Impact of Soot on NOx Adsorption over Cu-Modified Hydrotalcite

Mar 1, 2017 - modified hydrotalcite-derived lean NOx trap catalyst in a NO + O2 atmosphere. ... an efficient NOx adsorption in the lean phase is criti...
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Impact of Soot on NOx Adsorption over Cu-Modified HydrotalciteDerived Lean NOx Trap Catalyst Bo Li, Chonglin Song,* Gang Lv, Ke Chen, and Xiaofeng Cao State Key Laboratory of Engines, Tianjin University, Tianjin 300072, People’s Republic of China ABSTRACT: The impact of soot on NOx adsorption was studied over a Cumodified hydrotalcite-derived lean NOx trap catalyst in a NO + O2 atmosphere. Powder X-ray diffraction, scanning electron microscopy, Raman scattering spectroscopy, and X-ray photoelectron spectroscopy were used to characterize the surface properties of the pure catalyst and the soot/catalyst mixture. The adsorbed NOx species on the samples were evaluated by in situ diffuse reflectance Fourier transform spectroscopy. The soot coverage decreases the available adsorption sites on the surface of the catalyst, and a portion of active oxygen species are consumed by the soot oxidation during He pretreatment process. The NOx adsorption on two catalyst samples simultaneously undergoes two routes: the “nitrite route” and the “nitrate route”. The “nitrite route” is more dominant than the “nitrate route”. During NOx adsorption, the soot oxidation weakens the NO oxidation to NO2, and the released CO2 competes with NOx on the adsorption sites. Moreover, the temperature-programmed desorption tests indicate that the presence of soot reduces the NOx storage capacity of the catalyst and shifts the NO desorption peak to the lower temperature range by 50 °C.

1. INTRODUCTION Lean NOx trap (LNT) catalysts are one of the most efficient after-treatment devices to purify NOx from light-duty diesel engines. With the help of LNT catalysts, diesel engines can meet the increasingly stringent emission regulations.1 An LNT catalyst must periodically run at lean phases when the NOx are adsorbed on the alkaline sites and at rich phases when the trapped NOx species are released and reduced to N2.2 Because an efficient NOx adsorption in the lean phase is critical for the NOx reduction performance and fuel economy in the subsequent rich phase,3 considerable attention has been given to the factors affecting NOx adsorption, such as catalyst components,4−6 catalyst preparation methods,4,7 exhaust gas composition,4,8 and reaction temperature.9,10 Besides, there are several studies related to NOx adsorption mechanisms. For example, Nova et al.11,12 and Frozatti et al.13 studied the NO adsorption over conventional Pt−Ba/Al2O3 LNT catalyst under an excess oxygen condition, and suggested two parallel routes for NO adsorption process: the “‘nitrite route”’ and the “nitrate route”. Olsson et al.14 proposed that the NO2 trapping process involves a three-step mechanism: (i) NO2 is weakly adsorbed on BaO as a BaO−NO2 species; (ii) this species then decomposes to BaO2; and (iii) NO and barium peroxide reacts with the gas-phase NO2 to give barium nitrate. The studies concerning NOx adsorption mechanisms have been widely performed in a NOx and O2 atmosphere, without considering the presence of soot. During application, soot is inevitably deposited on the LNT catalyst, and it can affect the NOx adsorption process.15,16 Millet et al.17 showed a slower NOx storage rate on the LNT catalyst when soot was oxidized by NO2. Krishna et al.18 and Klein et al.19 observed that soot © XXXX American Chemical Society

oxidation modified the structure of the LNT catalyst, and the induced catalyst aging decreased the NOx storage capacity (NSC). Sullivan et al.20 asserted that soot presence was detrimental to the performance of a NOx trap, because it offered another reaction route for the utilization of NO2, rather than the desired formation of Ba(NO3)2. Although these studies provided some evidence about the impact of soot on the NOx adsorption, this information is insufficient to fully understand the contribution of soot to NOx adsorption. Thus, a comprehensive study is necessary. Hydrotalcite-derived mixed oxides are a promising support for the LNT catalyst, as they combine the desirable features including higher low-temperature activity, better dispersion and improved stability of Ba surface compounds, and enhanced performance for sulfur poisoning resistance.21 Fornasari et al.22,23 and Jeong et al.24 demonstrated that the hydrotalcitebased LNT catalysts with different Mg/Al atomic ratios improved the performances of storage−reduction of NOx, in comparison to a Toyota-type Pt−Ba/Al2O3 catalyst at the low temperatures ≤250 °C and achieved a better resistance to deactivation by SO2. Palomares et al.25 also found that at low temperatures, the 1 wt % vanadium-modified cobalt hydrotalcite-derived catalyst showed a high activity in the presence of H2O and SO2. Recently, more attention has been focused on the simultaneous removal of NOx-soot on the hydrotalcitebased LNT catalysts. Wang et al.26 showed that the Cu/Comodified hydrotalcite derived catalysts exhibited large surface Received: October 24, 2016 Revised: February 17, 2017 Published: March 1, 2017 A

