Steady-state Isotopic Transient Kinetic Analysis Research Articles

A Mechanistic Study of the Fischer–Tropsch Synthesis Using Transient Isotopic Tracing. Part-1: Model Identification and Discrimination

A steady state isotopic transient kinetic analysis (SSITKA) of the Fischer–Tropsch synthesis over Co/Ru/TiO2 catalyst is reported by using 13CO and D2. Besides a qualitative interpretation of the transients, model identification and discrimination are mainly based on numerical modeling. From this, it is concluded that two single-C species are present on the catalyst surface, Cα,ads and Cβ,ads. These species both participate in the formation of methane and of higher hydrocarbons. The heterogeneity of the catalyst surface is limited to these two single-C species. Only one type of chain-growth site is present in a low concentration compared to the surface concentrations of COads, Cα,ads, and Cβ,ads. The H-content of Cα,ads and Cβ,ads is assessed by comparing the simulated transient for the incorporation of the D-labeling into methane with experiments. In case the stepwise hydrogenations of Cads to CH4 are irreversible, Cα,ads and Cβ,ads are H-free species. In case these reactions are reversible, the H-exchange between Cads, CHads, CH2,ads, and CH3,ads is fast compared to the net formation of methane, and the H-content of Cα,ads and Cβ,ads cannot be assessed. The most probable mechanism for the Fischer–Tropsch reaction resulting from this study is used in a next paper (Part 2) for the quantification of the kinetic parameters. In that paper, a systematic analysis of these parameters yields detailed mechanistic insight into the Fischer–Tropsch synthesis reaction.

The Investigation of Mechanisms in Environmental Catalysis Using Time-Resolved Methods

The formation of nitrogen oxides (NOx) during a combustion process is difficult to avoid because of the large exotherm and the consequent problem of avoiding local high-temperature spikes. Consequently, for many applications, such as for automotive power generation, there will be a continuing need to use catalytic after-treatment to reduce harmful emissions. The investigation of the mechanisms of the key catalytic reactions in environmental catalysis can provide an insight into the action of the catalyst, and time-resolved methods offer a powerful means to study these processes under realistic conditions. The use of Temporal Analysis of Products (TAP) and Steady State Isotopic Transient Kinetic Analysis (SSITKA) methods to investigate the reduction of NOx under various experimental conditions is described. From a detailed analysis of the SSITKA profiles, it is shown that at low temperatures the mechanism for the formation of N2 and N2O from NO may differ from the conventional high-temperature mechanism. This is supported by density functional theory calculations, which show that the barrier to the formation of N2O from the reaction of N(ads) and NO(ads) may be too high to allow this process to occur at low temperatures. The alternative reaction of NO(ads) + NO(ads) = N2O(g) + O(ads) is shown to be much more favorable and is consistent with the SSITKA analysis. The remarkable effect of hydrogen as a reductant at low temperatures is described, and alternative interpretations of the role of hydrogen are discussed.

Effect of H 2 partial pressure on surface reaction parameters during CO hydrogenation on Ru-promoted silica-supported Co catalysts

Steady-state isotopic transient kinetic analysis (SSITKA), one of the most powerful techniques for the investigation of surface reactions, was used to study the effect of hydrogen partial pressure on the fundamental surface reaction parameters for methanation on ordered mesoporous silica (MCM-41) and amorphous SiO 2-supported CoRu catalysts. The abundances, coverages, and lifetimes of surface intermediates of the reaction were measured under reaction conditions and their dependence upon hydrogen partial pressure was determined. Although absolute hydrogen coverage under reaction conditions is not measurable due to the hydrogen isotope effect, relative hydrogen surface concentration as a function of P H 2 could be estimated from SSITKA parameters. Increasing the hydrogen partial pressure at a constant reaction temperature of 220 °C not only caused the expected increase in the relative surface concentration of hydrogen but also increased the abundance of surface methane intermediates ( N M), possibly due to increased hydrogenation. The impact of P H 2 on N M for MCM-41-supported CoRu catalysts was similar to that for SiO 2-supported ones, showing an approximately twofold increase in N M as P H 2 increased from 0.23 to 1.71 bar. The relative concentration of surface hydrogen, however, increased fourfold. The abundance of surface methane intermediates and the surface coverages were significantly higher for the MCM-41-supported CoRu catalysts. The average surface reaction residence time of the methane intermediates ( τ M) consistently decreased with increasing hydrogen partial pressure due to the fact that the pseudo first order rate constant (1/ τ M) contains the hydrogen surface concentration term. There was no difference, however, in the intrinsic site activity since the average surface reaction residence times of methane intermediates ( τ M) for SiO 2- and MCM-41-supported CoRu catalysts were essentially identical for a given partial pressure of hydrogen, regardless of Co loading. This also indicates that the type of silica support used (amorphous SiO 2 or MCM-41) did not have an impact on surface hydrogen concentration, contrary to the case for H 2 chemisorption at 100 °C. The increase in rate with increasing hydrogen partial pressure resulted due to the increase in methane surface intermediates and, more importantly, the increase in hydrogen surface concentration.

