Realistic Reaction Conditions Research Articles

06/01901 Genetic fuzzy control applied to the inverter of solid oxide fuel cell for power quality improvement: Jurado, F. and Valverde, M. Electric Power Systems Research, 2005, 76, (1–3), 93–105

The influence of the metal cluster size and the support on the reactivity of gold-based catalysts has been studied in the CO oxidation reaction. To overcome the structural complexity of the supported catalysts, gold nanoparticles synthesized from colloidal chemistry with precisely controlled size have been used. Those particles were supported over SiO2 and TiO2 and their catalytic activity measured in a flow reactor. The reaction rate was dependent on the particle size and on the support, suggesting two reaction pathways in the CO oxidation reaction. In parallel, ambient pressure photoelectron spectroscopy (APPS) has been performed under reaction conditions using bidimensional model catalysts prepared by supporting Au nanoparticles over planar polycrystalline SiO2 and TiO2 thin films. The nanoparticles were transferred from a water surface where they have been dispersed by means of the Langmuir–Blodgett (LB) technique. In this way, the catalytically active surface was characterized under real reaction conditions, revealing that during CO oxidation gold remains in the metallic state.

Estimation of kinetic triplet of cellulose pyrolysis reaction from isothermal kinetic results

This paper presents a new approach to estimate the Arrhenius parameters as well as the reaction model function of cellulose pyrolysis reaction. Reduced time plot (RTP) was employed to choose a proper form of reaction model function for cellulose pyrolysis reaction. A state-of-the-art thermobalance (TB) that is able to form real isothermal reaction conditions was introduced to construct RTPs from isothermal decomposition kinetic data. The reaction model function of cellulose pyrolysis reaction would be accounted for by Avrami-Erofeev function, n(1-α){-ln(1-a)}1-1/n where n is determined to be 3.69.

Enhancement of decane-SCR-NO x over Ag/alumina by hydrogen. Reaction kinetics and in situ FTIR and UV–vis study

Decane-SCR-NO x , oxidation of NO and decane by molecular oxygen, and these reactions enhanced by hydrogen over Ag/alumina were investigated by monitoring of gaseous product composition, surface intermediates (in situ FTIR), and the state of silver (in situ UV–vis) under realistic reaction conditions in the steady-state and transient modes. It has been shown that oxidation of NO or decane by oxygen is greatly affected by the presence of decane or NO. Monodentate nitrates are formed preferentially and are more reactive compared with the bidentate species. Oxidation of decane mostly yields surface acetates, and the presence of NO x favors the formation of formates (acrylates). The reaction steps most enhanced by the addition of hydrogen to the SCR-NO x reaction are the transformations of the intermediate CN species into NCO and oxidation of the hydrocarbon to formates (acrylates). Although NO–NO 2 oxidation is also enhanced by hydrogen, this effect does not contribute to the increased rate of the SCR-NO x reaction. A small part of very reactive Ag + (estimated to be <5%) is reduced to metallic charged Ag n δ + clusters ( n ⩽ 8 ) during both the decane- and H 2/decane-SCR-NO x reactions. The number of Ag n δ + clusters formed depends mainly on the level of NO conversion to nitrogen, regardless of whether the conversion level is attained by the addition of hydrogen or by an increased concentration of decane or oxygen in the feed. The time-resolved responses of NO x –N 2 conversion and of the number of Ag n δ + clusters to the addition/removal of hydrogen from the reactants indicate that the Ag n δ + clusters are mainly formed because of the reducing effect of adsorbed CH x O-containing reaction intermediates. The active sites are suggested to be single Ag + ions or small Ag 2O species. Hydrogen itself takes part in the SCR-NO x reaction. It is supposed that it dissociates, forms Ag-hydride, and enhances SCR-NO x by initiation radical reactions.

Removal of carbon monoxide from hydrogen-rich fuels by selective low-temperature oxidation over base metal added platinum catalysts

Various catalysts containing different catalytic materials, supports, and additives were tested for the preferential oxidation (PROX) of carbon monoxide from a hydrogen-rich gas stream. The results were analyzed based on three reactions involved in the PROX: oxidation of carbon monoxide, H 2–O 2 reaction, and methanation. The PROX reactions were performed in two reaction systems, one for catalyst screening and kinetic study and the other for simulation of the catalytic performance under real reaction conditions. The performances of PROX on different catalysts, varying active components, supports, and additives, were ranked in terms of carbon monoxide conversion and hydrogen consumption. Base metal added platinum catalysts exhibited excellent ability for the carbon monoxide removal. TPR results indicated that a new active species was formed resulting in the enhancement of catalytic activity. PtCo/Al 2O 3 was tested with a simulated steam-reformed fuel for confirmation of its high activity. The effect of operating conditions was analyzed on the PtCo/Al 2O 3, and the optimum conditions for PROX were obtained.

