INFRARED-SPECTROSCOPY OF ADSORBED CARBON-MONOXIDE ON PLATINUM NONACIDIC ZEOLITE CATALYSTS

Infrared spectroscopy of adsorbed carbon monoxide was investigated on a series of platinum (or palladium) zeolite catalysts plus platinum on silica. The platinum catalysts all showed a strong absorbance band around 2070 ± 20 cm-1 due to linear-bonded CO and a weaker band around 1850 ± 20 cm−1 due to bridge-bonded CO. Four platinum catalysts, Pt/K+ LTL, Pt/K+ MFI, Pt/K+ MTW, and Pt/K+ SiO2, had an additional absorbance near 1970 cm-1. The additional band is not observed for Pt/silica, Pt/La+3 LTL, or platinum supported on H+ LTL or H+ MFI, i.e., neutral and acidic supports. The additional band on the alkalized catalysts is speculated to result from an electrostatic attraction of linear-bonded CO with an alkali cation when Pt is on a nonacidic support. The electrostatic attraction increases the CO adsorption strength and lowers the stretching frequency. The additional linear CO absorption band was also observed on Pd/K+ LTL. The additional band, therefore, is not specific to platinum. Although the presence of this additional infrared band suggests a “new type of metal (catalytic) site,” no relation was found between the catalytic activity or selectivity for n-hexane dehydrocyclization and the additional CO absorbance band. © 1993 Academic Press, Inc.