Fast Reaction Step Research Articles

Effect of Hydroxyl Groups on Metal Anchoring and Formaldehyde Oxidation Performance of Pt/Al2O3.

Pt/Al2O3 catalysts showing excellent activity and stability have been used in various reactions, including HCHO oxidation. Herein, we prepared Pt-Na/Al2O3 catalysts with a Pt content of 0.05 wt % to reveal the key factors determining the anchoring of Pt as well as the catalytic activity and mechanism of HCHO oxidation. Pt-Na/nano-Al2O3 (denoted as Pt-Na/nAl2O3) catalysts with 0.05 wt % Pt content could completely oxidize HCHO to CO2 at room temperature, which is the lowest Pt content used in HCHO catalytic oxidation to our knowledge. After Na addition, terminal hydroxyl groups (denoted as HO-μter) on nano-Al2O3 were transformed to doubly bridging hydroxyl groups between Na and Al (denoted as HO-μbri(Na-Al)), which atomically dispersed Pt species. Pt anchoring further promoted the regeneration of HO-μbri(Na-Al) by activating O2 and H2O, oxidizing HCHO to CO2 directly by the fast reaction step ([HCOO-] + [OH]a → CO2 + H2O). Our study revealed that the HO-μbri(Na-Al) synergistically generated by HO-μter and Na species provided anchoring sites for Pt species.

Monitoring a Mechanochemical Reaction Reveals the Formation of a New ACC Defect Variant Containing the HCO3– Anion Encapsulated by an Amorphous Matrix

Amorphous calcium carbonate (ACC) is an important precursor in the biomineralization of crystalline CaCO3. In nature, it serves as a storage material or as a permanent structural element, whose lifetime is regulated by an organic matrix. The relevance of ACC in materials science is primarily related to our understanding of CaCO3 crystallization pathways and CaCO3/(bio)polymer nanocomposites. ACC can be synthesized by liquid–liquid phase separation, and it is typically stabilized with macromolecules. We have prepared ACC by milling calcite in a planetary ball mill. Phosphate “impurities” were added in the form of monetite (CaHPO4) to substitute the carbonate anions, thereby stabilizing ACC by substitutional disorder. The phosphate anions do not simply replace the carbonate anions. They undergo shear-driven acid/base and condensation reactions, where stoichiometric (10%) phosphate contents are required for the amorphization to be complete. The phosphate anions generate a strained network that hinders ACC recrystallization kinetically. The amorphization reaction and the structure of BM-ACC were studied by quantitative Fourier transform infrared spectroscopy and solid state 31P, 13C, and 1H magic angle spinning nuclear magnetic resonance spectroscopy, which are highly sensitive to symmetry changes of the local environment. In the first—and fast—reaction step, the CO32– anions are protonated by the HPO42– groups. The formation of unprecedented hydrogen carbonate (HCO3–) and orthophosphate anions appears to be the driving force of the reaction, because the phosphate group has a higher Coulomb energy and the tetrahedral PO43– unit can fill space more efficiently. In a competing second—and slow—reaction step, pyrophosphate anions are formed in a condensation reaction. No pyrophosphates are formed at higher carbonate contents. High strain leads to such a large energy barrier that any reaction is suppressed. Our findings aid in the understanding of the mechanochemical amorphization of calcium carbonate and emphasize the effect of impurities for the stabilization of the amorphous phases in general. Our approach allowed the synthesis of new amorphous alkaline earth defect variants containing the unique HCO3– anion. Our approach outlines a general strategy to obtain new amorphous solids for a variety of carbonate/phosphate systems that offer promise as biomaterials for bone regeneration.

Understanding the structure and dynamics of hydrogenases by ultrafast and two-dimensional infrared spectroscopy.

