Tuning SnO2 Surface Area for Catalytic Toluene Deep Oxidation: On

Subscriber access provided by University of Sunderland

Kinetics, Catalysis, and Reaction Engineering

Tuning SnO2 Surface Area for Catalytic Toluene Deep Oxidation: On the Inherent Factors Determining the Reactivity Yaqian Liu, Yang Liu, Yao Guo, Junwei Xu, Xianglan Xu, Xiuzhong Fang, Jianjun Liu, Weifan Chen, Hamidreza Arandiyan, and Xiang Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03401 • Publication Date (Web): 02 Oct 2018 Downloaded from http://pubs.acs.org on October 4, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

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.

Page 1 of 47 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

Industrial & Engineering Chemistry Research

Tuning SnO2 Surface Area for Catalytic Toluene Deep Oxidation: On the Inherent Factors Determining the Reactivity Yaqian Liu a †, Yang Liu a †, Yao Guo a, Junwei Xu a, Xianglan Xu a, Xiuzhong Fang a, Jianjun Liu c, Weifan Chen b

, Hamidreza Arandiyan d, Xiang Wang a *

a

College of Chemistry, Nanchang University, Nanchang, Jiangxi 330031, China

b

School of Materials Science & Engineering, Nanchang University, Nanchang 330031, P. R. China

c

Jiangxi Baoan New Material Technology Corporation, LTD, Pingxiang, Jiangxi 337000, China

d

Laboratory of Advanced Catalysis for Sustainability, School of Chemistry, The University of Sydney, Sydney

2006, Australia * Corresponding author. E-mail: [email protected] (X. Wang)

Keywords: SnO2 nano-materials, Toluene deep oxidation, Oxygen Vacancies, Surface acidic sites, Toluene adsorption capacity.

1

ACS Paragon Plus Environment

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

Page 2 of 47

Abstract: To understand the fundamental aspects of SnO2 catalytic materials and develop applicable catalysts for VOCs combustion, SnO2 samples possessing stable and varied surface areas are controllably constructed and applied to toluene deep oxidation. By improving SnO2 surface area, the Sn4+ cation exposure is improved, thus increasing its surface acidity, which benefits toluene molecule adsorption and activation. Furthermore, more surface defects can be generated, hence inducing the generation of more abundant surface active oxygen sites, which is favorable for further oxidizing the adsorbed and activated toluene intermediates. Therefore, the intrinsic activity of SnO2 is eventually promoted. The co-existence of both kinds of active sites is crucial for the reaction, and the concerted interaction between them controls the reaction performance. Catalysts with good activity, superior stability, potent water vapor tolerance can be achieved by controllably constructing SnO2 possessing large and stable surface areas, and supporting less amount of Pd onto it.

2


Page 3 of 47 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 Volatile organic compounds (VOCs) pollution is one of the main problems needs to be resolved since it is harmful to both human health and the environment. For instance, VOCs contribute to the formation of toxic photochemical smog. Catalytic oxidation is one of the most efficient ways to abate their pollution. To date, different kinds of catalysts have been tried for removing VOCs with varied properties.1-3 Supported noble metals such as Pt, Pd, Au and their alloys with other transition metals show very good low temperature activity for VOCs deep oxidation.4-7 However, the high cost and limited availability have restricted their widespread applications. Therefore, a variety of porous transition metal oxides with high surface areas, such as Co3O48, 9, Mn2O310-12, CeO213-15, CuO16 and their mixtures17-19, have been developed in the past several decades for VOCs combustion and proved to be efficient catalysts. The major reasons for the good catalytic performance are attributed to their porous structure and high surface areas, which can supply abundant surface active sites and benefit the diffusion of the reactants and their contact with the active sites.20-23 Arandiyan et al. synthesized 3DOM rhombohedral La0.6Sr0.4MnO3 catalysts by using a PMMA microsphere hard template and different surfactants.24,

25

It was found that the most active catalyst for methane deep

oxidation possessed the highest surface area, largest quantity of active oxygen species and best low temperature reducibility. Li et al. prepared a series of Cu xSn1-xOy solid solution nano-sheet catalysts for CO oxidation via a facile co-precipitation method and revealed that the activity of these catalysts is closely related to their surface areas and the abundance of loosely bonded surface oxygen centers. The best catalyst possesses the highest surface area and the most abundant surface active oxygen sites.26 More recently, our group has prepared a high surface area NiO catalyst with a template hydrothermal method, which possesses even rival activity to 1% Pd/Al2O3 for methane deep oxidation. It was found that the reaction is mainly diffusion controlled and the surface area determines the activity.27 Therefore, the surface area and texture property of a catalyst can influence its redox behaviors and eventually the activity significantly. Fabricating metal oxide catalysts with large surface areas that can be stabilized at elevated temperature is still a challenge, and could achieve highly active and cheap catalysts replacing noble metals for VOCs combustion. 3



Page 4 of 47

As an n-type semiconductor, SnO2 has been widely applied to gas sensing and electrode materials etc. It is also a promising catalytic material due to its abundant surface acidic sites, deficient surface oxygen and reducible lattice oxygen species.28-30 Over recent eight years, we have been studying the catalytic chemistry of SnO2 systematically for various reactions.31-34 It has been found that the mesoporous high surface area Cu-Sn oxide solid solutions with one dimensional nano-needle morphology exhibits superior activity for CO oxidation, which is even more active than Pd/SnO2 under the same condition. In addition, the Cu-Sn nano-needle catalysts also display good stability and remarkable water resistance. In another piece of work, we reported that SnO2 nano-rod catalysts with low surface area (1 m2 g-1) and without any active surface oxygen species behave similar to supported noble metal catalysts for CO oxidation. The well-defined reactive SnO2 (110) crystal facets are believed to be responsible for its special catalytic performance. By doping the crystalline lattice of SnO2 nano-rods with In, Cr and Al cations to generate a solid solution structure, the promoted catalysts exhibits much better toluene deep oxidation activity than the un-modified SnO2 nano-rod catalyst. The major reason is ascribed to the presence of both surface acidic sites and active oxygen species,35 which are able to adsorb and activate toluene molecules first, and then oxidize them mainly into CO2 via a Mars Van-Krevelen mechanism. As a matter of fact, a catalyst’s surface property and reactivity are intimately related to its surface area, which could directly influence both of its intrinsic and overall activity. Although SnO2-based catalytic materials have attracted growing interests for different reactions36-38, their application for VOCs deep oxidation has been limitedly documented, and some of the important fundamental aspects still lack deep understandings. Therefore, on the base of the former findings, hydrothermal method has been adopted to purposely construct SnO2 nano-materials with different and controllable surface areas in this study, with the objective to clarify the effects of specific surface area on the texture bulk and surface properties of SnO2 materials, and eventually their impact on the toluene deep oxidation performance. Indeed, it has been discovered that the toluene molecule adsorption capacity and the surface active oxygen amount improve with the increase of the surface areas, which significantly enhance the catalytic activity. By using different means, the physical chemical properties, the surface acidity and the surface mobile oxygen properties have been systematically investigated for the catalysts,

4


Page 5 of 47 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


which are correlated with the toluene oxidation performance. 2. Experimental details 2.1 Catalyst fabrication All the chemicals adopted for sample fabrication are of A.R. grade. Five typical SnO2 samples with varied surface areas were fabricated with a hydrothermal method and named according to their surface areas. For examples, a SnO2 sample having a surface area of 2 m2 g-1 is denoted as SnO2-2. The preparation of SnO2-2: 1.05 g SnCl4·5H2O was dissolved into 10 mL DDI water, then dripped slowly into 30 mL NaOH aqueous solution (1.08 mol L-1) under constant stirring for 30 minutes. Afterwards, 40 mL ethanol was add into the mixture, and kept stirring for another 30 minutes. The mixed solution was then put into a 100 mL Teflon-lined stainless steel autoclave for crystallizing at 200 oC for 24 h. After this, it was cooled down to room temperature, and the mixture was filtered to get the solid product, which was washed repeatedly by DDI water until Cl- free with a TDS below 20 ppm. The obtained solid product was dried at 110 oC overnight, followed by calcination at 450 oC in air atmosphere for 4 h with a heating rate of 2 oC min-1 and then grounded into powder. The preparation of SnO2-50: 1.75 g SnCl4·5H2O was dissolved into 10 mL DDI water, then 2.00 g H2NCONH2 was added into the solution under constant stirring for 1 h. Afterwards, 35 mL ethanol was added into the mixture, flowed by continuous ultrasonic dispersion for 30 min. The achieved mixture was named solution A. On the other hand, 0.54 g CTAB and 3.51 g NaCl were dissolved in 30 mL DDI water in a water bath kept at 40 oC under vigorous stirring. Then 30 mL NaOH solution (1.00 mol L-1) was added drop wise into the mixed solution, which was named solution B. Afterwards, solution B was dripped into solution A under constant stirring until the pH was ~9, which was then stirred for another 2 h. The solution was put into a 100

