Temporal Analysis Of Products Research Articles

New Mechanistic and Reaction Pathway Insights for Oxidative Coupling of Methane (OCM) over Supported Na2WO4/SiO2 Catalysts

AbstractThe complex structure of the catalytic active phase, and surface‐gas reaction networks have hindered understanding of the oxidative coupling of methane (OCM) reaction mechanism by supported Na2WO4/SiO2 catalysts. The present study demonstrates, with the aid of in situ Raman spectroscopy and chemical probe (H2‐TPR, TAP and steady‐state kinetics) experiments, that the long speculated crystalline Na2WO4 active phase is unstable and melts under OCM reaction conditions, partially transforming to thermally stable surface Na‐WOx sites. Kinetic analysis via temporal analysis of products (TAP) and steady‐state OCM reaction studies demonstrate that (i) surface Na‐WOx sites are responsible for selectively activating CH4 to C2Hx and over‐oxidizing CHy to CO and (ii) molten Na2WO4 phase is mainly responsible for over‐oxidation of CH4 to CO2 and also assists in oxidative dehydrogenation of C2H6 to C2H4. These new insights reveal the nature of catalytic active sites and resolve the OCM reaction mechanism over supported Na2WO4/SiO2 catalysts.

Understanding Reaction Networks through Controlled Approach to Equilibrium Experiments Using Transient Methods.

We report a combined experimental/theoretical approach to studying heterogeneous gas/solid catalytic processes using low-pressure pulse response experiments achieving a controlled approach to equilibrium that combined with quantum mechanics (QM)-based computational analysis provides information needed to reconstruct the role of the different surface reaction steps. We demonstrate this approach using model catalysts for ammonia synthesis/decomposition. Polycrystalline iron and cobalt are studied via low-pressure TAP (temporal analysis of products) pulse response, with the results interpreted through reaction free energies calculated using QM on Fe-BCC(110), Fe-BCC(111), and Co-FCC(111) facets. In TAP experiments, simultaneous pulsing of ammonia and deuterium creates a condition where the participation of reactants and products can be distinguished in both forward and reverse reaction steps. This establishes a balance between competitive reactions for D* surface species that is used to observe the influence of steps leading to nitrogen formation as the nitrogen product remains far from equilibrium. The approach to equilibrium is further controlled by introducing delay timing between NH3 and D2 which allows time for surface reactions to evolve before being driven in the reverse direction from the gas phase. The resulting isotopic product distributions for NH2D, NHD2, and HD at different temperatures and delay times and NH3/D2 pulsing order reveal the role of the N2 formation barrier in controlling the surface concentration of NHx* species, as well as providing information on the surface lifetimes of key reaction intermediates. Conclusions derived for monometallic materials are used to interpret experimental results on a more complex and active CoFe bimetallic catalyst.

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.

Ethanol Dehydrogenation: A Reaction Path Study by Means of Temporal Analysis of Products

Conventional fossil fuels such as gasoline or diesel should be substituted in the future by environmentally-friendly alternatives in order to reduce emissions in the transport sector and thus mitigate global warming. In this regard, iso-butanol is very promising as its chemical and physical properties are very similar to those of gasoline. Therefore, ongoing research deals with the development of catalytically-supported synthesis routes to iso-butanol, starting from renewably-generated methanol. This research has already revealed that the dehydrogenation of ethanol plays an important role in the reaction sequence from methanol to iso-butanol. To improve the fundamental understanding of the ethanol dehydrogenation step, the Temporal Analysis of Products (TAP) methodology was applied to illuminate that the catalysts used, Pt/C, Ir/C and Cu/C, are very active in ethanol adsorption. H2 and acetaldehyde are formed on the catalyst surfaces, with the latter quickly decomposing into CO and CH4 under the given reaction conditions. Based on the TAP results, this paper proposes a reaction scheme for ethanol dehydrogenation and acetaldehyde decomposition on the respective catalysts. The samples are characterized by means of N2 sorption and Scanning Transmission Electron Microscopy (STEM).

Effect of Formaldehyde in Selective Catalytic Reduction of NOx by Ammonia (NH3-SCR) on a Commercial V2O5-WO3/TiO2 Catalyst under Model Conditions.

