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View Article OnlineView Journal | View Issue

Activity and sulfu

aState Key Laboratory of Materials-Orient

University, Nanjing 210009, China. E-m

83587326; Tel: +86 25 58139922bCollege of Life Science and Pharmaceutica

Nanjing 210009, ChinacCollege of Materials Science and Enginee

210009, China

† The authors contributed equally to this

Cite this: RSC Adv., 2014, 4, 51280

Received 24th July 2014Accepted 1st October 2014

DOI: 10.1039/c4ra07538b

www.rsc.org/advances

51280 | RSC Adv., 2014, 4, 51280–5128

r resistance of CuO/SnO2/PdOcatalysts supported on g-Al2O3 for the catalyticcombustion of benzene

Q. Niu,†ab B. Li,†ab X. L. Xu,ab X. J. Wang,ab Q. Yang,ab Y. Y. Jiang,ab Y. W. Chen,ab

S. M. Zhuac and S. B. Shen*ab

In this paper, five types of catalysts, (i.e. SnO2/PdO/g-Al2O3, CuO/PdO/g-Al2O3, PdO/g-Al2O3, CuO/SnO2/

g-Al2O3 and CuO/SnO2/PdO/g-Al2O3) were prepared by multiple step impregnation for the catalytic

combustion of benzene. The catalysts were characterized by XRD, BET, H2-TPR and IR to investigate the

internal structural and textural changes. The results indicated that the addition of copper to Pd-

containing catalyst could promote the catalytic activity, and the addition of tin was beneficial for

promoting the sulfur resistance of catalysts but did not bring any benefit to activity. In addition, the CuO/

SnO2/PdO catalyst exhibited better sulfur resistance and catalytic activity than the other prepared

catalysts which was attributed to the addition of both copper and tin.

Volatile organic compounds (VOCs) that are emitted frommaterial industries are considered to be an important class ofair pollutants. Benzene is one of the most common types ofVOCs, and is stable and extensively applied in various elds,such as the petrochemical industry and the manufacture ofmotor fuels, paints, plastics, medications and detergents.1–3 Inaddition, catalytic combustion has been extensively investi-gated in the past few decades due to its practical applications inpollutant abatement, and this technology has been determinedto be more environmentally friendly than conventional amecombustion due to lower NOx, CO, and unburned hydrocarbonemissions as well as a higher energy efficiency.4–6

Currently, noble metal catalysts are being extensively usedfor the complete oxidation of VOCs.7–13 In regards to the activityand selectivity of the catalytic combustion catalysts, noblemetals are typically regarded as the most desirable catalysts. Inparticular, Pd-based catalysts offer several advantages such as ahigher activity, thermal stability, better performance and alower cost compared to other noble catalysts (Pt, Ru).7–9 The PdOcatalysts, which are the most active, exhibit a strong sensitivityto sulfur containing compounds, which is a serious drawback totheir use in VOCs exhaust aer treatment. Sulfur-containingcompounds can be readily converted to SOx and strongly adsorb

ed Chemical Engineering, Nanjing Tech

ail: zsbshen@gmail.com; Fax: +86 25

l Engineering, Nanjing Tech University,

ring, Nanjing Tech University, Nanjing

work.

5

on the surface of the active ingredient as a stable sulfate species,which decreases the number of active sites until saturation ofthe active sites surface by the sulfate species results in acomplete loss of catalytic activity for benzene oxidation.14,15

Researchers have reported that the addition of transitionmetals can enhance the sulfur resistance of PdO catalyst.16–19 Inaddition, the addition of transition metals with good thermalresistance (e.g. CuO) can act as a promoter, which is benecialfor a higher specic surface area and better catalytic activity.16

