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Electrochemical promotion of environmental catalysis

Final Activity Report Summary - EPOCENVIR (Electrochemical promotion of environmental catalysis)

From the end of the 20th century, the quality of breathable air is of ever-increasing concern. Even the more optimistic people today admit that pollution reaches levels which may be dangerous for health, and not only in urban zones. Air pollution is due to many factors, including the well-known greenhouse effect promoting gases (carbon dioxide, methane), particulate matter, and volatile organic compounds (VOCs). The removal of VOCs emitted from industrial and domestic processes is difficult, because they are present at trace level (some parts per million, ppm) in large air amounts.

Among these VOCs, we considered propene and toluene as representative of two important families. Of the total toluene released to the environment almost all is released to the air, mainly by the evaporation from gasoline and release through car exhausts. Regular exposure to 100 ppm can cause serious central nervous system problems.

VOCs are often eliminated by sorption (on active carbons for example) followed by incineration, with risk of harmful by-products formation. A cheap and efficient way of VOCs removing is complete catalytic oxidation. Supported Pt and Pd are efficient catalysts for such a purpose. However, due to the high cost and limited reserve of such noble metals, the quest for cheaper and more environmentally friendly catalytic materials are of ever-increasing importance for tomorrow's applications. Then, our goal was use the electropromotion of catalysis (EPOC, discovered by C.G. Vayenas and M. Stoukides in 1981) to improve the performance of as-cheap-as possible catalyst, operated at as-low-as possible temperatures.

Firstly, we considered complex oxides of the perovskite family, derived from LaCoO3 (LC). The addition of some elements such as Sr and Fe (forming LSCF), enabled us to improve the overall performance for VOCs catalytic combustion and to obtain an electronic conductivity enabling these compounds to be used as catalyst-electrodes in an EPOC system.

The more efficient LSCF samples were deposited as thin films by screen printing onto a solid electrolyte in order to be evaluated in EPOC. The optimised catalyst-electrode was very adherent to the support, electricity conducting and exhibited a quite good catalytic activity (about 20 % toluene conversion) if one considers the large amount of gas treated per second by a very small amount of catalyst (ghsv was some 10^7 h-1). Unfortunately, this optimised LSCF thin film catalyst could not be further promoted by electrochemistry, as it was previously observed for standard commercial compounds from the same family.

Then, we considered not-too-precious metals. Silver appeared as a very good candidate, because it is quite cheap (about 100 times cheaper than platinum), can be conductive in oxidative atmosphere and is active for the oxidation of hydrocarbons. Systems built from silver paint deposited on a classical solid electrolyte (YSZ) exhibited a fantastic EPOC behaviour. The toluene conversion at temperatures in the 300-350 degrees Celsius range can be multiplied by a factor of 5 or more, by applying only very small negative current (about 1 microA/cm2) to the catalyst-electrode. Moreover, the Faradaic efficiency (roughly the ratio: catalytic effect / electricity amount used) was incredibly high (about -40 000), the highest ever observed with this metal.

In order to comply with the real application, we developed an easy and cheap way to deposit the silver films, by a silver nitrate impregnation / H2 reduction method. Results obtained with such catalyst-electrodes were as good (or sometimes better) than those obtained with the commercial silver paint. Another advantage of such kind of deposits is their ability to be performed on solids with various, non regular and even badly accessible supports.

The present work has shown the ability to use electropromoted catalysis to successfully abate some aromatic VOCs such as toluene. It opens application to a wide range of VOCs to be treated with Pt, Rh or Pd - free systems, i.e. silver / YSZ / not precious metal or perovskite or few silver on perovskite / YSZ / not precious metal or perovskite assemblies, obtained by cheap processing methods.