Hydrogenases are valuable model enzymes for sustainable energy conversion approaches using H2, but rational utilization of these base-metal biocatalysts requires a detailed understanding of the structure and dynamics of their complex active sites. The intrinsic CO and CN- ligands of these metalloenzymes represent ideal chromophores for infrared (IR) spectroscopy, but structural and dynamic insight from conventional IR absorption experiments is limited. Here, we apply ultrafast and two-dimensional (2D) IR spectroscopic techniques, for the first time, to study hydrogenases in detail. Using an O2-tolerant [NiFe] hydrogenase as a model system, we demonstrate that IR pump-probe spectroscopy can explore catalytically relevant ligand bonding by accessing high-lying vibrational states. This ultrafast technique also shows that the protein matrix is influential in vibrational relaxation, which may be relevant for energy dissipation from the active site during fast reaction steps. Further insights into the relevance of the active site environment are provided by 2D-IR spectroscopy, which reveals equilibrium dynamics and structural constraints imposed on the H2-accepting intermediate of [NiFe] hydrogenases. Both techniques offer new strategies for uniquely identifying redox-structural states in complex catalytic mixtures via vibrational quantum beats and 2D-IR off-diagonal peaks. Together, these findings considerably expand the scope of IR spectroscopy in hydrogenase research, and new perspectives for the characterization of these enzymes and other (bio-)organometallic targets are presented.

Microwave-assisted selective cleavage of Cα[sbnd]Cβ bond for lignin depolymerization

Lignin is an alternative renewable feedstock for the production of low molecular weight aromatics. The cleavage of phenyl side-chain CαCβ bond is a valid depolymerization pathway to obtain aromatic monomers. The selective cleavage of the CαCβ bonds in two lignin model dimers and lignin with ferric sulfate as the catalyst was investigated to narrow the distribution of aromatic monomers. The CαCβ bonds in phenolic and non-phenolic dimers were selectively cleaved under microwave irradiation. The acidity promoted the etherification of Cα-hydroxyl groups with methanol solvent. Both the etherification and the decomposition of alkenyl products occurred in fast reaction steps, but the formation of alkenyl products was a decomposition-determining step. Density functional theory (DFT) calculations indicated that the phenolic dimer was more thermodynamically favorable to form alkenyl product than non-phenolic dimer, which resulted in the CαCβ bond cleavage reactivity of phenolic dimer being higher than that of non-phenolic dimer. Microwave heating contributed to 96.3% of CαCβ bond cleavage of organosolv lignin (OL), and a narrow distribution of aromatic monomers which mainly contained vanillin, syringaldehyde, methyl vanillate, and methyl syringate. This work highlights the prospect of microwave-assisted selective cleavage of the CαCβ bond to achieve the selective production of aromatic monomers from lignin.

KINETICS AND MECHANISMS OF THE ACID-BASE REACTION BETWEEN NH3 AND HCOOH IN INTERSTELLAR ICE ANALOGS

ABSTRACT Interstellar complex organic molecules are commonly observed during star formation, and are proposed to form through radical chemistry in icy grain mantles. Reactions between ions and neutral molecules in ices may provide an alternative cold channel to complexity, as ion-neutral reactions are thought to have low or even no-energy barriers. Here we present a study of the kinetics and mechanisms of a potential ion-generating, acid-base reaction between NH3 and HCOOH to form the salt NH HCOO−. We observe salt growth at temperatures as low as 15 K, indicating that this reaction is feasible in cold environments. The kinetics of salt growth are best fit by a two-step model involving a slow “pre-reaction” step followed by a fast reaction step. The reaction energy barrier is determined to be 70 ± 30 K with a pre-exponential factor 1.4 ± 0.4 × 10−3 s−1. The pre-reaction rate varies under different experimental conditions and likely represents a combination of diffusion and orientation of reactant molecules. For a diffusion-limited case, the pre-reaction barrier is 770 ± 110 K with a pre-exponential factor of ∼7.6 × 10−3 s−1. Acid-base chemistry of common ice constituents is thus a potential cold pathway to generating ions in interstellar ices.