DOI: 10.1021/acs.langmuir.6b03877 Langmuir XXXX, XXX, XXX−XXX

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The objective of this pretreatment is 2-fold: (1) to completely clear the surface and activate the catalyst and (2) to simulate the rich condition of O2 deficiency that the LNT catalyst experiences.8 In addition, if H2, HC, and CO that are commonly present in the real diesel exhaust are involved in the pretreatment atmosphere, they will work together with the soot and mutually interact with the results. Thus, helium was employed for the pretreatment of the catalyst samples. After pretreatment in a helium atmosphere at 500 °C for 1 h, the catalyst samples were removed to analyze the initial state of catalyst surface before the upcoming lean cycle and used for NOx adsorption and desorption tests. For the TPD tests, the catalyst samples were compressed and sieved to 20−40 mesh catalyst particles. 2.2. Catalyst Characterization. Micrographs of the catalyst morphology were acquired from scanning electron microscopy (SEM) (Model XL-30TMP, Philips, Eindhoven, The Netherlands), coupled with an energy-dispersive X-ray (EDX) detector (AMETEK, USA). The accelerating voltage of the electron beam was 200 kV. The Raman spectra were recorded by a Renishaw 2000 Raman microscope system (London, U.K.), using a laser wavelength of λ0 = 633 nm (He−Ne laser) at 5 mW. Wavelength calibrations were performed with a silicon wafer using the first-order phonon band of Si at 520 cm−1. XPS spectra were recorded using a Kratos Axis Ultra DLD spectrometer (Manchester, U.K.), employing a monochromatic Al Kα X-ray source (hv = 1486.6 eV), hybrid (magnetic/electrostatic) optics, and a multichannel plate and delay line detector. To subtract the surface charging effect, the C 1s peak was fixed at a binding energy of 284.6 eV. The crystal structure of catalyst sample was determined by an Rigaku Model D/MAX-2500 diffractometer operating at 200 mA and 40 kV, using Cu Kα as the radiation source (λ = 0.1790 nm). Data were recorded for 2θ values from 5° to 80° with a scanning rate of 2°/ min. 2.3. Time-Resolved In Situ DRIFTS. Time-resolved in situ DRIFTS was used to investigate the NOx adsorption and desorption process on the catalyst surface. The DRIFTS spectra were collected on a FTIR spectrometer (Model V70, Bruker, Karlsruhe, Germany) equipped with a mercury cadmium telluride detector. Considering that KBr has very poor catalytic activity for soot oxidation in the temperature range of 100−400 °C and presents a negligible interference effect on NOx adsorption,32 the catalyst samples were mixed with the diluents of KBr to lower the surface blackness of the catalyst sample and improve the signal-to-noise ratio. The catalyst sample/KBr dilution ratio was 1:20. At high temperatures, it was found that no significant difference was observed for the final NOx adsorption species. However, all of the information on the intermediates during NOx adsorption cannot be obtained via the in situ DRIFTS analysis, because of the faster reaction rates. In this study, therefore, the temperature for the isothermal NOx adsorption tests was fixed at 240 °C, because the largest part of NOx emissions of diesel engines is emitted in the temperature range of 180−250 °C during the EuroTest cycle.33 In the NOx adsorption process, the NOx gases, containing 1500 ppm of NO and 10% O2 with Ar balance, were introduced into the in situ catalyst sample cell that had been purged by Ar at 240 °C for 30 min. The gas flow-rate was 50 mL/min. The spectra were recorded at 1 min intervals with a resolution of 2 cm−1 and an accumulation of 64 scans for 100 min. At the end of the NOx adsorption test, the feed gas was switched to an Ar steam to purge the catalyst sample for 30 min. The desorption tests for the adsorbed NOx species were conducted by in situ DRIFTS. The temperature of the in situ cell was increased from 240 °C to 650 °C at intervals of 50 °C. The spectra were recorded at each temperature interval for 30 min to obtain the steady-state DRIFTS data. An online mass spectrometer (Model Dycor LC-D200; AMETEK) was used to monitor the slipped NOx gases during the adsorption and desorption tests. 2.4. Temperature-Programmed Desorption (TPD). TPD tests were performed using a Quantachrome ChemBet Pulsar system (Boynton Beach, FL, USA). For each run, a 50-mg catalyst sample was placed in the center of a cylindrical quartz tube reactor (6 mm internal diameter). Before testing, the catalyst was saturated in a 1500 ppm of NO + 10% O2/Ar atmosphere at 240 °C with a gas flow rate of 50 mL/min. We have performed the NO saturation at 240 °C for 120

