Insight into the Mechanism of Selective Catalytic Reduction of NOx by

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Insight into the Mechanism of Selective Catalytic Reduction of NOx by Propene over the Cu/Ti0.7Zr0.3O2 Catalyst by Fourier Transform Infrared Spectroscopy and Density Functional Theory Calculations Jie Liu,† Xinyong Li,*,†,‡ Qidong Zhao,† Ce Hao,† and Dongke Zhang*,‡ †

Key Laboratory of Industrial Ecology and Environmental Engineering and Key Laboratory of Fine Chemical, School of Environmental Sciences and Technology, Dalian University of Technology, Dalian 116024, People’s Republic of China ‡ Centre for Energy (M473), The University of Western Australia, 35 Stirling Highway, Crawley, Western Australia 6009, Australia S Supporting Information *

ABSTRACT: The mechanism of selective catalytic reduction of NOx by propene (C3H6-SCR) over the Cu/Ti0.7Zr0.3O2 catalyst was studied by in situ Fourier transform infrared (FTIR) spectroscopy and density functional theory (DFT) calculations. Especially, the formation and transformation of cyanide (−CN species) during the reaction was discussed. According to FTIR results, the excellent performance of the Cu/Ti0.7Zr0.3O2 catalyst in C3H6-SCR was attributed to the coexistence of two parallel pathways to produce N2 by the isocyanate (−NCO species) and −CN species intermediates. Besides the hydrolysis of the −NCO species, the reaction between the −CN species and nitrates and/or NO2 was also a crucial pathway for the NO reduction. On the basis of the DFT calculations on the energy of possible intermediates and transition states at the B3LYP/6-311 G (d, p) level of theory, the reaction channel of −CN species in the SCR reaction was identified and the role of −CN species as a crucial intermediate to generate N2 was also confirmed from the thermodynamics view. In combination of the FTIR and DFT results, a modified mechanism with two parallel pathways to produce N2 by the reaction of −NCO and −CN species over the Cu/Ti0.7Zr0.3O2 catalyst was proposed.

1. INTRODUCTION

On the basis of the mechanisms of HC-SCR proposed by different investigators over various catalysts, a consensus mechanism has been achieved, which is known as the “reduction mechanism”.5,14,15 In this mechanism, the generation of organic nitro compounds is the key step, which are produced by the reaction between nitrates and oxygenates rooting in the oxidation of NO and C3H6 in the presence of O2, and these organic nitro compounds are converted into N2, CO2, and H2O upon interaction with NO and O2. Among the various mechanisms proposed, −NCO species are widely accepted as an important organic nitro intermediate to yield N2 by the hydrolysis reaction or reaction with NO and/or O2 in the HC-SCR reaction.16,17 In contrast, there are some discrepancies about the reaction pathway of −CN species in different catalytic systems. Daturi and co-workers considered that −CN species on Ag/Al2O3 are stable against a further NO reduction reaction in the presence of O2.18 In the H2-assisted SCR reaction over the Ag/Al2O3 catalyst, Burch and co-workers found a sharp rise of the −NCO species signal but a rapid decrease of −CN species upon H2 introduction, and the

Owning to the high fuel efficiency and low emission of CO2, diesel engines have been widely considered as the future for transport and remote power generation.1,2 However, the abatement of NOx in lean-burn engine exhausts, which leads to the formation of acid rain and photochemical smog, remains a major challenge. As a potential alternative, the selective catalytic reduction of NOx by hydrocarbon (HC-SCR) has proven to be an effective and economical technique for the removal of NOx originating from mobile sources, which can eliminate NOx and unburnt hydrocarbons simultaneously.3−8 C3H6-SCR under lean-burn conditions has attracted a great deal of attention during the past few decades, although its current performance is not sufficient for commercial application, especially its low-temperature performance.6,7,9,10 By considering the efficacy and cost, copper-based catalysts have been explored widely to obtain more appropriate catalysts to elevate the low-temperature performance of the C3H6-SCR reaction.11,12 We have synthesized the Cu/Ti0.7Zr0.3O2 catalyst successfully in a recent work, which exhibited excellent lowtemperature SCR activity under lean-burn conditions compared to that of Cu/TiO2,13 while the mechanism for the activity promotion on this catalyst from the view of intermediates has not been clarified. © 2013 American Chemical Society

