Nitrogen oxides (NO_x) generated by internal combustion engines cause many environmental issues. Currently, by virtue of the NO_x remediation techniques, such as the three-way catalytic conversion (TWC), NO_x storage/reduction (NSR) and selective catalytic reduction (SCR) [1-5], they can be largely removed under normal operating conditions (typically above 200 ℃). However, the corresponding low efficiency during cold starts remains a big problem. Particularly, in the large semi-closed space such as the road tunnels and indoor parking areas where cold starts occur frequently, tens of ppms of NO_x (NO usually accounting for 90%) could accumulate as a result of incomplete NO_x removal. Hence, the removal of low-content NO at room temperature serves as one of the important but challenging targets. Considering that NO₂ can be readily absorbed in water or base solutions, to locate the highly efficient catalyst and achieve the catalytic oxidization of NO into NO₂ at mild condition is a crucial solution.

Currently platinum-based catalysts [6-9] have been widely used for NO oxidation on relatively high temperature, and the high costs restrict their applications. Transition metal oxides (TMOs) have also attracted great attention as promising catalysts for NO oxidation, such as the La_1-xSr_xCoO₃, La_1-xSr_xMnO₃, and Mn-mullite (Sm, Gd)Mn₂O₅ [10-13]; however, they are still not satisfactory at the room temperature. The activated carbon, especially modified carbon [14] and activated carbon fibers [15], were reported to be active for NO oxidation at room temperature, but the activity diminished quickly because of the irreversible NO₂ adsorption. Thus, exploring non-precious and room-temperature catalysts for dilute NO oxidation and understanding the catalytic mechanism are of great importance.

Recently, Cr-based catalysts [16-24] have drawn increasing attention owing to their superior ability in the complete oxidation of NO at relatively mild condition. Cai et al. studied the effect of Cr on the NO oxidation over the ceria-zirconia solid solutions, proposing that the well dispersed CrO_x leads to much improved catalytic activity [20]. Liu et al. found that chromium oxides showed high NO to NO₂ conversions at ambient temperature, and suggested that the amorphous phase structure of the prepared CrO_x is critical in obtaining high performance and long-time stability for NO oxidation [22]. These attractive findings represent much progress, because Cr-based oxides are significantly less expensive and more abundant. However, several important issues on Cr-based catalysts remain to be addressed. Firstly, as a metal oxide with multivalence, it is still controversial for the real active component, resulting from various stoichiometric Cr-oxide phases could be formed during the fabrication process, including CrO₃, Cr₂O₃ and even CrO₂ phases. Wang et al. reported that CrO_x-ZrO₂ mixed oxide showed enhanced activity for the low-content NO oxidation at ambient-temperature, and suggested that the surface Cr⁶⁺ species is more favorable than Cr₂O₃ to oxidize NO [25]. Meanwhile, very recently Cai et al. observed the existence of the active CrO₂ phase [26]. Theoretically, our previous work [27] have calculated the catalytic activities of Cr₂O₃ and the mono-chain CrO₃ phase, finding that the Cr₂O₃(001) with the highly unsaturated Cr_3c exposed exhibits strong adsorption ability and the overall activity is limited by the strong adsorption of NO₂ species (poisoning effect), while on Cr₂O₃(012) exposing the fivecoordinated Cr_5c, the reactant O₂ cannot be efficiently adsorbed/ activated and also limits the catalytic ability for NO oxidation; moreover, the monochain-CrO₃, which could be thermodynamically self-produced on Cr₂O₃(012), can give an enhanced NO oxidation activity in comparison with the bare Cr₂O₃(012) surface. Nevertheless, to the best of our knowledge, the catalytic ability of CrO₂ phase for NO oxidation has been not explicitly known either experimentally or theoretically.

