Structure sensitivity in the formic acid-driven catalytic transfer hydrogenation of maleic acid to succinic acid over Pd/C catalysts

A series of carbon-supported palladium catalysts (Pd/C) with average particle size in the range from 3 to 7 nm was prepared by varying the Pd loading. These materials and the influence of their physicochemical properties in the catalytic transfer hydrogenation (CTH) of maleic acid (MAc) to succinic acid (SAc) using formic acid (FAc) as H₂ donor were evaluated. The chemical, textural, and surface properties were studied by a number of characterization techniques (ICP-OES, XRD, TEM, and XPS). It was found that the intrinsic rate of SAc formation per surface Pd atom (turn-over frequency, TOF Pd_sur) is structure-sensitive. The TOF Pd_sur rate increases with the particle size following a volcano-type curve, reaching a maximum for an average particle size of ca. 6 nm. Assuming a cuboctahedral shape with a cubic close-packed structure, an approximation frequently adopted for Pd particles, it was found that, for the series of catalysts, neither the calculated TOF of Pd surface atoms at low coordination sites nor at high coordination are constant. This suggests that no geometric effect is responsible for the structure-sensitivity. Among the different Pd species present in the catalysts (i.e., metallic Pd (Pd⁰), palladium carbide (PdC_x), and oxidized Pd), it was observed that the relative number of surface Pd⁰ sites also follows a volcano-type curve, which indicates that Pd⁰ sites are necessarily involved in the most active centers. For particles with an average size larger than 4 nm, the TOF rate of surface Pd⁰ sites was constant and in the range of 0.12 s⁻¹. For smaller sizes, a more accurate determination of their Pd⁰ surface concentration is required to obtain reliable surface Pd⁰ TOF rates.