Selectivity of Palladium–Cobalt Surface Alloy toward Oxygen Reduction Reaction

Oxygen reduction reaction (ORR), O₂ + 4­(H⁺ + e^–) → 2H₂O, is one of the most important fundamental reactions occurring on the cathode catalytic surface of hydrogen fuel cells. Developing new catalysts by alloying metals other than the well-known but expensive Pt is the most feasible and economical way to improve the proficiency of fuel cells to a practicable level. In this paper, we employed density functional theory calculations to study the ORR mechanism on a promising and cheaper catalyst of PdCo(111) surface alloy. From the calculated enthalpy of mixing, we found that the alloy is most stable at about 30% Co; hence, the alloying substrates were sampled at this concentration of Co for exploring the ORR intermediates. We discovered on the PdCo substrates a new intermediate OHO that was not seen previously for Pt and Pd resulting in a new reaction pathway. From the detailed analysis on the reaction free energy diagrams, we gauged the ORR efficiency of the alloy versus Pt. The obtained results are in agreement with experiments in which the ORR activity of the alloy was found to be higher than that of Pt. We found that maximizing the number of Co atoms at the second atomic layer underneath a Pd skin provides the highest activity for the ORR

Kinetics of the Simplest Criegee Intermediate Reaction with Water Vapor: Revisit and Isotope Effect

The kinetics of the simplest Criegee intermediate (CH2OO) reaction with water vapor was revisited. By improving the signal-to-noise ratio and the precision of water concentration, we found that the kinetics of CH2OO involves not only two water molecules but also one and three water molecules. Our experimental results suggest that the decay of CH2OO can be described as d[CH2OO]/dt = −kobs[CH2OO]; kobs = k0 + k1[water] + k2[water]2 + k3[water]3; k1 = (4.22 ± 0.48) × 10–16 cm3 s–1, k2 = (10.66 ± 0.83) × 10–33 cm6 s–1, k3 = (1.48 ± 0.17) × 10–50 cm9 s–1 at 298 K and 300 Torr with the respective Arrhenius activation energies of Ea1 = 1.8 ± 1.1 kcal mol–1, Ea2 = −11.1 ± 2.1 kcal mol–1, Ea3 = −17.4 ± 3.9 kcal mol–1. The contribution of the k3[water]3 term becomes less significant at higher temperatures around 345 K, but it is not ignorable at 298 K and lower temperatures. By quantifying the concentrations of H2O and D2O with a Coriolis-type direct mass flow sensor, the kinetic isotope effect (KIE) was investigated at 298 K and 300 Torr and KIE(k1) = k1(H2O)/k1(D2O) = 1.30 ± 0.32; similarly, KIE(k2) = 2.25 ± 0.44 and KIE(k3) = 0.99 ± 0.13. These mild KIE values are consistent with theoretical calculations based on the variational transition state theory, confirming that the title reaction has a broad and low barrier, and the reaction coordinate involves not only the motion of a hydrogen atom but also that of an oxygen atom. Comparing the results recorded under 300 Torr (N2 buffer gas) with those under 600 Torr, a weak pressure effect of k3 was found. From quantum chemistry calculations, we found that the CH2OO + 3H2O reaction is dominated by the reaction pathways involving a ring structure consisting of two water molecules, which facilitate the hydrogen atom transfer, while the third water molecule is hydrogen-bonded outside the ring. Furthermore, analysis based on dipole capture rates showed that the CH2OO(H2O) + (H2O)2 and CH2OO(H2O)2 + H2O pathways will dominate in the three water reaction

Reactivity of Criegee Intermediates toward Carbon Dioxide

Recent theoretical work by Kumar and Francisco suggested that the high reactivity of Criegee intermediates (CIs) could be utilized for designing efficient carbon capture technologies. Because the <i>anti</i>-CH₃CHOO + CO₂ reaction has the lowest barrier in their study, we chose to investigate it experimentally. We probed <i>anti</i>-CH₃CHOO with its strong UV absorption at 365 nm and measured the rate coefficient to be ≤2 × 10^–17 cm³ molecule^–1 s^–1 at 298 K, which is consistent with our theoretical value of 2.1 × 10^–17 cm³ molecule^–1 s^–1 at the QCISD­(T)/CBS//B3LYP/6-311+G­(2d,2p) level but inconsistent with their results obtained at the M06-2X/aug-cc-pVTZ level, which tends to underestimate the barrier heights. The experimental result indicates that the reaction of a Criegee intermediate with atmospheric CO₂ (400 ppmv) would be inefficient (<i>k</i>_eff < 0.2 s^–1) and cannot compete with other decay processes of Criegee intermediates like reactions with water vapor (∼10³ s^–1) or thermal decomposition (∼10² s^–1)

