NO_x storage and soot combustion over well-dispersed mesoporous mixed oxides via hydrotalcite-like precursors

Abstract

A series of mixed oxides with highly dispersed redox components were prepared via hydrotalcite-like precursors in which Mg was partly substituted with copper and cobalt, which were employed for NO_x storage and soot combustion. The physico-chemical properties of the catalysts were characterized by XRD, TGA, IR, N₂ adsorption, H₂-TPR and in situ FTIR techniques. The results show the transition metal cations have isomorphously replaced Mg²⁺ in the layered structures forming a single hydrotalcite type phase. After calcination, the transition metal oxides exist in a highly dispersed form in the Mg(Al)O matrix and there is a cooperative effect between the copper and cobalt on the redox properties of the catalyst. The as-prepared oxide catalysts exhibit large surface areas, basic characters and improved redox properties, resulting in high performances in NO_x storage and soot combustion. Both the NO_x storage and desorption are catalytically accelerated due to the highly dispersed transition metal oxides. The presence of NO_x positively affects the activity of all the oxides catalysts for soot combustion, which may be related to the production of NO₂ during NO oxidation. NO₂-assisted mechanism and active oxygen mechanism may occur simultaneously in soot/NO/O₂ reaction.

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1. Introduction

Diesel engines with excellent fuel economy are receiving much attention as an effective method of reducing CO₂ emission to suppress global warming. However, on the other hand, suppression of diesel emissions is the major issue from the aspect of improving urban environment. Nitrogen oxides (NO_x) and soot particulates (PM) are considered to be the main pollutants emitted from diesel engines together with CO and HC causing serious problems to global environment and human health.¹ Emissions of HC and CO are low and can be easily oxidized to CO₂ by diesel oxidation catalysts (DOC). As fuel processing and engine design modifications alone will not meet the stringent legislations for both NO_x and soot, the after-treatment technologies for the removal of NO_x and soot are quite necessary.

For soot removal, filtration and controllable regeneration within the exhaust stream are among the most promising methods, while the key technology is oxidation catalysis.^2,3 In the past few decades, many materials have been applied in oxidation catalysis, with transition-metal oxides,^4–6 alkaline metal oxides,^7–10 perovskite-like type oxides,^11,12 noble metals^3,13 and ceria-based oxides^14–18 being the outnumbering materials. However, a cheap and efficient substitute with low-temperature activity and high selectivity is still desired. In spite of several drawbacks, there are now commercially available after-treatment technologies such as catalyzed soot filter (CSF),¹⁹ continuously regenerating trap (CRT)²⁰ and fuel borne additives (FBC), for decreasing soot emissions from various sources.

For the control of NO_x emissions, the NH₃/Urea selective catalytic reduction (SCR)²¹ and the NO_x storage-reduction (NSR),²² also called as lean NO_x trap (LNT), are widely accepted as the most hopeful strategies. The Urea-SCR has a high NO_x purification rate but has been essentially developed for heavy-duty trucks and buses due to deficiencies in the infrastructure with respect to urea supply. Meanwhile, a more confusing context characterizes light vehicles with a competition between SCR and NSR after-treatment systems. The mechanism of NSR catalyst is that the exhaust NO_x in lean conditions is stored on the catalyst surface, and the stored NO_x is reduced in rich conditions when the exhaust gas does not contain any oxygen. Both technologies suffer from strong kinetic and thermodynamic limitations, which make a suitable solution difficult. In addition, the main challenge is the reduction of the catalyst volume and the automatic control of urea injection in the particular case of urea SCR systems.²³

Considering the pollutants emitted from diesel engines as a whole, the most feasible removal method is the integration of NO_x traps and oxidation catalysts.²⁴ Based on this concept, the Toyota group has developed a practical catalyst system (Diesel Particulate-NO_x Reduction System: DPNR)²⁵ that is a world first simultaneously reduction system of PM and NO_x-a task that was previously considered impossible. The DPNR system has been applied to the diesel engine-powered light-duty trucks (Toyota Dyna, Hino Dutro) in Japan, and to a passenger vehicle (Toyota Avensis) in Europe. Similar with the NSR catalytic material (e.g., Pt–Ba/Al₂O₃ or Pt–K/Al₂O₃), DPNR catalysts work under cyclic conditions alternating a lean phase during which NO_x in the exhausts are stored as nitrates with a short rich phase, during which the stored NO_x are reduced to nitrogen. During these cycles, soot removal occurs as well through oxidation by active oxygen species generated in both the NO_x storage and reduction processes, together by excessive oxygen under lean conditions.

