THE INFLUENCE OF ALKALI-METAL IONS IN THE CHEMISORPTION OF CO AND CO2ON SUPPORTED PALLADIUM CATALYSTS - A FOURIER-TRANSFORM INFRARED SPECTROSCOPIC STUDY

Two series of palladium-based catalysts were compared on the basis of the adsorption of CO and CO2, monitored by Fourier transform infrared spectroscopy. The first series is represented by a silica-supported pa lladium catalyst and by some catalysts derived from it by addition of different amounts of sodium ion, 0 less than or equal to R less than o r equal to 25.6, where R is the atomic ratio Na/Pd. The second series consists of palladium catalysts supported on ''model'' and natural pum ices. The model pumices, obtained by sol-gel techniques, are silico-al uminates containing variable amounts of sodium so that the correspondi ng Pd catalysts have an R value in the range 0 less than or equal to R less than or equal to 6.1. In the Pd/natural pumice catalysts, change s of the atomic ratio R' = (Na + K)/Pd are achieved with different pal ladium loadings. Despite the analogous behaviour of the catalysts of b oth series when R = 0, the presence of increasing alkali metal ions in duces different behaviour towards the adsorption of CO. On increasing R in the Na-Pd/SiO2 series there is a progressive weakening of the C-O bond to produce eventually carbonates, whereas only a decrease of the amount of adsorbed CO occurs in the Pd/model pumice series (R less th an or equal to 6.1), Furthermore, only physisorbed CO bands are observ ed in Pd/natural pumice catalysts (R' less than or equal to 17). Diffe rent behaviour is also noticed towards the adsorption of CO2: the equi librium CO2(gas) reversible arrow COads + O-ads occurs in the Pd/SiO2 series, in contrast to the Pd/pumice series where only carbonate speci es on the surface of the support are detected. The results are interpr eted on the basis of geometric and electronic effects attributed to th e different localization of the alkali metal ions in the catalysts. De coration of palladium by the alkali metal ions is evident on increasin g R in the Pd/silica series; such a decoration does not occur in Pd/pu mice catalysts at any R or R' value. Accordingly the behaviours of bot h series of catalysts towards the chemisorption of CO and CO2 are inte rpreted assuming that geometric effects are predominant in the Pd/sili ca series, whereas in the Pd/pumice series electronic effects are the most important, geometric effects being practically absent. (C) 1996 A cademic Press, Inc.