Bimetallic Pt/Rh catalysts

Previous work has shown that oxidizing thermal treatment at high temperature (800-900°C) of bimetallic Pt-Rh catalysts prepared by coimpregnation would lead to the formation of Pt-Rh alloys with surface enrichment in rhodium oxides (3-7). In order to verify this hypothesis in our case, the coimpregnated Pt-Rh catalyst was characterized by temperature programmed reduction in hydrogen and by measure of the activity for the oxidation of a propane-propene mixture under lean conditions. 

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%. 

Ethanol oxidation was studied under lean conditions on platinum/alumina catalysts modified by rhodium and/or lanthanum oxide. The results on alumina support suggest that the formation of bimetallic Pt-Rh particles enhances the production of acetaldehyde, particularly after oxidizing thermal aging of the Pt-Rh/alumina catalyst prepared by coimpregnation. The addition of lanthanum oxide to alumina allows to avoid the formation of Pt-Rh alloy after high temperature treatment and therefore induces a decrease of the production of acetaldehyde. 

The kinetics of the CO+NO reactions has been studied at 300°C over a fresh and a deactivated bimetallic Pt-Rh/AhOs catalyst. Two kinetic models have been examined including competitive and non-competitive adsorptions of the reactants. The discrimination between these two assumptions has been achieved by using graphic and mathematical methods. From the comparison of kinetic and thermodynamic constants calculated from these methods with those previously obtained on RI1/AI2O3 and on Pt/AljOs, we believe that the kinetic data obtained on the fresh Pt-Rh/Al203 catalyst can be modelled by non-competitive adsorptions of the reactants assuming a preferential adsorption of NO on Rh and CO on Pt. By contrast NO and CO competitive adsorptions can only occur on the deactivated Pt-Rh/Al203 catalyst, which to assume that the active sur ce is mostly composed of Rh. 

This mechanism includes reversible adsorptions of NO and CO, steps (1) and (2), and the dissociation of adsorbed NO, step (3) as rate determining step. The values of tlie rate constant of step (3) and of the equilibrium adsorption constants of CO and NO determined on these different Pt catalysts were discussed in terms of changes in the adsorption properties of Pt induced by support effects [10]. Hence kinetics could be useful to state on the modifications in the extent of such interactions when Rh is added to Pt, in particular when the deactivation proceeds during the CO+NO reactions. This study reports kinetic data on a fresh and on an aged bimetallic Pt-Rh/AljO, catalyst which have been further interpreted with kinetic models including competitive adsorptions of NO and CO on a single kind of active site as well as non-competitive adsorptions in accordance with preferential adsorptions of the reactants on Pt and Rh sites as suggested by Van Slooten and Nieuwenhuys [11]. 

Up to now, most of the kinetic models proposed in the literature on Rh [12], Pt [13] and bimetallic Pt-Rh [14] catalysts assume (i) competitive adsorptions of NO and CO according to reaction steps (1) and (2) at equilibrium, (ii) the dissociation of adsorbed NO, step (3), as rate limiting, (iii) NO and CO as the most abundant adsorbed species on the active surface. With these assumptions, Eq. (8) is found ... 

The partial pressure dependencies of the CO+NO reaction rate on monometallic Pt/AljO, Rh/AljOj and bimetallic Pt-Rh/R, Pt-Rh/D catalysts can be discussed in terms of competitive as well as non-competitive adsorptions of the reactants (NO and CO). In the case of bimetallic Pt-Rh/AljOj catalysts these two kinetic models have been discriminated using graphic and mathematical methods. The comparison between kinetic and thermodynamic constants obtained from these two methods with those previously determined on monometallic Pt/Al203 and Rh/AljOj catalysts allows us to state on the role of RIi incorporation to kinetic behaviour of Pt. 

Mechanism for synergic promotion of hydrogen evolution in the dehydrogenation of organic hydrides such as cyclohexane on bimetallic Pt-M catalysts (M = Rh, Ir, Pd, Re, Mo, etc.). 

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... 

As legislations became stricter to involve NO control, the bimetallic Pt/Rh synthesis predominated in car catalysts formulations, while a double-bed concept was introduced. The engine run in slightly rich (in fuel) conditions, resulting in a reducing exhaust gas stream, enabling NO reduction over the... 

A comment regarding the dispersion of the Ru-Rh/Si02 and the Ru-Ir/Si02 is in order. For the case of the supported Pt-Ru catalysts. Increases in dispersion as a result of clustering were very large ( ). This effect was particularly noticeable for bimetallic particles which conform to the cherry model. Evidently, the formation of an inner core enriched in one of the two metals, followed by an outer layer enriched in the other metal, inhibits further crystal growth. For the alumina-supported Pt-Ru bimetallic clusters, the effect, although present, is considerably smaller. 

A significant volume of literature relates to our work. Concerning choice of support, Montassier et al. have examined silica-supported catalysts with Pt, Co, Rh Ru and Ir catalysts.However, these systems are not stable to hydrothermal conditions. Carbon offers a stable support option. However, the prior art with respect to carbon-supported catalysts has generally focused on Ru and Pt as metals.Additionally, unsupported catalysts have also been reported effective including Raney metals (metal sponges).Although the bulk of the literature is based on mono-metallic systems, Maris et al. recently reported on bimetallic carbon-supported catalysts with Pt/Ru and Au/Ru. In contrast, our work focuses primarily on the development of a class of rhenium-based carbon supported catalysts that have demonstrated performance equal to or better than much of the prior art. A proposed reaction mechartism is shown in Figure 34.2 °l... 

Hydrogen, preadsorbed on noble metals, is commonly used to prepare bimetallic catalysts by redox reaction. This requires the parent metal to chemisorb hydrogen (Pt, Pd, Rh, Ru, etc.) and to introduce a modifier that is reducible by hydrogen (Cu, Re, Ir, Rh, Pd, Pt, Au, etc.). All combinations of these metals have been prepared and characterized. For example, this technique has been used to prepare model Pt-Re reforming catalysts. Also, Pt-Rh and Pd-Rh were preformed to examine the interaction between platinum and rhodium in exhaust gas catalysts [8-10, 15-20, 21]. 

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