Supported Nanometric Pd Hierarchical Catalysts for Efficient Toluene

Article pubs.acs.org/IECR

Supported Nanometric Pd Hierarchical Catalysts for Efficient Toluene Removal: Catalyst Characterization and Activity Elucidation Chi He,†,‡ Lingling Xu,† Lin Yue,‡ Yanting Chen,† Jinsheng Chen,*,† and Zhengping Hao*,‡ †

Key Laboratory of Urban Environment and Health, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, P.R. China ‡ Department of Environmental Nano-materials, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, P.R. China S Supporting Information *

ABSTRACT: Micro-/mesoporous composite materials with coke-resistant ZSM-5 and three-dimensional large-pore KIT-6 as their micro- and mesoporous components, respectively, were synthesized using a simple in situ overgrowth approach. The hydrothermal stabilities and acidities of the biporous catalysts were found to be significantly enhanced. All composite materials acted as powerful catalysts for toluene oxidation. Among them, Pd/ZK-6% showed the highest catalytic activity, with 90% toluene decomposition at 203 °C, which is more than 30 °C lower than required for Pd/KIT-6. It can be anticipated that the specific surface area and acidity have a synergistic effect on the Pd dispersion. Overall, the catalytic activities were found to be primarily correlated with the specific surface area, support acidity, Pd dispersion, and toluene adsorption capability.

1. INTRODUCTION Environmental legislation has imposed increasingly stringent targets for permitted levels of atmospheric emissions over the past few years. Volatile organic compounds (VOCs), present at low concentrations in industrial and automotive exhaust gas streams, are considered to be major contributors to air pollution because of their toxic or malodorous natures, as well as their contributions to suspended particulate or photochemical smog formation.1 Many techniques can be used for VOC removal, such as adsorption, absorption, biofiltration, incineration, and catalytic oxidation. Catalytic oxidation, which can be applied effectively over a wide range of VOC concentrations and waste gas flow rates, presents an interesting solution for VOC elimination.2 However, hightemperature catalytic oxidation processes for the removal of low concentrations of VOCs require higher operating costs. Thus, the development of highly active catalysts that work at lower temperatures is of great importance. To date, various types of catalysts such as supported noble metals and metal oxides have been extensively explored in the deep oxidation of VOCs.3,4 However, good catalytic performance is observed for metal oxides only at relatively high temperatures, inevitably resulting in high energy consumption and rapid deactivation of catalysts.5 Thus, supported noble metals (e.g., Pd and Pt) are generally regarded as the most desirable catalysts for VOC oxidation considering their activity, selectivity, and stability. Among them, palladium-based catalysts offer several advantages such as superior activity, high thermal stability, good catalytic performance, and low cost compared with platinum-based catalysts.3,6 Moreover, it has been reported that the support plays an important role in improving the catalytic efficiency by offering a large specific surface area and pore volume to disperse the active phase or providing active acid sites to promote the oxidation process.7−9 Different types of materials such as Co3O4, Mn2O3, ZrO2, TiO2, Al2O3, CeO2, SiO2, and their © 2012 American Chemical Society

mixtures have been investigated as Pd/Pt supports with the aim of high catalytic activity.3,10−13 However, Pd and other metal nanoparticles are often unstable (sintering) on metal oxide supports at elevated reaction temperatures because of the low surface areas of metal oxides, which do not favor active-phase dispersion.14 Zeolite and zeolite-like materials (e.g., USY, Beta, MOR, ZSM-5, and FAU) have been widely used as adsorbents and catalysts because of their large internal surface areas, uniform pore sizes, excellent hydrothermal stabilities, and strong intrinsic acidities. However, the small pore diameters (usually Pd/ZSM-5 ≥ Pd/ZK-3% > Pd/ZK-24% ≫ Pd/KIT-6. On the other hand, coke is likely to form over catalysts with large amounts of acid sites, which can deactivate the catalyst by poisoning the active sites and blocking the inner pores and, 7215

