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A number of strategies for time-resolved analysis of heterogeneous oxidation reactions are being explored. They comprise the temporal analysis of products (TAP) reactor including micro-kinetic evaluation of TAP experiments, steady-state isotopic transient kinetic analysis, and time-resolved catalyst characterization by in-situ spectroscopic techniques. The present review demonstrates the potential of these methods for elucidating mechanistic and kinetic aspects of oxidative functionalization of C 2 –C 3 hydrocarbons, water–gas shift reaction, direct N 2 O decomposition, and high-temperature ammonia oxidation to nitric oxide (Ostwald process). Particular emphasis will be put on the application of isotopic labels and time-resolved analysis of gas-phase and surface species for identifying true surface intermediate species. Perspectives in further developments of time-resolved methods will be also indicated.

The oxidation of HCl to Cl2 (Deacon process) was studied over RuO2-based catalysts in bulk and supported forms. RuO2 (7 wt.%) was incorporated onto different TiO2 carriers (rutile, anatase, and P25) by deposition–precipitation of Ru(OH)2 followed by calcination. RuO2/TiO2-rutile exhibits the highest activity at ambient pressure in a continuous flow fixed-bed reactor. Insights into the mechanism of this experimentally demanding reaction on RuO2 and the influence of the TiO2-rutile carrier were gained by studies in the Temporal Analysis of Products (TAP) reactor at 623 K. HCl multipulsing in the absence of O2 leads to chlorination of the catalysts with no detectable gas-phase Cl2 production. The chlorine uptake of RuO2 amounts to 0.16 mmol Cl gcat-1, which enables to estimate that ca. 75% of the total surface Ru atoms (coordinatively unsaturated and bridge sites) are chlorinated, forming a RuO2−xClx oxychloride. RuO2 is also chlorinated by Cl2, but to a lesser extent compared to HCl. The chlorine uptake of fresh RuO2/TiO2-rutile per gram of sample was four times higher than that of fresh RuO2, namely due to chlorination of the high surface area carrier. Upon equilibration at ambient pressure under Deacon conditions, the TAP-derived chlorine uptake in the supported catalyst experienced a 6-fold decrease with respect to the fresh sample due to permanent chlorination. Pulsing of a mixture of O2 and HCl over the catalysts revealed that both gas-phase Cl2 evolution and H2O desorption are impeded processes. Pump–probe experiments of O2 and HCl evidenced the tight dependence of the net Cl2 production on both the oxygen and chlorine coverage. In addition, a limited contribution of lattice surface species could be detected by the production of small amounts of Cl2 at very low coverages. The equilibration procedure (ambient or TAP conditions) influences the reaction mechanism and the net Cl2 production, giving account of the reaction complexity due to the highly dynamic state of the catalyst surface. The Deacon process on RuO2−xClx-based catalysts can be mostly described by a Langmuir–Hinshelwood scheme with a minor 2D Mars–van Krevelen contribution.

The mechanism of CO oxidation reactions over undoped and gold-doped CuMnOX (Hopcalite) catalysts has been examined using a temporal analysis of products (TAP) reactor. Gold doping has been found to increase the activity of the mixed oxide catalyst significantly; however, using consecutive pulsing TAP experiments, the presence of gold was not found to affect the contribution of the Langmuir-Hinshelwood mechanism. Conversely, gold doping was found to promote the Mars van Krevelen mechanism. Using CO and O2 multi-pulse TAP experiments, the gold was found to modify the catalyst surface such that it stores much more oxygen that is active for the CO oxidation. The CO multi-pulse experiments indicated that two distinct types of active oxygen species were found to be involved in the CO oxidation. One type was observed in a similar amount on both doped and undoped catalysts and was associated with mixed oxide, while the second type was only found on the gold-doped catalyst and was therefore clearly associated with the presence of gold on the catalyst surface. The latter was found to be much less active than the oxygen inherent to the oxide but was at a concentration of approximately 10 times larger leading to the enhanced activity observed on gold doping. © 2010 Elsevier Inc. All rights reserved.

Temporal Analysis of Products (TAP) reactor system was used to investigate CO oxidation behavior on Au/SiO2 catalysts prepared via the physical vapor deposition method of magnetron sputtering. Au/SiO2 catalysts are a valuable model system for studying the reactivity of Au nanoparticles, as the SiO2 support plays little role in the reaction. The adsorption of CO on the catalyst was studied at different temperatures under TAP vacuum conditions. The heat of CO adsorption estimated from TAP results by moment analysis is −24.39 ± 3.70 kJ/mol. Multipulse TAP experiments showed the important dependency between the oxygen pretreatment pressure (UHV to 33.66 psia) and catalytic activity for CO oxidation. The observed kinetic behavior indicates that oxygen consumed by CO can be replenished by oxygen stored in a reservoir, which is filled during pretreatment. The oxygen pretreatment pressure determines the amount of oxygen stored in the reservoir. The kinetic behavior indicates that oxygen is stored in the gold subs...

The use of in situ approaches to study mechanisms of experimentally demanding and complex reactions under conditions of practical relevance opens doorways to discover new or improved heterogeneous catalysts. A representative example within this category of reactions is the gas-phase oxidation of HCl to Cl2 (Deacon process). Studies in the temporal analysis of products (TAP) reactor complemented by flow experiments at ambient pressure evidenced pronounced mechanistic differences between copper catalysts (CuO, CuCl, and CuCl2, that is, Deacon-like catalysts) and RuO2 (the basis of the recent Sumitomo’s catalyst for large-scale Cl2 production). RuO2 is 1 order of magnitude more active than the copper-based materials. HCl oxidation on RuO2 obeys a Langmuir−Hinshelwood mechanism in the presence of adsorbed species over a partially chlorinated surface. HCl oxidation on the copper catalysts is more complex: all the samples experience bulk changes, namely, chlorination, resulting in multiphase materials. Besides,...