Secondly, the favorable reaction mechanism behind the roomtemperature NO oxidation on the CrO₂ is not yet clear. The usually exposed (110) surface of CrO₂ give two kinds of active sites, the coordinately unsaturated Cr_cus and the lattice oxygens, which can result in the reaction routes following the Langmuir-Hinshelwood mechanism occurring on Cr_cus sites alone or the Mars-van Krevelen (MvK) one with the lattice oxygen involved. Which one is preferred for the low-content NO oxidation at room temperature has been not understood. Moreover, the NO oxidation process possibly involves kinds of intermediate species, which thus constitute a complex reaction network. Overall, it is therefore highly desirable to systematically explore the possible elementary steps.

Inspired by these issues, in this work, the most exposed (110) surface of rutile-type CrO₂ was adopted to investigate the reaction mechanism of NO oxidation by combing the first-principles density functional theory (DFT) calculations and microkinetic analysis. The optimal reaction route and the rate-determining step for the low-content NO oxidation at room temperature were quantitatively figured out. We believed these findings would provide a molecular-level insight into the activity origin and advantages/shortcomings of CrO₂ oxide in catalyzing NO oxidation. All the calculations in this work were performed within the framework of density functional theory (DFT) using the Vienna ab initio simulation package (VASP) [28, 29]. Generalized gradient approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) functional [30] was employed to describe the exchange and correlation energy (see details in the Supporting information).

As illustrated in Fig. 1a, the CrO₂(110) surface (serving as the most stable surface of rutile-type CrO₂) is mainly terminated by alternating rows of two-coordinated bridge oxygen (O_bri) and fivecoordinated Cr_5c linked by the three-coordinated in-plane O_3c, in which the coordinatively unsaturated Cr_5c cations (the Lewis acid site) and O_bri anions are usually considered to be vital for the catalytic activity, providing the basic adsorption sites. Specifically, considering that the adjacent Cr_5c-Cr_5c distance on the (110) surface is 2.955 Å, which implies that the NO oxidation could proceed along the Cr_5c sites, we first calculated the adsorption of key species X (X = NO, NO₂ and O₂) on the Cr_5c site and examined the possibility following the Langmuir-Hinshelwood (L-H) mechanism to catalyze NO oxidation.

It was found that NO can relatively strongly adsorb on the Cr_5c site in a preferential tilted η¹-N configuration (Fig. 1b, E_ads = - 1.14 eV) in comparison with the upright one (Fig. 1c, E_ads = - 0.88 eV). For NO₂, on examining four possible η¹-N-nitro, η¹-O-nitro, μ₂-N, O-nitrito and μ₂-O, O'-nitrito adsorption configurations (Figs. 1d–g, respectively), the bidentate μ₂-O, O'-nitrito configuration is identified as the most stable one, giving an E_ads of -1.18 eV (versus -0.58/-0.76/-0.82 eV for the other threes (Figs. 1d-f, respectively). By contrast, O₂ can only moderately adsorb at the Cr_5c site in either σ-bond (Fig. 1h) or π-bond configuration (Fig. 1i), with the adsorption energy being -0.57 eV and -0.49 eV, respectively. Moreover, the dissociation of the adsorbed O₂^* into atomic O^* at the Cr_5c sites is endothermic by 0.72 eV for the more stable O₂^* in the σ-bond configuration, and the corresponding barrier is as high as 2.02 eV, implying the infeasibility of obtaining the atomic O^* via O₂ dissociation at room temperature. In other words, the reactant O₂ has to participate in the subsequent reactions in the molecular state. Additionally, in light of the fact that the adsorption of NO/NO₂ at the Cr_5c site is much superior to that of O₂ (-1.14 eV vs. -0.57/-0.49 eV), in reality O₂ could be expected to exist in minority at the Cr_5c sites, owing to the competitive adsorption effect between NO^* and O₂^*. Therefore, one may expect the overall activity of NO oxidation following the L-H mechanism is unsatisfactory.

