Effect of the Preparation Method on the State of Nickel Ions in BEA Zeolites. A Study by Fourier Transform Infrared Spectroscopy of Adsorbed CO and NO, Temperature-Programmed Reduction, and X-Ray Diffraction

Abstract

The state of nickel ions in two BEA zeolites was studied by X-ray diffraction (XRD), temperature-programmed reduction (TPR), and Fourier transform infrared (FTIR) spectroscopy using CO and NO as probe molecules. One of the samples, NiAlBEA (0.9 wt % of Ni), was prepared by conventional ion exchange, and the other, NiSiBEA (1.0 wt % Ni), was prepared by a two-step postsynthesis method involving dealuminated SiBEA zeolite. No structural changes are observed with AlBEA after ion exchange with nickel. In contrast, the incorporation of Ni 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 the NiAlBEA sample Ni shows three TPR peaks in the region of 600−1100 K, whereas one sharp peak at 650 K with a shoulder at 750 K is observed with the NiSiBEA sample. Adsorption of CO at 100 K on NiAlBEA results in formation of Ni2+−CO species (2200 and 2214 cm-1). The latter are partially converted, at high coverages, to Ni2+(CO)2 dicarbonyls (2206−2204 cm-1). Reduction of the sample with CO leads to generation of Ni+ ions. The latter form, with CO, different carbonyl species that are in equilibrium: (i) Ni+−CO (2113 cm-1), (ii) Ni+(CO)2 (νs at 2138 cm-1 and νas at 2095 cm-1), and (iii) Ni+(CO)3 (2157, 2122, and 2113 cm-1). The polycarbonyl structures were proven by 12CO−13CO coadsorption. In line with these results, NO adsorption on NiAlBEA leads to formation of two kinds of nitrosyl species (1905 and 1897 cm-1). Adsorption of CO at 100 K on NiSiBEA leads to formation of several kinds of Ni2+−CO species observed in the 2196−2170 cm-1 region. The intensity of the bands was low, suggesting the majority of incorporated nickel ions are coordinatively saturated or inaccessible and thus not able to form carbonyls. Negligible amounts of Ni+ ions were present on the activated sample and formed various (poly)carbonyls. In this case the reduction hardly affected the state of the sample. Most of the nickel ions in NiSiBEA were not able to coordinate NO; the others formed nitrosyl complexes (1872−1845 cm-1) which were much less stable (up to 323 K) than those observed with NiAlBEA.