ZSM-5 catalysts

Chemical Engineering Journal 285 (2016) 536–543

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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Highly efficient catalytic removal of ethyl acetate over Ce/Zr promoted copper/ZSM-5 catalysts Shumin Li a, Qinglan Hao a, Ruozhu Zhao a, Deliang Liu a, Huazhen Duan a, Baojuan Dou b,⇑ a b

College of Chemical Engineering & Materials Science, Tianjin University of Science & Technology, Tianjin 300457, China College of Marine & Environmental Sciences, Tianjin University of Science & Technology, Tianjin 300457, China

h i g h l i g h t s
Ce/Zr promoter addition significantly improved the catalytic activity.
Synergistic effect of Cu–Ce–Zr increased active oxygen amount and oxygen mobility.
Both reducibility and acidity played crucial roles in catalytic performance.
Optimum CuCe0.75Zr0.25/Z provided superior stability with conversion above 99.5%.

a r t i c l e

i n f o

Article history: Received 3 June 2015 Received in revised form 18 September 2015 Accepted 29 September 2015 Available online 9 October 2015 Keywords: Catalytic oxidation CuO Ce/Zr promoter Ethyl acetate

a b s t r a c t A series of CuCexZr1xOy/ZSM-5 (x = 0, 0.25, 0.5, 0.75 and 1) catalysts with different Ce/Zr molar ratios were prepared by impregnation. The catalyst performances were tested for volatile organic compounds (VOCs) abatement in the fixed-bed reactor. Extensive characterizations, N2 adsorption/desorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction by hydrogen (H2-TPR), were undertaken in order to correlate the morphological, structural and surface properties of CuCexZr1xOy/ZSM-5 catalysts with their oxidation activities. The results showed that the CuO, CeO2, and ZrO2 species were highly dispersed on the surfaces of ZSM-5 support, and copper is mostly in Cu2+ state and Cu+ co-existed as well. Surface adsorbed oxygen, hydroxyl group, and oxygen vacancies were detected after the introduction of Ce and/or Zr, which increased with the increasing of Ce/Zr molar ratio. The specific surface area was not the key factor governing the catalytic activity, however, the remarkable relationship between reducibility and catalyst activity were observed. The excellent reducibility of the catalyst would lead to an improvement in catalytic performance. The optimum performance was obtained with CuCe0.75Zr0.25/Z catalyst, which offered complete conversion of ethyl acetate into CO2 at temperature as low as 270 °C, the onset temperature was 110 °C. Furthermore, this catalyst possessed superior stability, and no deactivation phenomenon was observed during the catalytic oxidation for 60 h at 270 °C. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Volatile organic compounds (VOCs) mean any organic compounds having an initial boiling point less than or equal to 250 °C measured at a standard pressure of 101.3 kPa [1]. It has been established that VOCs are the main air pollutants emitted from diverse industrial and domestic sources, which are harmful to environment and human health. Moreover, most of VOCs are great threats to the human health due to their carcinogenic, mutagenic, and teratogenetic nature [2–6]. The latest studies show that

⇑ Corresponding author. Fax: +86 22 60600300. E-mail address: [email protected] (B. Dou). http://dx.doi.org/10.1016/j.cej.2015.09.097 1385-8947/Ó 2015 Elsevier B.V. All rights reserved.

VOCs are also resulted in haze as well as NOx and SOx [7]. VOCs in the atmosphere are regarded as precursors for the formation of tropospheric ozone (O3). VOCs are capable of undergoing reactions with the hydroxyl radical (HO) to form photochemical oxidants which are responsible for the formation of secondary particulate matter in the atmosphere. These secondary fine particulate matter can further result in haze formation in the atmosphere [8,9]. In order to protect public health and environment, legislation on air quality has been revised to further reduce the harmful air pollutants [1,10–13]. Several countries have adopted more restrictions for emissions, not only for NOx and SOx but also for VOCs [1]. The removal of VOCs from industrial emissions to meet the increasingly stringent environmental air quality standards is becoming an urgent problem.

