Showing papers on "Reaction rate published in 2012"
TL;DR: It is shown that it is possible to close that 'two orders of magnitude' gap in the efficiency of the water oxidation with a rationally designed molecular catalyst [Ru(bda)(isoq)(2)] (H(2)bda = 2,2'-bipyridine-6,6'-dicarboxylic acid; isoq = isoquinoline).
Abstract: Across chemical disciplines, an interest in developing artificial water splitting to O(2) and H(2), driven by sunlight, has been motivated by the need for practical and environmentally friendly power generation without the consumption of fossil fuels. The central issue in light-driven water splitting is the efficiency of the water oxidation, which in the best-known catalysts falls short of the desired level by approximately two orders of magnitude. Here, we show that it is possible to close that 'two orders of magnitude' gap with a rationally designed molecular catalyst [Ru(bda)(isoq)(2)] (H(2)bda = 2,2'-bipyridine-6,6'-dicarboxylic acid; isoq = isoquinoline). This speeds up the water oxidation to an unprecedentedly high reaction rate with a turnover frequency of >300 s(-1). This value is, for the first time, moderately comparable with the reaction rate of 100-400 s(-1) of the oxygen-evolving complex of photosystem II in vivo.
1,051 citations
TL;DR: Direct photoionization mass spectrometric detection of formaldehyde oxide (CH2OO) as a product of the reaction ofCH2I with O2 enabled direct laboratory determinations of CH2OO kinetics, suggesting a substantially greater role of carbonyl oxides in models of tropospheric sulfate and nitrate chemistry than previously assumed.
Abstract: Ozonolysis is a major tropospheric removal mechanism for unsaturated hydrocarbons and proceeds via "Criegee intermediates"--carbonyl oxides--that play a key role in tropospheric oxidation models. However, until recently no gas-phase Criegee intermediate had been observed, and indirect determinations of their reaction kinetics gave derived rate coefficients spanning orders of magnitude. Here, we report direct photoionization mass spectrometric detection of formaldehyde oxide (CH(2)OO) as a product of the reaction of CH(2)I with O(2). This reaction enabled direct laboratory determinations of CH(2)OO kinetics. Upper limits were extracted for reaction rate coefficients with NO and H(2)O. The CH(2)OO reactions with SO(2) and NO(2) proved unexpectedly rapid and imply a substantially greater role of carbonyl oxides in models of tropospheric sulfate and nitrate chemistry than previously assumed.
603 citations
TL;DR: In this article, a comprehensive chemical kinetic model for all the four isomers of butanol (e.g., 1-, 2-, iso- and tert-butanol) is presented.
489 citations
TL;DR: Pd-Au bimetallic catalysts often display enhanced catalytic activities and selectivities compared with Pd-alone catalysts, often caused by two alloy effects, i.e., ensemble and ligand effects.
Abstract: Pd–Au bimetallic catalysts often display enhanced catalytic activities and selectivities compared with Pd-alone catalysts. This enhancement is often caused by two alloy effects, i.e., ensemble and ligand effects. The ensemble effect is a dilution of surface Pd by Au. With increasing surface Au coverage, contiguous Pd ensembles disappear and isolated Pd ensembles form. For certain reactions, for example vinyl acetate synthesis, this effect is responsible for reaction rate enhancement via the formation of highly active surface sites, e.g., isolated Pd pairs. The disappearance of contiguous Pd ensembles also switches off side reactions catalyzed by these sites. This explains the selectivity increase of certain reactions, for example direct H2O2 synthesis. The ligand effects are electronic perturbation of Pd by Au. Via direct charge transfer or by affecting bond lengths, the ligand effects cause the Pd d band to be more filled, moving the d-band center away from the Fermi level. Both changes make Pd more “atomic like” therefore binding reactants and products more weakly. For certain reactions, this eliminates a so-called “self-poisoning” effect and enhances activity/selectivity.
466 citations
TL;DR: A general theory of chemical kinetics, developed over the past 7 years, is presented, capable of answering questions about how reaction rate is a nonlinear function of the thermodynamic driving force, the free energy of reaction, expressed in terms of variational chemical potentials.
