Reactivity studies, bimetallic catalysts

In this chapter, SOMC/M will be used to study the reactivity of organometallic complexes with the surface of supported metals. In 1984, Travers [31] and Margit-falvi [32] simultaneously described this application of SOMC for the preparation of bimetallic catalysts. 

Table II. A Brief Summary of Literature Relating Generally to Reactivity—Selectivity-Poisoning Studies of Bimetallic Catalysts...

The recent accomplishments of near-edge X-ray absorption spectroscopy in catalysis studies are already quite impressive, in particular if one considers the limited availability of suitable X-ray spectrometers. Developments of catalytic interest have concerned the Shell Higher Olefin process, size effects, metal-support interaction, mono- and bimetallic catalysts (in particular the PtRe/Al203 system), the reactivity of supported metal catalysts, dynamical and in situ catalyst studies, and a variety of oxide and sulfide catalysts. Other catalytic problems are now coming within easy experimental reach, such as the study of sulfur poisoning and the nature of coking. 

Pd-only TWCs display limitations with respect to their ability to reduce NO and, particularly, in their selectivity towards N2 at low temperature.67,68 Modification of Pd by the introduction of a second, cheaper metal would appear to offer a viable solution from an economical and catalytic point of view.69 It is well known that the resulting bimetallic catalyst may display special features not anticipated by simple interpolation of the reactivity of the constituents. Although the complexity of TWC systems, where the metal components can be present over the alumina and/or the promoter makes the study of bimetallic systems rather difficult, the main physicochemical effects exerted by the second metal on the noble metal component allow a simple classification of bimetallic systems, somewhat independent of the specific kinetic and thermodynamic features of the metal-metal contact. First of all, catalysts are found where the introduction of the second metal (M) may generate a binary phase, either in the oxidised and/or reduced chemical states. This is typically the case for Cu70 71 or Cr.36,56 A classic explanation of the differential behavior with respect to the monometallic Pd system makes use of the interrelated structural (or ensemble) and electronic effects. This is typically applied to the zero-valent state but can also loosely embrace oxidised or partially reduced states where the noble metal displays catalytic activity.56 As previously mentioned, NO reduction (by CO) is... 

This paper deals with experimental studies of the reactivities of CO or alkanes with some platinum-based bimetallic catalysts under oxygen or hydrogen atmospheres, respectively. 

Methods of Controlled Surface Reactions (CSRs) and Surface Organometallic Chemistry (SOMC) were developed with the aim to obtain surface species with Sn-Pt interaction. In CSRs two approaches have been used (i) electrochemical, and (ii) organometallic. Characteristic feature of the organometallic approach is that both CSR and SOMC results in almost exclusively supported alloy type bimetallic nanoclusters. Studies on the reactivity of tin organic compounds towards hydrogen adsorbed on different transition and noble metals have revealed new aspects for the preparation of supported bimetallic catalysts. 

These studies of reaction 80 are of great interest for catalytic sciences, which try to understand how gold affects the reactivity of supported bimetallic catalysts . ... 

Very high olefin yields were reported by Schmidt and co-workers,who first proposed the oxidative dehydrogenation of hght alkanes over insulated noble metal coated monoliths at contact times of a few milhseconds. This new concept of catalytic reactor had been previously applied by the same group to methane partial oxidation and was extended to test the reactivity of C2-C6 alkane/air fuel-rich feeds. Ceramic foam monoliths (with 45 and 80 ppi) were mostly studied as supports of noble metals and bimetallic catalysts. 

Finally, we want to compare the main mechanistic findings of our study with the classic bifunctional mechanism, which is generally used to explain the improved CO oxidation reactivity of PtRu surfaces and catalyst particles [Watanabe and Motoo, 1975]. According to that mechanism, Ru acts as a promotor for the formation of oxygenated adspecies on bimetallic PtRu surfaces, which can then react with CO... 

Properties of supported catalysts by bimetallic substrates depend on the changes in geometry of the catalyst material by the strain of the substrate. Using a bimetallic substrate multiphes the possibilities to tune the catalyst to specific requirements. The chemistry of the nanosized overlayer is affected by the different orbital overlaps of atoms from the catalyst cluster and those from the substrate. Additionally, small supported metallic islands show low coordination and reduced near-neighbor distances thus their chemical properties are different with respect to those of flat surfaces. " Reactivity of several bimetallics were also studied by Balbuena et al., including bimetallics systems . Norskov et al. found several relations for the bimetallic systems considering local and nonlocal effects have also been reported. ... 

Abstract Thermally stable, ordered surface alloys of Sn and Pt that isolate threefold Pt, twofold Pt, and single-Pt atom sites can be produced by controlled deposition of Sn onto Pt single crystals and annealing. The strnctnre was established by characterization with several techniques, including ALISS, XPD, LEED, and STM. Chemisorption and catalysis studies of these well-defined, bimetallic surfaces also define the overall chemical reactivity of Pt-Sn alloys, clarify the role of a second-metal component in altering chemistry and catalysis on Pt alloys, and develop general principles that describe the reactivity and selectivity of bimetallic alloy catalysts. 

In this chapter, we will illustrate with a few selected examples how well-defined, ordered Pt-Sn surface alloys have been used to elucidate the overall chemical reactivity of Pt-Sn alloys, clarify the role of Sn in altering this chemistry and catalysis, and develop general principles for understanding the reactivity and selectivity of bimetallic alloy catalysts. Most studies have involved chemisorption under UUV conditions, but the use of these alloys as model catalysts for investigating catalysis at pressures up to one atmosphere will also be discussed. 

The modification of platinum catalysts by the presence of ad-layers of a less noble metal such as ruthenium has been studied before [15-28]. A cooperative mechanism of the platinurmruthenium bimetallic system that causes the surface catalytic process between the two types of active species has been demonstrated [18], This system has attracted interest because it is regarded as a model for the platinurmruthenium alloy catalysts in fuel cell technology. Numerous studies on the methanol oxidation of ruthenium-decorated single crystals have reported that the Pt(l 11)/Ru surface shows the highest activity among all platinurmruthenium surfaces [21-26]. The development of carbon-supported electrocatalysts for direct methanol fuel cells (DMFC) indicates that the reactivity for methanol oxidation depends on the amount of the noble metal in the carbon-supported catalyst. 

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