S. Li et al. / Chemical Engineering Journal 285 (2016) 536–543

Catalytic oxidation of VOCs is an environmentally friendly technology that requires low onset temperature, enjoys high conversion and causes less by-product [1,9,14–18]. Transition metal oxides can be used as catalysts for VOCs oxidation [19–22]. However, higher onset temperature (T10 > 300 °C) and lower activity of metal oxide catalyst for VOCs oxidation are the crucial problems to be solved [20–23]. It has been observed that copper oxide was more active for VOCs abatement than cobalt, manganese, nickel, and iron oxide on the same support [6,20–22]. Adding promoter to the metal oxide catalyst, and the metal-promoter interactions can significantly improve the catalyst activity and reduce T10 of VOCs abatement. CeO2 is potentially advantageous for catalytic applications [24]. Ce4+ and Ce3+ are the two stable oxidation states, hence oxygen could be easily stored and released by CeO2 via the redox shift between Ce4+ and Ce3+ [4,25]. The catalyst thermal stability could be enhanced by adding ZrO2 [25,26,28]. CuO–CeO2 catalysts showed good catalytic performance in CO preferential oxidation because of the interaction between copper and ceria [27]. The activities of Ni–CeO2, Co–CeO2 and Cu–CeO2 catalyst were higher than that of the metal oxides. The most active catalyst was Cu–CeO2 with complete conversion of ethyl acetate at 250 °C. Simultaneously, CO2 selectivity achieved 100% [9]. Madona et al. [28] obtained result was the inhibition of CO formation in favor of CO2 when Cu–ZrO2 catalyst was present in the process of the catalytic oxidation of propene and toluene. And the CO2 selectivity achieved 100% for the propene catalytic oxidation with 5% Cu–ZrO2 catalyst at 270 °C. Not only the unique features of oxygen storage capacity, but also the thermal resistance was observed with Ce/Zr based catalysts. The catalytic activity was improved in the presence of Ce/Zr mixed oxides. The Mn–Ce–Zr catalyst was synthesized by washcoating method for the VOC removal [29]. An optimum oxidation activity and CO2 selectivity toward n-butanol oxidation were obtained with Cu loadings about 5 wt%, and Ce/Zr moral ratio is 3:2. Yu et al. [30] found that the addition of ceria-zirconia mixed oxides significantly improved NO conversion and N2 yield due to dispersed copper species in proximity to ceria-zirconia. The ceria-rich CuCexZr1xO2/c-Al2O3 catalysts displayed better performance in activity and reducibility. NO reduction activity was correlated with the presence of Cu+ carbonyl species. The Cu–Ce–Zr/TiO2 catalysts exhibited the high NH3-SCR activity in NOx abatement over a wide temperature range, which is strikingly superior to that of Cu/TiO2 catalysts. The onset temperature T10 of Cu–Ce–Zr/TiO2 catalyst is at about 100 °C which is lower 55 °C than that of the Cu/TiO2 catalyst [31]. However, little attention has been focused on study of Cu–Ce–Zr mixed oxides loaded on supporter as the potential catalysts for the VOCs decomposition. ZSM-5 zeolite with obvious advantages of high specific surface area, uniform pore structure as well as high chemical stability would be beneficial to VOCs removal [2,3]. In the present study, the effect of adding zirconium and/or cerium oxide to Cu/ZSM-5 catalysts on the activity for ethyl acetate abatement was addressed. The correlation between the CuCexZr1xOy/ZSM-5 catalyst activity and structural characteristic, dispersion, reduction adsorption/desorption behaviors were investigated with XRD, XPS, H2-TPR, and NH3-TPD. The results provided a helpful understanding of a Ce/Zr-based catalyst for VOCs abatement.

2. Experimental 2.1. Catalyst preparation A series of CuCexZr1xOy/ZSM-5 (x = 0, 0.25, 0.5, 0.75 and 1, denoted as CuZr1/Z, CuCe0.25Zr0.75/Z, CuCe0.5Zr0.5/Z, CuCe0.75Zr0.25/ Z and CuCe1/Z, respectively) catalysts with different Ce/Zr molar

