Step Of CO Research Articles

Theoretical aspects of methyl acetate and methanol activation on MgO(100) and (501) catalyst surfaces with application in FAME production

Density functional theory (DFT) calculations were carried out to study the activation of methyl acetate and methanol on MgO(100) and MgO(501) surfaces and integrated in the context of transesterification, interesterification and glycerolysis reactions used in biodiesel industry. First results indicate the importance of including of dispersion forces in the calculations. On MgO(100) the reverse reactions steps of CO and CH dissociations and on MgO(501) the same reverse reaction step of CH dissociations of methyl acetate are energetically favorable, while the dissociation of CO bond into methoxide and acetate fragments on the edge of MgO(501) was found to be exothermic with a low activation energy. For methanol, the dissociation of OH bond on MgO(100) surface in the presence of the second coadsorbed methanol molecule becomes more energetically favoured compared to the isolated molecule, due to the fact that the methoxide fragment is stabilized by intermolecular hydrogen bonding. This is reflected by the decrease of the activation energy of the forward reaction step and the increase of the activation energy of the backward reaction step, increasing the probability to have dissociated molecules among the undissociated ones. These results represent a step forward for better understanding from atomistic point of view the paths of these reactions on these surfaces for the corresponding catalytic processes.

Promotion of CO oxidation on PdO(101) by adsorbed H2O

We investigated the influence of adsorbed H2O on the oxidation of CO on PdO(101) using temperature programmed reaction spectroscopy (TPRS), reflection absorption infrared spectroscopy (RAIRS) and density functional theory (DFT) calculations. We find that water inhibits CO adsorption on PdO(101) by site blocking, but also provides a more facile pathway for CO oxidation compared with the bare oxide surface. In the presence of adsorbed H2O, the oxidation of CO on PdO(101) produces a CO2 TPRS peak that is centered at a temperature ~50K lower than the main CO2 TPRS peak arising from CO oxidation on clean PdO(101) (~330 vs. 380K). RAIRS shows that CO continues to adsorb on atop-Pd sites of PdO(101) when H2O is co-adsorbed, and provides no evidence of other reactive intermediates. DFT calculations predict that the CO oxidation mechanism follows the same steps for CO adsorbed on PdO(101) with and without co-adsorbed H2O, wherein an atop-CO species recombines with an oxygen atom from the oxide surface lattice. According to DFT, hydrogen bonding interactions with adsorbed H2O species stabilize the carboxyl-like transition structure and intermediate that result from the initial recombination of CO and O on the PdO(101) surface. This stabilization lowers the energy barrier for CO oxidation on PdO(101) by ~10kJ/mol, in good agreement with our experimental estimate.

Numerical study of combustion and convective heat transfer of a Mach 2.5 supersonic combustor

In this paper, characteristics of combustion and convective heat transfer of a supersonic combustor at two fuel/air equivalence ratios of 0.9 and 0.46 were numerically studied. The numerical method of Favre averaged Navier–Stokes simulation with SST k-ω turbulence model and a multiple-step reaction mechanism of ethylene is introduced. The inlet Mach number of the combustor is 2.5 and inlet total temperature is 1650K, corresponding to Mach 6 flight conditions. Ethylene is injected at two locations upstream of a flame-holding cavity. The numerical method was validated by comparing the present results of wall pressures and heat fluxes to experiments and theoretical analysis. It is found that, due to injection of fuel at the bottom wall, fuel/air mixing and combustion occurs mainly in the vicinity of the bottom wall. High non-symmetry in distributions of the bottom and the top wall heat fluxes is observed. Peaks of wall heat flux at different locations and at varied fuel/air equivalence ratios are identified, which are caused respectively by effect of cavity and by shock structure formed upstream of the injection points. It is also found that heat flux peaks are strongly related to the reaction step of CO→CO2, contributing to major heat releasing.

The effect of the morphology of supported subnanometer Pt clusters on the first and key step of CO2 photoreduction.

