Pt/Rh catalyst

Ammonia bums in air with difficulty, the flammable limits being 16-25 vol%. Normal combustion yields nitrogen but, in the presence of a Pt or Pt/Rh catalyst at 750-900°C, the reaction proceeds further to give the thermodynamically less-favoured products NO and NO2 ... 

It is worth mentioning that the results discussed in Sections 3.1 and 3.2 provide a rationale behind the recent trends toward a close-coupled converter location as well as dual-brick converter design (i.e., a relatively small-voltmie catalyst brick with high Pd loadings followed by a larger Pt/Rh catalyst brick) in order to improve converter lightofF performance without excessive noble metal usage. 

Noble metal dispersions and surface areas Table 2 lists the apparent dispersions obtained from the CO methanation technique. No correlation is observed between dispersion and catalyst performance as measured by the CO/NOx crossover efficiencies. The C2 and C3 Pd-only TWCs, despite their extremely high CO/NOx crossover efficiencies, gave apparent dispersions of 3.5 and 3.0% after 75 and 120 h aging versus higher values of 5.9% for the Pd/Rh catalyst (E) and 4.3% for the Pt/Rh catalyst (G). both of which displayed low CO/NOx crossover efficiencies. Even between the two Pd/Rh catalysts, catalyst E h2is an apparent dispersion more than four times that of catalyst F, yet the two are nearly identical in their CO/NOx crossover efficiencies. 

Figure 8. Effect of Ce on the cycled dynamometer performance of Pt/Rh catalysts (2S)...

In another study by the same group, K, Ca and CaK promoters were added to Rh supported on MgAl204 spinel. Both modified and unmodified catalysts produced similar H2 and CO reformate concentrations of 23 and 25 vol%, respectively. The different modifiers did affect carbon production on the catalysts. The unpromoted Rh catalyst showed carbon levels of 0.03 wt% carbon, where the RhK catalyst had 0.02 wt% carbon, the RhCa 0.015 wt%, and the RhCaK only 0.01 wt%. Bimetallic Pt Rh with Li, Ba and Li-Ba modifiers supported on MgAl204 spinel were also examined for H2 and CO yields and carbon formation resistance, with similar results to previous work for the yields. The unpromoted Pt-Rh catalyst showed carbon levels of 0.01 wt% carbon, where the promoted Pt-Rh catalysts all showed reduced coking levels of 0.005 wt%. 

Gasolines contain a small amount of sulfur which is emitted with the exhaust gas mainly as sulfur dioxide. On passing through the catalyst, the sulfur dioxide in exhaust gas is partially converted to sulfur trioxide which may react with the water vapor to form sulfuric acid (1,2) or with the support oxide to form aluminum sulfate and cerium sulfate (3-6). However, sulfur storage can also occur by the direct interaction of SO2 with both alumina and ceria (4,7). Studies of the oxidation of SO2 over supported noble metal catalysts indicate that Pt catalytically oxidizes more SO2 to SO3 than Rh (8,9) and that this reaction diminishes with increasing Rh content for Pt-Rh catalysts (10). Moreover, it was shown that heating platinum and rhodium catalysts in a SO2 and O2 mixture produces sulfate on the metals (11). 

Sulfated catalyst activity was determined with the S02 free feedstream in the absence of water The light-off temperatures reported in Fig 3a for propene oxidation show that sulfation by SO2 induces the same effects on catalyst activity than SO2 in the feedstream in the course of the oxidation reaction (Fig 1b). Thus Pt-Rh catalyst activity is not affected by sulfation while monometallic platinum catalysts are far less active after sulfur storage with 20 ppm SC>2 We must note also that a small inhibiting effect appears after sulfur storage with 4 ppm SO2. 

Fi< . 33. Shift data for two adsorbates on Pt/Rh catalysts as a function of overall Rh concentration. Squares and right-hand scale. H circles and left-hand scale. CO. In both cases application of a simple two-site rapid-exchange model (as in Fig. 32) would lead to the conclusion that the surface is enriched in Rh. At least for the CO-covered samples, additional NMR experiments do not confirm this conclusion 9S, 105). 

The CO oxidation and NO reduction reactions have also been investigated in the presence of supported Pt-Rh catalysts. The results seem confusing Whereas Oh and Carpenter (151) and Lyman et al (153,156) reported... 

Hardacre el al. (7 75, 174) investigated the properties, structure, and composition of cerium oxide films prepared by cerium deposition on Pt(lll), finding that the activity for CO oxidation is enhanced on Pt(lll) that is partially covered by ceria. It was suggested that new sites at the Pt-oxide interface become available for reaction. A remarkable observation is the high activity for CO oxidation when the Pt(lll) sample is fully encapsulated by ceria (Pt was undetectable by XPS and AES). It was proposed that an ultrathin, disordered ceria film becomes the active catalyst. It was also demonstrated by XPS and AES that Pt dramatically increases the reducibility of cerium oxide that is in intimate contact with Pt. This result suggests that intimate contact between the noble metal and oxide phases is indeed crucial to facile oxygen release from ceria. High-resolution electron microscopy demonstrated the presence of direct contact between ceria and noble metal for supported Pt-Rh catalysts (775). Hardacre et al. (173,174) related the catalytic activity of the ceria phase to partially reduced cerium oxide. 

The procedure above is particularly usefiil for preparing supported noble metal (NM Pd, Pt, Rh) catalysts. T ugh obviously sensitive to the support surik e area, metal loading, and the specific experimental protocol, this procedure, at the laboratory scale, often leads to well dispersed metal systems with relatively narrow metal particle size distributions (97,117,183,235). 

Rhenium By-product from molybdenite Pt-Rh catalysts superalloys... 

Although the Pt/Rh catalyst generally retains good efficiency for hydrocarbon control, it is desirable to enhance the efficiency of catalysts for hydrocarbon removal in the light of the severe future U.S. hydrocarbon standards. Developments in palladium catalyst technology have resulted in a significant improvement in hydrocarbon control. These catalysts will see application in close-coupled converter systems, either as a single palladium catalyst or as a palladium catalyst in combination with rhodium [9] and in some... 

To summarize the laboratory data comparing Pd and Pt/Rh commercial catalysts, we found that an increase in the sulfur content in simulated exhaust results in a loss of both lightoff and warmed-up activity. The lightoff activity of the Pd is generally better than the Pt-Rh catalyst regardless of the sulfur content in the feedstream. Under warmed-up conditions, the loss of activity for HC, CO and NOx in the presence of sulfur was greater under slightly rich conditions than under lean conditions for both Pd and Pt-Rh catalysts. 

Other studies have also been conducted recently that indicate that Pd catalysts may be more sensitive to sulfur poisoning. One recent study compared several catalyst technologies including Pd and Pt/Rh catalysts [28]. 

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