So far, many kinds of metal oxides catalysts have been investigated as NO_x traps and oxidation catalysts, such as Pt–Ba/Al₂O₃,²⁶ Pt–K/Al₂O₃,²⁷ K/La₂O₃,²⁸ Ba, K/CeO₂ (ref. 29) and perovskites catalysts,^30,31 etc. For several years, Mg–Al hydrotalcite mixed oxides^22,32–34 were reported to offer potential advantages over Pt/BaO/Al₂O₃ in NO_x storage-reduction and assessed to be the new generation of NSR catalysts. In our previous studies,^6,35–38 hydrotalcites-derived oxides catalysts containing noble metals, transition metals or rare earth metals are catalytically active for NO_x storage, soot oxidation, and NO_x reduction by soot. In addition, Meng et al.^39–41 also reported a series of hydrotalcites-related oxides exhibited similar multifunctional activity for diesel soot and NO_x abatements. Hydrotalcites compounds can contain metal cations of more than two types, which provide a good platform for design of catalysts combining redox and basic properties. After calcinations at high temperatures, the derived oxides catalyst presents large surface areas, redox and basic properties, high metal dispersions and good thermal stability, which is promising to be one novel DPNR catalytic material.

With regard to the noble metal-free catalysts derived from hydrotalcites, transition metal oxides with bulk or spinel structure have attracted much attention as catalysts or catalyst supports for soot oxidation and NO_x storage because of the enhanced redox properties. In contrast, transition metal oxides with highly dispersed form draw a few interests. In fact, hydrotalcites-derived Mg/Al mixed oxide can act simultaneously as support for the dispersed metals with redox properties and as NO_x storage component. It is also known that the surface properties of the redox component and the storage component are critical to the efficacy of the NO_x storage-reduction catalysts. Motivated by the above considerations, a series of mixed oxides with highly dispersed redox components were prepared via hydrotalcite-like precursors in which Mg was partially substituted with copper and cobalt. The as-prepared oxides catalyst exhibited high performances on NO_x storage and soot combustion, which were discussed with their high surface areas, porous structures and improved redox properties.

2. Experimental

2.1 Catalyst preparation

The hydrotalcites precursors with different metal atomic ratio listed in Table 1 were prepared by co-precipitation of an aqueous solution of suitable metal nitrates (the molar ratio M²⁺/M³⁺was 3) with an aqueous solution of 2 M NaOH and 1 M Na₂CO₃. The two solutions were mixed under vigorous stirring at 25 °C with the pH maintained constant at 10.0 ± 0.5. The resulting slurry was aged in the mother liquor at 80 °C for 1 h. It was then filtered off and repeatedly washed with sufficient deionized water to ensure that the sodium content in the solid was lower than 0.05 wt%. The precipitate was then dried at 120 °C for 12 h to obtain hydrotalcites precursors. Samples are denoted according to the metal constituents in the initial mixture. Mg₇₅Al₂₅-HT, Cu₅Mg₇₀Al₂₅-HT, Co₅Mg₇₀Al₂₅-HT and Cu₅Co₅Mg₆₅Al₂₅-HT (marked as HT, CuHT, CoHT and CuCoHT, respectively) were similarly prepared.

Table 1 Chemical composition and structural parameters of the hydrotalcite precursors^a

Hydrotalcites precursors	Compositions (molar ratio)	FWHM (2θ)	a (Å)	c (Å)	Xs (nm)	Weight loss (%) (total/first stage)
a FWHM-Full width at half maximum of (003) plane; Xs-average crystallite size calculated from d(003) and d(110) planes using Debye-Scherrer equation.
HT	Mg/Al = 75/25	1.417	3.057	23.262	10.1	45.98/16.94
CuHT	Cu/Mg/Al = 5/70/25	1.515	3.063	22.788	8.8	44.59/15.85
CoHT	Co/Mg/Al = 5/70/25	1.572	3.067	23.308	9.3	45.70/17.70
CuCoHT	Cu/Co/Mg/Al = 5/5/65/25	1.554	3.064	23.309	8.9	45.32/16.74

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The corresponding mixed oxides were obtained by thermal decomposition of hydrotalcites precursors at 800 °C in air for 4 h, referred as to MgAl, CuMgAl, CoMgAl and CuCoMgAl, respectively.

2.2 Catalyst characterization

XRD was conducted with a BRUKER-AXS D8Adance X-ray Diffractometer using Cu Kα radiation, at 40 kV and 40 mA, in the scanning angle (2θ) range of 5° 80° at a scanning speed of 3° min⁻¹.

Thermogravimetric analysis was carried out using METTLER TOLEDO TGA/DSC1, with air as carrier gas (30 mL min⁻¹) at a heating rate of 10 °C min⁻¹ from 35 to 900 °C.