areas, basic characters, and improved redox properties, resulting in high performances in NOx storage and soot combustion. Li et al.27 discovered that the K-promoted hydrotalcite-based CoMgAlO catalysts decreased the activation energy of soot combustion from 207 kJ/mol to ∼160 kJ/mol and simultaneously achieved relatively larger nitrogen oxides storage capacity. We have developed a novel Cu-modified hydrotalcite-based LNT catalyst, which can expand the performance window to a lower temperature range by more than 70 °C, with respect to the Toyota-type Pt−Ba/Al2O3 LNT catalyst. In the present study, we address the impact of soot on NOx adsorption over this catalyst. The physicochemical characterizations for the catalyst sample are performed by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman scattering spectroscopy, and X-ray photoelectron spectroscopy (XPS). In a NO + O2 atmosphere, NOx-adsorbed species on the catalyst and soot/catalyst mixture are measured by in situ diffuse reflectance Fourier transform spectroscopy (DRIFTS) in an isothermal model. Moreover, the thermal stability of adsorbed NOx species and the NSC of the catalyst are studied by a temperature-programmed desorption (TPD) test with and without soot. From the results, the NOx adsorption mechanism is proposed, and the impact of soot on NOx adsorption process is discussed in detail.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The Cu-modified hydrotalcite-derived LNT catalyst was prepared using two steps. First, the Cu-modified hydrotalcite-like precursor (Cu0.6Mg2.4AlO) was prepared by a standard coprecipitation method, as described in the literature.22−28 The mixed aqueous solution of Cu(NO3)2·6H2O (Ar), Mg(NO3)2· 6H2O (Ar), and Al(NO3)3·9H2O (Ar) with the desired Cu/Mg/Al molar ratio were added dropwise into a beaker with deionized water under mechanical stirring at room temperature. At the same time, another aqueous solution containing Na2CO3 and NaOH (molar ratio = 1:1) was added dropwise to this beaker to maintain a constant pH of 10 ± 0.5. After continuously stirring for 2 h, the resulting slurry was aged at room temperature overnight, and then it was filtered and washed thoroughly with deionized water. The precipitate obtained was dried at 110 °C overnight and calcined at 500 °C in air for 5 h to obtain the mixed oxide support materials (CuO/MgO/Al2O3). Second, the LNT catalyst was synthesized by the widely used incipient wetness impregnation method using aqueous solutions of Pt(NH3)2(NO2)2 (5% Pt-containing solution, Heraeus) and barium acetate (99%) to impregnate the mixed oxides.29 To ensure a good dispersion and stability of the noble metal on the support, the mixed oxide support materials were first impregnated with the Pt-containing solution and then with the barium acetate solution. After impregnation, the catalyst was dried at 80 °C on a rotary evaporator and calcined at 500 °C for 5 h. The relative atomic weight loadings of Pt/BaO/(CuO−MgO−Al2O3) were designated as 1/20/75. A commercial Printex U soot (Degussa), with a specific surface area of 106 m2/g and a particle diameter of 25−50 nm, was used as a model substance for diesel soot particles.30 In reality, soot species covered on the LNT catalyst include not only the physically filtered soot particles but also the carbonaceous deposits generated during the reduction of NOx by HC and CO. These carbonaceous deposits mainly cover on the Pt sites or other surface active sites, and are in tight contact with the LNT catalyst surface.4 In addition, the results obtained in the tight contact mode are more reproducible than those in the loose contact mode.31 Thus, the soot and catalyst sample were mixed in a mortar, following a tight contact procedure,30,31 and the soot/catalyst ratio (w/w) was fixed at 1:9. For the pure catalyst, the soot added in the soot/catalyst mixture was replaced by quartz sand, following the same procedure. Before the NOx adsorption and desorption, the catalyst samples used were first pretreated in a helium atmosphere at 500 °C for 1 h. B

