Bond Dissociation Steps Research Articles

Theoretical assessments of CO2 activation and hydrogenation pathways on transition-metal surfaces

Carbon dioxide (CO2) hydrogenation on transition-metal active sites offers a promising carbon utilization route toward mitigating greenhouse gas emissions. C1 products are often formed in parallel during CO2 hydrogenation, prompting investigations into the intrinsic properties of transition metals that drive activity and product selectivity. In this work, close-packed surfaces of a selection of transition-metal catalysts (Ni, Co, Rh, Ru, Pd, and Pt) were studied with density functional theory (DFT) calculations to understand their fundamental reactivities for CO2 transformation reactions. Results indicate that CO2 conversion proceeds through CO* formation and hydrogenation to form C1 products (* denotes an adsorbed species). Ni, Co, Rh, and Ru favor CO/CH4 formation, while Pd and Pt favor CO/CH3OH formation. The ability of a metal to dissociate C-O bonds drives selectivity between CH4 and CH3OH, while competition between CO* desorption and surface hydrogenation describes CO selectivities. The C-O bond dissociation steps often impose the highest barrier along CH4 formation reaction profiles, suggesting their kinetic relevance for CH4 formation rates. The provided DFT-derived data sets detail a comprehensive reaction network of elementary steps relevant to C1 chemistries, ultimately offering a benchmark for insights into design strategies for materials that exploit transition-metal active sites in carbon capture or utilization processes.

Single Co-Atoms as Electrocatalysts for Efficient Hydrazine Oxidation Reaction.

Single-atom catalysts (SACs) have aroused great attention due to their high atom efficiency and unprecedented catalytic properties. A remaining challenge is to anchor the single atoms individually on support materials via strong interactions. Herein, single atom Co sites have been developed on functionalized graphene by taking advantage of the strong interaction between Co2+ ions and the nitrile group of cyanographene. The potential of the material, which is named G(CN)Co, as a SAC is demonstrated using the electrocatalytic hydrazine oxidation reaction (HzOR). The material exhibits excellent catalytic activity for HzOR, driving the reaction with low overpotential and high current density while remaining stable during long reaction times. Thus, this material can be a promising alternative to conventional noble metal-based catalysts that are currently widely used in HzOR-based fuel cells. Density functional theory calculations of the reaction mechanism over the material reveal that the Co(II) sites on G(CN)Co can efficiently interact with hydrazine molecules and promote the NH bond-dissociation steps involved in the HzOR.

Core Charge Density Dominated Size-Conversion from Au6 P8 to Au8 P8 Cl2.

The stimulus-response of metal nanoclusters is crucial to their applications in catalysis and bio-clinics, etc. However, its mechanistic origin has not been well studied. Herein, the mechanism of the AuI PPh3 Cl-induced size-conversion from [Au6 (DPPP)4 ]2+ to [Au8 (DPPP)4 Cl2 ]2+ (DPPP is short for 1,3-bis(diphenylphosphino)propane) is theoretically investigated with density functional theory (DFT) calculations. The optimal size-growth pathway, and the key structural parameters were elucidated. The Au-P bond dissociation steps are key to the size-growth, the easiness of which was determined by the charge density of the metallic core of the cluster precursors (i.e., "core charge density"). This study sheds light on the inherent structure-reactivity relationships during the size-conversion, and will benefit the deep understanding on the kinetics of more complex systems.

Low Temperature Activation of Methane on Metal-Oxides and Complex Interfaces: Insights from Surface Science.

