Reaction Rate Measurements Research Articles

Quantum-state-specific reaction rate measurements for the photo-induced reaction Ca+ + O2 → CaO+ + O

ABSTRACTAtoms and molecules often react at different rates depending on their internal quantum states. Thus, controlling which internal states are populated can be used to manipulate the reactivity and can lead to a more detailed understanding of reaction mechanisms. We demonstrate this control of reactions by studying the electronically excited state reaction . This reaction is exothermic only if Ca is in one of its excited electronic states. Using laser-cooling and electrodynamic trapping, we cool and trap Ca at millikelvin temperatures for several minutes. We can then change the fraction of time they spend in each of the two excited states by adjusting the detunings of the cooling lasers. This allows us to disentangle the reactions that begin with Ca in the 2P-state from the ones where Ca is in the 2D-state. Using time-of-flight mass spectrometry, we determine independent reaction rate constants for Ca in both electronically excited states.

Measurement of charge exchange between Na and Ca+ in a hybrid trap

We present measurements of the charge-exchange reaction rate between neutral sodium (Na) and ionized calcium (\ce{Ca+}) in a hybrid atom-ion trap, which is comprised of a Na magneto-optical trap concentric with a linear Paul trap. Once the Na and \ce{Ca+} are co-trapped, the reaction rate is measured by continuously quenching the reaction product \ce{Na+} from the ion trap, and then destructively measuring the decay of the remaining ion population. The reactants' electronic state and temperature are experimentally controlled, allowing us to determine the four individual reaction-rates between $\text{Na}[\text{S~or~P}]$ and $\text{Ca}^+[\text{S~or~D}]$ at different collision energies. With the exception of the largest reaction-rate channel ($\text{Na}[\text{S}]+\text{Ca}^+[\text{D}]$), our rates agree with classical Langevin rate limit. We have also found evidence of reactant collision-energy thresholds associated with two of the four entrance-channels.

Effects of WO3 nanoparticle size on ethylene-butene metathesis activity

WO3 nanoparticles (NPs) of 2.64 ± 1.11 nm, 16.6 ± 3.7 nm, and 54.3 ± 18.5 nm were investigated in the cross-metathesis reaction of ethylene and trans-2-butene to form propylene. Measurements of reaction rates normalized by exposed WO3 surface area showed that decreasing WO3 particle sizes resulted in increasing propylene formation rates. X-ray photoelectron spectroscopy of post-reaction catalysts showed that smaller WO3 NPs featured lower O/W surface atomic ratios, and thus higher degrees of surface unsaturation. Based on these results, high-temperature synthetic treatments aimed at increasing the degree of surface unsaturation of WO3 NPs showed significant improvements in metathesis activity when compared to baseline values. This work demonstrates that control of particle size for industrially-relevant catalysts drastically impacts reactivity.

Impact of Kinetic Uncertainties on Accurate Prediction of NO Concentrations in Premixed Alkane-Air Flames

ABSTRACT Accurate thermochemical mechanisms that can predict the formation of nitrogen oxides (NO) are important design tools for low-emissions engines. The lack of accurate direct measurements of reaction rates and the associated measurement scatter have resulted in recommended rate parameters for individual chemical reactions that have large uncertainty intervals. In an effort to quantify the impact of these parametric uncertainties on emissions predictions, forward uncertainty propagation is performed with five spectral methods. Sparse grids are identified as the optimal technique to rapidly construct accurate surrogate models. Subsequent polynomial expansions with sparse grids, performed in one-dimensional atmospheric laminar flames for only the 30 uncertain reactions that greatly affect NO formation, produce uncertainty intervals two orders of magnitude larger than nominal predictions. Primary uncertainty sources were identified with reaction pathway analyses to evaluate the contribution of individual formation routes and the uncertainties in prompt NO were found to propagate mostly from the CH chemistry. These results highlight the necessity of a comprehensive approach, using experimental measurements with uncertainty quantification and inference techniques, to reduce uncertainty and develop predictive NO models.

Kinetic Understanding of the Ultrahigh Ionization Efficiencies (up to 28%) of Excited-State CH2Cl2-Induced Associative Ionization: A Case Study with Nitro Compounds.

