Kiyoshi Nagai

A simple rate equation useful for adsorption systems: Analyses of thermal desorption spectra

Abstract Using the lattice gas model, a simple rate equation is derived by stressing upon the many body aspect among adsorbates within the absolute reaction rate theory. The rate equation incorporates two more excess modification factors — interaction among reactants and the excluded volume effect — than the conventional Polanyi-Wigner or Arrhenius equation. From the thermodynamic discussion, it is concluded that abnormal zero-order desorption spectra are always observed when the adsorbates are in the liquid-gas two-phase coexistence . Consequently, the possibility of constructing a phase diagram from thermal desorption spectra (TDS) is suggested. The present interpretation of the zero-order desorption is also successful to the associative desorption in the H/Zn(0001) system. Owing to the entropy effect among adsorbates particles (excluded volume effect), the shift in temperature of the TDS peak is obtained when the initial coverages are varied, even for the non-interacting adsorbates case. Comparisons of the calculated results with typical experimental spectra are reported. The difference between our equation and those of the precursor theories is explained.

Transformation of iridium(110) (1×1) into (1×2) and spatial distribution of reactive carbon dioxide desorption

The spatial distribution of the desorption flux of CO2 produced on Ir(110) (1×1) and (1×2) surfaces was studied by means of angle‐resolved thermal desorption and low‐energy electron diffraction. The distribution is collimated along the bulk surface normal on (1×1). It is sharp in the [001] direction and sharper in the [110] direction. This distribution is consistent with the model that the reactive desorption occurs on a short bridge site. On (1×2) surfaces, two‐directional desorption was observed, which was collimated along the axis at the polar angle of 26 deg in both [001] and [001] directions. The distribution in the [110] direction is collimated along the bulk surface normal. The reactive desorption was suggested to take place on a threefold hollow site on the declining terrace. The spatial distribution changed from the (1×1) type to the (1×2) type during the transformation of the surface structure. This structure change was confirmed by low‐energy electron diffraction.

Zero-order desorption kinetics based on phase equilibrium

Abstract The zero-order desorption kinetics is described for adsorbate systems in which three phases are in equilibrium and first-order desorption kinetics is assumed for the desorption from the topmost phase. The calculated results represent typical features of the observed zero-order desorption spectra. The possibility of specifying the phase boundaries from the thermal or isothermal desorption spectra is proposed. The relationship between the thermal or isothermal desorption processes and trajectories in the phase diagram is also discussed.

Zero-order desorption is always observed in phase equilibrium within adsorbates?

The zero-order desorption spectra are markedly well described by the desorption rate formula recently proposed by us. According to the formula the rate is proportional to the activity of adsorbates and the activity becomes constant independent of density when liquid-gas two-phase equilibrium occurs in an adsorbate layer. The zero-order desorption was also observed in another type of phase equilibrium, i.e., the phase equilibrium between adsorbate-induced-reconstructed and normally-adsorbed phases of the H/Ni(110) system. These two kinds of experimental findings of the zero-order desorption suggest that the zero-order desorption must generally be observed in any kind of phase equilibrium of first-order phase transitions. Taking this argument for granted, we can extract thermodynamic information about an adsorbate from conventional techniques of thermal or isothermal desorption experiments. As an example of such data analysis, the isothermal plot of chemical potential versus coverage is obtained from observed thermal desorption spectra.

Excluded volume effects on reaction rates: A desorption rate version of the langmuir isotherm

Abstract A reaction rate version of the Langmuir isotherm is derived exactly from the non-interacting lattice gas model within the framework of absolute rate theory. The rate has an excess multiplication factor arising from the excluded volume effect on the Arrhenius equation and gives a good fit to experimental thermal desorption spectra.

Velocity distributions of desorbing products in the oxidation of carbon monoxide on palladium (110) surfaces

Abstract The velocity distribution of desorbing CO 2 product from Pd(110) surfaces were measured by cross-correlation time-of-fight techniques combined with angle-resolved thermal desorption. Heating co-adlayers of CO and oxygen yields five peaks in the CO 2 formation spectrum in the range of 150 to 460 K. The translational temperature of CO 2 desorbing along the surface normal was estimated to be above 1600 K for all peaks. It increases with increasing the reactant coverage, and decreases with increasing the desorption angle.

Theory of angular and speed distributions of desorbed hydrogen from metal surfaces

Abstract A quantum theoretical approach for angular and speed distributions of recombinatively desorbed D2 from metal surfaces is proposed. The complex D‡2 is activated on the saddle point of the three-dimensional potential surface with respect to its center-of-mass coordinate and vibrates with frequencies v‡2 parallel to the metal surface. Through the interface of the metal with the vacuum these two vibration modes are coupled with translational motion along the reaction path and lead to a transmission coefficient in which momentum representation of the vibrational wave function enters. Angle-resolved speed distributions of the desorption rate are calculated on the basis of Eyrings theory of rate processes, incorporated with quantum mechanical effects. Theoretical predictions reproduce very well a variety of the characteristic features of the experimental results.

Spatial distribution of reactive carbon dioxide desorption on Ir(110)(1 × 2) reconstructed surfaces

Abstract The spatial distribution of the desorption flux of carbon dioxide produced on Ir (110)(1 × 2) reconstructed surfaces was studied by means of angle-resolved thermal desorption and low energy electron diffraction. A single CO 2 formation peak appeared around 380 K while heating the coadlayer of CO(a) and O(a). This CO 2 shows two-directional desorption collimated at about ± 26° off the bulk surface normal in the [001] direction. The orientation of the reaction site is well preserved in the spatial distribution of the product desorption.

Zero-order desorption kinetics observed in phase coexistence regions in adsorbates

Abstract The zero-order desorption spectrum (ZODS), which shows outstanding characteristics of a strictly constant desorption rate, has been observed coincidentally with the occurrence of the liquid-gas phase coexistence region in adsorbate layers. A different type of ZODS was also reported recently, which was observed in the phase-coexistence region between the adsorbate-induced-reconstructed and normally-adsorbed phases of the H / Ni(110) system . An attempt is made to see whether existing theories might reproduce the characteristics of ZODS. It is shown that they fail in the attempt. Finally, our thermodynamic interpretation is proposed, which qualitatively interprets well the characteristics of ZODS as well as those observed in the H/Ni(110) system. Our interpretation rests on a new type of desorption rate which was derived from a naive basis of transition state theory. In conclusion, a precise reinvestigation of the elementary rate, via experimental examinations, is desirable.