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Research Article Cite This: ACS Catal. 2018, 8, 5415−5424

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Insight into Room-Temperature Catalytic Oxidation of Nitric oxide by Cr2O3: A DFT Study Jiamin Jin,† Ningling Sun,† Wende Hu,† Haiyang Yuan,† Haifeng Wang,*,† and Peijun Hu*,†,‡ †

Key Laboratories 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, People’s Republic of China ‡ School of Chemistry and Chemical Engineering, Queen’s University Belfast, Belfast BT9 5AZ, U.K.

ACS Catal. 2018.8:5415-5424. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/10/19. For personal use only.

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ABSTRACT: Cr-based catalysts have drawn attention as promising room-temperature NO oxidation catalysts. However, the intrinsic active component and reaction mechanism at the atomic level remain unclear. Here, taking the Cr2O3, one of the most stable chromium oxides, as an object, we systematically investigated NO oxidation processes on Cr2O3(001) and -(012) surfaces by virtue of DFT+U calculations, aiming to uncover the activity-limiting factors and basic structure−activity relationship of the Cr2O3 catalyst. It was revealed that NO oxidation could not proceed via a Mars−van Krevelen mechanism involving the lattice oxygen on both surfaces. For the Cr2O3(001) surface exposing the isolated three-coordinated Cr3c, the reactions are inclined to occur through the Eley−Rideal route, in which the NO couples directly with the molecular O2* or atomic O* adsorbed at the Cr3c site to form two key intermediate species (ONOO* and NO2*) following a barrierless process. Nevertheless, the overall activity is limited by the irreversible adsorption of NO2 species on the highly unsaturated Cr3c. In contrast, on the (012) termination, which exposes the five-coordinated Cr5c, the NO2* can be easily released, but the reactant O2 cannot be efficiently adsorbed and also results in a limited overall activity at room temperature. To achieve a higher activity, a thermodynamically favored interface model of monochain CrO3 supported on Cr2O3(012) was proposed, which shows an improved O2 adsorption energy of −0.99 eV and thus an enhanced activity of Cr2O3(012), possibly accounting for the experimentally high activity of Cr-based catalysts usually involving the Cr3+/Cr6+ redox. This study demonstrated the catalytic ability of Cr2O3 for NO oxidation at room temperature, and the presented systematic picture may facilitate the further design of more active Cr-based catalysts. KEYWORDS: density functional theory, chromium oxides, NO oxidation, room temperature, catalytic mechanism, monochain CrO3

1. INTRODUCTION NO oxidation to NO2 is a key process in a number of NOx remediation technologies,1 including selective catalytic reduction (SCR)2−4 and NOx storage and reduction (NSR).5,6 Platinum-based catalysts7−10 have been widely used for NO oxidation; however, their high cost restrict their applications. Transition-metal oxides (TMOs) have recently attracted great attention as promising catalysts for NO oxidation, and previous studies have made much progress in finding highly efficient TMO catalysts. Perovskites such as La 1−x Sr x CoO 3 , La1−xSrxMnO3, and Mn-mullite (Sm,Gd)Mn2O5 are expected to be viable alternatives to precious-metal catalysts.11−14 These observations/suggestions are particularly noteworthy because these catalysts are much less expensive than catalysts based on Pt-group metals. However, they are still not satisfactory for NO oxidation at room temperature. It has been reported that the activated carbon, especially modified carbon15 and activated carbon fibers,16 are active for catalytic NO oxidation at room temperature, but the catalytic activity diminished quickly © 2018 American Chemical Society

because of the irreversible adsorption of NO2. Thus, exploring nonprecious and low-temperature catalysts for NO oxidation and understanding the catalytic mechanism are of great importance. Recently, there has been great interest in the development of Cr-based catalysts17−24 for the effective removal of NOx from stationary sources. Smirniotis et al.17,25 found that Cr oxides supported on Hombikat TiO2 showed superior performance in the low-temperature selective catalytic reduction (SCR) of NO with NH3 and pointed out the existence of a Cr2O3 crystal phase. Shiba et al.26 reported earlier that a TiO2-supported chromium component had a high ability for the oxidation of NO to NO2. Cai et al.27 studied the effect of Cr on the NO oxidation over ceria−zirconia solid solutions, proposing that the synergistic mixture of the well-dispersed CrOx and Received: January 8, 2018 Revised: April 6, 2018 Published: May 3, 2018 5415

DOI: 10.1021/acscatal.8b00081 ACS Catal. 2018, 8, 5415−5424

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Figure 1. Hexagonal unit cell of Cr2O3 (a) together with the rhombohedral primitive cell (b). Red spheres stand for O atoms; blue spheres represent Cr atoms. The Cr atoms denoting with ↑↓ stand for the spin arrangements. (c, d) Top views of the optimized surface configuration of the Cr2O3(001) and -(012) surfaces, respectively. This notation is used throughout the paper.