Texture properties of the prepared samples were determined from N₂ adsorption–desorption isotherms performed using a Micromeritics ASAP 2020 surface area analyzer after outgassing at 300 °C for 5 h prior to analysis. The specific surface areas were calculated with the Brunauer–Emmett–Teller (BET) equation on the basis of the adsorption data. Pore size distribution over the mesopore range was generated by the Barrett–Joyner–Halenda (BJH) analysis of the desorption branches, and values of the average pore size were calculated.

Infrared spectra were recorded on a Bruker Tensor 27 spectrometer over 400–4000 cm⁻¹ after 32 scans at a resolution of 4 cm⁻¹. The samples were prepared in the form of pressed wafers (2% of sample in KBr).

Temperature-programmed reduction with H₂ (H₂-TPR) experiments were performed in a quartz reactor with a thermal conductivity detector (TCD) to monitor the H₂ consumed. A 50 mg sample was pretreated in situ at 500 °C for 1 h in a flow of O₂ and cooled to room temperature in the presence of O₂. TPR was conducted at 10 °C min⁻¹ up to 900 °C in a 30 mL min⁻¹ flow of 5 vol% H₂ in N₂.

2.3 NO_x adsorption and desorption experiments

The NO_x storage performance was studied by isothermal adsorption of NO_x followed by temperature programmed desorption (TPD) of adsorbed species. The catalysts were finely ground, sized in 40–80 mesh for NO_x adsorption–desorption experiments. Thermal NO_x adsorption experiments were carried out in a quartz flow reactor (i.d. = 6 mm and L = 240 mm) using 50 mg of the catalysts. Catalyst was pretreated in situ at 500 °C for 1 h in N₂ and then cooled to 100 °C. When the temperature had stabilized at 100 °C, the flow gas was switched to 1050 ppm NO and 5% O₂ in He at a rate of 100 mL min⁻¹ for 60 min for thermal NO_x adsorption. Concentration of NO, NO₂ and NO_x from the reactor outlet were monitored by a chemiluminescence NO_x analyzer (Model 42i-HL, Thermo Electron Corporation).

After the isothermal NO_x adsorption, the flow gas was switched to pure N₂ (rate = 100 mL min⁻¹) to flush the catalysts until NO_x is not detected. The NO_x-TPD was conducted by heating the catalysts from 100 °C to 700 °C at a heating rate of 10 °C min⁻¹ with N₂ flowing at a rate of 100 mL min⁻¹. Concentrations of NO, NO₂ and NO_x from the reactor outlet were monitored by the chemiluminiscence NO_x analyzer, and the desorbed NO_x amount was thus calculated as the NO_x storage capacity of the catalyst.

2.4 In situ FTIR study of NO_x storage

The in situ FTIR spectra were recorded on a Bruker Tensor 27 spectrometer over 400–4000 cm⁻¹ after 32 scans at a resolution of 4 cm⁻¹. The self-supporting wafers of the oxide catalyst were loaded into an in situ infrared transmission cell which is capable of operating up to 500 °C and equipped with gas flow system. The wafers were pretreated in the IR cell at 400 °C in a flow of He for 30 min to remove any adsorbed species. After cooled to 100 °C, the background spectrum was recorded. The time-resolved IR spectra were recorded at 100 °C in the flow of 1000 ppm NO + 5 vol% O₂ in He (100 mL min⁻¹).

2.5 Catalyzed NO oxidation into NO₂

NO oxidation on the catalysts was investigated by a temperature-programmed oxidation (TPO) technique in the same experimental apparatus as used in NO_x storage experiments. The catalysts (40–80 mesh, 50 mg) were pretreated in situ at 500 °C for 1 h in He. After cooled down to room temperature, a feed gas containing 1000 ppm NO + 5 vol% O₂ in He (100 mL min⁻¹) was introduced and NO_x oxidation was started at a heating rate of 4 °C min⁻¹ until 700 °C. Concentrations of NO, NO₂ and NO_x from the reactor outlet were monitored by a chemiluminescence NO_x analyzer.

2.6 Catalytic combustion of soot

The model soot used in this study was Printex-U from Degussa with surface area of 93.5 m² g⁻¹. The mean agglomerate size measured using a Beckman Counter LS13320 laser particle size analyzer was about 177 nm⁹.

