EFFECT OF NO PRESSURE ON THE REACTION OF NO AND CO OVER RH(111)

We have examined the reaction of NO with CO over a Rh(111) catalyst under conditions of both low (0.8 Torr) and high (8 Torr) NO pressures. Our results show that the same three products are formed (CO2, N2O, and N-2) regardless of the NO pressure employed. However, the selectivity of the reaction and the E(a) for product formation are profoundly affected when the NO pressure is lowered by tenfold. For high NO pressures the selectivity is insensitive to either NO pressure (1 Torr < P-NO < 40 Torr) or reaction temperature (T < 673 K). In sharp contrast to this high-pressure behavior, when the NO pressure is lowered to 0.8 Torr, then the selectivity is a strong function of reaction temperature, with N2O being the major product below 635 K and N-2 the major product above 635 K. It is exactly this switch over in selectivity at elevated temperature that has always been seen over low-loaded Rh/Al2O3 catalysts but until this time not reported for single-crystal catalysts. On the basis of our new results we can say that Rh(111) has the characteristic selectivity behavior of practical Rh/Al2O3 catalysts. These results are encouraging because they show that the mechanistic insight we gain over Rh(111) is more broadly applicable to supported catalysts than was previously thought.