W. Kirstein

CO adsorption studies on pure and Ni-covered Cu(111) surfaces

Abstract The adsorption of CO on pure and Ni-covered Cu(111) surfaces has been studied by means of LEED, TDS, UPS and work function measurements during adsorption and desorption. Different Ni-coverages between 0.1 and 2 monolayers were obtained by Ni-evaporation controlled by a quartz micro balance and by AES. Near room temperature Ni grows in a layer-by-layer mode on Cu(111). The island structure of the surfaces with submonolayer Ni-coverages is clearly demonstrated by TDS und LEED results obtained after CO adsorption. As with surfaces of bulk Cu-Ni alloys CO adsorption on Cu(111) with submonolayer Ni-coverage is dominated by a site effect. Cu-, Ni-, and mixed adsorption sites can be distinguished. The CO induced work function changes for Ni- and Cu-site adsorption show the same sign as observed with the pure metals. Mixed site adsorption has only a minor influence on the work function. A “ligand effect” observed only for the Ni-site adsorption, and only at small Ni-coverages is discussed in detail. Studies on the adsorption kinetics reveal that the Cu-sites may serve as precursor sites for Ni-site adsorption. Detailed UPS studies demonstrate that the CO-induced emission maxima observed on Cu surfaces with submonolayer Ni-coverages can be interpreted as a superposition of the respective adsorption features observed with the pure metals, roughly separated by their work function difference.

CO adsorption studies on a stepped Cu(111) surface

Abstract The adsorption of CO on a stepped Cu(332) surface has been studied by means of TDS, LEED and work function measurements during adsorption and desorption. As can be deduced from the energy depending LEED spot splitting, a well prepared surface consists of regular (111) terraces with monatomic steps. TDS experiments reveal three different desorption states, two of them deriving from CO molecules originally bound to terrace sites, showing nearly the same behaviour as CO molecules adsorbed on an undisturbed Cu(111) surface. The third desorption maximum is assigned to CO desorbing from step sites showing a higher binding energy than the terrace states. No evidence for dissociative adsorption is found from the TDS experiments. After about 2 L exposure a faint streaky adsorption structure arises, interpreted as c(4 × 2) incoherent across the steps. At higher exposure no compression of this adsorption phase is observed. Measurements of the work function change during adsorption and desorption reveal that for a CO molecule on a step site the contribution to the surface dipole-change is considerably higher than for a CO on a terrace site. Based on this result, a possible explanation is given for the fact that the step sites have a higher adsorption energy than the terrace sites.

A study on CO2 dissociation on a stepped (332) copper surface

Abstract The adsorptional behaviour of CO 2 has been studied on a stepped (332) Cu surface at an adsorption temperature of 95 K. TDS experiments show that CO 2 is chemisorbed on this Cu surface. Most of the CO 2 dissociates and CO can be found in the TDS. The oxygen produced by the surface reaction does not desorb from the surface and can be detected by AES after TDS experiments. The oxygen stabilizes some CO 2 thus a CO 2 desorption state with considerable higher desorption energy develops as a function of CO 2 exposure. The different adsorption species have been identified spectroscopically, utilizing He II induced UPS.

Chemisorption of carbon monoxide on copper-nickel alloy surfaces

Abstract The adsorption of CO on Cu-Ni alloy surfaces has been studied at 300 and 120 K using LEED, AES, TDS, and work function measurement. The alloys have been prepared as thin (111) epitaxial films evaporated on mica, and as poly crystalline foils. At 300 K the alloy surfaces show an adsorption behavior similar to that of pure Ni: the work function increases to a saturation value which is higher for Ni-rich surfaces than for Cu-rich. The isosteric heat of adsorption (106 kJ/mole) is nearly as high as with pure Ni. At 120 K the alloys exhibit a more Cu-like adsorption behavior: the work function passes through a minimum which becomes deeper at higher Cu surface concentration. The isosteric heat of adsorption at low temperatures (50 kJ/mole) is nearly as low as for pure Cu. From TDS it can be shown, that the binding energy of the highest (Ni-like) adsorption states increases with increasing Ni surface concentration. At the (111) alloy surfaces no LEED superstructures due to CO adsorption could be observed.

