Jhbj Jozef Hoebink

An investigation on the reaction mechanism for the partial oxidation of methane to synthesis gas over platinum

The partial oxidation of methane to synthesis gas has been investigated by admitting pulses of pure methane, pure oxygen and mixtures of methane and oxygen to platinum sponge at temperatures ranging from 973 to 1073 K. On reduced platinum the decomposition of methane results in the formation of surface carbon and hydrogen. No deposition of carbon occurs during the interaction of methane with a partly oxidised catalyst. Oxygen is present in three different forms under the conditions studied: platinum oxide, dissolved oxygen and chemisorbed oxygen species. Carbon monoxide and hydrogen are produced directly from methane via oxygen present as platinum oxide. Activation of methane involving dissolved oxygen provides a parallel route to carbon dioxide and water. Both platinum oxide and chemisorbed oxygen species are involved in the oxidation of carbon monoxide and hydrogen. In the presence of both methane and dioxygen at a stoichiometric feed ratio the dominant pathways are the direct formation of CO and H2 followed by their consecutive oxidation. A Mars-van Krevelen redox cycle is postulated for the partial oxidation of methane: the oxidation of methane is accompanied by the reduction of platinum oxide, which is reoxidised by incorporation of dioxygen into the catalyst.

A Mechanistic Study of the Fischer–Tropsch Synthesis Using Transient Isotopic Tracing. Part-1: Model Identification and Discrimination

A steady state isotopic transient kinetic analysis (SSITKA) of the Fischer–Tropsch synthesis over Co/Ru/TiO2 catalyst is reported by using 13CO and D2. Besides a qualitative interpretation of the transients, model identification and discrimination are mainly based on numerical modeling. From this, it is concluded that two single-C species are present on the catalyst surface, Cα,ads and Cβ,ads. These species both participate in the formation of methane and of higher hydrocarbons. The heterogeneity of the catalyst surface is limited to these two single-C species. Only one type of chain-growth site is present in a low concentration compared to the surface concentrations of COads, Cα,ads, and Cβ,ads. The H-content of Cα,ads and Cβ,ads is assessed by comparing the simulated transient for the incorporation of the D-labeling into methane with experiments. In case the stepwise hydrogenations of Cads to CH4 are irreversible, Cα,ads and Cβ,ads are H-free species. In case these reactions are reversible, the H-exchange between Cads, CHads, CH2,ads, and CH3,ads is fast compared to the net formation of methane, and the H-content of Cα,ads and Cβ,ads cannot be assessed.The most probable mechanism for the Fischer–Tropsch reaction resulting from this study is used in a next paper (Part 2) for the quantification of the kinetic parameters. In that paper, a systematic analysis of these parameters yields detailed mechanistic insight into the Fischer–Tropsch synthesis reaction.

Acetylene and carbon monoxide oxidation over a Pt/Rh/CeO2/γ-Al2O3 automotive exhaust gas catalyst : kinetic modelling of transient experiments

The transient kinetics of acetylene (C2H2) conversion by oxygen over a commercial Pt/Rh/CeO2/γ-Al2O3 three-way catalyst have been modelled. Experiments to validate the model were carried out in a fixed-bed reactor with two separate inlets, enabling alternate feeding of acetylene and oxygen. Frequencies of feed composition cycling up to were applied. The experimental conditions resemble the cold-start period of an Otto engine. Two types of adsorbed acetylene species seem to exist. A selective catalyst deactivation for oxygen adsorption, due to deposition of carbonaceous deposits, was found. Ceria proved to have a significant influence on the acetylene oxidation. A kinetic model was developed for the conversion of acetylene to carbon monoxide, based on elementary reaction steps. This model was combined with the published kinetics for transient carbon monoxide oxidation to carbon dioxide over the same catalyst (Nibbelke, R. H., Chapman, M. A. J., Hoebink, J. H. B. J., & Marin, G. B. (1997). Kinetic study of the CO oxidation over Pt/V-Al2O3 and Pt/Rh/ CeO2/γ−Al2O3 in the presence of H2O and CO2. Journal of Catalysis, 171, 358–373.), in order to describe the total oxidation of acetylene quantitatively. The combined model is able to describe the results of the transient experiments on simultaneous acetylene and carbon monoxide oxidation. During a transient, both acetylene and carbon monoxide react mainly in a front moving through the reactor, carbon monoxide hardly influencing the acetylene oxidation.

