Temporal Analysis Of Products Research Articles

Isotopic Temporal Analysis of Products study of anaerobic NO reduction by NH3 on Pt catalysts

A Temporal Analysis of Products (TAP) study was carried out of the anaerobic NO+NH3 reaction system on Pt sponge and Pt/Al2O3 catalysts. Temperature, pulse timing, and feed composition were varied to quantify the conversion and product selectivities. Isotopically labeled nitric oxide (15NO) was used to follow reaction pathways to molecular nitrogen and nitrous oxide. A variation in the delay time between sequential pulses of 15NO and NH3 had a significant effect on the reaction path to molecular nitrogen. A mixed feed (no delay time) above the light-off temperature resulted in a product mixture nearly balanced between the mixed product 15NN, and the two single-source products 15N2, and N2. With increased delay time the selectivity of 15NN decreased significantly in favor of 15N2 and N2. The data suggest at least two major routes to nitrogen formation, one involving direct NO decomposition on reduced Pt sites and the other involving reaction between NHx species and NO. There is also kinetic evidence for a slower, third minor route involving an unidentified surface complex, possibly HNOad. The alumina support is shown to suppress this effect of delay time through ammonia adsorption, serving as a source-sink. The trends are interpreted with a mechanistic model constructed from a sequence of elementary steps.

In situ surface coverage analysis of RuO2-catalysed HCl oxidation reveals the entropic origin of compensation in heterogeneous catalysis

In heterogeneous catalysis, rates with Arrhenius-like temperature dependence are ubiquitous. Compensation phenomena, which arise from the linear correlation between the apparent activation energy and the logarithm of the apparent pre-exponential factor, are also common. Here, we study the origin of compensation and find a similar dependence on the rate-limiting surface coverage term for each Arrhenius parameter. This result is derived from an experimental determination of the surface coverage of oxygen and chlorine species using temporal analysis of products and prompt gamma activation analysis during HCl oxidation to Cl(2) on a RuO(2) catalyst. It is also substantiated by theory. We find that compensation phenomena appear when the effect on the apparent activation energy caused by changes in surface coverage is balanced out by the entropic configuration contributions of the surface. This result sets a new paradigm in understanding the interplay of compensation effects with the kinetics of heterogeneously catalysed processes.

Mechanistic study of the palladium-catalyzed ethyne hydrogenation by the Temporal Analysis of Products technique

The gas-phase hydrogenation of ethyne over Pd/Al2O3 is studied using the Temporal Analysis of Products (TAP) reactor, leading to valuable insights into the reaction mechanism. The exceptional time resolution of this transient technique enables to capture the hydrogenation of the carbon–carbon triple bond via the sequential addition of hydrogen. Hydrogen/deuterium exchange experiments demonstrate the dissociative and reversible adsorption of H2 on palladium. Conversion and selectivity change at early stages of the reaction as a consequence of the formation of a stable carbide layer. The alkene/alkane ratio was markedly higher in TAP compared to reported flow data, due to the mbar pressure operation of the technique. In this regime, the formation of subsurface hydrogen is unfavorable, enabling the assessment of the sole effect of reactants coverage on the product distribution, that is, in the absence of unselective hydride phases. Mechanistic aspects related to the use of carbon monoxide as a selectivity enhancer are also discussed.

Tailored Noble Metal Nanoparticles on γ-Al2O3for High Temperature CH4Conversion to Syngas

The simple deposition of tailored Rh or Pt nanoparticles (NP) on γ-Al2O3 results in active, selective, and stable catalysts for partial oxidation of methane (POM) to syngas. The NP were prepared by the ethylene glycol method in strong alkaline solution. This approach was found to be a promising way of providing active metallic NP for catalyzing the POM reaction. NP sizes determined by small angle X-ray scattering (SAXS) in solution and by transmission electron microscopy (TEM) on alumina were very close. This result highlights the possibility of easy pre-characterization of NP by the former method. Supported Rh NP are intrinsically more active in the POM reaction than Pt NP and also showed a superior performance compared with a conventionally prepared Rh catalyst, even if the latter had been pre-reduced. Mechanistic investigations in the temporal analysis of products (TAP) reactor indicate that the higher selectivity of well-defined Rh-NP is determined by their lower activity for consecutive CO transformations.