To give a more quantitative examination on the catalytic activity, the specific reaction network of NO oxidation with the L-H mechanism was explored on CrO₂(110), as depicted in Scheme 1. At the adjacent Cr_5c site around NO^*, O₂ can weakly co-adsorb with an adsorption energy of -0.24 eV (Fig. S1a in Supporting information), which is lower than the O₂-only adsorption (-0.57 eV), hinting that the preadsorption of NO inhibits the O₂ adsorption. They can then easily couple with each other into a more stable ON^*OO^* species (Fig. S1c in Supporting information) with a low barrier of 0.11 eV (Fig. S1b for the corresponding transition state (TS) with the forming O-N bond being 2.463 Å). Subsequently, the formed ON^*OO^* (IS') can break its O-O bond at a barrier of 0.81 eV (Fig. S1d for the TS), yielding the intermediate μ₂-N, O-nitrito NO₂^* and atomic O^*, which corresponds to a reaction enthalpy of -0.03 eV. The NO₂^* can readily desorb at an energy cost of 0.51 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 second μ₂-N, O-nitrito NO₂^*, and this progress is almost spontaneous; the barrier is rather low (0.11 eV, see TS in Fig. S1h with the forming O-N bond of 2.000 Å), and the reaction is much exothermic by -1.22 eV. Finally, the second NO₂^* desorbs into the gas phase, completing the catalytic cycle.

As shown by Figs. S2 and S3 (Supporting information), the energy profile, as well as the estimated free energy profile with the entropy effects of the reactant NO/O₂/NO₂ molecule included (TΔ S_NO, TΔS_O2 and TΔS_NO2 are 0.66 eV, 0.64 eV and 0.75 eV at T = 300 K, respectively; see details in Supporting information), were calculated, which reveal that the successive conversion from the coadsorbed NO^* and O₂^* into two NO₂ molecules can proceed smoothly without high barriers. However, the O₂ adsorption is weak (the adsorption free energy (ΔG_ads) including the entropy effect at room temperature is + 0.40 eV), and would be hindered specially due to the competitive effect resulting from the stronger NO adsorption (ΔG_ads= -0.48 eV) on CrO₂(110). Accordingly, the low coverage of O₂^* can be expected, which is quantitatively estimated to be only 4.85 ×10^-12 from the microkinetic analysis later. Overall, the conversion rate of NO oxidation at the Cr_5c sites only following the L-H mechanism is limited (see further the microkinetic results).

Owing to that the Cr_5c site cannot activate O₂ and provide reactive oxygen species to oxidize NO, we resorted to the participation of the surface lattice O_bri in oxidizing NO, i.e., the Mars-van Krevelen (MvK) mechanism. As illustrated in Scheme 2a, it was found that NO^* can easily couple with O_bri without an apparent barrier (E_a = 0.01 eV) to form an ONO_bri (namely NO₂#, # denoting the O_vac site) configuration (Fig. S4a in Supporting information) and the whole process is exothermic by -1.03 eV; in the optimized TS (Fig. S4b in Supporting information), the ON-O_bri bond distance is 2.621 Å. This indicates that the O_bri could be an important oxidative species in promoting NO oxidation on CrO₂(110).

Besides, it is worth discussing that the gaseous NO could also follow the Eley-Rideal mechanism to directly couple with O_bri and form the ONO_bri species, being exothermic by 2.17 eV (path Ⅱ in Scheme 2a); even when the NO molecule is arbitrarily put around O_bri at a long distance of ~3.0 Å, it was found that NO gradually descends and binds with O_bri spontaneously upon a typical structural optimization, and thus this step could be a barrierless process in term of the total energy (without considering the entropy effect).

Afterwards, the formed ONO_bri (NO₂#) could desorb with an O_vac left, i.e., NO₂# → NO₂ + O_vac, corresponding to a reaction enthalpy of 1.17 eV. In consideration of the entropy effect of NO₂ molecule, one can anticipate that this desorption process could occur at room temperature with a desorption free energy of 0.42 eV. Alternatively, the release of ONO_bri via the nitrate route (path Ⅲ in Scheme 2a) was also considered but seems to be less favorable. Specifically, ONO_bri could couple with another O_bri nearby to form a bidentate NO₃## species with a barrier of 0.91 eV and a reaction enthalpy of -0.53 eV; then, this NO₃## could transform into a monodentate configuration endothermic by 0.20 eV, but the cleavage of the NO₂-O_bri bond to release NO₂ into the gas phase is so difficult with a rather high barrier (E_a = 1.70 eV) and a large energy cost (ΔH = 1.51 eV). Accordingly, the overall reaction following path Ⅲ has an effective barrier as high as 1.37 eV (= 1.70 + 0.20 - 0.53 eV; Fig. S5 in Supporting information), and therefore is expected to be less favored relative to the direct desorption.