Received: Revised: Accepted: Published: 4528

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opposite manners of −NCO and −CN species were therefore considered to partially explain the large increase in activity when H2 is added.19 Furthermore, the role of −CN species as the precursor of −NCO species over Pt/Al2O3/MCM-41 has been studied by Jentys and co-workers.20 There are also some reports about the high activity of −CN species. Shimizu et al.21 considered −CN species as another important intermediate coexisting with −NCO species on the surface of Cu/Al2O3. Moreover, Captain and Amiridis22 regarded the −CN species as the sole intermediate to yield N2, while no −NCO species formed over Pt/SiO2. Up to date, the role of −CN species in the HC-SCR reaction is less clear than that of −NCO species, especially for the mixed oxide catalyst system, because of the complicated interactions between the components. For the multicomponent catalyst of Cu/Ti0.7Zr0.3O2, there are some uncertainties concerning the detailed generation and transformation mechanism of −CN species over the catalyst, and the investigation on the roles of −CN species in the SCR process should provide a better understanding of its excellent activity. Moreover, there are rare kinetic studies about the reaction channel of −CN species in the SCR reaction on the atomic level, which could make up the limitation of experimental information. In the present work, the formation and reaction pathways of the two potential intermediates (−NCO and −CN species) in the SCR process were clarified systematically by in situ Fourier transform infrared (FTIR) spectroscopy. The reaction channels of −CN species, including the free energy of intermediates and transition states, were analyzed specially by density functional theory (DFT) calculations, and the DFT results gave a deep insight into the roles of −CN species and its contribution to the excellent activity of the Cu/Ti0.7Zr0.3O2 catalyst. In addition, a modified reaction mechanism with two parallel pathways to produce N2 from −NCO and −CN species was proposed.

states were verified by intrinsic reaction coordinate (IRC) calculations, and the optimized transition-state structures were confirmed as saddle points on the potential energy surface as characterized by only one negative frequency. The thermal energy corrections of all species were calculated at 548 K.

3. RESULTS AND DISCUSSION 3.1. Activity of Various Catalysts. Panels a and b of Figure S1 of the Supporting Information show the NO conversion and NO conversion to both N2 and NO2 over various catalysts as a function of the reaction temperature ranging from 150 to 450 °C, respectively. The Cu/Ti0.7Zr0.3O2 sample exhibits an excellent low-temperature activity, reaching the highest NO conversion of 76.8% and N2 yield of 67.2% at 275 °C. By comparison, the Cu/TiO2 catalyst achieved the maximum NO conversion of 42.9% at 250 °C, corresponding to a 36.7% N2 yield, and NO was mainly oxidized to NO2 above 275 °C. The Cu component played a key role in the lowtemperature activity because the Ti0.7Zr0.3O2 support only achieved the maximum NO conversion of 39.3% at a much higher temperature of 400 °C, corresponding to a 16.1% conversion from NO to N2. NO2 was hardly generated over Zrcontaining catalysts, and thus, the Zr component was inferred to accelerate the reaction between NO2 and C3H6. The capacity of C3H6 oxidation during the SCR process decreased in the order: Cu/TiO2 > Cu/Ti0.7Zr0.3O2 > Ti0.7Zr0.3O2, as shown in Figure S1c of the Supporting Information. The oxygenated hydrocarbons deriving from partial oxidation of C3H6 have been proven as the important precursors for the formation of −NCO and/or −CN species, which are considered as the most important intermediates for HC-SCR.24,25 Furthermore, an appropriate catalyst should suppress the competitive pathway in which hydrocarbons react incipiently with the surplus oxygen.26 Hence, the unsatisfactory performance over Ti0.7Zr0.3O2 and Cu/TiO2 samples was attributed to the poor activity in C3H6 activation and the readily direct oxidation of C3H6, respectively, which led to the low utilization of C3H6 in the NO reduction process. 3.2. In Situ FTIR Analysis. 3.2.1. In Situ FTIR of NO + C3H6 + O2 Adsorption. Figure 1 shows the changes in the IR spectra during the exposure of the catalysts to a gas mixture containing 1000 ppm NO, 1000 ppm C3H6, and 2.5 vol % O2 in He as the balance in the temperature range of 150−350 °C. The surface of tested catalysts was covered by three kinds of nitrates: monodentate nitrates (1284 and 1499 cm−1), bidentate nitrates (1255, 1550, and 1582−1589 cm−1) and bridging nitrates (1255, 1594, and 1607 cm−1).27−29 The nitrates were formed by the adsorption of NO2 on basic oxygen sites provided by the solid catalysts, and the oxidation of NO to NO2 was a necessary step for the nitrate generation.4,30,31 The absence of characteristic bands assigned to NO2 may be caused by the quick reactions (i) with OH to form nitrates, (ii) with C3H6 and/or oxygenates to form organic intermediates, and (iii) with −CN species to form N2. C3H6 was mainly activated to be acetate [νs(COO−) around 1440 cm−1 and νas(COO−) around 1540 cm−1] and formate [νs(COO−) around 1352 cm−1 and δ(CH) around 1378 cm−1].20,32,33 Acetone [ν(CO) around 1668 cm−1]28 and unsaturated organic carboxylate compound [ν(CC) around 1628 cm−1]34 were detected over the Cu/ TiO2 sample. The intensity of nitrates and oxygenate species over Zr-containing samples was enhanced significantly, which should be attributed to the high capacity of adsorbing and activating NO and C3H6 caused by the Zr doping (see Figures

