U. Burghaus

Adsorption dynamics of CO on the polar surfaces of ZnO

Measurements of initial adsorption probabilities, S0, as well as the coverage dependence of the adsorption probability, S(ΘCO), of CO on Zn–ZnO [ZnO(0001)] and O–ZnO [ZnO(0001)] are presented. The samples have been characterized by He atom scattering, He atom reflectivity measurements, LEED, and XPS. Samples with different densities of defects were examined, either by investigating different samples with identical surface termination (for O–ZnO) or by inducing defects by ion sputtering at low temperatures (for Zn–ZnO). The influence of kinetic energy and impact angle (for Zn–ZnO) as well as adsorption temperature on the adsorption dynamics have been studied. For both polar surfaces the shape of the coverage dependent adsorption probability curves are consistent with a precursor mediated adsorption mechanism. Adsorbate assisted adsorption dominates the adsorption dynamics for high impact energies and low adsorption temperatures, especially for Zn–ZnO. The He atom reflectivity measurements point to the inf...

Adsorption dynamics of CO2 on Zn-ZnO(0001): a molecular beam study.

Presented are initial S(0) and coverage Theta dependent, S(Theta), adsorption probability measurements, respectively, of CO(2) adsorption on the polar Zn-terminated surface of ZnO, parametric in the impact energy E(i), the surface temperature T(s), the impact angle alpha(i), varied along the [001] azimuth, the CO(2) flux, and the density of defects, chi(Ar(+)), as varied by rare gas ion sputtering. S(0) decreases linearly from 0.72 to 0.25 within E(i)=0.12-1.33 eV and is independent of T(s). Above E(i)=0.56 eV, S(0) decreases by approximately 0.2 with increasing alpha(i). The shape of S(Theta) curves is consistent with precursor-mediated adsorption (Kisliuk shape, i.e., S approximately const) for low E(i); above E(i)=0.56 eV, however, a turnover to adsorbate-assisted adsorption (S increases with Theta) has been observed. The initial slope of S(Theta) curves decreases thereby with increasing alpha(i), chi(Ar(+)), and T(s), i.e., the adsorbate-assisted adsorption is most distinct for normal impact on the pristine surface at low T(s) and is independent of the CO(2) flux. The S(Theta) curves have been parametrized by analytic precursor models and Monte Carlo simulations have been conducted as well. The temperature dependence of the saturation coverage shows two structures which could be assigned to adsorption on pristine and intrinsic defect sites, respectively, in agreement with a prior thermal desorption spectroscopy study. The heat of adsorption E(d) for the pristine sites amounts to 34.0-5.4Theta, whereas for adsorption on the intrinsic defect sites E(d) of approximately 43.6 kJ/mol could be estimated. Thus, a kinetic structure-activity relationship was present.

Adsorption dynamics of CO on Cu(110): a molecular beam study

Molecular beam measurements of initial adsorption probabilities, S 0 , as well as the coverage dependence of the adsorption probability, S(Θ), of CO on Cu(110) are presented. The influence of kinetic energy, polar impact angle, α i , azimuthal orientation ([100] and [110]), and adsorption temperature, T s , on the adsorption dynamics have been studied. With regard to microkinetical models used to describe processes in heterogeneous catalysis, the dependence of the adsorption probability on the density of defects has been investigated. The surface has been characterized by measuring He atom angular distributions, He atom reflectivity curves, and LEED. The shape of the S(Θ) curves are consistent with a precursor mediated adsorption mechanism. For low temperatures T s ≤ 90 K, the adsorption dynamics are dominated by adsorbate assisted adsorption. Despite the pronounced difference in the surface corrugation along the different high-symmetry directions no significant differences in S 0 and S(Θ) were observed for the different azimuths studied.

The adsorption of hydrogen on the rutile TiO2(110) surface

The adsorption of hydrogen on a well defined single crystal rutile TiO2(110) surface has been investigated using helium atom scattering (HAS). Whereas the adsorption probability of molecular hydrogen was below the detection limit (S < 2 × 10−7), the hydrogen adlayers were prepared by exposing the clean surface to atomic hydrogen. The results reveal the formation of a H(1 × 1) TiO2(110) surface. In addition, the adsorption and desorption kinetics were studied by monitoring the reflectivity of the surface for helium atoms. The analysis of the data yielded two desorption maxima with activation energies for desorption of 99 kJ mol−1 and 162 kJ mol−1.

