Heterogeneous Reaction Pathways Research Articles

Chemistry of guanidinate-stabilised diboranes: transition-metal-catalysed dehydrocoupling and hydride abstraction.

Herein, we analyse the catalytic boron-boron dehydrocoupling reaction that leads from the base-stabilised diborane(6) [H2 B(hpp)]2 (hpp=1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinate) to the base-stabilised diborane(4) [H2 B(hpp)]2 . A number of potential transition-metal precatalysts was studied, including transition-metal complexes of the product diborane(4). The synthesis and structural characterisation of two further examples of such complexes is presented. The best results for the dehydrocoupling reactions were obtained with precatalysts of Group 9 metals in the oxidation state of +I. The active catalyst is formed in situ through a multistep process that involves reduction of the precatalyst by the substrate [H2 B(hpp)]2 , and mechanistic investigations indicate that both heterogeneous and (slower) homogeneous reaction pathways play a role in the dehydrocoupling reaction. In addition, hydride abstraction from [H2 B(hpp)]2 and related diboranes is analysed and the possibility for subsequent deprotonation is discussed by probing the protic character of the cationic boron-hydrogen compounds with NMR spectroscopic analysis.

Heterogeneous reaction of particulate chlorpyrifos with NO3 radicals: Products, pathways, and kinetics

Chlorpyrifos is a typical chlorinated organophosphorus pesticide. The heterogeneous reaction of chlorpyrifos particles with NO3 radicals was investigated using a vacuum ultraviolet photoionization aerosol time-of-flight mass spectrometer (VUV-ATOFMS) and a real-time atmospheric gas analysis mass spectrometer. Chlorpyrifos oxon, 3,5,6-trichloro-2-pyridinol, O,O-diethyl O-hydrogen phosphorothioate, O,O-diethyl ester thiophosphoric acid, diethyl hydrogen phosphate and a phosphinyl disulfide compound were identified as the main degradation products. The heterogeneous reaction pathways were proposed and their kinetic processes were investigated via a mixed-phase relative rate method. The observed effective rate constant is 3.4±0.2×10−12cm3molecule−1s−1.

New Directions in Advanced Modeling and in Situ Measurements Near Reacting Surfaces

Multidimensional numerical modeling and in situ spatially-resolved measurements of gas-phase thermoscalars over the catalyst boundary layer have fostered fundamental investigation of the heterogeneous and homogeneous chemical reaction pathways and their coupling at realistic operating conditions. The methodology for validating catalytic and gas-phase reaction mechanisms is firstly outlined for industrially-relevant fuels. Combination of advanced modeling and in situ near-wall species and velocity measurements is then used to address the intricate interplay between interphase fluid transport (laminar or turbulent) and hetero-/homogeneous kinetics. Controlling parameters of this interplay are the homogeneous ignition chemistry, flame propagation characteristics, competition between the catalytic and gaseous pathways for fuel consumption, diffusional imbalance of the limiting reactant, flow laminarization due to heat transfer from the hot catalytic walls, and fuel leakage through the gaseous reaction zone. Dynamic reactor operation and intrinsic flame dynamics driven by interactions between homogeneous kinetics and catalytic walls are outlined using detailed transient simulation. It is shown that the presence of catalytic reactions moderates flame instabilities. Future directions for transient modeling and for temporally-resolved in situ near-wall measurements are finally summarized.

Modelling the Fate of Sulphur During Pulverized Coal Combustion under Conventional and Oxy-fuel Conditions

