TiO2 Catalysts by

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Kinetics, Catalysis, and Reaction Engineering

High Catalytic Stability for CO Oxidation over Au/ TiO2 Catalysts by Cinnamomum Camphora Leaf Extract Mingming Du, Jiale Huang, Daohua Sun, Dan Wang, and Qingbiao Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02458 • Publication Date (Web): 22 Oct 2018 Downloaded from http://pubs.acs.org on October 23, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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Industrial & Engineering Chemistry Research

High Catalytic Stability for CO Oxidation over Au/TiO2 Catalysts by Cinnamomum Camphora Leaf Extract Mingming Dua, Jiale Huang b, Daohua Sunb, Dan Wangc, and, Qingbiao Lib,d* aOcean

College, Zhejiang University of Technology, Hangzhou, 310014, China

bDepartment

of Chemical and Biochemical Engineering, College of Chemistry and

Chemical Engineering, Xiamen University, Xiamen, 361005, China cDepartment

of International Trade, School of economics and management, Zhejiang

Sci-Tech University, Hangzhou, 310018, China dCollege

of Food and Biological Engineering, Jimei University, Xiamen, 361021, China

Corresponding

author.

Tel.:

+86

592-2189595;

E-mail:[email protected] and [email protected].

1

ACS Paragon Plus Environment

fax:

+86

592-2184822.

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: Au/TiO2 catalysts were synthesized through Cinnamomum camphora leaf extract. The obtained catalysts were characterized through TEM, XRD, DRUV−vis and were used for CO oxidation. The influence of various parameters (Au loading, biomass, calcination atmosphere, and support crystal form) on the catalytic performance in CO oxidation was studied in detail. The experimental result revealed that under the conditions of 1.0 wt.% Au loading , calcination atmosphere of air, TiO2 with mixed crystal of rutile and anatase, the Au/TiO2 catalyst exhibited optimum catalyitc performance, which remained it’s 90% CO conversion after 150 h.

Keywords: CO oxidation; Au catalyst; Cinnamomum camphora; Stability

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1. Introduction In 1987, high dispersion of Au catalysts through coprecipitation method exhibited high catalytic activity for CO oxidation reported by Haruta et al1. To date, CO oxidation has been performed over Au nanoparticles (NPs) supported on many materials2-6 like TiO2, Fe2O3, CeO2, Al2O3, SiO2, MCM-41, ZnO, ZrO2 and so on. Among those, the most studied catalyst is Au NPs supported on TiO2 due to remarkable catalytic activity for this reaction7-9. For Au/TiO2 catalysts, the current study is mainly focused on the influence of Au valence, Au particle size and Au–support interaction on the catalytic activity and stability2, 3, 10. Au NPs sizes influence the catalytic performance for CO oxidation11-13 and the most active Au NPs ranged from 2 to 5 nm8,

11,

which strongly depend on the preparation

method14. The deposition−precipitation (DP) was ofen used to prepare the Au/TiO2 catalysts, and the key factors determining the Au NPs sizes is the preparation parameter, such as temperature, pH, precursor concentration, and also the post-treatment parameter, such as calcination atmosphere, temperature2, 10. The post-treatment of the Au catalysts also produces different Au valence, and influences the interactions between Au and TiO2, which is important factors affecting catalytic activity11,

15, 16.

The role of metallic and

ionized Au species for CO oxidation is different. Metallic Au NPs might be benefit for CO adsorption and activation17, and oxygen mobility can be increased by ionized Au species, and facilitate

oxygen vacancy formation 18.

Au Catalyst deactivation is a serious problem17, 19, 20. If the interaction between TiO2 and Au is weak, the Au NPs usually aggregate and form larger Au NPs during the catalytic reaction10, losing their catalytic activity. Carbon deposition on the reactive sites of Au/TiO2 catalysts is another reason catalyst deactivation10. The Au/TiO2 catalyst prepared DP method usually suffered serious deactivation, the catalytic performance can decrease as high as abou 70% during 50 h10. In our previous studies, Au/TiO2 catalysts were prepared by biological method, and the 3



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effect of Au NPs size and valence on the catalytic performance of CO oxidation was studied in detail2, 16. In this work, various technological parameters and catalytic stability over bioreduction Au/TiO2 catalysts were further studied. 2. Experimental 2.1. Materials Cinnamomum camphora(CC) leaves from Peony Perfume&Chemicals Industry Co. Ltd of Xiamen were pretreated by the same method, reported in our earlier work1. HAuCl4.4H2O, sulfuric acid, TiO2 et al were obtained from Sinopharm Chemical Reagent Co. Ltd. 2.3. Catalysts synthesis The gold sol was prepared using the biosynthetic method by CC extract21,

22.

