Template-free synthesis of macroporous SiO2 catalyst supports for

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Kinetics, Catalysis, and Reaction Engineering

Template-free synthesis of macroporous SiO2 catalyst supports for diesel soot combustion Hui Wang, Zewen Chen, Xingyu Deng, Shengji Wu, Qinqin Yu, Wei Yang, and Jie Zhou Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02746 • Publication Date (Web): 09 Aug 2018 Downloaded from http://pubs.acs.org on August 10, 2018

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Template-free synthesis of macroporous SiO2 catalyst supports for diesel soot combustion Hui Wanga, Zewen Chena, Xingyu Denga, Shengji Wua,*, Qinqin Yub,*, Wei Yanga, Jie Zhoua a

College of Materials and Environmental Engineering, Hangzhou Dianzi University, Xiasha

University Park, Hangzhou, Zhejiang 310018, China. b

College of Chemistry, Chemical Engineering and Life Sciences, Wuhan University of

Technology, Wuhan 430070, China *Corresponding author: E-mail: [email protected], [email protected]

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ABSTRACT Catalytic combustion is considered as one of the most efficient method for soot removal. However, the limited contact area between soot and active site is a key restrictive factor for the present reaction, as soot particles (particle size > 25 nm) are usually difficult to enter the inner pores of general catalysts. In this work, we report a simple and economic strategy to synthesize macroporous SiO2 as the support for combustion catalysts so as to facilitate the mass transfer in the inner surface of the catalysts. The macroporous SiO2 could be synthesized rationally through controlling and adjusting the Si-OH dehydration condensation and neck growth during the sol-gel preparation process. The porosity of SiO2 influences catalytic activity greatly in the reaction of soot catalytic combustion. Furthermore, macroporous SiO2 supported Ag and K catalysts exhibit superior soot oxidation activity, showing important potentiality for future applications. Keywords: Soot combustion, Macroporous SiO2, Sol-gel method, Silver, Supported catalysts

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Introduction The emission control of soot particle has been regarded as one of the most challenging problems for diesel engines despite their better thermal performance and higher economic efficiency than gasoline engines1. Up to now, the combination of diesel particulate filter (DPF) and combustion catalysts appears to be the most efficient method for soot removal2-3. The key point is to develop highly efficient catalysts for soot combustion4. A great number of catalysts have been studied for soot combustion, and the effective ones mainly include perovskites5-6, perovskite-like oxides7, spinel oxides8, ceria-based materials9-11, transition metal oxides12, and alkali catalysts13 as well. Among them, supported catalysts have also been intensively investigated for soot combustion, due to the simple catalyst preparation process 14-15. It has been suggested that supported alkali oxides and Ag catalysts may favor the reaction of soot combustion16-17. Lietti L. et al. prepared γ-Al2O3 supported alkaline oxide catalysts and found that the combustion of soot was greatly enhanced by the catalyst18. Teraoka Y. et al. found that both K and Ag-based catalyst showed high performance for soot combustion under the loose contact condition19. On the other hand, in view of the catalytic removal of soot particles, usually only the outer surfaces of these catalysts are used in the reaction because the particle size of soot (> 25 nm) is generally much larger than the pore diameter of the catalysts20. Consequently, it is reasonable to develop catalysts with macroporous structure to increase the contact opportunity of the active centers and the soot particles so as to enhance the catalytic activity21. There have been several studies on the design of macroporous-structured catalysts for soot combustion. Einaga H. et al. synthesized macro-structure Ag/CeO2 fiber via the electrospinning method for 3


