Kinetics of NH₃ Desorption and Diffusion on Pt: Implications for the Ostwald Process

Abstract

We report accurate time-resolved measurements of NH 3 desorption from Pt(111) and Pt(332) and use these results to determine elementary rate constants for desorption from steps, from (111) terrace sites and for diffusion on (111) terraces. Modeling the extracted rate constants with transition state theory, we find that conventional models for partition functions, which rely on uncoupled degrees of freedom (DOFs), are not able to reproduce the experimental observations. The results can be reproduced using a more sophisticated partition function, which couples DOFs that are most sensitive to NH 3 translation parallel to the surface; this approach yields accurate values for the NH 3 binding energy to Pt(111) (1.13 ± 0.02 eV) and the diffusion barrier (0.71 ± 0.04 eV). In addition, we determine NH 3 ’s binding energy preference for steps over terraces on Pt (0.23 ± 0.03 eV). The ratio of the diffusion barrier to desorption energy is ∼0.65, in violation of the so-called 12% rule. Using our derived diffusion/desorption rates, we explain why established rate models of the Ostwald process incorrectly predict low selectivity and yields of NO under typical reactor operating conditions. Our results suggest that mean-field kinetics models have limited applicability for modeling the Ostwald process.

Velocity‐resolved Laser‐induced Desorption for Kinetics on Surface Adsorbates

“…The LID ion‐imaging‐based detection produces a signal that is proportional to the adsorbate surface density, which decays exponentially after the molecular beam dosing pulse. The derived 1 st order desorption rate constants are consistent with prior work, [2f,i] which, due to the advantages of LID, are easily extended to lower surface temperatures. This overcomes one of the limitations of VRK.…”

“… Rate constants obtained by monitoring the time‐dependent adsorbate surface density with laser‐induced desorption velocity‐resolved kinetics (black squares) for CO and NH 3 desorption from Pt (111). The LID results are compared to previous work using conventional VRK (blue), [2b,f] UV‐UV double resonance VRK (red), [2i] molecular beam relaxation spectroscopy (MBRS, magenta), [7] and thermal energy atom scattering (TEAS, green) [7] …”

“…VRK is capable of providing surface site specific rate constants; [2b] for example, reactions at terraces may appear with different product‐velocities than reactions at steps [3] and kinetic traces may depend dramatically on surface step‐density [2f] . Taking advantage of its kinetic competition with desorption, adsorbate diffusion has also been detected with VRK [2e,f] . While TPD, MBRS and VRK have proven highly useful, they cannot be used to monitor the concentrations of surface adsorbates directly, as they rely on desorption for their signals.…”

“…In this way, the product density and velocity are determined simultaneously. VRK is capable of providing surface site specific rate constants; [2b] for example, reactions at terraces may appear with different product‐velocities than reactions at steps [3] and kinetic traces may depend dramatically on surface step‐density [2f] . Taking advantage of its kinetic competition with desorption, adsorbate diffusion has also been detected with VRK [2e,f] .…”

Near-Ambient Pressure Velocity Map Imaging

“…These strengths of imaging detection have recently been applied to molecular beam surface scattering experiments, where they can be used to probe the dynamics of the molecule-surface interactions 36,37 Using pulsed molecular beams and lasers even allows pumpprobe measurements of the kinetics of surface processes and reactions by directly probing the time-dependent flux of molecules leaving the surface. [38][39][40] Wodtke and co-workers are developing high repetition rate imaging experiments that can probe the time dependence within a single MB pulse, and will allow changes in the surface during reaction to be investigated. 41 Typically, the pressure in the extraction region of an imaging system must be kept in the high vacuum region (below 1 × 10 −6 mbar) to avoid collisions of the particles with background gas and to prevent electrical arcing on the ion optics, or worse, the microchannel plate(s) and phosphor screen of the detector.…”