Surface chemistry and catalytic activity of La 1 − yM yCoO 3 perovskite (M = Sr or Th) : 1. Bulk and surface reduction studies

Abstract

The extent of bulk reduction of the mixed oxides in hydrogen, at temperatures between 250 and 500 °C, was measured using a gas recirculating batch reactor connected to a standard BET highvacuum system (10 −3 Pa). The hydrogen treatment was also performed in a chamber attached to an ESCA spectrometer to obtain information about the surface modifications produced upon reduction. In these mixed oxides cobalt is the only cation that can be reduced to its metallic state within the temperature range explored. Both systems included a trap which was either cooled with liquid air or left at room temperature to study the effect of the water arising from the hydrogen treatment upon both the rate and the extent of reduction. The bulk measurements have shown that the presence of water impairs the reduction process. The XPS studies have also substantiated this effect at the surface level. In the absence of water the metallic cobalt is segregated to the surface at reduction temperatures above 300 °C. When water is present, lower concentrations of Co 0 appear on the surface and the segregation of this element is impaired. When thorium partially replaces lanthanum that element is not affected by the hydrogen treatment at any temperature up to 500 °C. In the La 0.6Sr 0.4CoO 3 system, the SrO begins to segregate, whether or not water is present, at temperatures higher than 250 °C. This mixed oxide is in fact the easiest to reduce. After reduction, even with the trap cooled with liquid air, the XPS spectra show a sharp increase in hydroxyl concentration. This effect increases in the direction Th < La < Sr, as expected from the similar trend of their Pauling electronegativity values. At high extents of reduction it was found that these hydroxyl groups were able to reoxidize the surface Co 0 upon evacuation in the ESCA spectrometer. This effect is less pronounced in going from the Sr- to the Th-containing oxide, with the LaCoO 3 showing an intermediate behavior. The reduction model proposed for the lanthanum cobaltate by E. A. Lombardo, K. Tanaka, and I. Toyoshima ( J. Catal. 80, 1983) has been improved and extended to the other cobalt mixed oxides.