Spatial distribution of N2, N2O, and NO desorbing from a Pd(211) surface

The spatial distributions of NO, N-2, and N2O desorbing from Pd(211), Pd(S)-[3(111) x (100)], in temperature programmed desorption (TPD) and that of N-2 produced in the temperature programmed reaction (TPR) of NO + H-2 have been studied. The spatial distribution of N-2 desorbing at 510 K in the TPD had a maximum intensity at theta = -25- -30 deg (down the steps) in the [ 1 (1) over bar (1) over bar ] direction, which is almost normal to the (100) steps, and is described by 2.4 cos(12) (theta + 26 deg). Taking account of the fact that NO molecules are preferentially adsorbed on the (111) terraces of a Pd(211) surface at low coverage, it is deduced that the NO adsorbed on the (111) terraces undergoes migration to the (100) steps, where N-2 is produced. An isotopically labeled angle-resolved TPD experiment showed that N-2 was produced by the NO + N reaction. On the other hand, the desorption of NO is normal to the crystal surface. This phenomenon indicates that NO desorbs via a predesorption state, where NO molecules forget their memory of the previous adsorption sites. In contrast, the spatial distribution of N-2 given by the catalytic reaction in a flow of NO + H-2 at 650 K, was completely different from that of N-2 produced by NO + N reaction at around 500 K. It is concluded that the spatial distribution reflects the reaction or desorption mechanisms, especially the reaction coordinate on the surface. (C) 1999 American Institute of Physics.