Photocatalysis by morphologically tailored mesoporous silica (SBA-15) embedded with SnO2 nanoparticles: Experiments and model

doi:10.1016/j.apcata.2015.03.015

Applied Catalysis A: General

Volume 498, 5 June 2015, Pages 107-116

https://doi.org/10.1016/j.apcata.2015.03.015 Get rights and content

Highlights

•
Different morphologies of SBA-15 particles were synthesized by the same method.
•
Sphere-like SBA-15 with SnO₂ nanoparticles showed the highest photocatalysis.
•
New pore-level model developed has identified best host for catalysis reactions.

Abstract

Commonly obtained SBA-15 particles (fiber-like morphology), if used as a catalyst-host, has limitations in diffusion of reactant molecules (e.g. of a dye) inside the pores of the SBA-15 particle. The systematic synthetic approach developed here provides an easy control of SBA-15 particle morphology over a wide range (fiber, rod, and sphere), by only varying the HCl concentration. To assess its effect on molecular diffusion and reaction inside pores, SnO₂ nanoparticles of 3.5 nm diameter were synthesized in situ, inside the 6.3 nm diameter pores of SBA-15. These hybrids were tested for photocatalytic degradation of rhodamine B dye. Sphere-like morphology of SBA-15 with SnO₂ nanoparticle loading of 17.7 wt% showed the highest first-order degradation rate constant of 0.54 h⁻¹, compared to other morphologies (rod—0.51 h⁻¹, fiber—0.33 h⁻¹). On incorporating rates of diffusion, adsorption, and degradation-reaction of the dye in a single pore-level mathematical model of the SBA-15 particle, we have predicted the experimentally measured temporal variation of dye concentration for different SBA-15 morphologies and SnO₂-catalyst loadings. Therefore, the present work identifies the best SBA-15 particle morphology (sphere-like) for such reactions and provides a validated model to optimize such coupled problems of chemical transport and reaction.

Graphical abstract

Introduction

Water pollution caused by organic dyes is a serious global problem [1], [2]. Discharge of effluents with dyes is undesirable, not only in terms of the persisting nature of color, but also harmful to human beings because some of the dyes are reported to be toxic, carcinogenic, and mutagenic [3], [4], [5]. Most common techniques to remove dyes from wastewater are adsorption on activated carbon, reverse osmosis, coagulation by chemical agents, and ultrafiltration [6], [7], [8], [9], [10], [11]. However, these techniques transfer organic dyes to intermediate products, and do not completely convert to harmless substances [5], [12], [13]. So, it is necessary to develop a method for complete degradation of such compounds. Photocatalysis by semiconductor nanoparticles upon UV-ray irradiation involves generation of holes and electrons, and their migration to the external surface of the particle. Subsequently, these react on the particle-surface with contaminants. Photocatalytic performance of these semiconductor particles is strongly dependent on their size, location, and available specific surface area. Semiconductor particles such as TiO₂, SnO₂, ZnO, and WO₃ have been identified as potential photocatalysts in removal of organic contaminants, synthetic dyes, etc. However, these semiconductor nanoparticles when used directly in a solution without any catalyst-host tend to agglomerate, resulting in reduction in photocatalytic activity [13]. To overcome these difficulties, nanoparticles can be embedded within the conventionally synthesized, fiber-like mesoporous silica (SBA-15) host-particles, so as to increase the effective utilization of nanoparticle surface [14]. For example, Pt metal nanoparticles were embedded inside the pores of SBA-15 to increase the catalytic activity for CO oxidation [15]. Similarly, semiconductor nanoparticles (SnO₂, TiO₂, etc.) were embedded within the porous host-particles (zeolite, SBA-15, activated carbon, etc.), for a potentially higher photocatalytic activity against an organic dye [16], [17].

Among the various ordered mesoporous silica materials, SBA-15 has a high thermal and hydrothermal stability due to its large wall-thickness [18]. It is the most preferable host for loading various guest species. The morphology of conventionally synthesized SBA-15 is a fiber-like particle, which consists of many micron-sized rods aligned unidirectionally, having a few tens of micrometers length and a few micrometers diameter. This however, results in a large diffusion-path length for any reactant, like a dye-molecule, impacting the overall degradation rate.

Therefore, it is necessary to decrease the external particle size to shorten the diffusion path-length, requiring morphological changes during SBA-15 synthesis. There have been some investigations to understand how mass transport efficiency changes with various SBA-15 morphologies and pore diameters [19], [20]. Since controlling parameters for particle morphology and size are related to surface energy and cooperative organization between the block co-polymer micellar-template and the inorganic precursor, many different synthetic strategies for SBA-15 have been attempted in the literature. These are, addition of co-solvent and salt, variation of synthesis temperature and stirring rate, etc., which however, result in non-uniformity of pore-size distribution, and uncontrolled particle size and shape. Since the structural attributes are very critical for high adsorption, diffusion, and reaction rates, the above are not preferred routes for uniform uptake and release of molecules [21], [22].