537

ratios were prepared by impregnation. H/ZSM-5 with an atomic Si/Al ratio of 25 was supplied by Nankai University, Tianjin, P.R. China. Initially, appropriate amounts of copper acetate, zirconium and cerium nitrate were added to deionized water and mixed with 20 g of H/ZSM-5 powder at room temperature until water evaporation. The resulting precursor was dried at 105 °C for 24 h in air and then calcined at 550 °C for 4 h. The copper content of CuCexZr1xOy/ZSM-5 catalysts were fixed at 4 wt%, and the molar ratio of Cu/(Ce + Zr) is 1:1. In order to evaluate the catalytic activity, catalyst pellets were pressed by pressure of 20 MPa, then granulated and screened to a size of 20–40 meshes (420–590 lm). 2.2. Catalyst characterization N2 adsorption/desorption isotherms were measured at 196 °C using an Autosorb-Iq-MP instrument (Quantachrome). The samples were degassed at 300 °C for 4 h before measurements. BET surface area of samples was obtained according Brunauer– Emmett–Teller (BET) method, and average pore size was calculated by the Horvath–Kawazoe (HK) equation. X-ray diffraction (XRD) patterns were carried out in a XD-3automatic (PERSEE) equipped with multi-crystal X-ray diffractometer using Cu Ka (k = 0.1540560 nm, 40 kV, 20 mA) in the 2h range of 5° to 80° (scanning rate of 4°/min) and a step size of 0.02°. X-ray photoelectron spectroscopy (XPS) experiments were recorded on a Perkin–Elmer PHI-1600 ESCA spectrometer using a Mg Ka X-ray source. The binding energy (BE) was calibrated based on the line position of C 1s (285 eV). Temperature-programmed reduction experiments with hydrogen (H2-TPR) were investigated by PCA-140 instrument (Bolider). For the analysis, each sample (200 mg) was first pretreated in Ar at 500 °C (20 °C/min) for 60 min, and then cooled down to 30 °C. H2-TPR measurements were recorded in 5%H2/Ar (70 mL/ min) with a heating rate of 10 °C/min and a final temperature was 800 °C. The hydrogen consumption detected by thermal conductivity detector. Temperature-programmed desorption of ammonia (NH3-TPD) was performed on a PCA-140 instrument (Bolider) with a thermal conductivity detector (TCD). Prior to adsorption experiment, the sample (200 mg) was first pretreated in a quartz U-tube under an Ar stream (50 mL/min) from 50 °C to 500 °C for 60 min at a rate of 20 °C/min. And then the temperature was decreased to the room temperature. The adsorption step was performed by admitting 5% NH3/Ar (50 mL/min) at 100 °C to saturation. Subsequently, the sample was exposed to a flow of Ar for 30 min at 100 °C in order to remove reversibly and physically bound ammonia from the catalysts surface. Finally, desorption was carried out from 100 °C to 800 °C at a rate of 10 °C/min. 2.3. Catalyst tests The catalytic performance tests were investigated in a flowtype apparatus designed for continuous operation (Fig. 1), consisting of continuous flow gas supplying system, fixed-bed reactor and online multi-component analytical system. The streams with ethyl acetate were produced by bubbling air through the VOC saturators. And then the gas containing ethyl acetate was further diluted with another air stream to required concentration before reaching the fixed-bed. The gas flow rates of the two branches were metered by mass flow controllers. In each test, 0.8 g of the catalyst (20–40 mesh) was placed in the middle of the fixed-bed and the total flow rate was 400 mL/min, corresponding to a space velocity of 24,000 h1. The fixed-bed temperature was first raised to 80 °C with the gas stream passing and stabilized for 30 min. The effluent gas composition was analyzed

538

S. Li et al. / Chemical Engineering Journal 285 (2016) 536–543

Fig. 1. Schematic diagram of apparatus for catalyst evaluation. 1. Air compressor, 2. Mass flow controller, 3,4. Saturator, 5. Mixing chamber, 6. Fixed-bed reactor, 7. Six-way valve, 8. Gas chromatograph.

by a gas chromatograph (GC-7900, Shanghai Tianmei Co., China) equipped with TCD (TDX-01) for quantitative analysis of CO2, and with FID (HP-5) for analysis of ethyl acetate and by-product ethanol quantitatively.

The catalyst activity was measured by the conversion of ethyl acetate. The conversion was obtained by the following equation:

Con: ð%Þ ¼

C in  C out  100% C in

Cu/Z ZSM-5

C1  100% Con: ð%Þ  C in  n

ð2Þ

C2  100% Con: ð%Þ  C in

ð3Þ

0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/Po )

b CuCe1/Z

where Sc is CO2 selectivity, Ye is the yield of by-product ethanol, C1 is the outlet concentration of CO2, C2 is the outlet concentration of ethanol, and n is the number of carbon atoms in the VOC molecule (for ethyl acetate, n = 4).

Pore volume (a.u)

Y e ð%Þ ¼

CuZr 1/Z

ð1Þ

where Cin is the inlet molar flow rate of VOC, Cout is the outlet molar flow rate of VOC. The selectivity of CO2 and the yield of by-product ethanol can be obtained by the following equations:

Sc ð%Þ ¼

/Z Zr 0.25 CuCe 0.75 /Z Zr 0.5 CuCe 0.5 /Z Zr 0.75 CuCe 0.25

Volumn (cc/g)

2.4. The activity and selectivity

a r /Z CuCe 1Z 0

CuCe0.75Zr0.25/Z CuCe0.5Zr0.5/Z CuCe0.25Zr0.75/Z CuZr1/Z Cu/Z ZSM-5

3. Results and discussion 0.4

0.5

0.6

0.8

0.9

1.0

1.1

1.2

Pore size (nm)