The work function as a monitor for thermal desorption spectroscopy: CO desorption from Ni sites on a Ni-rich (110) Cu-Ni surface

Abstract A new method for the evaluation of thermal desorption spectra is described. Using this approach, based on the usual assumptions of transition-state theory, the coverage dependence of the Gibbs free activation energy G D (θ) of desorption reactions can be determined. As experimental input it suffices to measure any physical property which is proportional to θ during time-programmed desorption as a function of time and surface temperature. If the isosteric heat and the activation energy of adsorption are known, the activation entropy of desorption and the Arrhenius preexponential factor can be estimated from G D . Work-function data on CO desorption from Ni-rich (111) Cu-Ni alloy surface are used to discuss this method.

The interaction of carbon monoxide with the pure and potassium-promoted Cu(332) surface

The adsorption behaviour of the KCO coadsorbate system has been studied at an adsorption temperature of 95 K on the vicinal Cu(332) surface. Thermal desorption spectroscopy experiments show that the K adatoms disturb the electrostatic potential induced by the steps of the vicinal surface, and therefore the CO molecules adsorbed on step sites are the first to be influenced by the coadsorbed alkali. With K present on the surface in a metal-like form, a strong KCO interaction is evidenced by a stabilization of the adlayer up to temperatures >500 K. Work-function change measurements reveal a change in the bonding of CO towards the precovered surface: the results cannot be interpreted using a simple Blyholder model, but imply a direct interaction of potassium with the 1π orbital of CO. The existence of this direct interaction gives rise to an abolition of the (1π + 5σ) degeneration in the CO-induced states in the UV-photoelectron spectra.

A study on oxygen adsorption and coadsorption with carbonmonoxide on a stepped nickel surface

Oxygen adsorption and oxygen carbonmonoxide coadsorption have been studied on a stepped (775) Ni surface in the low coverage range (< 3 L) using thermal desorption spectroscopy (TDS), UV photoelectronspectroscopy (UPS), low energy electron diffraction (LEED), and workfunction change (ΔΦ) measurements. At the stepped surface the dissociative oxygen adsorption proceeds in nearly the same way as on the flat (111) surface with respect to LEED and ΔΦ. The predominant difference is that the p(2 × 2) adsorption structure exhibits the same energy depending splitting of the LEED spots as the pure stepped surface, thus indicating that the oxygen patches on the different terraces are in coherence across the steps. CO adsorption sites are influenced by preadsorbed oxygen at steps and terraces as well. With increasing oxygen precoverage the CO adsorption sites at the steps vanish, and new states develop. As the step sites vanish first, it can be concluded that oxygen adsorption starts at the steps. This gives a good explanation for the coherence of the oxygen layers on the terraces at higher coverages. The new CO states observed at higher oxygen precoverages will be compared to those known from the clean surface.

The interaction of hydrogen with Ni-rich (111)Cu-Ni surfaces

Abstract The hydrogen chemisorption on Ni-rich (111)CuNi alloy surfaces with different surface compositions has been studied by means of thermal desorption spectroscopy (TDS), low energy electron diffraction (LEED), and work function measurements during absorption and desorption. With respect to the interaction with hydrogen these alloy surfaces may, in a first approximation, be described as consisting of active Ni ensembles diluted by inert Cu. Desorption spectra observed after adsorption at 120 and 250 K show nearly the same overall features as those known from pure (111)Ni with two desorption states (β 1 , and β 2 ) and second order desorption kinetics indicating dissociative adsorption. Increasing Cu surface content leads to a rapid decrease of the adsorbed amount of hydrogen, indicating a strong ensemble effect, and to a higher relative intensity of the low temperature desorption maximum (β 1 ) as compared to the high temperature one (β 2 ). This result shows that the hydrogen adsorption on the Ni ensembles is influenced by the surrounding Cu atoms. This may be due to the fact that Cu prevents lateral ordering of the adsorbed hydrogen, at least on smaller Ni islands. These findings are in keeping with those of our LEED investigations and work function measurements. As with pure Ni the work function increases during the development of the β 2 -state and decreases if β 1 is filled. The initial work function increase rapidly becomes smaller for higher Cu surface content and even vanishes for higher concentrations. The (2 × 2) LEED adsorption structure associated to the β 2 -state can only be observed at small Cu content. Studies of the adsorption kinetics indicate that Cu atoms may form precursor sites for the adsorption on Ni sites.