In Situ Elucidation of the Active State of Co–CeO_x Catalysts in the Dry Reforming of Methane: The Important Role of the Reducible Oxide Support and Interactions with Cobalt

Abstract

The dry reforming of methane was systematically studied over a series (2-30 wt%) of Co (~5nm in size) loaded CeO2 catalysts, with an effort to elucidate the behavior of Co and ceria in the catalytic process using in-situ methods. For the systems under study, the reaction activity scaled with increasing Co loading, and a 10 wt% Co-CeO2 catalyst exhibiting the best catalytic activity and good stability at 500 °C with little evidence for carbon accumulation. The phase transitions and the nature of active components in the catalyst were investigated during pretreatment and under reaction conditions by ex-situ/in-situ techniques including X-ray diffraction (XRD) and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS). These studies showed a dynamical evolution in the chemical composition of the catalysts under reaction conditions. A clear transition of Co3O4 → CoO → Co, and Ce 4+ to Ce 3+ , was observed during the temperature programmed reduction under H2 and CH4. However, introduction of CO2, led to partial re-oxidation of all components at low temperatures, followed by reduction at high temperatures. Under optimum CO and H2 producing conditions both XRD and AP-XPS indicated that the active phase involved a majority of metallic Co with a small amount of CoO both supported on a partially reduced ceria (Ce 3+ /Ce 4+). We identified the importance of dispersing Co, anchoring it onto ceria surface sites, and then utilizing the redox properties of ceria for activating and then oxidatively converting methane while inhibiting coke formation. Furthermore, a synergistic effect between cobalt and ceria and the interfacial site are essential to successfully close the catalytic cycle.

Ceria-Based Catalysts Studied by Near Ambient Pressure X-ray Photoelectron Spectroscopy: A Review

“…Moreover, the C formed by total dissociation of methane did not react with ceria lattice oxygen atoms but with the O adatoms generated by the decomposition of CO2 instead. In another report, Zhang et al also investigated the dry reforming of methane over a series of ceria-supported powder catalysts with different cobalt loadings (2−30 wt %) in order to elucidate the interaction between Co and CeO 2 during the catalytic process, and thus optimise the design of a catalyst with improved activity [147]. Various in situ techniques were used to achieve so, such as in situ time-resolved XRD (TR-XRD).…”

Effect of Metal‐Support Interactions in Mixed Co/Al Catalysts for Dry Reforming of Methane

“…Noble metal catalysts such as Pt, Ru and Rh have demonstrated high performance for the DRM, although their high cost drives a need to develop transition metal alternatives. As such, supported Co catalysts have gained interest recently …”

Catalysts based on transition metals for applications in energy conversion

“…95,126,127 Finally, AP-XPS enables the correlation of reaction kinetics/mechanisms to surface composition and chemical/electronic states under catalytically relevant pressure, temperature, and atmosphere conditions. 120,123,126,127,129,130 112,131 It is also observed that, when exposed to both reactant gases (5 mTorr of CO2, and 25 mTorr of H2) at room temperature or higher temperatures (250, 350, and 450°C), cobalt still retained its metallic form. Contrary to Co3O4, the Zn LMM Auger electron spectra for both tested catalysts (figure 15d and 15e) demonstrate that ZnO support does not undergo any significant reduction process.…”

“…Also, an increase in the intensity of the peaks took place with the metal loadings. The absence of some characteristic peaks of Co3O4 and NiO for the samples with the two lowest metal loadings can be ascribed to the poor crystallinity of both cobalt and nickel at such low loadings 36,112. Furthermore, the sharp Co3O4 and NiO peaks in the highest metal loading catalysts indicate much larger average crystallite size of the Co and Ni oxide phases, implying the agglomeration of such phases during the synthesis process can occur 112,113.…”

“…The absence of some characteristic peaks of Co3O4 and NiO for the samples with the two lowest metal loadings can be ascribed to the poor crystallinity of both cobalt and nickel at such low loadings 36,112. Furthermore, the sharp Co3O4 and NiO peaks in the highest metal loading catalysts indicate much larger average crystallite size of the Co and Ni oxide phases, implying the agglomeration of such phases during the synthesis process can occur 112,113. In order to account for the formation of more distorted phase as well as any lattice shrinkage in the catalyts due to Ni and/or Co intrusion into ZnO lattices or vice-versa, further studies such as retvield refinement are currently being performed.…”