2. CALCULATION METHOD AND MODEL All of the spin-polarized calculations in this work were performed within the framework of density functional theory (DFT) using the Vienna ab initio simulation package (VASP).38,39 A generalized gradient approximation (GGA) with Perdew−Burke−Ernzerhof (PBE)40 functional was employed to describe the exchange and correlation energy. Electron−ion interactions were treated within the projected augmented wave (PAW) method.41 The valence electronic states were expanded in plane wave basis sets with a cutoff energy of 450 eV. The Cr[3d54s1], Cr[3p63d54s1], O[2s22p4], and N[2s22p3] states were treated as valence states for describing Cr3+ in Cr2O3, Cr6+ in CrO3, and O and N elements, respectively. We used a Monkhorst−Pack k-mesh of 8 × 8 × 3 for the bulk of Cr2O3. For both the Cr2O3(001) and -(012) surfaces, a p(2 × 2) supercell with four Cr2O3-unit layers was used, and the vacuum between slabs was 12 Å, in which a 2 × 2 × 1 k-point mesh was used. During structural optimization, the bottom two layers of the slab were fixed at bulk truncated positions and the top two layers and the adsorbates were fully relaxed. All of the adsorption geometries were optimized until the forces on all atoms were below 0.05 eV/Å, and the transition states were located with a constrained minimization technique.42−45 The DFT-D3(BJ) method was used to describe the dispersion effects in the system.46 Bulk Cr2O3 is an antiferromagnetic (AFM) semiconductor, which crystallizes in the hexagonal corundum structure (space group R3̅c; see Figure 1a). Three different AFM configurations: + + − −, + − − + , and + − + −, could exist for the Cr cations in the rhombohedral primitive cell (see Figure 1b), where + and − denote spin up and spin down, respectively. The tests showed that the equilibrium magnetic order of this material in its pure state is the AFM + − + − configuration (shown in Figure 1), which is consistent with previous studies.47 In addition, to correctly describe the strong correlation of Cr2O3 materials, the DFT+U method, proved to have a significantly improved description of the structure and magnetic and electronic properties of materials,48−50 has been adopted. To obtain a proper U term for the Cr 3d orbital, the electronic structure (the magnetic moment of Cr3+ and the band gap of Cr2O3 bulk) and the chemical properties such as the formation energy of Cr2O3 (referring to 1/2Cr2O3 + 1/4O2 → CrO2) and NO adsorption energy on the Cr2O3(001) surface, which reflect the Cr−O bond strength and the adsorption ability of the Cr3+ site, respectively, were calculated under different U

Ce0.5Zr0.5O2 exhibited higher catalytic activity in comparison to separate species. More recently, Liu et al.22 found that chromium oxides prepared by an ammonia precipitation method showed higher NO to NO2 conversions at ambient temperature and suggested that the amorphous-phase structure (neither Cr2O3 nor CrO3) of the prepared chromic oxide catalysts is critical for obtaining high performance and longterm stability for NO conversion. These attractive findings represent much progress, because Cr-based oxides are significantly less expensive and more abundant and, more importantly, they were demonstrated to possess excellent performance in the complete oxidation of NO at room temperature.22−24 However, several important issues on Crbased catalysts remain to be addressed. First, as chromium is a metal oxide with multivalence, the real active component for NO oxidation is still controversial, despite the fact that a Cr6+ species with high dispersion was suggested very recently to be more favorable than Cr2O3 to oxidize NO.28−30 Second, the favorable reaction path behind the room-temperature NO oxidation is not yet clear. Third, as the most common chromium oxide, what role Cr2O3 might play in NO conversion remains elusive. In particular, one may ask the following questions. (i) How easily can the involved molecules (O2, NO, NO2, etc.) in NO oxidation chemisorb and convert on Cr2O3? (ii) Is the catalytic activity surface sensitive or insensitive to Cr2O3? (iii) What are the advantages and shortcomings of Cr2O3 for catalyzing room-temperature NO oxidation? Could Cr2O3 cooperate with CrO3 to avoid the shortcomings and obtain an enhanced activity? Inspired by these questions, therefore, it is highly desirable to investigate the nature of Cr2O3 for NO oxidation theoretically. With the aim of revealing the aforementioned issues, in this work a molecular-level investigation of catalytic oxidation of the nitric oxide on Cr2O3 and at the Cr2O3/CrO3 interface is studied: two typically exposed surfaces of Cr2O3, (001) and (012) surfaces, are examined to address (i) the catalytic mechanism, (ii) the activity-limiting factor of Cr2O3, (iii) the basic structure−activity relationship of Cr2O3 on NO oxidation, and (iv) the possible Cr2O3/CrO3 interface model with improved activity. It was noted that first-principles studies have been used extensively to understand NO catalytic oxidation on metal,10,31,32 metal oxides,33−35 and other systems.36,37 However, to the best of our knowledge, this is the first computational study focusing on room-temperature NO oxidation catalyzed by chromium oxides. 5416

