Steady-state Isotopic Transient Kinetic Analysis Research Articles

Recent Approaches in Mechanistic and Kinetic Studies of Catalytic Reactions Using SSITKA Technique

One of the most useful techniques to obtain valuable information on catalyzed heterogeneous reactions at, or near to, molecular level is the Steady-State Isotopic Transient Kinetic Analysis (SSITKA). Kinetic parameters of catalyst-surface reaction intermediates, such as concentration, site coverage, reactivity, and rate constants can be obtained and processed to provide valuable information about the reaction mechanism. This technique has been extensively tested in a wide range of different surface-catalyzed reactions, where the influence of different parameters on the intermediates has been studied (i.e., supports, active phases, particle size, addition of promoters). Progresses in the coupling of spectroscopic techniques and advanced modeling could greatly improve the understanding of the surface reaction mechanism and provide more reliable kinetic models. This review compiles the main goals achieved up to date in heterogeneous catalytic systems using SSITKA and analyzes the perspectives of this techniq...

Applications of isotopic tracers in Fischer–Tropsch synthesis

In this review, five different applications of isotopic tracers to the Fischer–Tropsch synthesis reaction on cobalt, iron, and ruthenium catalysts are reviewed. By co-feeding molecules containing a radioactive 14C-label and monitoring the fate of the species in the product distribution, researchers have identified molecules that are candidates for chain initiation or reincorporation into the growing chain. Kinetic isotope effects resulting from H2/D2 switching have provided insights into the rate limiting steps involved in chain initiation and propagation. H2/D2 switching has also been employed in order to account for the holdup of heavier products in the reactor, thus allowing one to correct for the product accumulation effect on the ASF plot so that the true impact of olefin reincorporation on the product distribution can be assessed. The competitive adsorption of H2 and D2 on the catalyst surface has also been explored in order to identify if H–D partitioning effects could influence the probability for one isotope to be favored for reaction during FTS and thus influence KIE results. Finally, steady state isotopic transient kinetic analysis (SSITKA) has been utilized to obtain a number of important kinetically relevant parameters (e.g., number of active sites, rate constant, TOF, etc.), to quantify the pool of reactive intermediates on the catalyst surface, and explore how co-adsorbed molecules may influence the size or reactivity of the reactive pool.

The mechanism of the CH4/O2 reaction on the Pd–Pt/γ-Al2O3 catalyst: A SSITKA study

This work is mainly dedicated to the studies of the reaction mechanism of complete methane oxidation on the γ-alumina palladium–platinum catalyst by means of the steady state isotopic transient kinetic analysis (SSITKA) with 12CH4/13CH4 and 16O2/18O2 switches. Such kinetic parameters as the average surface life-time and the surface concentration of methane, oxygen, carbon dioxide and intermediates leading to its formation were determined. It was found that carbon dioxide is formed with two different rates. The average surface life-time of fast formed carbon dioxide change from 11.2s at 637K to 0.2–0.3s at higher temperatures, while the surface concentration of fast formed carbon dioxide and intermediates leading to it change from 3.1μmol/g at 637K to 1.8μmol/g at 980K. Carbon dioxide formed with a slow rate is present on the catalyst surface only above 637K. At higher temperatures the values of its average surface life-time are close to 1.7–1.8s, while the surface concentration of slow formed carbon dioxide and intermediates leading to it change from 14.8μmol/g at 755K to 16.3μmol/g at 980K. The coverage of the surface of the palladium–platinum active phase with fast formed carbon dioxide and intermediates leading to it is in the order of 0.16–0.17 in the range of 673–755K and 0.125 – at 980K. Much more higher is the surface coverage with slow formed carbon dioxide – 0.8 and 0.92 at 755K and 980K, respectively. It was also found that oxygen from the crystal lattice of the catalyst takes part in oxidation of methane, as well as that oxygen atoms are exchanged between the gas phase and the crystal lattice of the catalyst. The very long live-time of oxygen leading to formation of carbon dioxide, in the order of 80–380s, and very large formal coverage of the surface of palladium–platinum active phase with oxygen desorbed to the gas phase and oxygen leading to formation of carbon dioxide, scores of times exceeding the formal monolayer, clearly prove that a very large pool of subsurface oxygen participates in methane oxidation. It was also found that only at the lowest temperature of the reaction small amount of reversibly adsorbed methane resists with a very short life-time on the catalyst surface. It may be concluded that two different kinds of active centres (α—more active, but less numerous and β—less active, but more numerous) exist on the “working” surface of the catalyst and the process of methane oxidation proceeds simultaneously according to two different reaction mechanisms (Mars–van Krevelen and Langmuir–Hinshelwood). The extent of their participation in oxidation of methane is different and depends on the reaction temperature. Moreover, the process of methane oxidation proceeds not only simultaneously according to two different mechanism of the reaction, but also with different rates determined by the kind of active centres.

