Highly concentrated toluene decomposition on the dielectric barrier discharge (DBD) plasma–photocatalytic hybrid system with Mn-Ti-incorporated mesoporous silicate photocatalyst (Mn-Ti-MPS)

doi:10.1016/j.apsusc.2005.12.103

Applied Surface Science

Volume 253, Issue 2, 15 November 2006, Pages 535-542

https://doi.org/10.1016/j.apsusc.2005.12.103 Get rights and content

Abstract

This study investigates the Mn-Ti-incorporated mesoporous silicate (Mn-Ti-MPS) as a photocatalyst for highly concentrated toluene removal in a plasma–photocatalytic hybrid system. Various Mn-Ti-MPS [Ti/Si molar ratio = 1/4, Mn/Ti molar ratio = 0.01/1 (1 mol%), 0.05/1 (5 mol%) and 0.1/1 (10 mol%)] photocatalysts were successfully synthesized using a common hydrothermal method without causing any structural damage. In the X-ray diffraction (XRD) pattern, the main peaks of the TiO₂ anatase structure and MnO did not show. All samples displayed hexagonal specific peaks at 2.5° (d_1 0 0 plane), 4.1° (d_1 1 0 plane) and 4.7° (d_2 0 0 plane). This indicates that the Ti ions and Mn ions were well substituted into the Si ion sites in the framework of MCM-41. Their surface areas decreased compared with that of pure MCM-41, while the hexagonal straight pore size was distributed in a range of 2.5–3.5 nm. In the Mn-Ti-MPS, much more water and toluene molecules were absorbed compared to the Ti-MPS. From the X-ray photoelectron spectroscopy (XPS) result, it was determined that the hydrophilicity of the Mn-Ti-MPS was stronger than that of the Ti-MPS. Photocatalytic decomposition for highly concentrated toluene of 1000 ppm increased in the Mn-Ti-MPS when compared with the Ti-MPS, while toluene decomposition on 5 mol% Mn-Ti-MPS was remarkably enhanced to 80% in the plasma system. The conversion to CO₂, however, did not improve in the case of the plasma-only system. Nonetheless, in the plasma–photocatalytic hybrid system, the conversion to CO₂ for 5 mol% Mn-Ti-MPS reached 43% (in an 800 ppm toluene conversion).

Introduction

The world now faces a tremendous challenge regarding various environmental problems. As a result, extensive research is being done on advanced chemical, biochemical and physicochemical methods of elimination of hazardous chemical compounds from air and water. Many works have been done on the photocatalytic treatment of environmental pollutants using semiconductors like TiO₂ and metal/TiO₂ [1], [2], [3], [4], [5]. However, the application has been limited to industrial use because of poor performance. Furthermore, the anatase structural limitation has been suggested, so photocatalysis is moving toward the introduction of new structural titanium frameworks.

Microporous or mesoporous molecular sieves are used as catalysts and adsorbents in many chemical and petrochemical processes. Understanding the adsorption and diffusion of molecules within the pores of molecular sieves is important in achieving a more efficient use of and designing new applications for these materials. Hydrocarbon traps employed in many industrial processes [6], [7], [8] use zeolites where both the adsorption and diffusion of molecules play an important role. Therefore, many zeolite–catalyst configurations have been proposed. The general finding is that while the heavier exhaust HCs (aromatics) were adequately trapped by the zeolites, the light HC components of the exhaust stream were often desorbed from the HC trap before the catalyst reached a high-enough temperature for combustion to occur.

Most recently, there has been an increasing interest in semiconductor-loaded zeolites and mesoporous molecular sieves as potential photocatalysts owing to their unique pore structure and adsorption properties [9], [10], [11], [12], [13]. Titania has proved to be the most active photocatalytic semiconductor because it allows complete degradation of pollutants under ultraviolet irradiation. Some researchers studied the photodegradation of several organic compounds on various TiO₂-loaded zeolites, as well as molecular sieves of MCM-41 types. The Ti-loaded meso (nano) porous support in photocatalysis offered: (1) formation of ultrafine titania particles during sol–gel deposition; (2) increased adsorption especially for nonpolar compounds; (3) higher acidity that enhances electron abstraction; (4) less UV light scattering, since silica is the main component of zeolite. The combination of the effect of zeolites and TiO₂ in the photocatalytic destruction of aromatic compounds in an aqueous system was studied. However, the advantage of using a regular structure like that of zeolite in the gaseous system was not clearly identified. In addition, as a loading method for Ti on support, the impregnation was almost used up in these studies and the loading amounts were also very small.