2. EXPERIMENTAL SECTION 2.1. Materials. TiO2 and Ti0.7Zr0.3O2 supports were synthesized using a urea precipitation method. The Cu/TiO2 and Cu/Ti0.7Zr0.3O2 catalysts were prepared using the wet impregnation method with a Cu(NO3)2 solution, and the Cu loading was 5 wt %. For a detailed procedure, see the Supporting Information. 2.2. Catalytic Performance Test. The C3H6-SCR reaction was carried out in a fixed-bed quartz tube reactor in the temperature range of 150−350 °C. For a detailed procedure, see the Supporting Information. 2.3. In Situ FTIR. In situ FTIR spectra of reactant adsorption on catalysts were recorded using a FTIR spectrometer (Bulker VERTEX 70-FTIR). Prior to each experiment, the sample (∼20 mg) was pretreated in He stream (30 mL/min) at 250 °C for 1 h, which was followed by cooling to the desired temperature, and then the spectrum was collected and used as the background. All spectra were recorded over accumulative 32 scans with a resolution of 4 cm−1 in the range of 4000−400 cm−1. The concentrations of NO, C3H6, and O2 (if presented) in the gas mixture were 1000 ppm, 1000 ppm, and 2.5 vol %, respectively, with He as the balance, and the flow rate of mixed reactions was 30 mL/min. 2.4. Computational Methods. The calculations were carried out using the Gaussian 09 program packages.23 All of the reactants, products, intermediates, and transition states were optimized using the hybrid density functional B3LYP method and the 6-311 G (d, p) basis set. All of the transition 4529