Mechanistic and kinetic aspects of direct N2O decomposition over BaFeAl11O19 hexaaluminate were investigated in the Temporal Analysis of Products (TAP) reactor and compared with those previously determined for Fe-MFI zeolites. The catalysts were chosen due to their de-N2O operation in significantly different temperature regimes. Several micro-kinetic models were evaluated for describing the transient responses of N2O, N2, and O2 obtained in N2O pulse experiments at 823–973 K. Thorough discrimination between these models enabled us to conclude that the preferred models of N2O decomposition over BaFeAl11O19 and Fe-MFI zeolites differ in the reaction pathways leading to O2 and N2. Gas-phase N2 and O2 are simultaneously formed over BaFeAl11O19 upon interaction of gas-phase N2O with a bi-atomic surface oxygen (*–O2) species. Contrarily, the formation of O2 over Fe-MFI occurs via a sequence of three elementary heterogeneous steps and limits the overall rate of N2O decomposition. Despite the easy O2 formation, BaFeAl11O19 is less active for N2O decomposition below 973 K than the Fe-MFI zeolites due to the low coverage by *–O2. According to our quantitative micro-kinetic analysis, this species is formed when gas-phase N2O reacts with a mono-atomic oxygen (*–O) species. This reaction pathway is strongly influenced by the degree of isolation of iron species. The higher the degree of iron isolation in the catalyst, the lower the de-N2O activity.

A systematic study over Pt/BaO/Al 2 O 3 powder catalyst is carried out using Temporal Analysis of Products (TAP) to estimate the effective diffusivity of stored NO, in the barium phase. The prenitration of PtBaO/ Al 2 O 3 using sequential pulses of NO, followed by reduction with H 2 results in the evolution of N 2 and NH 3 . The reduction is carried out in the NO x transport limited regime in which diffusion of the stored NO, from BaO storage phase to Pt/BaO interface was determined to be the rate controlling process. The effluent profiles of N containing species (2N 2 + NH 3 ) were used to estimate stored NO, diffusivity in barium phase and the apparent activation energy. The activation energy (74-81 kJ/mol) is in good agreement with the estimated value of 75 kJ/mol from a recent NO, trap modeling study. 1 .

Reaction pathways governing HCN selectivity in the oxidative coupling of methane and ammonia were investigated over three commercial Pt–Rh gauzes (fresh, activated, and spent) in the temporal analysis of products reactor with submillisecond-time resolution using isotopic traces. These gauzes differed in the extent of reaction-induced restructuring as well as impurity content. The ability of the gauzes to form HCN in the dual interactions of CH4 with NH3 or NO was compared considering the morphological differences of the gauzes. It was found that the progressive structural changes increased the gauze activity for methane conversion and facilitate the stabilization of methane fragments on the catalyst surface, but did not influence significantly the surface residence time of N-containing species. The well-known increase in HCN selectivity within first hours on-stream in the Andrussow process was suggested to be likely due to restructuring-induced stabilization of surface methane fragments. The decrease in the HCN selectivity after a stable phase of operation is mainly related to consecutive oxidation of HCN over iron oxide accumulated on the catalyst surface under industrial conditions of the Andrussow process.

Primary O 2 –NH 3 –CH 4 interactions in the Andrussow process were investigated in the temporal analysis of products (TAP) reactor over three commercial Pt-Rh gauzes having been applied for different times on-stream in this reaction (fresh, activated and spent catalysts). The gauzes were characterized by scanning electron microscopy combined with energy dispersive X-ray analysis. It was established that they underwent severe morphological changes including deposition of various impurities with increasing time on-stream. Content and type of these impurities possess local variations. However, the near-to-surface Pt/Rh ratio of the deposit-free areas does not change with time on-stream. These areas are strongly faceted on the spent Pt-Rh gauze. Despite the gauzes differ in their morphology and in the content of impurities, the overall scheme of HCN production via oxidative coupling of CH 4 and NH 3 is valid for all the gauzes studied. Nitric oxide primarily formed via ammonia oxidation reacts further with methane and ammonia yielding hydrogen cyanide and nitrogen, respectively. The formed HCN is consecutively converted to N 2 . The surface restructuring, coverage by oxygen species and/or the presence of iron oxide influence the interplay between the reaction pathways leading to HCN and its further transformations.

Propane dehydrogenation over a Pt–Sn–K/γ–Al 2 O 3 catalyst has been studied in the TAP reactor. Qualitative analysis of the pulse responses reveals part of the reaction network. Dissociative adsorption of propane yields propylene and hydrogen. Propylene is further converted into coke. Hydrogen and propylene compete for the same active sites. The first order rate constant for the propane conversion has been evaluated by moment analysis, the thin-zone model and regression analysis of the full pulse response for different catalyst bed lengths and temperatures. This has shown that the three methods yield comparable results for both the values of the rate constant and the corresponding activation energies. However, the expression derived from the thin-zone model is the easiest to use and gives a reasonable order of magnitude estimate of the first order rate constant.

A systematic isotopic study over Pt/BaO/Al2O3 powder catalyst was carried out using temporal analysis of products (TAP) to elucidate the role of the Pt/BaO interface and spillover processes during NOX storage and reduction. The sequential pre-nitration of Pt/BaO/Al2O3 using NO and 15NO followed by reduction with H2 results in the preferential evolution of 15N-containing species during initial H2 pulses. The evolution shifts toward unlabeled N-containing species in later H2 pulses. The data suggest that NOX storage proceeds radially outward from the Pt crystallites and that some mobility of the stored NOX species exists. The evolution of N2 and 15NN during 15NO–H2 pump–probe on a pre-nitrated (using unlabeled NO) catalyst confirms the involvement of spillover processes at the Pt/BaO interface. The evolution of nitrogen takes place by NO decomposition as well as by the reaction of stored NOX with H2 to form adsorbed N and eventually N2. A significant fraction of N2 is also produced via NH3 serving as an intermediate. The results suggest that the local gradients at the Pt/BaO interface in the stored NOX are important and that these should be taken into account in NSR catalyst design and modeling.

For linear reaction networks, graph-theoretical methods are used to find two types of kinetic transfer functions in the Laplace domain for isothermal uniform open systems under transient regime. Rules are formulated to find the functional form of these transfer functions from properties of the reaction network, viz the number of components and the length of shortest paths. In many cases, some reaction intermediates are not observed. This is typical in heterogeneous catalysis, where the intermediates are bound to the catalytic surface. The mechanistic details are then manifested only through some transfer functions. This often causes the rate coefficients to be unidentifiable. A graph-theoretical rule is derived which allows assessing whether reaction networks are structurally locally identifiable. It is applied to the interaction of propane with a copper oxide/ceria-catalyst, as studied with a temporal analysis of products (TAP) setup.