Operando methodology: combination of in situ spectroscopy and simultaneous activity measurements under catalytic reaction conditions

Operando spectroscopy is a methodology that combines the spectroscopic characterization of a catalytic material during reaction with the simultaneous measurement of catalytic activity/selectivity. Operando spectroscopy is a new term that has been introduced into the catalysis literature at the beginning of the 21st century that underlines the importance of simultaneously combining fundamental spectroscopic surface phenomenon with catalytic performance, on the same sample, under realistic reaction conditions. The potential impact of operando spectroscopy on catalysis science lies in its ability to significantly assist in the establishment of fundamental molecular structure–activity/selectivity relationships for catalytic systems. It is critical that the design of the spectroscopic reaction cell, the catalytic reactor, also allows for the generation of catalytic performance data that is analogous to those achieved with conventional catalytic reactors. A case study of operando Raman-GC study is presented along with consideration of the methodology, spectroscopy and samples.

A general model of error-prone PCR

In this paper, we generalise a previously-described model of the error-prone polymerase chain reaction (PCR) reaction to conditions of arbitrarily variable amplification efficiency and initial population size. Generalisation of the model to these conditions improves the correspondence to observed and expected behaviours of PCR, and restricts the extent to which the model may explore sequence space for a prescribed set of parameters. Error-prone PCR in realistic reaction conditions is predicted to be less effective at generating grossly divergent sequences than the original model. The estimate of mutation rate per cycle by sampling sequences from an in vitro PCR experiment is correspondingly affected by the choice of model and parameters.

Fine tuning the “chiral sites” on solid enantioselective catalysts

A fundamental point in the mechanism of enantioselective hydrogenation over chirally modified metals is the nature of “chiral sites” developed by adsorption of the modifier on the metal surface. Despite considerable effort toward unraveling the adsorption mode of the modifier by surface science techniques, most of these spectroscopic measurements were done under conditions relatively far from those met under real reaction conditions. Here we applied a truly in situ “synthetic” approach, the systematic variation of the structure of the chiral modifier used for enantioselective hydrogenation over 5 wt% Pt/Al 2O 3. We have synthesized various O-alkyl, -aryl, and -silyl derivatives of cinchonidine (CD) and tested them in the enantioselective hydrogenation of ethyl pyruvate, ketopantolactone, 4,4,4-trifluoroacetoacetate, and 1,1,1-trifluoro-2,4-diketopentane. With increasing bulkiness of the ether group, the ee gradually decreased or even the opposite enantiomer formed in excess (up to 53% ee). We propose that the increasing bulkiness of the ether group prevents the strong, π-bonded adsorption of the quinoline ring of CD close to parallel to the Pt surface. In this tilted position the modifier adsorbs weaker via the quinoline N and also the position of the interacting function, the quinuclidine N, is shifted. This shift results in a different shape and size of the “chiral pocket” available for adsorption of the activated ketone substrate. The weaker adsorption of the bulky ether derivatives was proved by UV–vis spectroscopy and by the nonlinear behavior of modifier mixtures. The tilted adsorption mode was corroborated by the lower hydrogenation rate of the quinoline ring of the ether derivatives, relative to that of CD.

Towards real-time spectroscopic process control for the dehydrogenation of propane over supported chromium oxide catalysts

UV–Vis and Raman spectroscopy are applied under realistic reaction conditions to investigate the changes occurring to an industrial-like supported chromium oxide catalyst during the dehydrogenation of propane. Linking the catalytic activity to the simultaneously collected spectroscopic data has shown that coke is a key parameter to the activity of the catalyst. A small amount of coke improves its activity, most likely by an improved adsorption of propane on the catalyst, but the activity of the catalyst drops when the amount of coke exceeds a certain value. This point is visualized by a sharp increase in the overall absorbance of the sample as can be observed by UV–Vis spectroscopy and this information can be used as a signal for starting a regeneration cycle.

