16 research outputs found

    First-Principles Analysis of Potential-Dependent Proton Coupled Electron Transfer between Polypyridyl–Ruthenium Complexes and Oxygen-Modified Graphene Electrodes

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    Proton coupled electron transfer reactions which are pervasive throughout electrochemistry control a number of energy conversion strategies. First-principles density functional theoretical calculations are used herein to examine proton coupled electron transfer between a homogeneous mononuclear polypyridyl–ruthenium catalyst used in the catalytic oxidation of water and surface ketone groups on oxygen-modified graphene electrode surfaces. The potential-dependent interface energies were calculated for two proton transfer states: Ru<sup>III</sup>OH···OC–graphene and Ru<sup>IV</sup>O···HO–C–graphene. The reactivity for interfacial proton coupled electron transfer was found to be controlled by functional groups that terminate surface defect sites as well as graphene edge sites. The energy gap between the two proton transfer states becomes smaller as the number of surface ketone groups increases. Ab initio molecular dynamics simulations clearly show that increases in the number of surface ketone groups increase the hydrophilicity of the graphene basal plane. This significantly decreases the energy for proton transfer, thus providing a low-energy path that extends over a wide range of potentials. The surface ketone groups at the armchair edges of the graphene plane appear to be the most reactive oxygens on the graphene surface as they lead to direct reversible proton coupled electron transfer between the polypyridal–Ru complexes and the CO groups at the edges where the two proton transfer potential energy surfaces crossing at 0.2 V vs SHE. The graphene armchair edge helps to stabilize the Ru<sup>IV</sup>O···HO–C–graphene-edge proton transfer state which is an important step in water oxidation catalysis

    On the Yield of Levoglucosan from Cellulose Pyrolysis

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    Fast pyrolysis is a thermochemical process to fragment large biopolymers such as cellulose to chemical intermediates which can be refined to renewable fuels and chemicals. Levoglucosan (LGA), a six-carbon oxygenate, is the most abundant primary product from cellulose pyrolysis with LGA yields reported over a wide range of 5–80 percent carbon (%C). In this study, the variation of the observed yield of LGA from cellulose pyrolysis was experimentally investigated. Cellulose pyrolysis experiments were conducted in two different reactors: the Frontier micropyrolyzer (2020-iS), and the pulse heated analysis of solid reactions (PHASR) system. The reactor configuration and experimental conditions including cellulose sample size were found to have a significant effect on the yield of LGA. Four different hypotheses were proposed and tested to evaluate the relationship of cellulose sample size and the observed LGA yield including (a) thermal promotion of LGA formation, (b) the crystallinity of cellulose samples, (c) secondary and vapor-phase reactions of LGA, and (d) the catalytic effect of melt-phase hydroxyl groups. Co-pyrolysis experiments of cellulose and fructose in the PHASR reactor presented indirect experimental evidence of previously postulated catalytic effects of hydroxyl groups in glycosidic bond cleavage for LGA formation in transport-limited reactor systems

    Generalized Temporal Acceleration Scheme for Kinetic Monte Carlo Simulations of Surface Catalytic Processes by Scaling the Rates of Fast Reactions

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    A novel algorithm is presented that achieves temporal acceleration during kinetic Monte Carlo (KMC) simulations of surface catalytic processes. This algorithm allows for the direct simulation of reaction networks containing kinetic processes occurring on vastly disparate time scales which computationally overburden standard KMC methods. Previously developed methods for temporal acceleration in KMC were designed for specific systems and often require a priori information from the user such as identifying the fast and slow processes. In the approach presented herein, quasi-equilibrated processes are identified automatically based on previous executions of the forward and reverse reactions. Temporal acceleration is achieved by automatically scaling the intrinsic rate constants of the quasi-equilibrated processes, bringing their rates closer to the time scales of the slow kinetically relevant nonequilibrated processes. All reactions are still simulated directly, although with modified rate constants. Abrupt changes in the underlying dynamics of the reaction network are identified during the simulation, and the reaction rate constants are rescaled accordingly. The algorithm was utilized here to model the Fischer–Tropsch synthesis reaction over ruthenium nanoparticles. This reaction network has multiple time-scale-disparate processes which would be intractable to simulate without the aid of temporal acceleration. The accelerated simulations are found to give reaction rates and selectivities indistinguishable from those calculated by an equivalent mean-field kinetic model. The computational savings of the algorithm can span many orders of magnitude in realistic systems, and the computational cost is not limited by the magnitude of the time scale disparity in the system processes. Furthermore, the algorithm has been designed in a generic fashion and can easily be applied to other surface catalytic processes of interest