NO Adsorption and Reaction on Aged Pd–Rh Natural Gas Vehicle Catalysts: A Combined TAP and Steady-State Kinetic Approach

A combined temporal analysis of products (TAP) and steady-state kinetic study was achieved to characterize the surface reactivity of fresh and aged bimetallic Pd–Rh/Al2O3 Natural-Gas Vehicle catalysts. Single NO pulse TAP experiments were performed on a stabilized surface after exposure to successive NO pulses until to get a steady-state NO conversion. Outlet flow curves recorded during such experiments show fast reaction steps taking place on noble metal particles and a slow process during NO desorption ascribed to the involvement of spill-over effect of chemisorbed NO molecules from the metal to the support. This slow process attenuates on the aged sample likely due to an alteration of the metal/support interface induced by particle sintering at high temperature. Thermal aging also alters the surface composition of bimetallic Pd–Rh particles which leads to changes in the products distribution from NO dissociation. A similar selectivity behavior is observed from steady-state kinetic measurements during the NO/H2 reaction. Interestingly, a weak partial pressure dependency of the selectivity reflects a surface Rh enrichment of Pd–Rh particles during aging.

Kinetics of the epoxy–thiol click reaction initiated by a tertiary amine: Calorimetric study using monofunctional components

An analysis of the kinetics of the epoxy–thiol reaction in a model stoichiometric system of monofunctional reagents, 3-mercaptopropionate (BMP) and phenylglycidylether (PGE) is reported. Benzyldimethylamine (BDMA) was employed as initiator in amounts ranging from 0.5 to 2wt%. These formulations showed a kinetic behavior qualitatively similar to that of commercial adhesives and coatings formulated for a room-temperature cure. Isothermal DSC scans revealed the existence of a relatively long induction period preceding a fast autocatalytic reaction step. Dynamic DSC scans showed that the reaction was shifted to a lower temperature range by increasing the storage period of the initial formulation at 20°C. This unusual kinetic behavior could be modeled assuming that thiolate anions, slowly generated during the induction period, initiated a fast autocatalytic propagation/proton transfer reaction. The kinetic model included a pseudo-steady state for the initiator concentration and an equilibrium reaction between epoxy and OH groups generated by reaction. A reasonable fitting of isothermal and dynamic DSC runs was achieved in a broad range of temperatures and amine concentrations. In particular, both the length of the induction time and the effect of the storage period were correctly predicted.

Photoconversion of β-Lapachone to α-Lapachone via a Protonation-Assisted Singlet Excited State Pathway in Aqueous Solution: A Time-Resolved Spectroscopic Study.

The photophysical and photochemical reactions of β-lapachone were studied using femtosecond transient absorption, nanosecond transient absorption, and nanosecond time-resolved resonance Raman spectroscopy techniques and density functional theory calculations. In acetonitrile, β-lapachone underwent an efficient intersystem crossing to form the triplet state of β-lapachone. However, in water-rich solutions, the singlet state of β-lapachone was predominantly quenched by the photoinduced protonation of the carbonyl group at the β position (O9). After protonation, a series of fast reaction steps occurred to eventually generate the triplet state α-lapachone intermediate. This triplet state of α-lapachone then underwent intersystem crossing to produce the ground singlet state of α-lapachone as the final product. 1,2-Naphthoquinone is examined in acetonitrile and water solutions in order to elucidate the important roles that water and the pyran ring play during the photoconversion from β-lapachone to α-lapachone. β-Lapachone can also be converted to α-lapachone in the ground state when a strong acid is added to an aqueous solution. Our investigation indicates that β-lapachone can be converted to α-lapachone by photoconversion in aqueous solutions by a protonation-assisted singlet excited state reaction or by an acid-assisted ground state reaction.

Impact of Structural Perturbation of Aluminum Hydroxides by Tannate on Arsenate Adsorption