The impact of formaldehyde (HCHO, formed in vehicle exhaust gases by incomplete combustion of fuel) on the performance of a commercial V2O5-WO3/TiO2 catalyst in NH3-SCR of NOx under dry conditions has been analyzed in detail by catalytic tests, in situ FTIR and transient studies using temporal analysis of products (TAP). HCHO reacts preferentially with NH3 to a formamide (HCONH2) surface intermediate. This deprives NH3 partly from its desired role as a reducing agent in the SCR and diminishes NO conversion and N2 selectivity. Between 250 and 400 °C, HCONH2 decomposes by dehydration (major pathway) and decarbonylation (minor pathway) to liberate toxic HCN and CO, respectively. HCN was proven to be oxidized by lattice oxygen of the catalyst to CO2 and NO, which enters the NH3-SCR reaction.

Revisiting Activity- and Selectivity-Enhancing Effects of Water in the Oxidative Coupling of Methane over MnOx-Na2WO4/SiO2and Proving for Other Materials

The oxidative coupling of methane (OCM) to C2-hydrocarbons (C2H4 and C2H6) is attractive both from fundamental (selective C–H bond activation) and applied viewpoints. The main drawback hindering its commercialization is the low selectivity to C2-hydrocarbons due to their further oxidation to carbon oxides. In this respect, we focused on elucidating fundamentals of the previously reported positive effect of water on the activity and selectivity in the OCM reaction over MnOx-Na2WO4/SiO2 by means of steady-state kinetic and mechanistic tests at ambient pressure as well as temporal analysis of products with sub-millisecond resolution and isotopic tracers in vacuum. The obtained results cannot be rationalized by the earlier developed concepts explaining the role of water in the OCM reaction over this catalyst system. In addition, the selectivity-enhancing water effect was determined for MnOx/SiO2, MnOx/Al2O3, Na2WO4/SiO2, and Na2WO4/Al2O3, as well as SiO2-based materials with supported PbOx, ZrO2, or La2O3. No rate-improving effect was established for MnOx/Al2O3 and ZrO2/SiO2, while positive or negative effects were determined for PbOx/SiO2, MnOx/SiO2, and Na2WO4/SiO2 or La2O3/SiO2 and Na2WO4/Al2O3, respectively. Thus, testing other catalysts in the water presence seems to be promising in view of improving the OCM performance to an industrially attractive level. Owing to the developed protocol for catalyst testing under alternating water-free and water-containing OCM feeds, both reversible and irreversible water effects on the selectivity to C2-hydrocarbons over MnOx-Na2WO4/SiO2 were identified. The irreversible effect was related to water-induced redispersion of MnOx. Mechanistic insights into the reversible rate- and selectivity-improving effects were derived from kinetic analysis of the rates of methane conversion into individual reaction products at different water partial pressures and temperatures. Selectivity–conversion relationships enabled us to understand the specific reaction pathways in the course of the OCM reaction affected by water. Water was established to increase the rates of methane conversion into C2-hydrocarbons, CO, and CO2 with the strongest effect being determined for CO formation. For all the rates, the strength of the positive water effect decreases with an increase in temperature from 750 to 825 °C. In addition to the acceleration of methane conversion, water also helps to transform nonselective molecular-adsorbed oxygen species responsible for the direct oxidation of methane to carbon dioxide.

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.

A temporal analysis of products (TAP) study of C2-C4 alkene reactions with a well-defined pool of methylating species on ZSM-22 zeolite