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Langmuir min and found that, after 80 min, the NO concentration continued to slip at a very steady level, indicating that the catalyst samples have been saturated by NOx. Thus, 80 min was used as the NO saturation period. After saturation, the catalyst sample was cooled to 100 °C, and the feed gas was switched to He (150 mL/min) for 30 min at a flow rate of 150 mL/min to remove the gases in the tubes and weakly adsorbed NOx on the catalyst sample surface. After finishing these steps, the temperature was increased from 100 °C to 650 °C with a heating rate of 5 °C/min. The effluent NOx gases were monitored by the online mass spectrometer (Model Dycor LC-D200, AMETEK).

modified hydrotalcite precursor presents the typical patterns of the layered double hydroxide structure (hydrotalcite, JCPDS File Card No. 22-0700) with diffraction peaks at 2θ = 11.27°, 22.28°, 34.46°, 38.60°, 45.54°, 46.53°, 59.98°, and 61.88°, which confirms the desired the layer-structured hydrotalcite precursor to be obtained.22−30 For the pure LNT catalyst derived from the Cu-modified hydrotalcite precursor, the diffraction peaks at 2θ = 42.91° and 62.30° are attributed to the magnesium oxides (MgO, JCPDS File Card No. 45-0946), while the barium oxides are observed at 2θ = 40.58° and 58.96° (BaO, JCPDS File Card No. 30-0143). The diffraction peaks at 2θ = 35.54°, 38.70°, and 48.71° represent the tenorite planes of the orthorhombic structure of copper oxides (CuO, JCPDS File No. 48-1548). Moreover, the diffraction peaks at 2θ = 19.43°, 23.89°, 27.71°, 34.07°, 34.58°, 39.45°, 42.97°, 44.8°, 46.76°, and 68.12° correspond to BaCO3 with crystalline witherite, which mainly comes from the decomposition of barium acetate during catalyst calcination, because the calcination temperature (500 °C) is far lower than the decomposition temperature of BaCO3 (>1000 °C).8 Figure 2 shows the SEM images and EDX spectra for the pure catalyst and soot/catalyst mixture that were pretreated in a helium atmosphere at 500 °C for 1 h. A clear laminar structure and the stacking of the layers are observed on the pure catalyst surface in Figure 2a, which are representative characteristics for hydrotalcite-derived compounds.22,23,30 For the soot/catalyst mixture, the laminar surface in Figure 2b is partly covered by soot particles with the size of 25−50 nm. The examination of the EDX data for random points 1 and 2 highlights that the elemental concentration (at. %) of Pt, Cu, Ba, and Mg decreases after the addition of soot to the catalyst sample, indicating that part of reactive basic sites on catalyst surface are masked by the introduced soot. This masking effect will block

3. RESULTS AND DISCUSSION 3.1. Characterization of Catalyst Samples. The XRD patterns of the as-prepared Cu-modified hydrotalcite precursor and the derived LNT catalyst are shown in Figure 1. The Cu-

Figure 1. XRD patterns of the as-prepared Cu-modified hydrotalcite and the derived LNT catalyst.

Figure 2. SEM images and EDX data of the catalyst samples prior to NOx adsorption: (a) pure catalyst and (b) soot/catalyst mixture. C

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Langmuir the approach of the NOx molecules to the absorption sites and hinder the NOx adsorption. Raman spectra for the pure catalyst and soot/catalyst mixture are shown in Figure 3. For the pure catalyst, the Raman peak at

Figure 3. Raman spectra of the catalyst samples prior to NOx adsorption: (a) pure catalyst and (b) soot/catalyst mixture.