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soot combustion22. The catalysts exhibited improved catalytic performance for soot oxidation, due to the large pore size of the new support materials CeO2. Zhao Z. et al. prepared three-dimensionally ordered macroporous (3DOM) LaCoxFe1−xO3 perovskite by using colloid crystal template method with polymethyl methacrylate microspheres as the template23. The as-synthesized 3DOM LaCoxFe1−xO3 catalysts exhibited superiority for soot combustion in comparison with traditional non-macroporous LaCoxFe1−xO3 catalysts. Similarly, Men Y. et al. synthesized 3DOM CuO-CeO2 catalysts using the analogous template method and obtained good catalytic activity as well24. The increased activity of the 3DOM catalysts compared to their non-macroporous correspondents is attributed to the better utilization of the inner specific surface of the specific 3DOM porosity. Although 3DOM catalysts showed obvious superiority in catalyzing soot combustion, the catalyst preparation process was rather complicated. Controlling the sol-gel formation process conditions to modulate the porous structure of SiO2, in this work we reported a very simple strategy to synthesize macroporous SiO2 to act as an effective catalyst support of soot combustion catalyst. We mainly focused on the construction of fluent channels in the support for efficient mass transfer in the inner surface of the catalyst so as to maximize the utilization of inner-surface catalytic centers to obtain superior soot combustion capability. The synthesized macroporous SiO2 supported Ag and K catalysts exhibited superior soot oxidation activity, showing important potentiality for applications due to the high oxidation efficiency and great scalability for catalyst preparation.

2. Experimental 4



2.1. Materials Printex U carbon powder, provided by Evonik Degussa Corp., was used as the substitute for soot particles emitted from diesel engines. H-ZSM-5 and ultrastable Y-type (USY) zeolites were purchased from Nankai Catalyst Corp. Na2SiO3·9H2O of AR purity was purchased from Aladdin.

2.2. Catalyst preparation 2.2.1. Synthesis of SiO2 support with different porous structures Porous SiO2 support was synthesized via an effective and simple method. Briefly, 13.2 g Na2SiO3·9H2O was dissolved in 24.3 mL deionized water. After stirring for 0.5 h at room temperature, the pH of the solution was adjusted by dilute HNO3 (3mol/L) drop by drop to the desired values in the range of 1 to 7. The obtained mixture was then transferred into a Teflon-lined autoclave, and sealed at certain temperature for 20 h. After that, the precipitates were centrifuged, washed thoroughly with deionized water, and then dried at 100 oC. The obtained products were designated as SiO2-x-y, where x and y stand for the pH value and the aging temperature during the gel aging process, respectively.

2.2.2. Preparation of Ag/SiO2 catalyst Ag/SiO2 was prepared via an incipient wetness impregnation. Firstly, certain amounts of AgNO3 were dissolved in deionized water, and then SiO2-7-80 with a determined amount was introduced into the above solution. After dryness in air, the product was calcined in air at 500 o

C for 3 h. The obtained products were designated as 5%Ag/SiO2, when the loading amount of

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Ag is 5.0 wt.%. The CuO/SiO2 and K2CO3/SiO2 catalysts were prepared by the similar process.

2.2.3. Preparation of diesel soot simulated model The simulated diesel soot model was prepared by mixing Printex U with the prepared catalyst. The powders were mixed mechanically using an agate mortar for 10 min to achieve a tight contact (TC) mode of soot and the catalyst. The weight ratio of soot to catalyst is 1:20 in the experiment.

2.3. Catalyst characterization Nitrogen adsorption-desorption isotherms were measured at -195.8 oC using an ASAP 3020 apparatus (Micromeritics Instrument Corp.). Prior to the measurement, the samples were degassed at 300

o

C for 4 h. The specific surface area was analyzed by the

Brunauer-Emmett-Teller (BET) method. The cumulative pore volume and distribution of pore size were determined by the Barrett-Joyner-Halenda (BJH) method. The morphologies of the samples were observed by a scanning electron microscope (SEM; SU8000, Hitachi) at an accelerating voltage of 15 kV.