Reactions between alkenes and methanol or dimethyl ether (DME) on zeolite catalysts are involved in industrial processes that are highly relevant for the transition to renewable carbon sources, such as the Methanol-To-Hydrocarbons (MTH) process. In MTH chemistry, alkene methylation increases the length of product carbon chains, and its relative rate with respect to other reactions largely controls the overall selectivity. Experimental studies of alkene methylation present a considerable challenge because they are typically accompanied by cracking, hydrogen transfer, and aromatization reactions. Herein, the pulse-response Temporal Analysis of Products (TAP) methodology and complementary FTIR measurements were employed to isolate a well-defined population of surface species, consistent with Surface Methoxy Species (SMS) on Bronsted acid sites that are reactive in alkene methylation on a ZSM-22 (TON) zeolite. Their coverage was determined by TAP titration to be ca. 5% of the total amount of Brønsted acid sties, which was also indirectly suggested by FTIR data. C2-C4 alkenes were quantitatively reacted with SMS to precisely measure the intrinsic kinetic parameters of isolated alkene methylation steps. The rate constants increased and the activation energies decreased as functions of the carbon number (EC2H4 = 52 kJ/mol > EC3H6 = 32 kJ/mol > Ec-C4H8 = 16 kJ/mol). However, the rate constant for iso-butene was comparable to propene, despite its activation energy (Ei-C4H8 = 19 kJ/mol) being much lower than propene’s. This effect is in agreement with the increased steric hindrance predicted by DFT for isobutene adsorption and methylation in TON zeolites. Our results considerably extend previously available TAP data on alkene methylation reactions and furthermore validate ab initio models of these crucial steps in the complex MTH chemistry on acidic zeolites.

Comprehensive analysis of alternative downscaled soil moisture products

Recent advances in L-band passive microwave remote sensing provide an unprecedented opportunity to monitor soil moisture at ~40 km spatial resolution around the globe. Nevertheless, retrieval of the accurate high spatial resolution soil moisture maps that are required to satisfy hydro-meteorological and agricultural applications remains a challenge. Currently, a variety of downscaling, otherwise known as disaggregation techniques have been proposed as the solution to disaggregate the coarse passive microwave soil moisture into high-to-medium resolutions. These techniques take advantage of the strengths of both the passive microwave observations of soil moisture having low spatial resolution and the spatially detailed information on land surface features that either influence or represent soil moisture variability. However, such techniques have typically been developed and tested individually under differing weather and climate conditions, meaning that there is no clear guidance on which technique performs the best. Consequently, this paper presents a quantitative assessment of the existing radar-, optical-, radiometer-, and oversampling-based downscaling techniques using a singular extensive data set collected specifically for that purpose, being the Soil Moisture Active Passive Experiment (SMAPEx)-4 and -5 airborne field campaigns, and the OzNet in situ stations, to determine the relative strengths and weaknesses of their performances. The oversampling-based soil moisture product best captured the temporal and spatial variability of the reference soil moisture overall, though the radar-based products had a better temporal agreement with airborne soil moisture during the short SMAPEx-4 period. Moreover, the difference between temporal analysis of products against in situ and airborne soil moisture reference data sets pointed to the fact that relying on in situ measurements alone is not appropriate for validation of spatially enhanced soil moisture maps.

Influence of Precursors on the Induction Period and Transition Regime of Dimethyl Ether Conversion to Hydrocarbons over ZSM-5 Catalysts

ZSM-5 catalysts were subjected to step response cycles of dimethyl ether (DME) at 300 °C in a temporal analysis of product (TAP) reactor. Propylene is the major olefin and displays an S-shaped profile. A 44 min induction period occurs before primary propylene formation and is reduced upon subsequent step response cycles. The S-shaped profile was interpreted according to induction, transition-regime, and steady-state stages to investigate hydrocarbon formation from DME. The influence of precursors (carbon monoxide, hydrogen, dimethoxymethane, and 1,5-hexadiene) was studied using a novel consecutive step response methodology in the TAP reactor. The addition of dimethoxymethane, carbon monoxide, hydrogen, or 1,5-hexadiene reduces the induction period of primary olefin formation. However, while dimethoxymethane, carbon monoxide, and hydrogen accelerate the transition regime toward hydrocarbon pool formation, 1,5-hexadiene attenuates it. Heavier hydrocarbons obtained from 1,5-hexadiene compete for active sites...