289 cm−1 is assigned to the Ag modes of vibration for the O atoms in the CuO crystal, whereas the peaks at 334 and 618 cm−1 correspond to the Bg modes.34 The characteristic phonon frequencies of Cu2O are visible at 660, 218, and 154 cm−1.35 The band at 473 cm−1 is related to the breathing modes of O atoms around each metal cation, such as Mg2+ or Al3+ in the interlayer lattice,36 and the bands at 1048 cm−1 represent the spectra of surface BaO. 37 The band at 596−580 cm−1 corresponds to the oxygen vacancies in the metal oxide lattices.38 For the soot/catalyst mixture, the strong D and G peaks of soot are clear, whereas the aforementioned Raman signals for the pure catalyst are weak. This result is attributed to the soot coverage on the catalyst surface, as evidenced by the SEM measurements. At the same time, the decreased band intensity of breathing modes of O atoms at 473 cm−1 evidences that the O atoms around the metal cations in the catalyst interlayer lattice are consumed after the soot/catalyst mixture was pretreated in the helium atmosphere. To evaluate the soot effect, the height ratio of Raman peaks at 596 and 473 cm−1 (I 596 /I 473 ) was used to evaluate the changes in the concentration of oxygen vacancies.38 As listed in Table 1, the

Figure 4. XPS narrow spectra for (i) Cu 2p and (ii) O 1s for the catalyst sample prior to NOx adsorption: (a) pure catalyst and (b) soot/catalyst mixture.

the intense satellite peaks located in the range of 937.6−945.4 eV confirm the presence of more than two copper valence states. Through peak deconvolution and fitting process to the spectra peak of Cu 2p3/2, the Cu 2p3/2 peak can be fitted to two peaks, at 932.4 and 933.7 eV, corresponding to the Cu+ and Cu2+ ions, respectively. After a similar curve fitting processing was performed for the O 1s spectra, three discernible components of O species are displayed in Figure 4(ii). The peak with the lowest binding energy at 529.8 eV is associated with lattice oxygen (Olat) in the mixed metal oxides, whereas the peak located at 531.4 eV corresponds to adsorbed oxygen (Oads) species, such as O−, O2−, and O2−, which are mainly stored on the surface in the form of hydroxyl species and weakly bound oxygen.39 The amounts of surface copper and oxygen species were calculated from their relative peak areas and are shown in Figure 4. The ratios of Cu2+/Cu+ and Olat/ Oads obtained are listed in Table 1. The ratio of Olat/Oads for the soot/catalyst mixture (0.12) is slightly lower than that of the pure catalyst (0.18), indicating that the adsorbed oxygen (Oads) and/or lattice oxygen species (Olat) on the catalyst as the sources of activated oxygen species were consumed by soot after the pretreatment. Moreover, the soot/catalyst mixture exhibits a lower ratio Cu2+/Cu+ than the pure catalyst, suggesting that the soot presence changes the valence state of copper compounds; that is, part of the CuO species transform to Cu2O species after the pretreatment. A new peak with the binding energy at 535.1−533 eV, corresponding to oxygen that is double-bonded to carbon (CO),40 is observed on the soot/catalyst mixture. The appearance of this new peak further supports that soot oxidation occurs during pretreatment and consumes the surface active oxygen species on the catalyst. 3.2. NOx Adsorption. 3.2.1. NOx Adsorption on the Pure Catalyst. Figure 5 shows the in situ DRIFTS spectra of NOx adsorption on the pure catalyst in the 1500 ppm of NO + 10% O2/Ar atmosphere. Adsorbed nitro-species are gradually

Table 1. Surface Elements and Chemical States of the Catalyst Samples Prior to NOx Adsorption Raman Spectra

XPS Spectra

catalyst

I596/I473

Cu2+/Cu+

Olat/Oads

pure catalyst soot/catalyst

1.10 1.16

2.46 1.53

0.18 0.12

soot/catalyst mixture possesses a larger ratio of I596/I473 than the pure catalyst, indicating that the presence of soot increases the concentration of the oxygen vacancies on catalyst surface after the helium pretreatment. Hence, it is speculated that the soot consumes the O atoms in the catalyst interlayer lattice during the pretreatment process and creates more oxygen vacancies. Considering that the Raman peak of CuO at 334 cm−1 and Cu2O at 218 cm−1 disappear for the soot/catalyst mixture, the soot presence concurrently changes the copper oxide species. XPS analysis was used to investigate the surface oxygen species and valence state of elemental copper. In Figure 4(i), D