Abstract: Classical theories of chemical kinetics assume independent reactions in dilute solutions, whose rates are determined by mean concentrations In condensed matter, strong interactions alter chemical activities and create inhomogeneities that can dramatically affect the reaction rate The extreme case is that of a reaction coupled to a phase transformation, whose kinetics must depend on the order parameter -- and its gradients, at phase boundaries This Account presents a general theory of chemical kinetics based on nonequilibrium thermodynamics The reaction rate is a nonlinear function of the thermodynamic driving force (free energy of reaction) expressed in terms of variational chemical potentials The Cahn-Hilliard and Allen-Cahn equations are unified and extended via a master equation for non-equilibrium chemical thermodynamics For electrochemistry, both Marcus and Butler-Volmer kinetics are generalized for concentrated solutions and ionic solids The theory is applied to intercalation dynamics in the phase separating Li-ion battery material Li$_x$FePO$_4$
440 citations
TL;DR: It is determined that the dominant active sites are the low coordinated corner Au sites, which are 3 and 7 times more active than the perimeter Au sites for Au/Al( 2)O(3) and Au/TiO(2) catalysts, respectively.
Abstract: The water–gas shift (WGS) reaction rate per total mole of Au under 7% CO, 8.5% CO2, 22% H2O, and 37% H2 at 1 atm for Au/Al2O3 catalysts at 180 °C and Au/TiO2 catalysts at 120 °C varies with the number average Au particle size (d) as d–2.2±0.2 and d–2.7±0.1, respectively. The use of nonporous and crystalline, model Al2O3 and TiO2 supports allowed the imaging of the active catalyst and enabled a precise determination of the Au particle size distribution and particle shape using transmission electron microscopy (TEM). Further, the apparent reaction orders and the stretching frequency of CO adsorbed on Au0 (near 2100 cm–1) determined by diffuse reflectance infrared spectroscopy (DRIFTS) depend on d. Because of the changes in reaction rates, kinetics, and the CO stretching frequency with number average Au particle size, it is determined that the dominant active sites are the low coordinated corner Au sites, which are 3 and 7 times more active than the perimeter Au sites for Au/Al2O3 and Au/TiO2 catalysts, resp...
362 citations
TL;DR: A new way to determine the standard entropies of adsorbed molecules on single crystal surfaces from temperature programmed desorption data is presented, its accuracy is proved by comparison to enthalpy measured by equilibrium methods, and it can be extended to reliably predict prefactors in the Arrhenius rate constant for surface reactions involving such species.
Abstract: Adsorbed molecules are involved in many reactions on solid surface that are of great technological importance. As such, there has been tremendous effort worldwide to learn how to predict reaction rates and equilibrium constants for reactions involving adsorbed molecules. Theoretical calculation of both the rate and equilibrium constants for such reactions requires knowing the entropy and enthalpy of the adsorbed molecule. While much effort has been devoted to measuring and calculating the enthalpies of well-defined adsorbates, few measurements of the entropies of adsorbates have been reported. We present here a new way to determine the standard entropies of adsorbed molecules (Sad0) on single crystal surfaces from temperature programmed desorption data, prove its accuracy by comparison to entropies measured by equilibrium methods, and apply it to published data to extract new entropies. Most importantly, when combined with reported entropies, we find that at high coverage, they linearly track the entropy ...
333 citations
TL;DR: In this article, isothermal conduction calorimetry (ICC) is used to measure the kinetics of geopolymerisation of metakaolin by reaction with NaOH solution under a variety of conditions.
310 citations
TL;DR: To the best of the knowledge, this approach is the first example of specific engineering of successful hydrogen-bonding catalysis into a MOF material.
Abstract: A new urea-containing metal-organic framework (MOF) was synthesized to act as a heterogeneous catalyst. Ureas are well-known for self-recognition and aggregation behavior, resulting in loss of catalytic competency. The catalyst spatial isolation achievable in a porous MOF environment suggests a potentially general solution. The combination of a symmetrical urea tetracarboxylate strut, 4,4'-bipyridine, and Zn(NO(3))(2)·6H(2)O under solvothermal conditions afforded a new microporous MOF (NU-601). This material is indeed an effective hydrogen-bond-donor catalyst for Friedel-Crafts reactions between pyrroles and nitroalkenes, whereas a homogeneous urea is much less competent. The higher rates of reaction of small substrates relative to larger ones with NU-601 strongly suggest that catalysis primarily occurs within the pores of this new material rather than on its exterior. To the best of our knowledge, this approach is the first example of specific engineering of successful hydrogen-bonding catalysis into a MOF material.
274 citations
TL;DR: In this article, the NUI Galway mechanism was adopted in the sensitivity analysis of the ignition kinetics of lean mixtures of methane-hydrogen with various hydrogen volumetric contents.
