The Precursor Concept

The three-step precursor concept for manufacture of heterogeneous egg-shdl nanocatalysts was developed in the 1990s [8-11]. The catalyst precursor is manufactured by dipping the supports into an organic or aqueous media containing the dispersed precursor at ambient temperature to adsorb the pre-prepared particles. A standard procedure for the manufacture of an egg-shell Pt 3-nm nanocatalysf via adsorption of a pre-prepared Pt hydrosol on e.g. Degussa active carbon 196 [127, 128] is  

A combination of AFM, STM and XPS has revealed [14,96] the interaction of platinum hydrosols with oxide (sapphire, quartz) and graphite single crystal substrates. The metal core is immediately adsorbed e.g. on the HOPG surface when dipped into aqueous Pt colloid solutions at 20 °C. As shown in Fig. 2.17, the protecting shell (composed of either residual surfactants or al-organic residues) forms a carpetlike coat which cannot 

In order to remove excess protecting shell the supported colloidal pre-catalysts - as obtained in step 2 - are activated via heat treatment at 300 °C in an intermittent stream of oxidizing and reducing gases. We have coined the term conditioning (i.e. reactive annealing) for this process [126]. 

In the given example, 17 g of the desired egg-shell Pt nanocatalysr was isolated after the conditioning procedure. Elemental analysis showed a Pt-loading of 5 wt% on carbon while TEM confirmed the average particle size to be unchanged compared to the precursor (3 nm) and the zerovalent state of the Pt surface was confirmed by XPS. 

The example shows that the three-step preparation procedure described above produces true nanocatalysts having naked metal particles of defined size deposited on the support surface. Generally, carbon-supported colloidal pre-catalysts are conditioned at 300 C. However, individual heating and gas flow conditions may be optimized for every catalyst system on the basis of TGA-MS analysis data. For example, the optimum temperatures for conditioning supported nanometaUic pre-catalysts having tetraoclylammonium or aluminum-organic protective shells are 280 °C and 250 G respectively [96, 126]. 

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These problems of how to deal with the implications of applying the precursor concept stated in the previous Chapter in practice, will be further addressed in Section 5.2. The following sub-Section uses the identified precursors to model the control processes in the operational process. Subsequently, the ineffective elements of the control processes will be identified. 

Thus the precursor concept is extended in such a way that all deviation data can be processed to retrieve precursors unambiguously. 

Using this method, homogeneous alloys, segregated alloys, layered bi-metallics, and decorated particles are all readily accessible. An obvious advantage of the precursor concept over the conventional salt-impregnation method is that both the size and the composition of the colloidal metal precursors may be tailored independent of the support. Further, the metal particle surface may be modified by lipophilic or hydrophilic protective shells and coated by intermediate layers, e.g., of oxide. The modification of the precursor by dopants is also possible. 

Figure 10.3 Plasma polymerization by the precursor concept chemical structure of precursors determines structure of plasma polymers.

In an attempt to consider some extent of fragmentation of the monomer as well as to explain polymerization of simple organic molecules that are not considered monomers, plasma polymerization mechanisms are often explained by assuming plasma-induced precursors, which have polymerizable structures. The precursor concept is detailed in Figure 10.3. It is significantly different from the simple process described in Figure 10.2 however, it still depends on a simple deposition process from precursors to plasma polymer. This concept intuitively assumes that the structure of a plasma polymer can be predicted from the structures of precursors. 

The reactive species (preeursors in the precursor concept) are ereated not only by fragmentation of the monomer but also by fragmentation of the plasma polymer formed and of materials existing on the various surfaces that come into contact with the plasma. 

Figure 12 Scheme of the preparation of colloidal Pt/Ru/Al PEMFC anode catalysts (>20% metal on Vulcan XC72 ) via the precursor concept. (From Ref. 66.)... 

Inspection of Fig. 2.18 shows the dramatic increase in catalyst activity effected by conditioning monitored by H2-sorption. Using the precursor concept homo-... 

Metal chalcogenides, apart from their technological significance in industrial applications, have played an important role in the development of new synthetic concepts and methods in the area of solid-state chemistry. A great example is alkali metal intercalation into TiS2 (Chap. 6) first reported three decades ago, which highlighted the then-novel synthetic approach called soft chemistry chimie douce). This low-temperature process allows for new compounds to be obtained while retaining the structural framework of the precursor. Related to this concept is the... 

