.; Qu, L.; Fu, M.; Zeng, G.; Fan, C.; Ma, J.; Zhan, F. Catalytic Oxidation of NO with O2 over FeMnOx/TiO2: Effect of Iron and Manganese Oxides Loading Sequences and the Catalytic Mechanism Study. Appl. Surf. Sci. 2014, 300, 58-65. (11) Bhatia, D.; McCabe, R. W.; Harold, M. P.; Balakotaiah, V. Experimental and Kinetic Study of NO Oxidation on Model Pt Catalysts. J. Catal. 2009, 266, 106-119. (12) Arevalo, R. L.; Oka, K.; Nakanishi, H.; Kasai, H.; Maekawa, H.; Osumi, K.; Shimazaki, N. Oxidation of NO on Pt/M (M= Pt, Co, Fe, Mn): A First-Principles Density Functional Theory Study. Catal. Sci. Technol. 2015, 5, 882-886. (13) McClure, S. M.; Kim, T. S.; Stiehl, J. D.; Tanaka, P. L.; Mullins, C. B. Adsorption and Reaction of Nitric Oxide with Atomic Oxygen Covered Au(111). J. Phys. Chem. B 2004, 108, 15


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

17952-17958. (14) Wen, Y.; Zhang, C.; He, H.; Yu, Y.; Teraoka, Y. Catalytic Oxidation of Nitrogen Monoxide over La1-xCexCoO3 Perovskites. Catal. Today 2007, 126, 400-405. (15) Wang, W.; McCool, G.; Kapur, N.; Yuan, G.; Shan, B.; Nguyen, M.; Graham, U. M.; Davis, B. H.; Jacobs, G.; Cho, K.; Hao, X. K. Mixed-phase Oxide Catalyst Based on Mn-Mullite (Sm, Gd) Mn2O5 for NO Oxidation in Diesel Exhaust. Science 2012, 337, 832-835. (16) Tang, N.; Liu, Y.; Wang, H.; Wu, Z. Mechanism Study of NO Catalytic Oxidation over MnOx/TiO2 Catalysts. J. Phys. Chem. C 2011, 115, 8214-8220. (17) Li, K.; Tang, X.; Yi, H.; Ning, P.; Song, J.; Wang, J. Mechanism of Catalytic Oxidation of NO over Mn-Co-Ce-Ox Catalysts with the Aid of Nonthermal Plasma at Low Temperature. Ind. Eng. Chem. Res. 2011, 50, 11023-11028. (18) Chen, Z.; Liu, X.; Cho, K.; Chen, R.; Shan, B. Density Functional Theory Study of the Oxygen Chemistry and NO Oxidation Mechanism on Low-Index Surfaces of SmMn2O5 Mullite. ACS Catal. 2015, 5, 4913-4926. (19) 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. (20) Yang, J.; Zhang, J.; Liu, X.; Duan, X.; Wen, Y.; Chen, R.; Shan, B. Origin of the Superior Activity of Surface Doped SmMn2O5 Mullites for NO Oxidation: A First-Principles Based Microkinetic Study. J. Catal. 2018, 359, 122-129. (21) Junkaew, A.; Meeprasert, J.; Jansang, B.; Kungwan, N.; Namuangruk, S. Mechanistic Study of NO Oxidation on Cr-Phthalocyanine: Theoretical Insight. RSC Adv. 2017, 7, 88588865. (22) Jin, J.; Sun, N.; Hu, W.; Yuan, H. Y; Wang, H. F.; Hu, P. Insight into Room-Temperature Catalytic Oxidation of Nitric oxide by Cr2O3: A DFT Study. ACS Catal. 2018, 8, 5415-5424. (23) Wang, H.; Schneider, W. F. Adsorption and Reactions of NOx on RuO2(110). Catal. Today 2011, 165, 49-55. (24) Li, L.; Feng, X.; Nie, Y.; Chen, S.; Shi, F.; Xiong, K.; Ding, W.; Qi, X.; Hu, J.; Wei, Z.; Wan, L. J.; Xia, M. Insight into the Effect of Oxygen Vacancy Concentration on the Catalytic Performance of MnO2. ACS Catal. 2015, 5, 4825-4832. (25) Tompsett, D. A.; Parker, S. C.; Islam, M. S. Rutile (β-)MnO2 Surfaces and Vacancy Formation for High Electrochemical and Catalytic Performance. J. Am. Chem. Soc. 2014, 136, 1418-1426. 16


