Search

Recent analysis revealed that a pseudohomogeneous model of a uniformly active, adiabatic packed bed reactor can predict formation of stable hot zones in the cross-section of the reactor if the kinetic rate expression can lead to isothermal rate oscillations. This prediction was confirmed by simulations of CO oxidation. We show that hot zone formation in a shallow reactor can be predicted also for C2H4 hydrogenation, the kinetic model of which is structurally different from that of CO oxidation. Qualitatively different spatiotemporal temperature patterns may form under the same operating conditions. Their number increases as the reactor diameter is increased. An increase in the reactor diameter increases the time constant of the transversal heat dispersion and decreases the temperature synchronization among points on the same reactor cross-section. The interaction and conjugation among qualitatively different moving temperature patterns can lead to formation of complex motions. [ABSTRACT FROM AUTHOR]

Transversal hot zones have been observed in industrial and laboratory packed-bed reactors. Yet, previous modeling attempts failed to predict them without making the unrealistic assumption that the transversal heat dispersion exceeds that of the reactants. It is shown that the formation of transversal hot zones in a uniformly active catalytic reactor is strongly dependent on the reaction kinetics. For example, transversal spatiotemporal concentration and temperature patterns can be predicted to form in a shallow adiabatic packed-bed reactor using realistic parameters for a catalytic reaction, the rate of which may oscillate under constant ambient conditions. Various experimentally tested rate expressions, such as those describing the oxidation of CO, exhibit this feature. A large number of different types of stable, transversal patterns may form for a sufficiently large reactor diameter, most of which do not exhibit azimuthal symmetry. Surprisingly, the time-averaged effluent reactant concentration and the period of the different spatiotemporal patterns are rather similar and close to those obtained under a uniform oscillating state. A systematic procedure is presented for finding the initial conditions leading to formation of the different patterns. Numerical simulations show that, because of their homoclinic features, the spatiotemporal patterns have a long period (order of hours), in agreement with various reported laboratory experiments. [ABSTRACT FROM AUTHOR]

Cobalt ferrite CoFe2 O4 crystalline nanoparticles (50-100 nm) were produced by carbon combustion synthesis of oxides (CCSO). In this combustion synthesis process, the exothermic oxidation of carbon generates a thermal reaction wave that propagates through the solid reactants mixture of CoO and Fe2 O3 converting it to cobalt ferrite. The extensive emission of CO2 increased the porosity and friability of the product. The quenching front method combined with XRD and VSM characterization revealed that crystalline CoFe2 O4 particles formed in the early stage of the combustion, before the temperature reached its maximum. The maximum value of the coercivity of the quenched product within the front region was 940 Oe with a magnetization of 15 emu/g. The as-synthesized ferrites had hard magnetic properties with coercivity of 700 Oe and saturation magnetization of up to 47 emu/g. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Simulations of the oscillatory and steady-state behavior in CO oxidation on Pd(100) at atmospheric pressure has been done. • Available experimental results can be explained on the basis of the Langmuir-Hinshelwood mechanism. • The correlation between the TOF increase and oxide appearance does not prove that oxidized sites are catalytically active. • A partially oxidized catalyst may have practically the same reactivity as a completely non-oxidized catalyst. Abstract This article is a theoretical and numerical analysis of experimental results concerning the oscillatory and steady-state behavior during CO oxidation on Pd(100) and Pt(110) catalysts in continuous flow and batch reactors at near atmospheric pressure conditions. In the literature there is disagreement about which mechanism describes the reaction of CO oxidation. Some authors suggested that the oxidized phase is more active than the metallic phase with chemisorbed oxygen and that the Mars-van Krevelen mechanism and not the traditional Langmuir-Hinshelwood mechanism is required to describe the catalytic reaction. To study this disagreement in data interpretation, we apply a slight modification of a well-known kinetic model suggested by Sales, Turner, and Maple, which is based on the Langmuir-Hinshelwood mechanism. We show that in a high-activity state the calculated concentration of the oxide can exceed 0.9 ML. However, this does not mean that the oxide phase is more active than the metallic phase, because oxide-free sites of the partially oxidized catalyst may be responsible for the observed reaction rate. Available experimental results, including oscillatory behavior, can be explained on the basis of the Langmuir-Hinshelwood mechanism. We conclude that there is no need to introduce the Mars-van Krevelen mechanism to model CO oxidation on Pt-group metals. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Low dimensional model of hydrocarbon trap developed. • Model predicts observed oscillations and spatial effects. • Validation for the coupling mechanism between reaction and sorption. • Map developed shows when oscillations are expected. Abstract Dodecane oxidation on zeolite beta (BEA) was studied on a diesel oxidation catalyst (DOC) comprising Pt-Pd/Al 2 O 3. Oscillatory CO 2 formation under steady-state and transient conditions was studied by spatially-resolved mass spectrometry (SpaciMS) and coherent optical frequency domain reflectometry (c-OFDR) (Peng et al., 2018). The CO 2 oscillations were accompanied by local temperature fluctuations during the steady state and temperature-programmed oxidation. The coupling between hydrocarbon sorption and oxidation, the proposed underlying mechanism for the oscillatory behavior, is investigated using a monolith reactor model. Using independently measured dodecane oxidation and sorption kinetics, the model predicts most of the experimental features, including the existence and amplitude of the oscillations. The model also correctly predicts the existence of an observed hysteresis during temperature ramp-up/ramp-down experiments. A non-dimensional map provides a predictive guide for the existence of oscillations. In addition to providing understanding and prediction of the spatiotemporal phenomena, the model analysis suggests a way to decrease the hydrocarbon slip (emissions) during temperature ramp-down. [ABSTRACT FROM AUTHOR]