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Table 1. Formation Energies of Cr2O3 with Respect to the Reaction 1/2Cr2O3 + 1/4O2 → CrO2 (ΔH in eV)a U ΔH/eV Ead/eV mag moment/μB band gap/eV

0

1

2

3

4

5

exptl

−1.10 −1.79 2.62 1.59

−0.87 −1.34 2.74 2.03

−0.62 −0.99 2.82 2.46

−0.38 −0.79 2.88 2.84

−0.14 −0.56 2.94 3.13

0.05 −0.44 2.98 3.32

−0.29 −0.80 2.76 3.30

a The adsorption energies (Ead/eV) of NO on the Cr2O3 (001) surface, the magnetic moment of Cr3+, and band gap of Cr2O3 on a series of U values are also listed. For a comparison, the experimental values are also given.

Figure 2. Optimized adsorption structures of NO (a, b), molecular O2 (c, d), dissociated O2 (e), and NO2 (f−j) on the Cr2O3(001) surface. The value at the top right of each configuration is the corresponding adsorption energy. All energies are in eV.

3. RESULTS AND DISCUSSION 3.1. NO Oxidation on Cr2O 3(001) Surface. The optimized surface structure of Cr2O3(001) is shown in Figure 1c, from which one can see that the surface is terminated by three-coordinated Cr3c cations and three-coordinated lattice oxygens (O3c), and the distance of the adjacent Cr3c−Cr3c on the (001) surface is very long (5.051 Å), implying that the Cr3c site is basically isolated. Notably, the coordinatively unsaturated Cr3c cations are generally considered to be vital for the catalytic activity, providing basic adsorption sites for NO and O2. As a start, the adsorption energetics and geometries of NO on the Cr2O3(001) surface were explored. As shown in Figure 2a,b, NO on Cr2O3(001) prefers to adsorb via its N atom on the Cr3c in a bending configuration, corresponding to an adsorption energy of −0.79 eV. First, we considered whether the adsorbed NO (NO*) could react with the lattice oxygen (the Mars−van Krevelen path). It was found that the reaction NO* + O3c → NO2* requires a barrier as high as 1.81 eV, and the desorption energy of NO2 is very high (1.96 eV), which is too high for the occurrence of the reaction at room temperature, indicating that NO oxidation cannot proceed via the MvK path on the Cr2O3(001) surface. Second, we resorted to the coadsorption and reaction of NO and O2 on Cr2O3(001) via the Langmuir−Hinshelwood mechanism (LH), in which an important issue is to determine the adsorption manner of O2 and to identify whether the adsorbed O2 could dissociate into O atoms that then oxidize NO. As illustrated in Figure 2c,d, O2 can molecularly adsorb on the unsaturated Cr3c with the O−O bond parallel to the surface (Figure 2d, π-Cr3c-adsorption) or alternatively in a tilted manner (Figure 2c, σ-Cr3c-adsorption), giving adsorption energies of −0.90 and −0.95 eV, respectively, which show that O2 can bind more strongly than an NO molecule on the Cr2O3(001) surface. Moreover, the reactant O2 is typically richer than NO under the practical condition of NO oxidation, and therefore it can be expected that O2 could occupy the Cr3c

values. As shown in Table 1, in comparison with the experimental value for each term, a proper value of Ueff = 3.0 eV can be obtained on comprehensive consideration, which basically gives a minor error for all of the terms, especially the two chemical properties. For example, the calculated formation energy of Cr2O3 and the NO adsorption energy on the Cr2O3(001) surface are −0.38 and −0.79 eV at Ueff = 3.0 eV, respectively, being close to the respective experimental values of −0.29 and −0.80 eV. In addition, the identified Ueff value here is close to the findings of Shi et al.50 and Mosey et al.51 Therefore, the parameter of Ueff = 3.0 eV for Cr2O3 is utilized hereinafter. The adsorption energy, Ead, of an adsorbate on the surface is calculated as Ead = Esurf/adsorbate − Esurf − Eadsorbate, where Esurf/adsorbate and Esurf are the total energies of the surface slab with and without the adsorbate, respectively, and Eadsorbate is the energy of the NO or other small molecules under vacuum. On the basis of the above definition, a negative Ead indicates favorable (exothermic) adsorption. Notably, the Gibbs free energy change (ΔG) of each elementary step was estimated according to ΔG = ΔH − TΔS; the reaction enthalpy ΔH is approximated with the total energy difference (ΔE) including the zero-point energy correction (ΔZPE) and neglecting the small heat capacity correction and Δ(pV) term. Regarding the entropy effect term (TΔS), it is usually trivial for the surface reactions with no adsorption/desorption involved, due to the entropies of the surface species typically being small and also largely canceling between the initial state and the transition state or the final state. For the adsorption/desorption processes, the large entropy contributions of gaseous molecules (TΔS), including the vibrational, rotational, and translational entropies, have to be considered to estimate ΔG at a given temperature.52−55 5417