Ferrandon17 and Reyes19 both have reported that the addition ofcopper greatly improves the catalytic activity of the PdO catalyst.Tin oxide which exhibits superior performance and thermalstability, has been widely used for the catalytic oxidation ofmethane.20,21 However, tin oxide has not been extensivelyinvestigated for the catalytic combustion of VOCs in the pres-ence of SO2. H. Meng22 has determined that tin oxide improvedthe sulfur resistance of catalysts, because tin oxide is a type ofacidic oxide. This chemical property can reduce SO2 adsorptionon the surface of a catalyst. Therefore, we investigated a series ofcatalysts supported on g-Al2O3, including the SnO2/PdO, CuO/PdO, PdO, CuO/SnO2 and CuO/SnO2/PdO catalysts for thecatalytic combustion of benzene. In addition, the catalyticactivity in presence of SO2 was also studied. BET, XRD, H2-TPRand IR characterizations were used to investigate the structural,morphological and catalytic properties changes.

1 Experimental1.1 Catalyst preparation

The SnO2/PdO, CuO/PdO, PdO, CuO/SnO2 and CuO/SnO2/PdOcatalysts were manufactured using a multiple step impregna-tion method with Cu(NO3)2$6H2O, Pd(NO3)2$3H2O and

This journal is © The Royal Society of Chemistry 2014

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SnCl4$5H2O as precursors. 4.0 g of a commercial g-Al2O3

supporter was dipped into a solution mixture containingadequate amounts of an active component and distilled waterfor 2 h, dried at 80 �C for 2 hours and calcined in air up to 500 �Cfor 5 h to acquire desired catalysts. The catalysts contained 0.06wt% PdO; 10 wt% SnO2 and 10 wt% CuO based on the activeingredients. In order to describe the preparation processclearer, the total scheme of catalyst preparation was exhibited inFig. 1.

1.2 Catalyst characterization

Nitrogen adsorption and desorption isotherms were deter-mined at �196 �C using an ASAP-2020 analyzer (MicromeriticsInc.). Using these isotherms, the BET surface areas werecalculated, and the pore volumes were determined using theprocedure propose by BJH. XRD data were recorded on a Pan-alytical X'Pert Pro diffractometer at 40 kV and 40 mA with a stepsize of 0.0167�, and a scanning rate of 10� min�1 using Co Ka

radiation, which was then revised to Cu Ka. TPR experimentswere performed with a TPR2900Micromeritics system equippedwith a thermal conductivity detector. 50 mg samples wereplaced in a U-shape quartz tube and purged with a synthetic air(5% O2/He) steam at 50 ml min�1 at 773 K for1 h, followed bycooling to ambient temperature. Then, the reduction proleswere measured by passing a 5% H2/Ar ow at a rate of 25 mlmin�1 over the sample while heating at a rate of 5 �C min�1

from ambient temperature to 800 �C. IR spectra using KBrpellets of the samples were recorded on a Nicolet 410 FT-IRspectrometer at a resolution of 4 cm�1, and the amount ofsamples was 1.5 mg in 500 mg of KBr.

1.3 Catalytic combustion measurement

Catalytic activity tests were carried out in a xed-bed owreactor under atmospheric pressure. Approximately 2.0 g ofcatalysts were loaded in a quartz reactor and placed in themiddle of the reactor. The temperature was measured andcontrolled with a thermocouple in the range of 150–450 �C, at abenzene concentration of 1500 ppm and a GHSV of 20 000 h�1.The inlet and outlet gas compositions were analyzed aerstepwise changes in the reaction temperature using an on-linegas chromatograph (GC-2014, Shimadzu Corp) equipped withan FID detector and a Restek Rtx-1 column. The catalytic effi-ciency was evaluated based on the benzene consumption. Thecatalytic efficiency is determined as:

Fig. 1 The total scheme of the catalysts preparation process.

This journal is © The Royal Society of Chemistry 2014

h1 ¼C1Qsn1 � C2Qsn2

C1Qsn1

� 100% (1a)

where h1 was the conversion efficiency, C1 and C2 were the inletand outlet concentration of benzene, respectively, and Qsn1 andQsn2 were the inlet and outlet ux of air, respectively.

