A COMPARATIVE-STUDY OF THE COADSORPTION OF CO AND NO ON PD(100), PD(111), AND SILICA-SUPPORTED PALLADIUM PARTICLES WITH INFRARED REFLECTION-ABSORPTION SPECTROSCOPY

The coadsorption of nitric oxide and carbon monoxide on Pd(100), Pd(111), and a model silica-supported palladium catalyst has been investigated with infrared reflection-absorption spectroscopy (IRAS) in the temperature range 100-500 K. The palladium surfaces were in equilibrium with a 1 X 10(-6) Torr equimolar mixture of CO and NO during the FT-IRAS studies. The structure and coverage of the adsorbed layers strongly depend on the surface temperature and crystal orientation. On all surfaces, CO and NO form molecularly mixed overlayers, and the CO/NO coverage ratio increases with a decrease in surface temperature. On Pd(100), both CO and NO adsorb at the 2-fold bridging sites at all temperatures. The coverage for CO and NO is similar at temperatures above 400 K, whereas at 100 K the CO/NO ratio is similar to 5. On Pd(111), the majority surface species is NO, which adsorbs at the 3-fold hollow sites at >400 K. At lower temperatures, CO adsorbs at the bridging sites and the NO at the hollow sites converts to atop NO, At <200 K the overlayer consists of similar to 0.25 monolayer of NO on the atop sites and similar to 0.5 monolayer of CO on the nearby bridging sites. The adsorption behavior of a CO/NO mixture on the palladium particles is compared with that of pure CO and NO adlayers on palladium single crystal surfaces on the basis of the presence of (111) and (100) facets.