An integrated approach to Deacon chemistry on RuO2-based catalysts

Rationally designed RuO2-based Deacon catalysts can contribute to massive energy saving compared to the current electrolysis process in chemically recycling HCl to produce molecular chlorine. Here, we report on our integrated approach between state-of-the-art experiments and calculations. The aim is to understand industrial Deacon catalyst in its realistic surface state and to derive mechanistic insights into this sustainable reaction. We show that the practically relevant RuO2/SnO2 consists of two major RuO2 morphologies, namely 2–4 nm-sized particles and 1–3-ML-thick epitaxial RuO2 films attached to the SnO2 support particles. A large fraction of the small nanoparticles expose {110} and {101} facets, whereas the film grows with the same orientations, due to the preferential surface orientation of the rutile-type support. Steady-state Deacon kinetics indicate a medium-to-strong positive effect of the partial pressures of reactants and deep inhibition by both water and chlorine products. Temporal Analysis of Products and in situ Prompt Gamma Activation Analysis strongly suggest a Langmuir–Hinshelwood mechanism and that adsorbed Cl poisons the surface. Under relevant operation conditions, the reactivity is proportional to the coverage of a specific atomic oxygen species. On the extensively chlorinated surface that can be described as surface oxy-chloride, oxygen activation is the rate-determining step. DFT-based micro-kinetic modeling reproduced all experimental observations and additionally suggested that the reaction is structure sensitive. Out of the investigated models, the 2ML RuO2 film-covered SnO2 gives rise to significantly higher reactivity than the (101) surface, whereas the 1ML film seems to be inactive.

Direct decomposition of NO into N 2 and O 2 over Ba 3Y 3.4Sc 0.6O 9

Effects of dopant on direct decomposition of nitric oxide (NO) over Ba 3Y 4O 9 were investigated and it was found that Sc is partially substituted Ba 3Y 3.4Sc 0.6O 9 catalyst shows a high NO decomposition activity (N 2 yield: 92%) at 1123 K among the examined dopants. Sc is considered to contribute to the stabilization of the Ba 3Y 4O 9 crystal phase and also form the intrinsic oxygen defects. This catalyst was active in NO decomposition in the presence of O 2 and maintained its high activity at high space velocity values. Fourier transform infrared spectroscopy (FT-IR) measurement suggests that initial adsorption state of NO is a bent-type NO −. Moreover, the rate-determining step for NO decomposition seems to be the coupling of the NO species adsorbed on the surface. Doping with Sc effectively weakened the adsorption of NO species, resulting in the increased reactivity. FT-IR and temporal analysis of products (TAP) measurements suggested that N 2O may be an intermediate species in NO decomposition.

The Y-Procedure methodology for the interpretation of transient kinetic data: Analysis of irreversible adsorption

This paper deals with the analysis of transient kinetic data obtained in the Thin-Zone (TZ) configuration of the Temporal Analysis of Products (TAP) reactor. The Y-procedure that reconstructs the reaction rate and gas composition in the catalytic zone with no assumptions on the reaction mechanism ( Yablonsky et al., 2007) was applied to irreversible adsorption with coverage-dependent kinetic coefficients. We have used the Y-Procedure analysis to develop a novel framework for the interpretation of non-steady-state kinetic data. This framework was illustrated by multiple numerical examples concerning irreversible adsorption, and by experimental data corresponding to oxygen adsorption on polycrystalline platinum. The total concentration of active sites was estimated as 6 × 10 − 7 mol / g cat . It was found that for the surface coverage up to 0.8, kinetics followed the dissociative adsorption model with an intrinsic adsorption constant of 1.12 × 10 6 m 3 / mol / s . Kinetic parameters estimated via the Y-procedure agreed well with the values obtained via the moment analysis. The methodology developed in this paper will guide the systematic application of the Y-Procedure analysis to more complex catalytic systems.