1.4 The sulfur treatment of the catalysts

The sulfur treatment of the catalysts was also carried out in thexed-bed ow reactor. At the reaction temperature of 350 �C,100 ppm of SO2 was continuously added to the feed gas for 30hours to investigate the sulfur resistance of the prepared cata-lysts. Reactants and products were also analyzed by the on-linegas chromatograph (GC-2014, Shimadzu Corp).

2 Results and discussion2.1 BET analysis

The surface area, pore volume and average pore diameter of thecatalysts are summarized in Table 1. The catalysts exhibit highspecic surface areas, due to the use of the g-Al2O3 as support,which has a surface area of 288 m2 g�1. Because the porediameter is in the range of 2–50 nm, the catalysts are regardedas mesoporous materials. As expected, the surface areadecreased, as the active phase loading increased. (i.e. surfacearea is minimized when 10 wt% of SnO2 and 10 wt% of CuO aredeposited) and the very small amounts of PdO slightly affect thesurface area. The results from the physisorption studies indi-cate that in some cases, the sulfur treatment have an effect onthe specic surface area of the catalyst samples. According tothe results, the average pore diameter of the catalysts slightlyincreases aer the sulfur treatment. However, the surface areadecreases aer the sulfur treatment. The most pronounceddecrease aer the sulfur treatment is detected for the PdOcatalyst. Because the amount of PdO is too low to be measuredby BET, and g-Al2O3 is a well-known sulfatable support, thisdramatic variation in the surface area of the PdO catalyst is dueto an interaction between the g-Al2O3 support and the PdOactive phase.23

2.2 XRD analysis

In order to determine the phase of catalysts, Fig. 2 shows theXRD patterns of the catalysts before and aer sulfur treatment.A considering phenomenon is that none of the Pd-containing(A, A–S, B, B–S, D, D–S, C, C–S) samples exhibit a Bragg dif-fractive peak for Pd oxide. This result may be due to the lowloading of PdO loading (0.06 wt%) and PdO being welldispersed on the catalyst surface, which is consistent with theconclusion reported by F. L. Zhong et al.24 In samples of A, C andD, intense and sharp peaks, which appears near q¼ 26.8�, 34.1�,38.3�, 52.3�, 72.0� and 79.6� corresponding to crystalline SnO2 isregistered over Sn-containing catalysts.25,26 Noteworthily, themonoclinic CuO phase in sample B, C and D is observed at lowand weak diffraction angles. It is indicated that some of thecopper (apparently a very small amount) is incorporated in thecatalysts, which is in agreement with the work of V. R.

RSC Adv., 2014, 4, 51280–51285 | 51281

Table 1 The textural properties of non-sulfated and sulfated catalysts

Materials Surface area (m2 g�1) Mesopore volume (cm3 g�1) Pore diametera (nm)

g-Al2O3 288 0.75 10.49CuO/SnO2 213 0.59 11.05CuO/SnO2–S 202 0.55 11.09PdO 275 0.76 11.08PdO–S 226 0.68 11.15CuO/PdO 236 0.62 11.10CuO/PdO–S 201 0.57 11.16SnO2/PdO 226 0.59 11.11SnO2/PdO–S 210 0.53 11.15CuO/SnO2/PdO 199 0.54 11.13CuO/SnO2/PdO–S 172 0.51 11.19

a BJH adsorption average pore diameter.

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Choudhary.27 In addition, except for metal oxides are accumu-lated, no new crystal structure is formed as revealed in catalystby XRD analysis, these results suggest that the catalystsprepared by stepwise impregnation method are composed ofmixed phases.