5



Page 6 of 47

mL Teflon-lined stainless steel autoclave for crystallizing at 200 oC for 24 h. The left steps are the same to those used in the preparation of SnO2-2 sample described above. The preparation of SnO2-91: This catalyst was synthesized following the similar procedures adopted in the preparation of SnO2-50 except that the pH of solution A and B was controlled at ~8, and the crystallization temperature was improved to 180 oC and kept for 24 h. The preparation of SnO2-150: This catalyst was synthesized following the similar procedures adopted in the preparation of SnO2-91 except that the amount of CTAB used in solution B preparation was increased to 0.10 g, and the crystallization temperature was improved to 200 oC and kept for 24 h. The preparation of SnO2-217: 1.75 g SnCl4·5H2O was dissolved into 10 mL DDI water, and then 30 mL CTAB aqueous solution (0.05 mol L-1) was added into it under continuously stirring for 1 h in a water bath kept at 40 oC. Afterwards, ammonia aqueous solution (10 wt%) was added drop wise into the above mixture until the pH was ~11, and stirred continuously for another 18 h. The solution was then put into a 100 mL Teflon-lined stainless steel autoclave for crystallizing at 120 oC for 72 h. The other steps are the same to those used in the preparation of SnO2-2 sample described above except that the dried precipitate was first calcined at 350 oC in air atmosphere for 2 h with a heating rate of 2 oC min-1, and then heated up to 450 oC and calcined for another 4 h. 2.2 Activity tests and sample characterization The activity of the catalysts has been probed for toluene deep oxidation. To explore the effects of surface area on the bulk and surface properties of the SnO2 samples, different techniques have been adopted to characterize the physical chemical nature of the catalysts. The information about the used equipment, the operation condition, the experimental procedures and parameters is described in detail in the supporting information file.

6


Page 7 of 47 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. Results and discussion 3.1 N2 sorption and XRD study on the catalysts The texture properties have been measured for the catalysts by N2 sorption, with the results depicted in Figure S1 and Table 1. As expected, a series of SnO2 catalysts possessing surface areas of 2, 50, 91, 150, and 217 m2 g-1 have been successfully fabricated. Figure S1 (A) exhibits that SnO2-2 owns a type II isotherm without obvious capillary condensation step, indicating it has non-porous structure, that is in accordance to its extremely low surface area (2 m2 g-1). In contrast, SnO2-50, SnO2-91 and SnO2-150 exhibit a type IV isotherm with a H2-type capillary condensation step in the relative pressure (p/p0) range of 0.5-0.9, which is the characteristic for the existence of meso-pores formed by nano-particle assembly. Interestingly, SnO2-217 displays also type IV isotherm but with two capillary condensation loops. The H2-type hysteresis loop in the relative pressure (p/p0) range of 0.4-0.7 suggests the generation of a considerable amount of intraparticle meso-pores in the initially piled SnO2 grains. The second H3-type hysteresis loop in the higher relative pressure (p/p0) range of 0.7-1.0 is generally considered to be interparticle slit meso-pores formed by flake particle accumulation. Table 1 Texture properties of the catalysts measured by N2 sorption. Pore Size (nm)

SBET Sample

(m2 g-1)

Pore Volume

α pore

β pore

(m3 g-1)

Crystalline Size (nm)

Grain Size (nm)

SnO2-2

2

--

--

--

45.7

--

SnO2-50

50

8.9

--

0.15

8.7

9.2

SnO2-91

91

5.2

--

0.16

6.1

6.3

SnO2-150

150

5.1

--

0.21

3.9

4.2

SnO2-217

217

3.7

15.3

0.48

2.8

2.9

The pore size distribution profiles in Figure S1 (B) matches the information obtained in Figure S1 (A). Except for SnO2-2, all the catalysts possess a set of meso-pores with an average size below 10 nm, which are 7



labeled as α pore. In particular, SnO2-217 possesses also a set of extra meso-pores having a bigger mean size around 15 nm, which is labeled as β pore. Interestingly, with the increase of the surface area from 50 to 217 m2 g-1, the size of α meso-pores decrease from 8.9 to 3.7 nm, and the pore volumes increase significantly from 0.15 to 0.48 m3 g-1. Except that the large pore volume of SnO2-217 is partly contributed by the formation of β meso-pores, it is rational to conclude that the pore volume increase is mainly due to the generation of a larger amount of smaller size meso-pores with the increase of SnO2 surface area. XRD technique has been used to analyze the crystalline structure of the SnO2 catalysts. Figure 1 shows that all the samples display three strongest peaks at (110)=22.61 o, (101)=33.95

o

and (211)=51.65 o, which are

characteristic for tetragonal rutile SnO2 phase. As expected, the SnO2-2 exhibits very intensive diffraction peaks, substantiating its high crystallization during the hydrothermal process, which matches its extremely low surface areas. For clarification, the mean crystallite sizes are quantified in Table 1 for all the samples. Apparently, with the increasing of the surface areas from 2 to 217 m2 g-1, the sizes of the samples decline sharply from 45.7 to 2.8 nm, testifying that a SnO2 sample with a higher surface area is less crystallized with lower crystallinity. 

SnO2





(110)



(101)

 (211)



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

Page 8 of 47

SnO2-217 SnO2-150 SnO2-91 SnO2-50 SnO2-2

20

40

60

2θ/°

Figure 1 XRD patterns of the catalysts.

8


80

Page 9 of 47 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


3.2 SEM and TEM analysis of the catalysts Figure 2 displays the morphologies of the samples identified with SEM. Noticeably, SnO2-2 is composed of irregular SnO2 nano-rods having nearly square cross sections, and a mean cross sectional side length of 130 nm (Figure 2 A). In addition, the clean surfaces of the nano-rods indicate that its structure is compact without meso-pores, in good accordance to its extremely low surface areas and high crystallization without porous structure. As the surface areas of the samples increase, their morphologies have evidently changed (Figure 2 B, C, D, E). Apparently, the left four samples consist of irregular particles with different sizes. Starting from SnO2-91, the loosely assembled sponge-like structure with numerous visible pores is obviously observed, which might be due to the far less agglomeration of the initial particles during the crystallization process.

Figure 2 SEM images of the catalysts. (A) SnO2-2, (B) SnO2-50, (C) SnO2-91, (D) SnO2-150, (E) SnO2-217.

Therefore, the more exquisite morphologies and bulk structures of the SnO2 catalysts possessing different surface areas have been investigated further by TEM with higher magnification. As shown in Figure 3 (A), a single bar in the SnO2-2 nano-rod sample is in fact an original crystalline grain, in which no secondary particle aggregation can be observed. This indicates that SnO2-2 is highly crystallized, agrees well with the N2 sorption 9



Page 10 of 47

and XRD results. In contrast, the other four samples are all made up of irregular fine particles with mean sizes of 9.2, 6.3, 4.2, and 2.9 nm, respectively, as exhibited in Figure 3 (F). Apparently, with the increasing of the catalyst surface areas, the sizes of the accumulated particles are getting smaller, implying the availability of subtler initial particles. It is noted here that the average particle sizes measured by TEM are nearly the same to the crystallite sizes quantified by XRD for all the SnO2 samples within the experimental error, testifying the absence of the secondary aggregation of the initial crystallite sizes. In other words, the particles observed by TEM is actually the initial crystallites of the samples. In summary, SEM and TEM studies are in good agreement with the N2 sorption and XRD results, testifying the texture structure, porosity and morphology of the catalysts can be influenced significantly by the surface areas.