A diesel oxidation catalyst (DOC) comprising an alumina-based washcoat of Platinum (Pt) and Palladium (Pd) is used to catalyze the oxidation of a diesel exhaust containing hydrocarbons (HCs) with sufficiently high conversion. A large-pore zeolite such as Beta (BEA) is added to the DOC to trap diesel exhaust hydrocarbons during the cold start. There is a need to gain insight on how BEA affects the performance of the HC light-off and of the transient HC oxidation. We conducted steady-state and transient trapping experiments involving the n-dodecane in order to enable improved trap design and operating strategy. Simultaneous spatio-temporal measurements of concentration and temperature, respectively by spatially-resolved mass spectrometry and coherent optical frequency domain reflectometer, provide detailed insight into the reactor dynamics. Steady-state experiments reveal unexpected periodic oscillations of CO 2 and temperature in a feed temperature range (142–181 °C). The oscillatory amplitude increases with decreasing space velocity. Temperature programmed oxidation experiments reveal unusual multi-peak CO 2 generation and temperature excursion behavior. A lower ramp-rate increases the number of the additional peaks. In addition, at a low heating/cooling rate of ±1 °C/min, a counter-clockwise hysteresis behavior exists. The oscillations and multi-peak behavior indicates coupling between HC trapping and oxidation. Analysis of the behavior during the hysteresis suggests that HC trapping and oxidation dominate in different regions along the length of the monolith. Since the downstream is much hotter than the upstream, the HC is re-trapped by the upstream BEA and is oxidized downstream. This study provides insight into the spatiotemporal features of the transient HC trapping and oxidation of BEA/DOC catalysts. To our knowledge, it is the first report of oscillations in a hydrocarbon trap. [ABSTRACT FROM AUTHOR]

