Temporal Analysis Of Products Research Articles

Progress through Temporal Analysis of Products and Steady‐state Isotopic Transient Kinetic Analysis to Elucidate Oxidation, CO2 Hydrogenation and Lower Olefins Production Reactions

A prerequisite for the development or optimization of heterogenous catalysts is a comprehensive understanding of the reaction mechanism and kinetics at the level as elementary as possible. In comparison with steady‐state catalytic tests typically used for this purpose, non‐steady‐state (transient) studies provide deeper insights due to higher time resolution. For several decades, temporal analysis of products (TAP) and steady‐state isotopic transient kinetic analysis (SSITKA) have been successfully applied to the analysis of various heterogenous reactions. In this review, we highlight the contributions made by TAP and SSITKA studies between 2017 and 2023 to the understanding of reaction mechanisms and to the derivation of microkinetics. The versatility of these methods is illustrated by the examples of oxidative coupling of methane, CO/CO2 hydrogenation to hydrocarbons or methanol, metathesis of ethylene with 2‐butenes to propene, acrolein oxidation to acrylic acid, methanol conversion to olefins and non‐oxidative propane dehydrogenation to propene. We also provide our personal views on possible developments in this area.

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.

Uncovering the synergy between gold and sodium on ZrO2 for boosting the reverse water gas shift reaction: In-situ spectroscopic investigations

CO2 conversion to CO via the reverse water-gas shift (RWGS) reaction is a promising source of syngas for subsequent synthesis of liquid fuels and chemicals. Herein, we present the synthesis of catalysts containing Au supported on hydroxylated Na-modified ZrO2, with Au amounts ranging from 0.05 to 1 wt%. Systematic investigations reveal the formation of cooperative Au/Na sites at the interface. These sites cooperate synergistically to activate CO2 and generate a high surface density of carboxylate-like species, which serve as highly active intermediates for CO formation. It was found that the RWGS reaction on the catalyst with low Au loading proceeds mainly via a carboxylate pathway, with bidentate formate acting as spectators. At higher Au loading, the bidentate formate pathway contributes somewhat to CO formation alongside the carboxylate pathway. Based on temporal analysis of products, we emphasize the significant roles of H2 spillover and the metal-support interface in the RWGS reaction.

Lattice-Stabilized Chromium Atoms on Ceria for N2O Synthesis.

The development of selective catalysts for direct conversion of ammonia into nitrous oxide, N2O, will circumvent the conventional five-step manufacturing process and enable its wider utilization in oxidation catalysis. Deviating from commonly accepted catalyst design principles for this reaction, reliant on manganese oxide, we herein report an efficient system comprised of isolated chromium atoms (1 wt %) stabilized in the ceria lattice by coprecipitation. The latter, in contrast to a simple impregnation approach, ensures firm metal anchoring and results in stable and selective N2O production over 100 h on stream up to 79% N2O selectivity at full NH3 conversion. Raman, electron paramagnetic resonance, and in situ UV-vis spectroscopies reveal that chromium incorporation enhances the density of oxygen vacancies and the rate of their generation and healing. Accordingly, temporal analysis of products, kinetic studies, and atomistic simulations show lattice oxygen of ceria to directly participate in the reaction, establishing the cocatalytic role of the carrier. Coupled with the dynamic restructuring of chromium sites to stabilize intermediates of N2O formation, these factors enable catalytic performance on par with or exceeding benchmark systems. These findings demonstrate how nanoscale engineering can elevate a previously overlooked metal into a highly competitive catalyst for selective ammonia oxidation to N2O, paving the way toward industrial implementation.

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.

Effect of supports on the kind of in-situ formed ZnOx species and its consequence for non-oxidative propane dehydrogenation

Non-oxidative propane dehydrogenation (PDH) to propene is the basis of various large-scale processes suffering however from high costs or environmental incompatibility of currently applied Pt- or Cr-containing catalysts. Herein, we demonstrate that active and selective catalysts can be obtained from cheap and commercially available Zr- or Ti-based supports and ZnO without producing any liquid or solid waste. Catalytically active species formed in situ under PDH conditions are composed of isolated ZnOx as concluded from X-ray absorption spectroscopic analysis. The kind of support affects the geometry of such species that is probably decisive for catalyst activity. ZnOx on the surface of LaZrOx revealed the highest Zn-related TOF of propene formation. However, the following activity order in terms of space time yield of propene formation (STYC3H6) at 550 °C and about 50% equilibrium propane conversion using a feed with 40 vol% propane was obtained: ZnO//TiZrOx > ZnO//SiZrOx > ZnO//LaZrOx > ZnO//TiO2. The best-performing catalyst showed STYC3H6 of 2 kg kgcat−1 h−1 and was durable in 8 PDH/regeneration cycles. Temporal analysis of products with submillisecond resolution suggests that H2 formation should be the rate-determining step in the course of the PDH reaction.

