Poisoning reaction pathways

Covalent synthesis of complex molecules involves the reactive assembly of many atoms into subunits with aid of reagents and estabUshed as well as innovative reaction pathways. These subunits are then subjected to various reactions that will assemble the target molecule. These reaction schemes involve the protection of certain sensitive parts of the molecule while other parts are being reacted. Very complex molecules can be synthesized in this manner. A prime example of the success of this approach is the total synthesis of palytoxin, a poisonous substance found in marine soft corals (35). Other complex molecules synthesized by sequential addition of atoms and blocks of atoms include vitamin potentially anticancer KH-1 adenocarcinoma antigen,... 

Au/C was established to be a good candidate for selective oxidation carried out in liquid phase showing a higher resistance to poisoning with respect to classical Pd-or Pt-based catalysts [40]. The reaction pathway for glycerol oxidation (Scheme 1) is complicated as consecutive or parallel reactions could take place. Moreover, in the presence of a base interconversion between different products through keto-enolic equilibria could be possible. 

In summary, the total oxidation of propylene to C02 occurred at a higher rate than partial oxidation to propylene oxide and acetone total and partial oxidations occurred in parallel pathways. The existence of the parallel reaction pathways over Rh/Al203 suggest that the selective poisoning of total oxidation sites could be a promising approach to obtain high selectivity toward PO under high propylene conversion. 

In addition to the universal concern for catalytic selectivity, the following reasons could be advanced to argue why an electrochemical scheme would be preferred over a thermal approach (i) There are experimental parameters (pH, solvent, electrolyte, potential) unique only to the electrode-solution interface which can be manipulated to dictate a certain reaction pathway, (ii) The presence of solvent and supporting electrolyte may sufficiently passivate the electrode surface to minimize catalytic fragmentation of starting materials. (iii) Catalyst poisons due to reagent decomposition may form less readily at ambient temperatures, (iv) The chemical behavior of surface intermediates formed in electrolytic solutions can be closely modelled after analogous well-characterized molecular or cluster complexes (1-8). (v)... 

Up to now we have neglected any problems in the course of the reaction. In practice, however, catalytic systems exhibit all sorts of unwanted effects. Catalytic intermediates (or the active sites) can lose or gain activity as the reaction progresses, because catalysts are often sensitive to changes in acidity/basicity, temperature, pressure, and phase composition. Moreover, as the conversion increases, products and by-products can bind to the catalyst, thereby changing the preferred reaction pathway. Such processes are known as deactivation, sintering, inhibition, or poisoning. 

This book deals with four major areas of selectivity stereoselectivity clusters, alloys, and poisoning shape selectivity and reaction pathway control. An overview of the book and reviews of each of the four major areas are included as introductory chapters. Each review is followed by individual contributions by attendees of the symposium. 

This review encompasses the general area of selectivity in catalysis as well as the four major specific areas discussed in this book Stereoselectivity Clusters, Alloys and Poisoning Shape Selectivity and Reaction Pathway Control. Examples are taken from the literature for each of these four areas of recent articles that focus on selectivity in catalytic reactions. Specific reviews of the four areas listed above can be found in the overview chapters by D. Forster and coworkers, K. J. Klabunde, M. E. Davis and coworkers and H. C. Foley and M. Klein. 

This review is an overview of recent literature research articles that deal with selectivity in catalysis. Four specific areas including stereoselectivity clusters, alloys and poisoning shape selectivity and reaction pathway control will be discussed. This review is not meant to be a complete discussion of these areas. It represents a small fraction of the research presently underway and a very minor fraction of the available literature in this subject. The order of topics will follow the four major areas oudined above, however, there is no particular order for the articles discussed in each section. 

The above literature review gives a comparison of different ways to control selectivity for both homogeneous and heterogeneous catalytic reactions. There are several common features for the four areas of stereoselectivity metal clusters, alloys and poisoning shape selectivity and reaction pathway control. In fact, many times more than one of these areas may be involved in a catalytic system. Some common features for all of these areas include precise control of the structural and compositional properties of the catalysts. This paper serves as an overview for the other manuscripts in this book. Specific review chapters on each of the four areas can be found in reviews that follow by D. Forster et al., K. J. Klabunde et al., M. E. Davis et al., and H. C. Foley and M. Klein et al. 

