Deborah V. Cesar

Stability and selectivity of bimetallic Cu-Co/SiO2 catalysts for cyclohexanol dehydrogenation

Abstract Bimetallic Cu–Co/SiO 2 catalysts with different metal loading (5:5, 15:15, 35:35 – Cu:Co) were prepared by deposition–precipitation method and evaluated by using the cyclohexanol dehydrogenation. XRD results show that only the 15%Cu–15%Co/SiO 2 oxide precursor exhibits mainly a phase containing Cu and Co with spinel-like structure, while other bimetallic precursors present CuO and Co 3 O 4 as distinct phases. The Rietveld method was used to quantify the different phases on the precursors and to determinate how much Cu there was in the spinel form and as CuO. After reduction and passivation, the 15%Cu–15%Co/SiO 2 catalyst showed a Cu–Co alloy formation and a different H 2 chemisorption capacity. The H 2 uptake was 2.5 greater than 35%Cu–35%Co/SiO 2 catalyst and the same order of magnitude for the monometallic 35%Co/SiO 2 . Catalytic results showed that the 15%Cu–15%Co/SiO 2 catalyst presented the highest stability and selectivity to cyclohexanone, with the lowest phenol production.

Quantitative XPS analysis of silica-supported Cu–Co oxides

Abstract Copper–cobalt oxides with Cu/Co=5:5, 15:15 and 35:35 bulk ratio have been prepared by deposition–precipitation method at constant pH from copper and cobalt nitrate solutions. Different oxides were obtained by decomposition of the precursors at 673 K for 7 h in air and analyzed by X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). XRD data showed the formation of different oxide phases; for the bulk atomic ratio of 15Cu:15Co, a phase containing Cu and Co with spinel-like structure was observed, while the other bimetallic oxides presented CuO and Co3O4 as distinct phases. The XPS qualitative analysis has shown that all samples exhibited Cu2+ and Co3+ species at the surface. The Cu–Co spinel presented a displacement in Cu 2p binding energy value. A mathematical model was proposed from relative intensity ratios, which allowed the determination of the oxide particle thickness and the fraction of coverage at the support. This model described accurately the system and showed that cobalt improved the copper dispersion.

Quantitative XPS Analysis of Bimetallic Cu–Co Catalysts

The surfaces of bimetallic Cu-Co/SiO 2 catalyst with 1 Cu:2 Co atomic ratio has been studied focusing on the determination of bimetallic phase thickness and its distribution using a mathematical formalism from X-ray photoelectron spectroscopy (XPS) intensities. X-ray diffraction analysis indicated a Cu-Co alloy formation and XPS qualitative analysis showed that Cu 0 and Co 0 species were present on the surface. Atomic ratios of Cu/Si, Co/Si and Co/Cu from XPS and atomic absorption (A.A.) analyses suggested homogeneous particle formation with a slight Cu surface enrichment in the catalyst. A mathematical model was proposed from relative intensity ratios, which was able to determine the metallic particle thickness and its coverage fraction at the support. Considering that these values were comparable to those for the particle size obtained from transmission electron microscopy (TEM) measurements, it may be said that the model describes the evaluated system accurately.