Features in Vibrational Spectra Induced by Ar-Tagging for H₃O⁺Ar_<i>m</i>, <i>m</i> = 0–3

Understanding the spectral features for solvated hydronium has been hindered due to the strong and complex vibrational couplings that lead to broad bands in the aqueous phase. In this work, utilizing <i>ab initio</i> vibrational calculations, we determine how the vibrational couplings induced by the Ar microsolvation in H₃O⁺Ar_<i>m</i> <i>m</i> = 0–3 affect the observed spectra. With theoretical peak intensities and peak positions, we assign the experimental spectra. We also show that an increase in the number of Ar atoms results in an anticooperative blue shifting in the Ar-tagged OH stretching bands. This change in peak position of the OH stretching fundamental modulates the Fermi resonance with the bending overtone. This is observed as a distinct doublet feature at 3200 cm^–1 with varying intensities for H₃O⁺Ar₂ and H₃O⁺Ar₃. The coupling between the in-plane rotation of the hydronium and the bending modes of H₃O⁺ leads to the existence of a strong association bands around 1900 cm^–1

Effects of Co Content in Pd-Skin/PdCo Alloys for Oxygen Reduction Reaction: Density Functional Theory Predictions

Improving the slow kinetics of the oxygen reduction reaction (ORR) on the cathode of the proton exchange membrane fuel cells to achieve the performance at a practical level is an important task. PdCo alloys appeared as a promising electrocatalyst. Much attention has been devoted to the study of the effects of the Co content on the ORR activity of PdCo films and PdCo/C nanoparticles where the Co atoms can be at the topmost surface layer. While Pd-skin/PdCo alloys with the topmost layer formed only by Pd have been proved to provide a very high ORR activity and high durability, no researches are available in the literature for the effects of the Co content on the ORR activity of Pd-skin/PdCo alloys. Hence, the effects of the Co content on the ORR activity of Pd-skin/PdCo alloys are clarified in this work by using the density functional theory calculations and Nørskov’s thermodynamic model. Our results predicted that the ORR activity increases monotonically with the increase of the Co content. This behavior is particularly different compared to the Volcano behavior previously obtained in the literature for PdCo films and PdCo/C nanoparticles

Unimolecular Decomposition Rate of the Criegee Intermediate (CH₃)₂COO Measured Directly with UV Absorption Spectroscopy

The unimolecular decomposition of (CH₃)₂COO and (CD₃)₂COO was measured by direct detection of the Criegee intermediate at temperatures from 283 to 323 K using time-resolved UV absorption spectroscopy. The unimolecular rate coefficient <i>k</i>_d for (CH₃)₂COO shows a strong temperature dependence, increasing from 269 ± 82 s^–1 at 283 K to 916 ± 56 s^–1 at 323 K with an Arrhenius activation energy of ∼6 kcal mol^–1. The bimolecular rate coefficient for the reaction of (CH₃)₂COO with SO₂, <i>k</i>_SO₂, was also determined in the temperature range 283 to 303 K. Our temperature-dependent values for <i>k</i>_d and <i>k</i>_SO₂ are consistent with previously reported relative rate coefficients <i>k</i>_d/<i>k</i>_SO₂ of (CH₃)₂COO formed from ozonolysis of tetramethyl ethylene. Quantum chemical calculations of <i>k</i>_d for (CH₃)₂COO are consistent with the experiment, and the combination of experiment and theory for (CD₃)₂COO indicates that tunneling plays a significant role in (CH₃)₂COO unimolecular decomposition. The fast rates of unimolecular decomposition for (CH₃)₂COO measured here, in light of the relatively slow rate for the reaction of (CH₃)₂COO with water previously reported, suggest that thermal decomposition may compete with the reactions with water and with SO₂ for atmospheric removal of the dimethyl-substituted Criegee intermediate

