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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 Na 2 WO 4 /SiO 2 catalysts. The present study demonstrates, with the aid of in-situ Raman and chemical probe (H 2 -TPR, TAP and steady-state kinetics) experiments, that the long speculated crystalline Na 2 WO 4 active phase is unstable and melts under OCM reaction conditions, partially transforming to thermally stable dispersed surface Na-WO x sites. Kinetic analysis via temporal analysis of products (TAP) and steady-state OCM reaction studies demonstrate that ( i ) surface Na-WO x sites are responsible for selectively activating CH 4 to C 2 H x and over-oxidizing CH y to CO and ( ii ) molten Na 2 WO 4 phase is mainly responsible for over-oxidation of CH 4 to CO 2 and also assists in oxidative dehydrogenation of C 2 H 6 to C 2 H 4 . These new insights reveal the nature of catalytic active sites and resolve the OCM reaction mechanism over supported Na 2 WO 4 /SiO 2 catalysts.

Vehicles that run on natural gas must be equipped with an exhaust gas after-treatment system to burn their unavoidable CH4 slip, which must not be released into the environment as CH4 is a potent greenhouse gas. Pd is known as a highly active catalyst material for CH4 oxidation. For this study, Pd was deposited on a γ-Al2O3 support and the as-received samples were pre-reduced and pre-oxidized, respectively. Near-Ambient Pressure Photoelectron Spectroscopy and the Temporal Analysis of Products methodology were combined to understand that both electronic states, namely PdO and Pd°, are active for the CH4 oxidation reaction, whereby the superior activity of the pre-reduced Pd/γ-Al2O3 catalyst was ascribed to the spill-over of OH-groups from the support to Pd. It was found that the reaction products, CO, CO2 and H2, were strongly and, to a large extent, adsorbed on the catalyst surface and were involved in reversible chemical interactions on the catalyst surface during the CH4 oxidation reaction on the Pd/γ-Al2O3 catalyst.

Simultaneous reduction of N2O in the presence of co-existing oxidants, especially NO, from industrial plants, is a challenging task. This study explores the applications of a hydrocarbon reduced Rh/Zr stabilized La doped ceria (Rh/CLZ) catalyst in N2O abatement from oxidant rich industrial exhaust streams e.g. NO, CO2, and O2. The reaction mechanism was studied by the temporal analysis of products. The obtained results revealed that hydrocarbon pretreatment led to the creation of ceria oxygen vacancies and the formation of carbon deposits on the Rh/CLZ catalyst surface. These ceria oxygen vacancies are the active sites for the selective reduction of N2O into N2, while the dissociated O atoms from N2O fill the ceria oxygen vacancies. The oxidation of the deposited carbon via the lattice ceria oxygen generates new ceria oxygen vacancies, thereby extending the catalytic cycle. The reduction of N2O over C3H6 reduced Rh/CLZ is a process combining oxygen vacancy healing and deposited carbon oxidation. The results obtained from fixed-bed reactor experiments demonstrated that the hydrocarbon reduced Rh/CLZ catalyst provided a unique and extraordinary N2O abatement performance in the presence of co-existing competing oxidants (reactivity order: N2O ∼ NO > O2 > CO2 ∼ H2O).

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.

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.

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.

Environmentally friendly and low-cost catalysts are required for large-scale nonoxidative dehydrogenation of propane to propene (PDH) to replace currently used CrOx- or Pt-based catalysts. This work introduces ZnO-containing ZrO2- or MZrOx-supported (M = Ce, La, Ti or Y) catalysts. The most active materials outperformed the state-of-the-art catalysts with supported CrOx, GaOx, ZnOx, or VOx species as well as bulk ZrO2-based catalysts without ZnO. The space–time yield of propene of 1.25 kgC3H6·kg–1cat·h–1 at a propane conversion of about 30% with a propene selectivity of 95% was obtained over Zn­(4 wt %)/TiZrOx at 550 °C. For deriving key insights into the structure of active sites, reactivity, selectivity, and on-stream stability, the catalysts were characterized by XRD, HRTEM, EDX mapping, XPS, X-ray absorption, CO-TPR, CO2-TPD, NH3-TPD, pyridine-FTIR, operando UV–vis spectroscopy, Raman spectroscopy, TPO, and temporal analysis of products. In contrast with previous reports that used bulk ZrO2-based catalysts without ZnO, coordinatively unsaturated Zr cations are not the main active sites in the ZnO-containing catalysts. Supported ZnOx species were concluded to participate in the PDH reaction. The current X-ray absorption analysis proved that their structure is affected by the type of metal oxide used as a dopant for ZrO2 and by the crystallinity of ZrO2. Isolated tricoordinated Zn2+ species were concluded to show high activity and on-stream stability. Their intrinsic activity is enhanced when TiO2 and ZrO2 coexist in the support or when ZrO2 is promoted by TiO2. This is probably due to accelerating hydrogen formation in the course of the PDH reaction as concluded from temporal analysis of products with sub-millisecond resolution. The results of temperature-programmed oxidation of spent catalysts as well as ex situ Raman and operando UV–vis studies enabled us to conclude that the high on-stream stability of isolated tricoordinated Zn2+ species in the PDH reaction is related to their low ability to form coke. In general, the tendency for coke formation seems to increase with an increase in the degree of ZnOx agglomeration.