Acrolein Oxidation to Acrylic Acid on Mo/V/W‐Mixed Oxide Catalysts

AbstractOxidation of acrolein to acrylic acid on Mo/V/W‐mixed oxide catalysts has been studied by means of the isotopic exchange method steady‐state isotopic transient kinetic analysis (SSITKA). As a result of these isotopic exchanges performed with 16O2 and 18O2, some new mechanistic details could be obtained. Acrolein exchanges its oxygen with oxygen of the mixed oxide catalyst. Additionally, the kinetics of the different isotopologues is significantly influenced by oxygen exchange reactions between the surface and the bulk sites. From the SSITKA results, an extended reaction model has been deduced. Thus, the experimentally gained data of the individual isotopologue kinetics could be verified by modeling.

Novel catalytic and mechanistic studies on wastewater denitrification with hydrogen

The present work reports up-to-date information regarding the reaction mechanism of the catalytic hydrogenation of nitrates in water media. In the present mechanistic study, an attempt is made, for the first time, to elucidate the crucial role of several catalysts and reaction parameters in the mechanism of the NO(3)(-)/H(2) reaction. Steady-state isotopic transient kinetic analysis (SSITKA) experiments coupled with ex situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were performed on supported Pd-Cu catalysts for the NO(3)(-)/H(2) and NO(3)(-)/H(2)/O(2) reactions. The latter experiments revealed that the formation and surface coverage of various adsorbed active intermediate N-species on the support or Pd/Cu metal surface is significantly favored in the presence of TiO(2) in the support mixture and in the presence of oxygen in the reaction's gaseous feed stream. The differences in the reactivity of these adsorbed N-species, found in the present work, adequately explain the large effect of the chemical composition of the support and the gas feed composition on catalyst behaviour (activity and selectivity). The present study leads to solid mechanistic evidence concerning the presence of a hydrogen spillover process from the metal to the support. Moreover, this study shows that Cu clusters are active sites for the reduction of nitrates to nitrites.

Operando Infrared (IR) Coupled to Steady-State Isotopic Transient Kinetic Analysis (SSITKA) for Photocatalysis: Reactivity and Mechanistic Studies

This paper reports the coupling of operando infrared (IR) and steady-state isotopic transient kinetic analysis (SSITKA) for air purification studies from volatile organic campounds (VOCs) by photocatalysis. Methanol photo-oxidation has been used as a model reaction. A new photocatalyst (TiO2-L) has been studied and compared to the reference material TiO2-P25. We show that turnover frequency (TOF) values are not really available in heterogeneous photocatalysis where the number and/or efficiency of active sites can vary from one TiO2 material to another and from one experimental setup to another. The influence of methanol concentration and reaction temperature on photocatalytic activity and on selectivity was also investigated. TiO2-L is as active as TiO2-P25 at low methanol concentration and is more selective at high methanol concentrations. SSITKA clarified the role of the surface formate species in the reaction mechanism. Most of these formate species are spectators and are not the main intermediates in ...