dx.doi.org/10.1021/ie201243c | Ind. Eng. Chem. Res. 2012, 51, 7211−7222

Industrial & Engineering Chemistry Research

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lower than those of Pd/ZSM-5 (maximum value = 12 ppm) and Pd/KIT-6 (maximum value = 20 ppm). As indicated in Figure 9, the amounts of benzene generaed over the synthesized catalysts decreased in the order Pd/KIT-6 > Pd/ ZK-mixed > Pd/ZSM-5 ≫ Pd/ZK-24% > Pd/ZK-12% ≥ Pd/ ZK-3% > Pd/ZK-6%. Combined with the activity results, one can safely conclude that the amount of benzene formed can be interpreted in terms of the catalyst activities; that is, better catalytic activity allows for higher VOC oxidation efficiency (Figures 8 and 9). 3.4. Acidity and Reduction Characteristics of Various Catalysts. It is generally accepted that there are many factors that can influence the toluene oxidation activity, for example, the oxidation/reduction capability and dispersion of Pd atoms, the oxidation state of the catalyst, the metal−support interactions, and the support textural properties. Farrauto et al.53 reported that the active-phase valence state, the metal− supported interactions, and the catalyst particle size are interdependent parameters that strongly influence the catalytic activity. As for the effect of Pd particle size, some works have shown a correlation between the particle size and the catalytic activity,54 whereas others have reported no strong dependence between the Pd dispersion and catalytic activity.55 On the other hand, many researchers have found that Pd particles loaded on acidic supports are more easily oxidized than those supported on neutral or basic supports, as acidic supports with electrophilic character resulted in electron-deficient Pd atoms.9,41 In a previous work, we found that Pd atoms are prone to disperse on supports with more acid sites.27,56 Similarly, Okumura and co-workers9,41 revealed that the acid properties of the support have positive influences on the Pd dispersion, as confirmed using Pd K-edge X-ray absorption fine structure (XAFS) analysis. In the present work, the influences of the support acidity and palladium dispersion on toluene catalytic activity were studied. Figure 10 shows the NH3 TPD patterns of the synthesized catalysts. Catalyst acid strength is known to be directly proportional to the NH3 desorption temperature. Generally, two NH3 desorption peaks between 120 and 410 °C were found for all Pd/ZK-x samples, which means that there were two types of acid sites (i.e., weak and medium acid sites), whereas three NH3 desorption peaks centered at 239, 407, and 742 °C corresponding to weak, medium, and strong acid sites were observed for Pd/ZSM-5. Moreover, the desorption temperatures and quantitative numbers of moles of acid sites are summarized in Table 4. It can be observed that medium acid sites in the composite materials increased linearly from 0.03 to 0.24 mmol of NH3/gcat with increasing amount of ZSM5 added, although there was no certain trend for the number of weak acid sites. Overall, the total number of acid sites on the composites increases with the increasing of ZSM-5 content. The reducible properties of the prepared Pd/ZK-x catalysts were determined by H2 TPR, as shown in Figure 11. First, the blank test (reducible character of ZK-6%) shows that the TCD signal of H2 was not affect by the experimental conditions such as H2O desorption. All of the Pd2+ (PdCl2 or PdO) particles were reduced to metallic Pd at ambient temperature, as no H2 consumption peaks corresponding to the reduction of palladium oxides were found. However, one or two negative peaks (production of H2) in the temperature range of 39−63 °C attributed to the adsorbed hydrogen or the decomposition of PdHx were found for all samples.8 For Pd-based catalysts, the decomposition temperature and the intensity of these peaks

Figure 5. FT-IR spectra of (a) Pd/ZK-3%, (b) Pd/ZK-6%, (c) Pd/ ZK-12%, and (d) Pd/ZK-24%.