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surface comprise six consecutive elementary steps: (i) the coadsorption of NO and O2, NO + O2* → NO* + O2*, (ii) the reaction between the adsorbed NO* and O2*, NO* + O2* → NO2* + O*, (iii) the release of the first NO2, NO2* + O* → NO2 + O*, (iv) the coadsorption of O atom with another NO, NO + O* → NO* + O*, (v) the formation of the second NO2, NO* + O* → NO2* + *, and (vi) the desorption of the second NO2, NO2* → NO2 + *, completing the catalytic cycle. It was worth noting that, with respect to the initial coadsorption of NO and O2 molecules, they could adsorb at two adjacent Cr3c sites or alternatively at the same single-atom Cr3c site, owing to the high unsaturation of three-coordinated Cr3c. These two adsorption configurations lead to two different reaction routes, named LH-1 and LH-2, respectively, and Figure 3 illustrates their energy profiles (blue and green curves, respectively). For the LH-1 path, the adsorption energy of NO at Cr3c with O2* sitting at the nearest-neighboring Cr3c is −0.78 eV (see IS1 in Figure 3), and the as-formed coadsorption species can overcome a low barrier of 0.34 eV to form NO2* and atomic O* (see FS1 in Figure 3), which is exothermic by −2.21 eV. The optimized transition state structure (TS1) is shown in Figure 3, and one can see that the forming N−O bond is 2.287 Å and the O−O bond is elongated to 1.338 Å from 1.305 Å. Afterward, NO2* could desorb from Cr3c but is limited by a large energy requirement, as high as 1.30 eV. By comparison, the LH-2 path starting with the NO and O2 coadsorption at a single Cr3c atom seems more favorable. First, as NO coadsorbs with O2* at the same Cr3c, they would spontaneously transform to a four-membered-ring ON*OO* species (see IS2 in Figure 3) without an apparent barrier; more importantly, this process is strongly exothermic by −1.19 eV, indicating the greater stability of IS2 in comparison with IS1 (see Figure 3). Second, the ON*OO* intermediate can readily break its O−O bond, resulting in the coadsorbed NO2* and O*, which is slightly exothermic by −0.08 eV and has a low barrier of 0.39 eV (see TS2 in Figure 3 for the transition state, showing an elongated O−O bond of 1.825 Å). Moreover, the release of NO2*

site preferentially. It should be noted that the O2 dissociation is difficult from both of the two adsorption configurations, with the dissociation barriers being above 1.53 eV in which the distance of the O−O bond is activated to 1.90 Å at the TS, implying the infeasibility of obtaining O atoms via oxygen dissociation at room temperature. In other words, the reactant O2 has to participate in the subsequent reactions in the molecular state. As depicted in Scheme 1, the proposed reaction paths for NO oxidation via NO and O2 coadsorption on the Cr2O3(001) Scheme 1. Elementary Steps of NO Oxidation Based on the Langmuir−Hinshelwood Path (Blue for LH-1, Coadsorption of NO and O2 at Adjacent Reaction Sites, and Green for LH2, Cobinding at the Single Cr Atom) and the Eley−Rideal Path on the Cr2O3(001) Surface (Red)

Figure 3. Energy profiles of NO oxidation on Cr2O3(001) from three pathways: LH-1 (blue), LH-2 (green), and ER route (red). The solid and dashed lines represent the total energy profiles including the ZPE corrections and the Gibbs free energy profiles (including mainly the entropy contribution of gaseous molecules) at 300 K, respectively. The optimized structures of some important intermediates and transition states of each path are illustrated, and the key structural parameters are labeled. All length values are given in Å. 5418

