Size-dependent Surface Basicity of Nano-CeO2 and the Desorption

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Materials and Interfaces 2

Size-dependent Surface Basicity of Nano-CeO and the Desorption Kinetics of CO on its Surface 2

Zixiang Cui, Junzhen Gan, Jie Fan, Yongqiang Xue, and Rong Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01247 • Publication Date (Web): 18 Jul 2018 Downloaded from http://pubs.acs.org on July 23, 2018

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Industrial & Engineering Chemistry Research

Size-dependent Surface Basicity of Nano-CeO2 and the Desorption Kinetics of CO2 on its Surface Zixiang Cui∗, Junzhen Gan, Jie Fan, Yongqiang Xue*, Rong Zhang Department of Applied Chemistry, Taiyuan University of Technology, Taiyuan, Shanxi, 030024, P. R. China

ABSTRACT:

The desorption kinetics of nano-catalysts which is significantly

influenced by the particle size has crucial effects on catalytic performances. Herein, the desorption activation energy (Ed) and the desorption pre-exponential factor (A) for CO2 on nano-CeO2 with different particle sizes were obtained by temperature-programmed desorption (TPD). Meanwhile, we found that the particle size has a significant effect on surface basicity. The effects of particle size on Ed, A and surface basicity were discussed, as well as the relations between desorption kinetics and surface basicity. The results indicate that with the particle size decreasing, Ed, A, the strength and the amount of basic sites increase, and Ed, lnA and the amount of basic sites are linearly related with the diameter. Meanwhile, the strength and the amount of basic sites increase with the surface area increasing. The influence of particle size on desorption kinetics can be attributed to the effect of particle size on surface basicity. Key words: Size effect, Nano-CeO2, Desorption kinetics, Surface basicity

∗

Corresponding authors. E-mail addresses: [email protected] (Z. Cui), [email protected] (Y. Xue) 1

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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

1. INTRODUCTION Adsorption and desorption are essential steps in catalytic reactions,1-3 wherein, adsorption strength is one of the most important factors to determine the catalytic activity.4 At present, nanomaterials have been widely used as adsorbents and catalysts because of their unique adsorption properties compared with the corresponding bulk materials.5-12 The particle size has a great influence on the surface energy which influences the adsorption and the desorption properties of nanoparticles.13,14 The strong adsorption capacity of catalysts makes it easier for the reactants being adsorbed and activated, which is helpful to increase the concentration of reactants on the surface of catalysts. However, the excessively strong adsorption capacity leads to relatively difficult desorption of adsorbates, thereby limiting the efficient recovery of the catalysts. The preferred catalysts should have suitable adsorption and desorption properties, which are related to particle size. Wang et al .15 studied the desorption kinetics of water molecules on nano-SiO2, nano-HfO2 and nano-CeO2 with different particle sizes at different temperatures and found that with particle size decreasing, the desorption activation energy increases and nevertheless the rate of desorption decreases. Yean et al .16 studied the desorption of As3+ and As5+ on magnetite with the different sizes, and found that the particle size has quite a significant influence on the desorption of As3+ and As5+. The desorption of As3+ and As5+ on magnetite of 300 nm is irreversible and the ratio of the desorption amount to the adsorption amount is 20-25 %, whereas the ratio becomes only 1 % when the particle size is reduced to 20 nm. In addition, the acidity and basicity of the catalyst surface, including the strength, the

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distribution and the amount of acid and base centers have a decisive effect on the catalytic activity and the selectivity.17-21 The catalysts that possess suitable the strength and the amount of acid and base centers can improve the catalytic activity and the selectivity.22-26 The effects of particle size on surface basicity of the nano-catalysts and its influence regularities have not been reported by now, and the quantitative influence regularities of particle size on desorption kinetics and influence mechanism of nano-catalysts are still unclear. Therefore, study on the influence of particle size on the surface basicity of the nano-catalysts and desorption kinetics is very important for the design, preparation and application of the nano-catalysts employed. Herein, the desorption of acid gas CO2 on nano-CeO2 was taken as the research system. First, nano-CeO2 with different particle sizes were prepared, and then the desorption curves of CO2 on nano-CeO2 were obtained by TPD. Subsequently, the desorption kinetic parameters of CO2 on nano-CeO2 with different diameters were calculated, and the influence regularities of particle size and surface area on the surface basicity were discussed in detail.

