Temporal Analysis Of Products Reactor Research Articles

Model-Based Design of Experiments for Temporal Analysis of Products (TAP): A Simulated Case Study in Oxidative Propane Dehydrogenation.

Temporal analysis of products (TAP) reactors enable experiments that probe numerous kinetic processes within a single set of experimental data through variations in pulse intensity, delay, or temperature. Selecting additional TAP experiments often involves an arbitrary selection of reaction conditions or the use of chemical intuition. To make experiment selection in TAP more robust, we explore the efficacy of model-based design of experiments (MBDoE) for precision in TAP reactor kinetic modeling. We successfully applied this approach to a case study of synthetic oxidative propane dehydrogenation (OPDH) that involves pulses of propane and oxygen. We found that experiments identified as optimal through the MBDoE for precision generally reduce parameter uncertainties to a higher degree than alternative experiments. The performance of MBDoE for model divergence was also explored for OPDH, with the relevant active sites (catalyst structure) being unknown. An experiment that maximized the divergence between the three proposed mechanisms was identified and provided evidence that improved the mechanism discrimination. However, reoptimization of kinetic parameters eliminated the ability to discriminate between models. The findings yield insight into the prospects and limitations of MBDoE for TAP and transient kinetic experiments.

Simplifying the Temporal Analysis of Products reactor

The Temporal Analysis of Products (TAP) experiment provides an unparalleled level of kinetic insight into heterogenous catalytic materials, but due to the complex and expensive instrumentation required, its application has been limited to a small group of dedicated researchers. Herein we demonstrate through a series of designs that precisely defined TAP experiments can be performed on systems far smaller and simpler than previously imagined. The pulse reactors described in this work utilise readily available components and so can be assembled, operated, and maintained with minimal training. Using the case study of CO oxidation over a Pt/SiO2 catalyst we show that precise kinetic, mechanistic, and surface composition information is feasible using a single-valve design. With the developments outlined in this work we aim to decrease the activation barrier to TAP and open up the technique to a new generation of researchers.

Quantifying the impact of temporal analysis of products reactor initial state uncertainties on kinetic parameters

AbstractThe temporal analysis of products (TAP) reactor provides a route to extract intrinsic kinetics from transient measurements. Current TAP uncertainty quantification only considers the experimental noise present in the outlet flow signal. Additional sources of uncertainty such as initial surface coverages, catalyst zone location, inert void fraction, gas pulse intensity, and pulse delay, are not included. For this reason, a framework for quantifying initial state uncertainties present in TAP experiments is presented and applied to a carbon monoxide oxidation case study. Two methods for quantifying these sources of uncertainty are introduced. The first utilizes initial state sensitivities to approximate the parameter variances and provide insights into the structural certainty of the model. The second generates parameter confidence distributions through an ensemble‐based sampling algorithm. The initial state covariance matrix can ultimately be merged with the experimental noise covariance matrix, providing a unified description of the parameter uncertainties for a TAP experiment.

Internal calibration of transient kinetic data via machine learning

The temporal analysis of products (TAP) reactor provides a vast amount of transient kinetic information that may be used to describe a variety of chemical features including residence time distributions, kinetic coefficients, number of active sites, reaction mechanism, etc. However, as with any measurement device, the TAP reactor signal is convoluted with noise and drift is common. To reduce the uncertainty of the kinetic measurement and any derived parameters or mechanisms, proper preprocessing must be performed prior to any advanced type of analysis. This preprocessing includes baseline correction, i.e., a shift in the voltage response, and calibration, i.e., a scaling of the flux response based on prior experiments. The traditional methodology of preprocessing requires significant user discretion and reliance on separate calibration experiments that may drift over time. Herein we use machine learning techniques combined with physical constraints to understand the noise and drift that is being generated within and between experiments for enhancement of the chemical kinetic signal. As such, the proposed methodology demonstrates clear benefits over the traditional preprocessing approach by eliminating the need for separate calibration experiments or heuristic input from the user.

Site-specific scaling relations observed during methanol-to-olefin conversion over ZSM-5 catalysts

The conversion of methanol over ZSM-5 catalysts was studied using step response and temperature-programmed desorption and surface reaction analyses in a temporal analysis of products reactor, as well as quasi-elastic neutron scattering and Fourier transform infrared investigations, buttressed by archived 13C magic angle spinning nuclear magnetic resonance studies. The results were combined with micro-kinetic models that simulated the formation of the first C–C bond and primary olefin(s) from methanol. Dimethyl ether was the major surface oxygenate and a source of surface methoxy species. Propylene, the major olefin produced from dimethyl ether, was formed with a reaction barrier of ∼ 200 kJ mol−1, in agreement with archived density functional theory calculations. Propylene could form from dimethyl ether via a methoxymethyl mechanism under intrinsic kinetic conditions. Site-specific scaling relations between the barriers to methyl propenyl ether and methoxy methyl species formation and dimethyl ether desorption were observed. The active sites of the ZSM-5 catalysts can be locally optimised and selectively tuned to improve their activity during the conversion of methanol to olefin(s).