As for the formed O_vac, O₂ could adsorb and trigger a series of possible reaction paths. It was found that O₂ prefer to moderately adsorb at the O_vac site with the O-O bond parallel to the surface at an adsorption energy of -0.72 eV (Fig. S4c in Supporting information), in comparison with the upright adsorption configuration (Fig. S4d in Supporting information, E_ads= -0.60 eV). Notably, either one of these two O₂# configurations is difficult to dissociate at the O_vac with the barrier being above 1.31 eV, implying the reactant O₂# also has to participate in the following reaction in the molecular state. Accordingly, three possible reaction pathways starting from O₂# species were considered as illustrated by Scheme 2b, including the coupling reactions of O₂# with NO^* (path A), with the gaseous NO (path B) and with NO₂# (path C), respectively.

In path A, NO first adsorbs at the nearest Cr_5c around the O₂# with an adsorption energy of -0.86 eV, forming the co-adsorption configuration (IS1 in Fig. 2), which can quickly couple with each other into the ON^*OO# species (IS1' in Fig. 2) without an evident barrier (E_a = 0.21 eV, and also being exothermic by 0.80 eV). Second, ON^*OO# can overcome a barrier of 0.68 eV to break its O-O bond into NO₂^* and refill the O_vac simultaneously, corresponding to an enthalpy change of -0.22 eV; the optimized transition state (TS1') is shown in Fig. 2, and one can see that the O-O bond is elongated to 1.917 Å from the initial 1.403 Å in ON^*OO#. Third, NO₂^* could desorb from Cr_5c at ΔH = 0.82 eV and recovers the catalyst. The calculated free energy profile is illustrated in Fig. 2 (black line), and one can see that path A can efficiently occur without a significantly high barrier, revealing the feasibility of this path.

By comparison, in path B, the gaseous NO could directly couple with O₂# to form ONOO# (IS2 in Fig. 2) without prior adsorption at the nearest Cr_5c site, i.e., following the Eley-Rideal route. In term of total energy, this is a barrierless process and exothermic by -1.56 eV. Then, ONOO# can break its O-O bond to directly release NO₂ into the gas phase at an energy cost of 0.50 eV. Comparing path B with A, which one is more favored depends on the quantitative kinetics when the overall reaction arrives at the steady state with the entropy effects included (see further the kinetic details).

Further, it is worth noting that O₂# could also couple with NO₂# to form a five-membered-ring intermediate O₂NO#O# (IS3' in Fig. 2), corresponding to the nitrate formation/conversion route (path C in Scheme 2b). It requires a barrier of 0.52 eV, and the corresponding transition state (TS3) is shown in Fig. 2. Subsequently, the O-O bond cleavage in this O₂NO#O# intermediate would refill the O_vac and lead to a monodentate NO₃# species (FS3 in Fig. 2) with a barrier of 0.59 eV (TS3' in Fig. 2 for the corresponding TS). From the free energy profile (red line in Fig. 2), these two above steps can proceed readily; in particular, in comparison with the path A/B in Fig. 2, one can see that NO₃# formation following path C (O₂# +NO₂# → O₂NO#O# (IS3') → NO₃# + O#) is even energetically more favored with a lower energy profile, implying its easy formation. However, the following NO₃# dissociation to release NO₂ has to surmount a large energy cost as high as 1.59 eV, limiting the overall activity. In other word, the O_bri is too reactive to bind with NO₂, resulting in the difficult removal of NO₂ at room temperature, and the nitrate NO₃# species could be gradually accumulated on CrO₂(110) surface.