This paper presents an overview of the evolution, advancements, and capabilities of the temporal analysis of products (TAP) reactor system as a unique catalyst characterization tool. The origination of the TAP reactor based on molecular beam scattering experiments is briefly mentioned. The advancement in TAP reactor design from the TAP-1 system to the TAP-3 system is introduced to highlight its relevance as a valuable tool for elucidating mechanistic and kinetic aspects of adsorption, diffusion, and reaction in gas–solid systems. Since the invention of the TAP reactor system, a series of TAP microreactor configurations has been introduced with different amounts of catalyst packing starting from the one-zone microreactor to the most recent introduction, the single particle microreactor in which a single Pt particle is packed among 100,000 inert quartz particles. An advantage to decreasing the catalyst zone inside the microreactor is to eliminate non-uniformity in the active zone while still achieving high conversions (95%). Experimental designs and results coupling the TAP reactor to other experimental systems such as a time-of-flight mass spectrometer and atomic beam deposition system is also presented. Key results from recent TAP experiments are presented to show how the TAP reactor is used to answer fundamental questions in catalysis such as bridging the pressure gap between industrial catalysis and surface science, understanding the surface lifetimes of reactive adspecies in TAP pump-probe experiments, finding kinetic rate constants related to changes in catalyst composition and its performance.

The oxidative dehydrogenation of propane in the absence of O2 over the vanadia-based EL10V1 Eurocat catalyst is investigated in a Temporal Analysis of Products (TAP) reactor over a completely oxidized catalyst and at reduction degrees up to 0.47. Only CO2 and propene are detected as reaction products at temperatures from 723 to 823 K. The reduction of the catalyst with propane occurs through the formation of propene, the consecutive oxidation of propene to CO2 and the parallel total oxidation of propane. Four elementary reactions are considered as kinetically relevant: (1) the methyl C–H bond dissociation in propane during the formation of propene, (2) the methylene C–H bond breaking in propane to form surface isopropoxide, which finally transforms into CO2, (3) the C C double bond breaking in propene in the formation of surface formate species and (4) the oxidation of these surface formate species to CO2. The formation of propene via (1) is favored at the investigated conditions, while CO2 is largely produced by the sequential oxidation of propene (3–4), and to a lesser extent by the parallel route of the direct total oxidation of propane (2). Both the oxidative dehydrogenation and the direct total oxidation of propane reaction paths involve only one kinetically relevant step, (1) respectively (2), with an activation energy of 36 and 74 kJ mol−1 over a completely oxidized catalyst and of 15 and 40 kJ mol−1 over a partially reduced catalyst. The further oxidation of propene involves two kinetically relevant steps (3–4) of which only the double bond breaking (3) is slightly activated (5 kJ mol−1) over the completely oxidized catalyst, while both steps are non-activated when the catalyst is partially reduced.

The transient temporal analysis of products (TAP) pulse method has been applied to investigate the isomerization of n-butane on sulfated zirconias at very low pressure. By combining these results with findings from XRD, XPS, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), temperature-programmed desorption (TPD), and catalytic studies at atmospheric pressure, a wide range of pressure conditions can be accessed to study the influence of surface sulfate groups on the isomerization activity and the mechanism of this reaction. The catalytic activity of powder catalysts has been correlated to the surface site density of sulfates, with pyrosulfate structures playing a major role in the initiation of the reaction. The complex interplay between sulfate-free and sulfate-covered zirconia surfaces for the adsorption of alkanes has been investigated in detail, with TAP pulse experiments showing a monomolecular reaction pathway at low n-butane partial pressures. Moreover, TAP pulse experiments have allowed detection of the reaction products of this initiation process, including butene and water, and have shown their influence on the deactivation of sulfated zirconias. Heats of adsorption have been calculated from the TAP pulse experiments.

Following the recent finding [M. Kotobuki, R. Leppelt, D. Hansgen, D. Widmann, R.J. Behm, J. Catal. 264 (2009) 67] of a stable oxygen species on non-porous Au/TiO2 catalysts (TiO2: P25, Degussa), which is active for CO oxidation and which can be reversibly deposited and removed by exposure to O2 and CO pulses, respectively, we here present results of a TAP (temporal analysis of products) reactor study on the oxygen storage capacity (‘OSC’) for three Au/TiO2 catalysts supported on mesoporous TiO2 with different structural characteristics and its relation to the CO oxidation activity. The results are compared with the corresponding behavior of the non-porous Au/P25 supported catalyst. While the general reaction behavior of all catalysts is similar, there are distinct differences both in the absolute oxygen storage capacity and in its relation to the catalytic activity, depending on the nature of the support. The differences are associated with variations in the structure of the Au/TiO2 interface and its perimeter, which is proposed as the adsorption site for active oxygen storage.

Single pulse TAP experiments were chosen for obtaining estimates of intra-particle diffusion coefficients and better insight into adsorption–desorption dynamics and equilibria for isobutane and n -butane in commercially available beta and USY zeolite. This technique provides a unique way of directly estimating transport and sorption processes at extremely low surface coverage ( θ i → 0), in the absence of an inert carrier stream, with no external mass transfer resistance, and with a negligible thermal effect. The use of a thin zone TAP reactor configuration enables the use of small zeolite particles without causing high bed resistance. A theoretical model that considers transport and adsorption–desorption phenomena in the inter-particle and intra-particle space is developed and numerically solved. Numerical experiments demonstrate the ability of the model to represent the actual experimental response curves. It is shown that reliable values of intra-particle diffusivity and adsorption–desorption constants can be obtained from the TAP single pulse experiments when the three dimensionless constants of the model τ p i (the ratio of characteristic diffusion time in the micro-reactor to characteristic diffusion time in zeolite pore), K eq i (the ratio of characteristic desorption time to characteristic adsorption time), and k d i ∗ (the ratio of characteristic diffusion time in the micro-reactor to characteristic desorption time) fall within a prescribed range.

