METHANATION ON K-MODIFIED PT()SIO2 - THE IMPACT OF REACTION CONDITIONS ON THE EFFECTIVE ROLE OF THE PROMOTOR/

This paper reports on the first study that the authors know of of the effect of alkali promotion of Pt on methanation. Methanation was inves tigated on a 4.5 wt% Pt/SiO2 catalyst promoted with different amounts of K+ (K+/Pt = 0, 0.1, and 0.2) for two different temperature ranges ( 503-552 K and 573-665 K). The methanation rate was 10-70% lower on the promoted catalysts for reaction temperatures of 573 to 665 K. In this temperature range, the relative decrease in rate upon promotion was a function of K+ loading and did not vary with temperature, p(H2), p(CO ), or time-on-stream. In addition, there was no significant effect of K+-promotion on activation energy (ca. 29 kcal/mol) or methanation rea ction orders with respect to CO and H-2 (-0.1-0.0 and 0.4-0.6, respect ively). However, there was a decrease in the number of methane-destine d surface intermediates upon promotion as determined by steady-state i sotopic transient kinetic analysis (SSITKA). All these observations le ad to the conclusion that, in this higher reaction temperature range, K+ acts mainly as a site-blocking agent for methanation on Pt and does not change the reaction rate of the limiting step, probably hydrogena tion. Between 503 and 552 K, the activation energy and reaction orders with respect to (H)2 and CO were also not affected by K+. However, th e catalyst with a K+/Pt ratio of 0.1 showed the highest methanation ac tivity. In this lower temperature range and for all the catalysts, the apparent activation energies were also found to be lower, 18 vs. 29 k cal/mol, compared to those at higher temperatures. The reaction order with respect to CO was higher (0.2-0.3) in comparison with what was ob served in the higher temperature range(ca. -0.1-0.0). These results su ggest, that, in the low temperature range and for low loadings of K+, K+ affects the rate-determining step resulting in a rate increase grea ter than the decrease due to the blockage effect. Thus, K+ serves as a rate promoter at low reaction temperatures while its only effective f unction is site blockage at higher temperatures.