CO adsorption on supported and promoted Ag epoxidation catalysts

• Published in: Journal of catalysis Vol. 213; no. 2; pp. 305 - 320
• Main Authors: Müslehiddinoğlu, Jale, Vannice, M.Albert
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier Inc 25.01.2003; Elsevier
• Subjects: Catalysis; Catalytic reactions; Chemistry; Cs and Cl promoters; DRIFTS; Epoxidation; Exact sciences and technology; General and physical chemistry; Silver; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry; Epoxidation; Silver; Cs and Cl promoters; CO; DRIFTS; Chemisorption; Catalytic reaction; Fourier transformation; Diffuse reflection; Cesium chloride; Cesium; Transition metal; Modified catalyst; Experimental study; Supported catalyst; Infrared spectrometry; Gas solid adsorption; Heterogeneous catalysis; Aluminium Oxides; Carbon monoxide; Pretreatment
• ISSN: 0021-9517; 1090-2694
• DOI: 10.1016/S0021-9517(02)00014-3

CO was used to probe the nature of adsorption sites on Ag/ α-Al 2O 3 epoxidation catalysts and to investigate the effect of Cs and Cl promoters by employing diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and chemisorption measurements. In contrast to previous studies, IR absorption bands for CO chemisorbed on reduced, supported Ag crystallites were observed; however, CO adsorption occurred on only 3–7% of the total Ag surface at 300 K and coverage depended on both the pretreatment and CO pressure utilized. No irreversible CO adsorption occurred on the alumina, whereas linearly bonded CO was the dominant species on the metallic Ag sites. After a 30-min purge, the bands due to these chemisorbed forms of CO decreased in intensity while a band due to bridge-bonded CO increased in intensity, which implies that CO reoriented as the surface concentration of CO decreased. In the presence of Cs, similar behavior was observed and the band intensity of the bridge-bonded CO increased. After reduction at 673 K, cesium suboxides appeared to be formed based on the formation of carbonyl complexes at 2028, 1950, and 1869 cm −1. On reduced Ag catalysts, electronic effects of Cs and Cl were observed and adsorbed CO gave a lower frequency, i.e., 2018 and 2009 cm −1 for Cs-promoted samples reduced at 473 and 673 K, respectively, due to an increase in the electron density on surface Ag atoms, while this band occurred at a higher frequency of 2129 cm −1 with a CsCl-promoted Ag catalyst due to a net decrease in the electron density on surface Ag atoms. After CO adsorption on O-covered Cs-promoted and CsCl-promoted catalysts, a band between 1520 and 1491 cm −1 existed which was assigned to a COO − stretching mode in a carbonate species formed on composite AgCs x O y sites. These studies with CO provide evidence that reduction at 673 K following a calcination step can lead to redistribution of Cs atoms.