Pulsed flow technique

A square concentration pulse flow technique has been developed to study the kinetics of catalytic reactions over catalysts which change their stoichiometry in response to the reaction conditions. The technique makes it possible to obtain hysteresis-free kinetics data while greatly reducing the time during which the catalyst is exposed to the reaction mixture. 

In the literature on elimination reactions, it is found that mechanistic conclusions are quite frequently made on the basis of the values of the activation energy. This is a dubious practice, especially when the data have been obtained by the gas chromatographic (pulse-flow) technique, i.e. when there is a non-stationary state on the catalyst surface, and on the basis of supposed first-order kinetics. 

The application of selective chemisorption to supported Pt catalysts is well established but there have been valuable additional studies of the use of hydrogen in the pulse-flow technique and of CO adsorption using TPD and carbon monoxide. Recently the usual assumption about the stoicheiometry for hydrogen adsorption, Ptg/H = 1 has been questioned. For the Council of Europe Pt-Si02 catalyst, where a weak metal-support interaction was postulated, 1.75 hydrogen atoms per surface metal atom were found at 300 K in two adsorbed forms (the formation of jSa was activated). Recent work on selective chemisorption applied to metals of catalytic interest other than platinum will now be examined. 

The reduction behaviour of the catalysts was studied in an indigenously designed TPR unit. The metal dispersion was measured by oxygen titrations using dynamic pulse flow technique (Pulse chemisorb 2700, Micromeritics, USA). Acidity and acid strength distribution were determined through heats of adsorption of ammonia by Calvet C-80 microcalorimeter (Setaram, France) and by TPD of ammonia using Catalyst Data System, Baroda (India), TPD unit. 

The instruments include an ionization chamber, the charcoal-trap technique, a flow-type ionization chamber (pulse-counting technique), a two-filter method, an electrostatic collection method and a passive integrating radon monitor. All instruments except for the passive radon monitor have been calibrated independently. Measurements were performed... 

This chapter focuses on analytical CL methodologies, with emphasis on the kinetic connotations of typical approaches such as the stopped-flow, the continuous-addition-of-reagent (a new kinetic methodology) and the pulse perturbation technique developed for oscillating reactions, among others. Recent contributions to kinetic simultaneous determinations of organic substances using CL detection (kinetometric approaches included) are also preferentially considered here. 

Apart from the temperature-jump technique, other relaxation methods that have been used are those of ultrasonic absorption" " and electric-field pulse. Another technique that has been used for some of the more slowly included guest molecules is that of stopped-flow. ... 

Various NMR spectroscopic techniques have been applied to investigate the conversion of methanol on acidic zeolites in the low-temperature (r<523K) formation of DME and the high-temperature (T>523 K) formation of alkenes and gasoline. Techniques successfully applied were the stop-and-go method under batch reaction conditions 258,259), the pulse-quench method 113), and various flow techniques 46,49,74.207,260 263). This section is a summary of the recent progress in investigations of the mechanism of the MTO process by NMR techniques. 

The activity of Mo03 supported on a high surface silica carrier was studied by Vaghi et al. [331] using pulse and flow techniques at 400— 440°C. Oxidation activity and acrolein formation appear to be zero below 10 wt. % Mo03, but increase with the Mo03 content above 10%. The... 

Asbjdmsen (A6), 1961 Residence times in falling water films determined by a pulsed tracer technique. Mean residence time 2-7% greater than calculated from laminar film flow theory. 

The time involved in mixing places a limit on the dead time of flow techniques. The only way to increase the time resolution is to cut out the mixing by using a premixed solution of reagents that can be perturbed in some way to allow a measurable reaction to occur. A classic method from physical chemistry is flash photolysis, in which a particular bond in a reagent is cleaved by a pulse of light so that reactive intermediates are formed. This method was introduced in 1959... 

In the syringe-type pump the liquid is enclosed in a cylinder. A piston moves at a constant speed to push the liquid. Eluent compressibility induces time-consuming flow equilibrium. Nevertheless, the flow from a syringe pump is pulse free. For micro LC, flow rates of 50 yuL/min are utilized in spite of the drawback of column pressurization. With very low flow rates (in the nanoliter range) the use of pumps is tedious, and split-flow techniques are required. 

Rate studies of the reaction between cesium and water in ethylenediamine, using the stopped-flow technique, have been extended to all alkali metals. The earlier rate constant (k — 20 NT1 sec.-1) and, in some cases, a slower second-order process (k — 7 Af"1 sec.-1) have been observed. This is consistent with optical absorption data and agrees with recent results obtained in aqueous pulsed-radiolysis systems. Preliminary studies of the reaction rate of the solvated electron in ethylenediamine with other electron acceptors have been made. The rate constant for the reaction with ethylene-diammonium ions is about 105 NCl sec.-1 Reactions with methanol and with ethanol show rates similar to those with water. In addition, however, the presence of a strongly absorbing intermediate is indicated, which warrants more detailed examination. 

To study rapid reactions, traditional batch and flow techniques are inadequate. However, the development of stopped flow, electric field pulse, and particularly pressure-jump relaxation techniques have made the study of rapid reactions possible (Chapter 4). German and Japanese workers have very successfully studied exchange and sorption-desorption reactions on oxides and zeolites using these techniques. In addition to being able to study rapid reaction rates, one can obtain chemical kinetics parameters. The use of these methods by soil and environmental scientists would provide much needed mechanistic information about sorption processes. 

Another consideration in choosing a kinetic method is the objective of one s experiments. For example, if chemical kinetics rate constants are to be measured, most batch and flow techniques would be unsatisfactory since they primarily measure transport- and diffusion-controlled processes, and apparent rate laws and rate coefficients are determined. Instead, one should employ a fast kinetic method such as pressure-jump relaxation, electric field pulse, or stopped flow (Chapter 4). 

The pulse technique is in many ways similar to the flow technique except that the adsorbate is introduced by adding pulses (e.g. from a gas sample valve) into the carrier gas. The pulse volume is chosen so that the first few pulses will be completely adsorbed. Further pulses are introduced until no more gas is adsorbed. The quantity of gas adsorbed is calculated by summing up the amounts adsorbed in the successive pulses. This technique is only applicable for strongly retained adsorbates. 

The pulsed flow (PF) technique at the tubular electrode was introduced by Blaedel and Iverson [73]. The sensitivity of the electrode, over that achieved with steady flow, was increased by monitoring the change in current as the solution flow rate was switched between two well separated values (e.g., 9 ml min-1 and 0.5 ml min-1), in a manner similar to PRV. A/ in this case is given by equation (10.21). The time between flow rate pulses was of the order 10-30 s. 

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