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The kinetics of elementary reactions involved in the chain chlorination of cyclopropane are examined using a combination of absolute and relative rate constant measurements and first principles electronic structure calculations. Relative rate methods are used in a smog chamber FTIR apparatus to determine k(Cl + c-C3H6) = (1.15 ± 0.17) × 10^-13 and k(Cl + c-C3H5Cl) = (1.06 ± 0.18) × 10^-12 cm³ molecule^-1 s^-1 in 10−700 Torr of air, or N2, diluent at 296 K. Absolute rate coefficients for the reaction of Cl with cyclopropane are measured between 293 and 623 K by a laser-photolysis/CW infrared absorption method. The data can be represented in Arrhenius form as k(Cl + c-C3H6) = ((1.8 ± 0.3) × 10^-10 cm³ molecule^-1 s^-1)e^{-(2150±100)/T}. To support the experimental investigations, first principles electronic structure calculations are performed. Vibrational spectra of c-C3H5Cl, c-C3H4Cl2, and c-C3H3Cl3 are calculated and are presented. gem-C3H4Cl2 is calculated to be the kinetically and thermodynamically most favored dichlorocyclopropane. The experimental observations are consistent with the computational findings.

Alkaline earth oxides are leading candidates as NOx (x = 1, 2) sorbents; however, the understanding of adsorption and reaction of NOx on these surfaces is currently limited. In this paper, Mg(001) terrace cluster models and density functional theory (DFT) are used to characterize the adsorption properties of neutral and charged NO, NO2, and NO3. Neutral NO and NO2 molecules weakly adsorb on the MgO terrace, while neutral NO3 adsorbs more strongly through oxidation and significant acid−base coordination with the surface. Chemisorbed nitrite can be formed either from adsorption of Lewis acidic NO⁺ on Lewis base surface sites (Os^2-) or from adsorption of Lewis basic NO2^- on Lewis acid surface sites (Mgs²⁺). Similarly, chemisorbed nitrate can form from NO2⁺ adsorption on Os^2- sites or NO3^- adsorption on Mgs²⁺ sites. These charged adsorbates are not expected to exist in isolation on the MgO surface but rather are expected to form oppositely charged, cooperatively bound pairs with enhanced adsorption energy over the isolated neutrals. While results are demonstrated for adsorption on MgO, they have application to NOx adsorption on a wide range of oxide surfaces.

An extensive search for stable chemisorbed SO2 configurations on the Pt(111) surface has been performed using first-principles DFT−GGA calculations. The most energetically stable configurations, η²-Sb,Oa and η³-Sa,Oa,Oa at fcc sites, where η² and η³ mean that the number of atoms of the adsorbate coordinated to surface atoms are two and three, respectively, and the subscripts a and b stand for the atoms on atop sites and bridge sites, respectively, are consistent with experimental observations. It is found that strong sulfur−metal bonds are essential in stabilizing the molecular SO2 binding to the Pt(111) surface. The lateral dipole−dipole interactions among chemisorbed SO2 moieties are shown to be responsible for the strong coverage effect of the SO2 binding energy to the surface. These strong lateral interactions do not greatly affect the molecular structures or relative binding energy differences among different binding configurations. The projected density of states and the induced density of states are studied in detail to explain binding effects.

The chemisorption of molecules on metal oxide surfaces is generally considered to occur by either an acid/base or a redox mechanism, with the former dominating on nonreducible and insulating oxides such as MgO. NO, NO2, and the less familiar NO3 are atypical adsorbates in that their most potent Lewis acidic and basic forms are generated by one-electron oxidation or reduction of the parent molecules. In this work, first-principles density functional theory supercell calculations are used to probe the adsorption chemistry of the nitrogen oxides on an undefected MgO(001) surface. The isolated adsorbates are found to physisorb (NO, NO2) or weakly chemisorb (NO3) to the MgO terrace. In contrast, adsorbate partners located on neighboring surface acid (Mgs) and base (Os) sites form strongly chemisorbed products with features characteristic of nitrite (NO2^-) and nitrate (NO3^-). The origin of this new class of “cooperative” chemisorption is shown to be electron transfer between two NOx species to generate Lewis acid (NOx⁺) and base (NOx^-) pairs that strongly chemisorb to the MgO surface and that are further stabilized by lateral electrostatic attraction. The relatively low ionization potentials and large electron affinities of the NOx molecules are key to enabling the cooperative effect on MgO. Even more pronounced cooperative effects are expected for NOx adsorption on more basic or acidic oxides, including those used for NOx remediation. The effect is also likely to have a role in the heterogeneous chemistry of other odd-electron adsorbates, including the halogen oxides and HOx radicals.