Aer sulfur treatment, no signicant crystallographicchanges in the materials induced by the presence of SO2 areobserved in X-ray diffraction patterns. This phenomenon mayindicate that the resulting sulfate species are well dispersed onthe catalyst. In addition, the small amount of sulfate that isformed may have been below the detection limit.22

2.3 H2-TPR prolesof the catalysts

The H2-TPR proles of sulfated and non-sulfated catalystssupported on g-Al2O3 are shown in Fig. 3. A discernible and

Fig. 2 XRD patterns of the non-sulfated and sulfated catalysts (A:SnO2/PdO/Al2O3 catalyst, A–S: sulfated SnO2/PdO/Al2O3 catalyst, B:CuO/PdO/Al2O3 catalyst, B–S: sulfated CuO/PdO/Al2O3 catalyst, C:CuO/SnO2/Al2O3 catalyst, C–S: sulfated CuO/SnO2/Al2O3 catalyst, D:CuO/SnO2/PdO/Al2O3 catalyst, D–S: sulfated CuO/SnO2/PdO/Al2O3

catalyst, E: PdO/Al2O3 catalyst, E–S: sulfated PdO/Al2O3 catalyst).

51282 | RSC Adv., 2014, 4, 51280–51285

sharp peak at the high temperature hydrogen consumptiontemperature between 600 �C and 650 �C is due to the PdOreduction peak, which is attributed to a strong interaction withg-Al2O3 support.28 Researchers20,28,29 have reported that thereduction of copper oxide species resulted in a single peakcentered at approximately 300 �C, which is due to the reductionof Cu2+ to Cu0. This conclusion conrmed that the reductionpeaks at 269 �C, 296 �C, 280 �C, 295 �C, 301 �C and 320 �C in Cu-containing catalysts are associated with the consumption of H2

by surface and bulk CuO, respectively. In addition, the reduc-tion peaks of the tin oxide species are observed at 492 �C, 503�C, 504 �C, 514 �C, 557 �C and 563 �C, respectively.30 Incomparison to the PdO catalyst and CuO/PdO catalyst, thepalladium oxide reduction peak of the CuO/PdO catalystbecame smoother and shied to a lower temperature regionthan PdO catalyst, which indicates that the addition of coppercould enhance the redox ability and promote the lattice oxygenactivation in the PdO catalyst.

Aer sulfur treatment, it can be seen that the reductionpeaks of sulfated catalysts systematically shi to highertemperature regions, indicating that SO2 have a negative effecton the reduction ability of catalysts. In contrast of the PdOcatalyst and the SnO2/PdO catalyst, the temperature of PdOreduction peak in sulfated PdO catalyst have decreased 31 �C,and the temperature of PdO reduction peak in sulfuted SnO2/PdO catalyst have only decreased 15 �C. We can conclude thatSO2 have a greater effect on PdO catalyst than SnO2/PdOcatalyst.

2.4 IR spectroscopy

IR spectroscopy provides a tool for studying the sulfatespecies deposited on the catalysts upon reaction with theSO2-containing feed. It is well-known that sulfate species arecharacterized by IR bands in the 1040–1210 cm�1 range.Al2(SO4)3 exhibit a broad band at ca. 1190 cm�1, and the bandnear 1380 cm�1, which is observed in some catalyst samples,is used as evidence of surface aluminum sulfate.31 As shownin Fig. 4, sulfated catalysts exhibit an absorption band at1180 cm�1 and 1375 cm�1, indicating the existence of

This journal is © The Royal Society of Chemistry 2014

Fig. 3 H2-TPR profiles of the non-sulfated and sulfated catalysts (A:SnO2/PdO/Al2O3 catalyst, A–S: sulfated SnO2/PdO/Al2O3 catalyst, B:CuO/PdO/Al2O3 catalyst, B–S: sulfated CuO/PdO/Al2O3 catalyst, C:PdO/Al2O3 catalyst, C–S: sulfated PdO/Al2O3 catalyst, D: CuO/SnO2/Al2O3 catalyst, D–S: sulfated CuO/SnO2/Al2O3 catalyst, E: CuO/SnO2/PdO/Al2O3 catalyst, E–S: sulfated CuO/SnO2/PdO/Al2O3 catalyst).