Considering all the problems in loosing structural order due to the use of additives and alteration of parameters during the synthesis of SBA-15, the present study for the first time aims to demonstrate continuous morphological transition of SBA-15 from fiber to rod to sphere, which could be achieved by systematic tuning of only one variable, namely HCl concentration. All the other reactant concentrations are kept constant, without the addition of any external agent. Following that, in order to assess performance of these morphologically tailored SBA-15 hybrids on molecular diffusion and reaction, SnO₂ nanoparticles with a mean diameter of 3.5 nm were synthesized and embedded in three different SBA-15 morphologies (fiber, rod, and sphere) of pore diameter 6.3 nm. The focus is to study their photocatalytic activity against rhodamine B dye degradation. Finally, the present work also attempts a single pore-level mathematical model, incorporating diffusion, adsorption, and reaction leading to dye degradation. The overall objective is to predict our own experimental dye degradation data and suggest the best possible morphology of SBA-15, for such applications in water treatment.

Section snippets

Preparation of different morphologies of SBA-15

SBA-15 particles in the form of fiber were synthesized with a slight modification in the molar ratio of reactants [17]. 1 g of Pluronic 123 was dissolved in 145 ml of distilled water at 35 °C, followed by the addition of 15 ml HCl (35%). 3 g of TEOS was added to the above mixture and stirred at 200 rpm for 24 h. Then, the above mixture was transferred to a Teflon-coated closed vessel and heated at 100 °C for 24 h, without stirring. The solid product was filtered, washed thoroughly with water and

Mathematical Model

To understand the importance of diffusion and reaction in a SBA-15 particle of any morphology, a single pore-level model has been developed in this work. The system modeled is schematically depicted in Fig. 1. Experimentally, SnO₂-loaded SBA-15 particles were dispersed in rhodamine B dye solution with continuous stirring. Rhodamine B dye from the bulk, aqueous solution diffuses into the pores, adsorbs on the pore-interior, and reacts with embedded SnO₂ nanoparticles.

As shown in Fig. 1b, regular

Morphological transition in SBA-15 particles

The effect of HCl concentration (P123: HCl: H₂O: TEOS = 1: x: 50,000: 83.5, x = 835–2035) on SBA-15 particle morphology was investigated by using SEM and TEM images, as shown in Fig. 2. Fig. 2a shows the SEM image of sample S1 (x = 835), exhibiting fiber-like SBA-15 particles with a broad distribution of particle size. SBA-15 particles of fiber morphology have a mean length of 41 μm with a standard deviation of 12 μm, and a mean diameter of 3.4 μm with a standard deviation of 1.5 μm.

During synthesis, the

Conclusions

In summary, SBA-15 particles with different morphologies like fiber, rod, and sphere were synthesized by systematically increasing the concentration of only HCl. Tuning of HCl concentration in the synthesis very easily allowed us to have a wide range of particle shape and size, from tens of microns long SBA-15 (fiber) to submicron length scales (sphere). Subsequently, SnO₂ nanoparticles of controlled small size (of 3.5 nm) were synthesized in situ and embedded inside the bigger pores of SBA-15

Acknowledgements

Dr. Anees Y. Khan, Chemical Engineering Department, IIT Bombay, is gratefully acknowledged for many helpful suggestions on morphological changes in SBA-15. Seed research grant from IRCC, IIT Bombay and from Department of Science and Technology, Government of India, are gratefully acknowledged for financial support.

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Photocatalysis by morphologically tailored mesoporous silica (SBA-15) embedded with SnO2 nanoparticles: Experiments and model

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Preparation of different morphologies of SBA-15

Mathematical Model

Morphological transition in SBA-15 particles

Conclusions

Acknowledgements

Water Res.

Appl. Catal. A

Process Biochem.

Mutat. Res.

Chem. Eng. J.

Desalination

Desalination

J. Membr. Sci.

J. Hazard. Mater.

Desalination

Microporous Mesoporous Mater.

Appl. Catal. A

Microporous Mesoporous Mater.

Microporous Mesoporous Mater.

Microporous Mesoporous Mater.

Appl. Surf. Sci.

Catal. Commun.

J. Hazard. Mater.

Photocatalysis by morphologically tailored mesoporous silica (SBA-15) embedded with SnO₂ nanoparticles: Experiments and model