Reaction network for the total oxidation of toluene over CuO–CeO 2/Al 2O 3

The total oxidation of toluene was studied over a CuO–CeO 2/γ-Al 2O 3 catalyst in a Temporal Analysis of Products (TAP) setup in the temperature range 673–923 K in the presence and absence of dioxygen and at various degrees of reduction of the catalyst. The reaction rate significantly decreases over a mildly reduced catalyst. Under vacuum and at high-temperature, mild reduction also occurs in the absence of toluene. In the presence of dioxygen, the catalyst activity is determined by weakly bound surface lattice oxygen atoms and adsorbed oxygen species, the lifetime of which is close to 1 s. The weakly bound oxygen is highly reactive and is only found over a fully oxidized catalyst. The formation of products containing 18O during the isotope-exchange experiments with 18O 2 indicates that both lattice and adsorbed oxygen are involved in (a) reoxidation of mildly reduced copper oxide and (b) abstraction of hydrogen atoms and scission of C–C bonds. Isotopic labeling with C 6H 5– 13CH 3 and C 6H 5–CD 3 indicates the following reaction paths: adsorption of toluene on the active site, containing Cu 2+ with 4–5 adjacent surface lattice oxygen atoms; the simultaneous abstraction of H from the methyl and the phenyl group followed by the abstraction of the methyl carbon and next the destruction of the aromatic ring.

Mechanistic analysis of direct N2O decomposition and reduction with H2 or NH3 over RuO2

The mechanisms of direct N2O decomposition and N2O reduction by H2 or NH3 on RuO2 has been studied by means of steady-state catalytic tests at ambient pressure, transient experiments in a temporal analysis of products (TAP) reactor, and density functional theory (DFT) simulations. Bulk RuO2 is very active for N2O decomposition into N2 and O2, achieving full conversion at 723K. According to the DFT calculations, coordinatively unsaturated ruthenium (Rucus) sites are active for the decomposition and oxygen elimination from the surface is rate-limiting step. When pre-reducing RuO2 with H2, the de-N2O activity in the low-temperature region increases. This is attributed to the higher reactivity of surface oxygen vacancies created by H2 at bridge sites compared to Rucus sites. Co-feeding of hydrogen or ammonia with nitrous oxide strongly accelerates the rate of N2O elimination. TAP studies show that oxygen species formed upon N2O decomposition over RuO2 react with NH3 and H2 yielding N2, NO, N2O, and H2O. Both TAP experiments and DFT simulations show that the N-containing intermediates coming from ammonia adsorb much stronger on the catalyst surface than H-containing intermediates coming from hydrogen. This causes blockage of active sites for N2O decomposition by the former species, accounting for the higher efficiency of H2 as reductant for N2O. The presence of O2 in the feed cancels the reducing effect of H2 or NH3 due to the more favorable re-oxidation of reduced RuO2 by O2 compared to N2O.

Thermodynamic time-invariances: Theory of TAP pulse-response experiments

The theory of equilibrium relationships for non-equilibrium dependencies previously created for the batch reactor is further developed for the description of the Temporal Analysis of Products (TAP) experimental data. Corrections are necessary to account for the different diffusivity values affecting the gas transport over the inert zones in the reactor. These corrections can be performed in the Laplace domain, the Fourier domain, or, using special series of exponentially weighted integrals, directly in the time domain.

