Base metal catalyst, oxidation

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Catalysts. - Group VIII metals, conventional base metal catalysts (Ni, Co, and Fe) as well as noble metal catalysts (Pt, Ru, Rh, Pd) are active for the SR reaction. These are usually dispersed on various oxide supports. y-Alumina is widely used but a-alumina, magnesium aluminate, calcium aluminate, ceria, magnesia, pervoskites, and zirconia are also used as support materials. The following sections discuss the base metal and noble metal catalysts in detail, focusing on liquid hydrocarbon SR for fuel cell applications. 

Nickel and Other Base Metal Catalysts. Supported Ni is widely utilized as a catalyst for the industrial SR of hydrocarbons. The type of feedstocks and reaction conditions used for SR determine the choice of support, promoter, and loading of Ni. Typically, 15-25% nickel oxide loading is used in commercial SR catalysts. These supports must have high crush strength and stability so they can sustain severe reaction conditions. 

Recently, Takenaka et studied a series of base metal catalysts supported on various ceramic oxides for catalytic cracking of kerosene fuel. Yields of H2 and methane from a model kerosene fuel (52 wt% n-Ci2, 27 wt% diethylbenzene and 21 wt% t-butylcyclohexane) over various base metals at 600°C are shown in Figure 33. Ni/Ti02 showed the highest catalytic activity for the cracking reaction of kerosene fuel, and also maintained a better performance for the kerosene feed that contained benzothiophene. However, the catalytic performance of the... 

Hydrocarbon oxidation on base metal catalysts is also susceptible to lead poisoning, especially if the catalysts are exposed to relatively high temperatures, for at least part of their service time. It was noted above that lead retention, especially on base metal catalysts, also increases with temperature up to a certain point. This behavior is shown by the results of Yao and Kummer (81) in Fig. 18. One should note that the hydrocarbon used for testing catalyst activity, namely propylene, was quite reactive. With a less reactive test hydrocarbon one could expect a still sharper effect. The comparison with a reference production noble metal catalyst, given in Fig. 18, is quite instructive. 

There is no evidence in vehicle operation that the oxidation activity of noble metal catalysts suffers from poisoning by SOz (24, 28, 84), although Hunter claims (43) that Pt can be poisoned below 900°F. In contrast, severe deactivation of base metal catalysts has been observed in many instances. 

Sulfur oxides (S02 and S03) present in flue gases from upstream combustion operations adsorb onto the catalyst surface and in many cases form inactive metal sulfates. It is the presence of sulfur compounds in petroleum-based fuels that prevent the super-sensitive base metal catalysts (i.e., Cu, Ni, Co, etc.) from being used as the primary catalytic components for many environmental applications. Precious metals are inhibited by sulfur and lose some activity but usually reach a lower but steady state activity. Furthermore the precious metals are reversibly poisoned by sulfur compounds and can be regenerated simply by removing the poison from the gas stream. Heavy metals such as Pb, Hg, As, etc. alloy with precious metals and permanently deactivate them. Basic compounds such as NH3 can deactivate an acidic catalyst such as a zeolite by adsorbing and neutralizing the acid sites. 

Incorporation of the catalyst into the monolith during manufacture. This method always leads to poor utilization of the catalytic material some is buried within the crystalline matrix and is not accessible to reactants. As the catalytic material modifies the monolith s physical properties, either a limit has to be placed on the amount of catalytic material incorporated or some deterioration in monolith performance has to be accepted. This method of manufacture is reserved for the preparation of base-metal catalysts, particularly nickel and manganese catalysts. Care has to be taken to avoid the formation of spinels, etc., although in some cases the formation of a complex oxide structure is actively sought. 

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]. 

Neither of the base-metal oxide catalysts tested were active at low temperatures (see Figures 5 and 6), but CuO supported on alumina and silica exhibit rather low formation of acetaldehyde. This corresponds to the results presented by Rajesh and Ozkan (1993) who have tested the activity of catalysts containing either oxides of copper or chromium and also a combination of these two metal oxides supported on y-alumina pellets. The formation of acetaldehyde is slightly higher over CuO-Mn02. Catalysts supported on titania show the lowest light-off temperature for all the base-metal catalysts tested, but also the highest formation of acetaldehyde. 

The study which we have conducted shows that the support material has an effect on the soot combustion characteristics of a supported catalyst, with the best support being a novel support. Comparing base metal to noble metal catalysts w,e have found that certain base metal catalysts (i.e, CuO or C Og) are poisoned by sulfur, do not promote complete combustion of the soot, and initiate soot combustion at a higher temperature than noble metal catalysts. These results are consistent with a soot combustion mechanism in which the metal (especially the noble metals) initiates oxidation of easily combusted adsorbed hydrocarbons and thereby provides local exotherms which initiate the oxidation of the soot. 

The activities of fresh, supported platinum and base metal oxidation catalysts are evaluated in vehicle tests. Two catalysts of each type were tested by the 1975 FTP in four 600-4300 cm3 catalytic converters installed on a vehicle equipped with exhaust manifold air injection. As converter size decreased, base metal conversions of HC and CO decreased monotonically. In contrast, the platinum catalysts maintained very high 1975 FTP CO conversions (> 90% ) at all converter sizes HC conversions remained constant 70% ) at volumes down to 1300 cm3. Performance of the base metal catalysts with the 4300-cm3 converter nearly equalled that of the platinum catalysts. However, platinum catalysts have a reserve activity with very high conversions attained at the smallest converter volumes, which makes them more tolerant of thermal and contaminant degradation. 

These experiments provide a direct comparison of the initial activities of platinum and base metal catalysts. Differences in performance— produced by such variables as catalyst bed mass, exhaust gas space velocity, and catalyst temperature—are explained by the effect of converter size on warm-up rates and by the kinetic differences for oxidation reactions over the two types of catalysts. 

CO Control. Over base metal catalysts, the catalytic oxidation of CO is of the order of 0.7-1.0 in CO with little dependence on 02 concentration (8, 9, 11, 12). CO oxidation over platinum catalysts is of negative first order dependency on CO concentration (13) with some tendency toward a positive dependency at very high conversion (9). However, this last observation may have been the result of mass transfer limited kinetics (9). 

Supported platinum and base metal catalysts were evaluated in vehicle tests with converter volumes of 600-4300 cm3. The initial oxidation activity of the catalysts was determined as the vehicle was operated over the 1975 FTP. The ability of the base metal catalysts to control exhaust HC and CO emissions was strongly dependent on the catalyst volume. HC and CO conversion decreased quite rapidly as the converter size was decreased. 

Oxidation of CO and C2H4 by Base Metal Catalysts Prepared on Honeycomb Supports... 

We decided to evaluate the catalytic activity on a honeycomb of the best base metal catalyst that we know of in the absence of sulfur (a severe poison). If under the best conditions we could not provide sufficient catalytic activity (60% hydrocarbon oxidation activity in a typical... 

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