the mesostructured phase particles in most cases. Moreover, the pore structure of some typical catalysts was further explored by TEM (Figure 7). Pd/KIT-6 was found to exhibit an ordered three-dimensional cubic structure. Although the structural regularity decreased to some extent, the zeolite particles were tightly combined with the KIT-6 mesoporous phase in the biporous materials. These results further confirm the presence of a micro- and mesoporous hybridized structure. 3.3. Catalytic Performance of All Pd-Loaded Catalysts. First, a blank test was performed to examine the effect of the stainless steel reactor on toluene oxidation, and no toluene conversion was found below 400 °C. Figure 8 shows the lightoff curves and CO2 selectivity patterns of toluene oxidation over the synthesized catalysts. Generally speaking, the Pd/ZK-x catalysts (for which toluene complete oxidation occurred in the vicinity of 220 °C) were more active than Pd/KIT-6 (toluene complete oxidation at ∼250 °C) and Pd/ZSM-5 (toluene complete oxidation at ∼240 °C). In addition, the CO2 selectivities of the composite catalysts were also higher than those of microporous Pd/ZSM-5 and mesoporous Pd/KIT-6. Table 2 lists the temperatures at which 10%, 50%, and 90% conversions of toluene (i.e., T10, T50, and T90, respectively) occurred over all prepared catalysts. Among them, Pd/ZK-6% exhibited the highest catalytic activity, with 90% toluene decomposition at 203 °C, which was much higher than that of Pd/KIT-6 (T90 = 235 °C). According to the T90 values, the catalytic activity order of the synthesized catalysts was Pd/ZK6% (203 °C) > Pd/ZK-3% (208 °C) > Pd/ZK-12% (216 °C) > Pd/ZK-24% (221 °C) > Pd/ZSM-5 (222 °C) > Pd/ZK-mixed (229 °C) > Pd/KIT-6 (235 °C). In comparison with other active catalysts reported elsewhere, our catalysts performed better in the deep catalytic oxidation of toluene,7,13,27,49−52 as reported in Table 3. Moreover, the reaction products were analyzed online by mass spectrometry, and only a tiny amount of benzene was found for our catalysts, as revealed in Figure 9. The formation of benzene can be explained by the combustion mechanism in which the first step is cracking the C−C bonds before toluene is completely oxidized to CO2 and H2O.13 Notably, the concentration of byproduct benzene over the composite catalysts (maximum value ≈ 5 ppm) was much 7216



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Figure 6. Representative data on the synthesized catalysts. FE-SEM images: (a) Pd/ZSM-5, (b) Pd/KIT-6, (c) Pd/ZK-3%, (d) Pd/ZK-6%, (e) Pd/ ZK-12%, and (f) Pd/ZK-24%. SEM elemental mapping: (g) Pd/ZK-6%. EDS patterns: (h) Pd/ZK-3%, (i) Pd/ZK-6%, (j) Pd/ZK-12%, and (k) Pd/ ZK-24%.

increase linearly as a function of the Pd particle size.57 Pd/ZK6% only had a weak negative peak at the lowest temperature (39.7 °C), suggesting good Pd particle dispersion. In sharp contrast, Pd/ZK-24% exhibited a strong H2 production peak at the highest temperature (62.8 °C). Moreover, the Pd particle dispersion was also characterized by SEM elemental mapping,

TEM, and HAADF-STEM, as shown in Figures 6 and 7. The SEM elemental mapping revealed that Pd atoms were uniformly distributed throughout the support, and no obvious aggregation was noticed (Figure 6). The TEM images show that the Pd particles were better dispersed over the composite catalysts, with an average diameter of less than ca. 7 nm. The 7217



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Figure 8. Toluene catalytic activities and CO2 selectivities (50% and 90% conversions of toluene) over various Pd-supported catalysts. Figure 7. Transmission electron microscopy (TEM) and (insets) high-angle annular dark-field (HAADF)-scanning transmission electron microscopy (STEM) images of (a) Pd/ZSM-5, (b) Pd/KIT-6, (c) Pd/ZK-6%, (d) Pd/ZK-24%, and (e) Pd/ZK-mixed. (Pd particles are light spots in the STEM images.)

oxidation over Pd-loaded catalysts.58,59 This model is based on the assumption of a constant oxygen surface concentration on the catalyst, with reaction occurring through the interaction between reactant molecules and an oxidized portion of the Table 2. Characteristic Data and Catalytic Performances of Synthesized Catalysts