Fig. 4 IR spectra of non-sulfated and sulfated catalysts IR spectra ofnon-sulfated and sulfated catalysts (A: CuO/PdO/Al2O3 catalyst, A–S:sulfated CuO/PdO/Al2O3 catalyst, B: SnO2/PdO/Al2O3 catalyst, B–S:sulfated SnO2/PdO/Al2O3 catalyst, C: CuO/SnO2/Al2O3 catalyst, C–S:sulfated CuO/SnO2/Al2O3 catalyst, D: PdO/Al2O3 catalyst, D–S:sulfated PdO/Al2O3 catalyst, E: CuO/SnO2/PdO/Al2O3 catalyst, E–S:sulfated CuO/SnO2/PdO/Al2O3 catalyst).

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aluminum sulfate. In addition to aluminum sulfate, othersulfate substances, such as SnSO4 and CuSO4, are alsoformed in the sulfated catalyst. The PdO catalyst exhibits adeep sulfate adsorption peak, which suggests that the pres-ence of SO2 have a substantial impact on the PdO catalyst. Inthe Sn-containing catalyst samples, the sulfateadsorption peak is weak, and the peak corresponding to theSnO2/PdO catalyst is even weaker than that for the othercatalysts. This phenomenon demonstrates that the additionof SnO2 could prevent catalyst poisoning by forming a sulfatesubstance.

Fig. 5 Benzene conversion over the SnO2/PdO, CuO/PdO, CuO/SnO2, PdO and CuO/SnO2/PdO catalysts supported on g-Al2O3.

2.5 Catalyst activity

Fig. 5 shows the conversion for benzene oxidation over thecatalysts supported on g-Al2O3. From the catalyst activity, theCuO/SnO2/PdO and CuO/PdO catalyst possesses better activityfor benzene oxidation compared to the other catalysts. Inaddition, the activities are ranked in order as follows: CuO/SnO2

< SnO2/PdO < PdO < CuO/PdO z CuO/SnO2/PdO catalyst.Therefore, the catalysts containing a noble metal (PdO

catalyst; CuO/PdO catalyst; SnO2/PdO catalyst; CuO/SnO2/PdOcatalyst) exhibit better catalytic activity than the non-noblemetal containing catalyst (CuO/SnO2 catalyst), which conrmsthe excellent catalytic ability of Pd-containing catalysts. Theaddition of CuO slightly improves the catalytic activity. Theresults present herein provide evidence that the presence ofCuO affects the catalytic activity because the catalytic efficiencyof the CuO/PdO catalyst improves compared to the PdO catalyst.Such an effect has also been reported in the literature.17,19

Although the CuO/PdO catalyst exhibit good catalytic perfor-mance, its sulfur resistance is slightly inferior. According to theanalysis of catalytic activity and the stability test, the

This journal is © The Royal Society of Chemistry 2014

incorporation of SnO2 have no obvious effect on promoting thecatalytic activity. However, SnO2 incorporation is benecial forpromoting sulfur resistance of the PdO catalyst.

2.6 Stability of the catalysts in the presence of SO2

To simulate a long-term exposure of the catalysts to sulfurcompounds at very low concentrations in benzene, the catalystsare exposed to a dry reaction mixture containing 100 ppm ofSO2. The temperature for the stability test in the presence of SO2

is conrmed to be 350 �C to maintain consistent temperatureconditions.

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As shown in Fig. 6, during the sulfur treatment, the PdO andCuO/PdO catalysts exhibit a downward trend, and the PdOcatalyst, which is severely deactivated, decreased from 92.66%to 78.26%. The performance of the SnO2/PdO, CuO/SnO2 andCuO/SnO2/PdO catalysts is not affected by the presence of SO2.In addition, the SnO2/PdO catalyst exhibit a very stable perfor-mance, without appreciable deactivation during 30 h on stream.Therefore, the addition of SnO2 improves the sulfur resistanceof the PdO catalyst, which is consistent with the IR analysis. Theaddition of CuO does not promote the sulfur resistance of PdOcatalyst.

