Angle resolved intensity and velocity distributions of N2 desorbed by N2O decomposition on Rh(110).

The angle resolved intensity and velocity distributions of desorbing product N(2) were measured under a steady-state N(2)O+CO reaction on Rh(110) by cross-correlation time-of-flight techniques. Three-dimensional intensity distribution of N(2) has been constructed from the angle resolved intensity distributions in the planes along different crystal azimuths. N(2) desorption has been found to split into two lobes sharply collimated along 50-63 degrees off normal toward [001] and [001] directions, suggesting that N(2)O is decomposed through the transition state of N(2)O adsorbed with the molecular axis parallel to the [001] direction. From the velocity distribution analysis, each desorption lobe is found to consist of two components with different peak angles, ca. 50 degrees and 74 degrees off normal. In both lobe cases, desorption components have been interpreted by the model of two adsorption sites; N(2)O at on-top site emits N(2) to 50 degrees and that at bridge site emits to 74 degrees.

Polar- and azimuth-angle-dependent rotational and vibrational excitation of desorbing product CO2 in CO oxidation on palladium surfaces.

Clear polar and azimuth angle dependencies were found in rotational and vibrational energies of product CO2 in CO oxidation on Pd surfaces. On Pd(110)-(1x1), with increases in polar angle, both energies decreased in the [001] direction but remained constant in [110]. On the Pd(110) with missing rows, both energies increased in [001] but decreased in [110], indicating that the transition state changes with the geometry of the substrate. On Pd(111), the rotational energy greatly increased, but the vibrational energy decreased. Such angular dependence of internal energy provides new dimensions in surface reaction dynamics.

N2 emission in NO and N2O reduction on Rh(100) and Rh(110).

The angular distribution of desorbing N(2) was studied in both the thermal decomposition of N(2)O(a) on Rh(100) at 60-140 K and the steady-state NO (or N(2)O) + D(2) reaction on Rh(100) and Rh(110) at 280-900 K. In the former, N(2) desorption shows two peaks at around 85 and 110 K. At low N(2)O coverage, the desorption at 85 K collimates at about 66 degrees off normal towards the [001] direction, whereas at high coverage, it sharply collimates along the surface normal. In the NO reduction on Rh(100), the N(2) desorption preferentially collimates at around 71 degrees off normal towards the [001] direction below about 700 K, whereas it collimates predominantly along the surface normal at higher temperatures. At lower temperatures, the surface nitrogen removal in the NO reduction is due to the process of NO(a) + N(a) --> N(2)O(a) --> N(2)(g) + O(a). On the other hand, in the steady-state N(2)O + D(2) reaction on Rh(110), the N(2) desorption collimates closely along the [001] direction (close to the surface parallel) below 340 K and shifts to ca. 65 degrees off normal at higher temperatures. In the reduction with CO, the N(2) desorption collimates along around 65 degrees off normal towards the [001] direction above 520 K, and shifts to 45 degrees at 445 K with decreasing surface temperature. It is proposed that N(2)O is oriented along the [001] direction on both surfaces before dissociation and the emitted N(2) is not scattered by adsorbed hydrogen.

Internal-energy measurements of angle-resolved product CO2 in catalytic CO oxidation by means of infrared chemiluminescence.

Measurements of both vibrational and rotational energies of product CO(2) in CO oxidation on palladium surfaces have been successfully performed as a function of the desorption angle by means of infrared chemiluminescence. The remarkable angle dependences of both energies indicate facile energy partitioning in repulsive desorption and provide new dimensions in the study of surface reaction dynamics as well as additional insights into the product formation site. Details of the apparatus for energy analysis of angle-resolved products are described, especially on how to pick up extremely weak infrared emission signals.

Interplay of first-order kinetic and thermodynamic phase transitions in heterogeneous catalytic reactions.

Using the rate constants obtained on the basis of independent transient measurements and density functional theory calculations, we perform Monte Carlo (MC) simulations of the bistable kinetics of the N2O-CO reaction on Pd(110) at 450 K. In the absence of lateral interactions, the MC technique predicts a wide hysteresis loop in perfect agreement with the mean-field analysis. With attractive substrate-mediated lateral interactions resulting in the formation of (1 x 2) O islands and reducing the reaction rate inside islands, the hysteresis is found to be dramatically (about 5 times) narrower. This finding explains why the first-order kinetic phase transition experimentally observed in this reaction is not accompanied by hysteresis.