The impacts of the biomolecule‐induced structural perturbation of Al hydroxides and the resultant alteration of their surface reactivity toward the adsorption of nutrients and contaminants have received, to date, scant attention, in spite of their significance in determining the mineralogy and surface chemistry of these mineral colloids. This study investigated the equilibria and kinetics of As(V) adsorption on a crystalline Al hydroxide, a pure amorphous Al hydroxide and a short‐range ordered Al–tannate coprecipitate. Isotherms and kinetics of As(V) adsorption were conducted at pH 6.5; the kinetic experiments (0.083–24 h) were performed at 288, 298, 308, and 318 K. The adsorption data followed multiple second‐order kinetics, with an initial fast reaction step, followed by a slow reaction. While As(V) adsorption on the crystalline Al hydroxide was a rapid process, the poorly ordered minerals required longer contact intervals and greater activation energies. Compared with the pure amorphous Al hydroxide, the incorporation of tannate into the structural network of Al hydroxide decreased the adsorption rate, capacity, and affinity for As(V). These effects were attributable to the blocking of part of the adsorption sites by tannate, to the electrostatic repulsion induced by the net negative charge caused by the deprotonated organic molecules exposed on the surface of the Al–tannate coprecipitate, and to the steric hindrance of tannate, hampering access of the adsorbate to the micropores. These findings are of fundamental significance in understanding the sorption behavior and mobility of As as influenced by biomolecule‐induced structural perturbation of Al hydroxides in the environment.

Mechanisms of telomerase-dimer catalysis

There is evidence that human telomerase acts as a dimer [Wenz, C., Enenkel, B., Amacker, M., Kelleher, C., Damm, K., Lingner, J., 2001. Human telomerase contains two cooperating telomerase RNA molecules. EMBO J. 20, 3526–3534]. Three possible mechanisms have been proposed. We translate those proposals into three detailed mechanistic models for telomerase action, also introducing optional isomerizations with equilibrium constants inversely related to the number of bound nucleotides. To distinguish between these models by in situ experiments we propose a microscopic system which uses two-photon excitation of fluorescence in a volume of about 0.5 μm 3. A variety of detection strategies and experimental designs are considered; we focus on those best suited to observation of a small volume under limitations imposed by diffusion to and from the reacting micro-volume, and consequently restrict ourselves to constant flow. Numerical simulation is used to help identify an optimal experimental design. The detection of mechanistic changes hinges on linking fluorescence reporters to selected reaction components, either directly (chemically) or indirectly (via an indicator reaction). We show that rapid mixing experiments are better than chemical relaxation experiments, as the statistics of single molecule kinetics affects the latter more than the former. However, some fast reaction steps can only be revealed by chemical relaxation coupled with mixing experiments. We explore connections between our methods and studies of HIV and other systems with RNA to DNA transcription.

A Study of Charge-Transfer Reactions on (110) Single-Crystal Nickel Surfaces in Nickel Sulfamate Electrolyte Using Electrochemical Impedance Spectroscopy

Electrochemical impedance spectroscopy measurements under a small alternating current density were performed to investigate electrochemical reactions on (110) single-crystalline nickel surfaces in nickel sulfamate electrolyte. The reaction impedance shows a power law of the ac modulation frequency, and two exponents for the reaction resistance and reaction capacitance are found to approximately agree with the values predicted by a homogeneous charge-transfer reaction model. In addition, the Nyquist diagrams exhibit the absence of inductive loops for the frequency range from 50 mHz to 100 kHz. A condition for the existence of inductive loops derived in this study suggests that the electrocrystallization of Ni2+ ions may take place in the slow reaction step Ni2+ + e- → and in the fast reaction step + e- → Ni.

Kinetics of wet oxidation of black liquor over a Pt–Pd–Ce/alumina catalyst

The catalytic oxidation of industrial wastewater from paper and pulp mills has been investigated in a slurry reactor at a temperature range 433–463 K and at pressures from 1.5 to 2.2 MPa. Adding Ce on alumina support promotes the catalytic activity for oxidation of black liquor. Pt–Pd–Ce/alumina catalyst shows a promising activity for wet catalytic oxidation of black liquor. The oxidation reaction over a Pt–Pd–Ce catalyst is characterized by an initial fast reaction step followed by a slow reaction step. The rate of total organic carbon (TOC) reduction was described by first-order kinetics with respect to TOC concentration in black liquor for both initial and later reaction steps. The activation energies were determined to be 54.53 and 50.13 kJ/mol for the initial and later oxidation steps, respectively. Comparison of the data with the generalized kinetic model was also presented.

