Synthesis of Mg-Doped Ordered Mesoporous Pd ... - ACS Publications

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Synthesis of Mg-Doped Ordered Mesoporous Pd−Al2O3 with Different Basicity for CO, NO, and HC Elimination Yihong Xiao,† Xiaohai Zheng,†,‡ Xiaohua Chen,‡ Lilong Jiang,*,† and Ying Zheng†,‡ †

National Engineering Research Center of Chemical Fertilizer Catalyst, Fuzhou University, Fuzhou, Fujian 350002, China College of Materials Science and Engineering, Fujian Normal University, Fuzhou, Fujian 350007, China

‡

S Supporting Information *

ABSTRACT: Employing NH4HCO3 as pore-enlarging agent and P123 as a template agent, ordered mesoporous Mg-doped γAl2O3 with different basicity and large surface area have been successfully synthesized by a facile sol−gel approach by adjusting the content of magnesium and pH value. Specifically, the result alumina calcined at 1000 °C with large specific surface area (234.4 m2 g−1) and high pore volume (0.72 cm3 g−1) was obtained when the Mg content was 9 wt %. These materials with ordered mesostructure and advantageous structural properties were utilized as carriers of Pd−Al2O3 catalysts for the catalytic reaction of simulated exhaust automobile gases including CO, NO, and hydrocarbon (HC). The results revealed that the high catalytic performance of Pd/9Mg-OMA originated from its more appropriate basicity, greater PdO dispersion, and higher surface area. calcined at 1000 °C. Moreover, the application of the catalysts with OMA as carrier has also gained much attention. Li6 applied a facile route to prepared OMA-supported catalysts with Pt or Pd nanoparticles well-dispersed, which exhibits high thermal stability and high catalytic activity when applied in a CO oxidation reaction. The evaporation-induced self-assembly (EISA) process was employed by Bordoloi7 to achieve ordered mesoporous mixed cobalt−aluminum oxides with Co 2+ homogeneously distributed in an alumina matrix. The wellordered 15Co−Al2O3 composite turned out to be highly active in CO oxidation. The surface acid−base properties of the catalyst carrier will also strongly affect the performance of catalyst. Therefore, the alkaline earth metals (including Be, Mg, Ca, Sr, or Ba) were added to improve the surface acid−base properties and catalytic performances of the catalysts. The MgO is alkali metal oxides (Hammet constant H = +26.0), which is usually selected as carriers or basic modifiers for Pd supported catalysts.8

1. INTRODUCTION Since the environmental laws and regulations of exhaust emissions have become more and more strict, catalysts with more efficient catalytic performances are required, especially those with good acid−base properties and low temperature activity. Three-way catalysts (TWCs) have been widely used,1 which provides an efficient way to reduce automobile exhaust of carbon monoxide (CO), nitrogen oxides (NOx) and hydrocarbons (HC). The working temperature of gasoline engines is generally higher than 1000 °C, which makes the commonly used catalyst support γ-Al2O3 undergo a phase transition and sinter.2 Therefore, much attention has been paid to the improvement of the thermal stability of γ-Al2O3. Ordered mesoporous alumina (OMA) could effectively restrain the phase transformation from γ-Al2O3 to α-Al2O3 because of its excellent texture properties, such as large BET surface area, tunable pore sizes, and long-range ordering of the pore packing, which makes it extensively applied as catalyst carrier.3 Niesz4 first managed to synthesize OMA with P123 as the structuredirecting agent via a sol−gel-based self-assembly technique. Yuan et al.5 reported a modified sol−gel method to the synthesis of OMA with highly ordered mesoporous structure, but the specific surface area only remains 116 m2 g−1 after being © 2017 American Chemical Society

Received: Revised: Accepted: Published: 1687

September 29, 2016 December 6, 2016 January 25, 2017 January 25, 2017 DOI: 10.1021/acs.iecr.6b03799 Ind. Eng. Chem. Res. 2017, 56, 1687−1695