Excited-state CH2Cl2-induced associative ionization (AI) is a newly developed ionization method that is very effective for oxygenated organics. However, this method is not widely known. In this study, an unprecedented ionization efficiency and ultrafast reaction rate of AI toward nitro compounds were observed. The ionization efficiencies of o-nitrotoluene (o-NT), m-nitrotoluene (m-NT), and nitrobenzene (NB) were as high as (28 ± 3)%, (27 ± 2)%, and (13 ± 1)%, respectively (∼1-3 ions for every 10 molecules). The measured reaction rate coefficients of these nitroaromatics were (0.5-1.3) × 10-7 molecule-1 cm3 s-1 (∼300 K). These unusual rate coefficients indicated strong long-range interactions between the two neutral reactants, which was regarded as a key factor leading to the ultrahigh ionization efficiency. The detection sensitivities of the nitroaromatics, (1.01-2.16) × 104 counts pptv-1 in 10 s acquisition time, were obtained by an AI time-of-flight mass spectrometer (AI-TOFMS). These experimental results not only provide new insight into the AI reaction but also reveal an excellent ionization method that can improve the detection sensitivity of nitroaromatics to an unprecedented degree.

Cytometry of Reaction Rate Constant: Measuring Reaction Rate Constant in Individual Cells To Facilitate Robust and Accurate Analysis of Cell-Population Heterogeneity.

Robust and accurate analysis of cell-population heterogeneity is challenging but required in many areas of biology and medicine. In particular, it is pivotal to the development of reliable cancer biomarkers. Here, we prove that cytometry of reaction rate constant (CRRC) can facilitate such analysis when the kinetic mechanism of a reaction associated with the heterogeneity is known. In CRRC, the cells are loaded with a reaction substrate, and its conversion into a product is followed by time-lapse fluorescence microscopy at the single-cell level. A reaction rate constant is determined for every cell, and a kinetic histogram "number of cells versus the rate constant" is used to determine quantitative parameters of reaction-based cell-population heterogeneity. Such parameters include, for example, the number and sizes of subpopulations. In this work, we applied CRRC to a reaction of substrate extrusion from cells by ATP-binding cassette (ABC) transporters. This reaction is viewed as a potential basis for predictive biomarkers of chemoresistance in cancer. CRRC proved to be robust (insensitive to variations in experimental settings) and accurate for finding quantitative parameters of cell-population heterogeneity. In contrast, a typical nonkinetic analysis, performed on the same data sets, proved to be both nonrobust and inaccurate. Our results suggest that CRRC can potentially facilitate the development of reliable cancer biomarkers on the basis of quantitative parameters of cell-population heterogeneity. A plausible implementation scenario of CRRC-based development, validation, and clinical use of a predictor of ovarian cancer chemoresistance to its frontline therapy is presented.

H/D reactivity and acidity of Brønsted acid sites of MWW zeolites: Comparison with MFI zeolite

Acid site strength in MWW and MFI zeolites was investigated by isotopic exchange reaction between deuterated hydroxyl groups of zeolites and ethane molecules. The course of the reaction was monitored by FT-IR spectroscopy through time changes of integral intensities of the Si(OD)Al Brønsted acid sites represented by absorption bands at 2644, 2655, and 2657 cm−1 for MFI, MWW and MCM-36 zeolite, respectively. The rate of the reaction was described by pseudo first order kinetics and acid site strength was compared using rate constants characteristic of individual zeolites at constant temperature. The exchange reaction is significantly faster over MFI zeolite compared with MWW zeolites, suggesting a higher acid strength of Brønsted sites in MFI zeolite than in MWW. In our previous study (Arean et al. Phys. Chem. Chem. Phys. 16 (2014) 10,129), we found notable discrepancy in the relation between OH group frequency shift induced by adsorption of weak base molecules like CO and N2 and the change of adsorption enthalpy of these probe molecules for MWW zeolites, which calls into question the use of frequency shifts as a measure of acidity. Our reported results based on isotopic exchange reaction rate measurement correlate with the change of the probe molecules adsorption enthalpy involved in hydrogen bonding. Therefore, we believe that enthalpy change or H/D exchange activity measurement is more reliable method for assessment of acid strength than OH frequency shift probed by a weak base adsorbed at a low temperature.

