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 un der 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, an d N-2) regardless of the NO pressure employed. However, the selectivit y of the reaction and the E(a) for product formation are profoundly af fected when the NO pressure is lowered by tenfold. For high NO pressur es 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 To rr, then the selectivity is a strong function of reaction temperature, with N2O being the major product below 635 K and N-2 the major produc t above 635 K. It is exactly this switch over in selectivity at elevat ed temperature that has always been seen over low-loaded Rh/Al2O3 cata lysts but until this time not reported for single-crystal catalysts. O n the basis of our new results we can say that Rh(111) has the charact eristic selectivity behavior of practical Rh/Al2O3 catalysts. These re sults are encouraging because they show that the mechanistic insight w e gain over Rh(111) is more broadly applicable to supported catalysts than was previously thought.