Article

Industrial & Engineering Chemistry Research

of the samples were conducted by N2 adsorption−desorption experiments through an U.S. Micromeritics ASAP 2020 M analyzer. Differential thermal analysis (DTA) measurements were accomplished in a perkin-Elmer 1700 device under air atmosphere. Transmission electron microscopy (TEM) studies were performed with an FEI Tecnai G2 F20 S-Twin (Netherlands) transmission microscope operated at 200 kV. X-ray photoelectron spectroscopy (XPS) spectra were acquired in a Thermo ESCALAB 250 spectrometer (USA) using Al Kα X-ray radiation (hυ = 1486.6 eV), the data of which was corrected with the C1s binding energy (BE) at 284.8 eV. CO2 temperature-programmed desorption (CO2-TPD) studies were performed on an AutoChem 2920 instrument (Micromeritics) employing a thermocouple detector TCD detector to clarify the adsorption properties of CO2 on catalysts. The sample (0.3 g) was pretreated at 400 °C and held for 1 h in a He stream (10 °C min−1, 30 mL min−1) to remove the impurities and moisture, and then cooled down to 50 °C. Then a pure CO2 flow was pumped into the reactor for 1 h. Finally, the CO2-TPD measurements were performed from 50 to 800 °C at a heating rate of 10 °C min−1 under He flow. CO pulse chemisorption experiments were accomplished on the same apparatus as described for CO2-TPD. Approximately 0.3 g of each sample was pretreated in a pure He flow from 30 to 400 °C for 1 h at a heating rate of 10 °C min−1, then reduced in the gas flow of 10% H2/Ar (30 mL min−1) at 400 °C during 1 h followed by cooling down to 25 °C to carry out CO chemisorption. The dispersions, the metallic surface area, and particle diameter of PdO were recorded by pulse method. H2 temperature-programmed reduction (H2-TPR) measurements were accomplished in a chemisorption analyzer as was used for CO2-TPD. The catalyst (about 0.3 g) was pretreated in a flow of helium from 30 to 400 °C for 0.5 h to remove water vapor. The TPR profiles were acquired by pumping a 10% H2/ Ar (30 mL min−1) through the preheated catalyst with the temperature increasing from 30 to 900 °C (10 °C min−1). In situ diffuse reflectance infrared Fourier transform spectroscopy (in situ-DRIFTS) was performed by using a Nicolet-6700 FTIR spectrometer with an MCT detector. The spectra were obtained at 2 cm −1 resolution and an accumulation of 64 scans. The catalysts were pretreated with He at 300 °C for 0.5 h to remove impurities. (1) CO adsorption experiments were carried out under 1%CO/He for 1 h. The CO was then purged by helium for 0.5 h and the the spectra was again recorded. The sample was first reduced by 10% H2/Ar during 1 h at 300 °C. (2) CO2 chemisorbed experiments were taken after pure CO2 flow was then fed into the reactor for 30 min. The DRIFT spectra were collected at different temperatures. 2.4. Catalytic Performance Measurement. All tests were performed in an evaluation apparatus that simulated automobile exhaust. The reaction mixture containing CO (0.89%), O2 (0.76%), CO2 (11.5%), NO (800 ppm), HC (150 ppm, C3H6), and balance gas (N2) were introduced into the reactor at 45 000 h−1 GHSV, λ = 1. A 0.3 g sample of catalyst (2040 mesh) was held in a quartz tube (Φ 8 mm) by packing quartz wool at the end of the catalyst bed with 210 mL min−1 flow rate. The light-off (T50%) and complete conversion temperature (T100%) was recorded, and the CO, NO, and HC conversions were calculated by the following equations:

Moreover, the thermal stability of alumina can be improved by doping magnesium according to the reference.9 Arnby10 yielded catalysts modified by Mg with high thermal stability through incipient wetness impregnation by using platinum nitrate solution and Mg(NO3)2 as precursor compounds. The lowtemperature activity of Pt/γ-Al2O3 catalysts for carbon monoxide oxidation was slightly enhanced by adding magnesium. Yamaguchi11 prepared the Mg−Al composite oxides through the calcination of hydrotalcites. The composite oxide with the mole ratio of Mg/Al = 5 possessed the highest catalytic activity when acting as efficient catalysts for the CO2 reaction and styrene oxide because of the synergistic reaction of basic and acidic sites. Xu12 yielded ordered mesoporous OMxMgyAl composite oxides with high thermal stability through a one-pot EISA method; the catalysts exhibited high catalytic activities toward the reaction for CO2 reforming CH4. Herein, a series of thermally stable Mg-doped OMA with strong basicity and large BET surface area were facilely prepared through a simple sol−gel method. The increase of BET surface area, pore volume, and pore sizes can be attributed to the increase of micellar template sizes as well as the release of CO2 and NH3 in the calcination process by introducing of NH4HCO3. Ordered mesoporous Mg-doped Pd-alumina catalysts were prepared by the impregnation method with Mg-doped OMA as carriers. The effect of Mg content on catalytic properties and physicochemical characteristics of the catalysts were investigated. The results reveal that these materials with advantageous structural properties and different basicity show higher catalytic activity for conversion of CO, NO, and hydrocarbon (HC) compared with the undoped sample.

2. EXPERIMENTAL SECTION 2.1. xMg-OMA Supports. The required amounts of Mg(NO3)2 and approximately 1.00 g of EO20PO70EO20 triblock copolymer (Mn = 5800, Aldrich) were dissolved in a mixed solution containing anhydrous ethanol (20 mL) and hydrochloric acid (1.0 mL, 37 wt %) at room temperature. The other materials were then added into the solution in the molar ratio of NH4HCO3/acetic acid/aluminum isopropoxide (AIP) = 0.1:0.15:1 under vigorous agitation for at least 5 h, adjusting the final pH to 1.0−1.2. This solution was dried in air at 60 °C for 6 h and 80 °C for 8 h to evaporate the solvent, respectively. Afterward, the calcination was taken with the temperature increasing from 25 to 550 °C (1 °C min−1 ramping rate) for 4 h in air. Then they were calcined at required temperature (800 or 1000 °C) by using a heating rate of 10 °C min−1 during 1 h. The 0, 3, 6, 9, 12, and 15 wt % Mg doped samples were referred as xMg-OMA (x stands for the Mg content in wt %). 2.2. Catalyst Preparation. Supported Pd catalysts on xMgOMA were synthesized through incipient wetness impregnation route with 6 h impregnation in Pd(NO3)2 aqueous solution (0.028 g/mL) at room temperature. Then the catalyst precursors were dried at 110 °C for 6 h and this was followed by calcination at 800 or 1000 °C for 2 h by using a 10 °C min−1 ramping rate. Then the as-synthesized catalysts were crushed and sieved (20−40 mesh particles). The Pd loading was 0.5 wt % (theoretic value). All the as-prepared catalysts were designated as Pd/xMg-OMA in the following text. 2.3. Characterization Techniques. X-ray diffraction (XRD) measurements were collected on a Philips MPD diffractometer (X’Pert Pro) using Cu Kα radiation (λ = 0.15046 nm), operated at 40 mA and 40 kV. Texture properties