Elucidating the Role of Oxygen Species in Oxidative Coupling of Methane over Supported MnOx−Na2WO4‐containing Catalysts

AbstractThe present study of oxidative coupling of methane (OCM) over MnOx−Na2WO4/support catalysts demonstrated that the selectivity to C2H6 and C2H4 (C2‐hydrocarbons) is affected by the kind of support, co‐fed water, and the kind of oxidant (O2 vs. N2O). In addition to previous studies with MnOx−Na2WO4/SiO2, an enhancing water effect was obtained using catalysts based on TiO2‐ or ZrO2‐containing supports. However, a negative effect on methane conversion was established for SiO2−Al2O3‐supported catalysts. Temporal analysis of products with isotopic tracers suggests that the ability of MnOx−Na2WO4 to generate diatomic adsorbed oxygen species depends on the kind of support and is the key property for the water effect. The strength of the water effect on the activity decreases with an increase in the surface area of working catalysts. The kind of support also affects products selectivity due to its influence on the mobility/releasability of lattice oxygen in supported MnOx−Na2WO4. Among the prepared catalysts, MnOx−Na2WO4/TiO2 was found to be promising for H2O‐assisted OCM. The use of N2O instead of O2 further increases the selectivity to C2‐hydrocarbons to 84 % at 6.8 % CH4 conversion due to the formation of predominantly monoatomic oxygen species from N2O that selectively convert CH4 into C2H6.

Temporal analysis of products (TAP) reactor study of the dynamics of CO2 interaction with a Ru/γ-Al2O3 supported catalyst

As part of a comprehensive study on the reduction of COx on supported Ru catalysts we systematically investigated the dynamic interaction of CO2 with a Ru/γ-Al2O3 catalyst in a temporal analysis of products (TAP) reactor, focusing on the redox properties of the catalyst, specifically on the deposition of active oxygen (Oact) from CO2. This was investigated by exposing the pre-reduced catalyst to CO2 pulses and subsequent titration of the deposited Oact by CO pulses. CO2 is much less active for Oact deposition than O2. Reductive co-reactants reduce (CO) or increase (H2) the activity of CO2 for Oact deposition. 18O-marked CO2 pulses show facile oxygen exchange of CO2 with the catalyst, despite the inertness of the support. Based on excess effluent CO2 we conclude that surface carbonates can both build up on the surface upon interaction with CO or CO2 and also decompose upon O2 or CO2 pulsing, releasing additional CO2.

The Role of Adsorbed and Lattice Oxygen Species in Product Formation in the Oxidative Coupling of Methane over M2WO4/SiO2 (M = Na, K, Rb, Cs)

MnOx–Na2WO4/SiO2 is one of the best-performing catalysts in the oxidative coupling of methane (OCM) to C2 hydrocarbons (C2H6 and C2H4). The current mechanistic concepts related to the selectivity to the desired products are based on the involvement of crystalline Mn-containing phases, the molten Na2WO4 phase, surface Na–WOx species, and the associated lattice oxygen. Using in situ X-ray diffraction, operando UV–vis spectroscopy, spatially resolved kinetic analysis of product formation in steady-state OCM tests, and temporal analysis of products with isotopic tracers, we show that these phases/species are not categorically required to ensure high selectivity to the desired products. M2WO4/SiO2 (M = Na, K, Rb, Cs) materials were established to perform similarly to MnOx–Na2WO4/SiO2 in terms of selectivity–conversion relationships. The unique role of the molten Na2WO4 phase could not be confirmed in this regard. Our alternative concept is that the activity of M2WO4/SiO2 and product selectivity are determined by the interplay between the lattice oxygen of M2WO4 and adsorbed oxygen species formed from gas-phase O2. This lattice oxygen cannot convert CH4 to C2H6 but oxidizes CH4 exclusively to CO and CO2. Adsorbed monoatomic oxygen species reveal significantly higher reactivity toward overall CH4 conversion and efficiently generate CH3 radicals from CH4. These reactive intermediates couple to C2H6 in the gas phase and are oxidized, to a lesser extent, by the lattice oxygen of M2WO4 to CO and CO2. Adsorbed diatomic oxygen is involved in the direct CH4 oxidation to CO2. The electronegativity of alkali metal in M2WO4 was established to affect the catalyst ability to generate adsorbed oxygen species from O2. This knowledge opens the possibility to influence product selectivity by controlling the coverage by adsorbed and lattice oxygen via reaction conditions or catalyst design.

