Quantitative IR studies of the concentration of different nickel sites in NiZSM-5 zeolites

Quantitative IR studies were realized in order to determine the concentration of Ni2+ and Ni+ in two zeolites NiZSM-5 differing in Si/Al and nickel content. The NiZSM-5 zeolite of Si/Al = 11.5 contained most of Ni2+ in exchange positions whereas another one, of Si/Al = 40 contained most of Ni2+ in the form of oxides. Also the Ni+ ions in NiZSM-5 reduced by CO were investigated. The experimental procedure of quantitative studies was elaborated and the condition of stoichiometric reaction between probe molecule (CO or NO) and sorption site (Ni2+ or Ni+) was fulfilled: each CO or NO molecule introduced into the IR cell reacted with one Ni2+ forming Ni2+(CO) or Ni2+(NO) adducts. The procedure was also elaborated for quantitative studies of concentration of Ni+ in zeolite reduced. The absorption coefficients of IR bands of Ni2+(CO), Ni2+(NO) and Ni+(CO) at 2212, 1898, and 2109 cm−1, respectively were determined, and finally the concentrations of Ni2+ and Ni+ were calculated. In the zeolite of Si/Al = 11.5 the concentration of Ni2+ in exchange positions determined by our IR studies was nearly the same as that determined by chemical analysis. The values of the absorption coefficients turned out to be independent on the probe molecule (CO or NO). In the zeolite of Si/Al = 40, containing the most of Ni2+ in the form of oxides, the concentration of Ni2+ found by IR was 66% of concentration of Ni determined by chemical analysis demonstrating that dispersion of Ni2+ was low and some of Ni2+ was hidden inside oxide clusters being inaccessible to probe molecules. In reduced NiZSM-5 of Si/Al = 11.5 and 40 the concentration of Ni+ determined by IR studies was nearly the same as concentration of Ni determined by chemical analysis. In NiZSM-5 of Si/Al = 40 the dispersion of Ni increased distinctly upon the reduction, and all Ni+ cations became accessible to probe molecules or reactants.

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► Elaboration of the experimental procedure of IR quantitative studies of Ni sites.
► Determination of the absorption coefficient of Ni2+(CO), Ni+(CO), and Ni2+(NO) bands.
► Concentrations of Ni2+ and Ni+ detected by IR quantitative studies with CO and NO.