Figure 3 TEM images of the catalysts. (A) SnO2-2, (B) SnO2-50, (C) SnO2-91, (D) SnO2-150, (E) SnO2-217, (F) grain size distribution.

10


Page 11 of 47 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


3.3 Structure identification with Raman and HRTEM Raman technique has been adopted to investigate the surface structure of SnO2 catalysts possessing varied surface areas, with the patterns compared in Figure 4. SnO2 owns a tetragonal rutile phase structure and belongs to D144h space group. Theoretically, eight Raman vibration modes, that is, Γ = A1g + A2g + 2A2u + B1g + B2g + 2B1u + Eg + 4Eu, should be present for the structure. However, some modes such as A2g, B1g and Eu are Raman silent.

39, 40

Hence, only three basic Raman bands at 479, 633, 774 cm-1 corresponding to the Eg, A1g and B2g

vibrations are detected, as shown in Figure 4. For a pure metal oxide, the intensity reflects its crystallinity. The stronger the Raman peaks, the better it is crystallized

41

. SnO2-2 nano-rod sample owns the strongest peaks,

testifying its highest crystallinity amongst all the samples. With the improving of the surface areas, the intensity of the strongest 633 cm-1 typical peak decreases gradually in the sequence of SnO2-50, SnO2-91, SnO2-150 and SnO2-217, implying the declining of the crystallinity.

Figure 4 Raman spectra of the catalysts. (A) Complete profiles (B) Enlarged profiles from 450 cm-1 to 600 cm-1 and (C) Integrated areas of the peak at 570 cm-1 It is reported that the Raman mode at ~574 cm-1 is closely associated with the surface defects such as oxygen vacancies of SnO2.36, 42As illustrated in Figure 4 (C), with the increase of surface area, the integrated area of the 11



Page 12 of 47

peak evidently goes up in the order of SnO2-2 < SnO2-50 < SnO2-91 < SnO2-150 < SnO2-217, substantiating that the abundance of surface deficient oxygen species improves, which is believed to benefit the reaction performance. To understand better the surface feature of the catalysts, the fine structures have been further tested for the catalysts by HRTEM. Figure 5 depicts that the SnO2-2 nano-rod sample exposes preferentially the (101) facets with clear lattice fringes possessing a distance of 0.27 nm,30 which confirms that this sample consists of single nano-rod crystals.35 In comparison, the left four SnO2 samples expose naturally both the SnO2 (110) and (101) crystal facets with a lattice fringe distance of 0.34 and 0.27 nm, respectively, testifying that they are composed of polycrystalline nanoparticles. In summary, the HRTEM results confirm the measurements by N2 sorption, XRD, SEM, TEM and Raman, testifying that the SnO2 samples possess different morphologies and surface properties, which might eventually influence the catalytic performance of the final catalysts.

Figure 5 HRTEM images of the catalysts. (A) SnO2-2, (B) SnO2-50, (C) SnO2-91, (D) SnO2-150, (E) SnO2-217.

3.4 Activity tests 12


Page 13 of 47 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


Toluene deep oxidation has been selected to measure the activity of the SnO2 catalysts possessing varied surface areas. Figure 6 (A) depicts the toluene conversion versus reaction temperatures. It is observed that the overall activity improves with the order of SnO2-2 < SnO2-50 < SnO2-91 < SnO2-150 < SnO2-217. Notably, we formerly found that SnO2 sample with a nano-rod morphology is very active for CO oxidation due to the preferentially exposed (101) facets, which are good at adsorbing and activating CO molecules38. However, for toluene deep oxidation, SnO2-2 nano-rod catalyst with the smallest surface area displays obviously the lowest activity amongst all the catalysts, on which merely 43% conversion is obtained at 450 oC. In contrast, over SnO2-217 with the biggest surface area, 100% toluene conversion has already been achieved at 410 oC. This implies that the surface areas and the meso-porous structure of the catalysts could play vital roles for the reaction performance of toluene deep oxidation.

Figure 6 Toluene deep oxidation on the catalysts. (A) Toluene conversion, (B) Arrhenius plots, (C) toluene conversion rates (Rw and Rs) vis surface area at 250 oC. 13



Page 14 of 47

To comprehend the influence of surface area on the intrinsic activity, the Arrhenius plots have been collected for all the catalysts by controlling the toluene conversion lower than 20% to exclude any possible mass and heat transfer influence, with the results shown in Figure 6 (B). Moreover, the apparent activation energies (Ea) together with the differential toluene conversion rates at 250 oC are calculated from the Arrhenius plots in Table 2. For clarification, both the Rw and Rs, the reaction rates normalized by catalyst weight or surface area are plotted against the catalyst surface areas in Figure 6 (C). It is evident that with the increasing of the surface areas, both Rw and Rs improve in the sequence of SnO2-2 < SnO2-50 < SnO2-91 < SnO2-150 < SnO2-217. On the contrary, the Ea listed in Table 2 decline in the inversed ordered. These results demonstrate strongly that by enlarging the surface area of SnO2, toluene molecules become easier to be activated on its surface. As a consequence, the intrinsic activity has been enhanced, as particularly demonstrated by the improved specific rates Rs for the catalysts. Table 2 Reaction performance for toluene oxidation over SnO2 catalysts Sample

2

-1

SBET (m g )

Rw (mmol g-1 s-1)

Rs (mmol m-2 s-1)

-9 a

-10 b

(×10 )

(×10 )

Ea (kJ mol-1)

SnO2-2

2

0.15

0.75

121.1

SnO2-50

50

1.52

0.30

68.0

SnO2-91

91

15.60

1.71

57.1

SnO2-150

150

31.80

2.12

54.2

SnO2-217

217

64.90

2.99

47.6

a

The reaction rates measured at 250 oC and normalized by catalyst weights.

b

The reaction rates measured at 250 oC and normalized by catalyst surface areas.

Formerly, we found that for CH4 deep oxidation on high surface area individual NiO catalysts, the overall

14


Page 15 of 47

activity is diffusion controlled, since it increases with the increasing of the catalyst surface area, but the intrinsic activity based on Rs is constant in spite of the change of the NiO surface areas27. For pure SnO2 catalysts, it is apparent that different tendency is observed. With the increasing of the surface area, the R s reflecting the intrinsic activity also improves, as illustrated in Figure 6 (C), possibly due to the generation of more surface defects, as evidenced by Raman results. Therefore, the enhancing of the Rw and toluene conversion with the improved surface area is contributed not only by the surface areas of the samples, but also by the rising of the intrinsic activity. In other words, for the case of toluene deep oxidation, the reaction performance is controlled by both reactant diffusion and the improved intrinsic activity.

3.5 H2-TPR study on the redox properties of the catalysts

(A)

H2 Consumption (mmol g-1)

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


(B)

743

SnO2-2

631 629

SnO2-50

SnO2-2 SnO2-50

794 SnO2-91

625

SnO2-150

SnO2-91 SnO2-150

635 SnO2-217

549

773

SnO2-217

200

400

600

800

100

200

300

400 o

o

Temperature ( C)

Temperature ( C)

Figure 7 H2-TPR profiles of the catalysts. (A) Complete profiles and (B) Enlarged profiles from 100 oC to 400 oC.

To elucidate the reasons responsible for the activity change, the redox properties of the catalysts have been studied with H2-TPR. Figure 7 (A) exhibits that all the catalysts show a major peak above 500 oC, which can be

15



Page 16 of 47

assigned to the reduction of SnO2 to metallic Sn.43 The H2 uptake amount around stoichiometric 13.0 mmol g-1 in Table 3 for all the samples, which corresponds to the reduction of SnO2 lattice oxygen, confirms also this assignment. Due to the rigid single crystal lattice matrix structure and extremely low surface area, the major reduction peak of SnO2-2 nano-rod catalyst shows up at 743 oC. By increasing the surface area, this major peak shifts to lower temperature around 630 oC. Furthermore, the major peak of SnO2-217 splits into three parts at about 549, 635, and 773 oC, implying that lattice oxygen with more complex chemical environments has been formed, perhaps due to the complicated and abundant pore structures of this sample. Noticeably, SnO2-217 possesses the best activity amongst all the catalysts, which might be contributed partially by this lattice oxygen diversity.

