Adsorption of CO or NO on the (100) faces of NiO microcrystals occurs on the square lattice of Ni++ ions and yields an array of parallel oscillators perpendicular to the surface. A different adsorption energy is involved: NO adsorbs reversibly at r˙t˙, and irreversibly at 77 K; CO adsorbs readily only at 77 K. In both cases, strong repulsion among nearest neighbours causes at θ = 1/2 a c(2x2) structure to be formed. Dynamic coupling takes place among CO or NO oscillators causing a blue-shift of the stretching frequency: inductive interactions among adsorbed species brings about instead a red-shift. For θ < 1/2 adsorption on nearest neighbour sites occurs by the tilting of surrounding admolecules. When CO is adsorbed at 77 K on presorbed NO, dynamic coupling between the two kinds of molecules does not occur, and the inductive interaction of CO on NO and vice versa may be studied. It results: i) the presence of CO adparticles in the close neighborhood of a single NO molecule induces a discrete red-shift of the NO frequency, accompanied by a continuous one: two broad bands are identified, tentatively associated with a (4x4) and a (2x2) structures of the CO overlayer; ii) the integrated intensity of the NO bands remains appreciably constant, notwithstanding the substantial frequency shifts: CO undergoes continuous static shifts of lesser extent, because it is a much worse π acceptor than NO; iii) at θ < 1/2, as in the case of CO and NO alone, adsorption occurs by tilting of surrounding molecules.

Adsorbate-adsorbate interaction in mixed CO-NO overlayers on Ni(100): an IR investigation

GARRONE, Edoardo;SPOTO, Giuseppe;ZECCHINA, Adriano
1989

Abstract

Adsorption of CO or NO on the (100) faces of NiO microcrystals occurs on the square lattice of Ni++ ions and yields an array of parallel oscillators perpendicular to the surface. A different adsorption energy is involved: NO adsorbs reversibly at r˙t˙, and irreversibly at 77 K; CO adsorbs readily only at 77 K. In both cases, strong repulsion among nearest neighbours causes at θ = 1/2 a c(2x2) structure to be formed. Dynamic coupling takes place among CO or NO oscillators causing a blue-shift of the stretching frequency: inductive interactions among adsorbed species brings about instead a red-shift. For θ < 1/2 adsorption on nearest neighbour sites occurs by the tilting of surrounding admolecules. When CO is adsorbed at 77 K on presorbed NO, dynamic coupling between the two kinds of molecules does not occur, and the inductive interaction of CO on NO and vice versa may be studied. It results: i) the presence of CO adparticles in the close neighborhood of a single NO molecule induces a discrete red-shift of the NO frequency, accompanied by a continuous one: two broad bands are identified, tentatively associated with a (4x4) and a (2x2) structures of the CO overlayer; ii) the integrated intensity of the NO bands remains appreciably constant, notwithstanding the substantial frequency shifts: CO undergoes continuous static shifts of lesser extent, because it is a much worse π acceptor than NO; iii) at θ < 1/2, as in the case of CO and NO alone, adsorption occurs by tilting of surrounding molecules.
48
395
403
http://www.sciencedirect.com/science/bookseries/01672991/48
Ni-oxyde; CO; NO; adsorption; IR spectroscopy; surface characterization
E. Escalona Platero; E. Garrone; G. Spoto; A. Zecchina
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/2318/112499
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