CO conversion% = 1688

[CO]in − [CO]out × 100 [CO]in DOI: 10.1021/acs.iecr.6b03799 Ind. Eng. Chem. Res. 2017, 56, 1687−1695

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Industrial & Engineering Chemistry Research NO conversion% =

[NO]in ‐[NO]out × 100 [NO]in

HC conversion% =

[HC]in ‐[HC]out × 100 [HC]in

noteworthy that the undoped OMA calcined at 800 °C displayed six diffraction peaks assigned to the (220), (311), (222), (400), (440), and (511) reflections of the γ-phase alumina16 (JCPDS 10-0425) at around 32.9°, 37.1°, 39.5°, 45.5°, 60.7°, and 67.0°, respectively. As for Mg-doped OMA, the intensity of the diffraction peaks decreases with the Mg content gradually increasing. When the Mg content is 15 wt %, only one weak diffraction peak is observed in 15Mg-OMA. The results suggest that the crystal temperature of γ-alumina can be improved by Mg doping. When calcined at 1000 °C, all Mgdoped OMAs show remarkable MgAl2O4 diffraction peaks (JCPDS 87-0345).17 It can be noted that the position of the diffraction peaks shifts to lower 2θ values with increasing Mg content, the lattice parameters of the samples also increase, which indicates that the Mg species incorporated into the matrix of the alumina lattice.18 Besides, all samples heated at 1000 °C (Figure 2B) display higher intensity of diffraction peaks than those heated at 800 °C (Figure 2A), which suggests that all of them possess higher crystallinity. Table 1 gives the textural parameters of xMg-OMA calcined at 1000 °C. The BET surface area and pore volume of xMg-

3. RESULTS AND DISCUSSIONS 3.1. XRD and N2 Adsorption Measurements. The smallangle X-ray diffraction (SXRD) pattern of Mg-doped OMA shown in Figure 1 was used to assess the formation of ordered

Table 1. Textural Parameters of the Samples Prepared with Different Content of Mg Calcined at 1000 °C

Figure 1. Small-angle XRD patterns of the xMg-OMA materials calcined at (A) 800 °C and (B) 1000 °C (a, 0Mg-OMA; b, 3MgOMA; c, 6Mg-OMA; d, 9Mg-OMA; e, 12Mg-OMA; f, 15Mg-OMA).

without NH4HCO3

mesostructures. The samples calcined at 800 °C display two diffraction peak corresponding to the (100) and (110) reflections at around 0.71° and 1.08°, giving a hint that the obtained xMg-OMA possess a 2D hexagonal structure with a P6mm space group.13 With the rise of Mg content, the intensity of the (100) diffraction line of xMg-OMA first increases and then decreases. The phenomenon suggests that a moderate content of Mg is in favor of the higher ordering degree. Simultaneously, the peaks move toward relatively lower values with increasing Mg content, indicating that the segmental existence of Mg2+ in the Al2O3 lattice influences the cell parameter (a0) and d-spacing (d100) values, which were calculated in accordance with Meng14 (Supporting Information, Table S1). The result can be attributed to the incorporation of Mg species into the matrix of Al2O3 resulting in the formation of Mg−O−Al bonds. As for the samples calcined at 1000 °C (Figure 1B), the diffraction peaks show an obvious shift to a higher angle, which indicates a constriction of the grain size after heat treatment at high temperature.15 The wide-angle XRD patterns of xMg-OMA with different magnesium content were presented in Figure 2A. It was

with NH4HCO3

SBET

Vp

Dp

SBET

Vp

Dp

samples

m2 g−1

cm3 g−1

nm

m2 g−1

cm3 g−1

nm

Mg-OMA-0 Mg-OMA-3 Mg-OMA-6 Mg-OMA-9 Mg-OMA-12 Mg-OMA-15

162.5 184.2 192.1 212.3 189.6 178.9

0.48 0.53 0.51 0.56 0.49 0.42

11.7 10.5 9.4 9.8 9.1 10.6

183.6 197.3 211.7 234.4 207.8 191.5

0.52 0.58 0.66 0.72 0.63 0.57

12.1 11.3 10.9 10.5 10.8 11.3

OMA gradually increase with increasing Mg content (3−9 wt %) in comparison with those of undoped OMA. Meanwhile, 9Mg-OMA displays larger BET surface area (212.3 m2 g−1) and bigger pore volume (0.56 cm3 g−1) than others; the value is also higher than some other Mg-doped OMAs under the same calcined temperature reported by Pan.19 With the Mg content exceeding 9 wt %, both the BET surface area and pore volume of xMg-OMA were decreased, indicating that only a small amount of modifier can improve the textural parameters.20 To further improve the pore volume and BET surface area, the NH4HCO3 was introduced into the synthesis system of xMgOMA. It can be observed from Table 1 that all the samples with NH4HCO3 possess higher BET surface area and larger pore volume than those without NH4HCO3, the improvement of textural properties on the one hand can be attributed to the enlarged micellar template dimensions derived from the existence of additional NH4HCO3 molecules between the alumina precursor and P123. On the other hand the enlargement of the BET surface area and pore size is due to the change of the intracrystalline porosity during thermal decomposition (the release of CO2 and NH3).21 Moreover, the pore size of all samples also increases with with the addition of NH4HCO3; the results suggest that the pore size can be controlled in a wide range. Therefore, the Pd species may penetrate into the pore due to the larger pore size of the support.

