Base metal, automotive catalyst

Three-way automotive catalysts based on palladium, rather than the more expensive metals platinum and rhodium, have long been desired. However, Pd is more sensitive than R to poisoning by lead (Pb) compounds [1-4], Consequently, widespread commercial use of Pd-based automotive three-way catalysts (TWC) was delayed in the U.S. until the early 1990s, by which time residual Pb concentrations in unleaded gasoline had decreased to negligible levels. The past five years have witnessed... 

The kinetics of a mixed platinum and base metal oxide catalyst should have complementary features, and would avoid some of the reactor instability problems here. The only stirred tank reactor for a solid-gas reaction is the whirling basket reactor of Carberry, and is not adaptable for automotive use (84) A very shallow pellet bed and a recycle reactor may approach the stirred tank reactor sufficiently to offer some interest. 

Fig, 6. Change of the axial lead profile with temperature on a base metal oxidation catalyst. [From Rummer et al. (21).] (Reprinted with permission of the Society of Automotive Engineers.)... 

In contrast to lead, the possible poisoning by metallic elements, derived from the vehicle system, is not easily documented. Many formulations of automotive catalysts contain both base and noble metals, but the detailed effect of such combinations on the particular reactions is rarely known with precision. One study was concerned with the effect of Cu on noble metal oxidation catalysts, since these, placed downstream from Monel NOx catalysts, could accumulate up to 0.15% Cu (100). The introduction of this amount of Cu into a practical catalyst containing 0.35% Pt and Pd in an equiatomic ratio has, after calcination in air, depressed the CO oxidation activity, but enhanced the ethylene oxidation. Formation of a mixed Pt-Cu-oxide phase is thought to underlie this behavior. This particular instance shows an example, when an element introduced into a given catalyst serves as a poison for one reaction, and as a promoter for... 

The early days of automotive catalytic converter research and development, targeted the use of base metal catalysts. Numerous publications describe the results obtained with catalysts that contain, for example, the oxides of Cu, Cr, Fe, Co and Ni [7, 8]. 

Figure 40 shows the conversion of CO, HC and NO e at typical automotive catalyst operation conditions for a precious metal based catalyst on a ceramic monolith with an extremely low precious metal loading, and for a precious metal free catalyst in which the same ceramic monolith support was used but with a washcoat consisting of a typical base metal catalyst formulation. The extremely poor con-... 

Figure 40. Conversion of CO, HC and NO.v on a monolithic base metal catalyst and on a low loaded monolithic precious metal catalyst, both in the fresh state (space velocity 60000 Nll h gas temperature 723 K test procedure A/F scan-values shown at lambda-0.999 frequency I Hz amplitude 1 A/F unit). Reprinted with permission from ref [34], (f) 1991 Society of Automotive...

As mentioned before, the heat-up of the automotive catalyst only by the exhaust gas needs approx. 1 min. In order to shorten the start-up period an electrically heated pre-catalyst can be used which is located in front of the main catalyst (Figure 1). The presently used EHC is a two-brick design. It consists of a short metallic monolith which is heated by the car battery and a second, larger but unheated monolith. This second monolith enlarges the catalytic surface area and ensures the mechanical stability of the whole EHC construction. The design of this EHC was obtained by optimization based upon extensive experimental and simulation studies... 

Noble metal catalysts are highly active for the oxidation of carbon monoxide and therefore widely used in the control of automobile emissions. Numerous recent studies on noble metal-based three-way catalysts have revealed characteristics of good thermal stability and poison resistance(l). Incorporation of rare earth oxides as an additive in automotive catalysts has improved the dispersion and stability of precious metals present in the catalyst as active components(2). Monolith-supported noble-metal catalysts have also been developed(3). However, the disadvantages of noble metal catalysts such as relative scarcity, high cost and requirement of strict air/fuel ratio in three-way function have prompted attention to be focused on the development of non-noble metal alternatives. 

Certain factors are analyzed to determine their effects on automotive catalyst activity. At operating gas velocities, spherical catalysts were more active than monolithic catalysts at comparable catalyst volumes and metals loadings. Palladium was the most active catalyst metal. Platinum in a mixed platinum palladium catalyst stabilizes against the effects of lead poisoning. An optimum activity particulate catalyst would contain about 0.05 wt % total metals on a gamma-alumina base with a platinum content of 0.03-0.04 wt % and a palladium content of 0.01-0.02 wt %. A somewhat thick shell of metals located near the outer surface of the particle provides better catalyst activity than a shell type distribution of metals. 

The results obtained for the fresh and aged commercial Pt/Rh and Pd/Rh TWCs are shown in Table 2. The first column contains the dispersions and calculated spherical particle sizes (in parentheses) derived from the CO methanation technique based on an assumed adsorption stoichiometry of 1 CO per exposed noble metal atom. The arbitrary choice of a stoichiometric factor of 1, rather than the value of 0.7 suggested by the EmoPt-l catalyst, was made on the basis of several factors. The main reason is that the presence of Rh in these catalysts (16% and 10% of the noble metal weight in the Pt/Rh and Pd/Rh catalysts, respectively) is likely to increase the average stoichiometric factor above 0.7 due to the presence of gem-dicarbonyl species on Rh. Bimetallic Pt/Rh particles have been found in automotive catalysts, sometimes with surface enrichment by Rh [20,21] or even bulk enrichment of selected particles as... 

A chemisorption teclmique developed by Koinai et al., based on CO methanation, was successfrilly used to analyze noble metal dispersions of both fresh and vehicle-aged Pt/Rli and Pd/Rli commercial automotive three-way catalysts. The teclmique is relatively rapid (< 2 hours), extremely sensitive, and largely free from complications due to adsorption of CO on non-noble metal components of the washcoat (support, promoters, stabilizers, etc.). Particle sizes of the vehicle-aged catalysts, calculated by applying the spherical particle assumption to the dispersions measured by the CO methanation method, agreed well with particle sizes calculated from x-ray diffraction line-broadening data. These results indicate that the CO methanation teclmique can be applied routinely to obtain fast and accurate measurements of noble metal surface areas in automotive catalysts retrieved from tlie field, even tliose with metal dispersions ca. 2% or less. 

The recent interest in compact fuel cell units for automotive applications has led to development of WGS catalysts based on noble metals [134] [277], which in contrast to the iron- and copper-based catalysts can withstand exposure to air during start-up and shutdown. 

The unit was then improved for onboard methanol processing dedicated for the automotive drive train [573] but also for residential combined heat and power systems [575]. The catalyst bed was uniform and filled with a combined precious metal/base metal multi-component catalyst. The reaction produced 2.4 mol hydrogen per mole methanol [573], which corresponds to a S/C ratio of about 0.4 and an O/C ratio of about 0.6 in the feed. These conditions resulted in 58 vol.% dry hydrogen content of the reformate. The maximum temperature in the fixed bed was reduced to 400 °C... 

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