Rice–Ramsperger–Kassel–Marcus Simulation of Hydrogen Dissociation on Cu(111): Addressing Dynamical Biases, Surface Temperature, and Tunneling

The effects of dynamics, surface temperature, and tunneling on the dissociative chemisorption of hydrogen on Cu(111) are explored using a dynamically biased precursor-mediated microcanonical trapping (d-PMMT) model. Transition state vibrational frequencies were taken from recent generalized gradient approximation density functional theory (GGA-DFT) electronic structure calculations, and the model’s few remaining parameters were fixed by optimizing simulations to a limited number of quantum-state-resolved associative desorption experiments. The d-PMMT model reproduces a diverse variety of dissociative chemisorption and associative desorption experimental results and, importantly, largely captures the surface temperature dependence of quantum-state-resolved dissociative sticking coefficients. Molecular translational energy parallel to the surface was treated as a spectator degree of freedom. The efficacy of molecular rotational energy to promote dissociation, relative to normal translational energy, varied monotonically from −45% to 33% as the rotational energy increased. Efficacies for molecular vibrational energy and surface phonon energy were 60%. The efficacies did not vary with isotope change from H₂ to D₂. The thermal dissociative sticking coefficient for H₂/Cu­(111) is predicted to vary as <i>S</i>(<i>T</i>) = <i>S</i>₀ exp­(−<i>E</i>_a/<i>RT</i>) where <i>S</i>₀ = 0.075 and <i>E</i>_a = 49.2 kJ/mol over the 300 K ≤ <i>T</i> ≤ 1000 K temperature range. Dynamical effects are significant and suppress <i>S</i>(<i>T</i>) by ∼2 orders of magnitude as compared to statistical expectations. For thermal dissociative chemisorption of H₂/Cu­(111) at 1000 K, a temperature of catalytic interest, normal translational energy is calculated to provide 57% of the energy necessary to react, surface phonons 23%, molecular rotation 15%, and vibration 5%. Tunneling is calculated to account for 13% of <i>S</i>(<i>T</i>) at 1000 K and more than 50% at temperatures below 400 K. These results demonstrate that many aspects of gas-surface reactivity can be modeled using microcanonical transition state theory subject to a few dynamical constraints

Photoreduction of Hydrogen Cations on TiO₂ and Its Impact on Surface Band Bending and the Charge Carrier Recombination Rate: A Photoluminescence Study under High Vacuum Conditions

The interaction between hydrogen species and TiO₂ surfaces is important because of its relevance to hydrogen production in the photocatalytic splitting of water on TiO₂. In this study, the effect of the photocatalytic reduction of hydrogen cations, a half photoelectrochemical reaction run under high vacuum conditions, on the surface band bending of TiO₂ was investigated by photoluminescence (PL) spectroscopy. Exposure of TiO₂ to molecular hydrogen had no effect on the PL emission from TiO₂, but the PL intensity increased under UV irradiation when TiO₂ was exposed to hydrogen cations produced by ionization of molecular hydrogen by a cold cathode pressure gauge. The PL intensity increase, signaling a decrease in the upward band bending and increased electron–hole radiative recombination rate, is caused by charge transfer during photoreduction of hydrogen cations. Residual photoexcited holes left in TiO₂ due to the transfer of photoexcited electrons to hydrogen cations tend to accumulate at the TiO₂ surface, balancing the originally trapped electrons at the TiO₂ surface and thereby lowering the original upward band bending. These unusual observations point out that charge transfer during a photoelectrochemical reaction at a semiconductor interface alters the surface band bending and photoexcited electron–hole recombination rate in ways that are likely to impact the efficiency of photocatalytic devices

Angle-Resolved Thermal Dissociative Sticking of Light Alkanes on Pt(111): Transitioning from Dynamical to Statistical Behavior