The catalytic reactions for soot combustion were performed by a TPO technique in a fixed-bed flow reactor as described in our previous works.^36,38,42 Briefly, the soot was mixed with the catalyst in a weight ratio of 1 : 9 in an agate mortar for 10 min, which results in a tight contact. A 50 mg sample of the soot/catalyst mixture was pretreated in He (100 mL min⁻¹) at 200 °C for 1 h to remove surface-adsorbed species. After cooling down to room temperature, a gas flow with 5 vol% O₂ in He or 1000 ppm NO + 5 vol% O₂ in He (100 mL min⁻¹) was introduced and then TPO was started at a heating rate of 10 °C min⁻¹ until 800 °C. NO_x (NO and NO₂) and COx (CO and CO₂) in the effluent were online analyzed by the chemiluminescence NO_x analyser (42i-HL, Thermo Environmental) and a gas chromatograph (GC) (SP-6890, Shandong Lunan Ruihong Chemical Instrument Corporation, China), respectively. The characteristic temperatures from the TPO profiles, T₅ and T₅₀ are defined as the temperatures at which 5% and 50% of the soot is converted, respectively. The selectivity to CO₂ formation (S_CO2) is defined as the percentage outlet CO₂ concentration divided by the sum concentrations of the outlet CO₂ and CO. NO_x conversion is evaluated by maximum NO_x conversion (C_m) during each TPO process as described in our previous works.

3. Results and discussion

3.1 XRD analysis

Fig. 1 depicts the powder XRD patterns of the hydrotalcites precursors and their derived mixed oxides whilst Table 1 summarizes the chemical composition and relevant structural parameters. As shown in Fig. 1a, all the as-prepared precursors show sharp and symmetric reflections at lower 2θ values of 11.5°(003), 21.5°(006) and 34.5°(009), which are characteristic diffraction patterns of hydrotalcite with layered structures (JCPDS no. 22–0700). The well defined (110) and (113) diffraction peaks at 60.3° and 61.5° reveal a quite good dispersion of metal ions in the brucite-like layers.⁴³ No other phases were detected after transition metals incorporation, which suggests that magnesium and aluminum may be substituted by copper or cobalt in the brucite-like layers.

Fig. 1 XRD patterns of hydrotalcite precursors (a) and calcined samples (b) (phases: #-the periclase-type MgO, o-the spinel-type MgAl₂O₄).

The lattice parameters, a and c, typical of hydrotalcite structures with rhombohedral 3R symmetry were calculated for all samples and listed in Table 1. The “a” parameter corresponds to the average distance cation–cation in the layers of brucite type, which is obtained by a = 2 d(110). The “a” values are slightly increased after transition metals incorporation, which is in agreement with other research.⁴⁴ This is due to Mg²⁺ substitution by a larger cations Co²⁺ and Cu²⁺(rCo²⁺ = 0.74 Å, rCu²⁺ = 0.69 Å, rMg²⁺ = 0.65 Å), and suggests the formation of a single hydrotalcite type phase. In the case of “c” parameter, it is related to the thickness of the layer brucite type and interlayer distance, which is usually calculated using the relationship c = 3 d(003). It depends upon several factors such as the amount of interlayer water, the size of the interlayer anion and of the M²⁺–M³⁺ cations, and the strength of the electrostatic attractive forces between the layer and the interlayer.⁴⁵ The results shown in Table 1 must probably reflect the influence of these various factors.

The full width at half maximum (FWHM) of the (003) plane (2θ ≈ 11.5°) is included in Table 1 as a measure of crystallinity of the hydrotalcite phase in the c-axis direction. The FWHM value increases after Cu or/and Co doped, demonstrating that the crystallinity of the HT phase decreases due to the distortion induced by the dopants. The average crystallite size calculated from d(003) and d(110) planes using Debye-Scherrer equation varied in the range 80–100 Å.

As shown in Fig. 1b, the hydrotalcite phases were completely destroyed and new oxide derivatives were formed after calcination at 800 °C. For all calcined samples, three peaks with the 2θ angle centered at 36.9°, 42.9° and 62.3° were ascribed to the periclase-type MgO (JCPDS 45-0946, marked with #) with traces amount of spinel-type MgAl₂O₄ (JCPDS 47-0254, marked with o) at 2θ = 35.5°. Contrarily as it occurs with the calcined Cu- and Co-hydrotalcites,^42,46 no peaks assigned to the transition metal oxide were observed, indicating that the copper or cobalt oxides are well dispersed in the Mg(Al)O matrix.⁴⁵