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conversion (IS3 → FS3 versus IS2 → FS2). Moreover, another merit of the ER pathway at this stage is that NO2 can be directly released to the gas phase instead of the indirect energyconsuming NO2* desorption process in the LH-2 path. In other words, we can speculate that the ER path could be a preferable route for the stage (NO + O2 + * → NO2 + O*, i.e. the formation process of the first NO2) in NO oxidation. In addition, it was found that the gaseous NO can follow the ER route to readily couple with the left atomic O* from the above process, forming the second NO2* (a monodentate-ONO configuration; see Figure 2i) exothermically by −1.11 eV. Similar to the case of the surface O2* with ER path, this is also a barrierless process, which is confirmed by scanning the potential energy profile of NO2* formation (see red line in Figure 4). Furthermore, a natural population analysis has also been performed using the Periodic NBO software56 (see details in Tables S1 and S2) to understand these two barrierless processes of NO coupling with O* or O2*. It was found that either the surface O* or O2* on Cr2O3(001) carries an unpaired electron, being the radical-like species, which could readily couple with the open-shell NO molecule. In other words, it could be the radical-like chemistry that accounts for the origin of two such barrierless processes. To better understand the overall reaction behavior at a realistic temperature (T = 300 K), the entropy effects of the reactant NO/O2/NO2 molecule (TΔSNO, TΔSO2, and TΔSNO2 are 0.66, 0.64, and 0.75 eV, respectively) have been taken into consideration to estimate the free energy profile (see details in Calculation Method and Model). From the obtained free energy profiles (see dashed line in Figure 3), one can see that the whole free energy is downhill for the formation of the first gaseous NO2 and the second NO2* intermediate in the ER path, indicating that it is an easy process on the Cr2O3(001) surface. However, the adsorbed monodentate NO2* is strongly bound to the Cr3c site (−1.56 eV), resulting in its difficult removal and eventually limiting the overall activity of Cr2O3(001) toward NO oxidation at room temperature. According to the above discussions, an optimal pathway of NO oxidation on Cr2O3(001) can be summarized as follows. First, the NO reacts with the surface-adsorbed O2* from the gas phase (ER path), leading to the formation of cis-ONOO* species without an evident barrier. Second, the cleavage of the cis-ONO−O* bond results in O* formation and the first NO2 desorbing into the gas phase, with a dissociation barrier of 0.08 eV. Third, NO couples with the adsorbed atomic O* at the isolated Cr3c site spontaneously, resulting in the formation of the second NO2*. Finally, the NO2* desorbs, completing the catalytic cycle. The Cr2O3(001) surface can efficiently catalyze the first three steps, but the adsorbed NO2* binds with the highly unsaturated Cr3c sites so strongly that it becomes a poisoning species and limits the overall activity of Cr2O3(001) for NO oxidation at room temperature. 3.2. NO Oxidation on the Cr 2 O 3(012) Surface. Considering that the (012) facet is another typically exposed surface,57 we further investigated the NO oxidation on the Cr2O3(012) surface to explore the structural effect on the activity of Cr2O3. As depicted in Figure 1d, Cr2O3(012) exposes the three-coordinated lattice O3c and five-coordinated Cr5c, which constitute basic active sites; the adjacent Cr5c−Cr5c distance on Cr2O3(012) is 3.559 Å, being relatively shorter than that of Cr3c−Cr3c (5.051 Å) on Cr2O3(001). By viewing the coordination environment of the surface Cr site, one could infer that the NO2 adsorption could decrease at Cr5c, possibly

becomes easier at an energy release of 0.42 eV owing to the competing effect of atomic O* (see FS2 in Figure 3), in comparison to the strongly endothermic progress via the LH-1 path (1.30 eV). By comparing the energy profiles of such two LH pathways for NO oxidation (NO* + O2* → NO2 + O*; see Figure 3), one can clearly see that LH-2 is more favorable energetically. However, neither LH path can remove the left O* through reacting with NO. On one hand, O* cannot react with another NO* via the LH-1 path as a result of the long distance between the adjacent Cr cations; on the other hand, it is not plausible that NO cobinds with O* at the same Cr3c to follow the LH-2 path owning to the strong binding of O* (−1.56 eV). Therefore, an alternative pathway in which NO directly couples with the surface O*, as well as the O2*, from the gas phase (Eley−Rideal, ER path) is examined below. As illustrated in Scheme 1 (see the red line), the gaseous NO can spontaneously couple with the surface-adsorbed O2* to form a new monodentate cis-ONOO* configuration (see IS3 in Figure 3) when the NO molecule is arbitrarily put around the O2* at a distance of ∼3.0 Å or longer; the whole process is exothermic by −1.29 eV. To confirm this barrierless process, we examined the energy profile of ONOO* formation by putting the gaseous NO at a series of heights from the adsorbed O2*. As depicted in Figure 4 (blue line), one can see that the

Figure 4. Illustration of NO chemisorption processes on molecular (blue) and atomic (red) oxygen adsorbed on Cr2O3(001). The zero point (dashed) is aligned to the system energy when the NO molecule is in the gas phase.