2.4. Catalytic measurements The catalytic combustion of soot experiment was carried out on a thermogravimetric analysis (TGA) apparatus at atmospheric pressure. Typically, the soot combustion experiment was performed from 120 to 800 oC with a heating rate of 10 oC/min in a flowing mixed gas 6



(10% O2 and 90% N2, 40 mL/min). Prior to the reaction, the mixture of soot and catalyst was pretreated in the same flow at 120 °C for 20 min to avoid the influence of water in the mixture. The catalytic performance of the catalysts was evaluated and compared by the values of Tig (soot ignition temperature) and Tmax (the temperature at which the oxidation rate of the soot achieves maximum) measured on TGA.

2.5. Soot combustion activation energy The apparent activation energy of the soot combustion can be calculated by the Ozawa method from the TGA data using the following equation25: log(Φi)=B-0.4567(Ea/RTα,i) where Tα,i is the temperature to achieve α% soot conversion, Ea is the apparent activation energy, B is a constant value with regard to the reaction, R is ideal gas constant 8.3145 J/(mol·K) and Φi is the heating rate (10, 20 , and 30 oC/min heating rate were used in the experiment).

3. Results and discussion The limited contact area between soot and active site is a key restrictive factor for the present solid (reactant)-solid (catalyst) reaction, as soot (particle size > 25 nm) is difficult to enter the inner pores of general catalysts. Having a high amount of tangible active points between the soot and the active centers on the catalyst is vital for efficient soot oxidation. Thus, it is proposed that the pore diameter of the catalyst should be > 25 nm and with a wide distribution of pore sizes so as to increase the permeation of the different-sized soot particles 7


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and at the same time optimize the utilization of textural pores of the catalyst as well. In the present work, macroporous SiO2 used as a catalyst carrier is synthesized via a sol-gel method with Na2SiO3 as the silicon source. As no template is used, the synthesized SiO2 could have a generally random distribution of pore sizes. The porosity of SiO2 is controlled and modulated through careful adjustment of the preparation conditions so as to obtain a desirable porous texture for use in soot oxidation.

3.1 Modulating the porosity of SiO2 material 3.1.1 Controlling the kinetics of hydrolysis and condensation of the silicon source Figure 1 displays the isotherm curves and pore diameter distribution of the synthesized SiO2 via different adding sequence of the reactants. The SiO2 is labeled as SiO2-7-60, when Na2SiO3 solution was dropped into the HNO3 solution until the pH of the solution was 7. The mixture was in acidic conditions throughout the synthesis process. Correspondingly, the SiO2 is labeled as SiO2-H-7-60, when HNO3 solution was added into the Na2SiO3 solution until the pH of the mixture was 7. The solution was in basic conditions until the mixture is neutral. Figure 1(A) shows that the isotherm of N2 adsorption-desorption for SiO2-H-7-60 correspond to the isotherm of type IV, indicating its mesoporous structure. The isotherm for SiO2-7-60 indicates that the sample was mainly of macropores. Figure 1(B) displays that the pores in SiO2-7-60 are widely distributed, primarily in the range of 20-120 nm. Therefore, the different adding sequences of reactants exert a striking influence on the porous structure of the final products. The reason lies in the different condensation and hydrolysis kinetics in the different adding sequences of reactants. Na2SiO3 reacts with acid to give Si(OH)4, which then converts 8



to silica gel and forms linear branched chains under acidic conditions. Whereas in a basic environment, net-like structure of SiO2 easily forms due to the much easier cross-linking of silica colloidal particles in basic medium26-27. The as-produced SiO2 possesses much developed pore structure due to the facts that the condensation kinetics is faster than hydrolysis28.