N2O decomposition to gaseous N-2 and O-2 catalyzed by a commercial Fe-ZSM-5 has been studied by different transient techniques: (i) via the transient response methods at ambient pressure, (ii) via the temporal analysis of products (TAP) reactor under vacuum, and (iii) by temperature-programmed desorption (TPD) under vacuum. The catalyst was activated in He at 1323 K. Two main steps can be distinguished within the transient period of N2O decomposition under constant N2O feed at 603 K: the first step consists of molecular N-2 formation and surface atomic oxygen (O)(Fe). It follows a period of stoichiometric N2O decomposition to gaseous N-2 and O-2 with increasing conversion until steady state is reached. The observed rate increase is assigned to a slow accumulation on the surface of NOx,ads species formed from N2O and participating as co-catalyst in the N2O decomposition. The NOx,ads species accelerates the atomic oxygen recombination/desorption, which is the rate-determining step of N2O decomposition. The formation and accumulation of NOx,ads species during N2O interaction with the catalyst was confirmed by TAP studies. The amount of NOx,ads was found to depend on the number of N2O pulses injected into the TAP reactor. In the presence of adsorbed NOx on the catalyst surface (NOx,ads ) the desorption of dioxygen is facilitated. This results in a shift of the oxygen desorption temperature from 744 K to considerably lower temperatures of 580 K in TPD experiments. Pulses of gaseous NO had a similar effect leading to the formation NOx,ads thus facilitating the oxygen recombination/desorption.

A systematic study over Pt/Al2O3 powder and monolith catalysts is carried out using temporal analysis of products (TAP) to elucidate the transient kinetics of NO decomposition and NO reduction with H2. NO pulsing and NO–H2 pump-probe experiments demonstrate the effect of catalyst temperature, NO–H2 pulse delay time and H2/NO ratio on N2, N2O and NH3 selectivity. At lower temperature (150 °C) decomposition of NO is negligible in the absence of H2, indicating that N–O bond scission is rate limiting. At higher temperature NO decomposition occurs readily on reduced Pt but the rate is inhibited by surface oxygen as reaction occurs. The reduction of NO by a limiting amount of H2 at lower temperature indicates the reaction of surface NO with H adatoms to form N adatoms, which react with adsorbed NO to form N2O or recombine to form N2. In excess H2, higher temperatures and longer delay times favor the production of N2. The longer delay enables NO decomposition on reduced Pt with the role of H2 being a scavenger of surface oxygen. Lower temperatures and shorter delay times are favorable for ammonia production. The sensitive dependence on delay time indicates that the fate of adsorbed NO depends on the concentration of vacant sites for NO bond scission, necessary for N2 formation, and of surface hydrogen, necessary for hydrogenation to ammonia. A mechanistic-based microkinetic model is proposed that accounts for the experimental observations. The TAP experiments with the monolith catalyst show an improved signal due to the reduction of transport restrictions caused by the powder. The improved signal holds promise for quantitative TAP studies for kinetic parameters estimation and model discrimination.

Gubanova, Elena L. Sadykov, Vladislav A. van Veen, Andre C. Mirodatos, Claude; The temporal analysis of products (TAP) technique coupled with the oxygen TPD was used to elucidate the effects of the Pt-supported fluorite-like doped ceria-zirconia oxide chemical composition and the type of pretreatment on their oxygen bonding strength, mobility, and reactivity as related to catalytic properties in the partial oxidation of methane into synthesis gas. A rapid evolution of hydrogen under CH4 pulse observed for oxidized catalysts agrees with the direct route of the methane selective oxidation into syngas. This route is favored by the Pt-support interaction and a moderate bonding strength of surface oxygen species along with a high lattice oxygen mobility.

A simple method for determining the speed of oxygen release from Pt/CeO2–ZrO2 materials is proposed. We determined the speed of oxygen release from three differently prepared Pt(1 wt%)/CeO–ZrO2 catalysts by two methods: the method employing temporal analysis of products (TAP) reactor with CO, O2, and CO2 pulsed gases, and a conventional method with CO and O2 switch gases. Strong CO2 adsorption precluded correct analysis of oxygen release speed, when it was determined from the amount of CO2 formed. Therefore, instead of the amount of CO2 formed, we suggested the tmax value, defined as the position of maximal intensity of CO2 transient responses obtained upon low-intensity (1015–1016 molecules) CO pulsing in the TAP reactor. Shorter the tmax value, higher the rate of oxygen release, because the temperature dependence of the tmax determined in the present study resembles the previous results of oxygen release analysis [T. Tanabe, A. Suda, C. Descorme, D. Duprez, H. Shinjoh, M. Sugiura, Stud. Surf. Sci. Catal. 138 (2001) 135].

We developed a probabilistic theory of transport–reaction processes over various types of configurations of catalytic particles, particularly for a single particle. The theory describes a single pulse–response experiment which is similar to the single pulse temporal analysis of products (TAP) experiment. The analysis is based on the general theory of Brownian motion with “killing” and the Feynman–Kac formula. Different diffusion–reaction problems, particularly the problem “diffusion–very fast reaction” (infinite rate reaction), have been analyzed. In the latter case, the probability for a reactant to be converted equals a purely geometric characteristic, namely, the probability for a reactant molecule to hit at least one catalyst particle in a configuration of particles. Solving a boundary value problem, the probability of conversion can be found as a function of the apparent kinetic parameter. Based on experimental data on exit flow and conversion this parameter can be extracted. Experimental data in studies of CO oxidation over a single particle of Pt catalyst show a qualitative agreement with data from computational experiments based on the developed probabilistic theory. For two-particle catalyst configurations, it was found computationally a non-trivial dependence of the reactant conversion on some geometric characteristics, especially the distance between particles. A distance was found for which conversion is highest.

The mechanism of ammonia oxidation over Pt, Pd, and Rh wires has been investigated in the Temporal Analysis of Products (TAP) reactor at relevant temperatures in industrial ammonia burners. The results of primary (NH3+O2)(NH3+O2) and secondary (NH3+NONH3+NO) interactions with isotopically labeled ammonia at 1073 K enable to conclude that the overall reaction pathways to NO, N2O, and N2 are equivalent on the three noble metals. NO is a primary reaction product, while N2 and N2O originate from consecutive NO transformations. The extent of the secondary reactions determines the net NO selectivity. Rhodium is the most active catalyst for the unwanted reduction of NO by NH3, while platinum shows the lowest activity. This explains the superior NO selectivity attained over Pt and, therefore, its industrial application. The TAP-derived selectivity ranking was substantiated by Density Functional Theory calculations on the (100) facets of the noble metals. We proved experimentally that NO selectivity approaching 100% at complete NH3 conversion can be equivalently attained over Pt, Pd, and Rh by increasing the oxygen content in the feed. For a feed of O2/NH3=10O2/NH3=10, both N2O and N2 production are suppressed due to the impeded NO dissociation and favored NO desorption at high oxygen coverage.

