Ti

ARTICLE pubs.acs.org/JPCC

Sulfur-Resistant NO Decomposition Catalysts Derived from CoCa/TiAl Hydrotalcite-like Compounds Jie Cheng,† Xiaoping Wang,† Junjie Yu,† Zhengping Hao,*,† and Zhi Ping Xu*,‡ † ‡

Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, P. R. China Australian Research Council (ARC) Centre for Functional Nanomaterials, School of Engineering, The University of Queensland, Brisbane, QLD 4072, Australia

bS Supporting Information ABSTRACT: Co1.5M1.5/Al1xTix hydrotalcite-like compounds (where M = Co, Ca and x = 0, 0.1) were synthesized by a constant-pH coprecipitation. The derived oxides from hydrotalcites upon calcination at 800 °C for 4 h were all of spinel phase without crystalline TiO2 phase being detected. Substitution of partial Al for Ti significantly enhanced NO direct decomposition activity of these catalysts. In particular, catalyst Co3.0/Al0.9Ti0.1O (CATO) showed the highest NO direct decomposition percentage, up to 86% at 300 °C with GHSV of 30 000 h1 (800 ppm of NO and 8% O2 in N2 stream). CATO also showed the highest resistance to SO2 poisoning to NO direct decomposition, with the activity being only reduced by 16% in the presence of 64 ppm of SO2 in the mixed gas stream at 300 °C. The in-situ FT-IR spectra indicate different adsorption species over the catalysts, revealing NO surface storage/decomposition involves different adsorption reactions that determine the NO decomposition activity and resistance to SO2 poisoning.

1. INTRODUCTION Reduced CO2 emission and improved fuel efficiency make the diesel and lean-burn engines attractive compared to conventional gasoline engines. However, it is difficult to minimize gaseous pollutants such as NO2 and NO that account for more than 95% nitrogen emissions. The direct decomposition of nitric oxide (NO) to environment-friendly N2 and O2 would therefore be a desirable yet challenging NOx abatement process since it does not involve any other gaseous reducing agents and byproducts.1,2 Over the past two decades, several catalytic systems have been studied for NO decomposition, such as supported metals and oxides, metal-exchanged zeolites, perovskites, amorphous alloys, and membrane catalysts.36 Unfortunately, the low hydrothermal stability and low selectivity of NO decomposition to N2 and O2 are serious drawbacks, which limit the application of NO direct decomposition in practice. It is well-known that the presence of water vapor, O2, and CO2 normally affects the catalytic activity, and the most difficult problem to be solved for these catalysts is SO2 deactivation or poisoning.7,8 A promising candidate is mixed oxide catalyst derived from hydrotalcitelike compounds (HTlcs) that exhibit remarkably high catalytic ability in regard to thermally decomposing NO and capture NO under lean-burn conditions.9 HTlcs, also known as layered doubled hydroxides, are widely used as ion exchangers, adsorbent, catalyst supports, bioactive nanocomposites, and precursors of well-mixed oxides for various catalytic applications.10,11 The chemical composition of HTlcs r 2011 American Chemical Society

can be generally described as [MII1xMIIIx(OH)2]xþ(An)x/n 3 mH2O, where MII and MIII represent most divalent and trivalent metal cations or their combination and An any hydrated anion. It has been shown that monovalent and tetravalent cations, such as Liþ, Sn4þ, Zr4þ, and Ti4þ, could also be incorporated into the octahedral sites in the hydroxide layers.10 Therefore, HTlcs and derived oxides can be tailored with suitable acidbase strengths and redox properties for particular catalytic reactions. Previously, we investigated a number of well-mixed oxides derived from HTlcs as cost-effective NOx storage/reduction catalysts.1215 We noted that derived Co-containing oxides show excellent activity in NO direct decomposition. Two catalysts (CaCoAl-oxide and CaCoLaAl-oxide) can directly decompose 5575% NO into N2 and O2 at 300 °C.14 On the other hand, it is reported that the decomposition temperature of sulfates on titanium dioxide (TiO2) is lower than on Al2O3,16,17 which may improve the sulfur tolerance by incorporating Ti into HTlcs-derived oxide catalysts. To the best of our knowledge, Ti-based HTlcs derivatives have not been examined as catalysts in NOx abatement. Therefore, the objective of this research was to develop HTlcs-derived Co/Ti-containing oxide catalysts with

Received: December 18, 2010 Revised: February 28, 2011 Published: March 18, 2011 6651

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high activity for NO direct decomposition and high resistance to SO2 poisoning.

