Elsevier

Journal of Catalysis

Volume 230, Issue 1, 15 February 2005, Pages 158-165
Journal of Catalysis

Visible-light photooxidation of trichloroethylene by Cr–Al-MCM-41

https://doi.org/10.1016/j.jcat.2004.11.044Get rights and content

Abstract

In the present study, the photooxidation of gaseous trichloroethylene (TCE) was performed on Cr–Al-MCM-41 (predominantly a SiO2 framework) under visible light and UV light. This mesoporous Cr-containing MCM-41 material is a promising photocatalyst and exhibits excellent photoactivity. Moreover, the products of photooxidation under visible light are similar to those under UV-light illumination. The photocatalysis process occurs on isolated Cr6+ ions supported on a predominantly SiO2 framework. A possible mechanism leading to the photooxidation of TCE together with the detection of intermediates by gas chromatography-mass spectrometry is reported here.

Introduction

The increasing contamination of land, water, and air has raised serious environmental problems [1], [2], [3], [4], [5], [6], [7], [8], [9]. To reduce the impact of environmental pollution, many efforts have been made in green chemistry and purification technologies. Heterogeneous photocatalytic oxidation is a promising technology for the degradation of volatile organic compounds. The photocatalytic degradation of organic pollutants with the use of TiO2 has been demonstrated to be successful in various remediation systems of polluted water and air [1], [3], [4], [5], [6]. It is well known that in the presence of atmospheric oxygen and at room temperature, chlorinated molecules undergo progressive oxidation to complete mineralization, resulting in the formation of CO2, H2O, and HCl. Although TiO2 is very popular as a photocatalyst, it suffers from a lack of visible-light absorption. A chemical process for the degradation of chlorinated organics should yield innocuous products such as CO2 and HCl that are expected from complete mineralization. There has been active interest in the photooxidation of trichloroethylene (TCE) with the use of TiO2 [10], [11], [12], [13], [14], [15]. It is the most common and abundant pollutant in groundwater in the United States. Previous studies [11], [12], [13] have shown that a significant amounts of toxic by-products were formed in the photooxidation of TCE with the use of TiO2 both in aqueous solution and in the gas phase. The photocatalytic oxidation kinetics of trichloroethylene over titania exhibits complex dependences of the reaction rate on TCE, O2, and H2O partial pressure [16]. O2 is an electron-trapping molecule and was found to be indispensable for photooxidation reactions [1], [17], [18], [19]. Studies have suggested that the role of O2 may not be limited to electron trapping [1], [4], [17], [18], [19], [20], [21].

The photocatalytic degradation of TCE vapor has been investigated with various gas mixtures, and several reactor configurations have been used near ambient temperatures and pressure [22], [23], [24], [25], [26]. It was experimentally observed that TCE could be oxidized to CO2 and HCl directly in a photolytic way. However, Li et al. [27] indicated that ultraviolet light was necessary for TCE degradation. In studies of TCE oxidation on TiO2 in aqueous solution, Pruden and Ollis [28] suggested that hydroxyl radicals initiated the reaction and that dichloroacetaldehyde (DCAAD) is formed as an intermediate. Glaze et al. [13] argued that in addition to the OH. initiated oxidative reaction channels, a parallel reductive pathway involving conduction-band electrons of TiO2 also plays an important role. In studies of TiO2 photocatalyzed gaseous TCE oxidation, Anderson et al. [29] proposed a mechanism involving OH. and a monochloroacetate as an intermediate. Phillips and Raupp [10] suggested that OH. or hydroperoxide radical initiated the reaction that involved DCAAD as an intermediate. In contrast, Nimlos et al. [11], [30] suggested that TCE is oxidized in a chain reaction initiated by chlorine atoms. Thus, various mechanisms are currently proposed for TCE photooxidation on TiO2 surfaces.

Transition-metal-containing mesoporous silicates MCM-41 and aluminosilicates Al-MCM-41 with high surface area are applied in the field of catalysis and have recently been used as photocatalysts. The choice of Al-MCM-41 as the support in the present study was based on the fact that Al-MCM-41 exhibits higher hydrothermal stability compared with pure siliceous MCM-41. We are also interested in the use of these Al-MCM-41 supports as photocatalysts for the remediation of dyes and surfactants (reactions to be carried out in aqueous medium), and hence Al-MCM-41 was chosen as the support. In addition, the incorporation of Al into the framework of MCM-41 generally improves the acidity and ion-exchange capacity of MCM-41, which are crucial properties for catalysts. The use of Cr–Al-MCM-41 as a photocatalyst has already been reported for the decomposition of acetaldehyde under both visible light and UV irradiation [31]. In the present work, the photooxidation of gaseous trichloroethylene on Cr–Al-MCM-41 has been carried out under both visible light and UV radiation. Cr–Al-MCM-41 shows excellent photoactivity under UV-light illumination, but most importantly it also exhibits photoactivity in the visible region, thus making it a more versatile photocatalyst. We observe that under visible-light irradiation, the products are carbon dioxide and phosgene. A possible mechanism leading to the photooxidation of TCE together with the detection of intermediates by GC-MS is reported here. The main objective of this study is to emphasize the importance of our photocatalyst, which is effective for the degradation of volatile chlorinated organics under visible-light irradiation.

Section snippets

Synthesis

Commercial cetyltrimethylammonium bromide (CTAB) (Alfa Aesar), tetraethoxy ortho silicate (Aldrich), Cr(NO3)3 (Alfa Aesar), Al(NO3)3 (Alfa Aesar), diethylamine (Aldrich), and NH3 (25 wt%; Fisher) were used as received for Cr–Al-MCM-41 synthesis. Trichloroethylene (Aldrich) was used as received for photocatalytic measurements. Cr–Al-MCM-41 materials with different chromium content (Si/Cr=100, 80, 40, 20) were prepared by a co-assembly route with a procedure reported earlier [31]. For

XRD and N2 adsorption studies

A series of Cr–Al-MCM-41 with varying concentrations of Cr3+ was prepared by a co-assembly route [31]; these had surface areas greater than 1000 m2/g. XRD patterns of the calcined Cr–Al-MCM-41 (Si/Cr=100, 80, 40, and 20) (Fig. 1) are consistent with earlier reports [32], [33], [34] indicating a hexagonal structure for these materials. As the concentration of chromium ion in the mesoporous material was increased, there was a slight decrease in the crystallinity. For Cr–Al-MCM-41 (Si/Cr=20),

Possible mechanism for product formation

One may speculate about the possible mechanisms underlying the formation of detected products. Unlike TiO2, where the photochemistry is well explored for the oxidation of chlorinated volatile organics, the mechanism related to the use of Cr–Al-MCM-41 remains challenging and unexplored.

The DRS results, as discussed earlier, indicate that the chromium ions are highly dispersed and exist in an isolated state on the MCM-41 support. The absence of any peaks corresponding to Cr2O3 in Cr–Al-MCM-41 at

Conclusions

Photocatalytic oxidation of trichloroethylene over Cr–Al-MCM-41 yields complete mineralization products under UV and visible-light irradiation. The large surface area of this material is populated by the surface hydroxyl groups that participate in the oxidation step. The intermediate species dichloroacetyl chloride undergoes further oxidation to phosgene, carbon dioxide, and HCl. With prolonged illumination phosgene is further oxidized to CO2 and HCl. Under visible light the Cr6+/Cr5+ species

Acknowledgments

This work was supported by a MURI project (DAAD 19-01-10619) from the U.S. Army Research Office. We thank Dr. Alexander Bedilo for ESR measurements and Dr. Alexander Volodin for useful discussions.

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