Measuring reaction rates at equilibrium with the isotope doping method

Since the time of J. H. van’t Hoff [1], it has been known that chemical equilibrium is dynamic, meaning that at equilibrium, chemical reactions do not cease, but instead the forward and backward reaction rates are equal. The constant concentrations at equilibrium preclude the use of concentrations to measure reaction rates at equilibrium. Therefore, with the exception of a few special cases, no reaction rates at equilibrium have been published in the literature of chemistry, physics, or chemical engineering. Here we report dissolution and precipitation rates at equilibrium for quartz and barite with the isotope-doping method. Experimental data show that dissolution and precipitation rates are equal at equilibrium, indicating the principle of detailed balance (PDB) appear to be applicable at these experimental conditions. The PDB has been a cornerstone for irreversible thermodynamics and chemical kinetics for a long time, and its wide application in geochemistry has mostly been implicit and without experimental testing of its applicability. Nevertheless, many extrapolations based on PDB without experimental validation have far reaching impacts on society’s mega environmental enterprises. The isotope doping method appears to able to test its applicability for a variety of minerals at a wide range of conditions.

Utilization of ICT and Quality Management Techniques for Physical Chemistry Experiments in National Institute of Technology

Physical chemistry experiments utilized with Information and Communication Technology and quality management techniques were performed in National Institute of Technology, Kumamoto college. In the course of measurements of chemical reaction rate for decomposition of hydrogen peroxide, students can learn with e-learning materials and through the experiments, students can learn some quality management techniques.

Digitization of Color of Solution Using Arduino

아두이노 우노 (Arduino UNO) 보드와 발광다이 오드(LED)를 이용하여 실험실에서 흔히 볼 수 있는 화학용액의 색깔을 디지털화할 수 있는 장치를 고안하였다. 24 비트 RGB 형태로 디지털화한 색깔은 컴퓨터 화면에 나타내거나 블루투스를 통하여 스마트 폰으로 확인하였다. 컴퓨터 화면이나 스마트폰 화면에 재현된 색깔은 실제 용액의 색깔과 동일하였다. 아울러 넓은 pH 범위를 가지는 산-염기 용액에 대한 RGB 값으로 다중회귀분석한 결과 좋은 직선관계를 나타내주었다. 제안된 장치를 이용하여 검정곡선을 그려둔다면 정량적인 농도측정이나 색깔변화를 수반하는 화학반응 속도 측정 등에 이용할 것으로 기대한다. 나아가서 이 모든 과정은 최근에 관심을 많이 가지고 있는 메이커 운동 또는 메이커 문화 확산이나 중고등학교 소프트웨어 교육에도 도움이 될 것으로 기대한다.A simple device is designed using an Arduino Uno board and light emitting diodes (LED) to determine the digitized color of the chemical solution commonly found in the chemistry laboratories. The digitized color is represented using the 24 bits RGB format, and the three R,G,B values are concatenated to one value to transmitted to smartphone through a Bluetooth protocol as well as a PC monitor. Generated color displayed in computer monitor or smartphone using the RGB code is apparently identical with the color of the real solution. The color of acid-base solution with a wide range of pH values is well correlated with respect to the RGB values based on the multiple regression analysis. Using the device, variety of chemical experiments can be performed such as determination of the concentration of a solution using calibration curve, measurements of the specific chemical reaction rate where the color is changed. Furthermore, the procedures adapted in this work can be applied in spreading the maker movement (culture) and the software education in the secondary school.

One‐Shot Measurement of Effectiveness Factors of Chemical Conversion in Porous Catalysts

AbstractFrom the earliest days of heterogeneous catalysis, high surface area solids were extensively used for attaining largest possible densities of active sites and, correspondingly, for maximizing turnover. Since sites are active in the real sense of the word only if they are occupied by reactants, i. e., by molecules still to be converted, the relative fraction of pore volume occupied by reactants (the “effectiveness factor”) is a key number for the efficiency of a catalyst in a given reaction. Its determination, so far generally based on reaction rate measurements with purposefully varied catalyst particles, remained to date a challenging task since it must be based on additional assumptions. The “one‐shot determination” of effectiveness factors by IR microimaging, here exemplified on considering the catalytic hydrogenation of benzene to cyclohexane by platinum dispersed on nanoporous glass, is shown to open up a promising route to overcome these limitations.