The formation of the first C–C bond and primary olefins from methanol over zeolite and zeotype catalysts has been studied for over 40 years. Over 20 mechanisms have been proposed for the formation of the first C–C bond. In this quantitative multiscale perspective, we decouple the adsorption, desorption, mobility, and surface reactions of early species through a combination of vacuum and sub-vacuum studies using temporal analysis of products (TAP) reactor systems, and through studies with atmospheric fixed bed reactors. These results are supplemented with density functional theory calculations and data-driven physical models, using partial differential equations, that describe the temporal and spatial evolution of species. We consider the effects of steam, early degradation species, and product masking due to the inherent autocatalytic nature of the process, which all complicate the observation of the primary olefin(s). Although quantitative spectroscopic determination of the lifetimes, surface mobility, and reactivity of adspecies is still lacking in the literature, we observe that reaction barriers are competitive with adsorption enthalpies and/or activation energies of desorption, while facile diffusion occurs in the porous structures of the zeolite/zeotype catalysts. Understanding the various processes allows for quantitative evaluation of their competing energetics, which leads to molecular insights as to what governs the catalytic activity during the conversion of methanol to primary olefins over zeolite/zeotype catalysts.

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 NH_x* 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.

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 O 2 .

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.

Catalyst deactivation due to carbon deposition is a major issue in all reforming technologies. Because of the significant economic cost of catalyst replacement, catalyst regeneration is increasingly attracting attention. The regeneration mechanism of Ni catalysts, with respect to carbon removal, was investigated on support materials prepared by one-pot synthesis. The supports were classified based on their redox functionality: Al2O3, MgAl2O4 show no redox properties in contrast to MgFe0.09Al1.91O4 and CeZrO2 that have redox functionality. A Temporal Analysis of Products (TAP) setup was used to investigate the isothermal regeneration mechanism of Ni catalysts at 993 K by O2. Different mechanisms were distinguished depending on the redox functionality of the support material. Two consecutive steps occur on the support that have no redox properties (Al2O3 and MgAl2O4): metallic Ni is oxidized to form NiO (oxidation step), resulting in an initial local temperature increase of 50–60 K in total, enabling metal particle migration to carbon that was initially separated from the metal and subsequent oxidation through NiO lattice oxygen (reduction step). On the other hand, the mechanism of carbon removal by O2 from Ni catalysts on supports with redox properties does not require particle migration. Two parallel contributions are proposed: 1) Ni metal is oxidized to form NiO, where after lattice oxygen of NiO is used for the oxidation of carbon that is deposited upon the metals, 2) carbon oxidation through lattice oxygen that is provided by the support. No dependency of the carbon gasification mechanism on the exposed fraction of the metal (particle size in the nanoscale) or on the structure of the deposited carbon was concluded.

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., Unravelling the mystery of Au/CeO2 catalysts! Challenged by the complex nature of Au/CeO2 catalysts, operando X‐ray absorption spectroscopy in the near and extended Au LIII and Ce LIII energy edges is employed to decipher different aspects of the electronic and geometric features of Au NPs/clusters and CeO2 support. These results combined with findings from ex‐situ XPS, in situ FTIR/Raman spectroscopy and temporal analysis of products (TAP) reactor, provide a more fundamental insight into the performance of these catalysts.

