Catalytic Activity of Highly Dispersed Palladium. III. The Effect of Added Zirconium Oxide upon Palladium Metal Surface Revealed by a Combined Infrared Spectroscopic and Thermal Desorption Study of Carbon Monoxide Adsorption

Abstract

Abstract A combined infrared spectroscopic (IR) and thermal desorption (TD) study of CO adsorption was carried out to reveal the nature of Pd surface which is modified by the addition of ZrO2. The TD spectra of CO from SiO2- or α-Al2O3-supported Pd showed two broad peaks, α at 340 K and β at 530 K. The addition of ZrO2 in a range of atomic ratios of Zr/Pd=0.5–3.0 produced a new broad peak δ at 390–420 K and the β peak shifted toward higher temperature by 16–19 K. The IR spectra of CO on Pd/SiO2 catalysts, consisting of two broad bands: A (2030–2070 cm−1) and B (1880–1920 cm−1), were modified by the addition of ZrO2 to give rise to a new sharp band, Bh (1970–1985 cm−1) and to make the A band narrower. From the correspondence of the variation in intensities and position of the IR bands with those in the fraction of coverage of CO with temperature, the α peak was correlated to the A band, the δ peak to the Bh band, and the β peak to the B band. The changes in the TD peaks and the IR bands caused by added ZrO2 were semiquantitatively compared and are discussed in terms of the electronic and geometric effects; a high electron density state of Pd surface atoms as well as the formation of adsorption sites with specific structure is proposed. A brief study was made for the role of the sites in the catalytic hydrogenation of ethylene and cyclohexene.