Plasma-assisted catalysis total oxidation of trichloroethylene over gold nano-particles embedded in SBA-15 catalysts

https://doi.org/10.1016/j.apcatb.2007.05.030Get rights and content

Abstract

The oxidative decomposition of trichloroethylene (TCE) in dry air was investigated in non-thermal plasma at atmospheric pressure and room temperature, both in the absence and in the presence of gold containing mesoporous silica (GMS) catalysts. In the absence of catalyst, TCE removal reached 100% for average powers dissipated in the plasma above 3 W, for a TCE concentration of 430 ppmv. Carbon monoxide and carbon dioxide were the major reaction products with CO2 selectivity up to 25% and CO selectivity up to 70%. In the presence of gold containing mesoporous catalysts, the concentrations of CO and CO2 increased as compared to those obtained with plasma alone. The GMS catalysts can dissociate ozone produced in plasma to oxygen radicals that decompose TCE. Among these catalysts, the one containing the least amount of Au (0.5% GMS) showed the best catalytic performance. In the presence of ozone generated in the plasma, isolated gold cations might play a critical role for the catalytic behavior.

Introduction

Air pollution by volatile organic compounds (VOC) is an issue of major concern due to both environmental and medical reasons. Technologies for VOC removal have been recently reviewed in [1]. An attractive alternative for VOC removal from contaminated air streams is non-thermal plasma generated in electrical discharges, since it can be operated at room temperature and atmospheric pressure, over a wide range of gas flow rates and concentrations [1], [2], [3], [4], [5]. Moreover, the energy dissipated in the plasma is mostly used to accelerate the electrons and not spent on heating the entire gas stream, as in thermal or thermo-catalytic processes. The energetic electrons in the plasma are highly efficient in producing radicals and oxidizing agents, which can react with the VOC molecules decomposing them.

Various types of electrical discharges have been investigated for the oxidation of chlorinated hydrocarbons: pulsed corona discharges [5], [6], [7], [8], [9], atmospheric pressure glow discharges [10], dielectric barrier discharges [8], [9], [11], [12], [13], [14], [15], [16], dielectric packed-bed discharges [3], [4], [8], [12], [15], [17], surface discharges [13], [14], [18]. A recent review [19] summarizes non-thermal plasma techniques for the destruction of air pollutants. The authors thought that non-thermal plasma processing was one of the most hopeful technologies to remove toxic gas contaminants in air.

The main drawback of plasma activation is the relatively low selectivity towards total oxidation [6], [9], [16] and the formation of undesired by-products. The combination of non-thermal plasma and heterogeneous catalysis could overcome this drawback and improve the process selectivity and the energy efficiency of the plasma process.

For oxidative decomposition of VOC, two classes of catalysts have been reported, one being metal oxides (some of the most active ones are based on titanium, copper, cobalt, chromium and manganese [20], [21], [22], [23], [24], [25]) and the other one is metal oxide supported noble metal (Pd and Pt) catalysts [26], [27]. Generally, palladium catalysts are slightly more active, while platinum catalysts are more selective to CO2. In recent years, the unexpectedly high activity of gold as a low-temperature CO oxidation catalyst has initiated intensive research in the use of gold nano-particles as catalysts [28], [29]. Gold-based catalysts have demonstrated very interesting and promising activity, and different types of homogeneous and heterogeneous catalysts in the form of metal complexes or nano-particles have been developed for the oxidation of alcohols [30], [31], [32], [33], [34]. Generally, the catalytic properties of heterogeneous gold catalysts strongly depend on the particle size. The use of gold as a catalyst requires careful and unconventional preparation of the gold particle morphology, focusing on achieving a very small gold particle size. One of the key problematic issues that have hampered the applications of gold catalysts is the stability of the particles against sintering under reaction conditions [35]. Recently, in order to obtain highly active gold catalysts, a chemical grafting and reduction process was reported, in which amino, thiol or di-amino groups were grafted onto to the surfaces of mesoporous silica, then HAuCl4 was introduced via a neutralization reaction, followed by a reduction procedure [36], [37], [38]. This process has proven to be effective to some degree, but the gold nano-particles dispersed on the silica surface (in the channels or outside of the channels) are still problematic, since gold is mobile on the surface of silica these materials readily sinter under rigid reaction conditions. Recently, we reported a novel method to synthesize a catalytic system in which the gold nano-particles are highly dispersed and confined in the walls of mesoporous silica (GMS), not in the channels [39]. These GMS systems were fully characterized by conventional methods and demonstrated a gold nano-particle size of 3 nm. Thus, the channels will not be blocked and the gold nano-particles may not sinter under calcination or reaction conditions. To the best of our knowledge, there is not gold-based catalyst reported for the oxidative decomposition of TCE. However, there is a promise in the development of catalytic remediation of effluents containing TCE using Au-based catalysts [40], [41] Au-nano-particles covered by Pd showed a pretty good activity in hydrodechlorination of TCE.

In this work trichloroethylene (C2HCl3, TCE) was chosen as the target compound for total oxidation in a coupled plasma/heterogeneous catalytic system. Plasma remediation is very promising, since these compounds react with atomic oxygen, hydroxyl radicals, and ozone, which are effectively produced in non-thermal plasmas in air. The plasma was generated in a dielectric barrier discharge (DBD) operated in ac mode at 50 Hz frequency. Gold catalysts were placed downstream of the discharge reactor and the oxidation of TCE was compared in the plasma and in the plasma-catalytic system.

Section snippets

Catalyst preparation

Gold nano-particles confined in the walls of mesoporous silica (GMS) were synthesized by dissolving 20 g of Pluronic P123 (EO20PO70EO20, Mav = 5800, Aldrich) in 750 ml of a 2 M HCl solution, subsequently, a mixture of 41.6 g tetraethyl orthosilicate (TEOS) and 4.2 g 1,4-bis(triethoxysilyl)propane tetrasulfide were quickly added with vigorous magnetic stirring followed by the dropwise addition of the pre-designated amounts of aqueous HAuCl4 solution. The solution was stirred for 24 h at 313 K and then

Catalysts characterization

In the synthetic processes, tetraethyl orthosilicate (TEOS) and 1,4-bis (triethoxysilyl) propane tetrasulfide co-condense forming the thioether group in the framework and effectively anchoring HAuCl4. After HAuCl4 was introduced, the material was calcined to reduce the gold and remove the organic moieties yielding the highly dispersed and clear mesoporous silica supported gold nano-particles. The assembly process was investigated by diffuse reflectance infra-red Fourier transform (DRIFT)

Conclusions

The decomposition of trichloroethylene was investigated in the plasma generated in a dielectric barrier discharge, both in the absence and in the presence of Au catalysts placed downstream of the plasma reactor.

It was found that TCE removal reaches 100% for average powers dissipated in the plasma above 3 W. Carbon monoxide and carbon dioxide were the major reaction products detected from TCE oxidation in the plasma, with CO2 selectivities up to 25% and CO selectivities up to 70%. Other reaction

Acknowledgement

The work was partly financed by the Romanian Ministry for Education and Research (CEEX program). The authors also acknowledge financial support from the Swiss National Science Foundation (“SCOPES” program).

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