State defining experiment in chemical kinetics. Primary characterization of catalyst activity in a TAP experiment

This paper presents a new strategy, “state-by-state transient screening”, for kinetic characterization of states of a multicomponent catalyst as applied to TAP pulse-response experiments. The key idea is to perform an insignificant chemical perturbation of the catalytic system so that the known essential characteristics of the catalyst (e.g. oxidation degree) do not change during the experiment. Two types of catalytic substances can be distinguished: catalyst state substances, which determine the catalyst state, and catalyst dynamic substances, which are created by the perturbation. The general methodological and theoretical framework for multi-pulse TAP experiments is developed, and the general model for a one-pulse TAP experiment is solved. The primary kinetic characteristics, basic kinetic coefficients, are extracted from diffusion–reaction data and calculated as functions of experimentally measured exit-flow moments without assumptions regarding the detailed kinetic mechanism. The new strategy presented in this paper provides essential information, which can be a basis for developing a detailed reaction mechanism. The theoretical results are illustrated using furan oxidation over a VPO catalyst.

Thin-zone TAP reactor as a basis of �state-by-state transient screening

Thin-zone TAP reactor is presented as a basis of the new state-by-state transient screening approach which has been proposed by the authors for non-steady-state kinetic characterization of industrial catalysts. The general thin-zone TAP reactor model is described, and its mathematical status is justified analytically. It is shown that this model provides high enough accuracy to be applicable in the wide conversion interval (up to 90%), which is an important advantage of this approach compared with the traditional differential reactor.

Kinetics of phase transformations in a model with metastable fluid-fluid separation: A molecular dynamics study

By molecular dynamics (MD) simulations we study the crystallization process in a model system whose particles interact by a spherical pair potential with a narrow and deep attractive well adjacent to a hard repulsive core. The phase diagram of the model displays a solid-fluid equilibrium, with a metastable fluid-fluid separation. Our computations are restricted to fairly small systems (from 2592 to 10368 particles) and cover long simulation times, with constant energy trajectories extending up to 76x10(6) MD steps. By progressively reducing the system temperature below the solid-fluid line, we first observe the metastable fluid-fluid separation, occurring readily and almost reversibly upon crossing the corresponding line in the phase diagram. The nucleation of the crystal phase takes place when the system is in the two-fluid metastable region. Analysis of the temperature dependence of the nucleation time allows us to estimate directly the nucleation free energy barrier. The results are compared with the predictions of classical nucleation theory. The critical nucleus is identified, and its structure is found to be predominantly fcc. Following nucleation, the solid phase grows steadily across the system, incorporating a large number of localized and extended defects. We discuss the relaxation processes taking place both during and after the crystallization stage. The relevance of our simulation for the kinetics of protein crystallization under normal experimental conditions is discussed. (C) 2002 American Institute of Physics.

Characterization of a dielectric barrier discharge operating in an open reactor with flowing helium

A dielectric barrier discharge (DBD) generated by flowing helium between the parallel-plate electrodes of an open air reactor has been characterized using time resolved optical and electrical measurements. A sinusoidal voltage of up to 5 kV (peak to peak) of frequencies from 3 to 50 kHz has been applied to the discharge electrodes. The helium flow rate is varied up to 10 litre min(-1). The adjustment of flow rate allows the creation of uniform DBDs with optimized input power equal to 120 +/- 10 mW cm(-3). At flow rates from 4 to 6 litre min(-1) a uniform DBD is obtained. The maxima in the line intensities of N-2(+) and helium at 391.4 nm and 706.5 nm, respectively, 2 under those conditions indicate the importance of helium metastables and He-2(+) in sustaining such a discharge. The power efficiency and discharge 2 current show maxima when the DBD In He/air is uniform. The gas temperature during the discharge has been measured as 360 +/- 20 K.

Time dependence of rotational state populations of excited hydrogen molecules in an RF excited plasma reactor

We report on time-dependent population distributions of excited rotational states of hydrogen in a capacitively coupled RF discharge. The common model to obtain the gas temperature from the rotational distribution is not applicable at all times during the discharge cycle due to the time dependence of the EEDF. The apparent temperature within a cycle assumes values between 350 K and 450 K for the discharge parameters of this experiment. We discuss the optimum time window within the discharge cycle that yields the best approximation to the actual temperature. Erroneous results can be obtained, in principle, with time-integrated measurements; we find, however, that in the present case the systematic error amounts to only approximately 20 K. This is due to the fact that the dominant contribution to the average intensity arises during that time window for which the assumptions underlying the analysis are best fulfilled. A similar analysis can be performed for N+2 rotational bands with a small amount of nitrogen added to the discharge gas. These populations do not exhibit the time variations found in the case of H2.

Structure and kinetics of phosphonopyruvate hydrolase from Variovorax sp. Pal2: New insight into the divergence of catalysis within the PEP mutase/isocitrate lyase superfamily .