2. EXPERIMENT 2.1. Materials Synthesis. The CeO2 nanoparticles with different sizes were prepared using a hydrothermal method. 6.5 g Ce(NO3)3·6H2O was dissolved in 30 mL of distilled water. Then the resulting solution was dropped to another solution containing a certain amount of ethylene glycol and ammonium hydroxide. After stirring for 30 min, 90 mL of distilled water was added to the above solution. The resulting suspension was

3



transferred to a Teflon-lined stainless steel autoclave and hydrothermally treated at 160 °C for 24 h. The precipitates were recovered with the centrifuge and washed with distilled water, followed by being dried in air at 80 °C. At last, nano-CeO2 was obtained after the precursors calcined. The CeO2 nanoparticles with different sizes were prepared by controlling the hydrothermal temperature, the amounts of ammonium hydroxide and ethylene glycol.

2.2. Characterization. The crystal structures of samples were characterized by XRD-6000 (Cu Kα, k=0.154178 nm). And the average particle sizes of nano-CeO2 were calculated by using Scherrer’s formula. Figure 1 shows that there are no another peaks arising from impurity, indicating that the samples have high purity. And the average diameters of the prepared nano-CeO2 are 7.5 nm, 13.9 nm, 16.8 nm, 26.7 nm and 38.0 nm, respectively. The morphologies of samples were characterized by SEM (JEOL JSM-6701F). As shown in Figure 2, the morphologies of nano-CeO2 samples are approximately spherical.

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

38.0 nm

26.7 nm 16.8 nm 13.9 nm 7.5 nm 20

30

40

50

60

70

2 Theta (degree)

Figure 1. XRD patterns of nano-CeO2 with different diameters

4


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Figure 2. SEM images of nano-CeO2 with different diameters. (a: 7.5 nm, b: 13.9 nm, c: 16.8 nm, d: 26.7 nm, e: 38.0 nm)

2.3. Temperature-programmed Desorption of CO2. The desorption experiments

of

CO2

on

nano-CeO2

samples

were

performed

by

using

temperature-programmed desorption apparatus (FINESORB-3010). Typically, 100 mg of nano-CeO2 sample with a certain particles size was placed in a tubular quartz reactor. The sample was pre-treated at 200 °C for 10 min in a nitrogen flow, and then cooled down to room temperature in order to remove the adsorbed substance on the surface, CO2 adsorption was carried out for 30 min by passing CO2 gas. After that, the sample was flushed with nitrogen at 50 °C for 30 min to remove the physically adsorbed CO2. TPD experiments were carried out from 50 to 650 °C in a nitrogen atmosphere with the heating rates of 10, 11, 13, 15 and 17 °C /min respectively, and the desorption curves were obtained.

2.4. Calculation of Desorption Kinetics. According to the literatures,27,28 the desorption activation energy Ed and the desorption pre-exponential factor A can be 5



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obtained by the following equation:

 β  E E  ln  2  = − d − ln  d  RTm  AR   Tm 

(1)

where β is the heating rate, Tm is the desorption peak temperature. The Tm at different heating rate (β) can be obtained by the CO2-TPD. According to Eq. (1), the desorption activation energy Ed and the desorption pre-exponential factor

(

A can be obtained by the slope and the intercept of the fitted line of ln β Tm2

)

versus

Tm−1 , respectively.

3. RESULTS AND DISCUSSION 3.1. Effect of Particle Size on Desorption Kinetics. The desorption curves of nano-CeO2 with different diameters at different heating rates are shown in Figure 3.

6


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591.15 K

(a)

(b)

471.75 K

734.92 K

464.15 K

581.40 K

731.28 K

456.45 K

582.16 K

722.28 K

478.55K

17 K/min

17 K/min

471.75K

13 K/min 446.35 K

717.85 K

578.40 K

11 K/min 441.25 K

708.85 K

571.01 K

15 K/min

460.55K

Intensity

Intensity

15 K/min

452.45K

13 K/min

445.75K

11 K/min

10 K/min 10 K/min

400

500

600

700

400

800

500

(c)

(d)

468.75K 458.00K

700

15 K/min

17 K/min

Intensity

411.85K

13 K/min

441.85K

15 K/min

404.25K

13 K/min

396.25K

434.85K

500

800

428.25K 418.45K

17 K/min

448.15K

400

600

T/K

T/K

Intensity

600

700

11 K/min

11 K/min

10 K/min

10 K/min

800

400

500

T/K

600

700

800

T/K

(e) 420.85K 413.85K

17 K/min

405.85K 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


15 K/min 392.75K 13 K/min

387.95K

11 K/min 10 K/min

400

500

600

700

800

T/K

Figure 3. CO2-TPD of the nano-CeO2 with different diameters at different heating rates (a: 7.5 nm, b: 13.9 nm, c: 16.8 nm, d: 26.7 nm, e: 38.0 nm)

(

The relations between the ln β Tm2

)

and the Tm−1 for the nano-CeO2 with

different diameters are shown in Figure 4. There are good linear relations between

(

ln β Tm2

)

and Tm−1 , which indicates the error of the experiment is very small.