Table 3 Oxygen properties measured by XPS and H2-TPR techniques. Catalysts

(O22-+

O1s B.E.and amount (a.u) O2-

O22-

O-OH

O/Sn

O-OH)/ O2-

H2-TPR results Deficient

Lattice

oxygen

oxygen

(mmol g-1)

(mmol g-1)

SnO2-2

530.4/100

-

531.8/21

0.21

2.14

0

12.9

SnO2-50

531.0/56

531.2/9

532.1/16

0.45

2.43

0.4

13.1

SnO2-91

530.5/55

531.2/17

531.8/11

0.51

2.53

1.0

13.4

SnO2-150

530.7/47

531.3/33

532.2/13

0.98

2.63

1.1

13.0

SnO2-217

531.0/50

531.3/54

532.3/36

1.80

3.02

1.3

13.3

Furthermore, a shoulder peak is observed for all the catalysts below 400 oC, that can be ascribed to the reduction of loosely bonded surface deficient oxygen species of SnO2. To see the amount change of this part of oxygen more clearly, the H2-TPR profiles in the region of 100 to 400 oC are enlarged for the catalysts in Figure 7 (B). By increasing the surface area, the quantities of this part of oxygen species are obviously raised. As listed 16


Page 17 of 47 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


in Table 3, while SnO2-2 nano-rod catalyst does not own detectable deficient oxygen because of the very good crystallization, the amount of the deficient oxygen of all the other polycrystalline catalysts follow the rising order of SnO2-50 < SnO2-91 < SnO2-150 < SnO2-217. This suggests that the catalytic performance of the SnO2 catalysts might be closely related to the active surface deficient oxygen amount, considering the reaction follows Mars-Van Krevelen Mechanism44. Particularly, the quantified O/Sn atomic ratios are around stoichiometric 2/1 for all the samples, proving the Sn species is fully oxidized as Sn4+, well consistent with the Raman, HRTEM, XRD and XPS (Figure S2) results.

3.6 O2-TPD study on the SnO2 catalysts The nature of the surface active oxygen on SnO2 with different surface areas has been further explored by O2-TPD. As illustrated in Figure 8 (A), while oxygen desorption is negligible for SnO2-2 nano-rod catalyst, multiple oxygen desorption peaks are evidently observed on the profiles of the polycrystalline SnO2-50, SnO2-91, SnO2-150 and SnO2-217 catalysts, substantiating the presence of various surface active oxygen possessing different chemical environments. For the convenience of discussion, the peaks are classified into three categories, that is, ,  and γ group. The  peaks below 200 oC might be attributed to the desorption of loosely bounded surface oxygen; the  peaks between 200 and 400 oC are assigned to the oxygen ad-species, such as O2- and O22-; and the γ peaks above 400 oC can be attributed to the desorption of surface lattice oxygen species29, 30. Interestingly, although the quantities of  and γ oxygen do not follow a set order, the amounts of  oxygen and total oxygen improve with the increasing of the surface areas, in the order of SnO2-2< SnO2-50< SnO2-91< SnO2-150< SnO2-217, as depicted in Table S1 and Figure 8 (B). While all kinds of active oxygen species might devote to toluene deep oxidation, the  oxygen could play a more critical role for the reaction, since it matches the reaction temperature region in Figure 6 (A).

17



Page 18 of 47

Figure 8 O2 -TPD profiles of the catalysts. (A) Complete profiles, (B) The relative oxygen amount of the catalysts.

3.7 Oxygen properties analyzed by XPS technique XPS has been adopted to investigate the surface properties of the catalysts, particularly focusing on analyzing the surface oxygen property. As shown in Figure S2, Sn 3d spectrum of each sample shows two distinguished peaks at around 486.0 and 495.0 eV assigned to Sn 3d5/2 and Sn 3d3/2 in sequence, which are typical for Sn4+. With the increase of the surface area, these two peaks shift to higher binding energies, indicating the alteration of the chemical environment for Sn cations. In order to understand deeper the feature of the surface oxygen, the O1s signals of the SnO2 catalysts are compared in Figure 9 (A). The asymmetrical O1s spectrum of each sample can be deconvoluted into three symmetrical peaks at 531.0, 531.3 and 532.3 eV, attributed to lattice O2-, O22- and surface OH group.45, 46 This substantiates the existence of different kinds of oxygen species with varied chemical environments on the catalyst surfaces. The peak around 531.0 eV corresponding to OH group is observed for all the catalysts, and the peak around 532.0 eV corresponding to active O22- is observed only for the four polycrystalline catalysts. The lack of this peak for SnO2-2 fits well to its very high crystallinity and extremely low surface area.

18


Page 19 of 47 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


As listed in Table 3, the surface (O22-+O-OH)/O2- molar ratio increase apparently in the order of SnO2-2< SnO2-50< SnO2-91< SnO2-150< SnO2-217, following the same order of H2-TPR, O2-TPD and the activity results. Moreover, the calculated O/Sn atomic ratios are higher than the stoichiometric ratio of 2/1 for the samples, which rises also with the same order, testifying the generation of more surface oxygen species due to the improved surface defects. Therefore, the reaction rates and (O22-+O-OH)/ O2- molar ratios as a function of specific surface areas are depicted in Figure 9 (B) for the catalysts. Obviously, the (O22-+O-OH)/O2- molar ratio increases with the specific surface area, demonstrating the enlargement of the surface area can create more surface active oxygen sites. It has been commonly accepted that surface adsorbed oxygen species are active for the deep oxidation of hydrocarbons29. The higher its concentration, the better the VOCs total oxidation activity. Indeed, Figure 9 (B) depicts that the reaction rates improve together with the surface active oxygen amount. Therefore, XPS results have confirmed that the generation of more abundant surface oxygen sites by increasing the SnO2 surface area is a critical factor to decide its activity for toluene oxidation.

Figure 9 XPS analysis of the catalysts. (A) O1s spectra. (B) Rw and Rs at 250 oC vis (O22-+O-OH)/ O2- ratio.

19



Page 20 of 47

3.8 NH3-TPD study on the surface acidity of the catalysts A toluene molecule possessing a delocalizedπ66 bond, which is a strong electron donor and can be easily coordinated to any electron acceptors such as Lewis acidic sites47, 48. Through this way, the toluene molecule will be activated and form an adsorbed benzyl radical, which reacts subsequently with the active surface oxygen to form the deep oxidation products49. From this point of view, the surface acidity of a catalyst might also play a crucial role for the reaction performance besides the surface active oxygen sites. Therefore, the surface acidity has been measured with NH3-TPD for the catalysts. As depicted in Figure 10 (A), an NH3 desorption peak around 80 - 100 oC is observed on the profiles of all the samples, which can be assigned to the detaching of NH3 molecules adsorbed on the surface Lewis acidic sites of SnO2. It is well known that SnO2 owns surface Lewis acidity and SnO2-based catalysts have been used in quite some reactions requiring acidic sites50. As quantified in Table S2, with the increasing of the surface areas, the acidic site quantity improves evidently due to the larger exposure of the Sn4+ cations. Our former work has testified that the incorporation of Sn4+ cations into the framework of MFI zeolite with certain capacities can achieve Sn-MFI catalysts owning much improved acidity due to the highly distribution of the Sn4+ cations on the surface32. It is believed that the similar phenomenon has occurred to SnO2 samples having different surface areas in this study. Hence, it is rational to deduce that enlarging the surface area of SnO2 is favorable for toluene molecule adsorption and activation, which might eventually promote the toluene oxidation activity on the obtained catalysts. To confirm this and understand deeper the relation between SnO2 surface acidity and the intrinsic activity, the NH3 desorption quantities of all the catalysts have been plotted against the toluene conversion rates achieved at 250 oC. As depicted in Figure 10 (B), the activity improves linearly with the surface acidity on the catalysts, demonstrating strongly that the surface acidity is another critical element affecting the toluene oxidation activity over the SnO2 catalysts.

