The interaction of NO with Co2+/Co3+ redox centres in CoAPOs catalysts: FTIR and UV-VIS investigations

The interaction of NO with Co2+/Co3+ redox sites in CoAPO-18 and CoAPO-5 ca talysts was studied by means of FTIR and diffuse reflectance UV-Vis spectro scopy both at 298 and 85 K. Two families of Co2+ sites Were found in the Co APO-18 structure. (A) ions in framework [Co2+(OH)P], associated with Bronst ed acid sites which adsorb NO to produce dinitrosyls absorbing at 1903 and 1834 cm(-1) these dinitrosyl complexes are reactive, in that Co2+ is oxidiz ed to Co3+ and N2O is formed. (B) Structural defects Co2+ (Lewis acid sites ) which stabilize dinitrosyls absorbing at 1900 and 1813 cm(-1). The NO ads orption both on reduced and, more significantly, on oxidised CoAPO-18 also leads to the formation of NO2delta+ adsorbed species. It was found that the two kinds of dinitrosyl complexes have different reactivity in presence of oxygen. Both families of sites are also present in CoAPO-5 catalysts on wh ich, however, the redox reaction upon NO adsorption does not occur signific antly. (C) 1999 Elsevier Science B.V. All rights reserved.