Steady-state isotopic transient kinetic analysis of the Fischer–Tropsch synthesis reaction over cobalt-based catalysts

The paper presents a transient kinetic analysis of the Fischer–Tropsch synthesis reaction using the SSITKA technique in combination with a gas-chromatograph-mass-spectrometer (GCMS) analysis of the 13C-labeled and 18O-labeled hydrocarbon and alcohol reaction products. Experiments are performed on a Co/Ru/TiO2 catalyst and a fully metallic Co-sponge model catalyst at 498K and 1.2 bar. The experimental results are discussed in a qualitative way to obtain mechanistic information. The Co-sponge catalyst is used to study alcohol formation, since the TiO2 support disturbs the measurements on the Co/Ru/TiO2 catalyst. The formation of hydrocarbons proceeds via a two-pool mechanism, where two carbon pools contribute to methane formation and C–C coupling. Paraffins and olefins are both primary products, but the GCMS analysis demonstrates that readsorption of 1-olefins is an important step. The readsorption of iso- and 2-olefins is shown to be of less importance. The steady-state performance of the catalyst indicates the presence of a physisorbed hydrocarbon layer, even under process conditions where no wax build-up in the catalyst pores occurs. Although the Anderson–Schulz–Flory distribution gives rise to assume chain-length independence of the surface reactions starting at C3, this does not hold when discrimination between the paraffins and the olefins is made. Alcohol formation can be considered as a termination reaction that occurs via a CO insertion or a CHxO insertion mechanism.

Fixed bed reactor design for gas phase chain reactions catalysed by solids: the oxidative coupling of methane

Abstract A multitubular catalytic fixed bed reactor for the oxidative coupling of methane is designed. A heterogeneous two-dimensional reactor model taking into account axial dispersion of mass and enthalpy is applied. A reaction network consisting of 36 gas phase radical reactions and 10 catalytic reactions, based upon experiments with an Sn/Li/MgO catalyst at atmospheric pressure, temperatures of 923–1,023 K and methane-to-oxygen ratios of 2–12, is used. Intrapellet concentration profiles of molecules and, even more so, of radicals are shown to affect the selectivity towards ethane and ethylene. The importance of homogeneous reactions in the void space between the catalyst pellets is highlighted. The reactor dimensions are determined by heat removal considerations.

On a model-based control of a three-way catalytic converter

Though very important for the system performance, the dynamic behavior of the catalytic converter has mainly been neglected in the design of exhaust emission control systems. Since the major dynamic effects stem from the oxygen storage capabilities of the catalytic converter, a novel model-based control scheme, with the explicit control of the converter’s oxygen storage level is proposed. The controlled variable cannot be measured, so it has to be predicted by an on-line running model (inferential sensor). The model accuracy and adaptability are therefore crucial. A simple algorithm for the model parameter identification is developed. All tests are performed on a previously developed first principle model of the catalytic converter so that the controller effectiveness and performance can clearly be observed.