7



-9.2 -9.3 -9.4 -9.5

ln (β /T 2m )

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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-9.6 -9.7

7.5 nm 13.9 nm 16.8 nm 26.7 nm 38.0 nm

-9.8 -9.9 0.0021

0.0022

0.0023

0.0024

0.0025

0.0026

T-1 m

(

Figure 4. The relations between the ln β Tm

2

)

−1

and the Tm

for the nano-CeO2 with different

diameters

The desorption activation energies and the desorption pre-exponential factors of CO2 on nano-CeO2 with different diameters calculated by the slope and the intercept

(

of the fitted line of ln β Tm2

)

versus Tm−1 are given in Table 1.

Table 1 The desorption activation energies and pre-exponential factors of CO2 on nano-CeO2 with different diameters Tm/K d/nm

Ed/kJ·mol-1

A

10 K/min

11 K/min

13 K/min

15 K/min

17 K/min

7.5

441.25

446.35

456.45

464.15

471.75

22.46

63.78

13.9

445.75

452.45

460.55

471.75

478.55

20.82

34.83

16.8

434.85

441.85

448.15

458.00

468.75

19.74

29.99

26.7

396.25

404.25

411.85

418.45

428.25

17.76

29.56

38.0

387.95

392.75

405.85

413.85

420.85

14.51

10.49

It can be seen from Table 1 that the desorption activation energy and the desorption pre-exponential factor decrease with increase of the particle size, indicating that the desorption of CO2 on nano-CeO2 becomes easy as the particle size increases. As a clear notion, the functions of the desorption activation energy and the logarithm of desorption pre-exponential factor as the diameter were plotted, as shown in Figure 5

8


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and Figure 6. 24

22

E d /kJ.mol-1

20

18

16

14

5

10

15

20

25

30

35

40

d/nm

Figure 5. The relationship between the desorption activation energy and the diameter of nano-CeO2

4.0

3.5

lnA

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


3.0

2.5

5

10

15

20

25

30

35

40

d/nm

Figure 6. The relationship between the logarithm of the desorption pre-exponential factor and the diameter of nano-CeO2

Figure 5 and Figure 6 indicate that in the sizes range from 7.5 to 38.0 nm, the desorption activation energy and the logarithm of desorption pre-exponential factor present a good linear relationship with the diameter. With the particle size of nano-CeO2 decreasing, the desorption activation energy and the desorption pre-exponential factor increase, indicating that the different adsorption strength of CO2 on nano-CeO2 with different particle sizes. In order to obtain the reasons for the

9



influence of particle size on the desorption kinetics, we further studied the effect of particle size on the surface basicity of nano-CeO2.

3.2. Effect of Particle Size and Surface Area on Surface Basicity. The desorption curves of CO2 on nano-CeO2 with different diameters at 10 K/min are shown in Figure 7. 441.74 K 7.5 nm 448.15 K 13.9 nm

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

440.38 K 16.8 nm 399.79 K 26.7 nm 381.70 K

300

400

38.0 nm

500

600

700

800

900

1000

T/K

Figure 7. CO2-TPD of the nano-CeO2 with different diameters

Usually in CO2-TPD, the strengths of the basic sites are reported in terms of temperature range where the chemisorbed CO2 on the basic sites is desorbed, adsorbate on weaker sites desorb at lower temperature and that adsorbed on stronger sites desorb at higher temperature.29-31 We defined the weak basic sites (< 512 K), the moderate basic sites (512-654 K) and the strong basic sites (> 654 K). In addition, the amount of basic sites was estimated by integration of these peaks area.32 Figure 7 shows the surface basicity of nano-CeO2 with different diameters. The nano-CeO2 with 38 nm shows weak desorption peak. With the particle size decreasing, the area of desorption peaks, ascribed to the weak basic sites, moderate basic sites and strong basic sites respectively, increases gradually and the peaks shift to the temperature. 10