20


Page 21 of 47 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


Figure 10 NH3-TPD study on the catalysts. (A) NH3-TPD profiles. (B) Rw at 250 oC vis NH3 desorption amount.

3.9 Toluene adsorption capacities of the catalysts measured by Toluene-TPD The relationship between surface acidity and toluene adsorption capacity has been further explored with Toluene-TPD. Figure 11 (A) exhibits that except for SnO2-2 nano-rod sample, a major toluene desorption peak around 90 oC are observed on the profiles of all the polycrystalline catalysts (α peak), and a shoulder peak around 200 oC is also observed on the profile of the SnO2-217 (β peak). The existence of an additional small toluene desorption peak for SnO2-217 at higher temperature indicates the formation of a small amount of relatively stronger acidic sites, possibly due to its more complex and abundant porous structure. The toluene desorption quantities are also quantified for all the catalysts in Table S2, which follows the same sequence of surface acidity, that is, SnO2-2 < SnO2-50 < SnO2-91 < SnO2-150 < SnO2-217. This clearly demonstrates that the toluene adsorption capacity depends directly on the acidity of the catalysts, which is closely related to their surface areas. In other words, the surface Lewis acidic sites are the locations to adsorb and activate toluene molecules. Hence, the differential reaction rates obtained at 250 oC are plotted against the toluene adsorption capacities in Figure 11 (B). Apparently, the reaction rates increase almost linearly with the 21



Page 22 of 47

toluene adsorption capacities. This strongly indicates that the toluene adsorption and activation ability of SnO2, which are closely related to the surface acidity and influenced by the surface area, is another critical factor to control the reactivity of the catalysts.

Figure 11 Toluene-TPD study on the catalysts. (A) Toluene-TPD profiles. (B) Rw at 250 oC vis toluene desorption amount.

3.9 Evaluating the application potential of the catalysts 3.9.1 Reaction stability and water tolerance of the SnO2 samples The reaction stability of all the SnO2 catalysts with different surface areas has been tested at 370 °C. Figure S3(A) demonstrates that the samples display constant toluene conversion during 50 hours’ evaluation, indicating they are stable for the reaction. According to some previous reports, toluene deep oxidation on some catalysts was negatively affected by the presence of water vapor

49

. Therefore, SnO2-217, the best catalyst in

this study, has been subjected to the related tests. As shown in Figure S3(B), after the toluene conversion was stabilized at ~82% for 10 hours at 370 °C, 5% water vapor was introduced into the feed, which decreased the toluene conversion only marginally from 82% to ~80%, and will be completely restored after removing the

22


Page 23 of 47

water vapor. The repeated experiments showed the same phenomenon. In addition, after the water tolerance test, the toluene conversion on the catalyst is still constant in the following 20 hours. These results have testified that the presence of water vapor has only insignificant effects on the reactivity.

3.9.2 Testing the property of SnO2 as supports for Pd Former studies have testified that SnO2 is a superior support for noble metals to prepare active and stable catalysts 51-53. For instance, Fuller et al reported that by loading the same amount of Pd, Pd/SnO2 depicted much far better CO oxidation activity than Pd/SiO2 51. Furthermore, in the presence of water vapor, the CO oxidation activity on Pd/SnO2 was increased instead of being decreased, which was confirmed by our work 52. Therefore, SnO2 nano particles possessing different surface areas have been used as supports for 1 wt. % Pd, and adopted for toluene deep oxidation.

100 1% Pd/ SnO2-50 1% Pd/ SnO2-91 80

Toluene 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


1% Pd/ SnO2-150 1% Pd/ SnO2-217

60

40

20

0 140

160

180

200

220

240

260

280

300

o

Temperature ( C)

Figure 12 Toluene deep oxidation on 1% Pd/SnO2 catalysts.

Figure 12 shows that in contrast to the counterpart SnO2 samples, the overall activity of all the samples has been significantly enhanced because of the presence of Pd sites. In addition, the activity follows also the trend 23



Page 24 of 47

of 1% Pd/SnO2-50 < 1% Pd/SnO2-91 < 1% Pd/SnO2-150 < 1% Pd/SnO2-217. Apparently, with the increasing of the surface area, Pd sites can be better dispersed, which benefits the adsorption and activation of toluene molecules.49, 54 Furthermore, as demonstrated above, a SnO2 sample with a higher surface area owns more active deficient surface oxygen species, which thus has better ability to supply oxygen to the activated toluene molecules and facilitates the final oxidation process. Therefore, SnO2 with high surface areas are better supports for Pd to prepare deep oxidation catalysts. Through this way, a catalyst with competitive oxidation activity can be obtained with reduced amount of noble metals, thus saving the cost. The stability of the 1% Pd/SnO2 catalysts has been tested too, with the similar procedures to Figure S3. Figure S4 exhibits that these catalysts possess very stable performance in the presence or absence of water vapor.

3.10 Brief discussion on the critical factors determine the activity It has been commonly accepted that for VOCs combustion on polycrystalline metal oxide catalysts, it generally follows a Mars Van-Krevelen mechanism55. During the reaction, the reactants such as toluene in this study will adsorb first on the suitable sites to be activated, which will react with the active surface adsorbed or lattice oxygen species. In turn, the gas phase O2 molecules will re-oxidized the reduced sites. Therefore, the co-existence of both active surface oxygen species and appropriate sites to adsorb and activate the reactant molecules is of great importance for the combustion activity of a catalyst. SnO2 possesses several attractive merits, which make it a potential catalytic material for different reactions. For instance, it owns relatively abundant surface Lewis acidic sites that can effectively adsorb those electron donor reactants such as toluene in this work56. In addition, as an n-type semiconductor, it owns rich surface oxygen defects, which can induce the generation of a considerable amount of active deficient oxygen species 38; furthermore, its lattice oxygen is easily reducible. This implies that SnO2 as a catalyst is able to provide oxygen species to the activated reactant intermediates to advance the oxidation process smoothly. Last but not the least, 24


Page 25 of 47 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


the melting point of SnO2 is as high as 1630 oC, indicating that it has also good thermal stability, which is an important aspect for most of the environmental catalysts. In brief, SnO2 can provide active sites matching mostly the requirements of a good catalyst for VOCs combustion, especially toluene deep oxidation in this work. Therefore, with the target to understand the fundamental issues of SnO2-based materials for catalytic VOCs combustion and develop more applicable catalysts, individual SnO2 possessing different and stable surface areas have been purposely fabricated with a hydrothermal method by adjusting the preparing condition slightly in this study. N2 sorption and XRD results have indicated that SnO2 catalysts possessing different surface areas and mean crystallite sizes have been fabricated successfully. SEM and TEM results testify that except for SnO2-2 single-crystalline nano-rod sample, all the other catalysts are composed of SnO2 nano particles with varied average grain sizes. By increasing the specific surface area, more defects can be generated on the SnO2 surface, as evidenced by the Raman results. This indeed can induce the generation of a larger quantity of active surface oxygen sites, as has been proved by the H2-TPR, O2-TPD and XPS O1s results. For those polycrystalline SnO2 catalysts having relatively higher surface areas and oxygen vacancies, O2 molecules can enter into the oxygen vacancies. Therefore, the formation of bulk active electrophilic O22- can be described with the following equilibrium equation57, 58: (

)

(

)

(1)

The formed electrophilic O22- will then migrate to the catalyst surface. Through this pathway, very abundant surface active oxygen species can usually be produced, and the quantity improves with the amount of defects. Therefore, it is not difficult to understand that by increasing the SnO2 surface area, more active oxygen sites have been generated, which enhance the toluene deep oxidation activity significantly.