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the reaction to form −NCO species. On the other hand, different from the persistent consumption of nitrates with increasing the reaction temperature over Cu/TiO2 and Ti0.7Zr0.3O2, the characteristic band of bidentate nitrate centered at 1582 cm−1 over the Cu/Ti0.7Zr0.3O2 catalyst enhanced persistently and achieved the maximum intensity at 300 °C. Iglesias-Juez et al.30 have reported that the bidentate nitrate could easily react with C-containing intermediates to form potential intermediates for HC-SCR over the Ag/Al2O3 catalyst. Hence, it is proposed that the coexistent Cu and Zr components accelerated the transformation from monodentate and bridging nitrates to bidentate nitrates and, therefore, promoted the formation of −NCO species. Furthermore, the existence of −CN species may also facilitate the formation of −NCO species, because it has been reported that −CN species could transform into −NCO species by the reaction with O2 and/or NO2.35,38 Meanwhile, the −CN species has also been reported as an important intermediate to react with NO2 and O2 to form N2 quickly;36,39 thus, the main role of −CN species over Cu/Ti0.7Zr0.3O2 was still in dispute. 3.2.2. Generation of −CN Species. We further clarify the mechanism of −CN species formation. Panels a, b, and c of Figure 2 present the interaction of C3H6 with the adsorbed NOx species, the interaction of NOx with the adsorbed C3H6derived species, and the steady state of NO + C3H6 + O2 over Cu/Ti0.7Zr0.3O2 at 275 °C, respectively. According to Figure 2a, after the adsorption of NO + O2 for 40 min, the bidentate nitrate (1554 and 1577 cm−1), bridging nitrate (1608 cm−1), and NO+ (1852 and 1901 cm−1) were observed on the catalyst surface. During He purging, the bridging nitrate and the adsorbed NO+ species disappeared. Meanwhile, the intensity of the bidentate nitrate band at 1554 cm−1 was enhanced, which suggested that the bridging nitrate was unstable and tended to be converted into bidentate nitrate at 275 °C. After the addition of C3H6 and O2 mixture gas, the bands at 1554 and 1577 cm−1 shifted to the lower wavenumber of 1550 cm−1 and the higher wavenumber of 1580 cm−1, respectively, which suggested the reaction between nitrates and partial oxide hydrocarbons. In combination with the bands at 1275 and 1296 cm−1 attributed to the C−C stretching vibration,20 it is considered that the main adsorbed species on the catalyst surface changed to be acetate and formate species. Meanwhile, the band at 1636 cm−1 because of H2O as observed suggested the complete oxidation of C3H6.28 Furthermore, because no organic nitro intermediates were generated, neither the inorganic nitrates nor adsorbed NO+ was considered as the key factor for the formation of −NCO and −CN species. From Figure 2b, the adsorbed acetate (1442, 1540, and 1590 cm−1 37) and formate (1348 and 1376 cm−1) were observed to form on the catalyst surface in its exposure of C3H6 + O2. These oxygenates were stable because there was no obvious decrease in the intensity during He purging. When the gas was switched from C3H6 + O2 to NO + O2, NO+ (1852 and 1901 cm−1) appeared and the most remarkable change was the quick formation of −NCO (2204 and 2238 cm−1) and −CN (2153 cm−1) species. Meanwhile, the intensity of bands at 1348, 1376, 1440, 1540, and 1588 cm−1 decreased gradually. Hence, it was concluded that NO reacted rapidly with the surface-adsorbed acetate and formate species, which led to the formation and accumulation of −NCO and −CN species. Because no −NCO or −CN species was produced during the reaction between C3H6 and adsorbed nitrates, it was speculated that the oxygen-

Figure 1. IR spectra of surface species formed from the C3H6-SCR reaction over (a) Cu/TiO2, (b) Ti0.7Zr0.3O2, and (c) Cu/Ti0.7Zr0.3O2 at various temperatures.

S2 and S3 of the Supporting Information). In the range of 2300−1800 cm−1, the bands at 1845 and 1910 cm−1 are attributed to the adsorbed mononitrosyl (NO+).26 The characteristic bands of −NCO species at 2213 and 2240 cm−1 35,36 were detected over all of the samples. Moreover, an interesting phenomenon could be noted that the −CN species at 2147 cm−1 37 only appeared on the Cu/Ti0.7Zr0.3O2 sample, while it was not observed over either of the Cu/TiO2 and Ti0.7Zr0.3O2 samples. Hence, the −CN species obtained on Cu/ Ti0.7Zr0.3O2 should be induced by the coexistence of Cu and Zr components in the catalyst, which requires further investigation. Besides the unique emergence of −CN species on the catalyst, the excellent activity of Cu/Ti0.7Zr0.3O2 makes it reasonable to hypothesize that the formation of −CN species may contribute much to the promoted SCR performance. In comparison to Cu/TiO2 and Ti0.7Zr0.3O2 samples, the intensity of bands attributed to −NCO species over Cu/ Ti0.7Zr0.3O2 was enhanced distinctly. Because of the high concentration of nitrates and oxygenates adsorbed on the catalyst surface (see Figures S2 and S3 of the Supporting Information), more nitrates and oxygenates could take part in 4530

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Figure 2. IR spectral changes of surface species from transient C3H6-SCR over the Cu/Ti0.7Zr0.3O2 catalyst at 275 °C. (a) Pre-adsorbed nitrates react with propene and O2. (b) Pre-adsorbed propene-derived species react with NO and O2. (c) Full propene-SCR. (d) Time dependence of the integrated areas of the peaks of acetate, −CN, and −NCO species for the case in panel b.