3.1. Textural property and crystal structure Textural properties of CuCexZr1xOy/ZSM-5 catalysts were characterized by N2 adsorption–desorption measurement. Shown in Fig. 2a and b are the N2 adsorption and desorption isotherms and the corresponding pore size distributions of CuCexZr1xOy/ZSM-5 catalysts, respectively. Table 1 displays the specific surface area, pore volume, and average pore size of the samples. All CuCexZr1xOy/ZSM-5 samples exhibit typical I shape isotherms with the P/Po of microporous material according to the IUPAC classification (Fig. 2a) [14]. The pore size distribution curves of all samples center at the range of 0.4–0.6 nm, similar with ZSM-5 support (Fig. 2b). Table 1 shows that doping ZSM-5 with copper, ceria and zirconia lead to decrease in BET surface area and pore volume, which can be attributed to metallic species cover the external surface of ZSM-5, blocking a number of zeolite channels. The above

0.7

Fig. 2. N2 adsorption/desorption isotherms (a) and pore size distributions (b) of CuCexZr1xOy/ZSM-5 catalysts.

Table 1 Textural properties of CuCexZr1xOy/ZSM-5 catalysts. Catalysts

SBET (m2  g1)

Average pore size (nm)

Pore volume (cm3  g1)

ZSM-5 Cu/Z CuZr1/Z CuCe0.25Zr0.75/Z CuCe0.5Zr0.5/Z CuCe0.75Zr0.25/Z CuCe1/Z

501 430 401 384 365 339 398

0.54 0.53 0.51 0.53 0.54 0.53 0.51

0.35 0.26 0.40 0.24 0.32 0.29 0.27

539

S. Li et al. / Chemical Engineering Journal 285 (2016) 536–543

Cu2p3/2

shake-up

Cu2p1/2

CuCe1/Z

CuCe0.75Zr0.25/Z Intensity (a.u.)

results indicate that the addition of Ce/Zr promoter into Cu/ZSM-5 slightly influences the average pore size and pore size distribution. Fig. 3 shows the XRD spectra of the CuCexZr1xOy/ZSM-5 catalysts with different Ce/Zr molar ratios, together with pure ZSM-5. The inherent structure of ZSM-5 (2h = 7.50°, 8.1°, 23.1° and 24.5°) is observed for all catalysts, indicating that the catalysts still keep the microstructure orderly after the cerium and/or zirconium additions. No diffraction peaks attributed to either cerium or zirconium oxides are observed for the CuCe0.25Zr0.75/Z, CuCe0.5Zr0.5/Z and CuCe0.75Zr0.25/Z catalysts, suggesting that the cerium and zirconium dispersed uniformly on the surface of ZSM-5 support [5,26,27,31]. In addition, it can be seen that no diffraction peaks corresponding to CuO are detected on the CuCexZr1xOy/ZSM-5 catalysts. The result indicates that CuO is well dispersed as amorphous metal species on the ZSM-5 support or the localization of Cu ions in the Zr–Ce–O lattice. The amorphous CuO are aggregated into mini-crystals (<4 nm) that are too small to be detected by XRD [5]. With an increase of cerium content, two weak peaks of CeO2 (2h = 28.2°, 47.5°) are found for CuCe1/Z, which means the low crystallinity of CeO2.

+ Cu

2+ Cu

CuCe0.5Zr0.5/Z

CuCe0.25Zr0.75/Z CuZr1/Z Cu/Z 970

960

950

940

930

Binding energy (eV)

3.2. Surface state To obtain information about the surface composition and the elemental chemical states, XPS measurements were carried out for the CuCexZr1xOy/ZSM-5 catalysts. Fig. 4 depicts the XPS spectra in the Cu 2p region for the catalysts with different Ce/Zr molar ratios. All spectrum are characterized by two main peaks of Cu 2p1/2 (952.5–955 eV) and Cu 2p3/2 (930–935 eV), along with the shake-up satellite peaks centered at 937.5–947.5 eV. Peak deconvolution and fitting to experimental data show that the Cu 2p3/2 peak could be well fitted by two peaks at 932.5 and 934.7 eV, corresponding to the chemical states Cu2+ and Cu+, respectively [5,26,32]. As shown in Fig. 4, with an increase of

C u C e 1 /Z C u C e 0 .7 5 Z r 0 .2 5 /Z C u C e 0 .5 Z r 0 .5 /Z C u C e 0 .2 5 Z r 0 .7 5 /Z