    Mechanistic Insights on the Hydrogenation of α,β-Unsaturated Ketones and Aldehydes to Unsaturated Alcohols over Metal Catalysts

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    The selective hydrogenation of unsaturated ketones (methyl vinyl ketone and benzalacetone) and unsaturated aldehydes (crotonaldehyde and cinnamaldehyde) was carried out with H<sub>2</sub> at 2 bar absolute over Pd/C, Pt/C, Ru/C, Au/C, Au/TiO<sub>2</sub>, or Au/Fe<sub>2</sub>O<sub>3</sub> catalysts in ethanol or water solvent at 333 K. Comparison of the turnover frequencies revealed Pd/C to be the most active hydrogenation catalyst, but the catalyst failed to produce unsaturated alcohols, indicating hydrogenation of the CC bond was highly preferred over the CO bond on Pd. The Pt and Ru catalysts were able to produce unsaturated alcohols from unsaturated aldehydes, but not from unsaturated ketones. Although Au/Fe<sub>2</sub>O<sub>3</sub> was able to partially hydrogenate unsaturated ketones to unsaturated alcohols, the overall hydrogenation rate over gold was the lowest of all of the metals examined. First-principles density functional theory calculations were therefore used to explore the reactivity trends of methyl vinyl ketone (MVK) and benzalacetone (BA) hydrogenation over model Pt(111) and Ru(0001) surfaces. The observed selectivity over these metals is likely controlled by the significantly higher activation barriers to hydrogenate the CO bond compared with those required to hydrogenate the CC bond. Both the unsaturated alcohol and the saturated ketone, which are the primary reaction products, are strongly bound to Ru and can react further to the saturated alcohol. The lower calculated barriers for the hydrogenation steps over Pt compared with Ru account for the higher observed turnover frequencies for the hydrogenation of MVK and BA over Pt. The presence of a phenyl substituent α to the CC bond in BA increased the barrier for CC hydrogenation over those associated with the CC bond in MVK; however, the increase in barriers with phenyl substitution was not adequate to reverse the selectivity trend

    Selective Catalytic Oxidative-Dehydrogenation of Carboxylic AcidsAcrylate and Crotonate Formation at the Au/TiO<sub>2</sub> Interface

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    The oxidative-dehydrogenation of carboxylic acids to selectively produce unsaturated acids at the second and third carbons regardless of alkyl chain length was found to occur on a Au/TiO<sub>2</sub> catalyst. Using transmission infrared spectroscopy (IR) and density functional theory (DFT), unsaturated acrylate (H<sub>2</sub>CCHCOO) and crotonate (CH<sub>3</sub>CHCHCOO) were observed to form from propionic acid (H<sub>3</sub>CCH<sub>2</sub>COOH) and butyric acid (H<sub>3</sub>CCH<sub>2</sub>CH<sub>2</sub>COOH), respectively, on a catalyst with ∼3 nm diameter Au particles on TiO<sub>2</sub> at 400 K. Desorption experiments also show gas phase acrylic acid is produced. Isotopically labeled <sup>13</sup>C and <sup>12</sup>C propionic acid experiments along with DFT calculated frequency shifts confirm the formation of acrylate and crotonate. Experiments on pure TiO<sub>2</sub> confirmed that the unsaturated acids were not produced on the TiO<sub>2</sub> support alone, providing evidence that the sites for catalytic activity are at the dual Au–Ti<sup>4+</sup> sites at the nanometer Au particles’ perimeter. The DFT calculated energy barriers between 0.3 and 0.5 eV for the reaction pathway are consistent with the reaction occurring at 400 K on Au/TiO<sub>2</sub>

