Periodic activity experimental observation

A key feature of PdCys as precursors of Pd(0) nanoparticles is that reduction of Pd(II) -> Pd(0) involving C-Pd bond cleavage is required. This accounts for both the high temperatures invariably required and the induction period in the absence of reductants. Rosner et al. have developed a detailed kinetic model of a Heck reaction catalyzed by dimeric palladacycles (Rosner et al. 2001 a,b). This model explains the experimental observations and is consistent with an active species... 

Periodic activity catalytic reactions, 27 59-94 experimental observation, 27 64-73, 75-77, 87, 88... 

The paper is not equation oriented since after the period of theoretical investigation, only a small percentage of experimental papers published is completely supported with theory and very often only a qualitative explanation is presented. Hence in this paper we shall review the experimental information published in the literature concerning multiplicity of steady states and periodic activity in the systems catalyst-gas, making an attempt to explain qualitatively these phenomena on the basis of the theory developed.1 The number of experimental observations surveyed here which are not supported by a theory will surely indicate that there are many roads open for fundamental research in this area. 

A. Experimental Observation of Multiple Steady-State Phenomena and Periodic Activity... 

Experimental Observations of Multiply Steady States and Periodic Activity for a Single Catalyst Particle... 

The results of the calculations shown in Fig. 2.32 represent a complete quantitative solution of the problem, because they show the decrease in the induction period in non-isothermal curing when there is a temperature increase due to heat dissipation in the flow of the reactive mass. The case where = 0 is of particular interest. It is related to the experimental observation that shear stress is almost constant in the range t < t. In this situation the temperature dependence of the viscosity of the reactive mass can be neglected because of low values of the apparent activation energy of viscous flow E, and Eq. (2.73) leads to a linear time dependence of temperature ... 

Recently, we investigated the associative alkylation reaction of toluene with methanol catalyzed by an acidic Mordenite (see Figures 13 and 14) by means of periodic ab initio calculations." We observed that for this reaction some transition selectivity occurred, and induced sufficiently large differences in activation energies to explain the small changes in the para/meta/ortho distribution experimentally observed on large pore zeolites. Thepara isomer is the more valuable product as it is an important intermediate for terphthalic acid, an important polymer monomer." The steric constraints obtained for the transition state structures could be estimated from local intermediates for which the orientations of the toluene molecule were similar as the ones observed for the transition states (see Figure 14). 

Mechanistic studies performed with Freeh s pincer catalyst in the Heck reaction excluded catalytic cycles with the involvement of homogeneous palladium(O) species, as indicated by the results obtained from the (recently developed) dibenzyl-test, which is directly applicable under the reactions conditions applied [24aj. Dibenzyl formation was - in contrast to Heck reactions catalyzed by palladium(O) complexes of type [Pd(PR3)2, where Pd /Pd" cycles are operative - not detectable by gas chromatography-mass spectrometry (GC/MS) when reaction mixtures of aryl bromide, olefin, benzyl chloride ( 10 mol% relative to aryl bromide), catalyst, and base were thermally treated. On the other hand, experimental observations, such as quantitative poisoning experiments with metallic mercury and CS2, which were shown to eflfidently inhibit catalysis, as well as analysis of the reaction profiles showed sigmoidal-shaped kinetics with induction periods and hence indicated that palladium nanoparticles are the catalytically active form... 

Ninety-two percent of the oil is produced before breakthrough of the hydroxyl ion at T = 1.45 PV. The first evidence of an acid-base reaction occurs at the trailing edge of the primary oil bank at T = 0.78 PV. The oleic acid concentration in the oil phase is a minimum at T = 0.78 PV. The surface active oleates of calcium which are formed by the interfacial chemical reaction stimulate the production of a secondary oil bank. The oleic acid concentration in the oil phase peaks at the oil cut peak of the chemically stimulated oil bank at T = 1.72 PV during a period of minimum contact. The concentration of the oleic acid displays a diffuse minimum at the trailing edge of the secondary oil bank. Incremental recovery of acidic oil by an Emulsification and Coalescence mechanism in the secondary calcium hydroxide flood can be inferred from the following experimental observations ... 

The first term of Equation 13.18 is a hyperbolic function that represents the rapid initial activity decline as function of time t. As stated in the list of assumptions, it is considered that this happens during the first 100 h of operation, and the initial deactivation period is over after this time period. The choice of this simple function is based on numerous experimental observations showing that the deactivation curves during start-of-run follow the same pattern even with different types of heavy feeds in a wide range of temperatures [73]. The second term stands for the contribution of metals buildup during the whole cycle. This function is directly linked to the amount of metals-on-catalyst (MOC Xmoc) at axial position z and time t, which allows for establishing a time-evolving axial activity profile. 

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