Focus of the present study is on the fate of sulphur during coal combustion and modelling of the corresponding SOx formation mechanisms. The sulphur chemistry during coal combustion in general is briefly described and potential effects of the oxy-fuel conditions are explained. Details about the developed sulphur chemistry model which covers both heterogeneous and homogeneous reaction pathways are given. The model describes the sulphur transformation in a sequence of stages: the release of coal-bound sulphur, gas phase reactions of sulphuric species, and self-retention of sulphur oxides by coal ash.The model is evaluated against experimental data from IFK's semi-industrial scale furnace (500 kWth) firing lignite at conventional and oxy-fuel combustion conditions. Four reference cases are considered, i.e. air and oxy-fuel mode in both non-staged and staged operation. Based on the results from the basic combustion simulation with AIOLOS, the sulphur chemistry model has been applied in a subsequent post-processing step. The sulphur related results show that the general trends regarding the species concentrations may be predicted correctly. The specific characteristics and the effect of oxy-fuel conditions and oxidant staging are captured correspondingly within the simulation results. Yet, certain deficiencies concerning the quantitative prediction could be identified which necessitate further investigations.

Catalytic Abatement of Nitrous Oxide Coupled with Functionalization of C1–C3 Alkanes

AbstractMechanistic aspects of selective reduction of N2O by C1–C3 alkanes resulting in N2, olefins, and hydrogen over CaO‐based catalysts were studied by combining catalytic tests at ambient pressure with transient analysis of products in high vacuum with microsecond time resolution. In the frame of this reaction, oxygen species formed from N2O participate in selective (olefin production) and nonselective (COx formation) heterogeneous interactions. The coverage by these species and the interplay of exothermic heterogeneous (N2O decomposition and oxidative alkane conversions) and endothermic homogeneous (thermal nonoxidative dehydrogenation and cracking of hydrocarbons) reaction pathways determine the overall olefin selectivity.

A Kinetic Study of Ozone Decomposition on Illuminated Oxide Surfaces

The heterogeneous chemistry and photochemistry of ozone on oxide components of mineral dust aerosol, including α-Fe(2)O(3), TiO(2), and α-Al(2)O(3), at different relative humidities have been investigated using an environmental aerosol chamber. The rate and extent of ozone decomposition on these oxide surfaces are found to be a function of the nature of the surface as well as the presence of light and relative humidity. Under dark and dry conditions, only α-Fe(2)O(3) exhibits catalytic decomposition toward ozone, whereas the reactivity of TiO(2) and α-Al(2)O(3) is rapidly quenched upon ozone exposure. However, upon irradiation, TiO(2) is active toward O(3) decomposition and α-Al(2)O(3) remains inactive. In the presence of relative humidity, ozone decay on α-Fe(2)O(3) subject to irradiation or under dark conditions is found to decrease. In contrast, ozone decomposition is enhanced for irradiated TiO(2) as relative humidity initially increases but then begins to decrease at higher relative humidity levels. A kinetic model was used to obtain heterogeneous reaction rates for different homogeneous and heterogeneous reaction pathways taking place in the environmental aerosol chamber. The atmospheric implications of these results are discussed.

Synthesis, structural properties, and catalytic behavior of Cu-BTC and mixed-linker Cu-BTC-PyDC in the oxidation of benzene derivatives

Mixed-linker metal-organic frameworks based on the Cu-BTC structure have been synthesized in which the benzene-1,3,5-tricarboxylate (BTC) linkers have been partially replaced by pyridine-3,5-dicarboxylate (PyDC). X-ray-based techniques (powder XRD and XAS), thermal analysis, and infrared spectroscopy proved that a desired amount of PyDC (up to 50%) can be incorporated without changing significantly the crystal structure. The pyridine unit can be seen as a defect site in the local coordination environment of the dimeric copper units, which is significantly altering their electronic structure and the catalytic properties. Both Cu-BTC and the mixed Cu-BTC-PyDCs catalyze the demanding direct hydroxylation of toluene both in acetonitrile and in neat substrate. Different selectivity toward the desired ortho- and para-cresol and other oxidation products (benzaldehyde, benzyl alcohol, methylbenzoquinone) was observed for Cu-BTC and the Cu-BTC-PyDCs, respectively. Leaching tests and comparison with homogeneously dissolved Cu catalysts indicate mainly a heterogeneous reaction pathway.