Multi-product steady-state isotopic transient kinetic analysis of CO hydrogenation over supported molybdenum carbide

Isotopic transient analysis of steady state of CO hydrogenation catalyzed by alumina-supported Mo2C nanoclusters was performed at 573K, 1.2bar syngas with H2/CO=1. An isotope switch from 12CO to 13CO during the steady-state reaction produced a rapid transient associated with CO hydrogenation and a very slow transient associated with turnover of ineffective catalytic sites or exchange of carbidic carbon with reaction intermediates. Although the intrinsic turnover frequency for hydrocarbon formation was similar to that observed on late transition metals, the coverage of reaction intermediates on the Mo2C clusters was very low (≪1%), which accounts for the overall low activity of Mo2C. Promotion of Mo2C with Rb2CO3 lowered the coverage of intermediates even further, but increased the selectivity toward alcohols. Very strong interaction of the alcohols with the promoter suggests that Rb may form alkoxides at reaction conditions.

The water–gas shift reaction over CeO2/CuO: Operando SSITKA–DRIFTS–mass spectrometry study of low temperature mechanism

An inverse CeO2/CuO catalyst has been investigated by operando steady-state isotopic transient kinetic analysis (SSITKA) in combination with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) under 3% CO + 3% H2O reactant mixture at 473 K with the aim of determining intermediates involved in the water–gas shift reaction at relatively low temperatures. Among the various species detected in the infrared spectra which may be involved in the reaction, i.e. formates, copper carbonyls and carbonates, a particular type of carbonate species is identified as a reaction intermediate on the basis of detailed analysis of the spectra during isotopic exchange in comparison with the change in the corresponding isotopically labelled CO2 product.

The effect of La3+-doping of CeO2 support on the water-gas shift reaction mechanism and kinetics over Pt/Ce1−xLaxO2−δ

Platinum nanoparticles (dPt=1.0–1.2nm) supported on single CeO2 and La2O3 metal oxides and Ce0.8La0.2O2−δ solid solution were prepared to investigate for the first time the effect of La3+-doping of ceria on important mechanistic and kinetic aspects of the water-gas shift (WGS) reaction, namely: (i) the concentration and chemical structure of active adsorbed reaction intermediates present in the C-path and H-path of WGS at 250 and 300°C, (ii) the chemical nature of inactive species formed during WGS, and (iii) the prevailing mechanistic path among “redox” and “associative” both proposed in the literature. For this, steady-state isotopic transient kinetic analysis (SSITKA) experiments coupled with in situ DRIFTS and mass spectrometry were performed to follow the H-path (use of D2O) and C-path (use of 13CO) of the WGS. In addition, other transient isotopic experiments using operando methodology (use of DRFTS and mass spectrometry) were designed to follow with time on stream the reactivity toward water of the various adsorbed species formed under WGS. It is proposed that on Pt/Ce1−xLaxO2−δ (x=0.0, 0.2 and 1.0) the WGS reaction follows both the “redox” and “associative” mechanisms but the extent of participation of each mechanism to the overall WGS reaction rate depends on the support chemical composition. The WGS kinetic rate (μmol COg−1s−1) increased by a factor of 2.0 and 2.8 at 300°C on 0.5wt% Pt supported on Ce0.8La0.2O2−δ compared to CeO2 and La2O3, respectively. This was explained by (i) the larger concentration of active surface intermediates formed around each Pt nanoparticle (larger extent of reactive zone) and (ii) the higher reactivity of sites (k, s−1) responsible for CO2 and H2 formation on Pt/Ce0.8La0.2O2−δ compared to Pt/CeO2 and Pt/La2O3. Active OH groups is suggested to be formed on defect sites (Ce3+□s) of Ce0.8La0.2O2−δ as a consequence of the introduction of La3+ into the ceria lattice, the latter enhancing the concentration of labile oxygen and its surface mobility, important characteristics of the “redox” mechanism.