Fig. 4. Cu 2p X-ray photoelectronic spectra of CuCexZr1xOy/ZSM-5 catalysts.

cerium content, the binding energy of Cu 2p3/2 shifts to the slightly higher binding energy, confirming that some Cu+ is oxidized to Cu2+ [26]. The processed XPS experimental data also suggest that the Cu2+/Cu+ ratio in the CuCexZr1xOy/ZSM-5 catalysts increase with the cerium content increasing, from 0.85 to 2.96. The higher Cu2+/Cu+ ratio is, the more reduced Cu species exist in the catalyst [4,26]. The O 1s XPS spectra of the catalysts are shown in Fig. 5, in which two oxygen peaks of OI (531.6 eV) and OII (529.6 eV) are observed. The higher binding energy is related to lattice oxygen from copper, cerium and zirconium oxides, and the lower binding energy corresponds to regular lattice oxygen from the ZSM-5 zeolite structure [2,26]. The OII peak belongs to surface adsorbed oxygen, hydroxyl group, and oxygen vacancies [13,29] after the introduction of Ce/Zr. Furthermore, the OII peak shifts slightly to a lower binding energy with the cerium content increasing. Combining with the findings from XRD indicates that CeO2 is prone to enrichment on the ZSM-5 surface.

C u Z r 1 /Z

Intensity (a.u.)

C u /Z Z S M -5 35

40

45

50

CuCe1/Z CuCe0.75Zr0.25/Z CuCe0.5Zr0.5/Z CuCe0.25Zr0.75/Z CuZr1/Z

Intensity (a.u.)

30

CeO2

CuCe1/Z CuCe0.75Zr0.25/Z CuCe0.5Zr0.5/Z

O

CuCe0.25Zr0.75/Z CuZr1/Z

Cu/Z

O

ZSM-5

Cu/Z 10

20

30

40

50

60

70

2 Theta (o) Fig. 3. XRD spectra of CuCexZr1xOy/ZSM-5 catalysts.

80

540

538

536

534

532

530

528

Binding energy (eV) Fig. 5. O 1s X-ray photoelectronic spectra of CuCexZr1xOy/ZSM-5 catalysts.

S. Li et al. / Chemical Engineering Journal 285 (2016) 536–543

3.3. Temperature-programmed reduction by hydrogen Temperature-programmed reduction experiments were performed to evaluate the reducibility of the catalysts with different Ce/Zr molar ratios incorporation. Fig. 8 shows the H2-TPR results obtained over the CuCexZr1xOy/ZSM-5 samples, and Table 2 represents the amounts of H2 consumption for all the samples. Three main reduction peaks are evident in the all samples. The a peak (241 °C) is ascribed to the reduction of the copper species dispersed on the ZSM-5 support, and the b peak (275 °C) is associated with the reduction of the copper oxide adhering to the external

908.5

904.7

899.5

Intensity (a.u.)

CuCe1/Z

883.2

889.8 884.5

CuCe0.75Zr0.25/Z

CuCe0.5Zr0.5/Z

CuCe0.25Zr0.75/Z CuZr1/Z 925

920

915

910

905 900 895 Binding energy (eV)

3d5/2

CuCe0.75Zr0.25/Z

CuCe0.5Zr0.5/Z CuCe0.25Zr0.75/Z CuZr1/Z

188

186

184

182

180

178

Binding energy (eV) Fig. 7. Zr 3d X-ray photoelectronic spectra of CuCexZr1xOy/ZSM-5 catalysts.

CuCe1/Z CuCe0.75Zr0.25/Z CuCe0.5Zr0.5/Z CuCe0.25Zr0.75/Z CuZr1/Z

Cu/Z 100

200

300

400

500

600

700

800

o

Temperature ( C) Fig. 8. H2-TPR profiles of CuCexZr1xOy/ZSM-5 catalysts.

Table 2 H2 consumption and the acidity of strong acid sites for CuCexZr1xOy/ZSM-5.

902.1

917.3

3d3/2

CuCe1/Z

Intensity (a.u.)

