Autocatalysts

A gold catalyst with low temperature activity towards carbon monoxide and hydrocarbon oxidation could be suitable to combat cold start-up emission problems and removal of nitrogen oxides from lean-burn gasoline 

A gold-based material has been formulated for use as a three-way catalyst in gasoline and diesel applications.28 This catalyst, developed at Anglo American Research Laboratories in South Africa, consisted of 1% Au supported on zirconia-stabilized-Ce02, ZrC 2 and TiC 2, and contained 1% CoOx, 0.1% Rh, 2% ZnO, and 2% BaO as promoters. The catalytically active gold-cobalt oxide clusters were 40-140 nm in size. This catalyst was tested under conditions that simulated the exhaust gases of gasoline and diesel automobiles and survived 773 K for 157 h, with some deactivation (see Section 11.2.7). 

The formation of ionic gold trapped in an oxide lattice is thought to be responsible for the stability of some Toyota catalysts there was no reduction in T5o% conversion for propene after treatment at 1073 K for 5h (Table 14.1). A standard AU/AI2O3 catalyst under the same conditions suffered significant degradation.32 

Under diesel conditions, carbon monoxide and hydrocarbon oxidation is favoured. Under the highly oxidising conditions encountered in the diesel gas stream, reduction of nitric oxide is not expected. A nitric oxide conversion window is observed at temperatures between 493 and 623 with a T50 value of 523 K. However, large NO absorption bands are observed at temperatures above and below the conversion window.28 

Chemical Au content Initial After durability testb 

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The autocatalator model is in many ways closely related to the FONT system, which has a single first-order exothennic reaction step obeying an Arrhenius temperature dependence and for which the role of the autocatalyst is taken by the temperature of the system. An extension of this is tlie Sal nikov model which supports tliennokinetic oscillations in combustion-like systems [48]. This has the fonn ... 

Total reflects the contribution of recovered platinum from the autocatalyst industry. ... 

Numbers in parentheses represent the quantity of palladium recovered from this industry. Total reflects the contribution of recovered palladium from the autocatalyst industry. ... 

The main areas of commercial apphcation are automotive emission control catalysts (autocatalysts), oil refining, ammonia oxidation, hquid-phase ... 

Automotive Emission Control Catalysts. Air pollution (qv) problems caused by automotive exhaust emissions have been met in part by automotive emission control catalysts (autocatalysts) containing PGMs. In the United States, all new cars have been requited to have autocatalyst systems since 1975. In 1995, systems were available for control of emissions from both petrol and diesel vehicles (see Exhaust control, automotive). 

Meta.1 Oxides. Halogen-containing elastomers such as polychloropreae and chlorosulfonated polyethylene are cross-linked by their reaction with metal oxides, typically ziac oxide. The metal oxide reacts with halogen groups ia the polymer to produce an active iatermediate which then reacts further to produce carbon—carbon cross-links. Ziac chloride is Hberated as a by-product and it serves as an autocatalyst for this reaction. Magnesium oxide is typically used with ZnCl to control the cure rate and minimize premature cross-linking (scorch). 

P. Oser, Novel AutoCatalyst Concepts and Strategies for the Euture with Emphasis on Metal Supports, SAE 880319, Society of Automotive Engineers, Warrendale, Pa., 1988. 

Autocatalysts, based on monoliths, are probably the most extensively used catalytic reactors around a hundred million have been installed and are performing well in car exhaust systems [10-12]. Reduction of volatile organic carbon (VOC) emissions [13] and removal of NOj, from stationary sources [14, 15] are also... 

While conventional monoliths contain parallel channels, in practice, systems are often made from alternate layers that allow lighter structures with better mass transfer characteristics in gas-phase applications, see Figure 9.6 showing interconnected flow paths. They are usually made from metal, mostly Fecralloy , Kanthal , or stainless steel, and widely used in autocatalysts and in environmental... 

OS 92] [R 32 [P 72] Iodide and iodine are autocatalysts. When an electrical field is applied, iodide moves to the positive electrode and, by this means, the propagation of the reaction is accelerated [145]. As a net result, more iodide is generated for fronts which approach the positive electrode in turn, iodine is formed favorably for fronts propagating to the negative electrode. On svwtching the field off, the system comes back to the prior state. 