A Temporal Analysis of Products (TAP) reactor was used to study the mechanism of two green catalytic reactions, namely low-temperature CO oxidation on supported gold catalysts and volatile organic compounds (VOCs) elimination, with propane as model molecule, on vanadia-based catalysts. Single-pulse TAP experiments evidenced a weak and reversible adsorption of CO on Au/Ti(OH)(4)* catalyst and of propane on vanadia-based catalytic materials obtained by DC magnetron sputter deposition, while O-2 adsorption was irreversible on both catalysts. Alternating TAP pulse experiments provided evidence for the nature of oxygen species involved in the oxidation reactions. If the adsorbed oxygen was involved in the CO oxidation, the lattice oxygen was responsible for the propane oxidation. Multi-pulse TAP experiments assessed the degree of reduction of the catalyst while its influence upon the mechanism is determined by single-pulse experiments. The obtained results will allow the optimal formulation of industrial scale catalysts with applicability to a wide range of industrial processes.

A mechanistic scheme of N2O and N2 formation in the selective catalytic reduction of NO with NH3 over a Ag/Al2O3 catalyst in the presence and absence of H2 and O2 was developed by applying a combination of different techniques: transient experiments with isotopic tracers in the temporal analysis of products reactor, HRTEM, in situ UV/vis and in situ FTIR spectroscopy. Based on the results of transient isotopic analysis and in situ IR experiments, it is suggested that N2 and N2O are formed via direct or oxygeninduced decomposition of surface NH2NO species. These intermediates originate from NO and surface NH2 fragments. The latter NH2 species are formed upon stripping of hydrogen from ammonia by adsorbed oxygen species, which are produced over reduced silver species from NO, N2O and O2. The latter is the dominant supplier of active oxygen species. Lattice oxygen in oxidized AgOx particles is less active than adsorbed oxygen species particularly below 623 K. The previously reported significant diminishing of N2O production in the presence of H2 is ascribed to hydrogen-induced generation of metallic silver sites, which are responsible for N2O decomposition.

The catalytic partial oxidation of methane was studied over single channels of monolith catalysts Pt/PrCeZrO/α-Al2O3 and Pt/GdCeZrO/α-Al2O3 using the temporal analysis of products (TAP) and kinetic transients. Effects of catalyst composition, oxidation state, time offset between O2 and CH4 pulses on activity, selectivity and dynamics of product formation were elucidated. Realization of the direct pyrolysis-CH4 partial oxidation route was reliably established. This route is favored by optimum lattice/surface oxygen mobility and reactivity controlled by the dopant type (Gd, Pr) and oxidation state of the complex cerium/zirconium oxide.

Uniformity of change in catalyst activity during a multi-pulse TAP experiment with porous catalyst was theoretically analyzed for a TAP reactor with the ratio of the catalyst bed thickness to the reactor length of 1/30. The analysis was performed by simulation of an irreversible adsorption process. The catalyst change is described by the change in the occupied/unoccupied fractional surface coverages. The intraparticle uniformity is indicated by a small magnitude of Δθ p,max , the maximum difference between the occupied fractional surface coverages at the outermost and the innermost of the catalyst pellet during the multi-pulse experiment. In the interparticle region, the indicating quantity is Δθ b,max , the maximum difference between the pellet-outermost fractional surface coverages at the inlet and the outlet of the catalyst bed. It was found that Δθ p,max generally depends only on the effectiveness factor in the first pulse experiment, η. For η ≥ 0.94, Δθ p,max ≤ 0.05. In addition, Δθ b,max depends on...

This paper describes the development of a new technique, atomic beam deposition (ABD), for the fabrication of catalysts with different active sites capable of catalyzing different chemical reactions. Using ABD, submonolayer amounts of Te and Cu atoms were deposited on vanadyl pyrophosphate (VPO) catalyst particles, and precise non-steady-state kinetic characterization was performed using the temporal analysis of products (TAP) reactor immediately following deposition. Results from TAP Knudsen pulse response experiments determine if small changes in the metal surface composition would produce detectable changes in catalytic properties. Partial oxidation of 1-butene to furan was used as the test reaction. Furthermore, this paper also demonstrates the sensitivity of the TAP reactor by measuring oxygen uptake on a single 350 μm diameter polycrystalline Pt particle packed in a bed with 100,000 inert quartz particles of similar dimensions. The Pt experiments demonstrate the precise manipulation of surface oxygen content.

A temporal analysis of products (TAP) reactor was used to study relationships between the mechanism of direct N2O decomposition over metal-loaded zeolites and their resulting activity. Rh-ZSM-5 (prepared by incipient wetness) and Fe-ZSM-5 (prepared by liquid-ion exchange) were chosen as prototypic catalysts displaying low ( 650 K) temperature activity, respectively. Transient studies at the same contact time revealed the higher activity of Rh-ZSM-5 below 623 K and significantly stronger N2O adsorption over Rh species than over Fe species in the zeolites. Several microkinetic models were applied for simultaneous fitting the transient responses of N2O, N2, and O2. Classical reaction schemes failed to describe the experimental data. The preferred models of N2O decomposition over Rh-ZSM-5 and Fe-ZSM-5 differ in the reaction pathways of O2 formation. For both catalysts, free active metal sites (*) and those occupied by monoatomic oxygen species (*-O) from N2O participate in the decomposition of gas-phase N2O. Gas-phase O2 is formed directly on N2O interaction with *-O over Rh-ZSM-5, whereas the latter reaction over Fe-ZSM-5 leads to a surface bi-atomic oxygen species (O-*-O), followed by its transformation to *-O2. The latter species desorbs as molecular oxygen. Comparison of ion-exchanged and steam-activated Fe-ZSM-5 [J. Phys. Chem. B 110 (2006) 22586] revealed that the reaction mechanism is independent of the iron constitution induced by the preparation and activation routes, despite important differences in catalytic activity. Our quantitative microkinetic analysis demonstrated that both the stronger reversible N2O adsorption and, most importantly, the faster desorption of O2 are distinctive mechanistic features of Rh-ZSM-5, likely indicating its high de-N2O activity.