HAADF-STEM images (Figure 7, insets) can provide more detailed information about the Pd dispersion and indicate that the Pd particle dispersion is closely related to the support properties. In general, the composite samples had better Pd dispersion performance (Dc < 4 nm) than the single or mixed micro-/mesoporous phases. Moreover, H2 chemisorption was also performed to explore the actual Pd dispersion over various supports, and the results showed that Pd/ZK-6% had the best Pd dispersion with an average Pd diameter of ca. 2.2 nm, which was much smaller than those of Pd/ZSM-5 (Dc = 5.3 nm), Pd/ KIT-6 (Dc = 3.7 nm), and Pd/ZK-mixed (Dc = 4.1 nm). Generally speaking, the active phase is prone to disperse over supports with larger specific surface areas; however, the Pd dispersion in this work did not fully obey this rule (Tables 1 and 2). The acid sites on the supports could anchor Pd species through strong interactions between the acid sites and Pd2+.41−43 It could be anticipated that the specific surface area and acidity have a synergistic effect on Pd dispersion, and we propose that the superior activities of the Pd/ZK-x catalysts are partly ascribable to their higher Pd atom dispersion. Several models can be used to describe the kinetic process of hydrocarbon catalytic oxidation. Among them, the Mars−van Krevelen kinetic model is usually valid for VOC complete

sample Pd/ ZSM-5 Pd/ KIT-6 Pd/ZK3% Pd/ZK6% Pd/ZK12% Pd/ZK24% Pd/ZKmixed

Pda (wt %)

H/ Pdb

Dcc (nm)

T10d (°C)

T50d (°C)

T90d (°C)

S50e (%)

S90e (%)

0.48

0.21

5.3

173

206

222

94.3

96.4

0.49

0.30

3.7

167

213

235

90.8

93.4

0.49

0.41

2.7

166

193

208

94.7

97.4

0.48

0.49

2.2

162

191

203

98.2

99.5

0.50

0.47

2.4

167

194

210

95.1

98.8

0.47

0.35

3.2

168

200

221

96.2

97.9

0.49

0.27

4.1

171

211

229

91.5

94.5

a

Actual Pd contents obtained by ICP-OES analysis. bMolar ratio of adsorbed hydrogen atoms to total palladium atoms. cCalculated diameters of the palladium crystallites based on Pd dispersion. d Temperatures at which 10%, 50%, and 90% conversions of toluene were obtained. eCO2 selectivities at 50% and 90% conversions of toluene. 7218



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Table 3. Summary of Some Better-Known Catalysts for Toluene Oxidation

Table 4. Acidities of Synthesized Aluminosilicate Catalysts

conditions sample 0.5% 0.5% 1.0% 0.3% 0.5% 3.0% 0.5%

Pd/ZK-6% Pd/mesoZrO2b Pd/Co3AlO Pd/ZSM-5 Pd−1% Au/TiO2 Au/CeTi Pd/MACc

sample

CTola (ppm)

GHSV (h−1)

T90 (°C)

ref

1000 1000 800 650 1000 1000 1000

32 000 26 000 30 000 26 000 26 000 26 000 19 000

203 210 230 270 225 290 370

this work 7 13 27 49 50 52

Pd/ZSM-5 Pd/ZK-3% Pd/ZK-6% Pd/ZK-12% Pd/ZK-24% a

NH3 desorption peak (°C)

aciditya (mmol of NH3/gcat.)

I

I

II 239 136 172 169 125

407 280 333 302 403

III 742 − − − −

II 0.10 0.12 0.14 0.05 0.15

0.24 0.03 0.08 0.19 0.24

III

I + II + III

0.07 − − − −

0.41 0.15 0.22 0.24 0.39

Amounts of desorbed NH3 at different temperatures

a Toluene inlet concentration. bmesoZrO2 = meso-/macroporous zirconia. cMAC = mesoporous activated carbon.

Figure 11. H2 TPR patterns of (a) ZK-6%, (b) Pd/ZK-3%, (c) Pd/ ZK-6%, (d) Pd/ZK-12%, and (e) Pd/ZK-24%.

Figure 9. Yields of byproduct benzene over all synthesized catalysts.