Using density functional theory calculations, we investigate the influence of size-dependent cluster morphology on the synergistic catalytic properties of anatase TiO2(101) surfaces decorated with subnanometer Pt clusters. Focusing on the formation of the key precursor in the CO2 photoreduction reaction (bent CO2(-)), we find that flatter (2D-like) Pt clusters that "wet" the TiO2 surface offer significantly less benefit than 3D-like Pt clusters. We attribute the differences to three factors. First, the 3D clusters provide a greater number of accessible Pt-TiO2 interfacial sites with geometries that can aid CO2 bond bending and charge transfer processes. Second, binding competition among each Pt-CO2 bonding interaction mitigates maximum orbital overlaps, leading to insufficient CO2 binding. Third and also most interestingly, the 3D clusters tend to possess higher structural fluxionality than the flatter clusters, which is shown to correlate positively with CO2 binding strength. The preferred morphology adopted by the clusters depends on several factors, including the cluster size and the presence of oxygen vacancies on the TiO2 surface; this suggests a strategy for optimizing the synergistic effect between Pt clusters and TiO2 surfaces for CO2 photocatalysis. Clusters of ∼6-8 atoms should provide the largest benefit, since they retain the desired 3D morphology, yet are small enough to exhibit high structural fluxionality. Electronic structure analysis provides additional insight into the electronic motivations for the enhanced binding of CO2 on TiO2-supported 3D Pt clusters, as well as suppressed binding on flattened, 2D-like clusters.

Dissecting the steps of CO₂ reduction: 2. The interaction of CO and CO₂ with Pd/γ-Al₂O₃: an in situ FTIR study.

Alumina supported Pd catalysts with metal loadings of 0.5, 2.5 and 10 wt% were investigated by in situ FTIR spectroscopy in order to understand the nature of adsorbed species formed during their exposure to CO2 and CO. Exposing the annealed samples to CO2 at 295 K resulted in the formation of alumina support-bound surface species only: linear adsorbed CO2, bidentate carbonates and bicarbonates. Room temperature exposure of all three samples to CO produced IR features characteristic of both ionic and metallic Pd, as well as bands we observed upon CO2 adsorption (alumina support-bound species). Low temperature (100 K) adsorption of CO on the three samples provided information about the state of Pd after oxidation and reduction. Oxidized samples contained exclusively ionic Pd, while mostly metallic Pd was present in the reduced samples. Subsequent annealing of the CO-saturated samples revealed the facile (low temperature) reduction of PdO(x) species by adsorbed CO. This process was evidenced by the variations in IR bands characteristic of ionic and metallic Pd-bound CO, as well as by the appearance of IR bands associated with CO2 adsorption as a function of annealing temperature. Samples containing oxidized Pd species (oxidized, annealed or reduced) always produced CO2 upon their exposure to CO, while no CO2-related surface entities were observed on samples having only fully reduced (metallic) Pd.

Dissecting the steps of CO₂ reduction: 1. The interaction of CO and CO₂ with γ-Al₂O₃: an in situ FTIR study.

The adsorption of CO2 and CO was investigated on a pure γ-Al2O3 support material that has been used in Pd and Ru catalysts for the reduction of CO2. The adsorption of CO2 resulted in the formation of carbonates, bicarbonates and linearly adsorbed CO2 species. The amount and the nature of the adsorbed species were dependent on the annealing temperature of the alumina support. On γ-Al2O3 annealed at 473 K mostly bicarbonates formed, while no adsorbed CO2 was seen on this highly hydroxylated surface. With increasing calcination temperature the amount of both surface carbonates and linearly adsorbed CO2 increased, but still the most abundant surface species were bicarbonates. Surface carbonates and adsorbed CO2 can readily be removed from the alumina surface, while bicarbonates are stable to elevated temperatures. The interaction of CO with γ-Al2O3 is much weaker than that of CO2. At room temperature CO adsorbs only on Lewis acid sites, and can be readily removed by evacuation. At 100 K CO can probe different defect sites on the alumina surface. Under no conditions we have observed the formation of any carbonates or bicarbonates upon the interaction of CO with the pure alumina support. In co-adsorption experiments CO competes for adsorption sites with the linearly adsorbed CO2 on the 773 K-annealed γ-Al2O3 surface, but it does not result in the desorption of CO2, rather in the increased production of weakly held carbonates. After the removal of adsorbed CO, CO2 moves back to its original adsorption sites, i.e., Lewis acidic Al(3+) centers. The exposure of a CO2-saturated γ-Al2O3 to H2O did not affect any of the adsorbed surface species. The findings of this study will be used to rationalize the results of our ongoing in situ and in operando studies on the reduction of CO2 on supported Pd and Ru catalysts.