The diffusion of saturated and unsaturated hydrocarbons is of fundamental importance for many zeolite‐catalyzed processes. Transport of small alkenes in the confined zeolite pores can become hindered, resulting in a significant impact on the ultimate product selectivity and separation. Herein, intracrystalline light olefin/paraffin diffusion through the 8‐ring windows of zeolite SAPO‐34 is characterized by a complementary set of first‐principle molecular dynamics simulations, PFG‐NMR experiments, and pulse‐response temporal analysis of products measurements, yielding information at different length and time scales. Our results clearly show a promotional effect of the presence of Brønsted acid sites on the diffusion rate of ethene and propene, whereas transport of alkanes is found to be insensitive to the presence of acid sites. The enhanced diffusivity of unsaturated hydrocarbons is ascribed to the formation of favorable π–H interactions with acid protons, as confirmed by IR spectroscopy measurements. The acid site distribution is proven to be an important design parameter for optimizing product distributions and separations., The influence of the acid site density on the diffusivities of alkanes and alkenes through the 8‐ring windows of SAPO‐34 was probed using a complementary set of ab initio MD simulations and experimental techniques (PFG‐NMR and TAP pulse‐response measurements). The presence of Brønsted acid sites was found to have a clear promotional effect on alkene diffusion but it does not influence alkane diffusion.

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.

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 Bronsted 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.

Unique challenges to creating multiscale architectured materials arise from the gap between new material synthesis methods, fundamental scientific study, and their engineering applications. This chapter discusses bridging this gap and focuses on three examples that exemplify bottom-up design to build material platforms for the mechanistic study of electron, ion, or molecule transport, as well as a wide range of engineering applications. Specifically interfacial phenomena at atomic and molecular levels will be discussed in seemingly different systems, including: (1) nanoscale rare earth element (REE)-based catalysts for the water–gas shift reaction (WGS), (2) platinum atom cluster supported on phase-pure molybdenum carbide nanotube (beta-Mo2C) for hydrogen evolution reaction and oxygen reduction reaction, and (3) nanoscale carbon fibers for supercapacitors. Surface characterization techniques covered include the Brunauer–Emmett–Teller method, X-ray photoelectron spectroscopy, X-ray diffraction, high-resolution transmission electron microscopy, electrochemical cells, diffuse reflectance infrared Fourier transform spectroscopy, and temporal analysis of products. Engineering applications include: (1) dip-coating nanoscale REE catalysts onto tubular reactor surfaces for WGS, (2) membrane electrode assembly testing of platinum (Pt)/Mo2C catalysts as both anodes and cathodes of proton exchange membrane fuel cells, and (3) symmetrical supercapacitors for energy storage. Finally, the importance of a feedback loop between predictive modeling and experimental platforms that can be used to directly validate models is illustrated.

The Temporal Analysis of Products (TAP) technique is a pulse response methodology that reveals the time-dependent evolution of chemical processes between a gas phase molecule and a solid surface. By simplifying both the transport and kinetic processes the complexity of a multistep chemical processes can be distilled to where the underlying fundamental steps can be distinguished. Moreover, the technique is applicable to real materials (e.g. industrial catalysts) and hence the complexity of the solid is studied intact; a ‘top-down’ approach. In this short review we discuss key theoretical advancements in the interpretation of pulse response data; though not yet widely implemented, we will highlight the broader impacts of the Y-Procedure method [ 1•• ], momentary equilibrium phenomena [ 2•• ] and kinetic coherency discrimination [ 3•• ] over more commonly used analysis techniques. The conventional TAP approach for mechanism development is well-laid out in a recent work focused on the upgrading of ethanol to higher hydrocarbons. This includes detailed comparison of the time characteristics of reactant, intermediate and product pulses in both single pulse and pump/probe modes [ 4•• ]. This work calls to mind some of the challenges in the conventionally used residual gas analyzer for pulse response detection of homologous products. A new detection method using photoelectron photoion coincidence spectroscopy (PEPICO) was recently demonstrated for a TAP-like setup [ 5•• ]. This method presents the exciting opportunity to better resolve complex product spectra and even isomeric species. This enables the mechanistic detail of the TAP technique to be applied to more complex probe molecules as are needed for the study of biomass conversion processes. Finally, we discuss directions for the future development of instrumentation that can directly connect this rich kinetic data to features of catalyst structural and composition.