Elimination of Indoor Volatile Organic Compounds on Au/SBA-15 Catalysts: Insights into the Nature, Size, and Dispersion of the Active Sites and Reaction Mechanism

Gold catalysts, with different particle sizes ranging from 19 to 556 Å, and supported on SBA-15 mesoporous silica, were prepared by using deposition-precipitation, co-precipitation, and impregnation methods. All samples were characterised by TEM, EXAFS, XPS, XRD, CFR (Continuous Flow Reactor), and TPR. The sample which proved to have the highest activity was characterised by TAP (Temporal Analysis of Products) as well. XPS, wide-angle XRD, EXAFS, and H2-TPR measurements and data analysis confirmed that gold was present as Au0 only on all samples. The size of the Au nanoparticle was determined from TEM measurements and confirmed through wide-angle XRD measurements. EXAFS measurements showed that as the Au-Au coordination number decreased the Au-Au bond length decreased. TEM data analysis revealed a dispersion range from 58% (for the smallest particle size) to 2% (for the highest particle size). For Au particles’ sized lower that 60 Å, the Au dispersion was determined using a literature correlation between the dispersion and EXAFS Au-Au coordination number, and was in good agreement with the dispersion data obtained from TEM. The Au dispersion decreased as the particle size increased. CFR experiments validated the relationship between the size of the gold particles in a sample and the sample’s catalytic activity towards acetone oxidation. The lowest temperature for the acetone 100% conversion, i.e., 250 °C, was observed over the reduced catalyst sample with the smallest particle size. This sample not only showed the highest catalytic activity towards acetone conversion, but, at the same time, showed high reaction stability, as catalyst lifetime tests, performed for 25 h in a CFR at 270 °C for the as-synthesised sample, and at 220 °C for the reduced sample, have confirmed. TAP (Temporal Analysis of Products) measurements and data analysis confirmed a weak competitive adsorption of acetone and oxygen over the Au/SBA-15 sample. Based on TAP data, a combination of Eley–Rideal and Langmuir–Hinshelwood mechanisms for acetone complete oxidation was proposed.

The nature of VOx structures in HMS supported vanadium catalysts for non-oxidative propane dehydrogenation

Vanadium was introduced to hexagonal mesoporous SiO2 (HMS) to prepare supported vanadium-based catalysts (V-HMS). Their catalytic performance in the non-oxidative propane dehydrogenation to propylene (PDH) was modulated with V surface density varying from 0.12 to 15.08V atom·nm−2. VOx species with different structure were analyzed by UV–vis spectroscopy, NH3-TPD, H2-TPR and XPS. High propylene space time yield of 0.73 kg·h−1·kgcat−1 and excellent regeneration behavior were achieved. Correlating VOx polymerization degree with catalytic performance, VOx aggregates, mainly in low-polyvanadate form, were established to reveal high intrinsic activity and stability in comparison with their counterparts, including isolated or/and highly polymerized VOx as well as V2O5. Temporal analysis of products (TAP) identified hydrogen formation as the rate-limiting step in the PDH reaction. In-situ DRIFTS was employed to investigate the reaction mechanism. V species containing C=C bond, i.e., VOx-C3H5, was proposed to be an intermediate of gas-phase propylene.

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.

Parametric sensitivity analysis of the transient adsorption-diffusion models for hydrocarbon transport in microporous materials

The complex nature of hydrocarbon transport in microporous materials offers unique challenges for the description of transient kinetic data in catalysis by e.g. acidic zeolites and zeotypes. Currently, very few models in the literature capture such phenomena as surface barriers or hindered microporous diffusion - processes that are known to impact gas transport during the kinetic studies of zeolite catalyzed reactions. In this work we provide extended models that reflect the aforementioned phenomena during pulse-response experiments in the Temporal Analysis of Products (TAP) reactor. Systematic studies of models’ parametric sensitivity are presented to assist the design of future experiments. Numerical simulations are used to investigate how thermodynamic adsorption properties of realistic zeolites can affect the pulse-response shapes. For certain combination adsorption/diffusion parameters, microporous diffusion is shown to result in a characteristic bend in the mean residence time temperature dependency, which can be used as a fingerprint in model discrimination.

