Journal of Chemical Physics, Vol.110, No.14, 7031-7036, 1999
Spatial distribution of N-2, 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.