25



Page 26 of 47

In contrast, for the well crystallized SnO2-2 nano-rod sample having barely any oxygen vacancies, the surface electrophilic oxygen species can only be formed by the direct transformation of the adsorbed gas phase O2 molecules, as described in equation (2): (

)

(

) ↔

(

)↔

(

)

(

)

(

) (2)

Generally, through this pathway, only a minute quantity of surface active oxygen species can be formed.45 As listed in Table 2, while the apparent activation energy on SnO2-2 nano-rod sample is much higher than that on those polycrystalline powder samples, the apparent activation energy change of all the polycrystalline powder samples is within 10 kJ mol-1. This indicates strongly that toluene deep oxidation on SnO2-2 nano-rod sample could go through a different pathway from that on those polycrystalline samples. Furthermore, based on the slight change of the apparent activation energies, we believe that toluene deep oxidation on all the polycrystalline samples follows the same mechanism. Our previous work has demonstrated that a SnO2 nano-Rod single crystal sample possessing very small surface area and negligible active surface oxygen species depicts much higher intrinsic CO oxidation activity than those polycrystalline powder samples owning much higher surface areas and more abundant active surface oxygen sites.

38

The major reason is due to the difference in the reaction pathways. On those polycrystalline

powder SnO2 samples, CO oxidation proceeds via a Mars-Van Krevelen mechanism, which involves the consumption and regeneration of the surface lattice and adsorbed oxygen species, and is kinetically slow. In contrast, on SnO2 nano-Rod catalyst, CO oxidation shows the behavior similar to supported noble metal catalysts, therefore, it could follow a Langmuir-Hinshelwood or Eley-Rideal mechanism with the CO or O2 molecules adsorbed and activated on those low coordinated Sn sites, which does not involve the consumption and regeneration of the surface oxygen species, and is kinetically fast.

26


Page 27 of 47 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


Indeed, in this work, the same phenomenon could have occurred. In detail, on SnO2-2 nano-rod catalyst, toluene deep oxidation could follow a Langmuir-Hinshelwood or Eley-Rideal mechanism because of the scarce of surface active oxygen sites; while on those SnO2 powder samples with different surface areas, it should follow a Mars-Van Krevelen mechanism. Different from CO oxidation, the extremely low surface area of SnO2-2 nano-rod can only generate a minute amount of surface acidic sites (Fig. 10) and is not favorable for toluene molecules adsorption and activation (Fig. 11). Therefore, on this catalyst, the reaction proceeds with a much higher energy barrier. In comparison, on those polycrystalline SnO2 samples, their surface can provide abundant acidic sites for toluene molecule adsorption and activation; and also enough active surface oxygen species to oxidize the activated toluene molecules. Hence, toluene oxidation can proceed smoothly via Mars-Van Krevelen mechanism, and with a much lower energy barrier. With the increasing of the surface areas of the polycrystalline samples, more abundant surface acidic and active oxygen sites are generated, which can facilitate the reaction, and hence decreasing the apparent energies slightly. Base on above discussion, it is rational to propose that by improving the surface area of polycrystalline powder SnO2, more active catalysts can be obtained, as have been testified by the experimental results in this work. On the other hand, by increasing the SnO2 surface area, Sn4+ cations will become more exposed, hence improving the amount of surface acidic sites remarkably, as demonstrated by the NH 3-TPD results. As a consequence, the toluene adsorption capacity will be increased, which is beneficial to the toluene oxidation activity. In brief, the presence of both surface active oxygen species and acidic sites are believed to be critical for the activity of SnO2 catalysts for toluene deep oxidation. The surface acidic sites play the role to adsorb and activate the toluene molecules effectively, which will in turn be oxidized by the surface active oxygen sites. The concerted interaction of the two kinds of sites controls the activity of SnO2-based catalysts. By fabricating

27



Page 28 of 47

polycrystalline powder SnO2 with higher surface areas, the quantities of the two types of surface sites can be significantly improved, thus obtaining catalysts with remarkably promoted toluene oxidation activity.

4. Conclusions With the objective to understand the fundamental aspects of toluene deep oxidation on SnO2 and develop more applicable catalysts, a series of SnO2 samples with different and stable surface areas have been controllably constructed successfully and characterized in this work. It is discovered that: (1) By increasing the surface area of SnO2, more surface defects and oxygen vacancies can be generated, which induces the formation of larger quantities of surface active oxygen sites. Furthermore, the increase of SnO2 surface area also produces more abundant surface acidic sites, due to the improved exposing of Sn4+ cations, thus enhancing the toluene adsorption and activation ability. As a result, both the overall and intrinsic activity of SnO2 can be eventually promoted. (2) The co-existence of surface active oxygen species and acidic sites are of great importance for toluene deep oxidation on SnO2. The concerted interaction of the two kinds of sites is believed to determine the reaction performance. SnO2-217 possesses the highest surface area in all the catalysts, thus having the most abundant surface active oxygen species and acidic sites. As a consequence, it displays the best toluene oxidation activity as well as good reaction stability, and water vapor tolerance. (3) A SnO2 sample with a higher surface area can disperse Pd better, and owns also more abundant active deficient surface oxygen species. Therefore, it is also a good support to prepare competitive catalysts with less noble metals for toluene deep oxidation.

ASSOCIATED CONTENT The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. 28


Page 29 of 47 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


Supporting Information. Experimental details of SnO2-2, SnO2-50, SnO2-91, SnO2-150 and SnO2-217 characterization, performance evaluation. N2 adsorption-desorption profiles, XPS spectra of Sn 3d, On-stream toluene oxidation at 370 oC over SnO2 and 1% Pd/ SnO2 catalysts, a table listing the quantified O2-TPD results of the catalysts and a table listing the quantified toluene and NH3 desorption results.(PDF) AUTHOR INFORMATION Corresponding Author: *E-mail: [email protected] (X. Wang)

Notes The authors declare no competing financial interest.

Acknowledgments The authors acknowledge deeply the financial supporting by the National Natural Science Foundation of China (21567016, 21666020), the National Key Research and Development Program of China (2016YFC0209302),

the

Natural

Science

Foundation

of

Jiangxi

Province

(20181ACB20005,

20171BAB213013) , the Education Department of Jiangxi Province (GJJ150016, GJJ150085, KJLD14005), the Innovation Fund Designated for Graduate Students of Jiangxi Province (YC2018-B015) and for Graduate Students of Nanchang University (201802062). . References

(1) Abdelouahab-Reddam, Z.; El Mail, R.; Coloma, F.; Sepulveda-Escribano, A., Platinum Supported on Highly-dispersed Ceria on Activated Carbon for the Total Oxidation of VOCs. Appl. Catal. A. 2015, 494, 87. 29