3.2 Thermal gravimetric analysis (TGA and DTG)

Fig. 2 presents the weight loss rates of the precursors as a function of temperature during heating in air, revealing the transformation of hydrotalcites into the corresponding oxides. Generally, a similar thermal behavior was observed on the four prepared precursors which exhibit a two-stage thermal decomposition with a total mass loss of ∼45% as summarized in Table 1. Considering that the interlayer anions in the hydrotalcites precursors are carbonates through FTIR spectra (not shown here), the first stage occurred at 100–250 °C was ascribed to the release of interlayer and adsorbed water molecules with weight loss of 15.8–17.7%. The second stage of the weight loss (∼28%) at 250–500 °C consists of dehydroxylation of interlayer hydroxyl groups and decomposition of interlayer carbonate and traced nitrate, resulting in the collapse of the layered structure. After small amounts of Cu and/or Co introduced into hydrotalcite structure, the second weight loss shifts to low temperature range. The accelerated decomposition can be attributed to distortion of brucite-like sheets modified by substitution of Mg and Al cations by transition metals, resulting in weakening of anions bounding by hydrotalcite layers.⁴⁴

Fig. 2 DTG curves of the as-synthesized hydrotalcite precursors.

3.3 Textural characteristics

The textural properties of the mixed oxides obtained by nitrogen adsorption at −196 °C, are summarized in Table 2. The specific surface area reaching 186 m² g⁻¹ for the MgAl mixed oxide, decreased slightly upon introduction of the transition metal cations in the structure being in the range 166–185 m² g⁻¹. Such a reduction in the surface area may be related to the aggregation of metallic oxides blocking the smaller pores and/or causing some structural rearrangements.⁴⁷

Table 2 Textual properties and NO_x storage capacity of the oxide catalysts

Mixed oxides	SBET^a (m^2 g^−1)	Vp^b (cm^3 g^−1)	Dp^c (nm)	H2 uptake^d (mmol g^−1)	NSC^e (mg g^−1)
a BET surface area.b Total pore volume.c Average pore size.d H2 uptake at the range 100–500 °C in H2-TPR tests.e NOx storage capacity calculated from NOx-TPD tests.
MgAl	186.2	0.88	18.9	0	2.66
CuMgAl	166.6	0.87	20.9	0.501	3.30
CoMgAl	185.4	0.83	18.0	0.094	5.20
CuCoMgAl	179.1	0.97	21.7	0.565	3.40

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N₂ adsorption and desorption isotherms of MgAl, CuMgAl, CoMgAl and CuCoMgAl mixed oxides are plotted in Fig. 3. All the samples displayed type IV nitrogen adsorption/desorption isotherms (according to IUPAC classification) with a distinct hysteresis loop, characteristic of mesoporous materials. The hysteresis loop is ascribed to H3 type, which is usually given by adsorbents containing slit-shaped pores with a wide distribution of pore size. Applying the BJH method to the isotherm desorption branch, average pore size of 21 nm or smaller is assigned to the mixed oxides.

Fig. 3 N₂ Adsorption and desorption isotherms of hydrotalcites-derived mixed oxides.

The pore size distribution curves for the mixed oxides are plotted in the insets in Fig. 3, which display that most of the pores fall in meso size range (2 nm < r_p<50 nm). The pores present in the mixed oxides exhibit monomodal curves with the maximum at ≈11 nm, favoring gas molecule diffusion in the pores and thus ruling out the diffusion limitation in the adsorption and desorption process.⁴⁸

3.4 H₂-TPR

H₂-TPR was used to examine the redox properties of catalysts. Fig. 4 presents TPR profiles for mixed oxides catalysts. Differences in the redox properties for these samples can be anticipated to the nature of the transition metal and the crystal phases present in the compounds. For MgAl sample, no reduction of magnesium or aluminum species is observed until 800 °C, as expected. In the case of CoMgAl, the reduction proceeds largely in two stages. The first reduction zone with weak and broad peak from 200 to 450 °C corresponds to the reduction of Co₃O₄ into Co⁰ (Co₃O₄ → CoO → Co), while the second peak at above 600 °C is attributed to the reduction of CoAl₂O₄ to Co⁰.^49,50 Concerning CuMgAl solid, a sharp peak centered at 233 °C was detected, which can be ascribed to the reduction of the highly dispersed copper oxides species, including isolated copper ions, weak magnetic associates, and small two- and three-dimensional clusters.⁵¹ As well documented,^38,52,53 bulk CuO gave TPR signals at much higher temperatures (∼300 °C) than highly dispersed CuO species. Since such signals are absent in our TPR spectra, we can exclude the presence of bulk like CuO species, which is in consistent with XRD results. As for the CuCoMgAl sample, two reduction peaks were found at different temperatures. The first peak assigned to the highly dispersed CuO and Co₃O₄ species, while the second peak at above 550 °C is attributed to the reduction of CoAl₂O₄. It can be seen from Fig. 4 and Table 2 that the first reduction peak becomes shaper and shifts to lower temperature (225 °C) with a higher H₂ consumption compared with that of CuMgAl. The promoting effect of Cu–Co incorporation on reducibility at low temperature may be associated with the increase of the interaction among transition metal ions in the Mg(Al)O matrix. Evidence of a strong interaction between copper and cobalt in the mixed oxides catalysts have been given in investigation of Co_xO_y–CuO mixed oxides.⁵⁴

Fig. 4 H₂-TPR patterns of mixed oxides samples.