CO Hydrogenation on Ru-Promoted Co/MCM-41 Catalysts

A study of CO hydrogenation on MCM-41 and SiO 2-supported Ru-promoted Co catalysts has been performed using both global rate measurements and steady-state isotopic transient kinetic analysis (SSITKA). A significant increase in CO hydrogenation was observed when MCM-41 was used as the support in the order M1>M2>SiO 2 at methanation conditions, where M1 and M2 are small pore ( d p=3 nm) and larger pore ( d p=7 nm) MCM-41, respectively. TOF H based on H 2 chemisorption can be misleading since any suppression of hydrogen chemisorption, as probably occurred for CoRu/M1, results in larger values of TOF H being calculated. SSITKA results, on the other hand, provide a measure of the “true” intrinsic CO hydrogenation activity of the Co sites. SSITKA studies indicated that the higher CO hydrogenation rates for MCM-41-supported catalysts are due to their greater number of active sites, not to a change in the intrinsic site activity. N 2 physisorption and XRD results reveal that the structure of MCM-41 became less ordered and the surface area decreased after standard reduction and 24 h of FTS, probably due to an effect of water vapor produced during metal reduction and reaction. However, the Co surface was still accessible since there was no significant loss of activity with TOS. The extent of deactivation during the initial reaction period was similar for MCM-41- and SiO 2-supported CoRu catalysts. By providing high activity and unrestricted diffusion of FT reactants and products, CoRu/MCM-41 may be potentially useful for FTS as well as for other catalytic applications, although modified forms of MCM-41 will probably be required in order to increase its hydrothermal stability.

Importance of Product Readsorption during Isotopic Transient Analysis of Ammonia Synthesis on Ba-Promoted Ru/Ba X Catalyst

Steady-state isotopic transient kinetic analysis was used to evaluate the intrinsic turnover frequency (TOF intr) and the fractional surface coverage of nitrogen-containing species (θ NH x ) on a Ba-promoted Ru/Ba X zeolite (∼2 wt% Ru) catalyst during ammonia synthesis at 3 atm, 673 K, and a stoichiometric ratio of N 2 and H 2. The artifacts due to inter- and intraparticle readsorption of product ammonia on the kinetic parameters were removed by cofeeding ammonia to the system and varying the total flow rate. The values of TOF intr and θ NH x determined from the intrinsic lifetime of surface intermediates at zero ammonia pressure were 0.067 s −1 and 0.064, respectively. The zeolite-supported catalyst was compared to poorly active Ru/SiO 2 and highly active Cs–Ru/MgO catalysts. Although the TOF intr spanned two orders of magnitude, the value of θ NH x was fairly independent of support and/or promoter.

Isotopic Transient Kinetic Analysis of Cs-Promoted Ru/MgO during Ammonia Synthesis

Steady-state isotopic transient kinetic analysis was used to examine a cesium-promoted Ru/MgO catalyst (1.75 wt% Ru) for ammonia synthesis. For comparison, an unpromoted Ru/SiO 2 (22 wt% Ru) catalyst was also studied. Steady-state and isotopic transient measurements were conducted at temperatures between 603 and 673 K with a stoichiometric ratio of N 2 and H 2 at a total pressure of 3 atm. The fractional surface coverage of nitrogen-containing intermediates (θ NH x ) based on total H 2 chemisorption was found to range from 0.02 to 0.05 on both catalysts under the conditions used in the current study. However, the intrinsic turnover frequency of ammonia synthesis over Cs–Ru/MgO was two orders of magnitude greater than that of the unpromoted Ru/SiO 2. The combination of the Cs promoter and MgO support enhanced the intrinsic activity of Ru, presumably by lowering the barrier for dinitrogen dissociation.

