INFRARED-SPECTROSCOPY OF CARBON-MONOXIDE ADSORBED ON PT/L ZEOLITE

A series of 0.6 wt% Pt/MBaL zeolites, where M is Li, Na, K, Rb or Cs, were prepared and characterized by transmission electron microscopy, chemisorption, and infrared spectroscopy of adsorbed carbon monoxide. Greater than 90% of the exposed platinum in the samples is associated with small clusters, less than almost-equal-to 7 angstrom across, inside the zeolite channels. The remaining fraction of exposed platinum is on 100-500 angstrom crystallites outside the channels. Adsorption of carbon monoxide on the platinum at 25-degrees-C produces a broad infrared band whose maximum shifts from 2065 to 2025 cm-1 as the alkali cations in the zeolite are changed from Li to Cs. This shift is indicative of electron transfer between the cations and the platinum clusters. Heating the Pt/L catalysts to 225-degrees-C produces new infrared bands at 2020-2015, 1975, and 1935-1920 cm-1. The appearance of these low-frequency bands strongly suggests that the CO-covered platinum clusters change their structure during heating. We propose that the new structure is one in which the carbon monoxide molecules insert into spaces between the framework atoms of the L zeolite.