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Especially for the sample with 7.5 nm, a large amount of strong basic sites are produced. It is apparent conclusively that the decreased particle size is not only favourable for the increase of amount of basic sites, but also the enhancement of the strength of basic sites. The influence regularities can be explained by the following two aspects. With the particle size of nano-CeO2 decreasing, the more surface defects located at edges and corners of the crystal planes and the formation of more low-coordination oxygen anions, which lead to more strong basic sites.33,34 Meanwhile，oxygen has a good migration ability in the crystal and the electron delocalization formed by the oxygen vacancy can increase the electron density in the nano-CeO2 structure,35,36 and the strength and the amount of the basic sites increase.37 The strength of basic sites can be adjusted by changing the particle size, which has an important application prospect in the field of catalysis. In order to explore the quantitative effect of particle size and surface area on surface basicity, we obtained the amount of basic sites with different strength by integrating the peak areas of the desorption curves (in Figure 7) and the measured BET specific surface areas of nano-CeO2 with different diameters. The surface area and the amount of different basic sites on nano-CeO2 with different diameters are shown in Table 2.

11



Table 2 The surface area and the amount of different basic sites on nano-CeO2 with different diameters（（mmol/g）） BET, surface

Weak

Moderate

Strong

area/(m2/g)

654 K

7.5

192.1

0.1090

0.0412

0.1160

0.2662

13.9

81.2

0.1240

0.0327

─

0.1567

16.8

28.5

0.1037

0.0381

─

0.1418

26.7

13.7

0.0651

0.0288

─

0.0939

38.0

2.1

0.021

0.0130

─

0.0251

d/nm

Total

It can be seen from Table 2 that the nano-CeO2 with different diameters exhibit large differences in the surface area and the amount of different basic sites, and the surface area and the amount of different basic sites increase with the particle size decreasing. The variations of the amount of basic sites with diameter and surface area are illustrated in Figure 8 and Figure 9, respectively. 0.30

The amount of basic sites (mmol/g)

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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Total Weak Moderate

0.25

0.20

0.15

0.10

0.05

0.00 5

10

15

20

25

30

35

40

d/nm

Figure 8. The relationship between the amount of basic sites and the diameter of nano-CeO2

12


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0.30


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


Total Weak Moderate

0.25

0.20

0.15

0.10

0.05

0.00 0

15

30

45

60

75

90 105 120 135 150 165 180 195

Surface area/(m2/g)

Figure 9. The relationship between the amount of basic sites and the surface area of nano-CeO2

Figure 8 shows that there are linear relationships between the amount of different basic sites and diameter, and the amount of different basic sites increase with the particle size decreasing. Figure 9 indicates that with the surface area of nano-CeO2 increasing, the amounts of different basic sites first increase sharply and then the weak basic sites and the moderate basic sites gradually level off. When the surface area is 192.1 m2/g, the sharp increase in the total amount of basic sites is due to a large amount of the strong basic sites on the surface of the nano-CeO2 with 7.5 nm. Therefore, with the surface area increasing, the strength of basic sites is also enhanced. According to the above discussion, the particle size of catalysts can be used to adjust the surface basicity of catalysts to achieve the best catalytic performance.

3.3.

Effect

of

Particle

Size

and

Surface

Area

on

Adsorption/Desorption Capacity. The desorption/adsorption capacity of nano-CeO2 with different diameter were obtained by integrating the desorption peaks in Figure 7, and the variations of adsorption/desorption capacity with the reciprocal of 13



diameter and surface area are shown in Figure 10 and Figure 11, respectively. 12

10

V/mL

8

6

4

2

0 0.02

0.04

0.06

0.08

0.10

0.12

0.14

d-1/nm-1

Figure 10. The relationship between adsorption/desorption capacity and the reciprocal of the diameter of nano-CeO2 10

8

V/mL

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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6

4

2

0 0

20

40

60

80

100

120

140

160

180

200

Surface area/(m2/g)

Figure 11. The relationship between adsorption/desorption capacity and surface area of nano-CeO2

Figure 10 shows that there is a good linear relationship between the adsorption/desorption capacity and the reciprocal of the diameter of nano-CeO2, and the adsorption/desorption capacity increases with the particle size decreasing. Figure 11 indicates that the adsorption/desorption capacity increases with the surface area increasing. According to the above discussion, the strength and the amount of basic sites are both increase with the surface area increasing, which is conducive to the