CC

extract were prepared by the method reported in our earlier work1. Typical preparation of Au/TiO2 catalysts, 0.52 mL HAuCl4 of 0.04856 mol/L was put in 50 mL CC extract. After 1 h under stirring at 30 oC, pH value of the obtained gold colloid was adjusted to 2 using H2SO4 solution. 0.5 g TiO2 (anatase ， P25 or rutile) was added to above gold colloid. The solution was filtered after stirring for 1 h and completely washed using deionized water. 1.0Au/TiO2 catalyst was obtained after vacuum drying. Other Au/TiO2 catalysts with 0.5% and 2.0% of Au loading can be prepared by the same method through adjusting the amount of gold colloid. 2.4. Catalysts characterization The UV-visible diffuse reflectance (DRUV-Vis) spectra of Au/TiO2 catalysts were carried out on Varian Cary-5000 spectrometer. Transmission electron microscopy (TEM) images and energy dispersive x-ray (EDX) analysis of Au/TiO2 catalysts were carried out on Tecnai F30 microscope. X-ray diffraction (XRD) measurement was performed on X’Pert Pro X-ray diffractometer. The actual Au loadings of the samples were determined 4


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by atomic absorption spectroscopy (AAS). 2.5. Catalytic activity measurements Catalytic activity measurements are same as in our earlier work2,

16,

which were

performed on a fixed-bed stainless-steel reactor. The composition of gas feed is the mixed gas of CO, O2 and N2 with the concentration of 1/1/98 vol%. The space velocity is 18000 mL h−1 gcat−1 using 0.2 g catalyst. The analysis of reaction gas is same as in our earlier work2, 16. 3. Results and discussion 3.1. Effect of Au loadings Au NPs can been compeletely immobilized onto TiO2 support, therefore, it is easy to control Au loadings of the Au/TiO2 catalysts. Firstly, the Au NPs with about 3.3 ± 0.4 nm (as shown in Figure S1A) were prepared by CC extract. Figure S1B shows the XRD pattern of the obtained NPs. There are five typical diffraction peaks assigned to (111), (200), (220), (311) and (222) crystal faces of Au NPs(JCPDS 4-0784). Then, Au NPs were supported onto the TiO2 (anatase) to obtain the catalysts with 0.5, 1.0 and 2.0 wt% of Au loadings after calcined at 375 oC. As shown in Figure S2, compared to the TiO2(anatase), the 1.0Au/TiO2 catalyst obviously presented another band between 500 and 600 nm, which is typical for the surface plasmon resonance of metallic Au NPs (Au0 )2, proving the presence of Au0 on TiO2 support. Figure 1 shows the TEM images of the three catalysts. Mean diameters about 3.3 nm of Au NPs were highly dispersed on all catalysts. Increasing Au loading from 0.5 to 2.0 wt%, Au NPs number on the surfaces of TiO2 support also increase. As shown in Figure 2, the intensity of Au NPs XRD pattern at 44.5o of Au (200) plane also increase23. All catalysts gave the typical diffraction peaks at 2θ=25.28o, 37.80o and 48.04o ascribed to TiO2 anatase phase of

(101), (004) and (200)

crystal faces (JCPDS 21-1272). The catalytic performance of the catalysts with 0.5, 1.0 and 2.0 wt% of Au loadings 5



is shown in Figure 3, the 1.0Au/TiO2 catalyst gave better catalytic activity than that of the 0.5Au/TiO2 catalyst. This is because that increasing the Au loadings from 0.5 to1.0 wt% resulted in an increase in active sites of the catalysts. However, the catalytic activity decreased significantly while further increasing Au loadings from 1.0 to 2.0 wt%. The decrease in catalytic performance has nothing to do with the Au NPs sizes, as proved by TEM result. Au NPs sizes for theAu/TiO2 catalysts with 0.5, 1.0 and 2.0 wt% of Au loadings are almost the same. Increasing Au loadings of the catalysts, CC extract usage was also increased. Therefore, there might be much more residual biomass on 2.0Au/TiO2 catalyst surface, resulting in the loss catalytic activity. In order to affirm the side effects of residual biomass, the 1.0Au/TiO2 catalyst was not washed with deionized water in the process of preparing the catalyst. From the Figure 4 of the EDX spectra for the catalysts by washing or not, we can see that compared with the catalyst by fully washed with DI water, there are still many other elements, such as K, Na, Mg, S and Cl elements for the catalyst without washing, besides Ti, Au, C and O elements. These elements K, Na, Mg, S and Cl are from the residual biomass on the catalysts. As shown in Figure S3, the catalytic activity of the two catalysts was compared. Reaction temperature at which the 100% conversion (T100) of CO is 210 oC for the catalyst with washing, however, the T100 rise to 250 oC for the catalyst without washing. The result suggested that residual biomass on the catalysts is bad for catalytic performance. Calcination is another method to remove the residual biomass and improve the catalytic performance. Therefore, influence of calcination temperature and atmosphere on the catalytic activity was studied.

