Imaging and probing catalytic surface reactions on the nanoscale: Field Ion Microscopy and atom-probe studies of O2–H2/Rh and NO–H2/Pt

We present dynamic studies of surface reactions using video-Field Ion Microscopy (FIM) along with Pulsed Field Desorption Mass Spectrometry (PFDMS). Catalytic water formation is followed using rhodium and platinum 3D field emitter crystals for the oxidation of hydrogen with either oxygen (Rh) or NO (Pt). Strongly non-linear dynamics are observed with nanoscale spacial resolution. For both reactions quasi-oscillatory behaviour exists under certain conditions of temperatures and partial pressures. An influence of the probing electric field is observed and possibly essential in establishing oscillatory behaviour. Local chemical probing of selected surface areas with up to 400 atomic surface sites proves catalytic water formation to take place. Since water ions (H₂O⁺/H₃O⁺) cause image formation of the O₂–H₂ reaction on Rh, respective videos provide space-time resolved information on the catalytically active sites. Atom-probe data also reveal that the surface of the Rh sample reversibly switches from a metallic to an oxidized state during oscillations. As to the NO–H₂ reaction on Pt, fast ignition phenomena are observed to precede wave fronts. After catalytic water formation, NO molecules diffuse into emptied areas and cause high image brightness. Depending on the size of the Pt crystal, the reaction may ignite in planes or kinked ledges along the <100> zone lines. Thus FIM provides clear experimental evidence that kinks are more reactive than steps in the catalytic NO + H₂ reaction. Pt surface oxidation occurs and has probably been underestimated in previous FIM studies.