Styrene synthesis: in situ characterization and reactivity studies of unpromoted and potassium-promoted iron oxide model catalysts

Styrene synthesis over iron oxide model catalysts was studied by combining UHV characterization methods with in situ conversion measurements in a microflow reactor under realistic reaction conditions. Both unpromoted Fe 2O 3 and K-promoted model catalysts show a similar high starting activity while that of Fe 3O 4 is clearly lower. Water limits and K promotion slows down deactivation by coking and oxide reduction. The deactivation can be prevented and the high initial yield preserved by adding a small amount of oxygen to the feed. Both the presence of Fe 3+ and intermediate adsorption strength for ethylbenzene and styrene are essential for high conversion yields.

Competitive paths for methanol decomposition on Pt(111).

Periodic, self-consistent, Density Functional Theory (PW91-GGA) calculations are used to study competitive paths for the decomposition of methanol on Pt(111). Pathways proceeding through initial C-H and C-O bond scission events in methanol are considered, and the results are compared to data for a pathway proceeding through an initial O-H scission event [Greeley et al. J. Am. Chem. Soc. 2002, 124, 7193]. The DFT results suggest that methanol decomposition via CH(2)OH and either formaldehyde or HCOH intermediates is an energetically feasible pathway; O-H scission to CH(3)O, followed by sequential dehydrogenation, may be another realistic route. Microkinetic modeling based on the first-principles results shows that, under realistic reaction conditions, C-H scission in methanol is the initial decomposition step with the highest net rate. The elementary steps of all reaction pathways (with the exception of C-O scission) follow a linear correlation between the transition state and final state energies. Simulated HREELS spectra of the intermediates show good agreement with available experimental data, and HREELS spectra of experimentally elusive reaction intermediates are predicted.

Bis(mu-oxo)dicopper as key intermediate in the catalytic decomposition of nitric oxide.

The direct decomposition of NO into N2 and O2 is the simplest and most desirable approach to NO abatement. Although no catalysts are known that fulfill all criteria required for application, some materials show a high NO conversion and could be potential catalysts in the near future. 2] The copper-loaded ZSM5 zeolite is the most studied catalyst with a high and stable NO decomposition activity. Since its discovery in 1986, considerable research efforts have been made in order to unravel the reaction mechanism and the active sites in Cu-ZSM-5. A growing consensus is found that a redox process cycling between Cu ions and Cu -ELO species is present (ELO extra lattice oxygen). 4±6] However, the nature of the Cu -ELO species has so far not unequivocally been identified. 4±7] Here, we report on the elucidation of this catalysis by identifying the bis( -oxo)dicopper species as the key intermediate that allows the smooth formation and desorption of O2. The detection of the bis( -oxo)dicopper species and its role was made possible by in situ spectroscopy, that is, under real reaction conditions. Firstly, UV/Vis monitoring of the catalyst using optical fiber technology was combined with on-line GC analysis. Secondly, direct structural information of the catalyst under working conditions was obtained with XAS (X-ray absorption spectroscopy). 11] So far, the bis( -oxo)dicopper core had only been characterized in homogeneous, synthetic complexes. Its isomerized ( 2 : -peroxo)dicopper core is present in natural actin membrane skeleton mesh. In addition, the raft may keep the Lck concentrations high at and near the TCR cluster. Lck may partition into the TCR-cluster raft and/or the rafts with Lck may coalesce with those containing the TCR cluster, which would raise the Lck concentration in the neighborhood of the TCR cluster. These possibilities are now under investigation in our laboratories.

Determining the active site in a catalytic process: Operando spectroscopy is more than a buzzword

Determining the active site in a catalyst material and elucidating the related reaction mechanism remain intellectual challenges and are of paramount importance for the rational development of new or better catalysts. Identifying active sites should in principle be possible by studying the catalyst at its working place under real reaction conditions. This is the field of operando spectroscopy, a research area, which is under intense development and of high interest to the catalysis community. This feature article presents an attempt of a roadmap for future operando spectroscopy studies. Two case studies will illustrate the potential and limitations of the operando approach. A first study deals with operando vibrational spectroscopy to obtain mechanistic insight in the destruction of chlorinated hydrocarbons with steam over lanthanide oxide-based catalysts. The second case study elaborates on the use of operando electronic spectroscopy to elucidate the active alkane dehydrogenation site in supported chromium oxide catalysts.