2. EXPERIMENTAL SECTION 2.1. Material Preparation. Co3Al0.9Ti0.1-HT and Co1.5Ca1.5Al0.9Ti0.1-HT, denoted as CAT-HT and CCAT-HT, respectively, were prepared with a constant-pH coprecipitation method. Typically, mixed TiCl4/HCl solution was added into a salt solution containing a stoichiometric atomic ratio of Co(NO3)2 3 6H2O, Al(NO3)3 3 9H2O, and Ca(NO3)2 3 4H2O if applicable. The resulting solution and a mixed basic solution (NaOH and Na2CO3) were then simultaneously added dropwise into doubly distilled water at constant pH (10 ( 0.5) under vigorous mechanical stirring. The precipitate was aged in suspension at 60 °C for 4 h under stirring in static air, then filtered, and thoroughly washed with doubly distilled water, followed by drying at 120 °C overnight. In comparison, Co3Al-HT and Co1.5Ca1.5Al-HT, good precursors for NOx storage in our previous study1215 (denoted as CA-HT and CCA-HT, respectively), were prepared similarly. As-prepared HTlcs were calcined at 800 °C for 4 h to derive the corresponding mixed oxide catalysts that were named as CATO, CCATO, CAO, and CCAO, respectively. The oxide catalysts were made 2040 mesh in size by pressing, crushing, and sizing and kept in desiccators for subsequent tests. 2.2. Material Characterization. The X-ray diffraction patterns of as-prepared samples were recorded on a Rigaku powder diffractometer (D/max-RB) using Cu KR radiation (λ = 0.154 18 nm) in the 2θ range of 10°70° at scanning rate of 4°/min. The tube voltage and current were set at 40 kV and 30 mA, respectively. The surface area, the interparticle pore size, and volume of oxide catalysts were analyzed with N2 adsorption/desorption at liquid nitrogen temperature (77 K) in a Quantachrome NOVA 1200 gas absorption analyzer. The specific surface area was calculated with the BET equation, and the pore volume and pore size distribution were estimated with the BJH method from the adsorption isotherm. The thermal decomposition of HTlcs was investigated with thermogravimetry (TG, Seteram, Labsys). In a typical measurement, 2030 mg of HTlcs sample was heated in an Al2O3 crucible at a constant heating rate of 10 °C/min from 25 to 1000 °C, with air purging at a flow rate of 30 mL/min. In-situ FT-IR spectra were recorded on a Bruker Tensor 27 spectrometer in the range of 6004000 cm1 after 128 scans at a resolution of 4 cm1. Self-supporting pellets (∼50 mg, 2040 mesh) were prepared from the oxide catalysts and used directly in the IR flow cell. The IR cell, made of stainless steel and containing a KBr window, was connected to a vacuum apparatus at a residual pressure below 104 Pa. A K-type thermocouple was set in direct contact with the IR flow cell to monitor the temperature. Prior to the recording of an IR spectrum, the catalyst sample was degassed for 1 h at 350 °C to eliminate weakly adsorbed species on the sample surface. After the sample was cooled to 100 or 300 °C under vacuum, an IR spectrum of the treated sample was taken as the background. Then, a mixture gas stream (total flow 25 mL/min) containing 1300 ppm of NO, 100 ppm of NO2, and 8% O2 in N2 was introduced for NOx adsorption at 100 or 300 °C. Meanwhile, IR spectra were sequentially recorded at the time points of 1, 2, 5, 10, 20, 30, 40, 50, and 60 min.