Steady-state isotopic transient kinetic analysis of the H 2/D 2 exchange reaction as a tool for characterising the metal phase in supported platinum catalysts

The steady-state isotopic transient kinetic analysis (SSITKA) technique has been applied to the characterisation of the metal phase in Pt/CeO 2 and Pt/SiO 2 catalysts by studying the H 2/D 2 exchange process. The time scale of the exchange process is close to the delay in the transport of the gases through the reactor. The behaviour of the reactor has been studied by analysing the curves corresponding to blank experiments and it can be satisfactorily modelled with the mathematical equation corresponding to two continuous stirred-tank reactors (CSTRs) in series. The curves in the presence of catalyst can be modelled with a simple mathematical equation that yields the values of the total number of sites active in the isotopic exchange and the apparent first order rate constant for the H 2/D 2 exchange process. The results so derived are in good agreement with those obtained from the analysis of the TPD experiments following the ITK runs. Although the results suggest a normal equilibrium isotope effect and an inverse kinetic isotope effect, the measured isotope effects are moderate in both cases and include possible contributions from the uncertainties in the calibration of the HD signal.

The Impact of Pt and H2 on n-Butane Isomerization over Sulfated Zirconia: Changes in Intermediates Coverage and Reactivity

Previous steady-state isotopic transient kinetic analysis (SSITKA) studies in our laboratory showed that the deactivation of an unpromoted sulfated zirconia (SZ) catalyst during n-butane isomerization at 150°C is due to a loss of active sites as well as a small decrease in the average site activity (Kim, S. Y., Goodwin, Jr., J. G., and Galloway, D., Catal. Today63, 21 (2000)). The mechanism by which SZ deactivates is linked to coke/oligomer formation and, possibly, surface sulfur reduction. The use of Pt promotion of SZ and H2 addition to the reaction mixture has been shown to greatly decrease deactivation during n-butane isomerization. In this study, the mechanisms by which H2 addition and Pt promotion affect n-butane isomerization over SZ were studied using SSITKA. H2 probably inhibits n-butane isomerization activity of the unpromoted SZ catalyst through inhibition of butene formation. Little effect on the average residence time for reaction was observed. An improvement in catalyst stability was linked to a reduction in coke/oligomer formation. In the absence of H2 in the feed stream, the presence of Pt did not improve the activity or the stability of the catalyst. An enhanced catalytic activity and stability, however, was evident when both H2 addition and Pt promotion were used, but only at the higher reaction temperature of 250°C. This enhanced catalytic activity at 250°C upon both Pt promotion and H2 addition was a result of an increase in the concentration of active surface intermediates leading to isobutane. This concentration did not change greatly during reaction, thus showing the improvement also in catalyst stability. Although the concentration of active intermediates increased significantly, the average reaction rate (inverse seconds) of an intermediate actually decreased, but not enough to prevent an increase in overall activity. Only 4% (SZ)–16% (PtSZ) of the intermediate to strong acid sites appeared to be involved in the reaction after 5 min time-on-stream, suggesting a reason why NH3 adsorption characterization results do not correlate well with catalyst activity for SZ-based catalysts. The implications of the obtained results on the roles of Pt and H2 on SZ in enhanced catalyst activity and decreased deactivation are discussed.

Steady-state isotopic transient kinetic analysis of the Fischer–Tropsch synthesis reaction over cobalt-based catalysts

