Adsorbed Atoms and Molecules Destined for a Reaction |
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Authors: | Tanaka Ken-ichi Ikai Masamichi |
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Affiliation: | (1) Advanced Science Research Laboratory, Saitama Institute of Technology, 1690 Fusaiji, Okabe, Saitama, Japan;(2) Present address: Toyota Central R & D Labs, Inc., Nagakute, Aichi, 480-1192, Japan |
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Abstract: | The idea of an activation complex is popular for explaining reaction rates, but the characteristics of reactions and catalysis may not be explained in this way. A predestined state for each reaction composed of surface atoms and adsorbed species is responsible for these features. Two single Sn atoms trapped in adjacent half-unit cells of an Si(111) 7 × 7 surface is an example of a predestined state. An isolated Sn atom in a half-unit cell does not migrate to other half-unit cells at room temperature, but when two single Sn atoms are in adjacent half-unit cells they undergo rapid combination to form an Sn2 dimer. In addition, these two single Sn atoms replace the center Si adatoms and an Si4 cluster is formed. The spatial distribution of molecules desorbing from surfaces may reflect the predestined states for the desorption processes. The spatial distribution in the temperature-programmed desorption (TPD) of NO on Pd(110) and Pd(211) surfaces and that in the temperature-programmed reaction (TPR) of NO + H2 were studied. N2 desorbing from Pd(110) by the recombination of N atoms obeys cos6
– cos7
but the N2 produced by a catalytic reaction of NO with H2 obeys cos . In contrast, the N2 desorbing with NO at 490 K in the TPD of Pd(110) shows a sharp off-normal distribution expressed by cos46( – 38). The adsorption of NO on Pd(211) predominantly occurs on the (111) terrace but the spatial distribution suggests that the predestined states for the reaction and desorption are formed on both the (111) terrace and (100) step surfaces. |
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Keywords: | TPD and TPR on Pd(110) and Pd(211) spatial distribution of desorbed molecules off-normal desorption of N2 desorption-mediated reaction of NO catalytic reduction of NO Sn2 dimer on Si(111) 7 × 7 Si4 cluster on Si(111) 7 × 7 predestinated state for surface reaction |
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