Figure 1. XRD patterns of HTlcs samples (A) and derived oxide catalysts (B) where hydrotalcite-like phase was marked as “H” and spinel as “S”.

2.3. Thermal NOx Decomposition and Adsorption/Desorption. Thermal NOx adsorption experiments were carried

out in a quartz flow reactor (i.d. = 8 mm and L = 600 mm) using 1.0 g of one oxide catalyst (2040 mesh). The oxide catalyst was pretreated in a mixed gas of O2/N2 (8% v/v O2) at 500 °C at a constant gas hourly space velocity (GHSV) of ∼30 000 h1 for 1 h and then cooled to 300 °C. After the temperature was stabilized at 300 °C, the inlet gas stream was switched to 800 ppm of NO (or 800 ppm of NO plus 64 ppm of SO2) and 8% O2 in N2 and continued for 30 min for thermal NOx adsorption/decomposition with the same GHSV. Concentrations of NO, NO2, and NOx in the outlet stream were monitored with a chemiluminescence NONO2NOx analyzer (EC 9841, Ecotech Corp.). To determine the presence of other nitrogen species, He was used as the carrier gas and the outlet gas composition and concentration (NO, NO2, N2O, and N2) were simultaneously analyzed with the chemiluminescence NONO2NOx analyzer and a separate gas chromatograph (HP 6820 series, TCD detector, a molecular sieve 5A and a porapak Q column). After the thermal NOx adsorption, the inflow gas was then switched to pure N2 (GHSV = ∼30 000 h1) to flush the oxide catalyst for 20 min at 300 °C to remove the weakly adsorbed 6652

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Table 1. Crystallite Sizes, Textual Properties, and NO Storage/Direct Decomposition Capacity of Catalysts NO storagea

sample

NO direct decompositionb

spinel size

SSA

pore size

pore volume

[NO]

[NO þ SO2]

[NO þ

[NO]

[NO þ SO2]

[NO þ

(nm)

(m2/g)

(nm)

(cm3/g)

(mg/g)

(mg/g)

SO2]/[NO]

(%)

(%)

SO2]/[NO]

CAO

31.1

29.2

17.6

0.13

0.90

0.28

0.31

63.1

47.9

0.76

CATO CCAO

33.1 18.8

21.9 52.6

18.2 19.7

0.10 0.26

0.30 5.28

0.18 1.32

0.60 0.25

85.9 58.6

72.1 32.2

0.84 0.55

CCATO

22.9

52.0

21.7

0.28

6.01

4.03

0.67

77.0

38.9

0.51

The storage amount was deduced from the desorption from the catalyst that adsorbed NO at 300 °C for 30 min. b NO decomposition percentage: (1 [NOx]out/[NOx]in) 100. a

species. When the oxide catalyst was cooled to 100 °C, the temperature-programmed desorption (TPD) was conducted by heating the sample from 100 to 650 °C at a ramp of 10 °C/min with N2 flowing at the same GHSV. Concentrations of NO, NO2, and NOx in the outlet stream were similarly determined, and the adsorbed NOx amount was thus calculated.