On the other hand, extensive research has also been performed to develop a more effective and low-cost method of VOC decomposition. In recent years, several papers have established that these problems can be overcome by using discharge plasma as the driving light source for photocatalysis [14], [15]. Strong plasma is likely to elevate the excited rate of the electron on the surface of the TiO₂ catalyst, compared with UV irradiation (Fig. 1). Fig. 1 confirms the emission of the UV region energy, which is similar to that of a UV lamp, with a corresponding 3–4 eV. It is believed that photocatalysis is possible using plasma as a light source. Consequently, VOC decomposition can be enhanced. This system, however, continues to encounter some problems. One important issue in using the plasma process for removing VOC involves the treatment of inorganic and organic by-products that are emitted in the process.

The main objective of this study is to enhance the highly concentrated toluene (1000 ppm) decomposition activity in its gas phase using dielectric barrier discharge (DBD) plasma. In this study, the formation of secondary products was suppressed and the conversion to CO₂ to combine the photocatalytic system with the plasma system was enhanced. In this study, Mn (5 and 10 mol%)-Ti (25 mol%) incorporated into mesoporous silicates (Mn-Ti-MPS) were prepared using a common hydrothermal method, and their adsorption ability and photocatalytic decomposition for macromolecules (toluene) were investigated.

Section snippets

Catalyst preparation

Fig. 2 shows the general preparation method of Mn-Ti-incorporated mesoporous silicate. The hydrothermal method, which was already reported in many papers [9], [10], [11], [12], [13], was considered in this study. Ludox HS-40 colloidal silica (40 wt% SiO₂, Dupont), titanium tetra-isopropoxide (TTIP, 99.99%, Junsei Co.), KMnO₄ (99.9%, Junsei Co.) were added as silica, titanium and manganese sources, respectively, into the solution of sodium hydroxide (NaOH, 99.9%, Aldrich) and distilled water.

Characterization

Fig. 4 shows the XRD patterns of Mn- or Ti-incorporated mesoporous silicates (Ti-MPS, Mn-Ti-MPS). The Ti amount was 25 mol% per total silicon amount, while Mn was incorporated to 5 and 10 mol% into the Ti-MPS. All samples displayed hexagonal specific peaks at 2.5° (d_1 0 0 plane), 4.1° (d_1 1 0 plane) and 4.7° (d_2 0 0 plane). This indicates that the Ti and Mn ions were well substituted into the Si ion sites in the framework of MCM-41. The main peaks of the TiO₂ anatase structure and MnO did not show.

Conclusion

1.
Various Mn-Ti-MPS [Ti/Si molar ratio = 1/4, Mn/Ti molar ratio = 0.01/1 (1 mol%), 0.05/1 (5 mol%) and 0.1/1 (10 mol%)] photocatalysts were successfully synthesized using a common hydrothermal method without causing any structural damage.
2.
In the XRD pattern, the main peaks of the

Acknowledgement

This work was supported by Korea Research Foundation (KRF-2003-D00014). The authors are very grateful to it.

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Highly concentrated toluene decomposition on the dielectric barrier discharge (DBD) plasma–photocatalytic hybrid system with Mn-Ti-incorporated mesoporous silicate photocatalyst (Mn-Ti-MPS)

Abstract

Introduction

Section snippets

Catalyst preparation

Characterization

Conclusion

Acknowledgement

J. Photochem. Photobiol. A: Chem.

J. Mol. Catal. A: Chem.

Appl. Catal. B: Environ.

Appl. Catal. A: Gen.

Catal. Today

Stud. Surf. Sci. Catal.

Microporous Macroporous Mater.

J. Catal.

Catal. Today

Catal. Today