These TPR results together with those described previously by XRD show that transition metal oxide exist as highly dispersed form in the matrix and there is a cooperative effect between the copper and cobalt on reducibility of the catalyst. Such a cooperative effect is very likely originated by the intimate contact and by the good interdispersion of the different oxides forming the catalyst via hydrotalcites precursors.

3.5 NO oxidation

NO₂ is considered as an important intermediate in both NO_x storage and soot oxidation. NO_x storage materials are generally more effective in adsorbing NO₂ than NO, and NO₂ formation is a beneficial precursor step to adsorption. Besides, NO₂ has also been proved to be a more powerful oxidant for soot oxidation than NO and O₂. Soot-trapping followed by oxidation with the highly reactive NO₂ is the basis of the so-called Continuously Regenerating Trap (CRT) which is already a commercialized technology for decreasing particulate emissions from various sources. Fig. 5 shows the NO₂ formation profiles in the catalyzed NO oxidation reactions. It should be noted that in the absence of catalyst less than 3% of NO is oxidized to NO₂ because of the effect of temperature.⁵⁵ As can be seen from Fig. 5, NO can be readily oxidized to NO₂ on all catalysts even at low temperature with about 10% of NO converted at 100 °C. When the temperature exceeds 300 °C, the transition metal-containing catalysts accelerate the oxidation of NO with a maximum NO₂ level at 400–450 °C while MgAl does not present such behavior under the same conditions and only a weak and broad peak was observed at ∼550 °C. Above the maximum the NO₂ level decreases following the thermodynamic profile of the NO/NO₂ equilibrium. From the maximum NO₂ level in the TPO profiles the NO₂ production capacity decreases by the following order: CuCoMgAl > CuMgAl > CoMgAl > MgAl, which is in line with the reducibility of the catalysts. Sufficient NO₂ production capacity is needed in NO_x storage and NO₂-assisted soot oxidation.

Fig. 5 Catalyzed NO oxidation to NO₂ over the oxides samples.

3.6 NO_x storage and in situ FTIR spectra

NO_x storage experiment was carried out by isothermal adsorption of NO in the presence of O₂ at 100 °C over the hydrotalcites-derived mixed oxides. The stored NO_x species were investigated by in situ FTIR spectra as a function of time shown in Fig. 6. At low contact time, on MgAl sample ionic nitrite (bands at 1226 cm⁻¹) and bridged bidentate nitrite (bands at 1278 cm⁻¹) due to the adsorption of NO are simultaneously present with bridged bidentate nitrate (bands at 1675–1631 cm⁻¹, bands at 1310 cm⁻¹) attributed to NO₂ adsorption. On increasing the adsorption time, the bands due to ionic nitrite and bridged bidentate nitrite progressively increased in intensity, whereas those of bidentate nitrate grew slowly. The simultaneous presence of nitrite and nitrate leads to the overlapped peak of ionic nitrite with bridged bidentate nitrate at 1310 cm⁻¹. After 40 min of contact, the adsorption of NO_x reached saturation with nitrites as the predominant species on the Mg–Al sites.

Fig. 6 In situ IR spectra of NO_x adsorption at 100 °C over MgAl(a), CuMgAl(b), CoMgAl(c) and CuCoMgAl(d).

For CuMgAl sample, bridged bidentate nitrate (bands at 1310 cm⁻¹ and 1675–1631 cm⁻¹) and monodentate nitrate (1275 cm⁻¹) were formed in the NO_x adsorption at 100 °C, besides ionic nitrite (bands at 1219 cm⁻¹). The time-dependent spectra (Fig. 6b) clearly show that the increase in nitrate peak occurs simultaneously with the decrease of ionic nitrite bands, suggesting a redox conversion from ionic nitrite to nitrate species in the presence of well dispersed copper oxide. Similar results were observed on NO_x adsorption over CoMgAl and CuCoMgAl. Compared with MgAl sample, more nitrates together with ionic nitrite are the major NO_x species stored over the transition metal-containing mixed oxides. Thus, NO_x adsorption at low temperatures was enhanced due to the conversion of nitrites into nitrates on the highly dispersed Cu/Co oxides, which are then stored on the adjacent Mg–Al sites to form relatively stable Mg/Al nitrates and nitrites. Taking into account the above-mentioned NO oxidation results and the previous reports,^37,48,56 NO can be stored on Cu and/or Co incorporated hydrotalcite catalysts by two different pathways in the presence of O₂: (1) the NO oxidation to nitrites followed by oxidation to nitrates; (2) the NO oxidation to NO₂ followed by adsorption as nitrates. Both the two routes are promoted by the highly dispersed transition metal oxides species.