3. RESULTS AND DISCUSSION 3.1. Transformation of Hydrotalcites to Mixed Oxides. The X-ray diffraction patterns of HTlcs are shown in Figure 1A. All the samples show the typical peaks at 2θ ≈ 11°, 23°, and 34°, attributed to the (003), (006), and (009) crystal planes in the layered structure with rhombohedral symmetry (3R). In addition, the broad diffraction peaks at ∼35°, 38°, and 46° attributable to the (012), (015), and (018) crystal planes are characteristic of polytype 3R1 hydrotalcite (JCPDS 22-700). The well-defined (110) and (113) diffraction peaks at 60° and 61° reveal an even dispersion of cations in the hydroxide layer.18 These characteristics indicate that Ti4þ ions are well incorporated into the HT structure with the similar crystallinity. The XRD patterns of derived oxides (Figure 1B) show the complete transformation from the HT phase to the oxide spinel phase after calcination at 800 °C. The typical diffraction peaks at 2θ ≈ 31°, 36°, 39°, 45°, 55°, and 65° in Ti-containing and nonTi-containing samples are all attributed to the spinel phase (Co2AlO4, JCPDS 38-0814; CoAl2O4, JCPDS 44-0160 and 822246; Co3O4, JCPDS 74-2120).18 The crystallite size of the spinel phase is 1835 nm (Table 1), as estimated using the DebyeScherrer equation.19 The similarity in XRD patterns has further revealed that Ti ions are distributed in the oxide spinel phase, without TiO2 being segregated. In addition, the obtained mixed oxides have a moderate surface area (2252 m2/g) with the interparticle pore size of about 20 nm (Table 1). In general, thermal decomposition of HTlcs consists of two steps (see Supporting Information, Figure S1), i.e., dehydration of interlayer and adsorbed water molecules at 100250 °C and dehydroxylation of interlayer hydroxyl groups and decomposition of interlayer carbonate at 250350 °C.11 The latter step results in collapse of the layered structure and formation of the spinel phase. Seemingly, the introduction of Ti facilitates the decomposition steps, probably due to the lower thermal stability of TiOH bond and its weaker interaction with H2O with respect to those of AlOH bonds.20 3.2. NO Direct Decomposition over Oxide Catalysts. NOx outlet concentration profiles over oxide catalysts Co1.5M1.5/ Al1xTixO (M = Co, Ca and x = 0, 0.1) at 300 °C are presented in Figures 2 and 3. As expected, NO was completely trapped in

the initial period (60400 s) for all catalysts as almost no NOx was detected in the outlet stream. Afterward, NO was continuously trapped, a small amount of NO2 (3080 ppm) was detected, and NO concentration in the outlet gradually recovered to a certain level (100530 ppm) when the NOx concentration became steady after adsorption for 2030 min. It is interesting to note that the NO2 concentration in the outlet increased to a maximum and then decreased to a lower level. In comparison, the Ti-containing catalysts shortened the NOx complete trapping time and also decreased the NOx concentration in the outlet stream. For example, the NOx complete trapping time over CAO catalyst was 150 s with NOx being recovered to 290 ppm in the outlet stream (Figure 2). With Ti incorporated, the complete adsorption over CATO only lasted for 60 s while the NOx concentration in the downstream was only 105 ppm (Figure 2). A similar phenomenon was observed for catalysts CCAO and CCATO (Figure 3). Obviously, the steady outlet NOx concentration (NO þ NO2) was always lower than the feed NO (800 ppm) during the whole process, which suggests that the disappearing portion of NOx is transformed to nitrogen species other than NO and NO2, such as N2 and N2O.2 N2, but N2O, was detected with the GC when helium was used as the carrier gas. As shown in Figure 4, N2 was found to be about 200 ppm over CAO and about 300 ppm over CATO in the outlet stream during adsorption at 300 °C. The generation of N2 suggests part of NO directly decomposing to N2. More remarkably, the profile of the outlet nitrogen equivalent (TNout = 2N2 þ NO þ NO2) almost overlapped that of the inlet NOx after adsorption for 600 s (10 min) if the measurement error with GC was considered. This further reflects that the disappearing NOx is truly decomposed to N2. On the basis of this assumption, the NO direct decomposition percentage over Co1.5M1.5/TixAl1xO catalysts at the adsorption time of 30 min was calculated. As listed in Table 1, all catalysts directly decomposed 5886% NO at 300 °C, showing a high activity for NO direct decomposition in comparison with various other catalysts.2123 In contrast, Ti-containing catalysts decomposed about 20% more NO than the non-Ti-containing catalysts, indicating the facilitation effect of TiO component on NO direct decomposition. 3.3. NO Storage over Oxide Catalysts. Figures 5 and 6 show desorption profiles of adsorbed NOx from CAO, CATO, CCAO, and CCATO catalysts in 100650 °C. The adsorbed NOx was previously generated during 30 min adsorption at 300 °C over these catalysts. Obviously, NOx desorption started at around 300 °C and mainly occurred in 300500 °C (CAO and CATO) or in 400600 °C (CCAO and CCATO). In these cases, NO was the major desorbed species (2001000 ppm), with the 6653