Formation of Surface Impurities on Lithium-Nickel-Manganese-Cobalt Oxides in the Presence of CO2 and H2O.

Surface impurities involving parasitic reactions and gas evolution contribute to the degradation of high Ni content LiNixMnyCozO2 (NMC) cathode materials. The transient kinetic technique of temporal analysis of products (TAP), density functional theory, and infrared spectroscopy have been used to study the formation of surface impurities on varying nickel content NMC materials (NMC811, NMC622, NMC532, NMC433, NMC111) in the presence of CO2 and H2O. CO2 reactivity on a clean surface as characterized by CO2 conversion rate in the TAP reactor follows the order: NMC811 > NMC622 > NMC532 > NMC433 > NMC111. The capacity of CO2 uptake follows a different order: NMC532 > NMC433 > NMC622 > NMC811 > NMC111. Moisture pretreatment slows down the direct CO2 adsorption process and creates additional active sites for CO2 adsorption. Electronic structure calculations predict that the (012) surface is more reactive than the (1014) surface for CO2 and H2O adsorption. CO2 adsorption leading to carbonate formation is exothermic with formation of ion pairs. The average CO2 binding energies on the different materials follow the CO2 reactivity order. Water hydroxylates the (012) surface and surface OH groups favor bicarbonate formation. Water creates more active sites for CO2 adsorption on the (1014) surface due to hydrogen bonding. The composition of surface impurities formed in ambient air exposure is dependent on water concentration and the percentage of different crystal planes. Different surface reactivities suggest that battery performance degradation due to surface impurities can be mitigated by precise control of the dominant surfaces in NMC materials.

Influence of water vapor on the performance of Au/ZnO catalysts in methanol synthesis from CO2 and H2: A high-pressure kinetic and TAP reactor study

As part of an ongoing study on the performance and function of highly active Au/ZnO catalysts in the green synthesis of methanol, from H2 generated using renewable energies and CO2, we have systematically investigated the influence of water vapor on the reaction behavior. Water is a stoichiometric byproduct both in methanol formation and in the competing reverse water-gas shift (RWGS) reaction, and therefore its formation together with methanol formation is unavoidable. Changes in the reaction behavior, and possible physical or mechanistic reasons underlying these changes were addressed by high-pressure kinetic measurements, ex situ catalyst characterization and temporal analysis of products (TAP) reactor measurements. Already 2 % of water vapor led to a substantial but reversible lowering of both the methanol and the CO formation rate, and they decreased even more upon adding larger amounts. Possible reasons for this and consequences on the performance of Au/ZnO catalysts in this reaction are discussed.

Fundamental Aspects of Ceria Supported Au Catalysts Probed by In Situ/Operando Spectroscopy and TAP Reactor Studies.

The discovery of the activity of dispersed gold nanoparticles three decades ago paved the way for a new era in catalysis. The unusual behavior of these catalysts sparked many questions about their working mechanism. In particular, Au/CeO2 proved to be an efficient catalyst in several reactions such as CO oxidation, water gas shift, and CO2 reduction. Here, by employing findings from operando X‐ray absorption spectroscopy at the near and extended Au and Ce LIII energy edges, we focus on the fundamental aspects of highly active Au/CeO2 catalysts, mainly in the CO oxidation for understanding their complex structure‐reactivity relationship. These results were combined with findings from in situ diffuse reflectance FTIR and Raman spectroscopy, highlighting the changes of adlayer and ceria defects. For a comprehensive understanding, the spectroscopic findings will be supplemented by results of the dynamics of O2 activation obtained from Temporal Analysis of Products (TAP). Merging these results illuminates the complex relationship among the oxidation state, size of the Au nanoparticles, the redox properties of CeO2 support, and the dynamics of O2 activation.