In order to evaluate the effect of size on the adsorption energies on small Pt particles, we have performed a series of DFT-GGA calculations on Pt clusters of varying sizes, between 3 and 25 atoms, and a Pt(111) slab, with and without O and S adsorbates. We have found a significant variation in the energy of adsorption as a function of cluster size. In particular, energies of adsorption on the Pt10 cluster showed the largest deviation from those on the slab surface. Structural and energetic properties compare excellently to published experimental data. The Pt4 clusters with and without adsorbates were found to have reduced symmetry due to the Jahn−Teller effect. A simple rule between the cohesive energy and the size of small close-packed platinum particles is derived and confirmed by our first-principle calculations, the extrapolation of which excellently matches both experimental and computational bulk cohesive energies.

One of the main obstacles to the use of alkaline-earth-oxide-based NOx adsorbents for emissions control is the ready poisoning of these materials by SOx. To shed light on the mechanisms of poisoning, density functional theory (DFT) calculations and infrared spectroscopy experiments are used to study SOx adsorption on MgO as a model alkaline earth oxide. DFT plane-wave, pseudopotential results are presented for SOx adsorption on MgO(001) terraces and steps. Both SO2 and SO3 are found to exhibit both weakly bound physisorbed and strongly bound chemisorbed forms, with minimal activation barriers separating the two. Chemisorption is dominated by interactions between Lewis acidic sulfur and Lewis basic oxide anion sites to form surface sulfites and sulfates from SO2 and SO3, respectively. Within the generalized gradient approximation to DFT, the SO2 adsorption energy ranges from 25 kcal mol^-1 on terrace sites to 46 kcal mol^-1 on lower-coordinated step edge sites; analogous SO3 adsorption energies range from 49 to 76 kcal mol^-1. Surface sulfite is readily observed upon exposure of calcined MgO powder to SO2 and produces vibrational signatures consistent with that calculated for chemisorbed SO2. Sulfation is accomplished by exposure of the same powders to SO2 and O2 at elevated temperatures. Comparisons to calculated vibrational spectra indicate that both chemisorbed SO2 and SO3 are present under these conditions; further heating induces formation of a bulklike sulfate that is difficult to remove. The results demonstrate the high reactivity of SOx toward metal oxides and illustrate the challenges in developing sulfur-resistant oxide-based NOx traps.

Explicit atom (EA) and united atom (UA) models are compared for the description of the structure and dynamics of an alkane film adsorbed on an Al-terminated α-Al2O3 (0001) surface. Ab initio calculations for an Al2O3···CH4 cluster are used to parameterize the EA model. One UA model is derived by explicit averaging of the EA potential, while the second is rescaled from the first to reproduce the EA octane adsorption energy. The appropriate scale factor is found to be sensitive to relaxation of the α-Al2O3 surface. The three models are compared in constant NVT molecular dynamics simulations of an octane monolayer (21 molecules) adsorbed on an 80% relaxed α-Al2O3 (0001) surface. Overall, the UA models reproduce the EA structural predictions well, the primary difference being an underestimation of the average gauche angle and overestimation of the trans fraction in the UA simulations. The agreement between dynamic predictions is less satisfactory; the scaled UA model reproduces the EA trans ↔ gauche interconversion rate reasonably well, but fails to reproduce the overall rotational or translational dynamics. UA models must be used with caution when applied to alkane dynamics on a corrugated surface.