Mechanistic origin of the diverging selectivity patterns in catalyzed ethane and ethene oxychlorination

In the context of vinyl chloride monomer (VCM) production, an oxychlorination catalyst that allows direct VCM formation from gas-derived ethane instead of expensive oil-derived ethene is intensively sought after. A wide range of stable ethane oxychlorination catalysts for this purpose have recently been reported, yet they mainly yield ethene, while VCM remains a minor by-product. Strikingly, the same catalysts are active in ethene oxychlorination, resulting in selective VCM formation under equivalent reaction conditions. This work reveals the origin of these diverging selectivity patterns by combining quantitative catalytic tests, temporal analysis of products (TAP), and density functional theory (DFT) on iron phosphate. Ethane oxychlorination is found to proceed sequentially through ethyl chloride (EtCl) dehydrochlorination to ethene, while ethene oxychlorination directly yields VCM without formation of the intermediate dichloroethane (EDC) on iron phosphate. Furthermore, by co-feeding ethane in ethene oxychlorination, we demonstrate that the alkane suppresses the formation of VCM in ethene oxychlorination. The reason for this VCM inhibition is found in the hydrocarbon competition for a combination of the active, free and chlorinated iron centers. As ethane activation exhibits half of the barrier of ethene activation, the presence of ethane leads to active site depletion, hindering VCM formation. These observations are extended by ethane co-feeding tests in ethene oxychlorination over a wide range of known oxychlorination catalysts (EuOCl, LaOCl, CeO2, and CuCl2-KCl-LaCl3/γ-Al2O3), and corresponding DFT calculations, indicating that the described phenomenon is material independent. The gathered molecular-level understanding explains the major hurdle of using ethane as feedstock for vinyl chloride production.

Methods for determining the intrinsic kinetic characteristics of irreversible adsorption processes

To benchmark the performance of a different of catalytic materials it is essential to know the adsorption characteristics such as the kinetic order of adsorption, the apparent rate constant and the total number of active sites. It is advantageous to normalize the reaction rate to the number of active sites; a quantity commonly referred to as the turnover frequency (TOF). Methods such as chemisorption and SSITKA (steady state isotopic transient kinetic analysis) are conventionally used to quantify the number of active sites, both have advantages and drawbacks. The TAP (Temporal Analysis of Products) pulse response technique provides a distinct method for precise (10 nmol) quantification of active sites at elevated temperatures. Using irreversible reaction processes, mathematical techniques for analysis of pulsed active site titration are described herein. Whereas previous methods simply consider the total capacity for gas adsorption, these new methods take into account the dynamics and interdependence of the conversion, the number of available sites and the adsorption rate as they change over the course of the titration experiment. Moreover, these methods enable an independent fitting of the kinetic order and stoichiometry of the adsorption process directly from experimental data. In comparison with experimental data for the incremental oxidation of a reduced platinum, we found the analytical method of linear rectification was most robust and efficient. In fitting the kinetic order to experimental data collected between 125 and 175 °C we found a clear indication over the range of 45–95% conversion of a second order rate dependence and 2:1 balance between active sites and O2.

Ethanol dehydration pathways in H-ZSM-5: Insights from temporal analysis of products

Abstract Ethanol dehydration to ethene via direct and ether mediated paths is mechanistically investigated via transient experiments in a Temporal analysis of products, TAP-3E reactor over a temperature range of 473–573 K in Knudsen regime conditions. Pulse experiments of ethanol over H-ZSM-5 do not yield diethyl ether as a gas phase product. Cofeed experiments with diethyl ether and C-13 labeled ethanol show that ethene formation from diethyl ether is the preferential route. Kinetic parameters from ab initio based microkinetic modelling of ethanol dehydration are compared to the experimental data of the TAP reactor by an in-house developed reactor model code TAPFIT. Rate coefficients in ethene adsorption are in agreement with the ab initio based microkinetic modelling parameters. The experimental data from a diethyl ether feed are compared to the simulated responses from the ab initio based kinetic parameters and further optimized by regression analysis. Reaction path analysis with the optimized kinetic parameters identifies the preference of an ether mediated path under the applied transient experimental conditions.

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.