3.7 NO_x desorption from mixed oxide catalysts

Fig. 7 presents the desorption profiles of NO_x at 100–600 °C from the oxides catalysts after NO_x adsorption at 100 °C. The desorbed amounts of NO_x were calculated as the NO_x storage capacity and listed in Table 2. In general, the NO_x desorption follows a two-step process in which NO is the predominant species, with a small amount of NO₂ (less than 30 ppm over 100–600 °C). Thus, the decomposition of surface nitrates and nitrites tends to undergo the following reactions:

4NO₃⁻ → 4NO + 3O₂ + 2O_(s)²⁻

4NO₂⁻ → 4NO + O₂ + 2O_(s)²⁻

Fig. 7 NO_x desorption profiles from the catalysts after the NO_x adsorption at 100 °C.

Besides gaseous NO and O₂, a large amounts of basic oxygen ions were also formed on the catalyst surface during the desorption process.

For MgAl sample, the low temperature desorption exhibits a broak peak at 320 °C corresponding to various types of nitrites while the weak desorption at high temperature (520 °C) is related to various nitrates, which is consistent with the IR results. After Cu and/or Co incorporated, the two steps occurred at much lower temperatures and over a much narrower temperature range, presumably because of the catalytic activity of copper and/or cobalt for NO_x desorption. Obviously, the desorption peaks at both low and high temperature on transition metal-containing oxides are larger than those on MgAl sample, resulting in enhanced NO_x storage capacity. From the calibrated areas of the TPD peaks, a rather high NO_x storage capacity at 100 °C (5.20 mg g⁻¹) was observed on CoMgAl solid.

It can be seen from the in situ FTIR spectra and NO_x desorption results that both the storage and desorption are catalytically accelerated due to the highly dispersed transition metal oxides. It is suggested⁵⁷ that higher NO_x adsorption is due to a migration process of NO₃⁻ and NO₂⁻ from surface active metal oxides to adjacent Mg–Al sites to form relatively stable Mg/Al nitrates and nitrites. Thus, the role of well dispersed Cu or/and Co oxides in NO_x storage and decomposition in the present study may be similar to the noble metal (Pt) in Toyota NSR catalysts.⁵⁸

3.8 Soot oxidation with O₂ and NO + O₂

The soot conversion profiles obtained during catalytic tests performed CO₂ with O₂ and NO + O₂ are plotted as a function of temperature in Fig. 8, including the curves obtained with catalyst-soot mixture and SiO₂-soot mixture. The derived parameters of T₅, T₅₀ and S_CO2 under different conditions are summarized in Table 3.

Fig. 8 Catalytic oxidation of soot over hydrotalcites-derived mixed oxides in O₂ (a) and NO + O₂ (b).

Table 3 Catalytic performance of soot combustion over the oxides catalysts^a

Samples	C + O2			C + NO + O2
	T5 (°C)	T50 (°C)	SCO2 (%)	T5 (°C)	T50 (°C)	SCO2 (%)	Cm (%)
a T5 and T50 are defined as the temperatures at which 5% and 50% of the soot is converted, respectively; SCO2- selectivity to CO2 formation; Cm-maximum conversion of NOx during soot oxidations.
Blank	470	590	37.8	461	601	49.1	9.4
MgAl	422	577	70.9	320	563	88.3	17.2
CuMgAl	315	525	100	304	493	99.0	29.5
CoMgAl	344	528	100	330	486	99.1	25.3
CoCuMgAl	316	528	100	302	467	99.5	19.7

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As shown in Fig. 8a, the blank experiment with O₂ was performed mixing the soot with SiO₂, and the ignition temperature was 470 °C. In comparison with the non-catalyzed soot oxidation, soot conversion curves over the mixed oxides shift to lower temperature range with T₅ and T₅₀ decreased. MgAl sample exhibits modest activity with a shift of T₅ by 48 °C to lower temperature. After transition metals introduced, the soot oxidation activity was improved whilst the selectivity to CO₂ formation was increased to 100%, which may be related to the enhancement of reducibility of the catalysts as mentioned in TPR results. CuMgAl and CuCoMgAl show better catalytic performances with similar ignition temperatures at about 315 °C under the feed gas of O₂. Considering that the reducible oxygen species of CuMgAl measured in terms of TPR were less than that of CuCoMgAl, the accessible oxygen species was not the sole determining factor for the catalytic oxidation of soot, which has also been approved by isotopic TPO investigations on catalytic soot oxidation.^59,60 Also worth noting is the well dispersed transition metal oxides catalysts exhibit higher ignition activity than the bulk copper or cobalt oxides samples^6,38 derived from hydrotalcites under the same conditions, which could be ascribed to the improved reducibility, larger surface areas and pore volumes of the former.