6


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(a)

(b)

(c)

Figure 1. TEM images of Au catalysts with different Au loadings by CC extract; (a) 0.5%, (b) 1.0%, (c) 2.0%.

B:Au

A(224)

A(215)

A(116) A(220)

A(105) A(211)

A(200) B(200)

A(004)

A(204)

A（ TiO2（ anatase （

A(101)

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


d c b a

10

20

30

40

50

60

70

80

90

2θ/ o Figure 2. XRD patterns of Au catalysts with different Au loadings, a:0, b:0.5%, c:1.0%, d:2.0%.

7



100

CO conversion/%

80 60 40 20 0 0

50

100

150

200

250

300

350

o

Reaction temperature/ C

Figure 3.

Catalytic performance of Au catalysts for CO oxidation , ■: 0.5Au/TiO2, ●:

1.0Au/TiO2, ▲: 2.0Au/TiO2.

Ti

C

O

Cu Cu

Counts

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Si Al

Mg Na

0

Ti

SCl

2

Cu

Au

a

K

b 4

6

8

10

12

Energy / kev

Figure 4. EDX spectra of 1.0Au/TiO2 catalysts through washing(a) or not(b). 3.2. Effect of calcination temperature and atmosphere The calcination treatment can remove residual biomass on catalysts and expose active Au surface2. For the catalyst without calcination, there was almost no catalytic performance, due to excess amount of the residual biomass that cover active Au surface, 8


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as shown in Figure 5. After calcination at different temperatures in air atmosphere, the catalytic performance of the catalysts gradually increased, and the 1.0Au/TiO2 catalyst pretreated at 450 oC showed the best catalytic performance. The obtained 1.0Au/TiO2 catalyst were pretreated at different atmosphere (air, feed gas, N2 and H2) at 450 oC, and the catalytic performance was shown in Figure 6. The catalytic performance of 1.0Au/TiO2 catalyst increases gradually with the increase of oxygen content in the calcination atmosphere. The Au/TiO2 catalyst pretreated in air atmosphere gave the best catalytic performance, and the catalyst pretreated in hydrogen atmosphere gave the worst catalytic performance. Plant biomass molecules are mostly organic, which can be easily oxidized and removed at the calcination atmosphere with higher oxygen content. Those results suggested that the excessive residual biomass on the catalyst is bad for catalytic performance. 100 80 CO conversion /%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60 40 20 0 0

60

120 180 240 o Reaction temperature / C

300

Figure 5 Catalytic performance of 1.0Au/TiO2(anatase) catalysts for CO oxidation ■: uncalcined,●: 300 oC, ▲: oC, ▼: 450 oC, ◆: 550 oC.

9



100

CO conversion /%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80 60 40 20 0 0

60

120

180

240

300

360

o

Reaction temperature / C

Figure 6. Catalytic performance of 1.0Au/TiO2(anatase) catalysts for CO oxidation at different calcination atmosphere, ■: Air, ●: Feed gas, ▲: N2, ▼: H2, ★ :without calcination. 3.3. Effect of support crystal structure Crystal structure of TiO2 is another important factor affecting catalytic activity of Au catalysts. Effect of crystal structure (rutile, anatase and it’s mixed crystal) upon the catalytic performance for CO oxidation was explored. As shown in Figure 7. The Au/TiO2(Rutile) catalyst gave the lowest catalytic performance and T100 is 310 oC. T100 for the Au/TiO2(anatase) catalyst is 180 oC. The Au/TiO2(P25) catalyst with mixed crystal of rutile and anatase showed the best catalytic activity and T100 is 100 oC. Figure 8 displays typical DRUV–Vis spectra of Au catalysts with various crystal structure. All the catalysts showed the absorbance about 560 nm that is characteristic for metallic Au NPs plasmon resonance2. The Au/TiO2(P25) catalyst had the highest band intensity among the three catalysts. This might be caused by the strong interreaction among the Au NPs, rutile and anatase interface of TiO2, resulting in high catalytic performance.