Kinetics of cobalt(II) extraction from sulfate aqueous solution by sodium salt of di-decylphosphinic acid (DDPA)

Kinetics of Co(II) extraction from sulfate aqueous solution by the sodium salt of di-decylphosphinic acid (Na-DDPA) was studied using a modified Lewis cell to disclose the mechanism of extraction. Parameters affecting the extraction rate, such as stirring speed, temperature, interfacial area, Na-DDPA concentration and Co(II) concentration, were investigated, respectively. The effect of diffusion resistance on extraction rate was negligible when the stirring operation was conducted in a plateau region of 95-110 r/min. Extraction rate increased with the increase in the temperature or specific interfacial area. The activation energy E was calculated to be 32.75 kJ/mol. These suggested that rate controlling mechanism of Co(II) extraction by Na-DDPA was chemical reaction regime and the rate-determining step of Co(II) extraction was chemical reaction at the interface. The initial extraction rate also increased with the increase in the concentrations of Na-DDPA and Co(II). The extraction rate equation and mechanism of Co(II) extraction by Na-DDPA were proposed.

Reduction–oxidation network for flexible adjustment of cellular metabolism in photoautotrophic cells

Photosynthesis generates the energy carriers NADPH and ATP to be consumed in assimilatory processes. Continuous energy conversion and optimal use of the available light energy are only guaranteed when all reduction-oxidation (redox) processes are tightly controlled. A robust network links metabolism with regulation and signalling. Information on the redox situation is generated and transferred by various redox components that are parts of this network. Any imbalance in the network is sensed, and the information is transmitted in order to elicit a response at the various levels of regulation and in the different cellular compartments. Redox information within the chloroplast is derived from intersystem electron transport, the ferredoxin-NADP oxidoreductase (FNR)/NADPH branch of the redox network, the thioredoxin branch and from reactive oxygen species (ROS), resulting in a high diversity of responses that are able to adjust photosynthesis, as well as poising and antioxidant systems accordingly in each specific situation. Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) represents a central step in CO(2) reduction and in carbohydrate oxidation involving both forms of energy, namely NAD(P)H and ATP, with its various isoforms that are located in plastids, cytosol and nucleus. GAPDH is used as an example to demonstrate complexity, flexibility and robustness of the regulatory redox network in plants.

Comprehensive impedance model of cobalt deposition in sulfate solutions accounting for homogeneous reactions and adsorptive effects

A comprehensive physicochemical model for cobalt deposition onto a cobalt rotating disk electrode in sulfate-borate (pH 3) solutions is derived and statistically fit to experimental EIS spectra obtained over a range of CoSO 4 concentrations, overpotentials and rotation speeds. The model accounts for H + and water reduction, homogeneous reactions and mass transport within the boundary layer. Based on a thermodynamic analysis, the species CoSO 4(aq), B(OH) 3(aq), B 3O 3(OH) 4 −, H + and OH − and two homogeneous reactions (B(OH) 3(aq) hydrolysis and water dissociation) are included in the model. Kinetic and transport parameters are estimated by minimizing the sum-of-squares error between the model and experimental measurements using a simplex method. The electrode response is affected most strongly by parameters associated with the first step of Co(II) reduction, reflecting its control of the rate of Co deposition, and is moderately sensitive to the parameters for H + reduction and the Co(II) diffusion coefficient. Water reduction is found not to occur to any significant extent under the conditions studied. These trends are consistent with that obtained by fitting equivalent electrical circuits to the experimental spectra. The simplest circuit that best fits the data consists of two RQ elements (resistor-constant phase element) in parallel or series with the solution resistance.