Mechanistic Studies of the Water–Gas Shift Reaction over Pt/CexZr1–xO2 Catalysts: The Effect of Pt Particle Size and Zr Dopant

A series of y wt % Pt/CexZr1–xO2 catalysts (y = 0.1, 0.5, and 1.0; x = 0.3, 0.5, and 0.7) were synthesized and characterized to investigate the effect of CeO2 doping with Zr4+ and of Pt particle size (Pt/Ce0.5Zr0.5O2) on important mechanistic and kinetic aspects of the water–gas shift (WGS) reaction. These included the concentration (μmol·g–1 or θ (surface coverage based on Pts)) and chemical structure of active reaction intermediates present in the “carbon path” and “hydrogen path” of the WGS reaction in the 200–300 °C range and the prevailing mechanism among “redox” and “associative formate” largely considered in the literature. Toward this goal, steady-state isotopic transient kinetic analysis coupled with in situ DRIFTS and mass spectrometry experiments were performed for the first time using D2O and 13CO isotopic gases. A novel transient isotopic experiment allowed quantification of the initial transient rate of reaction of adsorbed formate (HCOO−) with water and that of adsorbed CO with water under ...

SSITKA analysis of CO hydrogenation on Zn modified cobalt catalysts

Cobalt and rhenium promoted cobalt catalysts with particle sizes in the range 7–40nm, supported on different modified aluminas, were investigated for CO hydrogenation. Steady-state isotopic transient kinetic analysis (SSITKA) was carried out at 483K, 1.85bar, and H2/CO/inert=15/1.5/33.5Nml/min. A loading-dependent adverse effect of zinc was found, and loss in methanation activity could be explained by an increased surface residence time of CHx (kCH4≡1/τCHx) and decreased selectivity to CH4. The surface concentration of CO (NCO) was largely unaffected by Zn and thus uncorrelated to activity. The relative effect of Zn on hydrocarbon formation decreased with increasing chain length, indicating that hydrogenation was more strongly affected than chain propagation. In situ measurements prior to and during CO hydrogenation suggested that the surface was largely covered by monomeric carbon species experiencing competitive adsorption from other species. As expected, no particle size effect was observed.

Synergy of Components in CuZnO and CuZnO/Al2O3 on Methanol Synthesis: Analysis at the Site Level by SSITKA

In the present study, the effects of the individual components and an Al2O3 support on CuZnO for methanol (MeOH) synthesis were investigated at the site level for the first time using steady-state isotopic transient kinetic analysis and reaction at 250 °C and 1.8 atm. The presence of ZnO was found to decrease the hydrocarbon (CH4 primarily) formation ability of Cu. By comparing the surface reaction parameters, it could be shown that Cu and ZnO catalysts (supported on Al2O3) exhibit lower MeOH formation rates compared to their combination in either CuZnO or CuZnO/Al2O3 due, especially, to lower intrinsic “site” activities. The synergy between Cu and ZnO was most obvious in the increase in MeOH TOFITK (a measure of site activity for MeOH formation), more than double that for Cu without ZnO. Al2O3 did not seem to impact MeOH synthesis in any way other than to increase dispersion of the CuZnO. However, it did furnish acid sites for the conversion of some MeOH to DME.

Mechanistic investigations on dimethyl carbonate formation by oxidative carbonylation of methanol over a CuY zeolite: an operandoSSITKA/DRIFTS/MS study

