COMPLEMENTARY STUDY BY CALORIMETRY AND INFRARED-SPECTROSCOPY OF ALKALI-METAL DOPED PD SIO2 SOLIDS - ADSORPTION OF HYDROGEN AND CARBON-MONOXIDE/

Alkali metal (Li, Na, K) promoted Pd/SiO2 catalysts were prepared by a dding nitrates to the Pd/SiO2 precursor and subsequent hydrogen reduct ion. The initial heats of adsorption of carbon monoxide and of hydroge n at 298 K and at 473 K and the variations of the differential heats w ith coverage were measured by calorimetry. The changes caused by the p resence of the alkali: decrease in the initial heat of hydrogen adsorp tion at 298 K in the presence of lithium and increase in the initial h eat of carbon monoxide adsorption at 298 K in the presence of potassiu m and sodium are very small (ca. 10%). These results, and the conclusi ons deduced from the infrared spectra of adsorbed carbon monoxide on t he same samples, are compared. They are discussed assuming that alkali cations interact with the oxygen atom of adsorbed carbon monoxide. Ca lorimetry clearly shows that adsorption heats of hydrogen and carbon m onoxide do not change appreciably when alkali cations are present on p alladium although infrared spectroscopy of adsorbed carbon monoxide an d catalytic properties are modified. Thus initial adsorption heats det ermined by calorimetry and catalytic activities are not directly relat ed.