A new configuration of catalyst loading in the Temporal Analysis of Products (TAP) reactor is presented. It is a single Pt particle packed in a bed with approximately 100,000 inert quartz particles. In TAP studies, the single particle configuration was proven to be sensitive enough to monitor and compare kinetic characteristics of the same catalyst particle at both high vacuum and atmospheric pressures. Catalytic CO oxidation reaction over platinum was used to illustrate these new possibilities. In the TAP single particle experiments under high vacuum conditions, a single particle provides high conversion (up to a total conversion of 95%) despite the catalyst particle occupies less than 0.1% of the total reactor volume. This phenomenon was proven in a variety of pulse response experiments. A mathematical probabilistic theory of this counterintuitive phenomenon will be published in the next paper. Also, an interesting similarity was found from both TAP vacuum and atmospheric pressure data, i.e. both sets of data are characterized by a “turning point” at the same temperature of 170 °C indicating a transition from an O 2 dominated regime to a CO dominated regime. This observation of pressure independent characteristic of the Pt particle creates a new possibility for bridging across the pressure gap.

CO multipulse temporal analysis of products (TAP) experiments were used to characterize a ceria-supported platinum catalyst after various oxidative and reductive pretreatments using O2, H2O, CO2, and H2. Based on the amount of CO consumed, using the final CO-saturated catalyst composition as the common state point, the oxidatively pretreated catalyst could be described using a general scale. From a kinetic analysis of the CO multipulse responses, two kinetic regimes corresponding to two types of active sites could be identified. As the temperature was raised, the number of the most active sites did not change while the amount of the less active site increased. Comparison of the number of active sites determined from the TAP data reported herein with that determined by a previous steady-state isotope transient kinetic analysis experiment showed excellent agreement. This correlation indicates that the (very fast response) TAP experiments can provide information regarding the number and type of active sites that are relevant to a catalyst under real reaction conditions.

Applying quantitative temporal analysis of products (TAP) techniques, we demonstrate that after calcination, a Au/CeO2 catalyst is only slightly active for CO oxidation. The activity is significantly increased on removal of about 7% of the surface oxygen, whereas overreduction leads to initially lower activity. Reductive activation can occur either during reaction, in the (oxidative) reaction gas, or by controlled prereduction. This is the first experimental verification of a CO oxidation rate enhancement by oxygen surface vacancies on a realistic oxide-supported Au catalyst. Consequences for the reaction mechanism are discussed.

The oxygen storage capacity including OSC, OSCC and dynamic OSC was measured for Ce–Pr oxides using a temporal analysis of products (TAP) reactor system. The exact features of reduction and re-oxidation were obtained by accurately measuring the uptake of CO and O, and the production of CO 2 . It was found that the reduction temperature of Ce–Pr oxides was lowered 50–100 °C in comparison to that of CeO 2 . The substitution of cerium by praseodymium in the Ce 1− x Pr x O 2− δ oxides enhanced both OSC and OSCC. However, OSCC was much influenced by the increase of praseodymium content more than OSC. It was interpreted that the addition of Pr significantly increased the amount of oxygen migrating from bulk to surface while the migration rate of bulk oxygen was slightly accelerated. On the basis of analyses of the Ce 3d, Pr 3d and O 1s X-ray photoelectron spectra, the promotion effect of Pr on oxygen storage capacity could be attributed to the Ce 4+ /Ce 3+ , Pr 4+ /Pr 3+ redox behaviour and the enrichment of oxygen vacancies in Ce 1− x Pr x O 2− δ .

A temporal analysis of products (TAP) reactor was used to study the mechanism of ammonia oxidation at high temperature (1073 K) over Fe2O3, Cr2O3, and CeO2. The results were compared with those obtained over the industrially applied Pt95–Rh5 alloy. Analysis of characteristic times of the transient responses of N2 and NO made it possible to gain insight into the sequence of their formation. A common feature of both metal oxide and noble metal catalysts is that NO is a primary product of NH3 oxidation, whereas N2 results mainly from secondary transformations of NO. The amount of N2O formed over the oxides was minimal. Multipulse NH3 experiments in the absence of gas-phase O2 give unequivocal evidence of the participation of surface lattice oxygen of the metal oxides in the reaction of NH3 to NO. The degree of reduction of the oxide surface determines the product distribution but does not alter the catalytic activity for NH3 conversion. Greater reduction favors the reaction channel to N2 over that to NO. The desired reaction follows a Mars–van Krevelen-type scheme involving the participation of lattice oxygen in the NH3 conversion to NO and regeneration of the so-formed vacancies by gas-phase O2 and bulk lattice oxygen. Replenishment of surface vacancies by the diffusion of bulk oxygen occurs to a significant extent over Fe2O3. However, when NH3 and O2 are fed together, dissociative adsorption of gas-phase oxygen can be considered the main mechanism for vacancy regeneration.

TAP (temporal analysis of products) technique was used to clarify the controversial mechanism for low-temperature CO oxidation on supported Au catalysts involving unidentified moisture effects on the performances. The unique TAP transient technique, along the use of a specially prepared, highly active Au/Ti(OH) 4 * catalyst, provided the information and characterization of each elementary step involving weak and reversible CO adsorption, strong and molecular O2 adsorption, and their surface reaction, which are suppressed by the coexistence of water vapor.