Wet oxidation of phenolic solutions over heterogeneous catalysts: Degradation profile and catalyst behavior

Removal of phenol by wet air oxidation (WAO) from dilute aqueous solutions was conducted in the presence of oxygen over a 1 wt% Pt/Al 2 O 3 and Mn/Ce (7/3) composite oxide catalysts. The reaction was carried out in a batch slurry reactor using pure oxygen as an oxidizing agent under mild reaction conditions, i.e., temperature from 80 to 175°C and oxygen pressure from 0.5 to 1.5 MPa. The platinum catalyst was found to be moderately active, while the Mn/Ce catalyst was remarkably effective in reducing both phenol concentration and total organic carbon. On both catalysts the kinetics of phenol removal was characterized by a fast reaction step followed by a slower one. During WAO reaction, a carbonaceous material was formed and deposited on the catalysts surface. This deposit was characterized by temperature-programmed oxidation coupled to mass spectrometry detection and X-ray photoelectron spectroscopy.

Equations for progress curves of some kinetic models of enzyme-single substrate-single slow binding modifier system.

A procedure is described by means of which the equations for progress curves for the kinetic models that include fast and slow reaction steps can be derived. It is based on combined assumptions of equilibrium and steady-state and uses Laplace transformation for solving the systems of differential equations. The progress curve equations and the significance of the corresponding parameters are given for some most frequently occurring models describing the influence of a slow binding modifier on a single substrate enzyme reaction.

Polypeptides with supersecondary structures as templates in rational catalyst design. Catalysis of self functionalization by designed helix–loop–helix motifs

Three designed polypeptides with 42 amino acids each, SA-42, RA-42 and PA-42, were engineered to catalyse acyl-transfer reactions of mono-p-nitrophenylfumarate I. The second-order rate constants of the peptides for the formation of p-nitrophenol were larger than that of the background reaction with factors of 331, 1750 and 1937, respectively. The background reaction is the reaction between I and trifluoroethoxide ion to form the trifluoroethyl ester. The second-order rate constant for the 4-methylimidazole catalysed reaction between I and trifluoroethoxide ion was 211 times larger than that of the background reaction and the similarity in rate constants suggests that the histidine residues in the polypeptides act as nucleophilic catalysts. The observed rate increases in RA-42 and PA-42 over that of SA-42 identifies the location of the reaction centre. The identity of the reaction products in the RA-42 catalysed reaction was established by NMR spectroscopy and mass spectrometry. The side chain of ornithine-15 was acylated by the fumaryl residue. Thus, in a fast and selective second reaction step the fumaryl group was transferred from histidine-11 to the side chain of ornithine-15. The ability of a designed helix–loop–helix motif, RA-42, to catalyse its own functionalization is thereby demonstrated for the first time.

Stopped-flow fluorescence kinetics of bovine alpha 2-antiplasmin inhibition of bovine midiplasmin.

In the conversion of bovine plasminogen to bovine plasmin not only the expected urokinase-catalysed cleavage of Arg-557-Val-558, and the following autocatalytic cleavage separating the N-terminal peptide 1-77 from the heavy chain of plasmin, but also a cleavage at Arg-342-Met-343 between kringles 3 and 4 is seen. Here, kinetic studies of the interaction of bovine alpha 2-antiplasmin with bovine plasmin were performed on isolated bovine midiplasmin (lacking kringles 1-3) and on bovine plasmin containing all of the activation products from the bovine plasminogen. A series of experiments using stopped-flow fluorescence fast kinetics as well as conventional techniques suggests a reaction model in accordance with the one known for the human system. First, a tight complex (K1 in the nanomolar range) is formed in a fast reaction step; and second, a tightening of this complex occurs in a slow reaction step. The final complex is indeed so tight (Ki < or = pM), that the reaction for many practical purposes is legitimately considered irreversible. The stopped-flow method allows for the determination of reliable values of the second-order rate constant for the fast association step. At pH 7.4 and 25 degrees C, k+1 = 1.7 x 10(6) M-1 s-1 was obtained in the absence and k+1 = 0.9 x 10(6) M-1.s-1 in the presence of the kringles 1-3 domain of bovine plasmin. In contrast to this, substantial reductions of k+1 were seen in the presence of concentrations of 6-amino-hexanoic acid corresponding to lysine-binding-site interactions and far too low to be attributed to active-site interactions with the bovine plasmins (for each, Ki = 42 mM). All in all, the data indicated that the lysine-binding site(s) not of kringle 1, but of midiplasmin (those of kringles 4 and 5) are regulating the inhibition reaction.