The catalytic applications of Moiseev s giant cationic palladium clusters have extensively been reviewed by Finke et al. [167], In a recent review chapter we have outlined the potential of surfactant-stabilized nanocolloids in the different fields of catalysis [53]. Our three-step precursor concept for the manufacture of heterogeneous egg-shell - nanocatalysts catalysts based on surfactant-stabilized organosols or hydrosols was developed in the 1990s [173-177] and has been fully elaborated in recent time as a standard procedure for the manufacture of egg-shell - nanometal catalysts, namely for the preparation of high-performance fuel cell catalysts. For details consult the following Refs. [53,181,387]. 

One potential solution to these problems, suggested some 20 years ago by Chantrell and Popper (1), involves the use of inorganic or organo-metallic polymers as precursors to the desired ceramic material. The concept (2) centers on the use of a tractable (soluble, meltable or malleable) inorganic precursor polymer that can be shaped at low temperature (as one shapes organic polymers) into a coating, a fiber or as a matrix (binder) for a ceramic powder. Once the final shape is obtained, the precursor polymer can be pyrolytically transformed into the desired ceramic material. With careful control of the pyrolysis conditions, the final piece will have the appropriate physical and/or electronic properties. 

These results clearly demonstrate the high potential of the single source precursor concept. In particular, the low deposition temperatures necessary for the stibinoalanes and the low carbon contents of the resulting material films are very promising results. However, the low vapor pressure of the heterocycles, which require sublimation temperatures about 130 °C, are a major drawback. 

With the developed concepts, a number of field experiments were conducted in the chemical process industry. A first experiment was carried out in a small company in The Netherlands. From this first trial, it was evident that the concepts of precursors, the model of the organisational control process and the structure of these concepts had to be adapted to obtain better and more reliable results. The improvements led to the development of a structured protocol of seven clearly defined stages. By applying this 7-stage protocol to the data of the small company, safety risks could pro-actively be identified and the accidents which the company had already experienced, could be explained. 

To verify the developed concepts underlying the structured 7-stage protocol in a reactive way, they were applied to an analysis of recent accidents in the Dutch chemical process industry. Despite the limitations in the information available from the accident database, it could be deduced that all accidents were preceded by precursors, and even that similar precursors had led to similar accidents, implying that companies had failed to learn from these re-occurring deviations which were in fact pre-warning signs of impeding accidents. 

A complete control model has now been derived, subsequently the link of the control model with the previous concepts of precursor and finally safety, has to be explained. The following Section will therefore discuss an approach of how to use all concepts in practice. 

In the following sub-Sections the practical approach derived in the previous Chapter will be applied on this case study. Therefore, the precursors present inside the operational process are identified. Then, the control of the operational process will be modelled and analysed to find the ineffective control elements. Furthermore, the latent conditions causing the ineffective control will be retrieved and their impact on the existing safety barriers will be identified. Finally, improvement opportunities for the company will be indicated, together with some recommendations and enhancements for the applied concepts and the proposed practical approach. 

Heterogeneous catalysts are readily obtained when pre-prepared nanometal colloids are deposited on supports [20], The so-called precursor concept for manufacture of heterogeneous... 

Figure 3.6 Schematic representation of the supported catalyst preparation from its precursor using precursor concept and conditioning. (Adapted from Bonnemann H. et at, Fuel Cells, 4, 1,2004. With permission from Wiley VCH.)...

According to Flory (10), the concept that proteins and carbohydrates are polymeric goes back to at least the work of Hlasiwetz and Habermann (11). In 1871 they proposed that these substances were made up of a number of species differing from one another with respect to the degree of molecular condensation. Flory also noted that Hlasiwetz and Habermann differentiate "soluble and unorganized" members of these substances, for example dextrin and albumin, from "insoluble organized" members, such as cellulose or keratin. This distinction is the precursor of the present day differentiation between non-crystalline and crystalline polymers. 

The prochirality concept is not necessarily an expression of a precursor-product relationship because there exist stereoselective reactions at pro-chiral elements that do not generate elements of chirality. An illustration of this is the reversible enzymatic dehydration of citric to cu-aconitic acid. In this process two prochiral centers of citric acid disappear and we obtain an achiral line of stereoisomerism that physically coincides with a prochiral plane of prostereoisomerism. 

Elements of this synthesis were used as the foundation for a clever synthesis of both enantiomers from a single chiral precursor 49, with a Diels-Alder reaction comprising the key step [64]. The critical concept was the recognition that the Diels-Alder reaction of 49 could proceed through two different transition states (Scheme 7), with the product from one transition state (50a) being the tricyclic alkene 44 which had been readily converted to the desired product 48 in the synthesis of the racemate (see above). The second transition state (50b) would provide intermediate 51 that, with slightly different synthetic manipulations, could be converted to the other enantiomer of 48. The full synthesis has been described in Vol. 1, Chap. 1, and so will not be reiterated here. 

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