Fig. 6 displays the Ce 3d XPS spectra of the catalysts. The main features at 883.2, 889.8, 899.5, 902.1, 908.5 and 917.3 eV are assigned to Ce4+ spectra, whereas those at 884.5 and 904.7 eV are ascribed to Ce3+ [4]. It is evident that the cerium is mostly in a Ce4+ and Ce3+ co-existed. The separation between the Ce 3d3/2 (883.2 eV eV) and Ce 3d5/2 (902.1 eV) signals is also in consistent with an expected value of 18.9 eV, these features are considered as fingerprints for the existence of Ce4+ [13]. Liu et al. [13] found that the enhancement of homogeneous CexZr1xO2 could ease the valence change of the Ce (Ce4+ $ Ce3+). The substitution of Ce4+ (r = 0.97 Å) by Zr4+ (r = 0.84 Å) and the spontaneous transformation of Ce4+ (r = 0.97 Å) to the larger Ce3+ (r = 1.10 Å) can compensate for this lattice contraction, and keep the catalyst morphology. The result is also confirmed in N2 adsorption–desorption analysis. Ce/Zr promoted copper/ZSM-5 catalysts still maintain the microstructure of ZSM-5. It is reported that the existence of Ce3+ in CeO2 implies the formation of an oxygen vacancy, increasing the catalyst activity [5]. Moreover, the generation of Cu+ together with Ce3+ is indicative of the redox equilibrium (Ce3+ + Cu2+ $ Ce4+ + Cu+), which is claimed to be the enhancement of reduced copper species [13]. XPS Zr 3d doublet spectra obtained from the CuCexZr1xOy/ZMS-5 catalysts are shown in Fig. 7. The peak areas and amplitudes of Zr 3d3/2 (181.9 eV) and Zr3d5/2 (184.3 eV) decrease as a function of zirconium content. Interestingly, the binding energy 184.3 eV of Zr 3d5/2 is higher than that in ZrO2 (182.9 eV), lower than that in Zr (180.0 eV). Hence, the XPS results prove the displacement of Zr4+ ions into the ceria lattice, which forms two kinds of zirconium species (Zr4+ and Zrd+). The existence of more reduced zirconium species is related to a significant amount of oxygen vacancies in the zirconia lattice. Similar findings have also been reported in zirconate [31].

H2 consumption (a.u.)

540

890

885

880

Fig. 6. Ce 3d X-ray photoelectronic spectra of CuCexZr1xOy/ZSM-5 catalysts.

Catalysts

H2 consumption (lmol  g1)

Acidity ðlmolNH3  g1 Þ

Cu/Z CuZr1/Z CuCe0.25Zr0.75/Z CuCe0.5Zr0.5/Z CuCe0.75Zr0.25/Z CuCe1/Z

3891 4409 4658 5349 6558 4638

1659 2357 3706 5314 8918 9186

surface of zeolite crystallites, whereas the c peak (430 °C) is generally proposed as the reduction of the bulk copper oxides [33,34]. The reduction peaks change with the Ce/Zr incorporation. The a peak and b peak shift to lower temperature, however, the c peak decreases gradually even disappears with the Ce/Zr molar ratio increased. The results can be explained by the fact that the synergistic effect of the Cu–Ce–Zr shifts the reduction peak to the lower temperature [5,16]. The more content of Ce there is in the catalyst, the greater synergistic effect there is in it. The Cu2+–O2–Ce4+connection in the catalyst could induce the redox potential of Cu

541

S. Li et al. / Chemical Engineering Journal 285 (2016) 536–543

bound NH3, which arises from NH3 physisorbed on Si–OH. The high-temperature peak around 450 °C is attributed to the NH3 adsorbed on the strong acid sites of Si–OH–Al. Upon incorporation of copper, cerium and/or zirconium, the uptake of both weakly and strongly bound NH3 decreases, however, the desorption peak of strong acid sites moves to the high temperature at about 725 °C, which mainly originates from metal oxide nanoclusters [26]. Table 2 also indicates the amount of acidity of strong acid sites. The acidity of strong acid sites on CuCexZr1xOy/ZSM-5 catalyst surface increases with the increasing of more Ce/Zr molar ratio. Combining with the results of H2-TPR, it suggests that there might be a correlation between H2 consumption and NH3 desorption with the addition of cerium and zirconium into the catalyst.

strong acid sites

weak acid sites

TCD signal (a.u.)

CuCe1/Z CuCe0.75Zr0.25/Z CuCe0.5Zr0.5/Z CuCe0.25Zr0.75/Z CuZr1/Z Cu/Z ZSM-5

3.5. Catalyst performances 300

400

500 600 o Temperature ( C)

700

800

Fig. 9. NH3-TPD profiles of CuCexZr1xOy/ZSM-5 catalysts.

species as well as act as a bridge for O transfer between Cu and Ce, then it enhances the reducibility for CuCexZr1xOy/ZSM-5 allowing an effective redox cycle during oxidation reactions [5]. The decreased bulk copper species results in intensity weakening of the c peak. It is worth noting that the total amount of H2 consumption increases from 4409 to 6558 lmol  g1 with the increasing of the Ce/Zr molar ratios from 0:1 to 3:1, while it decreases to 4638 lmol  g1 with further increasing the Ce/Zr molar ratio to 1:0. It can be obtained from the XPS analysis that the incorporation of cerium and zirconium into the catalyst promoted the amount of the surface oxygen species, which enhances the H2 consumption. In this study, CuCe0.75Zr0.25/Z exhibits the highest H2 consumption (6558 lmol  g1), and the H2 consumption is closely related to the activity of the catalyst (see Section 3.5).