Validation of selected (n,2n) dosimetry reactions in IRDFF-1.05 library

Spectrum-averaged cross sections (SACS) have been measured in the reference 252Cf(sf) neutron field for the following high-threshold (n,2n) neutron dosimetry reactions since they are especially important due to the high threshold which allows validation of upper parts of prompt fission neutron spectrum. This work includes 59Co(n,2n)58Co, 197Au(n,2n)196Au, 169Tm(n,2n)168Tm, 55Mn(n,2n)54Mn, 93Nb(n,2n)92 mNb and 89Y(n,2n)88Y and for the 59Co(n,p)59Fe threshold reactions. SACS were inferred from experimentally determined reaction rates by gamma spectrometry using a semiconductor high-purity germanium detector to measure irradiated samples. Measured reaction rates agree within quoted uncertainties with those calculated from the IRDFF-1.05 library, except for the reaction 55Mn(n,2n)54Mn, for which the measured value is underestimated by 16%. For this reaction the ENDF-B/VII.1 evaluation agrees with measured reaction rate within uncertainties.

Note: Calibration of majority isotopes for enriched and depleted uranium fission chambers by using the elimination method with fast neutrons.

Calibrated miniature pulse fission chambers (MPFCs) can be used for absolute measurement of the fission reaction rate in neutronic experiments. Absolute measurements require the effective number of fissionable atoms (ENFAs) in the coating of the chambers. An elimination method was proposed to calibrate the MPFCs embodying highly enriched and depleted uranium, using 14.75 MeV fast neutrons from the D-T reaction. The ENFAs of 235U in enriched uranium MPFC and 238U in depleted uranium MPFC are 8.39 × 1018 and 9.55 × 1018, with the uncertainties of 5.27% and 4.76%, respectively.

Quantifying hot carrier and thermal contributions in plasmonic photocatalysis

Photocatalysis based on optically active, "plasmonic" metal nanoparticles has emerged as a promising approach to facilitate light-driven chemical conversions under far milder conditions than thermal catalysis. However, an understanding of the relation between thermal and electronic excitations has been lacking. We report the substantial light-induced reduction of the thermal activation barrier for ammonia decomposition on a plasmonic photocatalyst. We introduce the concept of a light-dependent activation barrier to account for the effect of light illumination on electronic and thermal excitations in a single unified picture. This framework provides insight into the specific role of hot carriers in plasmon-mediated photochemistry, which is critically important for designing energy-efficient plasmonic photocatalysts.

Hot carriers reducing thermal barriers

Plasmonic Chemistry Plasmonic catalysts can generate hot charge carriers that can activate reactants and, in turn, reduce the overall barrier to a reaction. Zhou et al. studied the decomposition of ammonia to hydrogen on a copper alloy nanostructure that absorbed light and generated electrons that activated nitrogen atoms on ruthenium surface atoms (see the Perspective by Cortes). By measuring reaction rates at different wavelengths, light intensities, and catalyst surface temperatures, the light-induced reduction of the apparent activation barrier was quantified. Science , this issue p. [69][1]; see also p. [28][2] [1]: /lookup/volpage/362/69?iss=6410 [2]: /lookup/doi/10.1126/science.aav1133

Alite dissolution and C-S-H precipitation rates during hydration

An extensive dataset suitable for kinetic calculations was collected for the hydration of monoclinic alite at water to solid ratios of 10 and 0.5. The measurement methods used to investigate the hydration behavior are XRD, TGA, solid state NMR, BET, SEM and ICP-OES. A comparison of the two water to solid ratios shows only minor effects of the w/s-ratio on the reaction kinetics. Reaction rates for the two kinetic steps alite dissolution and C-S-H precipitation are calculated from this dataset and compared to the measured reaction rates. Computed reaction rates from C-S-H precipitation show a good match with the measured reaction rates, indicating that the computation incorporates all relevant factors related to C-S-H precipitation kinetics during hydration.

