Identification of a weak adsorption state of carbon monoxide on highly dispersed, alumina-supported platinum

Abstract

Temperature-programmed desorption of CO from 0.1 wt% Pt Al 2O 3 was measured as a thin wafer of the material was heated in vacuum. At a Pt dispersion of 100% ( CO Pt = 0.92 ), a relatively tall, narrow CO desorption peak was obtained at low temperatures (355 K at 0.93 K s −1) followed by a low, broad desorption band extending to 750 K. As the Pt dispersion decreased to 80% ( CO Pt = 0.70 ) and then 70% ( CO Pt = 0.64 ) as a result of pretreatment in O 2 and H 2, the fraction of CO that desorbed in the low-temperature peak decreased and the fraction of CO that desorbed in the broad desorption band increased. At Pt dispersions below 40%, only a broad desorption band was obtained, a result which is in agreement with data reported by others. The low-temperature peak is interpreted as desorption of CO from very small Pt particles (<20 atoms, <1.1 nm) supported on transitional Al 2O 3. The broad desorption band is interpreted as CO desorption from larger Pt particles. Results from experiments performed at different heating rates indicate that the enthalpy of CO adsorption in the state that corresponds to the low-temperature peak is 74 ± 4 kJ mol −1, an adsorption energy which is lower than energies obtained from measurements of CO desorption from Pt single crystal surfaces.