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This paper reports on the first study that the authors know of of the effect of alkali promotion of Pt on methanation. Methanation was investigated 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, \(p_{H_2 } ,p_{{\text{CO}}} \), or time-on-stream. In addition, there was no significant effect of K+-promotion on activation energy (ca. 29 kcal/mol) or methanation reaction orders with respect to CO and H2 (-0.1-0.0 and 0.4-0.6, respectively). However, there was a decrease in the number of methane-destined surface intermediates upon promotion as determined by steady-state isotopic transient kinetic analysis (SSITKA). All these observations lead 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 hydrogenation. Between 503 and 552 K, the activation energy and reaction orders with respect to H2 and CO were also not affected by K+. However, the catalyst with a K+/Pt ratio of 0.1 showed the highest methanation activity. In this lower temperature range and for all the catalysts, the apparent activation energies were also found to be lower, 18 vs. 29 kcal/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 observed in the higher temperature range (ca. -0.1-0.0). These results suggest, that, in the low temperature range and for low loadings of K+, K+ affects the rate-determining step resulting in a rate increase greater than the decrease due to the blockage effect. Thus, K+ serves as a rate promoter at low reaction temperatures while its only effective function is site blockage at higher temperatures. |
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