Mass spectrometry of atmospheric aerosols—Recent developments and applications. Part I: Off‐line mass spectrometry techniques

Many of the significant advances in our understanding of atmospheric particles can be attributed to the application of mass spectrometry. Mass spectrometry provides high sensitivity with a fast response time to probe chemically complex particles. This review focuses on recent developments and applications in the field of mass spectrometry of atmospheric aerosols. In Part I of this two-part review, we concentrate on off-line mass spectrometry techniques, which require sample collection on filters but can provide detailed molecular speciation. In particular, off-line mass spectrometry techniques utilizing tandem mass spectrometry experiments and high resolution mass analyzers provide improved insight into secondary organic aerosol formation and heterogeneous reaction pathways through detailed structural elucidation at the molecular level.

Contrasting the Individual Reactive Pathways in Protein Unfolding and Disulfide Bond Reduction Observed within a Single Protein

Identifying the dynamics of individual molecules along their reactive pathways remains a major goal of modern chemistry. For simple chemical reactions, the transition state position is thought to be highly localized. Conversely, in the case of more complex reactions involving proteins, the potential energy surfaces become rougher, resulting in heterogeneous reaction pathways with multiple transition state structures. Force-clamp spectroscopy experimentally probes the individual reaction pathways sampled by a single protein under the effect of a constant stretching force. Herein, we examine the distribution of conformations that populate the transition state of two different reactions; the unfolding of a single protein and the reduction of a single disulfide bond, both occurring within the same single protein. By applying the recently developed static disorder theory, we quantify the variance of the barrier heights, σ(2), governing each distinct reaction. We demonstrate that the unfolding of the I27 protein follows a nonexponential kinetics, consistent with a high value of σ(2) ∼ 18 (pN nm)(2). Interestingly, shortening of the protein upon introduction of a rigid disulfide bond significantly modulates the disorder degree, spanning from σ(2) ∼ 8 to ∼21 (pN nm)(2). These results are in sharp contrast with the exponential distribution of times measured for an S(N)2 chemical reaction, implying the absence of static disorder σ(2) ∼ 0 (pN nm)(2). Our results demonstrate the high sensitivity of the force-clamp technique to capture the signatures of disorder in the individual pathways that define two distinct force-induced reactions, occurring within the core of a single protein.

Protein Unfolding and Chemical Reactions Under Force: Complexity Versus Simplicity

Elucidating the dynamics of reactant molecules along the particular pathway that connects them with the resulting products is a cornerstone of modern chemistry. In general, for small systems composed of only a few molecules, the valley connecting reactants with products is narrow and the position of the transition state is highly localized. By contrast, in the case of more complex reactions involving bigger molecular systems such as proteins, the potential energy surfaces become rougher, which typically results in heterogeneous reaction pathways with multiple transition state structures. In both cases, detailed information on the energy landscapes has been mainly restricted to the results obtained using computational methods. Single molecule force-clamp spectroscopy can be now used to directly monitor the individual unfolding pathways of single proteins and the reactive trajectories corresponding to the chemical reduction of a single disulfide bond embedded within the core of a protein. Herein, we compare the effect of a mechanical constant force on the energy landscape of two different reactions; the unfolding of a single protein and the reduction of a single disulfide bond, both occurring within the same single protein. Our results demonstrate that the native state of the 27th immunoglobulin-like domain of titin is rough and probably composed of a set of similar structures with slightly different energy. Interestingly, shortening of the I27 protein by the presence of disulfide bonds has a strong effect on the measured heterogeneity in the unfolding pathways. Our results are quantitatively discussed within the framework of the static disorder theory. In sharp contrast with these results, the energy landscape of an SN2 chemical reaction, occurring within a much shorter length-scale, is smooth, implying a high degree of homogeneity within the individual reactive pathways connecting the reactants with the reaction products.