In addition, it is worth noting that NO could also adsorb at the O_vac site as a competitive adsorption species (NO# in Fig. S4e in Supporting information), which has a larger adsorption energy of -0.93 eV in comparison with that of O₂ (-0.72 eV). Fortunately, it was found that NO# can couple with the neighboring O_bri to form NO₂#, which requires a low barrier of 0.25 eV and is exothermic by 0.85 eV. Therefore, NO# species may not result in serious poisoning effect to block the O_vac sites and inhibit O₂ adsorption.

To figure out the favorable reaction mechanism of NO oxidation on CrO₂(110) and quantitatively estimate the contribution/ poisoning effect of different reaction routes, the microkinetic analysis was carried out at room temperature (T = 300 K, P_NO = 5.0 × 10^-5 atm (50 ppm), and P_O2 = 0.20 atm) (see details in the Supporting Information), in which all the above possible elementary steps were considered and the whole reaction network arrives at the steady state. The rate (r_i) of each elementary step i were calculated in Table 1, and the surface coverage (θ(X)) of the intermediate species X on Cr_5c or the lattice O_bri sites (denoted as ^* and #, respectively) are shown in Table S1 (Supporting information). The following features can be obtained.

First, with respect to the L-H mechanism (R1-R8) occurring on the Cr_5c sites, NO can efficiently adsorb at Cr_5c with a large coverage (θ(NO^*) = 9.75×10^-1), while the formation rate of O₂^* is small (2.70×10^-4 s^-1), according well with the weak adsorption of O₂ and extremely low coverage of O₂^* (θ(O₂^*) = 4.85×10^-12) on Cr_5c. The corresponding contribution to NO₂^* formation is low. Thus, it quantitatively demonstrates that the L-H mechanism on Cr_5c site alone (without the lattice O_bri involved) contributes little to the activity of CrO₂ for low-content NO oxidation at room temperature.

Second, the participation of surface lattice O_bri is necessary to trigger the overall low-content NO oxidation reaction, and the O_bri prefers to react with NO^* to form NO₂# (R9) rather than couple with the gaseous NO via the ER mechanism (R10) (4.89 ×10^-1 vs. 2.33×10^-9, s^-1). For the release of the formed NO₂#, at the steady state it is more likely to desorb as NO₂ directly rather than to be converted through the nitrate route (4.89 ×10^-1 vs. -3.43 ×10^-17, s^-1), i.e., NO₂# first reacting with another O_bri into the nitrate species and then releasing NO₂ by breaking the O₂N-O# bond. Besides, regarding the conversion of O₂#, it also tends to readily couple with NO^* via the MvK path. Therefore, one could realize that either the lattice O_bri or O₂# can easily oxidize NO^*, which largely contributes the catalytic activity of CrO₂(110) for NO oxidation at room temperature. Moreover, the Cr_5c site and the lattice O_bri site can efficiently adsorb NO and O₂, respectively, guaranteeing the surface coverage of each species, and thus the synergetic role in between is critical for the overall activity of CrO₂(110). For example, NO can predominately adsorb on the Cr_5c sites (θ(NO^*) = 9.75×10^-1) over O₂^* (θ(O₂^*) = 4.85×10^-12. Also, the high coverage of NO^* eventually facilitates the O#/O₂# to react with NO^* instead of the gaseous NO with the ER route (4.89 ×10^-1 vs. 2.33×10^-9/ 7.85 ×10^-6, s^-1), despite that ER route is barrierless in the total energy as mentioned above.

Third, the coverage of the nirtrate NO₃# species is very high (θ(NO₃#) = 9.65×10^-1), which results in the serious blocking effect for the lattice O_vac site and deactivates the CrO₂(110) to some extent.