A comparative study of Pt/γ-Al2O3, Au/α-Fe2O3 and CuO–CeO2 catalysts for the selective oxidation of carbon monoxide in excess hydrogen

Three different catalysts, namely Pt/γ-Al2O3, Au/α-Fe2O3 and CuO–CeO2 were prepared (by wet impregnation, coprecipitation and a sol–gel method, respectively) and their catalytic performance for the selective oxidation of carbon monoxide in the presence of excess hydrogen was evaluated and compared. The effects of the presence of CO2 and H2O in the reactant feed on the activity and selectivity of these catalysts, as well as their stability under realistic reaction conditions were also investigated.Regardless of whether CO2 or both CO2 and H2O are present in the reactant feed, the Au/α-Fe2O3 catalyst is superior to the other two for the selective CO oxidation at relatively low reaction temperatures (<80–120°C, depending on contact time and feed composition employed), while at higher reaction temperatures, best results are obtained with the CuO–CeO2 catalyst, which proved to be more active and remarkably more selective than the Pt/γ-Al2O3 catalyst. The Au/α-Fe2O3 catalyst was the most sensitive, while the Pt/γ-Al2O3 the most resistant towards deactivation caused by the presence of CO2 and H2O in the feed. Finally, while the Au/α-Fe2O3 catalyst lost a considerable portion of its activity during the first 80h under reaction conditions, the CuO–CeO2 and Pt/γ-Al2O3 catalysts exhibited a stable catalytic performance, at least during the time period tested in this work (7–8 days).

Towards in situ Raman Microscopy of Single Catalytic Sites

We describe analytical techniques that allow an observation of heterogeneously catalyzed reactions, close to realistic chemical reaction conditions, with a spatial resolution down to 100 nm. Vibrational spectroscopy and scanning-force microscopy are combined to accomplish this goal. Raman microscopy is used for the investigation of the structural changes during heterogeneous catalytic reaction. This results already in a resolution of 1–2 μm, while in combination with scanning near-field optical microscopy (SNOM), a spatial resolution of even 100 nm has been achieved. Surface-enhanced Raman scattering (SERS) improves the sensitivity of Raman spectroscopy by many orders of magnitude and thus allows the study of a much broader range of reactions. As a model reaction system, the Pd catalyzed hydrogenation of benzene to cyclohexane was chosen. Spectra measured under reactive conditions differ significantly from reference spectra of the reactants and products measured under nonreactive conditions. This is explained by a structural change of the adsorbates on the activated surface but also by the possible formation of reaction intermediates.

Following the hydrothermal crystallisation of zeolites using time-resolved in situ powder neutron diffraction

A specially designed hydrothermal cell has been used for the study of the crystallisation of some zeolites from amorphous aluminosilicate gels by time-resolved in situ neutron diffraction. The effect of sodium hydroxide concentration and temperature has been investigated. Following the reactions in situ has allowed the stability of zeolite phases to be monitored directly and efficiently under real reaction conditions. At the lowest sodium hydroxide concentrations used and at the lowest temperature applied (80°C) sodium zeolite A is the favoured product and after a period in excess of 10 h is the sole product. As the sodium hydroxide concentration or temperature is increased the initially formed sodium zeolite A is replaced by hydroxosodalite. At 140°C hydroxosodalite crystallises directly from the amorphous gel. By determining the changing areas of Bragg reflections with time, crystallisation curves can be obtained. The advantages of the in situ powder neutron diffraction technique over analogous X-ray powder diffraction methods are discussed.

Investigation of the formation of the Bi-2223 phase in multifilamentary Bi-2223/Ag tapes by in situ high temperature neutron diffraction

Results of in-situ diffraction experiments in the investigation of multifilamentary Bi-2223/Ag tape superconductors with neutron radiation are reported. The aim is to characterise the formation reaction of the Bi-2223 phase and the changes in the secondary phases assemblage under real reaction conditions without removing of the silver sheath of the composite. Furthermore, the mechanism of the evolution of a fibre texture of the superconducting phases Bi-2212 and Bi-2223 are discussed. Additionally, the results of the in-situ diffraction experiments will be related to the characterisation of the microstructure of quenched specimens done by scanning electron microscopy. Finally, a reaction model of the formation of the Bi-2223 phase within the composite conductor will be discussed.