266 citations
TL;DR: PTR-ToF-MS can successfully be used to estimate reaction rate coefficients between H(3)O(+) and VOC at PTR-MS working conditions and find good agreement with the corresponding nonthermal theoretical predictions.
Abstract: Proton transfer reaction - mass spectrometry (PTR-MS) has become a reference technique in environmental science allowing for VOC monitoring with low detection limits. The recent introduction of time-of-flight mass analyzer (PTR-ToF-MS) opens new horizons in terms of mass resolution, acquisition time, and mass range. A standard procedure to perform quantitative VOC measurements with PTR-ToF-MS is to calibrate the instrument using a standard gas. However, given the number of compounds that can be simultaneously monitored by PTR-ToF-MS, such a procedure could become impractical, especially when standards are not readily available. In the present work we show that, under particular conditions, VOC concentration determinations based only on theoretical predictions yield good accuracy. We investigate a range of humidity and operating conditions and show that theoretical VOC concentration estimations are accurate when the effect of water cluster ions is negligible. We also show that PTR-ToF-MS can successfully be used to estimate reaction rate coefficients between H(3)O(+) and VOC at PTR-MS working conditions and find good agreement with the corresponding nonthermal theoretical predictions. We provide a tabulation of theoretical rate coefficients for a number of relevant volatile organic compounds at various energetic conditions and test the approach in a laboratory study investigating the oxidation of alpha-pinene.
TL;DR: In this article, the authors analyzed the behavior of ilmenite as an oxygen-carrier in chemical-looping combustion (CLC) and found that the reaction rate is the sum of the individual reaction rates of H 2 and CO.
TL;DR: For the first time, nanoscale zero-valent iron (nZVI)-Fe(3)O(4) nanocomposites, prepared by an in situ reduction method, are employed for chromium(VI) removal in aqueous environment and the reaction mechanism was discussed in terms of the mutual benefit brought by the electron transfer from Fe(0) to Fe(3).
TL;DR: In this article, a comparative study of biomass catalysis using thermal heterogeneous and electrocatalysis in liquid-phase was performed using aqueous-phase heterogeneous catalysis under mild temperature (50°C) and pressure (10 bar O 2 ) conditions.
TL;DR: In this article, cubic, octahedral, and rhombic dodecahedral gold nanocrystals were employed as catalysts for the examination of facet-dependent catalytic activity toward NaBH4 reduction of p-nitroaniline to p-phenylenediamine at different temperatures.
Abstract: In this study, cubic, octahedral, and rhombic dodecahedral gold nanocrystals synthesized by a seed-mediated growth method were employed as catalysts for the examination of facet-dependent catalytic activity toward NaBH4 reduction of p-nitroaniline to p-phenylenediamine at different temperatures. Different amounts of the nanocrystal solutions were used so that all samples contain particles with the same total surface area. UV–vis absorption spectra monitored the reaction progress. Rhombic dodecahedra showed the best catalytic efficiency at all the temperatures examined. Nanocubes have higher reaction rates than those of octahedra from 25 to 36 °C, so the catalytic activity for the reduction reaction follows the order of {110} > {100} > {111}. However, the reaction rates for octahedra increase rapidly with rising temperature; their reaction rate surpasses that for the nanocubes at 40 °C. Rate constants and activation energies were determined, again showing that the activation energy is lowest for rhombic do...
TL;DR: In this article, transient isotopic labeling studies were performed under steady state conditions by using 15 N( 15 NO and 15 NH3) and 18 O( 18 O2) containing species to investigate the reaction mechanism of the low-temperature SCR of NO over Mn/TiO2.
TL;DR: In this paper, a chemical kinetic mechanism for C1 and C2 fuel combustion and PAH growth, previously validated for laminar premixed combustion, has now been modified and applied to opposed flow diffusion flames.
TL;DR: In this article, a simple experiment using a single crystal and its polycrystalline counterpart was conducted to demonstrate the sensitivity of dissolution rate to grain size, results that undermine the use of “classical” rate constants.
TL;DR: It is suggested that the presence of low-valent Fe (Fe) played a decisive role in improving the PROX activity, thus providing a clue for obtaining a highly effective Ir-based catalyst by incorporating materials containing, or easily forming, Fe species.