The collimation angle shift of desorbing product N2 in a steady-state N2O+CO reaction on Rh(110).

The angular distribution of desorbing product N2 was studied in N2O decompositions on Rh(110) in the temperature range of 60-700 K. The N2 desorption collimates along 62 degrees -68 degrees off normal toward either the [001] or [001] direction in a transient N2O decomposition below ca. 470 K or in the steady-state N2O+CO reaction above 540 K. In the steady-state reaction at the temperature from ca. 470 to 540 K, however, the collimation angle shifts from 62 degrees to 45 degrees with decreasing surface temperature. This angle shift is ascribed to the steric hindrance by coadsorbed CO because the N2 collimation in transient N2O decomposition at around 65 degrees is recovered in the range of 380-500 K by an abrupt CO pressure drop followed by the decrease in CO coverage. N2O is oriented along the [001] direction before dissociation. A scattering model of the nascent N2 by adsorbed CO is proposed, yielding smaller collimation angles.

Spatial distributions of desorbing products in steady-state NO and N2O reductions on Pd(110).

The angular and velocity distributions of desorbing product N(2) were examined over the crystal azimuth in steady-state NO+CO and N(2)O+CO reactions on Pd(110) by cross-correlation time-of-flight techniques. At surface temperatures below 600 K, N(2) desorption in both reactions splits into two directional lobes collimated along 41 degrees -45 degrees from the surface normal toward the [001] and [001] directions. Above 600 K, the normally directed N(2) desorption is enhanced in the NO reduction. Each product desorption component, as well as CO(2), shows a fairly asymmetric distribution about its collimation axis. Two factors, i.e., the anisotropic site structures and the reactant orientation and movements, are operative to induce such asymmetry, depending on the product emission mechanism.

Inclined N2 desorption in N2O reduction by D2 and CO on Pd(110).

Inclined N2 desorption was examined in the course of a catalyzed N2O + D2 (or CO) reaction on Pd(110) by angle-resolved mass spectroscopy combined with cross-correlation time-of-flight techniques. N2 desorption collimated at around 45 degrees off the normal toward the [001] direction in the temperature range of 400-800 K. Its collimation angle and kinetic energy were insensitive to both the surface temperature and surface conditions throughout the kinetic transition. It is proposed that this peculiar N2 desorption is induced by the decomposition of N2O oriented along the [001] direction.

Kinetics and dynamics of N2 formation in a steady-state N2O + CO reaction on Pd(110).

The N(2)O decomposition kinetics and the product (N(2) and CO(2)) desorption dynamics were studied in the course of a catalyzed N(2)O+CO reaction on Pd(110) by angle-resolved mass spectroscopy combined with cross-correlation time-of-flight techniques. The reaction proceeded steadily above 400 K, and the kinetics was switched at a critical CO/N(2)O pressure ratio. The ratio was about 0.03 at 450 K and reached approximately 0.08 at higher temperatures. Below it, the reaction was first order in CO, and negative orders above it. Throughout the surveyed conditions, the N(2) desorption sharply collimated along about 45 degrees off the normal toward the [001] direction. Desorbing N(2) showed translational temperatures in the range of 2000-5000 K. It is proposed that the decomposition proceeds in N(2)O(a) oriented along the [001] direction. On the other hand, the CO(2) desorption sharply collimated along the surface normal, showing a translational temperature of about 1600 K.

Collision-induced desorption in 193-nm photoinduced reactions in (O2+CO) adlayers on Pt(112).

The spatial distribution of desorbing O(2) and CO(2) was examined in 193-nm photoinduced reactions in O(2)+CO adlayers on stepped Pt (112)=[(s)3(111)x(001)]. The O(2) desorption collimated in inclined ways in the plane along the surface trough, confirming the hot-atom collision mechanism. In the presence of CO(a), the product CO(2) desorption also collimated in an inclined way, whereas the inclined O(2) desorption was suppressed. The inclined O(2) and CO(2) desorption is explained by a common collision-induced desorption model. At high O(2) coverage, the CO(2) desorption collimated closely along the (111) terrace normal.