Selective catalytic reduction of NO x over zeolite-coated structured catalyst packings

In this work, the performance of two configurations of in situ zeolite-coated structured catalyst packings is treated. Just to demonstrate the reactor concepts, the catalyst packings were tested on a bench scale in the selective catalytic reduction of NO with excess ammonia. The performance of Cu-ZSM-5-coated ceramic slabs that were placed in a parallel passage array (PPR) was compared with a Cu-ZSM-5 monolith. Practical correlations are given for pressure drop, diffusion and the overall performance of the reactor. According to a simple model, the catalyst effectiveness factor of the PPR turned out to be 17%, whereas that of the monolith was 95%. The plate thickness and the spacing of the parallel plates proved to be of paramount importance for optimum catalyst efficiency. For practical evaluation and comparison with known NO x reduction units, more realistic reaction conditions should be chosen.

Semiempirical molecular dynamics studies of C60/C70 fullerene oxides: C60O, C60O2 and C70O

The spectral analysis indicates that all isomers of C60O, C70O and C60O2 have an epoxide-like structure (an oxygen atom bridging across a C–C bond). According to the geometrical structure analysis, there are two isomers of fullerene monoxide C60O (the 5,6 bond and the 6,6 bond), eight isomers of fullerene monoxide C70O and eight isomers of fullerene dioxide C60O2. In order to simulate the real reaction conditions at 300K, the calculation of the different isomers of C60O, C60O2 and C70O fullerene oxides was carried out using the semiempirical molecular dynamics method with two different approaches: (a) consideration of the geometries and thermodynamic stabilities, and (b) consideration of the ozonolysis mechanism. According to the semiempirical molecular dynamic calculation analysis, the probable product of this ozonolysis reaction is C60O with oxygen bridging over the 6–6 bond (C2v). The most probable product in this reaction contains oxygen bridging across in the upper part of C70 (6–6 bond in C70O-2 or C70O-4) an epoxide-like structure. C60O2-1, C60O2-3 and C60O2-5 are the most probable products for the fullerene dioxides. All of these reaction products are consistent with the experimental results. It is confirmed that the calculation results with the semiempirical molecular dynamics method are close to the experimental work. The semiempirical molecular dynamics method can offer both the reaction temperature effect by molecular dynamics and electronic structure, dipole moment by quantum chemistry calculation.

Designer Nanostructures Of Catalysts in the Environmental-HREM

Abstract Major advances in the field of in situ environmental high resolution electron microscopy (EHREM) probe selective oxidation catalysis directly on the atomic scale. Dynamic gas-solid surface interactions are studied in real-time and under realistic reaction conditions to unravel atomic level insights into active sites and structure-property relationships in vital chemical and technological processes [1-3]. The recent developments include a pioneering approach with the controlled environmental cell (ECELL) facilities permanently mounted inside the EHREM [2]. Accessories have been added for simultaneous structural and compositional analyses of the reactor contents in real-time, and using atomic resolution imaging with transmission electron diffraction and parallel electron energy loss spectroscopy (PEELS). We are now developing innovative experimental methods that include very high temperature studies and combining in the same instrument facilities for both EHREM and environmental-SEM (ESEM), (EHREM-ESEM), with attractive possibilities for studying the science of selective oxidation catalysis. Alkane Catalysis, Chlorofluorocarbons and Nanotubes: In the domain of transition metal based oxides, discoveries of fundamental mechanisms underlying selective catalyzation have come from EHREM studies.

High-pressure soft X-ray absorption spectroscopy: application of a new in situ spectroscopic method in catalysis research

In situ investigations under real reaction conditions are necessary since the structure of catalysts can be very different from that found post mortem or ex situ. The object of this research was to demonstrate the use of an X-ray absorption spectroscopy (XAS) detector system that is suitable for in situ investigations in the soft X-ray range of 250≤ hν≤1000 eV under gas/solid reaction conditions close to those used in practice. As an example, the conversion reaction of methanol over copper catalysts is studied under reaction conditions ( T≥500 K, p≥0.5 mbar). A novel atomic oxygen species is found under reaction conditions characterized by chemical functionality that is significantly different from copper oxide surfaces found in the ex situ spectroscopic analysis which contain oxygen atoms interacting strongly with copper 3d states.