ConspectusThe abundance of cheap, natural gas has transformed the energy landscape, whereby revealing new possibilities for sustainable chemical technologies or impacting those that have relied on traditional fossil fuels. The primary component, methane, is underutilized and wastefully exhausted, leading to anthropogenic global warming. Historically, the manipulation of methane remained "clavis aurea," an insurmountable yet rewarding challenge and thus the focus of intense research. This is primarily due to an inability to dissociate C-H bonds in methane selectively, which requires a high energy penalty and is an essential prerequisite for the direct conversion of methane into a large set of value-added products. The discovery of such processes would promise an energy gainful use of natural gas benefiting several essential chemical processes associated with C1 chemistry. This first C-H bond dissociation step of the methane molecule appears in numerous catalytic mechanisms as the rate-determining step or most essential barrier sequence for all subsequent steps that follow in the production of C-C, C-O, or Cx-Hy-Oz bonds found in value added products. A main goal is to catalytically reduce the energy barrier for the first C-H bond dissociation to be able to achieve the activation of methane at low or moderate temperatures. As such there is great value in understanding the fundamental nature of the active sites responsible for bond breaking or formation and thus be able to facilitate better control of this chemistry, leading to the development of new technologies for fuel production and chemical conversion. Surface science studies offer enhanced perspectives for a careful manipulation of bonds over the last layer atoms of catalyst surfaces, an essential factor for the design of atomically precise catalysts and unravelling of the reaction mechanism. With the advent of new surface imaging, spectroscopy, and in situ tools, it has been possible to decipher the surface chemistry of complex materials systems and further our understanding of atomic active sites on the surfaces of metals, oxides, and carbides or metal-oxide and metal-carbide interfaces. The once considered near impossible step of C-H bond activation is now observed at low temperatures with high propensity over a collection of oxide, metal-oxide, and metal-carbide systems in a conventional or inverse configuration (oxide or carbide on metal). The enabling of C-H activation at low temperature has opened interesting possibilities for the specific production of chemicals such as methanol directly from methane, a step toward facile synthesis of liquid fuels. We highlight the most recent of these results and present the key aspects of active site configurations engineered from surface science studies which enable such a simple reactive event through careful manipulation of the last surface layer of atoms found in the catalyst structure. New concepts which help in the activation and conversion of methane are discussed.

Palladium Copper Chromium Ternary Nanoparticles Constructed In situ within a Basic Resin: Enhanced Activity in the Dehydrogenation of Formic Acid

AbstractPdCuCr ternary nanoparticles (NPs) responsible for the efficient production of H2 from formic acid are constructed in situ during the initial catalytic reaction within a macroreticular basic resin that possesses −N(CH3)2 functional groups. A high H2 production rate of 174 000 mL h−1 gpd−1 with a high turnover frequency of 830 h−1 based on Pd can be achieved, which is substantially higher than that obtained with bimetallic PdCu and PdCr and monometallic Pd NPs. Physicochemical characterization was performed by using X‐ray absorption fine structure analysis and high‐angle annular dark‐field scanning transmission electron microscopy. The kinetic isotope effect revealed not only the stabilization of highly dispersed NPs but also the boosting of the C−H bond dissociation step by a synergistic alloying effect induced by the introduction of Cr atoms, which plays an important role to achieve efficient catalytic activity. Moreover, this heterogeneous catalytic system exhibits high durability without agglomeration and leaching and has potential for applications in high‐pressure gas‐evolution systems.

Methane Oxidation over Palladium: On the Mechanism in Fuel-Rich Mixtures at High Temperatures

A kinetic modeling study on methane oxidation over reduced Pd for various fuel-rich conditions around the stoichiometric point of the partial oxidation at high temperatures (900–1100 K) is presented. A thermodynamically consistent detailed surface reaction mechanism is developed within the mean field approximation. The proposed kinetic model consists of 54 elementary-step based reactions including seven gas-phase species and 15 surface intermediates. Three different methane activation paths are implemented, comprising pyrolytic C–H bond dissociation steps, oxygen-assisted and dual-oxygen-assisted CH4 activation. In situ experimental measurements in a quasi-autothermally operated flow reactor, using the capillary sampling technique, are performed for model evaluation. The provided experimental data includes spatially resolved temperature and concentration profiles within a single catalytic channel of a Pd/Al2O3-coated monolith. Supplementary numerical simulations based on literature data for fuel-lean and fuel-rich conditions at high temperatures extend the model’s capability to predict a wide range of different experimental conditions.

Investigation of Size Sensitivity in the Hydrogen Production from Formic Acid over Carbon‐Supported Pd Nanoparticles

AbstractThe size effect in the hydrogen production from formic acid decomposition has been investigated using a series of carbon‐supported Pd NPs with different particle sizes ranging from 2.7 to 5.5 nm. A volcano type relationship between catalytic activity and NPs size was obtained. This result suggests that there is an optimum relative proportion of low‐ and high‐coordination surface atoms within a regular arrangement of the Pd NPs, which was achieved at 3.9 nm in diameter under the present experimental conditions. By performing calculations on the palladium crystallites, decomposition of formic acid is considered as a structure‐sensitive reaction, and high‐coordination sites principally act as catalytically active species in this size range. Kinetic isotope effect using HCOOD and DCOOH was well corresponded to the catalytic tendency. A kH/kD value obtained with Pd/C with optimal size was smallest among investigated, indicating the positive promoting effect in the rate‐determining C−H bond dissociation step from the Pd‐formate intermediate.