that acidic supports are favorable for Pd particle oxidation as the electrophilic character of acidic supports results in electrondeficient Pd atoms,9,41,60 which could accelerate the VOC oxidation process according to the Mars−van Krevelen kinetic model. This process probably plays an important role in our work (Figure 8, Tables 1 and 2). For instance, both the Pd dispersion (Dc = 5.3 nm) and the specific surface area (SBET = 324.1 cm2/g) of Pd/ZSM-5 are lower than those of Pd/KIT-6 (Dc = 3.7 nm, SBET = 768.8 cm2/g), whereas Pd/ZSM-5 (T90 = 222 °C) is more active than Pd/KIT-6 (T90 = 235 °C) in toluene oxidation. 3.5. Toluene Adsorption/Desorption Capabilities of the Composite Catalysts. It has been demonstrated that the catalytic oxidation activity of aromatic molecules is closely related to the interactions between the reactant and the catalyst.45 Becker and Forster61 proposed that the reactant adsorption strength (i.e., adsorption affinity) is one of the main factors in controlling the catalytic activity, which was also demonstrated by Barresi and Baldi.62 Generally, VOC molecules are first adsorbed by the absorbent and subsequently oxidized to CO2 and H2O. However, previous attention was directed toward understanding the role of the active Pd state and the support properties with regard to hydrocarbon catalytic oxidation.9,41 Herein, the toluene adsorption/desorption capabilities over the fresh catalysts were further investigated, as shown in Figure 12. Obviously, toluene is more easily desorbed from Pd/ZK-24% (maximum toluene desorption peak centered at 105 °C) than from the other catalysts. For Pd/

Figure 10. NH3 TPD profiles of various catalysts: (a) Pd/ZSM-5, (b) Pd/ZK-3%, (c) Pd/ZK-6%, (d) Pd/ZK-12%, and (e) Pd/ZK-24%.

catalyst. In our work, the oxidized Pd catalyst was reduced by toluene, which was simultaneously oxidized to CO2 and H2O, and then the catalyst redox center (Pd0) was oxidized by adsorption and dissociation of O2 to recover [Pd2+O2−] (Figure 2). Therefore, the oxidation step is crucial for the oxidation activity of Pd-supported catalysts. Previous research suggested 7219



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AUTHOR INFORMATION

Corresponding Author

*Tel./Fax: +86 59 26190765 (J.C.), +86 10 62923564 (Z.H.). E-mail: [email protected] (J.C.), [email protected] (Z.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Science and Technology Ministry Program of China (2008BAC32B03), the Science and Technology Project of Xiamen (3502Z20102015), the National Natural Science Foundation (20725723, 21107106), and the project supported by the Science Foundation of the Fujian Province (2011J05034).

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Figure 12. Toluene TPD patterns of prepared catalysts: (a) Pd/ZK3%, (b) Pd/ZK-6%, (c) Pd/ZK-12%, and (d) Pd/ZK-24%.

ZK-3% and Pd/ZK-6%, two desorption peaks in the range of 110−380 °C can be observed, ascribable to two-stage toluene desorption. In addition, the amount of toluene desorbed was found to be inversely proportional to the amount of ZSM-5 added. These results thus reveal that the toluene adsorption capability over the composite catalysts decreased in the order Pd/ZK-3% > Pd/ZK-6% > Pd/ZK-12% ≥ Pd/ZK-24%. Based on the catalytic activity sequence (Figure 8), one can reasonably infer that the toluene adsorption capability is closely related to the catalytic performance, that is, the higher the adsorption capability for toluene, the higher the catalytic activity.

4. CONCLUSIONS ZSM-5/KIT-6 biporous composite materials were successfully prepared by an in situ overgrowth approach. The prepared materials were integrated composites of ZSM-5 zeolite and KIT-6 and showed significant differences from their mechanically mixed counterpart. Both the specific surface areas and pore diameters of all composite catalysts were much higher than those of Pd/ZSM-5. The tetrahedrally coordinated Al in the framework could offer Brønsted acid sites, which are favorable for the dispersion of Pd active sites and the oxidation of metal Pd particles, and thus promote the VOC oxidation process. All of the composite materials were found to act as promising catalysts with superior catalytic activity and CO2 selectivity for toluene oxidation. Moreover, the hydrothermal stabilities and acidities of the composite catalysts were obviously enhanced compared with those of Pd/KIT-6. Generally, the catalytic activity can be interpreted in terms of the specific surface area, support acidity, Pd dispersion, and the toluene adsorption capability. Other potential applications of these materials in adsorption and separation processes can also be expected.

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ASSOCIATED CONTENT

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DTG patterns of the used composite catalysts. This material is available free of charge via the Internet at http://pubs.acs.org. 7220



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Supported Nanometric Pd Hierarchical Catalysts for Efficient Toluene

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