The paper presents a transient kinetic analysis of the Fischer–Tropsch synthesis reaction using the SSITKA technique in combination with a gas-chromatograph-mass-spectrometer (GCMS) analysis of the 13C-labeled and 18O-labeled hydrocarbon and alcohol reaction products. Experiments are performed on a Co/Ru/TiO 2 catalyst and a fully metallic Co-sponge model catalyst at 498 K and 1.2 bar. The experimental results are discussed in a qualitative way to obtain mechanistic information. The Co-sponge catalyst is used to study alcohol formation, since the TiO 2 support disturbs the measurements on the Co/Ru/TiO 2 catalyst. The formation of hydrocarbons proceeds via a two-pool mechanism, where two carbon pools contribute to methane formation and C–C coupling. Paraffins and olefins are both primary products, but the GCMS analysis demonstrates that readsorption of 1-olefins is an important step. The readsorption of iso- and 2-olefins is shown to be of less importance. The steady-state performance of the catalyst indicates the presence of a physisorbed hydrocarbon layer, even under process conditions where no wax build-up in the catalyst pores occurs. Although the Anderson–Schulz–Flory distribution gives rise to assume chain-length independence of the surface reactions starting at C 3, this does not hold when discrimination between the paraffins and the olefins is made. Alcohol formation can be considered as a termination reaction that occurs via a CO insertion or a CH x O insertion mechanism.

N-Butane isomerization on sulfated zirconia: active site heterogeneity and deactivation

The fast deactivation of sulfated zirconia (SZ) has limited its use in commercial processes such as n-butane isomerization. In order to investigate this deactivation, steady-state isotopic transient kinetic analysis (SSITKA) was utilized to study in situ changes in surface kinetic parameters for n-butane isomerization on a widely studied SZ at 150°C. Approximately 20% of the sulfate species was found to be n-butane adsorption sites, but only 1–2% of the sulfate species appeared to adsorb active surface reaction intermediates. The decrease in catalytic activity during deactivation could be attributed to the loss of active sites. The change in TOF ITK ∗ (TOF based on an average residence time of active surface intermediates) and the regeneration characteristics of the SZ catalyst suggest a possible active site heterogeneity. It appears that the high initial activity and the fast deactivation for TOS≤100 min were mainly due to the presence and deactivation of the more active sites, respectively. Following the loss of the more active sites, the less active sites provided the majority of the catalytic activity observed for TOS≥100 min . The less active sites appeared to be more easily regenerated than the more active sites as the catalytic activity at TOS≥100 min was recovered following regeneration at 315°C. Loss of active sites due to sulfur loss or migration seems unlikely. Site blockage by coke/oligomer formation appeared to be a significant contributor for catalyst deactivation for n-butane isomerization on SZ. The impact of sulfur reduction on catalyst deactivation cannot be ruled out at this point.

Transient Techniques in the Study of Lean-No x Reduction Over Supported Pt Catalysts

The use of concentration-transient methods in the elucidation of deNO x mechanisms over supported Pt catalysts is discussed with reference made to non steady state (TAP, reactant removal, NSSITKA (non steady state isotopic transient kinetic analysis)) and steady state (SSITKA (steady state isotopic transient kinetic analysis)) techniques. The reaction systems studied include the NO/C 3 H 6 /O 2 , NO/C 3 H 8 /O 2 , NO/H 2 /O 2 and NO/H 2 reactions over supported Pt catalysts (Al 2 O 3 and SiO 2 ) and Al 2 O 3 . The use of concentration-transient techniques can give information about the presence, concentration and reactivity of adsorbed surface intermediates. Extrapolations from concentration-transient techniques are then used to give information about various mechanistic steps on the catalysts involved in the conversion of reactants to products. This information can then be used to get a clearer understanding of how and why certain catalysts are active (inactive) for the reduction of NO x under lean burn conditions.

Steady-state isotopic transient kinetic analysis on Pd-supported hexaaluminates used for methane combustion in the presence and absence of NO

This paper presents a study for the combustion of methane in the presence and absence of NO over hexaaluminates (La 1− X Sr X MnAl 11O 19) and Pd-supported hexaaluminates (Pd/La 1− X Sr X MnAl 11O 19). All the catalysts were subjected to a reaction cycle comprising of (1) CH 4+O 2, (2) CH 4+O 2+NO, and (3) CH 4+O 2 reactions at temperatures between 300 and 700°C. Steady-state isotopic transient kinetic analysis (SSITKA) technique was employed to determine the surface lifetimes ( τ), average site activities ( k), and surface concentrations ( N) of each relevant species during the reactions (1) and (2), as described above, on La 0.6Sr 0.4MnAl 11O 19 and 0.9% Pd/La 0.6Sr 0.4MnAl 11O 19. The SSITKA experiments were performed at temperatures of 360–440°C. Steady-state results showed that the contribution of homogeneous gas phase reactions are appreciable in the presence of NO at temperatures above 650°C. Steady-state and SSITKA reactions on La 0.6Sr 0.4MnAl 11O 19 suggested that methane and NO oxidation occur on the catalyst surface at 440°C. Steady-state and SSITKA reactions on Pd/La 0.6Sr 0.4MnAl 11O 19 showed that the presence of NO 2 (formed via NO and O 2) boosted the site activity required for the formation of CO 2. In the reaction where NO is involved, NO acts indirectly as chain initiator in the form of NO 2, and also as chain terminator.