Figure 2. Wide-angle XRD patterns of the xMg-OMA materials calcined at (A) 800 °C and (B) 1000 °C (a, 0Mg-OMA; b, 3MgOMA; c, 6Mg-OMA; d, 9Mg-OMA; e, 12Mg-OMA; f, 15Mg-OMA). 1689

DOI: 10.1021/acs.iecr.6b03799 Ind. Eng. Chem. Res. 2017, 56, 1687−1695

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deficiency of γ-Al2O3 decreased, which improves the thermal stability of the obtained alumina. In view of the fine structure properties of xMg-OMA, the TEM analysis presented in Figure 5 was carried out to confirm

Taking the well textural properties of samples with adding NH4HCO3 into consideration, the N2 adsorption−desorption isotherms of xMg-OMA are presented in Figure 3. All the

Figure 5. TEM images of 9Mg-OMA calcined at (A) 800 °C and (B) 1000 °C.

Figure 3. Nitrogen adsorption−desorption isotherms (A) and pore size distributions (B) of the xMg-OMA materials with NH4NO3 calcined at 1000 °C (a, 0Mg-OMA; b, 3Mg-OMA; c, 6Mg-OMA; d, 9Mg-OMA; e, 12Mg-OMA; f, 15Mg-OMA).

the presence of well-defined hexagonal mesostructures with p6mm symmetry. It can be clearly observed that 9Mg-OMA displays the alignment of highly ordered hexagonal pores along [001] (insets) and the arrangement of parallel cylindrical channels along [110] after being calcined at 800 °C (Figure 5A),25 which is well consistent with the SXRD results (Figure 1A). After the calcined temperature increases to 1000 °C, its ordered mesoporous structure can still be well retained (Figure 5B). The results reveal that the obtained xMg-OMA possesses a highly ordered mesostructure and high thermal stability. 3.2. Catalytic Activity of the Catalysts. The completeconversion temperatures (T100%) of HC, CO, and NO for the catalysts Pd/xMg-OMA are presented in Table 2. It can be observed from the table that whether the catalysts are heated to 800 or 1000 °C, the complete conversion temperature of Pd/ xMg-OMA is lower than that of the undoped samples, indicating that the doping of Mg enhances the catalytic activities of Pd-supported alumina. It is also noticeable that the complete conversion temperatures of Pd/9Mg-OMA for all the target pollutants are lower than the others. Combined with the textural parameters (Table 1) and thermal stability (Figure 4) of supports as well as the Pd dispersion (Table 4) of all catalysts, the conclusion is drawn that the large surface area and high thermal stability of supports are in favor of the Pd dispersion, which makes the target pollutants in full contact with the active component and brings about the improvement of the catalytic performance,26,27 while excessive Mg content (12 and 15 wt %) may decrease the catalytic performance because it can weaken the interaction of Pd−Al2O3. Therefore, one can draw a conclusion that the excessive content of magnesium may produce a negative impact on the catalytic activity. After being calcined at 1000 °C, the catalytic activities of the catalysts suffered a certain degree of decrease. This phenomenon was suggested to be fairly related to the high temperature treatment, which makes the active sites of Pd evaporate and sinter.28 It is worth noting that the complete conversion temperature (T100%) of HC and NO for the catalysts Pd/9Mg-OMA are 326.3 and 342.8 °C, both being lower than the results over Pd−Ce0.7Zr0.3O2−Al2O329 and Pd/(Ce, Zr)Ox−Al2O330 three-way catalysts. On the other hand, the good catalytic activities can be attributed to the number of active centers exposed to the reactants on the mesopores surface and the stabilized active sites from the confinement effect of the mesostructure of alumina.31 The existence of confinement in the mesostructure of the xMg-OMA is

samples performed classic IV-type isotherms with a clear H1shaped hysteresis loops, a typical feature for mesoporous materials with regularly cylindrical pores according to the sharpness of the capillary condensation step.22 The pore size distributions for supports with adding NH4HCO3 calcined at 1000 °C, derived from the desorption branch by using Barrett− Joyner−Halenda (BJH) calculations, which shows that MgOMA-9 represented a narrower pore size distribution around 9.8 nm (Figure 3B). DTA analysis shown in Figure 4 was performed to study the thermal stability of as-synthesized xMg-OMA. There are two

Figure 4. DTA profiles of the xMg-OMA materials with NH4NO3 calcined at 1000 °C (a, 0Mg-OMA; b, 3Mg-OMA; c, 6Mg-OMA; d, 9Mg-OMA; e, 12Mg-OMA; f, 15Mg-OMA).

clear exothermic peaks in all curves. As for Mg-OMA-0, its DTA curve shows a clear peak located at 849 °C, which can be ascribed to the formation of the γ-phase alumina. Another exothermic peak centered at 1154 °C is assigned as the phase transformation of γ-alumina to stable α-Al2O3.23 The position of peaks for the Mg-doped OMA shifts to a higher temperature with increasing content of Mg. The results can be ascribed to the formation of MgAl2O4 delaying the migration of Al3+, which restrains the phase transformation of alumina.24 It is well known that γ-alumina possesses a spine structure, in which two kinds of vacancies (tetrahedral and octahedral) are randomly assigned on its surface. The ionic radius of Al3+ is smaller than that of Mg2+ and it can insert into the vacancies of γ-Al2O3, resulting in lattice expansion.18 As a result, the cationic 1690


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Industrial & Engineering Chemistry Research Table 2. Full-Conversion Temperature (T100%) for CO, HC, and NO of Catalysts 800 °C