Figure 2. Profiles of NO adsorption on catalysts CAO and CATO at 300 °C in the presence or absence of SO2.

amount of desorbed NO2 being ignorable (1040 ppm). Note that the desorption peak from Ti-containing CATO was much weaker (from 180 to 50 ppm) and shifted to a lower temperature (from 423 to 388 °C) than that from CAO (Figure 5C). This reveals that Ti incorporation into CAO destabilizes the adsorbed NOx on the catalyst surface. By contrast, the desorption peak from the Ti-incorporating catalyst CCATO was intensified

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Figure 3. Profiles of NO adsorption on catalysts CCAO and CCATO at 300 °C in the presence or absence of SO2.

(from 750 to 950 ppm) and shifted from 478 to 508 °C (Figure 6C) in comparison with CCAO. Very interestingly, there were two desorption events, at 478 and 580 °C, while desorption from Ti-incorporating catalyst CCATO occurred as one event between 478 and 578 °C. This indicates that Ti incorporation into CCAO mediates the interaction of adsorbed NOx with surface bonds, with slight strengthening. Our previous research revealed that N2 was also a desorbed species from oxide catalyst CoCaLaAlO,14 accounting for 6654



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Figure 4. Nitrogen species profiles before and after passing catalysts CAO (A) and CATO (B) in helium at 300 °C where total nitrogen coming in (TNin) was the NO concentration supplied and total nitrogen going out (TNout) was calculated by adding [NO], [NO2], and 2[N2], which were measured in the outlet stream.

2030% of the adsorbed NOx. If N2 desorption was not considered in the current case, the storage amount of NOx over these oxide catalysts could be obtained from the desorption profiles of NO and NO2 (Figures 5 and 6). As listed in Table 1, the NO storage amount of CCATO is higher than CCAO while that of CATO is smaller than CAO. In comparison, the storage capacity of CCAO and CCATO is much larger than CAO and CATO (Table 1). We used the in-situ IR to dynamically monitor formation of nitrogen species over these catalysts. As shown in Figure 7A for catalyst CAO as an example, the strong peak, initially located at 1539 cm1 and then slightly shifted to 1552 cm1, indicates formation of chelating bidentate nitrate due to adsorption of NO and then oxidation with O2 over the catalyst surface.2426 Some other minor species, such as NOþ (solid, 2346 cm1) and bridged bidentate nitrite (1234 cm1), were also present.27,28 When Ti was included (CATO catalyst), the formed species were quite similar, only with the strongest peak shifting to

15461563 cm1 (Figure 7B). Note that the spectral changes (intensity and peak position) took place just in the first 20 min, after which the IR spectra were very similar in the two cases. In the case of CCAO (Figure 8), the observation is much different. The initial weak peak at 13081316 cm1 became the strongest peak in the later adsorption stage, which can be assigned to the bidentate/monodentate nitrate.2528 This change, together with the disappearing of peaks at 1588 and 1060 cm1 (monodentate nitrites), suggests a redox conversion from nitrite to nitrate in the presence of Co oxide. On the contrary, the peak at 12971316 cm1 over CCATO was the highest one and gradually intensified with the adsorption time, indicating the Ti facilitation to direct formation of nitrate through oxidation of NO. 3.4. Influence of SO2 on NO Direct Decomposition and Storage. SO2 is a main cause for deactivation of deNOx catalysts. In order to study the sulfur poisoning of the catalyst, 64 ppm of SO2 was introduced to the inlet gas. The resultant NOx 6655



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Figure 5. NOx desorption profiles on catalysts CAO and CATO in the presence or absence of SO2.

