An electron spectroscopic investigation of the adsorption of NO on Ni(111)

The adsorption, decomposition, and desorption of NO on the close packed Ni(111) surface have been investigated by XPS, XPS satellites, XAES, UPS, and LEED between 125 and 1000 K. At adsorption temperatures below 300 K a single molecular species (v) is formed with about unit sticking coefficient, which is interpreted as bridge-bonded; its saturation coverage is about 85% of that of CO, i.e. 0.5 relative to surface Ni atoms. Adsorption at 300 to 400 K yields dissociative adsorption (β) followed by molecular adsorption; above 400 K only dissociated species are formed. Upon heating, a full molecular layer dissociates only after some NO desorption (at 380–400 K), while dilute layers (below half coverage) dissociate already above 300 K without NO desorption. Together with quantitative findings this shows that for dissociation of one v-NO, the space of two is required. N₂ desorption from the β-layer occurs above 740 K; the oxygen staying behind diffuses into the crystal above 800 K. Readsorption of NO onto a β-layer or onto an oxygen precoverage at 125 K leads, besides to an α₁-state similar to v-NO, to another molecular state (α₂) which is interpreted as linearly bound. The resulting total coverage is considerably higher than in a virgin layer. This shows that the blocking of dissociation in a full v-layer is probably not due to β requiring the same sites, but to kinetic hindrance; an influence of β-induced surface reconstruction cannot be excluded, however. The LEED results agree with a previous report and are well compatible with the other results.