To the Role of Catalyst-Solution Interface Properties in Catalytic Performance of Supported Pd Nanoparticles in Liquid-Phase Hydrogenation of Maleic Acid to Succinic Acid
Solid-solution interface structure may affect behaviour of heterogeneous chemical reactions, especially in the case of the reactions with electrolytes in polar solvents: under the conditions the solid surface usually forms an electric double layer as viscous, electrolyte-rich, nanostructured solvent film, which governs transportation of the reagents to the surface.
The present work proves that the liquid-phase hydrogenation of maleic acid (MA) to succinic one over palladium catalysts supported on selected non-porous supports (carbons, oxides, salts, 2-14 m2/g), which is structure- and support-insensitive in alcohol medium, demonstrates the opposite kinetic features in water. XPS studies of Pd/α-Al2O3, Pd/ZrO2, Pd/Sibunit and Pd/FW-70 catalysts allowed us to conclude that the electronic state of supported Pd particles being formed after the reduction of deposited PdO with hydrogen at 100-120°C doesn't vary with the support nature. No straight correlation between IEPs of the supports and catalytic activities of the corresponding catalysts was found, however, pH of MA aqueous solution considerably alters the reaction rate suggesting the ionization processes to play an important role in Pd catalytic performance. Moreover, every type of catalysts possesses its own limiting value of hydrogen consumption rate in aqueous MA, as if the process was governed by diffusion limitations at the vicinity of the catalyst surface. The latter is confirmed also by depressed activities of palladium on porous supports (Fig. 1 left). The obtained catalytic data are explained by the formation of a viscous solvent film at the catalysts surfaces which structure and permeability for reagents depend on the solvent and support nature.