1000 °C

catalysts

CO

HC

NO

CO

HC

NO

Pd/Mg-OMA-0 Pd/Mg-OMA-3 Pd/Mg-OMA-6 Pd/Mg-OMA-9 Pd/Mg-OMA-12 Pd/Mg-OMA-15

327.7 312.3 301.5 287.9 296.3 313.4

323.3 317.5 295.2 281.7 302.5 321.6

333.5 324.8 310.4 292.6 317.2 328.3

357.5 348.2 342.5 336.8 343.7 352.6

358.5 342.6 337.9 326.3 344.2 351.6

364.5 359.3 351.8 342.8 356.5 363.4

conducive to preventing particles of PdO from sintering under high temperature treatment and catalytic reaction. From this point of view, the Pd-supported catalysts prepared with xMgOMA as carrier can effectively improve the catalytic activity. Meanwhile, MgO is usually selected as basic supports or modifiers, which improves the surface acidity and basicity of catalysts and generally results in enhancing the catalytic activity. The characterizations of CO2-TPD and CO2 in situ-DRIFTS were taken below to further investigate their surface acidity and basicity. 3.3. Catalytic Stability of the Catalysts. The evaluation of the long-term stabilities of the catalysts was carried out under given conditions: 280 °C, 210 mL min−1, 0.3 g catalyst, GHSV = 45 000 h−1, λ = 1. Figure 6 illustrates the profiles of CO, HC,

Figure 7. CO2-TPD profiles of Pd/xMg-OMA with NH4NO3 calcined at 1000 °C (a, Pd/0Mg-OMA; b, Pd/3Mg-OMA; c, Pd/6Mg-OMA; d, Pd/9Mg-OMA; e, Pd/12Mg-OMA; f, Pd/15Mg-OMA).

OMA without the doping magnesium promotion had a strong desorption peak at ca. 154 °C, which would be assigned to the weakly chemical adsorption of CO2 in the framework and physisorption of CO2 on account of the high BET surface areas and pore volumes.33 It was also observed that the desorption peaks showed some shift to higher temperatures with magnesium content increasing from 3% to 9% while that of Pd/12Mg-OMA and Pd/15Mg-OMA suffered some decline. The results reveal that a suitable amount of the Mg modifier contributes to intensifying the basic intensity. As for the desorption peaks around 298 and 543 °C, they ought to be related to the chemisorbed CO2 over moderate and strong intensity basic sites.34 On the other hand, the amount of Mg obviously affected the intensities of these desorption peaks. The quantitative distribution of surface basic sites of different strengths of the catalysts derived from CO2-TPD profiles are summarized in Table 3. Among the Pd supported catalysts, the

Figure 6. Catalytic stability of the Pd/9Mg-OMA catalyst with NH4NO3 (under 280 °C) for CO, NO, and HC elimination: (A) calcined at 800 °C, (B) calcined at 1000 °C.

and NO conversions with time on stream over the Pd/9MgOMA catalyst. Pd/9Mg-OMA calcined at 800 °C exhibits high catalytic activities (e.g., around 99%, 96%, and 91% for the conversions of HC, CO, and NO, respectively) and stable catalytic behavior in the whole 30 h time on stream. It is worth noting that the Pd/9Mg-OMA calcined at 1000 °C showed stable activity during 30 h of reaction, the catalytic conversions of HC, CO, and NO were 90%, 85% and 80% after 5 h reaction. Besides, the conversion of NO on this catalyst reaches 80% after 3 h reaction and then decreases a little (5%) after 30 h time on stream. The results suggest that the Pd/9Mg-OMA catalyst exhibits favorable long-term stability during the whole 30 h studied, implying a promising catalyst for CO, NO, and HC elimination. 3.4. Acid−Base Properties. CO2-TPD analysis collected in Figure 7 was conducted to determine the basicity and base strength of the as-prepared catalysts. Commonly, it was known to us that CO2 adsorbed on the weak basic sites could be desorbed under low temperature and that desorption at higher temperature would occur at strong basic sites according to Pino.32 It can be seen that the TPD profiles for all the Pd/xMgOMA catalysts were similar in shape. It was observed that the

Table 3. Amount of CO2 Desorption from CO2-TPD Profiles amount of CO2 desorption (unit area/g) catalysts

1st peak

2nd peak

3rd peak

total


1.02 1.14 1.25 1.37 1.19 1.07

0.34 0.39 0.45 0.63 0.46 0.36

4.52 5.78 6.56 7.87 7.21 4.89

5.88 7.31 8.26 9.87 8.86 6.32

basicity of Pd/Mg-OMA-9 is larger than that of the others. It is interesting to find that the amount of CO2 desorption for all basic sites of Pd/Mg-OMA-x (x = 0, 3, 6, and 9) retained gradually increased with an increase of magnesium content. The phenomenon can be ascribed to the fact that Mg2+ has incorporated into the Al3+ lattice and created a surface defect to 1691