Figure 3. IR spectra of surface species from the transient C3H6-SCR reaction over the Cu/Ti0.7Zr0.3O2 catalyst at 275 °C. (a) Changes of surface species after C3H6 removal from the gas mixture of NO + C3H6 + O2 and the corresponding time-domain IR spectra. (b) Changes of surface species after NO removal from the gas mixture of NO + C3H6 + O2 and the corresponding time-domain IR spectra.

to the lower generation rate of −CN species compared to their consumption. In contrast, the spectral intensity of −NCO species had no obvious change after its maximum was achieved. Figure 2c shows the real-time change of the IR spectra over Cu/Ti0.7Zr0.3O2 at 275 °C under the reaction condition of NO + C3H6 + O2. The band at 1570 cm−1 was attributed to the mixed bands of bidentate nitrate and acetate, and the deflection of this band from single bidentate or acetate might be caused by

activated hydrocarbons were essential for the formation of −NCO and −CN species and the activation of C3H6 should be the crucial and rate-limiting step for the formation of −NCO and −CN species. With prolonging the reaction time, −CN species reacted with NO + O2 to form N2 and/or −NCO species persistently and the corresponding intensity of the −CN species band was decreased gradually and disappeared finally. The disappearance of −CN species might be attributed 4531

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3255, and 3160 cm−1,41,43 indicating the formation of NH3 on the catalyst surface. Furthermore, the band at 1634 cm−1 was attributed to H2O generated by the reaction between C3H6 and O2.28 In the C3H6 + O2 system, the −NCO species disappeared rapidly, accompanied by the generation of NH3. In contrast, the −NCO species was stable in the absence of C3H6, as seen in Figure 3a, and no obvious production of H2O and NH3 was observed meanwhile. Hence, the consumption of the −NCO species could be mainly caused by the reaction with H2O, which was considered as an important step to generate N2. In addition, according to previous reports,18,44 the decrease in the intensity of −NCO and −CN species bands may also be attributed to their reactions with O2 to produce N2 and the reaction of the −NCO species with C3H6 to produce the −CN species may lead to its consumption.20 Figure S4 of the Supporting Information presents the change of −NCO and −CN species in the atmospheres of NO, C3H6, or O2 alone to clarify the role of the single component in the reaction with −NCO and −CN species. As shown by Figure S4a of the Supporting Information, there was no consumption but a slight accumulation observed for −CN species and the intensity of −NCO species had no change. Hence, the −CN and −NCO species should be both inert in the NO flow, and the reaction between NO and adsorbed oxygenates led to some accumulation of −CN species. When the gas was switched to O2 (see Figure S4b of the Supporting Information), a moderate reactivity of −CN and −NCO species was observed, which could be attributed to their reaction with O2 to generate N2. According to the quite distinct behavior of −CN species in the atmosphere of NO or O2 alone from that under the NO + O2 condition, −CN species should actually react with nitrates and/ or NO2 originating from the NO oxidation by O2 but not react with NO alone. Furthermore, the weaker oxidizing ability of O2 compared to that of nitrates and/or NO2 led to the lower activity of the reaction between −CN species and O2. The inertness of −NCO species under the condition of NO + O2 indicated that the reaction between −CN species and nitrates and/or NO2 was preferential in the NO-rich atmosphere and the reaction between −NCO species and O2 was the second pathway to produce N2. Besides, because the −NCO species only exhibited a moderate reactivity in the atmosphere of O2, it was suggested that the consumption of the −NCO species in the stream of C3H6 + O2 should be mainly caused by the hydrolysis reaction. In the atmosphere of C3H6 alone (see Figure S4c of the Supporting Information), the consumption of −CN species may be caused by its reaction with stored nitrates to yield N2, while the consumption of −NCO species may be due to its reaction with C3H6 to produce −CN species.20 3.2.4. DFT Study of CN + NO2 and CN + NO Reaction Channels. To further confirm the roles of −CN species in the SCR reaction, the reaction channels as well as the free energy of the possible intermediates and transition states in the reactions of −CN + NO2 and −CN + NO were calculated. The results on the Gibbs free energy profiles are displayed in Figure 4. The geometries of all intermediates (P1-IM1, P1-IM2, P1-IM3, P1IM4, P2-IM1, P3-IM1, P3-IM2, and P4-IM1) and transition states (P1-TS12, P1-TS23, P1-TS34, P1-TS4, P2-TS, P3-TS12, P3-TS2, and P4-TS12) can be found in Figures S5 and S6 of the Supporting Information. With considering both NO2 and nitrates could react with −CN species to generate N2 and NO2 and NO2 is a necessary precursor for the formation of nitrates, only the reaction channel of −CN + NO2 is given to simplify