Page 30 of 47

(2) Debecker, D. P.; Farin, B.; Gaigneaux, E. M.; Sanchez, C.; Sassoye, C., Total Oxidation of Propane with a Nano-RuO2/TiO2 Catalyst. Appl. Catal. A. 2014, 481, 11. (3) Peng, Y.; Yang, S.; Shi, Z.; Meng, Z.; Zhou, R., Deep Oxidation of Chlorinated VOCs over CeO 2-based Transition Metal Mixed Oxide Catalysts. Appl. Catal. B. 2015, 162, 227-235. (4) Centeno, M. A.; Paulis, M.; Montes, M.; Odriozola, J. A., Catalytic Combustion of Volatile Organic Compounds on Au/CeO2/Al2O3 and Au/Al2O3 Catalysts. Appl. Catal. A. 2002, 234, 65. (5) Giraudon, J. M.; Elhachimi, A.; Wyrwalski, F.; Siffert, S.; Aboukais, A.; Lamonier, J. F.; Leclercq, G., Studies of the Activation Process over Pd Perovskite-type Oxides used for Catalytic Oxidation of Toluene. Appl. Catal. B. 2007, 75, 157. (6) Bedia, J.; Rosas, J. M.; Rodriguez-Mirasol, J.; Cordero, T., Pd Supported on Mesoporous Activated Carbons with High Oxidation Resistance as Catalysts for Toluene Oxidation. Appl. Catal. B. 2010, 94, 8. (7) Bai, B.; Li, J., Positive Effects of K+ Ions on Three-Dimensional Mesoporous Ag/Co3O4 Catalyst for HCHO Oxidation. ACS Catal. 2014, 4, 2753-2762. (8) Liotta, L. F.; Ousmane, M.; Di Carlo, G.; Pantaleo, G.; Deganello, G.; Marci, G.; Retailleau, L.; Giroir-Fendler, A., Total Oxidation of Propene at Low Temperature over Co3O4-CeO2 Mixed Oxides: Role of Surface Oxygen Vacancies and Bulk Oxygen Mobility in the Catalytic Activity. Appl. Catal. A. 2008, 347, 81. (9) Bai, B.; Arandiyan, H.; Li, J., Comparison of the Performance for Oxidation of Formaldehyde on Nano-Co3O4 , 2D-Co3O4 , and 3D-Co3O4 Catalysts. Appl. Catal. B. 2013, 142, 677. (10) Delimaris, D.; Ioannides, T., VOC Oxidation over MnOx-CeO2 Catalysts Prepared by a Combustion Method. Appl. Catal. B. 2008, 84, 303-312. (11) Bai, B.; Li, J.; Hao, J.; Bai, B.; Hao, J., 1D-MnO2 , 2D-MnO2 and 3D-MnO2 for Low-temperature Oxidation of Ethanol. Appl. Catal. B. 2015, 164, 241-250. (12) Li, J.; Qu, Z.; Qin, Y.; Wang, H., Effect of MnO2 Morphology on the Catalytic Oxidation of Toluene over Ag/MnO2 Catalysts. Appl. Surf. Sci. 2016, 385, 234. (13) Bonelli, R.; Albonetti, S.; Morandi, V.; Ortolani, L.; Riccobene, P. M.; Scire, S.; Zacchini, S., Design of Nano-sized FeOx and Au/FeOx Catalysts Supported on CeO2 for Total Oxidation of VOC. Appl. Catal. A. 2011, 395, 10. (14) Jin, B.; Wei, Y.; Zhao, Z.; Liu, J.; Yu, X.; Li, Y.; Li, J., Synthesis of three-dimensionally ordered macroporous Al–Ce mixed oxide catalysts with high catalytic activity and stability for diesel soot combustion. Cataly Today 2015, 258, 487. 30


Page 31 of 47 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) Hu, Z.; Liu, X.; Meng, D.; Guo, Y.; Guo, Y.; Lu, G., Effect of Ceria Crystal Plane on the Physicochemical and Catalytic Properties of Pd/Ceria for CO and Propane Oxidation. ACS Catal. 2016, 6, 2265. (16) Vu, V. H.; Belkouch, J.; Ould-Dris, A.; Taouk, B., Removal of Hazardous Chlorinated VOCs over Mn-Cu Mixed Oxide Based Catalyst. J. Hazard. Mater. 2009, 169, 758. (17) Arandiyan, H.; Dai, H.; Ji, K.; Sun, H.; Li, J., Pt Nanoparticles Embedded in Colloidal Crystal Template Derived 3D Ordered Macroporous Ce0.6Zr0.3Y0.1O2: Highly Efficient Catalysts for Methane Combustion. ACS Catal. 2015, 5, 1781. (18) Haidy Castano, M.; Molina, R.; Moreno, S., Cooperative Effect of the Co-Mn Mixed Oxides for the Catalytic Oxidation of VOCs: Influence of the Synthesis Method. Appl. Catal. A. 2015, 492, 48. (19) Hamidreza, A.; Dai, H.; Ji, K.; Sun, H.; Zhao, Y.; Li, J., Enhanced Catalytic Efficiency of Pt Nanoparticles Supported on 3D Ordered Macro‐/Mesoporous Ce0.6Zr0.3Y0.1O2 for Methane Combustion. Small 2015, 11, 2366. (20) Shaohua, X.; Hongxing, D.; Jiguang, D.; Yuxi, L.; Huanggen, Y.; Yang, J.; Wei, T.; Aisun, A.; Guangsheng, G., Au/3DOM Co3O4: Highly Active Nanocatalysts for the Oxidation of Carbon Monoxide and Toluene. Nanoscale 2013, 5, 11207. (21) Lin, S.; Yang, L.; Yang, X.; Zhou, R., Redox Properties and Metal–support Interaction of Pd/Ce0.67Zr0.33O2– Al2O3 Catalyst for CO, HC and NOx Elimination. Appl. Surf. Sci. 2014, 305, 642. (22) Lin, S.; Yang, L.; Yang, X.; Zhou, R., Redox Behavior of Active PdOx Species on (Ce,Zr)xO2–Al2O3 Mixed Oxides and Its Influence on the Three-way Catalytic Performance. Chem. Eng. J. 2014, 247, 42-49. (23) Tian, J.; Peng, H.; Xu, X.; Liu, W.; Ma, Y.; Wang, X.; Yang, X., High Surface Area La2Sn2O7 Pyrochlore as a Novel, Active and Stable Support for Pd for CO Oxidation. Catal. Sci.Technol. 2015, 5, 2270. (24) Arandiyan, H.; Dai, H.; Deng, J.; Liu, Y.; Bai, B.; Wang, Y.; Li, X.; Xie, S.; Li, J., Three-dimensionally Ordered Macroporous La0.6Sr0.4MnO3 with High Surface Areas: Active Catalysts for the Combustion of Methane. J. Catal. 2013, 327-339, 327. (25) Arandiyan, H.; Dai, H.; Deng, J.; Wang, Y.; Sun, H.; Xie, S.; Bai, B.; Liu, Y.; Ji, K.; Li, J., Three-dimensionally Ordered Macroporous La0.6Sr0.4MnO3 Supported Ag Nanoparticles for the Combustion of Methane. J. Phys. Chem. C 2014, 118, 14913. (26) Li, Y.; Peng, H.; Xu, X.; Peng, Y.; Wang, X., Facile Preparation of Mesoporous Cu–Sn Solid Solutions as Active Catalysts for CO Oxidation. RSC Adv. 2015, 5, 25755-25764. (27) Xu, X.; Li, L.; Yu, F.; Peng, H.; Fang, X.; Wang, X., Mesoporous high surface area NiO synthesized with 31



Page 32 of 47

soft templates: Remarkable for catalytic CH4 deep oxidation. Molecular Catalysis 2017, 441, 81-91. (28) Zhu, Y. G.; Wang, Y.; Xie, J.; Cao, G.-S.; Zhu, T.-J.; Zhao, X.; Yang, H. Y., Effects of Graphene Oxide Function Groups on SnO2 /Graphene Nanocomposites for Lithium Storage Application. Electrochim. Acta 2015, 154, 338-344. (29) Liu, C.; Xian, H.; Jiang, Z.; Wang, L.; Zhang, J.; Zheng, L.; Tan, Y.; Li, X., Insight into the improvement effect of the Ce doping into the SnO2 catalyst for the catalytic combustion of methane. Appl. Catal. B. 2015, 176-177, 542-552. (30) Jiang, W.; Pang, Y.; Gu, L.; Yao, Y.; Su, Q.; Ji, W.; Au, C.-T., Structurally defined SnO2 substrates, nanostructured Au/SnO2 interfaces, and their distinctive behavior in benzene and methanol oxidation. J. Catal. 2017, 349, 183-196. (31) Guo, Y.-n.; Cheng, J.-h.; Hu, Y.-y.; Li, D.-h., The effect of TiO2 doping on the catalytic properties of nano-Pd/SnO2 catalysts during the reduction of nitrate. Appl. Catal. B. 2012, 125, 21-27. (32) Peng, Y.; Peng, H.; Liu, W.; Xu, X.; Liu, Y.; Wang, C.; Hao, M.; Ren, F.; Li, Y.; Wang, X., Sn-MFI as Active, Sulphur and Water Tolerant Catalysts for Selective Reduction of NOx. RSC Adv. 2015, 5, 42789. (33) Sun, Q.; Xu, X.; Peng, H.; Fang, X.; Liu, W.; Ying, J.; Yu, F.; Wang, X., SnO 2-based Solid Solutions for CH4 Deep Oxidation: Quantifying the Lattice Capacity of SnO2 Using an X-ray Diffraction Extrapolation Method. Chin. J. Catal. 2016, 37, 1293-1302. (34) Haneda, M.; Ota, Y.; Doi, Y.; Hattori, M., Preparation, characterization, and activity of SnO 2 nanoparticles supported on Al2O3 as a catalyst for the selective reduction of NO with C3H6. J. Mater. Sci. 2016, 51, 10949-10959. (35) Liu, Y.; Guo, Y.; Liu, Y.; Xu, X.; Peng, H.; Fang, X.; Wang, X., SnO2 nano-rods promoted by In, Cr and Al cations for toluene total oxidation: The impact of oxygen property and surface acidity on the catalytic activity. Appl. Surf. Sci. 2017, 420, 186-195. (36) Rao, C.; Shen, J.; Wang, F.; Peng, H.; Xu, X.; Zhan, H.; Fang, X.; Liu, J.; Liu, W.; Wang, X., SnO2 promoted by alkali metal oxides for soot combustion: The effects of surface oxygen mobility and abundance on the activity. Appl. Surf. Sci. 2018, 435, 406-414. (37) Wang, S.; Huang, J.; Zhao, Y.; Wang, S.; Wang, X.; Zhang, T.; Wu, S.; Zhang, S.; Huang, W., Preparation, characterization and catalytic behavior of SnO2 supported Au catalysts for low-temperature CO oxidation. Journal of Molecular Catalysis A: Chemical 2006, 259, 245-252. (38) Wang, X.; Xiao, L.; Peng, H.; Liu, W.; Xu, X., SnO2 Nano-rods with Superior CO Oxidation Performance. 32