The rate‐limiting step for CO2 assimilation at different temperatures is influenced by the leaf nitrogen content in several C3 crop species

Effects of nitrogen (N) supply on the limiting step of CO(2) assimilation rate (A) at 380 µmol mol(-1) CO(2) concentration (A(380) ) at several leaf temperatures were studied in several crops, since N nutrition alters N allocation between photosynthetic components. Contents of leaf N, ribulose 1·5-bisphosphate carboxylase/oxygenase (Rubisco) and cytochrome f (cyt f) increased with increasing N supply, but the cyt f/Rubisco ratio decreased. Large leaf N content was linked to a high stomatal (g(s) ) and mesophyll conductance (g(m) ), but resulted in a lower intercellular (C(i) ) and chloroplast CO(2) concentration (C(c) ) because the increase in g(s) and g(m) was insufficient to compensate for change in A(380) . The A-C(c) response was used to estimate the maximum rate of RuBP carboxylation (V(cmax) ) and chloroplast electron transport (J(max) ). The J(max) /V(cmax) ratio decreased with reductions in leaf N content, which was consistent with the results of the cyt f/Rubisco ratio. Analysis using the C(3) photosynthesis model indicated that A(380) tended to be limited by RuBP carboxylation in plants grown at low N concentration, whereas it was limited by RuBP regeneration in plants grown at high N concentration. We conclude that the limiting step of A(380) depends on leaf N content and is mainly determined by N partitioning between Rubisco and electron transport components.

The effect of aqueous sulphate on basaltic glass dissolution rates

Steady-state dissolution rates of basaltic glass were measured in mixed-flow reactors at 50 °C and 3 < pH < 10 in HCl–NaCl–NH 4OH bearing solutions having an ionic strength of 0.01 M, or higher, as a function of aqueous sulphate concentration. Sulphate was added to reactive aqueous solutions in the form of Na 2SO 4. Measured dissolution rates increase with increased sulphate concentration in acid conditions, but little effect was found in basic conditions. At pH 5, the presence of 0.01 mol/kg sulphate doubled the dissolution rate while 0.10 mol/kg of sulphate tripled the dissolution rate compared to that measured in sulphate-free solutions. This rate increase is found to be consistent with that calculated using an equation previously proposed by Gislason and Oelkers (2003), where the observed rate increase stems from the formation of aqueous Al-sulphate complexes. As the release of divalent cations is thought to be the rate limiting step for CO 2 mineralization in basalt, adding aqueous sulphate to injected CO 2 could accelerate carbonization processes in this rock during carbon sequestration efforts.

Microkinetic analysis of CH 4 CPO tests with CO 2-diluted feed streams

Autothermal CH 4 catalytic partial oxidation (CPO) tests were performed in an adiabatic lab-scale reformer over 2 wt% Rh/α-Al 2O 3 catalysts supported on 600 cpsi cordierite honeycombs. The effect of adding either CO 2 or N 2 to CH 4/air mixtures was investigated at constant O 2/CH 4 ratio (0.56) and total flow rate (10 Nl/min). At increasing dilution the conversion of CH 4 and the gas temperatures decreased. Also, at equal CH 4 concentration, the addition of CO 2 decreased the temperatures and the conversion of CH 4 more importantly than N 2. Thus, in line with previous studies from this and other laboratories, no evidence of a direct dry reforming route was found; the observed thermal behavior appeared largely controlled by the heat capacity of the feed streams. However an important chemical effect of CO 2 addition, not previously appreciated in the literature, was shown by the evolution of the outlet H 2/CO ratio; at increasing N 2 dilution, the H 2/CO ratio slightly increased from about 2 to 2.5, but at increasing CO 2 dilution, the H 2/CO ratio decreased from about 1.7 to 0.7. These trends are in line with the thermodynamics of the reverse water gas shift (RWGS) reaction; such a reaction was thus extremely fast in all the experiments with CO 2, even at the highest values of dilution, while CH 4 conversion did not reach the equilibrium. A C 1 microkinetic model was used to analyze the results. A close match between calculated and measured temperatures, conversions and syngas composition was obtained under all conditions. Notably, the present microkinetic scheme, which incorporates steps for CO 2 adsorption and reactivity, was able to account for the observed net consumption of CO 2 and for the lowering of the H 2/CO ratio in the experiments with CO 2 co-feed. Surface coverages were analyzed for the various investigated conditions and the effects on the kinetics of methane steam reforming were evaluated in detail; the factors which control the onset of a more kinetically controlled regime at high degrees of dilution were highlighted.