3.1.2 Controlling the Si-OH groups condensation via pH adjustment As the surface of silica colloidal particle is almost completely covered by Si-OH group29-30, dehydration condensation is easily occurred between the neighboring Si-OH groups in the aging process, resulting in the excessive cross-linking of silica gel, as shown in Figure 2. H+ could catalyze the dehydration condensation of the Si-OH groups. This phenomenon could lead to contraction of SiO2 skeleton and further reduction of pore diameter. Thus, it is reasonable to control and optimize the porous structure of the SiO2 support for soot oxidation through the adjustment of the pH value during the gel aging process. Figure 3 displays the textural properties of the SiO2 support synthesized via different pH conditions. The N2 adsorption-desorption isotherm curves (Figure 3A) shows that with increasing the pH value of the aging gel, the porosity of the as-prepared SiO2 evolves from mesoporous to meso-macroporous to mainly macroporous. When the aging pH is 1, the isotherm of the as-prepared SiO2 corresponds to type IV, indicating its mesoporous structure. The pore-size peak is observed at approximately 7.0 nm (mesopore, between 2 and 20 nm). Increasing the aging pH value from 1 to 3.5, the porosity of the as-synthesized material transits from mesoporous to meso-macroporous. The pore size of SiO2 was distributed widely, ranging

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approximately from 2.5 nm to about 100 nm. When further improving the aging pH to 5.5 and 7, the pore size further increases correspondingly, as displayed by the even steeper adsorption jump of the N2 adsorption-desorption isotherm curves. The pores of the products are dominated by macropores, with the peaks of pore sizes at approximately 50 and 110 nm, respectively. In addition, BJH pore volumes of SiO2-5.5-60 and SiO2-7-60 are much larger than SiO2-1-60 and SiO2-3.5-60, as shown in table 1. Correspondingly, the BET surface areas of these two samples are much smaller. The pore distribution of SiO2-7-60 is rather wide, mainly in the range of 20 to 120 nm, which is important for soot oxidation, as the macropores with a variety of pore sizes could lead to easy diffusion and transferability of reactants in inner pores and high contact area between the catalyst and soot.

3.1.3 Controlling the neck-growth on the basis of Kelvin equation In the gel aging process, the silica gel undergoes the dissolution-precipitation process, during which the dissolved silica transport to the neck region between the silica colloidal particles31. This is known as the neck growth process, which could lead to contraction of SiO2 skeleton and further reduction of pore diameter. The mechanism of neck growth could be explained by the Kelvin equation32, p

S

RTIn p r =RTIn S r = 0

0

2γM ρr

where p0 and S0 are the partial pressure and solubility of a flat surface in the bulk phase, respectively. T is the temperature, γ is the surface tension, M is the molar mass of the solid, ρ is the molar mass, R is the ideal gas constant, and r is the radius of curvature. According to the Kelvin equation, the solubility of silica in the neck region is lower than 10



in the bulk phase because the curvature of the neck region is negative, r < 0, which is the driving force for the neck-growth of silica particles. At the same time, the gel aging temperature, T, is also an important parameter affecting the growth of silica particles during the aging process. Reducing the aging temperature could further decrease the dissolution of silica gel. In contrary, the neck growth could be restrained with increasing the temperature, due to the higher silica solubility, as shown in Figure 4. Thus, it is feasible to modulate the structure of SiO2 via controlling the skeleton contraction and neck-growth of silica gel so as to optimize the textural porosity of the final catalyst for soot oxidation. Figure 5 shows the porosity variations of the prepared-SiO2 support aged at different temperatures varying from 30 to 120 oC. It is obvious that the aging temperature has a considerable influence on the porous texture of the SiO2 material. The pore size distribution of room-temperature-aged sample is narrow, with a small half-peak width, indicating a rather even pore size distribution of the material. Increasing the aging temperature to 60 and 80 oC, the pore size increases greatly, the amount of mesopores decreases, and macropores with pore size > 50 nm predominates. Meanwhile, the pore size distribution gets very wide when the aging temperature is ≥ 60 oC. The pore size ranges from 20 to 120 nm and from 20 to 140 nm, respectively, when the aging temperature is 60 and 80 oC. This phenomenon presents that the neck-growth between silica colloidal particles is restrained with increasing the aging temperature. However, further increasing the temperature to 120 oC, the material becomes almost non-porous, which is unfavorable for soot oxidation. Accordingly, the BJH pore volume of SiO2-7-120 also remarkably decreases compared with other samples. Meanwhile, the effect of aging duration on the porosity of SiO2-7-80 is also investigated 11


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in this work. As shown in Figure 6, appropriate extension of aging duration is beneficial to the formation

of

macropores.