Abstract: The proton-exchange membrane fuel cell (PEMFC) has been regarded as one of the most promising candidates for the efficient use of hydrogen energy. However, small amounts of CO (0.3–1%) in the H2 stream from reforming processes must be selectively removed because CO is highly poisonous to the Pt anode of a PEMFC. The preferential oxidation of CO in a H2-rich gas (PROX) is presently the most effective approach to address this problem. Oxide-supported Au catalysts are highly active for the PROX reaction even at room temperature, but the lower stability and sensitivity to CO2 constrain their practical applications. Supported Pt catalysts, on the other hand, are less active and only a few have shown reasonable activity for conversion of CO at temperatures lower than 60 8C. Therefore, it is highly desirable to develop improved catalysts with better catalytic performance for the PROX reaction at lower temperatures. Ir has a higher melting point and surface energy than other metals with 5f orbitals, such as Pt and Au, and Ir can be well-dispersed on and strongly interact with the support. However, compared to Ptand Au-based catalysts, Ir-based catalysts have limited applications in heterogeneous catalysis and are rarely investigated for the PROX reaction, most probably because of its inferior activity. Although much effort has been made to improve the activity of Ir-based catalysts and remarkable progress has been achieved, their activities for the PROX reaction are still low at low temperatures. In fact, there is no report so far claiming that Ir-based catalysts can show high activity at temperatures below 80 8C; thus it remains a formidable challenge to utilize Ir-based catalysts for the PROX reaction at ambient temperatures. One basic task of modern catalysis is to rationally design catalysts based on the fundamental understanding of their reaction mechanisms. Especially, the contribution of support materials to the performance of the final catalysts should be taken into account. For the PROX reaction, the strong binding of CO to Ir poisons the surface so that O2 cannot competitively adsorb on the Ir surface and be activated at low temperatures, thereby prohibiting the conversion of CO to CO2. Therefore, weakening the adsorption strength of CO and/or promoting the activation of O2 at lower temperatures have become the crucial steps. Ferric oxide has proven effective for O2 activation and has been used extensively as an additive to Pt-based catalysts. Recently, we have designed a bimetallic catalyst by adding FeOx to a supported Ir catalyst, and the activity for the PROX reaction was improved. Further study of the catalytic reactions showed that the reaction rate of CO oxidation correlated well with the presence and amount of Fe, suggesting that Fe sites were indeed the active sites for O2 activation. [13] The coordinatively unsaturated Fe center was also recently identified as the site to activate O2, which helped the design of a highly active FeOx/Pt/SiO2 catalyst to totally convert CO at room temperature. All of these studies suggest that the presence of low-valent Fe (Fe) played a decisive role in improving the PROX activity, thus providing a clue for obtaining a highly effective Ir-based catalyst by incorporating materials containing, or easily forming, Fe species. Ferric hydroxide (Fe(OH)x) is a novel support material which has recently been adopted to stabilize various types of metal species for CO oxidation. It possesses a large surface area and a large amount of OH groups; these unique properties make Fe(OH)x a good candidate for generating highly dispersed metal clusters or even single-atom catalysts. Furthermore, the longer Fe O bonds in Fe(OH)x (compared to those in Fe2O3) make it easier to form Fe 2+
TL;DR: It is found that introduction of multiple electron-donating amino groups onto a simple o-(phenylazo)aniline platform dramatically accelerates its conversion to the emissive polycyclic product.
Abstract: An oxidative cyclization reaction transforms nonemissive azoanilines into highly fluorescent benzotriazoles. We have found that introduction of multiple electron-donating amino groups onto a simple o-(phenylazo)aniline platform dramatically accelerates its conversion to the emissive polycyclic product. Notably, this chemistry can be effected by μM-level concentrations of copper(II) ion in water (pH = 6–8) at room temperature to elicit >80-fold enhancement in the green emission at λem = 530 nm. Comparative kinetic and electrochemical studies on a series of structural analogues have established that the accelerated reaction rates correlate directly with a systematic cathodic shift in the oxidation onset potential of the azo precursors. In addition, single-crystal X-ray crystallographic analysis on the most reactive derivative revealed the presence of a five-membered ring intramolecular hydrogen-bonding network. An enhanced contribution of the quinoid-type resonance in such conformation apparently facilitate...
TL;DR: In this article, a detailed spectrokinetic study of the electrocatalytic oxidation of formic acid on Au and Pt electrodes using ATR-SEIRAS has been presented.