3.4. Temperature-programmed desorption of ammonia The NH3-TPD profiles of the CuCexZr1xOy/ZSM-5, Cu/Z and pure ZSM-5 samples are shown in Fig. 9. As shown in Fig. 9, the NH3-TPD profile of ZSM-5 displays two desorption peaks. The low-temperature peak around 200 °C is assigned to weakly

100

Conversion (%)

90

ZSM-5 Cu/Z CuZr1/Z

The investigation of catalytic performance was conducted in a fixed-bed reactor for the removal of typical VOCs, such as ethyl acetate, by a series of CuCexZr1xOy/ZSM-5 catalysts. Ethyl acetate can be completely oxidized to CO2 and H2O, whereas a small portion of ethyl acetate can be converted to intermediate products such as ethanol, acetaldehyde and acetic acid at low temperatures [4]. Fig. 10 shows the catalyst activity and the CO2 selectivity of CuCexZr1xOy/ZSM-5 toward ethyl acetate oxidation. The catalyst activity (Fig. 10a) is consistent with CO2 selectivity (Fig. 10b), implying the better catalyst performances corresponds to the higher selectivity to CO2. The catalyst activity and CO2 selectivity climb with the increasing of Ce/Zr molar ratio from 1 to 3. Furthermore, it can be seen from Fig. 10a that high catalytic activity corresponds to low value of T10 (the onset temperature), T50 (the temperature corresponding to 50% conversion of ethyl acetate to CO2), T90 (the temperature corresponding to 90% conversion of ethyl acetate to CO2) and T100 (the temperature of complete conversion) of ethyl acetate. CuCe0.75Zr0.25/Z exhibits the highest catalytic activity, and the corresponding T10, T50, T90 and T100 for oxidation of ethyl acetate are at 110 °C, 200 °C, 248 °C and 270 °C, respectively. Ethanol is a unique intermediate product in the preform of ethyl acetate removal to CO2. Fig. 11 shows yield of intermediate product ethanol as a function of reaction temperature (a) and at T10, T50 (b) for CuCexZr1xOy/ZSM-5 catalysts. It can be seen that the variations of the ethanol yield for CuCexZr1xOy/ZSM-5 samples exhibit the same trend, which initially increase remarkably, and then reach a maximum before a rapid decrease and even disappear with further

100

T90

90

80

CuCe0.25Zr0.75/Z

80

70

CuCe0.5Zr0.5/Z

70

60

CuCe0.75Zr0.25/Z

50

CuCe1/Z

60

T50

50

40

40

30

30

20 10

20

T10

a

b

0 100

150

200

250 o

Temperature ( C)

300

10 0

100

150

200

250

300

o

Temperature ( C)

Fig. 10. Ethyl acetate conversion (a) and CO2 selectivity (b) as a function of reaction temperature for CuCexZr1xOy/ZSM-5 catalysts.

CO 2 seletivity (%)

200

542

S. Li et al. / Chemical Engineering Journal 285 (2016) 536–543

b ethanol (%

)

4

a

CuCe0.25Zr0.75/Z 10

3 2

Yield of

ZSM-5 Cu/Z CuZr1/Z CuCe0.5Zr0.5/Z

1

CuCe0.75Zr0.25/Z

0

CuCe1/Z

T50

5

Temperature ( C)

-5

Z /Z u/ Z / 75 r 1 Z r 0. 5

300

Z 75

250

o

uZ

uC

200

/Z e1

150

e 0.

100

uC

uC

50

C 2 /Z e 0. .5 uC /Z Zr 0 C 5 25 e 0. r 0.

C

C C

0

M

C

T10

ZS

Yield of ethanol (%)

15

Fig. 11. Yield of intermediate product ethanol as a function of reaction temperature (a) and at T10, T50 (b) for CuCexZr1xOy/ZSM-5 catalysts.

