Effect of Activated Carbon Pretreatment Methods on the Performance of Pd/C Catalysts in the Hydrogenation of 4-Carboxybenzaldehyde

A commercially available shaped activated carbon (AC) was pretreated with HNO3, KOH, or CO2 and used to prepare catalysts with a nominal Pd loading of 0.5 wt.% relative to the dry support mass for 4-carboxybenzaldehyde (4-CBA) hydrogenation. The treatments produced distinct changes in pore structure, surface oxygen speciation, thermal behavior, Pd particle-size distribution, and adsorption properties. HNO3 treatment increased the relative C–O and carboxylic oxygen contributions and yielded the largest proportion of small Pd particles, with particles smaller than 5.7 nm accounting for more than 55% of the population. KOH activation produced the most extensive development of the accessible pore structure, increasing the external surface area from 238 to 767 m2·g−1 and the mesopore volume from 0.188 to 0.405 cm3·g−1. CO2 activation produced moderate pore development, whereas Pd/CO2-AC exhibited a large-particle tail extending to approximately 15 nm. Under identical hydrogenation conditions, Pd/HNO3-AC and Pd/KOH-AC gave comparable 4-CBA conversions of 85.7% and 84.3%, respectively, whereas Pd/AC and Pd/CO2-AC gave conversions of 70.2% and 73.2%. The different pretreatments produced distinct combinations of pore structure, surface oxygen speciation, Pd particle-size distribution, and ex situ adsorption behavior that were associated with the observed catalytic performance. These results show that no single measured property alone accounts for the conversion trend.