Kinetic instabilities during the NOx reduction with hydrogen on Pt crystals studied with field emission on the nanoscale

This paper reviews field emission studies of kinetic instabilities occurring during the catalytic reduction of nitric oxide (NO) and nitrogen dioxide (NO2) by hydrogen on three-dimensional platinum crystals. Emphasis is placed on revealing that both field ion microscopy (FIM) and field electron microscopy (FEM) can image such instabilities under truly in situ reaction conditions with a lateral resolution on the nanoscale. In particular, oscillatory behavior with rapid ignition from a state of low to a state of high catalytic activity is demonstrated for both NO and NO2 reduction. Results of a local chemical probing during FIM studies of the NO+H-2 reaction are also shown and provide clear evidence for the oscillatory behavior of water (detected as H2O+ and H3O+) formation and for diffusion supply of NO into surface regions emptied during the stage of high catalytic activity. The rapid ignition ("surface explosion") of the catalytic cycle is discussed on the basis of an autocatalytic mechanism of the NO decomposition. On the (001) plane of the Pt crystal small island formation is seen to occur during the low-activity state of the catalytic cycle. Islands have a size equivalent to similar to3 nm, move independently from each other, and do not merge when colliding. A tentative model is discussed associating islands with patches of hydroxyl groups. Very regular oscillatory behavior is demonstrated for the NO2 reduction using FEM. Advantages as well as shortcomings of the FEM/FIM experimental approach are discussed and an outlook on future studies using local chemical probing will be given wherever appropriate. (C) 2002 American Institute of Physics.