| An infrared cell acting as a differential, single-pass, plug flow reactor was built in order to obtain IR spectra and methanation rates simultaneously. Under steady-state reaction conditions, kinetic data on the methanation reaction between CO and H(,2) on SiO(,2)-, Al(,2)O(,3)-, SiO(,2)-Al(,2)O(,3)-, and TiO(,2)-supported Pt catalysts agreed well with results from a separate glass reactor system using larger catalyst loadings. The latter reactor was also used to obtain kinetic parameters and study more Pt catalysts. Catalysts were characterized by CO and H(,2) chemisorption and x-ray line broadening measurements.;No significant crystallite size effects were seen for particle sizes over a range of 1.2 nm to 160 nm. The support altered turnover frequencies more than 300-fold:;TiO(,2) Al(,2)O(,3) SiO(,2)-AlO(,3) SiO(,2). TiO(,2)-supported Pt was the most active Pt methanation catalyst, with turnover frequencies up to 0.076 sec('-1). IR spectra at 300(DEGREES)K indicated that H(,2) chemisorption is much more competitive with CO on TiO(,2)-supported Pt because hydrogen displaced CO from the Pt surface. For the SMSI-Pt/TiO(,2) catalyst, a combination of very low CO coverage, very high activity, and little IR-active CO was found under reaction conditions, implying that density of "active sites" was low.;A Langmuir-Hinshelwood kinetic model applicable for all Pt catalysts was proposed. It was assumed that an adsorbed CO molecule reacting with adsorbed hydrogen to form a surface carbon species was the rate-determining step in the methanation reaction. Results obtained from transition-state theory analysis indicated that density of active sites is lower than density of surface Pt atoms. It was indicated that the support markedly alters the adsorptive and catalytic behavior of Pt, which may be a result of lower heats of adsorption for H(,2) and CO.;Two major species of adsorbed CO were present on the Pt surface, with HF bands between 2050-2090 cm('-1) assigned to linearly bonded CO and LF bands between 1780 and 1860 cm('-1) to bridged-bonded CO. Separate extinction coefficients ((epsilon)) and integrated absorption intensities (A) at room temperature for linear and bridged CO were calculated for the first time. For all catalyst systems, the linear species had higher (epsilon) and A values, greater peak intensities, and higher binding energies than bridged species. TiO(,2)-supported Pt catalysts had higher (epsilon)(,HF) and A(,HF) values than other Pt catalysts: A(,HF) = 2.7 x 10('8) cm mole('-1) and (epsilon)(,HF) = 1.8 x 10('6) cm('2) mole('-1) for SMSI-Pt/TiO(,2) catalysts. All (epsilon)(,HF) and A(,HF) values for adsorbed CO were 2-3 times larger than those for gas-phase CO. |
