Pt–Sn catalysts supported on highly-dispersed ceria on carbon: Application to citral hydrogenation

Abstract

A novel kind of bimetallic Pt–Sn catalysts supported on carbon and promoted with highly-dispersed ceria were prepared by a co-impregnation method and tested in the liquid phase hydrogenation of citral after reduction at low (473 K) and high (773 K) temperatures. They were characterized by N 2 adsorption at 77 K, X-ray diffraction, temperature-programmed reduction (TPR), transmission electron microscopy (TEM) and CO chemisorption at room temperature. Results from N 2 adsorption, XRD and TEM indicated that impregnation method used here led to a highly-dispersed ceria on carbon. Conversely to massive CeO 2, TPR profile of CeO 2/C showed two broad overlapped peaks at low temperatures (803 and 903 K), which are indicative of the easier reduction of the supported promoter. Addition of platinum partially promotes the reduction of surface ceria, this indicating a close metal–promoter interaction. Ceria reducibility was increased with tin addition. The reduction treatment at 773 K produced a strong decrease in the CO adsorption capacity in all cases, although this loss was more relevant for the monometallic catalyst. After reduction at low temperature (473 K), the monometallic Pt/CeO 2/C was the most active catalyst, while the presence of tin reduced the catalytic activity. Reduction at high temperature (773 K) produced the decrease of activity in the case of monometallic sample while bimetallic catalysts, specially the one with Sn/Pt = 0.25, increased the citral conversion values. Reduction treatment at 773 K also increased selectivity to unsaturated alcohols (geraniol and nerol) of bimetallic samples, which presented in all cases higher values than their monometallic counterpart. The onset of new Pt–CeO x and Pt–SnO x sites after the reduction process is proposed to explain the increase in activity and selectivity of bimetallic catalysts.