Catalytic selective hydrogenation of acetic acid to acetaldehyde over the surface of the iron shell on Pd-Fe alloy nanoparticles

Since hydrogenation of acetic acid readily leads to ethanol formation, it is challenging to selectively obtain acetaldehyde in a high yield. The present study demonstrates the highly-selective synthesis of acetaldehyde by the hydrogenation of acetic acid catalyzed by the iron surface on Pd-Fe alloy nanoparticles derived from Pd/Fe2O3. The catalyst from Pd/Fe2O3 with low Pd loading amount provoked the deposition of carbonaceous species, including iron carbides, in the prolonged reaction, resulting in a low acetaldehyde yield. On the contrary, Pd/Fe2O3 with high Pd loading amount (28-44 wt%) prevented the formation of iron carbides. It led to the formation of aldehyde in an excellent yield, that was maintained even after 200 h. Further Pd loading increased the ethanol formation. In situ XAFS measurements showed that the reductive pretreatment under hydrogen flow of Pd/Fe2O3 formed the core-shell type Pd-Fe alloy nanoparticles. Acetic acid would convert to acetaldehyde with concomitant oxidation of the surface of the metallic iron shell, and H-2 would reduce the thus-partially oxidized shell to regenerate the initial metallic surface.