Remarkable effect of the preparation technique on the state of cobalt ions in BEA zeolites evidenced by FTIR spectroscopy of adsorbed CO and NO, TPR and XRD

The state of cobalt in two BEA zeolites was studied by XRD, TPR, and FTIR spectroscopy using CO and NO as probe molecules. One of the samples, CoAlBEA (0.4 wt % of Co), was prepared by conventional ion exchange and the other, CoSiBEA (0.7 wt % Co), by a two-step postsynthesis method involving dealuminated SiBEA zeolite. The introduction of Co into SiBEA leads to an increase of unit cell parameters of the BEA structure and to the consumption of silanol groups in vacant T-sites of the dealuminated zeolite. In contrast, no structural changes are observed after incorporation of cobalt into AlBEA by ion-exchange. The reduction temperature of cobalt in CoSiBEA zeolite (1130 K), is much higher than for CoAlBEA and indicates a strong interaction of cobalt ions with SiBEA. Low-temperature CO adsorption on CoAlBEA results in (i) H-bonded CO, (ii) Co3+-CO adducts (2208 cm(-1)) and (iii) a small amount of Co2+-CO complexes (2188 cm(-1)). In agreement with these results, NO adsorption leads to the appearance of (i) NO+ (2133 cm(-1), formed with the participation of the zeolite acidic hydroxyls), (ii) Co3+-NO (1932 cm(-1)), and (iii) a small amount of Co2+(NO)(2) dinitrosyls (nu(s) = 1898 and nu(as) = 1814 cm(-1)). Low-temperature CO adsorption on CoSiBEA leads to formation of two kinds of Co2+-CO adducts (2185 and 2178 cm(-1)). No Co3+ cations are detected. In line with these results, adsorption of NO reveals the existence of two kinds of Co2+(NO)(2) dinitrosyls (nu(s) = 1888 and nu(as) = 1808 cm(-1) and nu(s) = 1878 and nu(as) = 1799 cm(-1), respectively).