Real Time Observation of Chemical Reactions of Individual Metal Nanoparticles with High-Throughput Single Molecule Spectral Microscopy

Real time observation of chemical reactions of individual noble metal nanoparticles (MNPs) is fundamentally important to their controlled synthesis, chemical sensing and catalysis applications. Here, with a simple and high-throughput single-molecule darkfield spectral imaging technique, we demonstrate that the reaction-induced plasmonic resonance variations of multiple MNPs could be monitored in parallel. Oxidation kinetics of individual gold nanorods (AuNRs), either immobilized on a glass substrate or moving freely in homogeneous solution, was recorded successfully. Heterogeneous reaction pathways and intermediate states unobservable in ensemble UV-visible measurements were revealed. Interestingly, the oxidation rate of individual immobilized AuNRs was much slower than that of the bulk AuNR solution, which implies the existence of a novel self-catalysis mechanism. This high-throughput darkfield spectral imaging technique could be applied to chemical reaction kinetics and heterogeneous catalysis studies of other MNPs at single particle level.

Assessment of Long-Term Performance and Chromate Reduction Mechanisms in a Field Scale Permeable Reactive Barrier

An innovative full-scale implementation of a permeable reactive barrier, consisting of a double-row of cylinders filled with zerovalent iron shavings, for chromate remediation was monitored over four years. Solid samples were analyzed to elucidate (i) the relevant corrosion mechanisms and products, (ii) the pathways of chromate reduction and immobilization, and (iii) the long-term performance of the barrier situated in a hydrological and geochemical complex groundwater regime. Sampling and analysis of groundwater and reactive material revealed an oxidative iron corrosion zone evolving in the inflow and a zone of anaerobic iron corrosion in the center and outflow of the barrier. Chromate reduction was mainly confined to the inflow region. The formation and thickness of corrosion rinds depended on sampling time, position, and depth, as well as on the size, shape, and graphite content In the inflow, the corrosion rinds mostly consisted of goethite and ferrihydrite. X-ray absorption fine structure spectroscopy revealed two distinct Cr(III) species, most likely resulting from homogeneous and heterogeneous redox reaction pathways, respectively. The longevity and long-term effectiveness of the PRB appears to be primarily limited by reduced corrosion rates of the ZVI-shavings because of the thick layers of Fe-hydroxides.

Free Energy Landscapes for S−H Bonds in Cp*2Mo2S4 Complexes

An extensive family of thermochemical data is presented for a series of complexes derived from Cp*Mo(mu-S)(2)(mu-SMe)(mu-SH)MoCp* and Cp*Mo(mu-S)(2)(mu-SH)(2)MoCp*. These data include electrochemical potentials, pK(a) values, homolytic solution bond dissociation free energies (SBDFEs), and hydride donor abilities in acetonitrile. Thermochemical data ranged from +0.6 to -2.0 V vs FeCp(2)(+/o) for electrochemical potentials, 5 to 31 for pK(a) values, 43 to 68 kcal/mol for homolytic SBDFEs, and 44 to 84 kcal/mol for hydride donor abilities. The observed values for these thermodynamic parameters are comparable to those of many transition metal hydrides, which is consistent with the many parallels in the chemistry of these two classes of compounds. The extensive set of thermochemical data is presented in free energy landscapes as a useful approach to visualizing and understanding the relative stabilities of all of the species under varying conditions of pH and H(2) overpressure. In addition to the previously studied homogeneous reactivity and catalysis, Mo(2)S(4) complexes are also models for heterogeneous molybdenum sulfide catalysts, and therefore, the present results demonstrate the dramatic range of S-H bond strengths available in both homogeneous and heterogeneous reaction pathways.

Single-molecule nanocatalysis reveals heterogeneous reaction pathways and catalytic dynamics

Nanoparticles are important catalysts for many chemical transformations. However, owing to their structural dispersions, heterogeneous distribution of surface sites and surface restructuring dynamics, nanoparticles are intrinsically heterogeneous and challenging to characterize in ensemble measurements. Using a single-nanoparticle single-turnover approach, we study the redox catalysis of individual colloidal Au nanoparticles in solution, using single-molecule detection of fluorogenic reactions. We find that for product generation, all Au nanoparticles follow a Langmuir-Hinshelwood mechanism but with heterogeneous reactivity; and for product dissociation, three nanoparticle subpopulations are present that show heterogeneous reactivity between multiple dissociation pathways with distinct kinetics. Correlation analyses of single-turnover waiting times further reveal activity fluctuations of individual Au nanoparticles, attributable to both catalysis-induced and spontaneous dynamic surface restructuring that occurs at different timescales at the surface catalytic and product docking sites. The results exemplify the power of the single-molecule approach in revealing the interplay of catalysis, heterogeneous reactivity and surface structural dynamics in nanocatalysis.