the reaction between these two species. In comparison to the IR spectra of C3H6 + O2 co-adsorption (see Figure S3 of the Supporting Information), the intensity of formate and acetate strengthened greatly under the condition of NO + C3H6 + O2. Therefore, it was deduced that the partial oxidation of C3H6 occurred more readily in the presence of adsorbed nitrates and/ or NO2 than that in the O2 atmosphere alone and all O2 and nitrates and/or NO2 took part in the C3H6 oxidation. This observation is in good agreement with the reports that the prime role of the catalyst is to ensure preferential oxidation of the hydrocarbons by the nitrogen oxides (NO3−, NO+, and NO2) deriving from the oxidation of NO.26,35 In addition, because of the strong IR absorbance of −CN species existing on the catalyst surface, it was inferred that −CN species may play key roles in the promotion of SCR activity over Cu/ Ti0.7Zr0.3O2. The main roles and specific reaction channels of −CN species will be discussed below. Figure 2d presents the time dependence of the integrated areas of the peaks assigned to acetate, −CN, and −NCO species for the case in Figure 2b. Because the areas for formate could hardly be integrated accurately, the data are not presented, while a significant decrease in the intensity of format bands could be observed. The −NCO and −CN species accumulated, accompanied by the remarkable consumption of acetate and formate until the amount of −NCO and −CN species reached the maximum. After 20 min, the amount of −CN species decreased distinctly. Meanwhile, the rate of acetate consumption was much slower, and the change of −NCO species was minor. Therefore, NO + O2 should react with −CN species preferentially because of the much higher activity of −CN compared to acetate and formate, while their reaction with acetate and formate to form −CN species was inhibited. 3.2.3. Reaction Pathways of −NCO and −CN Species. To clarify the reaction pathways of −NCO and −CN species, the spectral changes caused by the removal of C3H6 or NO from the NO + C3H6 + O2 mixed gas are exhibited in Figure 3. The corresponding slight changes of the adsorbed species as a function of time are also given. Under the reaction condition of NO + C3H6 + O2 at 275 °C, the bands assigned to the −NCO species, −CN species, formate, and acetate were detected. When C3H6 was switched off (Figure 3a), the intensity of formate [νs(CH) around 2883 cm−1 and νas(COO) + δ(CH) around 2956 cm−1] and acetate [ν(CH) around 2933 and 2987 cm−1]28,37 decreased because of their reactions with NO. The rapid consumption of −CN species suggested its high activity in the mixed gas of NO + O2, while the −NCO species was rather stable in the atmosphere of NO + O2, because there was no evident change in its intensity. Similar results have been previously reported over the Pt/SiO2 catalyst.22 Bell et al.40 also reported that surface −CN species are reactive in O2 and NO2 environments to produce N2. When NO was removed (Figure 3b), the −NCO species vanished quickly. However, there was only a minor decrease in the intensity of the −CN species, indicating that the reactivity of the −CN species is much lower in the C3H6 + O2 atmosphere than in the NO + O2 atmosphere. In several previous studies,18,24,41,42 the hydrolysis of the −NCO species was considered as an important route to produce N2, in which the −NCO species reacted with H2O to form NH3 and NH3 further reacted with NO and/or NO2 to generate N2. In Figure 3b, the characteristic bands ascribed to N−H stretching vibration modes of coordinated NH3 were observed at 3356, 4532

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275 °C was not enough to overcome the high energy barrier of 66.2 kcal/mol. The DFT calculations provide sufficient and favorable supporting information for the role identification of the −CN species as an intermediate to react with nitrate and/or NO2. On the basis of the FTIR results and DFT calculations, a modified reaction mechanism of the C3H6-SCR reaction over the Cu/ Ti0.7Zr0.3O2 catalyst is proposed by Scheme 1. First, NO is Scheme 1. Proposed Mechanism of the C3H6-SCR Reaction over the Cu/Ti0.7Zr0.3O2 Catalyst

Figure 4. Gas-phase Gibbs free energy profile for the reaction of (a) CN + NO2 and (b) CN + NO at 275 °C.

