Deactivation diesel catalysts

The main causes of the deactivation of diesel catalysts are poisoning by lubrication oil additives (phosphorus), and by SOx, and the hydrothermal instability. The SCR by HC is less sensitive to SOx than the NO decomposition. The Cu-based catalysts are slightly inhibited by water vapor and SOx, and suffer deactivation at elevated temperature. Noble metal catalysts such as Pt-MFI undergo low deactivation under practical conditions, are active at temperatures below 573 K but the major and undesired reduction product is N20 (56). 

A more important deactivation aspect is poisoning. A diesel oxidation catalyst is poisoned by the same elements that poison three-way catalysts, except for lead, which is absent from diesel fuel and the diesel fuel supply chain. Table 25 compares the amount of sulfur, phosphorus and zinc offered to a diesel catalyst and to a three-way catalyst during their lifetime. From this table, it is apparent that a diesel oxidation catalyst has to deal with a considerably higher amount of sulfur during its lifetime than is the case with a three-way catalyst. This is because the diesel fuel specification allows for a higher sulfur content than the gasoline specification. Table 26 gives an overview of the maximum sulfur content in the diesel fuel specification of some selected countries. 

Apart from the hydrolysis step, the SCR-urea process is equivalent to that of stationary sources, and in fact the key idea behind the development of SCR-urea for diesel powered cars was the necessity to have a catalyst (1) active in the presence of 02, (2) active at very high space velocities ( 500.000 per hour based on the washcoat of a monolith) and low reaction temperatures (the temperature of the emissions in the typical diesel cycles used in testing are in the range of 120-200°C for over half of the time of the testing cycle), and (3) resistant to sulphur and phosphorus deactivation. V-Ti02-based catalysts for SCR-NH3 have these characteristics and for this reason their applications have also been developed for mobile sources. 

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 introduction of iron-zinc catalysts led to the low pressure nthesis of liquid and solid hydrocarbons from CO/Hj in 1925 [19. 20. However, it was found that these catalysts were deactivated rapidly and thus further investigations concentrated on nickel and cobalt catalysts. They led to the introduction of a standardized cobalt-based catalyst for llic normal-pressure synthesis of mainly saturated hydrocarbons at temperatures below 200 C. In 1936, the first four commercial plants went on stream. Until 1945 the Fischer-Tropscit synthesis was carried out in nine plants in Germany, one plant in France, four plants in Japan and one plant in Manchuria. The total capacity amounted to approximately one million tons of hydrocarbons per year in 1943. The catalysts used consisted of Co (1(X) parts), ThO (5 parts). MgO (8 parts), and kieselgur (200 parts) and were prepared by precipitation of the nitrates. These catalysts were used in fixed-bed reactors at normal or medium pressures (< 10 bar) and produced mainly saturated straightproduct obtained consisted of 46% gasoline. 23% diesel oil, 3% lubricating oil and 28% waxes (3.15). 

A Catalyst Deactivation Model for Residual Oil Hydrodesulfiirization and Application to Deep Hydrodesulfurization of Diesel Fuel... 

The concept of this model for catalyst deactivation is also applicable to hydrotreating of other petroleum fractions. An example of such applications for the case of deep hydrodesulfurization of diesel fuel is also presented. 

Catalyst Deactivation by Coke Deposition. The catalyst employed in deep desulfurization of diesel fuel is deactivated by coke deposition onto the catalyst. Coke deposition affects not only the surface activity but also the diffusivity of the reactants, because the pore diameter is relatively small in this case. The catalyst deactivation data suggest that the effectiveness factor is smaller than 1.0. 

The only indication of catalyst deactivation is a gradual deterioration in diesel quality. 

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