Platinum-bimetallic catalysts, surface

Surface Chemistry and Catalysis on Some Platinum-Bimetallic Catalysts... 

Nickel-platinum bimetallic catalysts showed higher activity during ATR than nickel and platinum catalysts blended in the same bed. It was hypothesized that nickel catalyzes SR, whereas platinum catalyzes POX and, when they are added to the same support, the heat transfer between the two sites is enhanced [59, 60]. Advanced explanations were reported by Dias and Assaf [60] in a study on ATR of methane catalyzed by Ni/y-Al203 with the addition of small amounts of Pd, Pt or Ir. An increase in methane conversion was observed, ascribed to the increase in exposed Ni surface area favored by the noble metal under the reaction conditions. 

Methylcyclopentane is a powerful probe molecule for the study of metal surfaces. The product distribution on platinum depends on the following factors particle size 491 reaction conditions 492-494 carbonaceous residues,492,493,495 and the extent of the interface between the metal and the support.492,493,495 The hydrogenolysis rate of methylcyclopentane depends on the hydrogen pressure.496,497 The rate exhibits a maximal value as a function of hydrogen pressure on EuroPt catalysts.498 The hydrogenolysis of methylcyclopentane has also been studied over Pt-Ru bimetallic catalysts.499... 

In this paper we report the application of bimetallic catalysts which were prepared by consecutive reduction of a submonolayer of bismuth promoter onto the surface of platinum. The technique of modifying metal surfaces at controlled electrode potential with a monolayer or sub-monolayer of foreign metal ("underpotential" deposition) is widely used in electrocatalysis (77,72). Here we apply the theory of underpotential metal deposition without the use of a potentiostat. The catalyst potential during promotion was controlled by proper selection of the reducing agent (hydrogen), pH and metal ion concentration. 

An example of the use of direct redox reactions in the preparation of bimetallic catalysts is the modification of copper catalysts by the addition of ruthenium, platinum, gold, or palladium [11-14], Assuming the metallic state for copper atoms on the surface, the redox reaction with the noble metal salts is... 

The essential application of LEISS analysis concerns supported bimetallic catalysts. For these solids, XPS analysis can no longer be considered as a surface analysis. The sizes of the particles present on the surface of these catalysts are around a nanometre, similar to the mean free path of the photoelectrons that determines the thickness probed by XPS. Certain pairs of elements do not lend themselves to LEISS analysis. This is the case, for example, with Pt-Rc for which the mass difference of the two metals is small compared to the resolution. One example of a pair that lends itself to LEISS analysis is provided by a study of Pt-Sn reforming catalysts. LEISS is ten times more sensitive for platinum than for tin. Despite this, an intensity of the tin peak at least comparable to that of platinum can be observed on a catalyst with a Pl/Sn ratio of three (Fig. 6.5), This study can be used to show a surface segregation of tin that is dilTicult to detect by XPS because of the small size of the platinum particles (2 nm). 

Since on pure platinum, methanol oxidation is strongly inhibited by poison formation, bimetallic catalysts such as PtRu or PtSn, which partially overcome this problem, have received renewed attention as interesting electrocatalysts for low-temperature fuel cell applications, and consequently much research into the structure, composition, and mechanism of their catalytic activity is now being undertaken at both a fundamental and applied level [62,77]. Presently, binary PtRu catalysts for methanol oxidation are researched in diverse forms PtRu alloys [55,63,95], Ru electrodeposits on Pt [96,97], PtRu codeposits [62,98], and Ru adsorbed on Pt [99]. The emphasis has recently been placed on producing high-activity surfaces made of platinum/ruthenium composites as a catalyst for methanol oxidation [100]. 

In this work, we investigated the preparation of bimetallic Pt-Re catalysts by surface redox reaction between hydrogen adsorbed on a parent monometallic platinum/alumina catalyst and the cation of the second metal (Re) according to the following reaction ... 

The alloy catalysts used in these early studies were low surface area materials, commonly metal powders or films. The surface areas, for example, were two orders of magnitude lower than that of platinum in a commercial reforming catalyst. Hence these alloys were not of interest as practical catalysts. The systems emphasized in these studies were combinations of metallic elements that formed continuous series of solid solutions, such as nickel-copper and palladium-gold. The use of such systems presumably made it possible to vary the electronic structure of a metal crystal in a known and convenient manner, and thereby to determine its influence on catalytic activity. Bimetallic combinations of elements exhibiting limited miscibility in the bulk were not of interest. Aspects of bimetallic catalysts other than questions related to the influence of bulk electronic structure received little attention in these studies. 

Thus the Shell workers conclude that Pt-Re bonds are present at the surface in platinum-rhenium catalysts, whether or not chemisorbed sulfur is present. A platinum-rhenium entity of the type envisioned by these investigators, if it indeed exists, could also be called a bimetallic cluster. 

While the use of a support or carrier stabilizes small metal crystallites against growth and surface area loss, reactions of support and metal may occur, especially if the catalyst is subjected to elevated temperatures The main factors affecting these phenomena are temperature, time of exposure and atmosphere. Most previously reported sintering studies were for supported platinum and particularly for Pt/AlgOj catalysts (1,2). Considerably less information is available on the sintering of other supported metal catalysts (3,4), and thus there is very little information on the behavior of bimetallic catalyst systems such as supported platinum-rhodium. 

A patent from Monsanto [100] disclosed the oxidation of alkaline solutions of amino alcohols and poly(ethylene glycol) in the presence of bimetallic catalysts prepared by deposition of copper on the surface of platinum. High yields to the corresponding carboxylates were reported. 

Augustine and Sachtler, on reduction, rhenium oxides migrate over the support surface and become reduced in contact with a prereduced platinum particle. In this way, each platinum particle can generate a mixed platinum-rhenium cluster that has the beneficial effects of bimetallic catalysts on selectivity and stability. 

Platinum (metal)- and acid (oxide)-catalyzed processes were developed to convert petroleum to high-octane fuels. Hydrodesulfurization catalysis removed sulfur from the crude to prevent catalyst deactivation. The discovery of microporous crystalline alumina silicates (zeolites) provided more selective and active catalysts for many reactions, including cracking, hydrocracking, alkylation, isomerization, and oligomerization. Catalysts that polymerize ethylene, propylene, and other molecules were discovered. A new generation of bimetallic catalysts that were dispersed on high-surface-area (100-400 m /g) oxides was synthesized. 

The interaction of CO with alloy or bimetallic surfaces is of special interest because of the importance of bimetallic catalysts in both the electrochemical and gas-phase oxidation of CO. Platinum-ruthenium alloys have long been known to be superior catalysts for the electrochemical CO oxidation, but the details of their catalytic action are still disputed. We will have more to say about the mechanism of the electrocatalytic CO oxidation on both metals and alloys in section III.8, in particular about the relevant ab initio quantum-chemical studies. Here, we will simply discuss how the chemisorption properties of CO on PtRu depend on the stmcture and composition of the bimetallic surface. 

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