Spatial distributions of desorbing products in basic catalytic processes and surface nano-structures.

Reviews of recent progress in angle-resolved measurements of desorbing surface reaction products are discussed. The angular and velocity distributions of desorbing products deliver information about the reaction site as well as the reaction mechanism when the products are repulsively desorbed. These distribution measurements can yield symmetry and orientation information of the reaction site for associative processes whereas, in dissociative desorption, the collimation of fragment desorption is related to the orientation of the intermediate species immediately before dissociation. These different collimations provide information on desorption steps whenever any step becomes rate determining.

A density-functional theory study of the interaction of N2O with Rh110.

The adsorption of nitrous oxide, N2O, on a Rh110 surface has been characterized by using density-functional theory. N2O was found to bind to the surface in two alternative forms. The first, less stable form is tilted with the terminal N atom attached to the surface, while the second, more stable form lies horizontally on the surface. Adsorption on the on-top site is more stable than that on the bridge site. The tilted form remains linear on adsorption, while the horizontal form is bent, with the terminal-nitrogen and oxygen atoms pointing towards the surface. At lower adsorbate coverage, Theta less than or similar to 1/4 ML (ML-monolayer), the adsorption of a few horizontal N2O configurations is dissociative, i.e., N2O-->N2(a)+O(a). The N2O-surface interaction is discussed in terms of the electronic structure analysis.

Surface-nitrogen removal in a steady-state NO + H2 reaction on Pd(110).

Surface-nitrogen removal steps were analyzed in the course of a catalyzed NO + H(2) reaction on Pd(110) by angle-resolved mass spectroscopy combined with cross-correlation time-of-flight techniques. Four removal steps, i.e., (i) the associative process of nitrogen atoms, 2N(a) --> N(2)(g), (ii) the decomposition of the intermediate, NO(a) + N(a) --> N(2)O(a) --> N(2)(g) + O(a), (iii) its desorption, N(2)O(a) --> N(2)O(g), and (iv) the desorption as ammonia, N(a) + 3H(a) --> NH(3)(g), are operative in a comparable order. Above 600 K, process (i) is predominant, whereas the others largely contribute below 600 K. Process (iv) becomes significant at H(2) pressures above a critical value, about half the NO pressure. Hydrogen was a stronger reagent than CO toward NO reduction and relatively enhanced the N(a) associative process.

Inclined N2 desorption in a steady-state NO + CO reaction on Pt(100).

The angular and velocity distributions of desorbing products N2 and CO2 were studied in a steady-state NO + CO reaction on Pt(100). From the observation of the inclined N2 desorption, a contribution of the intermediate N2O decomposition pathway was first proposed on this surface. On the other hand, CO2 desorption collimated along the surface normal.

Inclined N2 desorption in a steady-state N2O + CO reaction on Pd(110).

The angular and velocity distributions of desorbing products were analyzed in the course of a catalyzed N2O + CO reaction on Pd(110). The reaction proceeded steadily above 450 K, and the N2 desorption merely collimated sharply along 45 degrees off the surface normal toward the [001] direction. It is proposed that this peculiar N2 desorption is induced by the decomposition of adsorbed N2O oriented along the [001] direction. On the basis of the observation of similar inclined N2 desorption in both NO + CO and N2O + CO reactions, the N2 formation via the intermediate N2Oa dissociation was confirmed in catalytic NO reduction.

Desorption of products in 193 nm photo-induced reactions in (O2 + CO) adlayers on Pt(112).

The spatial distributions of desorbing products were examined in 193 nm photo-induced reactions in O2 + CO adlayers on stepped Pt(112) = [(s)3(111) x (001)]. At high coverage of O2(a) and CO(a), both O2 and CO2 desorption collimated closely along the (111) terrace normal. The results were compared with those in thermal CO oxidation, and the origin of the collimation angle shift in the latter is discussed. On the other hand, at low CO(a) coverage, O2 and CO2 desorption collimated in inclined ways in the plane along the surface trough. At these collimation positions, the kinetic energy of desorbing 02 and CO2 was maximal, confirming the hot-atom collision mechanism.