Gas-surface reactivity involving small molecules can exhibit significant dynamical deviations away from statistical behavior that complicate quantitative modeling of catalytic processes. This study examines dissociative chemisorption of light alkanes to determine if a transition toward statistical behavior can be identified at some threshold level of molecular complexity. Angle-resolved thermal dissociative sticking coefficients (ar-DSCs) were measured for methane, ethane, and propane on Pt(111) using effusive molecular beams at a temperature of 700 K. The ar-DSCs were peaked around the direction of the surface normal, and the angular variation flattened with increasing alkane size from roughly cos¹²ϑ to only a 20% variance with angle. Precursor-mediated microcanonical trapping models of the gas-surface reactivity were used to simulate the ar-DSCs while maintaining consistency with other nonequilibrium supersonic and effusive molecular beam experiments. This work indicates that dissociative chemisorption of ethane is dynamically biased similar to methane, wherein the mean vibrational efficacy for promoting reaction relative to normal translational energy is 0.40 and molecular translations parallel to the surface and rotations are spectator degrees of freedom. In contrast, propane’s reactivity shows no discernible evidence for dynamical deviations away from statistical behavior

Graphene Growth on Pt(111) by Ethylene Chemical Vapor Deposition at Surface Temperatures near 1000 K

The nucleation and growth of graphene islands on a Pt(111) surface were examined at temperatures near 1000 K. Graphene was grown by chemical vapor deposition of ethylene, and a low-energy electron microscope (LEEM) was used to image the growing graphene islands with resolution of 10 nm and to perform spatially localized electron diffraction. It is shown that graphene grows bidirectionally over the step edges of Pt, and its formation can induce substantial changes in the platinum surface morphology. Average size and density of graphene islands strongly depend on surface temperature during deposition. Postdosing Auger electron spectroscopy was employed as a complementary macroscopic technique to measure the total carbon deposited as a result of ethylene dissociative sticking and decomposition. The initial dissociative sticking coefficient <i>S</i>₀(<i>T</i>_g = 300 K, <i>T</i>_s) for ethylene was found to decrease with increasing surface temperature until a temperature of <i>T</i>_s = 850 K was reached whereupon it increased with temperature

Extreme value statistics of the halo and stellar mass distributions at high redshift: are JWST results in tension with ?CDM?

The distribution of dark matter halo masses can be accurately predicted in the lambda cold dark matter (?CDM) cosmology. The presence of a single massive halo or galaxy at a particular redshift, assuming some baryon and stellar fraction for the latter, can therefore be used to test the underlying cosmological model. A number of recent measurements of very large galaxy stellar masses at high redshift (z > 8) motivate an investigation into whether any of these objects are in tension with ?CDM. We use extreme value statistics to generate confidence regions in the mass–redshift plane for the most extreme mass haloes and galaxies. Tests against numerical models show no tension, neither in their dark matter halo masses nor their galaxy stellar masses. However, we find tentative >3s tension with recent observational determinations of galaxy masses at high redshift from both Hubble Space Telescope and James Webb Space Telescope, despite using conservative estimates for the stellar fraction (f* ~ 1). Either these galaxies are in tension with ?CDM, or there are unaccounted for uncertainties in their stellar mass or redshift estimates

<i>De Novo</i> Assembly of Human Herpes Virus Type 1 (HHV-1) Genome, Mining of Non-Canonical Structures and Detection of Novel Drug-Resistance Mutations Using Short- and Long-Read Next Generation Sequencing Technologies - Fig 5

<p><b>Mapping of 454-Roche (A) and Nanopore-MinION (B) reads over the oriL palindrome sequence (id38, 62403 – 62547bp in strain 17).</b> Long MinION reads (in red, blue, green) assist the bridging of discontinuous 454 contigs. (C): genome annotation.</p

Detection of non-canonical contigs.

<p>Similarity plots between 2 representative non-canonical contigs (middle panel of each screenshot) and the reference genome (strain 17) (top panel of each screenshot). Plots below the base line indicate inverted sequences (plus/minus strand alignment). The coordinates of the edges of each rearranged fragment are below while the coordinates of the disrupted/nearby genes are above the reference plots. Confirmative 454-mapping alignments and coverage of the respective supporting reads are shown below each contig.</p

Evaluation of solo 454 and hybrid 454/MinION de-novo assemblies.

<p>Contigs Generated by 454 reads are in red while hybrid contigs generated by 454 and MinION reads are in blue. Left column: Cumulative assembly length plotted against the number of the contigs of each individual assembly. Right column: NGx value resulted by the alignment of the contigs on the reference HHV-1 genome.</p

Phenotypic drug susceptibility testing in HHV-1 clinical samples.

<p>Natural polymorphisms and drug resistance mutations in TK and Pol genes.</p