Structure of the Membrane Domain of Human Erythrocyte Anion Exchanger 1 Revealed by Electron Crystallography

The membrane domain of human erythrocyte anion exchanger 1 (AE1) works as a Cl −/HCO 3 − antiporter. This exchange is a key step for CO 2/O 2 circulation in the blood. In spite of their importance, structural information about AE1 and the AE (anion exchanger) family are still very limited. We used electron microscopy to solve the three-dimensional structure of the AE1 membrane domain, fixed in an outward-open conformation by cross-linking, at 7.5-Å resolution. A dimer of AE1 membrane domains packed in two-dimensional array showed a projection map similar to that of the prokaryotic homolog of the ClC chloride channel, a Cl −/H + antiporter. In a three-dimensional map, there are V-shaped densities near the center of the dimer and slightly narrower V-shaped clusters at a greater distance from the center of the dimer. These appear to be inserted into the membrane from opposite sides. The structural motifs, two homologous pairs of helices in internal repeats of the ClC transporter (helices B + C and J + K), are well fitted to those AE1 densities after simple domain movement.

Pd-Based Membrane Reactor for Syngas Upgrading

In H2 production, the reformer downstream upgrading is a fundamental step for CO (ca. 10%) reduction. H2 (ca. 50%) presence limits CO conversion (ca. 25%) significantly (by thermodynamics) in a traditional reactor (TR). A Pd−Ag membrane (60 μm thick) removing H2 from the reaction side shifted the water−gas shift reaction toward a further product formation, and a very high (ca. 90%) CO conversion was measured, significantly exceeding the TR equilibrium value. In the meantime, a pure H2 stream, suitable for a proton-exchange membrane fuel cell (PEMFC), is recovered on the permeate side because no sweep gas is used. The H2 permeation was driven by feed pressure.

Synthesis, crystal structure, and electrochemistry of [Co{(Me-sal)2dien}(N3)] and [Co{(Me-sal)2dpt}(N3)

The structure, spectroscopic, and electrochemical properties of [Co{(Me-sal)2dien}(N3)] and [Co{(Me-sal)2dpt}(N3)], where (Me-sal)2dien = 2,2′-[1,1′-(3-azapentane-1,5-diyldinitrilo)diethylidyne] diphenolate and (Me-sal)2dpt = 2,2′-[1,1′-(4-azapentane-1,7-diyldinitrilo)diethylidyne] diphenolate, have been investigated. These complexes have been characterized by elemental analyses, IR, UV–Vis, and 1H-NMR spectroscopy. The crystal structures of these complexes have been determined by X-ray diffraction. Complex 1 crystallizes in the triclinic space group P 1, with a = 7.8443(4) Å, b = 11.0660(5) Å, c = 11.6216(6) Å, α = 73.360(1)°, β = 76.965(1)°, γ = 84.436(1)° and Z = 2. Complex 2 crystallizes in the monoclinic space group P21/n, with a = 12.1985(13) Å, b = 10.9332(12) Å, c = 15.2808(16) Å, β = 76.965(1)° and Z = 4. The coordination geometry around cobalt(III) in both complexes is a distorted octahedron. The electrochemical reduction of these complexes at a glassy carbon electrode in acetonitrile indicates that the first reduction corresponding to CoIII–CoII is electrochemically irreversible, accompanied by dissociation of the axial Co–N(N3) bond. The second reduction step of Co(II/I) leads to decomposition of the complex. These observations are rationalized based on the structure-function relations.

Entrapment of Carbon Dioxide in the Active Site of Carbonic Anhydrase II

The visualization at near atomic resolution of transient substrates in the active site of enzymes is fundamental to fully understanding their mechanism of action. Here we show the application of using CO(2)-pressurized, cryo-cooled crystals to capture the first step of CO(2) hydration catalyzed by the zinc-metalloenzyme human carbonic anhydrase II, the binding of substrate CO(2), for both the holo and the apo (without zinc) enzyme to 1.1A resolution. Until now, the feasibility of such a study was thought to be technically too challenging because of the low solubility of CO(2) and the fast turnover to bicarbonate by the enzyme (Liang, J. Y., and Lipscomb, W. N. (1990) Proc. Natl. Acad. Sci. U. S. A. 87, 3675-3679). These structures provide insight into the long hypothesized binding of CO(2) in a hydrophobic pocket at the active site and demonstrate that the zinc does not play a critical role in the binding or orientation of CO(2). This method may also have a much broader implication for the study of other enzymes for which CO(2) is a substrate or product and for the capturing of transient substrates and revealing hydrophobic pockets in proteins.