N ‐butane isomerization on sulfated zirconia: isotopic transient kinetic analysis of reaction at the site level

By using SSITKA (steady‐state isotopic transient kinetic analysis), n‐butane (n‐C4) isomerization on sulfated zirconia (SZ) has been studied for the first time at the site level. Accurate measures of the average residence time and the concentration of the most active surface intermediates leading to isobutane (iso‐C4) were able to be determined. As has previously been observed, a fast initial deactivation of the catalyst followed by a slow steady‐state deactivation was observed over 400 min time‐on‐stream (TOS). It was shown that even though a large amount (∼100 μmol/g) of n‐C4 $$\left( {N_{n - C4} } \right)$$ was adsorbed on the catalyst, the concentration of active surface intermediates leading to iso‐C4 $$\left( {N_{iso - C4}^* } \right)$$ was only ∼10 μmol/g at 30 min TOS. The continuous decrease in $$\left( {N_{iso - C4}^* } \right)$$ indicated that the decrease in catalytic activity was due to loss of active sites. An increase in the average residence time of active surface intermediates leading to iso‐C4 $$\left( {\tau _{iso - C4}^* } \right)$$ was observed only between 30 and 100 min TOS. This suggests a possible presence of two types of active sites for n‐C4 isomerization on SZ under the reaction condition studied. It is suggested that the more‐active sites contributed to the high initial activity, while the majority of active sites after 100 min TOS were the less active sites.

A Steady-State Isotopic Transient Kinetic Analysis of the NO/O2/H2 Reaction over Pt/SiO2 Catalysts

The NO/O2/H2 reaction under strongly oxidising conditions has been studied over a 5% Pt/SiO2 catalyst using Steady-State Isotopic Transient Kinetic Analysis (SSITKA). The 14N-containing reactants and products were monitored following a 14NO/O2/H2→15NO/O2/H2 switch. N2O was found to be the isotopically first product and N2 the isotopically second. It was found that there was a constant desorption of NO from the catalyst surface under steady-state conditions. Recently introduced transformations (IDIMP, TRIMP, and the semilogarithmic plot of the α function versus time) of the product profiles, which concentrate on the isotopic distribution of the product molecules following the 14NO→15NO switch, are used to analyse the mechanism of N2 formation from this reaction. These show that N2 formation is predominantly through an “impact” route in which gaseous or physisorbed NO reacts with a reduced N species on the catalyst in a modified Eley–Rideal mechanism. A second, less active, mode of N2 formation is through the interaction of two equivalent species on the surface, each of which gives one N atom to the N2 molecule. The presence of O2 results in a change in the relative contribution of each type of N2 production.

A Transient Kinetic Study of the Mechanism of the NO/C3H6/O2Reaction over Pt–SiO2Catalysts: Part 2: Steady-State Isotopic Transient Kinetic Analysis

Steady-state isotopic transient kinetic analysis (SSITKA) of the NO/C3H6/O2reaction, using14NO →15NO switches, has been carried out over a 1% Pt–SiO2catalyst at four different temperatures. The results indicate that below the temperature of the maximum NOxconversion there are no NO adsorption/desorption processes taking place under these steady-state conditions. Moreover, less than 15% of the catalyst surface is covered with N2or N2O precursors. N2O is the “isotopically first” product while N2is the second. The surface lifetime of N2O precursors is relatively short while the surface lifetime of N2precursors is significantly longer. The production of N2and N2O are both increased by raising the temperature. In the case of N2O, this seems to be solely due to an increase in the concentration of active sites on the catalyst while in the case of N2the increase in the number of active sites is accompanied by an increase in their intrinsic activity. At temperatures above the maximum in the NOxconversion there is a discernible amount of NO desorption taking place under steady-state conditions. This is rationalised by the formation of NO2 (ads)species on the now-oxidised Pt surface. The SSITKA results are found to support the previously proposed reaction mechanism for the NO/C3H6/O2reaction over Pt-based catalysts. Finally, a general form for the calculation of concentrations of adsorbed species from any SSITK analysis is presented.

