Acetic Acid Reduction by H_2 on Bimetallic Pt-Fe Catalysts

摘要

Vapor-phase acetic acid hydrogenation was studied over a family of supported Pt-Fe catalysts. These catalysts were characterized by Mossbauer spectroscopy, successive H_2-O_2-H_2-O_2 titration cycles at 300 K, and DRIFTS (diffuse reflectance FTIR spectroscopy) to determine the Fe phases present during the titration reactions as well as prior to and under reaction conditions, to count surface metal atoms and estimate the average surface composition of the bimetallic particles, and to observe surface species formed after acetic accid adsorption, respectively. Although the metallic mole fraction of Pt in the bimtallic Pt-Fe/SiO_2 catalysts varied from 0.04 to 0.64, the from 0.39 to 0.70. The addition of small amounts of Pt to Fe/SiO_2 catalysts (i) increased Fe reducibility during the reduction pretreatment, (ii) enhanced activity more than 10-fold and turnover frequencies 10-to 100-fold, (iii) eliminated the induction period,(iv) lowered the apparent activation energy by 8-10 kcal/mol, and (v) still maintained a high selectivity to accetaldehyde of over 70%. The addition of Pt to an Fe/C catalyst prevented the severe deactivation that has been associated with iron carbide formation. Under reaction conditions, the best bimetallic catalyst contained both Fe~O and Fe~(2+) phases,which is a combination that seems to be required for stable selective acetaldehyde formation. A model consistent with our previous studies of Pt and Fe catalysts, which invokes one type of active site on a reduced metal surface and another type on a metal oxide phase, successfully describes this reaction on a bimetallic catalyst.It is proposed that the reaction sequence involves a rate-determining step between a hydrogen atom from the metallic site and an acyl species on the FeO surface.