Temperature-Dependent Rate Coefficients for the Reaction of CH₂OO with Hydrogen Sulfide

The reaction of the simplest Criegee intermediate CH₂OO with hydrogen sulfide was measured with transient UV absorption spectroscopy in a temperature-controlled flow reactor, and bimolecular rate coefficients were obtained from 278 to 318 K and from 100 to 500 Torr. The average rate coefficient at 298 K and 100 Torr was (1.7 ± 0.2) × 10^–13 cm³ s^–1. The reaction was found to be independent of pressure and exhibited a weak negative temperature dependence. <i>Ab initio</i> quantum chemistry calculations of the temperature-dependent reaction rate coefficient at the QCISD­(T)/CBS level are in reasonable agreement with the experiment. The reaction of CH₂OO with H₂S is 2–3 orders of magnitude faster than the reaction with H₂O monomer. Though rates of CH₂OO scavenging by water vapor under atmospheric conditions are primarily controlled by the reaction with water dimer, the H₂S loss pathway will be dominated by the reaction with monomer. The agreement between experiment and theory for the CH₂OO + H₂S reaction lends credence to theoretical descriptions of other Criegee intermediate reactions that cannot easily be probed experimentally

Dependence of Adenine Raman Spectrum on Excitation Laser Wavelength: Comparison between Experiment and Theoretical Simulations

We acquired the Raman spectra of adenine in powder and aqueous phase using excitation lasers with 532, 633, and 785 nm wavelengths for the region between 300 and 1500 cm^–1. In comparison to the most distinct peak at 722 cm^–1, the peaks between 1200 and 1500 cm^–1 exhibited a characteristic increase in cross-section with decreasing excitation wavelength in both phases. This trend can be reproduced by different density functional theory (DFT) calculations for the adenine molecule in the gas phase as well as in the aqueous phase. Furthermore, from the calculation on the π-stacked dimer, hydrogen-bonded dimer, and trimer, we find that this trend toward excitation laser wavelength is not sensitive to the packing. When comparing the Raman spectra given by different excitation wavelength, one should take care in analyzing the cross-section, and present day DFT calculations are able to capture general trends in the excitation laser wavelength dependence of the Raman activity

Strong Negative Temperature Dependence of the Simplest Criegee Intermediate CH₂OO Reaction with Water Dimer

The kinetics of the reaction of CH₂OO with water vapor was measured directly with UV absorption at temperatures from 283 to 324 K. The observed CH₂OO decay rate is second order with respect to the H₂O concentration, indicating water dimer participates in the reaction. The rate coefficient of the CH₂OO reaction with water dimer can be described by an Arrhenius expression <i>k</i>(<i>T</i>) = <i>A</i> exp­(−<i>E</i>_a/<i>RT</i>) with an activation energy of −8.1 ± 0.6 kcal mol^–1 and <i>k</i>(298 K) = (7.4 ± 0.6) × 10^–12 cm³ s^–1. Theoretical calculations yield a large negative temperature dependence consistent with the experimental results. The temperature dependence increases the effective loss rate for CH₂OO by a factor of ∼2.5 at 278 K and decreases by a factor of ∼2 at 313 K relative to 298 K, suggesting that temperature is important for determining the impact of Criegee intermediate reactions with water in the atmosphere

Strong Negative Temperature Dependence of the Simplest Criegee Intermediate CH₂OO Reaction with Water Dimer

The kinetics of the reaction of CH₂OO with water vapor was measured directly with UV absorption at temperatures from 283 to 324 K. The observed CH₂OO decay rate is second order with respect to the H₂O concentration, indicating water dimer participates in the reaction. The rate coefficient of the CH₂OO reaction with water dimer can be described by an Arrhenius expression <i>k</i>(<i>T</i>) = <i>A</i> exp­(−<i>E</i>_a/<i>RT</i>) with an activation energy of −8.1 ± 0.6 kcal mol^–1 and <i>k</i>(298 K) = (7.4 ± 0.6) × 10^–12 cm³ s^–1. Theoretical calculations yield a large negative temperature dependence consistent with the experimental results. The temperature dependence increases the effective loss rate for CH₂OO by a factor of ∼2.5 at 278 K and decreases by a factor of ∼2 at 313 K relative to 298 K, suggesting that temperature is important for determining the impact of Criegee intermediate reactions with water in the atmosphere