Ammonia carbon supported

The concept of mechanical fixation of metal on carbon makes catalytic applications at high temperatures possible. These applications require medium-sized active particles because particles below 2nm in size are not sufficiently stabilised by mechanical fixation and do not survive the high temperature treatment required by the selective etching. Typical reactions which have been studied in detail are ammonia synthesis [195, 201-203] and CO hydrogenation [204-207]. The idea that the inert carbon support could remove all problems associated with the reactivity of products with acid sites on oxides was tested, with the hope that a thermally wellconducting catalyst lacking strong-metal support interactions, as on oxide supports, would result. 

Obviously, the number of [Me(NH3) ] complexes adsorbed depends both on the concentration of the acidic sites and on the nature of the X anion. Hence, solutions of ammonia complexes of metal hydroxides and oxidized carbon supports (Cox) are used to prepare catalysts via adsorption [12,24,64,65,114,117-120]. Equilibrium (7) is significantly shifted to the left in the case of the adsorption of metal salts, [Me(NH3) ]Xz (X = Cr,NOj), due to releasing strong acids HX. Eor this reason, the most frequently used method for supporting these salts is incipient wetness impregnation [41,42,121-127]. The completeness of the adsorption processes usually is not stated in this case, which makes it difficult to classify the prepared Me/C specimens as impregnated or adsorbed catalysts. 

The possible complete replacement of Pt or Pt alloy catalysts employed in PEFC cathodes by alternatives, which do not require any precious metal, is an appropriate final topic for this section. Some nonprecious metal ORR electrocatalysts, for example, carbon-supported macrocyclics of the type FeTMPP or CoTMPP [92], or even carbon-supported iron complexes derived from iron acetate and ammonia [93], have been examined as alternative cathode catalysts for PEFCs. However, their specific ORR activity in the best cases is significantly lower than that of Pt catalysts in the acidic PFSA medium [93], Their longterm stability also seems to be significantly inferior to that of Pt electrocatalysts in the PFSA electrolyte environment [92], As explained in Sect. 8.3.5.1, the key barrier to compensation of low specific catalytic activity of inexpensive catalysts by a much higher catalyst loading, is the limited mass and/or charge transport rate through composite catalyst layers thicker than 10 pm. 

Platinum catalysts were prepared by an ion-exchange method [16,17]. Oxidised sites on the surface of an activated carbon support (CECA SOS) were created by pre-treatment with sodium hypochlorite (3%) the associated protons were subsequently exchanged with Pt(NH3)4 " ions, in an aqueous ammonia solution, and reduction was carried out on the dry catalyst under a flow of hydrogen at 300°C. A surface redox reaction was subsequently employed to deposit the bismuth whereby the catalyst was suspended in a glucose solution, under an inert nitrogen atmosphere, and the required volume of a solution of BiONOs, dissolved in hydrochloric acid (IM), was added [18]. 

DINITROGEN MONOXIDE (10024-97-2) May form explosive mixture with flammable and reactive gases, including anhydrous ammonia, carbon monoxide, chlorine trifluoride, hydrogen, hydrogen sulfide, nitryl fluoride, phosphine. Nonflammable but supports combustion as temperature increases above 572°F/300°C, it becomes both a strong oxidizer and self-reactive. Pyrophoric at elevated temperatures. Reacts, possibly violently, with aluminum, ammonia, boron, hydrazine, lithium hydride, sodium, tungsten carbide. 

The early development of catalysts for ammonia synthesis was based on iron catalysts prepared by fusion of magnetite with small amounts of promoters. However, Ozaki et al. [52] showed several years ago that carbon-supported alkali metal-promoted ruthenium catalysts exhibited a 10-fold increase in catalytic activity over conventional iron catalysts under the same conditions. In this way, great effort has been devoted during recent years to the development of a commercially suitable ruthenium-based catalyst, for which carbon support seems to be most promising. The characteristics of the carbon surface, the type of carbon material, and the presence of promoters are the variables that have been studied most extensively. 

It has been claimed that carbon-supported ruthenium-based catalysts for ammonia synthesis show some important drawbacks, such as high catalyst cost and methanation of the carbon snpport under industrial reaction conditions. This has stimnlated the research for alternative catalysts, although the use of carbon snpports is a common feature. One example of these new catalysts is provided by the work of Hagen et al. [61], who reported very high levels of activity with barinm-promoted cobalt catalysts snpported on Vulcan XC-72. It was demonstrated that althongh cobalt had received little attention as a catalyst for ammonia synthesis, promotion with barium and the nse of a carbon support resulted in very active catalysts with very low NH3 inhibition. 

A pulsed microwave-induced small-scale production of HCN has been reported in the reaction of methane with ammonia (see Scheme 13) [26]. The process uses a series of Pt/Al203, RU/AI2O3 and carbon-supported catalysts with conversions exceeding 90%. 

Today, we know that it is possible to produce these Fe- and/or Co-based electrocatalysts by adsorbing related metal-N4 macrocycles on a carbon support and heat-treating this material at about 600°C, the optimum temperature in terms of activity. More stable, but less active catalysts are, however, obtained for heat-treatment temperatures > 800 C. Similar catalysts may also be obtained with cheaper metal and nitrogen precursors (like metal salts and ammonia, for instance). For aU these catalysts, it is generally now believed that two types of catalytic sites are obtained simultaneously, but not in the same proportions. 

Fig. 9.2 Schematic illustration of the micropore filling technique and active site formation during NPMC synthesis, (a) Two adjacent graphitic crystallites hosting a slit micropore in the BP carbon support, (b) cross section view of the empty micropore, (c) micropore after being filled with 1,10-phenanthroline and iron acetate precursors, and (d) active site formation and nitrogen-doped graphitic carbon deposition after subsequent heat treatments in argon and ammonia (from [28] with permission from AAAS)...

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