Washcoat diesel catalysts

Figure 1 5. Conversion of carbon monoxide, gaseous hydrocarbons and sulfur dioxide reached over a diesel catalyst with and without measures to suppress the formation of sulfates, as a function of the exhaust gas temperature (monolith catalyst with 62 cells cm dedicated diesel washcoat formulations with platinum at a loading of 1.76 g I" diesel engine test bench light-off test at a space velocity of 120000 N1 F h diesel engine bench aging procedure for 100 h at a catalyst inlet temperature of 773 K).

Common cell densities used for heavy-duty diesel applications with vanadia SCR catalysts are 300 cpsi [19, 36] and 400 cpsi [6, 18, 35, 37, 40]. For these cell densities, the wall thicknesses for cordierite substrates range typically from 4 mil (100 pm) to 8 mil (200 pm) [18, 19, 37, 41]. In Fig. 3.12, with wall thickness is here considered the total wall thickness resulting from the substrate including the catalyst washcoat. The washcoat thickness for a coated vanadia SCR catalyst depends on the washcoat loading and could range from 20 to 100 pm [37]. For a washcoated SCR catalyst, the wall thickness could of course be reduced either by decreasing the inert substrate wall thickness or reduce the washcoat thickness as long as this does not effect the catalyst performance. 

Tao T, Xie Y, Dawes S, Melscoet-Chauvel I, Pfeifer M, Spurk P C (2004) Diesel SCR NOx Reduction and Performance on Washcoated SCR Catalysts. SAE Technical Paper 2004-01-1293... 

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. 

The microstructure of diesel filters not only affects physical properties like CTE, strength, and structural modulus, but it has a strong bearing on filter/catalyst interaction, which, in turn, affects the performance and durability of catalyzed filter. The coefficient of thermal expansion, strength, fatigue, and structural modulus of the diesel filter, which also depend on cell orientation and temperature, have a direct impact on its mechanical and thermal durability [21-25]. Finally, since all of the physical properties are affected by washcoat formulation, washcoat loading, and washcoat processing, they must be evaluated before and after the application of washcoat to assess filter durability. 

Figure 103. The amount of soluble organic fraction (SOF) adsorbed on an aged diesel oxidation catalyst, as a function of the washcoat formulation (monolith catalyst with 62cellscm", dedicated diesel washcoat formulations with platinum at a loading of l.76gl diesel engine bench aging for 50 h diesel fuel containing 0.15 wt. % sulfur). Reprinted with permission from ref. (68], C 1991 Society of Automotive Engineers, Inc.

Figure 109. Conversion of carbon monoxide and gaseous hydrocarbons reached over a diesel oxidation catalyst at various settings of the exhaust gas temperature, for a model gas composition with and without SO2 (monolith catalyst with 62cells cm dedicated diesel washcoat formulations with platinum loading of 1.76gl" in the fresh state model gas light-off test at a space velocity of 50000Nir h model gas simulates the exhaust gas composition of an IDI passenger car diesel engine at medium load and speed).

Figure 111. Emission of aldehydes, acrolein and various polynuclear aromatic hydrocarbons of two passenger cars equipped with an IDI/NA and with a DI/NA diesel engine, once without and once with a diesel oxidation catalyst, in the US-FTP 75 vehicle test cycle (monolith catalyst with 62 cells cm dedicated diesel washcoat formulation with a platinum loading of 1.76 g 1 in the fresh state vehicle dynamometer tests according to the US-FTP 75 vehicle test procedure, with passenger cars equipped with a DI/NA and with an IDI/NA diesel engine of displacement 2.0 1). Reprinted with permission from ref [70], 1990 Society of Automotive Engineers, Inc.

The current paper offers an alternative systems approach that broadens the temperature window for managing NOx in a full lean environment. The system has a trap component which adsorbs NOx over a temperature range where current lean NOx catalysts are not active. The trapped NOx is periodically desorbed and presented to a downstream lean NOx catalyst when conditions are optimal for its reduction. The predominant species present in the exhaust is NO. The principle is to oxidize NO to NOx above 150 C to enhance its adsorption. The trapped or stored NOx is desorbed by an exotherm generated within the washcoat by oxidation of a small amount of injected hydrocarbon, i.e. diesel fuel while maintaining the environment lean and not significantly modifying the bulk gas temperature. The injection temperature is controlled to allow for efficient downstream reduction of the NOx over a lean NOx catalyst i.e. 200-250 C for Pt or above 400°C for Cu/ZSM-5. 

Diesel oxidation catalyst technology using base metal oxides in the washcoat for removal and conversion of particulate SOF and gas phase HC s has been demonstrated in steady state and transient engine tests. 

Catalyst A was prepared on a 46 cells/cm ceramic monolith. It is based upon a washcoat optimized for Diesel engine operation, but with the usual Pt content of 1,77 g/dm. Catalyst A had a washcoat and a precious metal kind and loading... 

Allied Signal Ib, fully catalytic Recuperative single-shaft 50 kW Diesel or JET Proprietary washcoat on Coming EX-22 substrate NR + Pre-heated air from heat exchanger, no pilot-flame, low outlet temperature prevents catalyst overheating 130, 131... 

Greaser et al. [76] modeled autothermal diesel reforming in a ceramic monolithic reactor and verified the modeling results with experimental data as shown in Figure 14.10. The model revealed that axial heat conduction plays an important role even in ceramic monoliths. The catalyst temperature was found to be 25°C hotter than the gas phase at the reactor inlet according to the calculations. At the positions of highest reaction rates, the catalyst utilization was as low as 20%. Transport limitations in the washcoat were assumed to be the root cause. The... 

Examples of space velocities used for heavy-duty diesel applications with vanadia SCR catalysts are in the range 20,000-70,000 h [6, 18, 34, 35]. Havenith et al. [6] used 51 dm of washcoated vanadia/alumina SCR catalyst volume, corresponding to a space velocity of 28,000 h for a 12 1 heavy-duty engine, van Helden et al. [35] used 34 dm of washcoated vanadia SCR catalyst volume (space velocity 45,000 h ) for a 12.0 and a 12.6 1 heavy-duty engine. Hofmann et al. [36] used the same SCR catalyst volume (34 dm ), but with a fully extruded vanadia SCR catalyst for a 12 1 heavy-duty diesel engine. 

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