Progress in carbon dioxide capture and separation research for gasification-based power generation point sources

The purpose of the present work is to investigate novel approaches, materials, and molecules for the abatement of carbon dioxide (CO 2) at the pre-combustion stage of gasification-based power generation point sources. The capture/separation step for CO 2 from large point sources is a critical one with respect to the technical feasibility and cost of the overall carbon sequestration scenario. For large point sources, such as those found in power generation, the carbon dioxide capture techniques being investigated by the Office of Research and Development of the National Energy Technology Laboratory possess the potential for improved efficiency and reduced costs as compared to more conventional technologies. The investigated techniques can have wide applications, but the present research is focused on the capture/separation of carbon dioxide from fuel gas (pre-combustion gas) from processes such as the Integrated Gasification Combined Cycle (IGCC) process. For such applications, novel concepts are being developed in wet scrubbing with physical sorption, chemical sorption with solid sorbents, and separation by membranes. In one concept, a wet scrubbing technique is being investigated that uses a physical solvent process to remove CO 2 from fuel gas of an IGCC system at elevated temperature and pressure. The need to define an “ideal” solvent has led to the study of the solubility and mass transfer properties of various solvents. Pertaining to another separation technology, fabrication techniques and mechanistic studies for membranes separating CO 2 from the fuel gas produced by coal gasification are also being performed. Membranes that consist of CO 2-philic ionic liquids encapsulated into a polymeric substrate have been investigated for permeability and selectivity. Finally, processes based on dry, regenerable sorbents are additional techniques for CO 2 capture from fuel gas. An overview of these novel techniques is presented along with a research progress status of technologies related to membranes and physical solvents.

Intrinsic Deuterium Kinetic Isotope Effects in Glutamate Mutase Measured by an Intramolecular Competition Experiment

Kinetic isotope effects (KIEs) provide a powerful tool to interrogate transition states of both enzymic and non-enzymic reactions, provided that one can measure the intrinsic KIE on the chemical step of interest, that is, the isotope effect undiminished by other isotope-insensitive steps that may contribute to the overall rate of reaction. For small molecules reacting in solution the KIE measured usually represents the intrinsic value; however, for enzymes this is seldom the case. Here we report the first measurement of the intrinsic KIE in an adenosylcobalamin enzyme (AdoCbl, coenzyme B12) for hydrogen atom transfer from substrate to coenzyme, which is a key step in the mechanism of this class of enzymes. For the B12 enzyme glutamate mutase the intrinsic deuterium KIE for hydrogen transfer from the substrate, (2S,3S)-3-methylaspartate, to 5’-deoxyadensosine is 4.1. This value is well within the semiclassical limit for a deuterium isotope effect and is much smaller than the anomalously large KIEs previously measured in other B12 enzymes and non-enzymatic model reactions, which were attributed to extensive hydrogen tunneling. Glutamate mutase one is of a group of AdoCbl-dependent enzymes that catalyze unusual isomerization reactions that formally involve a 1,2 hydrogen atom migration and proceed through a mechanism involving carbon-based free radical intermediates (Scheme 1). Radicals are generated by homolysis of the reactive cobalt–carbon bond of the coenzyme to form cob(II)alamin, a cobalt(II) intermediate, and the 5’-deoxyadenosyl radical. The adenosyl radical then abstracts the migrating hydrogen from the substrate to form 5’-deoxyadenosine and the substrate radical. The substrate radical next undergoes rearrangement to give the product radical, which is then quenched by hydrogen transfer from 5’deoxyadenosine to give the product and regenerate the 5’deoxyadenosyl radical. Finally, recombination of the adenosyl radical and cob(II)alamin to reform the coenzyme completes the catalytic cycle. Our interest in the mechanisms by which enzymes generate free radicals, as exemplified by dependent glutamate mutase, led us to undertake an extensive set of KIE measurements to examine how hydrogen abstraction from the substrate and coenzyme homolysis are coupled together. KIE measurements using deuteriumand tritium-labeled substrates and coenzyme have proved especially informative probes of the key steps of Co C bond homolysis and hydrogen atom abstraction from substrate. Pre-steady-state measurements on a number of enzymes have shown that hydrogen abstraction is kinetically coupled to Co C bond homolysis, as evidenced by the appearance of a kinetic isotope effect on cobalt–carbon bond homolysis when the enzymes are reacted with deuterated substrates. This observation implies that the 5’-dA radical is a highenergy intermediate that only has a fleeting existence. Furthermore, the KIEs reported for several AdoCbl enzymes are extremely large (ranging from 10 to 50), which has generally been attributed to hydrogen tunneling. In particular, extensive hydrogen tunneling in methylmalonyl-CoA mutase has been deduced from the temperature dependence of the deuterium isotope effect on hydrogen transfer and Co C bond homolysis. The KIEs discussed above were all measured indirectly by UV/Vis stopped-flow spectroscopy, using the spectroscopic changes associated with Co C bond homolysis as a convenient reporter of the kinetics. Although we originally reported a very large primary deuterium isotope effect, suggestive of tunneling, for the formation of cob(II)alamin and 5’-dA upon reaction of hologlutamate mutase with deuterated substrates (kH/kD= 28 with glutamate and 35 with methylaspartate), [11]