SSITKA Investigation of Palladium Precursor and Support Effects on CO Hydrogenation over Supported Pd Catalysts

CO hydrogenation on supported Pd catalysts was investigated using steady-state isotopic transient kinetic analysis (SSITKA) in order to explore the differences in catalytic behavior which have been previously reported by others to result from using different supports and Pd precursors. In this study, two different precursors [PdCl2and Pd(NO3)2] and three supports [SiO2, Al2O3, and SiO2–Al2O3] were investigated. The dispersion of the Pd particles on the prepared catalysts, as evaluated by CO chemisorption, was not affected by either the Pd precursor or the support used. In addition, for a given support the catalysts exhibited almost the same selectivities during CO hydrogenation toward methane, methanol [MeOH], and dimethyl ether [DME]. However, catalysts prepared using PdCl2showed higher overall activities than those prepared using Pd(NO3)2for a given support. The nature of the support altered significantly the activity and the selectivity of the Pd. The order of activities at steady-state for a given precursor was Al2O3> SiO2–Al2O3> SiO2. The activity of the Pd/SiO2–Al2O3was lower than expected probably due to the relatively high level of S (500 ppm) present in the support as an impurity. It was found that Pd/SiO2produced essentially only MeOH. The use of acidic supports [Al2O3and SiO2–Al2O3] resulted in the formation of DME due to the secondary reaction of MeOH on acid sites of the supports. DME production was found to be limited by the amount of MeOH formed. Pd on the acidic supports also produced significant amounts of methane. SSITKA results showed [after considering readsorption effects] that the Pd precursor used, or more specifically its anion [Cl−and NO3−], had a slight effect on theintrinsic activityof the sites producing methane (1/τmethane) [Cl−> NO3−] and a significant effect on thesurface coverageof the intermediates leading to methane (Nmethane) [Cl−> NO3−]. On the other hand, the precursor used had little or no effect on theintrinsic activityof the sites producing MeOH (1/τMeOH) or thesurface coverageof MeOH intermediates (NMeOH). The nature of the support altered only slightly theintrinsic activitiesof the sites producing methane and MeOH. However, the number ofsurface intermediatesleading to methane and MeOH were significantly affected by the support used and were in the order Al2O3> SiO2–Al2O3and Al2O3> SiO2–Al2O3> SiO2, respectively. It is, thus, the effect of the support on the concentration of active sites/intermediates on Pd that determines the impact of the support on the reaction rate.

Effect of Ca2+ Exchange on Adsorption of N2−O2 Mixtures by NaCaX Zeolite

Steady-state isotopic transient kinetic analysis (SSITKA) was used in this study to compare the adsorption behavior of pure N2 and O2 with the adsorption of binary N2−O2 mixtures at different PN2/PO2 ratios on a series of NaCaX zeolites with different degrees of Ca2+ exchange. A sharp increase in relative N2 selectivity was observed when more than 49% of Na+ was exchanged with Ca2+. For the single adsorbate case, there was also a sharp increase in N2 uptake at this point. This was probably due to Ca2+ cations starting to be in accessible locations in the zeolite for adsorption and the stronger field-quadrupole interactions between N2 and Ca2+. A similar increase was observed for O2 uptake for zeolite samples with more than 71% of the Na+ exchanged. It is suggested that, during the exchange process, the first Na+ cations to be replaced are the ones located at sites I and I‘ which are inaccessible to either of the adsorbate molecules. At exchange degrees higher than 49%, the drastic increase in adsorption uptake, particularly for N2, suggests that the Ca2+ also replaces Na+ cations located at sites II and III in the supercages of the X zeolite. The differences in molecular interactions between adsorbate molecules and cationic sites are reflected by differences in the isosteric heats of adsorption of N2 which were found to be around 3.8 kcal/mol on the Na zeolite with no Ca2+ and 6.3 kcal/mol on the one having 94% of the Na+ replaced by Ca2+. For the zeolite with 94% of Na+ exchanged, the N2 uptake in the presence of O2 at a PN2/PO2 ratio of 1/4 was significantly smaller than the one measured for pure N2. Similarly, for the same zeolite, the O2 uptake in the presence of N2 at a PN2/PO2 ratio of 4/1 was significantly smaller than the one measured for pure O2. These phenomena are attributed to competition between N2 and O2 adsorption. They were not observed at lower levels of Na+ replacement (higher Na+ content), probably due to lower adsorbate coverages under the conditions studied which minimized the competitive adsorption effects.

