Dehydrogenation of isopropyl alcohol on carbon-supported Pt and Cu–Pt catalysts

Abstract

The kinetics of isopropyl alcohol (IPA) dehydrogenation over carbon-supported Pt and three bimetallic Cu Pt catalysts were examined and compared with previous results for carbon-supported Cu. Adsorption of H 2 and CO, dissociative chemisorption of N 2O, and the titration of adsorbed O atoms by H 2 and CO were used to determine surface compositions and metal dispersion. Monometallic platinum catalysts had a higher specific activity than monometallic Cu catalysts and the bimetallic catalysts; for example, at 448 K and 14 Torr IPA, for an activated carbon heat treated at 1223 K as the support, the turnover frequency (TOF) on the Pt catalyst was 0.11 s −1, the TOF on the Cu catalyst was 0.020 s −1, and the TOFs on the three bimetallic catalysts were between 0.006 and 0.043 s −1. Reaction orders were typically between 0 and 1 2 for IPA and slightly negative for acetone and H 2. Increasing the Cu loading reduced the number of surface Pt atoms, and the kinetic behavior became similar to that of monometallic copper catalysts. The apparent activation energy for acetone production from IPA on Pt/C was 6.8 kcal mol −1, compared with 21 kcal mol −1 for the monometallic Cu/C catalysts. Apparent activation energies for the Cu Pt/C catalysts (7–9 kcal mol −1) were comparable to those of Pt/C at lower temperatures and decreased further to ∼ 3 kcal mol −1 above 443 K. This decrease is not due to diffusional limitations and can be explained by a decrease in the surface coverage of IPA. A Langmuir–Hinshelwood model invoking cleavage of the alcohol O H bond to form a surface isopropoxide species as the rate-determining step fit the data well and accounted for the shift in the apparent activation energy. Physically meaningful values of enthalpies and entropies of adsorption were obtained from the fitting parameters contained in the L-H rate expression. Additional evidence for adsorbed IPA and acetone was provided by infrared spectra of the catalysts obtained under reaction conditions.