Chemisorption of nitrogen on PdCu(1 1 0) single-crystal alloy

Thermal desorption spectroscopy (TDS) was mainly used to study the chemisorption of nitrogen on specially prepared Pd-rich surfaces of the PdCu(1 1 0) single-crystal alloy. The TD spectra showed two desorption maxima below 500 K; the α state and the β state with desorption maxima at 130 and 445 K, respectively. The β state is characteristic for the alloy surface and was not observed on pure constituent metals. With higher doses the TD spectra display an unexpected behaviour. Although the top layer consists entirely of Pd (∼100% Pd), the presence of Cu atoms as impurities in the subsurface region (5–15% Cu) was found to influence the interaction of the Pd atoms in the top layer with adsorbed nitrogen leading to the formation of strongly adsorbed nitrogen species with desorption rate maxima at 445, 580 and 680 K, respectively. The enhanced formation of these species was attributed to the electronic interaction of surface Pd with Cu atoms present in subsurface region

Thermal desorption spectroscopy analysis of oxygen from Pd-rich surfaces of PdCu(110) single crystal

The PdCu(110) single crystal alloy, with Pd:Cu=1:1 in the bulk, was prepared by sputtering and annealing at low temperatures (ad=115 K oxygen desorption was proceeding. The desorption rate increased with doses up to saturation at ~9 L. With further oxygen doses a decrease in intensity was recorded. By comparing such behaviour with that from hydrogen desorption, it follows that for doses higher than the saturation dose, the binding energies decrease. The influence of subsurface oxygen on this behaviour has been studied by LEED. The type of crystal lattice and the ratio of prior to adsorption determined the type of adsorbate structures obtained. Oxygen adsorption on the clean surface that had (1×1) structure produced c(2×4), (1×2) or (2×2) patterns. The original (1×1) structure could be restored only by leaving the alloy for few hours under vacuum at room temperature or by applying the CO cleaning procedure. Oxygen adsorption on the clean surface that had the (1×2) structure did not produce any changes on this structure. The influence of small Cu-concentrations on the adsorption properties of this Pd-rich surface, in the case of fcc (110) or when Cu together with Pd atoms builds the bcc(110), is discussed. These properties differ drastically from those for pure Pd(110) or from PdCu(111) planes

Interaction of CO with clean and oxygen covered PdCu(110) single crystal alloy

Compositional changes that may occur upon CO chemisorption on PdCu (1 1 0) single crystal alloy have been studied by means of work function change measurements using the Kelvin probe technique. Furthermore, various surfaces of this alloy with different surface compositions were investigated with respect to their reactivity in the CO oxidation applying the same technique. It was found that CO chemisorption, in the case of Cu-rich surfaces, induces the segregation of Pd to the surface. High CO pressures (10-4–10-3 mbar) also cause the surface segregation of still unidentified contamination characterized by an AES signal at 180 eV. In the CO oxidation experiments the PdCu alloy showed lower reactivities compared to pure Pd, even though the outermost layer was entirely composed of Pd atoms. The reactivity was found to decrease further with increasing Cu concentrations. The lower reactivity of the PdCu alloy was attributed to the charge transfer from Pd to Cu observed in this system resulting in decreasing the adsorption enthalpy (and probably the sticking coefficient) of CO on “modified” Pd of the alloy surface

CO chemisorption on PdCu(110): a work function study

The work function change (ΔΦ) that occurs upon CO chemisorption on PdCu(110) single crystal was studied using the Kelvin probe technique. The ΔΦ measurements were carried out for various surface compositions ranging from 100% Pd to 100% Cu, which were prepared by sputtering and annealing the sample at different conditions. Auger Electron Spectroscopy (AES) and Thermal Desorption Spectroscopy (TDS) were used to characterise the surface before and after each ΔΦ measurement. The results obtained in this study prove that the ΔΦ technique is very sensitive to changes in surface composition and can be therefore successfully used to determine quantitatively the top layer composition of PdCu(110) single crystal alloy. The ΔΦ results reported here indicate a charge transfer from Pd to Cu in PdCu alloys. As a result of this transfer process, the back donation of electrons from Pd to adsorbed CO is reduced and the dipole moment of adsorbed CO on Pd becomes smaller

Sticking probability of CO on an oxygen covered Pd{110}-surface under reaction conditions

The adsorption of carbon monoxide into an oxygen overlayer on a Pd(110) surface and its conversion to CO2 was studied by quantitative mass spectrometric methods and video recorded LEED. For temperatures with negligible desorption of CO (T≤420 K) the sticking probability of CO into the oxygen adsorbate is reduced by approximately 1/2 compared to a CO adsorbate on a clean Pd(110) surface of the same coverage. The evaluation reveals that each oxygen adatom effectively blocks more than ten sites for CO adsorption, which is explained by the influence of adsorbed oxygen on the transition probability of the CO precursor to the chemisorbed state. Initially due to a locally higher sticking probability CO is adsorbed at defects of the c(2×4) oxygen overlayer. After consumption of less ordered oxygen regions further reaction is restricted to the domain boundaries of the reactands and propagates preferably in an unidirectional fashion along the surface troughs with a constant turnover number until whole domains of oxygen have been consumed by the reaction

Anomalous electron energy-loss spectra of Ni(430) and a disordering of atomic steps

The authors present energy-loss spectra (ELS) in the range 0-20 eV taken as a function of temperature (90-800 K) and primary energy (100-400 eV) for a Ni(430), or (7/2-(110)*1/2-(100)), surface. A strong primary-energy dependence of ELS and pronounced differences within the ELS of the low-energy electron diffraction beams were observed. These characteristics arise from the presence of ordered atomic steps on the surface, as shown by a model calculation. They also observed temperature-dependent variations of the ELS, and interpreted them as being induced by a structural transition of the surface

Adsorption and desorption of oxygen and nitrogen on PdCu(110) single crystal alloy surfaces

LEED, AES and TDS were used to study the chemisorption of oxygen and nitrogen on specially prepared Pd-rich alloy surfaces. CuPd ratios of ⩽ 0.4 were determined by AES for a four-layer surface region (SR); the top layer (TL) composition was ~ 100% Pd, obtained by CO TDS as characterisation method. This enabled the average ratio of CuPd in the next three subsurface layers (3SSL) to be evaluated. Oxygen and nitrogen were adsorbed at 115 K and TDS were obtained for temperatures up to ~ 500 K. Oxygen desorption from the alloy surfaces showed one peak at 144 K. The TDS saturated at ~ 9 L. Different types of adsorbate structures were obtained depending on the CuPd (SR) ratio and the lattice type (fcc or bcc). Oxygen adsorption on the (1 × 1) clean surface developed two structures, depending on the ratio of the CuPd (SR). For values ⩽ 0.1, a c(2 × 4) structure appeared at room temperature (RT), while a reconstructed (1 × 2) pattern appeared at 120 K for a CuPd(SR) of ~ 0.4. Nitrogen adsorption on surfaces with a CuPd(SR) of ⩽ 0.1 showed only a (1 × 1) surface structure. TDS of nitrogen with one maximum at 130 K saturated at ~ 1 L

Adsorption of O2 on Pd{110}

We studied the adsorption of oxygen on Pd(110) in the temperature range from 100 to 600 K. The experimental apparatus used a video LEED system to monitor changes in surface crystallography and two mass spectrometers to study adsorption, desorption and reaction with CO. The sticking coefficient for dissociative oxygen adsorption is 0.86 ± 0.03 at T > 160 K. For T 0.23 a commensurate c(2 × 4) phase is formed and saturates at θ ≈ 0.48