    Mechanistic Insights into the Catalytic Oxidation of Carboxylic Acids on Au/TiO<sub>2</sub>: Partial Oxidation of Propionic and Butyric Acid to Gold Ketenylidene through Unsaturated Acids

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    The partial oxidation of model C<sub>2</sub>–C<sub>4</sub> (acetic, propionic, and butyric) carboxylic acids on Au/TiO<sub>2</sub> catalysts consisting of Au particles ∼3 nm in size was investigated using transmission infrared spectroscopy and density functional theory. All three acids readily undergo oxidative dehydrogenation on Au/TiO<sub>2</sub>. Propionic and butyric acid dehydrogenate at the C2–C3 positions, whereas acetic acid dehydrogenates at the C1–C2 position. The resulting acrylate and crotonate intermediates are subsequently oxidized to form β-keto acids that decarboxylate. All three acids form a gold ketenylidene intermediate, Au<sub>2</sub>CCO, along the way to their full oxidation to form CO<sub>2</sub>. Infrared measurements of Au<sub>2</sub>CCO formation as a function of time provides a surface spectroscopic probe of the kinetics for the activation and oxidative dehydrogenation of the alkyl groups in the carboxylate intermediates that form. The reaction proceeds via the dissociative adsorption of the acid onto TiO<sub>2</sub>, the adsorption and activation of O<sub>2</sub> at the dual perimeter sites on the Au particles (Au–O–O-Ti), and the subsequent activation of the C2–H and C3–H bonds of the bound propionate and butyrate intermediates by the weakly bound and basic oxygen species on Au to form acrylate and crotonate intermediates, respectively. The CC bond of the unsaturated acrylate and crotonate intermediates is readily oxidized to form an acid at the beta (C3) position, which subsequently decarboxylates. This occurs with an overall activation energy of 1.5–1.7 ± 0.2 eV, ultimately producing the Au<sub>2</sub>CCO species for all three carboxylates. The results suggest that the decrease in the rate in moving from acetic to propionic to butyric acid is due to an increase in the free energy of activation for the formation of the Au<sub>2</sub>CCO species on Au/TiO<sub>2</sub> with an increasing size of the alkyl substituent. The formation of Au<sub>2</sub>CCO proceeds for carboxylic acids that are longer than C<sub>2</sub> without a deuterium kinetic isotope effect, demonstrating that C–H bond scission is not involved in the rate-determining step; the rate instead appears to be controlled by C–O bond scission. The adsorbed Au<sub>2</sub>CCO intermediate species can be hydrogenated to produce ketene, H<sub>2</sub>CCO­(g), with an activation energy of 0.21 ± 0.05 eV. These studies show that selective oxidative dehydrogenation of the alkyl side chains of fatty acids can be catalyzed by nanoparticle Au/TiO<sub>2</sub> at temperatures near 400 K

    Localized Partial Oxidation of Acetic Acid at the Dual Perimeter Sites of the Au/TiO<sub>2</sub> CatalystFormation of Gold Ketenylidene

