| Acetic acid was chosen to probe the kinetic behavior of carboxylic acid hydrogenation over supported Pt, Fe, and Pt-Fe catalysts in an effort to develop a better fundamental understanding of the structural and mechanistic details of this reaction. The direct reduction of acetic acid by hydrogen was studied in the vapor phase under conditions of 423–573 K, 100–700 Torr H2, and 7–50 Torr acetic acid in a differential, fixed-bed reactor. In this study of Pt dispersed on TiO2, η-Al2 O3, Fe2O3, and SiO2, activity and selectivity during acetic acid hydrogenation were found to be strongly dependent on the oxide support, with the titania-supported Pt being the most active catalyst. Based on a DRIFTS study coupled with TPD and TPR experiments, acetic acid hydrogenation was proposed to occur via addition of hydrogen atoms from the Pt surface to an acyl species adsorbed on an adjacent reaction site on the oxide surface, which serves as the major route to produce acetaldehyde and, subsequently, ethanol.; Fe catalysts were highly selective for acetic acid hydrogenation to acetaldehyde, and the reaction typically went through an induction period of several hours before stable catalytic activity was achieved. Iron crystallite size was found to affect the catalytic behavior rather significantly because the TOF increased from 3 to 60 ms−1 and the apparent activation energy decreased from 27 to 16 kcal/mol as the average iron crystallite size increased from 10 to 4000 nm. Fe particles, as characterized using Mössbauer spectroscopy, underwent significant phase transformations under reaction conditions leaving a mixed Fe-FeO phase within the particles at steady state, and it was proposed that acetic acid hydrogenation was a process involving both metallic and oxidic phases of iron, with acetate species formed after adsorption of acetic acid on FeO as the active intermediate.; Addition of Pt to Fe to form bimetallic Pt-Fe particles resulted in activity enhancement for acetic acid hydrogenation, with specific activities equal to or greater than those with Pt/TiO2, while high selectivity for acetaldehyde was maintained, particularly by catalysts with Pt/Fe ratios less than 0.5. Besides providing sites that facilitate H2 dissociation, Pt-Fe clusters also appeared to be responsible for creating sites for acetic acid adsorption to form reactive acyl species, and thereby giving rise to hydrogenation of adsorbed acyl species as the main pathway to produce acetaldehyde.; The present study provided evidence derived from kinetic modeling that acetic acid hydrogenation over supported Pt, Fe, and bimetallic Pt-Fe catalysts can be described by a Langmuir-Hinshelwood-type mechanism invoking two types of sites, one on the metal surface, whether it is Pt or Fe, to dissociate H2, and the other on the oxide surface to molecularly adsorb acetic acid and then produce reactive surface acyl or acetate species. |