TAP studies of Pt–Sn–K/γ-Al 2O 3 catalyst for propane dehydrogenation

A Pt–Sn–K/γ-Al 2O 3 catalyst was prepared by incipient wet impregnation and studied with a temporal analysis of products (TAP) reactor for propane dehydrogenation. Previously to TAP analyses, the catalyst samples were used for propane dehydrogenation in two different reactors: a conventional fluidized bed reactor (FBR) where the catalyst was deactivated in time; and a two zone fluidized bed reactor (TZFBR) where propane and oxygen are fed at different levels, providing separated zones for the reaction and catalyst regeneration thus allowing a continuous mode operation. Several operating conditions were also investigated: different times on stream in the FBR, and in TZFBR at steady state the temperature, the oxygen flow rate and the location of the propane feeding point. Samples of catalyst were obtained at several axial positions in the bed. The amount of coke deposited on the catalyst surface has been determined by thermogravimetric analysis. TAP results show that activation energies for propane conversion are markedly different for catalysts used in FBR and TZFBR; these differences have been related to the different nature of coke present on the catalyst surface.

Mechanism–Performance Relationships of Metal Oxides in Catalyzed HCl Oxidation

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

Mechanistic origins of the promoting effect of tiny amounts of Rh on the performance of NiO x/Al 2O 3 in partial oxidation of methane

The influence of Rh concentration (0.005–0.15 wt.%) in NiO x (3 wt.%)/Al 2O 3 on the mechanism and kinetics of partial oxidation of methane (POM) to syngas was elucidated by combining steady-state catalytic tests over 120 h on stream with catalysts characterization and temporal analysis of products. All bi-metallic catalysts showed POM performance close to the thermodynamic one. The reaction mechanism does not depend on the nature of metals but is influenced by their reduction state. From a kinetic point of view, rhodium species (both reduced and oxidized) possess significantly higher activity toward methane conversion than the nickel ones. Rhodium fulfills a double role in the bi-metallic catalysts. Irrespective of its concentration, it promotes the reduction of Ni 2+ in inactive NiAl x O y to metallic Ni directly under POM conditions. This is a reason for the higher activity and selectivity of low Rh-loaded (0.005 wt.%) NiO x (3 wt.%)/Al 2O 3 catalyst compared to its respective mono-metallic counterparts. Owing to the high activity of rhodium species, they alone determine the POM performance of higher Rh-loaded catalysts.

Microkinetic modeling of the NO + H2 system on Pt/Al2O3 catalyst using temporal analysis of products

A systematic experimental and modeling study of the NO+H2 reaction system on Pt/Al2O3 catalyst was carried out using temporal analysis of products (TAP). A microkinetic model was developed that explains the transient product distribution during various experimental TAP protocols. NO pulsing experiments show the inhibiting effect of oxygen poisoning and the kinetic limitations posed by N–O bond scission. The NO–H2 pump–probe experiments demonstrate the effect of temperature, H2/NO ratio (⩾1), and pulse delay time between consecutive NO and H2 pulses on N2, N2O, and NH3 selectivity. The simulations reveal the competition between surface N–N recombination and N and/or NO reaction with H to form N2 and NH3, respectively. The developed mechanistic-based microkinetic model accounts for all of the experimental data including the aforementioned selectivity versus pulse delay trends.

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

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

Time of flight mass spectrometry for quantitative data analysis in fast transient studies using a Temporal Analysis of Products (TAP) reactor

A Time of flight (ToF) mass spectrometer suitable in terms of sensitivity, detector response and time resolution, for application in fast transient Temporal Analysis of Products (TAP) kinetic catalyst characterization is reported. Technical difficulties associated with such application as well as the solutions implemented in terms of adaptations of the ToF apparatus are discussed. The performance of the ToF was validated and the full linearity of the specific detector over the full dynamic range was explored in order to ensure its applicability for the TAP application. The reported TAP-ToF setup is the first system that achieves the high level of sensitivity allowing monitoring of the full 0-200 AMU range simultaneously with sub-millisecond time resolution. In this new setup, the high sensitivity allows the use of low intensity pulses ensuring that transport through the reactor occurs in the Knudsen diffusion regime and that the data can, therefore, be fully analysed using the reported theoretical TAP models and data processing.