A kinetic study of the effect of ultrasound power on the sonohydrolysis of tetraethyl orthosilicate

A kinetic study of the ultrasound-stimulated and acid-catalyzed sonohydrolysis of tetraethyl orthosilicate (TEOS) in solventless TEOS–water heterogeneous mixtures was carried out by means of a calorimetric method as a function of the ultrasound power. The hydrolysis reaction starts in acidulated heterogeneous water–TEOS mixtures after an induction period under ultrasonic stimulation. The ultrasound power seems to play a role on the dynamical coupling of the system originating a continuum upward shifting of the base line during the induction period of sonication. The rate in which the base line is upward shifted diminishes with the power. The best coupling between the ultrasound and the reactant heterogeneous mixtures for this experimental setup was found to occur at 50 W, for which the gelation time was found to be a minimum. The kinetics of the heterogeneous TEOS sonohydrolysis was studied on the basis of a dissolution and reaction modeling. The heterogeneous reaction pathway as deduced from the kinetic study was drawn in a ternary diagram as a function of the ultrasound power.

Heterogeneous–homogeneous interactions in catalytic microchannel reactors

AbstractMicromachining of chemical reactors enables the manufacture of microchannel reactors with unsually large surface‐to‐volume ratios. This can strongly affect the coupling between heterogeneous (wall) reactions and homogeneous (gas‐phase) reactions, ultimately leading to a complete quenching of homogeneous reactions. Using a 2D boundary‐layer model coupled with detailed reaction kinetics for surface and gas‐phase reactions, we investigate the ignition behavior of hydrogen/air mixtures in a Pt‐coated microchannel. The influence of temperature, pressure, reactor diameter, and fuel‐to‐air ratio is studied. We find that a purely kinetic radical scavenging by the catalytic surface can indeed result in a complete suppression of gas‐phase reactions. However, the attainability of “intrinsic safety” in microchannel reactors is strongly dependent on a fine interplay between homogeneous and heterogeneous reaction pathways in the individual reaction system. In particular, the strong dependency of homogeneous reactions on pressure leads to a breakdown of intrinsic reactor safety at sufficiently high reactor pressure. A generalized equation for the boundary of safe reactor operation is derived for the current reaction system. © 2006 American Institute of Chemical Engineers AIChE J, 2006

Impact of heterogeneous sulfate formation at mineral dust surfaces on aerosol loads and radiative forcing in the Goddard Institute for Space Studies general circulation model

Heterogeneous chemical reactions between sulfate precursors on the surface of mineral dust aerosols affect the atmospheric aerosol cycle and the Earth radiation budget. Heterogeneous reactions of sulfur dioxide with mineral dust particles enhance sulfate formation by producing internally mixed sulfate dust aerosols. The anthropogenic sulfate forcing is estimated to be reduced to −0.18 W/m2 because of the reduced load of externally mixed sulfate aerosols, compared to −0.25 W/m2 when heterogeneous surface reactions are excluded. Sulfate coating on mineral dust particles increases wet deposition of dust, causing a positive anthropogenic forcing due to less sulfate coating at preindustrial times. However, heterogeneous reaction pathways are highly uncertain, which is reflected in the wide spread of reaction pathways and uptake probability coefficients in the literature. We undertake a series of sensitivity experiments with the Goddard Institute for Space Studies climate model to investigate the impact of the uncertainty in uptake mechanisms and dust aerosol size distributions on the simulated sulfate cycle. The results of this study are very sensitive to both tested variables. For example, doubling the clay emissions (particles whose radii are less than 1 μm) leads to a sevenfold increase in heterogeneous sulfate production.

