An official website of the United States government
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.
more »
« less
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.
more »
« less
- 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
More Like this
-
-
Hydrogen-rich cation radicals (GATT + 2H) + ˙ and (AGTT + 2H) + ˙ represent oligonucleotide models of charged hydrogen atom adducts to DNA. These tetranucleotide cation radicals were generated in the gas phase by one-electron reduction of the respective (GATT + 2H) 2+ and (AGTT + 2H) 2+ dications in which the charging protons were placed on the guanine and adenine nucleobases. We used wavelength-dependent UV/Vis photodissociation in the valence-electron excitation region of 210–700 nm to produce action spectra of (GATT + 2H) + ˙ and (AGTT + 2H) + ˙ that showed radical-associated absorption bands in the near-UV (330 nm) and visible (400–440 nm) regions. Born–Oppenheimer molecular dynamics and density-functional theory calculations were used to obtain and rank by energy multiple (GATT + 2H) dication and cation-radical structures. Time-dependent density functional theory (TD-DFT) calculations of excited-state energies and electronic transitions in (GATT + 2H) + ˙ were augmented by vibronic spectra calculations at 310 K for selected low-energy cation radicals to provide a match with the action spectrum. The stable product of one-electron reduction was identified as having a 7,8-dihydroguanine cation radical moiety, formed by intramolecular hydrogen atom migration from adenine N-1–H. The hydrogen migration was calculated to have a transition state with a low activation energy, E a = 96.5 kJ mol −1 , and positive activation entropy, Δ S ‡ = 75 J mol −1 K −1 . This allowed for a fast isomerization of the primary reduction products on the ion-trap time scale of 150 ms that was substantially accelerated by highly exothermic electron transfer.more » « less
-
ABSTRACT: We report the generation and spectroscopic study of hydrogen-rich DNA tetranucleotide cation radicals (GATC+2H)+• and (AGTC+2H)+•. The radicals were generated in the gas phase by one-electron reduction of the respective dications (GATC +2H)2+ and (AGTC+2H)2+ and characterized by collision-induced dissociation and photodissociation tandem mass spectrometry and UV−vis photodissociation action spectroscopy. Among several absorption bands observed for (GATC+2H)+•, the bands at 340 and 450 nm were assigned to radical chromophores. Timedependent density functional theory calculations including vibronic transitions in the visible region of the spectrum were used to provide theoretical absorption spectra of several low-energy tetranucleotide tautomers having cytosine-, adenine-, and thymine- based radical chromophores that did not match the experimental spectrum. Instead, the calculations indicated the formation of a new isomer with the 7,8-H-dihydroguanine cation radical moiety. The isomerization involved hydrogen migration from the cytosine N-3−H radical to the C-8 position in N-7-protonated guanine that was calculated to be 87 kJ mol−1 exothermic and had a low-energy transition state. Although the hydrogen migration was facilitated by the spatial proximity of the guanine and cytosine bases in the low-energy (GATC+2H)+• intermediate formed by electron transfer, the reaction was calculated to have a large negative activation entropy. Rice−Ramsperger−Kassel−Marcus (RRKM) and transition state theory kinetic analysis indicated that the isomerization occurred rapidly in hot cation radicals produced by electron transfer with the population-weighed rate constant of k = 8.9 × 103 s−1. The isomerization was calculated to be too slow to proceed on the experimental time scale in thermal cation radicals at 310 K.more » « less
-
null (Ed.)In this study, we elucidate the reaction kinetics for the simultaneous hydrodeoxygenation of xylitol to 1,2-dideoxypentitol and 1,2,5-pentanetriol over a ReOx-Pd/CeO2 (2.0 weight% Re, 0.30 weight% Pd) catalyst. The reaction was determined to be a zero-order reaction with respect to xylitol. The activation energy was elucidated through an Arrhenius relationship as well as non-Arrhenius kinetics. The Arrhenius relationship was investigated at 150–170 °C and a constant H2 pressure of 10 bar resulting in an activation energy of 48.7 ± 10.5 kJ/mol. The investigation of non-Arrhenius kinetics was conducted at 120–170 °C and a sub-Arrhenius relation was elucidated with activation energy being dependent on temperature, and ranging from 10.2–51.8 kJ/mol in the temperature range investigated. Internal and external mass transfer were investigated through evaluating the Weisz–Prater criterion and the effect of varying stirring rate on the reaction rate, respectively. There were no internal or external mass transfer limitations present in the reaction.more » « less
-
The vibrational predissociation dynamics of H2/D2···I35Cl(B,v′=3) complexes containing both para- and ortho-hydrogen prepared in different intermolecular vibrational levels were investigated. The Δv = −1 I35Cl(B,v = 2,j) rotational product-state distributions measured for excitation to the lowest-energy T-shaped levels of these complexes are mostly bimodal. The rotational distributions measured for excitation of the H2···I35Cl(B,v′=3) complexes are colder than those of the D2···I35Cl(B,v′=3) complexes, and there are only slight differences between those measured for the para- and ortho-hydrogen containing complexes. Excitation of the delocalized bending levels results in slightly colder rotational product-state distributions. The distributions suggest the dynamics result from more than impulsive dissociation off of the inner repulsive wall of the lower-energy H2/D2 + I35Cl(B,v = 2) potential surfaces of the products. The depths of these potentials and the energies available to these products also contribute to the dynamics. The formation of the Δv = −2, I35Cl(B,v = 1) product channel was only identified for excitation of levels within the ortho(j = 0)-D2 + I35Cl(B,v′=3) potential. The formation of this channel occurs via I35Cl(B,v′=3) vibrational to D2 rotational energy transfer forming the ortho(j = 2)-D2 + I35Cl(B,v = 1,j) products.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- The DOI is not currently available.