With

increasing

the

aging

duration

too

long,

the

dissolution-precipitation process is stretched, which results in over sufficient neck-growth. The SEM images of the SiO2-7-60 and SiO2-1-30 catalysts are presented in Figure 7. The particle sizes of SiO2 primary particle (derived from silica colloidal particle) estimated from the SEM image are about 40–50 nm on both two catalysts. From the images of Figure 7A and 7B, SiO2 primary particle is clearly visible on the SiO2-7-60 catalyst and the surface of SiO2 is rough. From the images of Figure 7C and 7D, SiO2 primary particle is vaguely visible and the surface of SiO2 tends to be smooth. It can be indicated that the aggregation and growth of silica colloidal particle was restrained under the condition of higher pH and temperature in the process of gel aging due to the inhibition of Si-OH dehydration condensation and neck growth, whereas silica colloidal particles aggregate and grow rapidly under the condition of lower pH and temperature. The phenomenon is in accord with the results from Nitrogen adsorption-desorption isotherms.

3.2. Soot catalytic combustion evaluated via TGA 3.2.1 Commercial catalysts Figure 8 shows the DTG curves for catalytic combustion of soot as a function of reaction temperature over various commercial catalysts. As solid acid catalysts，ZSM-5 and USY Zeolites exhibit good catalytic activity in many reactions. However, these two classic catalysts have little effect on soot catalytic combustion in comparison with the result from soot combustion without catalysts. Notably, the commercial SiO2 shows better activity. This 12



phenomenon indicates that the porosity of catalyst determine catalytic activity in the reaction of soot catalytic combustion, because ZSM-5 and USY are typical microporous materials and the commercial SiO2 is a mesoporous material. The result enlightens us to further control the porosity of SiO2 through the preparation conditions so as to obtain a desirable porous texture for soot catalytic combustion.

3.2.2. SiO2 with different porous structures Figure 9(A) DTG curves for soot catalytic combustion over SiO2-H-7-60 and SiO2-7-60 as a function of reaction temperature. The Tmax values of SiO2-7-60 and SiO2-H-7-60 are 523 and 543 ◦C, respectively. The macroporous structure of SiO2-7-60 is more beneficial than the mesoporous structure of SiO2-H-7-60 for diffusion and transferability of soot. Figure 9(B) exhibits the DTG curves for catalytic combustion of soot over various SiO2-x-60 catalysts as a function of reaction temperature. Obviously, the catalytic activities of the SiO2-x-60 catalysts for soot catalytic combustion increase with increasing of the pH in catalyst synthesis process. Additionally, the Tmax values are listed in Table 1. The Tmax values of SiO2-1-60, SiO2-3.5-60, SiO2-5.5-60 and SiO2-7-60 are 616, 600, 539 and 523 oC, respectively. The porosity of the as-prepared SiO2-x-60 evolves from mesoporous to meso-macroporous to mainly macroporous when the aging pH is from 1 to 7. Consequently, SiO2-7-60 shows the best catalytic activity, due to the meso-macropores with a variety of pore sizes, which could lead to easy diffusion and transferability of reactants in inner pores and high contact area between the catalyst and soot. The catalytic activities of SiO2-7-y are shown in Figure 9(C). The results demonstrate that 13


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the variations of the aging temperature have an obvious effect on the soot oxidation. As shown in Table 1, the Tmax values of SiO2-7-30, SiO2-7-60, SiO2-7-80 and SiO2-7-120 are 532, 523, 510 and 539 oC, respectively. According to the porosity data, SiO2-7-60 and SiO2-7-80 possess macroporous structure in comparison with SiO2-7-30 and SiO2-7-120. Therefore, SiO2-7-60 and SiO2-7-80 show better catalytic activity The catalytic activities of SiO2-7-60 catalysts are shown in Figure 9(D). With the aging time increasing, the catalytic activities of the as-prepared catalysts are gradually increasing. When the aging time is 20h, the catalyst exhibits the highest catalytic activity. This result is consistent with the result of characterization.

