V. G. Milt, M. A. Ulla and E. A. Lombardo

Instituto de Investigaciones en Catálisis y Petroquímica, INCAPE (FIQ, UNL-CONICET), Santiago del Estero 2829, 3000 Santa Fe, Argentina.

Phone/Fax: 54-342-4571192. E-mail: nfisico@fiqus.unl.edu.ar

The catalytic combustion of natural gas, applicable to gas turbines, allows the efficient burning of lean fuel-air mixtures with a minimum pollutant formation as compared to conventional flame combustion.

Co₃O₄ is an excellent catalyst for methane oxidation but it easily sinters at temperatures as low as two thirds the melting temperature (m.p.=1170K), and at temperatures close to 1270K it becomes volatile. Therefore, there has been an intense research effort trying to stabilize Co in different matrixes. For this application, the matrixes should be stable under severe hydrothermal conditions. Furthermore, the transition metal cation must be tightly anchored to the surface to avoid the sintering of the active phase. Within limits, the surface area should be as high as feasible.

In this presentation, we will discuss catalytic and characterization data obtained by our group, and elsewhere, for a number of cobalt-containing mixed oxides and for cobalt supported on a variety of solids such as magnesium oxide, lanthanum oxide, zirconia and lanthanum-doped zirconia. Different techniques have been used to synthesize these catalysts. Among them, we have particularly used the explosion method for bulk-mixed oxides and the atomic layer epitaxial growth technique (ALE) for supported systems, as well as the traditional impregnation procedure for comparison purposes.

For the ALE method, a flow-type reactor operated at low pressure (P@ 1330 Nm^-2) was used. Cobalt acetylacetonate was sublimed and chemisorbed on the surface of the support at 440K using N₂ as carrier. Calcination at 770K in air yielded the oxide species on the surface of the support. This reaction cycle was repeated several times in order to increase the amount of supported cobalt. For the explosion method, a citric acid solution was added to an equimolar solution of the corresponding metal nitrates. After drying at 380K overnight, a precursor was obtained, which was pressed with benzoic acid to form a pellet containing a filament. After the ignition of the pellet in a calorimetric bomb, loaded with oxygen, a porous, expanded and crystalline solid was obtained.

The rate data measured at 770K of ca. 25 different cobalt-containing catalysts varies between 0.02 and 0.7 m mol.s^-1.m^-2 and 0.09 and 20 m mol.s^-1.g^-1. For comparison one of the best-substituted hexaaluminates (LaMnAl₁₁O_19-a ) reaches 6.3 m mol.s^-1.g^-1.

The use of several characterization techniques provides clues to understand the causes underlying these large variations in catalytic activity our group and others have observed. The presence of both low temperature (420-470K) reduction peaks (TPR) and Co²⁺/Co³⁺ couples (XPS) on the catalyst surface and the absence of stable carbonates (TPR, XPS) are the common features of the best formulations. In some cases the presence of segregated Co₃O₄ in oxides (XRD) explains the widely different rate data reported in the literature for mixed oxides such as LaCoO₃ perovskites.

From the literature survey and our own data it is concluded that the best results are obtained using the ALE technique to react the cobalt b -diketonate with either ZrO₂ or La₂O₃-doped ZrO₂. The presence of lanthanum lowers the initial activity of these solids, but does it affect durability?

Stability tests were performed keeping the catalysts on stream for over 150h at 970K. The cobalt supported on La₂O₃/ZrO₂ catalysts maintained their activities after this test while those supported on ZrO₂ decrease their activity by 60% after this period. The factors that influence the activity and stability of methane combustion catalysts will be thoroughly discussed.

Collaboration Opportunities (J. Petunchi, E. Lombardo, E. Miró)

Our group specializes in:

1. Materials such as Perovskites, Zeolites, VPO and supported oxides (Fe, Co/MgO, Co/La₂O₃/ZrO₂, etc.)
2. Reactions such as the SCR of NO_x with hydrocarbons, methane combustion, n-C₄H₁₀ to maleic anhydride.
3. Just starting: Production of H₂ and syngas in membrane reactors.
4. Development of new ceramic materials, active and stable for catalytic soot combustion in diesel exhaust filters.

1. In Environmental Catalysis with Prof. Martin Schmal, NUCAT-COPPE, Rio de Janeiro.
2. In membrane reactors with Prof. Jesús Santamaría, Department of Chemical Engineering, Universidad de Zaragoza, Spain.

Our group has expertise in XPS, Raman and Infrared Spectroscopy. TGA and TP techniques are currently being used as additional characterization tools. Besides conventional characterization techniques, we have a home-made system for Atomic Layer Epitaxial (ALE) attachment of different elements on refractory oxides.

We are particularly interested in collaborating in the areas of :

1. Development of new types of membranes for H₂ separation.
2. Zeolites washcoated or grown over cordierite monoliths (SCR of NOx with hydrocarbons).
3. Catalysts for CO₂ reforming with high activity and low carbon deposition at 500ºC.