energy monotonously declines as the ON···OO bond distance decreases, implying that there is indeed no total energy barrier. Then, the intermediate cis-ONOO* can readily break its ONO−O bond and release the NO2 into the gas phase, with a rather low barrier of 0.08 eV and a reaction energy change of −0.40 eV. The corresponding transition state (TS3) is shown in Figure 3, in which the ONO−O bond increases to 1.711 Å from the original 1.442 Å. These results demonstrate that the ER path could exist in the system. Specifically, comparing the ER path with the LH-2 path from the energy profiles in Figure 3, one can see that the identified cis-ONOO* species (IS3) is 0.10 eV more stable than the four-membered-ring species (IS2) in the LH-2 path, and the cleavage barrier of the ONO−O bond in cis-ONOO* is lower than the latter barrier (0.08 eV vs 0.39 eV), which results in a smaller effective barrier for ONOO 5419

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Figure 5. Optimized adsorption structures of NO (a), molecular O2 (b, e, f), dissociated O2 (g), and NO2 (c, d, h) on the Cr2O3(012) surface. The value at the top right of each configuration is the corresponding adsorption energy. All of the energies are given in eV.

Figure 6. Reaction profile of NO oxidation by O2 on the Cr2O3(012) surface, following the Langmiur−Hinshelwood mechanism. The solid and dashed lines represent the total energy profiles including the ZPE corrections and the Gibbs free energy profiles (including mainly the entropy contribution of gaseous molecules) at 300 K, respectively. The optimized structures of some key intermediates and transition states of each path are illustrated, with the important bond length information. All of the values are given in Å.

revealing that the NO oxidation also cannot proceed via the MvK route on the (012) surface. With respect to the adsorption of an O2 molecule, it is found that O2 would preferably adsorb with a double σ-bond on Cr2O3(012) (Figure 5f) at an adsorption energy of −0.65 eV, in comparison with the other two adsorption configurations, the single σ-bond (Figure 5b, −0.41 eV) and π-bond (Figure 5e, −0.31 eV), on the (001) surface. In addition, the dissociation of a double σbond adsorbed O2 into O atoms is exothermic (−0.58 eV) but gives a barrier as high as 1.31 eV, which implies that the dissociation is difficult at ambient temperature. In light of the fact that the NO binding energy at one Cr5c site is better than the energy of O2 occupying two Cr5c sites (−0.75 vs −0.65 eV), it is worth noting that in reality O2 could exist in minority and prefer to be with the single σ-bond configuration to participate in the catalytic oxidation, owing to the competitive adsorption effects between NO* and O2*. At the adjacent Cr5c site around NO*, O2 can weakly coadsorb with an adsorption energy of −0.41 eV, named the IS1 state (see Figure 6); IS1 can quickly transform into a more stable ON*OO* species (IS1′) without an apparent barrier.

giving rise to an improved activity for NO oxidation in comparison with Cr2O3(001). As shown in Figure 5, Cr2O3(012) indeed has a weaker adsorption ability on NO2 as expected, due to the increased coordination number of the surface Cr5c site. The most stable configuration of NO2* on the (012) surface is μ(N,O) nitrito (Figure 5h) with an adsorption energy of −1.07 eV, which is much weaker than that on the (001) surface (−1.56 eV). Especially, for the monodentate-ONO, which is the most stable case on the (001) surface, the corresponding binding energy on the (012) surface is only −0.77 eV (Figure 5d). Thus, it is reasonable to suggest that NO2 adsorption might not be the limiting factor on the (012) surface. To evaluate the overall activity of Cr2O3(012), the adsorption of reactants NO and O2 and their possible interactions on Cr2O3(012) was investigated. It is found that NO tends to adsorb at Cr5c at the N-end with an adsorption energy of −0.75 eV (vs −0.32 eV with the O-end; see Figure 5a). Then we explored the MvK path, and the formation of ONO3c was identified to be endothermic by 0.98 eV. Moreover, the desorption of ONO3c requires an energy as high as 2.47 eV, 5420

