Skeletal Ru/Cucatalysts prepared from crystalline and quasicrystalline ternary alloy precursors: characterization by X-ray absorption spectroscopy and COoxidation

Abstract

The Ru/Cu system is of historical significance in catalysis. The early development and application of X-ray absorption spectroscopy (XAS) led to the original ‘bimetallic cluster” concept for highly-immiscible systems. This work explores alkali leaching of Al-based ternary crystalline and quasicrystalline precursors as a potential route to bulk Ru/Cu alloys. Single-phase ternary alloys at 3 trial compositions; Al₇₁Ru₂₂Cu₇, Al_70.5Ru₁₇Cu_12.5, and Al₇₀Ru₁₀Cu₂₀, were prepared by arc melting of the pure metal components. After leaching, the bimetallic residues were characterized principally by transmission XAS, “as-leached” and after annealing in H₂ (and passivation) in a thermobalance. XRD and BET revealed a nanocrystalline product with a native structure of hexagonal Ru. XPS surface analysis of Ru₂₂Cu₇ and Ru₁₇Cu_12.5 found only slight enrichment by Cu in the as-leached forms, with little change upon annealing. Ru₁₀Cu₂₀ was highly segregated as-leached. XANES data showed preferential oxidation of Cu in Ru₂₂Cu₇, implying that it exists as an encapsulating layer. TG data supports this view since it does not show the distinct two-stage O₂ uptake characteristic of skeletal Ru. Cu K-edge EXAFS data for Ru₂₂Cu₇ were unique in showing a high proportion of Ru neighbours. The spacing, d_CuRu = 2.65 Å, was that expected from a hypothetical (ideal) solid solution at this composition, but this is unlikely in such a bulk-immiscible system and Ru K-edge EXAFS failed to confirm bulk alloying. Furthermore its invariance under annealing was more indicative of an interfacial bond between bulk components, although partial alloying with retention of local order cannot entirely be ruled out. The XAS and XPS data were reconciled in a model involving surface and bulk segregation, Cu being present at both the grain exterior and in ultra-fine internal pores. This structure can be considered as the 3-dimensional analogue of the classical type. Preliminary studies in CO and H₂ oxidation were made in a DRIFTS flow reactor with on-line MS, and their activities and selectivities were compared against skeletal Ru and Cu controls, Ru/Al₂O₃, and Au/Fe₂O₃. All samples were active in CO oxidation above ∼50 °C, showing light-off temperatures in the range 60–70 °C. Ru₂₂Cu₇ and Ru₁₇Cu_12.5 also showed good selectivities (vs. H₂ oxidation), attributed tentatively to Ru-modified Cu surfaces of varying thickness. These compositions are promising candidates to test in a (PROX) fuel processor to supply purified (CO-free) H₂ to a PEM fuel cell.