Copper-ceria stability

Since classical Cu/ZnO catalysts exhibited a poor stability while the addition of alumina resulted in much better systems, it was tempting to add alumina to Cu-Ce intermetallic compounds. Jennings et al. (1992a), prepared ternary Cu-Ce-Al alloys of various compositions and also tried a variety of other metals (Ca, Cr, Mn, Pd, Zn). Among these ternary alloys aluminum-containing catalysts were the best. In spite of lower initial activities as compared to binary alloys, they exhibited a much better long-term stability. It is believed that the role of aluminum is to stabilize the disperse copper-ceria phases responsible for methanol synthesis activity, although the mechanism for such a process remains unclear. 

This equilibrium has a buffer-like effect stabilizing the presence of cationic copper species in the structure even in a highly reductive atmosphere. The above scheme of copper oxide-ceria interactions indicates clearly that the catalyst is mutually promoted, i.e., both copper and ceria cooperate in the redox mechanism. 

The present work was focused on the synthesis of nanocrystalline components for electrochemical cells via the cellulose-precursor technique. This method was used to prepare nanostructured intermediate-temperature (IT) SOFC anodes made of a series of cermets comprising gadolinia-doped ceria (CGO), yttria-stabilized zirconia (YSZ), Gdi.86Cao.i4Ti207.5 (GCTO) pyrochlore, metallic nickel and copper. Perovskite-type SrFcojAlo.sOs.s (SFA) powder, also obtained via the cellulose precursor, was applied onto membranes of the same composition to enhance specific surface area and electrocatalytic activity in the reactors for methane conversion [3]. 

Direct oxidation (or direct utilization) The fuel is oxidized directly in the SOFC without external reformation. The SOFC has been shown to have the capabihty for direct oxidation of different types of fuels [4, 36-38]. To address the carbon deposition issue associated with nickel commonly used in the anode composition, other metals such as copper have been tested. The abihty of copper to resist carbon formation leads to the development of a composite anode composed of a ceria support and a copper phase [38]. The key technical challenges in the development of direct-oxidation SOFCs relate to the anode, especially the electrode s performance, stability, and direct-oxidation capabihty. 

Because of the excellent catalytic performance of the ceria-based C03O4 and CuO catalysts, a combination of the two systems was investigated by preparing, through co-precipitation, mixed Cu-Co-Ce-O oxides with different compositions. The mixed oxides had a larger surface area with respect to the Co-Ce-O and Cu-Ce-O composite oxides. Moreover, the addition of cobalt to the Cu-Ce-O catalyst enhanced thermal stability because the particle size was unchanged on calcination at 850 °C. According to the H2-TPR patterns, the peaks due to cobalt oxide reduction and those for copper oxide reduction were reciprocally affected and shifted towards lower temperatures. The interaction between cobalt and... 

Figure 6.14 Stability of ceria-catalyzed copper cermet under different fuel conditions contrasts with rapid irreversible deactivation of nickel cermet [43].

Ceria, titania, and zirconia supported ruthenium and eopper catalysts were tested in the production of n-butanal by n-butanol oxidation. These eatalysts were characterized by means of X-ray diffraction (XRD), N2 adsorption-desorption isotherms, temperature-programmed reduction (TPR), and X-ray photoelectron spectroscopy (XPS) techniques. The activity tests were performed in a fixed bed reaetor at 0.1 MPa and 623 K and pure mixture of reactants, air and n-butanol, in stequiometric proportion was introduced to the reactor. The rathenium catalysts showed a higher activity and stability than the copper catalysts, nevertheless the copper system showed a higher selectivity toward butyraldehyde production by n-butanol oxidation. 

Replacement of most or all of the Ni is also investigated using an anode catalyst, such as doped ceria, that is stable in contact with YSZ in SOFC fuel conditions [10]. Ceria offers improved stability, though its electronic conductivity is much less than that of Ni. As the electronic current passing from the metal support to the active electrode area is carried by the ceria catalyst coating, cell performance may be limited by ceria electronic conductivity. This situation can be improved by addition of a conductive component to the anode backbone, such as copper or stainless steel particles. Ris0 has demonstrated the latter choice and promising durability of the cell is reported. 

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