TAPsolver: A Python package for the simulation and analysis of TAP reactor experiments

An open-source, Python-based Temporal Analysis of Products (TAP) reactor simulation and processing program is introduced. TAPsolver utilizes algorithmic differentiation for the calculation of highly accurate derivatives, which are used to perform sensitivity analyses and PDE-constrained optimization. The tool supports constraints to ensure thermodynamic consistency, which can lead to more accurate descriptions of microkinetic models. The mathematical and structural details of TAPsolver are outlined, as well as validation of the forward and inverse problems against well-defined prototype problems. Benchmarks of the code are presented, and a case study for extracting kinetic parameters that do not violate thermodynamics from experimental TAP measurements of CO oxidation on supported platinum particles is presented. TAPsolver will act as a foundation for future development and dissemination of TAP data processing techniques.

Probability theory for inverse diffusion: Extracting the transport/kinetic time-dependence from transient experiments

Detailed kinetic data is an essential component of process and catalyst development. Transient experiments can provide abundant kinetic information describing the evolution of the reaction rate with respect to the gas concentration, but require more complex analysis than steady-state experiments as they must contend with the separation of transport and kinetic information as a function of time. With the Temporal Analysis of Products (TAP) reactor and Fourier domain based transformations via the Y-Procedure, kinetic information may be separated from transport without any mechanistic assumptions, but there is a strong influence of experimental noise, Yablonsky et al. [1]. As an alternative, this work is focused on connecting steady-state reactor assumptions to the TAP experiment through the use of the residence time distribution. The proposed residence time distribution for the TAP reactor, and the following algorithm referred to as the G-Procedure, is shown to measure time-dependent rate and concentration information in the active zone of the reactor while being mechanistic and diffusional model free. We compare the differences of the Y- and G-procedure in the form of estimation of concentration and rate on synthetic and experimental data consisting of platinum oxidation and ammonia decomposition. The results show improved accuracy in the estimation of kinetic information using the G-Procedure as well as successfully maintaining an estimate of the reaction rate in the presence of large amounts of experimental noise. This is especially important in dealing with low-level response signals typical to that of minor products. Embodied in a probabilistic framework, we believe this methodology provides a useful tool for separating transport and kinetic processes in general and may be applied more broadly beyond the specialized configuration of the TAP reactor in the future.

Transient kinetic studies and microkinetic modeling of primary olefin formation from dimethyl ether over ZSM‐5 catalysts

AbstractThe formation of primary olefins from dimethyl ether (DME) was studied over ZSM‐5 catalysts at 300°C using a novel step response methodology in a temporal analysis of products (TAP) reactor. For the first time, the TAP reactor framework was used to conduct single‐ and multiple‐step response cycles of DME (balance argon) over a shallow bed with the continuous flow panel. Propylene is the major primary olefin and portrays an S‐shaped profile with a preceding induction period when it is not observed in the gas phase. Methanol and water portray overshoot profiles due to their different rates of generation and consumption. DME effluent shows a rapid rise halfway to its steady‐state value leading to a slow rise thereafter because of its high desorption rates followed by subsequent reactions involving DME in further steps during the induction period. To analyze the experimental data quantitatively, nine reaction schemes were compared, and kinetic parameters were obtained by solving a transient plug flow reactor model with coupled dispersion, convection, adsorption, desorption, and reaction steps. The methoxymethyl pathway involving dimethoxyethane and methyl propenyl ether gives the closest match to experimental data in agreement with recent density functional theory studies. Gaseous dispersion coefficients of ca. 10−9 m2 s−1 were obtained in the TAP reactor. The novel experimental data validated against the transient kinetic model suggests that after the formation of initial species, the bottleneck in propylene formation is the transformation of the initial C–C bond, that is dimethoxyethane formed initially from DME and methoxymethyl groups. DME adsorption on ZSM‐5 catalyst generates surface methoxy groups, which further react with the feed to give methoxymethyl groups. These methoxymethyl groups are regenerated through a series of reactions involving intermediates such as dimethoxymethane and methyl propenyl ether before propylene formation.

Dynamic changes of Au/ZnO catalysts during methanol synthesis: A model study by temporal analysis of products (TAP) and Zn LIII near Edge X-Ray absorption spectroscopy

Small gold nanoparticles supported on ZnO have been identified as highly active and selective catalysts for the green synthesis of methanol from CO2 and H2. Furthermore, they can serve also a model system for the mechanistic understanding of methanol synthesis on the industrial Cu/ZnO catalyst. The dynamic changes in the structure of Au/ZnO upon exposure to methanol synthesis gas mixtures were studied using a combination of TAP reactor and near edge X-ray absorption spectroscopy (XANES) measurements at the Zn LIII edge, both in CO2/H2 and CO/H2 gas mixtures. TAP measurements indicated that CO can create significant amounts of O-vacancy defects in ZnO at 240°C, while CO2 can re-oxidize a pre-reduced catalyst or maintain this state in the presence of strongly reducing gases (CO and H2). Furthermore, CO2 present as reactant or resulting from the reactive removal of surface lattice oxygen by interaction with CO can be deposited on the pre-reduced Au/ZnO surface as stable adsorbed carbon containing species, e.g., as surface carbonates, which decompose at T≥250°C. In situ XANES measurements at the Zn LIII edge revealed that ZnO is significantly reduced during reaction, both in CO2/H2 and CO/H2 gas mixtures, but with the extent of the reduction being more pronounced in CO/H2 than in CO2/H2. These results will be critically discussed in the light of previous findings on the role of ZnO reduction in the activity of methanol synthesis catalysts.