Page 33 of 47 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


J. Mater. Chem. A 2014, 2, 5616-5619. (39) Sun, S. H.; Meng, G. W.; Zhang, G. X.; Gao, T.; Geng, B. Y.; Zhang, L. D.; Zuo, J., Raman scattering study of rutile SnO2 nanobelts synthesized by thermal evaporation of Sn powders. Chem. Phys. Lett. 2003, 376, 103-107. (40) Bonu, V.; Das, A.; Sivadasan, A. K.; Tyagi, A. K.; Dhara, S., Invoking forbidden modes in SnO 2 nanoparticles using tip enhanced Raman spectroscopy. J. Raman. Spectrosc. 2015, 46, 1037-1040. (41) Diéguez, A.; Romano-Rodrı́guez, A.; Vilà, A.; Morante, J. R., The complete Raman spectrum of nanometric SnO2 particles. J. Appl. Phys. 2001, 90, 1550-1557. (42) Liu, L. Z.; Li, T. H.; Wu, X. L.; Shen, J. C.; Chu, P. K., Identification of oxygen vacancy types from Raman spectra of SnO2 nanocrystals. J. Raman. Spectrosc. 2012, 43, 1423-1426. (43) Liu, Y.; Guo, Y.; Peng, H.; Xu, X.; Wu, Y.; Peng, C.; Zhang, N.; Wang, X., Modifying Hopcalite catalyst by SnO2 addition: An effective way to improve its moisture tolerance and activity for low temperature CO oxidation. Appl. Catal. A. 2016, 525, 204-214. (44) Batzill, M.; Diebold, U., The surface and materials science of tin oxide. Prog. Surf. Sci. 2005, 79, 47-154. (45) Babu, B.; Neelakanta Reddy, I.; Yoo, K.; Kim, D.; Shim, J., Bandgap tuning and XPS study of SnO 2 quantum dots. Mater. Lett. 2018, 221, 211-215. (46) Kwoka, M.; Ottaviano, L.; Passacantando, M.; Santucci, S.; Czempik, G.; Szuber, J., XPS study of the surface chemistry of L-CVD SnO2 thin films after oxidation. Thin Solid Films 2005, 490, 36-42. (47) Menon, U.; Galvita, V. V.; Marin, G. B., Reaction network for the total oxidation of toluene over CuO– CeO2/Al2O3. J. Catal. 2011, 283, 1-9. (48) Farmanzadeh, D.; Valipour, A., Adsorption of benzene and toluene molecules on surface of pure and doped cadmium oxide nanosheets: A computational investigation. Appl. Surf. Sci. 2018, 450, 509-515. (49) Peng, R.; Li, S.; Sun, X.; Ren, Q.; Chen, L.; Fu, M.; Wu, J.; Ye, D., Size effect of Pt nanoparticles on the catalytic oxidation of toluene over Pt/CeO2 catalysts. Appl. Catal. B. 2018, 220, 462-470. (50) Song, X.; Sayari, A., Sulfated Zirconia-Based Strong Solid-Acid Catalysts: Recent Progress. Catal. Rev. 1996, 38, 329-412. (51) Fuller, G. C. M., Water-promoted oxidation of carbon monoxide over tin(IV) oxide-supported palladium. Nature 1977, 269, 585-586. (52) Wang, X.; Tian, J.; Zheng, Y.; Xu, X.; Liu, W.; Fang, X., Tuning Al2O3 Surface with SnO2 to Prepare Improved Supports for Pd for CO Oxidation. ChemCatChem 2014, 6, 1604-1611. 33



Page 34 of 47

(53) Kamiuchi, N.; Mitsui, T.; Yamaguchi, N.; Muroyama, H.; Matsui, T.; Kikuchi, R.; Eguchi, K., Activation of Pt/SnO2 catalyst for catalytic oxidation of volatile organic compounds. Catal. Today 2010, 157, 415-419. (54) Wang, X.; Gorte, R. J.; Wagner, J. P., Deactivation Mechanisms for Pd/Ceria during the Water–Gas-Shift Reaction. J. Catal. 2002, 212, 225-230. (55) Si, W.; Wang, Y.; Zhao, S.; Hu, F.; Li, J., A Facile Method for in Situ Preparation of the MnO2/LaMnO3 Catalyst for the Removal of Toluene. Environ. Sci. Technol. 2016, 50, 4572-4578. (56) Lu, M.; Huang, R.; Wu, J.; Fu, M.; Chen, L.; Ye, D., On the performance and mechanisms of toluene removal by FeOx/SBA-15-assisted non-thermal plasma at atmospheric pressure and room temperature. Catal. Today 2015, 242, 274-286. (57) Sung Han Lee; Da Woon Jung; Jung Bae Kim; Kim, Y.-R., Effect of altervalent cation-doping on catalytic activity of neodymium sesquioxide for oxidative coupling of methane. Appl. Catal. A. 1997, 164, 156-169. (58) A.G. anshits; E.N. Voskresenskaya; kurteeva, L. I., Role of defect structure of active oxiddes in oxidative couping methane. Catal. Lett. 1990, 6, 67-76.

For Table of Contents Only

34


Page 35 of 47 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


Figure 1 XRD patterns of the catalysts 118x84mm (96 x 96 DPI)



Figure 2 SEM images of the catalysts 236x115mm (148 x 150 DPI)


Page 36 of 47

Page 37 of 47 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


Figure 3 TEM images of the catalysts 199x133mm (150 x 150 DPI)



Figure 4 Raman spectra of the catalysts 261x102mm (146 x 146 DPI)


Page 38 of 47

Page 39 of 47 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


Figure 5 HRTEM images of the catalysts 203x137mm (134 x 134 DPI)



Figure 6 Toluene deep oxidation on the catalysts 200x157mm (117 x 117 DPI)


Page 40 of 47

Page 41 of 47 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


Figure 7 H2-TPR profiles of the catalysts 127x97mm (96 x 96 DPI)



Figure 8 O2 -TPD profiles of the catalysts 268x107mm (138 x 150 DPI)


Page 42 of 47

Page 43 of 47 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


Figure 9 XPS analysis of the catalysts 274x116mm (101 x 101 DPI)



Figure 10 NH3-TPD study on the catalysts 260x111mm (150 x 150 DPI)


Page 44 of 47

Page 45 of 47 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


Figure 11 Toluene-TPD study on the catalysts 179x76mm (150 x 150 DPI)



Figure 12 Toluene deep oxidation on 1% Pd/SnO2 catalysts 1268x970mm (96 x 96 DPI)


Page 46 of 47

Page 47 of 47 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


Figure Abstract for TOC 309x188mm (96 x 96 DPI)


Tuning SnO2 Surface Area for Catalytic Toluene Deep Oxidation: On

Recommend Documents