Preparation of ultra-active alumina of designed porous structure by successive hydrothermal and thermal treatments of porous anodic Al2O3 films

A porous anodic alumina film was prepared by the anodic oxidation of Al metal sheet in a thermostated and vigorously stirred bath of H2SO4 15% (w/v) at a temperature of 25 degrees C and a current density of 15 mA cm(-2). It had a geometric surface area of 33 cm(2), a surface density of pores congruent to 1.269 x 10(11) cm(-2) and the maximum limiting thickness and porosity achieved at these conditions which are 50.3 mu m and 0.42, respectively. This oxide was tried in the catalytic test reaction of the decomposition of HCOOH at temperatures 270-390 degrees C. Then, the oxide was treated hydrothermally in H2O at 100 degrees C for 5 h and tried in the same test reaction. The procedure of hydrothermal treatment and catalysis experiment was repeated 40 times. In all cases the oxide showed an almost exclusively dehydrative catalytic effect, congruent to 98-100%. Both the total activity of the alumina film with the aforementioned constant geometric surface area and its specific activity referred to the unit of oxide mass gave a maximum in the first and a minimum about the fourth hydrothermal treatment; then, they increased strongly with the order of hydrothermal treatment. Despite the decrease of the oxide mass during hydrothermal treatment, the final promotion of the total catalytic activity of oxide was 13.7-10.6 times that of non-treated oxide for temperatures 330-390 degrees C, The corresponding promotion of specific activity was 31.5-24.5 times that of the non-treated oxide. The results of the present study showed that the successive hydrothermal and thermal treatments of porous anodic Al2O3 films produce more and more active alumina catalysts. In this way ultra-active alumina catalysts or supports can be prepared. (C) 1999 Elsevier Science B.V. All rights reserved.