Page 16 of 18

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

ACS Catalysis

(26) Yuan, H. Y.; Sun, N. N.; Chen, J. F.; Jin, J. M.; Wang, H. F.; Hu, P. Insight into the NH3assisted Selective Catalytic Reduction of NO on β-MnO2(110): Reaction Mechanism, Activity Descriptor and Evolution from the Pristine State to Steady State. ACS Catal. 2018, 8, 92699279. (27) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 1169-1186. (28) Perdew, J. P.; Ruzsinszky, A.; Csonka, G. I.; Vydrov, O. A.; Scuseria, G. E.; Constantin, L. A.; Zhou, X.; Burke, K. Restoring the Density-Gradient Expansion for Exchange in Solids and Surfaces. Phys. Rev. Lett. 2008, 100, 136406. (29) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758-1775. (30) Alavi, A.; Hu, P.; Deutsch, T.; Silvestrelli, P. L.; Hutter, J. CO Oxidation on Pt (111): An Ab Initio Density Functional Theory Study. Phys. Rev. Lett. 1998, 80, 3650. (31) Oxford, G. A.; Chaka, A. M. First-Principles Calculations of Clean, Oxidized, and Reduced β-MnO2 surfaces. J. Phys. Chem. C 2011, 115, 16992-17008. (32) Kiejna, A.; Ossowski, T.; Pabisiak, T. Surface Properties of the Clean and Au/Pd Covered Fe3O4 (111): DFT and DFT + U Study. Phys. Rev. B 2012, 85, 125414. (33) Wang, H. F.; Kavanagh, R.; Guo, Y. L.; Guo, Y.; Lu, G.; Hu, P. Origin of Extraordinarily High Catalytic Activity of Co3O4 and Its Morphological Chemistry for CO Oxidation at Low Temperature. J. Catal. 2012, 296, 110-119. (34) Li, S. C.; Jacobson, P.; Zhao, S. L.; Gong, X. Q.; Diebold, U. Trapping Nitric Oxide by Surface Hydroxyls on Rutile TiO2(110). J. Phys. Chem. C 2012, 116, 1887-1891. (35) Paik, Y.; Osegovic, J. P.; Wang, F.; Bowden, W.; Grey, C. P. 2H MAS NMR Studies of the Manganese Dioxide Tunnel Structures and Hydroxides Used as Cathode Materials in Primary Batteries. J. Am. Chem. Soc. 2001, 123, 9367-9377. (36) Franchini, C.; Podloucky, R.; Paier, J.; Marsman, M.; Kresse, G. Ground-state Properties of Multivalent Manganese Oxides: Density Functional and Hybrid Density Functional Calculations. Phys. Rev. B 2007, 75, 195128. (37) Mellan, T. A.; Maenetja, K. P.; Ngoepe, P. E.; Woodley, S. M.; Catlow, C. R. A.; GrauCrespo, R. Lithium and Oxygen Adsorption at the β-MnO2(110) Surface. J. Mater. Chem. A. 2013, 1, 14879-14887. (38) Xu, J.; Li, X.; Liu, W.; Sun, Y.; Ju, Z.; Yao, T.; Wang, C.; Ju, H.; Zhu, J.; Wei, S.; Xie, Y. 17


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Carbon Dioxide Electroreduction into Syngas Boosted by a Partially Delocalized Charge in Molybdenum Sulfide Selenide Alloy Monolayers. Angew. Chem. Int. Ed. 2017, 129, 9249-9253. (39) http://webbook.nist.gov/chemistry/, 12 Oct, 2017. (40) Yuan, H. Y.; Chen, J. F.; Guo, Y.; Wang, H. F.; Hu, P. Insight into the Superior Catalytic Activity of MnO2 for Low-content NO Oxidation at Room Temperature. Unpublished (41) Michaelides, A.; Liu, Z. P.; Zhang, C. J.; Alavi, A.; King, D. A.; Hu, P. Identification of General Linear Relationships between Activation Energies and Enthalpy Changes for Dissociation Reactions at Surfaces. J. Am. Chem. Soc. 2003, 125, 3704-3705. (42) Wang, H. F.; Guo, Y.; Lu, G.; Hu, P. An Understanding and Implications of the Coverage of Surface Free Sites in Heterogeneous Catalysis. J. Chem. Phys. 2009, 130, 224701. (43) Cheng, J.; Hu, P. Utilization of the Three-Dimensional Volcano Surface to Understand the Chemistry of Multiphase Systems in Heterogeneous Catalysis. J. Am. Chem. Soc. 2008, 130, 10868-10869.

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Activity Trend for Low-Concentration NO Oxidation at Room

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