Degradation of model pollutants in gas phase by plasmonic catalysis on gold nanoparticles
The use of plasmonic nanoparticles for photocatalysis allows simple and cost-effective exhaust air purification. Nanoparticle-coated porous glass plates are used for this purpose. Nanoparticle-based catalytic degradation of model pollutants has been successfully demonstrated. The gold nanoparticles are activated with light in the visible wavelength range (sunlight), making cost-intensive tempering obsolete.
By L. Stolle // F. Garwe // R. Müller // T. Krech // B. Oberleiter // T. Rainer // W. Fritzsche // A. Stolle
Plasmonically-active metal nanoparticles, especially gold nanoparticles, are interesting materials for numerous applications, such as for use in plasmon-based sensors, surface-enhanced Raman spectroscopy (SERS), and as active centers in catalysis. They offer numerous advantages over other catalysts, especially in the latter field of research. Plasmonically-induced catalysis is a photocatalytic process. Thus, high energy input is not necessary to reach or maintain temperatures for maximum conversion, as is necessary with thermally-activated catalysts. However, it is possible to achieve this particularly efficiently when sunlight is used as an energy source. The result is a “switchable” catalyst that can be activated instantaneously by irradiation with resonant wavelengths or deactivated by removal of the light source, depending on its use. One advantage over photocatalysts such as TiO2or ZnO2is the absorption that occurs at wavelengths in the visible (VIS) range. Depending on the size and shape of the gold nanoparticles, wide ranges of the light spectrum can be used for excitation.
How can gold nanoparticles be used for plasmon catalysis? Project No. 6 of the „Wachstumskern“ PADES (FKZ 03WKCN06) dealt with this question and others like it. PADES – which refers to Particle Design Thuringia – is an association of regional research institutions and companies that are dedicated to the topics of particle design and application in several joint projects funded by the Federal Ministry of Education and Research (BMBF). In this project (6), four partners from industry and research have come together to develop metal nanoparticles for plasmonically-induced exhaust air purification and test them in model systems. The metal nanoparticles were to be applied to porous glass, if possible, in order to increase the catalyst surface and thus be able to apply large numbers of nanoparticles as active centers. Leibniz-IPHT was entrusted with the synthesis and characterization of the nanoscale catalysts, HEGLA-Boraident from Halle/Saale was entrusted with the production of the porous glass and carrier plates for the catalysts, the Institute for Technical and Environmental Chemistry (ITUC) of the Friedrich Schiller University of Jena was entrusted with testing the catalysts obtained in a laboratory reactor using the model reaction of the plasmonically-induced degradation of ethanol, and Jenoptik Automatisierungstechnik GmbH Jena was entrusted primarily with the simulation of the test system and also catalytic testing. The first step was the development of a laboratory test system and a photoreactor based on the use of gold nanoparticles on a 15×15 cm glass support coated with porous glass and nanoparticles in an area of 10×10 cm. It was recognized that the designed system could achieve good utilization of the incoming light of the xenon arc lamp used since its beam path directly hits the catalyst plate. However, it was challenging to evenly coat such a large surface with gold nanoparticles, especially because the porous glass is an uneven substrate. In addition, numerous steps are necessary to obtain such catalyst plates, including the following: coating a suitable glass plate with porous glass, synthesizing the gold nanoparticles in the desired size, activating the porous glass to firmly bind the nanoparticles, and carefully coating the glass with the nanoparticles. Taken together, these processes require a lot of time and materials since different chemicals, especially a large volume of gold particle solution, are required.
In order to simplify the production of the catalyst by reducing the number of steps and, subsequently, the material required, porous glass with the required amount of HAuCl4was ground to a fine powder at low power in a vibrating ball mill and applied as a catalyst precursor to any glass plate by laser sintering. Laser irradiation is supposed to generate particles from the metal salt. Initially, feasibility studies of this process were carried out on the surface of the slides. The sintered metal-glass layers exhibited good homogeneity, as well as uniform, intensive wine-red coloration. This indicates the formation of gold nanoparticles. After these successful tests, several of the 15×15 cm plates required for the test reactor were able to be coated very uniformly using the same process (see Figure 1). It was found that the initial porosity of the porous glasses used (1-2, 50, 100, 200, and 370 nm) could not be maintained and that pores in the µm range were formed. However, the essential project of particle formation was fulfilled. Applying different methods of analysis (UV/VIS, XRD, SEM), the formation of gold nanospheres with a diameter of 30 nm and a high particle density on the surface of the catalyst plates was able to be proven.
After testing the catalyst plates produced in this way using the degradation reaction of the model contaminant ethanol, it was possible to prove that the plates are catalytically active. A conversion from ethanol to CO2of up to 30% is achieved before a degree of stable degradation of approx. 5% is achieved, combined with the formation of 2% of CO2(see Figure 2). Full conversion was not observed under the selected process conditions and catalysts used.
It was possible to develop a fast and effective method of producing catalysts for plasmon catalysis in the gas phase. In addition, this method is extremely inexpensive if one considers the cost factor of gold chloric acid, which is in the cent range here (€ 0.15-0.76). The catalytic activity was sufficiently high.
Funded by: BMBF
Object slides coated with gold particles (top left) and 15×15cm glass plates (right), as well as REM image of the particles (white) on the glass surface (bottom left).