Interplay of phase boundary anisotropy and electro-auto-catalytic surface reactions on the lithium intercalation dynamics in LiXFePO4 plateletlike nanoparticles

Experiments on single crystal Li$_X$FePO$_4$ (LFP) nanoparticles indicate rich nonequilibrium phase behavior, such as suppression of phase separation at high lithiation rates, striped patterns of coherent phase boundaries, nucleation by binarysolid surface wetting and intercalation waves. These observations have been successfully predicted (prior to the experiments) by 1D depth-averaged phase-field models, which neglect any subsurface phase separation. In this paper, using an electro-chemo-mechanical phase-field model, we investigate the coherent non-equilibrium subsurface phase morphologies that develop in the $ab$- plane of platelet-like single-crystal platelet-like Li$_X$FePO$_4$ nanoparticles. Finite element simulations are performed for 2D plane-stress conditions in the $ab$- plane, and validated by 3D simulations, showing similar results. We show that the anisotropy of the interfacial tension tensor, coupled with electroautocatalytic surface intercalation reactions, plays a crucial role in determining the subsurface phase morphology. With isotropic interfacial tension, subsurface phase separation is observed, independent of the reaction kinetics, but for strong anisotropy, phase separation is controlled by surface reactions, as assumed in 1D models. Moreover, the driven intercalation reaction suppresses phase separation during lithiation, while enhancing it during delithiation, by electro-autocatalysis, in quantitative agreement with {\it in operando} imaging experiments in single-crystalline nanoparticles, given measured reaction rate constants.

Optical Control of Reactions between Water and Laser-Cooled Be+ Ions.

We investigate reactions between laser-cooled Be+ ions and room-temperature water molecules using an integrated ion trap and high-resolution time-of-flight mass spectrometer. This system allows simultaneous measurement of individual reaction rates that are resolved by reaction product. The rate coefficient of the Be+(2S1/2) + H2O → BeOH+ + H reaction is measured for the first time and is found to be approximately two times smaller than predicted by an ion-dipole capture model. Zero-point-corrected quasi-classical trajectory calculations on a highly accurate potential energy surface for the ground electronic state reveal that the reaction is capture-dominated, but a submerged barrier in the product channel lowers the reactivity. Furthermore, laser excitation of the ions from the 2S1/2 ground state to the 2P3/2 state opens new reaction channels, and we report the rate and branching ratio of the Be+(2P3/2) + H2O → BeOH+ + H and H2O+ + Be reactions. The excited-state reactions are nonadiabatic in nature.

The 19F(α, p)22Ne Reaction at Energies of Astrophysical Relevance by Means of the Trojan Horse Method and Its Implications in AGB Stars

Abstract The main source of 19F in the universe has not yet been clearly identified and this issue represents one of the unanswered questions of stellar modeling. This lack of knowledge can be due to the 19F(α, p)22Ne reaction cross-section that has proven to be difficult at low energies: direct measurements stop only at about ∼660 keV, leaving roughly half of the astrophysical relevant energy region (from 200 keV to 1.1 MeV) explored only by R-matrix calculations. In this work, we applied the Trojan Horse Method to the quasi-free three-body 6Li(19F, p22Ne)d reaction performed at E beam = 6 MeV in order to indirectly study the 19F(α, p)22Ne reaction in the sub-Coulomb energy region. In this way, we obtained the cross-section and the reaction rate in the temperature region of interest for astrophysics and free from electron screening effects. A brief analysis of the impact of the new measured reaction rate in AGB star nucleosynthesis is also presented.

Determination of thorium fission rate based on 135I in thorium oxide cylinder bombarded by D-T fusion neutrons

Aiming at developing the activation method and provide reference for the validation of thorium fission parameters, a dedicated integral experiment was carried out in a thorium oxide cylinder bombard by D-T fusion neutrons. Thorium fission rate in the axial direction of the cylinder was determined by detecting the 1260.409 keV gamma emitted from fission product 135I, with experimental uncertainties of 6.2%. MCNP calculations employing ENDF/B-VII.0, ENDF/B-VII.1, JENDL-4.0, CENDL-3.1 library data were in good agreement with measured thorium fission rates within experimental uncertainties. The agreement of measured reaction rates of monitors with the calculations on the whole demonstrated the reliability of this experiment in some degree, and could provide reference for the analyses of thorium fission rates. Neutron spectra and the contribution of neutrons in each sub-energy range to thorium fission rate were also analyzed. The activation method to determine thorium fission rate was developed and the data obtained in this work could provide reference for the validation of thorium fission parameters.