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intensity after CO2 adsorption at 30 °C or evacuation at 180 °C. The results suggest that the strength of the basic sites increases when the appropriate Mg content was added in, which is consistent with the results of CO2-TPD analysis. 3.6. CO Pulse Chemisorption. The CO pulse chemisorption was performed to test Pd dispersion of the catalysts, the results of which are summarized in Table 4. The dispersion, metallic surface area, and particle sizes of active component were calculated using average CO/Pd adsorption stoichiometries of 1:1.39 As for the catalysts treated at 800 °C, the metallic surface area and the dispersion of Pd gradually increase with the Mg content increasing, while the active particle sizes gradually decrease. Pd/9Mg-OMA shows a higher dispersion value (34.3%) and may be attributed to the strong interaction between the Mg-doped OMA support and Pd. On one hand, the incorporation of Mg species into the Al2O3 lattice with the formation of Mg−O−Al causes the defects of Al2O3 to increase making these strong sites for Pd-supported catalysts. From another point of view, the mesopore barriers are beneficial to protect the Pd particles located at the pore surface of Pd/xMgOMA from growing.40 Besides, the high BET surface area and highly ordered mesostructure of Pd/Mg-OMA-9 may be another reason for the high dispersion of active component, leading to the higher catalytic activity than that of other catalysts. 3.7. In Situ DRIFTS Spectra of Chemisorbed CO. Representative in situ-DRIFTS spectra of CO adsorption are presented in Figure 9 to reveal the chemical state of the PdOx

make up for the formation of a positive electric charge, which makes the adjacent surface oxygen ion become unsaturated, bringing about the generation of surface magnesium aluminate with high basicity.35 With the magnesium content further increasing, the amount of CO2 desorption suffered some decline because the charge compensation effect on the surface became less important. The result can be attributed to the formation of bulk magnesium aluminate spinel of the Pd/xMgOMA (x = 12 and 15) as shown in WXRD patterns (Figure 2B). It is noteworthy that the decrease of BET surface area of 12Mg-OMA and 15Mg-OMA could be another important cause for the decline of basicity.36 Therefore, it can be deduced that Pd/9Mg-OMA possesses higher basicity compared to the catalysts with other magnesium content due to the formation of basic surface magnesium aluminate and high BET surface area. 3.5. In Situ DRIFTS Spectra of Chemisorbed CO2. Figure 8 presents the in situ DRIFTS spectra obtained on Pd/

Figure 8. In situ DRIFTS spectra of chemisorbed CO2 on Pd/xMgOMA with NH4NO3 calcined at 1000 °C after adsorption and evacuation at (A) 30 °C and (B) 180 °C. (a, Pd/0Mg-OMA; b, Pd/ 3Mg-OMA; c, Pd/6Mg-OMA; d, Pd/9Mg-OMA; e, Pd/12Mg-OMA; f, Pd/15Mg-OMA).

xMg-OMA after CO2 was adsorbed at 30 °C and evacuated at 180 °C. There are three surface species of the adsorbed state CO2 reflecting the different types of surface alkaline sites that were detected on the Pd supported catalysts. Bicarbonate formation (weak-strength basic sites) involves surface hydroxyl groups and shows an asymmetric O−C−O stretches mode at 1410 cm−1. Unidentate and bidentate carbonate (strongstrength alkaline sites) formation requires surface alkaline oxygen atoms.37 It shows an asymmetric O−C−O stretches centered in 1510 cm−1. Asymmetric O−C−O stretches around 1635 cm−1 can be ascribed to bidentate carbonates (mediumstrength alkaline sites).38 After evacuation at 180 °C, bicarbonate on the surface of the catalysts disappears while the unidentate and bidentate carbonates are still retained. It was also observed that Pd/9Mg-OMA possesses higher band

Figure 9. In situ-DRIFTS spectras of chemisorbed CO on Pd/xMgOMA with NH4NO3 calcined at 1000 °C: (A) absorbed CO for 1 h and (B) purged by helium. (a, Pd/0Mg-OMA; b, Pd/3Mg-OMA; c, Pd/6Mg-OMA,; d, Pd/9Mg-OMA; e, Pd/12Mg-OMA; f, Pd/15MgOMA).

species. In the DRIFT spectra of CO adsorption (Figure 9A), the main bands in the range of the C−O stretches frequency could be assigned to linear carbonyl groups with Pd 2+ complexes (Pd2+−CO) at 2040−2230 cm−1; another band for

Table 4. Metallic Dispersion, Metallic Surface Area, and Active Particle Diameter of Catalysts Calcined at Different Temperatures 800 °C

1000 °C

catalysts

metal dispersion (%)

metallic surface area (m2 g−1)

active particle diameter (nm)

metal dispersion (%)

metallic surface area (m2 g−1)

active particle diameter (nm)


23.4 27.5 31.4 34.3 28.6 25.5

45.6 49.7 53.3 58.9 54.4 52.1

5.5 5.2 4.9 4.4 4.7 5.3

17.3 18.4 21.4 24.8 23.6 19.3

33.5 35.8 38.4 40.3 37.5 34.1

7.7 7.2 6.9 6.2 6.5 6.8

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Industrial & Engineering Chemistry Research Pd/0Mg-OMA centered at 1915 cm−1 are associated with bridging carbonyl ligands with Pd+ complexes (Pd2+−CO).41 It can be observed that the wave numbers of catalysts gradually shift to higher positions with the increase of Mg content from 3 to 9 wt % and the Pd/9Mg-OMA shows a clear band at 1948 cm−1. This phenomenon may be due to the higher BET surface area of Mg-doped OMA, and smaller PdO particle sizes make it more likely to interact with Mg-modified support.42 It is also found that the band at about 1988 cm−1 of Pd/9Mg-OMA displays a clearly stronger intensity than other catalysts. One possible reason is that there is a relatively much more number of active sites on the mesopore surface of Pd/9Mg-OMA highly dispersed,43 which is well consistent with dispersion of the Pd species shown in Table 4. After getting rid of CO from the catalysts, the bands related to bridge-bound carbonyl CO still are maintained while the the intensity of the peaks of linearly bound CO weakened. The reducibility of Pd-supported catalysts is one of the influences on catalytic activities.44 The H2-TPR profiles of Pd/ xMg-OMA catalysts are established in Figure 10. One peak

