First-cycle dynamics, catalyst

Temperature Increase Dynamics after the First Cycle. As with the start up of the bed, subsequent temperature cycles resulted in the formation of a mild hot spot. The occurrence of this temperature fluctuation is undesirable since the past history of the catalyst may be altered. The adsorption of thiophene upon the active hydrogenation sites was assumed to be irreversible and therefore unaffected by temperature. However, as will become apparent later, the effect of temperature may have altered the poison coverage/or profile. Lyubarski, et. al [73 determined that, as a result of the hydrogenation of thiophene and subsequent hydrogenolysis to butane, the adsorption capacity of a suported Ni... 

In this paper we will first describe a fast-response infrared reactor system which is capable of operating at high temperatures and pressures. We will discuss the reactor cell, the feed system which allows concentration step changes or cycling, and the modifications necessary for converting a commercial infrared spectrophotometer to a high-speed instrument. This modified infrared spectroscopic reactor system was then used to study the dynamics of CO adsorption and desorption over a Pt-alumina catalyst at 723 K (450°C). The measured step responses were analyzed using a transient model which accounts for the kinetics of CO adsorption and desorption, extra- and intrapellet diffusion resistances, surface accumulation of CO, and the dynamics of the infrared cell. Finally, we will briefly discuss some of the transient response (i.e., step and cycled) characteristics of the catalyst under reaction conditions (i.e.,... 

It was mentioned at the beginning of this chapter that alkaloids were among the first catalysts to be used for asymmetric hydrocyanation of aldehydes. More recent work by Tian and Deng has shown that the pseudo-enantiomeric alkaloid derivatives 5/6 and 7/8 catalyze the asymmetric addition of ethyl cyanoformate to aliphatic ketones (Scheme 6.6) [50]. It is believed that the catalytic cycle is initiated by the alkaloid tertiary amine reacting with ethyl cyanoformate to form a chiral cyanide/acylammonium ion pair, followed by addition of cyanide to the ketone and acylation of the resulting cyanoalkoxide. Potentially, the latter reaction step occurs with dynamic kinetic resolution of the cyano alkoxide intermediate... 

Autocatalysis is a distinctive phenomenon while in ordinary catalysis the catalyst re-appears from the reaction apparently untouched, additional amounts of catalyst are actively produced in an autocatalytic cycle. As atoms are not interconverted during chemical reactions, this requires (all) the (elementary or otherwise essential) components of autocatalysts to be extracted from some external reservoir. After all this matter was extracted, some share of it is not introduced in and released as a product but rather retained, thereafter supporting and speeding up the reaction(s) steadily as amounts and possibly also concentrations of autocatalysts increase. At first glance, such a system may appear doomed to undergo runaway dynamics ( explosion ), but, apart from the limited speeds and rates of autocatalyst resupply from the environment there are also other mechanisms which usually limit kinetics even though non-linear behavior (bistability, oscillations) may not be precluded ... 

In this chapter, modeling of monolith reactors will be considered from a first-principles point of view, preceded by a discussion of the typical phenomena in monoliths that should be taken into account. General model equations will be presented and subsequently simplified, depending on the subprocesses that should be described by a model. A main lead will be the time scales at which these subprocesses occur. If they are all small, the process operates in the steady state, and all time-dependent behavior can be discarded. Unsteady-state behavior is to be considered if the model should include the time scale of reactor startup or if deactivation of the catalyst versus time-on-stream has to be addressed. A description of fully dynamic reactor operation, as met when cycling of the feed is applied, requires that all elementary steps of a kinetic model with their corresponding time scales are incorporated in the reactor model. 

In the BZ reaction, malonic acid is oxidized in an acidic medium by bromate ions, with or without a catalyst (usually cerous or ferrous ions). It has been known since the 1950s that this reaction can exhibit limit-cycle oscillations, as discussed in Section 8,3. By the 1970s, it became natural to inquire whether the BZ reaction could also become chaotic under appropriate conditions. Chemical chaos was first reported by Schmitz, Graziani, and Hudson (1977), but their results left room for skepticism—some chemists suspected that the observed complex dynamics might be due instead to uncontrolled fluctuations in experimental control parameters. What was needed was some demonstration that the dynamics obeyed the newly emerging laws of chaos. 

The first step in analyzing the performance of a catalyst in an emission control system is to determine "what the catalyst sees" in terms of temperature, exhaust composition, and exhaust flow rate variations during the driving cycle. The nature of the conditions that a catalyst is exposed to is not only a function of the driving cycle and the vehicle type, but also is dependent upon the air-fuel control system. Tests which record the dynamic conditions have to be repeated and evaluated statistically since the detailed results of each test will vary as a result of random test-to-test variations. 

Since the 1980s, SSITKA has been widely used to understand the formation mechanism of methane as the first paraffin in the chain. The study of the dynamics of the entire complex of reactions involved in the Fischer-Tropsch process became possible only after the development of the GC-MS technique with high resolution time. A review of field suggests that the cycle of papers by van Dijk et al. [18-21] describes the results that were obtained using the full potential of the SSITKA technique. First, a comparison of C, O, and H labeling on different Co-based catalyst formulations and in different conditions was made. For the first time, a substantial part of the product spectrum (both hydrocarbons and alcohols) was included in the isotopic transient analysis. After the qualitative interpretation of the experimental data, extensive mathematical modeling was performed for the identification and discrimination of reaction mechanisms. 

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