NO Binding Energies to and Diffusion Barrier on Pd Obtained with Velocity-Resolved Kinetics

Borodin, D.; Rahinov, Igor; Fingerhut, Jan; Schwarzer, Michael; Hörandl, Stefan; Skoulatakis, Georgios; Schwarzer, Dirk; Kitsopoulos, Theofanis N.; Wodtke, Alec M.

doi:10.1021/acs.jpcc.1c02965

Cited by 8 publications

(30 citation statements)

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“… 37 Furthermore, Q ⧧ is easily computed using tabulated gas-phase vibrational frequencies and rotational constants. We carried out this procedure in a similar way to a recent report for NO desorption from Pd; 21 also, see section S6.3 of the SI .…”

Section: Further Analysis and Discussionmentioning

confidence: 99%

“…The hindered translational partition function exploits an established interpolation scheme, ensuring its proper behavior at low and high temperatures. 21,33,35 We treat NH 3 rotation around its symmetry axis as a free rotation, justified by our DFT results and in agreement with previous theoretical work. 36 The remaining DOFs are described by harmonic oscillators.…”

Section: Further Analysis and Discussionmentioning

confidence: 99%

“…Here, the partition functions for T x , y are described using a model potential, parametrized using DFT-calculated hindered translational frequencies (see Table ) and the experimentally derived terrace hopping barriers (see section ). The hindered translational partition function exploits an established interpolation scheme, ensuring its proper behavior at low and high temperatures. ,, We treat NH 3 rotation around its symmetry axis as a free rotation, justified by our DFT results and in agreement with previous theoretical work . The remaining DOFs are described by harmonic oscillators.…”

Section: Further Analysis and Discussionmentioning

confidence: 99%

“…We also used a Pt(332) crystal with a step density of 16.7%. Similar to previous work, ,, a 20–50 μs long pulsed molecular beam of NH 3 (0.5–2% NH 3 in He) passed from the source chamber through two differential pumping chambers before entering a surface-scattering chamber, with a base pressure of 2 × 10 –10 mbar. The incidence kinetic energy of NH 3 in the beam was ∼0.25 eV.…”

Section: Methodsmentioning

confidence: 99%

“…For example, the KB mechanism includes a 0.65 eV adsorption barrier for NO on Pt, in contradiction to our current understanding that NO–Pt adsorption is barrierless. 20 , 21 Beyond this, the rate parameters used to describe NH 3 desorption in the KB model include an unphysically low prefactor in the Arrhenius expression, suggesting the entropy of Pt-adsorbed NH 3 is higher than that of the gas-phase molecule. Clearly, there is a pressing need for reliable information on site-specific binding energies and entropies of NH 3 and other molecules on Pt surfaces.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2021

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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.

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Section: Further Analysis and Discussionmentioning

confidence: 99%

Section: Further Analysis and Discussionmentioning

confidence: 99%

Section: Further Analysis and Discussionmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

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

et al. 2022

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Most experimental methods for studying the kinetics of surface reactions – for example, temperature programmed desorption (TPD), molecular beam relaxation spectrometry (MBRS) and velocity‐resolved kinetics (VRK) – employ detection schemes that require thermal desorption. However, many adsorbates – for example reaction intermediates – never leave the surface under reaction conditions. In this paper, we present a new method to measure adsorbate concentrations on catalytic surfaces and demonstrate its utility for studying thermal desorption kinetics. After a short‐pulsed molecular beam deposits CO or NH3 on Pt (111), the surface is irradiated with an ultrashort laser pulse that induces desorption. Another tightly focused ultrashort laser pulse ionizes the gas‐phase molecules by a non‐resonant multiphoton process and the ions are detected. This two‐laser signal is then recorded as a function of time after the dosing molecular beam pulse and decays exponentially. First‐order thermal desorption rate constants are obtained over a range of temperatures and found to be in good agreement with past reports. Ion detection is done mass selectively with ion‐imaging, dispersing the gas phase molecules by their velocities. Since laser‐induced desorption (LID) produces hyperthermal gas phase molecules, they can be detected with little or no background. This approach is highly surface‐specific and exhibits sensitivity below 10−4 ML coverage. Because the signals are linearly proportional to adsorbate concentration, the method can be employed at lower temperatures than VRK, whose signal is proportional to reaction rate.

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Binding Energy and Diffusion Barrier of Formic Acid on Pd(111)

et al. 2022

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Velocity-resolved kinetics is used to measure the thermal rate of formic acid desorption from Pd(111) between 228 and 273 K for four isotopologues: HCOOH, HCOOD, DCOOH, DCOOD. Upon molecular adsorption, formic acid undergoes decomposition to CO2 and H2 and thermal desorption. To disentangle the contributions of individual processes, we implement a mass-balance-based calibration procedure from which the branching ratio between desorption and decomposition for formic acid is determined. From experimentally derived elementary desorption rate constants, we obtain the binding energy 639 ± 8 meV and the diffusion barrier 370 ± 130 meV using the detailed balance rate model (DBRM). The DBRM explains the observed kinetic isotope effects.

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NO Binding Energies to and Diffusion Barrier on Pd Obtained with Velocity-Resolved Kinetics

Cited by 8 publications

References 42 publications

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

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

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

Binding Energy and Diffusion Barrier of Formic Acid on Pd(111)

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NO Binding Energies to and Diffusion Barrier on Pd Obtained with Velocity-Resolved Kinetics

Cited by 8 publications

References 42 publications

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

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

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

Binding Energy and Diffusion Barrier of Formic Acid on Pd(111)

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Product

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

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