Elucidation of Pathways for NO Electroreduction on Pt(111) from First Principles

AbstractThe mechanism of nitric oxide electroreduction on Pt(111) is investigated using a combination of first principles calculations and electrokinetic rate theories. Barriers for chemical cleavage of NO bonds on Pt(111) are found to be inaccessibly high at room temperature, implying that explicit electrochemical steps, along with the aqueous environment, play important roles in the experimentally observed formation of ammonia. Use of explicit water models, and associated determination of potential‐dependent barriers based on Bulter–Volmer kinetics, demonstrate that ammonia is produced through a series of water‐assisted protonation and bond dissociation steps at modest voltages (<0.3 V). In addition, the analysis sheds light on the poorly understood formation mechanism of nitrous oxide (N2O) at higher potentials, which suggests that N2O is not produced through a Langmuir–Hinshelwood mechanism; rather, its formation is facilitated through an Eley–Rideal‐type process.

Elucidation of Pathways for NO Electroreduction on Pt(111) from First Principles.

The mechanism of nitric oxide electroreduction on Pt(111) is investigated using a combination of first principles calculations and electrokinetic rate theories. Barriers for chemical cleavage of N-O bonds on Pt(111) are found to be inaccessibly high at room temperature, implying that explicit electrochemical steps, along with the aqueous environment, play important roles in the experimentally observed formation of ammonia. Use of explicit water models, and associated determination of potential-dependent barriers based on Bulter-Volmer kinetics, demonstrate that ammonia is produced through a series of water-assisted protonation and bond dissociation steps at modest voltages (<0.3 V). In addition, the analysis sheds light on the poorly understood formation mechanism of nitrous oxide (N2 O) at higher potentials, which suggests that N2 O is not produced through a Langmuir-Hinshelwood mechanism; rather, its formation is facilitated through an Eley-Rideal-type process.

Selectivity of chemisorbed oxygen in C–H bond activation and CO oxidation and kinetic consequences for CH 4–O 2 catalysis on Pt and Rh clusters

Rate measurements, density functional theory (DFT) within the framework of transition state theory, and ensemble-averaging methods are used to probe oxygen selectivities, defined as the reaction probability ratios for O * reactions with CO and CH 4, during CH 4–O 2 catalysis on Pt and Rh clusters. CO 2 and H 2O are the predominant products, but small amounts of CO form as chemisorbed oxygen atoms (O *) are depleted from cluster surfaces. Oxygen selectivities, measured using 12CO– 13CH 4–O 2 reactants, increase with O 2/CO ratio and O * coverage and are much larger than unity at all conditions on Pt clusters. These results suggest that O * reacts much faster with CO than with CH 4, causing any CO that forms and desorbs from metal cluster surfaces to react along the reactor bed with other O * to produce CO 2 at any residence time required for detectable extents of CH 4 conversion. O * selectivities were also calculated by averaging DFT-derived activation barriers for CO and CH 4 oxidation reactions over all distinct surface sites on cubo-octahedral Pt clusters (1.8 nm diameter, 201 Pt atoms) at low O * coverages, which are prevalent at low O 2 pressures during catalysis. CO oxidation involves non-activated molecular CO adsorption as the kinetically relevant step on exposed Pt atoms vicinal of chemisorbed O * atoms (on *–O * site pairs). CH 4 oxidation occurs via kinetically relevant C–H bond activation on *– * site pairs involving oxidative insertion of a Pt atom into one of the C–H bonds in CH 4, forming a three-centered HC 3–Pt–H transition state. C–H bond activation barriers reflect the strength of Pt–CH 3 and Pt–H interactions at the transition state, which correlates, in turn, with the Pt coordination and with CH 3 * binding energies. Ensemble-averaged O * selectivities increase linearly with O 2/CO ratios, which define the O * coverages, via a proportionality constant. The proportionality constant is given by the ratio of rate constants for O 2 dissociation and C–H bond activation elementary steps; the values for this constant are much larger than unity and are higher on larger Pt clusters (1.8–33 nm) at all temperatures (573–1273 K) relevant for CH 4–O 2 reactions. The barriers for the kinetically relevant C–H bond dissociation step increase, while those for CO oxidation remain unchanged as the Pt coordination number and cluster size increase, and lead, in turn, to higher O * selectivities on larger Pt clusters. Oxygen selectivities were much larger on Rh than Pt, because the limiting reactants for CO oxidation were completely consumed in 12CO– 13CH 4–O 2 mixtures, consistent with lower CO/CO 2 ratios measured by varying the residence time and O 2/CH 4 ratio independently in CH 4–O 2 reactions. These mechanistic assessments and theoretical treatments for O * selectivity provide rigorous evidence of low intrinsic limits of the maximum CO yields, thus confirming that direct catalytic partial oxidation of CH 4 to CO (and H 2) does not occur at the molecular scale on Pt and Rh clusters. CO (and H 2) are predominantly formed upon complete O 2 depletion from the sequential reforming steps.