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ACS Catalysis Subsequently, the formed ON*OO* can break its O−O bond by overcoming a barrier of 0.69 eV, yielding the intermediates ON*O and O*, which corresponds to a reaction enthalpy of −0.59 eV. The ON*O can readily desorb at an energy cost of 0.54 eV and form the first NO2 molecule. With respect to the remaining O* atom, it can readily react with another NO* adsorbed nearby to form the μ-(N,O)-nitrito NO2* (see Figure 5h) because of the short Cr−Cr distance on the (012) surface. This progress is almost spontaneous; the barrier is rather low (0.10 eV, the forming O−N bond is 2.069 Å at the TS2), and the reaction is greatly exothermic by −0.76 eV. Finally, the second NO2 desorbs into the gas phase, completing the catalytic cycle. The free energy profile in Figure 6 reveals that the successive conversion from the coadsorbed NO* and O2* into two NO2 molecules can proceed smoothly without high barriers. However, O2 adsorption is very weak (+0.23 eV for the adsorption free energy (ΔGads) including the entropy effect at room temperature), especially due to the competitive effect resulting from the stronger adsorption of NO (ΔGads= −0.09 eV) to repel O2* occupation on Cr2O3(012). Accordingly, a low coverage of O2* can be expected, which would lead to a limitation of the overall activity of Cr2O3(012). From the free energy profile (see dashed lines in Figure 6), one could further ascertain that the O2 activation would be the rate-determining step in the whole progress of NO oxidation on the Cr2O3(012) surface. In addition, it is worth mentioning that, due to the fact that only a small amount of O2 could adsorb on the Cr2O3(012) surface, the subsequent Eley−Rideal process through the gaseous NO random collision with the surface O2* can hardly be expected to occur. In addition, on the (012) surface, it is unlikely that NO and O2 coadsorb at a single Cr5c atom, which rules out the possibility of the LH-2 process as occurs on the (001) surface. Now, we are in a position to comparatively discuss the NO oxidation activities on the (001) and (012) surfaces. In brief, the (001) surface exposing the isolated Cr3c site can efficiently adsorb and activate the reactant O2, and NO can favorably attack O2* and trigger subsequent reactions through the ER path. However, the overall reaction would be depressed by the strongly adsorbed NO2* species on Cr3c. In contrast, as a result of the surface exposed Cr5c site, the (012) surface has a weaker adsorption ability, which could guarantee the efficient desorption of the formed NO2 species; however, in the LH path the reactant O2 binds too weakly at Cr5c and thus results in a low coverage, limiting the overall conversion rate of NO oxidation. Therefore, it is the structural coordination feature of Cr3c versus Cr5c that determines their different activity constraints. In addition, the different adsorption abilities of Cr3c and Cr5c sites can be rationalized from their energy levels of 3d electrons. As the projected density of states shows in Figure 7, the Cr 3d electrons dominate the valence electron maximum (VBM) for both the (001) and (012) surfaces, and the 3d electrons of Cr3c on (001) are more localized near the Fermi level (and thus a higher reactivity) in comparison with that of Cr5c on (012), verified by higher d-band centers of −1.23 and −2.20 eV for the 3d majority electrons, respectively. 3.3. Synergetic Mechanism between Cr2O3 and CrO3. On the basis of the above results, to boost the catalytic activity of Cr2O3 for low-temperature NO oxidation, one possible strategy is to increase the surface O2 coverage on Cr2O3(012) or to weaken the binding strength of NO2 on the Cr2O3(001)

Figure 7. Projected density of states (DOS) for the Cr 3d electrons on the Cr2O3(001) (blue) and Cr2O3(012) surfaces (red) with respect to the Fermi levels.

surface. It is worth noting that, under realistic conditions of NO oxidation, the presence of the excess oxygen could potentially oxidize Cr2O3 into CrO3 species thermodynamically. In addition, some evidence that the Cr6+ species could promote or even dominate the overall activity of Cr2O3 catalysts has been given.22,23,28−30 Here, we computationally explore the possible synergetic mechanism between Cr2O3(012) and CrO3, aiming to enhance the O2 adsorption on Cr2O3(012) and thus improve the whole catalytic performance. In order to obtain a quantitative estimation of the presence of CrO3, a simple phase diagram was calculated by considering the thermodynamic equilibrium between the two states, Cr2O3 and CrO3. The crystal structure of CrO3 is illustrated in Figure 8a, which is a stacked configuration of one-dimensional CrO3

Figure 8. (a) Bulk structure of CrO3 oxide, which shows a chainstacked configuration. (b) Proposed model structure of monochainCrO3/Cr2O3. (c) O2 adsorption on the interface site of Cr6+/Cr3+. Green, blue, and red (pink) indicate Cr6+, Cr3+, and O, respectively, while yellow denotes the adsorbed O2.

chains through weak van der Waals interactions.58,59 At a given temperature and O2 partial pressure, the Gibbs free energy change from the Cr2O3 to CrO3 can be written as 1 bulk bulk ΔG(T , pO ) = GCrO (T , pO ) − GCr O (T , pO2 ) 3 2 2 2 2 3 3 − μO (T , pO ) 2 (1) 4 2 bulk where Gbulk CrO3(T, pO2) and GCr2O3(T, pO2) are the Gibbs free energies for CrO3 and Cr2O3, respectively. μO2(T, pO2) is the chemical potential of gas-phase O2 at the given temperature T

5421

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ACS Catalysis and partial pressure pO2. The chemical potential μO2(T, pO2) was calculated on the basis of the thermodynamic equation pO μO (T , pO ) = μO (T , p°) + RT ln 2 2 2 2 p° pO = μO (0 K, p°) − TSO2(T , p°) + RT ln 2 2 p°

8c), which may provide an alternative pathway to supply O2 species on Cr2O3(012). In addition, at this interface site, the product NO2 would adsorb moderately with an adsorption energy of −1.09 eV, implying the relatively easy desorption once generated: i.e., no evident poisoning effect. Therefore, an enhanced activity for low-temperature NO oxidation catalyzed by Cr2O3(012) could be expected at the mc-CrO3/Cr2O3(012) interface. On the other hand, our model could also provide a possible explanation for the experimental XPS observation that Cr6+ could exist and improve the catalytic activity of Cr2O3. Additionally, such a supported monochain CrO3 itself may also facilitate the NO oxidation process as an individual site, a study of which is under way in our follow-up work.

