SUPPORTED METAL-CATALYSTS - PREPARATION, CHARACTERIZATION, AND FUNCTION .3. THE ADSORPTION OF HYDROCARBONS ON PLATINUM CATALYSTS

Adsorption of ethene in the absence and presence of carbon monoxide has been investigated over Pt/alumina (I), Pt/silica (I), and Pt/molybdena (I) catalysts prepared by impregnation, anda Pt/molybdena (C) catalyst prepared by co-crystallization in thetemperature range 188-296 K. Static 14C-ethene radiotracer adsorption measurements show that on each catalyst only a fraction of the adsorbed ethene is reversibly adsorbed at 293 K; this fraction increases in the order Pt/alumina < Pt/silica ≪ Pt/molybdena. Dynamic flow adsorption studies show that, on Pt/alumina and Pt/silica, retention of ethene is accompanied by self-hydrogenation to ethane. Retention, but no self-hydrogenation, is observed with both Pt/molybdena catalysts. Following reduction/activation and thermal treatment in helium at 573 K, the Pt/silica surface retains appreciable quantities of hydrogen, which reacts with adsorbed ethene producing ethane. Preadsorption of carbon monoxide totally blocks the surface of Pt/alumina, Pt/silica and Pt/molybdena (C) for ethene adsorption. However, preadsorption of ethene only partially poisons the surface of Pt/alumina and Pt/silica for carbon monoxide adsorption. With Pt/molybdena (C), preadsorption of ethene promotes a threefold increase in the capacity of the surface for carbon monoxide chemisorption. On Pt/molybdena (I) carbon monoxide and ethene are adsorbed noncompetitively at separate sites. With Pt/alumina and Pt/silica. coadsorption of ethene and carbon monoxide results in an enhancement in the amounts of ethane formed, relative to those formed when ethene alone is adsorbed. This is interpreted in terms of the chemisorption of carbon monoxide causing the release of hydrogen retained on the catalyst surface following the reduction activation/thermal desorption procedure. This effect is more pronounced with Pt/silica than with Pt/alumina, the amounts of hydrogen released relative to the total Pt content of the catalyst (Pt(total): H) being, respectively, 2.38 and 1.06. FTIR examination of ethene adsorbed on Pt/silica shows bands ascribable to surface alkyl (-CH3 and CH2) species. Evidence for an interaction between the retained hydrocarbon and the -OH groups of the silica support has also been obtained. No hydrocarbon bands were obtained from species retained by the Pt/alumina catalyst during ethene adsorption. However, DRIFT spectra of ethene retained by the Pt/molybdena (I) catalyst suggest the formation of small amounts of aromatic residues. © 1993 Academic Press, Inc.