Effect of Pd surface structure on the activation of methyl acetate

The activation of methyl acetate (CH 3COOCH 3) has been studied using periodic density functional theory calculations to probe the effect of Pd surface structure on this reaction. The adsorption of methyl acetate, dehydrogenated derivatives including enolate (CH 2COOCH 3) and methylene acetate (CH 3COOCH 2), and dissociation products including acetate, acetyl, ketene, methoxy, formaldehyde, CO, C, O, and H; and the C–H and RCO–OR′ C–O bond dissociation in methyl acetate, enolate, and methylene acetate, are calculated on Pd(1 1 1) terrace, step edge, and kink; and on Pd(1 0 0) terrace and step edge. The adsorption of most species is not strongly affected from (1 1 1)- to (1 0 0)-type surfaces, but is clearly enhanced by step/kink compared to the corresponding terrace. For a given reactant, going both from terrace to step edge and from (1 1 1)- to (1 0 0)-type surfaces stabilizes the transition states of C–O bond dissociation steps. Going from terrace to step edge also stabilizes the transition states of C–H bond dissociation steps, but going from (1 1 1)- to (1 0 0)-type surfaces does not clearly do so. Our results suggest that compared to the Pd(1 1 1) terrace, the Pd(1 0 0) terrace is more selective for the C–O bond dissociation that is desirable for alcohol formation, whereas step edges are more selective for C–H bond dissociation.

Trends of Water Gas Shift Reaction on Close-Packed Transition Metal Surfaces

The mechanism of the water gas shift reaction (WGSR) on the close-packed transition metal surfaces of Co, Ni, Cu (from the 3d row), Rh, Pd, Ag (from the 4d row), Ir, Pt, and Au (from the 5d row) has been systematically examined by periodic density functional theory (DFT) calculations. The comparison of potential energy surface (PES) concludes that WGSR activity is influenced by two kinds of elementary steps: O−H bond dissociation and C−O bond formation. Activation barriers (Ea) and reaction energies (ΔH) on a series of metal surfaces show good BEP relationship; however, their energetic trends are opposite in these two kinds of steps. In O−H bond dissociation steps, trends of Ea and ΔH are groups 9 < 10 < 11 and 3d < 4d < 5d. On the other hand, C−O bond formation steps on the surfaces of the lower-right metals in the d block (Cu, Ag, Pt, Au) have relatively lower Ea and ΔH, which is responsible for their high WGSR activity of metal/oxide catalysts. In addition, the fundamental of energetic trends has been examined from the analyses of adsorption energy, density of state (DOS), and charge density. The result shows that the surfaces of upper-left d-block metals (Co, Ni, Rh) with higher energy and smaller delocalization of their d orbitals yield a stronger adsorption energy with higher induced charges that will stabilize dissociating fragments to lower the barrier and retard desorptions to lift the barrier in O−H bond dissociation and C−O bond formation steps, respectively. The prediction of energetic trends in the present work is also appropriate for other catalytic reactions, such as ethanol decomposition and CO oxidation, and can help us scientifically design a better catalyst for the desired reaction.

On the Parallel Mechanism of the Dissociation of Energy-Selected P(CH3)3+ Ions

Energy selected trimethyl phosphine ions were prepared by threshold photoelectron photoion coincidence (TPEPICO) spectroscopy. This ion dissociates via H, CH(3), and CH(4) loss, the latter two involving hydrogen transfer steps. The ion time-of-flight distribution and the breakdown diagram are analyzed in terms of the statistical RRKM theory, which includes tunneling. Ab initio and DFT calculations provide the vibrational frequencies required for the RRKM modeling. CH(3) loss could produce both the P(CH(3))(2)(+) by a simple bond dissociation step, and the more stable HP(CH(2))CH(3)(+) ion by a hydrogen transfer step. Quantum chemical calculations are extensively used to uncover the reaction scheme, and they strongly suggest that the latter product is exclusively formed via an isomerization step in the energy range of the experiment. The data analysis, which includes modeling with the trimethyl phosphine thermal energy distribution, provides accurate onset energies for both H (E(0K) = 1024.1 +/- 3.5 kJ/mol) and CH(3) (E(0K) = 1024.8 +/- 3.5 kJ/mol) loss reactions. From this analysis, we conclude that the Delta(f)H(298K) degrees [HP(CH(2))(CH(3))(+)] = 783 +/- 8 kJ/mol and Delta(f)H(298K) degrees [P(CH(2))(CH(3))(2)(+)] = 711 +/- 8 kJ/mol.