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Langmuir M(NO2 )2 + 2O* → M(NO3)2

where O* refers to the surface reactive oxygen on the catalyst and M represents the metal or alkaline metal distributed on the catalyst surface (including elemental barium, magnesium, copper, or aluminum). In these reactions, the nitrites as intermediates are generated at the initial NOx adsorption stage and then oxidized to nitrates.11,12 Moreover, as shown in Figure 5, among the nitrate species, the monodentate nitrates are dominant in the initial stage of NOx adsorption. However, after 20 min from the beginning of NOx adsorption, the band intensity of the free ionic nitrates exceeds that of the monodentate nitrates and continues to increase as the NOx adsorption progresses. Therefore, these nitrate ad-species that are generated experience the ionization process from the monodentate nitrates to free ionic nitrates during the NOx adsorption process. 3.2.2. NOx Adsorption on the Soot/Catalyst Mixture. Figure 6 shows the in situ DRIFTS spectra of NOx adsorption

Figure 5. In situ DRIFTS spectra of NOx adsorption on the pure catalyst in NO + O2. Recorded at constant temperature of 240 °C.

generated on the catalyst surface, because various absorption bands are observed in the range of 1000−1800 cm−1. The adsorption bands at 1220 cm−1 are ascribed to bridged bidentate nitrites. The adsorption bands at 1332 and 1043 cm−1 are assigned to monodentate nitrates,41 and those at 1352 cm−1 and 1375−1416 cm−1 correspond to free ionic nitrates and bulk free ionic nitrates, respectively.42 There are chelating and bridging nitrates present, which are denoted by the absorption bands at 1521 and 1629 cm−1, respectively. At higher wavenumbers, the bands at 1767 cm−1 are ascribed to the adsorbed N2O4, which are believed to be the addition compounds, for example, Cu(NO3)2·N2O4 and Mg(NO3)2· N2O 4.41,43 The specified assignments of these various adsorption bands are listed in Table 2. Table 2. DRIFTS Assignments for Surface-Adsorbed Nitro Species NOx species free ionic nitrate bridged bidentate nitrite

monodentate nitrate

bridging nitrate chelating bidentate nitrate absorbed N2O4

band position (cm−1)

vibration mode

1352, 1394

ν(N−O)

1220

ν(NO2,s)

11 12

1043, 1332 1415−1425

ν(NO2,s) ν(NO2,as)

41 42

1629

ν(NO)

43

1550−1620

ν(NO)

1767

ν(NN)

Figure 6. In situ DRIFTS spectra of NOx adsorption on the soot/ catalyst mixture in NO + O2. Recorded at a constant temperature of 240 °C.

ref

on the soot/catalyst mixture. As well as the nitrite and nitrate species observed in Figure 5, new adsorption bands at 1378 and 1581 cm−1, which are assigned to carbonate and carboxylate species, respectively,41,43 appear in the presence of soot. The carboxylate species is likely to be attributed to soot oxidation through the following reaction: C + O* → C(O) → CO2

(3)

where O* is the available active oxygen species, including the surface-adsorbed oxygen, lattice oxygen, NOx, O2 in the feed gases, and the adsorbed NOx species.3 As a weak Lewis acid, the CO2 generated in the soot oxidation may compete with NOx and form carbonates on the adsorption sites on the catalyst surface.35−37 To elucidate the impact of soot on NOx adsorption, bridged bidentate nitrite and monodentate nitrate are employed as the representatives of nitrite and nitrate ad-species, respectively. The band intensities of bridged bidentate nitrite and monodentate nitrate in Figures 5 and 6 are summarized and illustrated in Figure 7. The peak values of the bridged bidentate nitrite and monodentate nitrate for the soot/catalyst mixture are lower than those for the pure catalyst. At the same time, adding soot to the catalyst sample causes the maximum peak value of bridged bidentate nitrite to appear later, by ∼8 min.