Oxygen Adsorption and Low-Temperature CO Oxidation on a Nanoporous Au Catalyst: Reaction Mechanism and Foreign Metal Effects

To further our understanding of the role of trace impurities of the second metal in the catalytic performance of unsupported, nanoporous Au (NPG) catalysts, in particular for the activation of O2, we have prepared a NPG catalyst by electrochemical leaching of Cu from a AuCu alloy and investigated its behavior in the CO oxidation reaction. The structural and chemical properties of the as-prepared catalyst as well as that after reaction for 1000 min were characterized by scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The nature of the surface oxygen species and the oxygen storage capacity were investigated and quantified by multi-pulse experiments in a temporal analysis of products (TAP) reactor. The catalytic behavior in the low-temperature CO oxidation reaction was evaluated both in a TAP reactor under dynamic vacuum conditions and in a conventional micro-reactor under atmospheric pressure. We discuss implications of these results and of similar data obtained previously on a Ag-containing NPG catalyst on the reaction mechanism and on the role of the second metal in the reaction and its impact on the reaction characteristics.

Mechanistic Insights into the Desorption of Methanol and Dimethyl Ether Over ZSM-5 Catalysts

The desorption of methanol and dimethyl ether has been studied over fresh and hydrocarbon-occluded ZSM-5 catalysts with Si/Al ratios of 25, 36 and 135 using a temporal analysis of products reactor. The catalysts were characterized by XRD, SEM, N2 physisorption and pyridine FT-IR. The crystal size increases with Si/Al ratio from 0.10 to 0.78 µm. The kinetic parameters were obtained using the Redhead method and a plug flow reactor model with coupled convection, adsorption and desorption steps. ZSM-5 catalysts with Si/Al ratios of 25 and 36 exhibit three adsorption sites (low, medium, and high temperature sites), while there is no difference between medium and high temperature sites at a Si/Al ratio of 135. Molecular adsorption on the low temperature site and dissociative adsorption on the medium and high temperature sites give a good match between experiment and the plug flow reactor model. The DME desorption activation energy was systematically higher than that of methanol. Adsorption stoichiometry shows that methanol and DME form clusters onto the binding sites. When non-activated re-adsorption is accounted for, a local equilibrium is reached only on the low and medium temperature binding sites. No differences were observed, other than in site densities, when extracting the kinetic parameters for fresh and activated ZSM-5 catalysts at full coverage.Graphical

Partial Oxidation of Methane to Syngas Over γ-Al2O3-Supported Rh Nanoparticles: Kinetic and Mechanistic Origins of Size Effect on Selectivity and Activity

A series of supported Rh/γ-Al2O3 catalysts with an overall metal loading of 0.005 wt % was synthesized by impregnation of γ-Al2O3 with a toluene solution containing colloidally prepared well-defined (1.1, 2.5, 2.9, 3.7, and 5.5 nm) Rh nanoparticles (NP). The size of NP was not found to change after their deposition on γ-Al2O3 and even after performing partial oxidation of methane (POM) to synthesis gas at 1073 K for 160 h on stream. Apparent CO formation turnover rates and CO selectivity strongly decrease with an increase in this size. Contrarily, the overall scheme of POM is size-independent, i.e. CO and H2 are mainly formed through reforming reactions of CH4 with CO2 and H2O at least under conditions of complete oxygen conversion. The size effect on the activity and selectivity was related to the kinetics of interaction of CH4, O2, and CO2 with Rh/γ-Al2O3 as concluded from our microkinetic analysis of corresponding transient experiments in the temporal analysis of products reactor. The rate constants of...