(2)

where μO2(0 K, p°) was calculated from DFT including the zero-point energy and the entropies were taken from the experimental data.60 When ΔG(T, pO2) < 0, the oxidation formation is favored. Therefore, we can obtain the phase diagram of Cr2O3/CrO3, illustrated in Figure 9 (the black solid line). We can see that under the typical experimental condition (T = 300 K, pO2/p° = 0.2), the Cr2O3 species are not inclined to form CrO3 thermodynamically.

4. CONCLUSION In summary, the reaction mechanism of NO oxidation catalyzed by two common Cr2O3 facets, i.e. (001) and (012), at room temperature has been studied, aiming at shedding light on the catalytic ability of Cr2O3 and the inherent activitylimiting factors. It was found that the (001) termination with the highly unsaturated Cr3c exposed exhibits strong adsorption ability and the overall activity would be limited by the strong adsorption of NO2 species (poisoning effect); the whole reaction tends to follow the Eley−Rideal mechanism. In contrast, on the (012) termination which exposes the fivecoordinated Cr5c, the reactant O2 cannot be efficiently adsorbed and activated, implying the limited catalytic performance for NO oxidation, which is in good accordance with the experimental findings. To boost the catalytic activity of Cr2O3 catalyst, we proposed an interface structure by depositing a monochain CrO3 on Cr2O3(012), which could be self-evolved on Cr2O3(012) thermodynamically under the typical reaction conditions. Significantly, it has been found that the interface Cr6+@Cr3+ sites could show a much improved adsorption ability of O2 and can enhance the activity of Cr2O3(012). This study presents a systematic picture on the catalytic activities of Cr2O3 on NO oxidation at room temperature, and the provided insight could facilitate the design of more efficient Cr-based catalysts.

Figure 9. Phase diagram of Cr oxides against partial pressures of O2 (pO2) and temperatures T. The black line represents thermodynamic boundaries between Cr2O3 and CrO3, and the red line indicates that between Cr2O3 and mc-CrO3/Cr2O3. The dashed lines show the typical experimental conditions.

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ASSOCIATED CONTENT

S Supporting Information *

Nevertheless, owing to the weakly stacked configuration of CrO3, we further examined the possibility of a CrO3 monochain (maintaining the local structure of CrO359 without destroying the evident chemical bond) supported by Cr2O3(012) (denoted as mc-CrO3/Cr2O3(012)), which could be regarded as the initially evolved CrO3 species on Cr2O3. The optimized mc-CrO3/Cr2O3(012) interface structure is shown in Figure 8b. Thermodynamically, it is interestingly found that such an interface structure can be favorably formed under the practical O2-rich conditions (Cr2O3 + O2 → mc-CrO3/Cr2O3(012)), as shown by the phase diagram in Figure 9, implying that mcCrO3/Cr2O3(012) could be thermodynamically self-produced on Cr2O3(012). At the mc-CrO3/Cr2O3(012) interface, our calculations show that the reactant NO can readily couple with the terminal oxygen of CrO3 and leave a vacant Cr6+ site, which can, together with the original Cr5c on Cr2O3(012), constitute a new active site Cr6+@Cr3+. Significantly, this interface Cr6+@Cr3+ site can adsorb an O2 molecule with an improved adsorption energy of −0.99 eV (see the adsorption configuration in Figure

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.8b00081. Natural population analysis of the NO coupling with O* or O2* on Cr2O3001) surface and summary table of the optimized structures along with their total energy and the imaginary frequency for each TS (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*H.W.: e-mail, [email protected]; tel, +86-21-64253453. *P.H.: e-mail, [email protected]. ORCID

Haifeng Wang: 0000-0002-6138-5800 Peijun Hu: 0000-0002-6318-1051 Notes

The authors declare no competing financial interest. 5422

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ACKNOWLEDGMENTS This project was supported by the National Natural Science Foundation of China (21333003, 21622305), the Young Elite Scientist Sponsorship Program by CAST (YESS20150131), the Shanghai Shuguang scholar program (17SG30), and the Fundamental Research Funds for the Central Universities (WJ1616007).

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