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    Chemisorbed acetate species derived from the adsorption of acetic acid have been oxidized on a nano-Au/TiO<sub>2</sub> (∼3 nm diameter Au) catalyst at 400 K in the presence of O<sub>2</sub>(g). It was found that partial oxidation occurs to produce gold ketenylidene species, Au<sub>2</sub>CCO. The reactive acetate intermediates are bound at the TiO<sub>2</sub> perimeter sites of the supported Au/TiO<sub>2</sub> catalyst. The ketenylidene species is identified by its measured characteristic stretching frequency ν­(CO) = 2040 cm<sup>–1</sup> and by <sup>13</sup>C and <sup>18</sup>O isotopic substitution comparing to calculated frequencies found from density functional theory. The involvement of dual catalytic Ti<sup>4+</sup> and Au perimeter sites is postulated on the basis of the absence of reaction on a similar nano-Au/SiO<sub>2</sub> catalyst. This observation excludes low coordination number Au sites as being active alone in the reaction. Upon raising the temperature to 473 K, the production of CO<sub>2</sub> and H<sub>2</sub>O is observed as both acetate and ketenylidene species are further oxidized by O<sub>2</sub>(g). The results show that partial oxidation of adsorbed acetate to adsorbed ketenylidyne can be cleanly carried out over Au/TiO<sub>2</sub> catalysts by control of temperature

    Consequences of Metal–Oxide Interconversion for C–H Bond Activation during CH<sub>4</sub> Reactions on Pd Catalysts

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    Mechanistic assessments based on kinetic and isotopic methods combined with density functional theory are used to probe the diverse pathways by which C–H bonds in CH<sub>4</sub> react on bare Pd clusters, Pd cluster surfaces saturated with chemisorbed oxygen (O*), and PdO clusters. C–H activation routes change from oxidative addition to H-abstraction and then to σ-bond metathesis with increasing O-content, as active sites evolve from metal atom pairs (*–*) to oxygen atom (O*–O*) pairs and ultimately to Pd cation-lattice oxygen pairs (Pd<sup>2+</sup>–O<sup>2–</sup>) in PdO. The charges in the CH<sub>3</sub> and H moieties along the reaction coordinate depend on the accessibility and chemical state of the Pd and O centers involved. Homolytic C–H dissociation prevails on bare (*–*) and O*-covered surfaces (O*–O*), while C–H bonds cleave heterolytically on Pd<sup>2+</sup>–O<sup>2–</sup> pairs at PdO surfaces. On bare surfaces, C–H bonds cleave via oxidative addition, involving Pd atom insertion into the C–H bond with electron backdonation from Pd to C–H antibonding states and the formation of tight three-center (H<sub>3</sub>C···Pd···H)<sup>⧧</sup> transition states. On O*-saturated Pd surfaces, C–H bonds cleave homolytically on O*–O* pairs to form radical-like CH<sub>3</sub> species and nearly formed O–H bonds at a transition state (O*···CH<sub>3</sub><sup>•</sup>···*OH)<sup>⧧</sup> that is looser and higher in enthalpy than on bare Pd surfaces. On PdO surfaces, site pairs consisting of exposed Pd<sup>2+</sup> and vicinal O<sup>2–</sup>, Pd<sub>ox</sub>–O<sub>ox</sub> , cleave C–H bonds heterolytically via σ-bond metathesis, with Pd<sup>2+</sup> adding to the C–H bond, while O<sup>2–</sup> abstracts the H-atom to form a four-center (H<sub>3</sub>C<sup>δ−</sup>···Pd<sub>ox</sub>···H<sup>δ+</sup>···O<sub>ox</sub>)<sup>⧧</sup> transition state without detectable Pd<sub>ox</sub> reduction. The latter is much more stable than transition states on *–* and O*–O* pairs and give rise to a large increase in CH<sub>4</sub> oxidation turnover rates at oxygen chemical potentials leading to Pd to PdO transitions. These distinct mechanistic pathways for C–H bond activation, inferred from theory and experiment, resemble those prevalent on organometallic complexes. Metal centers present on surfaces as well as in homogeneous complexes act as both nucleophile and electrophile in oxidative additions, ligands (e.g., O* on surfaces) abstract H-atoms via reductive deprotonation of C–H bonds, and metal–ligand pairs, with the pair as electrophile and the metal as nucleophile, mediate σ-bond metathesis pathways

