Kinetics and dynamics of the initial adsorption of nitric oxide on Ir(111)

Abstract

The interaction of nitric oxide ͑NO͒ with an Ir͑111͒ surface has been studied with supersonic molecular beam techniques and electron energy loss spectroscopy. Initial adsorption probability S 0 , measurements as a function of incident kinetic energy E i , surface temperature T s , and angle of incidence i reveal that separate mechanisms govern adsorption at low and high kinetic energy. This distinction is reflected in measurements of the initial molecular adsorption probability where a decrease in the value of S 0 with increasing T s ͑between 77 and 300 K͒ is observed at low kinetic energy ͑E i Ͻ0.45 eV͒, but no surface temperature dependence is detected at high kinetic energy in this temperature range. We present a model describing both the molecular and dissociative chemisorption of NO on Ir͑111͒. At low kinetic energy, NO adsorbs initially as a physically adsorbed species. From this state, desorption to the gas phase or conversion to a molecularly chemisorbed state on the surface are competing processes which depend on surface temperature. The molecularly chemisorbed state is the precursor to dissociation for elevated surface temperatures. At high kinetic energy, NO adsorption occurs directly into the molecularly chemisorbed well, with the probability of trapping as a physically adsorbed species near zero and with undetectable direct dissociation. Indeed, after exposure of the Ir͑111͒ surface at 77 K to a high kinetic energy ͑1.3 eV͒ beam, surface vibrational spectroscopy measurements show only features attributable to molecularly chemisorbed NO. The success of this model in describing our measurements is demonstrated by the separate calculation from low and high kinetic energy data of rate constants corresponding to forward and reverse conversion from the molecularly chemisorbed well. Additionally, we discuss attempts to promote dissociation on the surface with vibrational energy and with a combination of translational and surface thermal energy.

Molecularly chemisorbed intermediates to oxygen adsorption on Pt(111): A molecular beam and electron energy-loss spectroscopy study

“…A schematic and more detailed description of the apparatus is available elsewhere. 20 The sample was cleaned by Ar ϩ ion sputtering, a 20 min oxidation cycle at 600 K, and several successive oxygen TPD cycles. A rather lengthy anneal ͑ϳ30 min͒ at 1300 K was required to decompose the oxide formed by high temperature oxidation.…”

“…Langmuir originally used the concept of mobile adsorption precursors nearly 70 years ago to explain the coverage independence observed in the adsorption probability of cesium on a tungsten filament, 13,14 and today the notion of trapping-mediated chemisorption via a physically adsorbed precursor is well-established as governing low translational energy ͑less than ϳ0.1 eV for oxygen͒ adsorption in a great many gas/transition metal systems. [15][16][17][18][19][20][21][22][23][24] In fact, a decrease in adsorption probability with increasing incident energy typically signals the predominance of a physically adsorbed precursor mechanism.…”

Thickness dependent crystallization kinetics of sub-micron amorphous solid water films

“…The specimen was maintained at 86.5 K during dosing to ensure rapid diffusion of CHF 2 Cl across the ice surface to unoccupied sites 60 and permit adsorption of only a single saturated layer, whose formation was identified by monitoring CHF 2 Cl uptake kinetics using the King and Wells reflectivity technique. 52,65,66 At the conclusion of the dose, the ice was quenched to 77 K and then heated at 0.5 K/s to 120 K, during which an angle-integrated TPD spectrum was acquired with a quadrupole mass spectrometer. The film was then partially crystallized by isothermal annealing for a prescribed period of time at a temperature between 136 and 140 K. ͑We have estimated local temperature increases within the film owing to heat evolved during CI nucleation and growth, and predict their magnitude to be negligibly small in our experiments, supporting the assumption of isothermal crystallization.͒ After cooling to 77 K, a new CHF 2 Cl TPD spectrum was acquired as described above.…”

Real-Time Observation of Nonadiabatic Surface Dynamics: The First Picosecond in the Dissociation of NO on Iridium

“…The pathway calculated by DFT involves bending in an entirely analogous fashion to the motion on the atop site [19,26]. The DFT minimum energy route starts from the hollow site, while experiments indicate that NO adsorbed on atop sites dissociates at low coverage on iridium [9,27]. We consider the bending motion from the atop site a viable path to dissociation, of which we monitor the first picosecond.…”