Isotopic transient response

Steady-state isotopic transient kinetic analysis (SSITKA) involves the replacement of a reactant by its isotopically labelled counterpart, typically in the form of a step or pulse input function. Producing an input function with isotope-labelled reactants permits the monitoring of isotopic transient responses, while maintaining the total concentration of labelled plus nonlabelled reactants, adsorbates, and products at steady-state conditions. It is assumed that there are no effects due to differences in kinetic behavior of the isotopic species from unmarked atomic species. However, for instance, deuterium substitution exhibits isotopic effects that can not be neglected. 

Fig. 4. Typical normalized isotopic transient responses in product species P following an isotopic switch in reactant R— R. An inert tracer, I, is introduced to determine the gas-phase holdup of the reactor system.

Equations (11) and (12) enable the generation of the total isotopic transient responses of a product species given (a) the transient response that characterises hypothesized catalyst-surface behaviour and (b) an inert-tracer transient response that characterises the gas-phase behaviour of the reactor system. Use of the linear-convolution relationships has been suggested as an iterative means to verify a model of the catalyst surface reaction pathway and kinetics. I This is attractive since the direct determination of the catalyst-surface transient response is especially problematic for non-ideal PFRs, since a method of complete gas-phase behaviour correction to obtain the catalyst-surface transient response is presently unavailable for such reactor systems.1 1 Unfortunately, there are also no corresponding analytical relationships to Eqs. (11) and (12) which permit explicit determination of the catalyst-surface transient response from the measured isotopic and inert-tracer transient responses, and hence, a model has to be assumed and tested. The better the model of the surface reaction pathway, the better the fit of the generated transient to the measured transient. 

The following isotopic labeling experiment was performed in order to quantify the contribution of the direct and indirect reaction routes to CO formation After steady-state reaction with CH4/02/He was achieved, an abrupt switch of the feed from CH4/02/He to an isotopic mixture of CH4/1 02/ C 02/He was made, in which the partial pressures of CH4 and 62 were kept exactly the same as in the ordinary CH4/02/He mixture, so as not to disturb the steady-state condition. However, C 02 was added to the isotopic mixture in an amount corresponding to approximately 10-15% of the CO2 produced during reaction of the mixture. The purpose was to measure the production of C 0 due to reforming of CH4 with C 02 only (indirect reaction scheme) under steady-state conditions of the working catalyst surface. Figure 3 shows the transient responses of and C O... 

Transient response experiments have revealed that the formation of N2 and N2O during NO reduction by H2 over Rh proceeds without the intervention of H2 By contrast, the formation of NH3 and H2O involves the reactions of dissociatively chemisorbed H2 with N and 0 atoms, respectively. The results obtained from experiments involving the reduction of adsorbed NO and isotopic substitution of NO for NO can be interpreted on the basis of the reaction mechanism presented in Fig. 11. Key elements of this mechanism are that NO is adsorbed reversibly into a molecular state, that reduction is initiated by the dissociation of molecularly adsorbed NO, and that all products are formed via Langmuir-Hinshelwood process. 

Acid Form - Pseudoliquid Phase Behavior. Owing to a high affinity for polar molecules, large quantities of molecules such as alcohols and ether are absorbed within the bulk phase of crystalline heteropolyacids. The amounts of pyridine, methanol, and 2-propanol absorbed correspond to 50-100 times that which can be adsorbed on the surface, while nonpolar molecules like ethylene and benzene are adsorbed at the surface only. Catalytic reactions of polar molecules occiu both on the surface and in the bulk, so that the solid heteropolyacid behaves as a highly concentrated solution, called a pseudoliquid phase . The dehydration of alcohols, various conversions of methanol and dimethyl ether to hydrocarbons in gas-solid systems, and the alkylation of phenol and pinacol rearrangements can all occur in the pseudoliquid. The transient response using isotopically labeled 2-propanol provides evidence for the pseudoliquid phase behavior of H3PW12O40. This behavior influences the selectivity, for example, the aUcene/aUcane ratio, in the conversion of dimethyl ether. 

There are macroscopic (uptake measurements, liquid chromatography, isotopic-transient experiments, and frequency response techniques), and microscopic techniques (nuclear magnetic resonance, NMR and quasielastic neutron spectrometry, QENS) to measure the gas diffusivities through zeolites. The macroscopic methods are characterized by the fact that diffusion occurs as the result of an applied concentration gradient on the other hand, the microscopic methods render self-diffusion of gases in the absence of a concentration gradient [67]. 

The following sections provide a kinetic analysis of the transient responses based on an atomic state for the chemisorbed oxygen, 0(s). We show that this approach allows us to account for the qualitative features of the results described above, the temperature dependence of the rate of isotope scrambling under steady-state conditions, and results ftom temperature programmed desorption (TPD) experiments performed at very low pressure. The steady-state exchange and TPD experiments are described in Sec. 3.1.3.. The kinetics of isotope exchange of O2 (gas) with oxide materials have been reviewed by Ceilings and Bouwmeester. Readers are referred to this work and references therein for a more comprehensive discussions of the mechanisms and kinetics involved in more complex systems. 

