Expanded lattice ruthenium pyrochlore oxide catalysts II. Catalyst surface investigations by electron microscopy, X-ray photoelectron spectroscopy, and temperature-programmed reduction and oxidation

• Published in: Journal of catalysis Vol. 127; no. 1; pp. 421 - 444
• Main Authors: Felthouse, Timothy R., Fraundorf, Philip B., Friedman, Robert Mark, Schosser, Claire L.
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 1991
• ISSN: 0021-9517; 1090-2694
• DOI: 10.1016/0021-9517(91)90235-V

Five expanded lattice ruthenium pyrochlore oxide powders with the general formula A 2+ x Ru 2− x O 7− y ( A = Pb: x = 0.06, 0.15, 0.62; y = 0.5 and A = Bi: x = 0.39, 0.86; 0 < y ≤ 0.5) were investigated using high-resolution electron microscopy (HREM), X-ray photoelectron spectroscopy (XPS), and temperature-programmed reduction/oxidation (TPR/TPO) to ascertain factors that contribute to their low-temperature catalytic oxidation activity toward 1,2-diols and alcohols in aqueous alkaline solution. HREM data find small crystallites that vary from about 25 to 200 Å in diameter. Image analysis techniques applied to one 100-Å crystallite of Pb 2.62Ru 1.38O 6.5 reveal a 5% range of lattice spacings about the exterior portions of the particle. XPS data were collected in both the valence-band and core-level regions. XPS valence-band spectra for samples with higher levels ( x > 0.15) of Ru-site substitution by the A-site atoms display a less intense band near the Fermi energy level indicating a reduced Ru 4 d character compared to the more stoichiometric analogues. Core level XPS bands contain contributions from two different valence states for each of the Pb, Bi, and Ru atoms in the A 2+ x Ru 2− x O 7− y series. XPS-derived compositions show a higher A/Ru ratio for Pb 2.62Ru 1.38O 6.5 than the bulk whereas the other four oxides have A/Ru ratios that are similar in the surface (XPS) and bulk (XRD) compositions. TPR using H 2 of the A 2+ xRu 2− xO 7− y oxides reveals remarkable reactivity below 200°C that is in line with loosely bound oxygen in these oxides. A sample of Pb 2.62Ru 1.38O 6.5 with a lead-rich surface shows markedly less reactivity toward H 2 below 100°C than the other oxides. No distinct differences are seen in TPR data for all five expanded lattice ruthenium pyrochlore oxides between the surface and bulk oxygen. Reduced oxides reoxidize reversibly with oxygen after up to 20 mol% reduction (based on oxygen content) but at rates that are slow compared to H 2 reduction. The lack of a correlation between the TPR/TPO data and the liquid-phase catalytic activities toward 1,2-diols and alcohols is explained through differences in the active site preferences by the various substrates.