The effects of alkali promoters on the dynamics of hydrogen chemisorption and syngas reaction kinetics on Ru/SiO2 surfaces

Abstract

The dynamics of chemisorbed hydrogen on unpromoted and promoted Ru/SiO 2 catalysts was studied by means of single pulse and selective excitation 1 H NMR spectroscopy. Dynamic NMR studies indicated a reduced mobility of hydrogen in the presence of alkali promoters (Na and K) at high loadings (65 atomic %). On unpromoted Ru/SiO 2 catalysts, the line due to hydrogen-on-metal was homogeneously broadened at pressures above 0.5 Torr H 2 . Similar behavior was observed on promoted Ru/SiO 2 catalyst with 66 atomic % K. The line due to hydrogen-on-metal was inhomogeneously broadenced at least up to 200 Torr H 2 on promoted Ru catalyst with 66 atomic % K. A similar behavior was observed on a 65 atomic % Na promoted catalyst up to pressures of 735 torr and temperatures up to 630 K. A homogeneous lineshape indicates that there is fast exchange of hydrogen among different Ru particles whereas an inhomogeneous line indicates that such an inter-particle motion is restricted. The exchange parameter of hydrogen motion was determined from a multisite exchange model. It was determined that this exchange parameter on unpromoted Ru catalysts was 20 fold higher than that on a K/Ru catalyst with 66% K at a given hydrogen pressure. The mechanism for this inhibited mobility was postulated as follows: Alkali blocked the low coordination sites needed for dissociative chemisorption of hydrogen and the kinetics of adsorption-desorption was thereby slowed down significantly. Good quantitative agreement was obtained when the exchange parameters are used to determine the effects of alkali promoters on olefin selectivities in Fischer Tropsch synthesis reaction.