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Many computational studies of catalytic surface reaction kinetics have demonstrated the existence of linear scaling relationships between physical descriptors of catalysts and reaction barriers on their surfaces. In this work, the relationship between catalyst activity, electronic structure, and alloy composition was investigated experimentally using a AgxPd1−x Composition Spread Alloy Film (CSAF) and a multichannel reactor array that allows measurement of steady-state reaction kinetics at 100 alloy compositions simultaneously. Steady-state H2 −D2 exchange kinetics were measured at atmospheric pressure on AgxPd1−x catalysts over a temperature range of 333−593 K and a range of inlet H2 and D2 partial pressures. X-ray photoelectron spectroscopy (XPS) was used to characterize the CSAF by determining the local surface compositions and the valence band electronic structure at each composition. The valence band photoemission spectra showed that the average energy of the valence band, ε̅v, shifts linearly with composition from −6.2 eV for pure Ag to −3.4 eV for pure Pd. At all reaction conditions, the H2 −D2 exchange activity was found to be highest on pure Pd and gradually decreased as the alloy was diluted with Ag until no activity was observed for compositions with xPd < 0.58. Measured H2 −D2 exchange rates across the CSAF were fit using the Dual Subsurface Hydrogen (2H′) mechanism to extract estimates for the activation energy barriers to dissociative adsorption, ΔEads ‡ , associative desorption, ΔEdes ‡ , and the surface-to-subsurface diffusion energy, ΔEss, as a function of alloy composition, xPd. The 2H′ mechanism predicts ΔEads ‡ = 0−10 kJ/mol, ΔEdes ‡ = 30−65 kJ/mol, and ΔEss = 20−30 kJ/mol for all alloy compositions with xPd ≥ 0.64, including for the pure Pd catalyst (i.e., xPd = 1). For these Pd-rich catalysts, ΔEdes ‡ and ΔEss appeared to increase by ∼5 kJ/mol with decreasing xPd. However, due to the coupling of kinetic parameters in the 2H′ mechanism, we are unable to exclude the possibility that the kinetic parameters predicted when xPd ≥ 0.64 are identical to those predicted for pure Pd. This suggests that H2 −D2 exchange occurs only on bulk-like Pd domains, presumably due to the strong interactions between H2 and Pd. In this case, the decrease in catalytic activity with decreasing xPd can be explained by a reduction in the availability of surface Pd at high Ag compositions.
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Kinetic Parameter Estimation for Catalytic H2–D2 Exchange on Pd
Kinetic parameters have been estimated for the H2– D2 exchange reaction on a thin film Pd catalyst by fitting reaction data from T = 333 to 593 K over a range of inlet partial pressures, Pin H2 and Pin D2 . A rigorous approach to estimating the 95% confidence regions of the kinetic parameters reveals some of the issues and complexities that are not routinely considered in the estimation of kinetic parameter uncertainty from catalytic data. Three different mechanistic models were used to assess the influence of subsurface hydrogen, H′: the traditional Langmuir–Hinshelwood (LH) mechanism, the Single Subsurface Hydrogen (1H′) mechanism, and the Dual Subsurface Hydrogen (2H′) mechanism. The fitting was performed by fixing the preexponential factors for all Arrhenius rate constants and equilibrium constants to their transition state theory values. The diffusion of H and D atoms from the surface into the subsurface was constrained to be endothermic (i.e. ΔE ss > 0) and represented as an equilibrium process. Performance of the fitting routine was evaluated on a noiseless simulated dataset (created using ΔE‡ ads = 0, ΔE‡ des = 43, and ΔE ss = 25 kJ/mol) and the same simulated dataset with the inclusion of 3% Gaussian noise. In both cases, the solver was able to return the chosen values of ΔE‡ ads , ΔE‡ des , and ΔE ss . Mapping of the behavior of the residual sum of squared errors, 2 , about its global minimum within 3D ( ads , des , ss ) parameter space allowed quantification and visualization of the 95% confidence regions using 2D error ellipses for each pair of fitting parameters. For the experimental dataset on the Pd catalyst, fitting to the LH model predicted that H2– D2 exchange is adsorption rate limited, with ΔE‡ ads = 51.1 ± 0.6 kJ/mol with 95% confidence. On the other hand, fitting to both the 1H′ and 2H′ models led to predictions of ΔE‡ ads = 0, consistent with the current understanding that the barrier to H2 dissociation on Pd is low. Thus, the results detailed herein provide supporting evidence for a non-LH mechanism for H2– D2 exchange on Pd while also illustrating the issues associated with quantification of uncertainty in kinetic parameter estimation.
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- Award ID(s):
- 1954340
- PAR ID:
- 10586309
- Publisher / Repository:
- Springer Nature
- Date Published:
- Journal Name:
- Catalysis letters
- ISSN:
- 1572-879X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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