7 research outputs found
Thermal Activation of Methane by MgO+: Temperature Dependent Kinetics, Reactive Molecular Dynamics Simulations and Statistical Modeling
The kinetics of MgO + + CH 4 was studied experimentally using the variable ion source, temperature adjustable selected ion flow tube (VISTA-SIFT) apparatus from 300 − 600 K and computationally by running and analyzing reactive atomistic simula- tions. Rate coefficients and product branching fractions were determined as a function of temperature. The reaction proceeded with a rate of k = 5 . 9 ± 1 . 5 × 10 − 10 ( T/ 300 K) − 0 . 5 ± 0 . 2 cm 3 s − 1 . MgOH + was the dominant product at all temperatures, but Mg + , the co-product of oxygen-atom transfer to form methanol, was observed with a product branching fraction of 0 . 08 ± 0 . 03( T/ 300 K) − 0 . 8 ± 0 . 7 . Reactive molecular dynamics simulations using a reactive force field, as well as a neural network trained on thousands of structures yield rate coefficients about one order of magnitude lower. This underestimation of the rates is traced back to the multireference character of the transition state [MgOCH 4 ] + . Statistical modeling of the temperature-dependent kinetics provides further insight into the reactive potential surface. The rate limiting step was found to be consistent with a four-centered activation of the C-H bond, consistent with previous calculations. The product branching was modeled as a competition between dissociation of an insertion intermediate directly after the rate- limiting transition state, and traversing a transition state corresponding to a methyl migration leading to a Mg-CH 3 OH + complex, though only if this transition state is stabilized significantly relative to the dissociated MgOH + + CH 3 product channel. An alternative non-statistical mechanism is discussed, whereby a post-transition state bifurcation in the potential surface could allow the reaction to proceed directly from the four-centered TS to the Mg-CH 3 OH + complex thereby allowing a more robust competition between the product channels
Temperature and Isotope Dependent Kinetics of Nickel-Catalyzed Oxidation of Methane by Ozone
The temperature dependent kinetics of Ni+ + O3 and of NiO+ + CH4/CD4 are measured from 300 to 600 K using a selected-ion flow tube apparatus. Together, these reactions comprise a catalytic cycle converting CH4 to CH3OH. The reaction of Ni+ + O3 proceeds at the collisional limit, faster than previously reported at 300 K. The NiO+ product reacts further with O3, also at the collisional limit, yielding both higher oxides (up to NiO5+ is observed) as well as undergoing an apparent reduction back to Ni+. This apparent reduction channel is due to the oxidation channel yielding NiO2+∗ with sufficient energy to dissociate. 4NiO+ + CH4 (CD4) (whereas 4NiO+ refers to the quartet state of NiO+) proceeds with a rate constant of (2.6 ± 0.4) × 10-10 cm3 s-1 [(1.8 ± 0.5) × 10-10 cm3 s-1] at 300 K and a temperature dependence of ∼T-0.7±0.3 (∼T-1.1±0.4), producing only the 2Ni+ + 1CH3OH channel up to 600 K. Statistical modeling of the reaction based on calculated stationary points along the reaction coordinate reproduces the experimental rate constant as a function of temperature but underpredicts the kinetic isotope shift. The modeling was found to better represent the data when the crossing from quartet to doublet surface was incomplete, suggesting a possible kinetic effect in crossing from quartet to doublet surfaces. Additionally, the modeling predicts a competing 3NiOH+ + 2CH3 channel to become increasingly important at higher temperatures
Kinetics of Cations with C<sub>2</sub> Hydrofluorocarbon Radicals
Reactions
of the cations Ar<sup>+</sup>, O<sub>2</sub><sup>+</sup>, CO<sub>2</sub><sup>+</sup>, and CF<sub>3</sub><sup>+</sup> with
the C<sub>2</sub> radicals C<sub>2</sub>H<sub>5</sub>, H<sub>2</sub>C<sub>2</sub>F<sub>3</sub>, C<sub>2</sub>F<sub>3</sub>, and C<sub>2</sub>F<sub>5</sub> were investigated using the variable electron
and neutral density attachment mass spectrometry technique in a flowing
afterglow–Langmuir probe apparatus at room temperature. Rate
coefficients for observed product channels for these 16 reactions
are reported as well as rate coefficients and product branching fractions
for the 16 reactions of the same cations with each of the stable neutrals
used as radical precursors (the species RI, where R is the radical
studied). Reactions with the stable neutrals proceed by charge transfer
at or near the collisional rate coefficient where energetically allowed;
where charge transfer is endothermic, bond-breaking/bond-making chemistry
occurs. While also efficient, reactions with the radicals are more
likely to occur at a smaller fraction of the collisional rate coefficient,
and bond-breaking/bond-making chemistry occurs even in some cases
where charge transfer is exothermic. It is noted that unlike radical
reactions with neutral species, which occur with rate coefficients
that are generally elevated compared to those of stable species, ion–radical
reactivity is generally decreased relative to that of reactions with
stable species. In particular, long-range charge transfer appears
more likely to be frustrated in the ion–radical systems
Surface Plasmon-Driven Water Reduction: Gold Nanoparticle Size Matters
Water reduction under two different
visible-light ranges (λ
> 400 nm and λ > 435 nm) was investigated in gold-loaded
titanium
dioxide (Au-TiO<sub>2</sub>) heterostructures with different sizes
of Au nanoparticles (NPs). Our study clearly demonstrates the essential
role played by Au NP size in plasmon-driven H<sub>2</sub>O reduction
and reveals two distinct mechanisms to clarify visible-light photocatalytic
activity under different excitation conditions. The size of the Au
NP governs the efficiency of plasmon-mediated electron transfer and
plays a critical role in determining the reduction potentials of the
electrons transferred to the TiO<sub>2</sub> conduction band. Our
discovery provides a facile method of manipulating photocatalytic
activity simply by varying the Au NP size and is expected to greatly
facilitate the design of suitable plasmonic photocatalysts for solar-to-fuel
energy conversion