which may be one reason for the high catalytic activity of Pd/ 9Mg-OMA because the ordered structure was beneficial to full access between reaction gas and active component. For the catalyst calcined at 1000 °C, the ordered mesostructure still could be maintained, though accompanied by some damage, which fully demonstrates that the obtained catalyst possesses high thermal stability. It is interesting to find that there is a particle on the catalyst surface (marked by a circle), the interplanar spacing of which (ca. 0.26 nm) is determined to be corresponding to the PdO (101) lattice fringes.47 Besides, the particle diameter estimated from the image was found to be 6.4 nm, which is close to the value obtained from the results of the CO pulse chemisorption (Table 4). 3.9. XPS Spectra Characterization. The XPS study on the Pd/0Mg-OMA, 9Mg-OMA, and Pd/9Mg-OMA shown in Figure 12 was carried out to investigate the chemical state of

Figure 10. H2-TPR profiles of Pd/xMg-OMA with NH4NO3 calcined at 1000 °C (a, Pd/0Mg-OMA; b, Pd/3Mg-OMA; c, Pd/6Mg-OMA; d, Pd/9Mg-OMA; e, Pd/12Mg-OMA; f, Pd/15Mg-OMA).

centering at 82 °C was observed on Pd/0Mg-OMA, which can be assigned to the PdO species on the mesopores surface of the catalyst carrier easily reduced under lower temperature.45 It is also found that the reduction peak gradually shifted to lower temperature with increasing Mg content, which indicates that the presence of magnesium improves the reducibility of the PdO species.46 This behavior is attributable to the high dispersion of Pd and the strong interaction between Mg-doped OMA carrier and PdO. These results indicate that the coexistence of Mg and Pd result in a synergetic effect, which improves the reducibility of Pd/xMg-OMA catalysts. 3.8. Pd Dispersion and Chemical State. Figure 11 gives the TEM images of the Pd/9Mg-OMA catalyst calcined at 800 and 1000 °C. After heating at 800 °C, the catalyst possessed a highly ordered alignment of mesopores along [001] direction,

Figure 12. XPS profiles of samples calcined at 1000 °C (a, 9Mg-OMA; b, Pd/9Mg-OMA; c, Pd/0Mg-OMA).

Mg, O, Al, and Pd. The binding energy (BE) of the Mg 2p spectra of 9Mg-OMA and Pd/9Mg-OMA are 50.8 and 51.1 eV; both of them are assigned to Mg2+ in Mg−O−Al bonds.48 The BE value of Al 2p for 9Mg-OMA and Pd/9Mg-OMA are 73.6 and 73.9 eV, both of them are lower than pure alumina (74.2 eV). The BE value of O 1s for 9Mg-OMA (531.9 eV) and its catalyst (532.1 eV) are slightly different from that of pure MgO (529.8 eV), γ-alumina (531.1 eV) and PdO (530.7 eV). The Mg−O−Al bonds are usually suggested to be connected with the formation of the spinel crystalline phase (MgAl2O4) and strengthen the interaction between Mg and Al in the catalysts. This interaction is present in the crystal lattice of the samples, in which Mg occupies tetrahedral sites and Al occupies octahedral sites.49 XPS spectra of Pd/0Mg-OMA and Pd/ 9Mg-OMA presented a Pd 3d5/2 peak located at around 337.1 and 337.3 eV, which corresponds to the BE values of the PdO species.50 The migration to the higher binding energy can be attributed to the interaction between support and active component resulting from the introduction of Mg. The turnover frequency (TOF) values of the Pd-supported catalysts were shown in Table 5. The dispersion of active metal was determined from the results of CO chemisorption (Table 4). It can be clearly observed from the catalysts calcined at 800

Figure 11. TEM images of Pd/9Mg-OMA with NH4NO3 calcined at (A) 800 °C and (B) 1000 °C. 1693


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Industrial & Engineering Chemistry Research Notes

Table 5. TOF Values of Catalysts Calcined at Different Temperatures 800 °C

The authors declare no competing financial interest.

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1000 °C

catalysts

CO

HC

NO

CO

HC

NO


54.8 57.6 63.2 60.7 59.5 55.1

35.6 37.8 40.4 43.5 41.7 36.9

8.7 9.2 9.6 10.1 9.7 9.1

38.2 39.3 43.6 47.1 44.3 41.9

24.3 27.9 29.4 31.3 28.4 25.5

6.7 7.1 7.4 7.8 7.5 7.1

ACKNOWLEDGMENTS The authors are grateful for financial support from the National Key Research and Development Program (2016YFC0203903), the Project (class A) in Fujian Province Department of Education (No. JA14060) and industry−university−institute cooperation projects of Fujian Province (No. 2014H6015).