In-situ UV/vis spectroscopy in combination with transient isotopic analysis was applied for elucidating the governing factors, which determine the superior performance of N 2 O as compared to O 2 in the oxidative dehydrogenation of propane (ODP) over VO X /MCM-41, VO X /MCM-48, and VO X /SiO 2 catalytic materials. Characterization studies applying in-situ steady-state UV/vis spectroscopy evidenced higher reduction degree of surface VO x species at 773 K in a C 3 H 8 -N 2 O (C 3 Hg/N 2 O = 1/1) feed than in a C 3 H 8 -O 2 (C 3 H 8 /O 2 = 2/1) flow at controlled conversions of propane (

The effect of moisture on the behavior of the active supported gold catalysts developed by us, Au/Ti(OH)∗4 (I) and Au/Fe(OH)∗3 (II), during CO activation and CO oxidation in the temperature range 298–473 K was investigated by the temporal analysis of products (TAP) technique. The oxidation reaction over the Au/Ti(OH)∗4 catalyst was totally suppressed by the presence of water, independent of the water content of 200 and 1000 ppm, the carbon monoxide-to-oxygen ratio, and the temperature. In contrast, the presence of water vapors showed no effect on CO oxidation over the Au/Fe(OH)∗3 catalyst. For both catalysts, the carbon monoxide adsorption feature was unalterable by the presence of water, namely CO molecules reversibly adsorbed on the catalyst surface, but the amount of CO adsorbed decreased by increasing water content. When a reaction mixture at a 1:1 ratio of CO to O2 was pulsed on the catalyst in the absence of water, the CO response curve showed two peaks, whereas in the presence of water, the TAP response curve showed only one peak. At a 1:3 CO-to-O2 ratio, the CO response curve showed only one peak, which shifted to a shorter time and narrowed in the absence of water. The shift was more pronounced on catalyst (II). The TAP experiments provided indirect evidence that CO and water adsorbed together (CO weakly and water strongly) and water blocked [irreversibly for Au/Ti(OH)∗4 and reversibly for Au/Fe(OH)∗3], the adsorption sites for oxygen.

Pt supported on CeO 2 and 10 wt.% La 3+ -doped CeO 2 catalysts have been prepared, characterised and tested for soot oxidation by O 2 in TGA. The reaction mechanism has been studied in a TAP reactor with labelled O 2 . Isotopic oxygen exchange between molecular O 2 and ‘O’ on the support/catalyst was observed and soot oxidation is being carried out by lattice oxygen. TAP studies further show that Pt improves O 2 adsorption and, therefore, 5 wt.% Pt-containing catalysts are more active for soot oxidation than the counterpart supports. In addition, CeO 2 doping by La 3+ leads to an improved support, since La 3+ stabilises the structure of CeO 2 when calcined at high temperature (1000 °C) and minimises sintering. In addition, La 3+ improves the Ce 4+ /Ce 3+ reduction as deduced from H 2 -TPR experiments and favours oxygen mobility into the lattice. A synergetic effect of Pt and La 3+ is observed, Pt-containing La 3+ -doped CeO 2 being the most active catalyst for soot oxidation by O 2 among the samples studied.

This manuscript concisely reviews the significance of the temporal analysis of products (TAP) technique in catalysis research from its introduction to the scientific community in the late 1980s. Evolutionary aspects of this time-resolved transient pulse method are presented, highlighting its relevance for elucidation of mechanistic and kinetic aspects of adsorption, diffusion, and reaction in gas–solid systems. The high-temperature ammonia oxidation over noble metal catalysts is used to underline key advantages of the TAP reactor in mechanistic studies, narrowing the often-encountered pressure and materials gaps between techniques operating at ambient pressure and in ultra-high vacuum. Perspectives to further enhance the capabilities of this technique are briefly put forward.

Mechanistic and kinetic aspects of the direct decomposition of N 2 O over steam-activated Fe-silicalite were investigated by transient experiments in vacuum (N 2 O peak pressure of ca. 10 Pa) using the temporal analysis of products (TAP) reactor in the temperature range of 773–848 K. The transient responses of N 2 O, N 2 , and O 2 obtained upon N 2 O decomposition were fitted to different micro-kinetic models. Through model discrimination it was concluded that both free iron sites and iron sites with adsorbed mono-atomic oxygen (* O) species are active for N 2 O decomposition. Oxygen formation occurs via decomposition of bi-atomic (* O 2 ) oxygen species adsorbed over the iron site. This bi-atomic oxygen species originates from another bi-atomic oxygen species (O * O), which is initially formed via interaction of N 2 O with iron site possessing mono-atomic oxygen species (* O). Based on our modeling, the recombination of two mono-atomic oxygen (* O) species or direct O 2 formation via reaction of N 2 O with * O can be excluded as potential reaction pathways yielding gas-phase O 2 . The simulation results predict that the overall rate of N 2 O decomposition is controlled by regeneration of free iron sites via a multi-step oxygen formation at least below 700 K.

The pulsing of argon in a temporal analysis of products (TAP) reactor and reactor modeling of the response curves were used to measure the effective intraparticle diffusivities in porous materials. The diffusivity that can be measured is limited: (1) at the low end by intraparticle diffusion being too slow such that just a small fraction of the pulse gets into the pores to give an indistinguishable tail, which only measures that the diffusivity is smaller than an upper limit and (2) at the high end by intraparticle diffusion being too fast such that it gives a constant concentration in the pores, which only measures that the diffusivity is larger than a lower limit. The limits and range are slightly different for different particle and bed dimensions. A 9 mm long packed bed has a sensitive range of about 300-fold where there are discernible changes in the normalized pulse shape due to diffusivity changes. If small particles of about 50 μm are used, the range is from 1 × 10−10 to 3 × 10−8 m2/s, and if large particles of about 500 μm are used, the range is from 2 × 10−9 to 5 × 10−7 m2/s.

A systematic mechanistic study of NO storage and reduction over Pt/Al 2 O 3 and Pt/BaO/Al 2 O 3 is carried out using Temporal Analysis of Products (TAP). NO pulse and NO/H 2 pump-probe experiments at 350 °C on pre-reduced, pre-oxidized, and pre-nitrated catalysts reveal the complex interplay between storage and reduction chemistries and the importance of the Pt/Ba coupling. NO pulsing experiments on both catalysts show that NO decomposes to major product N 2 on clean Pt but the rate declines as oxygen accumulates on the Pt. The storage of NO over Pt/BaO/Al 2 O 3 is an order of magnitude higher than on Pt/Al 2 O 3 showing participation of Ba in the storage even in the absence of gas phase O 2 . Either oxygen spillover or transient NO oxidation to NO 2 is postulated as the first steps for NO storage on Pt/BaO/Al 2 O 3 . The storage on Pt/Ba/Al 2 O 3 commences as soon as Pt–O species are formed. Post-storage H 2 reduction provides evidence that a fraction of NO is not stored in close proximity to Pt and is more difficult to reduce. A closely coupled Pt/Ba interfacial process is corroborated by NO/H 2 pump-probe experiments. NO conversion to N 2 by decomposition is sustained on clean Pt using excess H 2 pump-probe feeds. With excess NO pump-probe feeds NO is converted to N 2 and N 2 O via the sequence of barium nitrate and NO decomposition. Pump-probe experiments with pre-oxidized or pre-nitrated catalyst show that N 2 production occurs by the decomposition of NO supplied in a NO pulse or from the decomposition of NOx stored on the Ba. The transient evolution of the two pathways depends on the extent of pre-nitration and the NO/H 2 feed ratio.