As NOx adsorption proceeds, the band intensity of bridged bidentate nitrites (1220 cm−1) shows a decline after an initial increase. Along with the increase and decline in the band intensity of the bridged bidentate nitrites, the band intensities of monodentate nitrates (1332 cm−1), free ionic nitrates (1352 cm−1), and bulk free ionic nitrates (1394 cm−1) gradually increase. This finding suggests that the NO adsorption on the catalyst follows the “nitrite route”, as shown in the following reactions: MO + 2NO + O* → M(NO2 )2

(2)

(1) E

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For the pure catalyst, as shown in Figure 8a, there is no NO2 detected until 10 min after the onset of NOx adsorption. Bearing in mind that the nitrate species in Figure 5 were found in the same period, the generated NO2 at this initial stage is believed to efficiently adsorb on the catalyst and directly form as nitrate ad-species through reactions 6 and 7:3,45

Correspondingly, the appearance of monodentate nitrates is delayed by more than 10 min. The above results indicate that the presence of soot weakens the nitrite formation in reaction 1 and postpones the transformation from nitrites to nitrates in reaction 2. 3.3. Slipped NOx during NOx isothermal adsorption. Figure 8 displays the NO and NO2 concentration slipped from

(5)

(7)

MO + NO2 → MO−NO2

(8)

MO−NO2 → MO2 + NO

(9)

MO2 + 2NO2 → M(NO3)2

(10)

(11)

Reactions 6−11 for the formation of nitrate ad-species, referenced as the “nitrate route” by Nova et al.,11−13 contribute slightly to the NOx adsorption, because (i) a small amount of NO2 is present in Figure 7, and (ii) the low band intensity of nitrates is shown in Figure 5 at the initial stage of NOx adsorption process. Moreover, reactions 8−10 are also available to account for the behavior of slipped NO concentration during the initial 10 min of NOx adsorption. The initial increase in NO concentration can be assigned to the acceleration of reaction 9, whereas the subsequent decrease is because NO2 adsorption through reactions 8−10 is weakened by the gradual saturation of nitro ad-species on the catalyst surface, reducing the generation of NO. During the subsequent NOx adsorption process, the NO concentration decreases to a steady level of ∼1300 ppm, after a rapid increase to the inlet value of 1500 ppm. At the same time, the NO2 concentration increases to a constant value of ∼200 ppm. These observations illustrate that the catalyst is saturated by NOx, and the NSC maximum is reached when the slipped NOx concentrations are at steady levels. In addition, part of NO2 can be converted to N2O4 through reaction 12, and N2O4 will bond with the NO3− groups to form additional nitrato compounds, such as Cu(NO3)2·N2O4 and Mg(NO3)2·N2O4,43 which is verified by the appearance of adsorption bands for these nitrato compounds in Figure 5 (1767 cm−1). 2NO2 → N2O4

the in situ cell during the DRIFT tests. The presence of NO2 in the slip gases evidences that NO is oxidized to NO2 on the catalyst in the course of NOx adsorption. The detected NO2 can be produced through two steps:44 (1) NO adsorption on vacant sites on Pt and (2) desorption as NO2. NO−Pt + O*−Pt → NO2 + Pt

MO + NO2 → M(NO2 )2 + M(NO3)2

BaCO3 + 2NO2 + O* → Ba(NO3)2 + CO2

Figure 8. Slip NOx concentration in the NOx adsorption process for (a) pure catalyst and (b) a soot/catalyst mixture.

(4)

(6)

At the same time, reactions 8−10 can occur on the catalyst, as described by Olsson et al.14 The MO−NO2 species in reactions 8 and 9 refer to the intermediate NOx species that are generated through the loosely adsorbed of NO2 on the metal oxides (for example, BaO−NO2).4 Furthermore, the detection of CO2 in the slipped gases (not shown) indicates that the BaCO3 participates in the NO2 adsorption through reaction 11:4

Figure 7. Variations in band intensity of the ad-species during NOx adsorption: (a) bridged bidentate nitrite and (b) monodentate nitrate.

NO + Pt → NO−Pt

MO + NO2 + O* → M(NO3)2

(12)

For the soot/catalyst mixture, as shown in Figure 8b, the slipped NO2 concentration is much lower than that of the pure catalyst. Two factors are responsible for this behavior: (i) the NO adsorption on oxygen adatoms in reaction 4 decreases as a consequence of the consumption of surface active O* by the soot oxidation in reaction 3, reducing the NO2 formation from reaction 5; and (ii) the generated NO2 acts as a stronger oxidizer (O*) and participates in the soot oxidation in reaction 3,46 decreasing the slipped NO2 concentration.