Figure 6. NOx desorption profiles on catalysts CCAO and CCATO in the presence or absence of SO2.

concentration profiles in the presence of SO2 over the catalysts are also shown in Figures 2 and 3. In general, the presence of SO2 in the feed prohibited the catalyst activity. For example, the existence of SO2 increased the recovered NOx concentration by 100300 ppm in the outlet stream, thus meaning that NO direct decomposition over these catalysts has been partly prohibited by SO2. The presence of SO2 severely affects the catalytic stability of CCAO and CCATO (Figure 3). If there was no SO2 in the inlet

stream, the concentration of gaseous NO and NO2 became steady after 1020 min adsorption. However, the presence of SO2 resulted in a continuous increase of NOx concentration until 30 min. Further data analysis reveals that the resistance of these catalysts to SO2 poisoning varies significantly. As listed in Table 1, the presence of SO2 decreased the NO decomposition percentage at the end point of adsorption. If the ratio in respect with the case without SO2 introduced was regarded as the indicator of the 6656



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Figure 7. In-situ IR spectra of NO adsorption on catalysts CAO and CATO at 300 °C in the presence or absence of SO2.

resistance (Table 1), CAO and CATO were highly resistant to SO2 poisoning. In comparison, incorporation of Ti into CAO increased the resistance to SO2 poisoning, i.e., enhancing the ratio from 0.76 to 0.84. However, introduction of Ti into CCAO did not much affect the resistance (Table 1). The negative impact of SO2 on NO adsorption has been also reflected by the IR peak changes in the dynamic monitoring during the NO adsorption at 300 °C over catalysts in the presence of 64 ppm of SO2. Basically, the introduction of SO2 in the feed stream weakened the peak intensities of nitrates and nitrites and also led to a new peak ascribed to sulfate. In particular for CAO, the peak at 1539 cm1 (bidentate nitrate) was shifted to 1576 cm1 with much weaker intensity (Figure 7C) at the end of adsorption in comparison with that with SO2 absence (Figure 7A), while monodentate nitrate (featured with IR peaks at 1490 and 1350 cm1) seemed to be the most abundant species, and bidentate sulfate (peak at 11491173 cm1) was also formed.26,27,2932 Similar observation can be made for catalyst CATO, seemingly with even more monodentate nitrate (IR peak at 1362 cm1, Figure 7D). Therefore, the presence of

SO2 affects the adsorbed NOx in both adsorption species and strength over catalysts CAO and CATO. Furthermore, the formation of sulfate on the catalyst surface reduced the active sites available for NOx adsorption, thus decreasing the NOx storage capacity, as also reflected by the NOx desorption peak area in Figure 6C and the values in Table 1. For catalyst CCAO (Figure 8C), the strong peak at 1291 cm1 is ascribed to bidentate nitrate, which overlapped with the peak at 1162 cm1 that is due to the formed sulfate. A slightly different observation can be made for catalyst CCATO. The peak at 1305 cm1 was quickly intensified within 10 min, while the shoulder at 11501200 cm1 started to show up at 20 min and became intense with the adsorption time (Figure 8D). Obviously, the peak of sulfate is continuously intensified over CCAO and CCATO, and much stronger than that over CAO and CATO, revealing that more SO2 is adsorbed over Cacontaining catalysts. It is worth noting that there were a few negative peaks between 1350 and 1600 cm1 developing with the adsorption time in dynamic IR spectra of Ca-containing catalysts (CCAO and CCATO) (Figure 8C,D). These negative 6657



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Figure 8. In-situ IR spectra of NO adsorption on catalysts CCAO and CCATO at 300 °C in the presence or absence of SO2.