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(1) Prasad, V. S.; Aghalayam, P. Microkinetic modeling of the effects of oxygen on the catalytic reduction of NO on Pt and Rh in automotive after treatment. Ind. Eng. Chem. Res. 2016, 55, 9362−9371. (2) Cho, K. M.; Park, S.; Seo, J. G.; Youn, M. H.; Nam, I.; Baeck, S. H.; Chung, J. S.; Jun, K. W.; Song, I. K. Effect of calcination temperature of alumina supports on the wax hydrocracking performance of Pd-loaded mesoporous alumina xerogel catalysts for the production of middle distillate. Chem. Eng. J. 2009, 146, 307−314. (3) Abidli, A.; Hamoudi, S.; Belkacemi, K. Synthesis, characterization and insights into stable and well organized hexagonal mesoporous zinc-doped alumina as promising metathesis catalysts carrier. Dalton Trans. 2015, 44, 9823−9838. (4) Niesz, K.; Yang, P.; Somorjai, G. A. Sol-gel synthesis of ordered mesoporous alumina. Chem. Commun. 2005, 15, 1986−1987. (5) Yuan, Q.; Yin, A. X.; Luo, C.; Sun, L. D.; Zhang, Y. W.; Duan, W. T.; Liu, H. C.; Yan, C. H. Facile synthesis for ordered mesoporous γaluminas with high thermal stability. J. Am. Chem. Soc. 2008, 130, 3465−3472. (6) Li, Z. X.; Shi, F. B.; Li, L. L.; Zhang, T.; Yan, C. H. A facile route to ordered mesoporous-alumina-supported catalysts, and their catalytic activities for CO oxidation. Phys. Chem. Chem. Phys. 2011, 13, 2488− 2491. (7) Bordoloi, A.; Sanchez, M.; Noei, H.; Kaluza, S.; Großmann, D.; Wang, Y.; Grünert, W.; Muhler, M. Catalytic behaviour of mesoporous cobalt-aluminum oxides for CO oxidation. J. Catal. 2014, 2014, 807545. (8) Han, S. J.; Bang, Y.; Lee, H.; Lee, K.; Song, I. K.; Seo, J. G. Synthesis of a dual-templated MgO-Al2O3 adsorbent using block copolymer and ionic liquid for CO2 capture. Chem. Eng. J. 2015, 270, 411−417. (9) Kumar, P. A.; Reddy, M. P.; Hyun-Sook, B.; Phil, H. H. Influence of Mg addition on the catalytic activity of alumina supported Ag for C3H6-SCR of NO. Catal. Lett. 2009, 131, 85−97. (10) Arnby, K.; Törncrona, A.; Skoglundh, M. Influence of ammonia on CO and methanol oxidation over Pt/γ-Al2O3 catalysts modified by Mg. Appl. Catal., B 2004, 49, 51−59. (11) Yamaguchi, K.; Ebitani, K.; Yoshida, T.; Yoshida, H.; Kaneda, K. Mg-Al mixed oxides as highly active acid-base catalysts for cycloaddition of carbon dioxide to epoxides. J. Am. Chem. Soc. 1999, 121, 4526−4527. (12) Xu, L.; Song, H.; Chou, L. Ordered mesoporous MgO-Al2O3 composite oxides supported Ni based catalysts for CO2 reforming of CH4: Effects of basic modifier and mesopore structure. Int. J. Hydrogen Energy 2013, 38, 7307−7325. (13) Li, L. L.; Duan, W. T.; Yuan, Q.; Li, Z. X.; Duan, H. H.; Yan, C. H. Hierarchical γ-Al2O3 monoliths with highly ordered 2D hexagonal mesopores in macroporous walls. Chem. Commun. 2009, 41, 6174− 6176. (14) Meng, Y.; Gu, D.; Zhang, F.; Shi, Y.; Yang, H.; Li, Z.; Yu, C.; Tu, B.; Zhao, D. Ordered mesoporous polymers and homologous carbon frameworks: amphiphilic surfactant templating and direct transformation. Angew. Chem. 2005, 117, 7215−7221. (15) Wu, Q.; Zhang, F.; Yang, J.; Li, Q.; Tu, B.; Zhao, D. Synthesis of ordered mesoporous alumina with large pore sizes and hierarchical structure. Microporous Mesoporous Mater. 2011, 143, 406−412. (16) Semeykina, V. S.; Parkhomchuk, E. V.; Polukhin, A. V.; Parunin, P. D.; Lysikov, A. I.; Ayupov, A. B.; Cherepanova, S. V.; Kanazhevskiy,

°C that the TOF values first increased as the Mg loading on the support increased and then decreased when the Mg content was more than 9 wt %. As for catalysts with different Mg content calcined at 1000 °C, the TOF values of HC, NO, and CO were observed to decrease when the content of Mg was larger than 9 wt %. The results may be attributed to a great deal of active component nanoparticles on the surface of Pd/9MgOMA,51 which is consistent with the results of catalytic performance (Table 2) and Pd dispersion (Table 4) for the catalysts with different content of Mg.

4. CONCLUSIONS The preparation of ordered mesoporous Mg-doped γ-aluminas with different basicities via a facile sol−gel approach through the use of P123 as template and NH4HCO3 as pore-enlarging agent was reported. The as-prepared thermally stable OMA supports with favorable textural and structural characteristics were utilized as the Pd-based catalysts toward the catalytic converters of HC, CO, and NO elimination. It was observed that the introduction of magnesium effectively enhanced the BET surface area and thermal stability. It is interesting to confirm that Pd/9Mg-OMA catalyst with moderate basic modifier and advantageous texture parameters displays better catalytic performance than the other catalysts. From the above discussions, these favorable advantages make ordered mesoporous Mg-doped Pd-alumina a potential catalyst for purification of automobile exhaust and the treatment of dyecontaining wastewater.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b03799. Interplanar spacing and lattice parameter of samples prepared with different Mg content calcined at different temperatures (Table S1); small-angle (A) and wide-angle (B) XRD patterns for the catalysts prepared with different content of Mg calcined at 1000 °C (Figure S1); interplanar spacing and lattice parameter of catalysts prepared with with NH4HCO3 and different Mg content calcined at different temperatures (Table S2) (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Fax: +86 591 83707796. Tel: +86 591 83731234-8201. Email: [email protected]. ORCID

Lilong Jiang: 0000-0002-0081-0367 1694


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