Addition of transient kinetics capabilities to an infrared reflection absorption spectroscopy system through synchronized gas pulsing and data acquisition

Surface science methodologies for understanding thermodynamic aspects of surface processes are at an advanced level. However, instrumentation and approaches for extracting kinetic parameters from elementary steps are far less accessible. In this work, we present an approach combining the use of a fast gas pulsing valve synchronized with data acquisition to enable surface transient kinetics studies using infrared reflection absorption spectroscopy. This methodology applies to the study of reversible processes and borrows concepts and ideas from molecular beam scattering and temporal analysis of products. A temporal resolution of ~67 ms is achieved, and this is illustrated through the study of CO adsorption and desorption on a Pd(111) crystal in the presence and absence of background O2. The same approach can be extended to other surface spectroscopies, such as X-ray photoelectron spectroscopy, to obtain spectra with high temporal resolution and signal-to-noise ratio and enable future multimodal surface transient kinetic studies aiming at elucidating reaction mechanisms.

Truth is, we all are transients: A perspective on the time-dependent nature of reactions and those who study them

Understanding and mastery of Transient Kinetics in the context of multi-scale transport-reaction systems employed in catalysis is one of the ultimate frontiers in chemical science and engineering. In this perspective, we will highlight key results enabled by experiments in the Temporal Analysis of Products (TAP) Reactor Systems spanning several decades of research, several length-scales of space and time, as well as several ensembles of individual (re)actors, ranging from a single 350 µm-sized catalytic particle to a bed fully packed with them. Conclusions are drawn and new horizons are discerned.

Mechanistic pathways and role of oxygen in oxidative coupling of methane derived from transient kinetic studies

Oxidative coupling of methane (OCM) is a promising industrial process to upgrade natural gas to high value chemicals. In this study, Temporal Analysis of Products (TAP) and steady-state experiments were conducted to distinguish how the composition of surface and gas phase oxygen influence mechanistic details of the selective conversion of CH4 to C2H4 over the Mn-Na2WO4/SiO2 catalyst. The results from TAP studies indicate that methane activation on this catalyst proceeds predominantly via a short-lived, transient surface oxygen species and there is a competition for this species to form either CO or methyl radicals on the surface. This active species has a total lifetime of 3 s and is identified to have a dioxygen (e.g., O22- or O2-) form. We show that the concentration of the transient surface oxygen species significantly impacts the OCM performance. Oxygen attributed to the catalyst lattice (in a singular form e.g., O-), is found to activate methane to a lesser degree, but exclusively forms CO2. Evidence for these surface pathways for methyl radical, CO and CO2 formation identified by TAP are also validated through steady-state experiments. By distinguishing different catalyst oxygen species and their role in selective/nonselective pathways, important screening criteria have been identified for the advancement of superior catalyst formulations.

Sorption of n-butane on sulfated zirconia investigated using a combination of in-situ DRIFT spectroscopy, XPS and TAP

The sorption of n-butane on sulfated zirconia (SZ) was investigated using in-situ Diffuse Reflection Infrared Fourier Transform Spectroscopy (DRIFTS), Photoelectron spectroscopy (XPS) and Temporal Analysis of Products (TAP) measurements. The combination of different spectroscopic techniques like XPS and DRIFTS with transient TAP experiments offer the chance to comprehensively investigate and characterize catalytic sorption sites. Using XPS, DRIFTS, and TAP it was shown that both Brønsted centers (Zr-OH) and the very reactive disulfate groups are involved in the adsorption of n-butane and thus are mainly responsible for the isomerization reaction of n-butane to iso-butane over SZ. Moreover, all methods enable to bridge the pressure gap ranging from UHV to ambient pressure. Especially, adsorption of alkanes with XPS in-situ for different adsorption times has been done for the first time.

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.

New Mechanistic and Reaction Pathway Insights for Oxidative Coupling of Methane (OCM) over Supported Na2 WO4 /SiO2 Catalysts.

The 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 Na2 WO4 /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 Na2 WO4 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 C2 Hx and over-oxidizing CHy to CO and (ii) molten Na2 WO4 phase is mainly responsible for over-oxidation of CH4 to CO2 and also assists in oxidative dehydrogenation of C2 H6 to C2 H4 . These new insights reveal the nature of catalytic active sites and resolve the OCM reaction mechanism over supported Na2 WO4 /SiO2 catalysts.

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.