A global kinetic model is used to study the light-off behavior of a mixture of CO and HCs ( C 2 H 4 , C 7 H 8 , C 6 H 14 & C 2 H 6 ) on Pt, Pd and combined catalysts. The light-off curves are clustered around 550 K on Pt whereas species light-off temperatures range from 400–600 K on Pd. The self poisoning of CO is more pronounced on Pt than on Pd. As long chain HCs such as hexane have a better light-off performance on Pt than on Pd, it is beneficial to use dual layer or dual brick catalysts for optimising HC conversion on a C1 basis. Dual brick catalyst with Pd upstream showed superior performance compared to the configuration with Pd downstream. Pt in 50% of washcoat and Pd in the remaining 50% of washcoat has a better overall light-off performance than other fractions of Pt, Pd. It is found that the light-off behavior of all the species is nearly same for both dual layer and dual brick configurations at low space velocities. However, the strong diffusional limitations in the washcoat at high space velocities result in improved light-off performance of dual layer configuration. The more practical C1 basis conversion, which is the weighted average conversion of HCs, is investigated for different configurations. It is also shown that the exotherm generated by the oxidation of CO can be used to improve the C 1 basis conversion of HCs. [ABSTRACT FROM AUTHOR]

A global kinetic model is developed for the oxidation of mixtures of CO, C 2 H 4 (ethylene), C 7 H 8 (toluene), C 6 H 14 (hexane), and C 2 H 6 (ethane) over Pt-Pd/γ-Al 2 O 3 diesel oxidation catalysts with different Pt/Pd molar ratios (100:0, 75:25, 50:50, 25:75, 0:100). The kinetic model is based on bench flow reactor data obtained by temperature-programmed oxidation, ramping up the feed gas temperature at a fixed rate of 8 °C/min. The kinetic model is developed in stages starting with pure CO oxidation, followed by oxidation of the individual hydrocarbons (HCs) and then mixtures over all Pt/Pd ratios. Examination of the inhibition terms containing the effect of CO, HCs, and NO shows that CO has the most dominant inhibition effect on light-off over the range of conditions investigated. The kinetic model is validated with experimental data at different feed concentrations and is found to simulate the light-off behavior for all catalyst compositions with acceptable accuracy. Comparison of the light-off temperatures and trend investigation for kinetic parameters of CO, C 2 H 4 , and C 7 H 8 in the mixture over all Pt/Pd ratios is also presented. Considering trade-offs between having a low light-off temperature, low deactivation impact on the catalyst, and higher conversion of HCs within the temperature domain of interest, the Pt/Pd (1:1) molar ratio is found to be the optimal catalyst under the conditions examined. [ABSTRACT FROM AUTHOR]

The impact of the capillary probe of a spatially-resolved mass spectrometer system (SpaciMS) on the measured reactant conversion is reported using propylene oxidation over Pt/Al 2 O 3 washcoated monoliths. The findings suggest that the invasive nature of SpaciMS depends on its configuration and application. Using monoliths with a range of cell densities (100–600 cells per square inch, CPSI), the concentration profiles of propylene sampled with probes of two different outer diameters (170 and 363 μm) are compared with the temperature measured using coherent optical frequency domain reflectometry (c-OFDR). The comparison indicates that flow blockage has a negligible effect if the limiting propylene is depleted in the downstream reactor section. Suction by the probe compensates for the blockage for certain combinations of the channel diameter and probe size. In such cases the profile measured by a probe with the larger outer diameter (363 μm) is similar to that measured by a smaller capillary (170 μm). The experiments reveal that the axial position of the probe does not influence the flow profile in a 100 CPSI monolith channel, nor does it affect the amount of flow deflected to surrounding channels for a 600 CPSI monolith. Under some conditions the results are impacted by transverse concentration gradients. Their existence complicates the interpretation of the SpaciMS data. The difference between the location of propylene depletion and temperature maximum provides a useful metric for capturing the collective impacts of flow deflection and transverse gradients. The complexity of the flow, transport and reaction suggests that at least three dimensionless groups are needed to bound the operating regions in which the presence of the probe has minimal impact on the measured concentration. [ABSTRACT FROM AUTHOR]