400

100 T90

80

300

60 T50

200

40 100 20 T10

0

o Temperature ( C)

Conversion (%)

increase of temperature. It can be found that higher catalytic activity leads to lower yield of ethanol byproduct. By increase the Ce/Zr moral ratio, the temperatures corresponding to the maximum of ethanol yield move toward lower values. In particular, for CuCe0.75Zr0.25/Z, with the best activity, the temperature corresponding to the maximum of ethanol yield is around 200 °C, which is 20 °C lower than those of the other catalysts. In addition, it can be seen that the maximum yield of ethanol brings to catalytic activity and CO2 selectivity increasing slowly with the increasing of reaction temperature. Obviously, CuCe0.75Zr0.25/Z exhibits the best catalytic behavior with the highest Cu2+/Cu+ ratio, however, its specific surface area (339 m2  g1) is lower than those of other samples. The content of CuO in the prepared CuCexZr1xOy/ZSM-5 catalysts is only 4 wt %, which was dispersed uniformly on the catalyst surface, therefore excess specific surface area is not necessary to enhance the activity of catalysts. The obtained results show that the specific surface area is not the key factor governing the catalytic activity, and the similar results have been found in the previous studies [5,8,14,21]. Furthermore, from the results of H2-TPR and NH3-TPD, it can be concluded that the catalytic activity for VOCs oxidation has high affinity for the reducibility and the strong acidity sites [4,14,16]. Combined with results of XPS, with Ce/Zr molar ratio increases, the good reducibility owing to the increase of the amount and the kinds of reactive oxygen species, can provide a facile redox process that would lead to an enhanced catalytic oxidation [4,6,16,29]. In particular, CuCe0.75Zr0.25/Z with the highest Ce/Zr molar ratio shows the highest H2 consumption (6558 umol  g1), resulting in the highest catalytic activity and CO2 selectivity. On the other hand, with Ce/Zr molar ratio increases, oxygen species can be stored/released by ceria via the redox shift between Ce4+/Ce3+ [26], leading to the increase of surface mobile oxygen, which is beneficial to the redox equilibrium (Ce3+ + Cu2+ $ Ce4+ + Cu+) shifting to the right in these samples. Moreover, the synergistic effect in Cu–Ce–Zr of CuCexZr1xOy/ZSM-5 can improve the surface mobile oxygen significantly [4], contributing to the improvement in reducibility of the catalyst. Consequently, the values of T10, T50, T90 and T100 decrease with the increase of Ce/Zr molar ratio. For the obtained CuCe0.75Zr0.25/Z catalyst, the onset temperature is as low as 110 °C, which is lower 40 °C compared with Cu–Ce1xSmxOd

CuCe0.75Zr0.25/Z

0 0

10

20

30

40

50

60

Time on stream (h) Fig. 12. Stability of CuCe0.75Zr0.25/Z catalyst as a function of time on stream.

catalysts with T10 of 150 °C [4], and 90 °C compared with the Au–CuO with T10 of 200 °C [6]. The stability of CuCe0.75Zr0.25/Z catalyst at 270 °C is displayed in Fig. 12. The CuCe0.75Zr0.25/Z catalyst shows a superior stability for catalytic removal ethyl acetate in 60 h, and the conversion is always remained above 99.5% at 270 °C. And no carbon deposition in the used catalyst is observed. The obtained results demonstrate that the catalyst stability could be improved by adding ZrO2 [25,26,28], on the other hand, take into consideration the characterization results of XPS and H2-TPR, augment of the amount and the kinds of surface mobile oxygen can suppress the carbon deposition [14]. 4. Conclusions (1) A series of CuCexZr1xOy/ZSM-5 catalysts with different Ce/ Zr molar ratios were prepared by impregnation. The catalysts still keep the microstructure orderly after the cerium and/or zirconium additions. Active species CuO and Ce/Zr promoter well dispersed on the ZSM-5, and copper is mostly in Cu2+ state and Cu+ co-existed. Higher Cu2+/Cu+ ratio in the

S. Li et al. / Chemical Engineering Journal 285 (2016) 536–543

catalyst results in higher catalytic activity. With Ce/Zr molar ratio increasing, oxygen could be easily stored/released by ceria via the redox shift between Ce4+/Ce3+. (2) The synergistic effect of Cu–Ce–Zr can increase the amount of the active oxygen and improve the oxygen mobility. Both reducibility and acidity of strong acid sites play crucial roles in the CuCexZr1xOy/ZSM-5 catalytic performance. (3) CuCe0.75Zr0.25/Z exhibits the highest catalytic activity and superior stability for ethyl acetate oxidation, with T10 of 110 °C and T100 of 270 °C, respectively. The temperature corresponding to the maximum of ethanol yield is 20 °C lower than those of the other catalysts. No deactivation phenomenon was detected during the catalytic oxidation in 60 h, and the conversion is always remained above 99.5% at 270 °C.

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