Pulse intensities in vacuum experiments range from 10 to 10 molecules per pulse with a pulse width of 250 ps and a pulse frequency of between 0.1 and 50 pulses per second. Such a spectrum of time resolution is unique among kinetic methods. Possible experiments include high-speed pulsing, both single-pulse and multipulse response, steady-state isotopic transient kinetic analysis (SSITKA), temperature-programmed desorption (TPD), and temperature-programmed reaction (TPR). 

To gather information for the role of O2 in the H2-SCR mechanism, similar SSITKA experiments with the use of 02 were conducted at 140 °C. Figure 26.8 presents the transient response curves of N2 0, N2 0, and Ar obtained on Pt/Lao.sCeo.sMnOs (Figure 26.8a) and Pt/Si02 (Figure 26.8b) catalysts after the switch N0/H2/ 02/Ar/He N0/H2/ 02/He was made at 140 °C. As seen in Figure 26.8, the concentration of N2 0 produced by both catalysts is reduced after the isotopic switch, whereas the continuous evolution of N2 0 is noticed. The sum of the steady-state concentrations of N2 0 and N2 0 formed under H2-SCR in the isotopic gas mixture is the same as the steady-state concentration... 

Figure 26.8 Dimensionless transient response curves of N2 0, N2 0, and Ar obtained following the isotopic switch NO/H2/ 02/Ar/He N0/H2/ 02/Ar/He at 140 °C over...

Temperature-prograimned reduction, oxidation and desorption (TPR, TPO, TPD), belong probably to the most widely used in situ techiuques for the characterization of oxidation catalysts and are discussed in more detail in Section 19.4. While TPD (with ammonia as the probe molecule) is frequently used to examine surface acid sites, TPR and TPO (with H2 or O2, respectively) provide information on the redox properties of oxide catalysts being crucial for their performance in catalytic oxidation reactions. Important information on reaction mechanisms can be obtained when the catalysts are heated in the presence of reactants combined with mass spectrometric product analysis. This is called temperature-programmed reaction spectroscopy (TPRS). As far as reaction mechanisms and kinetics are concerned, transient techniques which reflect the response of the catalytic system to a sudden change of reactant are inevitable tools. Two such techiuques, namely the temporal analysis of products (TAP) reactor and steady-state isotopic transient kinetic analysis (SSITKA) will be described in more detail in Section 19.5. 

F nre 3.12 Transient response curves for (a) Oj and (b) 0 0 gaseous oxygen isotopic species obtained during TPIE over fresh Ceo.5Zro.5O2 solids and Ceo,5Zro 5O2 solids contaminated with P, P-Zn, or P-Ca. Adapted with permission from Christou et alP Copyright 2011 Elsevier. 

The distribution of the isotopic labels is dependent upon the steady-state transferrates between pools. Analytic expressions in SSITKA can be rather comphcated. As an example, transient responses and kinetic parameters are presented below for an irreversible reaction with reversible adsorption R Xi —> X,- P, where Xj and X,- are intermediates, R is the reactant, and P is the products. The multiple in-series pool system gives... 

Figure 20.5. A graphical representation of the time evolution of transients for the Norrish type-I a-cleavage 43 and 46 amu fragments from acetone and from acetone-de- The representative sets of data points ( for 43 amu, for 46 amu fragments) are modeled with simple buildup and decay response functions, I(t) = 4[exp(—t/t2) — exp(—f/x])] the time constants of buildup and decay are Ti and T2, respectively. A modest isotope effect on the characteristic time for formation of these acyl radicals (60 and 80 fs, respectively) and a more prominent —CH3/—CD3 effect on decays through loss of CO (420 and 670 fs, respectively) were recorded. ...

The plug flow reactor is increasingly being used under transient conditions to obtain kinetic data by analysing the combined reactor and catalyst response upon a stimulus. Mostly used are a small reactant pulse (e.g. in temporal analysis of products (TAP) [16] and positron emission profiling (PEP) [17, 18]) or a concentration step change (in step-response measurements (SRE) [19]). Isotopically labeled compounds are used which allow operation under overall steady state conditions, but under transient conditions with respect to the labeled compound [18, 20-23]. In this type of experiments both time- and position-dependent concentration profiles will develop which are described by sets of coupled partial differential equations (PDEs). These include the concentrations of proposed intermediates at the catalyst. The mathematical treatment is more complex and more parameters are to be estimated [17]. Basically, kinetic studies consist of ... 

The oxidation of carbon monoxide has been studied by both the usual step-response and isotopic experiments and by the TAP system (2/7). The general conclusion is that the fast response of the TAP system did not produce any additional mechanistic information to that obtained from step-response experiments. A number of the points discussed in previous paragraphs are mentioned, and it is suggested that the final pattern of multipulse response experiments be termed a pseudo-steady state. A factor not mentioned is that transient IR experiments are valuable with the step-response method but not compatible with the TAP system. 

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