10


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100

CO conversion /%

80 60 40 20 0

0

50

100

150

200

250

300

350

o

Reaction temperature / C

Figure 7. Catalytic performance of 1.0Au/TiO2 catalysts for CO oxidation at different crystal form of TiO2, ■: Rutile, ●: Anatase, ▲: mixed crystal(P25).

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


c b a

200

300

400

500

600

700

800

Wavelength / nm

Figure 8. DRUV-Vis spectra of the 1.0Au/TiO2 catalysts with various crystal structure, a: rutile, b: mixed crystal(P25), c: anatase. 3.4 Catalytic stability We also study the catalytic stability of the Au/TiO2(P25) catalyst prepared by Cinnamomum camphora leaf extract, as shown in Figure 9. The Au/TiO2(P25) catalyst 11



still remain it’s 90% CO conversion during 150 h at 100 oC, suggesting the high catalytic stability. Figure S4 displays

typical DRUV–Vis spectra for the catalysts after testing.

Compared to the fresh catalyst, The absorbance peak of Au NPs for the Au/TiO2 catalyst didn’t change after reaction of 150 h, suggesting no significant change of Au NPs size, as evidenced by TEM images in Figure 10. Mean diameters about 3.8 nm of Au NPs were still highly dispersed on catalyst. Therefore, 10% loss of catalytic performance might be due to carbon deposition. After reaction, the Au/TiO2 catalyst can recover it’s initial catalytic performance by calcination at 300 oC in air. Au Catalyst deactivation is a universal problem, which might be due to Au NPs aggregation or carbon deposition for CO oxidation 17, 19, 20. The Au/TiO2 catalyst prepared DP method usually suffered serious deactivation, the catalytic performance can decrease as high as about 70% during 50 h10, 19, 24

. Compared to the previous studies, the Au/TiO2 catalyst prepared by Cinnamomum

camphora leaf extract showed remarkable catalytic stability during 150 h.

100

80

CO conversion/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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60

40

20

0

0

15

30

45

60

75

90

105

120

135

150

Time on stream /h

Figure 9. Catalytic stability of the Au/TiO2 catalysts for CO oxidation

12


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Figure 10. TEM image of the 1.0Au/TiO2 after CO oxidation of 150 h. 3.5 Comparison of the biosynthesis catalyst with similar catalysts Catalytic performance of the biosynthesis Au/TiO2 catalysts and analogous supported Au catalysts from earlier reports is summarized

in Table 1. Apparently, the catalytic

activity of the biosynthesis catalysts is comparable and superior (under certain condition) to that of catalysts obtained through traditional chemical synthesis (DP, and so on.), especially for the catalytic stability. Table 1 Comparison of Au/TiO2 catalysts for CO oxidation

Catalyst

Au loading

T100a

(wt%)

(°C)

Stability Time

Deactivation

(h)

(%)

Ref.

Au/TiO2(P25)

1.0

100

150

10

This work

Au/TiO2(P25)

4.0

~210

24

~39

25

Au/TiO2(P25)

3.0

~230

48

~88

10

Au/Y-TiO2

3.0

~100

48

~11

10

Au/TiO2(P25)

1.0

~130

/

/

6

Au/TiO2(Rutile)

5.0

~200

/

/

26

13



a

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Au@TiO2

/

>275

/

/

27

Au-Ir/TiO2(P25)

4.0

~50

18

~60

24

Au/TiO2

3.0

~30

30

~70

19

Au/TiO2

2.9

~80

16.7

~12

28

T100 : the temperatures for 100% CO conversion.

4. Conclusions In summary, the Au/TiO2 catalysts for CO oxidation were biosynthesized using Cinnamomum camphora leaves extract. The obtained catalysts gaved both high activity and catalytic stability. Under this conditions of Au loading of 1.0 wt.%, calcination atmosphere of air, TiO2 with mixed crystal of rutile and anatase, the Au/TiO2 catalysts exhibited optimum catalyitc performance. The temperature for 100% CO conversion is 100 oC ， and the Au/TiO2 catalyst still remain it’s 90% CO conversion during 150 h, suggesting the high catalytic stability. AUTHOR INFORMATION Corresponding Authors *Tel.: +86 592-2189595; fax: +86 592-2184822. E-mail: [email protected] and [email protected]. . Notes The authors declare no competing financial interest. Acknowledgments This work was supported by the Natural Science Foundation (LQ16B060004 and LQ18G030018) from Zhejiang province, National Nature Science Foundation (Nos.41673088, 21606198 and 21476187) and China Postdoctoral Science Foundation (2016M592014). Supporting Information TEM image and XRD pattern of Au NPs. Catalytic performance of Au/TiO2 catalysts

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TiO2 Catalysts by

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