Hydrogen spillover and substrate–support hydrogen bonding mediate hydrogenation of phenol catalyzed by palladium on reducible metal oxides

Abstract

Substrate–support interactions play an important role in the catalytic hydrogenation of phenolic compounds by ceria-supported palladium (Pd/CeO₂). Here, we combine surface contrast solution NMR methods and reaction kinetic assays to investigate the role of substrate–support interactions in phenol (PhOH) hydrogenation catalyzed by titania-supported palladium (Pd/TiO₂). We show that PhOH adsorbs on the catalyst via a weak hydrogen-bonding interaction between the –OH group of the substrate and one oxygen atom on the support. Interestingly, we observe that the addition of 20 mM inorganic phosphate results in a ∼2-fold destabilization of the PhOH–support interaction and a corresponding ∼2-fold inhibition of the catalytic reaction, suggesting an active role of the PhOH–TiO₂ hydrogen bond in catalysis. A comparison of the data measured here with the results previously reported for a Pd/CeO₂ catalyst indicates that the efficiency of the Pd-supported catalysts is correlated to the amount of PhOH hydrogen bonded to the metal oxide support. Since CeO₂ and TiO₂ have similar ability to uptake activated hydrogen from a noble metal site, these data suggest that hydrogen spillover is the main mechanism by which Pd-activated hydrogens are shuttled to the PhOH adsorbed on the surface of the support. Consistent with this hypothesis, Pd supported on a non-reducible metal oxide (silica) displays negligible hydrogenation activity. Therefore, we conclude that basic and reducible metal oxides are active supports for the efficient hydrogenation of phenolic compounds due to their ability to hydrogen bond to the substrate and mediate the addition of the activated hydrogens to the adsorbed aromatic ring.