Spatial distributions of desorbing products in basic catalytic processes and surface nano‐structures

Abstract

Reviews of recent progress in angle-resolved measurements of desorbing surface reaction products are discussed. The angular and velocity distributions of desorbing products deliver information about the reaction site as well as the reaction mechanism when the products are repulsively desorbed. These distribution measurements can yield symmetry and orientation information of the reaction site for associative processes whereas, in dissociative desorption, the collimation of fragment desorption is related to the orientation of the intermediate species immediately before dissociation. These different collimations provide information on desorption steps whenever any step becomes rate determining.

Spatial distributions of desorbing products in steady-state NO and N2O reductions on Pd(110)

“…This signal is due to the inclined N 2 desorption in the intermediate N 2 O decomposition. 2,4 The N 2 signal at = 0°, on the other hand, increased slowly and peaked at about 600 K. This comes from the associative desorption of N͑a͒. 9,10 The AR CO 2 signal at the normal direction increased at around 500 K and decreased above 570 K with increasing T S , more slowly than the N 2 signal at = 41°.…”

“…The N 2 O density above 470 K is very low because of the small heat of adsorption, about 37 kJ mol −1 . Thus, the intermediate N 2 O is decomposed in a situation similar to ␤ 4 -N 2 , which suggests that the collimation angle of N 2 emission is affected by the coadsorbed species in the steady-state NO reduction rather than N 2 O reaction.…”

“…7,8 The inclined N 2 desorption in N 2 O decomposition on Pd͑110͒ is useful to analyze the removal pathways of surface nitrogen because the concomitant associative desorption of N͑a͒ emits N 2 sharply along the surface normal and other desorbing products N 2 O and NH 3 show a cosine distribution. 2,4,9,10 Measurements of the angular and velocity distributions will provide information on the reaction pathways whenever any step becomes rate determining because these distributions do not involve the reaction rate and are always controlled by their own desorption steps. 4 The distributions of desorbing products in the NO reduction have been analyzed with several relaxation methods, such as modulated molecular beams, 11 angle-resolved ͑AR͒ temperatureprogrammed desorption ͑TPD͒, or AR-pressure jumps.…”

The collimation angle shift of desorbing product N2 in a steady-state N2O+CO reaction on Rh(110)

“…The distance from the ionizer to the chopper blade is 377 mm, and the time resolution was set at 20 s. The other apparatus used for N 2 O pressure jump ͑PJ͒ experiments at T S = 60-400 K has a cryoplate in the reaction chamber yielding a pumping rate of about 9 m 3 / s and allows cooling a sample below 50 K. 23 A Rh crystal with ͑110͒ surfaces was rotated to change the desorption angle ͑: polar angle͒ in the plane along the ͓001͔ direction. 2 The crystal was cleaned by repeated cycles of Ar + ion bombardments, oxygen treatments at 850 K, and annealing to 1200 K until a sharp ͑1 ϫ 1͒ LEED structure or the reproducible oxygen temperature-programmed desorption ͑TPD͒ curve was observed. The cleanliness of the surface was verified by the absence of CO and CO 2 desorption in TPD after oxygen exposure.…”

“…1 Its decomposition largely shares the N 2 emission in the NO reduction. 2 Furthermore, it has provided the first example of collimated fragment desorption in thermal decompositions on metal surfaces. 3,4 We deliver the first report of the collimation angle of desorbing N 2 on Rh͑110͒ in a wide surface-temperature range ͑60-700 K͒.…”