TAP reactor studies of the oxidizing capability of CO2 on a Au/CeO2 catalyst – A first step toward identifying a redox mechanism in the Reverse Water–Gas Shift reaction

As a first step toward elucidating the mechanism of the Reverse Water–Gas Shift (RWGS) reaction on a supported Au/CeO2 catalyst, in particular the role of the redox mechanism, we have investigated the ability and activity of CO2 for re-oxidizing a pre-reduced Au/CeO2 catalyst surface by quantitative temporal analysis of products (TAP) measurements. It is demonstrated that a surface-reduced Au/CeO2 catalyst can be (partially) re-oxidized by exposure to CO2 pulses and that the surface oxygen deposited this way can be reactively removed again. The probability for oxygen deposition from CO2, however, is significantly lower than that from O2. Both the low reaction probability and the temperature dependence of oxygen deposition from CO2 point to an activated step. Implications of these results for the mechanism of the RWGS reaction are discussed.

Highly selective ammonia oxidation to nitric oxide over supported Pt nanoparticles

Tailored Pt nanoparticles were prepared in strong alkaline ethylene glycol solution and deposited on α-Al2O3, γ-Al2O3, and CeO2–TiO2. The resulting supported catalytic materials were applied for ammonia oxidation between 473 and 1073K in the temporal analysis of products reactor. Although the catalysts possessed only 0.005wt.% Pt, they were active and selective for nitric oxide formation. The NO selectivity obtained over Pt/CeO2–TiO2 between 873 and 1073K was around 98% and exceeded that of industrially relevant Pt wire. Both alumina-supported catalysts were less selective. Pt/CeO2–TiO2 also showed the highest NO selectivity in steady-state ambient pressure tests. The superior performance of Pt/CeO2–TiO2 is related to a synergy effect between the noble metal and the support since they provide adsorbed and lattice oxygen species for selective NH3 oxidation to NO in a complementary fashion.

Mechanism–Performance Relationships of Metal Oxides in Catalyzed HCl Oxidation

The gas-phase oxidation of HCl to Cl2 over heterogeneous catalysts, known as the Deacon process, is a sustainable way for chlorine recycling in the chemical industry. Mechanistic aspects of this reaction over metal oxides (Cr2O3, CeO2, and MnO2) have been gathered using the temporal analysis of products (TAP) reactor and compared with the outcome of previous studies over RuO2 and CuO. The intrinsic features of the TAP technique enable investigation of this demanding reaction in a safe manner and under highly controlled conditions. We have correlated the catalytic activity measured isothermally in a continuous-flow reactor at ambient pressure with mechanistic descriptors derived from the transient responses of reaction products (Cl2 and H2O) in the TAP reactor. The order of activity was RuO2 > Cr2O3 > CeO2 ∼ CuO > MnO2. The oxides with lowest activity, MnO2 and CuO, exhibited bulk chlorination detected by X-ray diffraction and a highly impeded Cl2 evolution. Chlorination of Cr2O3 and CeO2 during reaction c...

Dynamic studies of CO oxidation on nanoporous Au using a TAP reactor

The oxidation of CO on nanoporous Au (NPG), in particular the activation of molecular O 2, was investigated by a combination of kinetic and temporal analysis of products (TAP) measurements. Continuous reaction measurements in a flow of reaction gas, at atmospheric pressure, show a catalytic behavior of the NPG, with the activity decreasing to 33% of the initial activity over 1000 min on stream. In contrast, during simultaneous pulsing of CO and O 2, the formation of CO 2 on the NPG catalyst rapidly decreased to values below the detection limit after reactive removal of the surface oxygen species present after sample preparation. Possible mechanisms explaining this discrepancy are discussed, using further information from multi-pulse TAP experiments, which revealed that molecular O 2 can be activated and stored on NPG catalyst at room temperature, though with a low probability.

Support effects in the Au-catalyzed CO oxidation – Correlation between activity, oxygen storage capacity, and support reducibility

The oxygen storage capacity (OSC) and its correlation with the activity for the CO oxidation reaction and the reducibility of the support material were investigated for four different metal oxide-supported Au catalysts with similar Au loading and Au particle sizes (Au/Al 2O 3, Au/TiO 2, Au/ZnO, Au/ZrO 2), which were prepared by deposition of pre-formed Au colloids. Temporal Analysis of Products (TAP) reactor measurements show that the OSC and the activity for CO oxidation, measured under identical conditions, differ significantly for these catalysts and are correlated with each other and with the reducibility of the respective support material, pointing to a distinct support effect and a direct participation of the support in the reaction. Activity measurements performed under ambient conditions show a similar trend of the activity as the TAP reactor measurements, supporting that the conclusions drawn from the TAP reactor measurements are valid also under continuous reaction conditions. Moreover, the rapid formation and accumulation of carbon-containing surface species during reaction is demonstrated, which can severely reduce the activity for CO oxidation. Implications of these results on the CO oxidation mechanism over metal oxide-supported catalysts are discussed.