This paper presents a new approach for precisely tailoring the surface of complex catalytic particles at the atomic level. We describe a new apparatus of original construction that combines an atomic beam deposition (ABD) system with a temporal analysis of products (TAP-2) reactor system. Catalyst samples were prepared by directly adding metal atoms and metal oxide clusters in sub-monolayer amounts to the surface of micron-sized particles. Precise kinetic characterization, (non-steady-state and steady-state) was performed using pulse response methods, temperature-programmed reaction (TPR), etc. The new approach is illustrated by the example of CO oxidation over a series of Pd/PdO catalysts. Catalysts of different loadings were prepared by direct atomic deposition on quartz particles. The Pd deposits were characterized using XPS, SEM and TEM. CO2 production during TPR experiments exhibited oscillatory behavior. The goal of combining ABD with TAP experiments is to develop a method of establishing direct, reproducible correlations between changes in surface composition and changes in catalyst activity.

The reaction pathways of N2 and N2O formation in the direct decomposition and reduction of NO by NH3 were investigated over a polycrystalline Pt catalyst between 323 and 973 K by transient experiments using the temporal analysis of products (TAP-2) reactor. The interaction between nitric oxide and ammonia was studied in the sequential pulse mode applying 15 NO. Differently labelled nitrogen and nitrous oxide molecules were detected. In both, direct NO decomposition and NH3–NO interaction, N2O formation was most marked between 573 and 673 K, whereas N2 formation dominated at higher temperatures. An unusual interruption of nitrogen formation in the 15 NO pulse at 473 K was caused by an inhibiting effect of adsorbed NO species. The detailed analysis of the product distribution at this temperature clearly indicates different reaction pathways leading to the product formation. Nitrogen formation occurs via recombination of nitrogen atoms formed by dissociation of nitric oxide or/and complete dehydrogenation of ammonia. N2O is formed via recombination of adsorbed NO molecules. Additionally, both products are formed via interactions between adsorbed ammonia fragments and nitric oxide. # 2006 Elsevier B.V. All rights reserved.

Transient isotopic studies in the temporal analysis of products (TAP) reactor evidenced the importance of the lifetime of oxygen species generated upon N 2 O decomposition on extraframework iron sites of Fe-silicalite for methane oxidation at 723 K. Fe-silicalite effectively activates CH 4 when N 2 O and CH 4 are pulsed together in the reactor. However, these oxygen species gradually become inactive for methane oxidation as the time delay between the N 2 O and CH 4 pulses is increased from 0 to 2 s.

The mechanism of the CO2 reforming of methane reaction over the Pt/ZrO2 catalyst was investigated using a temporal analysis of products (TAP) reactor system. For comparative purposes, the reaction pathway using a Pt/Al2O3 catalyst was also examined. A reaction sequence is suggested for both catalysts. Over both catalysts, methane decomposition takes place over platinum. The main difference between the two catalysts concerns the carbon dioxide dissociation. Over Pt/Al2O3 this step is assisted by hydrogen. Over Pt/ZrO2 this step takes place over the zirconia support and involves surface vacancies. Moreover, large pools of formate and carbonate species are present on the zirconia. Transient studies conducted to determine the origin of carbon species accumulated during CO2 reforming revealed that more than 99% of the carbon was derived from the methane molecule over both catalysts. Over the Pt/ZrO2 catalyst, only a single very reactive carbon species was detected, while over the Pt/Al2O3 a second less active species was also formed.

Transient isotopic studies in the Temporal Analysis of Products (TAP) reactor evidenced a clear mechanistic regularity between methane and propane for N 2 O reduction over Fe-silicalite in the temperature range of 623–773 K. The lifetime of the oxygen species generated by N 2 O decomposition on extraframework iron sites was found to be crucial for alkane oxidation. When N 2 O and CH 4 or C 3 H 8 were pulsed together over Fe-silicalite, oxygen species deposited on iron sites effectively activated the hydrocarbon. However, these oxygen species become inactive for alkane oxidation if the pulses of N 2 O and the alkane were separated by more than 0.1 s. Transient studies have indicated that C 3 H 8 is more efficient for N 2 O reduction than CH 4 , as expected from the facilitated activation of the C H bond upon increasing the carbon number in alkanes. The performance derived from the TAP reactor agrees well with experiments under flow conditions at ambient pressure.

Mechanistic aspects of low-temperature (423–723 K) selective catalytic reduction of NO with NH3 (NH3-SCR) over an Ag(1.7 wt%)/Al2O3 (2Ag/Al2O3) catalyst in the presence and absence of O2 and H2 were studied using a transient low-pressure (peak pressure < 10 Pa) technique, the temporal analysis of products (TAP) reactor, in combination with isotopic traces. Preoxidized 2Ag/Al2O3 showed very low activity in the NH3-SCR reaction. The activity increased tremendously after ex situ reduction of 2Ag/Al2O3 in a hydrogen flow (5 vol% H2 in Ar) at 373 K for 30 min. This observation was related to the creation of reduced Ag species, which catalyze O2 and NO dissociation, yielding adsorbed oxygen species. O2 is a better supplier of oxygen species. Oxygen species played a key role in NH3 dehydrogenation, yielding reactive NHx fragments that are important intermediates for nitrogen formation via a coupling reaction between NO and NH3. This reaction pathway predominated over direct NO decomposition to N2 in the presence of O2. In addition to generation of active oxygen species, gas-phase oxygen accelerated transformation of surface N-containing intermediates into gas-phase reaction products. The role of hydrogen in the NH3-SCR reaction is to transform oxidized Ag species into reduced species that are active sites for O2 and NO adsorption. Our findings suggest that the reduction of oxidized Ag is responsible for the boosting effect of H2 in the NH3-SCR reaction, and also that H2 helps decrease total N2O production.