According to the above discussions, the favorable reaction pathways of low-content NO oxidation by CrO₂(110) at room temperature (Figs. S2 and S3) can be summarized as follows. First, following the Mars-van Krevelen mechanism, the adsorbed NO^* at Cr_5c couples with the surface lattice O_bri to form the ONO_bri species without an evident barrier, which desorbs as NO₂ and form the lattice vacancy O_vac. Second, O₂ adsorbs at O_vac and couples with NO^*, leading to the intermediate ON^*OO# spontaneously, which can break its O-O bond with a low barrier of 0.68 eV, yielding NO₂^* and refilling the O_vac. Third, the NO₂^* desorbs, completing the catalytic cycle. Fourth, the alternative coupling of O₂# (the O₂ adsorbed at O_vac) with NO₂# would generate the stable nitrate species, which could hardly decompose and results in the surface accumulation (θ(NO₃#) = 9.65 ×10^-1), constituting the crucial poisoning effect toward the surface lattice O_vac sites. Nevertheless, following these reaction steps, the overall rate (turnover frequency, TOF) of room-temperature NO oxidation into NO₂ on CrO₂(110) is calculated to be as high as 9.78 × 10^-1 s^-1. This quantitatively demonstrates the high activity of CrO₂ and that CrO₂ could be one of the real active sites of CrO_x-based catalysts. Interestingly, the very recent experimental result has uncovered the existence of CrO₂ in efficiently promoting NO oxidation [26].

Furthermore, the "degree of rate control" (X_{RC, i}, see details in Supporting Information) of each elementary step i was calculated in Table 1, aiming to quantitatively reveal the relative importance of each elementary step on affecting the overall catalytic activity. It can be found that the steps O₂# + NO^* ON^*OO# (R16) and NO₂# NO₂(g) + # (R11) have the largest X_RC of 0.486 and 0.493, respectively, indicating that decreasing either of their kinetic barriers would critically improve the overall activity of NO oxidation. Notably, the step O₂# + NO₂# O₂NO#O# (R20), which constitutes the formation of nirtrate NO₃# (the poisoning species) to a largest extent (Table 1 and Table S1), gives a large negative vale of -0.465, and implies that increasing its barrier would interestingly accelerate the overall NO oxidation via decreasing the NO₃# formation (and weakening the poisoning effect); accordingly, accelerating the decomposition of NO₃#, i.e. decreasing the barrier of the step NO₃# NO₂(g) + O# (R14), would be also significant in increasing TOF with a X_RC = 0.486.

In summary, the reaction mechanism of low-content NO oxidation catalyzed by CrO₂(110) surface at room temperature has been studied, by performing DFT calculations together with the microkinetic analysis, aiming to shed light on the catalytic ability of CrO₂ and the inherent activity-limiting factors. The main result can be summarized as follows:

(ⅰ) CrO₂(110) was revealed to have a potentially high activity in catalyzing NO oxidation at room temperature. Mechanically, the Langmuir-Hinshelwood path occurring at the Cr_5c site alone is less active, while the overall reaction preferentially proceeds following the Mars-van Krevelen mechanism with the lattice O_bri involved and the optimal reaction route is identified. Specifically, NO adsorbs at Cr_5c and couples with the lattice O_bri to form the ONO_bri species, which directly desorbs as NO₂ and leaves an O_vac; then, O₂ adsorbs at O_vac and couples with NO^* into ON^*OO# species, which can break its O-O bond with a low barrier to yield NO₂^* and refill O_vac; finally, the NO₂^* desorbs and completes the catalytic cycle.

(ⅱ) The nitrate NO₃# species, mainly generated from the reaction of O₂# with NO₂#, was found to hardly decompose and results in the serious surface accumulation, which constitutes the crucial poisoning effect and deactivates the CrO₂(110) catalyst. Originally, it indicates that the O_bri on CrO₂(110) is too reactive and bind NO₂ tightly, and eventually limits the overall activity.

(ⅲ) Kinetically, the four steps, i.e., the formation of ON^*OO#, NO₂# desorption, NO₃# formation and its decomposition, constitute the rate-limiting steps of NO oxidation network on CrO₂(110).

This study extensively explored, at the atomic level, the catalytic ability of CrO₂ for NO oxidation at room temperature, and the theoretical understanding may facilitate the further design of more active Cr-based catalyst.

Acknowledgments

This project was supported by the National Natural Science Foundation of China (NSFC, Nos. 21333003, 21622305), National Ten Thousand Talent Program for Young Top-notch Talents in China, The Shanghai Shuguang Scholar Program (No. 17SG30).

Appendix A. Supplementary data

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cclet.2018.12.018.