Pressure-jump kinetic experiments

The apparatus s step change from ambient to desired reaction conditions eliminates transport effects between catalyst surface and gas phase reactants. Using catalytic reactors that are already used in industry enables easy transfer from the shock tube to a ffow reactor for practical performance evaluation and scale up. Moreover, it has capability to conduct temperature- and pressure-jump relaxation experiments, making this technique useful in studying reactions that operate near equilibrium. Currently there is no known experimental, gas-solid chemical kinetic method that can achieve this. 

Kinetics of Molybdenum Adsoiption. Zhang and Sparks 41) examined molybdate adsorption on goethite using pressure jump relaxation experiments. Molybdate adsorption was proposed to occur via two steps, an initial outer-sphere complex and subsequent replacement of a water molecule by formation of an inner-sphere complex of Mo04, based on optimized fits using the triple layer model. Forward rate constants were on the order of 4x10 L mol s and 40 s for the first and second reaction steps. 

Temperature jump studies were performed in combination with stopped-flow experiments94 in response to criticism as to the reliability of temperature jump experiments26,142 and divergent interpretation of the kinetics. In particular, the suggestion from pressure jump experiments that the association step is diffusive in nature... 

Kinetic experiments were conducted using a pressure-jump apparatus with conductivity detection. Details of the apparatus and its operation can be found in Appendix A. Sample equilibration time can have an effect on the kinetic results (e.g., slow processes (on the order of hours-days) occurring concurrently but not monitored in the time frame of the p-jump technique (milllseconds-seconds)) hence, it is important to run kinetic experiments on samples with similar equilibration history. All samples were equilibrated between 3 and 4 hours for the p-jump kinetic studies. The temperature of the p-jump apparatus, which includes sample and reference solution cells, was maintained at 25.0°C 0.1°C. 

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). 

Use of pressure-jump relaxation and other relaxation techniques have been shown to offer much in the study of sorption measurements on soil components (Sparks and Zhang, 1991 Sparks, 1995). An especially attractive approach for ascertaining sorption mechanisms on soils would be to combine relaxation approaches with in situ surface spectroscopic techniques. However, there are a few examples in the literature of studies where sorption reactions on soil components have been hypothesized via kinetic experiments and verified in separate spectroscopic investigations (Fuller et al., 1993 Waychunas et al., 1993 Fendorf et al., 1997 Grossi et al., 1997 Scheidegger et al., 1997). 

An important turning point in reaction kinetics was the development of experimental techniques for studying fast reactions in solution. The first of these was based on flow techniques and extended the time range over which chemical changes could be observed from a few seconds down to a few milliseconds. This was followed by the development of a variety of relaxation techniques, including the temperature jump, pressure jump, and electrical field jump methods. In this way, the time for experimental observation was extended below the nanosecond range. Thus, relaxation techniques can be used to study processes whose half lives fall between the range available to classical experiments and that characteristic of spectroscopic techniques. 

Experiments for investigating the lamellar-Hn transition kinetics have been performed, for example, on DOPE dispersions. The F,p-phase diagram of DOPE in excess water is depicted in Fig. 21. Figures 22, 23 show the diffraction patterns and lattice parameters at 20 °C after a pressure jump from 300 to 110 bar. Clearly, the (001) reflection of the L phase and the (10) reflection of the developing Hu phase can be identified. In this case, a two-state mechanism is observed. Interestingly, we find that successive pressure jumps lead to an acceleration of the phase transition kinetics. The half transit time decays from 8.5 s for the first pressure jump to... 

Elucidation of the kinetics and mechanisms of mineral-fluid interactions requires high-resolution X-ray scattering measurements on rapid time scales. Time series analyses are desired for addressing the evolution of structure and composition at the interface, on time scales as small as milliseconds or less. The high brilliance of the third-generation synchrotron sources affords new opportunities for such time-resolved studies, because we can observe in real time the processes of adsorption/desorption and complex formation at mineral-fluid interfaces. For example, experiments using a pressure-jump relaxation techniques yield rates of adsorption and desorption of protons and hydroxide at the surface of metal oxides in the range of milliseconds to seconds (reviewed by Casey and... 

Adsorption kinetics will be discussed first. Figure 11A shows the equilibration of the alumina system after the addition of HCl [16]. Figure IIB shows that the adsorption of Cu, with liberation of protons [47], takes about 24 h to reach equilibrium. Figure 11C gives kinetic data for the adsorption of Co, Fe, and Ni ions [54], evidently in equilibrium after about 1 h. On the other hand, pressure jump experiments of Cu adsorption used a time scale of 10 -10 s [55]. These results are mentioned to demonstrate that reactions taking place in the adsorption system may have rates that differ by several orders of magnitude. 

The kinetics of intercalation and deintercalation of alkali metal ions were investigated in pressure-jump experiments while monitoring the electrical conductivity of the samples (32). These studies indicate biphasic kinetics whose magnitudes are in milliseconds the rates of the fast and slow components increased with increased concentrations of the metal ions. The forward and reverse rates depend on the interlayer distances, and the fast and slow components have been attributed to the ingress of ions into the galleries and interlayer diffusion, respectively. Similar biphasic kinetics on millisecond-second time scales were also observed in pressure-jump experiments for the deprotonation-reprotonation of a-ZrP (33). In the latter case, the slow and fast components have been attributed to deprotonation from the surface and from the interlayer regions of the solid, respectively. 

Modem techniques of fast kinetics make it possible to study almost any reaction. Many of the techniques are relatively similar some are quite complicated. The major difficulty is in properly defining the problem so that the results of the study are significant. Modem equipment and modem techniques can not create order out of chaos. Presently it is possible to purchase commercially much of the equipment necessary to make many of these fast measurements. Flash photolysis systems are commercially available. Temperature jump, pressure jump, stopped flow, fast flow and quenched flow instruments can be bought. In fact attachments to stopped flow equipment are commercially available to do many additional types of experiments. 

Flynn and Dickens (124) described a TG method in which the magnitude of a rate-forcing variable such as temperature, pressure, gaseous flow rate, gaseous composition, and so on is jumped by discrete steps. This method can be used to determine kinetic relationships between the rate of mass-loss and thejpmped variahlft. The method avoids the disparate effects of separate experimental histories in methods in which two or more experiments are compared and also the necessity for guessing the complex rate versus the extent of reaction relationship. 

Whenever a chemical equilibrium is subjected to a perturbation, most commonly a change in temperature, pressure, pH, or other concentrations, the system will start to relax back to a new equilibrium state. The kinetics of this relaxation can be followed. Methods for quickly inducing a perturbation followed by monitoring the relaxation are referred to as jump techniques. Changes in temperature, pH, and pressure can often be done fast enough that reactions with half-lives in the microsecond range can be followed. For example, the equilibrium positions of Bransted acid-base reactions are controlled by the pH, and therefore pH jump experiments are particularly useful with these reactions. 

---