Abstract: We present a detailed spectrokinetic study of the electrocatalytic oxidation of formic acid on Au and Pt electrodes using ATR-SEIRAS that has allowed us to unveil the mechanisms of both the direct (in which adsorbed CO is not involved) and the indirect (through adsorbed CO) paths of the reaction with unprecedented detail. Au electrodes were used to study the mechanism of the direct path without the interference of the indirect path, and the observed quadratic dependence of the reaction rate on the formate coverage was then shown to apply also to Pt. The direct path consists of three steps, namely, (i) the electroadsorption of formate (corresponding to the first electron transfer), (ii) the purely chemical bimolecular decomposition of adsorbed formate, and (iii) the second electron transfer. The dehydration of HCOOH to adsorbed CO, that is then oxidized to CO2 in the indirect path, was studied on Pt at E < 0.4 V vs the reversible hydrogen electrode (RHE), at which potentials the dehydration reaction is the...
TL;DR: In this article, state-of-the-art and perspectives of ultrasound-assisted biodiesel production from different oil-bearing materials using acid, base and enzyme catalysts are critically discussed.
Abstract: In the present paper state-of-the art and perspectives of ultrasound-assisted (UA) biodiesel production from different oil-bearing materials using acid, base and enzyme catalysts are critically discussed. The ultrasound action in biodiesel production is primarily based on the emulsification of the immiscible liquid reactants by microturbulence generated by radial motion of cavitation bubbles and the physical changes on the surface texture of the solid catalysts generating new active surface area. The importance of ultrasound characteristics and other process variables for the biodiesel yield and the reaction rate is focused on. UA transesterification is compared with other techniques for biodiesel production. Several different developing methods reducing the biodiesel production costs such as the optimization of process factors, the development of the process kinetic models, the use of phase transfer catalysts, the application of the continuous process, the design of novel types of ultrasonic reactors and the in situ ultrasound application in transesterification of oily feedstocks are also discussed.
TL;DR: In this paper, the kinetics of reduction with CH4, H2 and CO and oxidation with O2 of two NiO-based oxygen-carriers for chemical-looping combustion (CLC), prepared by impregnation, NiO18-αAl and NiO21-γAl, have been determined.
TL;DR: In this paper, the reaction rate constants and activation energies of n -hexane cracking over ZSM-5 zeolites with various crystal sizes and Si/Al ratios were evaluated.
TL;DR: In this article, the site requirements for the methane total combustion, partial oxidation, and decomposition on LaFeO3 perovskite catalysts during the chemical looping process were addressed by a kinetic study through varying the O content and the crystal size.
TL;DR: In this paper, the combustion profiles of the torrefied sample and the low-temperature carbonized sample blended with coal were examined in order to analyze the characteristics of co-combustion.
TL;DR: In this article, a density functional theory (DFT)-parameterized cluster expansion (CE) was used to incorporate accurate adsorbate-adsorbate interactions into a surface kinetic model.
TL;DR: In this paper, the authors combine measured rate data with physical property measurements and molecular simulation in order to gain a more fundamental understanding of mixed solvent effects for this heterogeneously catalysed reaction.
TL;DR: In this article, a modified Langmuir-Hinshelwood expression was developed to link the rate of thermal degradation of a biomass to its potassium or sodium content, and an apparent first-order kinetics derived.
TL;DR: A series of y wt % Pt/CexZr1-xO2 catalysts were synthesized and characterized to investigate the effect of CeO2 doping with Zr4+ and of Pt particle size on important mechanistic and kinetic aspects of the water-gas shift (WGS) reaction.
Abstract: A series of y wt % Pt/CexZr1–xO2 catalysts (y = 0.1, 0.5, and 1.0; x = 0.3, 0.5, and 0.7) were synthesized and characterized to investigate the effect of CeO2 doping with Zr4+ and of Pt particle size (Pt/Ce0.5Zr0.5O2) on important mechanistic and kinetic aspects of the water–gas shift (WGS) reaction. These included the concentration (μmol·g–1 or θ (surface coverage based on Pts)) and chemical structure of active reaction intermediates present in the “carbon path” and “hydrogen path” of the WGS reaction in the 200–300 °C range and the prevailing mechanism among “redox” and “associative formate” largely considered in the literature. Toward this goal, steady-state isotopic transient kinetic analysis coupled with in situ DRIFTS and mass spectrometry experiments were performed for the first time using D2O and 13CO isotopic gases. A novel transient isotopic experiment allowed quantification of the initial transient rate of reaction of adsorbed formate (HCOO−) with water and that of adsorbed CO with water under ...