Molecular dynamics simulations and selected experiments have been carried out to study the growth of Cu films on (010) bcc Ta and the deposition of CuxTa1−x alloy films on (111) fcc Cu. They indicate that fcc Cu films with a (111) texture are always formed when Cu is deposited on Ta surfaces. These films are polycrystalline even when the Ta substrate is single crystalline. The grains have one of two different orientations and are separated by either orientational or misfit dislocations. Periodic misfit dislocations and stacking faults develop within these grains to release structure difference induced misfit strain energy. The Cu film surface roughness was found to decrease with increase in the adatom energy for deposition. When CuxTa1−x is deposited on Ta, the films always have a higher Cu composition than that of the vapor mixture. This arises from a surface segregation phenomenon. When the Cu and Ta fractions in the films are comparable, amorphous structures form. The fundamental origins for the segregation and amorphization phenomena are discussed.
DFT-GGA periodic slab calculations were used to examine the adsorption and hydrogenation of ethylene to a surface ethyl intermediate on the Pd(111) surface. The reaction was examined for two different surface coverages, corresponding to (2×3) [low coverage] and (√3×√3)R 30° [high coverage] unit cells. For the low coverage, the di-σ adsorption of ethylene (−62 kJ/mol) is 32 kJ/mol stronger than the π-adsorption mode. The intrinsic activation barrier for hydrogenation of di-σ bonded ethylene to ethyl, for a (2×3) unit cell, was found to be +88 kJ/mol with a reaction energy of +25 kJ/mol. There appeared to be no direct pathway for hydrogenation of π-bonded ethylene to ethyl, for low surface coverages. At higher coverages, however, lateral repulsive interactions between adsorbates destabilize the di-σ adsorption of ethylene to a binding energy of −23 kJ/mol. A favorable surface geometry for the (√3×√3)R 30° coverage is achieved when ethylene is π-bound and hydrogen is bound to a neighboring bridge site. At high coverage, the hydrogenation of di-σ bound ethylene to ethyl has an intrinsic barrier of +82 kJ/mol and a reaction energy of −5 kJ/mol, which is only slightly reduced from the low coverage case. For a (√3×√3)R 30° unit cell, however, the more favorable reaction pathway is via hydrogenation of π-bonded ethylene, with an intrinsic barrier of +36 kJ/mol and an energy of reaction of −18 kJ/mol. This pathway is inaccessible at low coverage. This paper illustrates the importance of weakly bound intermediates and surface coverage effects in reaction pathway analysis.
Remarkable advances in computer hardware over the past 15 years have produced an avalanche of sophisticated software for chemists and chemical engineers. The authors are using these powerful tools to estimate rate and equilibrium constants for complex mixtures. The linear free energy relationship (LFER) they use provides a correlation of a rate or equilibrium constant with a property of a molecule or intermediate for a family of reactions. The computational quantum chemistry (CQC) calculations thus provide an estimate of rate or equilibrium coefficients for reactants that have not been studied experimentally. The mixtures the authors studying are the complex feeds used for catalytic hydrocracking. Current environmental and performance demands make it necessary to estimate the properties of these mixtures as a function of the severity of their reactions. In this light, reactivity is just another property, albeit the special one that ties various mixture compositions to operating conditions. The role of CQC in the study of these mixtures is the subject of this paper.
The unique interfacial sites of Au nanoparticles supported on TiO2 are known to catalyze the activation of oxygen and it's addition to small molecules including H2, CO, NO and propylene. Herein we extend these ideas and show that the unique Au-Ti dual perimeter sites that form at the Au/TiO2 interface can also catalyze more demanding C-H and C-O bond activation reactions involved in the deoxygenation organic acids such as acetic acid. We have shown previously that acetic acid can be partially oxidized on a Au/TiO2 catalyst to form a novel gold ketenylidene (Au2==C==C==O) intermediate. In the present work we use in situ infrared spectroscopy and first-principle density functional theory (DFT) to examine the mechanism and the kinetics by which this reaction proceeds. The reaction was found to be localized at the dual perimeter sites of the Au/TiO2 catalyst, where 02 was activated. In contrast to Au/TiO2, no ketenylidene formation was observed on a similar Au/SiO2 catalyst or a TiO2 blank sample. The reaction involves the activation of multiple C-H bonds as well as the C-O bond in the adsorbed CH3COO species. C-O bond scission is postulated to occur at the TiO2 sites, while C-H bond scission occurs on Au sites, both near the active Au-Ti4+ dual perimeter sites. 18O2 isotopic labeling indicated that the O moiety of the ketenylidene species originates from the acetic acid during the oxidation process involving molecular O2. The rate-limiting step was found to be the C--O bond scission resulting in an apparent overall activation energy of 1.72 eV as determined from DFT calculations. This is in very good agreement with the experimentally measured apparent activation energy of 1.7 +/- 0.2 eV. A deuterium kinetic isotope effect of approximately 4 indicates that C-H bond activation is kinetically involved in the overall acetate oxidation reaction.
