Surface Chemistry and Catalytic Activity of La1 yMyCoO3 Perovskite (M=Sr or Th). -2 Hydrogenation of CO2

M. A, ULLA; R. MIGONE; J. O. PETUNCHI; E. A. LOMBARDO

JOURNAL OF CATALYSIS

ACADEMIC PRESS INC ELSEVIER SCIENCE

Año: 1987 vol. 105 p. 107 - 119

0021-9517

The partial substitution of LaIII by either SrII or ThIV in lanthanum cobaltate perovskite affects both the rate of hydrogenation of carbon dioxide and the distribution of products. The reaction of cyclopropane with hydrogen was used as a parallel test reaction in order to ascertain the nature of the active sites developed on these solids upon reduction. The active site density was calculated from the measured amount of chemisorbed hydrogen at 25 °C on the oxides reduced at temperatures between 250 and 500 °C. The catalytic activity was measured in a standard recirculation system using a H2: CO2 = 4:1 ratio, total pressure of 160 Torr, and 280 °C reaction temperature. The total conversion of CO2 into products (activity) on LaCoO3 is little affected by the extent of reduction of the sample. More sensitive to this parameter are both the rate of methanation and the production of C+ 2 compounds. Reduced La0.8Th0.2CoO3 is very stable giving a constant activity, the highest of all the solids assayed and almost exclusively producing methane. La0.6Sr0.4CoO3 shows a sharp maximum in activity and high selectivity to methane when prereduced at 300 °C. However, when reduced at increasing temperatures the overall activity sharply drops while the selectivity to higher hydrocarbons increases very rapidly. In all cases the unreduced solids present induction periods, which indicate that the oxide is being reduced in situ by the reacting mixture. In runs designed to measure the extent of deactivation due to coke deposition it was found that the degree of activity decay was inversely correlated with the methanation selectivity. The test reaction was conducted in the same system at H2: cyclopropane = 1:1 ratio, P = 170 Torr, and 250 °C. The overall activity and the product distribution toward isomerization, hydrogenation, and hydrogenolysis is very sensitive to both the nature of the solid involved and the extent of reduction. The initial rate of formation of hydrogenation plus hydrogenolysis products when plotted vs extent of reduction produces curves which are similar to those observed in methanation activity. To gain further insight into both the matrix and promoter effect a series of catalysts were prepared containing different combinations of Co, Sr, La, and Th supported on either celite or La2O3. The matrix effect is most important in the Srsubstituted oxide, less so in LaCoO3, and unimportant in the Th-containing perovskite. The promoter effect for C2+ production follows the order Sr > La ≫> Th. This and prevous studies made on crystalline mixed oxides, together with data available in the literature, allowed us to propose a model to interpret the effect of lanthanum replacement upon the catalytic activity, Selectivity, and stability of these solids.