| The synthesis of hydrocarbons from CO and H(,2) provides one route to convert heavy, hydrogen-deficient materials into clean fuels and chemicals. Earlier studies had shown that both metal crystallite size effects and metal-support interactions may possibly alter the rate of methanation over Pt and Pd surfaces. This study was conducted to determine the effects of metal-support interactions and crystallite size on the state of CO adsorption and on the catalytic behavior of Pd catalysts. The possibility that certain forms of adsorbed CO were more reactive was also to be examined.;catalysts were characterized by CO and H(,2) chemisorption and by x-ray diffraction measurements. IR spectra and kinetic data were simultaneously obtained in the IR reactor, and all kinetic data were checked in another differential reactor system using larger samples. This latter reactor was also used to obtain kinetic parameters and to study additional Pd catalysts. Infrared spectra of CO adsorbed on these supported Pd catalysts revealed the presence of at least two forms of CO on the Pd surface, which is.;consistent with previous studies, and bands above 2000 cm('-1) were assigned to linearly bonded CO while the bands between 2000 cm('-1) and 1880 cm('-1) were assigned to multiply coordinated CO. Separate extinction coefficients at room temperature for both linear and bridged CO have been calculated for the first time, and the average values are (epsilon)(, ) = 7.5 x 10('-19) molecule('-1) cm('2) and (epsilon)(,b) = 6.2 x 10('-18) molecule('-1) cm('2).;No significant crystallite size effects were apparent for particles larger than (TURN) 3nm, but the support utilized altered turnover frequencies more than 100-fold in the following manner TiO(,2) > Al(,2)O(,3) (TURNEQ) SiO(,2)-Al(,2)O(,3) > SiO(,2). TiO(,2)-supported Pd is the most active Pd methanation catalyst studied to.;For this study, a unique IR cell, which acts as a differential single-pass, plug-flow reactor, was designed and built. Palladium was dispersed on four different metal oxides--SiO(,2), (eta)-Al(,2)O(,3), SiO(,2)-Al(,2)O(,3) and TiO(,2)--and all.;date with a turnover frequency as high as 46 x 10('-3) s('-1). IR spectra at 300 K indicated that H(,2) chemisorption is much more competitive with CO on TiO(,2)-supported Pd. For the SMSI Pd/TiO(,2) catalyst, a combination of very low CO coverage and high activity was found, which is quite unusual, and no IR visible CO was detected under reaction conditions. This result implies that the density of "active sites" on the Pd surface is very low.;A Langmuir-Hinshelwood kinetic model for the methanation reaction on these Pd catalysts was proposed which satisfactorily describes the kinetic data and is also consistent with the interpretation of the IR spectra. The hydrogen-assisted rupture of the C-O bond was proposed as the rate-determining step in the methanation reaction. Results obtained from a transition-state theory analysis also indicated that the density of active sites is much lower than that of surface Pd atoms. All results indicated that the support can have a marked influence on the adsorption and reactivity of CO, and this may be due to a lower heat of adsorption for both CO and H(,2). |