We hope to have indicated in this brief report the excellent match possible between the capabilities of current imaging SIMS technology and some of the characterization needs of the catalysis community. In cases where heterogeneity, poisoning, and diffusion limitations are present, imaging SIMS offers a way to gain a novel view of catalyst composition, performance, and reaction pathways. 

In electrocatalysis, notable cases of formation of strongly bound species that are not, however, the kinetically involved intermediates in the main reaction pathway arise in the electrochemical oxidations of HCOOH, HCHO, and CH3OH at Pt anodes for those reagents, a self-poisoning intermediate, variably identified as chemisorbed CO, in bridged or linear double bonding to the electrode, or the species- C—OH, is involved (43) this species is not a principal kinetically involved intermediate in, for example, HCOOH oxidation, which proceeds at unpoisoned sites by the mechanism discussed in Section V,B,3. 

Evidently, however, another species arises in a side, self-poisoning, reaction and extensively covers the surface, inhibiting the progress of the above main reaction in the sequence of steps shown (89-91) In situ IR spectroscopy shows that this species is principally chemisorbed CO, bridged or linearly bonded to surface metal atoms. Its behavior is similar to that observed with CO directly chemisorbed at a Pt electrode from the gas phase. However, the mechanism of its catalytic formation from HCOOH is unclear. It is well known that CO can be formed from HCOOH by dehydration, but such conditions do not obtain at a Pt electrode in excess liquid water. Hence a catalytic pathway for adsorbed CO formation has to be considered. The species C=0 or C—OH are not to be regarded as the kinetically involved intermediates in the main reaction sequence (Section IV). Because the poisoning species seems to be formed in the presence of coadsorbed, H steps such as... 

Vogel, W. Lundquist, J. Ross, P. Stonehart, P. Reaction pathways and poisons—II The rate controlling step for electrochemical oxidation of hydrogen on Pt in acid and poisoning of the reaction by CO. Electrochim. Acta 1975, 20 ), 79-93. 

Well-defined Rh/AliOs catalysts selectively poisoned by Ge a new tool to study reaction pathways... 

Acetic acid decomposes on the clean surface at elevated temperature to produce gas phase CO2 and hydrogen and leaves C (henceforth Ca for adsorbed carbon) on the surface [6,7]. However, this carbon has a surprising property, that is, it can modify the reaction pathway on the surface, yet does not affect the activity for adsorption very significantly. The Ca forms a well-ordered c(2x2) structure which is identified by LEED. As shown in fig 2 the carbon acts as a poison in one sense and in one regime of temperature, that is, it deactivates the surface for acetate decomposition in such a way that the acetate TPD peak is shifted from 360-390K to 455K when the c(2x2) layer is preformed before dosing the acetic acid onto the surface. The overall reaction is -... 

The presence of oxygen can open up a number of additional reaction pathways that can control the actual surface chemistry. Madix has demonstrated that adsorbed atomic oxygen can behave as a nucleophillic center and attack surface bound hydrocarbon intermediates or as a Brqnsted base for hydrogen transfer reactions [63]. Chemisorbed atomic oxygen can also act as a poison on different transition metal surfaces. 

Thus, considering the results obtained by TPR and the high interaction existing between Ni and Co in the non-stoichiometric spinel matrix, the formation would be expected, to a certain extent, of a Ni-Co alloy in the Ni-Co-Zn-Al system after reduction in H2 at 500°C. This type of compound would lead to an important reduction in the number of three nickel atom arrangements on the cluster surface, responsible for the production of ethane. The consequence of this, once the hydrogenol)4 ic sites generated by addition of Co are poisoned, would be the suppression of the reaction pathway shown in Scheme 1. Therefore, ethylene... 

The simplest approach to a quantitative analysis of deactivation phenomena is to treat the overall reaction/catalyst/deactivation assembly in terms of reaction pathways very similar to the Type I and Type III nearly complex reactions of Chapter 1. If we let S be an active site on the surface, independent chemical poisoning in the simplest example can be represented by... 

---