Cerium-based bimetallic oxides of the type 3NiO·CeO2, Co3O4·CeO2, and 1.5Fe2O3·CeO2 were prepared by two sol–gel methods (epoxide addition and Pechini methods) and by the electrospinning technique, aiming at the synthesis of compounds with different morphologies (nanoparticles or nanofibers) and the evaluation as catalysts for the methanation of CO2. The results obtained show a catalytic performance that depends on the type of d block element, method, and catalyst morphology. Regardless of such parameters, the best results were those obtained over the nickel-based catalysts prepared by the electrospinning technique that are remarkably more active than a commercial rhodium catalyst used as a reference and tested in the same conditions. Moreover, the cobalt-based catalysts obtained either as nanoparticles (Pechini method) or fibers (electrospinning technique) present also a good activity, comparable to that measured over the reference catalyst. All catalysts were very stable to deactivation during at least 60–70 h of reaction, which is clearly an advantage that could make them an alternative to be considered for future developments in this area of knowledge.

Cobalt-lanthanide bimetallic oxide nanofibers (5Co 3 O 4 .3LnCoO 3 , Ln=La, Pr; 4Co 3 O 4 .Ln 2 O 3 , Ln=Sm, Gd, Dy, Yb and 2Co 3 O 4 .CeO 2 ) were prepared by electrospinning technique and for the first time evaluated as catalysts for the hydrogenation of CO 2 . Depending on the lanthanide, the reaction products are different: lighter lanthanides (La, Pr, Sm and Gd) produce mainly CO and are more active to reverse water gas shift (RWGS), whereas the catalysts with Ce, Dy and Yb are more active and selective to methane. Lanthanide intrinsic properties such as ionic radii and basicity strongly correlate with the nanofibers' catalytic performance: lower lanthanide ionic radii and higher basicity favour the catalysts activity. Moreover, the bimetallic oxide nanofibers catalytic behavior also seems to point to the existence of a synergetic interaction between cobalt and lanthanide. The cobalt-lanthanide bimetallic oxides present a high deactivation resistance for at least 60 h in the gaseous stream, which is an advantage for any catalytic application. Compared with a commercial catalyst (5 wt.% Rh/Al 2 O 3 ) tested in the same conditions, cobalt-ytterbium, dysprosium and cerium bimetallic oxide nanofibers present an activity 5 to 13 times higher at 350 °C.

Nanostructured Cobalt-Actinide (An=Th, U) bimetallic oxides were for the first time prepared by the epoxide addition method and electrospinning technique aiming the synthesis of aerogels and nanofibers, respectively. Tested as catalysts for the methanation of CO 2 and regardless of the actinide element, the yield and selectivity towards CH 4 are high (> 40 and 95 %, respectively), which was explained by their physicochemical properties such as reducibility and basicity and the existence of a synergic effect between cobalt and actinide species. The aerogels present the best catalytic behavior, namely those with thorium and the catalytic activity increases with the Co/An molar ratio. Both aerogels and nanofibers also present a high resistance to deactivation for at least 75 h in the gaseous stream, which is an advantage for any catalytic application. Moreover, at 300 °C they are 2 to 4 times more active than two reference catalysts (5 wt % Rh/Al 2 O 3 and NiO/Al 2 O 3 ) tested in the same conditions, which to our best knowledge is a significant result.

No abstract

Nickel– and cobalt–cerium bimetallic oxides were used as catalysts for the methanation of CO2 under pressure. The catalysts’ activity increases with pressure and an increase of just 10 bar is enough to double the yield of methane and to significantly improve the selectivity. The best results were those obtained over nickel–cerium bimetallic oxides, but the effect of pressure was particularly relevant over cobalt–cerium bimetallic oxides, which yield to methane increases from almost zero at atmospheric pressure to 50–60% at 30 bar. Both catalyst types are remarkably competitive, especially those containing nickel, which were always more active than a commercial rhodium catalyst used as a reference (5wt.% Rh/Al2O3) and tested under the same conditions. For the cobalt–cerium bimetallic oxides, the existence of a synergetic interaction between Co and CoO and the formation of cobalt carbides seems to play an important role in their catalytic behavior. Correlation between experimental reaction rates and simulated data confirms that the catalysts’ behavior follows the Langmuir–Hinshelwood–Hougen–Watson kinetic model, but Le Chatelier’s principle is also important to understand the catalysts’ behavior under pressure. A catalyst recycle study was also performed. The results obtained after five cycles using a nickel–cerium catalyst show insignificant variations in activity and selectivity, which are important for any type of practical application.