Reconstruction of Pt₁₃ clusters into Pt₂(CO)_m on CO addition in NaY Zeolite

Abstract

Pt₁₃ clusters with extraordinarily high magnetization are interesting samples in the area of CO chemistry. For the first time paramagnetic platinum carbonyl samples were prepared and characterized by electron paramagnetic resonance (EPR) spectroscopy to improve the understanding of the CO adsorption effect on the structure of small Pt particles with X-ray absorption spectroscopy and Fourier transform infrared (FTIR) methods. In contrast to existing studies on the reduced Pt cluster-CO interaction in the literature, the structure of the paramagnetic Pt sample before CO adsorption is entirely known. The well-defined Pt₁₃ cluster in NaY zeolite reconstructs into small Pt₂(CO)_m aggregates under CO atmosphere at room temperature. X-Band EPR spectroscopy shows that the multiplet spectrum of the Pt₁₃ cluster at g_iso = 2.35 disappears, and a new spectrum belonging to the Pt₂(CO)_m sample appears at g_iso = 1.98. Extended X-ray absorption fine structure spectroscopy supports the Pt₁₃ cluster destruction upon admission of CO and the formation of a dimeric platinum carbonyl complex with a nearest-neighbor coordination number that decreases from ca. 5.8 to 0.9. FTIR spectroscopy is a very sensitive technique for the C-O vibrational frequency. It reveals that Pt₂(CO)_m species have one type of linear and two types of bridge bonded CO groups. Initially, a weak band at 2108 cm^-1 shows that Pt has a small positive charge, which is converting into zero charge upon CO desorption-adsorption cycles. The surprisingly sharp bands in the CO bridge bond region facilitate assignment, in contrast to the bands of CO bonds of other neutral platinum carbonyl samples.