Using the "memory effect" of hydrotalcites for improving the catalytic reduction of nitrates in water

The role of the support in the catalytic hydrogenation of nitrates in the liquid phase using a Pd-Cu/Mg/Al hydrotalcite is studied. The material has been characterized by X-ray diffraction, infrared spectroscopy, elemental analysis, and surface area. It is shown that the use of calcined hydrotalcites as support reduces the problems associated with mass transfer limitations observed on Pd-Cu/Al2O3, by introducing a new concept of active supports. In this case by taking advantage of the "memory effect" of calcined hydrotalcites, the nitrates are forced to be located between the positively charged layers of the hydrotalcite and therefore close to the reductive active sites. The nitrates are reduced to nitrites that remain in the same position, and these are further reduced to nitrogen or in a much lower extent to ammonia. These final compounds due to their inadequate charge are released to the solution, reducing the problems related with diffusion limitations that strongly affect the selectivity of the reaction. This has been proved by studying separately the adsorption and reaction steps with different characterization techniques. (C) 2003 Elsevier Inc. All rights reserved.