Determination of reactivity distributions from steady-state isotopic transient data: A model study

The potential of different mathematical methods for the recovery of reactivity distributions, f( k), from steady-state isotopic transient kinetic analysis (SSITKA) data has been tested and compared using a model catalyst system that consisted of a mechanical mixture of two catalysts with catalytic sites of known and different reactivity. Fitting one or two exponential functions to the measured transients confirmed the presence of two different pools of reaction intermediates, but the use of this method for determination of f( k) is limited to simple catalytic systems. For more complicated systems, the Tikhonov/Fredholm method might be superior to the inverse Laplace transform method.

Isotopic Transient Study of Multicomponent N2 and O2 Adsorption on CaX Zeolite

Steady-state isotopic transient kinetic analysis, previously used extensively for the study of surface-catalyzed reactions, was used for the first time to compare the adsorption behavior of pure N 2 and O 2 with the adsorption of binary N2/0 2 mixtures on a CaX zeolite. By variation of the partial pressure of N 2 or 0 2 in the feed stream, N 2 and 0 2 adsorption isotherms at temperatures between 303 and 338 K were able to be measured at equilibrium conditions using isotopic switches between 4 N 2 (or 6 0 2 D and 5 N 2 (or 8 0 2 1. The N 2 uptake was about 1 order of magnitude higher than the 0 2 uptake on Call, mainly due to the higher quadrupole moment of N 2 . N 2 adsorption isotherms in the presence of a constant partial pressure of 0 2 in the gas phase were also measured between 303 and 338 K. The N 2 uptake in the presence of O 2 was significantly smaller than the one measured for pure N 2 . However, the isosteric heats of adsorption of N 2 calculated from the N2/0 2 data were very similar to those obtained from the pure N 2 data (around 6.3 kcavmol). On the basis of these observations, it is suggested that O 2 does not affect the interactions between N 2 molecules and the Ca 2 adsorption sites. On the other hand, when the O 2 partial pressure was varied at constant N 2 partial pressure, a systematic decrease in N 2 uptake could be observed with increasing O 2 uptake. Thus, the smaller N 2 uptake in the presence of O 2 would appear only to be due to the competitive adsorption of N 2 and O 2 molecules. Additionally, N 2 and O 2 uptakes were measured from mixtures having a constant N 2 to O 2 ratio of 4/1 where only the overall adsorbate pressure (P N2 + Po 2 ) was varied. While the amount of N 2 adsorbed increased with an increase in P N2 , no significant change in nitrogen selectivity, u N2 , was able to be observed. This suggests, that the competition for adsorption between N 2 and O 2 depends solely on the P N /P O2 ratio rather than on the absolute value of P N2 .

N2 Adsorption in LiX Zeolite: Isotopic Transient Analysis

Steady-state isotopic transient kinetic analysis (SSITKA) is a very powerful technique used in heterogeneous catalysis to measure coverages and reactivities of surface intermediates. This paper reports the results of a study by SSITKA of N2 adsorption behavior (physisorption) in a LiX zeolite around room temperature. By varying the partial pressure of N2 in the feed stream, N2 adsorption isotherms at temperatures between 303 and 338 K were able to be measured at equilibrium conditions with SSITKA using isotopic switches between 14N2 and 15N2. The amount of N2 adsorbed per total Li cation content varied between (1 and 6.0) × 10-2 molecules/cation. The isosteric heats of adsorption of N2 as a function of coverage were calculated from the measured isotherms using the Clausius−Clapeyron equation. The obtained values (around 5.0 kcal/mol) are in good agreement with ones obtained from data collected using traditional techniques. On the basis of the results of this study, application of SSITKA to the investigation of adsorption from multicomponent sorbing mixtures offers exciting possibilities.