Phosphonopyruvate (P-pyr) hydrolase (PPH), a member of the phosphoenolpyruvate (PEP) mutase/isocitrate lyase (PEPM/ICL) superfamily, hydrolyzes P-pyr and shares the highest sequence identity and functional similarity with PEPM. Recombinant PPH from Variovorax sp. Pal2 was expressed in Escherichia coli and purified to homogeneity. Analytical gel filtration indicated that the protein exists in solution predominantly as a tetramer. The PPH pH rate profile indicates maximal activity over a broad pH range.The steady-state kinetic constants determined for a rapid equilibrium ordered kinetic mechanism with Mg+2 binding first (Kd =140 ± 40 M), are kcat = 105 ± 2 s-1 and P-pyr Km = 5 ± 1 M. PEP (slow substrate kcat = 2 × 10-4 s-1), oxalate, and sulfopyruvate are competitive inhibitors with Ki values of 2.0 ± 0.1 mM, 17 ± 1 M, and 210 ± 10 M, respectively. Three PPH crystal structures have been determined, that of a ligand-free enzyme, the enzyme bound to Mg2+ and oxalate (inhibitor), and the enzyme bound to Mg2+ and P-pyr (substrate). The complex with the inhibitor was obtained by cocrystallization, whereas that with the substrate was obtained by briefly soaking crystals of the ligand-free enzyme with P-pyr prior to flash cooling. The PPH structure resembles that of the other members of the PEPM/ICL superfamily and is most similar to the functionally related enzyme, PEPM. Each monomer of the dimer of dimers exhibits an (/)8 barrel fold with the eighth helix swapped between two molecules of the dimer. Both P-pyr and oxalate are anchored to the active site by Mg2+. The loop capping the active site is disordered in all three structures, in contrast to PEPM, where the equivalent loop adopts an open or disordered conformation in the unbound state but sequesters the inhibitor from solvent in the bound state. Crystal packing may have favored the open conformation of PPH even when the enzyme was cocrystallized with the oxalate inhibitor. Structure alignment of PPH with other superfamily members revealed two pairs of invariant or conservatively replaced residues that anchor the flexible gating loop. The proposed PPH catalytic mechanism is analogous to that of PEPM but includes activation of a water nucleophile with the loop Thr118 residue.

Quantitative analysis of the reactivity of formate species seen by DRIFTS over a Au/Ce(La)O2 water–gas shift catalyst: First unambiguous evidence of the minority role of formates as reaction intermediates

The reactivity of the species formed at the surface of a Au/Ce(La)O2 catalyst during the water������¢���¯���¿���½���¯���¿���½gas shift (WGS) reaction were investigated by operando diffuse reflectance Fourier transform spectroscopy (DRIFTS) at the chemical steady state during isotopic transient kinetic analyses (SSITKA). The exchanges of the reaction product CO2 and of formate and carbonate surface species were followed during an isotopic exchange of the reactant CO using a DRIFTS cell as a single reactor. The DRIFTS cell was a modified commercial cell that yielded identical reaction rates to that measured over a quartz plug-flow reactor. The DRIFTS signal was used to quantify the relative oncentrations of the surface species and CO2. The analysis of the formate exchange curves between 428 and 493 K showed that at least two levels of reactivity were present. ������¢���¯���¿���½���¯���¿���½Slow formates������¢���¯���¿���½���¯���¿���½ displayed an exchange rate constant 10- to 20-fold slower than that of the reaction product CO2. ������¢���¯���¿���½���¯���¿���½Fast formates������¢���¯���¿���½���¯���¿���½ were exchanged on a time scale similar to that of CO2. Multiple nonreactive readsorption of CO2 took place, accounting for the kinetics of the exchange of CO2(g) and making it impossible to determine the number of active sites through the SSITKA technique. The concentration (in mol g������¢���¯���¿���½���¯���¿���½1) of formates on the catalyst was determined through a calibration curve and allowed calculation of the specific rate of formate decomposition. The rate of CO2 formation was more than an order of magnitude higher than the rate of decomposition of formates (slow + fast species), indicating that all of the formates detected by DRIFTS could not be the main reaction intermediates in the production of CO2. This work stresses the importance of full quantitative analyses (measuring both rate constants and adsorbate concentrations) when investigating the role of adsorbates as potential reaction intermediates, and illustrates how even reactive species seen by DRIFTS may be unimportant in the overall reaction scheme.

Studies of switching kinetics in ferroelectric thin films

Experimental studies are reported concerning the importance of interfacial capacitance (including electrode screening, space-charge layers, and/or chemically discrete dead layers). on domain switching behaviour in thin films of ferroelectric lead zirconate-titanate (PZT), strontium bismuth tantalate (SBT), and barium strontium titanate (BST). Emphasis is placed upon studies at applied field values very near the coercive field E, asymmetry in fatigue for positive and negative polarity coercive fields, and in the case of BST, of the coexistence of ferroelectric and paraelectric phases Studies of dielectric loss show important correlations between tan 6 and fatigue (polarization decrease) as a function of bipolar switching cycles N. This is a priori not obvious, since the former is a linear response and the latter, a nonlinear response. Modelling of enlarged interfacial,space-charge layers in PZT films and chemically distinct dead (paraelectric) layers in BST films shows contradictory tendencies of coercive-voltage changes with the growth of passive layers.