Solid-state 13C NMR experiments and quantum chemical Density Functional Theory (DFT) calculations of acetone adsorption were used to study the location of protons in anhydrous 12-tungstophosphoric acid (HPW), the mobility of the isolated and hydrated acidic protons, and the acid strength heterogeneity of the anhydrous hydroxyl groups. This study presents the first direct NMR experimental evidence that there are two types of isolated protons with different acid strengths in the anhydrous Keggin HPW. Rotational Echo DOuble Resonance (REDOR) NMR experiments combined with quantum chemical DFT calculations demonstrated that acidic protons in anhydrous HPW are localized on both bridging (Oc) and terminal (Od) atoms of the Keggin unit. The CP/MAS NMR experiments revealed that the isolated acidic protons are immobile, but hydrated acidic protons are highly mobile at room temperature. The isotropic chemical shift of the adsorbed acetone suggested that the acid strength of the H(H2O)n+ species in partially hydrated HPW is comparable to that of a zeolite, while the acidity of an isolated proton is much stronger than that of a zeolite. Isolated protons on the bridging oxygen atoms of anhydrous HPW are nearly superacidic.
Alkaline earth metal ions accelerate the breaking of cellulose bonds and control the distribution of products in the pyrolysis of lignocellulose to biofuels and chemicals. Here, the activation of cellulose via magnesium ions was measured over a range of temperatures from 370 to 430 ⁰C for 20 to 2000 milliseconds and compared with activation of cellulose via calcium, another naturally-occurring alkaline earth metal in lignocellulose materials. The experimental approach of pulse heated analysis of solid/surface reactions (PHASR) showed that magnesium significantly catalyzes cellulose activation with a second order rate dependence on the catalyst concentration. An experimental barrier of 45.6 ± 2.1 kcal mol-1 and a pre-factor of 1.18 x 1016 (mmol Mg2+ / g CD)-2 * s-1 was obtained for the activation of α-cyclodextrin (CD), a cellulose surrogate, for catalyst concentrations of 0.1 to 0.5 mmol Mg+2 per gram of CD. First principles density functional theory calculations showed that magnesium ions play a dual role in catalyzing the reaction by breaking the hydrogen bonds with hydroxymethyl groups and destabilizing the reacting cellulose chain, thus making it more active. The calculated barrier of 47 kcal mol-1 is in agreement with the experimentally measured barriers and similar to that for calcium ion catalysts (~50 kcal mol-1).
Precise control of electron density at catalyst active sites enables regulation of surface chemistry for optimal rate and selectivity to products. Here, an ultrathin catalytic film of amorphous alumina (4 nm) was integrated into a catalytic condenser device that enabled tunable electron depletion from the alumina active layer and correspondingly stronger Lewis acidity. The catalytic condenser had the following structure: amorphous alumina/graphene/HfO2 dielectric (70 nm)/p-type Si. Application of positive voltages up to +3 V between graphene and the p-type Si resulted in electrons flowing out of the alumina; positive charge accumulated in the catalyst. Temperature programmed surface reaction of thermocatalytic isopropanol dehydration to propene on the charged alumina surface revealed a shift in the propene formation peak temperature of up to ΔT(peak)~50 ⁰C relative to the uncharged film, consistent with a 16 kJ/mol (0.17 eV) reduction in the apparent activation energy. Electrical characterization of the thin amorphous alumina film by ultraviolet photoelectron spectroscopy (UPS) and scanning tunneling microscopy (STM) indicates the film is a defective semiconductor with an appreciable density of in-gap electronic states. Density functional theory calculations of isopropanol binding on the pentacoordinate aluminum active sites indicate significant binding energy changes (ΔBE) up to 60 kJ/mol (0.62 eV) for 0.125 e- depletion per active site, supporting the experimental findings. Overall, the results indicate that continuous and fast electronic control of thermocatalysis can be achieved with the catalytic condenser device.
This chapter contains sections titled: Introduction The Electrified Aqueous Metal Interface Electronic Structure Methods and Models for the Electrocatalytic Interface Applications of Density Functional Theory to Electrochemical Systems Summary and Conclusions References