A theoretical study of reactivity and regioselectivity in the hydroxylation of adamantane by ferrate(VI).

The conversion of adamantane to adamantanols mediated by ferrate (FeO(4)(2)(-)), monoprotonated ferrate (HFeO(4)(-)), and diprotonated ferrate (H(2)FeO(4)) is discussed with the hybrid B3LYP density functional theory (DFT) method. Diprotonated ferrate is the best mediator for the activation of the C-H bonds of adamantane via two reaction pathways, in which 1-adamantanol is formed by the abstraction of a tertiary hydrogen atom (3 degrees ) and 2-adamantanol by the abstraction of a secondary hydrogen atom (2 degrees ). Each reaction pathway is initiated by a C-H bond cleavage via an H-atom abstraction that leads to a radical intermediate, followed by a C-O bond formation via an oxygen rebound step to lead to an adamantanol complex. The activation energies for the C-H cleavage step are 6.9 kcal/mol in the 1-adamantanol pathway and 8.4 kcal/mol in the 2-adamantanol pathway, respectively, at the B3LYP/6-311++G level of theory, whereas those of the second reaction step corresponding to the rebound step are relatively small. Thus, the rate-determining step in the two pathways is the C-H bond dissociation step, which is relevant to the regioselectivity for adamantane hydroxylation. The relative rate constant (3 degrees )/(2 degrees ) for the competing H-atom abstraction reactions is calculated to be 9.30 at 75 degrees C, which is fully consistent with an experimental value of 10.1.

The Relationship between the Electronic and Redox Properties of Dispersed Metal Oxides and Their Turnover Rates in Oxidative Dehydrogenation Reactions

The mechanistic connections among propane oxidative dehydrogenation (ODH) rates, H2 reduction rates, and the electronic transitions responsible for the absorption edge in the electronic spectra of dispersed metal oxides were explored for VOx, MoOx, WOx, and NbOx samples consisting predominately of two-dimensional oxide domains supported on Al2O3, ZrO2, and MgO. For a given active oxide, propane turnover rates increased in parallel with the reduction rate of the oxide catalyst using H2, but propane ODH rates differed significantly among different metal oxide samples with similar H2 reduction rates. For all catalysts, ODH turnover rates increased monotonically as the energy of the absorption edge in the UV–visible spectrum decreased. These results, taken together with the respective mechanisms for electron transfer during C–H bond activation and during the ligand-to-metal charge-transfer processes responsible for the UV–visible edge, suggest that the stability of activated complexes in C–H bond dissociation steps depends sensitively on the ability of the active oxide domains to transfer electrons from lattice oxygen atoms to metal centers. The electronic transitions responsible for the UV–visible absorption edge are mechanistically related to the redox cycles involving lattice oxygens responsible for oxidative dehydrogenation turnovers of alkanes. As a result, the details of near-edge electronic spectra provide useful guidance about intrinsic reaction rates on active oxides typically used for these reactions.

Stereospecific multi-step elimination of methanol involving 1,4-methoxyl migration from the MH+ ion of 2,3-cis-3-methoxytricyclo[6.2.2.02,7]dodeca-9-ene

The eilmination of methanol from the MD + ion of 2,3-cis-3-methoxytricyclo[6.2.2.0 2,7 ]dodeca-9-ene, endo-2, upon chemical ionization (CI) gives rise to both [MD - MeOD] + and [MD - MeOH] + ions. Only the [MD - MeOH] + ion is formed under collision-induced dissociation (CID) conditions. This is in contrast with the behavior of the 2,3-trans-stereoisomer exo-2 and with saturated analogs which undergo exclusive elimination of MeOD. The unusual elimination of MeOH from endo-2 indicates transfer of the external deuteron in the MD + ion from the oxygen atom to the interior of the organic moiety and a back transfer of a hydrogen from the organic moiety to the oxygen atom prior to the C-O bond dissociation step. A deuterium labeling study showed that the hydrogen atom involved in this elimination process originates at position 3 (formal 1,1-elimination). These results suggest a multi-step mechanism for this unique stereospecific methanol elimination, initiated by a proton transfer from the methoxy group to the double bond followed by a 1,4-methoxyl migration from C-3 to C-10. The proposed mechanism finds support in the Cl and CID study of deuterium-labeled analogs which have a methoxy group at position 10.