Reaction-transport simulations of non-oxidative methane conversion with continuous hydrogen removal — homogeneous–heterogeneous reaction pathways

Detailed kinetic-transport models were used to explore thermodynamic and kinetic barriers in the non-oxidative conversion of CH 4 via homogeneous and homogeneous–heterogeneous pathways and the effects of continuous hydrogen removal and of catalytic sites on attainable yields of useful C 2–C 10 products. The homogeneous kinetic model combines separately developed models for low-conversion pyrolysis and for chain growth to form large aromatics and carbon. The H 2 formed in the reaction decreases CH 4 pyrolysis rates and equilibrium conversions and it favors the formation of lighter products. The removal of H 2 along tubular reactors with permeable walls increases reaction rates and equilibrium CH 4 conversions. C 2–C 10 yields reach values greater than 90% at intermediate values of dimensionless transport rates ( δ=1–10), defined as the ratio hydrogen transport and methane conversion rates. Homogeneous reactions require impractical residence times, even with H 2 removal, because of slow initiation and chain transfer rates. The introduction of heterogeneous chain initiation pathways using surface sites that form methyl radicals eliminates the induction period without influencing the homogeneous product distribution. Methane conversion, however, occurs predominately in the chain transfer regime, within which individual transfer steps and the formation of C 2 intermediates become limited by thermodynamic constraints. Catalytic sites alone cannot overcome these constraints. Catalytic membrane reactors with continuous H 2 removal remove these thermodynamic obstacles and decrease the required residence time. Reaction rates become limited by homogeneous reactions of C 2 products to form C 6+ aromatics. Higher δ values lead to subsequent conversion of the desired C 2–C 10 products to larger polynuclear aromatics. We conclude that catalytic methane pyrolysis at the low temperatures required for restricted chain growth and the elimination of thermodynamics constraints via continuous hydrogen removal provide a practical path for the direct conversion of methane to higher hydrocarbons. The rigorous design criteria developed are being implemented using shape-selective bifunctional pyrolysis catalysts and perovskite membrane films in a parallel experimental effort.

Methane oxidation over noble metal gauzes: an LIF study

The contributions of homogeneous and heterogeneous reactions to high-temperature catalytic methane oxidation were studied over three different gauze catalysts (Pt, Pt-10%Rh, and Ni) using laser-induced fluorescence (LIF) spectroscopy to measure OH · concentrations in the boundary layers downstream of the gauzes. OH · concentrations were found to decrease in the order Ni > Pt-10%Rh > Pt, which could be correlated with the catalytic activity of the metals with Pt being the most active oxidation catalyst, followed by Pt-10%Rh, and then Ni, which is essentially inert in excess oxygen. The experimental LIF results were compared to one-dimensional reaction–diffusion simulations, confirming that the differences in the measured OH · concentrations can be explained by the different degrees of catalytic methane conversion by the three gauze catalysts. No evidence for a direct interaction of homogeneous and heterogeneous reaction pathways through the radical pool was observed. Rather, the heterogeneous and the homogeneous reactions appeared to be spatially decoupled.

Measurement of the rate constants for the reactions of the IO• radical with sulfur-containing compounds H2S, (CH3)2S, and SO2

The reactions of iodine monoxide (IO•) with sulfur-containing compounds, which are important for the atmospheric chemistry, are studied. An attempt is made to distinguish between the heterogeneous and homogeneous reaction pathways. It is shown that, under the experimental conditions, the reactions proceed on the wall and generate iodine atoms into the gas phase. It is found that, at room temperature, the rate constants for the gas-phase reactions of IO• with (CH3)2S and H2S are lower than 2.5 × 10−14 and 8.0 × 10−14 cm3 molecule−1 s−1, respectively; the rate constant for the gas-phase reaction of iodine monoxide with SO2 ≤ 5.6 × 10−15 cm3 molecule−1 s−1.