Catalytic ceramic filters for flue gas cleaning. 1. Preparation and characterization
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This paper deals with the preparation and the characterization of ceramic porous filters whose pores were deposited with a γ-Al2O3 layer via the so-called "urea method", in order to increase their specific surface area. Once activated with a suitable catalytic principle, these filters can find a potential application in flue gas cleaning according to a combined action: mechanical particulate removal + catalytic abatement of chemical pollutants (nitrogen oxides volatile organic compounds, etc.). Both the obtained filters and the bulk γ-Al2O3 powder synthesized through the above method, were characterized from either a structural (BET surface area measurement Hg porosimetry, differential thermal analysis-thermal gravimetry analysis, X-ray diffraction, scanning electron miscroscopy (SEM) observation, gas permeation) or a catalytic viewpoint in this last context, isopropyl alcohol dehydration was chosen as a model reaction since it is directly catalyzed by γ-Al2O3 (thanks to its acidic properties) with no need of further catalytic activation. A reaction mechanism is proposed for the test reaction, based on the existence of two types of active sites (A and B). On A-sites isopropyl alcohol gives an intermediate adsorbate and decomposes provided vicinal B-sites are available for water adsorption. A kinetic rate expression is worked out on the basis of experimental runs performed on a batch-operated differential reactor. The knowledge of this rate expression and of the structural characterization parameters are of major importance for the interpretation and the modeling of the results of a pilot-plant study on the above filters, performed using the same model reaction and presented in part 2 of this series. The urea method is demonstrated to be a reliable tool to deposit a γ-Al2O3 layer all over the pore walls of the filter, markedly increasing its specific surface area. Drawbacks of the procedure employed are though the occurrence of pore blocking after a few deposition cycles and the occasional presence of cracks in the deposited layer.