oxidized by O2 to form nitrates and NO2, although the latter is not significantly observed in the present study. Meanwhile, C3H6 is activated to form formate and acetate, and this process is greatly promoted by the nitrates and/or NO2, while the direct combustion of C3H6 also occurs in the process. Then, the most important intermediates in the HC-SCR reaction, −NCO and −CN species, are formed by the reaction between nitrates and oxygenates. The formation of −CN species over the Cu/ Ti0.7Zr0.3O2 sample is induced by the coexistence of Cu and Zr components, and the C3H6 activation is considered as the prerequisite for the generation of −NCO and −CN species. Besides the effective generation of nitrates and/or NO2 as well as the activation of C3H6, the excellent SCR performance of the Cu/Ti0.7Zr0.3O2 catalyst is mainly attributed to the coexistence of two separate pathways to generate N2 from the −NCO and −CN species: (i) the hydrolysis of the −NCO species, in which NH3 forms in the reaction between the −NCO species and H2O and then further reacts with NO and/or NO2 and (ii) the reaction between the −CN species and nitrates and/or NO2. In addition, the reaction between O2 and the −NCO and/or −CN species is also a possible pathway to form N2. However, pathways i and ii are more likely or dominant because of the easy hydrolysis of −NCO species as well as the stronger oxidizing property of nitrates and/or NO2 compared to that of O2. Furthermore, the generation of the −NCO species from the −CN species in pathway ii is minor, because it is a noncompetitive reaction with respect to the N2 production from a kinetics viewpoint.

the calculations. In Figure 4a, P1 represents the channel when the N atom of −CN species attacks the N atom of NO2. In path P1, P1-IM2 could generate −NCO species and NO directly according to surmounting an energy barrier of 21.2 kcal/mol. On the other hand, P1-IM2 could be converted to N2 and CO2 by a multistep process described as P1-IM2 → P1-TS23 → P1IM3 → P1-TS34 → P1-IM4 → P1-TS4 → N2 + CO2, in which the highest energy barrier is 23.1 kcal/mol from P1-IM3 to P1TS34. Taking into account the much more stable products of N2 + CO2 (−202.7 kcal/mol) compared to −NCO + NO (−69.0 kcal/mol) and the similar highest energy barrier to overcome in the two channels, the channel to generate N2 and CO2 should be more potential. P2 represents the reaction pathway when the C atom of −CN species attacks the N atom of NO2. As presented in path P2, P2-IM1 should surmount a high barrier of 68.5 kcal/mol (P2-IM1 → P2-TS) to generate −NCO + NO, which makes it relatively shallow feasibility. In other words, from the view of thermodynamics, N2 and CO2 are the most possible products in the reaction between −CN species and NO2. The theoretical calculation confirms the role of −CN species as an important intermediate to produce N2, and the formation of −NCO species from −CN species is not a competitive reaction pathway in the present study. Figure 4b shows two possible reaction channels P3 and P4 in the reaction between −CN species and NO. P3 and P4 represent the channels that C and N atoms of −CN species attack the O atom of NO, respectively. The barriers are 53.5 and 66.2 kcal/ mol for P3-IM1 → P3-TS12 and P3-IM2 → P3-TS2, respectively, in path P3. Meanwhile, the barriers are 8.2, 38.7, and 66.2 kcal/mol for CN + NO → P4-IM1, P4-IM1 → P4TS12, and P3-IM2 → P3-TS2, respectively. In paths P3 and P4, the highest energy barriers are both 66.2 kcal/mol for P3-IM2 → P3-TS2, which also indicates the stability of intermediate P3IM2. Hence, from the thermodynamics view, the reason for the inertness of −CN species in the NO atmosphere, as presented in Figure 4a, is that the energy provided in the experiment at

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ASSOCIATED CONTENT

* Supporting Information S

Additional details concerning SCR activity over three catalysts, in situ FTIR spectra of NO + O2 adsorption, in situ FTIR spectra of C3H6 + O2 adsorption, changes in IR spectra of −NCO and −CN species in the NO, O 2 , and C 3 H 6 atmospheres, and the optimized geometries of reactants, products, intermediates, and transition states. This material is available free of charge via the Internet at http://pubs.acs.org. 4533

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-411-8470-8084 (X.L.); +61-8-6488-7600 (D.Z.). Fax: +86-411-8470-8084 (X.L.); +61-8-6488-7235 (D.Z.). E-mail: [email protected] (X.L.); dongke. [email protected] (D.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported financially by the National Basic Research Program of China (2011CB936002), the National Natural Science Foundation of China (NSFCRGC21061160495 and 51178076), the National High Technology Research and Development Program of China (863 Program) (2010AA064902), the Excellent Talents Program of Liaoning Provincial University (LR2010090), and the Key Laboratory of Industrial Ecology and Environmental Engineering, China Ministry of Education, and the Australia Research Council under the Linkage Project Scheme (LP0989368).

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