    Theoretical Insights into the Effects of KOH Concentration and the Role of OH<sup>–</sup> in the Electrocatalytic Reduction of CO<sub>2</sub> on Au

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    The active and selective electrochemical reduction of CO2 to value-added chemical intermediates can offer a sustainable route for the conversion of CO2 to chemicals and fuels, thus helping to mitigate greenhouse gas emissions and enabling intermittent energy from renewable sources. Alkaline solutions are often the preferred media for the electrocatalytic CO2 reduction reaction (CO2RR) as they provide high current densities and low overpotentials while suppressing the hydrogen evolution side reaction. Recent experiments carried out on Au and Ag in KOH, as well as other electrolytes, including KHCO3, K2CO3, and KCl, showed that increasing electrolyte concentration lowered onset potentials, increased Faradaic efficiencies to CO, and improved current densities. Herein, we carry out potential-dependent ab initio molecular dynamic (AIMD) simulations along with density functional theory (DFT) calculations using explicit KOH electrolyte and H2O solution molecules to examine the influence of OH– anions and the KOH electrolyte on the elementary steps and their corresponding energetics in the mechanism for CO2 reduction. The simulations indicate that the first electron transfer step to CO2 to form the adsorbed *CO2(•−) radical anion is rate-limiting, while the subsequent proton and electron transfer steps are facile and downhill in energy at reducing potentials. The OH– anions present in the solution can adsorb on the Au cathode down to potentials as low as ∼ −3 V (SCE). This enables the OH– anions to transfer electrons to the Au cathode and into antibonding 2π* orbitals of CO2, thus facilitating the rate-determining adsorption and electron transfer to CO2 to form the adsorbed *CO2(•−) radical anion. Increasing the concentration of the K+OH– electrolyte reduces the barrier for the electrocatalytic reduction of CO2 and thus improves the current density, consistent with the reported experimental results. The *CO2(•−) radical anion that forms subsequently undergoes facile proton transfer from a vicinal water molecule in solution to form the hydroxy carbonyl (*HOCO) intermediate that readily undergoes subsequent proton and electron transfer from a second water molecule to form CO and OH– at a potential of ∼ −1.2 V SCE. While the formation of formate (HCOO–) is thermodynamically favorable, the direct hydrogenation of *CO2(•−) as well as the intramolecular proton transfer via *HOCO to form HCOO– are kinetically unfavored. The presence of OH– anions near the surface also facilitates the formation of bicarbonate (HCO3–) at lower potentials. The bicarbonate that forms can be converted to the reactive *HOCO intermediate at more negative potentials that subsequently reacts to form CO and regenerate OH–. The results discussed herein help provide a more detailed understanding of the interplay between the OH–, K+, H2O, and reaction intermediates on the Au surface in the electric double layer and their influence on the onset potential, electrocatalytic activity, and selectivity for CO2RR

    Inhibition at Perimeter Sites of Au/TiO<sub>2</sub> Oxidation Catalyst by Reactant Oxygen

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    TiO<sub>2</sub>-supported gold nanoparticles exhibit surprising catalytic activity for oxidation reactions compared to noble bulk gold which is inactive. The catalytic activity is localized at the perimeter of the Au nanoparticles where Au atoms are atomically adjacent to the TiO<sub>2</sub> support. At these dual-catalytic sites an oxygen molecule is efficiently activated through chemical bonding to both Au and Ti<sup>4+</sup> sites. A significant inhibition by a factor of 22 in the CO oxidation reaction rate is observed at 120 K when the Au is preoxidized, caused by the oxygen-induced positive charge produced on the perimeter Au atoms. Theoretical calculations indicate that induced positive charge occurs in the Au atoms which are adjacent to chemisorbed oxygen atoms, almost doubling the activation energy for CO oxidation at the dual-catalytic sites in agreement with experiments. This is an example of self-inhibition in catalysis by a reactant species
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