Silica-Supported Palladium Particles with Infrared Reflection-Absorption Spectroscopy

The coadsorption of nitric oxide and carbon monoxide on Pd( loo), Pd( 11 l ) , 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 l P 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. O n all surfaces, CO and NO form molecularly mixed overlayers, and the CO/NO coverage ratio increases with a decrease in surface temperature. On Pd(loo), both CO and NO adsorb a t the 2-fold bridging sites a t all temperatures. The coverage for CO and NO is similar a t temperatures above 400 K, whereas a t 100 K the CO/NO ratio is -5. On Pd( l1 l ) , the majority surface species is N O , which adsorbs at the 3-fold hollow sites a t >400 K. A t lower temperatures, CO adsorbs a t the bridging sites and the NO a t the hollow sites converts to atop NO. A t <200 K the overlayer consists of -0.25 monolayer of NO on the atop sites and -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 (1 11) and (100) facets.