Alloy catalysts surface enrichment

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. 

The surface properties of three types of methanation catalysts obtained by oxidation of selected Intermetallics were examined In relation to their CO conversion activity. The first type (Ni Si, N1 A1 ) which corresponds to active phase-supporl iX the coXventionally prepared catalyst Is little affected by the oxidation treatment. The surface Nl is oxidized and relatively more abundant In the active solids. The second type (active phase-promoter ex Ni Th ) is extensively decomposed on oxidation. The transformation of these alloys Is accompanied by a surface enrichment in Nl. 

Current views on the surface enrichment of one component over another in alloy systems are, surprisingly, more a consequence of gas titration and Auger electron spectroscopy than XPS and UPS. There is little doubt, however, that looking to the future XPS will provide important clues regarding the mechanism of bimetallic catalysts, the significance of promoters. 

Pt-Rh/AROs catalysts are widely used in automotive-exhaust emission control. In these systems, Pt is generally used for the oxidation of CO and hydrocarbons and Rh is active for the reduction of nitric oxide to N2. HRTEM and AEM show two discrete particle morphologies and Pt-Rh alloy particles (Lakis et al 1995). EM studies aimed at understanding the factors leading to deactivation, surface segregation of one metal over the other and SMSI are limited. There are great opportunities for EM studies, in particular, of surface enrichment, and defects and dislocations in the complex alloy catalysts as sites for SMSI. 

Mechanism. The interaction of antimony with supported nickel particles was studied with X-ray diffraction (XRD) (10) These studies suggested that a high level of antimony is present on the surface of Ni-Sb alloys. Further studies of the Ni-Sb alloy, using X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES), confirmed the presence of an antimony enriched surface (.11) The surface enrichment of antimony on the Ni-Sb alloy would be expected to significantly alter the catalytic activity of nickel, as indeed occurs when antimony is added to nickel laden cracking catalysts. 

A third factor controlling surface composition is the atmosphere in which the catalyst is used. The surface will be enriched in that component of the alloy that has the highest heat of adsorption of the gas. In an oxygen atmosphere the surface of a nickel-gold catalyst becomes enriched with nickel rather than gold. 57 In the presence of CO the surface of a palladium-silver alloy becomes enriched with palladium while, normally, silver would be the predominant surface component. 58 This enrichment of the surface by palladium should also be observed in a hydrogen atmosphere. 

Surface Enrichments.—Alloy catalysts may or may not have the same composition on their surfaces as they do in the bulk. In the case of 22 atom % Pd-Au alloy the surface and bulk compositions, as measured by Maire et al. using AES, are identical for clean surfaces. Now pretreatments in O2 have an effect on catalytic activities for Pd-Au and Pt-Au alloys. In the case of Pd-Au the O2 pretreatment induces surface enrichment of Pd. 

In addition to Pt—Au/C catalysts, several other Pt-based alloy catalysts, such as PtCo/C, PtNi/C, PtV/C, and PtPd/C were also reported, and the mechanism of enhancing ORR activity was investigated using both RDE and RRDE techniques. The mechanism of ORR improvement by alloying is ascribed to (1) increase in the catalyst surface roughness,(2) decrease in the coverage of surface oxides and an enrichment of the Pt-active sites of the catalyst surface,(3) increase in the d-orbital vacancy, which strengthened the Pt—O2 interaction, and (4) decrease in the Pt—Pt distance and the Pt—Pt coordination numbers. Table 7.4 lists the properties of PtCo/C and PtNi/C catalysts and their ORR performance parameters measured by RDE techniques for comparison. 

Note finally that, as mentioned in the Introduction, the corrosion of the substrate may also damage irreversibly a microstructured device under the severe conditions of fuel processing reactions. For example, under water vapor pressure, many detrimental effects can occur, such as surface migration of Ni in stainless-steel alloys, surface oxidation of metals (Fe to Fe203), surface enrichment with trace elements able to alloy/react with the coated catalyst (Sn, Pb, Cl ions) and poison it or surface substrate restructuring. 

Surface enrichment is one of the major reasons resulting in the lower activity poor selectivity or declining of activity of catalysts. Therefore, in the catalyst investigations, the phenomena of enrichment on the surfaces are extremely important and have been widely studied. Many researchers like Overbary, Ponec and Sachtler d3s. j g detailed description for the alloy catalysts and did the theoretical discussion. Menon also gave a commendatory for the metal oxides and the supported metal catalysts. 

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