Here, O* refers to the active oxygen species that dissociates from O2 or NOx over Pt sites on the catalyst.3 F

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Langmuir Associated with the low slipped NO2 concentration, the NO generated from reaction 9 is reduced and finally consumed out by soot through reaction 3. Therefore, NO cannot be detected during the initial 10 min of NOx adsorption in Figure 8b. Moreover, the decrease in NO2 also reduces the yield of nitrate ad-species in reaction 11 and N2O4 in reaction 12, which is reflected by the decrease in the absorbance band intensity of nitrate ad-species (1043 cm−1, 1332 cm−1, etc.) and nitrato compounds (1767 cm−1) in Figure 6. 3.4. Thermal Stability of Adsorbed NOx Species and NSC. TPD experiments were performed to evaluate the thermal stability of adsorbed NOx species and the NSC of the pure catalyst and soot/catalyst mixture. The NO profiles obtained in the TPD-MS system are shown in Figure 9. The NO2 desorbed

Figure 9. NO-TPD profiles of the catalyst sample after NOx saturation at 240 °C in the presence and absence of soot. Figure 10. DRIFTS spectra of the catalyst sample from 240 °C to 650 °C after NOx saturation at 240 °C: (a) pure catalyst and (b) soot/ catalyst mixture.

is not listed, because it was under the detection limit. For the pure catalyst, the desorbed NO showed a low and broad peak in the temperature region of 200−350 °C and a high and sharp peak in the temperature region of 350−550 °C. However, the peak value of NO concentration for the soot/catalyst mixture was larger than that of the pure catalyst in the low-temperature region, whereas the NO peak in the high-temperature region shifts to a lower temperature by ∼50 °C. These results suggest that the presence of soot decreases the thermal stability of the NOx adsorbed species. To characterize the thermal stability of adsorbed NOx species in detail, in situ DRIFTS measurements for the catalyst samples after NOx saturation at 240 °C were conducted from 240 °C to 650 °C. The DRIFTS spectra are shown in Figure 10. For the pure catalyst, as shown in Figure 10a, the bands of monodentate nitrate (1043, 1332, and 1415 cm−1), chelating bidentate nitrates (1521 cm−1), and the additional nitrato compounds (1767 cm−1) continuously decrease from 240 °C and disappear at 450 °C. The disappearance for these adsorbed species may be related to the NO desorption peak at 200−350 °C in Figure 9. With respect to the free ionic nitrates, their band at 1352 cm−1 sharpens and shifts to the higher wavenumbers with the increase in temperature, indicative of their bulk ionic characteristic. From 450 °C to 650 °C, the band intensity of bulk free ionic nitrates (1375−1416 cm−1) decreases. Concurrently, new bands at 1240 and 1080 cm−1 show an increase in band intensity and are assigned to free nitrite ions (NO2−) and chelated nitrito groups, respectively, which can be produced during the decomposition of bulk ionic nitrates with a high thermal stability.47 Hence, the NO desorption peak at 350−550 °C occurs because of the decomposition of free ionic and bulk ionic nitrates. For the soot/catalyst mixture, as shown in Figure 10b, the bands at

1043, 1332, 1415, 1521, and 1767 cm−1 disappear when increasing the temperature to 350 °C. As the temperature increases further, the band intensity of bulk free ionic nitrates at 1375−1416 cm−1 decrease at 400 °C, and, simultaneously, the free nitrite ions (NO2−) and chelated nitrito group appear. In comparison to the pure catalyst, the soot/catalyst mixture reduces the temperature for the yield or decomposition of the above surface NOx ad-species by 50 °C, which is concordant with the TPD results in Figure 9 that the presence of soot shifts the NO desorption peak to the lower temperature range, by 50 °C. Because only NO and low amounts of N2 were detected during TPD tests for the pure catalyst and soot/catalyst mixture, the NSC was calculated using the relative peak areas of the NO profiles in Figure 9. To support this calculation, a nitrogen balance was performed on the pure catalyst and the soot/catalyst mixture, and it is found that there is a slight difference between the desorbed NO and the adsorbed NOx during TPD. In addition, NO-TPD test was repeated three times for each sample, and the relative standard deviations (RSDs) about the NSC values are all