Platinum species in the pores of NaX, NaY and NaA zeolites studied using EPR, XAS and FTIR spectroscopies

The results of X-band EPR, X-ray absorption and Fourier transform infrared spectroscopy on Pt(NH3)(4)(2+) exchanged NaX, NaY and NaA zeolites reveal after oxygen calcination at 573 K that diamagnetic Pt2+ is not the only product. Calcination provides Pt3+ cations, but depending on the heating rate, the decomposition of amino groups during calcination also produces hydrogen that reduces Pt3+ to Pt2+ and Pt+. NaX (Si/Al = 1.23) has a more negative framework charge than NaY (Si/Al = 2.31), so Pt3+ can be stabilized only in NaX, whereas lower oxidation states of Pt such as Pt+ can be stabilized in both, NaX and NaY, and neither of the paramagnetic Pt cations are stabilized in NaUSY (Si/Al = 3). The autoreduction process allows controlling the number of Pt3+ and Pt+ in the NaX zeolite by changing the calcination heating rate: a heating rate of 1.25 K min(-1) gives only Pt+, but 0.5 K min(-1) gives a Pt3+/Pt+ ratio close to 1. The structure of the support is also important for the synthesis of Pt species. While isolated paramagnetic Pt ions were stabilized in faujasite zeolites ( NaX and NaY), a paramagnetic Pt dimer was obtained in a Linde type A zeolite (LTA, Si/Al = 1) by applying the same preparation methods. The fraction of paramagnetic Pt species which were characterized by X-band EPR spectroscopy amounts to 2-18% of the total Pt in the zeolites, the remaining Pt must be diamagnetic.