peaks are suggested to be due to decarbonation of Ca-containing CCAO and CCATO that would be carbonated during the operation.33 When NOx and SO2 are oxidized and adsorbed as nitrate and sulfate, the acidity associated with nitric/sulfuric acid will convert carbonate into CO2 that will be released into the outlet stream, resulting in the negative peaks. A more severe influence of SO2 in the stream is that the formed sulfate competitively occupies the basic sites of CCAO and CCATO catalysts. Since the formed sulfate is more thermally stable than nitrites and nitrates, thus the basic sites available for storing the various nitrites and nitrates become fewer and fewer.34,35 As a consequence, the NOx storage capacity decreases, and moreover, the catalytic activity for NO decomposition reduces because less adsorbed NOx is available for decomposition on the catalyst surface. 3.5. NO Reactions over Catalyst Surfaces. The nitrogen species formed on the oxide catalysts and their thermal desorption products provide some information relevant to NO storage/ decomposition mechanism. As described previously, there are various nitrogen species formed during the adsorption, such as monodentate/bidentate nitrates (MNO3) and nitrites

(MNO2), and NOþ (probably regarded as adsorbed NO). To simplify the expression, we proposed the NOx surface reaction network, as presented in Figure 9. In this network, gaseous NO is first adsorbed (step 1). The adsorbed NO is then oxidized to nitrite (step 2) and further to nitrate (step 3). As-formed nitrate can be decomposed to NO2 (step 4) and released to the stream (step 5), which may explain the conversion of NO to NO2, as observed in all adsorption tests (Figures 2 and 3). In this reaction line, the existence of NO(ad) (tentatively regarded as NOþ), MNO2, and MNO3 has been confirmed by in-situ IR. Of course, the adsorbed NO can also be nitrogenized to N2O (step 6). The as-formed N2O could be either released to the gas phase (step 7) or decomposed to gaseous N2 (steps 8 and 9). However, in-situ IR spectra did not show the characteristic peak of adsorbed N2O at 2010 cm1,14 and we did not detected the gaseous N2O in the outlet stream; thus, it is less likely for these reactions (steps 69) to take place at 300 °C. The other most likely reaction line is the decomposition of adsorbed NO (step 10) to adsorbed N that is then combined to form N2 (step 11) and released as N2 into the stream. Since there is only 105 ppm of NO left in the outlet stream over catalyst CAO 6658



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Figure 9. Schematic outline of NOx reaction network over oxide catalysts. Number indicates the transformation between the two species arrows connect, MO stands for surface metaloxygen species, and “ad” and “g” in the parentheses mean the adsorbed and gaseous species.

in the steady adsorption/reaction (Figure 2), the disappearing NO (695 ppm) is thus decomposed to N2. Therefore, these reactions (steps 1012) would be predominant during the NO steady adsorption/reaction over catalysts CAO and CATO. This reaction network may also explain the influence of SO2. With regard to the NO direct decomposition, SO2 could compete with NO to occupy the similar active sites and reduce the chance for NO adsorption (step 1), which would decrease the NO decomposition percentage, as observed in the research. On the other hand, the formation of sulfate/sulfite with metal cations would lead to less storage sites for nitrate and nitrite, which decreases the NO storage capacity. Since catalysts CAO and CATO have much weaker alkalinity than catalysts CCAO and CCATO, so-formed CoSO4 species on CAO/CATO is less thermally stable than CaSO4 on CCAO/CCATO.36,37 Therefore, the interference of SO2 with NO decomposition/storage over catalysts CAO/CATO is much weaker, without losing the catalyst stability; e.g., they have a higher sulfur resistance (Figure 2). In contrast, CaSO4 on CCAO/CCATO is continuously formed, resulting in less storage sites and active sites and decreasing the activity and catalyst stability (Figure 3).

4. CONCLUSIONS Co1.5M1.5/TixAl1x hydrotalcite-like compounds (M = Co, Ca and x = 0, 0.1) were successfully prepared with a constant-pH coprecipitation, and its calcination at 800 °C in the air derived the mixed Co1.5M1.5/TixAl1xO oxide with a good Ti dispersion. The derived oxide catalysts exhibit very good performance for NO direct decomposition. Ti incorporation enhances the NO decomposition activity while has an opposite effect on the storage capacity of catalysts CCAO and CAO. SO2 shows the negative impact on NO adsorption and direct decomposition. In particular, the Ti containing catalyst CATO has displayed a very high sulfur resistant capacity for NO direct decomposition, showing a potential cost-effective catalyst in de-NOx process. ’ ASSOCIATED CONTENT

bS

Supporting Information. Figure S1. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel þ86-10-62849194, Fax þ86-10-62923564, e-mail [email protected] (Z.P.H.); Tel 61-7-33469973, Fax 61-733656074, e-mail [email protected] (Z.P.X.).