Membrane transport models with fast and slow reactions: General analytical solution for a single relaxation

Membrane transport models are usually expressed on the basis of chemical kinetics. The states of a transporter are related by rate constants, and the time-dependent changes of these states are given by linear differential equations of first order. To calculate the time-dependent transport equation, it is necessary to solve a system of differential equations which does not have a general analytical solution if there are more than five states. Since transport measurements in a complex system rarely provide all the time constants because some of them are too rapid, it is more appropriate to obtain approximate analytical solutions, assuming that there are fast and slow reaction steps. The states of the fast steps are related by equilibrium constants, thus permitting the elimination of their differential equations and leaving only those for the slow steps. With a system having only two slow steps, a single differential equation is obtained and the state equations have a single relaxation. Initial conditions for the slow reactions are determined after the perturbation which redistribute the states related by fast reactions. Current and zero-trans uptake equations are calculated. Curve fitting programs can be used to implement the general procedure and obtain the model parameters.

The elimination of fast variables in complex chemical reactions. III. Mesoscopic level (irreducible case)

The master equation for a complex chemical reaction cannot always be reduced to a simpler master equation, even if there are fast and slow individual reaction steps. Nevertheless the elimination of intermediates can be carried out with the help of theΩ-expansion. This is illustrated with a well-known complex reaction: the dissociation of N2O5. It is shown that the intrinsic fluctuations in the N2O5 decay are larger than those implied by the master equation suggested by the macroscopic rate law.

ChemInform Abstract: A Kinetic Study of the Oxidation of Indigo Carmine with Acidic Bromate

The oxidation kinetics of indigo carmine (disodium 3,3′-dioxobi-indolin-2,2′-ylidene-5,5′-disulphonate) with potassium bromate have been studied in aqueous sulphuric acid, by monitoring the absorbance of indigo carmine (IC) at 610 nm. The reaction involves competitive and consecutive reaction steps—an initial slow step followed by a rapid one for depletion of IC. For the initial stages the reaction order is four- first-order with respect to IC and bromate ion and second order with H+ ion. For the fast reaction step the studies are limited to qualitative treatment due to the complex nature of the reaction. The rate of depletion of IC increased with time and with the increase in HOBr concentration. Hypobromous acid, the reaction intermediate, may possibly compete with bromate ion for IC to give an intermediate, which is further oxidised to yield the final product, isatin-5-monosulphonic acid. The stoicheiometric ratio of IC to bromate is 3:2. The dual role of bromine ion as an inhibitor and autocatalyst in the reaction mechanism is discussed. Computer simulations were performed using the proposed mechanism. The results of the computer simulations are similar to the experimental observations.

A kinetic study of the oxidation of indigo carmine with acidic bromate

The oxidation kinetics of indigo carmine (disodium 3,3′-dioxobi-indolin-2,2′-ylidene-5,5′-disulphonate) with potassium bromate have been studied in aqueous sulphuric acid, by monitoring the absorbance of indigo carmine (IC) at 610 nm. The reaction involves competitive and consecutive reaction steps—an initial slow step followed by a rapid one for depletion of IC. For the initial stages the reaction order is four- first-order with respect to IC and bromate ion and second order with H+ ion. For the fast reaction step the studies are limited to qualitative treatment due to the complex nature of the reaction. The rate of depletion of IC increased with time and with the increase in HOBr concentration. Hypobromous acid, the reaction intermediate, may possibly compete with bromate ion for IC to give an intermediate, which is further oxidised to yield the final product, isatin-5-monosulphonic acid. The stoicheiometric ratio of IC to bromate is 3:2. The dual role of bromine ion as an inhibitor and autocatalyst in the reaction mechanism is discussed. Computer simulations were performed using the proposed mechanism. The results of the computer simulations are similar to the experimental observations.