The interaction of a number of nitrogen containing molecules (NO, NH3 and N2H4) with copper has been investigated by electron spectroscopy. Nitric oxide is shown to be molecularly adsorbed in a linearly bonded configuration at 85 K ; this adlayer is unstable above 120 K, resulting in an exclusively oxygen adlayer at 295 K, the nitrogen being desorbed. At 85 K N(1s) peaks observed at 401 and 406 eV are associated with NOs-(ads) and NOd+(ads) species, the former dissociating and the latter desorbing on thermal activation of the adlayer.Interaction of nitric oxide with Cu at 295 K results in dissociative chemisorption with both oxygen and nitrogen retained at the surface. The surface concentration suggests that each nitrogen and oxygen adatom is bridge-bonded to two surface copper atoms. Estimates of absolute surface coverages are made by comparing the intensities of the N(ls) and O(1s) peaks with the Cu(2p) substrate intensity, while valence level spectroscopy (He I and I1 radiation) supplements core-level spectroscopy for discriminating between molecular and dissociated surface species.Ammonia dissociates at 295 K on copper while with oxidized copper the surface oxide is replaced by an imide-type surface. With hydrazine, adsorption is molecular over the temperature range 85 to 295 K, in contrast to our observations with iron.
The adsorption of nitric oxide on Cu(lO0) and Cu(l11) surfaces has been studied by combining X-ray and U.V. photoelectron spectroscopy with low energy electron diffraction. At 80K two molecular states, characterised by N(1s) values of 399.5 and 401 eV, have been assigned to '' bent " and " linear " configurations, respectively, The bent form dissociates slowly at 80 K while the h e a r species desorbs above z 170 K. Assignments of the " bent " and " linear " forms were facilitated by recourse to the known sterochemistry of metal nitrosyl complexes where a correlation between formal charge on the ligand and sterochemistry has been established. Furthermore the results are compared with the adsorption of NO on both clean nickel and nickel whose surface reactivity has been controlled by pre-exposure to oxygen.The N-adatoms arising from dissociation at 80 K are mobile and highly reactive forming N20 which remains on the surface at this temperature. There is no evidence for dinitrogen desorption.The N 2 0 was characterised by both its X-ray and U.V. photoelectron spectra. Cu(100) and Cu(ll1) behave similarly, Cu(ll1) being less active than Cu(100) in dissociation. At 80 K with Cu(100) there is evidence from LEED for the formation of an ordered (2/2xd/2)R45" structure superimposed on an increase in the background intensity of the scattered electrons. On warming the adlayer to 290 K well ordered (2/Zx1/2)R45"-0 structures are present. No ordered structures were observed with Cu(l11) at either 80 or 290 K and this is compatible with previous studies of chemisorbed oxygen on Cu(ll1). At 290 K NO chemisorbed dissociatively on Cu(100) and Cu(l11) both fragments being retained at the surface. With Cu(100) the symmetry of the adlayer conformed to a ( 2 / 2 x 4 5 ~4 5 0 structure. From a curve fitting analysis of the O(1s) data for the adlayer formed on Cu(100) concentrations of the individual surface species O(a), NO(a) and N20(a) present at 80 K and on thermal activation to 110, 133 and 290 K were calculated.A detailed understanding of the mechanism of inherently simple surface processes is essential for the development of models for more complex heterogeneously catalysed reactions. This is clear from the impact that the early studies of Langmuir (see Suits)l and J. K. Roberts had on the future development of the subject. X-ray photoelectron spectroscopy (X.P.S.) offers a new dimension to the study of molecular processes occurring at solid surfaces in that it allows the chemical identification of all elements (other than hydrogen) ; furthermore, the measured core-electron binding energies are sensitive to the chemical environment of the atom in question. Ultraviolet photoelectron spectroscopy (u.P.s.) provides an important adjunct to X.P.S. in that it gives more direct information on the electron density of states close to the Fermi level of the substrate and also reflects changes in the latter after gas adsorption. By making direct comparison with the photoelectron spectrum of the gaseous molecule (the " finger print " appro...
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