Aqueous phase hydrogenation of phenol catalyzed by Pd and PdAg on ZrO₂

The impact of particle size of ZrO₂-supported Pd and of alloying with Ag was explored for hydrogenation of phenol in aqueous phase. Kinetic assessments were performed in a batch reactor, on monometallic Pd/ZrO₂ samples with different Pd loadings (0.5%, 1% and 2%), as well as on a 1% PdAg/ZrO₂ sample. The catalytic activity normalized to accessible Pd (turnover frequency, TOF) increased with the particle size of Pd. Reaction orders in phenol and H₂ and lower activation energies suggest that smaller particles bind the reacting substrates more strongly, leading to higher surface coverages by phenol. However, surprisingly, smaller Pd particles exhibited lower TOFs. The lower activity of the small Pd particles is attributed to lower activation entropies for the strongly bound species. The presence of Ag increased the catalyst activity by decreasing the apparent energy of activation and increasing the coverages of phenol and H_2, without negatively affecting the activation entropy.