Starting from an initial state where half the system has species A and the other half B, a reaction front will develop as the autocatalyst B consumes the fuel A in the reaction. The front will move with velocity c. The reaction-diffusion equation can be solved in a moving frame, z = x — ct, to determine the front profile and front speed,... 

A recent discovery that RNA will act as a self-catalyst, called a ribozyme, leads to a simple three-step model for self-replication - this might include a surface. In the model (Figure 8.18), the template molecule T is self-complementary and is able to act as an autocatalyst. In the first step, it reversibly binds with its constituents A and B, forming the termolecular complex M. The termolecular complex undergoes irreversible polymerisation and becomes the duplex molecule D. Reversible dissociation of D gives two template molecules T, which can initiate new replication. The model preserves the order of the moieties on the template (the direction of the arrow) and the backbone, which may be on the surface... 

The oxidation of formaldehyde by chlorite, C102, has been studied in aqueous solution.In the presence of excess chlorite, formaldehyde was oxidized to CO2, with CIO2 also being formed. This compound was also obtained as an oxidation product when HCHO was in excess, in which case the latter was oxidized only as far as formic acid. The first step of the reaction produces HOCl, which acts as an autocatalyst, catalysing the formation of CIO2 and the further oxidation of HCO2H to CO2. The build-up of CIO2 is due to the fact that HOCl reacts much more rapidly... 

Steam is invariably present in a real exhaust gas of motor vehieles in relatively high concentration due to the fuel combustion. The influence of water vapor on catalytic performances should not be ignored when dealing with the aim to develop a practical TWCs. Cu/ZSM-5 catalysts once were regarded as suitable substitutes to precious metal catalysts for NO elimination[78], nevertheless, they are susceptible to hydrothermal dealumination leading to a permanent loss of activity[79], Perovskites have a higher hydrothermal stability than zeolites[35]. Although perovskites were expected to be potential autocatalysts in the presence of water[80], few reports related to the influence of water on the reactants adsorption, the perovskite physicochemical properties, and the catalytic performance in NO-SCR were previously documented. The H2O deactivation mechanism is also far from well established. 

When (5)-2-a]kynyl-5-pyrimidyl alkanol 2c with >99.5% ee was employed as an asymmetric autocatalyst, (5)-2c with >99.5% ee composed of both the newly formed 2c and the initially used 2c was obtained. The yield of the newly formed 2c was >99%. To make use of the advantage of asymmetric autocatalysis—that the structures of the asymmetric autocatalyst and the product are the same—the 2c obtained in the first round was used as an asymmetric autocatalyst for the following round. Again, the product (5)-2c and the initial autocatalyst had an ee of >99.5% and the yield of the newly formed (S)-2c was >99%. The product 2c was therefore used as an asymmetric autocatalyst for the following round. Even after the 10th round, the yield of 2c was >99% and the ee was >99.5%. Thus, 2-aIkynyl-5-pyrimidyl alkanol 2c served as a virtually perfect asymmetric autocatalyst. Moreover, the amount of (S)-2c automultiplied by a factor of 60 million during the 10 rounds ... 

Obtained alcohol was used as an asymmetric autocatalyst for the next round. 

Asymmetric autocatalysis using (5)-pyrimidyl alkanol 2a with only 2% ee afforded (5)-2a with an increased ee of 10%, [Eq. (9.4)]. The (5)-2a obtained with 10% ee was then used as an asymmetric autocatalyst for the following asymmetric autocatalysis. (5)-Pyrimidyl alkanol 2a with an increased ee of 57% was obtained. The subsequent consecutive asymmetric autocatalysis and the use of that product as an asymmetric autocatalyst for the following round gave (5)-pyrimidyl alkanol 2a with 81 % and 88% ee, respectively. Thus, the overall process was the asymmetric autocatalysis of (5)-2a starting from a low ee of 2% with significant amplification of chirality to 88% ee, with the increase in the amount without need for other chiral auxiliary. ° This stands as the first example of an asymmetric autocatalysis with amplification of ee. In addition, one-pot asymmetric autocatalysis of pyrimidyl alkanol 2b also significantly increased the chirality from 0.28 to 87% ee. ... 

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