Adsorption of CO2 on oxidized, defected, hydrogen and oxygen covered rutile (1 × 1)-TiO2(110)

Presented are initial, S(0) and coverage, Theta, dependent S(Theta), adsorption probability measurements of CO(2) as a function of impact energy, E(i) = 0.12-1.3 eV, adsorption temperature, T(s) = 85-300 K, hydrogen and oxygen pre-exposure, as well as density of defects, Gamma, as varied by annealing (T = 600-900 K) and Ar(+) ion sputtering (dose chi(Ar) at 600 eV at 85 K) of a rutile (1 x 1) TiO(2)(110) surface. The defect densities were qualitatively characterized by thermal desorption spectroscopy (TDS) of CO(2). The CO(2) TDS curves consisted of two structures that can be assigned to adsorption on pristine and oxygen vacancy sites, in agreement with earlier studies. S(0) decreased linearly with E(i) and was independent of T(s). The adsorption dynamics were dominated by the effect of precursor states leading to Kisliuk-like shapes over the E(i) and T(s) range studied. Oxygen vacancy sites reduced S(0) of CO(2). Preadsorbed oxygen blocked preferentially defect sites, which led to an increase in S(0). Hydrogen preadsorption results in physical site blocking with decreased S(0) as H-preexposure increased, while the shape of S(Theta) curves was conserved. In contrast to oxygen, hydrogen does not adsorb preferentially on defect sites. The adsorption probability data were parameterized by analytic functions (Kisliuk model) and by Monte Carlo simulations (MCSs).

Adsorption of hydrogen on the polar O–ZnO surface: a molecular beam study

The interaction of atomic and molecular hydrogen with the polar oxygen-terminated ZnO(000-1) surface has been studied by He atom scattering, LEED, and He atom reflectivity measurements. Whereas the sticking coefficient S0 for molecular hydrogen is so low that only an upper limit can be given (S0 < 3 × 10−7), adsorption of atomic hydrogen leads to the formation of a H(1 × 1) adlayer. Prolonged exposure to H atoms induces a disordering of the surface. The desorption temperature of the adsorbed H atoms was determined by monitoring the reflectivity of the surface for the He atoms. Assuming that the activation energy for desorption is equal to the H atom binding energy and using a pre exponential factor of 1021 cm2/(mol s) we yield a value of 163 kJ/mol for the H atom binding energy.

Adsorption of CO on the copper-precovered ZnO(0001) surface : A molecular-beam scattering study

Auger electron spectroscopy (AES), thermal-desorption spectroscopy (TDS), and, in particular, molecular-beam scattering techniques have been combined to address particle size effects in the adsorption of CO on Cu-on-ZnO(0001). AES and TDS lead to a Cu coverage, theta(Cu), calibration. The TDS curves, as a function of theta(Cu) and CO exposure, revealed three structures at 150, 220, and 260-280 K, in agreement with prior studies. However, a unique assignment of the TDS structures to a particular Cu face was not possible. An enhancement of the initial adsorption probability, S0, with respect to the support indicates the effect of the Cu nanoparticles. Despite that the shape of S0 versus impact energy curves was independent of theta(Cu) and agreed with Cu single-crystal reference systems, distinct particle size effects were present with regard to the adsorption mechanism. It was possible to observe a crossover from Langmuir-type adsorption dynamics to more precursor-assisted adsorption dynamics with increasing theta(Cu). Thus, a dynamic structure-activity relationship was evident, i.e., the energy-transfer mechanism depends on the Cu morphology.

Transient study of carbon monoxide oxidation on Pd(111) and Pd(110)

Abstract We report measurements of CO oxidation by CO molecular beam titration on Pd(110) and Pd(111) surfaces pre-covered with atomically bonded oxygen. The results are semi-quantitatively discussed by computer simulations with rate equations of the Langmuir–Hinshelwood type.

Adsorption dynamics of CO2 on copper-precovered ZnO(0001)–Zn: A molecular-beam scattering and thermal-desorption spectroscopy study

Initial, S(0), as well as coverage-dependent adsorption probability measurements, S(Theta), have been conducted at normal impact angle and as a function of the impact energy of CO(2), E(i), adsorption temperature, T(s), and copper precoverage, Theta(Cu) (at 300 K). S(0), which decreased from approximately 0.4 exponentially to approximately 0.05 with E(i) was independent of Theta(Cu). Astonishingly, S(0) for Cu on ZnO(0001)-Zn is smaller than for the clean support which indicates a chemical modification of the support by the Cu deposits. S(Theta) curves consist of two regimes, a Kisliuk-type and Langmuirian-type section. The first is consistent with capture zone models; the second may indicate direct adsorption of CO(2) on the Cu cluster. The thermal-desorption (TDS) curves for Cu on ZnO(0001)-Zn consist of two structures with binding energies of 26 and approximately 40 kJmol (nu=1 x 10(13) ls). The TDS results indicate that CO(2) populates predominantly the Cu deposits and the rim along the Cu nanoparticles. No indications for CO(2) dissociation could be obtained with Auger electron spectroscopy.

Adsorption probabilities of CO on O–ZnO: A molecular beam study

We present measurements of initial adsorption probabilities, S0, as well as its coverage dependence, S(ΘCO), of CO on oxygen terminated ZnO(0001). The impact energies of the CO (48 meV<Ei<750 meV) and surfaces temperature (75 K<TS< 220 K) have been varied. The shape of the coverage dependent adsorption probability curves indicates the presence of precursor mediated adsorption. The heat of adsorption has been determined to (7–2 ΘCO) kcal/mol by assuming a pre-exponential factor of 1×1013 l/s.