’ ACKNOWLEDGMENT This work was financially supported by the Chinese National Science Fund for Distinguished Young Scholars (No. 20725723) and the Natural Science Foundation of China (No. 20877088). Dr. Xu also appreciated the support from the ARC Centre of Excellence for Functional Nanomaterials, funded by the Australia Research Council under its Centre of Excellence Scheme. ’ REFERENCES (1) McMichael, A. J., Campbell-Lendrum, D. H., Corvalan, C. F., Ebi, K. L., Githeko, A. K., Eds.; Climate Change and Human Health: Risks and Responses; World Health Organization: Geneva, 2003. (2) Garin, F. Appl. Catal., A 2001, 222, 183–219. (3) Dedecek, J.; Capek, L.; Wichterlova, B. Appl. Catal., A 2006, 307, 156–164. (4) Shi, Y.; Pan, H.; Li, Z. J.; Zhang, Y. T.; Li, W. Catal. Commun. 2008, 9, 1356–1359. (5) Silveira, E. B.; Perez, C. A. C.; Baldanza, M. A. S.; Schmal, M. Catal. Today 2008, 133, 555–559. (6) Komvokis, V. G.; Marnellos, G. E.; Vasalos, I. A.; Triantafyllidis, K. S. Appl. Catal., B 2009, 89, 627–634. (7) Nakatsuji, T.; Yasukawa, R.; Tabata, K.; Ueda, K.; Niwa, M. Appl. Catal., B 1999, 21, 121–131. (8) Huang, H. Y.; Long, R. Q.; Yang, R. T. Energy Fuels 2001, 15, 205–213. (9) Yu, J. J.; Cheng, J.; Ma, C. Y.; Wang, H. L.; Li, L. D.; Hao, Z. P.; Xu, Z. P. J. Colloid Interface Sci. 2009, 333, 423–430. (10) Cavani, F.; Trifir, F.; Vaccari, A. Catal. Today 1991, 11, 173–301. (11) Braterman, P. S.; Xu, Z. P.; Yarberry, F. In Handbook of Layered Materials; Auerbach, S. M., Carrado, K. A., Dutta, P. K., Eds.; Marcel Dekker, Inc.: New York, 2004; p 373. (12) Yu, J. J.; Jiang, Z.; Zhu, L.; Hao, Z. P.; Xu, Z. P. J. Phys. Chem. B 2006, 110, 4291–4300. (13) Yu, J. J.; Tao, Y. X.; Liu, C. C.; Hao, Z. P.; Xu, Z. P. Environ. Sci. Technol. 2007, 41, 1399–1404. (14) Yu, J. J.; Wang, X. P.; Li, L. D.; Hao, Z. P.; Xu, Z. P.; Lu, G. Q. Adv. Funct. Mater. 2007, 17, 3598–3606. (15) Yu, J. J.; Wang, X. P.; Tao, Y. X.; Hao, Z. P.; Xu, Z. P. Ind. Eng. Chem. Res. 2007, 46, 5794–5797. (16) Huang, H. Y.; Long, R. Q.; Yang, R. T. Appl. Catal., B 2001, 33, 127–136. (17) Macleod, N.; Lambert, R. M. Catal. Commun. 2002, 3, 61–65. (18) Xu, Z. P.; Zeng, H. C. J. Phys. Chem. B 2000, 104, 10206–10214. (19) Cullity, B. D. Elements of X-ray Diffraction, 2nd ed.; AddisonWesley: Boston, MA, 1978; p 278. (20) Zhang, W. H.; Guo, X. D.; He, J.; Qian, Z. Y. J. Eur. Ceram. Soc. 2008, 28, 1623–1629. 6659



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