Recently, there has been increasing evidence that CO oxidation on TiO2 supported Au catalysts proceeds predominantly via a Au-assisted Mars-van Krevelen mechanism for reaction temperatures of 80 °C and above. We here present results of a combined experimental and theoretical study, aiming at the identification of activated steps in this reaction. O2 multipulse experiments, performed in a temporal analysis of products (TAP) reactor at different temperatures between -80 and +240 °C, revealed that the replenishment of surface lattice oxygen vacancies at perimeter sites, at the perimeter of the interface between TiO2 support and Au nanoparticles, proceeds with essentially constant efficiency, independent of the reaction temperature. Hence, this reaction step is barrier-free. Previous studies (Widmann and Behm Angew. Chem. Int. Ed. 2011, 50, 10241) had shown that the preceding step, the formation of a surface lattice oxygen vacancy at these sites, is activated, requiring temperatures above room temperature. Density functional theory based calculations, performed on a Au nanorod supported on a TiO2 anatase (101) substrate confirmed that the presence of the Au nanorod leads to a significant reduction of the vacancy formation energy at these sites, resulting in a barrier of only ∼0.9 eV for vacancy formation by reaction with adsorbed CO. The reverse process, replenishing the vacancies by reaction with O2, was found to be activated in the case of individual vacancies but essentially barrier-free for the case of pairs of neighbored vacancies. Consequences of these findings for the mechanism of the CO oxidation reaction on these catalysts, which can be considered as a model system for Au catalysts supported on reducible oxides, are discussed.

We report a MgFexAl2-xO4 synthetic spinel, where x varies from 0 to 0.26, as support for Ni-based catalysts, offering stability and carbon control under various conditions of methane reforming. By incorporation of Fe into a magnesium aluminate spine!, a support is created with redox functionality and high thermal stability, as concluded from temporal analysis of products (TAP) experiments and redox cycling, respectively. A diffusion coefficient of 3 x 10(-17) m(2) s(-1) was estimated for lattice oxygen at 993 K from TAP experiments. X-ray diffraction (XRD) and extended X-ray absorption fine structure (EXAFS) modeling identified that the incorporation of iron occurs as Fe3+ in the octahedral sites of the spinel lattice, replacing aluminum. Simulation of the X-ray absorption near edge structure (XANES) spectrum of the reduced support showed that 60 +/- 10% of iron was reduced from 3+ to 2+ at 1073 K, while there was no formation of metallic iron. A series of Ni/MgFexAl2-xO4 catalysts, where x varies from 0 to 0.26, was synthesized and reduced, yielding a supported Ni-Fe alloy. The evolution of the catalyst structure during H-2 temperature-programmed reduction (TPR) and CO2 temperature-programmed oxidation (TPO) was examined using time-resolved in situ XRD and XANES. During reforming, iron in both the support and alloy keeps control of carbon accumulation, as confirmed by O-2-TPO on the spent catalysts. By fine tuning the amount of Fe in MgFexAl2-xO4, a supported alloy was obtained with a Ni/Fe molar ratio of similar to 10, which was active for reforming and stable. By comparison of the performance of Ni-based catalysts with Fe either incorporated into or deposited onto the support, the location of Fe within the support proved crucial for the stability and carbon mitigation under reforming conditions.

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.

Aiming at a better mechanistic understanding of Mars-van Krevelen type catalytic reactions , we have systematically investigated possible correlations between the composition of the reaction gas atmosphere, the concentration of surface oxygen vacancies (surface oxidation state), and the catalytic activity of a Au/TiO 2 catalyst in the CO oxidation reaction . In the meantime, there is considerable experimental and theoretical evidence that this reaction follows a Au-assisted Mars-van Krevelen mechanism at reaction temperatures ≥80 °C. Employing quantitative pulse experiments in a temporal analysis of products (TAP) reactor we found that both the surface oxidation state and the activity of the catalyst under steady-state conditions depend sensitively on the composition of the reaction gas atmosphere, specifically on the CO:O 2 ratio, and that there is a strict correlation between these quantities. A simple kinetic model is introduced, which allows to quantitatively determine the ratio of the effective reaction rate constants for reduction and oxidation of the TiO2 support surface. These results and ideas are considered to be of general relevance for the understanding of Mars-van Krevelen type reactions and specifically of oxidation reactions on supported Au catalysts, and they lend further support to previous proposals of a Au-assisted Mars-van Krevelen mechanism for this reaction.

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 (TAP) 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.

This paper presents a straightforward procedure for non-steady-state kinetic characterization of solid active materials, particularly catalysts, by extracting quantities called reactiviti es which are obtained from pulse-response data using the Temporal Analysis of Products (TAP) methodology. This method provides a new ‘state characterization’ of materials to understand their physicokinetic behavior and does not require knowledge of mechanism. In this paper, three reactivities are presented as explicit functions of pulse response data and then the numerical values of reactivities are calculated from experimental moment data of pulse response curves. Two procedures based on the availability and reliability of reactant and product data are distinguished here. The developed theoretical procedure is illustrated by the experimental example of CO oxidation over a supported Au/SiO 2 catalyst pretreated with oxygen. We consider the presented procedure as an efficient tool for precise non-steady-state kinetic characterization of many active materials, e.g. catalysts, adsorbents, sensors, electrodes etc.

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.