TAP studies of ammonia decomposition over Ru and Ir catalysts

The Temporal Analysis of Products (TAP) technique has been used to investigate the mechanism involved in the catalytic decomposition of NH(3) over a series of catalysts consisting of activated carbon supported Ru (promoted and non-promoted with Na) and over an activated carbon supported Ir. An extensive study of the role played by both the support and the promoter in the "side reactions" and in the stability and surface lifetime of the NH(x) species has been performed. It was suggested that the N(2) produced during the first steps of the reaction over the activated carbon supported Ru catalysts promoted with Na forms a Na-N-Ru complex at the promoter-transition metal crystallite interface. This study also suggests that the Na promoter prevents the diffusion of hydrogen from the metal to the support via spill-over. A similar effect was observed after the thermal treatment at high temperature of the carbon catalyst support. Finally large differences in multi-pulse TAP results have been detected between Ru and Ir catalysts implying that the NH(3) decomposition reaction mechanism must be different on both metals.

CO bond cleavage on supported nano-gold during low temperature oxidation

The oxidation of CO by Au/Fe(2)O(3) and Au/ZnO catalysts is compared in the very early stages of the reaction using a temporal analysis of products (TAP) reactor. For Au/Fe(2)O(3) pre-dosing the catalyst with (18)O labelled water gives an unexpected evolution order for the labelled CO(2) product with the C(18)O(2) emerging first, whereas no temporal differentiation is found for Au/ZnO. High pressure XPS experiments are then used to show that CO bond cleavage does occur for model catalysts consisting of Au particles deposited on iron oxide films but not when deposited on ZnO films. DFT calculations, show that this observation requires carbon monoxide to dissociate in such a way that cleavage of the CO bond occurs along with dynamically co-adsorbed oxygen so that the overall process of Au oxidation and CO dissociation is energetically favourable. Our results show that for Au/Fe(2)O(3) there is a pathway for CO oxidation that involves atomic C and O surface species which operates along side the bicarbonate mechanism that is widely discussed in the literature. However, this minor pathway is absent for Au/ZnO.

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

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

Transient mechanistic study of the gas-phase HCl oxidation to Cl 2 on bulk and supported RuO 2 catalysts

The oxidation of HCl to Cl 2 (Deacon process) was studied over RuO 2-based catalysts in bulk and supported forms. RuO 2 (7 wt.%) was incorporated onto different TiO 2 carriers (rutile, anatase, and P25) by deposition–precipitation of Ru(OH) 2 followed by calcination. RuO 2/TiO 2-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 RuO 2 and the influence of the TiO 2-rutile carrier were gained by studies in the Temporal Analysis of Products (TAP) reactor at 623 K. HCl multipulsing in the absence of O 2 leads to chlorination of the catalysts with no detectable gas-phase Cl 2 production. The chlorine uptake of RuO 2 amounts to 0.16 mmol Cl g cat - 1 , which enables to estimate that ca. 75% of the total surface Ru atoms (coordinatively unsaturated and bridge sites) are chlorinated, forming a RuO 2− x Cl x oxychloride. RuO 2 is also chlorinated by Cl 2, but to a lesser extent compared to HCl. The chlorine uptake of fresh RuO 2/TiO 2-rutile per gram of sample was four times higher than that of fresh RuO 2, 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 O 2 and HCl over the catalysts revealed that both gas-phase Cl 2 evolution and H 2O desorption are impeded processes. Pump–probe experiments of O 2 and HCl evidenced the tight dependence of the net Cl 2 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 Cl 2 at very low coverages. The equilibration procedure (ambient or TAP conditions) influences the reaction mechanism and the net Cl 2 production, giving account of the reaction complexity due to the highly dynamic state of the catalyst surface. The Deacon process on RuO 2− x Cl x -based catalysts can be mostly described by a Langmuir–Hinshelwood scheme with a minor 2D Mars–van Krevelen contribution.