AMMONIA ADSORPTION AND OXIDATION ON CU MG/AL MIXED-OXIDE CATALYSTS PREPARED VIA HYDROTALCITE-TYPE PRECURSORS/

Cu/Mg/Al catalysts obtained by controlled calcination of hydrotalcite- type (HT) anionic clays may be new interesting and cheap catalysts for the selective catalytic reduction (SCR) of NO by NH3. In this paper t he ammonia adsorption and oxidation on CuxMg0.710-xAl0.290 catalysts ( x = 0.022, 0.046, and 0.072, as atomic ratio), obtained by calcination for 14 h at 923 K of HT precipitates, have been investigated and comp ared with those of the corresponding Mg0.710Al0.290 sample. The presen ce of copper strongly increases the SCR activity and the selectivity t o nitrogen in ammonia oxidation, while the Mg/Al catalyst did not show SCR activity in these conditions and formed significant amounts of ni trogen oxides by ammonia oxidation. All samples adsorbed coordinativel y ammonia on medium-week Lewis acid sites, while no Bronsted acidity w as found, showing that protonic acidity is not necessary for both SCR and ammonia oxidation. With an increase in the copper content, the amm onia gave rise by oxidation to adsorbed hydrazine (likely via amide in termediates) and other adsorbed species, tentatively identified as imi do or nitroxyl fragments and nitrogen anions. These surface species we re probably involved in either selective or unselective ammonia oxidat ion, this last occurring via a Mars-van Krevelen-type mechanism. In or der to have more information on the SCR activity of the Cu/Mg/Al catal ysts, the NO adsorption also was investigated, showing that on the PI/ Ig/Al-mixed oxide free surface, NO disproportionates to nitrogen dioxi de and to a species identified as hyponitrite anions. On the other han d, over the Cu-containing centers NO gave rise mainly to surface nitro syl, being also oxidized to nitrates. On the basis of these data, it w as hypothesized that on the Cu/Mg/Al catalysts the SCR took place betw een NO or nitrosyls and amide species, which were likely common interm ediates in either SCR and ammonia oxidation.