Standard States for Adsorption on Solid Surfaces: 2D Gases, Surface Liquids, and Langmuir Adsorbates

Standard states are utilized to compare thermodynamic data obtained from different experiments and calculations, and this ability to compare thermodynamic data plays an important role in science and society. For molecules adsorbed on surfaces, there are currently no universally accepted standard states. Here, standard states are proposed for the different types of molecular adsorbate phases, with the intent to enable physical insight to be gained by tabulating experimental/calculated values, such that comparison between different systems and existing societal tabulations of chemical standard state tabulated values can be done directly. A “density based” standard state is proposed for 2D gases, and a “relative coverage based” standard state is proposed for immobile adsorbates and nonislanding 2D liquids. These units are chosen based upon the units which the activity depends on. The standard states recommended here are chosen due to the entropies associated with them, such that physical insight can be gained by direct comparison to existing tabulated data. For 2D gases adsorbed on solid surfaces, the recommended standard state is σ° = 1.39 × 10^–7 mol m^–2. For immobile adsorbates and nonislanding liquid states on solid surfaces, the recommended standard state is θ_A° = 0.5 (which implies a standard state for the surface sites of of θ_S° = 1 – θ_A° = 0.5). With the standard states recommended here, tabulated values at a common temperature are expected to display the following approximate hierarchy for decreasing entropy: 3D gas > 2D gas > liquid > surface liquid > solid > lattice confined. Recommended standard states are also provided in the Supporting Information for cases with dissociative adsorption

Correction to “Comment on ‘Equilibrium Constants and Rate Constants for Adsorbates: 2D Ideal Gas, 2D Ideal Lattice Gas, and Ideal Hindered Translator Models’”

Correction to “Comment on ‘Equilibrium Constants and Rate Constants for Adsorbates: 2D Ideal Gas, 2D Ideal Lattice Gas, and Ideal Hindered Translator Models’

Correction to “Comment on ‘Equilibrium Constants and Rate Constants for Adsorbates: 2D Ideal Gas, 2D Ideal Lattice Gas, and Ideal Hindered Translator Models’”

Correction to “Comment on ‘Equilibrium Constants and Rate Constants for Adsorbates: 2D Ideal Gas, 2D Ideal Lattice Gas, and Ideal Hindered Translator Models’

Below-Room-Temperature C–H Bond Breaking on an Inexpensive Metal Oxide: Methanol to Formaldehyde on CeO₂(111)

Upgrading of primary alcohols by C–H bond breaking currently requires temperatures of >200 °C. In this work, new understanding from simulation of a temperature-programmed reaction study with methanol over a CeO₂(111) surface shows C–H bond breaking and the subsequent desorption of formaldehyde, even below room temperature. This is of particular interest because CeO₂ is a naturally abundant and inexpensive metal oxide. We combine density functional theory and kinetic Monte Carlo methods to show that the low-temperature C–H bond breaking occurs via disproportionation of adjacent methoxy species. We further show from calculations that the same transition state with comparable activation energy exists for other primary alcohols; with ethanol, 1-propanol, and 1-butanol explicitly calculated. These findings indicate a promising class of transition states to search for in seeking low-temperature C–H bond breaking over inexpensive oxides

Adsorption of isophorone and trimethyl-cyclohexanone on Pd(111): A combination of infrared reflection absorption spectroscopy and density functional theory studies

Understanding the interaction of α,β-unsaturated carbonyl compounds with late transition metals is a key prerequisite for rational design of new catalysts with desired selectivity towards C = C or C = O bond hydrogenation. The interaction of the α,β-unsaturated ketone isophorone and the saturated ketone TMCH (3,3,5-trimethylcyclohexanone) with Pd(111) was investigated in this study as a prototypical system. Infrared reflection–absorption spectroscopy (IRAS) and density functional theory calculations including van der Waals interactions (DFT + vdW^surf) were combined to form detailed assignments of IR vibrational modes in the range from 3000 cm^{− 1} to 1000 cm^{− 1} in order to obtain information on the binding of isophorone and TMCH to Pd(111) as well as to study the effect of co-adsorbed hydrogen. IRAS measurements were performed with deuterium-labeled (d₅-) isophorone, in addition to unlabeled isophorone and unlabeled TMCH. Experimentally observed IR absorption features and calculated vibrational frequencies indicate that isophorone and TMCH molecules in multilayers have a mostly unperturbed structure with random orientation. At sub-monolayer coverages, strong perturbation and preferred orientations of the adsorbates were found. At low coverage, isophorone interacts strongly with Pd(111) and adsorbs in a flat-lying geometry with the C = C and C = O bonds parallel, and a CH₃ group perpendicular, to the surface. At intermediate sub-monolayer coverage, the C = C bond is strongly tilted, while the C = O bond remains flat-lying, which indicates a prominent perturbation of the conjugated π system. Pre-adsorbed hydrogen leads to significant changes in the adsorption geometry of isophorone, which suggests a weakening of its binding to Pd(111). At low coverage, the structure of the CH₃ groups seems to be mostly unperturbed on the hydrogen pre-covered surface. With increasing coverage, a conservation of the in-plane geometry of the conjugated π system was observed in the presence of hydrogen. In contrast to isophorone, TMCH adsorbs in a strongly tilted geometry independent of the surface coverage. At low coverage, an adsorbate with a strongly distorted C = O bond is formed. With increasing exposure, species with a less perturbed C = O group appear