Transient Kinetic Experiments within the High Conversion Domain: The Case of Ammonia Decomposition

In the development of catalytic materials, a set of standard conditions is needed where the kinetic performance of many samples can be compared. This can be challenging when a sample set covers a broad range of activity. Precise kinetic characterization requires uniformity in the gas and catalyst bed composition. This limits the range of convecting devices to low conversion (generally <20%). While steady-state kinetics offer a snapshot of conversion, yield and apparent rates of the slow reaction steps, transient techniques offer much greater detail of rate processes and hence more information as to why certain catalyst compositions offer better performance. In this work, transient experiments in two transport regimes are compared: an advecting differential plug flow reactor (PFR) and a pure-diffusion temporal analysis of products (TAP) reactor. The decomposition of ammonia was used as a model reaction to test three simple materials: polycrystalline iron, cobalt and a bimetallic preparation of the two. These materials presented a wide range of activity and it was not possible to capture transient information in the advecting device for all samples at the same conditions while ensuring uniformity. We push the boundary for the theoretical estimates of uniformity in the TAP device and find reliable kinetic measurement up to 90% conversion. However, what is more advantageous from this technique is the ability to observe the time-dependence of the reaction rate rather than just singular points of conversion and yield. For example, on the iron sample we observed reversible adsorption of ammonia and on cobalt materials we identify two routes for hydrogen production. From the time-dependence of reactants and product, the dynamic accumulation was calculated. This was used to understand the atomic distribution of H and N species regulated by the surface of different materials. When ammonia was pulsed at 550 °C, the surface hydrogen/nitrogen, (H/N), ratios that evolved for Fe, CoFe and Co were 2.4, 0.25 and 0.3 respectively. This indicates that iron will store a mixture of hydrogenated species while materials with cobalt will predominantly store NH and N. While much is already known about iron, cobalt and ammonia decomposition, the goal of this work was to demonstrate new tools for comparing materials over a wider window of conversion and with much greater kinetic detail. As such, this provides an approach for detailed kinetic discrimination of more complex industrial samples beyond conversion and yield.

Alcohol Synthesis from CO2, H2, and Olefins over Alkali‐Promoted Au Catalysts—A Catalytic and In situ FTIR Spectroscopic Study

Au/TiO2 and Au/SiO2 catalysts containing 2 wt % Au and different amounts of K or Cs were tested for alcohol synthesis from CO2 , H2 , and C2 H4 /C3 H6 . 1-Propanol or 1-butanol/isobutanol were obtained in the presence of C2 H4 or C3 H6 . Higher yields of the corresponding alcohols were obtained over TiO2 -based catalysts in comparison with their SiO2 -based counterparts. This is caused by an enhanced ability of the TiO2 -based catalysts for CO2 activation, as concluded from in situ fourier-transform infrared (FTIR) spectroscopy and temporal analysis of products (TAP) studies. The synthesized carbonate and formate species adsorbed on the support do not hamper CO2 conversion into CO and the hydroformylation reaction. The transformation of Auδ+ to active Au0 sites proceeds during an activation procedure. As reflected by CO adsorption and scanning transmission electron microscopy, the accessible Au0 sites are influenced by the amount of alkali dopants and the support. FTIR data and TAP tests reveal a very weak interaction of C2 H4 with the catalyst, suggesting its quick reaction with CO and H2 after activation on Au0 sites to form propanol and propane.

Kinetic Modeling of the Metal/Support Interaction for CH4 Reaction over Oxidized Pd/Al2O3

This paper deals with the kinetics of CH4 dissociation on model Pd/Al2O3 catalysts. The adsorption and conversion of CH4 over oxidized catalysts, fresh or aged, were studied with a temporal analysis of products (TAP) reactor. Single CH4 pulse TAP experiments were performed on a stabilized surface. The experiments are discussed in the light of a selected mechanism involving CH4 decomposition into CO2 taking into account an interaction between the metallic active sites and the support. TAP experiments over the oxidized catalysts confirm the involvement of the metal/support interface, with a spill-over effect. Optimized kinetics parameters validate this interaction of OH species adsorbed on Pd sites with alumina, showing a relatively good agreement between experimental and calculated outlet gas composition. Both fresh and aged catalysts follow the chosen mechanism with the same kinetic constants at 400 °C only an alteration of the catalytic surface areas explain the loss of activity.