The simultaneous combination of steady state isotopic transient kinetic analysis (SSITKA) with diffuse reflectance Fourier transform spectroscopy (DRIFTS) and mass spectrometric (MS) analysis was applied to study the oxidative carbonylation of methanol (MeOH) to dimethyl carbonate (DMC) on a CuY zeolite catalyst prepared by incipient-wetness impregnation of commercial zeolite NH(4)-Y. The interaction of the catalyst with different reactants and reactant mixtures (O(2), CO, CO/O(2), MeOH/O(2), MeOH/CO, and MeOH/CO/O(2)) was studied in detail using (16)O(2)/(18)O(2) as well as (12)CO/(13)CO containing gas mixtures. DMC is produced via a monodentate monomethyl carbonate (MMC) species as intermediate which is formed by the concerted action of adsorbed methoxide and CO with gas phase MeOH. Adsorbed bidentate MMC species were found to be inactive. Lattice oxygen supplied by CuO(x) species is involved in the formation of MMC. Gas phase oxygen is needed to re-oxidize the catalyst but favours also the oxidation of CO to CO(2) and unselective oxidation reactions of MeOH to methyl formate, dimethoxymethane, and CO(2). The appropriate choice of reaction temperature and of the oxygen content in the reactant gas mixture was found to be indispensable for reaching high DMC selectivities.

Catalytic effects of ruthenium particle size on the Fischer–Tropsch Synthesis

This work investigates the catalytic consequences of Ru cluster size in the Fischer–Tropsch Synthesis (FTS). Ru/Al 2O 3 catalysts with different metal particles size have been obtained by treating the solid in pure H 2 at increasing temperatures and times. Steady-state isotopic transient kinetic analysis (SSITKA) has been carried out at 523 K, 5.5 kPa CO, 55 kPa H 2, and 124.5 kPa inert in order to determine surface residence times and coverage of reversibly bonded CO and CH x intermediates as a function of Ru particle size (4–23 nm). We have found that FTS with Ru-based catalysts is a highly structure-sensitive reaction when Ru < 10 nm. In this range, turnover frequency of CO consumption (TOF CO) increases as the particle size increases, reaching a constant value for Ru particles larger than 10 nm. The lower intrinsic activity shown by Ru clusters <10 nm may be related to the stronger CO adsorption and concomitant partial blocking of active sites, as suggested by the decreased CO surface residence time as the Ru cluster size increases in the range below 10 nm.

Studies of Macroporous Structured Alumina Based Cobalt Catalysts for Fischer–Tropsch Synthesis

Macroporous structured (MPS)-Al2O3 support was synthesised from Al2O3 nanoparticles with polystyrene beads as a sacrificing agent. A MPS-Al2O3 with uniformly spherical and interconnected pores throughout the sample was obtained. The MPS-Al2O3 was characterized and used as support for cobalt Fischer–Tropsch catalysts. Surface kinetic parameters and intrinsic activity of the catalysts were determined by steady-state isotopic transient kinetic analysis (SSITKA) at 483 K, 1.85 bar and H2/CO = 10. The catalyst was compared with conventional α- and γ-Al2O3 supported cobalt catalysts. The results show that the MPS-Al2O3 support seems to combine the advantages of α-Al2O3 and γ-Al2O3 as support materials, with activity and selectivity comparable to conventional Co/γ-Al2O3 and Co/α-Al2O3, respectively. SEM micrograph of macroporous structured alumina (MPS-Al2O3) with a Al2O3/polystyrene ratio of 0.4.

A Highly Active and Selective Manganese Oxide Promoted Cobalt-on-Silica Fischer–Tropsch Catalyst

A highly active and selective manganese oxide-promoted silica-supported cobalt catalyst for the Fischer–Tropsch reaction is reported. Co/MnO/SiO2 catalysts were prepared via impregnation of a cobalt nitrate and manganese nitrate precursor, followed by drying and calcination in an NO/He flow. The catalysts were studied with STEM–EELS, infrared spectroscopy measurements of adsorbed CO and Steady-State Isotopic Transient Kinetic Analysis experiments. Based on those experiments, a relation between C5+-selectivity and surface-coverages of CH x -intermediates on cobalt was found.