Reductive Cleavage Mechanism of Methylcobalamin: Elementary Steps of Co−C Bond Breaking

Density functional theory has been applied to the investigation of the reductive cleavage mechanism of methylcobalamin (MeCbl). In the reductive cleavage of MeCbl, the Co-C bond is cleaved homolytically, and formation of the anion radical ([MeCbl]*-) reduces the dissociation energy by approximately 50%. Such dissociation energy lowering in [MeCbl]*- arises from the involvement of two electronic states: the initial state, which is formed upon electron addition, has dominant pi*corrin character, but when the Co-C bond is stretched the unpaired electron moves to the sigma*Co-C state, and the final cleavage involves the three-electron (sigma)2(sigma*)1 bond. The pi*corrin-sigma*Co-C states crossing does not take place at the equilibrium geometry of [MeCbl]*- but only when the Co-C bond is stretched to 2.3 A. In contrast to the neutral cofactor, the most energetically efficient cleavage of the Co-C bond is from the base-off form. The analysis of thermodynamic and kinetic data provides a rationale as to why Co-C cleavage in reduced form requires prior departure of the axial base. Finally, the possible connection of present work to B12 enzymatic catalysis and the involvement of anion-radical-like [MeCbl]*- species in relevant methyl transfer reactions is discussed.

Spatially resolved UV–vis microspectroscopy on the preparation of alumina-supported Co Fischer–Tropsch catalysts: Linking activity to Co distribution and speciation

The preparation steps of Co/ γ-Al 2O 3 Fischer–Tropsch (FT) catalysts have been studied using spatially resolved UV–vis microspectroscopy. The speciation of the different Co phases inside the support bodies changed by varying the composition of the impregnation solutions and the applied treatment, that is, subsequent impregnation, aging, drying, and calcination. The activity in the FT synthesis reaction of the reduced catalysts has been correlated with the applied preparation procedure. The highest activity was observed when γ-Al 2O 3 was impregnated with an aqueous CoCO 3 solution containing 1 equivalent citric acid. The formation of a Co citrate complex inside the support after impregnation was favored due to the higher pH of this solution compared with a similar solution involving Co(NO 3) 2. After calcination, Co 3O 4 was observed with an eggshell distribution, and no unfavorable CoAl 2O 4 was observed. The Co citrate complex formation is thought to restrict the extent of interaction of Co with alumina to form CoAl 2O 4, thereby allowing a higher degree of reduction and a concomitant higher catalytic activity. This study demonstrates the power of monitoring catalyst preparation with spatially resolved UV–vis microspectroscopy. The understanding of the physicochemical processes occurring at the different stages of the preparation process allows fine-tuning of the catalyst properties, which is ultimately expected to enable the development of improved supported catalysts.