During sulfur poisoning, when SO2 is added into the reac-tion, SO2 is converted to SO3 at a certain temperature and reactswith the active sites to form sulfate species, which results incatalyst poisoning. In previous studies,17,32,33 the formation ofsulfate on the catalyst surface is the primary source of the loss ofcatalytic activity. In addition, the sulfur poisoning mechanismof PdO/g-Al2O3 is more complex, because g-Al2O3 is a sulfatablesupport. PdO oxidises SO2 to SO3, SO3 is trapped by PdO andspills to g-Al2O3 sites surrounding the PdO particles by surfacediffusion. At saturation of these sites, SO3 poisons the PdO dueto palladium sulfate formation. Therefore, as shown in Fig. 5,the PdO/g-Al2O3 catalyst exhibit sulfur poisoning trend, becausesurface sulfate is formed on the sulfated catalysts.

M. A. Fraga have concluded that the effect of the addition ofSnO2 on the PdO catalyst improves the thermal stability of thecatalyst but does not bring any benet to combustion activity.21

This conclusion can also explain why SnO2 acquires goodstability in the presence of SO2. The thermal stability of thesulfate substance have the decisive function upon the variationof the catalyst during the SO2 treatment. In the study by C. J.Zhou,34 Cr2(SO4)3 is unstable and easily decomposed at certaintemperature, which decreases the SO2 adsorption on the cata-lyst surface. In the same manner, SnO2 is known to form astable sulfate substance which decomposes at 350 �C, and SnO2

Fig. 6 Influence of 100 vol ppm SO2 addition on methane conversionover SnO2/PdO, CuO/PdO, CuO/SnO2, PdO and CuO/SnO2/PdOcatalysts supported on g-Al2O3.

51284 | RSC Adv., 2014, 4, 51280–51285

is a type of acidic oxide than that can also reduce SO2 adsorp-tion on the surface of the catalyst. These properties may explainwhy the addition of SnO2 results in favorable sulfur stability(Fig. 6).

3 Conclusion

In this study, SnO2/PdO, CuO/PdO, CuO/SnO2, PdO and CuO/SnO2/PdO catalysts supported on g-Al2O3 are investigated andcharacterized using BET, H2-TPR, XRD and IR. The resultsreveals that the addition of copper and tin to the PdO catalystpromotes catalytic ability and sulfur resistance. The CuO/SnO2/PdO and CuO/PdO catalysts both exhibit better catalyticcombustion than the other catalysts, which indicates that CuOaddition improves the catalytic activity of the PdO catalyst.During the sulfur treatment, the CuO/SnO2, CuO/SnO2/PdO andSnO2/PdO catalysts exhibit good sulfur stability, indicating thefavorable stability performance of the Sn-containing catalyst.The surface area and pore diameter decreases aer SO2 treat-ment, which indicates the inuence of SO2 on the physico-chemical structural variation in the catalysts. In addition, SO2

also have a negative effect on the reduction performanceaccording to the H2-TPR analysis. In the IR analysis, the pres-ence of a sulfate substance is conrmed in the sulfated cata-lysts, and more of the sulfate substance is formed in the PdOcatalyst.

In conclusion, the Pd-containing catalysts exhibit goodcatalytic activity, and the addition of CuO improved the catalyticability of the PdO catalyst. The enhancement of the sulfurresistance of the PdO catalyst is due to the addition of SnO2,because the chemical properties of SnO2 result in a reduction inSO2 adsorption on the surface of catalyst. Among the preparedcatalysts, the CuO/SnO2/PdO catalyst exhibit the highest activityand excellent resistance to sulfur compounds, and the incor-poration of copper and tin promotes catalytic activity and sulfurresistance of the PdO catalyst.

Acknowledgements

This work is nancially supported by the Natural ScienceFoundation of China (no. 51172107 and no. 51272105), theNational Key Technology R&D Program of China (no.2012BAE01B03-3), the Natural science research project in theUniversity of Jiangsu Province (no. 14KJB430014) and the theProgram for Postgraduate Research Innovation in the Universityof Jiangsu Province (no. CXLX13_441 and no. CXZZ13_0453).

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