3.2.3 macroporous SiO2 supported metal catalysts In order to further improve the catalytic activity, we introduce the active center on the macroporous SiO2-7-80 by the incipient wetness impregnation method. Figure 10 exhibits the DTG curves for soot oxidation over various supported catalysts as a function of reaction temperature. The Tig and Tmax values for Ag/SiO2, K/SiO2 and Cu/SiO2 were lower than those for SiO2. Among these catalysts, the Ag/SiO2 catalyst showed the best catalytic activity. When the loading amount of Ag is increased from 5% to 10%, the Tmax and Tig values are decreased from 448 to 437 oC and from 321 to 286 oC, respectively. The results indicated that the catalytic activity for soot oxidation is much improved by the deposition of Ag particles on SiO2, which is in agreement with references22. Meanwhile, the activation energy is calculated using the Ozawa method from the TGA data. The activation energies of soot non-catalytic oxidation and soot catalytic oxidation over 5%Ag/SiO2 are evaluated at 50% soot conversion 14



under the TC mode. The activation energy is decreased from 160.2 to 114.1 kJ mol−1, respectively. Thus, the catalysis efficiency is significantly enhanced.

4. Conclusions The macroporous SiO2 could be synthesized through a very simple and economic strategy. The porosity of SiO2 is modulated through controlling the Si-OH dehydration condensation and neck growth. Among all the as-synthesized SiO2, SiO2-7-80 exhibits largest pore structure and the pore size ranges from 20 to 140 nm. Meanwhile, SiO2-7-80 shows the highest catalytic activity for soot combustion due to the easy diffusion and transferability of reactants in inner pores. Furthermore, the macroporous SiO2-7-80 supported Ag catalysts exhibit superior soot oxidation activity. Consequently, the results show the important potential of macroporous SiO2 used as an effective catalyst carrier for soot oxidation in industrial

application.

Acknowledgements Financial support from the Zhejiang Province Nature Science Foundation of China (Grant LQ17B060006) is greatly acknowledged.

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Table 1. The BET specific surface area, BJH pore volume, and Tmax value of SiO2 Sample Sample SiO2- x-60 SiO2- 1-60 SiO2- 3.5-60 SiO2- 5-60 SiO2- 7-60 Sample SiO2- 7-y SiO2- 7-30 SiO2- 7-60 SiO2- 7-80 SiO2- 7-120

BET surface area (m²/g)

BJH pore volume (cm³/g)

Tmax (oC)

622.1 435.1 180.9 149.6

0.51 0.87 1.12 1.08

616 600 539 523

168.1 149.6 144.1 65.5

1.08 1.09 0.97 0.23

532 523 510 539

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Figure 1. (A) Isotherm curves and (B) pore diameter distribution of the synthesized SiO2 via different adding sequence of the reactants.

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Figure 2. Effect of H+ on dehydration condensation between the neighboring Si-OH groups in the aging process

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Figure 3. (A) Isotherm curves and (B) pore diameter distribution of the synthesized SiO2 via different pH conditions.

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Figure 4. Effect of temperature on the neck growth between silica colloidal particles

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Figure 5. (A) Isotherm curves and (B) pore diameter distribution of the synthesized SiO2 via different temperatures.

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Figure 6. (A) Isotherm curves and (B) pore diameter distribution of the synthesized SiO2-7-80 via different aging duration.

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Figure 7. SEM images of the (A) (B) SiO2-7-60 and (C) (D) SiO2-1-30 catalysts

Figure 8. DTG curves for soot combustion as a function of reaction temperature over various commercial catalysts.

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Figure 9. DTG curves for soot combustion as a function of reaction temperature over various SiO2.

Figure 10. DTG curves for soot combustion as a function of reaction temperature over various SiO2-7-80 supported catalysts. 26


Template-free synthesis of macroporous SiO2 catalyst supports for

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