14


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adsorption of acidic CO2 on nano-CeO2. In addition, we investigated the adsorption mechanism of CO2 on nano-CeO2 with different particle sizes. Figure 12 is the mechanism of adsorption of CO2 on CeO2 with different particle sizes. With particle size decreasing, nano-CeO2 has plentiful oxygen vacancies on the surface of the material.38,39 The two neighboring Ce4+ ions essentially reduce to two Ce3+ ions so as to make the system charge neutral after formation of an oxygen vacancy.40 Therefore, the concentration of Ce3+ ions increases with decreasing particle size of nano-CeO2. Numberous researches indicated that the reduced Ce3+ ions are contributed to adsorb CO2 to form a carbonate CO22- species,41-44 and found that the carbonate species formed on the Ce3+ sites have a higher thermal stability than those on the Ce4+ sites. Furthermore, with decreasing particle size of nano-CeO2, the strength and the amount of basic sites and the specific surface area increase also contribute to adsorb CO2. Therefore,

with

the

particle

size

decreasing,

the

amount

of

the

the

adsorption/desorption capacity increases, which is consistent with the results in literature.39

Figure 12. The mechanism of adsorption of CO2 on CeO2 with different particle sizes

15



3.4 The relationship between the kinetic parameters and the amount of basic sites. From Figure 13 and Figure 14, it is clear that the desorption activation energy, the logarithm of desorption pre-exponential factor and the total amount of basic sites on nano-CeO2 present the same change trend and have a similar linear relationship with diameter. With the particle size of nano-CeO2 decreasing, the desorption activation energy, the logarithm of desorption pre-exponential factor and the total amount of basic sites increase. We can explain the influence regularity of particle size on the desorption activation energy and the desorption pre-exponential factor by the effect of particle size on the basic sites. With the decrease of particle size of nano-CeO2, the strength and the amount of basic sites increases, which can lead to enhancement of the adsorption of acid gas CO2 on nano-CeO2. Therefore, the desorption activation energy increases with the decrease of particle size. The effect of particle size for desorption pre-exponential factor is that the amount of basic sites increases with decreasing particle size, the probability of effective collision between CO2 and basic sites increases, leading to desorption pre-exponential factor increases.

16


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0.5

25

Basic sites 0.4 20

E d/kJ.mol-1


Ed/kJ.mol-1

0.3

0.2 15

0.1

0.0

10

5

10

15

20

25

30

35

40

d/nm

Figure 13. The desorption activation energy and the basic sites vary with the diameter of nano-CeO2 lnA Basic sites

4

0.4

3

lnA


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


0.2 2

0.0

1 5

10

15

20

25

30

35

40

d/nm

Figure 14. The logarithm of desorption pre-exponential and the basic sites vary with the diameter of nano-CeO2

4. CONCLUSIONS In summary, particle size has a significant effect on the desorption kinetics of CO2 on nano-CeO2; the desorption activation energy and the desorption pre-exponential factor increase with the particle size decreasing. Moreover, in the sizes range from 7.5 nm to 38.0 nm, the desorption activation energy and the logarithm of the desorption pre-exponential factor are linearly related to the diameter, respectively.

17



Especially, the strength and the amount of basic sites on nano-CeO2 are remarkably influenced by particle size and surface area. With the particle size decreasing and the surface area increasing, the strength and the amount of basic sites increase. We discovered that the influence regularities of particle size on the desorption kinetics of nano-CeO2 can be attributed to the effect of particle size on the amount of basic sites. The influence regularities of particle size on the desorption kinetics of CO2 on nano-CeO2 and the surface basicity can provide an important reference for the design, preparation and application of other nano-catalysts.

AUTHOR INFORMATION Corresponding Authors ∗E-mail: [email protected] (Zixiang Cui) ∗E-mail: [email protected] (Yongqiang Xue)

Author Contributions These two authors contributed equally to this project.

Note The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS The project is supported by the National Natural Science Foundation of China (Nos. 21573157 and 21373147).

REFERENCES (1) Fang, L.; Xu, C.; Li, J.; Borggaard, K.; Wang, D. The Importance of Environmental Factors and Matrices in the Adsorption, Desorption, and Toxicity of Butyltins: a Review. Environ. Sci. Pollut. Res. Int. 2017, 24, 1–15. (2) Liao, G.; Chen, J.; Zeng, W. Facile Preparation of Uniform Nanocomposite Spheres 18


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Size-dependent Surface Basicity of Nano-CeO2 and the Desorption

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