Experiments were conducted to determine the impact of key operating variables (ceria loading, space velocity, cycle time, and rich pulse intensity) on the oxygen storage and release process of a Pt/Rh/CeO 2 /BaO monolithic catalyst during periodic lean (oxygen)–rich (propylene) operation. The concentrations were measured by spatially-resolved capillary inlet mass spectrometry (SpaciMS), and the temperature profile was measured by coherent optical frequency domain reflectometry (c-OFDR), providing detailed insight into the spatio-temporal features of the reaction system. The experiments revealed that the addition of ceria increased the breakthrough time of the propylene during a lean-to-rich transition due to an increased oxidation rate. Hydrogen was formed in the upstream and was consumed in the downstream section of the catalyst by reaction with ceria. Increasing the space velocity increased the upstream hydrogen formation rate and decreased its downstream oxidation rate. Increasing the lean and rich durations (while keeping their duration ratio constant) increased the oxygen uptake but resulted in propylene breakthrough and decreased the solid temperature. Increasing the rich period (at a fixed rich feed and total cycle time) allowed the catalyst to release more oxygen. A comparison of the ceria-containing catalyst to one without ceria revealed a more efficient complete oxidation for the former, while a comparison of the periodic to stationary operation revealed that less than half the propylene was converted to CO 2 by the former. [ABSTRACT FROM AUTHOR]

Highlights: [•] New method of measuring internal catalytic reactor temperature. [•] Method achieves±3mm precision and sampling frequency of 0.9Hz. [•] Method applied to transient exothermic catalytic reaction in a monolith channel. [•] Method demonstrates wrong-way temperature rise for Pt-catalyzed NH3 oxidation. [•] Temperature rise due to feed temperature decrease exceeds that due to flow increase. [ABSTRACT FROM AUTHOR]

• NOx storage functionality is essential for lean NOx reduction with lean/rich cycling. • Ceria redox pathway only has a secondary effect on NOx conversion under excess O 2. • Promotion by fast cycling is due to better usage of NOx and oxygen storage sites. • Excess O 2 enhances or inhibits NOx reduction on Pt/CeO 2 /Al 2 O 3 below or above 400 °C. • C 3 H 6 enhances NOx reduction on Pt/CeO 2 /Al 2 O 3 in stoichiometric case with fast cycling. NOx reduction under net lean and near-stoichiometric conditions was carried out on Pt/Al 2 O 3 , Pt/CeO 2 /Al 2 O 3 and Pt/BaO/Al 2 O 3 washcoated monoliths to compare performance features and identify reaction pathways. The impact of the storage components (BaO, CeO 2) on the NOx conversion and byproduct (NH 3 and N 2 O) yields was quantified for a range of feed temperatures, reductant types (H 2 , and C 3 H 6), O 2 feed concentrations, and cycle times. The NOx storage functionality is essential for NOx reduction under net lean conditions while the oxygen storage functionality promotes NOx reduction for near-stoichiometric conditions. NOx conversion by H 2 under lean conditions is dependent on the NOx storage capacity of the catalyst, with Pt/CeO 2 /Al 2 O 3 and Pt/BaO/Al 2 O 3 exhibiting the highest NOx conversion below and above 300 °C, respectively. High NOx conversion is achieved over Pt/CeO 2 /Al 2 O 3 for anaerobic rich feeds at temperatures above 400 °C. Increasing the O 2 feed concentration enhances NOx conversion over Pt/CeO 2 /Al 2 O 3 below 400 °C but inhibits NOx conversion above 400 °C. The former is attributed to promotion of NO oxidation leading to NOx storage while the latter is attributed to O 2 inhibition of NO decomposition/reduction. Shorter cycle times increase the NOx conversion with C 3 H 6 as reductant over Pt/BaO/Al 2 O 3 under lean conditions and over PCA for the near-stoichiometric feed. The findings confirm that improved NOx storage utilization is mainly responsible for NOx conversion enhancement. A ceria redox pathway has only a secondary effect on NOx conversion under excess O 2. [ABSTRACT FROM AUTHOR]