A comparison of the NO-CO reaction over Rh(100), Rh(110) and Rh(111)

Reaction rates and product selectivities were measured over the Rh(100) sur face as a function of temperature, and CO and NO partial pressures. These r esults are compared with our prior studies of the NO-CO reaction on the Rh( 111) and Rh(110) surfaces. The only products detected for all three surface s were CO2, N2O, and N-2. Furthermore, for the Rh(100) surface we have foun d a significant change in the apparent activation energy (E-a) with reactio n temperature. For the Rh(100) surface it was found that the E-a can change by a factor of 2.3 in the temperature range investigated here, from 528 to 700 K, with the lower values obtained at higher temperatures. In contrast, E-a's were found to remain constant over the same temperature range for th e Rh(110) and Rh(111) surfaces. The results observed for the Rh(100) surfac e suggest that reaction kinetics are dominated by variations in NO coverage s. At low temperatures, the surface is fully saturated with NO, and dissoci ation is limited by the availability of vacancy sites through NO desorption . At high temperatures, the surface is still primarily covered with NO, how ever, the number of vacancy sites has increased substantially. In this case , we propose that the apparent activation energy is now reflecting NO disso ciation kinetics rather than those for NO desorption.