Comparison of chemisorption close to ambient vs. under reaction conditions for Group VIII metal catalysts

A comparison of the relationship of H 2 or CO chemisorption measurements at 25–100 °C to similar results obtained under CO hydrogenation conditions by steady-state isotopic transient kinetic analysis (SSITKA) is made for the first time for a wide variety of Group VIII metal catalysts. The ratio N T ∗ / N chem (amount of chemisorption by SSITKA vs. by static chemisorptions) was found to be almost always ca. unity for Co catalysts. SSITKA can, thus, be used as a complementary characterization technique to TEM, XRD, and static chemisorption for better understanding of Co metal catalyst dispersion and metal surface site availability for Co catalysts with a wide variety of promoters/supports. Unfortunately, application of SSITKA chemisorption measurements under reaction conditions for characterizing metal dispersion for the other metals is limited at this time. However, the results do suggest some possibilities for Ru and Rh.

Effects of Reaction Temperature and Support Composition on the Mechanism of Water–Gas Shift Reaction over Supported-Pt Catalysts

The present work reports on a detailed study of the effect of support chemical composition and reaction temperature on the mechanism of the water–gas shift (WGS) reaction over supported-Pt catalysts. The effect of the same parameters on the chemical composition and surface concentration of active reaction intermediates was also determined for the first time, information that allowed elucidating also the site location of these intermediates, e.g., support versus metal or support–metal interface. The above-mentioned mechanistic information was rigorously provided by the application of steady-state isotopic transient kinetic analysis (SSTIKA) experiments coupled with mass spectrometry (MS) and DRIFTS techniques, and by other transient isotopic experiments designed. It was found that on Pt/CeO2 a switch of the WGS reaction mechanism from “redox” to a combination of redox and “associative formate with −OH group regeneration” is obtained after increasing the reaction temperature from 473 to 573 K. On the other ...

Quantification of Reactive CO and H2 on CuOx-CeO2 during CO Preferential Oxidation by Reactive Titration and Steady State Isotopic Transient Kinetic Analysis

The molar quantity of adsorbed CO and H2 present on the surface of a mixed CuOx-CeO2 catalyst during CO preferential oxidation in H2 at 353 K was quantified using both the reactive titration method and the steady-state isotopic-transient kinetic analysis (SSITKA). For the reactive titration method, either CO or H2 was replaced by He during steady state reaction while monitoring the residual transient product formation of CO2 or H2O produced from the surface adsorbed CO or H2 via catalytic oxidations. For SSITKA, 12CO was replaced by 13CO during steady state reaction while monitoring the transient product formation of 12CO2. The amount of adsorbed reactive CO increases with increasing CO partial pressure or decreasing H2 partial pressure, while the amount of reactive H2 decreases with increasing CO partial pressure or decreasing H2 partial pressure, showing that the adsorbed CO and H2 compete for active redox sites and prohibit the other’s adsorption. Using reactive CO and H2 amounts, two models of coverag...

“Redox” vs “associative formate with –OH group regeneration” WGS reaction mechanism on Pt/CeO 2: Effect of platinum particle size

A series of x wt.% Pt/CeO 2 catalysts ( x = 0.1–2.0) was prepared to investigate for the first time the effect of Pt particle size on important kinetic and mechanistic aspects of the water–gas shift (WGS) reaction, namely the concentration (μmol g −1) and chemical structure of active adsorbed reaction intermediates present in the “carbon-path” and “hydrogen-path” of the WGS reaction at 300 °C. For this, steady-state isotopic transient kinetic analysis (SSITKA) coupled with in situ DRIFTS and mass spectrometry experiments was performed using D 2O and 13CO. Α novel transient isotopic experiment performed allowed to quantify the initial transient rates of reactions of adsorbed formate (–COOH) and CO by water, based on which it was concluded that formate should not be considered as an important intermediate. According to the present work, it is proposed that the WGS reaction on ceria-supported Pt at 300 °C occurs largely via the “ redox” mechanism, and to a lesser extent via the “ associative formate with –OH group regeneration” mechanism. The TOF (s −1) of WGS was found to vary only slightly with Pt particle size (1.3–8.0 nm), while the specific reaction rate based on the length of periphery of Pt–CeO 2 interface (μmol cm −1 s −1) was found to significantly increase with increasing platinum loading and mean particle size in the 250–300 °C range.