It can be seen from Fig. 8b and Table 3 that the presence of NO positively affects the activity of all the oxides catalysts over the entire soot conversion range. Moreover, the Cu/Co incorporated catalysts also show improved activity for NO_x removal, with the largest C_m (maximum conversion of NO_x) of about 30% obtained over CuMgAl sample. The promotion effect of NO on soot combustion at low temperature is more obvious on MgAl with T₅ decreased from 422 to 320 °C, compared with transition metal incorporated samples. Interestingly, a bimodal CO₂ formation curve was observed on MgAl sample with a minor CO₂ peak at about 360 °C and a larger one centered at 570 °C. The two peaks profile of the TPO results in NO + O₂ were also reported in our previous work,^36,61 which suggests that the first peak in the low temperature range corresponds to the reaction of carbon with NO₂, whereas at higher temperatures oxidation with O₂ is dominating. Interestingly, MgAl catalyst shows higher ignition activity (value of T₅), namely lower combustion temperature (around 300 °C) in NO + O₂ than that of CoMgAl catalyst, which can be ascribed to its higher NO₂ production at the same temperature range. The promotion effect of Cu and/or Co incorporation are manifested by the disappearance of higher temperature combustion peak with lower T₅₀ and soot depletion below 500 °C. Similarly, CuMgAl and CuCoMgAl also show close ignition performance with T₅ of about 300 °C, while CuCoMgAl exhibits lower T₅₀ value of 467 °C implying a cooperative effect between the copper and cobalt in the mixed oxides.

With regard to the promotion effect of NO_x adsorbed species on soot oxidation, the catalytic activity for soot oxidation in NO + O₂ seems to be related to the production of NO₂, which is a stronger oxidant than NO and O₂. In Fig. 9, the temperature at which 50% of the soot is converted (T₅₀) in catalytic tests has been plotted as a function of maximum level of NO₂ reached by each catalyst in the catalyzed NO oxidation reactions. Obviously, the most active catalyst (CuCoMgAl) in terms of T₅₀ is also the most effective for NO conversion to NO₂ and vice versa. An approximate linear relationship between the soot combustion and NO₂ production has been obtained, which demonstrates that the combustion of soot over the hydrotalcites-derived Cu/Co oxides catalysts mainly occurs through the NO₂-assisted mechanism. A similar relationship was also obtained previously with a set of ceria catalysts.¹⁵ Taking into account the mechanism for soot combustion on K/MgAlO oxides in our previous works,^35,36,62 the catalytic combustion of soot under NO + O₂ on well dispersed transition metal oxides catalysts may be initiated by the attack of NO₂ to the soot surface, and once the soot surface is partially oxidized and the temperature is high enough, some other oxidizing species such as molecular O₂, surface nitrates/nitrites or active oxygen species released by Cu/Co oxides, are also able to oxidize soot along with NO₂. Thus, two different reaction mechanisms, active oxygen mechanism and NO₂-assisted mechanism, may occur simultaneously on the soot/NO/O₂ reaction in the present study.

Fig. 9 Relationship between mixed oxides-catalyzed NO oxidation to NO₂ and soot combustion (T₅₀ parameter).

4 Conclusions

In the present study, a series of mixed oxides with redox components were prepared via hydrotalcite-like precursors in which a small amount of Mg was substituted with copper and cobalt. The transition metal oxides exist as highly dispersed form in the matrix and there is a cooperative effect between the copper and cobalt on reducibility of the catalyst. The as-prepared oxides catalysts exhibit large surface areas, basic characters and improved redox properties, resulting in high performances on NO_x storage and soot combustion. Both the NO_x storage and desorption are catalytically accelerated due to the highly dispersed transition metal oxides. The presence of NO positively affects the catalytic activity for soot oxidation, in which NO₂-assisted mechanism and active oxygen mechanism may occur simultaneously.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (no. 21007019 and 21277059), the Development Program of the Science and Technology of Shandong Province (no. 2014GSF117039), Jinan Science and Technology Development Plan (no. 201303066), and National Science & Technology Pillar Program (2014BAK13B02).

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