A mechanistic study of the Fischer-Tropsch synthesis using transient isotopic tracing. Part 2: Model quantification

A quantitative mechanistic model for the low-pressure Fischer–Tropsch synthesis reaction on a Co/Ru/TiO2 catalyst is presented. Although the Fischer–Tropsch synthesis is operated at dry conditions, the presence of a physisorbed state is essential in the mechanism. The monolayer coverage of the C3 to C20 hydrocarbons in the physisorbed state is low at 0.3%. The most abundant chemisorbed surface species are COads and two single-C species, Cα,ads and Cβ,ads. The fractional surface coverage of growing hydrocarbon chains is low at 1.4%. With increasing H2/CO feed ratio, the surface concentrations of Hads and free sites increase. The rate coefficient for chain initiation is one order of magnitude lower than that for chain growth. The rate coefficient of ethene readsorption is one order of magnitude higher than that for the readsorption of higher 1-olefins. Chain branching and bond shift are important secondary reactions at atmospheric pressure, transforming reactive 1-olefins into unreactive internal and isoolefins and thus decreasing the asymptotic chain growth probability. As is to be expected, the termination to paraffin is represented by a hydrogenation reaction. The termination to olefin, however, appears to be a desorption reaction rather than a hydrogenation or a dehydrogenation reaction. The growing hydrocarbon chain is therefore represented by a CiH2i species.The quantitative mechanistic model as presented in this paper in combination with additional assumptions is used to successfully predict the characteristics of the product distribution of the high-pressure Fischer–Tropsch reaction.

A quantitative analysis of transient kinetic experiments: the oxidation of CO by O2 over Pt

Abstract Transient kinetic experiments allow to assess quantitatively the rate coefficients of individual elementary steps. Experiments were performed in the temperature range 300–400 K with temporal analysis of products (TAP) for CO oxidation over polycrystalline platinum, initially fully precovered with O adatoms, at varying degrees of CO and O coverage. Experimental evidence for the existence of islands was found. The results were described with an island model, that accounts for CO chemisorption from a precursor state, followed by either CO incorporation into CO islands or by a surface reaction with O adatoms from adjacent O islands. CO chemisorption was not activated and its rate coefficient was estimated as 40 s −1 , while the surface reaction was relatively fast. Indications for the size of the CO and O islands were found.

Competing reactions in three-way catalytic converters: modelling of the NOx conversion maximum in the light-off curves under net oxidising conditions.

Automobile exhaust gas conversion was simulated with a reactor model based on first principles. The monolithic reactor was modelled as adiabatically operating with a uniform flow distribution over the channels and with constant heat and mass transfer coefficients. The kinetic rate equations in the model were constructed from elementary step kinetics of the individual global reactions. The model predictions for light-off curves compare quite well with experimental data from the literature. Light-off is in the sequence hydrogen, carbon monoxide, propene, but the light-off temperatures do not differ very much. The nitric oxide conversion as function of the reactor feed temperature passes through a maximum at high hydrocarbon conversion, but does not reach the level of 50%, defined as light-off. Hydrogen is the major oxygen consumer in the front part of the reactor, while a slow reaction proceeds between CO and NO. At sufficiently high feed temperatures thermal reactor ignition occurs when the degree of CO surface coverage drops considerably. The corresponding increase of the O adatoms coverage causes increased reaction rates and a sudden temperature rise. Beyond the ignition point oxygen is mainly consumed by propene and by unconverted carbon monoxide. NO is reduced by unconverted hydrogen, while NO reduction by the hydrocarbon is not significant. Catalysts capable of increasing the NO surface coverage or the NO dissociation, or leading to a lower oxygen sticking coefficient would show a higher NO conversion maximum. The results indicate that well-known reaction mechanisms are capable to describe the behaviour of automotive exhaust gas converters, if mutual interactions of gaseous components and surface species are taken into account via elementary step kinetics.

NO reduction by CO over automotive exhaust gas catalysts in the presence of O2

The reduction of NO by CO in absence and presence of O2 has been investigated by transient experiments at automotive cold-start conditions over Pt/Rh/CeO2/γ-Al2O3, and derived model catalysts. A high-resolution magnetic sector mass spectrometer was used for distinguishing CO/N2 and CO2/N2O. Mechanistic comparisons are made between the catalyst formulations. A kinetic scheme of elementary reaction steps is proposed, which highlights the various contributions of the catalyst constituents.