Co adsorption-desorption properties of cation-exchanged NaX zeolite and supported ruthenium

The binding states of carbon monoxide over cation-exchanged NaX zeolites and over corresponding Ru-containing samples have been investigated using thermal desorption spectroscopy. Exchange of sodium with cations such as Li +, Ca 2+, Mg 2+, and La 3+ gave rise to additional CO adsorption states, a higher isosteric heat of CO adsorption, an increased density of acid sites, and an increased amount of adsorbed CO, depending on the nature and ionic radius of the chargebalancing cation. The charge-balancing cations at the zeolite surface (e.g., Ca 2+, La 3+) function as additional CO adsorption sites in conjunction with surface acid centers (e.g., the A1 3+ center) and metal sites. In addition to surface sites, CO is also found to be held in structural cavities and macropores of the zeolite matrix. The programmed heating of both the metal-free and Ru-containing zeolites subsequent to room temperature CO adsorption gave desorption peaks due to release of CO at temperatures less than ~500 K while the higher temperature peaks were constituted mainly of CO 2. Electron spectroscopy results have revealed that the exposure of these samples to CO and subsequent thermal treatment resulted in the formation of surface carbonaceous species. The nature of CO adsorption states giving rise to CO 2 formation is discussed.