ISOTOPIC TRANSIENT STUDY OF LA PROMOTION OF CO AL(2)O(3)OR CO HYDROGENATION/

An investigation of La3+ promotion of 20 wt% Co/Al2O3 [(La/Co)(atomic) = 0, 0.05, 0.10, and 1.0] for CO hydrogenation at 463-503 K and 1.8 a tm has been carried out in order to develop a better understanding of the mechanism and effects of promotion. Chemisorption results show tha t at lower La3+ loadings there was a more significant effect on H-2 ad sorption blockage. La3+ promotion enhanced the selectivity for higher hydrocarbons during CO hydrogenation, as has previously been observed for La3+-promoted Co catalysts at lower metal loadings, while the over all activity went through a maximum for La/Co = 0.05. Steady-state iso topic transient kinetic analysis (SSITKA) with carbon tracing was used to decouple the effects of La3+ on methane-producing sites during CO hydrogenation. The SSITKA results indicate that both a measure of the average methanation site/intermediate activity (1/tau(M)) and the conc entration of active surface intermediates leading to CH4 (N-M) increas ed upon initial La3+ promotion. At higher La3+ content the concentrati on of active surface intermediates leading to CH4 decreased; however, the average site/intermediate activity remained essentially constant, resulting in a lower rate of methane formation. Deconvolution results indicate that La3+ promotion affected methane site activity by increas ing the reactivity of the most active pool of intermediates. Increasin g the H-2/CO inlet ratio at constant temperature and CO partial pressu re led to an increase in the observed average site/intermediate activi ty, indicating a dependence on the concentration of surface hydrogen. One can conclude that while La3+ species block some of the Co surface, promotion results in the creation of more sites/intermediates on the surface having higher activities and probably results in a modificatio n of the hydrogen surface concentration. (C) 1995 Academic Press, Inc.